Introducing high-valence molybdenum to stimulate lattice oxygen in a NiCo LDH cathode for chloride ion batteries

Layered double hydroxides (LDHs) have been intensively investigated as promising cathodes for the new concept chloride ion battery (CIB) with multiple advantages of high theoretical energy density, abundant raw materials and unique dendrite-free characteristics. However, driven by the great compositional diversity, a complete understanding of interactions between metal cations, as well as a synergetic effect between metal cations and lattice oxygen on LDH host layers in terms of the reversible Cl-storage capability, is still a crucial but elusive issue. In this work, we synthesized a series of chloride-inserted trinary Mo-doped NiCo₂-Cl LDH (x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5) with gradient oxygen vacancies as enhanced cathodes toward CIBs. The combination of advanced spectroscopic techniques and theoretical calculations reveals that the Mo dopant facilitates oxygen vacancy formation and varies the valence states of coordinated transition metals, which can not only tune the electronic structure effectively and promote Cl-ion diffusion, but improve the redox activity of LDHs. The optimized Mo_0.3NiCo₂-Cl LDH delivers a reversible discharge capacity of 159.7 mA h g^-1 after 300 cycles at 150 mA g^-1, which is almost a triple enhancement compared to that of NiCo₂Cl LDH. The superior Cl-storage of trinary Mo_0.3NiCo₂Cl LDH is attributed to the reversible intercalation/deintercalation of chloride ions in the LDH gallery along with the oxidation state changes in Ni⁰/Ni²⁺/Ni³⁺, Co⁰/Co²⁺/Co³⁺ and Mo⁴⁺/Mo⁶⁺ couples. This simple vacancy engineering strategy provides critical insights into the significance of the chemical interaction of various components on LDH laminates and aims to effectively design more LDH-based cathodes for CIBs, which can even be extended to other halide-ion batteries like fluoride ion batteries and bromide ion batteries.