2D-Nb₂CT_z-Supported, 3D-Carbon-Encapsulated, Oxygen-Deficient Nb₂O₅ for an Advanced Li-Ion Battery

Exploring advanced electrode materials with rapid lithium-ion charging/discharging kinetic properties is significant for the development of modern electric transportation. Herein we report a powerful synergistic engineering of carbon encapsulation and oxygen deficiency to construct Nb2O5 through a two-step method of pregelation and annealing treatment. The yielded Nb2O5 with sufficient oxygen vacancies are supported by 2D highly conductive Nb2CTz (T = O, OH, and F) MXene and further encapsulated by 3D carbon layers (2D/3D Nb2O5–x). Such an exquisite architecture is proved to efficiently overcome the intrinsic weakness of slow ion transfer, low electrical conductivity, and long-term cycling instability in metal oxides, in this case Nb2O5. Consequently, 2D/3D Nb2O5–x composites share an improved average diffusion coefficient from 1.34 × 10–12 cm–2 s–1 to 3.02 × 10–12 cm–2 s–1, a facilitated Li+ ion diffusion pathway, and shortened relaxation time constant (τ0) from 8.9 to 6.1 ms. In an optimized 2D/3D Nb2O5–x electrode, it delivers a high capacity of 245 mAh g–1 at 0.1 C (1 C = 270 mA g–1), 85 mAh g–1 at a high rate of 5 C, and an excellent long-term durability with 92.7% capacity retention during 1250 cycles. These results clearly demonstrate the significance of tailoring the microstructure and composition of metal oxides as used in high-rate and long-cycling lithium-ion storage.