Recent progress in solid-state electrolytes for alkali-ion batteries

Accepted Manuscript Review Recent progress in solid-state electrolytes for alkali-ion batteries Cheng Jiang, Huiqiao Li, Chengliang Wang PII: DOI: Reference:

S2095-9273(17)30527-3 https://doi.org/10.1016/j.scib.2017.10.011 SCIB 246

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Science Bulletin

Received Date: Revised Date: Accepted Date:

8 August 2017 29 September 2017 10 October 2017

Please cite this article as: C. Jiang, H. Li, C. Wang, Recent progress in solid-state electrolytes for alkali-ion batteries, Science Bulletin (2017), doi: https://doi.org/10.1016/j.scib.2017.10.011

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Review Received 8 August 2017 Received in revised form 29 September 2017 Accepted 10 October 2017

Recent progress in solid-state electrolytes for alkali-ion batteries Cheng Jiang,a Huiqiao Li,b,* Chengliang Wanga,* a

School of Optical and Electronic Information, Huazhong University of Science and

Technology, Wuhan 430074, China b

School of Materials Science and Engineering, Huazhong University of Science and

Technology, Wuhan 430074, China. *Email: [email protected], [email protected] Abstract: Solid-state electrolytes have a lot of advantages, including the inhibition of alkali metal dendrite growth, the elimination of liquid electrolyte leakage, the improvement of safety, the enhancement of energy density and power density, and the potential application in flexible electronics. Therefore, solid-state electrolytes have become one of the hottest topics in energy-storage research area. An up-to-date review on solid-state electrolytes is of not only scientific significance but also technological imperative. Here, recent progress in solid-state electrolytes for alkali ion batteries is summarized. Through this comprehensive review and the comparison of different solid-state electrolytes, we hope it can give a clear figure of the state-of-art status and the development trend of the future solid-state electrolytes.

Keywords: Solid-state electrolytes, Lithium-ion batteries, Sodium-ion batteries, Energy storage

1

Introduction The electrochemical energy storage is one of the most imperative issues for human being in

this century, due to the exhausting fossil energy, the increasing energy demand, the climate warming and the tidal phenomena of renewable energies [1]. It is recognized that the alka1

li-ion batteries (AIBs) are one of the most appropriate candidates for energy storage, because of their advantages including high energy density, rechargeability, low self-discharging, non-memory effect, and wide operating temperature range etc. [2, 3]. The AIBs can be extensively used as energy storage facilities or power supply appliances for electric vehicles, portable electronics and large scale grid energy. The schematic configuration of a traditional AIB cell is shown in Fig.1a. It mainly contains four parts, namely a cathode, an anode, the electrolyte and a separator. Alkali metals (especially for lithium) are ideal anode materials for AIBs, due to their unparalleled theoretical specific capacity [4, 5]. However, the alkali metals are suffering fatal safety issues. They are sensitive to moisture, oxygen and even nitrogen (for Li), leading to combustion or explosion. The discovery of graphite for lithium-ion batteries (LIBs) significantly has reduced the high risk of direct usage of alkali metals and has made a great success in portable electronics in the past two decades. Nevertheless, the possibility of combustion and explosion still exist because of the utilization of flammable organic electrolyte and the potential formation of dendrites of alkali metals on the anodes during the charging (or unintentional overcharging, crash) process that leads to short circuit [6].

Fig. 1 (Color online) The schematic diagram of (a) traditional battery cell based on liquid electrolytes and (b) solid-state battery.

On the other hand, the energy density (weight or volumetric energy density) of the batteries is highly dependent on the capacities of the two electrode materials and the weight (or volume) of the whole cell. Although kinds of routes are reported for enhancing the specific capacity of the two electrodes, the energy density is also confined by the weight (or the volume) of the inactive materials (conductive additives, binder, electrolyte and the separator). Thereinto, the weight (or the volume) of the electrolyte and the separator take a major role [7]. Under this situation, the solid-state electrolytes, which functionalize the electrolyte as well as the separator, are raised with expect to improve the safety and enhance the energy density. A cell of all solid-state battery is presented in Fig.1b, in which the solid-state electrolyte replaces the separator and the electrolytes in traditional AIBs. The solid-state electrolytes pos2

sess various advantages. For instance, the solid-state electrolytes can inhibit the growth of dendrites of alkali metals on the anodes during the charging process, leading to enhanced safety of the batteries. In this case, even alkali metals (e.g., Li anode could provide a capacity of 3,860 mAh g-1 or 2,061 mAh cm-3) [8] can be adopted as anodes, which can significantly enhance the capacity and the energy density. Secondly, the dissolution of electrode materials often occurs in liquid organic electrolytes, which leads to significant performance degradation of the batteries. However, the solid-state electrolytes would eliminate the dissolution of the electrode materials and hence can enhance the long-term cycleability. Meanwhile, the materials with high capacity but high dissolution (e.g., S can deliver a capacity of 1,672 mAh g-1 ) in liquid electrolyte can be adopted, which can further improve the energy density and power density [9]. Another, the combination of electrolyte and the separator into solid-state electrolytes can significantly increase the energy density due to the lessened weight. Moreover, the solid-state electrolytes are able to simplify the fabrication, eliminate the leakage of liquid electrolyte (safety issue), and bear the shock and vibration. In addition, the development of flexible solid-state electrolytes [10, 11] make it possible to achieve bendable and wearable electronic devices [12]. During the charging process, the alkali ions, are extracted from the cathode and migrate to the anode through the electrolyte and then insert into the anode. The route is reversed during the discharging process. Hence, a good solid-state electrolyte should have the following characteristics: (1) high ionic conductivity at room temperature, (2) electron insulating, (3) smooth surface and dense structure, (4) high mechanical strength, (5) high chemical and thermal stability, (6) wide electrochemical window and operation temperature range, (7) good interfacial compatibility, and (8) flexibility for flexible applications. In additional to these requirements, the practical application of solid-state electrolytes should meet other criteria, including the facile fabrication, the low cost and the possibility of mass production etc. In recent years, much progress has been made on the aforementioned aspects in order to optimize the properties of solid-state electrolytes. Various materials have been proposed as solid-state electrolytes, including Li-ionic conductors, NASICON (Na superionic conductors) and polymer electrolytes etc. By using “solid-state electrolyte” as keywords to search the title in Web of Science citation database, more than 3,000 papers have been published and around half of them were published after 2010, indicating that this topic is one of the hottest research areas. On the other hand, the safety issue and the demand of batteries with high energy density for electric vehicles and flexible portable electronics desiderate the development of high performance solid-state electrolytes. Therefore, an up-to-date review in regard to the solid-state electrolytes for alkali-ion batteries is of not only scientific significance but also technological imperative [6]. Thus, in the present review, an overview of AIBs (for LIBs and sodium-ion batteries 3

(SIBs)) based on the solid-state electrolytes will be given. The materials for solid-state electrolytes will be classified into two parts: inorganic (Section 2) and organic (Section 3) materials, along with different conduction mechanisms as starting section for each part. The inorganic-organic composite will be highlighted in the end (Section 4). Through this comprehensive summary and comparison of different solid-state electrolytes, we hope it can give a clear figure of the state-of-art status and development trend of the future solid-state electrolytes.

2

Inorganic solid-state electrolytes Inorganic ceramic materials for solid-state electrolytes mainly contain four crystalline types:

NASICON, LISICON, perovskite and garnet [13-16]. Amorphous/glassy materials were also reported [17]. These inorganic materials are regarded as promising candidates for solid-state electrolytes due to the electrochemical stability, thermal stability, efficient suppression of Li/Na dendrites, considerable ionic conductivity and negligible electric conductivity. 2.1 Ionic conduction mechanism of inorganic solid-state electrolytes During the charging/discharging process, the ions transport in the electrolytes and insert into/extract from the electrodes under the effect of different sorts of gradient forces, including concentration gradient, electrical field gradient and chemical potential gradient. Hence, the ionic transport (or ionic conductivity) property in solid-state electrolytes can be described by two parameters, diffusion coefficient (D) and ionic transference number (ti). Kinds of defects exist even in crystals or crystal grains. If the materials contain point defects, such as ion vacancies or interstitial ions, the ions can transport in the material via the motion of these kinds of point defects. The point defects can be intrinsic or doped defects. The formation of intrinsic defects is motivated by the thermal energy, and the amount of defects depends on the Arrhenius-type equation:
 =
exp −





,

(1)

where ND stands for the number of defects, the N expresses the amount of ion pairs, EF is the energy which is required to form the defects, k is the Boltzmann constant, and T is the temperature. The conductivity (σ) of a material is determined by different free charge carriers inside, and can be calculated by:  = ∑   

,

(2)

 describes the density,  is the charge, and   stands for the mobility of the charge carriers. The relation between the mobility and the diffusion coefficient is: =



.



(3)

The total conductivity is calculated by the ionic and electric conductivities: 4

σ =  + 

.

(4)

The solid-state electrolytes for AIBs should be electrons insulator, in order to prevent the internal short circuit in batteries. Meanwhile, high ionic conductivity is necessary [18, 19]. The parameter, ionic transference number is used to describe the contribution of ions and electrons to the total conductivity, and is defined by  =



 =



, (5)



, (6)



where ti and te are ionic and electronic transference numbers, respectively. The total transference number is 1 and for a good solid-state electrolyte, the ionic transference number ti should be close to 1. In this case, the conductivity is only correlative to the ionic conductivity and is proportional to the number of ions and diffusion coefficient:  = ∑    ≈

! 



=

!

"

.



(7)

This is the Nernest-Einstein relation. Because the amount of defects depends on the Arrhenius-type relation, the behavior of ionic conductivity also follows the Arrhenius-type equation [20]: σ = # exp (−

%

&

),

(8)

where σ0 is the pre-exponential factor, Ea is the activation energy and R is the ideal gas constant. When the ions migrate from one point defect to another, hopping of ions between adjacent defects is one of the most possible means. The hopping of single particle among the available sites in solid can be explained by random-walk theory, and its diffusion coefficient is calculated by )

( = *  +  (1 − -)./

,

(9)

where d equals to 1 (2 or 3) for one (two or three) dimensional motion, l and νh are the jumping distance and frequency respectively, c is the concentration of ions and z is the number of nearest neighbors. There is no doubt that the jumping frequency is highly dependent on the temperature and the migration free energy. The ions can transport through the vacancies (when the size of the ions is similar to the vacancies) or the interstitial ions (when the size of the ions is much smaller than the lattice ions). Therefore, there are a lot of different ionic transport mechanisms proposed for the solid-state electrolytes, as shown in Table 1. The schematic diagrams of typical ionic transport mechanism of inorganic materials, i.e., the vacancy(defect)-mediated and interstitial-mediated mechanism are presented in Fig. 2 [21, 22]. The Schottky (an anion vacancy along with a cation vacancy) and Frenkel (a vacancy along with an interstitial ion) point defects are two main point defects, which corresponds to the vacancy-mediated and interstitial-mediated ionic 5

transport respectively. The ionic conductivity is highly dependent on the Schottky and Frenkel point defects [21]. Regardless of the transport media (either vacancies, mobile cations or anions) [19, 23], in order to acquire high ionic conductivity, it is necessary to fulfill the following requirements: (1) enough defects (vacancies or interstitial ions) are required for occupation (or substitution) of the mobile ions; (2) the migration barrier energy should be as low as possible to ensure hopping of the mobile ions between adjacent available sites; (3) enough connected available sites are essential for the formation of continuous pathways between the cathode andthe anode [24]. Table 1

Ionic conduction mechanisms in inorganic crystalline electrolytes [21]

Mechanism Vacancy (defect)-mediated Non-vacancy (defect)-mediated

Description Self-diffusion in metals and substitution alloys. Diffusion via aggregates of vacancies Solute atoms considerably smaller than the host atoms, and atoms are incorporated into interstitial sites of the host lattice to form an interstitial solid solution Solute atoms similar in size to host atoms involving simultaneous motion of several atoms. Usually substitutional solid solutions are formed A collective mechanism important for radiation-induced diffusion. At least two atoms move simultaneously; however, this mechanism is negligible for thermal diffusion Solute atoms are dissolved on both interstitial and substitutional sites and diffuse via interstitial or substitutional exchange mechanisms

Vacancy Divacancy Interstitial

Collective

Interstitialcy

Interstitial-substitutional exchange

Fig.2 (Color online) Schematic diagrams of typical ionic transport mechanism of inorganic materials. (a) Vacancy(defect)-mediated mechanism. (b) Interstitial-mediated mechanism of ion transport in inorganic materials.

The transport theories varied in different systems. And in real solid materials, different de6

fects and grain boundaries also exist, which makes the transport of ions more complex. Hence, in the following section, different kinds of materials for solid-state electrolytes will be discussed. 2.2 Crystalline materials Considering the ionic hopping transport in solid-state electrolytes as mentioned above, crystalline materials have been considered as the promising candidates of solid-state electrolytes for alkali-ion batteries. The reason can be ascribed to the lack of grain boundaries and the long-range ordered structures of single crystal materials. Grain boundary conductivity may vary one or several orders of magnitude lower than bulk conductivity. Such low grain boundary conductivity and the short-range ordered structure may lead to negative influence on the total ionic conductivity for polycrystalline materials. Nevertheless, they can also provide relatively considerable ionic conductivity, compared to the amorphous or glassy materials. 2.2.1 Garnets Garnets were one of the promising crystalline materials for all-solid-state AIBs. The ideal garnet-structured materials are those which have a general formula of A3B2X3O12 (A = Ca, Mg, Y, La or rare earth; B = Al, Fe, Ga, Ge, Mn, Ni or V; X = Si, Ge, Al) [13]. The garnet structure were revealed in 1969 [25] and after that a series of garnet-typed materials were discovered. The first reported Li-ion conducting garnets were Li5La3M2O12 (M= Nb, Ta) which showed ambient ionic conductivity about 10-6 S cm-1 [26]. And in 2005, the Li6SrLa2Bi2O12 with the ionic conductivity of 2.0×10-5 S cm-1 at room temperature was presented by Thangadurai et al. [27]. Afterwards, in 2007, Murugan et al. [28] reported the Li7La3Zr2O12 (LLZ) based garnets with ionic conductivity of 5×10-4 S cm-1 at room temperature, which became the benchmark of garnet based solid-state electrolytes. LLZ was considered as the prospective solid-state electrolytes due to its remarkable characteristics, for example, low activation energy, considerable ionic conductivity, excellent thermal performance [29], rich Li+ concentration [28], and the acceptable elastic property [30]. There are two types of crystal structures for LLZ, cubic [31] and tetragonal structure [32], whose refined structures were shown in Fig. 3. The tetragonal structure was acquired by slightly distorted by the cubic structure framework. The cubic LLZ has an ionic conductivity which is two orders of magnitudes higher than that of tetragonal LLZ. For the cubic LLZ, the structure of cubic LLZ was made of dodecahedral LaO8 and octahedral ZrO6 . There were two types of crystalographic sites for Li ions and located in the tetrahedral 24d (Li1 atoms) and distorted octahedral 96h site (Li2 atoms), respectively, as revealed in Fig. 3a. The disordering and abundant vacancies of Li atoms at the Li2 site were regarded as the main factor deter7

mining the Li-ion conduction. The basic unit of the Li arrangement can be simplified as a loop structure shown in Fig. 3c. Two adjacent loops were linked by sharing of Li1 sites, and a three dimensional (3D) network of the Li-ion migration pathway is formed in the structure, which is shown in Fig. 3e [31]. The tetrahedral Li1O4 and distorted octahedral (Li2)2O6 shared a face, shown in inset of Fig. 3a, leading to a very short Li-Li distance and good Li-ion conduction property. However, the tetrahedral LLZ presented completely ordered Li atoms and had three types of crystalographic sites for Li atoms (Fig. 3b). All of these three sites were fully occupied, leading to longer Li-Li distance (Fig. 3d) and lower Li-ionic conductivity.

Fig.3 (Color online) Crystal structure of cubic (a) and tetragonal (b) Li7La3Zr2O12 . The loop structures constructed by Li atomic arrangement in cubic (c) and tetragonal (d) LLZ. (e) 3D network structure of the Li atomic arrangement in cubic Li7La3Zr2 O12. Reprinted with permission from ref. [31], Copyright © 2011 The Chemical Society of Japan. In 2015, Guo and co-workers [33] reported batteries by using Ta-doped lamellar LLZ electrolyte. The batteries were composed of composite carbon-coated LiFePO4 cathodes mixed with poly(vinylidene fluoride) (PVDF):lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and Ketjen Black, garnet electrolyte and Li anode. At 60 °C, the battery showed the first discharge capacity of 150 mAh g−1 at current density of 0.05 C and maintained about 93% capacity after 100 cycles. The performance of the batteries could be further optimized at 100 °C. Beside of the pursuit of higher ionic conductivity by substitutions or doping, the interfacial engineering is also important for garnet-related electrolytes, because of their poor interfacial compatibility with Li or Na and the potential growth of dendrite through garnets grain boundaries. Poor interfacial compatibility between the electrode and solid-state electrolyte would also lead to large interfacial resistance for Li+ transport and increase in the total cell impedance. The low interfacial compatibility would also result in loose contact between the electrolyte and the electrodes and hence make it difficult to integrate an electrolyte directly into solid-state cells. Even worse, the garnet-related electrolytes are instable when exposing in wet air 8

[34, 35]. It was found that impurity La(OH)3 secondary phase would form at grain boundary, when LLZ was exposed to air. The Li6.5La3Zr1.5Ta0.5O12 (LLZT) garnet is also sensitivity to air [36], forming a +

Li -insulating LiCO3 layer on the surface of garnets which led to a large interfacial resistance versus electrode materials [37]. In order to inhibit the effect of humidity on the electrolyte and improve the interfacial compatibility, LiF, which has strong ionic bond and insolubility in water, was used to engineer the interface between LLZT and electrodes. In order to improve the wetting by Li metal and suppress the growth of dendrite, polymer buffer layer was utilized to optimize the interface versus electrodes materials. The mixing of polymer buffer layer with electrode is capable to further enhance the interfacial contact, through forming a stagger interface. By using Li metal and LiFePO4 as anode and cathode, the mixture of cross-linked polyethylene oxide (PEO) and LiTFSI polymer as a buffer layer and LLZT-2 wt% LiF as solid-state electrolytes, the interfacial resistance is reduced significantly and a lower over-potential (0.2 V at 80 µA cm-2) was obtained. Moreover, a coulombic efficiency of 99.8%-100% was achieved because of the lack of formation of solid electrolyte interface (SEI) layers. In addition, at the current density of 80 mA cm-1, 80% capacity (initial capacity 142 mAh g-1 ) maintained after 100 cycles. So as to reduce the interfacial resistance and improve the performance of all solid-state batteries, Van den Broek et al. [38] developed another method, by using sacrificial organic templates to realize the interface-engineering. In a short word, the organic material starch was used as sacrificial material to form porous interface and thereby increase the interfacial contact between the electrode (Li4Ti5O12 and Li) and the electrolyte (c-Li6.25Al0.25La3Zr2O12). In order to further improve the interfacial compatibility, the electrode layer Li4Ti5O12 was fabricated by mixing with c-Li6.25Al0.25La3Zr2O12 electrolyte, PVDF binder and super P conductive additive. The results showed that the unmodified battery had a much higher polarization than the modified one. What’s more, the charge/discharge capacities were in the range of 15-20 mAh g-1 at the current rate of 2 A kg-1 and considerable cycle stability was also presented. In addition, when a rate test started at the current rate of 2 A kg-1, the discharge capacity was 16 mAh g-1, and then through higher current rate from 4 to 8 A kg-1 and finally recover current rate of 2 A kg-1, the discharge capacity still maintained 14 mAh g-1. Generally speaking, this method improved performance of solid LIBs successfully. Generally speaking, impressive bulk ionic conductivity, high mechanical strength and wide electrochemical window of garnet type electrolytes make them a kind of promising materials for solid-state electrolytes. However, at the same time, non-negligible grain boundary resistance, rigid structure, poor interfacial compatibility, and air sensibility still limit the performance and practical application of garnet-based solid-state electrolytes. 9

2.2.2 Perovskite Perovskite structure becomes popular because of its potential application in high performance [39-41]. A typical perovskite structure has a formula of ABO3 (A and B are metal ions), where the size of A cation is similar to that of oxygen ion and they form approximate cubic close packing together. B cations occupy one of the fourth octahedral sites. The concept of perovskite solid-state electrolytes had been raised as early as 1970s, which were mainly utilized in fuel cells [42]. The typical perovskite structure used for LIBs follows a Li3xLa2/3−xTiO3 formula, in which Li+ and La3+ located at A sites. The La3+ stabilizes the structure and the Li+ acts as mobile ions. The ionic conductivity is originated from the high concentration of A site vacancies. Although perovskite structures own impressive ionic conductivity (>10-3 S cm-1) at room temperature [43], they are not appropriate for lithium metal batteries, because the metallic Li would reduce Ti4+ in perovskite based materials. Such risk also exists for LIBs due to the possible formation of Li dendrites. The perovskite solid-state electrolytes also suffer the grain boundary effect. The ionic conductivity along grain boundaries may be several orders of magnitude lower than the bulk conductivity, and hence leads to lower total conductivity [44]. In 2014, Ma et al. [15] reported the atomic-scale origin of crystallographic grain boundaries, which have a cell thickness of 2-3 units and were composed of Ti-O binary compounds. Such depletion of lithium ions in the crystallographic grain boundaries limited the lithium ion transportation and accommodation, leading to low grain boundary conductivity. Furthermore, Wu and Guo [44] found that the grain boundary conductivity decreases with the grain size and the introduction of electrons at the grain boundaries can enhance the grain boundary conductivity. Therefore, they claimed that the low grain boundary conductivity in Li3xLa0.67-xTiO3 was ascribed to lithium ion depletion in the space-charge layer adjacent to the grain boundaries. Inspired by the fabulous bulk ionic conductivity of perovskite structure materials [45, 46], the anti-perovskite structure was also proposed as solid-state electrolytes in lithium-rich which showed super ionic conductivity [47]. The anti-perovskite structure with a typical X-O2-A+3 (X=F, Cl, Br, I and A=Li) structure is lithium-rich, lightweight, thermal stable (decomposition temperature nearly 400 °C) and highly ionic conductive (>10-3 S cm-1). Such anti-perovskite structure (Li2(OH)Cl) has been reported as potential solid-state electrolyte for LIBs in 2003 with an ionic conductivity of 10-4 S cm-1 at 50 °C. And it showed that the cubic phase is essential to the high conductivity [48]. The crystal structure of Li2(OH)Cl was shown in Fig. 4a. The formation of SEI layers on the Li2(OH)Cl surface is supposed to be efficient to stabilize the metallic lithium anode without significantly reducing the conductivity [50].

10

Fig.4 (Color online) (a) Crystal structure of cubic Li2OHCl. Charge and discharge voltage profiles (b) and cycling performance (c) at 65 °C of a Li/LiFePO4 all-solid-state battery at current density of 0.2 C with Li2(OH)0.9F0.1 Cl solid electrolyte. Reprinted with permission from ref. [49], Copyright © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. The fluorine doped structure (Li2(OH)0.9F0.1Cl) was reported later. The authors claimed that the fluorine doping can stabilize the cubic structure and thereby enhance the lithium ion conductivity [49]. This anti-perovskite structure displayed a potential application in all-solid-state batteries, in which LiFePO4 and Li were used as cathode and anode respectively. In order to reduce the interfacial resistance and prevent the formation of Li dendrite, the cathode was embedded in a Li-conducting membrane consisting of a polymer binder and super P conductor. When charged/discharged at the current density of 0.2 and 0.5 C at 65 °C, no obvious capacity loss was observed. At 0.2 C, it showed capacity of 125 and 108 mAh g-1 at 0.5 C (Fig.4b). As mentioned above, impressive bulk ionic conductivity can be obtained from perovskite structured electrolytes, but Ti-based perovskites face the challenge of potential reduction of Ti4+ by metallic Li. The perovskite electrolytes also suffer the risk of possible formation of Li dendrites if using Li metallic anodes. Even more, the presence of grain boundaries leads to especially large grain boundary resistance to total impedance (bulk resistance and grain boundary impedance), namely, the total ionic conductivity would some orders of magnitude lower than bulk ionic conductivity due to the grain boundary effect. In this regard, how to decrease the grain boundary resistance (e.g., through interfacial engineering) is a large concern for perovskite electrolytes.

11

2.2.3 LISICON LISICON represents lithium super ionic conductor. It was regarded as a kind of promising materials for solid-state electrolytes due to great thermal stability and wide electrochemical window. The first proposed LISICON was Li14Zn(GeO4)4, which belongs to Li2+2xZn1-xGeO4 system and can be considered as the Li4GeO4 -Zn2GeO4 solid solution [51]. It revealed an acceptable conductivity of 1.3×10-1 S cm-1 at 300 °C. But conductivity of only 1×10-7 S cm-1 was achieved at room temperature, which may be attributed to the trapping of the mobile ions by the sub-lattice at ambient temperature during the formation of defects [52]. Moreover, Li14Zn(GeO4)4 is apt to react with Li metal and atmospheric CO2, and the conductivity would decrease with time . Under this situation, lots of efforts have been done to improve the ionic conductivity of LISICON-based electrolytes. Thio-LISICON was raised with regard to enhance the ionic conductivity [53]. Compare with LISICON, in thio-LISICON framework, the oxide is replaced by sulfur, therefore, higher ionic conductivity above 1×10-4 S cm-1 at room temperature can be achieved because of the larger radius and better polarization ability of S2-. Remarkably, it has been reported that the Li10GeP2S12 had the ionic conductivity of 1.2×10-2 S cm-1, which was comparable to commercial liquid electrolytes [18]. In addition, thio-LISICON solid electrolyte had good mechanical property for application, and it was easy to reduce grain boundary resistance through orthodox cold-press of electrolyte power [54]. In 2001, Kanno and Murayama [55] synthesized a series of thio-LISICON. Therein, the Li3.25Ge0.25 P0.75S4 showed ambient ionic conductivity of 2.2×10-3 S cm−1, electrical insulation and good electrochemical stability. In 2013, it was reported that the Li10SnP2S12 was prepared by replacing Ge with Sn [56]. It presented 7×10-3 S cm−1 grain Li-ion conductivity at 27 °C, and the Li10SnP2S12-based solid-state battery (cathode: LiCoO2; anode: Li metal) showed a discharge capacity over 120 mAh g-1 and coulombic efficiency around 100%. Later, the similar solid-state electrolyte with a composition of Li3.833Sn0.833As0.166S4 was reported by Sahu et al. [57] which showed considerable conductivity of 1.39 mS cm−1 at 25 °C. Furthermore, Zhou et al. [58] reported a LISICON-based Li11AlP2S12 with a thio-LISICON analogous structure. The prepared solid-state electrolyte gave a conductivity of 8.02×10-4 S cm-1 at room temperature and electrochemical window over 5 V (vs. Li+/Li), which was ascribed to the thio-LISICON analogous structure and the Al substitution. Obviously, LISICON solid electrolytes attracted much attention due to the exceptional stability and easy process of synthesis; however, relatively low ionic conductivity becomes its bottleneck of development. In this regard, thio-LISICON with higher ionic conductivity over 10-2 S cm-1 was proposed. As for thio-LISICON, despite the high bulk ionic conductivity, these kinds of materials tend to react with lithium anode and oxide cathodes, which is a trou12

ble of further research. 2.2.4 NASICON Sodium super ionic conductor (NASICON) is another kind of materials with high ionic conductivity and can be adopted as solid-state electrolytes. It has the same crystal structure as (Ⅱ)

(Ⅱ)

(Ⅳ)

A123242) M3 M4 M6364 Si) P:6) O1 . For electrolyte of sodium-ion batteries, the A is Na, the M is replaced by divalent, trivalent, or tetravalent cations, and the P can be Si or As [59, 60]. In the 1960s, Hagman et al. [61] firstly studied NASICON. And in 1976, Hong and Goodenough [62, 63] pioneered the NASICON-type material Na1+xZr2SixP3-xO12 with a three dimensional framework which showed total conductivity higher than 10-3 S cm-1. It was concluded that the radius of M should be close to Zr so that higher ionic conductivity can be obtained [64]. And the highest ionic conductivity and a monoclinic distortion crystallographic lattice can be observed when the NASICON has 3-3.5 mol Na per formula. Guin et al. [60] reported the promising NASICON solid-state electrolyte Na3+xSc2SixP3−xO12, in which the relationship between the crystal structure and ionic conductivity were discussed. The radius of Sc was close to that of Zr, and the trivalent Sc enhanced the amount of Na in each formula unit. As a result, the Na3+0.4 Sc2Si0.4P3-0.4O12 showed the highest conductivity of 6.9×10-4 S cm-1 at room temperature. Park et al. [65] presented the Na3.3Zr2Si2PO12, a derivative from the common NASICON Na3Zr2Si2PO12 with 10 at% excessive Na, indicating that the grain boundary diffusion and the grain diffusion were the main limitation of Na ionic transport in NASICON structure at low and high temperatures respectively. In addition, the excessive Na led to a rise of conductivity to 1.2×10-1 S cm-1 at 300 °C. In 2013, Okada’s group [66] reported a solid-state symmetrical battery that was based on Na3Zr2Si2PO12 electrolyte. Na3V2(PO4)3 was used as the active materials for both anode and cathode. At room temperature, a discharge capacity of 68 mAh g-1 at 1.2 µA cm-2 was displayed within a voltage window of 0.01 and 1.9 V. Although NASICON revealed considerable ionic conductivity, the high calcination temperature and presence of voids limited its practical application. Recently, additives 60Na2O-10Nb2O5-30P2O5 glass was proposed to mix with NASICON, which led to an ionic conductivity of 10-6 S cm-1 at room temperature [67]. Beside of application in sodium-ion batteries, NASICON-type Li-ion conductors are also popular. The Li+ conductor with NASICON structure is provided with formula of LiM2(PO4)3. In general, the M site was occupied by Ge, Zr or Ti. In 2013, Kotobuki et al. [68] synthesized Li1.5Al0.5Ti1.5(PO4)3 by a co-precipitation method, and the Li1.5Al0.5 Ti1.5(PO4 )3 presented an attractive ionic conductivity of 1.4 × 10−3 S cm−1 at room temperature. Recently, Arbi et al. [69] proposed the solid electrolyte based on NASICON Li1+xAlxM2−x(PO4)3, in which the M site was occupied by Ti or Ge. Li1+xAlxTi2−x(PO4)3 showed a higher room-temperature ionic conductivity than that of Li1+xAlxGe2−x(PO4)3 and the highest 13

conductivity reached to 3.4 × 10−3 S cm−1 when x was 0.2 or 0.4. Li et al. [70] utilized the LiZr2(PO4)3 as electrolyte in Li-ion batteries which is also of NASICON structure. A considerable ionic conductivity of 2×10−4 S cm−1 was obtained for the solid electrolyte LiZr2(PO4)3 with rhombohedral structure. In addition, as shown in Fig.5, the all solid-state Li/LiZr2 (PO4)3/LiFePO4 cells showed a long-term cycle stability, and at current rate of 50 µA cm-1, the battery exhibited discharge capacity about 160 mAh g-1.

Fig.5 (Color online) (a) A cyclic voltammogram of LiZr2(PO4)3 when the scanning rate was 0.5 mV s–1. (b) The impedance plots of Li/LiZr2(PO4)3/LiFePO4 cell. Charge and discharge voltage profiles (c) and cycling performance (d)

at 80 °C of Li/LiZr2(PO4)3/LiFePO4 battery

at different current densities. Reprinted with permission from ref. [70], Copyright © 2016 United States National Academy of Sciences

In general, NASICON type materials are becoming more and more popular in the field of energy storage. Its abundant resources, low cost and atmospheric stability are all attractive. On the other hand, considerable ionic conductivity of 10−4 S cm−1 is achievable. Besides, like other typical inorganic solid-state electrolytes, NASICON has huge interfacial resistance and large grain boundaries. Meanwhile, the Ti4+ is apt to be reduced by metallic Li. 2.3 Glassy materials Glassy materials are considered as ideal candidates for solid-state electrolytes, because they have a lot of advantages, such as the wide options of various compositions, the isotropic properties, and the facile formation of films [17]. The ionic diffusion process of glassy materials is similar to that of crystal materials. The ions at local sites are activated to adjacent sites, and then diffuse on a macroscopic scale [71]. 14

In terms of common glassy materials, short and medium-range order still exist in the amorphous structure. Electrolytes of glassy systems denoted as Li2O-MOx formula [72-74] (the M could be Si, P, B, etc.) held high degree of disorder structure or network structure. In 1966, Li2O-SiO2-B2 O3 was reported, showing an ionic conductivity of 4×10-4 S cm-1 at 350 °C, which revealed that the glassy structure could provide high ionic conductivity. However, in order to realize practical application of glassy materials for alkali ion battery at ambient temperature, extensive improvements should be achieved. The sulfide glass-ceramic electrolytes were proposed to enhance the ionic conductivity under lower temperature. Sulfide glass-ceramic electrolytes have been studied comprehensively for dozens of years, due to its outstanding ionic conductivity, wide electrochemical window and good mechanical characteristics [75, 76]. In recent years, there were still lots of reports about superior lithium ionic conduction by using sulfide glassy materials [76, 77]. In 2016, Mo et al. [78] presented 95(0.7Li2S-0.3P2S5)-5Li3PO4 (mol%) glassy electrolyte, which gave ionic conductivity of 2.513 × 10−3 S cm−1 at 100 °C. The batteries consisted of LiCoO2/95(0.7Li2S-0.3P2 S5)-5Li3PO4/Li provided a discharge capacity of 144 mAh g−1 at 0.5 C, and considerable cycle stability could be obtained. The sulfide glassy electrolytes were also utilized in sodium-ion batteries. The glassy state of Na3 PS4 was presented in 2012 [79]. Later, silicon- or halogen-doped Na3PS4 were proposed to increase the ionic conductivity, which reaches the order of magnitude of 1×10-3 S cm-1 . In 2016,

rechargeable

solid-state

sodium-ion

battery

based

on

Cl-doped

Na3PS4

(t-Na2.9375PS3.9375Cl0.0625) was raised [80]. Excellent cycling performance and good capability room temperature were obtained. Afterwards, in order to improve the stability of Na3PS4 under moisture and achieve better electrochemical stability and higher capacity, the Na3 P0.62As0.38S4 was reported by Yu et al. [81], showing high conductivity of 1.46 mS cm-1 at room temperature, additionally, at the current density of 4.8 mA g−1, a discharge capacity about 163 mAh g−1 at 80 °C in the Na-Sn/Na3P0.62As0.38S4/TiS2 cell was revealed. Besides sulfide glassy electrolytes, glassy electrolytes such as LiTaO3 and LiTiO3 were also studied. In 1978, Negran’s group [82] reported the rapid quenched LiTaO3, which exhibited ionic conductivity as high as 10-5 S cm-1 and negligible electronic conductivity as low as 10-11 S cm-1 at room temperature. There is no doubt that the interface between the electrolyte and the electrode will determine the contact resistance and the ionic transport between them and tremendously influence the total ionic conductivity. Therefore, the methods of coating were also studied with the purpose of improving the interfacial compatibility. For example, Sun’s group [83, 84] widely studied the atomic layer deposition (ALD) of LiTaO3 solid-state electrolytes through combining subcycles of Li2O (LiOt-Bu−H2O) and Ta2O5 (Ta(OEt)5−H2 O). Parameters such as morphology, binding energy, impedance, etc. were analyzed, showing us the preference of ALD in con15

trast to traditional methods such as physical vapor deposition (PVD) and chemical vapor deposition (CVD). And the impedance data showed that the thin film possessed an ionic conductivity of 2 × 10−8 S cm-1 at ambient temperature. Another impressive glass oxide electrolyte is lithium phosphorous oxynitride (LiPON). In 1992, an amorphous thin film with the composition of LixPOyNz was firstly produced by magnetron sputtering [85]. It was reported that the moisture stability, hardness and glass transition temperature etc. of Li2O-P2O5 could be increased remarkably when the nitrogen was introduced [86, 87]. Therefore, the LiPON exhibits excellent compatibility to Li metal, attractive electrochemical window of 5.5 V versus Li+/Li, and negligible electric conductivity [88]. And the thin film electrolyte can compensate the negative impact of the non-ideal ionic conductivity about only 10−6 S cm-1. In 2014, Li et al. prepared an all-solid-state thin film Li-ion battery fabricated with Pt/LiCoO2/LiPON/SnxNy/Pt. It showed thickness of only 7.3 µm, and good cycle stability can be obtained. And in this work, it was concluded that the absorptive oxygen and defect polarization of LiPON can be reduced and the ionic can be improved through the heating treatment. Recently, Li et al. [89] proposed a kind of all-solid-state battery based on LiPON electrolyte. The battery fabricated with LiNi0.5Mn1.5O4 cathode, LiPON solid electrolyte and Li anode exhibited about 90% capacity retention after 10,000 cycles and a coulombic efficiency of 99.98%, even more, long-term cycle stability can also be obtained. Compared to crystalline solid-state electrolytes, the glass type solid-state electrolytes possess facile and various fabrication and especially low grain boundary resistance. However, most of them show low total ionic conductivity (e.g., the representative thin films of LiPON gave a conductivity of 10−6 S cm-1). Although sulfide glass-ceramic electrolytes display enhanced ionic conductivity comparable to other type inorganic solid-state electrolytes, these kinds of electrolytes are hygroscopic; moreover, the limited voltage window is a crucial problem which limits its development.

3

Polymer electrolytes Compared to inorganic materials, organic materials possess a lot of goodness, for instance,

lightweight, easy to produce on a large scale, degradability, and substantial flexibility. As a result, more and more attention has been drawn on how to acquire polymer electrolytes with great properties. Solid polymers stand for mixture of a kind of alkali salt and neutral polar polymer or a polymeric salt. The polymer electrolytes has been studied since 1973 [90]. In order to improve the ionic conductivity, the processibility of thin films and the mechanical flexibility, kinds of strategies have been adopted including the molecular design of polymers with low crystallinity, the blending of alkali salt with high dissociation constant, the introduction of slight liquid electrolyte forming gel polymer electrolyte (GPE), the mixing of inorganic electrolyte with high ionic conductivity (will be discussed in Section 4) etc. [91]. 16

3.1 Ionic conduction mechanism of polymer solid-state electrolytes The ionic conduction mechanism differs in organic and inorganic electrolytes. The transportation of alkali ions in polymers is proved to be originated from the segmental movement of polymer chains, mainly depending on the amorphous phase of polymer [92]. The schematic diagram of ionic transport in polymer solid-state electrolytes is shown in Fig.6.

Fig.6 (Color online) Schematic diagram of ionic transport in polymer solid-state electrolytes, through the segmental movement of polymer chains. Reprinted with permission from ref. [93], Copyright © 2015 The Royal Society of Chemistry. The hopping of alkali ions requests free vacancies. The motion of polymer segment can produce free space for alkali ions. At the same time, an appropriate coordination between ions and polar groups can provide absorption/desorption of the ions on the polymers, which have been confirmed to be essential to high ionic conductivity. The polar groups lead to moment of dipole. During charging/discharging, the electrical field force, concentration gradient or chemical potential gradient affects the dipole and drives the movement of polymer chains. As a result, long-term continuous hopping of alkali ions is achieved [94, 95]. The motion of the side chains and the hopping of the ions are dependent on the temperature, the glass transition temperature and the activation energy. Low glass transition temperature is conducive to form amorphous structure and hence higher ionic conductivity under ambient temperature. Beside of the Arrhenius-type equation, Vogel-Tammann-Fulcher (VTF) equation was also proposed  = # < 61/exp >

6 %

&( 6 ? )

@,

(10)

where σ0 is the pre-exponential factor, Ea is the activation energy, R is the ideal gas constant and T0 is a parameter correlated to the glass transition temperature (Tg). VTF equation is based on the quasi thermodynamic model that is always used to model the ion conductivity in polymer systems. VTF model is different from Arrhenius-type equation in the relationship between logσ and 1,000/T, which displays as non-linear curve compared with linear curve for the latter [96-98]. In Fig. 7 [18], the relationship between temperature and ion conductivity of 17

different electrolytes was drawn. It is verified that the majority of inorganic electrolytes obey the Arrhenius equation; meanwhile the polymer electrolytes obey both the Arrhenius equation and the VTF equation.

Fig.7 (Color online) The relationship between ionic conductivity and temperature of different kinds of electrolytes. Typically, inorganic electrolytes follow Arrhenius equation; while organic electrolytes obey either Arrhenius equation or VTF equation. Reprinted with permission from ref. [18], Copyright © 2011 Nature Publishing Group. 3.2 Solid polymer electrolytes In 1973, PEO was applied as solid polymer electrolytes (SPEs), due to the possibility of ionic conducting, the well film-forming ability and low Tg [90]. In the following decades, the PEO became more and more popular [99-101]. However, PEO also suffers the low ionic conductivity and high crystallinity with Tg around 60 °C. As mentioned above, in polymer electrolytes, the ionic transportation is decided by the segmental movement of polymer chains at the temperature above T g [92]. The high Tg results in even lower ionic conductivity under room temperature. Therefore, various polymer hosts for solid-state polymer electrolyte were also proposed, including polyacrylonitrile (PAN), polymethyl methacrylate (PMMA) and PVDF etc. These materials suffer even lower ionic conductivity than that of PEO. Facing these challenges, various methods such as blending, cross-linking, copolymerzation were suggested to reduce the crystallinity and the Tg of the host and to increase the ionic conductivity. For example, different types of fillers like inorganic nanoparticles, organic plasticizers, and cross-linkers have been proposed [102] to reduce the Tg, soften the polymer hosts and increase the amorphous phase, aiming at enhancing the ionic movement [92, 94, 103]. 3.2.1 Tuning of blended alkali salts Typically, lithium or sodium salts are prerequisite for solid-state polymer electrolyte. So first of all, it is important to choose proper alkali salts so as to optimize performance of solid-state polymer electrolyte. Traditional lithium salts including LiClO4, LiPF6, and LiAsF6 etc. [104, 105] have been employed for solid-state polymer electrolyte. However, these electrolyte systems appeared ionic conductivity below 10-4 S cm-1 at room temperature. For further enhancing the ionic conductivity, per-fluoroalkyl sulfonic-type conducting salts like LiTFSI, 18

lithium bis(fluorosulfonyl)imide (LiFSI) and lithium trifluoromethanesulfonate (LiTf) etc. were proposed due to their high solubility, high ionic conductivity, high electrochemical stability and large anions. The large size of anions and the strong electron-withdrawing groups facilitate the dissociation of the lithium salt and increase of lithium ionic transference number. In

previous

researches,

ionic

liquids

for

solid-state

electrolytes

such

as

trifluoromethylsulfonylimide [106-109], tetraphenyl borate [110], and tetraperfluorinated phenyl borate [111] have been recognized. Recently, Zhou’s group [96] reported a kind of lithium

salt

with

polyanion,

poly[(4-styrenesulfonyl)(trifluoromethyl(S-trifluoromethylsulfonylimino)sulfonyl)imide] (PSsTFSI–), which was used together with PEO as the polymer electrolyte. The LiPSsTFSI/PEO electrolyte exhibited quite high Li-ion transport number, 0.91. And it showed ionic conductivity of 1.35×10−4 S cm−1 at 90 °C. In the same year, Polu and Rhee [112] presented the ionic liquid doped PEO based solid polymer electrolytes. The room-temperature ionic

liquid

used

in

this

report

was

1-ethyl-3-methyllimidazolium

bis(trifluoromethylsulfonyl)imide (EMImTFSI) and the difluoro(oxalato)borate (LiDFOB) was chosen as the lithium salt. The electrolytes was the mixture of PEO, EMImTFSI and LiDFOB, which revealed considerable ionic conductivity, especially in the presence of 40% EMImTFSI, in addition, there was an ionic conductivity as high as 1.85×10-4 S cm-1 at 30 °C. The attractive ionic conductivity may be attributed to the decrese of PEO crystallinity which was induced by the addition of LiDFOB and EMImTFSI. For the battery fabricated with Li/LiFePO4, it presented 155 mAh g-1 initial specific capacity in the presence of 40% EMImTFSI and about 86.5% capcity remained at rate of 0.1 C after 50 cycles. Similarly, for sodium-ion batteries, the PEO-NaClO4 and PEO-NaPF6 electrolyte was prepared in 1988 and 1995 respectively [113, 114]. They all showed ionic conductivity of ~10-6 S cm-1. Recently, in 2015, Boschin and Johansson systematically investigated the NaX (X: TFSI, FSI)-PEO based solid electrolyte [115]. The results demonstrated that the NaTFSI(PEO)n SPE with n = 9 showed a higher ionic conductivity (4.5×10-5 S cm-1 at 20 °C) than NaFSI(PEO)n materials at temperature lower than 40 °C. 3.2.2 Mixing with inorganic fillers Besides tuning of salts, inorganic fillers including Al 2O3, SiO2, and TiO2 were usually added in solid-state polymer electrolytes to compensate the poor mechanical strength [116-118]. Meanwhile, these fillers can improve the ionic conductivity through forming pathway in the interface for ionic migration [93, 112]. The mechanism can be explained by Lewis’s acid-base theory. The fillers and Li ions are Lewis acids; while the anions and the polymer electrolytes are Lewis base. The fillers have three effects in the composites: (1) the inorganic fillers act as the crosslinking centers for polymer segment and X- anions and prevent the reorganization 19

and the crystallization of local polymer chains. The inhabitation of the crystallization and the decrease of Tg favors fast ionic transport. (2) The inorganic fillers, acting the crosslinking centers with the electrolyte anionic species, which reduces ionic coupling of anions with Li ions. As a result, salt dissociation is promoted and ionic conductivity can be further enhanced. (3) At the same time, the Lewis acid inorganic fillers as cross-linkers can enhance the connections among the polymers and thereby improve the mechanical strength [119]. Such “mixing with fillers” strategy is also considered as “composite polymer electrolytes” in many literatures. However, due to the mechanism of this filler-effect mentioned above, we classify the mixing of polymers with inorganic fillers into the SPE part; while composite polymer electrolytes (CPEs) in this review are assigned to the composites consisting of inorganic solid-state electrolytes with solid polymer electrolytes, both of which are ion-conductive electrolyte materials. In 2014, Chen’s group [120] reported a kind of poly(ethylene glycol) (PEG)/ poly(methacrylate) (PMA) based composite polymer electrolyte. It was concluded that the electrolyte with 3 wt% SiO2 gave an acceptable ionic conductivity of 2.6 ×10-4 S cm-1 at room temperature, which was shown in Fig. 8a. In addition, the pillar[5]quinone (P5Q) was prepared as the cathode. As described in Fig.8c, d, the battery depended on the P5Q and PEG/PMA-SiO2 gel electrolyte exhibited room-temperature capacity of 418 mAh g-1 when the current density was 0.2 C, and about 94% capacity retention can be obtained after 50 cycles. Then Ni’mah et al. [121] studied the influence of TiO2 filler on PEO polymer electrolyte in Na2/3Co2/3Mn1/3O2/Na half-cell. The nanocomposite electrolyte composed of 5% nanosized TiO2, PEO and NaClO4 with the ratio of EO:Na=20 gave a maximum ionic conductivity of 2.62×10-4 S cm-1 and at the current density of 0.1 C, the Na2/3Co2/3Mn1/3O2/Na half-cell presented discharge capacity of 50 mAh g-1 at 60 °C. At the same time, good cycle stability can be observed. Armand’s group [122] reported PEO/poly(ethylene glycol)dimethyl ether (PEGDME) electrolyte grafted by Al2O3 nanoparticles. The all-solid-state Li-ion battery fabricated with metallic lithium anode, PEO/PEGDME electrolytes and LiFePO4 cathode. In order to improve the interfacial compatibility, the cathode material was mixed with super P conductive additive and salt composed electrolyte (PEO and LiPSTFSI). The batteries showed discharge capacity of about 120 mAh g-1 at the rate of C/2 rate after 130 cycles, and the coulombic efficiency was around 95%. Much stronger chemical/mechanical interactions between the filler (SiO2) and the PEO chains were produced by in situ hydrolysis of tetraethyl orthosilicate in PEO solution [123]. The enhanced interactions were proved to be beneficial to the efficient Lewis acid-base interaction, the inhibition of the PEO crystallization, the dissociation of LiClO4 and the subsequent high ionic conductivity (one order of magnitude higher than that by just mechanical mixing). 20

Fig.8 (Color online) (a) The relationship between fillers contents and the ionic conductivity of the electrolytes at room temperature. (b) Temperature dependence of the ionic conductivity of the electrolytes with 3 wt% fillers. (c) Discharge-charge profiles of different cycles at 0.2 C rate. (d) Rate capability of P5Q/PMA/PEG-LiClO4-SiO2 GPE/Li battery under room temperature. Reprinted with permission from ref. [120], Copyright © 2014 American Chemical Society.

3.2.3 Blending with polymers or organic plasticizers Another way to optimize polymer host is to blend with polymers or organic plasticizers, which has the advantage of facile fabrication and well compatibility between polymeric host. The blending of flexible organic or polymeric materials could inhibit the crystallinity and improve ionic conductivities and dimensional stability of electrolytes. Blending components like PMMA, polyvinyl alcohol (PVA), PVDF, PEG etc. have been proposed for many years. Recently, Hashaikeh’s group [124] used a celllulose network to blend with PEO for good mechanical stability at high temperature while keeping the electrochemical properties of PEO. It was concluded that the PEO with 15% network cellulose presented better mechanical property and thermal stability than those of individual PEO. The blending polymer quasi-solid electrolyte, which is made up of PEG (or tetraglyme), PEO, network cellulose and LiClO4 [125] with a PEG/PEO/network cellulose ratio of 7:2:1 and EO/Li ratio of 12:1, provided an ionic conductivity around 10-4 S cm-1 at room temperature. As mentioned in last section, PEO-PEGDME blending polymer electrolyte was used in solid-state Li-ion batteries, in which the PEGDME was plasticizer for optimization of PEO-based electrolyte [122]. For sodi21

um-ion batteries, as early as 2001, Chandrasekaran and Selladurai [126] reported the PEO-PEG-NaClO3 (3:6:1) systems with an increased ionic conductivity of 3.4 × 10-6 S cm-1 which was higher than the PEO-NaClO3, and the activation energy was decreased to 0.417 eV. In addition, the energy density of Na/electrolyte/MnO2 cell was 350 Wh kg-1 at 35 °C. Recently, a PEO-based polymer solid electrolyte was prepared by blending PEO with sodium carboxymethyl cellulose (NaCMC) [127]. The reports demonstrated that NaCMC was able to improve mechanical property without compromise of ionic conductivity and thermal stability. In addition, the PEO-NaCMC revealed lower charge transfer resistance, suggesting a better interfacial compatibility between electrolyte and sodium electrode. Beside of the polymers, small molecular plasticizers were also reported, which could inhibit the crystallinity of the polymeric electrolyte and enhance the mechanical stability. For example, a star polyether (star-PEO), which has a very low Tg of -93 °C was used as the filler. The electrolyte prepared with PEO, star-PEO and LiClO4 provided an ionic conductivity of 6×10-5 S cm-1 at 0 °C, which was two orders of magnitude higher than that without star-PEO under the same condition [128]. Other organic plasticizers, such as succinonitrile, were also widely studied [129]. 3.2.4 Copolymerization Instead of various additives, molecular design of polymer hosts is one efficient way to improve the property of solid-state polymer electrolyte. Therein, block copolymer was one of the most available methods to optimize the property of polymer solid-state electrolytes, because block copolymer comprising different covalently bonded polymers can combine advantages of different parts. In general, one block A is PEO-based polymer which provides acceptable ionic conductivity, the other block B supports other functionalities, mainly provides mechanical strength and reduces the crystallinity of block A. In 2013, Bouchet et al. [130] reported a polyanionic

BAB

triblock

copolymers

electrolyte

using

poly(styrene

trifluoromethanesulphonylimide of lithium) P(STFSILi) as B block and linear PEO as central A block to pursue considerable ionic conductivity without compromise of mechanical property. The P(STFSILi)-PEO-P(STFSILi) materials gave an attractive ionic transference number above 0.85. And at 60 °C, it displayed ionic conductivity of 1.3×10-5 S cm-1. In addition, not only the mechanical strength and electrochemical stability, but also great cycle stability of Li/BAB/LiFePO4 battery was presented. Rolland et al. [131] presented the chemically anchored liquid-PEO electrolyte which revealed conductivity of 1×10-5 S cm-1 through block copolymer engineering. The engineered electrolyte exhibited great electrochemical stability and considerable compatibility to Li metal. The graft copolymerization is one meaningful way to enhance the performance of the polymers, in which the polymer backbone is used to enhance the mechanical stability and the 22

side chains (normally PEO-based chains) provide the ionic conductivity. The efficient motion of side chains is beneficial to high ionic conductivity. For example, Daigle et al. [132] reported the synthesis of a comb-like copolymers, in which the poly(oligo-oxyethylene methacrylate) was grafted to the copolymer polystyrene-co-poly(4-vinylanisole). At room temperature, the highest conductivity could reach 6.8×10-5 S cm-1. And a conductivity of 2.54×10-4 S cm-1 could be revealed at 60 °C. For the battery fabricated with LiFePO4, solid polymer electrolyte (SPE) and Li anode, it showed a discharge capacity of 146 mAh g-1 at 80 °C when the current density was C/24. Another interesting work showed that the side chains with similar length may result in local crystallization and a deliberate adjustment of side chains with different length can lead to higher conductivity [133]. Crosslinking copolymers which form a two- or three-dimensional network through crosslinking between different polymers are promising candidates for solid-state electrolytes. The crosslinking copolymers have the advantages of low crystallinity, high mechanical strength and high ionic conductivity. For example, the reported interpenetrating poly(ether-acrylate) (ipn-PEA) network [134] developed via photopolymerization of ion-conductive PEO and branched acrylate showed an ionic transference number of 0.65, a room-temperature conductivity of 2.2×10-4 S cm-1 and a mechanical strength of 12 GPa, with the absence of alkali salt LiPF6. As revealed in Fig.9, the electrolyte is quite electrochemically stable until 4.5 V (vs. Li/Li+) and the Li/ipn-PEA electrolyte/LiFePO4 battery delivered a capacity about 141 and 66 mAh g-1 at 0.5 and 5 C respectively, in addition, a light emitting diode (LED) device can be lighted by the Li/ipn-PEA electrolyte/LiFePO4 curved pouch cell.

Fig.9 (Color online) Rate capabilities (a) and charge and discharge voltage profiles (b) for a Li/ipn-PEA electrolyte/LiFePO4 battery at different rate. (c) Cycling performances for the solid battery at a rate of 1 C. (d) The voltage of a Li/ipn-PEA electrolyte/LiFePO4 pouch cell after being cut. The LED device can be lighted before (e) and after (f) the bending test. All the tests were carried out at room temperature (25°C). Reprinted with permission from ref. [134], Copyright © 2016 American Chemical Society. 23

Normally, the strategies mentioned above are adopted together for increasing the transference number, enhancing the ionic conductivity and mechanical strength. For example, with the absence of alkali salt LiClO4, the polymer plasticizer PEG and the inorganic filler SiO2, the polymer poly(methacrylate) (PMA) [120] produced a maximum ionic conductivity of 2.6 ×10-4 S cm-1 at room temperature. In this case, the soluble organic molecules can be used as cathode, delivering an initial capacity of 418 mAh g-1 with average voltage of 2.6 V and capacity retention of 94.7% after 50 cycles at 0.2 C. Another example [135] can be found by using crosslinking copolymer as polymer host which was obtained by polymerization of polyethylene (PE) with PEO segments. The addition of LiTFSI gave an ionic conductivity of 2.5×10-5 S cm-1 at 25 °C. The further introduction of plasticizer PEGDME will enhance the conductivity to 2×10-4 S cm-1. In terms of polymer electrolytes, impressive interfacial compatibility, considerable flexibility, and outstanding convenience for packaging and large-scale production can be realized due to the natural advantages of polymer materials. Yet, non-ideal ionic conductivity below 10-3 S cm-1 at room temperature is the main challenge which limits its practical applications, especially for the high power density applications. In addition, it is imperative to pursue considerable mechanical strength, wide electrochemical window and good chemical stability for solid polymer electrolytes. 3.3 Gel polymer electrolytes Although various methods have been attempted to increase the ionic conductivity, the solid polymer electrolytes usually showed an ionic conductivity lower than 10-3 S cm-1 at ambient temperature, as mentioned above. The high working temperature for giving a high conductivity will extremely restrict their application. Another, the Li+/Na+ transference number in PEO-based SPE is inherently low, which is around 0.2~0.5 [99, 136, 137]. In order to increase the ionic transference number and conductivity, liquid electrolyte was added into the SPE, forming a gel polymer electrolyte. The qusi-solid state property is helpful for enhancing the performance of the electrolytes, because they possess the advantages of both liquids (diffusive property) and solids (cohesive property). To date, many types of polymer host for gels have been presented, such as PEO, PVDF-HFP, PMMA, and so on. In order to form a GPE, a suitable uptake of liquid electrolyte is of great importance. Therefore, porous structures were often utilized. For example, porous membrane of electrospun polyacrylonitrile nanofibers was fabricated and used as the polymer host, which showed a high electrolyte uptake of >390% and a high ionic conductivity of 2×10-3 S cm-1 [138]. According to the last section, one of the most efficient methods to produce GPEs is the utilization of cross-linked polymers, due to the entangled chains [139, 140]. Cross-linked poly24

mers can be achieved through chemical cross-linking or physical entanglement. The cross-linked polymers transform from liquid to solid state at a critical condition [141]. Based on this strategy, an extremely high room-temperature ionic conductivity of 10-2 S cm-1 was obtained by using pentaerythritol tetraacrylate (PETEA)-based GPE. The GPE was prepared by gelation of PETEA monomer with azodiisobutyronitrile initiator and liquid electrolyte of 1 mol L-1 LiTFSI in DOL/DME (1:1 by volume) [142]. The obtained Ea by using VTF equation (2.94×10-2 eV) is quite close to that of liquid electrolyte. Previously, it has been reported that the electrolytes with considerable ion transference number and conductivity could suppress the polarization in batteries [143]. So, numerous ways to enhance the transference number have been utilized. For example, the using of lithium salt with large size of anions as mentioned above can be adopted. The composite GPE (soaked in 1 mol L-1 LiPF6) based on electrospun PVDF and lithium polyvinyl alcohol oxalate borate (LiPVAOB) provided a high transference number of 0.58, twice as that in commercial separator (0.27) [144]. And the ionic conductivity at ambient temperature reached up to 0.26 mS cm-1. The calculated Ea by using Arrhenius equation was as low as 0.022 eV. As a consequence, the ceramic-polymer GPEs appealed to more interests. However, the ceramic-polymer GPEs always cannot keep mechanical property, so improvement to constitute a strong skeleton is essential. Tsao et al. [145] presented silica cross-linker to achieve cross-linked polymer, which allow PEO polymer to form the channels for Li-ions. The electrolyte membrane was produced by mixing polyetheramine, bisphenol A diglycidyl ether and silica cross-linker and then soaked in liquid electrolyte to form the GPE. The batteries were fabricated with LiFePO4 cathode, Li anode and GPEs. For comparison, three types of GPES were proposed: (1) Pristine gel polymer electrolytes (PGPES); (2) ceramic gel polymer electrolytes (CGPES), ceramic was used as filler; and (3) ceramic-cross-linked gel polymer electrolytes (SGPES). From the data presented, the SGPES with 20% silica cross-linker had the highest tLi+=0.5. Even more, the SGPES with 20% silica cross-linker showed excellent electrochemical stability up to 5.0 V, which may be attributed to the enhanced stability by silica particles. In terms of discharge capacity, the SGPES with 20% silica cross-linker showed 162, 148, 96 mAh g-1 at 0.1, 1 and 5C respectively, which was superior to other GPES. In addition, after 100 cycles, almost 97% columbic efficiency still remained. Another report about cross-linked GPEs was using mesoporous nanoparticle SiO2 containing methacrylate groups (MA-SiO2) as cross-linking sites [146]. The MA-SiO2 dispread uniformly onto the PAN membrane and directly reacted with the GPEs precursor (tri(ethylene glycol) diacrylate (TEGDA)). The resulted cross-linked GPEs provided acceptable Li-ion conductivity and great interfacial adhesion to the electrodes. The addition of core-shell SiO2 nanostructures into the GPE increased the Li transference number and the room-temperature ionic conductivity can increase to 1.4×10-3 S cm-1 [147]. The ceramic fillers also reinforced the mechanical strength 25

and the interfacial properties. Although the addition of liquid electrolyte into the SPE forming GPE produced high ionic conductivity, it makes the development of SPE go back to the beginning. The addition of liquid electrolyte brings back the electrolyte leakage, the flammability and safety issues. Meanwhile, even though slight liquid electrolyte is soaked, the mechanical strength of SPE becomes deteriorated. In order to reduce the flammability and enhance the safety, ionic liquids are utilized in GPEs. Ionic liquids are molten salts at room temperature, which have numerous advantages for batteries, for instance, significant ability, non-flammability, non-volatility, high ionic conductivity etc. [148]. As far as 2005, Watanabe’s group [149] prepared transparent gel polymer film through polymerization of methyl methacrylate (MMA) in 1-ethyl-3-methyl imidazolium bis(trifluoromethane sulfonyl)imide (EMITFSI) ionic liquid with some cross linkers. The gel electrolyte can give an ionic conductivity of 10-2 S cm-1 at room temperature, and acceptable cation transport number of 0.703 when the mole fraction of EMITFSI was 0.5 mol L-1. Ravi et al. [150] proposed environmental friendly (biodegradable), transparent, and flexible GPEs using 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4) ionic liquid. The GPEs were composed of poly(e-caprolactone) (PCL), lithium tetrafluoroborate (LiBF4 ) and EMIMBF4. As a result, 85PCL:15LiBF4 system doped with 40% EMIMBF4 possessed an ionic conductivity of 2.83×10-4 S cm-1, and the transference number reached approximately 0.43. Importantly, it was still electrochemically stable till 3.7 V. A new ionic liquid 1-butyl-1-methylpyrrolidinium bis(fluorosulfonyl)imide (P14FSI) were utilized in GPEs [151]. The GPEs constituted of cross-linked polyoxyethylene-based methacrylate copolymer, LiTFSI salts and the ionic liquid. Quasi-solid-membranes were obtained with acceptable mechanical property, transparent appearance and optimized Li-ion conductivity of 4×10-4 S cm-1 at room temperature. The batteries assembled with LiNi0.33Mn0.33Co0.33O2, metallic Li electrodes and GPEs exhibited considerable specific capacity during 50 cycles, and extraordinary columbic efficiency. On the basis of cross-linking methods, Lu et al. [152] presented the GPEs with 3D conductive and substantial network via ring-opening polymerization. Diglycidyl ether of bisphenol-A (DEBA) was chosen as the supporting framework, which was cross-linked with diamino-poly (propylene oxide) (DPPO) to ensure fast ionic transport. PVDF-HFP embedded in the network was used in GPEs to support flexibility. Electrolyte uptake is also one important parameter to judge whether the GPEs is suitable for LIBs, and the results suggested that the 3D-GPE possessed good uptake ability. Compared to liquid electrolyte, 3D-GPE revealed low volatility and thereby high stability until about 150 °C and better prevention to flammability than liquid electrolytes. It is distinct that the commercial separator sustained drastic shrinkage, and in sharp contrast, the 3D-GPE maintained original shape. The LiFePO4/3D-GPE/Li battery delivered an initial discharge capacity of 154.3 mAh g−1 at 0.3 C and columbic efficiency 26

about 94%, which were comparable to the LiFePO4/liquid electrolyte/Li battery. After 200 cycles, almost 99.3% capacity remained and the columbic efficiency approached 99.5%. In addition, at different charge/discharge rate, the LiFePO4/3D-GPE/Li battery showed higher capacity than original LiFePO4 / liquid electrolyte/ Li battery, especially at high rate more than 8 C. At last, the excellent adhesion of 3D-GPE and Li anode provided smooth interface, which suppressed the growth of Li dendrite and kept electrochemical stability of battery. The electrochemical performance of Li-metal batteries was shown in Fig. 10.

Fig.

10

(Color

online)

The

lithium

deposition/stripping

3D-GPE

and

separa-

tor–liquid-electrolyte based symmetrical cells at current densities of 0.5 (a) and 2.5 (b) mA cm-2. Cycling performances of the 3D-GPE and separator-liquid-electrolyte based LiFePO4/Li batteries at 0.3 (c) and 4 (d) C respectively. (e) Rate performances of the 3D-GPE and separator liquid-electrolyte based LiFePO4/Li batteries. (f) EIS of the 3D-GPE and separator–liquid-electrolyte based LiFePO4/Li batteries after 1, 100, and 200 cycles at rate of 0.3C. Reprinted with permission from ref. [152], Copyright © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. For sodium-ion batteries, Gao [153] presented a composite gel polymer, the key point was the glass fiber which was utilized to get better mechanical strength. PVDF-HFP was adopted as the polymer host, whose hydrophobic nature of PVDF-HFP is detrimental to the performance of batteries at high current density [145, 154]. In this case, hydrophilic polydopamine membrane was selected, by coating the memebrane on the surface of PVDF-HFP. When fabricated with Na2MnFe(CN)6 cathode and Na metal anode, the ionic conductivity of the gel was comparative to the liquid electrolytes, and the composite electrolytes enhanced the maximum stress to 20.9 MPa. The battery based on PVDF-HFP/GF/polydopamine displayed a considerable capacity of 131 mAh g-1, and great cylce stability at different current density can be obtained. And very recently, they reported a gel polymer electrolyte based on cross-linked PMMA which was synthesized by in situ radical polymerization [155]. The cross-linked PMMA electrolyte exhibited ionic conductivity of 6.2×10-3 S cm-1 at room temperature. The sodium-ion full-cell assembled with Sb/gel electrolyte/Na3V2 (PO)4 showed capacity about 27

106.8 mAh g-1 at 0.1 C and 61.1 mAh g-1 at 10 C discharge capacity. In addition, excellent cycleability and highly reversible electrochemical reaction can be observed in the full-cell. Compared to all-solid-state electrolytes, gel polymer electrolytes provide better ionic conductivity even comparable to commercial liquid electrolytes. At the same time, the quasi-solid state of GPE provides opportunities of achieving solid-state batteries and thereby benefits the suppression of Li dendrites growth and safety. However, the remaining challenge is that the inevitable presence of liquid electrolytes still cannot avoid the risk of leakage and combustion completely, which leads to difficulty of packing and achieving flexible and foldable alkali-ion batteries by using gel polymer electrolytes.

4

Composite solid-state electrolytes The composite polymer electrolytes (CPE), which contain inorganic solid-state electrolytes

and polymer electrolytes, are intriguing for solid-state batteries, because they can integrate the merits of inorganic (high ionic conductivity and stability) and polymeric solid electrolytes (high mechanical property and good interfacial compatibility) and compensate the shortages of them. The CPE mentioned here are identified as composite solid-state electrolytes, in which both the inorganic and polymeric materials are ionic-conductive (or active) electrolytes [84, 156] rather than SPE with addition of inorganic fillers or plasticizers. The mixing of SPEs with inorganic fillers or plasticizers is used to enhance the performance of the SPEs, such as inhibit the crystallization of solid polymer electrolytes, decrease T g, facilitate the ionic dissociation and thereby benefit the ionic conductivity, and enhance the mechanical strength. However, the CPE mentioned here are proposed for quite different purposes, i.e., utilizing the merits of both inorganic and polymeric electrolytes. An example was reported by Zhao et al. [157], the solid-state electrolyte consisted of PEO and Li10GeP2S12, concluding that the membrane with 1 wt% Li10GeP2S12 revealed 1.18×10-5 S cm-1 at room temperature. Moreover, at 60 °C the LiFePO4 /Li battery exhibited about 160 mAh g-1 at 0.1 C, and presented 92.5% capacity retention after 50 cycles. Even more, the interfacial compatibility between electrolyte/electrode was also pursued, which is important for ionic transport, mechanical stability and inhabitation of the growth of Li dendrites. In order to improve the interfacial compatibility, the concept of sandwich structure was definitely raised by Zhou et al. (Figure 11a, b) [158]. The ceramic as the middle layer provides the high ionic conductivity and two polymer layers as the buffer layer on both sides of the ceramic is conducive to improve the interfacial compatibility and inhibit the growth of Li dendrites. In this work, the cross-linked poly(ethylene glycol) methyl ether acrylate (CPMEA) was utilized with Al2O3 as buffer layers and the ceramic materials was NASICON-type Li1.3Al0.3Ti1.7 (PO4)3 (LATP), which owns high ionic conductivity and moisture stability. LATP layer also played the role to prevent short circuit if the Li dendrites punc28

tured the polymer layer. The LiFePO4/CPMEA-LATP/Li sandwich battery displayed comparable discharge capacity to LiFePO4/CPMEA/Li battery at 65 °C. Moreover, the sandwich-electrolyte based battery showed better cyclic stability that the capacity maintained at 102 mAh g-1 after 640 cycles; on the contrary, the capacity of LiFePO4/CPMEA/Li battery showed a rapid fading to 70 mAh g-1 after 325 cycles. In one previously mentioned work, garnet structure LLZT [37] was adopted as the inorganic electrolyte and cross-linked PEO with LiTFSI was the polymer buffer layer for wetting the Li metal and suppressing the Li-dendrite growth. In order to improve the interface between LLZT and electrodes or polymer electrolyte, LiF was added, which can also increase the stability of LLZT against moisture and CO2 in air.

Fig.11 (Color online) (a) Illustration the sandwich-structured electrolyte based battery. (b) Chemical structure of cross-linked polymer CPMEA. Reprinted with permission from ref. [158], Copyright © 2016 American Chemical Society. (c) Ceramic particles are randomly dispersed in the polymer matrix. (d) Vertically aligned and connected ceramic channels. Reprinted with permission from ref. [159], Copyright © 2017 American Chemical Society.

A similar report was also concerning interfacial engineering in hybrid ceramic-polymer electrolytes [160]. In order to modify the interfaces, the conformal SiO2 coating was spread on the

Li2O-Al2O3-SiO2-P2O5-TiO2 -GeO2

(LiCGC),

then

silanated

with

(CH3CH2O)3 -Si-(OCH2CH2)-OCH3, in the presence of Li salt (LiTFSI). During the silanation 29

process, a relatively smooth polymer layer could be obtained. The modified electrolytes showed the lowest total resistance. In addition, the (PEG-silane/LiTFSI)-SiO2-LICGC electrolyte showed comparable conductivity of 1.5×10-4 S cm-1 at 30 °C to individual LICGC. Good cycle stability was also presented. It can be understood that the composite electrolyte, regardless of blending or sandwiched structure, the total ionic conductivity is usually compromised by the polymer matrix. In 2017, Zhai et al. [159] reported a new structure of composite electrolyte, in which Li1+xAlxTi2−x(PO4 )3 (LATP) nanoparticles vertically aligned forming an ionic conductive channel inside the PEO, by templating-based method (Figure 11c, d). Different from randomly dispreading LATP nanoparticles, the aligned geometry of LATP nanoparticles (PEG as plasticizer) presented ionic conductivity of 0.52×10-4 S cm-1 at room temperature, which was comparable to individual LATP (1×10-4 S cm-1). Meanwhile, thermal and electrochemical stability were better than pure PEO. Generally speaking, the composite polymer electrolytes integrate the merits of organic and inorganic materials and compensate their shortcomings, so that the composite electrolyte based solid-state alkali batteries can exhibit good performance in both ionic conductivity and mechanical property. Such composite solid-state electrolytes are considered as a direction of the future research in the field of solid-state electrolytes. However, regardless the sandwich structure or the vertical arrays design, although impressive performance can be observed, as for practical application, there are still a lot of challenges on how to enhance ionic conductivity in order of magnitude, how to simplify the fabrication process for industrial manufacture and so on.

5

Conclusion and outlook Alkali-ion batteries are playing necessary role in our daily life, whereas, it is imperative to

solve the safety problems of liquid-electrolyte based alkali-ion batteries. Even more, flexible devices are calling for flexible power supply. As a result, over the past several years, lots of efforts on solid-state electrolytes for alkali-ion battery have been made. The solid-state electrolytes have the advantages of eliminating the electrolyte leakage, the flammability and the growth of alkali metal dendrites, therefore leading to higher safety. Moreover, the dissolution problem of the electrode materials in the liquid electrolyte can be solved. In this case, materials with high energy density and power density (e.g., Li as anode, sulphur and small molecular organic materials) can be used as electrodes. In addition, the application of solid-state electrolytes made the flexible electronics possible. In the past decades, a large variety of solid-state electrolytes with considerable performance have been reported. In short words, each has its own advantages and disadvantages. The basic characteristics of different independent materials can be drawn: 30

(1) The inorganic solid-state electrolytes including glass, garnet, perovskite/anti perovskite, LiSICON and NaSICON electrolytes have high ionic conductivity (as high as 10-2-10-3 S cm-1), high thermal stability, high electrochemical stability, and good mechanical strength, although the rigidity leads to the loss of flexibility and the syntheses and purifications often require tough conditions leading to high expenses. Moreover, the utilization of inorganic solid-state electrolytes always suffers the poor interfacial compatibility between the electrolytes and the electrodes. (2) As for polymer electrolytes, the polymers have good flexibility and well compatibility with electrodes. Meanwhile, the polymers can be prepared by artificial synthesizing and hence massive and inexpensive production can be expected. However, they always have the shortcomings of low room-temperature ionic conductivity (lower than 10-3 S cm-1). Although the addition of slight liquid electrolyte forming GPE leads to high ionic conductivity (10-3-10-4 S cm-1), the liquids brings the leakage, flammability and safety problems back. (3) Composite electrolytes seem to be a good way by compromising of ionic conductivity, flexibility and mechanical stability, leading to acceptable overall performance, and therefore are regarded as potential candidates for future research and applications. The sandwiched structures consisting of an inorganic solid-state electrolyte for high ionic conductivity and two organic solid-state electrolytes as buffer layers for improving interfacial compatibility provide an opportunity for combining both advantages, although attention should to be paid because the total ionic conductivity probably would be restricted by the buffer layers. Another method by using vertical inorganic solid-state electrolyte arrays was proved to be efficient to maintaining the high ionic conductivity of inorganic solid-state electrolytes; however, this method has been doubtful for practical large-scale applications. In another word, there is still a long way to go. However, it is worth noting that the demo of various electronic devices such as phones, watches and laptops with all-solid-state LIBs have been exhibited and all-solid-state LIBs have been also applied in electric cars. In 2015, BOSCH presented a low-cost electric car with all-solid-state LIBs which can drive as far as 322 km. Sakti3 also declared that a breakthrough has been made in all-solid-state LIBs for electric car and 800 km could be afforded. Such exciting news predicts a bright future of solid-state electrolytes and the practical applications of all-solid-state alkali-ion batteries. Great change is going on!

Conflict of Interest: The authors declare that they have no conflict of interest. Acknowledgments This work was supported by the National 1000-Talents Program, the National Natural Science Foundation of China (51203067, 51773071), Wuhan Science and Technology Bureau 31

(2017010201010141), and the Fundamental Research Funds for the Central Universities (HUST: 2017KFYXJJ023).

References 1

Armand M, Tarascon JM. Building better batteries. Nature 2008; 451: 652-657.

2

Su L, Jing Y, Zhou Z. Li ion battery materials with core-shell nanostructures. Nanoscale 2011; 3: 3967-3983.

3

Tang M, Li H, Wang E, et al. Carbonyl polymeric electrode materials for metal-ion batteries. Chin Chem Lett 2017: DOI: 10.1016/j.cclet.2017.09.005.

4

Xu W, Wang J, Ding F, et al. Lithium metal anodes for rechargeable batteries. Energy Environ Sci 2014; 7: 513-537.

5

Tarascon J M, Armand M. Issues and challenges facing rechargeable lithium batteries. Nature 2001; 414: 359.

6

Quartarone E, Mustarelli P. Electrolytes for solid-state lithium rechargeable batteries: recent advances and perspectives. Chem Soc Rev 2011; 40: 2525-2540.

7

Arora P, Zhang Z. Battery separators. Chem Rev 2004; 104: 4419-4462.

8

Lin D, Liu Y, Cui Y. Reviving the lithium metal anode for high-energy batteries. Nat Nanotechnol 2017; 12: 194-206.

9

Guo Y, Li H, Zhai T. Reviving lithium-metal anodes for next-generation high-energy batteries. Adv Mater 2017; 29: 1700007.

10

Wang C, Jiang C, Xu Y, et al. A Selectively permeable membrane for enhancing cyclability of organic sodium-ion batteries. Adv Mater 2016; 28: 9182-9187.

11

Li X, Cheruvally G, Kim J K, et al. Polymer electrolytes based on an electrospun poly(vinylidene fluoride-co-hexafluoropropylene) membrane for lithium batteries. J Power Sources 2007; 167: 491-498.

12

Zhu B, Wang H, Leow W R, et al. Silk fibroin for flexible electronic devices. Adv Mater 2016; 28: 4250-4265.

13

Thangadurai V, Narayanan S, Pinzaru D. Garnet-type solid-state fast Li ion conductors for Li batteries: critical review. Chem Soc Rev 2014; 43: 4714-4727.

14

Alonso J A, Sanz J, Santamaría J, et al. On the location of Li+ cations in the fast Li-cation conductor La0.5Li0.5TiO3 perovskite. Angew Chem Int Ed 2000; 31: 619.

15

Ma C, Chen K, Liang C, et al. Atomic-scale origin of the large grain-boundary resistance in perovskite Li-ion-conducting solid electrolytes. Energy Environ Sci 2014; 7: 1638-1642.

16

Stramare S, Thangadurai V, Weppner W. Lithium lanthanum titanates: A Review. Chem Mater 2003; 15: 3974-3990.

17

Ren Y, Chen K, Chen R, et al. Oxide electrolytes for lithium batteries. J Am Ceram Soc 2015; 98: 3603-3623. 32

18

Kamaya N, Homma K, Yamakawa Y, et al. A lithium superionic conductor. Nat Mater 2011; 10: 682.

19

Sun C, Liu J, Gong Y, et al. Recent advances in all-solid-state rechargeable lithium batteries. Nano Energy 2017; 33: 363-386.

20

Zhou D, He YB, Liu R, et al. In situ synthesis of a hierarchical all-solid-state electrolyte based on nitrile materials for high-performance lithium-ion batteries. Adv Energy Mater 2015; 5: 1500353.

21

Park M, Zhang X, Chung M, et al. A review of conduction phenomena in Li-ion batteries. J Power Sources 2010; 195: 7904-7929.

22

Mehrer H. Diffusion in solids. 2007; 87: 841-845.

23

Manthiram A, Yu X, Wang S. Lithium battery chemistries enabled by solid-state electrolytes. Nat Rev Mater 2017; 2: 16103.

24

Kumar P P, Yashonath S. Ionic conduction in the solid state. J Chem Sci 2006; 118: 135-154.

25

Kasper H M. Series of rare earth garnets Ln3+3 M2Li+3O12 (M=Te, W). Inorg Chem 1969; 8: 1000-1002.

26

Thangadurai V, Kaack H, Weppner W J F. Novel fast lithium ion conduction in garnet-type Li5 La3M2O12 (M = Nb, Ta). J Am Ceram Soc 2003; 86: 437-440.

27

Thangadurai V, Weppner W. Li6ALa2Ta2O12 (A=Sr, Ba): novel garnet-like oxides for fast lithium ion conduction. Adv Funct Mater 2005; 15: 107-112.

28

Murugan R, Thangadurai V, Weppner W. Fast lithium ion conduction in garnet-type Li7La3Zr2O12. Angew Chem Int Ed 2007; 46: 7778-81.

29

Kokal I, Somer M, Notten P H L, et al. Sol-gel synthesis and lithium ion conductivity of Li7 La3Zr2O12 with garnet-related type structure. Solid State Ionics 2011; 185: 42-46.

30

Yu S, Schmidt R D, Garcia-Mendez R, et al. Elastic properties of the solid electrolyte Li7La3Zr2O12 (LLZO). Chem Mater 2016; 28: 197-206.

31

Awaka J, Takashima A, Kataoka K, et al. Crystal structure of fast lithium-ion-conducting cubic Li7La3Zr2O12. Chem Lett 2011; 40: 60-62.

32

Awaka J, Kijima N, Hayakawa H, et al. Synthesis and structure analysis of tetragonal Li7La3Zr2O12 with the garnet-related type structure. J Solid State Chem 2009; 182: 2046-2052.

33

Du F, Zhao N, Li Y, et al. All solid state lithium batteries based on lamellar garnet-type ceramic electrolytes. J Power Sources 2015; 300: 24-28.

34

Jin Y, McGinn P J. Li7 La3Zr2O12 electrolyte stability in air and fabrication of a Li/Li7La3Zr2O12/Cu0.1V2O5 solid-state battery. J Power Sources 2013; 239: 326-331. 33

35

Ahn CW, Choi JJ, Ryu J, et al. Electrochemical properties of Li7La3Zr2O12-based solid state battery. J Power Sources 2014; 272: 554-558.

36

Li Y, Han JT, Vogel S C, et al. The reaction of Li6.5La3Zr1.5Ta0.5O12 with water. Solid State Ionics 2015; 269: 57-61.

37

Li Y, Xu B, Xu H, et al. Hybrid polymer/garnet electrolyte with a small interfacial resistance for lithium-ion batteries. Angew Chem Int Ed 2016; 56: 753-756.

38

Van den Broek J, Afyon S, Rupp J L M. Interface-engineered all-solid-state li-ion batteries based on garnet-type fast Li+ conductors. Adv Energy Mater 2016; 6: 1600736.

39

Jeon N J, Noh J H, Kim Y C, et al. Solvent engineering for high-performance inorganic-organic hybrid perovskite solar cells. Nat Mater 2014; 13: 897-903.

40

Liu M, Johnston M B, Snaith H J. Efficient planar heterojunction perovskite solar cells by vapour deposition. Nature 2013; 501: 395-398.

41

Snaith H J, Abate A, Ball J M, et al. Anomalous hysteresis in perovskite solar cells. J Phys Chem Lett 2014; 5: 1511-1515.

42

Bonanos N, Knight K S, Ellis B. Perovskite solid electrolytes: structure, transport property, and fuel cell applications. Solid State Ionics 1995; 79: 161-170.

43

Inaguma Y, Liquan C, Itoh M, et al. High ionic conductivity in lithium lanthanum titanate. Solid State Commun 1993; 86: 689-693.

44

Wu J F, Guo X. Origin of the low grain boundary conductivity in lithium ion conducting perovskites: Li3xLa0.67-xTiO3. Phys Chem Chem Phys 2017; 19: 5880-5887.

45

Matsui M, Price G D. Simulation of the pre-melting behaviour of MgSiO3 perovskite at high pressures and temperatures. Nature 1991; 351: 735-737.

46

Wang Y, Richards W D, Ong S P, et al. Design principles for solid-state lithium superionic conductors. Nat Mater 2015; 14: 1026-1031.

47

Zhao Y, Daemen L L. Superionic conductivity in lithium-rich anti-perovskites. J Am Chem Soc 2012; 134: 15042-15047.

48

Schwering G, Hönnerscheid A, Van W L, et al. High lithium ionic conductivity in the lithium halide hydrates Li3-n (OH)nCl (0.83 < or = n < or = 2) and Li3-n(OH)nBr (1 < or = n < or = 2) at ambient temperatures. ChemPhysChem 2003; 4: 343.

49

Li Y, Zhou W, Xin S, et al. Fluorine-doped antiperovskite electrolyte for all-solid-state lithium-ion batteries. Angew Chem Int Ed 2016; 55: 9965-9968.

50

Hood Z D, Wang H, Samuthira Pandian A, et al. Li2OHCl crystalline electrolyte for stable metallic lithium anodes. J Am Chem Soc 2016; 138: 1768-1771.

51

Hong Y P. Crystal structure and ionic conductivity of Li14Zn(GeO4)4 and other new Li+ superionic conductors. Mater Res Bull 1978; 13: 117-124. 34

52

Robertson A D, West A R, Ritchie A G. Review of crystalline lithium-ion conductors suitable for high temperature battery applications. Solid State Ionics 1997; 104: 1-11.

53

Kanno R, Hata T, Kawamoto Y, et al. Synthesis of a new lithium ionic conductor, thio-LISICON-lithium germanium sulfide system. Solid State Ionics 2000; 130: 97-104.

54

Tatsumisago M, Nagao M, Hayashi A. Recent development of sulfide solid electrolytes and interfacial modification for all-solid-state rechargeable lithium batteries. J Asia Ceram Soc 2013; 1: 17-25.

55

Kanno R and Murayama M. Lithium ionic conductor thio-LISICON: the Li2S-GeS2-P2S5 System. J Electrochem Soc 2001; 148: A742-A746.

56

Bron P, Johansson S, Zick K, et al. Li10SnP2S12: an affordable lithium superionic conductor. J Am Chem Soc 2013; 135: 15694-15697.

57

Sahu G, Lin Z, Li J, et al. Air-stable, high-conduction solid electrolytes of arsenic-substituted Li4SnS4. Energy Environ Sci 2014; 7: 1053-1058.

58

Zhou P, Wang J, Cheng F, et al. A solid lithium superionic conductor Li11 AlP2S12 with a thio-LISICON analogous structure. Chem Commun 2016; 52: 6091-6094.

59

Che H, Chen S, Xie Y, et al. Electrolyte design strategies and research progress for room-temperature sodium-ion batteries. Energy Environ Sci 2017; 10: 1075-1101.

60

Guin M, Tietz F, Guillon O. New promising NASICON material as solid electrolyte for sodium-ion batteries: correlation between composition, crystal structure and ionic conductivity of Na3+xSc2SixP3−xO12. Solid State Ionics 2016; 293: 18-26.

61

Hagman L O, Kierkegaard P, Karvonen P, et al. The crystal structure of NaM2IV(PO4)3; MeIV = Ge, Ti, Zr. Acta Chem Scand 1968; 22: 1822-1832.

62

Goodenough J B, Hong Y P, Kafalas J A. Fast Na+-ion transport in skeleton structures. Mater Res Bull 1976; 11: 203-220.

63

Hong Y P. Crystal structures and crystal chemistry in the system Na1+xZr2SixP3−xO12. Mater Res Bull 1976; 11: 173-182.

64

Shannon R. Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta Crystallogr A 1976; 32: 751-767.

65

Park H, Jung K, Nezafati M, et al. Sodium ion diffusion in nasicon (Na3 Zr2Si2PO12 ) solid electrolytes: effects of excess sodium. ACS Appl Mater Interfaces 2016; 8: 27814-27824.

66

Noguchi Y, Kobayashi E, Plashnitsa L S, et al. Fabrication and performances of all solid-state symmetric sodium battery based on NASICON-related compounds. Electrochim Acta 2013; 101: 59-65.

35

67

Honma T, Okamoto M, Togashi T, et al. Electrical conductivity of Na2O-Nb2O5-P2O5 glass and fabrication of glass–ceramic composites with NASICON type Na3 Zr2Si2PO12 . Solid State Ionics 2015; 269: 19-23.

68

Kotobuki M, Koishi M, Kato Y. Preparation of Li1.5Al0.5Ti1.5(PO4)3 solid electrolyte via a co-precipitation method. Ionics 2013; 19: 1945-1948.

69

Arbi K, Bucheli W, Jiménez R, et al. High lithium ion conducting solid electrolytes based on NASICON Li1+xAlxM2−x (PO4)3 materials (M = Ti, Ge and 0 ≤ x ≤ 0.5). J Eur Ceram Soc 2015; 35: 1477-1484.

70

Li Y, Zhou W, Chen X, et al. Mastering the interface for advanced all-solid-state lithium rechargeable batteries. Proc Natl Acad Sci USA 2016; 113: 13313-13317.

71

Hagenmuller P, Solid electrolytes: general principles, characterization, materials, applications. New York: Academic Press;1978.

72

Charles R J. Some structural and electrical properties of lithium silicate glasses. J Am Ceram Soc 2006; 46: 235-238.

73

Martin S W. Ionic conduction in phosphate glasses. J Am Ceram Soc 1991; 74: 1767-1784.

74

Nagamedianova Z, Sánchez E. Synthesis and characterization of new glasses based on Li2S-P2S5 -Sb2S3 system. J Mater Sci Mate in Electron 2007; 18: 547-552.

75

Sakuda A, Hayashi A, Tatsumisago M. Sulfide solid electrolyte with favorable mechanical property for all-solid-state lithium battery. Sci Rep 2013; 3: 2261.

76

Holzmann T, Schoop L M, Ali M N, et al. Li0.6[Li0.2Sn0.8S2]—a layered lithium superionic conductor. Energy Environ Sci 2016; 9: 2578-2585.

77

Kato Y, Hori S, Saito T, et al. High-power all-solid-state batteries using sulfide superionic conductors. Nat Energy 2016; 1: 16030.

78

Mo S, Lu P, Ding F, et al. High-temperature performance of all-solid-state battery assembled with 95(0.7Li2S-0.3P2S5)-5Li3PO4 glass electrolyte. Solid State Ionics 2016; 296: 37-41.

79

Hayashi A, Noi K, Sakuda A, et al. Superionic glass-ceramic electrolytes for room-temperature rechargeable sodium batteries. Nat Commun 2012; 3: 856.

80

Chu I H, Kompella C S, Nguyen H, et al. Room-temperature all-solid-state rechargeable sodium-ion batteries with a Cl-doped Na3 PS4 superionic conductor. Sci Rep 2016; 6: 33733.

81

Yu Z, Shang S L, Seo J H, et al. Exceptionally high ionic conductivity in Na3 P0.62As0.38S4 with improved moisture stability for solid-state sodium-ion batteries. Adv Mater 2017; 29: 1605561.

36

82

Glass A M, Nassau K, Negran T J. Ionic conductivity of quenched alkali niobate and tantalate glasses. J Appl Phys 1978; 49: 4808-4811.

83

Liu J, Banis M N, Li X, et al. Atomic layer deposition of lithium tantalate solid-state electrolytes. J Phys Chem 2013; 117: 20260-20267.

84

Li X, Liu J, Banis M N, et al. Atomic layer deposition of solid-state electrolyte coated cathode materials with superior high-voltage cycling behavior for lithium ion battery application. Energy Environ Sci 2014; 7: 768-778.

85

Bates J B, Dudney N J, Gruzalski G R, et al. Electrical properties of amorphous lithium electrolyte thin films. Solid State Ionics 1992; 53: 647-654.

86

Marchand R. Nitrogen-containing phosphate glasses. J Non-Cryst Solids 1983; 56: 173-178.

87

Larson R W, Day D E. Preparation and characterization of lithium phosphorus oxynitride glass. J Non-Cryst Solids 1986; 88: 97-113.

88

Yu X H, Bates J B, Jellison GE. A stable thin-film lithium electrolyte: lithium phosphorus oxynitride. J Electrochem Soc 1997; 144: 524-532.

89

Li J, Ma C, Chi M, et al. Solid electrolyte: the key for high-voltage lithium batteries. Adv Energy Mater 2015; 5: 1401408.

90

Fenton D E, Parker J M, Wright P V. Complexes of alkali metal ions with poly(ethylene oxide). Polymer 1973; 14: 589.

91

Stephan A M. Review on gel polymer electrolytes for lithium batteries. Eur Polym J 2006; 42: 21-42.

92

Berthier C, Gorecki W, Minier M, et al. Microscopic investigation of ionic conductivity in alkali metal salts-poly(ethylene oxide) adducts. Solid State Ionics 1983; 11: 91-95.

93

Xue Z, He D, Xie X. Poly(ethylene oxide)-based electrolytes for lithium-ion batteries. J Mater Chem A 2015; 3: 19218-19253.

94

Nitzan A, Ratner M A. Conduction in polymers: dynamic disorder transport. J Phys Chem 1994; 98: 1765-1775.

95

Oleg Borodin, Grant D S. Mechanism of ion transport in amorphous poly(ethylene oxide)/LiTFSI from molecular dynamics simulations. Macromolecules 2014; 39: 1620-1629.

96

Ma Q, Zhang H, Zhou C, et al. Single lithium-ion conducting polymer electrolytes based on a super-delocalized polyanion. Angew Chem Int Ed 2016; 55: 2521-2525.

97

Suo L, Hu YS, Li H, et al. A new class of solvent-in-salt electrolyte for high-energy rechargeable metallic lithium batteries. Nat Commun 2013; 4: 1481.

98

Etacheri V, Marom R, Elazari R, et al. Challenges in the development of advanced Li-ion batteries: a review. Energy Environ Sci 2011; 4: 3243. 37

99

Fauteux D, Lupien M D, Robitaille C D. Phase diagram, conductivity, and transference number of PEO-NaI electrolytes. J Electrochem Soc 1987; 134: 2761-2767.

100

Karmakar A, Ghosh A. Dielectric permittivity and electric modulus of polyethylene oxide (PEO)–LiClO4 composite electrolytes. Curr Appl Phys 2012; 12: 539-543.

101

Władysław W, Dorota R, Aldona Z, et al. Effect of salt concentration on the conductivity of PEO-based composite polymeric electrolytes. J Phys Chem B 1998; 102: C67-C73.

102

Fullerton S, Maranas J. Structure and mobility of PEO/LiClO4 solid polymer electrolytes. J Phys Chem C 2010; 114: 9196-9206.

103

Stephan A M, Nahm K S. Review on composite polymer electrolytes for lithium batteries. Polymer 2006; 47: 5952-5964.

104

MacGlashan G S, Andreev Y G, Bruce P G. Structure of the polymer electrolyte poly(ethylene oxide)6:LiAsF6. Nature 1999; 398: 792-794.

105

Lightfoot P, Mehta M A, Bruce P G. Crystal structure of the polymer electrolyte poly(ethylene oxide)3:LiCF3SO3. Science 1993; 262: 883-885.

106

Kaskhedikar N, Paulsdorf J, Burjanadze M, et al. Ionic conductivity of polymer electrolyte membranes based on polyphosphazene with oligo(propylene oxide) side chains. Solid State Ionics 2006; 177: 703-707.

107

Matsumoto K, Endo T. Synthesis of networked polymers by copolymerization of monoepoxy-substituted lithium sulfonylimide and diepoxy-substituted poly(ethylene glycol), and their properties. J Polym Sci A Polym Chem 2011; 49: 1874–1880.

108

Shaplov A S, Vlasov P S, Armand M, et al. Design and synthesis of new anionic “polymeric ionic liquids” with high charge delocalization. Polymer Chem 2011; 2: 2609-2618.

109

Matsumoto K, Endo T. Preparation and properties of ionic-liquid-containing poly(ethylene glycol)-based networked polymer films having lithium salt structures. J Polym Sci A Polym Chem 2011; 49: 3582-3587.

110

Choi U H, Liang S, O’Reilly M V, et al. Influence of solvating plasticizer on ion conduction of polysiloxane single-ion conductors. Macromolecules 2014; 47: 3145–3153.

111

Liang S, Choi U H, Liu W, et al. Synthesis and lithium ion conduction of polysiloxane single-ion conductors containing novel weak-binding borates. Chem Mater 2012; 24: 2316–2323.

112

Polu A R, Rhee H W. Ionic liquid doped PEO-based solid polymer electrolytes for lithium-ion polymer batteries. Int J Hydrogen Energy 2016; 42: 7212-7219.

113

West K, Zachau-Christiansen B, Jacobsen T, et al. Poly(ethylene oxide)-sodium perchlorate electrolytes in solid-state sodium cells. British Polym J 1988; 20: 243-246. 38

114

Hashmi S A, Chandra S. Experimental investigations on a sodium-ion-conducting polymer electrolyte based on poly(ethylene oxide) complexed with NaPF6. Mater Sci Eng B 1995; 34: 18-26.

115

Boschin A, Johansson P. Characterization of NaX (X: TFSI, FSI)-PEO based solid polymer electrolytes for sodium batteries. Electrochim Acta 2015; 175: 124-133.

116

Chen-Yang Y W, Chen H C, Lin F J, et al. Polyacrylonitrile electrolytes: 1. A novel high-conductivity composite polymer electrolyte based on PAN, LiClO4 and α-Al2O3. Solid State Ionics 2002; 150: 327-335.

117

Liu Y, Lee J Y, Hong L. In situ preparation of poly(ethylene oxide)-SiO2 composite polymer electrolytes. J Power Sources 2004; 129: 303-311.

118

Croce F, Appetecchi G B, Persi L, et al. Nanocomposite polymer electrolytes for lithium batteries. Nature 1998; 394: 456-458.

119

Croce F, Persi L, Scrosati B, et al. Role of the ceramic fillers in enhancing the transport properties of composite polymer electrolytes. Electrochim Acta 2001; 46: 2457-2461.

120

Zhu Z, Hong M, Guo D, et al. All-solid-state lithium organic battery with composite polymer electrolyte and pillar[5]quinone cathode. J Am Chem Soc 2014; 136: 16461-16464.

121

Ni'mah Y L, Cheng M Y, Cheng J H, et al. Solid-state polymer nanocomposite electrolyte of TiO2/PEO/NaClO4 for sodium ion batteries. J Power Sources 2015; 278: 375-381.

122

Lago N, Garciacalvo O, Jm L D A, et al. All-solid-state lithium-ion batteries with grafted ceramic nanoparticles dispersed in solid polymer electrolytes. ChemSusChem 2015; 8: 3039.

123

Lin D, Liu W, Liu Y, et al. High ionic conductivity of composite solid polymer electrolyte via in situ synthesis of monodispersed SiO2 nanospheres in poly(ethylene oxide). Nano Lett 2016; 16: 459-465.

124

Samad Y A, Asghar A, Lalia B S, et al. Networked cellulose entrapped and reinforced PEO-based solid polymer electrolyte for moderate temperature applications. J Appl Polym Sci 2013; 129: 2998-3006.

125

Lalia B S, Samad Y A, Hashaikeh R. Ternary polymer electrolyte with enhanced ionic conductivity and thermo-mechanical properties for lithium-ion batteries. Int J Hydrogen Energy 2014; 39: 2964-2970.

126

Chandrasekaran R, Selladurai S. Preparation and characterization of a new polymer electrolyte (PEO:NaClO3) for battery application. J Solid State Electron 2001; 5: 355-361. 39

127

Colò F, Bella F, Nair J R, et al. Cellulose-based novel hybrid polymer electrolytes for green and efficient Na-ion batteries. Electrochim Acta 2015; 174: 185-190.

128

Stowe M K, Liu P, Baker G L. Star poly(ethylene oxide) as a low temperature electrolyte and crystallization inhibitor. Chem Mater 2005; 17: 6555-6559.

129

Wu X L, Xin S, Seo H H, et al. Enhanced Li+ conductivity in PEO-LiBOB polymer electrolytes by using succinonitrile as a plasticizer. Solid State Ionics 2011; 186: 1-6.

130

Bouchet R, Maria S, Meziane R, et al. Single-ion BAB triblock copolymers as highly efficient electrolytes for lithium-metal batteries. Nat Mater 2013; 12: 452-457.

131

Rolland J, Brassinne J, Bourgeois J P, et al. Chemically anchored liquid-PEO based block copolymer electrolytes for solid-state lithium-ion batteries. J Mater Chem A 2014; 2: 11839.

132

Daigle J C, Vijh A, Hovington P, et al. Lithium battery with solid polymer electrolyte based on comb-like copolymers. J Power Sources 2015; 279: 372-383.

133

Zuo X, Liu X M, Cai F, et al. A novel all-solid electrolyte based on a co-polymer of poly-(methoxy/hexadecal-poly(ethylene glycol) methacrylate) for lithium-ion cell. J Mater Chem 2012; 22: 22265.

134

Zeng X X, Yin Y X, Li N W, et al. Reshaping lithium plating/stripping behavior via bifunctional polymer electrolyte for room-temperature solid li metal batteries. J Am Chem Soc 2016; 138: 15825-15828.

135

Khurana R, Schaefer J L, Archer L A, et al. Suppression of lithium dendrite growth using cross-linked polyethylene/poly(ethylene oxide) electrolytes: a new approach for practical lithium-metal polymer batteries. J Am Chem Soc 2014; 136: 7395-7402.

136

Evans J, Vincent C A, Bruce P G. Electrochemical measurement of transference numbers in polymer electrolytes. Polymer 1987; 28: 2324-2328.

137

Lu N, Ho Y M, Fan C W, et al. A simple method for synthesizing polymeric lithium salts exhibiting relatively high cationic transference number in solid polymer electrolytes. Solid State Ionics 2007; 178: 347-353.

138

Raghavan P, Manuel J, Zhao X, et al. Preparation and electrochemical characterization of gel polymer electrolyte based on electrospun polyacrylonitrile nonwoven membranes for lithium batteries. J Power Sources 2011; 196: 6742-6749.

139

Liao C, Sun X G, Dai S. Crosslinked gel polymer electrolytes based on polyethylene glycol methacrylate and ionic liquid for lithium ion battery applications. Electrochim Acta 2013; 87: 889-894.

140

Gerbaldi C, Nair J R, Ahmad S, et al. UV-cured polymer electrolytes encompassing hydrophobic room temperature ionic liquid for lithium batteries. J Power Sources 2010; 195: 1706-1713. 40

141

Winter H H, Chambon F. Analysis of linear viscoelasticity of a crosslinking polymer at the gel point. J Rheol 1986; 30: 367-382.

142

Liu M, Zhou D, He Y B, et al. Novel gel polymer electrolyte for high-performance lithium-sulfur batteries. Nano Energy 2016; 22: 278-289.

143

Lu Y, Tikekar M, Mohanty R, et al. Stable cycling of lithium metal batteries using high transference number electrolytes. Adv Energy Mater 2015; 5: 1402073.

144

Zhu Y, Xiao S, Shi Y, et al. A composite gel polymer electrolyte with high performance based on poly(vinylidene fluoride) and polyborate for lithium ion batteries. Adv Energy Mater 2014; 4: 375-379.

145

Tsao C H, Hsiao Y H, Hsu C H, et al. Stable lithium deposition generated from ceramic-cross-linked gel polymer electrolytes for lithium anode. ACS Appl Mater Interfaces 2016; 8: 15216-15224.

146

Shin W K, Cho J, Kannan A G, et al. Cross-linked composite gel polymer electrolyte using mesoporous methacrylate-functionalized SiO2 nanoparticles for lithium-ion polymer batteries. Sci Rep 2016; 6: 26332.

147

Lee Y S, Lee J H, Choi J A, et al. Cycling characteristics of lithium powder polymer batteries assembled with composite gel polymer electrolytes and lithium powder anode. Adv Funct Mater 2013; 23: 1019-1027.

148

Khutia M, Joshi G M. Dielectric relaxation of PVC/PMMA/NiO blends as a function of DC bias. J Mater Sci Mater Electron 2015; 26: 5475-5488.

149

Susan M A, Kaneko T, Noda A, et al. Ion gels prepared by in situ radical polymerization of vinyl monomers in an ionic liquid and their characterization as polymer electrolytes. J Am Chem Soc 2005; 127: 4976-4983.

150

Ravi M, Song S, Wang J, et al. Ionic liquid incorporated biodegradable gel polymer electrolyte for lithium ion battery applications. J Mater Sci Mater Electron 2015; 27: 1370-1377.

151

Chaudoy V, Ghamouss F, Luais E, et al. Cross-linked polymer electrolytes for li-based batteries: from solid to gel electrolytes. Ind Eng Chem Res 2016; 55: 9925-9933.

152

Lu Q, He Y B, Yu Q, et al. Dendrite-free, high-rate, long-life lithium metal batteries with a 3D cross-linked network polymer electrolyte. Adv Mater 2017; 29: 1604460.

153

Gao H, Guo B, Song J, et al. A composite gel-polymer/glass-fiber electrolyte for sodium-ion batteries. Adv Energy Mater 2015; 5: 1402235.

154

Cao C, Tan L, Liu W, et al. Polydopamine coated electrospun poly(vinyldiene fluoride) nanofibrous membrane as separator for lithium-ion batteries. J Power Sources 2014; 248: 224-229. 41

155

Gao H, Zhou W, Park K, et al. A Sodium-ion battery with a low-cost cross-linked gel-polymer electrolyte. Adv Energy Mater 2016; 6: 1600467.

156

Zhang X, Liu T, Zhang S, et al. Synergistic coupling between Li6.75 La3Zr1.75Ta0.25O12 and poly(vinylidene fluoride) induces high ionic conductivity, mechanical strength, and thermal stability of solid composite electrolytes. J Am Chem Soc 2017: DOI: 10.1021/jacs.7b06364.

157

Zhao Y, Wu C, Peng G, et al. A new solid polymer electrolyte incorporating Li10GeP2S12 into a polyethylene oxide matrix for all-solid-state lithium batteries. J Power Sources 2016; 301: 47-53.

158

Zhou W, Wang S, Li Y, et al. Plating a dendrite-free lithium anode with a polymer/ceramic/polymer sandwich electrolyte. J Am Chem Soc 2016; 138: 9385-9388.

159

Zhai H, Xu P, Ning M, et al. A Flexible solid composite electrolyte with vertically aligned and connected ion-conducting nanoparticles for lithium batteries. Nano Lett 2017; 17: 3182-3187.

160

Chinnam P R, Wunder S L. Engineered interfaces in hybrid ceramic-polymer electrolytes for use in all-solid-state Li batteries. ACS Energy Lett 2017; 2: 134-138.

42