Intrinsic pseudocapacitive Na0.44MnO2 prepared by novel ion-exchange method for high-rate and robust sodium-ion storage

Sodium-ion batteries are promising new-generation energy storage devices due to the low cost and rich resource of sodium. Among various cathodes, tunnel-type Na0.44MnO2 with large S-shaped Na+ transport tunnels is considered an appropriate cathode for fast-charging batteries, yet still suffering from sluggish Na+ kinetics. Herein, a novel ion-exchange method is developed for the first time to synthesize Na0.44MnO2 from K0.5MnO2 precursor. By precisely adjusting the synthesis condition, the amount of residual K+ and the size of Na0.44MnO2 are well controlled. The presence of trace K+ in the Na0.44MnO2 structure is demonstrated to broaden the Na+ transport tunnels while the minimized size shortens the Na+ migration distance. Besides, band-like defects with distorted polyhedrons further promote Na+ transport. The as-prepared Na0.44MnO2 with intrinsic pseudo-capacitance characteristic exhibits excellent rate performance of 79.0 mA h g(-1) at 20 C between 2 and 4 V. Long-term cycling tests present superior stability of 98.1% retention after 1000 cycles at 20 C and 96.3% retention after 200 cycles at 0.5 C. This work provides a novel way for large-scale production of high-rate and robust Na0.44MnO2 cathode for fast-charging energy storage devices.