Valence Engineering Boosts Kinetics and Storage Capacity of Layered Double Hydroxides for Aqueous Magnesium‐Ion Batteries

The kinetics and storage‐capacity of NiCoMg‐ternary layered double hydroxide (NiCoMg‐LDH) are successfully boosted by valence engineering. As the cathode for aqueous magnesium‐ion batteries (AMIBs), the assembled NiCoMg‐LDH//active carbon (AC) delivers a high specific discharge capacity (121.0 mAh·g−1 at 0.2 A·g−1), long‐term cycling stability (85% capacity retention after 2000 cycles at 1.0 A·g−1) and an excellent performance at −30 °C. Moreover, NiCoMg‐LDH//perylenediimide (PTCDI) is assembled, achieving a high specific discharge capacity and long‐term cycling stability. X‐ray absorption spectra (XAS)/X‐ray photoelectron spectroscopy (XPS) analyses and Density functional theory (DFT) calculations disclose that the electrons are redistributed due to the 3d orbital overlap of Co/Ni atoms in NiCoMg‐LDH, which obviously reduces the valence states of Co/Ni atoms, enhances Mg─O bond strength and degree of hybridization of Co/Ni 3d and O 2p orbitals. Hence, the electronic conductivity is significantly enhanced and the electrostatic repulsion between Mg2+ and host layers is greatly reduced, giving rise to the improved diffusion kinetics and storage‐capacity of Mg2+. Furthermore, in situ Raman/X‐ray diffraction (XRD) and ex situ XPS reveal corresponding energy‐storage mechanism. This paper not only demonstrates the feasibility of LDHs as cathode for AMIBs, but also offers a new modification method of valence engineering for high‐performance electrode materials. A promising valence engineering strategy is proposed to regulate LDH (layered double hydroxides) in AMIBs (aqueous magnesium‐ion batteries). By reducing the valence state of cations in LDHs layers, the electrostatic repulsion between layers and Mg2+ is significantly reduced. Hence, the diffusion kinetics and Mg2+‐storage capacity are distinctly improved. Meanwhile, AMIBs based on LDHs can achieve high energy and power densities.