Sodium Stoichiometry Tuning of the Biphasic‐Na_xMnO₂ Cathode for High‐Performance Sodium‐Ion Batteries

Abstract Sodium‐ion batteries (SIBs) are promising alternatives for large‐scale energy storage owing to the rich resource and cost effectiveness. However, there are limitations of suitable low‐cost, high‐rate cathode materials for fast charging and high‐power delivery in grid systems. Herein, a biphasic tunnel/layered 0.80Na 0.44 MnO 2 /0.20Na 0.70 MnO 2 (80T/20L) cathode delivering exceptional rate performance through subtly regulating the sodium and manganese stoichiometry is reported. It delivers a reversible capacity of 87 mAh g −1 at 4 A g −1 (33 C), much higher than that of tunnel Na 0.44 MnO 2 (72 mAh g −1 ) and layered Na 0.70 MnO 2 (36 mAh g −1 ). It proves that the one‐pot synthesized 80T/20L is able to suppress the deactivation of L‐Na 0.70 MnO 2 under air‐exposure, which improves the specific capacity and cycling stability. Based on electrochemical kinetics analysis, the electrochemical storage of 80T/20L is mainly based on pseudocapacitive surface‐controlled process. The thick film of 80T/20L cathode (a single‐side mass loading over 10 mg cm −2 ) also has superior properties of pseudocapacitive response (over 83.5% at a low sweep rate of 1 mV s −1 ) and excellent rate performance. In this sense, the 80T/20L cathode with outstanding comprehensive performance could meet the requirements of high‐performance SIBs.