Insights into the Electrochemical Stability and Lithium Conductivity of Li₄MS₄ (M = Si, Ge, and Sn)

Abstract

The design of solid electrolytes with a wide electrochemical stability window and high Li-ion conductivity is a prerequisite for the realization of all solid-state Li batteries, which promises to enable extraordinary levels of safety for the battery system and may potentially revolutionize the energy storage field. Among all the promising inorganic solid electrolytes, Li4MS4 (M = Si, Ge, and Sn) with a crystal structure of Pnma symmetry have recently been recommended due to their good air stability and ionic conductivity. Here, we employ ab initio simulations to conduct a systematic investigation of the electrochemical stability and Li conductivity of Li4MS4 compounds. Our computation results reveal that the edge-sharing and face-sharing characteristics of LiS4 and LiS6 polyhedra not only facilitate the formation of a percolating Li diffusion network but would severely destabilize the crystal structure as well, thus resulting in a rather narrow electrochemical window. Although the stronger M–S bonds manifested in Li4SiS4 can benefit the overall stability, unfortunately, it also contributes to a more rugged energy landscape that inhibits Li diffusion. Li4SnS4 with a less densely packed lattice exhibits a substantially lower energy for Li ions to be accommodated at interstitial sites, which is a trigger for high Li conductivity in the bulk material, reaching 11 mS/cm at room temperature. The fast Li diffusion occurs through the concerted migration of multiple Li ions at lattice and interstitial sites. These findings open up new possibilities for the rational design of Li4MS4 (M = Si, Ge, and Sn) solid electrolytes for next-generation Li batteries.

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

“…Likewise, the fractions of all three silicon compounds decrease with each deposition step as they get reduced as well as superimposed by the deposited lithium and formed Li 2 S. It is noticeable that no Si 0 (99.5 eV) or Li–Si alloy (≤97.5 eV) was detected during the reaction, which could be due to the detection limit of the experimental setup. Another reason could be the higher stability of the SiS 4 4– tetrahedron compared to the PS 4 3– tetrahedron , so that the phosphorus tetrahedra are reduced first and the silicon tetrahedra remain stable, which is also predicted by the DFT calculations presented below.…”

“…No practical stability windows have yet been reported for SEs containing silicon. While PS 4 3– is always reduced at 1.71 V vs Li + /Li in theoretical calculations independent of the SE it is contained in, − it is reasonable to assume that the onset of reduction of SiS 4 4– is independent of the SEs and starts at lower potentials than for the PS 4 3– or GeS 4 4– ion, as predicted in theoretical works. , …”

“…Theoretical (thermodynamic) stability windows have been calculated for a range of SEs including β-Li 3 PS 4 (LPS), Li 6 PS 5 Cl (LPSCl), and LGPS using density functional theory (DFT) calculations to determine which phases are thermodynamically most stable. − These studies found that thiophosphate SEs decompose already at a potential of 1.71 V vs Li + /Li due to the reduction of P 5+ ions to P 0 . Upon decreasing the potential further, P 0 is reduced forming various Li–P phases until Li 3 P is formed at 0.87 V vs Li + /Li. − If the SE does not contain phosphorus, the stability window is shifted to different reduction potentials. , The reduction of Li 4 GeS 4 or Li 4 SiS 4 , for example, starts at 1.65 V or 1.38 V vs Li + /Li, respectively, indicating that the GeS 4 4– and SiS 4 4– tetrahedra are more stable against reduction than the PS 4 3– anions. The reduction of the anions is accompanied by the formation of Li 2 S in all electrolytes. − The t-Li 7 SiPS 8 studied in this paper can be considered a solid solution of Li 3 PS 4 and Li 4 SiS 4 .…”

“…Upon decreasing the potential further, P 0 is reduced forming various Li–P phases until Li 3 P is formed at 0.87 V vs Li + /Li. − If the SE does not contain phosphorus, the stability window is shifted to different reduction potentials. , The reduction of Li 4 GeS 4 or Li 4 SiS 4 , for example, starts at 1.65 V or 1.38 V vs Li + /Li, respectively, indicating that the GeS 4 4– and SiS 4 4– tetrahedra are more stable against reduction than the PS 4 3– anions. The reduction of the anions is accompanied by the formation of Li 2 S in all electrolytes. − The t-Li 7 SiPS 8 studied in this paper can be considered a solid solution of Li 3 PS 4 and Li 4 SiS 4 . It contains two complex anions with slightly different reduction potentials.…”

Recent Advances and Strategies toward Polysulfides Shuttle Inhibition for High‐Performance Li–S Batteries

“…While the M 4+ metal ions show instability against Li anode, a stable interlayer is needed between Li metal and such SSEs. [131] Wang et al infused LITFSI and LiNO 3 into glass fiber to produce solid-state plastic crystal electrolytes (PCEs). [132] The PCEs could work as an interlayer to suppress the reaction between Li anode and sulfide-based SSEs.…”