Multi-ion doped P2/O3 biphasic layered oxides cathodes NaxNi0.2Mn0.55Cu0.14TiyZnzO2 for sodium-ion batteries

Layered oxides are the most commonly used cathode materials in sodium ion batteries, typically presented in two structure variations: the sodium-poor P2 and the sodium-rich O3. The P2 structure is generally more stable in comparison to the O3 structure; however, it has lower capacity and initial coulombic efficiency owing to the low content of Na ⁺. Conversely, the O3 structure offers higher operating voltage and capacity, but it is not practical due to its structural instability. To amalgamate the properties of both structures, in our work, we prepared Na _xNi _0.2Mn _0.55Cu _0.14Ti _yZn _zO ₂ (x = 0.67,0.9,1, y + z = 0.11) as the cathode material by a straightforward sol-gel method. Among them, the specific capacity of Na _0.9Ni _0.2Mn _0.55Cu _0.14Ti _yZn _zO ₂ (P2/O3-NMCTZ) is 82.8 mAh·g ⁻¹ during the initial discharge cycle at 1 A·g ⁻¹. After 500 cycles, the specific capacity is maintained at 65.4 mAh·g ⁻¹ with a capacity retention rate of 78.9%. Besides, the material shows a strong pseudo-capacitance effect (89.72%, 90.67%, 93.14%, 93.51%, 94.98% and 96.86%). These results demonstrate the outstanding electrochemical performance of the multi-ion doped P2/O3 hybrid materials, providing new ideas for constructing layered oxides cathode for sodium-ion batteries.