Multifunctional and Radii-Matched High-Entropy Engineering Toward Locally-Regulable Metal Oxide Layers in...

Layered oxides, one of the most fascinating cathodes for sodium-ion batteries (SIBs), have appropriate voltage window and feasible preparation process, however, cycling stability is the biggest challenge. Element doping is the most rational strategy to address this problem, but six-coordinated octahedral radii and different radii in different valence states of these doping elements and the functions of these elements need to be taken into account. Hence, an example of P2/O3-type Na_0.7Mn_0.53Ni_0.26Fe_0.15Mg_0.01V_0.01Co_0.01Cu_0.01Zn_0.01Sn_0.01O₂ (high-entropy-doped layered oxides, HEO) has been designed in consideration of moderate six-coordinated octahedral radii and stabling the metal-oxygen bond. A reversible capacity of 126.9 mAh g^-1 can be achieved. Even tested at 1000 mA g^-1, an improved rate performance of 72.9 mAh g^-1 can be observed with a capacity retention rate of 66.5% after 1000 cycles. Potential-based in situ electrochemical impedance spectroscopy measurements and corresponding distribution of relaxation time profiles prove the effect of multiple elemental combination. Concomitantly, in situ XRD results reveal the P2/O3 biphasic clamping reaction mechanism of HEO. Density functional theory results reveal that the multielement doping can modify the localization of electrons and enhance the structural stability. This work provides an idea of designing HEO cathode for SIBs by crystal structure modulation.