Ultrafast Charge-Discharge Capable and Long-Life Na3.9 Mn0.95 Zr0.05 V(PO4 )3 /C Cathode Material for Advanced Sodium-Ion Batteries.

ABSTRACT

Na4 MnV(PO4 )3 /C (NMVP) has been considered an attractive cathode for sodium-ion batteries with higher working voltage and lower cost than Na3 V2 (PO4 )3 /C. However, the poor intrinsic electronic conductivity and Jahn-Teller distortion caused by Mn3+ inhibit its practical application. In this work, the remarkable effects of Zr-substitution on prompting electronic and Na-ion conductivity and also structural stabilization are reported. The optimized Na3.9 Mn0.95 Zr0.05 V(PO4 )3 /C sample shows ultrafast charge-discharge capability with discharge capacities of 108.8, 103.1, 99.1, and 88.0 mAh g-1 at 0.2, 1, 20, and 50 C, respectively, which is the best result for cation substituted NMVP samples reported so far. This sample also shows excellent cycling stability with a capacity retention of 81.2% at 1 C after 500 cycles. XRD analyses confirm the introduction of Zr into the lattice structure which expands the lattice volume and facilitates the Na+ diffusion. First-principle calculation indicates that Zr modification reduces the band gap energy and leads to increased electronic conductivity. In situ XRD analyses confirm the same structure evolution mechanism of the Zr-modified sample as pristine NMVP, however the strong ZrO bond obviously stabilizes the structure framework that ensures long-term cycling stability.