High Rate Capability from a Graphite Anode through Surface Modification with Lithium Iodide for All-Solid-State Batteries

All-solid-state batteries (ASSBs) have been attracting attention as a potential paradigm for batteries in the future, as they are safer because they do not leak and are stable at high temperatures compared to lithium-ion batteries (LIBs) that use liquid electrolytes; further, the use of a bipolar structure is expected to improve energy density. For ASSBs, graphite is one of the most promising practical anode materials because of its superior power density in LIBs. However, the power density of ASSBs is unsatisfactory for practical applications and is lower than that of LIBs. One reason for this is the slow lithium ion transport at the interface between the graphite anode and solid electrolyte (SE). Because of the low redox potential for lithium ion intercalation into graphite (close to the lithium reduction potential), sulfide SEs undergo reductive decomposition, which impedes lithium ion transport at the interface with graphite. To address this problem, we attempted to coat LiI, which is stable at the lithium deposition potential, directly onto the graphite surface and examined the effect on the sulfide SE and electrochemical performance. The electrochemical measurements showed that the graphite composite without LiI showed a discharge capacity of 248 mA h g^–1, while that with 5 wt % LiI showed a relatively high discharge capacity of approximately 348 mA h g^–1. Impedance spectroscopy and S and P K-edge X-ray absorption spectroscopy indicated that the LiI-coated graphite composites displayed a stable interface behavior, in contrast to the uncoated graphite composite, after the lithiation process.