Enhanced Elastic Migration of Magnesium Cations in alpha-Manganese Dioxide Tunnels Locally Tuned by Aluminium Substitution

The harsh conditions of large hydrated ion radius of Mg2+ cations and the strong electrostatic interaction with the host material put forward higher requirements for high-performance aqueous magnesium ion (Mg2+) energy storage devices. Herein, substituted aluminium ions (Al3+) doped alpha-MnO2 materials are prepared. The introduction of Al3+ cations adjust the local chemical environment inside the tunnel structure of alpha-MnO2 and precisely regulates the diffusion behavior of inserted Mg2+ cations. The shortened oxygens' distance and abundant oxygen defects result in a substantially enhanced elastic migration pattern of Mg2+ cations driven by strengthened electrostatic attraction, which brings the lower diffusion energy barrier, improved reaction kinetics, and adaptive volume expansion as evidenced by Climbing Image-Nudged Elastic Band density function theory calculations coupled with experimental confirmation in X-ray photoelectron spectroscopy, electron paramagnetic resonance, and galvanostatic intermittent titration technique. As a result, this rationally designed cathode exhibits a high reversible capacity of 197.02 mAh g(-1) at 0.1 A g(-1) and stable cycle performance of 2500 cycles with 82% retention. These parameters are among the best of Mg-ion capacitors reported to date. This study offers a detailed insight into the local tunnel structure tunning effect and opens up a new path of modification for tunnel-type structural materials.