In the search for novel solid electrolytes for solid-state batteries, thiophosphate ionic conductors have been in recent focus owing to their high ionic conductivities, which are believed to stem from a softer, more polarizable anion framework. Inspired by the oft-cited connection between a soft anion lattice and ionic transport, this work aims to provide evidence on how changing the polarizability of the anion sublattice in one structure affects ionic transport. Here, we systematically alter the anion framework polarizability of the superionic argyrodites LiPSX by controlling the fractional occupancy of the halide anions (X = Cl, Br, I). Ultrasonic speed of sound measurements are used to quantify the variation in the lattice stiffness and Debye frequencies. In combination with electrochemical impedance spectroscopy and neutron diffraction, these results show that the lattice softness has a striking influence on the ionic transport: the softer bonds lower the activation barrier and simultaneously decrease the prefactor of the moving ion. Due to the contradicting influence of these parameters on ionic conductivity, we find that it is necessary to tailor the lattice stiffness of materials in order to obtain an optimum ionic conductivity.

Solid-state batteries with inorganic solid electrolytes are currently being discussed as a more reliable and safer future alternative to the current lithium-ion battery technology. To compete with state-of-theart lithium-ion batteries, solid electrolytes with higher ionic conductivities are needed, especially if thick electrode configurations are to be used. In the search for optimized ionic conductors, the lithium argyrodites have attracted a lot of interest. Here, we systematically explore the influence of aliovalent substitution in Li 6+x P 1−x Ge x S 5 I using a combination of X-ray and neutron diffraction, as well as impedance spectroscopy and nuclear magnetic resonance. With increasing Ge content, an anion site disorder is induced and the activation barrier for ionic motion drops significantly, leading to the fastest lithium argyrodite so far with 5.4 ± 0.8 mS cm −1 in a cold-pressed state and 18.4 ± 2.7 mS cm −1 upon sintering. These high ionic conductivities allow for successful implementation within a thick-electrode solid-state battery that shows negligible capacity fade over 150 cycles. The observed changes in the activation barrier and changing site disorder provide an additional approach toward designing better performing solid electrolytes.

The local structure phase diagram of (Li2S)x(P2S5)100−x thiophosphates derived from pair distribution function crystallization experiments.

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...