| Summary: | <p>Chloride ion batteries (CIBs) are an example of a promising new emerging rechargeable battery technology, that exhibits large theoretical volumetric energy density performance and good safety. However, unsatisfactory capacity and poor cycling lifetime of the cathode currently hinder the development of CIBs. Herein, we report the use of an Ni<small><sup>2+</sup></small>Fe<small><sup>3+</sup></small>-based layered double hydroxide (LDH) intercalated by chloride ions as a promising cathode material for CIBs. [Ni<small><sub>2</sub></small>Fe(OH)<small><sub>6</sub></small>]Cl·1.37H<small><sub>2</sub></small>O (NiFe–Cl LDH) exhibits a high maximum capacity of 350.6 mA h g<small><sup>−1</sup></small> and a long lifetime of over 800 cycles (at 101.1 mA h g<small><sup>−1</sup></small>) at a current density of 100 mA g<small><sup>−1</sup></small>, which is superior to most currently reported CIB cathodes. <em>In situ</em> X-ray absorption near-edge structure (XANES) and <em>ex situ</em> X-ray photoelectron spectroscopy (XPS) reveal the valency changes of the Fe<small><sup>2+</sup></small>/Fe<small><sup>3+</sup></small> and Ni<small><sup>2+</sup></small>/Ni<small><sup>3+</sup></small> redox pairs within the metal hydroxide layers of the LDH during electrochemcial cycling. <em>In situ</em> XRD reveals that 2D anion diffusion within the LDH results in only ∼3% structural change. Oxygen K-edge soft X-ray absorption spectroscopy (SXAS) reveals the oxygen atoms within the MO<small><sub>6</sub></small> octahedra reversibly participate in the electrochemical reaction. In view of the extensive chemical variation, low-cost, and ease-of-preparation of LDH-based materials we regard LDHs as a promising materials platform for application as cathode materials in chloride ion batteries.</p>
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