Exploration and application of high entropy lithium argyrodites as solid electrolytes for all-solid-state batteries

Abstract

Solid-state batteries (SSBs) are a potentially safe, next-generation energy storage technology. Commercial viability of SSBs relies on the development of solid electrolytes with high ionic conductivity, high (electro)chemical stability, and good processability. A recent innovative approach to modify materials, potentially resulting in improved properties, is the “high-entropy” concept, characterized by ΔSconf > 1.5R (where ΔSconf and R represent the configurational entropy and ideal gas constant, respectively). However, the beneficial influence of a high configurational entropy on ion diffusion remains largely elusive given the absence of systematic studies. Therefore, this doctoral dissertation aimed to apply the high-entropy concept to solid electrolytes, in particular focusing on lithium argyrodites with the goal to achieve superionic conduction (~10 mS cm−1 at room temperature). The scientific objective was to understand the relationship between configurational entropy and charge-transport properties and to evaluate their potential as solid electrolytes for solid-state batteries. The first part of this thesis mainly focuses on altering configurational entropy via composition of lithium argyrodites by multiple cation and anion substitutions, with the general formula Li6+x[M1aM2bM3cM4d]S5I (M = P, Si, Ge, and Sb) as well as Li5.5PS4.5ClxBr1.5−x. Both strategies enabled ionic conductivities of more than 10 mS cm−1 at room temperature, owing to increased configurational entropy, i.e., occupational disorder. Then, the second part of this work presents a detailed electrochemical performance evaluation of high-entropy lithium argyrodites and the commercially available lithium argyrodite Li6PS5Cl. The outcome shows that solely multi-anion substituted lithium argyrodites possess an enhanced electrochemical stability as compared to the reference solid electrolyte Li6PS5Cl and thus lead to an increased solid-state battery performance, especially at high current rates. In contrast, multi-cation substituted lithium argyrodite solid electrolytes suffer from poor oxidative stability and thus might only be applicable as separator layer in all-solid-state batteries. Altogether, our results indicate the possibility of improving ionic conductivity and electrochemical stability in ceramic ion conductors via entropy engineering, i.e., inducing complex substitution, overcoming compositional limitations for the design of advanced electrolytes and opening up new avenues in the field.