Investigation of the Reversible Intercalation/Deintercalation of Al into the Novel Li<sub>3</sub>VO<sub>4</sub>@C Microsphere Composite Cathode Material for Aluminum-Ion Batteries

The Li<sub>3</sub>VO<sub>4</sub>@C microsphere composite was first reported as a novel cathode material for rechargeable aluminum-ion batteries (AIBs), which manifests the initial discharge capacity of 137 mAh g<sup>–1</sup> and and remains at 48 mAh g<sup>–1</sup> after 100 cycles with almost 100% Coulombic efficiency. The detailed intercalation mechanism of Al into the orthorhombic Li<sub>3</sub>VO<sub>4</sub> is investigated by ex situ X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) of Li<sub>3</sub>VO<sub>4</sub>@C electrodes and the nuclear magnetic resonance aluminum spectroscopy (<sup>27</sup>Al NMR) of ionic liquid electrolytes in different discharge/charge states. First-principle calculations are also carried out to investigate the structural change as Al inserts into the framework of Li<sub>3</sub>VO<sub>4</sub>. It is revealed that the Al/Li<sub>3</sub>VO<sub>4</sub>@C battery goes through electrochemical dissolution and deposition of metallic aluminum in the anode, as well as the insertion and deinsertion of Al<sup>3+</sup> cations in the cathode in the meantime. The rechargeable AIBs fabricated in this work are of low cost and high safety, which may make a step forward in the development of novel cathode materials based on the acidic ionic liquid electrolyte system.