Anisotropy of V3O7 nanobelts enables ultralong cycling life of magnesium ion battery

Magnesium ion batteries (MIBs) are a promising alternative to lithium-ion batteries, which suffer from the short cycling life and sluggish Mg2+ diffusion kinetics of cathodes. Nano morphologies are used to shorten Mg2+ diffusion path for diffusion kinetics acceleration, but the cycling life is still unsatisfactory. Herein, the anisotropy of layered V3O7⋅1.9H2O nanobelts is utilized to stabilize their structure during discharging/charging. The V3O7⋅1.9H2O nanobelts grow along the preponderant migration direction of Mg2+, and the resulted axial migration of Mg2+ enables the stress caused by Mg2+ insertion to be decentralized in large zone, thus improving the cycling stability of V3O7⋅1.9H2O nanobelts. The inserted Mg2+ cations bond with O atoms in adjacent V3O8 layers of V3O7⋅1.9H2O, further stablizing the layered structure. Meanwhile, the axial migration of Mg2+ significantly reduces the charge transfer resistance at electrode/electrolyte interface, which accelerates the Mg2+ diffusion kinetics. Thus, the symmetric RMB assembled from V3O7⋅1.9H2O nanobelts exhibits an ultralong cycling life of 11,000 cycles at 4 A g−1, alongside a high specific capacity of 137 mAh g−1 at 0.05 A g−1. According to our knowledge, this ultralong cycling life surpasses those of reported full RMBs. This strategy provides insight into the design of cathode materials with improved cycling lives.