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

Abstract

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.