Green and Scalable Synthesis of Na3V2(PO4)3 Cathode and the Study on the Failure Mechanism of Sodium-Ion Batteries

Na3V2(PO4)3 material has been extensively studied and modified, but there are few studies to evaluate its performance in the full-cell system with hard carbon as the anode and analyze the failure mechanism. In this work, a green and scalable synthesis method is successfully developed to prepare more than 1 kg of Na3V2(PO4)3 with good processability and electrochemical performance (initial discharge capacity of 110.9 mA h g–1 with the initial Coulombic efficiency of 98.7%, the capacity retention rate of 84.45% at 10 C after 5000 cycles, and the rate capability of 80.7 mA h g–1 at 50 C). The full-cell containing Na3V2(PO4)3 and hard carbon shows a working voltage of 3.3 V with an energy density of 225.3 W h Kg–1 (based on electrodes weight). The failure mechanism of full-cell and Na3V2(PO4)3 is the loss of active sodium ions. The "sodium deficiency" state of the waste cathode can neither damage the crystal structure nor can it significantly increase the cathode impedance and deteriorate the Na+ diffusion dynamic conditions. Finally, replenishing sodium can regenerate the waste Na3V2(PO4)3, indicating that the "pre-sodization" and "replenishing sodium" are the key ways to prolong the cycle life of full-cells and to recover the spent sodium-ion batteries.