A New Lithium Thioborate–Lithium Iodide Solid-State Electrolyte with High Ionic Conductivity for Lithium Metal Batteries

In the pursuit of high-performance solid-state batteries (SSBs), which can have excellent safety and energy density, it is critical to understand the cyclability of emerging classes of solid-state electrolytes (SSEs). Lithium thioborates (LBS) are an understudied class of materials with promising applications in SSBs with lithium metal anodes. We investigate the electrochemistry, structure, and cyclability of the LBS SSE with stoichiometric Li 10 B 10 S 20 with lithium iodide (LiI) as an additive. LBS-LiI exhibits an outstanding ionic conductivity of 1.0 mS cm –1 due to increased LBS crystallinity and favorable modification of the LBS grain boundaries with LiI. LiI improves the cycling stability against lithium metal anodes, limits dendrite growth with a high critical current density of 2.0 mA cm –2, and cycles well in cells with LiNi 0.6 Mn 0.2 Co 0.2 O 2 (NMC) 622 cathodes and indium anodes. Our work highlights LiI as a grain boundary modification in LBS solid electrolytes and demonstrates the promising cyclability of LBS-LiI in SSBs.