The transition metal oxides are extensively utilized as cathode materials in Lithium ion batteries. These oxides occur as different polymorphs that have varied structures, stabilities and affinities for Li⁺ and consequently, it is desirable to develop heuristics for the choice of the optimal polymorph. In order to address these issues, exploring MnO₂ can be useful as it has more than 10 polymorphic structures, besides being earth abundant inexpensive transition metal oxide. While some of these polymorphic structures have been studied in detail, we present analysis for less explored Ramsdellite MnO₂ (R-MnO₂) structure, which is part of inter growth structure of most electrochemically active polymorph ϒ-MnO₂. Highly crystalline R-MnO₂ have been synthesized and experimentally obtained discharge features are compared against density functional-cluster expansion calculations to obtain molecular insights into the intercalation process. The material shows voltage fading beyond Li concentration of 0.5 in R-MnO₂ and the computations suggests that changes in Li-coordination environment from tetrahedral to octahedral beyond x=0.5 is critical towards rationalizing trajectories in voltage fading. Voltage fading is also correlated to Mn 2p and O 1s XPS spectra and the modulation of experimentally obtained valence band spectra compares well with the spectra obtained computationally. Volume expansion of about ~20% at full lithiation is accompanied by Jahn-Teller distortion of MnO₆, which further rationalizes changes in the valence band-edge. Computations reveal a diffusional barrier of ~ 0.2 eV, which is lesser compared to the barriers in other polymorphs of MnO₂ and structural rationale for the smaller barrier is developed. Finally, the results are presented in a more general framework of other polymorphic structures of Cobalt Oxide and Nickel Oxide in light of their importance to the state of the art cathode materials for Lithium Ion batteries.

Keywords: R-MnO₂, Lithium ion battery, Cluster expansion, DFT, XPS, Jahn-Teller effect