Controllable synthesis of a Na-enriched NaV(PO) cathode for high-energy sodium-ion batteries: a redox-potential-matched chemical sodiation approach.

Exploring a sodium-enriched cathode ( NaV(PO), which differs from its traditional stoichiometric counterpart NaV(PO) that can provide extra endogenous sodium reserves to mitigate the irreversible capacity loss of the anode material ( 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 NaV(PO) precursor into Na-enriched NaV(PO). The spontaneous sodiation reaction enables a fast reduction of one-half V ions from V to V, followed by the insertion of one Na ion into the NASICON framework, which only takes 90 s to obtain the phase-pure NaV(PO) product. When paired with a hard carbon anode, the resulting NaVP‖HC full cell exhibits a high energy density of 251 W h kg, which is 58% higher than that of 159 W h kg for the NaVP‖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.