Na_2/3Ni_1/3Mn_2/3O₂ 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.

研究了具有P2相的Na _2/3 Ni _1/3 Mn _2/3 O ₂作为钠离子电池的阴极材料。在半电池中，它在1.5-4.5 V范围内可提供228 mAh g ^-1的高放电容量，远高于172 mAh g ^-1的理论值。金属K边缘X射线吸收近边缘光谱结果表明，Mn离子在碱化/脱碱过程中保持4 +氧化态，电荷补偿归因于Ni ²⁺ / Ni ⁴⁺氧化还原。软X射线吸收光谱法的结果表明，带电电极的Ni离子从本体到表面的价态存在梯度，并且带电后O K边缘峰的积分强度发生变化，强烈暗示了部分电荷补偿发生在氧气位点。另外，在放电电极上观察到表面上的Mn离子的还原，这表明碳酸酯类电解质与正极材料反应，导致容量快速下降。通过利用离子液体（IL）电解质（Pyr ₁₄ TFSI中的1 M NaTFSI ）减少界面反应，可保持约200 mAh g ^-1的放电容量。