Review—Manganese-Based P2-Type Transition Metal Oxides as Sodium-Ion Battery Cathode Materials

Sodium transition metal oxides (NaMO2) with a P2 structure exhibit good Na+ ion conductivity and are promising sodium-ion battery cathode materials. Manganese-based compounds have a high working potential vs. Na+/Na, and high capacity. Yet, the layered nature of these materials means that they are prone to structural rearrangements at high voltage/low Na contents, the phase transformations and Na+ ion/vacancy ordering transitions resulting in capacity fade and poor reversibility. This review discusses the latest advances in the field and focuses mainly on recent work on NayMn1-xMxO2 (x, y ≤ 1, M = Ni, Mg, Li) compounds. We compare the different lithium and sodium transition metal layered oxides (P2, O3, etc.) and describe the structures and mechanisms observed on alkali (de)intercalation. The strategies used to enhance the electrochemical properties and stabilize the structural framework of sodium transition metal oxides are reviewed. We show how X-ray diffraction and 7Li/23Na solid-state Nuclear Magnetic Resonance can be combined to provide a detailed insight into the structural and electronic processes occurring upon electrochemical cycling of these materials.