Multi-electron reaction and fast Al ion diffusion of δ-MnO₂ cathode materials in rechargeable aluminum batteries via first-principle calculations

Rechargeable aluminum batteries with multi-electron reaction have a high theoretical capacity for next generation of energy storage devices. However, the diffusion mechanism and intrinsic property of Al insertion into MnO₂ are not clear. Hence, based on the first-principles calculations, key influencing factors of slow Al-ions diffusion are narrow pathways, unstable Al-O bonds and Mn³⁺ type polaron have been identified by investigating four types of δ-MnO₂ (O3, O'3, P2 and T1). Although Al insert into δ-MnO₂ leads to a decrease in the spacing of the Mn-Mn layer, P2 type MnO₂ keeps the long (spacious pathways) and stable (2.007–2.030 Å) Al-O bonds resulting in the lower energy barrier of Al diffusion of 0.56 eV. By eliminated the influence of Mn³⁺ (low concentration of Al insertion), the energy barrier of Al migration achieves 0.19 eV in P2 type, confirming the obviously effect of Mn³⁺ polaron. On the contrary, although the T1 type MnO₂ has the sluggish of Al-ions diffusion, the larger interlayer spacing of Mn-Mn layer, causing by H₂O could assist Al-ions diffusion. Furthermore, it's worth to notice that the multilayer δ-MnO₂ achieves multi-electron reaction of 3|e|. Considering the requirement of high energy density, the average voltage of P2 (1.76 V) is not an obstacle for application as cathode in RABs. These discover suggest that layered MnO₂ should keep more P2-type structure in the synthesis of materials and increase the interlayer spacing of Mn-Mn layer for providing technical support of RABs in large-scale energy storage.