Probing the interfacial chemistry of solid-state lithium batteries

Solid State Ionics 343 (2019) 115068

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Probing the interfacial chemistry of solid-state lithium batteries Christofer Sångeland, Jonas Mindemark, Reza Younesi, Daniel Brandell

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Department of Chemistry - Ångström Laboratory, Uppsala University, Box 538, 751 21 Uppsala, Sweden

ARTICLE INFO

ABSTRACT

Keywords: Li-battery Solid electrolyte Solid state battery Interface Solid electrolyte interphase Photoelectron spectroscopy Modelling

This review aims to give a brief overview of the current state-of-the-art in the analysis of the interfacial chemistry in solid-state batteries. Despite generally regarded as being decisive for the ultimate success of these energy storage devices, this surface chemistry has so far only been explored to a rather limited extent in the scientific literature, but constitutes a research area which is currently undergoing rapid progress due to the growing interest in solid-state electrolyte materials and their corresponding battery applications. The review discusses the technical difficulties in performing these interfacial analyses for both ceramic and solid polymer electrolyte systems, and describes ways to overcome them using different methodologies: electrochemical techniques (primarily impedance spectroscopy), photoelectron spectroscopy, microscopy, and other less familiar experimental techniques. Modelling studies of the solid electrolyte–electrode interface are also included. It is concluded that especially the interfacial chemistry of polymer electrolytes has indeed been an understudied area. Furthermore, the review shows that analytical techniques employed so far have been largely complimentary to each other, but that joint studies and the development of novel analytical tools exploiting large-scale facilities will boost this research over the coming years.

1. Introduction It is not difficult to see the advantages with solid-state batteries for high energy-density storage systems for both automotive and stationary storage applications. Firstly, the only highly flammable component – the liquid electrolyte – is replaced by a more or less non-flammable counterpart, thereby contributing radically to increased safety of the battery system [1]. Secondly, since the mechanical properties of solid electrolytes can generally be considered sufficient, there is no need for a costly and sometimes unreliable separator in the cell [2,3]. Thirdly, since solid electrolytes enables the use of lithium metal anodes – which has a significantly higher specific capacity and lower redox potential compared to conventional graphite or lithium titanate anodes – and bipolar stacking battery architecture, the energy density will become proportionally higher [3–6]. The current interest in solid-state batteries – seen from the rapid increase in publications, patents and research funding [7] – can therefore be expected to signal the beginning of a trend, and much development can be foreseen in this technical and scientific area over the coming years. There are two major candidates for all-solid-state Li- and Li-ion batteries: ceramic electrolytes (CEs), such as garnet oxides, phosphosulfides/thiophosphates, perovskites or different types of glasses such as LiPON [8,9], and solid (solvent-free) polymer electrolytes (SPEs),

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based conventionally on polyethers (typically poly(ethylene oxide) — PEO) or polyesters [10,11]. Their properties differ significantly; while the CEs can possess comparably high bulk conductivity, sometimes higher than their liquid counterparts over a wide temperature window [4,12], this is not the case for SPEs which must be operated at elevated temperatures to achieve sufficiently high enough conductivity (~10−2 S cm−1). On the other hand, SPEs are often soft and non-brittle materials and exhibit isotropic ion conductivity, which are appealing properties for battery devices where the electrode particles change in volume and orientation during operation. It is often stated that the chemistry and physics of the interfaces between the electrode and electrolytes is a problematic issue – if not the most problematic issue – to tackle in solid-state batteries [13,14]. Li-ion battery electrodes are generally porous, which means that the solid electrolyte still needs to “wet” the entire electrode surface to achieve good power performance and storage capacity. Moreover, most electrode materials undergo volume expansion and contraction during battery cycling which both generates a mechanical strain on the electrolyte which must be accommodated, or it can lead to loss of contact between the electrode and electrolyte materials. That soft SPEs here possess properties which better resemble those of liquid electrolytes can often lead to the use of solid electrolyte composites between ceramic and polymer electrolyte materials [15].

Corresponding author. E-mail address: [email protected] (D. Brandell).

https://doi.org/10.1016/j.ssi.2019.115068 Received 30 June 2019; Received in revised form 11 September 2019; Accepted 12 September 2019 0167-2738/ © 2019 Elsevier B.V. All rights reserved.

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Fig. 1. The different methods used to analyze the SEI/CEI interphase (striped grey), electrode (green), and solid-state electrolyte (blue) in solid-state batteries. The absence of blue indicates that the electrolyte has to be partially or completely removed before analyzing the interphase, which is often the case for ex situ methods. The information depth for each method is highlighted in yellow. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Figures adapted with permission from the following sources (from left to right) [14,20–24]: Copyright 2018, the American Chemical Society; Copyright 2014, the American Chemical Society; Copyright 2015, Elsevier; Copyright 2014, Royal Society of Chemistry; Copyright 2015, the American Chemical Society; Copyright 2018, the American Chemical Society.

Irrespective of electrolyte type, solid-state batteries intrinsically involve different problems compared to their liquid-electrolyte counterparts. While both liquid and solid electrolytes can, in principle, be chemically and electrochemically stable towards often very reactive electrodes, this is normally not the case, with extensive decomposition reactions taking place at the interfaces. While the dissolution of the decomposition products back into the electrolyte is often highly problematical in for the liquid case [16], diffusion into the solid counterparts is universally slow. Instead, two other situations can be distinguished: [17] either an ionically conductive but electronically insulating film is formed which contributes to a well-passivated interface, or the decomposition products constitute a mixed ion/electron conductor where continuous electrolyte decomposition of electrolyte will proceed during battery operation. Considering the crucial role the interfacial chemistry has for the realization of solid-state batteries, it is remarkable that this is not a research area which has been explored more, although significant work has been done in recent years [18]. In this review, we therefore summarize the attempts we have found in the scientific literature from a methodological viewpoint. The reason for this lack of attention is partly that the solid-state electrolyte battery field has earlier been strikingly smaller than the conventional liquid-electrolyte field, and partly due to the difficulties involved in transferring the use of conventional interfacial analytical techniques to solid-state systems. The electrode interfaces and build-up of a Solid Electrolyte Interphase (SEI) layer on their surfaces are frequently analyzed in Li-ion batteries after cell

disassembly and removal of electrolyte — the main analytical technique being X-ray photoelectron spectroscopy (XPS) [19]. This is less feasible for solid-state systems, where the electrolyte is difficult to remove, especially after battery operation. The removal of either SPEs or CEs from an electrode – especially a porous composite electrode – can easily cause considerable damage to the ultrathin interphase layers formed. This is also probably why different in-situ spectroscopic techniques have been explored widely for solid-state batteries, or the extensive application of (in-situ) electrochemical impedance spectroscopy (EIS). This review is structured according to the different methodologies applied for analyzing the interfacial chemistry of solid-state batteries: electrochemical methods such as EIS, microscopy techniques, ex-situ and post-mortem photoelectron spectroscopy, in-situ photoelectron spectroscopy, alternative experimental techniques, and finally materials modelling (see Fig. 1). We have deliberately excluded all work on ‘quasi-solid’ or ‘plasticized’ systems, where liquid components are included in the solid electrolyte or electrode matrices to improve wettability or bulk conductivity, since it is highly likely that these liquid additives will decompose at the electrode surfaces and solubilize any decomposition products, and therefore ultimately control the interfacial chemistry. However, we have not made any distinction here between thin-film, metallic or porous electrode systems. While these generally target different battery applications and the materials involved often differ, their interfacial chemistries on a molecular scale are largely similar. The ambition with this review is to give a starting point for future exploration of this potentially rich area of research. 2

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2. Electrochemical techniques

typical cycling conditions (Fig. 3) [14,38,44,45]. In the case of Li10GeP2S12 (LGPS), for example, interfacial decomposition results in the deterioration of the charge-transfer resistance and rapid increase in the overall cell resistance in Li | LGPS | Li cells [38]. Just as in the case of polymer electrolytes, time-resolved impedance spectroscopy is a powerful tool to monitor reactive changes, although it does not provide direct chemical information about the interphase or the degradation reactions [17]. The initial reaction may be so rapid that the major part of the resistance is formed immediately on cell assembly and before measurements can be initiated. The initial part of the resistance evolution can therefore not be observed directly [17,38]. A typical scenario in studying symmetric Li/Li or Na/Na cells is the observation of three distinct dielectric responses corresponding to ion transport, charge transfer impedance and interfacial (degradation layer) impedance. In contrast, only the ionic transport component is observed when Au blocking electrodes are used [14]. As in the case of liquid- and polymer-electrolyte cells, a Warburg impedance is observed at very low frequencies [46]. Compared to polymer electrolytes, the complexity of the impedance spectrum for CEs may be further compounded by the presence of grain boundaries, requiring additional components to complete the equivalent circuit (Fig. 4) [28,38,47]. In some systems, the grain boundary contribution is small and may be difficult to separate out from the bulk electrolyte resistance [46,48], whereas deterioration of the grain boundaries on cycling results in this feature (visible as a high-frequency semicircle in the Nyquist plot) becoming more prominent (Fig. 4) [20]. Another important difference compared to liquid systems is the presence of constriction resistances, i.e., because of its stiffness, only a fraction of the electrolyte surface is actually in contact with the electrode. Unless addressed and mitigated, this will complicate interpretation of the impedance response [35]. The separation of the different contributions to the overall resistance enables a detailed analysis of the individual changes in resistance at the anode and cathode interfaces, and separate from, e.g., grain boundary contributions. Such analysis has been applied to the interface between β-Li3PS4 and an LiNi0.8Co0.1Mn0.1O2 (NCM-811) cathode to reveal a strong and irreversible increase in resistance during the 1st cycle. This appeared not to be dependent on the state-of-charge but rather on the applied potential, indicating irreversible interfacial degradation by oxidation of the solid electrolyte [49]. Monitoring the time-resolved evolution of the interfacial resistance can help characterize the interphase if reasonable assumptions are made about its constituents. For example, continuous SEI growth during cycling with an LGPS electrolyte will eventually create such a large SEI resistance that the cells are essentially destroyed [45], while use of the sulfidic glass ceramic Li7P3S11 is reported to lead to a stabilization of the interfacial impedance after ca. 12 h, suggesting the formation of a non-growing interphase [28]. Assuming Li2S to be the main component of this interphase, its thickness could be estimated to 2.3 nm from the resistance increase and the conductivity of Li2S [28]. Information can also be gained about the characteristics of the degradation reaction from the growth of the interfacial resistance over time. For a diffusion-controlled reaction, the temporal evolution of the resistance R will follow the equation:

Analogous to what happens in liquid electrolytes, an ionically conducting passivation film can be formed when a polymer or ceramic electrolyte is placed in contact with Li metal [25,26]. Early studies of SPEs attributed this largely to degradation of impurities, but the close similarities to the case of liquid electrolytes (where similar impurities are not present) clearly show that the degradation process indeed also involves the polymer host and electrolyte salt [27]. The solid–solid interphases in polymer and ceramic electrolyte batteries are both limited in thickness and difficult to access due to their buried nature. This makes in situ electrochemical techniques particularly relevant for the study of interfaces in solid-state systems [28]. The evolution of interfacial degradation or passivation layers in solid-state batteries can be seen directly during electrochemical cycling as an increase in cell resistance. This is observable from cycling curves as increased polarization, or can be determined specifically using techniques such as Intermittent Current Interruption (ICI), which has been applied in solid polymer cells to monitor the development of the intracell resistance over time [29,30]. While this gives an indication of the emergence of interfacial resistances, these techniques do not distinguish their different origins. The individual contributions from different resistance processes and can instead be distinguished through EIS. Using time-resolved EIS, the formation of interphase layers can be monitored through changes in resistance and capacitance at the electrode interface. This has been applied to polymer electrolytes based on a variety of polyether and non-polyether host materials both during electrochemical cycling and passive contact with Li metal [31–33]. The simplicity of this method and the possibility to run the analysis in parallel with electrochemical cycling makes EIS attractive, although ambiguities in the data can make interpretation complex. Typically, an equivalent circuit model is fitted to the impedance data. It should be noted, however, that a good fit to the model does not prove that the model is valid and physically relevant [34]. Peled et al. suggested a complex circuit model for ionic conduction in SEI layers formed on Li metal in polymer electrolyte systems based on a mosaic-type SEI model [35,36]. However, such structural detail is typically difficult to distinguish in the experimental data, and the complex circuit can be reduced to a single element consisting of a resistor and a capacitor (or constant phase element) connected in parallel to represent the contribution from the interfacial layer [35]. Typically, the Nyquist plot (−Z″ vs. Z′) of EIS data for a polymer electrolyte in contact with Li metal displays a series of semicircles or arcs (Fig. 2) which can be modelled as a series of parallel resistors and capacitors or constant phase elements. The high-frequency (> 105 Hz) response corresponds to the bulk electrolyte resistance, and is not affected by aging [37]. This response can be easily perturbed by inductive effects due to the electrical connections and care should be taken when interpreting this part of the spectrum to avoid modelling such artifacts [37,38]. The response in the middle-frequency domain (105–102 Hz) was originally attributed to charge-transfer resistance at the interface [39–41], but has later been shown to depend on temperature and aging, suggesting that it rather corresponds to interphase layer resistance [25,26,37,42]. In certain cases, the observation of an additional small charge-transfer resistance has been observed at an early stage at low frequencies [27], but it may not always be possible to separate it from the passivation layer resistance, and they will merge into a single RC unit in the equivalent circuit [43]. Other authors have similarly reported an unidentified small resistance at medium frequencies (102–0.1 Hz) which is difficult to reproduce and has been attributed tentatively to reactions with impurities [37]. A Warburg impedance may be observed at sufficiently low frequencies (0.1–10−4 Hz). This does not depend on aging and arises from bulk ion transport in the electrolyte [37]. As stated, but contrary to popular belief, ceramic electrolytes are not necessarily stable vs. Li or Na metal under either passive contact or

R=

1 • A

½ el ion

•

2•MSEI•µA0 F 2•

SEI •x

•t ½ = k •t ½

leading to a t1/2 dependence [50]. This relation agrees well with experiment for Li6PS5X (X = Cl, Br, I) in symmetric Li/Li cells [50]. Interfacial decomposition in LGPS and Li7P3S11 has also been reported to be diffusion-controlled [28,38], while the degradation of Na3PS4 is controlled by an interfacial reaction [17]. While the fitting of EIS data to an equivalent circuit model presents a challenge due to signal overlap with other parts of the impedance spectra in cells using Li- or Na-metal electrodes, the problems are more severe for porous electrodes [51]. Combination with physical (Newman 3

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Fig. 4. Evolution of the Nyquist plot from the 1st (inset) to the 100th charge cycle. The equivalent circuit shown in the inset is for a model all-solid-state battery with an LGPS electrolyte. The HF (high frequency) semicircle is attributed to grain boundaries, while the MF (medium frequency) and LF (low frequency) responses correspond to interfacial processes. The Warburg impedance is represented by the series of constant phase element CPEW [20]. Reprinted with permission from ref. [20]. Copyright 2018, American Chemical Society. Fig. 2. a) Equivalent circuit and b) Nyquist plot of a typical impedance response from a Li | PEO:LiTFSI | Li cell [37]. Reprinted with permission from ref. [37]. Copyright 2003, The Electrochemical Society.

theory) cell modelling and EIS simulation have highlighted the difficulties in obtaining reliable information on the interfacial chemistry when using EIS for more complex battery chemistries, where porosity and tortuosity are taken into account [52]. Solid electrolytes will be no exception in this context. 3. Microscopy Electron microscopy has the potential to give both chemical and morphological information about electrolyte–electrode interphases, but has only been applied sparingly to solid electrolytes. Using an operando scanning electron microscopy (SEM) setup, Hovington et al. have studied cross-sections of Li | LiFePO4 and Li | Li1.2V3O8 cells with a polyether:LiTFSI electrolyte [24]. The SEM setup allowed recording of both dimensional changes and the formation of interfacial degradation layers on cycling. In the Li | Li1.2V3O8 cell, dissolution of V was detected crossing over the electrolyte and depositing in the passivation layer at the Li interface. With extended cycling, this layer was found to grow to

Fig. 3. Evolution of the impedance response for a Li | LGPS | Li cell [38]. Reprinted with permission from ref. [38]. Copyright 2016, American Chemical Society. Fig. 5. Cross-section of a Li | SPE | Li1.2V3O8 cell, with the interfacial degradation layers highlighted [24]. Reprinted with permission from ref. [24]. Copyright 2015, American Chemical Society. 4

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a respectable 5.4 μm (Fig. 5). In contrast, any equivalent dissolution of Fe could not be observed for LiFePO4. In the ceramic system β-Li3PS4 together with an NCM-811 cathode, Koerver et al. used SEM to reveal contact loss at the electrolyte–electrode interface due the limited plasticity of the solid electrolyte, resulting in irreversible changes in morphology detrimental to battery performance. This was correlated with a marked capacity loss in the first cycle not seen for liquid electrolytes [49]. In the argyrodite system Li6PS5Cl, Auvergniot et al. used SEM coupled with scanning Auger microscopy (SAM) mapping to correlate elemental Mn, S and Cl signals at the interface with an LiMn2O4 (LMO) electrode. After 22 cycles, no clear overlap could be detected between S and Cl signals, indicating oxidation of the Li6PS5Cl into S-containing species and LiCl, thereby spatially separating S and Cl [53]. The relative scales of features resolvable with SEM means that it is more suitable for studying the type of larger morphological changes shown in the above examples, while transmission electron microscopy (TEM) can more precisely image the very interface or SEI layer, while also providing compositional and phase information. To this end, Sakuda et al. have studied samples taken from LiCoO2 | Li2S-P2S5 | Li-In cells after charging to 3.6 V, prepared by focused ion beam (FIB) milling using TEM and STEM/EDX (Scanning TEM/Energy Dispersive X-ray Spectroscopy). This revealed Co in the solid electrolyte, which had diffused from the active cathode material up to 50 nm from the interface [54]. Using EDX, an amorphous (as indicated by the absence of XRD reflections) interfacial layer comprising mainly Co and S was also identified in this system (Fig. 6). Similar diffusion of transition metal atoms from the active cathode material to the solid electrolyte interphase region was also reported based on TEM/EDX analysis for systems

comprising NCM with a 80Li2S·19P2S5·1P2O5 glass ceramic electrolyte, and LiMn2O4 with a Li2S–P2S5 electrolyte. In this latter system Mn was detected up to ~250 nm from the interface [55,56]. Electron microscopy can also be used to reveal more subtle structural changes. Through a combination of atomic-resolution STEM and spatially resolved EELS (Electron Energy Loss Spectroscopy), Ma et al. have imaged the interface formed in situ when cubic Li7La3Zr2O12 (LLZO) was brought into contact with a Li-coated W tip. This created a distinct interfacial layer ~6 nm thick which formed instantly and stopped growing immediately after formation. The oxidation states of the transition metals in LLZO were then slightly reduced, necessitating the inclusion of additional Li+ to maintain charge balance in the structure [57]. 4. Post mortem XPS Post mortem XPS is probably the second most common technique (after EIS) to study solid-state electrolyte-electrode interphases experimentally. XPS, also known as Electron Spectroscopy for Chemical Analysis (ESCA) or Photoelectron Spectroscopy (PES), allows the user to probe the chemical composition of the uppermost atomic layers of an interface with a high degree of elemental sensitivity. The first use of XPS to study the interphase formed between a solid-state electrolyte and an electrode was demonstrated by Ismail et al. in 2001 [58]. They compared the composition, ionic resistance and thickness of the Solid Electrolyte Interphase (SEI) formed on lithium metal using two different salts (LiTFSI and LiBF4) in an ethylene oxide/propylene oxide copolymer matrix (see Fig. 7). XPS combined with argon sputtering could show that the lithium metal surface was covered by a native passivation bilayer composed of an outer layer of Li2CO3 and LiOH and an inner layer of Li2O. The lithium anode was then placed in contact with the SPEs for a prolonged time (more than one month at 60 °C) and separated to expose the anode–electrolyte interphase. In the LiTFSI case, the native layer was covered by a thin film of decomposed TFSI species and polymer residues. Small traces of LiF were also observed in the outer layer, which increased gradually with sputtering depth. The formation of LiF was considered to appear due to the reaction between HF contaminants and the native layer. Alternatively, LiF could have been formed by the reaction between lithium metal and the TFSI ion. This explanation is perhaps less plausible since it would require the bulky TFSI anion to migrate through the entire native layer. In the case of LiBF4, LiF replaced the native passivation layer to a larger degree, which also resulted in a higher and less stable interfacial resistance. However, LiF formation is not necessarily an exclusively negative phenomenon. For example, using XPS and EIS, Li et al. found that inserting a LiF layer between LiFePO4 (LFP) and a poly(propylene carbonate) (PPC) polymer electrolyte not only reduced the interfacial resistance but also prevented polymer electrolyte decomposition during overcharging [59]. This shows that the composition of the SEI or CEI (Cathode Electrolyte Interphase) is not the only feature to be considered, but also its morphology, location, and compatibility with the surrounding materials. Interestingly, Li et al. found that the same positive effects could be achieved by initially subjecting the cell to a higher C-rate, 0.5C, for five cycles prior to cycling at 0.1C. According to the authors, the decomposition of PPC was suppressed due to slower kinetics at higher C-rates compared to competing reactions, which could therefore promote the formation of a stable CEI. As illustrated by the previous two examples, the dynamic interaction between materials at the electrode–electrolyte interface in solidstate batteries is complex. One strategy to overcome this problem is to create simplified representations of the optimal design, where a limited number of materials are involved. For example, Koerver et al. used XPS to complement in-situ EIS and post mortem SEM analyses of the interphase between β-Li3PS4 and NCM-811, and opted to use no electronically conductive material in the composite cathode to isolate the reaction between the active material and the ceramic thiophosphate

Fig. 6. (a) A cross-sectional HAAD (High-Angle Annual Dark-field) TEM micrograph and (b) atomic composition from an EDX line-trace across the cathode–electrolyte interface of a LiCoO2 | Li2S-P2S5 | In cell after initial charging [54]. Reprinted with permission from ref. [54]. Copyright 2010, American Chemical Society. 5

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Fig. 7. A schematic illustration of the lithium-SPE interphase cross-section after contact with (a) a LiTFSI-network polymer (mono-acrylated and tri-acrylated copolymer of ethylene oxide and propylene oxide) and (b) a LiBF4-network polymer [58]. Reprinted from ref. [58], Copyright (2001), with permission from Elsevier.

electrolyte [49]. This presented three challenges: firstly, electrolytes are electronically insulating, which means that they are prone to charging during radiation exposure and thus require a charge neutralizer [48]. Secondly, the participation of electronically conductive carbon additives in interface degradation processes cannot be ignored. For example, Zhang et al. have shown that the presence of carbon additives (Ketjen black, graphite, DENKA-acytelene black, C65 and SPLi) all resulted in a large charge-transfer resistance across the interface and even impeded the function of the LiNb0.5Ta0.5O3 protective coating which had been applied to the LiCoO2 (LCO) particles [60]. Lastly, it has been shown in liquid-electrolyte batteries that small changes in composite electrode composition can result in massively different SEI layers with quite different properties [61]. In other words, the interphase between β-Li3PS4 and NCM-811 could potentially look very different for an energy-density optimized composite that has carbon additives and a lower electrolyte content. However, whether or not this applies for CEs containing no salt anions remains an open question. Irrespective of this, the XPS analysis done by Koerver et al. [49] showed that no chemical degradation occurred as a result of electrolyte–cathode contact alone. However, significant degradation occurred during the first charge-cycle when P and S were oxidized to form polysulfides, similar to those found in Li-S batteries [62]. It was suggested that the CEI layer formed was not passivating, which would explain the 1–2% capacity fade over 50 cycles following the first charge–discharge. Post mortem XPS was used by Zhang et al. to study the interface between LGPS and LCO and Li4Ti5O12 (LTO) [20]. XPS peak shifts identified as GeS2, Li2Sx, P–S–(S)x, GeO2 and polymerized PS4 groups associated with LGPS oxidation were observed at the cathode–ceramic electrolyte interface. It was proposed that the existence of compounds such as CoS (detected using EELS) and GeS2 in the interphase may have provided electronic pathways that led to continued oxidation of LGPS, despite the formation of an interphase layer. At the anode–ceramic electrolyte interface, on the other hand, the formation of Li2S along with polymerized PS4 groups helped form a more stable passivation layer. It was suggested that the poor electronic conductivity of Li2S was sufficient to prevent further reduction of the ceramic electrolyte, despite the presence of 10 wt% carbon in the composite anode. The severe decomposition observed at the cathode interface is in accord with previous literature, since the operating potential window of the LCO | LGPS | Li-In cell is 2.0 to 3.6 V vs. In/Li-In, and it has been shown that Li can be removed and inserted electrochemically from LGPS at potentials above 1.8 and below 1.6 V vs. In/Li-In, respectively [63]. In their study on LGPS-LCO and LGPS-LTO interfaces, Zhang et al. stated that assigning peak-shifts to specific sulfur and phosphorus compounds was highly challenging as a result of the high degree of overlap in the XPS spectra. In an effort to overcome this problem, Dietrich et al. used XPS combined with XANES (X-ray Absorption NearEdge Spectroscopy) to characterize the electrochemical decomposition compounds which form at the interfaces in the amorphous lithium

thiophosphate systems (xLi2S–(100 − x)P2S5) [64]. They concluded that P-S end-groups were formed during PS4 depolymerisation and that P-S-P bridging groups shifted peaks towards lower binding energies. It was also found that the Li content significantly affected the observed peak shifts. The interface stability between argyrodite Li6PS5Cl vs. LiCoO2, NCM-111, and LMO was studied by Auvergniot et al., since all three cathode materials exhibited poor coulombic efficiency during the initial cycles with this electrolyte [53]. In addition to XPS, the interfaces were also analyzed using Auger Electron Spectroscopy (AES) and Scanning Auger Microscopy (SAM). In contrast to the standard approach of starting the XPS analysis at the interface between the ceramic electrolyte pellet and the composite cathode pellet, the XPS analysis was here first performed at the cathode composite–current collector interface. A depth-profile analysis was then performed by mechanically removing material with a scalpel rather than using argon sputtering. They found that NCM-111 and LMO reacted on contact with Li6PS5Cl, while LCO did not. The argyrodite was oxidized during cycling into sulfur, lithium polysulfides, P2Sx species, phosphates and LiCl. Based on the peak intensity arising from decomposition products, the authors ranked the cathode materials from least to most reactive in the sequence: LCO, NCM-111 and LMO. Finally, this unconventional experimental XPS approach revealed that interfacial reactions varied with distance from the current collector. It was concluded that the complete delithiation of Li6PS5Cl close to the current collector because electron conduction was the limiting process. This is a likely explanation due to the high ceramic electrolyte content in the cathode composite: 57 wt%. Unlike ceramic materials, polymer electrolytes are more difficult to separate from the electrodes because of their adhesive properties and superior wetting abilities. This problem can be overcome if the cells are cycled more “gently”. For example, Xu et al. cycled membranes of a polymer electrolyte consisting of PEO and LiTFSI-salt sandwiched between a graphite electrode and a lithium electrode at a discharge rate of C/50 at 50 °C (well below the optimal operational temperature) to facilitate post-cycling separation [23]. As a second precaution, the cells were only annealed at 50 °C for 12 h prior to cycling to ensure a good interfacial contact without compromising separatability. Post mortem XPS revealed that LiOH and species attributed to LiTFSI decomposition products (Li3N, Li2S, Li2SO3 and Sx) were key components of the graphite SEI (see Fig. 8). The presence of LiOH was attributed to large trace amounts of water in the polymer membrane (up to 675 ppm from Karl Fischer titration), most likely due to the hydrophilic nature of PEO and LiTFSI. Similar species were observed in the Li-metal SEI layer, with the exception of LiTFSI decomposition products. It was claimed that the decomposition of LiTFSI was most likely catalyzed by the graphite surface. This inspired the authors to investigate SPEs based on poly (trimethylene carbonate) (PTMC), which is a more hydrophobic yet ionically conductive polymer [65]. As expected, in the absence of water, no LiOH could was found in the graphite SEI layer. Li2CO3, often 6

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Fig. 8. The SEI chemical composition observed in a graphite half-cell with: (a) a SPE and (b) a conventional liquid electrolyte [23]. Copyright 2014. Reproduced with permission from the Royal Society of Chemistry.

formed in the presence of H2O, was similarly absent in the graphite SEI. Instead, the SEI comprised organic ROLi and ROCO2Li species, along with LiTFSI decomposition products. Both electrolytes exhibited negligible CEI formation, confirming that the electrochemical stability window of PTMC extends up to 5 V vs. Li+/Li, as reported by Sun et al. [66]. To facilitate separation after long-term cycling, a PTMC oligomer could also be added as a lubricant between the electrode and electrolyte without seemingly affecting the composition of the degradation layer. The surface sensitivity of XPS can be both an advantage and a disadvantage. In the work of Wang et al., post mortem XPS was used to study the interface between NASICON-type electrolytes LiY0.15Zr1.85(PO4)3 (LYZP) and Na3Zr2(PO4)(SiO4)2 (NZPS) with lithium and sodium, respectively [14]. Zn reduction and Li2O formation were detected at the Li–NASICON interface. Contrary to time-resolved EIS data, which indicated an initial increase in interfacial resistance over time, no reaction products were observed at the Na–NASICON interfaces. It was later revealed by ToF-SIMS (Time-of-Flight Secondary Ion Mass Spectrometry) that Na had diffused into the NZPS structure, where it then reduced Zn and weakened the PeO bond, as had also occurred at the LYZP surface. This shows that post mortem XPS characterisation alone is not sufficiently powerful to provide answers regarding the interfacial chemistry in solid-state batteries. There is also an uncertainty regarding the preservation of the “true” interface during post mortem handling and analysis, especially if the solid electrolyte and electrode are mechanically difficult to separate and/or sensitive to radiation damage. Argon sputtering may be used to characterize the SEI/CEI at different depths, but little is known about how sputter damage and selective sputtering can alter the “true” interface [14,48,58]. A more precise alternative, albeit less accessible than argon sputtering, is to use a synchrotron source, where the kinetic energy can be tuned to obtain signals deeper into the SEI/CEI [67].

sodium on the surface of the sample inside an XPS measurement chamber. The chemical stability of a solid electrolyte exposed to metallic lithium/sodium can then be investigated. For example, using an XPS analysis chamber connected to a series of external deposition chambers via an UHV tunnel, lithium phosphorus oxynitride (LiPON) was shown to decompose to Li3PO4, Li3P, Li3N and Li2O after exposure to metallic lithium [68]. Janek et al. used an Ar+ sputtering source inside the XPS chamber to deposit lithium/sodium on the surface of the electrolyte sample – thereafter subjecting it to XPS measurements (see Fig. 9) [22]. Several different solid electrolytes such as lithium lanthanum titanate (LLTO) [22], Li1+xAlxM2−x4+ (PO4)3 with M = Ge or Ti and Ta (LAGP and LATTP) [69], β″-Al2O3 and Na3PS4 [17], have been studied with this method. In the case of LLTO, the formation of Ti3+ and Ti2+ along with Ti metal was detected on the surface of the 100 μm thick ceramic pellet. The deposition of lithium was also accompanied by a colour change from white to black. In the case of LAGP and LATTP, a shoulder towards lower binding energies was observed following lithium deposition in the Ti 2p and Ge 3d spectra, indicating the reduction of Ti4+ and Ge4+. These results could confirm the ex situ XPS measurements, where reduction of Ti and Ge was also observed. It was concluded that lithiation of the LAGP and LATTP CEs, followed by the reduction of Ti and Ge, led to the formation of a mixed electronic/ ionic conductive interphase (MCI), which continued to grow and ultimately caused a short-circuit. In the cases of β″-Al2O3 and Na3PS4, the former appeared stable in the presence of sodium metal, while the latter also exhibited the formation of an MCI. Work so far has been focused on the stability of solid electrolytes at low potentials, i.e. when exposed to metallic lithium/sodium, however, Guhl et al. have sputtered thin films of NCO on NASICON and Na-β″-Al2O3 to study the initial reaction between the CEs and NCO using XPS [70]. According to their findings, insignificant degradation between NCO and NASICON was observed in the form of slight zirconium and scandium reduction. In contrast, severe side-reactions between NCO and Na-β″-Al2O3 resulted in a CaOAl2O3 mixed interphase that led to a higher cell polarization. Finally, concerns regarding these types of experiments have been raised; gas residues or surface contaminants could interfere with the measurements and the kinetics at low temperatures typically used to prevent sulfur, a common component in CEs, decontamination at UHV could also seriously influence the observations: the side reactions may be too fast or too slow to be studied under these conditions [22]. In the second method, current or potential is applied to a cell during

5. In situ XPS measurements In situ XPS measurements have been developed in an effort to obtain a more reliable analysis of interfacial reactions in solid electrolyte systems. The in situ XPS measurements performed can be divided straightforwardly into two categories: i) in situ lithiation/sodiation of solid electrolytes, and ii) in situ electrochemical cells combined with XPS measurements. The former approach relies on deposition of metallic lithium/ 7

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Fig. 9. The experimental set-up developed by Wenzel et al. which allows deposition of lithium on the ceramic electrolyte inside the analysis chamber (a) followed by XPS analysis (c). The geometry used to determine the angle of the target relative to the sample is shown in (b), and the possible reaction paths are illustrated in (d) [22]. Reprinted from ref. [22], Copyright (2015), with permission from Elsevier.

the XPS analysis of the solid electrolyte. For example, using the experimental set-up shown in Fig. 10, β-Li3PS4 mixed with conductive carbon was exposed to a potential of ± 10 V during the XPS analysis [47]. Koerver et al. could thus observe the partially reversible formation of Li4P2S8, Li4P2S7 and Li2P2S6 when the Li3PS4 CE was oxidized, and irreversible formation of Li4P2S6 and Li2S species during Li3PS4 reduction. It was hypothesized that the formation of these species led to an increase in interfacial resistance. Wu et al. have taken things further by designing a cell which even allows stack pressure to be applied [71]. This was achieved using a perforated plate and reinforcing the cathode composite with a stainless steel mesh. The in situ XPS measurements on the cathode composite – comprising of LCO, (Li2S)3–P2S5 (LPS) and vapour-grown carbon fibre (VGCF) – showed the formation of degradation products at 2.1 V vs. InLix.

low mass and low signal cross-section of alkali metals render them difficult to detect using TEM or XPS. Wang et al. have used high-resolution ToF-SIMS to create 3D maps of the interphase between the electrode and electrolyte in symmetrical alkali metal-NASICON cells [14]. In spite of the lower current density compared to the symmetrical lithium cell, sodium cells short-circuited after only eight cycles. This was attributed to the thin sodium dendrites revealed in the 3D maps. Based on these observations and thermodynamic calculations, it was postulated that dendrites could grow easily in the narrow inflexible crevices along the grain boundaries, given that sodium did not react with the electrolyte. Furthermore, narrower dendrites have a larger tip curvature, which enhances the electric field and exacerbates the growth, resulting in cell failure after just a few cycles. ToF-SIMS has also been used to investigate the stability of Li6PS5Cl argyrodite ceramic electrolyte against NCM-622 [72]. According to the 3D maps (see Fig. 11) the gradual capacity fading observed in the Li-In | Li6PS5Cl | NCM-622 cells over 100 cycles was due to the formation of a phosphateand sulphate-based species (POXx− and SOXx−) around the cathode active material. Similar species were also observed at the CE-cathode interface in Li-In | Li2S-P2S5 | NCA [73]. It was shown that the ionic resistance and instability of this interphase was significantly reduced by

6. Less common techniques The assumed compatibility between solid-state electrolytes and alkali metal electrodes enabling higher energy densities has been one of the long-standing arguments in favour of all-solid-state batteries. However, it is a challenge to study interphase composition, since the

Fig. 10. a) A solid-state battery mounted in the XPS sample holder with the backside of the composite cathode facing the X-ray source and detector; b) an image of the measurement spot (marked in yellow) next to the current-collector as seen using Secondary Electron Imaging (SXI) [47]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Copyright 2017. Reproduced with permission from the Royal Society of Chemistry. 8

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Fig. 11. (A) Local fragments of NiO2− (blue) and POx− (green) observed on the pristine CE, the uncycled composite cathode, and the composite cathode after 100 cycles. The signal arising from POx− at the NCM622-CE interface increases after 100 cycles. (B) 3D map of the CEI layer formed at the NCM622-CE interface after 100 cycles showing the presence of NiO2− (blue), POx− (green), and SOx− (red) [74]. Reprinted with permission from ref. [74]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article) Copyright 2019, American Chemical Society.

introducing a 4 nm thick diamond-like coating (DLC) on the NCA powder, thereby significantly improving capacity retention over 100 cycles. As these three works illustrate, ToF-SIMS can be employed to complement the elemental sensitivity of XPS with spatial resolution that allows for local compositional characterisation. An alternative technique to probe decomposition reactions indirectly is Differential Electrochemical Mass Spectrometry (DEMS), which allows detection and identification of gases evolved from decomposition reactions within the battery. For example, Li et al. have observed CO2 gas formation originating from SPE decomposition [59]. DEMS has also been used to study gas evolution from Li-In | β-Li3PS4 |NCM-622 and Li4Ti5O12 | β-Li3PS4 |NCM-622 [74]. Spikes in CO2 and O2 were detected at 4.5 V vs Li+/Li and slowly decreased subsequent cycles. The gas evolution of CO2 was attributed to the presence of Li2CO3 on the surface of the NCM-622 and the formation of O2 was attributed to the extraction of oxygen from the cathode material at high potentials. Surprisingly, no H2S gas was detected; however, SO2 gas was released clearly indicating electrolyte decomposition. Other less common techniques for interfacial studies of all-solid-

state batteries include FTIR and NMR spectroscopy. By depositing thin (< 1 μm) films of a PEO:LiCF3SO3 electrolyte on an ATR crystal and then covering with Li metal, Le Granvalet-Mancini et al. were able to use FTIR to study the interfacial degradation, utilizing the fact that the IR beam penetrated the thin films to reach the Li-metal interphase. This revealed a rapid emergence of vibration modes associated with ]CeOeCe and CeF bonds, supporting a model of initial anion degradation to form SO32− and CF3 radicals. This was later followed by modes associated with LieOeR degradation species [75]. NMR spectroscopy, on the other hand, can be used to probe ion dynamics and the kinetics of ion transport over the solid electrolyte–electrode interface. To this end, Yu et al. used 7Li-7Li 2D-EXSY to characterize the spontaneous exchange between a Li6PS5Br electrolyte and Li2S active material. It was found that the interfacial charge transfer changed considerably after cycling, with less facile ion transport over the interface, which in turn suggested poor wetting in the solid–solid system. An increased activation energy for the ion transfer post-cycling also suggested the formation of an interfacial layer consisting of oxidation products from the electrolyte [76]. 9

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7. Modelling

a recent study by Verners et al. [80], who simulated a graphite–SPE interface using a cross-linked poly(propylene oxide) (PPO) doped with LiPF6 as electrolyte and a reactive (ReaxFF) force field. This could be attributed to the formation of a charged interface at the (unlithiated) graphite. The authors were primarily interested in the viscoelastic and mechanical properties of the SPE film, and could conclude that the salt concentration gradients formed in the system contributed to an improved shear strength. Ebadi et al. have also used DFT calculations to simulate potential decomposition of a range of different SPE polymer hosts on Li-metal. It was shown that polynitriles, polyesters and polycarbonates were reactive on the surface, while polyethers, polyimines and polyachohols were more inert [81]. Interfaces between CEs and electrodes have been studied more intensively by computational techniques than their SPE counterparts. Recently, Yokokawa showed that profound insights could be achieved for the broader sulfide electrolyte/oxide electrode system through classical thermodynamic calculations [82]. By exploring a large variety of compounds in the Li-Co-S-O and Li-Co-P-S-O systems through the construction of multidimensional chemical potential diagrams, phenomena such as formation of resistive interphase layers and Co diffusion could be predicted. This pinpointed the instability of especially sulfide-based CEs, which has also been the conclusion of several DFT studies. For example, Mo et al. estimated the electrochemical stability of the ‘superionic conductor’ material LGPS, while also exploring its transport properties [83]. It was found that the material decomposes spontaneously and surprisingly easily, forming either Li2S at the anode or P2S5 at the cathode, thereby questioning the experimental findings of the ESW [12]. Han et al. came to similar conclusions regarding the LGPS stability [63], and could identify a number of possible decomposition products at different potentials, while also the oxide CE LLZO was shown to possess questionable stability at higher potentials. The large discrepancies between experiment (LSV and CV) and computation could be explained by the limited electrode/electrolyte contacts using conventional setups. Tian et al. arrived at similar conclusions for Nabased CEs such as Na3PS4 and Na3PSe4, and suggested screening the electrochemical stability of CEs through a range of other experimental methodologies (XRD, XPS, DSC) in combination with DFT [84]. Also using a DFT-based approach, Richards et al. investigated systematically a large range of CE materials (see Fig. 12) [85], which also highlighted the comparatively poor stability of the sulfide electrolytes, especially with respect to high-voltage cathodes, and that their decomposition products are largely ionically insulating which would contribute to a high interfacial resistance. Primarily, it could be concluded that the anion in the CE is decisive for the interface stability — where sulfides are especially problematic. LiPON was also calculated to be unstable against Li-metal but, on the other hand, formed an ionically conductive passivating layer on top of the anode, thereby explaining why it could serve as a useful coating layer. DFT calculations by Chu et al. [86], on the other hand, suggested that the sulfide superconductor Li7P3S11 could actually passivate LiFePO4 and LiMn2O4 electrodes by forming electronically insulating decomposition phases, while the CE was considerably more problematical with LiCoO2. In another DFT electrolyte screening study focusing particularly on Na-ion batteries, Tang et al. observed similar instabilities for the thiophosphates [87], where oxide instabilities was the main driving force for interfacial reactions at the cathode. The Na-metal anode was also shown to be problematical for most electrolytes investigated, while Na2Ti3O7 displayed a much more stable surface chemistry. They also computationally investigated a range of binary oxides for stabilization of the interface, and could challenge the frequent use of Al2O3 by finding improved properties for a number of compounds, where HfO2 seemed particularly promising. In a series of articles [88–90], Zhu and co-workers have investigated the thermodynamics of CEs using DFT to explore strategies for interfacial stabilization. The stability of CEs was primarily shown to be related to kinetic factors – rather than thermodynamic stability – where the formation of passivating layers led to

Different forms of modelling and simulation can provide highly useful insights into the stability and reactivity at the electrode–solid electrolyte interface. Firstly, calculations of thermodynamic and electrical properties of electrolytes give an estimate of the chemical potential at the interface, and thereby the reactivity with respect to electrodes, while calculations of the electrolyte HOMO and LUMO levels can serve as a basis for estimating the electrochemical stability window (ESW). More advanced computational methods and physical models of the interfaces allow the true ESW to be estimated with higher precision. For example, while several of these thermodynamic parameters can be extracted from classical or semi-empirical methods, ab initio and especially Density Functional Theory (DFT) are today commonly used for this purpose. Moreover, DFT can be used to estimate the mixing energy, thereby allowing prediction of specific chemical stabilities between different electrode and electrolyte materials. Secondly, possibilities exist to predict surface reactivity and interfacial transport properties if the surface is modelled explicitly on an atomic scale. Again, DFT and ab initio methods are highly useful for estimating interface, adsorption or vacancy energies, and can be employed to model surface reactivity and possible decomposition products. Transport processes, on the other hand, are accessible through Molecular Dynamics (MD) simulations. With the increase in computational power and methodology development, there exist today powerful hybrid methods between electronic structure calculations and MD; typically ab initio MD (AIMD), DFT-MD, ReaxFF, etc. — where the sophisticated interplay between reactivity and transport can be explored at an atomic level. The main computational method used to study the SPE–electrode interface has been MD, while surprisingly little effort has been given to estimate the electrochemical stability of these materials. Few examples exist of such MD studies – all focusing on polyether systems. Aabloo et al. made the first study in this subarea of electrolyte modelling by MD-simulating a PEO–V2O5 interface [77]. Using a relatively small MDbox (a 20 Å thick PEO layer on a 15 Å thick slab of V2O5), a few interesting conclusions could be drawn: both polymer and oxide mobility decreased significantly in the interphase region, primarily as a result of the PEO ether oxygens bonding strongly to the V-atoms in the oxide surface, thereby forming immobile local structures. A similar observation was made by Borodin et al. in MD studies of the PEO | TiO2 system [78] using a quantum chemistry-generated force field. Close to the oxide surface, the PEO layer became significantly perturbed and formed a more dense region stretching ca. 15 Å out from the surface. This clearly hindered PEO conformational and structural relaxations in this part of the material, implying a reduced ionic transport over the interface. Interestingly, this slowing-down effect could be related primarily to the surface structure of TiO2 – locking the polymer into potential wells – rather than the densification of PEO itself, thereby opening for possibilities to tailor the electrode surface for improved polymer mobility. In simulating the Li-metal | PEO8LiTSFI system, Ebadi et al. have pioneered MD studies of the Li–SPE interphase and the inclusion of an electrolyte salt into their model [79]. These studies also suggested slower dynamics of the polymer and salt-ions at the metal surface, which could also be attributed to charge accumulation in this region. Especially the TFSI anions had an energetically favorable interaction with the Li-metal surface which, in turn, also attracted Li+ in a doublelayer arrangement, thereby cross-linking the PEO to reduce its mobility. On the other hand, salt-depleted regions further into the bulk exhibited an increased diffusivity of both PEO and salt-ions, making the global conductivity in the wider electrolyte interface region insignificantly lower than in the bulk electrolyte system. It could also be envisioned that, if Li plating and stripping processes were included in the model, this charge accumulation would change dramatically. A similar ‘doublelayer’ phenomenon with associated decreased mobility was observed in 10

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oxide, which is a comparatively poor Li conductor, was stable at the metal surface in this idealized example, it was seen that the sulfide decomposed, in accordance with the thermodynamic calculations discussed above. The poor stability of sulfide electrolytes with respect to Li-metal was also highlighted in recent AIMD simulations by Cheng et al. for a Li–Li6PS5Cl interface. Severe P-S bond-breaking was observed [93], followed by the formation of Li-P and Li-S bonds. After 500 ps of simulation, also at lower temperatures (298 K), significant parts of the surface had decomposed and simulated the XRD patterns indicated the formation of mainly Li2S, Li3P and LiP. It could be concluded that, given sufficient time and the availability of metallic Li, the entire electrolyte was likely to decompose into LixP and LiyS phases (see Fig. 13). A stabilizing coating layer between a solid electrolyte and a Libattery cathode has also been studied by Sumita et al. [94,95], using a surface of Li3PO4 and the (010) surface for LiFePO4. DFT-MD calculations could show that this interface was chemically stable and did not forming any impurity phases, but that it contained Li-vacancy sites into which the mobile ions could migrate. This was predicted to lower the interfacial resistance. The large interfacial resistance on the cathode side of CE-based batteries is generally attributed to lattice mismatch, structural changes in the electrode surface related to diffusion of transition metals (e.g., Co) into the electrolyte, and the formation of a so-called ‘space-charge layer’. The latter, which result from Li depletion in the electrolyte close to the electrode, was studied using a DFT model of LiCoO2 | β-Li3PS4 by Haruyama et al. [21] Extensive Li adsorption on the ridge side of the CoO6 octahedra was seen after structural optimization, thereby generating a deformed interphase with a clear space-charge layer. The low vacancy formation energy for the Li atoms in the Li3PS4 layer close to the interface contributed to the formation of these structures. It was also shown, however, that this effect could be mitigated by introducing an interlayer of LiNbO3 between the sulfide electrolyte and the oxide electrode, since the oxide surfaces matched well and could thereby give a homogeneous Li distribution. It was also predicted that this barrier layer could hinder Co diffusion into the sulfide electrolyte, which otherwise occurred due to an energetically favorable mixing of Co and P [96]. DFT-MD simulations could also show that the formation of a space-charge layer for sulfide electrolytes is especially problematical during charging, where lithium depletion close to the cathode is extensive and is thereafter followed by electrolyte oxidation [97].

Fig. 12. Electrochemical stability ranges for various electrolytes estimated from DFT calculations [85]. Reprinted with permission from ref. [85]. Copyright 2015, American Chemical Society.

8. Summary and outlook

high overpotentials. This could explain the large experimentally observed ESWs (using CV or LSV) of these materials. Electrode coating strategies with have also been addressed computationally for different materials, where especially nitride-based materials (e.g., Li3N) displayed unique stability properties against Li-metal. Miara et al., on the other hand, studied the stability of LLZO and LLTO (Li5La3Ta2O12, which was found to be the more stable) against different cathodes materials. They found that the interphase was more sensitive to decomposition at the high sintering temperatures used, than to electrochemical decomposition during cycling [91]. The stability trend found for the different cathodes also highlights important differences between oxide and sulfide CEs. More specific surface reaction processes can be studied than by using pure thermodynamic and kinetic models alone if an explicit physical interface is included into the DFT or ab initio calculations, and using atomistic models for both the bulk and interface materials. This applies for simulations based on both classical force field methods and on DFT. This means that the issue of surface matching must be taken into account, and can be decisive when the both the electrode and electrolyte materials are solid and crystalline. These approaches are also considerably more computationally demanding. An example of modelling such as physical interface is the study by Lepley et al. who used DFT to compare the interfaces between metallic Li and the (010) surfaces of the analogous compounds Li3PS4 and Li3PO4 [92]. While the

The discovery and the subsequent engineering of the SEI layer has been a crucial step towards realising practical LIBs. This applies equally well for solid-state electrolyte LIBs. It is very evident that the stability of the electrode–electrolyte interfaces are key in determining battery performance, especially as the dimensions of these electrolytes approach practical thicknesses and the bulk ionic conductivity is no longer the major bottleneck. In this review, we have summarized the techniques used to probe electrolyte–electrode interphases. Electrochemical impedance spectroscopy has provided a fundamental understanding of how changes in interface impedance can be detrimental to battery performance. However, the technique offers little insight into the actual morphology and chemical composition of these interphases. Microscopy and surface-sensitive analytical techniques, especially XPS, have played and will continue to play an integral role in deriving this information. Unfortunately, both techniques are plagued by substantial uncertainties as to whether or not the true interphase is preserved following post mortem sample preparation. To circumvent this problem, reactions between electrolyte and electrode have been studied in real-time using in situ XPS. However, the high surface sensitivity of XPS is in itself a limitation and it becomes a challenge to design samples and experimental condition which mimic real life batteries. Here, the application of novel characterisation techniques available at large-scale facilities such as synchrotrons and neutron sources are likely to move the field forward. 11

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Fig. 13. AIMD simulations showing the degradation of a Li | Li6PS5Cl interface. Li-metal is blue, Li ions are green, S is yellow, P is purple, and Cl is dark green [93]. Reprinted with permission from ref. [93]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Copyright 2017, American Chemical Society.

Computational modelling also constitute a powerful array of techniques which facilitate the prediction of reactivity and transport processes occurring at the interface, where the exploitation of more advanced multi-scale techniques and machine-learning approaches are envisioned to generate both a better in-depth understanding and opportunities to tailor the interphases in solid-state systems [98]. Much of the existing literature has so far addressed interphases in systems involving CEs, which leaves plenty of room for development in the SPE field. Based on the literature cited in this review, it is evident that there is no single technique capable providing all of the information we need to understand the dynamic processes occurring at the electrolyte-electrode interface. We can therefore foresee an increase in multiple techniques carefully tuned to specific systems of interest, to achieve a more complete picture of how these chemistries and materials behave in juxtaposition with one another.

[10] [11] [12] [13] [14] [15]

Acknowledgements

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This work has been supported by the European Research Council (ERC), grant no. 771777 ‘FUN POLYSTORE’. The authors would also like to thank Prof. Em. J.O. Thomas, Uppsala University, for fruitful discussions.

[17]

[18]

References

[19]

[1] T. Inoue, K. Mukai, Are all-solid-state lithium-ion batteries really safe?—verification by differential scanning calorimetry with an all-inclusive microcell, ACS Appl. Mater. Interfaces 9 (2) (2017) 1507–1515, https://doi.org/10.1021/ acsami.6b13224. [2] X.-B. Cheng, R. Zhang, C.-Z. Zhao, Q. Zhang, Toward safe lithium metal anode in rechargeable batteries: a review, Chem. Rev. 117 (15) (2017) 10403–10473, https://doi.org/10.1021/acs.chemrev.7b00115. [3] J.R. Nair, L. Imholt, G. Brunklaus, M. Winter, Lithium metal polymer electrolyte batteries: opportunities and challenges, Electrochem. Soc. Interface 28 (2) (2019) 55–61, https://doi.org/10.1149/2.F05192if. [4] Y. Kato, S. Hori, T. Saito, K. Suzuki, M. Hirayama, A. Mitsui, M. Yonemura, H. Iba, R. Kanno, High-power all-solid-state batteries using sulfide superionic conductors, Nat. Energy 1 (4) (2016) 16030, https://doi.org/10.1038/nenergy.2016.30. [5] A. Mauger, M. Armand, C.M. Julien, K. Zaghib, Challenges and issues facing lithium metal for solid-state rechargeable batteries, J. Power Sources 353 (2017) 333–342, https://doi.org/10.1016/j.jpowsour.2017.04.018. [6] F. Zheng, M. Kotobuki, S. Song, M.O. Lai, L. Lu, Review on solid electrolytes for allsolid-state lithium-ion batteries, J. Power Sources 389 (2018) 198–213, https://doi. org/10.1016/j.jpowsour.2018.04.022. [7] Reisch, Solid-state batteries inch their way to market, CEN Glob. Enterp. 95 (46) (2017) 19–21, https://doi.org/10.1021/cen-09546-bus. [8] C. Sun, J. Liu, Y. Gong, D.P. Wilkinson, J. Zhang, Recent advances in all-solid-state rechargeable lithium batteries, Nano Energy 33 (2017) 363–386, https://doi.org/ 10.1016/j.nanoen.2017.01.028. [9] Z. Gao, H. Sun, L. Fu, F. Ye, Y. Zhang, W. Luo, Y. Huang, Promises, challenges, and

[20]

[21]

[22]

[23] [24]

[25] [26]

12

recent progress of inorganic solid-state electrolytes for all-solid-state lithium batteries, Adv. Mater. 30 (17) (2018) 1705702, , https://doi.org/10.1002/adma. 201705702. Q. Zhang, K. Liu, F. Ding, X. Liu, Recent advances in solid polymer electrolytes for lithium batteries, Nano Res. 10 (12) (2017) 4139–4174, https://doi.org/10.1007/ s12274-017-1763-4. J. Mindemark, M. Lacey, T. Bowden, D. Brandell, Beyond PEO—alternative host materials for Li+-conducting solid polymer electrolytes, Prog. Polym. Sci. 81 (2018) 114–143, https://doi.org/10.1016/j.progpolymsci.2017.12.004. N. Kamaya, K. Homma, Y. Yamakawa, M. Hirayama, R. Kanno, M. Yonemura, T. Kamiyama, Y. Kato, S. Hama, K. Kawamoto, et al., A lithium superionic conductor, Nat. Mater. 10 (9) (2011) 682–686, https://doi.org/10.1038/nmat3066. K. Takada, Progress and prospective of solid-state lithium batteries, Acta Mater. 61 (3) (2013) 759–770, https://doi.org/10.1016/j.actamat.2012.10.034. S. Wang, H. Xu, W. Li, A. Dolocan, A. Manthiram, Interfacial chemistry in solid-state batteries: formation of interphase and its consequences, J. Am. Chem. Soc. 140 (1) (2018) 250–257, https://doi.org/10.1021/jacs.7b09531. J. Zhang, N. Zhao, M. Zhang, Y. Li, P.K. Chu, X. Guo, Z. Di, X. Wang, H. Li, Flexible and ion-conducting membrane electrolytes for solid-state lithium batteries: dispersion of garnet nanoparticles in insulating polyethylene oxide, Nano Energy 28 (2016) 447–454, https://doi.org/10.1016/j.nanoen.2016.09.002. R. Mogensen, D. Brandell, R. Younesi, Solubility of the solid electrolyte interphase (SEI) in sodium ion batteries, ACS Energy Lett 1 (6) (2016) 1173–1178, https://doi. org/10.1021/acsenergylett.6b00491. S. Wenzel, T. Leichtweiss, D.A. Weber, J. Sann, W.G. Zeier, J. Janek, Interfacial reactivity benchmarking of the sodium ion conductors Na3PS4 and sodium β-alumina for protected sodium metal anodes and sodium all-solid-state batteries, ACS Appl. Mater. Interfaces 8 (41) (2016) 28216–28224, https://doi.org/10.1021/ acsami.6b10119. R.C. Xu, X.H. Xia, S.Z. Zhang, D. Xie, X.L. Wang, J.P. Tu, Interfacial challenges and progress for inorganic all-solid-state lithium batteries, Electrochim. Acta 284 (2018) 177–187, https://doi.org/10.1016/j.electacta.2018.07.191. P. Verma, P. Maire, P. Novák, A review of the features and analyses of the solid electrolyte interphase in Li-ion batteries, Electrochim. Acta 55 (22) (2010) 6332–6341, https://doi.org/10.1016/j.electacta.2010.05.072. W. Zhang, F. Richter, S.P. Culver, T. Leichtweiß, J.G. Lozano, C. Dietrich, P.G. Bruce, W.G. Zeier, J. Janek, Degradation mechanisms at the Li10GeP2S12/ LiCoO2 cathode interface in an all-solid-state lithium ion battery, ACS Appl. Mater. Interfaces (2018), https://doi.org/10.1021/acsami.8b05132. J. Haruyama, K. Sodeyama, L. Han, K. Takada, Y. Tateyama, Space–charge layer effect at interface between oxide cathode and sulfide electrolyte in all-solid-state lithium-ion battery, Chem. Mater. 26 (14) (2014) 4248–4255, https://doi.org/10. 1021/cm5016959. S. Wenzel, T. Leichtweiss, D. Krüger, J. Sann, J. Janek, Interphase formation on lithium solid electrolytes — an in situ approach to study interfacial reactions by photoelectron spectroscopy, Solid State Ionics 278 (2015) 98–105, https://doi.org/ 10.1016/j.ssi.2015.06.001. C. Xu, B. Sun, T. Gustafsson, K. Edström, D. Brandell, M. Hahlin, Interface layer formation in solid polymer electrolyte lithium batteries: an XPS study, J. Mater. Chem. A 2 (20) (2014) 7256–7264, https://doi.org/10.1039/C4TA00214H. P. Hovington, M. Lagacé, A. Guerfi, P. Bouchard, A. Mauger, C.M. Julien, M. Armand, K. Zaghib, New Lithium metal polymer solid state battery for an ultrahigh energy: nano C-LiFePO4 versus nano Li1.2V3O8, Nano Lett. 15 (4) (2015) 2671–2678, https://doi.org/10.1021/acs.nanolett.5b00326. D. Fauteux, Formation of a passivating film at the lithium-PEO-LiCF3SO3 interface, Solid State Ionics 17 (2) (1985) 133–138, https://doi.org/10.1016/0167-2738(85) 90061-X. D. Fauteux, Lithium electrode/PEO-based polymer electrolyte interface behavior

Solid State Ionics 343 (2019) 115068

C. Sångeland, et al.

[27] [28]

[29]

[30]

[31]

[32]

[33] [34] [35] [36] [37] [38]

[39] [40] [41]

[42] [43] [44] [45]

[46]

[47] [48]

[49]

[50]

between 60° and 120 °C, J. Electrochem. Soc. 135 (9) (1988) 2231, https://doi.org/ 10.1149/1.2096244. P. Bruce, F. Krok, Characterisation of the electrode/electrolyte interfaces in cells of the type Li/PEO_LiCF3SO3/V6O13 by ac impedance methods, Solid State Ionics 36 (3–4) (1989) 171–174, https://doi.org/10.1016/0167-2738(89)90164-1. S. Wenzel, D.A. Weber, T. Leichtweiss, M.R. Busche, J. Sann, J. Janek, Interphase formation and degradation of charge transfer kinetics between a lithium metal anode and highly crystalline Li7P3S11 solid electrolyte, Solid State Ionics 286 (2016) 24–33, https://doi.org/10.1016/j.ssi.2015.11.034. M.J. Lacey, Influence of the electrolyte on the internal resistance of lithium−sulfur batteries studied with an intermittent current interruption method, ChemElectroChem 4 (8) (2017) 1997–2004, https://doi.org/10.1002/celc. 201700129. A. Bergfelt, M.J. Lacey, J. Hedman, C. Sångeland, D. Brandell, T. Bowden, εCaprolactone-based solid polymer electrolytes for lithium-ion batteries: synthesis, electrochemical characterization and mechanical stabilization by block copolymerization, RSC Adv. 8 (30) (2018) 16716–16725, https://doi.org/10.1039/ C8RA00377G. I. Aldalur, H. Zhang, M. Piszcz, U. Oteo, L.M. Rodriguez-Martinez, D. Shanmukaraj, T. Rojo, M. Armand, Jeffamine® based polymers as highly conductive polymer electrolytes and cathode binder materials for battery application, J. Power Sources 347 (2017) 37–46, https://doi.org/10.1016/j.jpowsour.2017.02.047. Eriksson, T.; Mindemark, J.; Yue, M.; Brandell, D. Effects of nanoparticle addition to poly(ε-caprolactone) electrolytes: crystallinity, conductivity and ambient temperature battery cycling. Electrochim. Acta. doi:https://doi.org/10.1016/j.electacta. 2019.01.117. Z. Li, J. Mindemark, D. Brandell, Y. Tominaga, A concentrated poly(ethylene carbonate)/poly(trimethylene carbonate) blend electrolyte for all-solid-state Li battery, Polym. J. (2019), https://doi.org/10.1038/s41428-019-0184-5. F. Croce, B. Scrosati, Interfacial phenomena in polymer-electrolyte cells: lithium passivation and cycleability, J. Power Sources 43 (1–3) (1993) 9–19, https://doi. org/10.1016/0378-7753(93)80097-9. E. Peled, D. Golodnitsky, G. Ardel, V. Eshkenazy, The Sei model—application to lithium-polymer electrolyte batteries, Electrochim. Acta 40 (13–14) (1995) 2197–2204, https://doi.org/10.1016/0013-4686(95)00163-9. E. Peled, Advanced model for solid electrolyte interphase electrodes in liquid and polymer electrolytes, J. Electrochem. Soc. 144 (8) (1997) L208, https://doi.org/10. 1149/1.1837858. R. Bouchet, S. Lascaud, M. Rosso, An EIS study of the anode Li/PEO-LiTFSI of a Li polymer battery, J. Electrochem. Soc. 150 (10) (2003) A1385, https://doi.org/10. 1149/1.1609997. S. Wenzel, S. Randau, T. Leichtweiß, D.A. Weber, J. Sann, W.G. Zeier, J. Janek, Direct observation of the interfacial instability of the fast ionic conductor Li10GeP2S12 at the lithium metal anode, Chem. Mater. 28 (7) (2016) 2400–2407, https://doi.org/10.1021/acs.chemmater.6b00610. J. Weston, B. Steele, Effects of inert fillers on the mechanical and electrochemical properties of lithium salt-poly(ethylene oxide) polymer electrolytes, Solid State Ionics 7 (1) (1982) 75–79, https://doi.org/10.1016/0167-2738(82)90072-8. J. Weston, B. Steele, Effects of preparation method on properties of lithium salt-poly (ethylene oxide) polymer electrolytes, Solid State Ionics 7 (1) (1982) 81–88, https://doi.org/10.1016/0167-2738(82)90073-X. P. Ravn Sørensen, T. Jacobsen, Conductivity, charge transfer and transport number—an ac-investigation of the polymer electrolyte LiSCN-poly(ethyleneoxide), Electrochim. Acta 27 (12) (1982) 1671–1675, https://doi.org/10.1016/00134686(82)80162-X. A. Teyssot, M. Rosso, R. Bouchet, S. Lascaud, Evolution of the electrode–electrolyte interface in a lithium–polymer battery, Solid State Ionics 177 (1–2) (2006) 141–143, https://doi.org/10.1016/j.ssi.2005.10.026. G.B. Appetecchi, S. Scaccia, S. Passerini, Investigation on the stability of the lithium-polymer electrolyte Interface, J. Electrochem. Soc. 147 (12) (2000) 4448, https://doi.org/10.1149/1.1394084. J.M. Whiteley, J.H. Woo, E. Hu, K.-W. Nam, S.-H. Lee, Empowering the lithium metal battery through a silicon-based superionic conductor, J. Electrochem. Soc. 161 (12) (2014) A1812–A1817, https://doi.org/10.1149/2.0501412jes. M. Sakuma, K. Suzuki, M. Hirayama, R. Kanno, Reactions at the electrode/electrolyte interface of all-solid-state lithium batteries incorporating Li–M (M = Sn, Si) alloy electrodes and sulfide-based solid electrolytes, Solid State Ionics 285 (2016) 101–105, https://doi.org/10.1016/j.ssi.2015.07.010. W. Zhang, D.A. Weber, H. Weigand, T. Arlt, I. Manke, D. Schröder, R. Koerver, T. Leichtweiss, P. Hartmann, W.G. Zeier, et al., Interfacial processes and influence of composite cathode microstructure controlling the performance of all-solid-state lithium batteries, ACS Appl. Mater. Interfaces 9 (21) (2017) 17835–17845, https:// doi.org/10.1021/acsami.7b01137. R. Koerver, F. Walther, I. Aygün, J. Sann, C. Dietrich, W.G. Zeier, J. Janek, Redoxactive cathode interphases in solid-state batteries, J. Mater. Chem. A 5 (43) (2017) 22750–22760, https://doi.org/10.1039/C7TA07641J. M. Zarabian, M. Bartolini, P. Pereira-Almao, V. Thangadurai, X-ray photoelectron spectroscopy and AC impedance spectroscopy studies of Li-La-Zr-O solid electrolyte thin film/LiCoO2 cathode interface for all-solid-state Li batteries, J. Electrochem. Soc. 164 (6) (2017) A1133–A1139, https://doi.org/10.1149/2.0621706jes. R. Koerver, I. Aygün, T. Leichtweiß, C. Dietrich, W. Zhang, J.O. Binder, P. Hartmann, W.G. Zeier, J. Janek, Capacity fade in solid-state batteries: interphase formation and chemomechanical processes in nickel-rich layered oxide cathodes and lithium thiophosphate solid electrolytes, Chem. Mater. 29 (13) (2017) 5574–5582, https://doi.org/10.1021/acs.chemmater.7b00931. S. Wenzel, S.J. Sedlmaier, C. Dietrich, W.G. Zeier, J. Janek, Interfacial reactivity

[51]

[52]

[53]

[54]

[55]

[56] [57]

[58] [59]

[60]

[61]

[62] [63] [64]

[65]

[66] [67]

[68] [69]

[70]

[71]

[72]

13

and interphase growth of argyrodite solid electrolytes at lithium metal electrodes, Solid State Ionics 318 (2018) 102–112, https://doi.org/10.1016/j.ssi.2017.07.005. C. Fleischer, W. Waag, H.-M. Heyn, D.U. Sauer, On-line adaptive battery impedance parameter and state estimation considering physical principles in reduced order equivalent circuit battery models part 2. Parameter and state estimation, J. Power Sources 262 (2014) 457–482, https://doi.org/10.1016/j.jpowsour.2014.03.046. S. Srivastav, C. Xu, K. Edström, T. Gustafsson, D. Brandell, Modelling the morphological background to capacity fade in Si-based lithium-ion batteries, Electrochim. Acta 258 (2017) 755–763, https://doi.org/10.1016/j.electacta.2017. 11.124. J. Auvergniot, A. Cassel, J.-B. Ledeuil, V. Viallet, V. Seznec, R. Dedryvère, Interface stability of argyrodite Li6PS5Cl toward LiCoO2, LiNi1/3Co1/3Mn1/3O2, and LiMn2O4 in bulk all-solid-state batteries, Chem. Mater. 29 (9) (2017) 3883–3890, https://doi. org/10.1021/acs.chemmater.6b04990. A. Sakuda, A. Hayashi, M. Tatsumisago, Interfacial observation between LiCoO2 electrode and Li2S–P2S5 solid electrolytes of all-solid-state lithium secondary batteries using transmission electron microscopy, Chem. Mater. 22 (3) (2010) 949–956, https://doi.org/10.1021/cm901819c. H. Kitaura, A. Hayashi, K. Tadanaga, M. Tatsumisago, Electrochemical performance of all-solid-state lithium secondary batteries with Li–Ni–Co–Mn oxide positive electrodes, Electrochim. Acta 55 (28) (2010) 8821–8828, https://doi.org/10.1016/ j.electacta.2010.07.066. H. Kitaura, A. Hayashi, K. Tadanaga, M. Tatsumisago, All-solid-state Lithium secondary batteries using LiMn2O4 electrode and Li2S–P2S5 solid electrolyte, J. Electrochem. Soc. 157 (4) (2010) A407, https://doi.org/10.1149/1.3298441. C. Ma, Y. Cheng, K. Yin, J. Luo, A. Sharafi, J. Sakamoto, J. Li, K.L. More, N.J. Dudney, M. Chi, Interfacial stability of Li metal–solid electrolyte elucidated via in situ electron microscopy, Nano Lett. 16 (11) (2016) 7030–7036, https://doi.org/ 10.1021/acs.nanolett.6b03223. I. Ismail, A. Noda, A. Nishimoto, M. Watanabe, XPS study of Lithium surface after contact with lithium-salt doped polymer electrolytes, Electrochim. Acta 46 (10−11) (2001) 1595–1603, https://doi.org/10.1016/S0013-4686(00)00758-1. J. Li, S. Dong, C. Wang, Z. Hu, Z. Zhang, H. Zhang, G. Cui, A study on the interfacial stability of the cathode/polycarbonate interface: implication of overcharge and transition metal redox, J. Mater. Chem. A 6 (25) (2018) 11846–11852, https://doi. org/10.1039/C8TA02975J. W. Zhang, T. Leichtweiß, S.P. Culver, R. Koerver, D. Das, D.A. Weber, W.G. Zeier, J. Janek, The detrimental effects of carbon additives in Li10GeP2S12-based solidstate batteries, ACS Appl. Mater. Interfaces 9 (41) (2017) 35888–35896, https:// doi.org/10.1021/acsami.7b11530. F. Jeschull, J. Maibach, K. Edström, D. Brandell, On the electrochemical properties and interphase composition of graphite: PVdF-HFP electrodes in dependence of binder content, J. Electrochem. Soc. 164 (7) (2017) A1765–A1772, https://doi.org/ 10.1149/2.0121709jes. X. Liang, C. Hart, Q. Pang, A. Garsuch, T. Weiss, L.F. Nazar, A highly efficient polysulfide mediator for lithium–sulfur batteries, Nat. Commun. 6 (1) (2015), https://doi.org/10.1038/ncomms6682. F. Han, Y. Zhu, X. He, Y. Mo, C. Wang, Electrochemical stability of Li10GeP2S12 and Li7La3Zr2O12 solid electrolytes, Adv. Energy Mater. 6 (8) (2016) 1501590, , https:// doi.org/10.1002/aenm.201501590. C. Dietrich, R. Koerver, M.W. Gaultois, G. Kieslich, G. Cibin, J. Janek, W.G. Zeier, Spectroscopic characterization of lithium thiophosphates by XPS and XAS — a model to help monitor interfacial reactions in all-solid-state batteries, Phys. Chem. Chem. Phys. (2018), https://doi.org/10.1039/C8CP01968A. B. Sun, C. Xu, J. Mindemark, T. Gustafsson, K. Edström, D. Brandell, At the polymer electrolyte interfaces: the role of the polymer host in interphase layer formation in Li-batteries, J. Mater. Chem. A 3 (26) (2015) 13994–14000, https://doi.org/10. 1039/C5TA02485D. B. Sun, J. Mindemark, K. Edström, D. Brandell, Polycarbonate-based solid polymer electrolytes for Li-ion batteries, Solid State Ionics 262 (2014) 738–742, https://doi. org/10.1016/j.ssi.2013.08.014. S. Malmgren, K. Ciosek, M. Hahlin, T. Gustafsson, M. Gorgoi, H. Rensmo, K. Edström, Comparing anode and cathode electrode/electrolyte interface composition and morphology using soft and hard X-ray photoelectron spectroscopy, Electrochim. Acta 97 (2013) 23–32, https://doi.org/10.1016/j.electacta.2013.03. 010. A. Schwöbel, R. Hausbrand, W. Jaegermann, Interface reactions between LiPON and lithium studied by in-situ X-ray photoemission, Solid State Ionics 273 (2015) 51–54, https://doi.org/10.1016/j.ssi.2014.10.017. P. Hartmann, T. Leichtweiss, M.R. Busche, M. Schneider, M. Reich, J. Sann, P. Adelhelm, J. Janek, Degradation of NASICON-type materials in contact with lithium metal: formation of mixed conducting interphases (MCI) on solid electrolytes, J. Phys. Chem. C 117 (41) (2013) 21064–21074, https://doi.org/10.1021/ jp4051275. C. Guhl, P. Kehne, Q. Ma, F. Tietz, P. Komissinskiy, W. Jaegermann, R. Hausbrand, Interfaces in solid-state sodium-ion batteries: NaCoO2 thin films on solid electrolyte substrates, Electrochim. Acta 268 (2018) 226–233, https://doi.org/10.1016/j. electacta.2018.02.109. X. Wu, C. Villevieille, P. Novák, M. El Kazzi, Monitoring the chemical and electronic properties of electrolyte–electrode interfaces in all-solid-state batteries using operando X-ray photoelectron spectroscopy, Phys. Chem. Chem. Phys. 20 (16) (2018) 11123–11129, https://doi.org/10.1039/C8CP01213J. F. Walther, R. Koerver, T. Fuchs, S. Ohno, J. Sann, M. Rohnke, W.G. Zeier, J. Janek, Visualization of the interfacial decomposition of composite cathodes in argyroditebased all-solid-state batteries using time-of-flight secondary-ion mass spectrometry, Chem. Mater. 31 (10) (2019) 3745–3755, https://doi.org/10.1021/acs.chemmater.

Solid State Ionics 343 (2019) 115068

C. Sångeland, et al. 9b00770. [73] H. Visbal, Y. Aihara, S. Ito, T. Watanabe, Y. Park, S. Doo, The effect of diamond-like carbon coating on LiNi0.8Co0.15Al0.05O2 particles for all solid-state lithium-ion batteries based on Li2S–P2S5 glass-ceramics, J. Power Sources 314 (2016) 85–92, https://doi.org/10.1016/j.jpowsour.2016.02.088. [74] T. Bartsch, F. Strauss, T. Hatsukade, A. Schiele, A.-Y. Kim, P. Hartmann, J. Janek, T. Brezesinski, Gas evolution in all-solid-state battery cells, ACS Energy Lett 3 (10) (2018) 2539–2543, https://doi.org/10.1021/acsenergylett.8b01457. [75] M. Le Granvalet-Mancini, Characterization of the passivation layer at the polymer electrolyte/lithium electrode interface, Solid State Ionics 135 (1–4) (2000) 283–290, https://doi.org/10.1016/S0167-2738(00)00448-3. [76] C. Yu, S. Ganapathy, E.R.H. van Eck, H. Wang, S. Basak, Z. Li, M. Wagemaker, Accessing the bottleneck in all-solid state batteries, lithium-ion transport over the solid-electrolyte-electrode Interface, Nat. Commun. 8 (1) (2017), https://doi.org/ 10.1038/s41467-017-01187-y. [77] A. Aabloo, M. Klintenberg, J.O. Thomas, Molecular dynamics simulation of a polymer–inorganic interface, Electrochim. Acta 45 (8–9) (2000) 1425–1429, https://doi.org/10.1016/S0013-4686(99)00354-0. [78] O. Borodin, G.D. Smith, R. Bandyopadhyaya, O. Byutner, Molecular dynamics study of the influence of solid interfaces on poly(ethylene oxide) structure and dynamics, Macromolecules 36 (20) (2003) 7873–7883, https://doi.org/10.1021/ma0346005. [79] M. Ebadi, L.T. Costa, C.M. Araujo, D. Brandell, Modelling the polymer electrolyte/ Li-metal interface by molecular dynamics simulations, Electrochim. Acta 234 (2017) 43–51, https://doi.org/10.1016/j.electacta.2017.03.030. [80] O. Verners, A.V. Lyulin, A. Simone, Salt concentration dependence of the mechanical properties of LiPF6/poly(propylene glycol) acrylate electrolyte at a graphitic carbon interface: a reactive molecular dynamics study, J. Polym. Sci. Part B Polym. Phys. 56 (9) (2018) 718–730, https://doi.org/10.1002/polb.24584. [81] M. Ebadi, C. Marchiori, J. Mindemark, D. Brandell, C.M. Araujo, Assessing structure and stability of polymer/lithium-metal interfaces from first-principles calculations, J. Mater. Chem. A 7 (14) (2019) 8394–8404, https://doi.org/10.1039/ C8TA12147H. [82] H. Yokokawa, Thermodynamic stability of sulfide electrolyte/oxide electrode interface in solid-state lithium batteries, Solid State Ionics 285 (2016) 126–135, https://doi.org/10.1016/j.ssi.2015.05.010. [83] Y. Mo, S.P. Ong, G. Ceder, First principles study of the Li10GeP2S12 lithium super ionic conductor material, Chem. Mater. 24 (1) (2012) 15–17, https://doi.org/10. 1021/cm203303y. [84] Y. Tian, T. Shi, W.D. Richards, J. Li, J.C. Kim, S.-H. Bo, G. Ceder, Compatibility issues between electrodes and electrolytes in solid-state batteries, Energy Environ. Sci. 10 (5) (2017) 1150–1166, https://doi.org/10.1039/C7EE00534B. [85] W.D. Richards, L.J. Miara, Y. Wang, J.C. Kim, G. Ceder, Interface stability in solidstate batteries, Chem. Mater. 28 (1) (2016) 266–273, https://doi.org/10.1021/acs. chemmater.5b04082. [86] I.-H. Chu, H. Nguyen, S. Hy, Y.-C. Lin, Z. Wang, Z. Xu, Z. Deng, Y.S. Meng, S.P. Ong,

[87]

[88]

[89] [90] [91] [92] [93] [94]

[95] [96]

[97]

[98]

14

Insights into the performance limits of the Li7P3S11 superionic conductor: a combined first-principles and experimental study, ACS Appl. Mater. Interfaces 8 (12) (2016) 7843–7853, https://doi.org/10.1021/acsami.6b00833. H. Tang, Z. Deng, Z. Lin, Z. Wang, I.-H. Chu, C. Chen, Z. Zhu, C. Zheng, S.P. Ong, Probing solid–solid interfacial reactions in all-solid-state sodium-ion batteries with first-principles calculations, Chem. Mater. 30 (1) (2018) 163–173, https://doi.org/ 10.1021/acs.chemmater.7b04096. Y. Zhu, X. He, Y. Mo, Origin of outstanding stability in the lithium solid electrolyte materials: insights from thermodynamic analyses based on first-principles calculations, ACS Appl. Mater. Interfaces 7 (42) (2015) 23685–23693, https://doi.org/10. 1021/acsami.5b07517. Y. Zhu, X. He, Y. Mo, First principles study on electrochemical and chemical stability of solid electrolyte–electrode interfaces in all-solid-state Li-ion batteries, J. Mater. Chem. A 4 (9) (2016) 3253–3266, https://doi.org/10.1039/C5TA08574H. Y. Zhu, X. He, Y. Mo, Strategies based on nitride materials chemistry to stabilize Li metal anode, Adv. Sci. 4 (8) (2017) 1600517, , https://doi.org/10.1002/advs. 201600517. L.J. Miara, W.D. Richards, Y.E. Wang, G. Ceder, First-principles studies on cation dopants and electrolyte|cathode interphases for lithium garnets, Chem. Mater. 27 (11) (2015) 4040–4047, https://doi.org/10.1021/acs.chemmater.5b01023. N.D. Lepley, N.A.W. Holzwarth, Y.A. Du, Structures, Li+ mobilities, and interfacial properties of solid electrolytes Li3PS4 and Li3PO4 from first principles, Phys. Rev. B 88 (10) (2013) 104103, https://doi.org/10.1103/PhysRevB.88.104103. T. Cheng, B.V. Merinov, S. Morozov, W.A. Goddard, Quantum mechanics reactive dynamics study of solid Li-electrode/Li6PS5 Cl-electrolyte interface, ACS Energy Lett 2 (6) (2017) 1454–1459, https://doi.org/10.1021/acsenergylett.7b00319. M. Sumita, Y. Tanaka, M. Ikeda, T. Ohno, Theoretically designed Li3PO4 (100)/ LiFePO4 (010) coherent electrolyte/cathode interface for all solid-state Li ion secondary batteries, J. Phys. Chem. C 119 (1) (2015) 14–22, https://doi.org/10.1021/ jp5060342. M. Sumita, Y. Tanaka, M. Ikeda, T. Ohno, Theoretical insight into charging process in a Li3PO4 (100)/LiFePO4 (010) coherent interface system, Solid State Ionics 285 (2016) 59–65, https://doi.org/10.1016/j.ssi.2015.09.029. J. Haruyama, K. Sodeyama, Y. Tateyama, Cation mixing properties toward Co diffusion at the LiCoO2 cathode/sulfide electrolyte interface in a solid-state battery, ACS Appl. Mater. Interfaces 9 (1) (2017) 286–292, https://doi.org/10.1021/ acsami.6b08435. M. Sumita, Y. Tanaka, M. Ikeda, T. Ohno, Charged and discharged states of cathode/sulfide electrolyte interfaces in all-solid-state lithium ion batteries, J. Phys. Chem. C 120 (25) (2016) 13332–13339, https://doi.org/10.1021/acs.jpcc. 6b01207. A.A. Franco, A. Rucci, D. Brandell, C. Frayret, M. Gaberscek, P. Jankowski, P. Johansson, Boosting rechargeable batteries R&D by multiscale modeling: myth or reality? Chem. Rev. 119 (7) (2019) 4569–4627, https://doi.org/10.1021/acs. chemrev.8b00239.