Enhancing d-p orbital hybridization through oxygen vacancies boosting capacity and kinetics of layered double hydroxides for durable aqueous magnesium-ion batteries

Abstract

Layered double hydroxides (LDHs) are potential cathode materials for aqueous magnesium-ion batteries (AMIBs). However, the low capacity and sluggish kinetics significantly limit their electrochemical performance in AMIBs. Herein, we find that oxygen vacancies can significantly boost the capacity, electrochemical kinetics, and structure stability of LDHs. The corresponding structure-performance relationship and energy storage mechanism are elaborated through exhaustive in/ex-situ experimental characterizations and density functional theory (DFT) calculations. Specially, in-situ Raman and DFT calculations reveal that oxygen vacancies elevate orbital energy of O 2p and electron density of O atoms, thereby enhancing the orbital hybridization of O 2p with Ni/Co 3d. This facilitates electron transfer between O and adjacent Ni/Co atoms and improves the covalency of Ni–O and Co–O bonds, which activates Ni/Co atoms to release more capacity and stabilizes the Ov-NiCo-LDH structure. Moreover, the distribution of relaxation times (DRT) and molecular dynamics (MD) simulations disclose that the enhanced d-p orbital hybridization optimizes the electronic structure of Ov-NiCo-LDH, which distinctly reduces the diffusion energy barriers of Mg²⁺ and improves the charge transfer kinetics of Ov-NiCo-LDH. Consequently, the assembled Ov-NiCo-LDH//active carbon (AC) and Ov-NiCo-LDH//perylenediimide (PTCDI) AMIBs can both deliver high specific discharge capacity (182.7 and 59.4 mAh g⁻¹ at 0.5 A g⁻¹, respectively) and long-term cycling stability (85.4% and 89.0% of capacity retentions after 2500 and 2400 cycles at 1.0 A g⁻¹, respectively). In addition, the practical prospects for Ov-NiCo-LDH-based AMIBs have been demonstrated in different application scenarios. This work not only provides an effective strategy for obtaining high-performance cathodes of AMIBs, but also fundamentally elucidates the inherent mechanisms.