Isothermal Microcalorimetry Analysis of Li/β-MnO₂ Discharge

Abstract

Despite widespread use over several decades, the lithium/manganese dioxide (Li/MnO2) discharge mechanism is not completely understood owing to the structural complexity of the material. However, an improved understanding could lead to broader adoption as a primary and even secondary cathode material. Here, we examine the discharge of single-phase β-MnO2 using isothermal microcalorimetry for the first time. Equilibrium voltage and entropy changes are characterized over the entire discharge range and used to rationalize the results. These measurements are supplemented by electrochemical impedance and X-ray diffraction data that give the clearest picture of the β-MnO2 lithiation process to date. We find that the first half of discharge is dominated by a two-phase reaction to form Li0.5MnO2 followed by single-phase insertion to a composition of Li1.0MnO2, which confirms prior first-principles calculations. The tetragonal β-MnO2 lattice undergoes asymmetric expansion from Jahn-Teller distorted Mn3+ to form an orthorhombic LiMnO2 phase which retains the 1 × 1 tunnel structure. Microcalorimetry results suggest the presence of parasitic reactions occurring during the second half of discharge, which could arise from decomposition of electrolyte or release and reaction of residual water retained in the structure.