Discharging of Ramsdellite MnO₂ Cathode in a Lithium-Ion Battery

We report an application of our unbiased Monte Carlo approach to investigate thermodynamic and electrochemical properties of lithiated manganese oxide in the ramsdellite phase (R-MnO₂) to uncover the mechanism of lithium intercalation and understand charging/discharging of R-MnO₂ as a cathode material in lithium-ion batteries. The lithium intercalation reaction was computationally explored by modeling thermodynamically significant distributions of lithium and reduced manganese in the R-MnO₂ framework for a realistic range of lithium molar fractions 0 < <i>x</i> < 1 in Li _<i>x</i> MnO₂. We employed interatomic potentials and analyzed the thermodynamics of the resultant grand canonical ensemble. We found ordered or semiordered phases at <i>x</i> = 0.5 and 1.0 in Li _<i>x</i> MnO₂, verified by configurational entropy changes and simulated X-ray diffraction patterns of partially lithiated R-MnO₂. The radial distribution functions show the preference of lithium for homogeneous distributions across the one-dimensional 2 × 1 ramsdellite channels accompanied by alternating reduced/oxidized manganese ions. The occupation of the interstitial sites in the channels is correlated with the calculated voltage profile, showing a sharp voltage drop at <i>x</i> = 0.5, which is explained by the energy penalty of shifting lithium ions from stable tetrahedral to unstable octahedral sites. To facilitate this work, our in-house software, Knowledge Led Master Code (KLMC) was extended to support massive parallelism on high-performance computers.