Correlating Voltage Profile to Molecular Transformations in Ramsdellite MnO2 and Its Implication for Polymorph Engineering of Lithium Ion Battery Cathodes

Transition metal oxides are the primary choice for 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. MnO2 is the most extensively used cathode material for lithium ion battery, and it exists in more than ten polymorphic forms. Among them, T-MnO2 the most electrochemically active polymorph, is a mixture of pyrolusite and ramsdellite polymorphs. Here we have focused on the less explored ramsdellite MnO2 (R-MnO2). Highly crystalline R-MnO2 has been synthesized, and the experimentally obtained discharge features are compared with the calculations based on density functional theory followed by cluster expansion to obtain molecular insights into the intercalation process. R-MnO2 shows voltage fading beyond x = 0.5 (in LixMnO2). Computational results suggest that change in Li environment from tetrahedral to octahedral beyond x = 0.5 (in LixMnO2) causes the voltage fading. This change in Li environment is also correlated with experimentally obtained Mn 2p, O ls, and valence band XPS spectra. The shift in the d-band center, plotted for valence band spectra at different lithium concentrations, is adduced for the migration of lithium ion from the tetrahedral site to the less favorable octahedral site beyond x = 0.5. Volume expansion of about 20% at full lithiation is accompanied by Jahn-Teller distortion of MnO6. Further, computations reveal the diffusional energy barrier to be 200 meV in the limit of dilute Li concentration (x = 0.125) and 481 meV in the limit of dilute vacancy concentration (x = 0.875). A structural rationale for the energy barrier is developed. The experimental and computational studies provide insight into the lithiation mechanism in R-MnO2 and are relevant to rationalize the high performance of the intergrowth structure of r-MnO2. We discuss a wider implication of the study by correlating the voltage profile with structural changes in MnO2 polymorphs, which gives general design principles to make better cathodes through polymorphic engineering.