Revealing the reversible mechanism of lithium hydride and its role in accelerating graphite anode failure

Abstract

The generation of LiH and its decomposition products such as H₂ pose safety concerns for lithium ion batteries. However, the reversible mechanism underlying LiH formation and its impact on battery cycling stability remain unclear. Here, we quantify the formation and evolution of LiH on practical graphite anodes during cycling. It is disclosed for the first time that reversible LiH formation/removal on the graphite anode during lithiation/delithiation even wherein there is no Li metal plating. By comparing LiH formed with chemically synthesizing LiC_x, theoretical simulations and a series of spectroscopic results, we propose a new formation mechanism of LiH which is induced by highly lithiated graphite such as LiC₆. For the reversibly formed LiH, the Li and H element comes from Li⁺ in graphite and H₂O adsorbed on graphite surface, respectively. In addition, we conducted the heating-mass spectrometry titration (H-MST) experiment to quantify its thermal stability at 60 °C, unveiling a dynamic equilibrium between LiH formation and decomposition, e.g. metallic Li resulting from LiH decomposition can re-intercalate into graphite, leading to the regeneration of LiH and further production of H₂ supposed with rich-sourced hydrogen species from the electrolytes. The continuous accumulation of H₂ poses a potential safety hazard for a long-term cycling of LIBs. We believe that this new discovery on LiH in this work provides a unique insight for the understanding and managing of thermal safety of LIBs caused by existences of hydrogen.