Improving Energy Density and Structural Stability of Manganese Oxide Cathodes for Na-Ion Batteries by Structural Lithium Substitution

We report excellent cycling performance for P2-Na_0.6Li_0.2Mn_0.8O₂, an auspicious cathode material for sodium-ion batteries. This material, which contains mainly Mn⁴⁺, exhibits a long voltage plateau on the first charge, similar to that of high-capacity lithium and manganese-rich metal oxides. Electrochemical measurements, X-ray diffraction, and elemental analysis of the cycled electrodes suggest an activation process that includes the extraction of lithium from the material. The "activated" material delivers a stable, high specific capacity up to ∼190 mAh/g after 100 cycles in the voltage window between 4.6-2.0 V versus Na/Na⁺. DFT calculations locate the energy states of oxygen atoms near the Fermi level, suggesting the possible contribution of oxide ions to the redox process. The addition of Li to the lattice improves structural stability compared to many previously reported sodiated transition-metal oxide electrode materials, by inhibiting the detrimental structural transformation ubiquitously observed with sodium manganese oxides during cycling. This research demonstrates the prospect of intercalation materials for Na-ion battery technology that are active based on both cationic and anionic redox moieties.