Controllable synthesis of a Na-enriched Na 4 V 2 (PO 4 ) 3 cathode for high-energy sodium-ion batteries: a redox-potential-matched chemical sodiation approach.

Exploring a sodium-enriched cathode ( i.e. Na 4 V 2 (PO 4 ) 3 , which differs from its traditional stoichiometric counterpart Na 3 V 2 (PO 4 ) 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 Na 3 V 2 (PO 4 ) 3 precursor into Na-enriched Na 4 V 2 (PO 4 ) 3 . The spontaneous sodiation reaction enables a fast reduction of one-half V ions from V 3+ to V 2+ , followed by the insertion of one Na + ion into the NASICON framework, which only takes 90 s to obtain the phase-pure Na 4 V 2 (PO 4 ) 3 product. When paired with a hard carbon anode, the resulting Na 4 VP‖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 Na 3 VP‖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.