Discharging\nof Ramsdellite MnO<sub>2</sub> Cathode\nin a Lithium-Ion Battery

We report an application of our unbiased Monte Carlo\napproach to\ninvestigate thermodynamic and electrochemical properties of lithiated\nmanganese oxide in the ramsdellite phase (R-MnO<sub>2</sub>) to uncover\nthe mechanism of lithium intercalation and understand charging/discharging\nof R-MnO<sub>2</sub> as a cathode material in lithium-ion batteries.\nThe lithium intercalation reaction was computationally explored by\nmodeling thermodynamically significant distributions of lithium and\nreduced manganese in the R-MnO<sub>2</sub> framework for a realistic\nrange of lithium molar fractions 0 < <i>x</i> < 1\nin Li<sub><i>x</i></sub>MnO<sub>2</sub>. We employed interatomic\npotentials and analyzed the thermodynamics of the resultant grand\ncanonical ensemble. We found ordered or semiordered phases at <i>x</i> = 0.5 and 1.0 in Li<sub><i>x</i></sub>MnO<sub>2</sub>, verified by configurational entropy changes and simulated\nX-ray diffraction patterns of partially lithiated R-MnO<sub>2</sub>. The radial distribution functions show the preference of lithium\nfor homogeneous distributions across the one-dimensional 2 ×\n1 ramsdellite channels accompanied by alternating reduced/oxidized\nmanganese ions. The occupation of the interstitial sites in the channels\nis correlated with the calculated voltage profile, showing a sharp\nvoltage drop at <i>x</i> = 0.5, which is explained by the\nenergy penalty of shifting lithium ions from stable tetrahedral to\nunstable octahedral sites. To facilitate this work, our in-house software,\nKnowledge Led Master Code (KLMC) was extended to support massive parallelism\non high-performance computers.