A high-voltage symmetric sodium ion battery using sodium vanadium pyrophosphate with superior power density and long lifespan

Sodium ion batteries have been considered as promising alternatives to lithium ion batteries for large-scale renewable energy and smart grids applications due to their low cost and rich resources. However, critical drawbacks such as low energy density and poor stability are hindering their development and application. In this work, a stable symmetric sodium ion cell using sodium vanadium pyrophosphate Na6·88V2·81(P2O7)4 as the positive and negative electrodes is fabricated. Since the bipolar Na6·88V2·81(P2O7)4 possesses high sodium-ion diffusion ability and stable structure framework, it demonstrates promising rate capability and cycling performance as both the positive and negative electrodes. The symmetric sodium ion cell, with Na6·88V2·81(P2O7)4 as the active material in both the positive and negative electrodes, exhibits a high operating voltage plateau of ≈3.0 V, distinct rate capability (e.g. 45 mAh g−1 at 10 C) and excellent cycling performance (e.g. 71.1% capacity retention after 1000 cycles at 2 C). The results of this work represent a step toward the development of symmetric sodium ion batteries with high operating voltage, good rate capability and long lifespan.