Revisiting the Role of Hydrogen in Lithium‐Rich Antiperovskite Solid Electrolytes: New Insight in Lithium Ion and Hydrogen Dynamics

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. The relationship between H and electrochemical performance of solid electrolytes has yet to be explored. The present work clarifies that H strongly affects Li‐ion dynamics, crystal stability, electronic conductivity, and electrochemical stability of Li‐rich antiperovskite solid electrolytes, and thus the cycling and rate performance of the as‐assembled all‐solid‐state Li metal batteries.