Enhanced structural stability in Mn–rich layered transition metal oxide positive electrode materials via germanium substitution

Manganese–rich layered oxides are among the most promising positive electrode candidates for sodium–ion batteries due to their abundant resources and relatively high theoretical specific capacity. However, conventional P2–type layered transition metal oxides suffer from structural instability, particularly at high voltages, resulting in poor capacity retention. Here, we investigate the effect of Ge substitution on the structural and electrochemical properties of Na_0.67Ni_0.33Mn_0.67O₂ at high voltage. X–ray and neutron diffraction analyses reveal that the P2 phase and honeycomb ordering are preserved after Ge⁴⁺ substitution. Meanwhile, the enlarged sodium layer spacing facilitates Na⁺ diffusion, enhancing ionic transport and transfer kinetics, which leads to improved rate capability. Particularly, Na_0.67Ni_0.33Mn_0.62Ge_0.05O₂ (NNMO–Ge_0.05) exhibits a high–rate capability and excellent cycling stability, retaining 81% of its capacity after 500 cycles at 150 mA g^–1 between 2 - 4.2 V, compared with the undoped sample (69%). Moreover, it also shows excellent electrochemical performance among recently reported NNMO–based materials. ²³Na solid–state nuclear magnetic resonance and operando synchrotron X–ray diffraction characterizations demonstrate that the enhanced performance arises from stabilization of the P2 phase and suppression of the detrimental O2 phase at high voltage. Soft X–ray TM L –edge absorption spectroscopy and O K –edge resonant inelastic X–ray scattering analyses suggest that charge compensation is predominantly governed by reversible Ni redox, with limited direct participation of lattice oxygen. This work provides a simple and effective strategy for improving the performance of Mn–rich layered transition metal oxide positive electrode materials through Ge substitution.