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 Na0.78Ni0.31Mn0.67Nb0.02O2, 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.