A high-capacity P2 Na2/3Ni1/3Mn2/3O2 cathode material for sodium ion batteries with oxygen activity

Na2/3Ni1/3Mn2/3O2 with a P2 phase is investigated as a cathod material for sodium ion batteries. It delivers a high discharge capacity of 228 mAh g−1 within 1.5–4.5 V in half cells, which is much higher than the theoretical value of 172 mAh g−1. Metal K-edge X-ray absorption near edge spectroscopy results show that the Mn ions remain in 4 + oxidation state during sodiation/desodiation and the charge compensation is due to the Ni2+/Ni4+ redox. Soft X-ray absorption spectroscopy results reveals a gradient in the valence state of Ni ions from bulk to surface for the charged electrode, and a change in the integrated intensity of O K-edge peak after charging, strongly suggesting that part of the charge compensation takes place at the oxygen sites. In addition, the reduction of Mn ions on the surface is observed on the discharged electrode, which indicates that the carbonate-based electrolyte reacts with the cathode material, resulting in a fast capacity drop. By utilizing an ionic liquid (IL) electrolyte (1 M NaTFSI in Pyr14TFSI) to reduce the interfacial reactions, the discharge capacity of ∼200 mAh g−1 is retained.