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.