The Conversion Chemistry for High-Energy Cathodes of Rechargeable Sodium Batteries.

Herein, the Cu2P2O7/carbon-nanotube nanocomposite is reported as a cathode material based on a conversion reaction for rechargeable sodium batteries (RSBs). The nanocomposite electrode exhibits the large capacity of 355 mAh g-1, which is consistent with the 4 mol Na+ storage per formula unit determined by first-principles calculation. Its average operation voltage is approximately 2.4 V (vs Na+/Na). Even at 1800 mA g-1, a capacity of 223 mAh g-1 is maintained. Moreover, the composite electrode exhibits acceptable capacity retention of over 75% of the initial capacity for 300 cycles at 360 mA g-1. The overall conversion reaction mechanism on the Cu2P2O7/carbon-nanotube nanocomposite is determined to be Cu2P2O7 + 4Na+ + 4e- → 2Cu + Na4P2O7 based on operando/ex situ structural and physicochemical analyses. The high energy density of the Cu2P2O7/carbon-nanotube nanocomposite (720 Wh kg-1) supported by this conversion chemistry indicates a high possibility of application of this material as a promising cathode candidate for RSBs.