Green Synthesis of Cu3P to Achieve Low-Temperature and High Initial Coulombic Efficiency Sodium Ion Storage

Abstract

Conversion-type transition metal phosphides (TMPs) are competitive anode materials to overcome the volumetric energy density limits of hard carbon for sodium-ion batteries (SIBs). However, the application of TMPs is generally constrained by their low initial coulombic efficiency (ICE), unsatisfied cycling stability and poor low-temperature (LT) performance. Herein, a green synthesis method is reported to prepare carbon quantum dots modified Cu₃P nanoparticles anchored on carbon fibers (CF@Cu₃P-CQDs) as anode for high-energy and LT SIBs. It is disclosed that such a structure enables good interface contact between electrodes/electrolytes, thus prompting the formation of a uniformly fine solid electrolyte interphase and hence a record-high ICE of 93% with a volumetric capacity of 1343 mAh·cm⁻³. Distribution of relaxation time analysis unveils that the rapid Na⁺ transfer between electrode/electrolyte interfaces and Na⁺ diffusion ability in CF@Cu₃P-CQDs underlies the main reason for its high-rate capability (369–101 mAh·g⁻¹ @0.1-50 C) and LT performance (368/350 mAh·g⁻¹ @ 0.1C under −20/−40 °C). Promisingly, the CF@Cu₃P-CQDs are directly used toward three cathode materials (namely P2-type Na_0.78Ni_0.31Mn_0.67Nb_0.02O₂, carbon coated Na₃V₂(PO₄)₃, and low-cost Na₄Fe₃(PO₄)₂P₂O₇) without pre-sodiation process to assemble full-cells. This work sheds light on the fundamental understanding of electron/ion transfer kinetics of TMPs during de/sodiation and lays a foundation for the practical application of TMPs.