Enhancing the Lithium Storage Performance of the Nb₂O₅ Anode via Synergistic Engineering of Phase and Cu Doping

Nb₂O₅ has been viewed as a promising anode material for lithium-ion batteries by virtue of its appropriate redox potential and high theoretical capacity. However, it suffers from poor electric conductivity and low ion diffusivity. Herein, we demonstrate the controllable fabrication of Cu-doped Nb₂O₅ with orthorhombic (T-Nb₂O₅) and monoclinic (H-Nb₂O₅) phases through annealing the solvothermally presynthesized Nb₂O₅ precursor under different temperatures in air, and the Cu doping amount can be readily controlled by the concentration of the precursor solution, whose effect on the lithium storage behaviors of the Cu-doped Nb₂O₅ is thoroughly investigated. H-Nb₂O₅ shows obvious redox peaks (Nb⁵⁺/Nb⁴⁺ and Nb⁴⁺/Nb³⁺) with much higher capacity and better cycling stability than those for the widely investigated T-Nb₂O₅. When introducing appropriate Cu doping, the optimized H-Cu_0.1-Nb₂O₅ electrode shows greatly enhanced conductivity and lower diffusion barrier as revealed by the theoretical calculations and electrochemical characterizations, delivering a high reversible capacity of 203.6 mAh g^-1 and a high capacity retention of 140.8 mAh g^-1 after 5000 cycles at 1 A g^-1, with a high initial Coulombic efficiency of 91% and a high rate capacity of 144.2 mAh g^-1 at 4 A g^-1. As a demonstration for full-cell application, the H-Cu_0.1-Nb₂O₅||LiFePO₄ cell displays good cycling performance, exhibiting a reversible capacity of 135 mAh g^-1 after 200 cycles at 0.2 A g^-1. More importantly, this work offers a new synthesis protocol of the monoclinic Nb₂O₅ phase with high capacity retention and improved reaction kinetics.