Aqueous Magnesium-Ion Battery Anode with 75,000 Cycles Lifespan

Aqueous magnesium-ion batteries (AMIBs) are garnering growing interest due to their abundant resources, inherent safety, and low cost. However, the strong Mg²⁺-H₂O interaction results in bulky hydrated metal ions that diffuse sluggishly within host materials, leading to structural degradation and poor cycling performance. Here, we report a strain delocalization strategy to preserve structural robustness and achieve ultrastable cycling stability of AMIBs by using self-ordered Ta-doped MoO₃ (MoTaO_x) nanotube array electrodes. Oxygen vacancies within MoTaO_x can facilitate the accommodation and dissociation of interlayer H₂O molecules, leading to the formation of a Ta-OH···OH₂ configuration. This reduces the kinetic energy barrier for Mg²⁺ diffusion, resulting in the uniform magnesiation of MoTaO_x, where the rigid Ta-O bonds further enable delocalization of mechanical strain throughout the host, conferring a shear strain tolerance of ∼95% during magnesiation. The MoTaO_x electrode exhibits stable operation over 75,000 cycles and delivers a cumulative capacity of 7.2 kAh g^-1, significantly surpassing previous reports. These findings elucidate the sluggish H₂O co-intercalation-induced localized strain as a degradation pathway and establish vacancy-pinned, water-regulated magnesiation for delocalization of strain as a viable design principle for developing long-lifespan AMIBs with high capacity.