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

Thiophosphate solid electrolytes containing metalloid ions such as silicon or 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 achieve specific energies competitive with liquid lithium-ion batteries (LIBs), it is also well known that most of the metalloid ions used in promising thiophosphate solid electrolytes are reduced in contact with an LMA. This reduction reaction and its products formed at the solid electrolyte|LMA interface can compromise the performance of an SSB due to impedance growth. To study the reduction of these metalloid ions and their impact more closely, we used the recently synthesized Li7SiPS8 as a member of the tetragonal Li10GeP2S12 (LGPS) family. Stripping/plating experiments and the temporal evolution of the impedance of symmetric Li|Li7SiPS8|Li transference cells show a severe increase in cell resistance. We characterize the reduction of Li7SiPS8 after lithium deposition with in situ X-ray photoelectron spectroscopy, time-of-flight secondary-ion mass spectrometry, and solid-state nuclear magnetic resonance spectroscopy. The results indicate a continuous reaction without the formation of elemental silicon. For elucidating the reaction pathways, density functional theory calculations are conducted followed by ab initio molecular dynamics simulations to study the interface evolution at finite temperature. The resulting electronic density of states confirms that no elemental silicon is formed during the decomposition. Our study reveals that Li7SiPS8 cannot be used in direct contact with the LMA, even though it is a promising candidate as both a separator and a catholyte material in SSBs.