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.