Microstructure evolution and self-discharge degradation mechanism in Li/MnO₂ primary batteries

Li/MnO₂ 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 MnO₂ 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 MnO₂ 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 Mn²⁺/Mn³⁺/Mn⁴⁺ on the surface of the original 20 nm MnO₂ 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.