Na₃V₂(PO₄)₃/C Positive Electrodes with High Energy and Power Densities for Sodium Secondary Batteries with Ionic Liquid Electrolytes That Operate across Wide Temperature Ranges

Abstract

Positive electrode materials with a wide operating temperature range and high energy and power densities are required for the construction of practical sodium secondary batteries. High performance is expected at elevated temperatures because electrode reactions and ion diffusion are enhanced under such conditions. In the present study, carbon‐coated Na superionic conductor‐type Na₃V₂(PO₄)₃ is prepared via a sol–gel method and investigated as a positive electrode material for sodium secondary batteries operating under low‐to‐intermediate temperature conditions. The Na₃V₂(PO₄)₃/C material utilizes fully the safety and large liquid‐phase temperature range of Na[bis(fluorosulfonyl)amide]–[1‐ethyl‐3‐methylimidazolium][bis(fluorosulfonyl)amide] ionic liquid electrolytes. Electrochemical testing of the Na₃V₂(PO₄)₃/C composite reveals superior cycling and rate performance at 253–363 K. Cycling tests at 1C (117 mA g⁻¹) confirm capacity retention of 99% and Coulombic efficiency of over 99.9% after 300 cycles at 298 and 363 K. Even faster‐current cycling at 20C results in capacity retention of 89.2% after 5000 cycles at 363 K. The high rate capability at 363 K (50.1 mA h g⁻¹ at 58500 mA g⁻¹) leads to excellent power and energy densities that exceed those of NaCrO₂ and Na₂FeP₂O₇.