Instability of the Li7SiPS8 Solid Electrolyte at the Lithium Metal Anode and Interphase Formation

Thiophosphate solid electrolytes containing metalloid ions such as siliconor germanium show a very high lithium-ion conductivity and the potential to enable solid-state batteries (SSBs). While the lithium metal anode (LMA) is necessary to achievespecific energies competitive with liquid lithium-ion batteries (LIBs), it is also well knownthat most of the metalloid ions used in promising thiophosphate solid electrolytes arereduced in contact with an LMA. This reduction reaction and its products formed at thesolid electrolyte|LMA interface can compromise the performance of an SSB due toimpedance growth. To study the reduction of these metalloid ions and their impact moreclosely, we used the recently synthesized Li7SiPS8as a member of the tetragonalLi10GeP2S12(LGPS) family. Stripping/plating experiments and the temporal evolution of the impedance of symmetric Li|Li7SiPS8|Litransference cells show a severe increase in cell resistance. We characterize the reduction of Li7SiPS8after lithium deposition with insitu X-ray photoelectron spectroscopy, time-of-flight secondary-ion mass spectrometry, and solid-state nuclear magnetic resonancespectroscopy. The results indicate a continuous reaction without the formation of elemental silicon. For elucidating the reactionpathways, density functional theory calculations are conducted followed by ab initio molecular dynamics simulations to study theinterface evolution atfinite temperature. The resulting electronic density of states confirms that no elemental silicon is formed duringthe decomposition. Our study reveals that Li7SiPS8cannot be used in direct contact with the LMA, even though it is a promisingcandidate as both a separator and a catholyte material in SSBs