Oxalate-Based High-Capacity Conversion Anode for Potassium Storage

Conversion anode materials have been applied in lithium and sodium secondary batteries owing to their high capacities; however, there are limited reports on their use in potassium-ion batteries. Herein, we introduce cobalt oxalate as a high-capacity anode material for potassium storage. Carbon nanotubes are encapsulated by long CoC2O4 sticks (∼5 μm in length) to establish a facile electron transport path, resulting in a specific charge (oxidation) capacity of 394 mAh g–1 (80 mA g–1, 0.2C) with a capacity retention of 73% over 200 cycles. Moreover, the composite electrode is active at a rate of 3C (1.2 A g–1), with a charge capacity of 161 mAh g–1. In situ X-ray diffraction, X-ray absorption spectroscopy, and time-of-flight secondary-ion mass-spectroscopy studies reveal the occurrence of the conversion reaction CoC2O4 + 2K+ + 2e– → Co + K2C2O4 on reduction, which reversibly occurs on oxidation. Microscopic studies demonstrate that the conversion reaction occurs on the carbon nanotubes where the CoC2O4 sticks are suited, indicating that the carbon nanotubes assist in facile electron transfer and enable the reversibility of the conversion reaction.