A cobalt-free high-entropy P2/O3 layered oxide cathode with suppressed oxygen redox for sodium-ion batteries

O3-type layered transition metal oxides are regarded as promising cathode candidates for sodium-ion batteries (SIBs) owing to their appealing properties and cost-effectiveness. Nevertheless, the complex phase transformations and sluggish sodium ion diffusion rate significantly hinder their cycling stability and rate performance. Here, a Co-free Mg-substituted high-entropy P2/O3-Na 0.7 Mn 0.41 Cu 0.1 Fe 0.1 Ti 0.09 Ni 0.25 Mg 0.05 O 2 biphasic layered oxide is designed to address these challenges. Ex-situ X-ray diffraction and X-ray photoelectron spectroscopy analyses demonstrate that Mg doping effectively restrains the detrimental phase evolution, which reduces the Mn 3+ content and its Jahn-Teller effect, resulting in improved Na + ions diffusion kinetics. Moreover, the replacement of Ni with Mg strengthens the TM-O bond, which in turn suppresses the oxygen redox reactions within the high voltage range and reduces the oxygen deficiencies. As a result, the as-prepared high-entropy biphasic cathode achieves a remarkable capacity of 138.7 mAh g −1 at a rate of 0.2C, and maintains a capacity of 103.8 mAh g −1 at 5C with 81.1 % capacity retention after 200 cycles. Notably, the full cell paired with hard carbon anode demonstrates superior capacity retention of 87.5 % over 200 cycles at 2C. This work broadens the correlation between high-entropy structure and performance, providing novel perspectives on the construction of advanced high-entropy cathodes for high-performance SIBs.