Engineering crystal-facet modulation to obtain stable Mn-based P 2-layered oxide cathodes for sodium-ion batteries

The undesirable phase transformation of Mn-based P 2-layered oxide cathodes is a tremendous challenge in commercializing Mn-based oxide cathodes for sodium-ion batteries. In this work, Na 0.67 MnO 2 cathode with stable P 2-type structure was successfully synthesized by modulating its coordination numbers to suppress the preferred orientation growth of (0 0 1) crystal plane, which was realized to maintain a stable P 2-type structure in the whole state of charging and discharging. Specifically, designing Mn 2+ six coordination sites to lower the high surface energy of (0 0 1) 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 0.67 MnO 2 cathode. Na 0.67 MnO 2 cathodes derived from the precursor of Mn 2+ six coordination sites (C-Na 0.67 MnO 2 ) have more exposed {0 1 0} crystal face and enlarged sodium-ion diffusion channels and structure integrity compared to Na 0.67 MnO 2 cathode prepared by the precursor of Mn 2+ four coordination sites (O-Na 0.67 MnO 2 ). Therefore, C-Na 0.67 MnO 2 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 P 2-layered oxide cathodes with stable sodium-ion diffusion channels and lamellar structure.