Switchable and Strain‐Releasable Mg‐Ion Diffusion Nanohighway Enables High‐Capacity and Long‐Life Pyrovanadate Cathode

Rechargeable magnesium batteries (RMBs) suffer from low capacity and poor cyclability of cathode materials, which is due to the sluggish Mg2+ diffusion kinetics and large lattice strain. Here, a layer‐interweaving mechanism in lamellar cathode to simultaneously facilitate Mg2+ diffusion and release Mg2+‐insertion strain is reported. In the Cu3V2O7(OH)2·2H2O (CVOH) cathode, Mg2+ diffusion highways are generated by the vertical interweaving of CVOH layers and V6O13 layers that nucleate in CVOH during discharging, which are switchable by Mg2+ insertion/extraction. These highways enhance the Mg2+ diffusion coefficient by three orders of magnitude and release 50% Mg2+‐insertion strain. This enables CVOH to exhibit a high capacity of 262 mAh g−1 at high current density of 250 mA g−1 in aqua, and extremely low capacity loss of 0.0004% per cycle in the activated carbon//CVOH cell. This work inspires designing the magnesiation phase transformation of electrodes to resolve both kinetic and strain issues for high‐performance RMBs. Discharging of lamellar‐structured Cu3V2O7(OH)2·2H2O (CVOH) cathode generates Mg2+ diffusion highways in CVOH due to the production of V6O13 layers interweaving vertically with CVOH layers. The Mg2+ diffusion highways lead to the high specific capacity and excellent rate capability of CVOH. Their formation also releases the strain induced by Mg2+ insertion and stabilizes the CVOH structure, leading to its ultralong cycling life.