Lithium Superionic Conduction in BH₄‐Substituted Thiophosphate Solid Electrolytes

Abstract

Compared with conventional liquid electrolytes, solid electrolytes can better improve the safety properties and achieve high-energy-density Li-ion batteries. Sulfide-based solid electrolytes have attracted significant attention owing to their high ionic conductivities, which are comparable to those of their liquid counterparts. Among them, Li thiophosphates, including Li-argyrodites, are widely studied. In this study, Li thiophosphate solid electrolytes containing BH 4 − anions are prepared via a simple and fast milling method even without heat treatment. The synthesized materials exhibit a high ionic conductivity of up to 11 mS cm −1 at 25 °C, which is much higher than reported values. To elucidate the mechanism behind, the thiophosphate local structure, whose effect on the ionic conductivity remains unclear to date, is investigated. Raman and solid-state NMR spectroscopies are performed to identify the thiophosphate local structure in the sulfide samples. Based on the analysis results, the ratios of the different thiophosphate units in the prepared electrolyte samples are determined. It is found that the thiophosphate local structure can be varied by changing the amount of LiBH 4 and the milling conditions, which significantly impact the ionic conductivity. The all-solid-state cell with the prepared solid electrolyte exhibits superior cycle and rate performances.

Borohydride and halide dual-substituted lithium argyrodites

“…As LiBH 4 vigorously reacts with P 2 S 5 during milling, Li 2 S and P 2 S 5 were milled in the first step to form β-Li 3 PS 4 , and then LiBH 4 and LiX (X = Cl, Br, I) were added in the second step. As shown in a previous study, 19 milling with Li 2 S and P 2 S 5 produces β-Li 3 PS 4 , as confirmed by the XRD results in Fig. 1a.…”

“…18 Very recently, we have synthesized electrolytes by ball milling LiBH 4 and Li 3 PS 4 and obtained electrolytes with a high ionic conductivity of up to 11 mS cm −1 at 25 °C. 19 We also demonstrated that the changes in conductivity can be attributed to local structural variations caused by adopting different ball milling conditions. Inappropriate ball milling conditions can lead to the formation of non-stoichiometric thiophosphate units such as P 2 S 7 4− and P 2 S 6 4− , which negatively impacts the ionic conductivity.…”

“…21 The phase appearing in the XRD pattern was determined to be an argyrodite-type phase, and the diffraction peak shifted to a lower angle with increasing LiBH 4 , indicating that BH 4 − occupied 4a and/or 4d sites in the argyrodite structure. 15,18,19…”

“…S7, ESI†). Several theoretical studies 18,19,35,36 have been conducted to elucidate the possible mechanisms behind the enhanced ionic conductivity of BH 4 -substituted argyrodite. Fang et al 35 demonstrated through molecular dynamics (MD) simulations and density functional theory (DFT) calculations that BH 4 − substituted argyrodite can exhibit up to 177 mS cm −1 of ionic conductivity at room temperature.…”

“…Inappropriate ball milling conditions can lead to the formation of non-stoichiometric thiophosphate units such as P 2 S 7 4− and P 2 S 6 4− , which negatively impacts the ionic conductivity. 19…”