Deciphering the function mechanism of high-valence tantalum doping in O3-types layered cathode for sodium-ion battery

O3-types layered cathode materials in sodium-ion batteries (SIBs) suffer from the obvious lattice distortion induced by the complex phase transitions during Na⁺ intercalation/deintercalation process, leading to severe structural collapse and performance degradation. Herein, a series of high valence tantalum (Ta⁵⁺) doped Na(Ni_0.4Fe_0.2Mn_0.4)_1−xTa_xO₂ (x = 0/0.0025/0.005/0.01) secondary spherical particles are firstly developed, where Ta⁵⁺ doping enables the refined primary grain with a tightly stacked rod-like morphology. Comprehensive structural analysis via Neutron powder diffraction (NPD) and Synchrotron radiation X-ray diffraction (SXRD) reveals an expanded NaO₂ slab and a reduction in Na site vacancy. The potential charge compensation mechanism is further illustrated by X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS), unveiling a partial reduction from Ni³⁺ to Ni²⁺ with Ta⁵⁺ doping. In situ X-ray diffraction (in situ XRD) suggests that the decorated sample undergoes a volume change as low as 0.8 %, in contrast with the pristine one (1.5 %). Thus, the optimized sample with x  = 0.005 retains an enhanced capacity retention up to 70.4 % at 1 C after 300 cycles in half-cell and delivers a high energy density of 251 Wh kg⁻¹ (0.1 C) and with a good capacity retention of 81.0 % at 1 C after 200 cycles in full-cell. Our findings provide new insights into the mechanism of high valence Ta⁵⁺ doping in stabilizing layered oxides cathode materials for SIBs.