Synthesis Mechanisms of over-Stoichiometric Rocksalt-Type Li Superionic Conductors

Oxides materials with a face-centered cubic ( fcc ) anion sublattice are generally unsuitable for use as solid-state electrolytes as such structural framework typically exhibits high migration energy barrier, preventing a facile Li transport [1,2]. A recent discovery involving a class of over-stoichiometric rocksalt (ORX) compounds has shown promise for achieving fast Li conduction in oxide materials by introducing Li-Li face-sharing configurations in a spinel-like arrangement (s-phase) [3]. In general, it is challenging to create rocksalts in which the cation to anion ratio is larger than 1. As a result, the fast-conducting s-phase has been observed to be formed only in nano-sized domains that originate through a specific cooling process from a high-temperature disordered rocksalt. To further optimize the ORX-based Li superionic conductors, we conducted a systematic study combining first-principles calculations and experiments to understand the formation mechanism of the s-phase. Taking the reported ORX-Li 17 In 9 SnO 24 material as a prototype compound, we explored the detailed thermodynamic landscape in the Li-In-Sn-O (LISO) space, comparing the stability of several competing phases at different synthesis temperatures. Our insights into the synthesizability of s-phase have led to large improvements in Li conductivity in ORX-LISO compounds compared to the previous report [3]. In this presentation, we will present the key thermodynamic and kinetic mechanisms governing the formation of the unusual s-phase and discuss how a combination of equilibrium and non-equilibrium routes can be applied to synthesize a general ORX compound for solid-state electrolyte application. [1] Z. Zhang et al., Energy Environ . Sci . 11 , 1945–1976 (2018). [2] K. J. Kim, M. Balaish, M. Wadaguchi, L. Kong and J. L. M. Rupp, Adv . Energy Mater . 11 , 2002689 (2020). [3] Y. Chen, Z. Lun, X. Zhao et al., Nat . Mater . 23 , 535–542 (2024).