Controllable synthesis of a Na-enriched Na4V2(PO4)3 cathode for high-energy sodium-ion batteries: a redox-potential-matched chemical sodiation approach

Exploring a sodium-enriched cathode (i.e. Na₄V₂(PO₄)₃, which differs from its traditional stoichiometric counterpart Na₃V₂(PO₄)₃ that can provide extra endogenous sodium reserves to mitigate the irreversible capacity loss of the anode material (i.e. hard carbon), is an intriguing presodiation method for the development of high energy sodium-ion batteries. To meet this challenge, herein, we first propose a redox-potential-matched chemical sodiation approach, utilizing phenazine-sodium (PNZ-Na) as the optimal reagent to sodiate the Na₃V₂(PO₄)₃ precursor into Na-enriched Na₄V₂(PO₄)₃. The spontaneous sodiation reaction enables a fast reduction of one-half V ions from V³⁺ to V²⁺, followed by the insertion of one Na⁺ ion into the NASICON framework, which only takes 90 s to obtain the phase-pure Na₄V₂(PO₄)₃ product. When paired with a hard carbon anode, the resulting Na₄VP‖HC full cell exhibits a high energy density of 251 W h kg^-1, which is 58% higher than that of 159 W h kg^-1 for the Na₃VP‖HC control cell. Our chemical sodiation methodology provides an innovative approach for designing sodium-rich cathode materials and could serve as an impetus to the development of advanced sodium-ion batteries.