# Improved Air Stability of Li Argyrodites Through PS4 3− Rotation Suppression by Al and Se Co‐Substitution for All‐Solid‐State Batteries

## TL;DR

A new Li argyrodite material with improved air stability and higher lithium conductivity is developed for use in all-solid-state batteries.

## Contribution

Al and Se co-substitution in Li argyrodites suppresses PS4 3− rotation, enhancing air stability and Li+ conductivity for solid-state batteries.

## Key findings

- Al/Se co-substituted Li6PS5Cl shows 4.91 mS cm−1 Li+ conductivity at 30°C with 22% reduction after dry-air exposure.
- Surface degradation is reduced due to suppressed rotation of P[S2SeO]3− and P[S2O2]3− tetrahedra.
- Improved electrochemical performance is observed in NCM|(Li-In) cells after air exposure.

## Abstract

Sulfide‐based solid electrolytes, particularly Li argyrodites, hold significant promise for practical all‐solid‐state batteries (ASSBs); however, their poor stability under humid conditions presents a critical challenge. Despite numerous efforts to address this issue, a comprehensive mechanistic understanding of moisture‐induced degradation remains limited. Herein, we introduce an Al and Se co‐substituted argyrodite, Li6‐3

x
Al
x
PS5‐1.5

x
Se1.5

x
Cl, which enhances both the Li+ conductivity and air stability. The optimized composition (x = 0.05) exhibits an improved Li+ conductivity of 4.91 mS cm−1 at 30 °C and a 22% conductivity reduction after dry‐air exposure (dew point: −40 °C for 5 h), compared with 3.71 mS cm−1 and a 42% decrease for the unsubstituted sample. Reduced surface degradation is validated by comprehensive experimental analyses. Complementary calculations indicate less favorable H2O adsorption and further reveal that Al and Se co‐substitution inhibits the rotation of P[S2SeO]3− and P[S2O2]3−; tetrahedra via preferential surface‐oriented Se2− and Al─O interactions, which otherwise promote H2O‐induced degradation, thereby minimizing moisture interactions. Finally, the improved electrochemical performance of the co‐substituted argyrodite is validated by its enhanced capacity retention following air exposure in NCM|Li6PS5Cl|(Li‐In) cells. This study highlights rotational dynamics as an overlooked mechanism underlying moisture‐induced degradation, and demonstrates that targeted co‐substitution is a viable strategy for advancing practical ASSBs.

Al/Se co‐substitution in Li6PS5Cl achieves enhanced dry‐air stability and improves Li+ conductivity, retaining 85% of its high Li+ conductivity (4.91 mS cm−1 at 30 °C) after dry‐air exposure (dew point −40 °C for 5 h). The stability enhancement is experimentally validated and underpinned by theoretical analyses indicating suppressed rotation of P[S2SeO]3−. Consequently, dry‐air‐exposed Al/Se‐substituted Li6PS5Cl enables NCM||(Li‐In) cells to deliver superior electrochemical performance.