Investigation of the Reversible Intercalation/Deintercalation of Al into the Novel Li₃VO₄@C Microsphere Composite Cathode Material for Aluminum-Ion Batteries

The Li₃VO₄@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^-1 and and remains at 48 mAh g^-1 after 100 cycles with almost 100% Coulombic efficiency. The detailed intercalation mechanism of Al into the orthorhombic Li₃VO₄ is investigated by ex situ X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) of Li₃VO₄@C electrodes and the nuclear magnetic resonance aluminum spectroscopy (²⁷Al 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 Li₃VO₄. It is revealed that the Al/Li₃VO₄@C battery goes through electrochemical dissolution and deposition of metallic aluminum in the anode, as well as the insertion and deinsertion of Al³⁺ 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.