Unveiling the Reactivity and the Li‐Ion Exchange at the PEO‐Li₆PS₅Cl Interphase: Insights from Solid‐State NMR

Li 6 PS 5 Cl (LPSCl) argyrodites offer high room temperature ionic conductivity (>1 mS cm −1 ) and are among the most promising solid electrolytes. However, their chemical instability against Li metal compromises the long‐term cyclability. Using PEO‐LiTFSI as an interlayer or as a matrix for composite electrolytes is a promising strategy to address this issue. Nevertheless, the interphase of PEO‐LiTFSI and LPSCl requires further detailed investigations. This work explores the interfacial reactions between these phases using solid‐state nuclear magnetic resonance. Results show that PEO facilitates the formation of a complex with LiCl and Li 3 PS 4 from LPSCl, resulting in an interphase material with limited local mobility, thus impeding ion transport. Although the addition of Br as a dopant can improve the ionic conductivity of LPSCl by inducing disorder and generating the Li vacancies, it makes the LPSCl more susceptible to PEO and increases the extent of the interfacial reaction. 6 Li– 6 Li EXSY experiments demonstrate spontaneous Li‐ion exchange between the PEO and the LPSCl, yet this exchange is significantly hindered by reaction products within the PEO‐LPSCl interphase, attributable to their sluggish local dynamics. This study sheds light on the complex interfacial interaction between PEO‐LiTFSI and sulfide argyrodite, providing insights into designing solid electrolytes for the new generation of electrochemical devices.