Enhanced electrochemical performance of NASICON-type sodium ion cathode based on charge balance theory

The variable components and relatively high Na⁺ intercalation voltage makes sodium ion superconductors (NASICON) as ideal cathode materials for secondary sodium-ion batteries. Na₃V₂(PO₄)₃, as one of the most typical NASICON-type cathode, needs urgent improvement to enable its wide application. Based on charge balance theory, via utilizing Al³⁺and SiO₄⁴⁻ to replace V³⁺ and PO₄³⁻ in Na₃V₂(PO₄)₃, respectively, Na_3+xV_1.5Al_0.5(PO₄)_3-x(SiO₄)_x (0 ≤ x ≤ 0.5) cathode materials were prepared in this work. In comparison with the initial state, the operation potential was apparently promoted because of the reversible access of V⁴⁺/V⁵⁺ redox couple at 4.0 V vs. Na⁺/Na; the higher specific capacity can be achieved owing to much more conductive Na⁺ involved in the redox reaction of V species. Furthermore, the structural stability and diffusion kinetics is dramatically enhanced, which are further validated by ex-situ XRD and GITT analysis. Specifically, Na_3.3V_1.5Al_0.5(PO₄)_2.7(SiO₄)_0.3 can offer highly reversible capacities of 127.3 and 181.5 mAh g⁻¹ within the region of 2.5–4.4 V and 1.4–4.4 V, respectively, making itself a promising sodium ion cathode material. Na-ion full cells paired with Na_3.3V_1.5Al_0.5(PO₄)_2.7(SiO₄)_0.3 cathode and hard carbon anode are assembled, rendering a specific capacity of 154.3 mAh g⁻¹ at 0.5 C based on the mass of cathode.