Engineering crystal-facet modulation to obtain stable Mn-based P2-layered oxide cathodes for sodium-ion batteries

The undesirable phase transformation of Mn-based P2-layered oxide cathodes is a tremendous challenge in commercializing Mn-based oxide cathodes for sodium-ion batteries. In this work, Na_0.67MnO₂ cathode with stable P2-type structure was successfully synthesized by modulating its coordination numbers to suppress the preferred orientation growth of (001) crystal plane, which was realized to maintain a stable P2-type structure in the whole state of charging and discharging. Specifically, designing Mn²⁺ six coordination sites to lower the high surface energy of (001) crystal plane is an effective way to reduce nucleation rates, which leads to the production of few grain boundaries and the suppression of layer-to-layer stacking in the crystal growth stage. Due to their fewer grain boundaries and skeleton structure with layer-to-layer stacking, the interlaminar stress and intragranular fatigue cracks can be alleviated in the long-life cycling performance of Na_0.67MnO₂ cathode. Na_0.67MnO₂ cathodes derived from the precursor of Mn²⁺ six coordination sites (C-Na_0.67MnO₂) have more exposed {010} crystal face and enlarged sodium-ion diffusion channels and structure integrity compared to Na_0.67MnO₂ cathode prepared by the precursor of Mn²⁺ four coordination sites (O-Na_0.67MnO₂). Therefore, C-Na_0.67MnO₂ cathode delivers an initial capacity of 106.8 mAh/g and has excellent capacity retention of 94.8 % after 150 cycles at 80 mAh/g. The rational design strategy endows Mn-based P2-layered oxide cathodes with stable sodium-ion diffusion channels and lamellar structure.