In-situ X-ray studies of high-entropy layered oxide cathode for sodium-ion batteries

High-entropy oxides (HEOs) are promising electrode materials for sodium-ion batteries (SIBs) owing to their remarkable electrochemical properties and excellent cycling stability. A high-entropy O3-type layer-structured NaCu0.1Ni0.3Fe0.2Mn0.2Ti0.2O2 (NCNFMT) was synthesized using the Pechini method, as a new cathode material for SIBs. Herein, density functional theory calculations were used to understand the formation energy of NCNMFT, proving its synthesizability. Battery testing revealed a reversible capacity of 130 mAh g−1, with a capacity retention of up to 87% after 100 cycles at 0.1C, and excellent long-term cycling stability (approximately 71% after 500 cycles at 0.5C). The prepared material also exhibited an outstanding Coulombic efficiency of 98% and a high energy efficiency of 95%. Furthermore, the high entropy effect was investigated using energy dispersive spectroscopy and X-ray absorption fine structure to prove that the material contains a single phase with well-distributed metal ions. The operando synchrotron X-ray diffraction demonstrated that the phase transition was delayed until the extraction of 0.32 sodium ion from NCNFMT, which indicates the stabilization of the O3-phase induced by the high entropy effect. Furthermore, the quasi-solid-state NCNFMT||polyethylene oxide-Na3.2Ca0.1Zr1.9Si2PO12||Na cells exhibited excellent performance with a high discharge capacity of 112 mAh g−1 and retention of 78% after 100 cycles. The above results demonstrate the advantages of HEOs as potential cathode materials for SIBs.