Sodium Stoichiometry Tuning of the Biphasic-Na x MnO 2 Cathode for High-Performance Sodium-Ion Batteries.

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.