Mitigation of Jahn-Teller distortion and Na/vacancy ordering in a distorted manganese oxide cathode material by Li substitution.

Layered manganese-based oxides are promising candidates as cathode materials for sodium-ion batteries (SIBs) due to their low cost and high specific capacity. However, the Jahn-Teller distortion from high-spin Mn induces detrimental lattice strain and severe structural degradation during sodiation and desodiation. Herein, lithium is introduced to partially substitute manganese ions to form distorted P'2-NaLiMnO, which leads to restrained anisotropic change of Mn-O bond lengths and reinforced bond strength in the [MnO] octahedra by mitigation of Jahn-Teller distortion and contraction of MnO layers. This ensures the structural stability during charge and discharge of P'2-NaLiMnO and Na/vacancy disordering for facile Na diffusion in the Na layers with a low activation energy barrier of ∼0.53 eV. It exhibits a high specific capacity of 192.2 mA h g, good cycling stability (90.3% capacity retention after 100 cycles) and superior rate capability (118.5 mA h g at 1.0 A g), as well as smooth charge/discharge profiles. This strategy is effective to tune the crystal structure of layered oxide cathodes for SIBs with high performance.