Na3VMn0.5Ti0.5(PO4)3/C with active Na+ hopping sites for high-rate and durable sodium-ion batteries

Abstract

This study established the carbon-coated Na3VMn0.5Ti0.5(PO4)3/C (NVMTP/C) as a high-energy cathode material of the NASICON family for next-generation sodium-ion batteries (SIBs). When subjected to electrochemical sodium storage testing, the NVMTP/C cathode provided a stable specific capacity of 160.3 mAh g-1 with three electron reactions (average working voltage ∼3.4 V) during the Na+ (de)insertion routine, achieving a superior energy output of 544 Wh kg−1 among the NASICON family of SIBs. Benefiting from the characteristic NASICON structure and established multiple redox couples, the cathode delivered excellent rate capability (82 mAh g-1 at 17 C, 1 C = 176 mAh g-1), in addition to excellent cycling permanence at low and high current surges (104 mAh g-1 after 500 cycles at 0.85 C and 84 mAh g-1 after 3000 cycles at 11.34 C, respectively). Computational studies based on first principle calculations and density of states (DoS) attributed the superior electrochemical properties of NVMTP/C cathode to its substantial Na+ hopping sites facilitating the unhindered multi-electron transfer reactions (V2+/3+, V3+/4+, Ti3+/4+, and V4+/5+) reversibly at their respective redox potentials. The detailed DoS analysis revealed that the varying electronic configuration due to the co-swapping of Ti and Mn significantly uplifts the electronic conductivity of the NVMTP/C cathode. Further calculation studies clearly indicate lower activation energy for active Na+ hopping and hence faster Na-diffusion thus suggesting that the present NVMTP/C cathode can have definite commercial prospects.