Microstructure evolution and self‐discharge degradation mechanism in Li/MnO ₂ primary batteries

Abstract

Li/MnO 2 primary batteries are widely used in industry for their high specific capacity and safety. However, a deep comprehension of the Li + insertion mechanism and the high self‐discharge rate of the batteries is still needed. Here, the storage mechanism of Li + in the tunnel structure of MnO 2 as well as the dissolution and migration of Mn‐ions were investigated based on multi‐scale approaches. The Li/Mn ratio (at%) is determined at about 0.82 when the discharge voltage decreases to 2 V. The limited Li‐ions transport rate in the bulk MnO 2 restrains the reduction reaction, resulting in a low practical specific capacity. Moreover, utilizing spherical aberration‐corrected transmission electron microscopy (TEM) coupled with electron energy loss spectroscopy (EELS), the presence of a mixed valence state layer of Mn 2+ /Mn 3+ /Mn 4+ on the surface of the original 20 nm MnO 2 particles was identified, which could contribute to the initial dissolution of Mn‐ions. The battery separator exhibited channels for Mn‐ions migration and diffusion and aggregated Mn particles. We put forward the discharge and degradation route in the ways of Mn‐ions trajectories, and our findings provide a deep understanding of the high self‐discharge rates and the capacity decay of Li‐Mn primary batteries.