Design of fluorine-substituted high-entropy phosphates as cathode materials towards high-performance Na-ion batteries

Abstract

Phosphate cathodes have been considered a potential choice for Na-ion batteries owing to their elevated thermal stability and appropriate working voltage. However, inherently slow diffusion kinetics and inadequate conductivity significantly result in their weak rate performance and cycling stability. To solve these problems, high-entropy (HE) Na_3.4(Fe_0.2Mn_0.2V_0.2Cr_0.2Ti_0.2)₂(PO₄)_3−xF_x (x = 0.02, 0.04 and 0.06) cathode materials were constructed using the fluorine-substitution strategy to enhance the kinetics, leading to good cycling stability. Fluorine doping improves the structural stability and Na-ion insertion/extraction reversibility, thus facilitating the transfer of sodium ions. Due to the rational design of the composition, Na_3.4(Fe_0.2Mn_0.2V_0.2Cr_0.2Ti_0.2)₂(PO₄)_2.98F_0.02 exhibits excellent rate performance with a reversible discharge capacity of 160.3, 140.5, 110.9, 97.6, 78.7, and 70.1 mA h g⁻¹ at 0.05, 0.1, 0.5, 1, 3, and 5C, respectively. Even at a high current density of 20C, the initial discharge capacity of the best sample is as high as 95.5 mA h g⁻¹, and it retains relatively high discharge specific capacity (63.0 mA h g⁻¹), with a capacity retention rate of 66% after 1000 cycles. This work confirms that F doping effectively enhances the reversible capacity of high-entropy cathodes under high current density and provides a viable approach for constructing high-performance HE phosphate cathode materials for sodium-ion batteries.