On the Functionality of Coatings for Cathode Active Materials in Thiophosphate‐Based All‐Solid‐State Batteries

Abstract

achievable by SSBs. Meanwhile, polymer-, oxide-, and sulfide-based ionic conductors are being heavily investigated as the solid electrolyte (SE) separator. [9][10][11][12][13][14][15] Nevertheless, recent estimates [16,17] show that only batteries possessing sulfide-based SEs will be leading contenders for room-temperature applications.The sulfides, which are better denoted as thiophosphates, provide the highest lithium-ion conductivity, a relatively low E modulus, and can be processed at low temperatures. [2,3,18] Currently, only thiophosphates allow for the preparation of thick cathode composites with sufficient rate capability. [19,20] Unfortunately, thiophosphates also have a rather narrow electrochemical stability window, i.e., the onset of oxidative decomposition begins even before 3 V versus Li + /Li due to S(0)/S(−2) redox reactions, while reductive decomposition is theoretically expected at potentials of about 1.7 V versus Li + /Li due to P(+5)/P(−3) redox reactions. [21,22] Importantly, the perfect SE does not actually exist. The perfect SE would combine the ionic transport properties of thiophosphates, the mechanical properties of polymers, and the oxidation stability of oxides. Therefore, in order to exploit the superionic transport properties of thiophosphates, the implementation of protective coatings to overcome the intrinsic electrochemical instabilities is most certainly a necessity. The thermodynamic prerequisites for coatings (Figure 1) have already been treated in a number of theoretical papers, [18,21,[23][24][25] which provide the initial design guidelines.Beyond the thermodynamic considerations, it is well known that within SSBs, thiophosphate SEs are oxidized and decomposed in direct contact with cathode active materials (CAMs), in particular at high potentials during charging. [2,[26][27][28] Several years ago, Takada summarized the early work on SSBs and described the need for coatings against the formation of space charge layers at the interface between high-voltage cathodes and thiophosphate SEs. [29] Haruyama et al. concluded from density functional theory calculations that space charge layers between LiCoO 2 (LCO) and thiophosphate-based SEs are responsible for high impedances and that the addition of a buffer layer reduces such effects. [30] However, experimental evidence for such claims has not yet been reported. While these considerations are certainly valuable, from a current perspective, effects arising from the space charge layer are likely overstated and the role of oxidative degradation of the SE is understated. [31] Though the The last decade has seen considerable advancements in the development of solid electrolytes for solid-state battery applications, with particular attention being paid to sulfide superionic conductors. Importantly, the intrinsic electrochemical instability of these high-performance separators highlights the notion that further progress in the field of solid-state batteries is contingent on the optimization of component material interfaces in order to se...

Enabling a Durable Electrochemical Interface via an Artificial Amorphous Cathode Electrolyte Interphase for Hybrid Solid/Liquid Lithium‐Metal Batteries

“…Usually, the coating materials exteriorly grow on the surface of cathode particle and reach crystalline state after high‐temperature calcination, behaving moderate compatibility with cathode. Hence, brittle failure easily takes place in this thin layer during mechanical assembling or long‐term cycling, resulting in continuous electrolyte decomposition and low Coulombic efficiency (CE) . Fortunately, the above drawbacks can be significantly conquered when an amorphous cathode electrolyte interphase (CEI) has been in situ deposited at the interface, owing to its excellent structure compatibility and plasticity .…”

“…Hence, brittle failure easily takes place in this thin layer during mechanical assembling or long‐term cycling, resulting in continuous electrolyte decomposition and low Coulombic efficiency (CE) . Fortunately, the above drawbacks can be significantly conquered when an amorphous cathode electrolyte interphase (CEI) has been in situ deposited at the interface, owing to its excellent structure compatibility and plasticity . However, there is still a lack of systematic studies on how the amorphous CEI manipulates the cathode interface in a hybrid solid/liquid battery system .…”

“…To elucidate the origin of the interfacial kinetics enhancement, COMSOL Multiphysics concentration field simulation was performed to detect ions distribution. The Li + will spontaneously migrate from the solid polymer interface to the cathode interface (Figure c), when they contact together, driven by the enormous discrepancy of chemical potential . At the same time, a steep potential drop is formed at the interface (NCM622/PDXL), which sharply raises Li ions diffusion impedance, deteriorates interfacial dynamics.…”

“…[12] Fortunately, the above drawbacks can be significantly conquered when an amorphous cathode electrolyte interphase (CEI) has been in situ deposited at the interface, owing to its excellent structure compatibility and plasticity. [11][12][13] However, there is still a lack of systematic studies on how the amorphous CEI manipulates the cathode interface in a hybrid solid/liquid battery system. [14] Thus, it is of great significance to elaborate an amorphous buffer layer via an in situ conversion reaction, and further expound the interfacial stability mechanism in a system closer to practical application.…”

Anisotropically Electrochemical–Mechanical Evolution in Solid‐State Batteries and Interfacial Tailored Strategy

“…As the discharging process proceeds, volume expansion of FeS 2 particles occurs and the active FeS 2 particles begin to separate from the solid‐state electrolyte. It is well‐known that most conversion materials undergo huge volume expansion upon lithium‐ion insertion, which is responsible for the contact loss between FeS 2 and solid‐state electrolytes . Meanwhile, the heterogeneous phase transformation from different orientations also results in the sum of the axial strains within the FeS 2 particles, which further deteriorate the contact loss.…”