Keplerate-type polyoxometalate {Mo72Fe30} nanoparticle anodes for high-energy lithium-ion batteries

Abstract

Li-ion batteries (LIBs), with high energy densities, have been explored in recent years. Polyoxometalates (POMs) have been proposed as promising anodes, with high capacity. Herein, we report a Keplerate-type POM [{Mo₆O₁₉}⊂{Mo₇₂Fe₃₀O₂₅₄ (CH₃COO)₁₂(H₂O)₉₆}]·150H₂O ({Mo₇₂Fe₃₀}) nanoparticle as a high energy anode material for LIBs for the first time. Employing synchrotron radiation X-rays, we conducted in operando and ex situ measurements to investigate the charge storage mechanisms of {Mo₇₂Fe₃₀}. The in operando X-ray absorption near edge structure (XANES) reveals that a single {Mo₇₂Fe₃₀} molecule can store 377 electrons. The {Mo₇₂Fe₃₀} anode exhibits a high capacity of ∼1250 mA h g⁻¹ at 100 mA g⁻¹, with 92% capacity retention after 100 cycles, and it also demonstrates an excellent rate performance (868 mA h g⁻¹ at 2000 mA g⁻¹). Li-ions can react with {Mo₇₂Fe₃₀} through surface-capacitive reactions and diffusion processes, and the Li-ion diffusion coefficient of {Mo₇₂Fe₃₀} is up to 10⁻¹⁰ cm² s⁻¹, resulting in high rate performance. Furthermore, a full-cell utilizing {Mo₇₂Fe₃₀} as an anode and LiFePO₄ as a cathode was assembled, and it delivered a high energy density of 258 W h kg⁻¹, which successfully demonstrates that {Mo₇₂Fe₃₀} is a promising anode material for LIB applications.