Hybrid electrolytes for lithium metal batteries

Journal of Power Sources 392 (2018) 206–225

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Hybrid electrolytes for lithium metal batteries Marlou Keller a b

a,b

, Alberto Varzi

a,b

, Stefano Passerini

T

a,b,∗

Helmholtz Institute Ulm (HIU), Helmholtzstrasse 11, 89081 Ulm, Germany Karlsruhe Institute of Technology (KIT), P.O. Box 3640, 76021 Karlsruhe, Germany

H I GH L IG H T S

G R A P H I C A L A B S T R A C T

hybrid electrolytes • Polymer/inorganic for lithium metal batteries. and characterization • Preparation methods of hybrid electrolytes. conduction mechanism in hybrid • Ionic electrolytes. of hybrid electrolytes Li metal • Impact cells energy performance.

A R T I C LE I N FO

A B S T R A C T

Keywords: Hybrid solid electrolyte Ionic conduction mechanism Polymer-inorganic interface Lithium metal All-solid-state battery Energy density

This perspective article discusses the most recent developments in the field of hybrid electrolytes, here referred to electrolytes composed of two, well-defined ion-conducting phases, for high energy density lithium metal batteries. The two phases can be both solid, as e.g., two inorganic conductors or one inorganic and one polymer conductor, or, differently, one liquid and one inorganic conductor. In this latter case, they are referred as quasisolid hybrid electrolytes. Techniques for the appropriate characterization of hybrid electrolytes are discussed emphasizing the importance of ionic conduction and interfacial properties. On this view, multilayer systems are also discussed in more detail. Investigations on Lewis acid-base interactions, activation energies for lithium-ion transfer between the phases, and the formation of an interphase between the components are reviewed and analyzed. The application of different hybrid electrolytes in lithium metal cells with various cathode compositions is also discussed. Fabrication methods for the feasibility of large-scale applications are briefly analyzed and different cell designs and configurations, which are most suitable for the integration of hybrid electrolytes, are determined. Finally, the specific energy of cells containing different hybrid electrolytes is estimated to predict possible enhancement in energy with respect to the current lithium-ion battery technology.

1. Introduction The early development of lithium metal batteries in the 1970's has strikingly evidenced the incompatibility of the highly reactive lithium metal anode with liquid electrolytes based on flammable organic solvents. Indeed, upon repeated cycling, the formation of lithium dendrites led to internal short circuit of the cells with risk of thermal runaway and, ultimately, explosion. In the 1990s, lithium metal was successfully ∗

replaced by the lithium-ion battery (LIB) technology, whose rockingchair mechanism avoids the plating of metallic lithium thus effectively eliminating the uncontrolled Li dendrite growth at the negative carbonaceous electrode in normal operation conditions. Since then, graphite is the state-of-the-art anode material, offering reversible intercalation at relatively low potentials (< 0.2 V vs. Li/Li+). Although, a substantial reduction of energy density is the price to pay for the improved safety due to the one order of magnitude lower capacity of

Corresponding author. Helmholtz Institute Ulm (HIU), Helmholtzstrasse 11, 89081 Ulm, Germany. E-mail addresses: [email protected] (M. Keller), [email protected] (A. Varzi), [email protected] (S. Passerini).

https://doi.org/10.1016/j.jpowsour.2018.04.099 Received 26 February 2018; Received in revised form 19 April 2018; Accepted 27 April 2018 Available online 06 May 2018 0378-7753/ © 2018 Elsevier B.V. All rights reserved.

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graphite compared to Li metal (theoretically, 372 mAh g−1 vs. 3860 mAh g−1). In spite of this, LIBs offer by far the best performance among all rechargeable battery chemistries operating at room temperature. Nowadays, however, the increasing demand of powerful consumer electronics, stationary storage and electric vehicles drives the development of evermore high-energy density batteries. Among the next generation battery technologies, safe lithium metal batteries based on a solid-state approach appear to be the most appealing systems. Although inorganic solid electrolytes (ISEs) notoriously suffer from poor ionic conductivity compared to liquids, recent research efforts have yield to a variety of inorganic conductors with extremely high conductivity at room temperature [1–5]. NASICON-like materials [6–8] (e.g., Li1.3Al0.3Ti1.7(PO4)3), perovskites [9,10] (e.g., La0.57Li0.29TiO3), garnets [11–17] (e.g., Li7La3Zr2O12) and sulfidic electrolytes [2,18–23] (e.g., Li7P3S11) have been intensively explored. NASICON-like materials are stable in atmosphere, show high oxidation voltages and high ionic conductivities but are brittle and unstable with lithium metal [6,24–26]. Perovskites reveal high mechanical strength and high oxidation potential, but high grain boundary resistances [27] and, furthermore, they are unstable in contact with lithium [3,28–30]. Garnets are stable in contact with lithium metal, but react with moisture to form decomposition products depositing at their interface [31–33]. Sulfidic electrolytes reveal the highest ionic conductivities but, at the same time, also the most severe sensitivity towards air and moisture [2,18,20]. Issues such as high resistances at the electrode/electrolyte interface and engineering challenges like the large scale manufacturing of such batteries still hinder their imminent application. Differently, the easily processable solid polymer electrolytes are already applied with success in lithium metal batteries [34], but must operate at elevated temperatures (above 60 °C) due to ionic conductivity limitations. Hence, the necessary continuous heat supply limits their efficiency. Even more detrimental, the most promising polymer, polyethylene oxide (PEO), is electrochemically unstable at high potentials, limiting the operative voltage and thus the energy density of such systems. The development of hybrid electrolytes comprising inorganic and organic ionic conductors might be the ultimate solution to benefit from each component's virtues. The combination of inorganic conductors with ionic liquids, organic liquids or solid polymer electrolytes (SPEs) may provide many advantages such as, for example, improved charge transfer at the electrode/electrolyte interface [35–37]. Additionally, the flexibility of polymers would ensure easy manufacturing and buffer volume changes occurring during cell operation, while maintaining intimate contacts between the cell components.

standard separators [45], they do not allow the use of lithium metal as they cannot suppress the reaction between the liquid electrolyte and Li, leading to dendrite formation. Therefore, they are out of the topic of this perspective article. The aim of this review is to describe the different classes of hybrid electrolytes suitable for lithium metal batteries, and to highlight the characterization techniques most suited to study such hybrid systems. Special focus is put on the ion conduction mechanism, which is still under discussion, and on interfacial properties that severely influence the overall battery performance. Also, the application of hybrid electrolytes in cells with a cathode and a lithium anode is reviewed and discussed. The fabrication methods for large scale application of hybrid electrolytes are discussed, too. Finally, energy density calculations are performed to show that, compared to state-of-the-art LIBs, higher energy densities are achievable by the use of hybrid electrolytes in combination with lithium metal anodes. 3. Hybrid electrolytes and their synthesis 3.1. Hybrid all-solid-state electrolytes 3.1.1. Inorganic/inorganic Inorganic/inorganic hybrid electrolytes such as those composed of, e.g., soft sulfide glasses and stiff oxides, can be made to take advantages of their different characteristics, e.g., avoid sintering at high temperature [38,47]. They are prepared by ball milling of compliant nanoporous β-Li3PS4 (β-LPS) and hard oxide garnets [38,47] in spite of their different nature, e.g., the oxides are hard acids and β-LPS is a soft base [47]. Indeed, their chemical stability was experimentally confirmed by X-ray diffraction (XRD) [47]. However, the eventual presence of a submicrometric amorphous interphase due to reactions between the different electrolytes cannot be excluded and would need further investigation, e.g., by means of X-ray photoelectron spectroscopy. The combination of hard oxides and compliant β-LPS enables cold pressing of dense pellets even when using only 10 wt% of the softer component (i.e., β-LPS). The highly energetic sintering step to densify pure Li7La3Zr2O12 (LLZO) pellets is therefore avoided since β-LPS can effectively fill the porosity [47]. Interestingly, the ionic conductivity of this hybrid electrolyte can exceed the conductivity of the individual parental materials. Such phenomenon is attributed to the space-charge layer at the β-LPS/LLZO interface, leading to a redistribution of interstitials and vacancies across the interface [38]. The choice of highly conductive parental materials [38], as well as their volume ratio [47], can significantly influence the overall ionic conductivity of the hybrid. The fraction of the oxide should not largely exceed 30 wt% (unfortunately, volume ratios are rarely reported) to avoid the less conductive oxide blocking the motion of lithium ions thus decreasing the overall ionic conductivity of the hybrid electrolyte [47]. In fact, two opposite effects are present: (i) the space-charge effect effectively enhancing the ion mobility and (ii) the blocking effect of the less conductive inorganic electrolyte. The combination of sulfides and oxides into a hybrid electrolyte appears very promising for enhanced ionic conductivities, improved interfacial stability with lithium [38,47] and ease of processing. However, to the best of our knowledge, only two reports are available on this topic so far. The integration of the high-density oxide (5.1 g cm−3) into the low-density sulfide (2.1 g cm−3) brings the disadvantage of an increased weight with to the sulfidic electrolyte alone, which impacts the energy density of the whole battery (see section 9, Specific Energy Calculations). Indeed, the solid electrolyte membrane should be as thin as possible to limit the weight of inactive materials in the cell. Additionally, it is still to be proved if a large scale processing of such thin solid membranes is feasible, since so far only pellets were investigated at laboratory scale. A possible approach to tackle this issue is to use a slurry coating process analogous to the manufacturing of sheet-type cells, however, this was reported to be successful with sulfidic

2. Classification of hybrid electrolytes The term “hybrid electrolyte” covers a very broad class of composites. These could be divided into two main categories. Hybrid all-solidstate electrolytes (HSEs) include combinations of (at least) two, welldefined, lithium-ion conductive solid phases. These can be both inorganic [38], or a combination of inorganic and polymeric phases [39]. Differently, hybrid quasi-solid electrolytes (QSEs) integrate an inorganic conductor with an ionic liquid [35,40] or organic liquid [41] phase. An overview of the possible hybrid electrolyte combinations is given in Fig. 1. It should be mentioned that gel electrolytes, which are constituted by a liquid electrolyte (salt + solvent) embedded in a polymeric matrix could also be considered as “hybrid”. However, in these systems the ion conduction occurs only in the liquid phase, whereas the polymer simply ensures the mechanical stability [42]. As they are already extensively discussed in several previous reviews [42,43], these will not be covered herein. Soaked hybrid membranes comprising an inorganic conductor, an inert polymer and a liquid electrolyte will also be omitted [44–46]. In fact, although these hybrid electrolytes have been applied in lithiumion batteries, showing superior safety performance compared to 207

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Fig. 1. Overview of single electrolytes and combination of these resulting in different types of hybrid electrolytes. The dimensions are not true to scale to highlight the different electrolytes. In a practice, the cathode would be substantially thicker than the electrolyte layer.

components to avoid the evolution of large interfacial resistances caused by resistive layers formed by decomposition products. If possible, solvent-free preparation methods would be preferred to avoid altogether the introduction of liquid components. Alternatively, the hybrid electrolytes should be carefully dried under high vacuum conditions in order to minimize the amount of trapped solvent molecules. A proper comparison of performance of this class of hybrid electrolytes is made difficult by the lack of information available in literature. For example, the densities of the single materials is not always reported, making difficult to calculate volume fractions from weight fractions (and vice versa), as well as to estimate the concentration of lithium ions in the two components (polymer and inorganic electrolytes). Recently, the role of nanostructures and alignment of the inorganic conductors in the polymer electrolyte matrix are under intense investigation, since vertically aligned inorganic nanowires were produced by ice-templating [64] or electrospinning [66]. Differently, three dimensional nanostructured pre-percolated frameworks can be derived from hydrogels [67]. The polymer matrix can be then infiltrated into the inorganic conductor network with the help of a solvent that is afterwards evaporated [51]. However, these methods have low production rates and electrospinning typically involves unfriendly solvents

electrolytes only [48], and required the addition of a binder that may detrimentally affect the ionic conductivity. Also an in-depth investigation of the chemical interfacial stability of this kind of hybrid electrolyte with cathode materials has not been conducted yet. In fact, it is reasonable to expect these materials being limited by the stability of the sulfidic component [30].

3.1.2. Inorganic/polymer Several hybrid electrolytes, composed of a solid polymer electrolyte and an inorganic conductor, have been investigated [39,49–67]. The preparation methods are rather easy and include their intimate mixing in an inert solvent followed by casting [56], or a solvent-free mixing followed by hot-pressing [39]. In both cases the careful pre-drying of all components is crucial, and, preferably, the materials are processed in dry atmosphere, such as a glovebox or dry room. In the available literature, however, the processing parameters like environment (air [68], dry room or glove box), the materials pre-drying conditions and the solvent-removal steps are often omitted or described with insufficient details [49]. For PEO-based electrolytes, e.g., solvent and water residues can lead to overestimate the ionic conductivity of the hybrid [57,69]. Also, the lithium metal anode requires extremely dry 208

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(i.e., free of porosity and grain boundaries) with a Young's (elastic) modulus (which can be determined from the linear high strain region in a stress-strain compression experiment [64], or from linear rheology measurements [78]) higher than 6 GPa [79], given that it adheres to the lithium metal surface [80,81]. However, the adhesiveness of solid electrolytes vanishes when the modulus exceeds a few MPa [82] and the poor adhesion can lead to increasing interfacial resistances upon cycling. Polymer electrolytes show very good adhesion but low moduli (usually below 0.1 GPa) [83]. On the other hand, the moduli of inorganic solid electrolytes usually exceed 20 GPa, but they show poor adhesion to lithium metal [83]. Thus, hybrid electrolytes might provide the optimal compromise of high modulus and good surface adhesion to Li metal to effectively suppress Li dendrites. Indeed, the Young's modulus of a hybrid electrolyte is always higher than that of the pure polymer [64], but lower than that of the inorganic component [78,83]. Even without reaching the theoretical dendrite suppression criterion (Young's modulus of 6 GPa [79]), a glass-polymer hybrid electrolyte was reported to have substantially improved adhesion properties compared to pure sulfidic glass [78]. Fig. 2b depicts mechanical stability and adhesion properties of different solid electrolytes. The mechanical stability increases with inorganic content whereas the adhesion properties follow the opposite trend. A smooth adhesive polymer electrolyte that is sandwiched between the lithium anode and the inorganic electrolyte might also effectively help to overcome these opposite trends.

for the precursor solution [70]. Although not very appealing for largescale production, these approaches might be helpful to understand the conduction mechanism within the hybrid electrolytes. 3.2. Hybrid quasi-solid electrolytes An inorganic ionic conductor wetted with a liquid electrolyte is defined as a hybrid quasi-solid electrolyte. The liquid component ensures improved interfacial contact with the inorganic materials, and enables fast Li+-ion transfer between the electrode material and the inorganic conductor. However, the chosen combination is critical as the two components must be chemically stable upon intimate contact [36,41,71,72]. The ionic conductivity of a quasi-solid electrolyte is usually higher than that of the parent inorganic component [35]. The opposite is true regarding the ionic conductivity of the liquid electrolyte [35], due to the increased tortuosity of the highly conductive, liquid phase. Both observation point out that the Li+-ion transfer between the liquid and solid phase is kinetically sluggish [73] and, consequently, Li+ ions mainly migrate through the maze of the liquid electrolyte. Different methods have been used to manufacture QSEs. Solid-liquid composites can be obtained by ball milling the components under argon atmosphere [35,71] or by a wet route [40], in which the components are mixed in an inert solvent that is removed afterwards. The latter is especially interesting when using ionic liquid-based electrolytes as they have an almost negligible vapor pressure. Thus, the inert solvent, having a high vapor pressure, can be easily removed. After mixing, pellets can be produced by simple cold-pressing of the mixture [35,41,71]. However, a residual porosity might still be present in the pellets [41]. It is important to mention that, often, the liquid component compromises the thermal stability and, thus, the safety of the hybrid electrolyte [41]. In order to avoid misleading expectations, the term “solid state” should be avoided if a liquid component is used in the system.

4.2. Thermal properties Differential scanning calorimetry (DSC) is the most common technique used to determine melting point (Tm) [39], glass transition temperature (Tg) and crystallization behavior of electrolytes (Fig. 2c). The crystallization degree is a particularly meaningful parameter for hybrid electrolytes containing polymers, e.g., PEO-containing electrolytes, since they offer substantially higher ionic conductivities in their amorphous state (i.e., at temperatures > Tm). The crystallization degree Δxc can be estimated from the melting enthalpy ΔHm of the electrolyte (which corresponds to the peak area) divided by the melting enthalpy of a 100% crystalline polymer, e.g., ΔHPEO ranges from 203 J g−1 [75] to 205 J g−1 [50,75]. Note that the heat flow should be normalized to the mass of the polymer inside the hybrid electrolyte [50,75]. The thermal stability (Fig. 2d) is analyzed by means of thermogravimetric analysis (TGA). The decomposition of the polymeric phase generally occurs below 300 °C, whereas most inorganic electrolytes are stable up to much higher temperatures [39,51]. The inorganic electrolyte, acting as a ridged backbone, helps to stabilize the integrity of the hybrid electrolyte at elevated temperatures [64]. Even if the polymer electrolyte component is degraded, the electrodes remain separated by the inorganic electrolyte, yet, depending on the initial content of the inorganic electrolyte. If liquid electrolytes are used, the thermal stability is, of course, significantly decreased.

4. Characteristics of hybrid electrolytes and methods for their determination A complementary characterization using different techniques is required to understand and optimize the performance of hybrid electrolytes. In the following subsections, the most relevant concepts and methods used to characterize hybrid electrolytes will be discussed. Morphological and structural characterization methods can be found in the supporting information (S-1). 4.1. Mechanical properties The electrolyte layer has to withstand enormous stress during cell manufacturing as well as upon electrochemical cycling, since the electrode materials can undergo severe volume changes. Stress-strain experiments (Fig. 2a) allow to evaluate the mechanical properties of a membrane, determine the tensile strength and the elongation-at-break. PEO-based polymer electrolytes show a tensile strength ranging from 0.32 MPa [62] to 1.5 MPa [75] and an elongation-at-break value between 140% [62] and 400% [75]. Note that both parameters are heavily dependent on the molecular weight of the PEO [76]. The addition of an inorganic filler (whether ionically conductive or not) increases the tensile strength but reduces the elongation-at-break [62,75]. A common strategy to increase the performance of hybrid electrolytes is the addition of plasticizer molecules such as polyethylene glycol [53,64] or succinonitrile (SN) [75,77]. While the elongation-at-break is improved, the tensile strength is necessarily decreased compared to the hybrid electrolyte without plasticizer [75]. Besides purely practical matters like ease of handling and manufacturing, the mechanical stability of the electrolyte membrane is fundamental to avoid the risk of Li dendrite growth. Theoretically, dendrite growth can be suppressed by a solid and dense electrolyte membrane

4.3. Chemical and electrochemical stability All electrolytes are thermodynamically unstable in contact with lithium, including solid electrolytes [30,74,84]. However, they are often kinetically stabilized, which experimentally results in a wider electrochemical window [74]. Furthermore, if the decomposition products are lithium ion conductive and electronically insulating, a solid electrolyte interphase (SEI) is formed, similarly to liquid electrolytes [85–87]. The same is true also at the positive electrode [88]. This protects the electrolyte from further decomposition by shielding it from the extreme chemical potentials at the electrodes (Fig. 2e) [74]. However, these kinetic hindrances are difficult to predict and need to be experimentally investigated [89]. The chemical stability of an electrolyte can be evaluated by contacting it with the electrode of interest. By measuring 209

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Fig. 2. a) Typical stress-strain curve from which the maximum tensile strength and the elongation at break are determined, b) scheme of the mechanical stability (the lightning indicates the occurrence of mechanical stress generated, e.g., by dendrite growth) and the adhesion to lithium metal of different electrolytes, c) DSC measurements with the determination of Tg, Tm, ΔHm and the crystallinity Δxc of a polymer-containing electrolyte, d) thermal stability of electrolyte systems, e) interfacial resistance of an electrolyte in a symmetrical Li/Li cell, f) schematic illustration of the electrochemical stability window (adapted from Ref. [74]), g) typical linear sweep voltammetry experiment to probe the anodic electrochemical stability.

cases, the interfacial resistance of hybrid electrolytes can even be smaller than that of the pure polymer phase [39,61–63,95]. The improved interfacial resistance was attributed to the scavenging ability of the inorganic particles [39], or associated with Lewis acid-base reactions [62,75]. The inorganic electrolyte particles might also react with traces of water or residual solvents trapping them on their particle surface, thus preventing their reaction with lithium metal, which would lead to increased interfacial resistance. In general, the electrolyte experiences a large potential gradient, separating the lithium metal (i.e., the negative electrode) and the cathode material (i.e., the positive electrode), especially upon charging of the cell. Therefore, the electrochemical stability of hybrid electrolytes becomes of interest for their application. This is commonly evaluated by linear sweep voltammetry (LSV). Usually, the electrolyte is sandwiched between a lithium metal electrode and a (flat) blocking working electrode (e.g., stainless steel or nickel). However, a working electrode composed of high-surface area carbon, such as the one used as conductive additive for the real electrode) may give a more realistic compromise to determine the anodic (i.e., high potential) electrochemical stability limit [96]. The cell is then polarized with a constant sweep rate and the current density measured. In a LSV test, hybrid electrolytes usually show enhanced anodic stability in comparison to the parental polymer electrolytes (Fig. 2g) [49,61,62]. Different explanations for this improved anodic stability have been proposed in literature, such as (i) excellent oxidative stability of the inorganic electrolyte at high potentials [61,97], (ii) shift to higher potentials of the lithium salt anion decomposition due to their immobilization on the solid electrolyte surface through Lewis acid-base interactions [62,75], (iii) dipole-dipole interactions of the inorganic solid (e.g., LAGP) and the polymer (e.g., PEO) that alter the latter electron transition energy level and elevate its oxidative decomposition potential [76], and/or (iv) interactions of the inorganic electrolyte with both the polymer

electrochemical impedance spectroscopy of, e.g., symmetric Li/Li cells, the interfacial resistance between lithium and the electrolyte can be determined (Fig. 2f), which usually consists of contributions from both, charge transfer and SEI. Contact issues can further increase the internal resistance of the cell, therefore applying external pressure to the cell may be critical to obtain reproducible and reliable data. The long-term stability of the interface can be also evaluated by monitoring the impedance response of, e.g., symmetric cells at open circuit condition for several days or weeks (to allow the fair comparison of different systems, the interfacial resistance should be reported in Ω cm2, which is not always available) [90]. The interfacial resistance between inorganic electrolytes and lithium metal is found to range from several Ω cm2 to even MΩ cm2, depending on the chemical nature of the inorganic electrolyte [25,91,92]. Such high interfacial resistances may have different reasons. In the case of garnet electrolytes, their dramatically low lithium wettability causes microscopic contact gaps at the interface, which intrinsically leads to high impedance [92]. Differently, Li1+xAlyTi2-y(PO4)3 (LATP) reacts in contact with lithium, forming an interphase that strongly deteriorates the mechanical integrity of the pellet. Repeated micro-crack formation and, ultimately, the pulverization of the interface leads to the observed dramatic increase of impedance over time [25]. Soft and flexible polymer electrolytes enable better contact and interfacial properties instead. The interfacial resistance is commonly in the order of several Ω cm2 at 60 °C for PEObased systems [90]. Differently, polyacrylonitrile (PAN)-based polymer electrolytes are unstable against lithium metal, resulting in a continuously increasing interfacial resistance [93,94]. Additionally, it strongly depends on the purity of the used polymer electrolyte. Most polymer-inorganic hybrid electrolytes show interfacial properties closer to that of the polymer electrolyte component. In fact, contact and chemical compatibility issues typical of the inorganic component are mitigated by the surrounding polymer matrix. In some 210

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Fig. 3. Depictive conduction mechanism schemes of a) polymer-rich hybrid electrolytes, b) inorganic-rich hybrid electrolytes and c) aligned inorganic nanostructures in a polymer electrolyte matrix. d) Arrhenius-plot, e) scheme of the chemical potential of lithium-ions and calculated lithium-ion concentrations within a hybrid electrolyte, f) schematic illustration the determination of conduction pathways by sNMR and a 6Li/7Li replacement strategy, g) stripping/plating experiment with required parameter information.

available. For polymer-rich HSEs (inorganic content < 50 vol% [104]), the probability that inorganic particles pathways are available is obviously rare. In general, Li+-ions can move through i) mixed polymer electrolyte – inorganic particles pathways, ii) along polymer/inorganic particles boundaries, iii) solely through the polymer electrolyte phase (polymerrich hybrids), and iv) solely through the inorganic electrolyte phase (inorganic-rich hybrids). The overall Li+-ion conductivity depends on the individual Li+-ion conductivities of the two materials, their volume fractions and contribution to the Li+ pathways. In case of a synergistic effect, the total lithium ion conductivity is higher than the sum of the single contributions. The volume fractions of the single components can be determined by the manufacturing parameters of the hybrid electrolyte (X vol% inorganic electrolyte + Y vol% polymer electrolyte = 100 vol%). However, they need to be corrected by taking into account the volume fraction of the interphase resulting upon mixing (correction approaches are explained in the supporting information S-2). The ionic conductivity of the electrolytes can easily be obtained from electrochemical impedance spectroscopy (EIS) measurements. Usually, the electrolyte is sandwiched between two inert blocking electrodes in order to maximize the charge transfer resistance. However, an intimate contact between the electrodes and the electrolyte is essential to avoid contact issues that might affect the bulk resistance (Rb) determination from which the ionic conductivity (σ) is

(crosslinking sites for EO-segments) and the lithium salt anion (like in (ii)) shifting their decomposition to more anodic potentials [63]. Note that the anodic stability in composite cathode might be different as transition metal oxides can catalyze the oxidative decomposition of the electrolyte [98,99]. For example, a PEO-based hybrid electrolyte might appear stable from the LSV on an inert electrode, but not in combination with a layered oxide, due to the catalyzed decomposition of the polymer [100]. At the lithium metal anode, the cathodic decomposition of hybrid electrolytes is mainly attributed to the reductive decomposition of the polymer electrolyte (especially, PEO and the salt anion) which, generally, completely surround the inorganic electrolyte and avoid its direct contact with lithium [63,101,102].

4.4. Ionic conductivity The ionic conductivity is still the major research focus in the field of hybrid electrolytes. Here, inorganic-rich [39,76], polymer-rich [62,95,103] and hybrid electrolytes with (aligned) nanostructures [51,64,66] are distinguished due to potentially different Li+ conduction mechanism (Fig. 3a–c). Hybrid electrolytes containing more than 50 vol% [104,105] of inorganic particles are classified as inorganic-rich HSEs. The probability that the inorganic particles form a 3D percolation network is rather high, but is dependent on several parameters such as particle size and distribution. If a percolation network exists, an additional Li+ pathway via the inorganic conductor interconnections is 211

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Table 1 Composition, ionic conductivity of liquid, polymer, inorganic and hybrid electrolytes (molar weight Mw in g mol−1, RT = room temperature). Organic phase

Inorganic phase

Ionic conductivity

Liquid electrolytes LP30 (1.0 M LiPF6 in EC/DMC = 50/50 vol%) [106] Pyr14TFSI + 0.3 M LiN(SO2CF3)2 (LiTFSI) [107] Polymer electrolytes PEO10LiTFSI [108]

–

1.12 × 10−2 S cm−1 at 25 °C 1.0 × 10−3 S cm−1 at 20 °C

PEO20LiTFSI [109]

10 wt%PEO SiO2

PEO20LiTFSI-[Pyr14TFSI]2 [109]

–

PAN-(LiClO4)0.2 [110] PAN-(LiClO4)0.6 [111] Inorganic electrolytes – – –

– 7.5 wt% β-Al2O3

ca. 10−6 S cm−1 at 20 °C, 2 × 10−4 S cm−1 at 60 °C 2 × 10−6 S cm−1 at 20 °C 4 × 10−4 S cm−1 at 60 °C 2 × 10−5 S cm−1 at 20 °C, 8 × 10−4 S cm−1 at 60 °C 6.5 × 10−7 S cm−1 at RT 5.7 × 10−4 S cm−1 at 30 °C

Li6.4La3Zr1.4Ta0.6O12 [15] Li1.3Ti1.7Al0.3(PO4)3 [8] Li9.54Si1.74P1.44S11.7Cl0.3 [18] β-Li3PS4 [23] Li0.34La0.51TiO2.94 [10]

7.2 × 10−4 S cm−1 at 25 °C 1.2 × 10−3 S cm−1 at 25 °C 2.53 × 10−2 S cm−1 at 25 °C 1.6 × 10−4 S cm−1 at RT 2 × 10−5 S cm−1 at RT

β-Li3PS4 + Li7La3Zr2O12 70:30 wt% [38] β-Li3PS4 + Li6ZnNb4O14 90:10 wt% [47] 20 wt% Li6.4La3Zr2Al0.2O12 (3D-network) [51]

5.4 × 10−4 S cm−1 at RT 2.44 × 10−4 S cm−1 at RT 2.5 × 10−4 S cm−1 at RT

70 wt% Li1.5Al0.5Ge1.5(PO4)3 (D50 = 10.6 μm) [77]

1.1 × 10−4 S cm−1 at RT

70 wt% Li7La3Zr2O12 [39]

1.1 × 10−5 S cm−1 at 30 °C

60 wt% Al-doped Li6.75La3Zr1.75Ta0.25O12 [112] 40 vol% Li7La3Zr2O12 [50]

2.48 × 10−4 S cm−1 at 30 °C 5 × 10−8 S cm−1 at 30 °C

20 wt% Li6La2BaTa2O12 [68]

2 × 10−4 S cm−1 at 30 °C

7.5 wt% Li7La3Zr2O12 [113]

5.5 × 10−4 S cm−1 at 30 °C

10 wt% Li6.4La3Zr1.4Ta0.6O12 [53]

1.2 × 10−4 S cm−1 at 30 °C

18 or 30 wt% Li6.4La3Zr1.4Ta0.6O12 [115] 15 wt% y Li1.3Al0.3Ti1.7(PO4)3 (EO/(x + y) = 8) [60]

2 × 10−6 S cm−1 at 22 °C 8 × 10−6 S cm−1 at RT

70 wt% Li1.5Al0.5Ge1.5(PO4)3 [61]

1 × 10−5 S cm−1 at 25 °C

20 wt% Li1.5Al0.5Ge1.5(PO4)3 [62,63]

2.1 × 10−5 S cm−1 at 25 °C

40 wt% Li1+xAlxTi2−x(PO4)3 (vertically aligned and connected nanoparticles) [64] 10 wt% Li1.3Al0.3Ti1.7(PO4)3 (ca. 65 nm) [103] 70 wt% Li1.3Al0.3Ti1.7(PO4)3 [116]

6.8 × 10−6 S cm−1 at RT

5 × 10−5 S cm−1 at 30 °C

99 wt% Li1.5Al0.5Ge1.5(PO4)3 [76]

2 × 10−5 S cm−1 at 30 °C

3 vol% Li2O-Al2O3-SiO2-P2O5-TiO2-GeO2 (Ohara®) [95]

1 × 10−6 S cm−1 at 30 °C

1 wt% Li10GeP2S12 [97]

1.2 × 10−5 S cm−1 at 25 °C

2 vol% β-Li3PS4 [114]

3.5 × 10−5 S cm−1 at 30 °C

99.5 mol% 70 Li2S × 30 P2S5 [117]

6.8 × 10−5 S cm−1 at RT

12.7 vol% Li6.4La3Zr1.4Ta0.6O12 (D50 = 43 nm) [118] 77 wt% 75 Li2S × 25 P2S5 [78]

2.1 × 10−4 S cm−1 at 30 °C 1 × 10−4 S cm−1 at 30 °C

5 wt% Li6.75La3Zr1.75Ta0.25O12 [55]

5.2 × 10−4 S cm−1 at 40 °C

70 wt% Li1.5Al0.5Ge1.5(PO4)3 [119]

1.5 × 10−6 S cm−1 at 25 °C

3 wt% Li0.33La0.557TiO3 (oriented nanowires) [66]

6 × 10−5 S cm−1 at 30 °C

– Hybrid solid electrolytes – PEOxLiTFSI (PEO: MW = 6 × 105) 21 wt% PEO18LiClO4 and 9 wt% SN (PEO: Mn = 2 × 105) PEO15LiTFSI (PEO: MW = 4 × 106) PEO12LiTFSI (PEO: MW = 3 × 105) PEO20LiClO4 (PEO: MW = 105) PEO8LiClO4 (PEO: MW = 5 × 106) PEO8LiTFSI (PEO: MW = 6 × 105) PEO8LiTFSI (PEO: MW = 6 × 105) PEO15LiClO4 (PEO: Mn = 6 × 105) PEO-xLiClO4 (PEO: Mn = 106) PEO18LiClO4 (PEO: Mn = 2 × 105) PEO18LiTFSI (PEO: MW = 6 × 105) PEO8LiClO4 (PEO: MW = 6 × 105) PEO5LiClO4 (PEO: MW = 9 × 105) 15 wt% PEO, 15 wt% boronized polyethylene glycol (BPEG) (PEO: MW = 4 × 106), (BPEG: MW = 1800), LiTFSI (Li/EO = 1/20) PEO8LiTFSI (PEO: MW = 5 × 105) PEO20LiFSI (PEO: MW = 5 × 105) PEO18LiTFSI (PEO: MW = 6 × 105) PEO18LiTFSI (PEO: MW = 6 × 105) PEO (PEO: MW = 400) PEO (PEO: Mv = 106) PFPE-diol25LiTFSI (MW = 103) poly(propylene carbonate) (PPC)-LiTFSI (4:1 wt) (PPC: MW = 5 × 104) PBA6LiClO4 poly(1,4-butylene adipate) (PBA) (PBA: MW = 12 × 103) PAN-LiClO4 (2:1 wt)

–

1.7 × 10−4 S cm−1 at 20 °C

(continued on next page)

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Table 1 (continued) Organic phase

Inorganic phase

Ionic conductivity

PAN-LiClO4 (2:1 wt) (PAN: Mn = 1.5 × 105) PAN-LiClO4 (2:1 wt) (PAN: Mn = 1.5 × 105) PEO10LiTFSI (PEO: Mv = 6 × 105) Hybrid quasi-solid electrolytes different ILs (90-10 mol%) 19 wt% Py14TFSI + 1 wt% LiTFSI Li(triglyme)1TFSI (solvate ionic liquid) 40 wt% PEO18LiClO4 (PEO: MW = 4 × 105) 20 wt% tetraethylene glycol dimethyl ether 1 g PEO (PEO: Mv = 2 × 106) 0.2 mL [BMIM]TF2N P(VDF-HFP)-LiTFSI-EMITFSI (5:5:7 in wt)

15 wt% Li0.33La0.557TiO3 (nanowire) [65]

2.4 × 10−4 S cm−1 at RT

5 wt% Li7La3Zr2O12 (nanowire) [54]

1.3 × 10−4 S cm−1 at 20 °C

44 wt% (16.2 vol%) Li0.35La0.55TiO3 (3D framework) [67]

8.8 × 10−5 S cm−1 at RT

Glassy Li2S-P2S5 [71] 80 wt% LLZO [35] Li10GeP2S11 [41] 40 wt% LLZO [120]

maximum 1 × 10−3 S cm−1 at 25 °C

15 vol% Li6.4La3Zr1.4Ta0.6O12 (D90 = 228 nm) [121]

1.7 × 10−5 S cm−1 at 20 °C

50 wt% Li1+xAlxGe2−x(PO4)3 [122]

9.2 × 10−4 S cm−1 at RT

5 × 10−3 S cm−1 at 30 °C 5.15 × 10−5 S cm−1 at RT (decreases with time)

LiTFSI plasticizes PEO better than LiClO4 [129]. Differently, PAN-based polymer electrolytes, although considered amorphous, show lower ionic conductivities compared to PEO-based systems at room temperature. However, if high amounts of lithium salts and inert fillers are used, the ionic conductivity can be increased. Nonetheless, PAN-based polymer electrolytes are usually studied as gel-type polymer electrolytes that contain an organic electrolyte as swelling agent. This allows the Li+-ion transfer to mainly occur through the highly conductive liquid phase [111]. For hybrid electrolytes, the additional inorganic/organic interface has to be considered when describing the ion transport mechanism. A similar interface is well known from composite polymer electrolytes integrating inert fillers, such as Al2O3, TiO2 or SiO2. Surface-related ion transport phenomena, including (i) increased ion pair dissociation, (ii) enhanced Li+ surface transport, (iii) anion attraction on the surface due to Lewis acid–base interactions, and (iv) PEO chain promoted surface transport known from composite electrolytes, are frequently mentioned to explain the higher conductivity of hybrid electrolytes with respect to their parental polymer electrolyte [63, 130]. In fact, the surface chemistry of inorganic particles and their interplay with the polymer electrolyte may significantly influence the ionic conductivity. Smaller inorganic particles increase the interfacial regions with the polymer and more efficiently prevent a dense packing of the polymer segments [131]. Even though the crystallization behavior of the polymer electrolyte fraction is kinetically hindered, the ionic conductivity of ceramic/polymer hybrids depends on the temperature (T). Hence, the values of conductivity frequently depend on the equilibration time used prior to perform the conductivity (e.g., by EIS) measurement, as well as by the temperature scanning direction (i.e., heating or cooling). Due to the influence of the thermal history, the measurement procedure must carefully be reported. In order to fit the temperature dependence of hybrid electrolytes, both Arrhenius [49] and VTF [55] equations are used (see supporting information S-3). Being composed of more than one electrolyte phase, hybrid electrolytes may have different activation energies. Additionally, the activation energy for the charge transfer between the different phases needs to be considered too. More details about this aspect are given in section 5.6 (Interfacial Properties). Hybrid electrolytes are reported to show higher ionic conductivities compared to composite electrolytes with similarly shaped inert fillers [49,54,61,66,103]. Nonetheless, the complexity of these hybrid polymer-inorganic systems is testified by the contrasting ionic conductivity values reported in the literature. Synergistic effects [49,61,62,103] as wells as reduced ionic conductivities [39,50,95] with respect to the individual components have been reported for rather similar hybrid electrolytes (Fig. 3d). The controversial discussion about the ion conduction mechanism is still ongoing. It is now clear that the

calculated (taking into account contact area (A) and thickness (l) of the electrolyte). The ionic conductivity of different classes of electrolytes is reported in Table 1. The ionic conductivity of the new inorganic conductors [8,15,18,20,23] is usually much higher compared to that of polymer electrolytes at room temperature. However, they rarely exceed the conductivity of liquid electrolytes. Li9.54Si1.74P1.44S11.7Cl0.3 [18] is the only solid electrolyte showing a higher ionic conductivity than conventional carbonate-based electrolytes (e.g., LP30) at 25 °C. Inorganic crystalline conductors enable Li+-ion diffusion by direct interstitial hopping, interstitial knock-off, and direct vacancy hopping [1]. For these site-to-site motions, well-defined values of activation energy are required. Thus, ceramic as well as glassy materials show Arrhenius-type ionic conductivity behavior. The crystal structure determines the spatial arrangement of the immobile framework as well as both the sub-lattice and pathways of Li+-ions. The interactions between the immobile framework and the mobile Li+-ions strongly influence the mobility of the latter. The ionic conductivity of the inorganic electrolyte can be altered, e.g., introducing defects or dopants [1,123,124]. For example, the Li+-ion to vacancy ratio greatly influences the ionic conductivity. Element doping or substitution can tune the point defect concentration as well as the bottleneck size for Li+-ions diffusion and the lattice volume. The activation energy, as a measure of the ease of ion movement [125], generally ranges between 0.2 and 0.6 eV in ionic crystalline solids [5] and from 0.2 to 1 eV [125] in inorganic glasses, mostly depend on the concentration and nature of the mobile cation in the latter. The transference number in ionic solids is generally very close to unity. Differently, polymer electrolytes, e.g., those consisting of PEO and a lithium salt, enable Li+ diffusion by segmental polymer chain motion coupled with a hopping mechanism of the coordinated Li+ ions. Thus, the ionic conductivity of polymer electrolytes usually follows the VTF (Vogel, Tammann, and Fulcher) or, exceptionally, the Arrhenius behavior depending on their composition [126]. In PEO-based electrolytes, Li+-ions hop between coordination sites, i.e., ether oxygens. This hopping is enabled by the relaxation process of the amorphous polymer [127]. The activation energy is related to the energy barrier for the cooperative ion transfer from one coordination site to another, the dissociation energy of the salt, and the dielectric constant of the polymer matrix [125]. The Li+ ion transport is therefore much higher in amorphous than in crystalline regions. However, PEO-lithium salt complexes are highly crystalline at room temperature due to their high melting temperature (Tm) [128]. By lowering Tg and increasing the degree of amorphicity, the relaxation processes can be facilitated and the ionic conductivity increased. Generally, higher amorphicity is also obtained by increasing the lithium salt concentration, the polymer chain length, or the size and Lewis basicity of the counter anion, e.g., 213

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et al. in 1995 [137]. This model can predict the interplay between ionic conductivity and volume concentration of the added filler. In 2013, Kalnaus et al. introduced a computational method to estimate the effective conductivity and mechanical properties based on finite element analyses [104]. Diffusivity coefficients of hybrid electrolytes can also be computed by means of molecular dynamics simulations. The increased Li+-ion diffusivity, by the addition of an inorganic electrolyte into a polymer electrolyte matrix, can be attributed to the larger free volume within the hybrid electrolyte compared to the pure polymer electrolyte [55]. A more comprehensive summary of modeling approaches for composite and hybrid electrolytes is found elsewhere [138]. Given the growing interest in this field, more theoretical studies are expected to appear in the near future. As previously mentioned, the shape of the inorganic component particles also have a crucial role on the conductivity. The ionic conductivity of hybrid electrolytes including nanowires is enhanced when compared to those involving nanoparticles [54,64]. Inorganic conductive nanofiber-based membranes produced by electrospinning and infiltrated by polymer electrolytes show high ionic conductivity [51,54]. This is attributed to fast Li+-ion pathways through the interwoven ceramic nanofiber network and along the continuous fiber/ polymer interface [51]. As suggested by solid-state nuclear magnetic resonance (sNMR) spectroscopy, Li+ ions preferentially move through the polymer interphase that is structurally altered by the inorganic electrolyte [54]. Also, an increased ionic conductivity is observed by vertically aligned ceramic fillers [64,66]. This is attributed to the fast Li+-ion transportation along the nanowire surface, without any crossing junctions [66]. In spite of that, the high ionic conductivity of the sintered ceramic conductor is not reached by any of these systems, possibly due to the poor Li+ transport across the polymer/ceramic interfaces [64]. For lithium battery applications, the Li+-ion transference number or the limiting current density (as determined from potentiostatic measurements on symmetric Li cells) are more meaningful than conductivity values. Indeed, the anion conductivity does not contribute to the cell operation. In this view, high Li+-ion transference numbers can actually alleviate polarization effects and, in turn, delay dendrite formation [126,139]. Multiplying the ionic conductivity and the cation transference number, the Li+ ion conductivity can be calculated [140]. For inorganic single ion conductors the lithium transference number is ideally unity, whereas in polymer electrolytes it is far below 0.5 [141] (i.e. close to 0.1–0.2 [128]). Lithium transference numbers for polymer electrolytes are usually obtained by the Bruce-Evans-Vincent method [142]. However, this method assumes that ion-pairing, electrolyte instabilities at the lithium electrode and convection are negligible, which is highly questionable for most electrolytes, or at least needs practical confirmation. A recent review explains these issues in more detail and discusses alternative measurement possibilities [143]. For the large majority of PEO-containing hybrid electrolytes, the lithium transference number is well below 0.5 (0.22–0.26 at 80 °C [97], 0.208–0.378 at 60 °C [62], 0.208–0.385 at 60 °C [63] or 0.14–0.20 at 40 °C [75]). However, an increase compared to the pure polymer electrolyte is observed in all cases. This is attributed to Lewis acid-base interactions of the inorganic electrolyte and the salt anion (see 5.6.1 Lewis Acid-base Interactions) [63]. Especially, the poly(propylene carbonate)-LiTFSI (4:1 wt) containing 5 wt% Li6.75La3Zr1.75Ta0.25O12 shows the promising Li transference number of 0.75, which is higher than that of the pure polymer electrolyte (0.6) [55]. Also in this case, the increase is attributed to the interactions of the inorganic nanoparticles with the polymer chains and the TFSI− anion. Li+ ion pathways can be investigated with the help of high-resolution 6Li sNMR analysis (Fig. 3f) combined with electrochemical experiments [52,54,120]. Stripping/plating experiments with symmetrical 6Li/hybrid electrolyte (7Li)/6Li cells are conducted to partially replace 7Li by 6Li thus leaving a 6Li trail in the hybrid electrolyte that should highlight the preferential Li+-ion pathways (i.e., after the

ionic conductivity is highly dependent on size [118], shape [51,64–66], volume fraction [49,50] and chemical nature [62] of the inorganic particles. Doping of the inorganic conductor appears to have only a minor contribution instead [54]. The processing route (tape-casting allowing solvent leftovers vs. solvent-free hot-pressing) and the composition of the polymer electrolyte [76,132] also influence the ionic conductivity. For PEO-based electrolytes, higher ionic conductivity values are achievable lowering the EO/Li ratio (higher salt concentrations) and the polymer molecular weight [103]. Inorganic-rich hybrid electrolytes (inorganic content > 50 vol%) show intermediate ionic conductivities with respect to their single components [61] with an increased ionic conductivity of the hybrid electrolyte compared to the pure polymer electrolyte [49,61,76]. The enhancement is attributed to the high ionic conductivity of the ceramic electrolyte accompanied by the reduction in crystallinity of the polymer electrolyte [61]. It was also proposed that the Li+ transport mainly takes place through the inorganic particles, with Li+-ions crossing the polymer electrolyte only at the thinnest spots where the resistance is lower [76]. Polymer-rich hybrid electrolytes (inorganic content < 50 vol%) show decreased ionic conductivities compared with their parent (but sintered) inorganic conductors, but improved ionic conductivities compared to their pure polymer electrolyte parent. According to some groups, this is solely attributed to the decreased crystallinity of the polymer matrix caused by the presence of the conductive filler [133]. Even if a percolation network is not established, others report that the active fillers can also participate to the Li+-ion transport through two extra pathways, i.e., through the polymer-inorganic phase boundaries and across the interface [62,63]. So far, the ionic conductivity of the boundaries can only be estimated. Two different studies show an inherent boundary ionic conductivity of 2.0 × 10−4 S cm−1 [103] and 1.26 × 10−2 S cm−1 (30 °C) [66]. However, some others show reduced [39,50] ionic conductivities also compared to the pure polymer electrolyte parent. This is probably due to the kinetically sluggish Li+-ion transfer across the polymer/inorganic interfaces [134], making the inorganic conducting particles to increase the tortuosity of the ionic paths in the polymer electrolyte matrix. Additionally, the cross-sectional area of the polymer electrolyte is decreased, leading to an overall lower ionic conductivity [39,50]. In a hybrid electrolyte, both phases are ionically conductive and share the same charge carrier (i.e., Li+ ions). However, the chemical potential of the charge carrier is different in the two environments due to differences in the Li+-ion coordination and concentration (Fig. 3e) [50,103]. Generally, the lithium ion concentration is higher in inorganic electrolytes, even by more than one order of magnitude (see example in Fig. 3e and supporting information S-4 for calculations). This might lead to the diffusion of Li+ ions from the inorganic into the polymer electrolyte to balance the concentration gradient (and, thus, chemical potential gradient) between the two materials upon intimate contact [55]. The space-charge effect [135,136], well known from solid-solid ionic interfaces, is hypothesized to take place, but has not been fully confirmed for hybrid electrolytes containing polymer electrolytes, yet. Still, the chemical potential difference between the two materials is under discussion, and believed to influence the structural properties of the interphase as a result of Li+ redistribution. A clear evidence whether the decreased concentration of mobile Li+ within the inorganic electrolyte (as they diffuse into the polymer) has a negative [50] or a positive [55,66,103] effect on the overall conductivity is still missing. It also remains unclear if the ionic conductivities of the single phases change upon their intimate contact, as a result of lithium ion migration. Certainly, the redistribution of Li+ across the inorganicpolymer interface needs intensive investigation. Here, in combination with experimental work, mathematical modeling might help to better understand the hybrid electrolytes and greatly accelerate the progress of the field. The effective medium theory was the first tool used for modeling hybrid electrolytes (i.e. PEO-NaI-NASICON) by Przyluski 214

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experiment, the 6Li peak intensity in the sNMR spectrum should be increased for those local environments through which 6Li ions can effectively move). The comparison of the 6Li peak intensity prior and after polarization allows the localization of the effective Li+-ion pathways once the chemical shift of lithium in the electrolyte components is known (e.g., 2 ppm in LLZO and −0.2 ppm in PEO18LiClO4, although it should be mentioned that an additional peak is observed at around 1.4 ppm associated with the inorganic/organic interphase) [52]. A first sNMR study revealed that Li+ ions preferably move through the inorganic LLZO particles, and minimizes the existence of Li+ ion pathways through the polymer electrolyte (PEO18LiClO4) and along the inorganic/polymer interphase [52]. However, in that study, only 20 vol % LLZO was used, which is below the percolation threshold of inorganic particles. This implies that lithium ions have to travel through the polymer phase and the interphase to reach the LLZO, making the result of the study ambiguous. Additionally, the study was not supported by ionic conductivity measurements, failing to explain why the Li+ ions preferentially move through the LLZO matrix. In a second study, 2 vol% LLZO nanowires were integrated into a PAN-based polymer phase. In this case, the Li+ ions were found to preferentially move along the inorganic/polymer interphase, due to the increased LiClO4 dissociation in the polymer near the LLZO particles [54]. In the most recent study, a LLZO–PEO–TEGDME (40/40/20 wt%) system was analyzed [144]. The 6 Li/7Li replacement analysis revealed that the Li+ ions preferably moved through the gelified TEGDME- PEO electrolyte, underlining the strong dependence of the conduction mechanism on the composition of the hybrid electrolyte.

overvoltage [55]. A lifetime assessment was conducted for a hybrid electrolyte composed of PAN-LiClO4 (2:1 wt) and 5 wt% nanowire Li7La3Zr2O12. After 535 stripping/plating cycles the cell fails, whereas a cell without inorganic nanowires reveals only 492 cycles which is about 92% of the lifetime of the hybrid electrolyte [54]. Hybrid electrolytes comprising two inorganic phases showed, interestingly, low overvoltages at room temperature. However, less than 40 cycles were reported [38,47]. Hybrid electrolytes seem to effectively suppress dendrite formation for some cycles under well selected temperature and current density conditions. However, to the best of our knowledge no study is so far dedicated only to this topic, and in-depth analysis is still needed to assess dendrite formation on a long term and investigate their formation mechanisms. 4.6. Interfacial properties The interfacial properties between the components in hybrid electrolytes significantly influence important performance parameters, such as the ionic conductivity. Of main interest are i) the existence of Lewis acid-base interactions between the two components and ii) the transfer of charge carrier ions across the phase boundary. 4.6.1. Lewis acid-base interactions Fourier transform infrared spectroscopy (FTIR) can be used to investigate the interaction of the inorganic particles with the polymer phase in composite [150] and hybrid [55,60,103,151] electrolytes. When comparing the spectrum of the hybrid electrolyte with those of the parental electrolytes, chemical reactions between the components are revealed by additional bands [117]. The complexation of Li-salts in polyethylene oxide (PEO)-based electrolytes is proven by band shifting associated with modified vibrations of the involved bonds [103]. With regard to the inorganic electrolyte, it may also interact with the polymer phase through different mechanisms. For example, Lewis acidbase interactions between LATP (a Lewis acid) and the salt anion (e.g., TFSI− a Lewis base) may result in the formation of an “ion-ceramic complex”, which decreases the anion mobility and promotes the Li-salt dissociation [63]. Even more complicated is the case of garnets, which, by showing Lewis acid (Li+, La3+, Zr4+, possibly hydroxyl groups on the surface from side reactions) and basis (O2−, lithium vacancy) characters, may interact with both, the oxygen atom of PEO and the salt anions, also leading to an increased number of mobile lithium ions [151]. Fig. 4a shows an overview of Lewis bases (electron donors) and Lewis acids (electron acceptors). So far, the quantification of Lewis acidity or basicity of the ceramic electrolyte is omitted, although it would be interesting to verify whether the increased ionic conductivity in presence of Lewis acidic surface groups as in Al2O3 is occurring in hybrid electrolytes, too [152]. More in-depth investigations are needed to better understand the interaction of different phases in hybrid electrolytes. This knowledge would help to modify the components of the hybrid electrolytes and improve their properties like ionic conductivity, lithium transference number and electrochemical stability.

4.5. Deposition and dissolution of lithium The application of lithium in ASSBs brings along several challenges that need to be addressed. In fact, the lithium electrode shows infinite volume change, tends to deposit in dendritic form and is highly reactive [83]. Low current densities upon lithium deposition (charge), high current densities upon stripping (discharge) and an applied pressure perpendicular to the Li metal electrode are often beneficial to avoid dendrite formation [145]. Commonly, to evaluate the stripping and plating properties of the lithium metal anode, symmetrical Li/electrolyte/Li cells are assembled and subjected to reversing constant current steps (Fig. 3g). The overpotential is a measure of the overall resistance (i.e., bulk electrolyte, diffusion and interfacial resistances). The bulk resistance is dependent on the ionic conductivity, the electrolyte thickness and the contact area taken into account when calculating the applied current density. Ideally, the thickness should be the same when comparing different systems, but at least reported. If the applied current density exceeds the limiting current density, increasing overvoltage and dendrite growth will deteriorate the stripping plating behavior. Note that current densities used in real application can well exceed 1 mA cm−2 [146]. Surface structuring methods might help to overcome this problem and at the same time might suppress dendrite growth [147]. PEO-based hybrid electrolytes have moderate overvoltage at elevated temperatures (T = 60 °C), but low current densities (I ≤ 0.1 mA cm−2) [39]. The addition of a plasticizer (i.e., succinonitrile) can effectively reduce the overpotential [77], however, the study reported only 14 stripping/plating cycles, leaving the question of dendrite prevention open. A promising low temperature stripping/plating performance is revealed by a LLZO 3D-network infiltrated with a PEObased polymer electrolyte [51]. Even at 15 °C and 0.2 mA cm−2, as well as at room temperature and 0.5 mA cm−2, several hundred stripping/ plating cycles were achieved with an overvoltage around 400 mV, but without any dendrite formation [51]. Impressive stripping/plating performance is also shown by poly(propylene carbonate)-LiTFSI (4:1 wt) containing 5 wt% Li6.75La3Zr1.75Ta0.25O12 [55], possibly due to its previously discussed high Li+ ion conductivity. Sequential stripping and plating for 1000 h at 25 °C and 0.1 mA cm−2 reveals only 12 mV of

4.6.2. Lithium-ion transfer across the phase boundary The lithium-ion transfer across an organic/inorganic interface includes distinct processes: i) Li+ ion migration in the polymer electrolyte, ii) desolvation of the Li+ ion at the interface, iii) transport of the Li+ ion through an eventual interphase layer (e.g., Li2CO3 on the surface of the inorganic particle) and, finally, iv) transport of the Li+ ion through the inorganic electrolyte. Generally, the desolvation of the Li+ ions at the interface (ii) is considered to be the rate-determining step in the whole transfer process [73,153]. The activation energy for the desolvation process is determined by the chemical potential difference of the Li+ ions in the organic electrolyte and the transition state [148]. The chemical potential itself is influenced by the activity, which includes the molar fraction of the Li+ ions in the electrolyte phase and the 215

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Fig. 4. a) Lewis acids and basis in hybrid electrolytes, b) activation energies [73] for lithium-ion transfer through the interfaces of different hybrid systems (adapted from Ref. [148]), b) electrical potential profile upon DC application on a symmetrical Li cell with a multilayer electrolyte (partially adapted from Ref. [37]) and indication of processes within the multilayer electrolyte, d) electrical potential profile upon DC application on a symmetrical Li cell with a hybrid electrolyte, e) symmetrical Li cell with a multilayer (organic liquid/inorganic solid) electrolyte with illustrated activation energies and resistances (adapted from Ref. [149]), f) electrical potential profile upon DC application on cathode/lithium cell with a multilayer electrolyte.

realized by a careful selection of lithium concentrations within the phases and controlling the interactions between the components. However, controlling the lithium concentration is not easy, especially, in the inorganic electrolytes. The activation energy associated to the transfer of the Li+ ions between the organic and inorganic electrolyte components can be estimated by electrochemical impedance spectroscopy studies on macroscopic model systems using the symmetrical sandwich-type cell setup (i.e., polymer electrolyte/inorganic electrolyte/polymer electrolyte sandwich between two blocking electrodes) as shown in Fig. 4c [73,157,159,160]. The activation energy for the interface between a LLZO pellet and a PEO20LiClO4 membrane were found to be 0.9 eV and 1.0 eV for molten and solid PEO, respectively [159]. Also, the activation energy of Li+-ion transport in the PEO-based polymer electrolyte depends on the temperature being above (0.42 eV) or below (1.8 eV) the melting point. Thus, at elevated temperatures, i.e., above the melting point of the polymer electrolyte, the activation energy for the Li+-ion transfer across the interface is higher than that of the Li+-ion transport within the each electrolyte. At temperatures below the melting point of the polymer electrolyte, the activation energy of Li+-ion transport in the solid and less amorphous polymer electrolyte is much higher. This suggests that an interphase transport might become more likely [159]. Unfortunately, these cell model systems consider a macroscopic scale. Contrary, in the hybrid electrolyte the two phases are (ideally) homogenously mixed at the microscopic scale (Fig. 4d). The organic-inorganic interface of the macroscopic model system experiences a much higher potential difference than the same interface in the microscopic system (Fig. 4c and d, respectively). Thus, the lithium ion transfer across the interface of the macroscopic model system might take place, but the same is not occurring in the microscopic system. Theoretical calculations are necessary to evaluate the difference between these two systems.

Table 2 Activation energies for interfacial lithium-ion transfer between two phases (LLTO = Li0.35La0.55TiO3). Interface

Activation Energy (kJ/ mol)

LLTO/1 M LiCF3SO3 in propylene carbonate [73] LLTO/PEO20LiCF3SO3 [73] LLTO/0.7 M LiClO4 in dimethyl carbonate [154] LLTO/1 M LiClO4 in dimethyl carbonate:ethylene carbonate (1:1 vol) [154] LLTO/1 M LiBF4 in 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4) [153] LLZO/PEO20LiClO4 [159]

56.2 97.4 32 51 25 87 (molten PEO) 96 (solid PEO)

strength of the (electrostatic) interaction between the Li+ ion and the solvent molecules (in case of liquid electrolyte), the counter anions, the polymer or host structure (activity coefficient). Fig. 4b shows the activation energies for the Li+-ion transfer through the interfaces of different hybrid systems in a simplified manner (please, note that the transition state is randomly chosen and the drawing is not in scale). The activation energy for the Li+-ion transfer from a solvent-coordinating electrolyte into an inorganic electrolyte reflects the desolvation energy of the Li+-ion from the solvent (polymer) [154] and thus the solvation ability of the solvent (polymer) [155]. The rate performance of the ion transfer might be increased by lowering the interactions between the Li+ ion and the organic electrolyte [73]. Polymer electrolytes, like PEO, show strong interactions with the Li+ ions, thus the activation energy is higher for the ion transfer across the polymer/inorganic electrolyte interface compared to that of liquid/inorganic electrolyte interface [73]. For ionic liquids, the activation energies for the Li+-ion transfer through the ionic liquid/inorganic electrolyte interface depend on both, the cation and – slightly more – the anion of the ionic liquid [156]. Activation energies of several systems are summarized in Table 2. In summary, the interfacial resistance between a ceramic and a polymer electrolyte depends on the chemistry of the single phases [157,158]. The lithium transfer across the interface is enhanced by equalizing the chemical potential of the single phases. This can be

5. Related developments 5.1. Multilayer cells The combination of liquid and solid electrolytes in a multi216

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conductors in pure polymers without a conducting salt. It was proven that garnet Li6.4La3Zr1.4Ta0.6O12 (LLZTO) embedded in an insulating PEO polymer can effectively prevent lithium dendrite growth [118]. The Li+ ions from the garnet diffuse into the polymer, while lithium vacancies are formed on the garnet electrolyte surface. The creation of such a highly ionically conductive interphase is larger for smaller particle sizes. The total ionic conductivity is 2.1 × 10−4 S cm−1 at 30 °C for 12.7 vol% LLZTO (D50 = 43 nm) [118] in PEO. A sulfide glass (1.2 Li2S-1.6 LiI-B2S3) with 10.9 vol% micronized polyethylene wax showed a promising ionic conductivity of ca. 8.7 × 10−5 S cm−1 at 25 °C [170]. A reduction of ionic conductivity compared to the pure sulfide glass is observed, but the addition of the ionically conductive PEO-based polymer electrolyte did not improve the performance. The authors concluded that a pure binding polymer can be used, according to its mechanical and chemical properties [170]. A mixture of 77.5Li2S-22.5P2S5 inorganic electrolyte and 30 wt% wellmalleable polyimine shows good dendrite prevention abilities due to the self-healing of the polymer [171]. 3D micro-ordered LAGP–epoxy polymer electrolytes show superior mechanical properties but reduced ionic conductivities compared to sintered LAGP pellets. Such an electrolyte with gyroidal architecture reveals a total ionic conductivity of 1.6 × 10−4 S cm−1 at room temperature, which is lowered according to the lower volume fraction of the ceramic in the hybrid [172].

electrolyte system enabled new types of batteries. The seawater battery, e.g., works with a set-up comprising seawater/NASICON/ionic liquid (or organic electrolyte) in which the Na3Zr2Si2PO12 (NASICON) solid electrolyte is separating the aqueous and non-aqueous compartments [161,162]. Also Li-air [163], Li-S [164] and Li-Se [165] cells can profit from multi-layer electrolyte systems. The integration of an inorganic single ion conductor prevents chemical cross-talk [166], such as polysulfide shuttling, and can suppress dendrite growth [165,167]. However, the liquid-solid interface introduces additional potential drops in the cell, which can dramatically decrease the gravimetric energy. The resistive contributions within a Li/liquid electrolyte/solid electrolyte/ liquid electrolyte/Li cell (Fig. 4e) derive from Ref. [149] i) interface (charge transfer + SEI) resistance between the lithium electrode and the liquid electrolyte; ii) the bulk resistance of the liquid electrolyte; iii) the interface resistance between the liquid and the solid electrolyte; iv) the bulk resistance of the solid electrolyte; and v) the grain-boundary resistance of the solid electrolyte. Additionally, solid and liquid electrolytes are not always chemically stable with each other, leading to their partial decompositions and formation of highly resistive solid-liquid electrolyte interphases (SLEI) with a finite thickness [149]. The presence of water traces in the liquid electrolyte might result in the growth of such SLEI, with subsequent increase of internal cell resistance [149,168]. In such a system, the two competing driving forces are the solvation energy of the solid electrolyte in the liquid electrolyte, and the lattice formation energy [149]. Strong Lewis acids (e.g., Li+, Al3+, Ge4+ in Li1+xAlyGe2-y(PO4)3 (LAGP)) might facilitate decomposition reactions of the liquid electrolyte [149]. When a solid and a liquid electrolytes are in contact, a redistribution of charge carriers (i.e., Li+ ions) across the interface takes place, which results in a potential difference at the interface. This potential difference can lead to high local fields which may promote the electrolyte decomposition reactions [149]. Investigations of such dissolution and interphase formation phenomena are still rare, and many electrolyte combinations have not been investigated, especially, the combination of inorganic and polymer electrolytes. The charge-transfer polarization is investigated with direct current (DC) polarization of a multiphase system [168]. The charge-transfer kinetics derives from a thermally activated Butler-Volmer like process superimposed with the ohmic resistance attributed to the solid-liquid electrolyte interface. This study confirms that the rate determining step is most likely the desolvation/solvation of lithium ions (via a transition state) as well as it confirms the interfacial electrolyte degradation and formation of an SLEI [168]. To enhance the lithium ion transfer across the interface, micro- or nano-structuring of the inorganic electrolyte has been proposed. This could enlarge the contact area and reduce the interfacial resistance between the liquid and the solid electrolyte [149]. Surface modifications might counteract the SLEI formation if its formation mechanism is better understood [149]. Fig. 4f depicts a sandwich-type multilayer electrolyte (SPE/ISE/ SPE) between a cathode and a lithium anode. Such a setup can prevent continuous reduction of unstable inorganic electrolytes at the lithium metal interface, enable flexibility to buffer volume changes upon electrochemical cycling, and significantly improve wettability and contacts [158]. The adhesive polymer electrolyte reduces the interfacial resistance and enables a more homogenous lithium ion flux at the interface to suppress dendrite nucleation [37]. Complementary, the inorganic electrolyte can physically block dendrites and provide mechanical support [169]. The multilayer systems reduces the doublelayer electric field at the lithium metal/polymer interface since the transfer of the salt anions is blocked by the inorganic conductor [37]. This, in turn, reduces the (electro-)chemical decomposition of the polymer electrolyte, increasing the cell coulombic efficiency [37].

6. Cathode composites and full cell application The integration of the solid electrolyte into the composite cathode of ASSBs is essential, which is the main difference from conventional LIBs in which the liquid electrolyte is filled into the cell after assembly. The integration of a polymer phase into the cathode composite ensures a good contact and helps to buffer volume changes occurring during charge and discharge, also preventing contact losses upon consecutive cycling. Contrary, a pure inorganic electrolyte might show worse contacts to the active material, depending on its ductility. An ideal composite cathode should offer high ionic and electronic conductivities, which can be evaluated, e.g., using the Au/cathode/Au set-up, by EIS and DC potentiostatic polarization. EIS can supply information about the composite cathode ionic, interfacial and contact resistances. On the other hand, the DC potentiostatic polarization allows the determination of the electronic conductivity of the composite cathode. Different compositions or mixing techniques can be evaluated with such a combination of techniques to obtain optimal composite cathodes [112]. Several reports show that lithium cells with hybrid electrolytes outperform analogous cells with the parental polymer electrolyte in terms of delivered specific capacity and rate capability [49,55,61,63]. However, the number of reported cycles is still limited to 200, in most of the cases, requiring more prolonged investigations. Long-term cycling performance tests are crucial to evaluate the compatibility of hybrid electrolytes with cathode materials as well as with the lithium metal anode. Interestingly, multilayer cell setups allow longer cycling performances. An LiFePO4(LFP)/LAGP/PEO18LiTFSI+1 wt% (75% Li2S-24% P2S5-1% P2O5)/Li cell shows long-term performance over 1000 cycles at 1C and 60 °C [173]. An LFP/cross-linked poly(ethylene glycol) methyl etheracrylate/LATP/cross-linked poly(ethylene glycol) methyl etheracrylate/Li cell was demonstrated capable of cycling at different C-rates for more than 600 cycles [37]. PEO-based polymer electrolytes are electrochemically stable with LFP. This electrolyte/electrode combination is already commercially used in the Bolloré Bluecar. However, PEO-based polymer electrolytes are electrochemically unstable above 4.0 V in combination with layered oxides [100,174], explaining the preferential use of LFP as cathode material (Table 3). To increase the energy density compared to today's LIBs, positive materials offering higher capacities and average voltage are required. Nickel-rich layered cathode materials can fulfill such requirements, but their integration with hybrid electrolytes is particularly

5.2. Inert polymers A recent trend in literature is the integration of inorganic 217

218 70 wt%

PEO8LiTFSI-10 wt% Li6.4La3Zr1.4Ta0.6O12

PEO8LiTFSI7.5 wt% Li7La3Zr2O12

PEO-12.7 vol% Li6.4La3Zr1.4Ta0.6O12

PEO18LiTFSI-20 wt% Li1.5Al0.5Ge1.5(PO4)3

PEO18LiClO4-70 wt% Li1.5Al0.5Ge1.5(PO4)3

PEO8LiTFSI-99 wt% Li1.5Al0.5Ge1.5(PO4)3

21 wt% PEO18LiClO4-70 wt% Li1.5Al0.5Ge1.5(PO4)3-9 wt% SN

PEO18LiTFSI-2 vol% β-Li3PS4

PEO18LiTFSI-1 wt% Li10GeP2S1210 wt% SN

15 wt% PEO-15 wt% BPEG-LiTFSI (Li:EO = 1:20)-70 wt% Li1.3Al0.3Ti1.7(PO4)3 PPC-LiTFSI (4:1 wt)-5 wt% Li6.75La3Zr1.75Ta0.25O12

Li/HSE/LiFePO4

Li/HSE/LiFePO4

Li/HSE/LiFe0.15Mn0.85PO4

Li/HSE/LiFePO4

Li/HSE/LiFePO4

Li(protected with PEO8LiTFSI)/HSE/ LiFe0.2Mn0.8PO4

Li/HSE/LiFePO4

Li/HSE/LiFePO4

Li/HSE/LiFePO4

Li/HSE/LiFePO4

Li/HSE/LiFePO4

50 wt%

PEO12LiTFSI-60 wt% Al-doped Li6.75La3Zr1.75Ta0.25O12

Li/HSE/LiFePO4

80 wt%

50 wt%

70 wt%

70 wt%

55 wt%

70 wt%

80 wt%

75 wt%

80 wt%

85 wt%

56 wt%

PEO15LiClO4-52.5 wt% Li7La3Zr2O12

Li/HSE/LiNi0.6Co0.2Mn0.2O2

Amount cathode material in the composite cathode

HSE/QSE

Anode/hybrid electrolyte (HSE/QSE)/cathode material

10 wt% HSE

20 wt% PEO20LiTFSI

10 wt% PEO18LiClO4

30.86 wt% PEO 4.14 wt% LiClO4 5 wt% SN 20 wt% PEOxLiClO4

20 wt% PEO8LiTFSI

30.86 wt% PEO 4.14 wt% LiClO4

20 wt% HSE

10 wt% PVdF/SN/ LiClO4

10 wt% Carbon black

30 wt% Acetylene black

20 wt% Super P

10 wt% Super P

10 wt% Super P

10 wt% Super P

10 wt% Super P

10 wt% Super P

10 wt% Super P

20 wt% Super P

15 wt% Super P

–

5 wt% HSE

10 wt%LFP In2O5Sn

5 wt% Super P

Conductive agent and its amount in the composite cathode

15 wt% PEO12LiTFSI

15 wt% PEO 21.5 wt% LLZO 7.5 wt%LiClO4

Electrolyte in the cathode and its amount in the composite cathode

-/n.a.

Acetonitrile

-/Acetonitrile

-/Acetonitrile

-/Acetonitrile

-/Tetrahydro-furan

-/n.a.

-/Acetonitrile

-/Acetonitrile

-/Acetonitrile

5 wt% PVdF/Nmethylpyrrolidinone (NMP)

-/n.a.

-/Acetonitrile

Binder and its amount in the composite cathode/processing solvent

29 μm

0.4–0.6 mg cm−2

n.a.

2.2 mg cm−2; 55 μm

8.0 mg cm−2

2.16 mg cm−2

3.6 mg cm−2

n.a.

n.a.

n.a.

2-3 mg cm−2

(continued on next page)

1st discharge: 166 mAh g−1 at 55 °C and 3.4 mA g−1 within 2.5–4.3 V (average dcv≈ 3.7 V) 1 cycle coulombic efficiency: 87.8% [49] 1st discharge: 152 mAh g−1 at 60 °C and 0.1 mA cm−2 within 2.3–3.8 V (average dcv≈ 3.2 V) C-rate test, 10 cycles, energy density discussed (334 Wh L−1) [112] 1st discharge: 149 mAh g−1 at 55 °C and 0.2C within 2.9–3.8 V (average dcv≈ 3.4 V) C-rate test, temperature test, 100 cycles [53] 1st discharge: 150 mAh g−1 at 60 °C and 0.1C within 2.9–4.0 V (average dcv≈ 3.4 V) C-rate test, 100 cycles at 0.5C [113] 1st discharge: 118 mAh g−1 at 60 °C and 0.156 mA cm−2 within 2.5–4.3 V (average dcv≈ 4.0 V) C-rate test, 200 cycles, energy density discussed [118] 1st discharge: 108 mAh g−1 at 60 °C and 1C within 3.0–3.8 V (average dcv≈ 3.4 V) 50 cycles, C-rate test [63] 1st discharge: 138 mAh g−1 at 55 °C and 0.2C within 2.6–4.0 V (average dcv≈ 3.4 V) 100 cycles, C-rate test, temperature test, coulombic efficiency: 96.5% [61] 1st discharge: 162 mAh g−1 at 50 °C and 0.1C within 2.5–4.5 V (average dcv≈ 3.7 V) 200 cycles, C-rate test, coulombic efficiency: 92.4% [76] 1st discharge: 128 mAh g−1 at 25 °C and 0.2C within 2.6–4.0 V (average dcv≈ 3.4 V) 100 cycles [77] 1st discharge: 153 mAh g−1 at 60 °C and 15 mA g−1 within 2.8–3.8 V (average dcv≈ 3.4 V) 325 cycles, C-rate test [114] 1st discharge: 158 mAh g−1 at 40 °C and 0.1C within 2.8–4.0 V (average dcv≈3.4 V) C-rate test, 100 cycles [75] 1st discharge: 158 mAh g−1 at 60 °C and 0.1C within 2.5–3.9 V (average dcv≈ 3.4 V) C-rate test [116] 1st discharge: 155 mAh g−1 at 20 °C and 17 mA g−1 within 2.5–4.0 V (average dcv≈ 3.4 V) C-rate test, 200 cycles, also works at 0 °C and 160 °C [55]

2.0 mg cm−2; 40 μm

∼ 70 μm

Performance parameters

Cathode mass loading; thickness

Table 3 Overview of lithium cells with hybrid electrolytes containing detailed information about cathode composition, mass loading and cell performance (V vs. Li/Li+, dcv = discharge voltage).

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2.2 mg cm−2 10 wt% carbon black –

10 wt% P(VDF-HFP)/NMP

n.a. 10 wt% Super P 35 wt% LiTFSI

5 wt% PVdF/acetonitrile

10 wt% Carbon black

Processing of inorganic powders into extremely thin and dense membranes with sufficient mechanical stability is one of the main stumbling blocks to the integration of solid electrolytes in cells. For lab scale investigations, ceramic electrolyte powders (e.g., LATP, LLZO) are easily pressed into thick pellets and sintered at high temperatures. However, for large-scale industrial production, this approach is not suitable. However, recent engineering efforts have shown 4 cm2 flexible films of LLZO produced from flame spray pyrolysis [175]. Still, high temperatures are crucial for the sintering process. Contrarily, sulfidic electrolytes are much softer and do not need sintering. However, still, mainly pellet-type cells are investigated while the research for tapecasting methods has just begun. Differently, processing methods for the flexible polymer-based electrolytes are already established. Comparably low energy consuming processes, like melt-extrusion or tape-casting followed by calandering, can be applied for large-scale production. The flexible nature of polymers also enables roll-to-roll and lamination processes, in which the polymer electrolyte can easily replace the commercial separator. The same manufacturing processes might be applied for polymer-inorganic hybrid electrolytes, inorganic-inorganic (e.g., sulfide-oxide mixed electrolytes) and liquid-inorganic hybrid electrolytes, thus circumventing the energy intensive sintering. Tape-casting and coldpressing procedures from current LIB manufacturing can be easily

1 g PEO-0.2 mL [BMIM]TF2N15 vol% Li6.4La3Zr1.4Ta0.6O12

P(VDF-HFP)-LiTFSI-EMITFSI (5:5:7 in wt)-50 wt% Li1+xAlxGe2−x(PO4)3

Li/QSE/LiFePO4

Li/QSE/LiFePO4

80 wt%

PEO8LiClO4-15 wt% Li7La3Zr2O12 Li/HSE/(S/LLZO@C in which active S = 61–64 wt%)

50 wt%

PBA6LiClO4-70 wt% Li1.3Al0.3Ti1.7(PO4)3 Li/HSE/LiNi0.6Co0.2Mn0.2O2

80 wt% (48.8–51.2 wt%S)

10 wt% HSE

-/Acetonitrile

1.2 mgS cm−2

1st discharge: 170 mAh g−1 at 55 °C and 0.2C within 3.0–4.2 V (average dcv≈ 3.7 V) C-rate test, temperature test, 100 cycles [119] 1st discharge: 1030 mAh gS−1 at 37 °C and 0.05 mA cm−2 within 1.5–2.7 V (average dcv≈2.1 V) 200 cycles [133] 1st discharge: 133 mAh g−1 at 25 °C and 0.1C within 2.8–3.8 V (average dcv≈ 3.4 V) C-rate test, 150 cycles (88% capacity retention) [121] 1st discharge: 158 mAh g−1 at °C and 0.05C at RT within 2.7–3.85 V (average dcv≈ 3.4 V) C-rate test, 50 cycles [122] ∼6 mg cm−2 10 wt% Super P 27.56 wt% PBA 2.44 wt% LiClO4

HSE/QSE

60 wt%

Conductive agent and its amount in the composite cathode

n.a.

Performance parameters

challenging. There is, in fact, only one study on hybrid electrolytes using LiNi0.6Mn0.2Co0.2O2 (NMC-622) as active material in combination with PEO, where only the first cycle is reported [49]. The use of hybrid (inorganic-polymer) electrolytes led to enhanced anodic stability [49,61,62] (measured via LSV tests) suggesting its stability with NMC materials. However, the experimental tests of a hybrid electrolyte in intimate contact with the cathode material, is lacking. Also the electrolyte that is integrated in the cathode, if there is any, is not always of exactly the same nature as the hybrid electrolyte layer in the lithium cell. Frequently, only the polymer electrolyte component of the hybrid electrolyte is used in the composite cathode. This omits the advantage of the higher anodic stability of the hybrid electrolyte. Lithium metal cells, so far, have been primarily realized using hybrid solid electrolytes, including NASICON-type (e.g., LAGP, LATP) or garnet-type (e.g., LLZO) inorganic electrolytes and PEO-based polymer electrolytes (Table 3). Recent research efforts are focusing on new types of polymers (e.g., PPC [55] and PBA [119]) and plasticizing additives (e.g., succinonitrile) [77]. On the other hand, perovskite-type inorganic electrolytes are not used in hybrid electrolytes for lithium cells, due to their (electro-)chemical instability with lithium metal [28]. Demonstrator cells also showed the flexibility and safety of the hybrid electrolytes, e.g., upon bending [112,118] and cutting of the cell [112] even during operation. Table 3 summarizes the so far investigated cathode compositions and their performance in lithium metal cells. The weight ratio of the active cathode material ranges from around 50 wt% to 85 wt%, whereas the electrolyte content is in the range of 0–40 wt% and the conductive carbon content is in the range of 5–30 wt% (but mostly 10 wt%). While the active cathode material content needs further increase to enable higher energy densities, presently, the electrode mass loading and the processing (solvents) of the composite cathodes are not always clearly reported in literature, thus making difficult to compare and even reproduce the results. Reported mass loadings range from 0.4 mg cm−2 to 8.0 mg cm−2. Note that the high mass loading cathodes contain a comparably higher weight fraction of inactive material to ensure better electronic connections. To conclude, the cathode composition needs further improvement. Especially, when reducing the fraction of electrolyte and conductive agent, an intimate sub-micrometer mixing level of the compounds is necessary to provide large contact areas to enable Li+ ions and electron transfer. 7. Large scale fabrication methods

Anode/hybrid electrolyte (HSE/QSE)/cathode material

Table 3 (continued)

Amount cathode material in the composite cathode

Electrolyte in the cathode and its amount in the composite cathode

Binder and its amount in the composite cathode/processing solvent

Cathode mass loading; thickness

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Fig. 5. a) Densities of active and inactive cell components [29,176–180], b-f) theoretical calculations of the specific energy on electrode level based on the parameters listed in Table S1 in the supporting information for different hybrid electrolytes as a function of volume ratio and separator thickness. For (b), cathode composition with LPS (Table S1) was considered and for the polymer-containing systems (c–f), cathode composition with polymer (Table S1) was considered.

parameters listed in Table S1. It is immediately evident as the copper current collector has, by far, the highest density. Thus, its avoidance, which can be attained with bipolar cell configurations, can significantly increase the specific energy of the cell (see supporting information S-5 for the theoretical specific energy calculations). The specific energy on electrode level is depicted as a function of volume ratios within different hybrid system for several electrolyte thicknesses in Fig. 5b–f. Our calculations show the highest specific energies for pure polymer electrolyte systems as the density of the polymer electrolyte is the lowest. Obviously, high inorganic electrolyte fractions result in higher cell weights, leading to the need that the inorganic-to-organic electrolyte ratio should be carefully chosen to obtain satisfactory specific energy densities [176]. Especially, the high

adapted. Nonetheless, after pressing or calandering, inorganic electrolytes might become brittle which could exclude roll-to-roll processing. This impacts the cell design, excluding rolled and spirally wound cell formats, in favor of stacked configuration. In summary, engineers are challenged to find cost-efficient processes that allow the commercial application of (hybrid) solid electrolytes.

8. Cell specific energy calculations The specific energy calculations are based on the density of the cell components (Fig. 5a). Lithium and LiNi0.8Mn0.1Co0.1O2 were considered as electrode materials to estimate the energy density of cells incorporating different hybrid electrolytes using Equation S(6) and the 220

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Appendix A. Supplementary data

densities of garnets and perovskites substantially affects the specific energy of the corresponding cells. The maximum specific energies in a polymer electrolyte based system are 526 Wh kg−1 to 479 Wh kg−1 for 1 μm and 50 μm separator thickness, respectively. For LLZO (or LLTO)polymer hybrid electrolytes, the specific energies range in the order of 370–520 Wh kg−1 depending on the volume fractions and separator thickness. Given their lower density, the use of LATP and LPS as inorganic components in hybrid electrolytes leads to increased specific energies. The polymer-free cell (LPS-LLZO as hybrid electrolyte and LPS in the cathode) shows the lowest energy densities (346–495 Wh kg−1). The calculated specific energies are slightly higher compared to the values obtained by Placke et al. [176] for 100 vol% LPS (or LLZO) and a separator thickness of 20 μm due to different cathode compositions. The specific energy on electrode level of today's electric vehicle LIBs is ca. 340 Wh kg−1 [146]. So, all hybrid electrolyte-based systems show superior specific energies making these electrolyte very interesting candidates for future ASSB application. However, the feasibility of such idealized cathode composition and such very thin separators needs to be proven. Also, according to Gallager et al., the specific energy on electrode level should be increased to approximately 700 Wh kg−1 to obtain a specific energy of ca. 225 Wh kg−1 at the battery level [146] which is not reached by any of these systems.

Supplementary data related to this article can be found at http://dx. doi.org/10.1016/j.jpowsour.2018.04.099. References [1] B. Zhang, R. Tan, L. Yang, J. Zheng, K. Zhang, S. Mo, Z. Lin, F. Pan, Mechanisms and properties of ion-transport in inorganic solid electrolytes, Energy Storage Mater 10 (2018) 139–159, http://dx.doi.org/10.1016/j.ensm.2017.08.015. [2] Z. Ma, H.-G. Xue, S.-P. Guo, Recent achievements on sulfide-type solid electrolytes: crystal structures and electrochemical performance, J. Mater. Sci. 53 (2018) 3927–3938, http://dx.doi.org/10.1007/s10853-017-1827-6. [3] L. Fan, S. Wei, S. Li, Q. Li, Y. Lu, Recent progress of the solid-state electrolytes for high-energy metal-based batteries, Adv. Energy Mater. (2018), http://dx.doi.org/ 10.1002/aenm.201702657. [4] A. Varzi, R. Raccichini, S. Passerini, B. Scrosati, Challenges and prospects on the role of solid electrolytes for the revitalization of lithium metal batteries, J. Mater. Chem. A (2016), http://dx.doi.org/10.1039/C6TA07384K. [5] J.C. Bachman, S. Muy, A. Grimaud, H.-H. Chang, N. Pour, S.F. Lux, O. Paschos, F. Maglia, S. Lupart, P. Lamp, L. Giordano, Y. Shao-Horn, Inorganic solid-state electrolytes for lithium batteries: mechanisms and properties governing ion conduction, Chem. Rev. 116 (2016) 140–162, http://dx.doi.org/10.1021/acs. chemrev.5b00563. [6] M. Monchak, T. Hupfer, A. Senyshyn, H. Boysen, D. Chernyshov, T. Hansen, K.G. Schell, E.C. Bucharsky, M.J. Hoffmann, H. Ehrenberg, Lithium diffusion pathway in Li1.3Al0.3Ti1.7(PO4)3 (LATP) superionic conductor, Inorg. Chem. 55 (2016) 2941–2945, http://dx.doi.org/10.1021/acs.inorgchem.5b02821. [7] D. Rettenwander, A. Welzl, S. Pristat, F. Tietz, S. Taibl, G.J. Redhammer, J. Fleig, A microcontact impedance study on NASICON-type Li 1+x Al x Ti 2−x (PO 4 ) 3 (0 ≤ x ≤ 0.5) single crystals, J. Mater. Chem. A 4 (2016) 1506–1513, http://dx. doi.org/10.1039/C5TA08545D. [8] F. Ma, E. Zhao, S. Zhu, W. Yan, D. Sun, Y. Jin, C. Nan, Preparation and evaluation of high lithium ion conductivity Li1.3Al0.3Ti1.7(PO4)3 solid electrolyte obtained using a new solution method, Solid State Ion 295 (2016) 7–12, http://dx.doi.org/ 10.1016/j.ssi.2016.07.010. [9] Y. Inaguma, M. Nakashima, A rechargeable lithium–air battery using a lithium ion-conducting lanthanum lithium titanate ceramics as an electrolyte separator, J. Power Sources 228 (2013) 250–255, http://dx.doi.org/10.1016/j.jpowsour.2012. 11.098. [10] Y. Inaguma, C. Liquan, M. Itoh, T. Nakamura, T. Uchida, H. Ikuta, M. Wakihara, High ionic conductivity in lithium lanthanum titanate, Solid State Commun. 86 (1993) 689–693, http://dx.doi.org/10.1016/0038-1098(93)90841-A. [11] S. Ramakumar, C. Deviannapoorani, L. Dhivya, L.S. Shankar, R. Murugan, Lithium garnets: synthesis, structure, Li+ conductivity, Li+ dynamics and applications, Prog. Mater. Sci. 88 (2017) 325–411, http://dx.doi.org/10.1016/j.pmatsci.2017. 04.007. [12] Y. Wang, W. Lai, High ionic conductivity lithium garnet oxides of Li7−xLa3Zr2−xTaxO12 compositions, Electrochem. Solid-State Lett. 15 (2012) A68–A71, http://dx.doi.org/10.1149/2.024205esl. [13] Y. Li, J. Tao Han, C. An Wang, H. Xie, J.B. Goodenough, Optimizing Li + conductivity in a garnet framework, J. Mater. Chem. 22 (2012) 15357–15361, http:// dx.doi.org/10.1039/C2JM31413D. [14] D. Wang, G. Zhong, O. Dolotko, Y. Li, M.J. McDonald, J. Mi, R. Fu, Y. Yang, The synergistic effects of Al and Te on the structure and Li+-mobility of garnet-type solid electrolytes, J. Mater. Chem. A 2 (2014) 20271–20279, http://dx.doi.org/10. 1039/C4TA03591G. [15] X. Tong, V. Thangadurai, E.D. Wachsman, Highly conductive Li garnets by a multielement doping strategy, Inorg. Chem. 54 (2015) 3600–3607, http://dx.doi. org/10.1021/acs.inorgchem.5b00184. [16] R. Murugan, V. Thangadurai, W. Weppner, Fast lithium ion conduction in garnettype Li7La3Zr2O12, Angew. Chem. Int. Ed. 46 (2007) 7778–7781, http://dx.doi. org/10.1002/anie.200701144. [17] C. Deviannapoorani, L. Dhivya, S. Ramakumar, R. Murugan, Lithium ion transport properties of high conductive tellurium substituted Li7La3Zr2O12 cubic lithium garnets, J. Power Sources 240 (2013) 18–25, http://dx.doi.org/10.1016/j. jpowsour.2013.03.166. [18] Y. Kato, S. Hori, T. Saito, K. Suzuki, M. Hirayama, A. Mitsui, M. Yonemura, H. Iba, R. Kanno, High-power all-solid-state batteries using sulfide superionic conductors, Nat. Energy 1 (2016) 16030, http://dx.doi.org/10.1038/nenergy.2016.30. [19] Y. Seino, T. Ota, K. Takada, A. Hayashi, M. Tatsumisago, A sulphide lithium super ion conductor is superior to liquid ion conductors for use in rechargeable batteries, Energy Environ. Sci. 7 (2014) 627–631, http://dx.doi.org/10.1039/C3EE41655K. [20] N. Kamaya, K. Homma, Y. Yamakawa, M. Hirayama, R. Kanno, M. Yonemura, T. Kamiyama, Y. Kato, S. Hama, K. Kawamoto, A. Mitsui, A lithium superionic conductor, Nat. Mater. 10 (2011) 682–686, http://dx.doi.org/10.1038/nmat3066. [21] D.A. Weber, A. Senyshyn, K.S. Weldert, S. Wenzel, W. Zhang, R. Kaiser, S. Berendts, J. Janek, W.G. Zeier, Structural insights and 3D diffusion pathways within the lithium superionic conductor Li10GeP2S12, Chem. Mater. 28 (2016) 5905–5915, http://dx.doi.org/10.1021/acs.chemmater.6b02424. [22] M. Murayama, R. Kanno, M. Irie, S. Ito, T. Hata, N. Sonoyama, Y. Kawamoto, Synthesis of new lithium ionic conductor thio-LISICON—lithium silicon sulfides system, J. Solid State Chem. 168 (2002) 140–148, http://dx.doi.org/10.1006/jssc. 2002.9701.

9. Summary and outlook Generally, research activities on hybrid electrolytes are behind those on liquid electrolytes. Considerable fundamental research is still required to better understand the transport phenomena within the hybrid electrolytes at the atomic scale. Model cell systems, such as those incorporating multilayer electrolytes, can help answering these questions. In-depth investigations on how to tune the activation energy that is required to transfer Li+ ions between the two electrolytes are also urgently needed. Adjusting the Li+-ion concentration and coordination within the two electrolytes, in order to level the chemical potential and activity of the Li+-ions within them, might also possibly reduce the activation energy and enable a better transfer. Additionally, Lewis acidbasis interactions among all components need further assessment. Especially, the possibility to tune the surface Lewis acid and basis centers of the inorganic ion conductors by modifications needs to be systematically studied, as well as their effect on the interphase with the polymer electrolyte. In general, little is known about interphases between inorganic and organic conductors. The existence of a space charge effect for polymer-inorganic hybrid electrolytes is hypothesized, but needs further in-depth clarification. For liquid-solid systems, the existence of an interphase resulting from the degradation phenomena is confirmed [149]. If similar reactions occur also in polymer-inorganic systems needs scientific verification. Finally, the combined answers to these fundamental questions can elucidate the conductivity mechanism in hybrid electrolytes. Although many fundamental questions remain unanswered, the potential ease of fabrication and integration in cells as well as the compatibility with lithium metal make polymer/inorganic hybrid electrolytes very attractive candidates for future all-solid-state batteries.

Declarations of interest None.

Acknowledgements The authors would like to acknowledge financial support from the German Federal Ministry of Education and Research within the FELIZIA project (03XP0026F), as well as from the Helmholtz Association. 221

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M. Keller et al.

[47] Z.D. Hood, H. Wang, Y. Li, A.S. Pandian, M. Parans Paranthaman, C. Liang, The “filler effect”: a study of solid oxide fillers with β-Li3PS4 for lithium conducting electrolytes, Solid State Ion 283 (2015) 75–80, http://dx.doi.org/10.1016/j.ssi. 2015.10.014. [48] A. Sakuda, K. Kuratani, M. Yamamoto, M. Takahashi, T. Takeuchi, H. Kobayashi, All-solid-state battery electrode sheets prepared by a slurry coating process, J. Electrochem. Soc. 164 (2017) A2474–A2478, http://dx.doi.org/10.1149/2. 0951712jes. [49] J.-H. Choi, C.-H. Lee, J.-H. Yu, C.-H. Doh, S.-M. Lee, Enhancement of ionic conductivity of composite membranes for all-solid-state lithium rechargeable batteries incorporating tetragonal Li7La3Zr2O12 into a polyethylene oxide matrix, J. Power Sources 274 (2015) 458–463, http://dx.doi.org/10.1016/j.jpowsour.2014.10.078. [50] F. Langer, I. Bardenhagen, J. Glenneberg, R. Kun, Microstructure and temperature dependent lithium ion transport of ceramic–polymer composite electrolyte for solid-state lithium ion batteries based on garnet-type Li7La3Zr2O12, Solid State Ion 291 (2016) 8–13, http://dx.doi.org/10.1016/j.ssi.2016.04.014. [51] K. (Kelvin) Fu, Y. Gong, J. Dai, A. Gong, X. Han, Y. Yao, C. Wang, Y. Wang, Y. Chen, C. Yan, Y. Li, E.D. Wachsman, L. Hu, Flexible, solid-state, ion-conducting membrane with 3D garnet nanofiber networks for lithium batteries, Proc. Natl. Acad. Sci. 113 (2016) 7094–7099, http://dx.doi.org/10.1073/pnas.1600422113. [52] J. Zheng, M. Tang, Y.-Y. Hu, Lithium ion pathway within Li7La3Zr2O12Polyethylene oxide composite electrolytes, Angew. Chem. 128 (2016) 12726–12730, http://dx.doi.org/10.1002/ange.201607539. [53] L. Chen, Y. Li, S.-P. Li, L.-Z. Fan, C.-W. Nan, J.B. Goodenough, PEO/Garnet composite electrolytes for solid-state lithium batteries: from “Ceramic-in-Polymer” to “Polymer-in-Ceramic”, Nano Energy (2017), http://dx.doi.org/10.1016/j. nanoen.2017.12.037. [54] T. Yang, J. Zheng, Q. Cheng, Y.-Y. Hu, C.K. Chan, Composite polymer electrolytes with Li7La3Zr2O12 garnet-type nanowires as ceramic fillers: mechanism of conductivity enhancement and role of doping and morphology, ACS Appl. Mater. Interfaces 9 (2017) 21773–21780, http://dx.doi.org/10.1021/acsami.7b03806. [55] J. Zhang, X. Zang, H. Wen, T. Dong, J. Chai, Y. Li, B. Chen, J. Zhao, S. Dong, J. Ma, L. Yue, Z. Liu, X. Guo, G. Cui, L. Chen, High-voltage and free-standing poly(propylene carbonate)/Li 6.75 La 3 Zr 1.75 Ta 0.25 O 12 composite solid electrolyte for wide temperature range and flexible solid lithium ion battery, J. Mater. Chem. A 5 (2017) 4940–4948, http://dx.doi.org/10.1039/C6TA10066J. [56] J. Płcharski, W. Weiczorek, PEO based composite solid electrolyte containing nasicon, Solid State Ion 28 (1988) 979–982, http://dx.doi.org/10.1016/01672738(88)90315-3. [57] K.M. Nairn, A.S. Best, P.J. Newman, D.R. MacFarlane, M. Forsyth, Ceramicpolymer interface in composite electrolytes of lithium aluminium titanium phosphate and polyetherurethane polymer electrolyte, Solid State Ion 121 (1999) 115–119, http://dx.doi.org/10.1016/S0167-2738(98)00537-2. [58] K. Nairn, M. Forsyth, H. Every, M. Greville, D.R. MacFarlane, Polymer-ceramic ion-conducting composites, Solid State Ion 86 (1996) 589–593, http://dx.doi.org/ 10.1016/0167-2738(96)00212-3. [59] Y.-J. Wang, Y. Pan, L. Chen, Ion-conducting polymer electrolyte based on poly (ethylene oxide) complexed with Li1.3Al0.3Ti1.7(PO4)3 salt, Mater. Chem. Phys. 92 (2005) 354–360, http://dx.doi.org/10.1016/j.matchemphys.2004.12.004. [60] Y.-J. Wang, Y. Pan, D. Kim, Conductivity studies on ceramic Li1.3Al0.3Ti1.7(PO4) 3-filled PEO-based solid composite polymer electrolytes, J. Power Sources 159 (2006) 690–701, http://dx.doi.org/10.1016/j.jpowsour.2005.10.104. [61] Y.-C. Jung, S.-M. Lee, J.-H. Choi, S.S. Jang, D.-W. Kim, All solid-state lithium batteries assembled with hybrid solid electrolytes, J. Electrochem. Soc. 162 (2015) A704–A710, http://dx.doi.org/10.1149/2.0731504jes. [62] S. Chen, Y. Zhao, J. Yang, L. Yao, X. Xu, Hybrid solid electrolytes with excellent electrochemical properties and their applications in all-solid-state cells, Ionics (2016) 1–9, http://dx.doi.org/10.1007/s11581-016-1905-9. [63] Y. Zhao, Z. Huang, S. Chen, B. Chen, J. Yang, Q. Zhang, F. Ding, Y. Chen, X. Xu, A promising PEO/LAGP hybrid electrolyte prepared by a simple method for all-solidstate lithium batteries, Solid State Ion 295 (2016) 65–71, http://dx.doi.org/10. 1016/j.ssi.2016.07.013. [64] H. Zhai, P. Xu, M. Ning, Q. Cheng, J. Mandal, Y. Yang, A flexible solid composite electrolyte with vertically aligned and connected ion-conducting nanoparticles for lithium batteries, Nano Lett. 17 (2017) 3182–3187, http://dx.doi.org/10.1021/ acs.nanolett.7b00715. [65] W. Liu, N. Liu, J. Sun, P.-C. Hsu, Y. Li, H.-W. Lee, Y. Cui, Ionic conductivity enhancement of polymer electrolytes with ceramic nanowire fillers, Nano Lett. 15 (2015) 2740–2745, http://dx.doi.org/10.1021/acs.nanolett.5b00600. [66] W. Liu, S.W. Lee, D. Lin, F. Shi, S. Wang, A.D. Sendek, Y. Cui, Enhancing ionic conductivity in composite polymer electrolytes with well-aligned ceramic nanowires, Nat. Energy 2 (2017), http://dx.doi.org/10.1038/nenergy.2017.35 nenergy201735. [67] J. Bae, Y. Li, J. Zhang, X. Zhou, F. Zhao, Y. Shi, J. Goodenough, G. Yu, A 3D nanostructured hydrogel framework-derived high-performance composite polymer Li-Ion electrolyte, Angew. Chem. (2018), http://dx.doi.org/10.1002/ ange.201710841. [68] P. Buvana, K. Vishista, D. Shanmukaraj, R. Murugan, Lithium garnet oxide dispersed polymer composite membrane for rechargeable lithium batteries, Ionics (2016) 1–8, http://dx.doi.org/10.1007/s11581-016-1830-y. [69] G.B. Appetecchi, F. Croce, G. Dautzenberg, M. Mastragostino, F. Ronci, B. Scrosati, F. Soavi, A. Zanelli, F. Alessandrini, P.P. Prosini, Composite polymer electrolytes with improved lithium metal electrode interfacial properties I. Elechtrochemical properties of dry PEO-LiX systems, J. Electrochem. Soc. 145 (1998) 4126–4132, http://dx.doi.org/10.1149/1.1838925. [70] H.-G. Wang, S. Yuan, D.-L. Ma, X.-B. Zhang, J.-M. Yan, Electrospun materials for

[23] Z. Liu, W. Fu, E.A. Payzant, X. Yu, Z. Wu, N.J. Dudney, J. Kiggans, K. Hong, A.J. Rondinone, C. Liang, Anomalous high ionic conductivity of nanoporous βLi3PS4, J. Am. Chem. Soc. 135 (2013) 975–978, http://dx.doi.org/10.1021/ ja3110895. [24] P. Hartmann, T. Leichtweiss, M.R. Busche, M. Schneider, M. Reich, J. Sann, P. Adelhelm, J. Janek, Degradation of nasicon-type materials in contact with lithium metal: formation of mixed conducting interphases (MCI) on solid electrolytes, J. Phys. Chem. C 117 (2013) 21064–21074, http://dx.doi.org/10.1021/ jp4051275. [25] H. Chung, B. Kang, Mechanical and Thermal Failure Induced by Contact between a Li1.5Al0.5Ge1.5(PO4)3 Solid Electrolyte and Li Metal in an All Solid-state Li Cell, (2017), http://dx.doi.org/10.1021/acs.chemmater.7b02301. [26] H. Kitaura, A. Hayashi, K. Tadanaga, M. Tatsumisago, Electrochemical performance of all-solid-state lithium secondary batteries with Li–Ni–Co–Mn oxide positive electrodes, Electrochim. Acta 55 (2010) 8821–8828, http://dx.doi.org/10. 1016/j.electacta.2010.07.066. [27] C. Ma, K. Chen, C. Liang, C.-W. Nan, R. Ishikawa, K. More, M. Chi, Atomic-scale origin of the large grain-boundary resistance in perovskite Li-ion-conducting solid electrolytes, Energy Environ. Sci. 7 (2014) 1638–1642, http://dx.doi.org/10. 1039/C4EE00382A. [28] C.H. Chen, K. Amine, Ionic conductivity, lithium insertion and extraction of lanthanum lithium titanate, Solid State Ion 144 (2001) 51–57, http://dx.doi.org/10. 1016/S0167-2738(01)00884-0. [29] Y.-H. Cho, J. Wolfenstine, E. Rangasamy, H. Kim, H. Choe, J. Sakamoto, Mechanical properties of the solid Li-ion conducting electrolyte: Li0.33La0.57TiO3, J. Mater. Sci. 47 (2012) 5970–5977, http://dx.doi.org/10. 1007/s10853-012-6500-5. [30] Y. Zhu, X. He, Y. Mo, First principles study on electrochemical and chemical stability of solid electrolyte–electrode interfaces in all-solid-state Li-ion batteries, J. Mater. Chem. A 4 (2016) 3253–3266, http://dx.doi.org/10.1039/C5TA08574H. [31] A. Sharafi, E. Kazyak, A.L. Davis, S. Yu, T. Thompson, D.J. Siegel, N.P. Dasgupta, J. Sakamoto, Surface chemistry mechanism of ultra-low interfacial resistance in the solid-state electrolyte Li7La3Zr2O12, Chem. Mater. 29 (2017) 7961–7968, http://dx.doi.org/10.1021/acs.chemmater.7b03002. [32] M. Wang, J. Sakamoto, Correlating the interface resistance and surface adhesion of the Li metal-solid electrolyte interface, J. Power Sources 377 (2018) 7–11, http:// dx.doi.org/10.1016/j.jpowsour.2017.11.078. [33] K. (Kelvin) Fu, Y. Gong, B. Liu, Y. Zhu, S. Xu, Y. Yao, W. Luo, C. Wang, S.D. Lacey, J. Dai, Y. Chen, Y. Mo, E. Wachsman, L. Hu, Toward garnet electrolyte–based Li metal batteries: an ultrathin, highly effective, artificial solid-state electrolyte/ metallic Li interface, Sci. Adv. 3 (2017) e1601659, , http://dx.doi.org/10.1126/ sciadv.1601659. [34] C. Sun, J. Liu, Y. Gong, D.P. Wilkinson, J. Zhang, Recent advances in all-solid-state rechargeable lithium batteries, Nano Energy 33 (2017) 363–386, http://dx.doi. org/10.1016/j.nanoen.2017.01.028. [35] H.W. Kim, P. Manikandan, Y.J. Lim, J.H. Kim, S. Nam, Y. Kim, Hybrid solid electrolyte with the combination of Li7La3Zr2O12 ceramic and ionic liquid for high voltage pseudo-solid-state Li-ion batteries, J. Mater. Chem. A (2016), http:// dx.doi.org/10.1039/C6TA07268B. [36] F. Aguesse, W. Manalastas, L. Buannic, J.M. Lopez del Amo, G. Singh, A. Llordés, J. Kilner, Investigating the dendritic growth during full cell cycling of garnet electrolyte in direct contact with Li metal, ACS Appl. Mater. Interfaces 9 (2017) 3808–3816, http://dx.doi.org/10.1021/acsami.6b13925. [37] W. Zhou, S. Wang, Y. Li, S. Xin, A. Manthiram, J.B. Goodenough, Plating a dendrite-free lithium anode with a polymer/ceramic/polymer sandwich electrolyte, J. Am. Chem. Soc. 138 (2016) 9385–9388, http://dx.doi.org/10.1021/jacs.6b05341. [38] E. Rangasamy, G. Sahu, J.K. Keum, A.J. Rondinone, N.J. Dudney, C. Liang, A high conductivity oxide–sulfide composite lithium superionic conductor, J. Mater. Chem. A 2 (2014) 4111–4116, http://dx.doi.org/10.1039/C3TA15223E. [39] M. Keller, G.B. Appetecchi, G.-T. Kim, V. Sharova, M. Schneider, J. Schuhmacher, A. Roters, S. Passerini, Electrochemical performance of a solvent-free hybrid ceramic-polymer electrolyte based on Li7La3Zr2O12 in P(EO)15LiTFSI, J. Power Sources 353 (2017) 287–297, http://dx.doi.org/10.1016/j.jpowsour.2017.04.014. [40] S.-W. Baek, I. Honma, J. Kim, D. Rangappa, Solidified inorganic-organic hybrid electrolyte for all solid state flexible lithium battery, J. Power Sources 343 (2017) 22–29, http://dx.doi.org/10.1016/j.jpowsour.2017.01.030. [41] D.Y. Oh, Y.J. Nam, K.H. Park, S.H. Jung, S.-J. Cho, Y.K. Kim, Y.-G. Lee, S.-Y. Lee, Y.S. Jung, Excellent compatibility of solvate ionic liquids with sulfide solid electrolytes: toward favorable ionic contacts in bulk-type all-solid-state lithium-ion batteries, Adv. Energy Mater. 5 (2015) 1–7, http://dx.doi.org/10.1002/aenm. 201500865. [42] A. Arya, A.L. Sharma, Polymer electrolytes for lithium ion batteries: a critical study, Ionics 23 (2017) 497–540, http://dx.doi.org/10.1007/s11581-016-1908-6. [43] I. Osada, H. de Vries, B. Scrosati, S. Passerini, Ionic-liquid-based polymer electrolytes for battery applications, Angew. Chem. Int. Ed. 55 (2016) 500–513, http://dx.doi.org/10.1002/anie.201504971. [44] Y.J. Lim, H.W. Kim, S.S. Lee, H.J. Kim, J.-K. Kim, Y.-G. Jung, Y. Kim, Ceramicbased composite solid electrolyte for lithium-ion batteries, ChemPlusChem 80 (2015) 1100–1103, http://dx.doi.org/10.1002/cplu.201500106. [45] Y.-C. Jung, S.-K. Kim, M.-S. Kim, J.-H. Lee, M.-S. Han, D.-H. Kim, W.-C. Shin, M. Ue, D.-W. Kim, Ceramic separators based on Li+-conducting inorganic electrolyte for high-performance lithium-ion batteries with enhanced safety, J. Power Sources 293 (2015) 675–683, http://dx.doi.org/10.1016/j.jpowsour.2015.06.001. [46] K. Yoshima, Y. Harada, N. Takami, Thin hybrid electrolyte based on garnet-type lithium-ion conductor Li7La3Zr2O12 for 12 V-class bipolar batteries, J. Power Sources 302 (2016) 283–290, http://dx.doi.org/10.1016/j.jpowsour.2015.10.031.

222

Journal of Power Sources 392 (2018) 206–225

M. Keller et al.

[71]

[72]

[73]

[74]

[75]

[76]

[77]

[78]

[79]

[80]

[81]

[82]

[83]

[84]

[85]

[86]

[87]

[88]

[89]

[90]

[91]

[92]

[93]

[94]

(1994) 538–542, http://dx.doi.org/10.1021/cm00040a032. [95] X. Judez, H. Zhang, C. Li, G.G. Eshetu, Y. Zhang, J.A. González-Marcos, M. Armand, L.M. Rodriguez-Martinez, Polymer-rich composite electrolytes for allsolid-state Li–S cells, J. Phys. Chem. Lett. (2017) 3473–3477, http://dx.doi.org/ 10.1021/acs.jpclett.7b01321. [96] M. Wetjen, M.A. Navarra, S. Panero, S. Passerini, B. Scrosati, J. Hassoun, Composite poly(ethylene oxide) electrolytes plasticized by N-Alkyl-N-butylpyrrolidinium bis(trifluoromethanesulfonyl)imide for lithium batteries, ChemSusChem 6 (2013) 1037–1043, http://dx.doi.org/10.1002/cssc.201300105. [97] Y. Zhao, C. Wu, G. Peng, X. Chen, X. Yao, Y. Bai, F. Wu, S. Chen, X. Xu, A new solid polymer electrolyte incorporating Li10GeP2S12 into a polyethylene oxide matrix for all-solid-state lithium batteries, J. Power Sources 301 (2016) 47–53, http://dx. doi.org/10.1016/j.jpowsour.2015.09.111. [98] M. Gauthier, T.J. Carney, A. Grimaud, L. Giordano, N. Pour, H.-H. Chang, D.P. Fenning, S.F. Lux, O. Paschos, C. Bauer, F. Maglia, S. Lupart, P. Lamp, Y. Shao-Horn, Electrode–electrolyte interface in Li-Ion batteries: current understanding and new insights, J. Phys. Chem. Lett. 6 (2015) 4653–4672, http://dx. doi.org/10.1021/acs.jpclett.5b01727. [99] W. Li, B. Song, A. Manthiram, High-voltage positive electrode materials for lithium-ion batteries, Chem. Soc. Rev. 46 (2017) 3006–3059, http://dx.doi.org/10. 1039/c6cs00875e. [100] M. Wetjen, G.-T. Kim, M. Joost, G.B. Appetecchi, M. Winter, S. Passerini, Thermal and electrochemical properties of PEO-LiTFSI-Pyr14TFSI-based composite cathodes, incorporating 4 V-class cathode active materials, J. Power Sources 246 (2014) 846–857, http://dx.doi.org/10.1016/j.jpowsour.2013.08.037. [101] B. Sun, C. Xu, J. Mindemark, T. Gustafsson, K. Edström, D. Brandell, At the polymer electrolyte interfaces: the role of the polymer host in interphase layer formation in Li-batteries, J. Mater. Chem. A 3 (2015) 13994–14000, http://dx.doi. org/10.1039/C5TA02485D. [102] C. Xu, B. Sun, T. Gustafsson, K. Edström, D. Brandell, M. Hahlin, Interface layer formation in solid polymer electrolyte lithium batteries: an XPS study, J. Mater. Chem. A 2 (2014) 7256–7264, http://dx.doi.org/10.1039/C4TA00214H. [103] W. Wang, E. Yi, A.J. Fici, R.M. Laine, J. Kieffer, Lithium ion conducting poly (ethylene oxide)-based solid electrolytes containing active or passive ceramic nanoparticles, J. Phys. Chem. C 121 (2017) 2563–2573, http://dx.doi.org/10. 1021/acs.jpcc.6b11136. [104] S. Kalnaus, W.E. Tenhaeff, J. Sakamoto, A.S. Sabau, C. Daniel, N.J. Dudney, Analysis of composite electrolytes with sintered reinforcement structure for energy storage applications, J. Power Sources 241 (2013) 178–185, http://dx.doi.org/10. 1016/j.jpowsour.2013.04.096. [105] A. Bunde, W. Dieterich, Percolation in composites, J. Electroceramics 5 (2000) 81–92, http://dx.doi.org/10.1023/A:1009997800513. [106] M. Wakihara, Recent developments in lithium ion batteries, Mater. Sci. Eng. R Rep. 33 (2001) 109–134, http://dx.doi.org/10.1016/S0927-796X(01)00030-4. [107] G.B. Appetecchi, M. Montanino, A. Balducci, S.F. Lux, M. Winterb, S. Passerini, Lithium insertion in graphite from ternary ionic liquid-lithium salt electrolytes: I. Electrochemical characterization of the electrolytes, J. Power Sources 192 (2009) 599–605, http://dx.doi.org/10.1016/j.jpowsour.2008.12.095. [108] G.-T. Kim, G.B. Appetecchi, F. Alessandrini, S. Passerini, Solvent-free, PYR1ATFSI ionic liquid-based ternary polymer electrolyte systems: I. Electrochemical characterization, J. Power Sources 171 (2007) 861–869, http://dx.doi.org/10.1016/j. jpowsour.2007.07.020. [109] V. Sharova, G.-T. Kim, G.A. Giffin, A. Lex-Balducci, S. Passerini, Quaternary polymer electrolytes containing an ionic liquid and a ceramic filler, Macromol. Rapid Commun. (2016) 1188–1193, http://dx.doi.org/10.1002/marc.201600025. [110] C.R. Yang, J.T. Perng, Y.Y. Wang, C.C. Wan, Conductive behaviour of lithium ions in polyacrylonitrile, J. Power Sources 62 (1996) 89–93, http://dx.doi.org/10. 1016/S0378-7753(96)02414-7. [111] Y.W. Chen-Yang, H.C. Chen, F.J. Lin, C.C. Chen, Polyacrylonitrile electrolytes, Solid State Ion 150 (2002) 327–335, http://dx.doi.org/10.1016/S0167-2738(02) 00457-5. [112] R.-J. Chen, Y.-B. Zhang, T. Liu, B.-Q. Xu, Y.-H. Lin, C.-W. Nan, Y. Shen, Addressing the interface issues in all-solid-state bulk-type lithium ion battery via an allcomposite approach, ACS Appl. Mater. Interfaces 9 (2017) 9654–9661, http://dx. doi.org/10.1021/acsami.6b16304. [113] F. Chen, D. Yang, W. Zha, B. Zhu, Y. Zhang, J. Li, Y. Gu, Q. Shen, L. Zhang, D.R. Sadoway, Solid polymer electrolytes incorporating cubic Li7La3Zr2O12 for all-solid-state lithium rechargeable batteries, Electrochim. Acta 258 (2017) 1106–1114, http://dx.doi.org/10.1016/j.electacta.2017.11.164. [114] S. Chen, J. Wang, Z. Zhang, L. Wu, L. Yao, Z. Wei, Y. Deng, D. Xie, X. Yao, X. Xu, In-situ preparation of poly(ethylene oxide)/Li3PS4 hybrid polymer electrolyte with good nanofiller distribution for rechargeable solid-state lithium batteries, J. Power Sources 387 (2018) 72–80, http://dx.doi.org/10.1016/j.jpowsour.2018.03. 016. [115] S.H.-S. Cheng, K.-Q. He, Y. Liu, J.-W. Zha, M. Kamruzzaman, R.L.-W. Ma, Z.M. Dang, R.K.Y. Li, C.Y. Chung, Electrochemical performance of all-solid-state lithium batteries using inorganic lithium garnets particulate reinforced PEO/ LiClO4 electrolyte, Electrochim. Acta 253 (2017) 430–438, http://dx.doi.org/10. 1016/j.electacta.2017.08.162. [116] L. Yang, Z. Wang, Y. Feng, R. Tan, Y. Zuo, R. Gao, Y. Zhao, L. Han, Z. Wang, F. Pan, Flexible composite solid electrolyte facilitating highly stable “soft contacting” Li–Electrolyte interface for solid state lithium-ion batteries, Adv. Energy Mater. 7 (2017), http://dx.doi.org/10.1002/aenm.201701437. [117] A. Hayashi, T. Harayama, F. Mizuno, M. Tatsumisago, Mechanochemical synthesis of hybrid electrolytes from the Li2S–P2S5 glasses and polyethers, J. Power Sources 163 (2006) 289–293, http://dx.doi.org/10.1016/j.jpowsour.2006.06.018.

lithium and sodium rechargeable batteries: from structure evolution to electrochemical performance, Energy Environ. Sci. 8 (2015) 1660–1681, http://dx.doi. org/10.1039/C4EE03912B. K. Minami, A. Hayashi, M. Tatsumisago, Characterization of solid electrolytes prepared from Li2S – P2S5 glass and ionic liquids, J. Electrochem. Soc. 157 (2010) A1296–A1301, http://dx.doi.org/10.1149/1.3489352. B. Xu, H. Duan, H. Liu, C. Wang, S. Zhong, Stabilization of garnet/liquid electrolyte interface using superbase additives for hybrid Li batteries, ACS Appl. Mater. Interfaces 9 (2017) 21077–21082, http://dx.doi.org/10.1021/acsami. 7b05599. F. Sagane, T. Abe, Y. Iriyama, Z. Ogumi, Li+ and Na+ transfer through interfaces between inorganic solid electrolytes and polymer or liquid electrolytes, J. Power Sources 146 (2005) 749–752, http://dx.doi.org/10.1016/j.jpowsour.2005.03.075. Y. Zhu, X. He, Y. Mo, Origin of outstanding stability in the lithium solid electrolyte materials: insights from thermodynamic analyses based on first-principles calculations, ACS Appl. Mater. Interfaces 7 (2015) 23685–23693, http://dx.doi.org/10. 1021/acsami.5b07517. B. Chen, Z. Huang, X. Chen, Y. Zhao, Q. Xu, P. Long, S. Chen, X. Xu, A new composite solid electrolyte PEO/Li10GeP2S12/SN for all-solid-state lithium battery, Electrochim. Acta 210 (2016) 905–914, http://dx.doi.org/10.1016/j. electacta.2016.06.025. C. Wang, Y. Yang, X. Liu, H. Zhong, H. Xu, Z. Xu, H. Shao, F. Ding, Suppression of lithium dendrite formation by using LAGP-PEO (LiTFSI) composite solid electrolyte and lithium metal anode modified by PEO (LiTFSI) in all-solid-state lithium batteries, ACS Appl. Mater. Interfaces 9 (2017) 13694–13702, http://dx.doi.org/ 10.1021/acsami.7b00336. Y.-C. Jung, M.-S. Park, C.-H. Doh, D.-W. Kim, Organic-inorganic hybrid solid electrolytes for solid-state lithium cells operating at room temperature, Electrochim. Acta 218 (2016) 271–277, http://dx.doi.org/10.1016/j.electacta. 2016.09.141. I. Villaluenga, K.H. Wujcik, W. Tong, D. Devaux, D.H.C. Wong, J.M. DeSimone, N.P. Balsara, Compliant glass–polymer hybrid single ion-conducting electrolytes for lithium batteries, Proc. Natl. Acad. Sci. 113 (2016) 52–57, http://dx.doi.org/ 10.1073/pnas.1520394112. C. Monroe, J. Newman, The impact of elastic deformation on deposition kinetics at lithium/polymer interfaces, J. Electrochem. Soc. 152 (2005) A396–A404, http:// dx.doi.org/10.1149/1.1850854. Y. Ren, Y. Shen, Y. Lin, C.-W. Nan, Direct observation of lithium dendrites inside garnet-type lithium-ion solid electrolyte, Electrochem. Commun. 57 (2015) 27–30, http://dx.doi.org/10.1016/j.elecom.2015.05.001. L. Cheng, W. Chen, M. Kunz, K. Persson, N. Tamura, G. Chen, M. Doeff, Effect of surface microstructure on electrochemical performance of garnet solid electrolytes, ACS Appl. Mater. Interfaces 7 (2015) 2073–2081, http://dx.doi.org/10. 1021/am508111r. G.M. Stone, S.A. Mullin, A.A. Teran, D.T. Hallinan, A.M. Minor, A. Hexemer, N.P. Balsara, Resolution of the modulus versus adhesion dilemma in solid polymer electrolytes for rechargeable lithium metal batteries, J. Electrochem. Soc. 159 (2012) A222–A227, http://dx.doi.org/10.1149/2.030203jes. D. Lin, Y. Liu, Y. Cui, Reviving the lithium metal anode for high-energy batteries, Nat. Nanotechnol. 12 (2017) 194–206, http://dx.doi.org/10.1038/nnano. 2017.16. W.D. Richards, L.J. Miara, Y. Wang, J.C. Kim, G. Ceder, Interface stability in solidstate batteries, Chem. Mater. 28 (2016) 266–273, http://dx.doi.org/10.1021/acs. chemmater.5b04082. E. Peled, The electrochemical behavior of alkali and alkaline earth metals in nonaqueous battery systems—the solid electrolyte interphase model, J. Electrochem. Soc. 126 (1979) 2047–2051, http://dx.doi.org/10.1149/1.2128859. E. Peled, D. Golodnitsky, G. Ardel, Advanced model for solid electrolyte interphase electrodes in liquid and polymer electrolytes, J. Electrochem. Soc. 144 (1997) L208–L210, http://dx.doi.org/10.1149/1.1837858. S. Wenzel, T. Leichtweiss, D. Krüger, J. Sann, J. Janek, Interphase formation on lithium solid electrolytes—an in situ approach to study interfacial reactions by photoelectron spectroscopy, Solid State Ion 278 (2015) 98–105, http://dx.doi.org/ 10.1016/j.ssi.2015.06.001. D. Aurbach, B. Markovsky, G. Salitra, E. Markevich, Y. Talyossef, M. Koltypin, L. Nazar, B. Ellis, D. Kovacheva, Review on electrode–electrolyte solution interactions, related to cathode materials for Li-ion batteries, J. Power Sources 165 (2007) 491–499, http://dx.doi.org/10.1016/j.jpowsour.2006.10.025. B.V. Lotsch, J. Maier, Relevance of solid electrolytes for lithium-based batteries: a realistic view, J. Electroceramics (2017) 1–14, http://dx.doi.org/10.1007/ s10832-017-0091-0. G.B. Appetecchi, S. Passerini, Poly(ethylene oxide)- LiN ( so 2 CF 2 CF 3 ) 2 polymer electrolytes ii. Characterization of the interface with lithium, J. Electrochem. Soc. 149 (2002) A891–A897, http://dx.doi.org/10.1149/1. 1483098. S. Wenzel, S.J. Sedlmaier, C. Dietrich, W.G. Zeier, J. Janek, Interfacial reactivity and interphase growth of argyrodite solid electrolytes at lithium metal electrodes, Solid State Ion (2017), http://dx.doi.org/10.1016/j.ssi.2017.07.005. C. Yang, K. Fu, Y. Zhang, E. Hitz, L. Hu, Protected lithium-metal anodes in batteries: from liquid to solid, Adv. Mater. 29 (2017), http://dx.doi.org/10.1002/ adma.201701169. F. Croce, F. Gerace, G. Dautzemberg, S. Passerini, G.B. Appetecchi, B. Scrosati, Synthesis and characterization of highly conducting gel electrolytes, Electrochim. Acta 39 (1994) 2187–2194, http://dx.doi.org/10.1016/0013-4686(94)E0167-X. G. Dautzenberg, F. Croce, S. Passerini, B. Scrosati, Characterization of PAN-Based gel electrolytes. Electrochemical stability and lithium cyclability, Chem. Mater. 6

223

Journal of Power Sources 392 (2018) 206–225

M. Keller et al.

[118] J. Zhang, N. Zhao, M. Zhang, Y. Li, P.K. Chu, X. Guo, Z. Di, X. Wang, H. Li, Flexible and ion-conducting membrane electrolytes for solid-state lithium batteries: dispersion of garnet nanoparticles in insulating polyethylene oxide, Nano Energy 28 (2016) 447–454, http://dx.doi.org/10.1016/j.nanoen.2016.09.002. [119] M.-S. Park, Y.-C. Jung, D.-W. Kim, Hybrid solid electrolytes composed of poly(1,4butylene adipate) and lithium aluminum germanium phosphate for all-solid-state Li/LiNi0.6Co0.2Mn0.2O2 cells, Solid State Ion 315 (2018) 65–70, http://dx.doi. org/10.1016/j.ssi.2017.12.007. [120] J. Zheng, H. Dang, X. Feng, P.-H. Chien, Y.-Y. Hu, Li-ion transport in a representative ceramic–polymer–plasticizer composite electrolyte: Li7La3Zr2O12–polyethylene oxide–tetraethylene glycol dimethyl ether, J. Mater. Chem. A 5 (2017) 18457–18463, http://dx.doi.org/10.1039/C7TA05832B. [121] H. Huo, N. Zhao, J. Sun, F. Du, Y. Li, X. Guo, Composite electrolytes of polyethylene oxides/garnets interfacially wetted by ionic liquid for room-temperature solid-state lithium battery, J. Power Sources 372 (2017) 1–7, http://dx.doi.org/ 10.1016/j.jpowsour.2017.10.059. [122] Q. Guo, Y. Han, H. Wang, S. Xiong, Y. Li, S. Liu, K. Xie, New class of LAGP-based solid polymer composite electrolyte for efficient and safe solid-state lithium batteries, ACS Appl. Mater. Interfaces 9 (2017) 41837–41844, http://dx.doi.org/10. 1021/acsami.7b12092. [123] V. Thangadurai, S. Narayanan, D. Pinzaru, Garnet-type solid-state fast Li ion conductors for Li batteries: critical review, Chem. Soc. Rev. 43 (2014) 4714–4727, http://dx.doi.org/10.1039/C4CS00020J. [124] J.B. Goodenough, Ceramic solid electrolytes, Solid State Ion 94 (1997) 17–25, http://dx.doi.org/10.1016/S0167-2738(96)00501-2. [125] P.G. Bruce, Solid State Electrochemistry, Cambridge University Press, 1997. [126] J. Mindemark, M.J. Lacey, T. Bowden, D. Brandell, Beyond PEO—alternative host materials for Li+-conducting solid polymer electrolytes, Prog. Polym. Sci. (2018), http://dx.doi.org/10.1016/j.progpolymsci.2017.12.004. [127] M.A. Ratner, P. Johansson, D.F. Shriver, Polymer electrolytes: ionic transport mechanisms and relaxation coupling, MRS Bull. 25 (2000) 31–37, http://dx.doi. org/10.1557/mrs2000.16. [128] M. Marcinek, J. Syzdek, M. Marczewski, M. Piszcz, L. Niedzicki, M. Kalita, A. Plewa-Marczewska, A. Bitner, P. Wieczorek, T. Trzeciak, M. Kasprzyk, P. Łężak, Z. Zukowska, A. Zalewska, W. Wieczorek, Electrolytes for Li-ion transport – review, Solid State Ion 276 (2015) 107–126, http://dx.doi.org/10.1016/j.ssi.2015. 02.006. [129] A. Vallée, S. Besner, J. Prud'Homme, Comparative study of poly(ethylene oxide) electrolytes made with LiN(CF3SO2)2, LiCF3SO3 and LiClO4: thermal properties and conductivity behaviour, Electrochim. Acta 37 (1992) 1579–1583, http://dx. doi.org/10.1016/0013-4686(92)80115-3. [130] S. Srivastava, J.L. Schaefer, Z. Yang, Z. Tu, L.A. Archer, 25th anniversary article: polymer–particle composites: phase stability and applications in electrochemical energy storage, Adv. Mater. 26 (2014) 201–234, http://dx.doi.org/10.1002/ adma.201303070. [131] F. Croce, G.B. Appetecchi, L. Persi, B. Scrosati, Nanocomposite polymer electrolytes for lithium batteries, Nature 394 (1998) 456–458, http://dx.doi.org/10. 1038/28818. [132] C. Wang, X.-W. Zhang, A.J. Appleby, Solvent-free composite PEO-ceramic fiber/ mat electrolytes for lithium secondary cells, J. Electrochem. Soc. 152 (2005) A205–A209, http://dx.doi.org/10.1149/1.1828952. [133] X. Tao, Y. Liu, W. Liu, G. Zhou, J. Zhao, D. Lin, C. Zu, O. Sheng, W. Zhang, H.W. Lee, Y. Cui, Solid-state lithium–sulfur batteries operated at 37 °C with composites of nanostructured Li7La3Zr2O12/Carbon foam and polymer, Nano Lett. 17 (2017) 2967–2972, http://dx.doi.org/10.1021/acs.nanolett.7b00221. [134] S. Xin, Y. You, S. Wang, H.-C. Gao, Y.-X. Yin, Y.-G. Guo, Solid-state lithium metal batteries promoted by nanotechnology: progress and prospects, ACS Energy Lett. 2 (2017) 1385–1394, http://dx.doi.org/10.1021/acsenergylett.7b00175. [135] J. Maier, Ionic conduction in space charge regions, Prog. Solid State Chem. 23 (1995) 171–263, http://dx.doi.org/10.1016/0079-6786(95)00004-E. [136] J. Maier, Nanoionics: ion transport and electrochemical storage in confined systems, Nat. Mater. 4 (2005) 805–815, http://dx.doi.org/10.1038/nmat1513. [137] J. Przyluski, M. Siekierski, W. Wieczorek, Effective medium theory in studies of conductivity of composite polymeric electrolytes, Electrochim. Acta 40 (1995) 2101–2108, http://dx.doi.org/10.1016/0013-4686(95)00147-7. [138] N.J. Dudney, W.C. West, J. Nanda, Handbook of Solid State Batteries, World Scientific, 2015. [139] M. Doyle, T.F. Fuller, J. Newman, The importance of the lithium ion transference number in lithium/polymer cells, Electrochim. Acta 39 (1994) 2073–2081, http:// dx.doi.org/10.1016/0013-4686(94)85091-7. [140] H. Zhang, C. Li, M. Piszcz, E. Coya, T. Rojo, L.M. Rodriguez-Martinez, M. Armand, Z. Zhou, Single lithium-ion conducting solid polymer electrolytes: advances and perspectives, Chem. Soc. Rev. 46 (2017) 797–815, http://dx.doi.org/10.1039/ C6CS00491A. [141] L. Long, S. Wang, M. Xiao, Y. Meng, Polymer electrolytes for lithium polymer batteries, J. Mater. Chem. A (2016), http://dx.doi.org/10.1039/C6TA02621D. [142] P.G. Bruce, J. Evans, C.A. Vincent, Conductivity and transference number measurements on polymer electrolytes, Solid State Ion 28 (1988) 918–922, http://dx. doi.org/10.1016/0167-2738(88)90304-9. [143] K.M. Diederichsen, E.J. McShane, B.D. McCloskey, Promising routes to a high Li+ transference number electrolyte for lithium ion batteries, ACS Energy Lett. 2 (2017) 2563–2575, http://dx.doi.org/10.1021/acsenergylett.7b00792. [144] J. Zheng, Y.-Y. Hu, New insights into the compositional dependence of Li-Ion transport in polymer–ceramic composite electrolytes, ACS Appl. Mater. Interfaces (2018), http://dx.doi.org/10.1021/acsami.7b17301. [145] J.-G. Zhang, W. Xu, W.A. Henderson, Lithium metal anodes and rechargeable

[146]

[147]

[148]

[149]

[150]

[151]

[152]

[153]

[154]

[155]

[156]

[157]

[158]

[159]

[160]

[161]

[162]

[163] [164]

[165]

[166] [167]

[168]

[169]

[170]

224

lithium metal batteries, Lithium Met. Anodes Recharg. Lithium Met. Batter, Springer, 2017, pp. 45–152. K.G. Gallagher, S.E. Trask, C. Bauer, T. Woehrle, S.F. Lux, M. Tschech, P. Lamp, B.J. Polzin, S. Ha, B. Long, Q. Wu, W. Lu, D.W. Dees, A.N. Jansen, Optimizing areal capacities through understanding the limitations of lithium-ion electrodes, J. Electrochem. Soc. 163 (2016) A138–A149, http://dx.doi.org/10.1149/2. 0321602jes. R. Zhang, N.-W. Li, X.-B. Cheng, Y.-X. Yin, Q. Zhang, Y.-G. Guo, Advanced micro/ nanostructures for lithium metal anodes, Adv. Sci. 4 (2017), http://dx.doi.org/10. 1002/advs.201600445. F. Sagane, T. Abe, Z. Ogumi, Sodium-ion transfer at the interface between ceramic and organic electrolytes, J. Power Sources 195 (2010) 7466–7470, http://dx.doi. org/10.1016/j.jpowsour.2010.04.054. M.R. Busche, T. Drossel, T. Leichtweiss, D.A. Weber, M. Falk, M. Schneider, M.L. Reich, H. Sommer, P. Adelhelm, J. Janek, Dynamic formation of a solid-liquid electrolyte interphase and its consequences for hybrid-battery concepts, Nat. Chem. 8 (2016) 426–434, http://dx.doi.org/10.1038/nchem.2470. W. Wieczorek, Z. Florjanczyk, J.R. Stevens, Composite polyether based solid electrolytes, Electrochim. Acta 40 (1995) 2251–2258, http://dx.doi.org/10.1016/ 0013-4686(95)00172-B. C. Yang, B. Liu, F. Jiang, Y. Zhang, H. Xie, E. Hitz, L. Hu, Garnet/polymer hybrid ion-conducting protective layer for stable lithium metal anode, Nano Res. (2017) 1–10, http://dx.doi.org/10.1007/s12274-017-1498-2. F. Croce, L. Persi, B. Scrosati, F. Serraino-Fiory, E. Plichta, M.A. Hendrickson, Role of the ceramic fillers in enhancing the transport properties of composite polymer electrolytes, Electrochim. Acta 46 (2001) 2457–2461, http://dx.doi.org/10.1016/ S0013-4686(01)00458-3. T. Abe, F. Sagane, M. Ohtsuka, Y. Iriyama, Z. Ogumi, Lithium-ion transfer at the interface between lithium-ion conductive ceramic electrolyte and liquid electrolyte-a key to enhancing the rate capability of lithium-ion batteries, J. Electrochem. Soc. 152 (2005) A2151–A2154, http://dx.doi.org/10.1149/1.2042907. Y. Yamada, F. Sagane, Y. Iriyama, T. Abe, Z. Ogumi, Kinetics of lithium-ion transfer at the interface between Li0.35La0.55TiO3 and binary electrolytes, J. Phys. Chem. C 113 (2009) 14528–14532, http://dx.doi.org/10.1021/jp9043539. Y. Ishihara, K. Miyazaki, T. Fukutsuka, T. Abe, Kinetics of lithium-ion transfer at the interface between Li4Ti5O12 thin films and organic electrolytes, ECS Electrochem. Lett. 3 (2014) A83–A86, http://dx.doi.org/10.1149/2.0011408eel. F. Sagane, T. Abe, Z. Ogumi, Electrochemical analysis of lithium-ion transfer reaction through the interface between ceramic electrolyte and ionic liquids, J. Electrochem. Soc. 159 (2012) A1766–A1769, http://dx.doi.org/10.1149/2. 027211jes. T. Abe, M. Ohtsuka, F. Sagane, Y. Iriyama, Z. Ogumi, Lithium ion transfer at the interface between lithium-ion-conductive solid crystalline electrolyte and polymer electrolyte, J. Electrochem. Soc. 151 (2004) A1950–A1953, http://dx.doi.org/10. 1149/1.1804813. Y. Li, B. Xu, H. Xu, H. Duan, X. Lü, S. Xin, W. Zhou, L. Xue, G. Fu, A. Manthiram, J.B. Goodenough, Hybrid polymer/garnet electrolyte with a small interfacial resistance for lithium-ion batteries, Angew. Chem. Int. Ed. (2016) 753–756, http:// dx.doi.org/10.1002/anie.201608924. F. Langer, M.S. Palagonia, I. Bardenhagen, J. Glenneberg, F.L. Mantia, R. Kun, Impedance spectroscopy analysis of the lithium ion transport through the Li7La3Zr2O12/P(EO)20Li interface, J. Electrochem. Soc. 164 (2017) A2298–A2303, http://dx.doi.org/10.1149/2.0381712jes. W.E. Tenhaeff, X. Yu, K. Hong, K.A. Perry, N.J. Dudney, Ionic transport across interfaces of solid glass and polymer electrolytes for lithium ion batteries, J. Electrochem. Soc. 158 (2011) A1143–A1149, http://dx.doi.org/10.1149/1. 3625281. J.-K. Kim, F. Mueller, H. Kim, S. Jeong, J.-S. Park, S. Passerini, Y. Kim, Eco-friendly energy storage system: seawater and ionic liquid electrolyte, ChemSusChem 9 (2016) 42–49, http://dx.doi.org/10.1002/cssc.201501328. S.T. Senthilkumar, M. Abirami, J. Kim, W. Go, S.M. Hwang, Y. Kim, Sodium-ion hybrid electrolyte battery for sustainable energy storage applications, J. Power Sources 341 (2017) 404–410, http://dx.doi.org/10.1016/j.jpowsour.2016.12.015. A. Manthiram, L. Li, Hybrid and aqueous lithium-air batteries, Adv. Energy Mater. 5 (2015). Y.-Z. Sun, J.-Q. Huang, C.-Z. Zhao, Q. Zhang, A review of solid electrolytes for safe lithium-sulfur batteries, Sci. China Chem. (2017), http://dx.doi.org/10.1007/ s11426-017-9164-2. Y. Zhou, Z. Li, Y.-C. Lu, A stable lithium–selenium interface via solid/liquid hybrid electrolytes: blocking polyselenides and suppressing lithium dendrite, Nano Energy 39 (2017) 554–561, http://dx.doi.org/10.1016/j.nanoen.2017.07.038. J. Janek, W.G. Zeier, A solid future for battery development, Nat. Energy 1 (2016) 16141, http://dx.doi.org/10.1038/nenergy.2016.141. K. (Kelvin) Fu, Y. Gong, S. Xu, Y. Zhu, Y. Li, J. Dai, C. Wang, B. Liu, G. Pastel, H. Xie, Y. Yao, Y. Mo, E. Wachsman, L. Hu, Stabilizing the garnet solid-electrolyte/ polysulfide interface in Li–S batteries, Chem. Mater. 29 (2017) 8037–8041, http:// dx.doi.org/10.1021/acs.chemmater.7b02339. M. Schleutker, J. Bahner, C.-L. Tsai, D. Stolten, C. Korte, On the interfacial charge transfer between solid and liquid Li+ electrolytes, Phys. Chem. Chem. Phys. 19 (2017) 26596–26605. W. Liu, R.J. Milcarek, R.L. Falkenstein-Smith, J. Ahn, Interfacial impedance studies of multilayer structured electrolyte fabricated with solvent-casted PEO10–LiN (CF3SO2)2 and ceramic Li1.3Al0.3Ti1.7(PO4)3 and its application in all-solidstate lithium ion batteries, J. Electrochem. Energy Convers. Storage 13 (2016), http://dx.doi.org/10.1115/1.4035294 021008-021008-6. S. Skaarup, K. West, P.M. Julian, D.M. Thomas, Mixed phase solid electrolytes with

Journal of Power Sources 392 (2018) 206–225

M. Keller et al.

[171]

[172]

[173]

[174]

[175]

nonconducting polymer binder, Solid State Ion 40 (1990) 1021–1024, http://dx. doi.org/10.1016/0167-2738(90)90177-S. J.M. Whiteley, S. Hafner, C. Zhu, W. Zhang, S.-H. Lee, Stable lithium deposition using a self-optimizing solid electrolyte composite, J. Electrochem. Soc. 164 (2017) A2962–A2966, http://dx.doi.org/10.1149/2.0331713jes. S. Zekoll, C. Marriner-Edwards, A.K. Ola Hekselman, J. Kasemchainan, C. Kuss, D.E.J. Armstrong, D. Cai, R.J. Wallace, F.H. Richter, J.H.J. Thijssen, P.G. Bruce, Hybrid electrolytes with 3D bicontinuous ordered ceramic and polymer microchannels for all-solid-state batteries, Energy Environ. Sci. (2018), http://dx.doi. org/10.1039/C7EE02723K. Z. Zhang, Y. Zhao, S. Chen, D. Xie, X. Yao, P. Cui, X. Xu, An advanced construction strategy of all-solid-state lithium batteries with excellent interfacial compatibility and ultralong cycle life, J. Mater. Chem. A 5 (2017) 16984–16993, http://dx.doi. org/10.1039/C7TA04320A. Y. Xia, T. Fujieda, K. Tatsumi, P.P. Prosini, T. Sakai, Thermal and electrochemical stability of cathode materials in solid polymer electrolyte, J. Power Sources 92 (2001) 234–243, http://dx.doi.org/10.1016/S0378-7753(00)00533-4. E. Yi, W. Wang, J. Kieffer, R.M. Laine, Flame made nanoparticles permit processing of dense, flexible, Li + conducting ceramic electrolyte thin films of cubic-Li 7

[176]

[177]

[178]

[179] [180]

225

La 3 Zr 2 O 12 (c-LLZO), J. Mater. Chem. A 4 (2016) 12947–12954, http://dx.doi. org/10.1039/C6TA04492A. T. Placke, R. Kloepsch, S. Dühnen, M. Winter, Lithium ion, lithium metal, and alternative rechargeable battery technologies: the odyssey for high energy density, J. Solid State Electrochem. 21 (2017) 1939–1964, http://dx.doi.org/10.1007/ s10008-017-3610-7. S. Duluard, A. Paillassa, L. Puech, P. Vinatier, V. Turq, P. Rozier, P. Lenormand, P.L. Taberna, P. Simon, F. Ansart, Lithium conducting solid electrolyte Li1.3Al0.3Ti1.7(PO4)3 obtained via solution chemistry, J. Eur. Ceram. Soc. 33 (2013) 1145–1153, http://dx.doi.org/10.1016/j.jeurceramsoc.2012.08.005. G.-T. Kim, S.S. Jeong, M.-Z. Xue, A. Balducci, M. Winter, S. Passerini, F. Alessandrini, G.B. Appetecchi, Development of ionic liquid-based lithium battery prototypes, J. Power Sources 199 (2012) 239–246, http://dx.doi.org/10. 1016/j.jpowsour.2011.10.036. A.A. Teran, N.P. Balsara, Thermodynamics of block copolymers with and without salt, J. Phys. Chem. B 118 (2014) 4–17, http://dx.doi.org/10.1021/jp408079z. M.E. Spahr, D. Goers, A. Leone, S. Stallone, E. Grivei, Development of carbon conductive additives for advanced lithium ion batteries, J. Power Sources 196 (2011) 3404–3413, http://dx.doi.org/10.1016/j.jpowsour.2010.07.002.