Li2OHX (X = Cl or Br) with an antiperovskite structure possess the advantages of low melting point, low cost, and ease of scaling‐up, which show great promise for applications in all‐solid‐state Li metal batteries (ASSLMBs). However, Li‐ion transport mechanisms in Li2OHX are still debated and the influence of H on the electrochemical performance of Li2OHX is yet to be explored. Herein, combining the theoretical calculations and experimental measurements, it is found that H affects Li‐ion transport, crystal stability, electrochemical stability, and electronic conductivity of Li2OHX. Compared with H‐free Li3OCl, although H helps to generate vacancy‐like defects, the electrostatic repulsive force between H and Li‐ion leads to an increase in both the activation energy and the diffusion length (space compensation effect), resulting in special Li ion transport trajectories along the Li‐O plane. Decreasing H content reduces the electronic conductivity and enhances the reduction‐resistant ability of Li2OHX, promoting the cycling stability and rate performance of Li∣Li2OHX∣Li symmetric cells and the ASSLMBs. This work delivers a new insight into the role of H in antiperovskite Li2OHX and can serve as guidance for solid electrolyte design.

Lithium-rich antiperovskites (LiRAPs) hold great promise to be the choice of solid-state electrolytes (SSEs) owing to their high ionic conductivity, low activation energy, and low cost. However, processing sheet-type solid-state Li metal batteries (SSLiB) with LiRAPs remains challenging due to the lack of robust techniques for battery processing. Herein, we propose a scalable slurry-based procedure to prepare a flexible composite electrolyte (CPE), in which LiRAP (e.g., Li2OHCl0.5Br0.5, LOCB) and nitrile butadiene rubber (NBR) serve as an active filler and as a polymer scaffold, respectively. The low-polar solvent helps to stabilize the LiRAP phase during slurry processing. It is found that the addition of LOCB into the NBR polymer enhances the Li ion conductivity for 2.3 times at 60°C and reduces the activation energy (max. 0.07 eV). The as-prepared LOCB/NBR CPE film exhibits an improved critical current of 0.4 mA cm−2 and can stably cycle for over 1000 h at 0.04 mA cm−2 under 60°C. In the SSLiB with the sheet-type configuration of LiFePO4(LFP)||LOCB/NBR CPE||Li, LFP exhibits a capacity of 137 mAh/g under 60 at 0.1°C. This work delivers an effective strategy for fabrication of LiRAP-based CPE film, advancing the LiRAP-family SSEs toward practical applications.