The effect of Sn substitution on the structure and oxygen activity of Na0.67Ni0.33Mn0.67O2 cathode materials for sodium ion batteries

A series of Na0.67Ni0.33Mn0.67-xSnxO2 (x = 0, 0.01, 0.03, 0.05) materials with mixed P2/P3 phases are synthesized with a conventional solid-state reaction method and investigated as cathode materials for sodium ion batteries. The effects of Sn substitution on the structure and electrochemical performance of the Na0.67Ni0.33Mn0.67O2 are systematically investigated. The substituted samples show smaller particle sizes compared to the pristine one and the P2:P3 phase ratio highly depends on the substitution amount. The best electrochemical performance is obtained by Na0.67Ni0.33Mn0.66Sn0.01O2, and it delivers a discharge capacity of 245 mA h g−1 in 1.5–4.5 V (vs. Na|Na+), which is the highest result for Na0.67Ni0.33Mn0.67O2 materials reported so far. The ex situ X-ray absorption spectroscopy and X-ray photoelectron spectroscopy measurements reveal that the oxygen ions participate in the redox reactions within the wide voltage range of 1.5–4.5 V. The increased capacity can be attributed to the smaller particle size, which results in more oxygen activity and then higher capacity.