Mg<sub>2</sub>Nb<sub>34</sub>O<sub>87</sub> Porous Microspheres for Use
in High-Energy, Safe, Fast-Charging, and Stable Lithium-Ion Batteries
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Abstract
M–Nb–O
compounds are advanced anode materials for lithium-ion batteries (LIBs)
due to their high specific capacities, safe operating potentials,
and high cycling stability. Nevertheless, the found M–Nb–O
anode materials are very limited. Here, Mg<sub>2</sub>Nb<sub>34</sub>O<sub>87</sub> is developed as a new M–Nb–O material.
Mg<sub>2</sub>Nb<sub>34</sub>O<sub>87</sub> porous microspheres (Mg<sub>2</sub>Nb<sub>34</sub>O<sub>87</sub>-P) with primary-particle sizes
of 30–100 nm are fabricated based on a solvothermal method.
Mg<sub>2</sub>Nb<sub>34</sub>O<sub>87</sub> has an open 3 × 4
× ∞ Wadsley–Roth shear structure and a large unit-cell
volume, leading to its largest Li<sup>+</sup> diffusion coefficients
among all the developed M–Nb–O anode materials. In situ
X-ray diffraction analyses reveal its high structural stability and
intercalating characteristic. These architectural, conductivity, and
structural advantages in Mg<sub>2</sub>Nb<sub>34</sub>O<sub>87</sub>-P lead to its most significant intercalation pseudocapacitive contribution
(87.7% at 1.1 mV s<sup>–1</sup>) among the existing M–Nb–O
anode materials and prominent rate capability (high reversible capacities
of 338 mAh g<sup>–1</sup> at 0.1C and 230 mAh g<sup>–1</sup> at 10C). Additionally, this new material exhibits a safe operating
potential (∼1.68 V), an ultrahigh initial Coulombic efficiency
(94.8%), and an outstanding cycling stability (only 6.9% capacity
loss at 10C over 500 cycles). All of these evidences indicate that
Mg<sub>2</sub>Nb<sub>34</sub>O<sub>87</sub>-P is an ideal anode material
for high-energy, safe, fast-charging, and stable LIBs