pH-manipulated large-scale synthesis of Na3(VOPO4)2F at low temperature for practical application in sodium ion batteries

Abstract

Na₃(VOPO₄)₂F (NVOPF) is an ideal cathode for high-energy-density sodium ion batteries (SIBs), due to its impressive output voltage and stable structure. However, the high production costs and complex operation processes associated with conventional synthetic methods impede its commercial applications. Herein, a simple but efficient method for large-scale low-cost synthesis of NVOPF at low temperatures is proposed. The optimal sample obtained at a suitable pH delivers a high discharge capacity of 119.6 mA h g⁻¹ at 0.2C and remarkable cycling performance with a decay rate of merely 0.0043% per cycle over 1400 cycles at 1C. Coin-type full cells composed of NVOPF and commercial hard carbon achieve a distinguished energy density of 426.2 W h kg⁻¹ calculated based on the cathode material. Most importantly, 2.1 A h soft-pack batteries with 100 mm in length and 60 mm in width were assembled based on the as-synthesized kilogram-scale NVOPF, which can stably cycle at 1C for 400 cycles with a capacity retention of 93.2% and an average coulombic efficiency of 99.96%. Moreover, the soft-pack battery exhibits fascinating performance in a wide temperature range, including superior rate capability at −20 °C (86.4% at 5C compared to 1C) as well as stable cycling performance at 50 °C (72.5% capacity retention after 400 cycles at 1C). This paper proposes a low-temperature co-precipitation method for synthesizing NVOPF, which significantly reduces the cost of production and contributes a solution for the industrial-scale production of NVOPF.