Development of Novel Cathode with Large Lithium Storage Mechanism Based on Pyrophosphate-Based Conversion Reaction for Rechargeable Lithium Batteries

A conversion-reaction-based nanosized Cu₂P₂O₇–carbon composite is investigated as a novel cathode material with superior capacity for lithium-ion batteries. To overcome the sluggish kinetics of the conversion reaction, the nanosized Cu₂P₂O₇–carbon composite is prepared by high-energy ball-milling of Cu₂P₂O₇ and conductive carbon to achieve simultaneous nanosizing and carbon mixing. The nanosized Cu₂P₂O₇–carbon composite exhibits a large specific capacity of ≈355 mAh g⁻¹ with an average operation voltage of ≈2.8 V (vs Li⁺/Li). Moreover, even at 10C (1C = 355 mA g⁻¹), the composite delivers a capacity of ≈215 mAh g⁻¹, corresponding to ≈60% of its theoretical capacity. For 400 cycles at 1C, the nanosized Cu₂P₂O₇–carbon composite exhibits capacity retention of ≈72% compared with the initial capacity as well as high Coulombic efficiency of more than 99%. The reversible conversion reaction mechanism of the nanosized Cu₂P₂O₇–carbon composite under the Li-cell system is confirmed using various techniques, including operando/ex situ X-ray diffraction, X-ray absorption near edge structure spectroscopy, extended X-ray absorption fine structure spectroscopy, and transmission electron microscopy. It is verified that Cu₂P₂O₇ is converted into Li₄P₂O₇ and metallic Cu⁰ on discharge and reversibly recovered to Cu₂P₂O₇ on charge.