Synthesis and investigation of electrochemical performance of mixed valent Li4FeMoO6 as positive electrode material in rechargeable lithium ion batteries

Abstract

Lithium rich layered cathode materials containing additional lithium in metal layer are of current interest as they exhibit capacities exceeding 250 mAh g−1. Here, we report Li4FeMoO6, a new lithium rich cathode material containing mixed valent cations with a stoichiometry of Li4Fe2+0.45Fe3+0.55Mo5+0.55Mo6+0.45O6. XRD data shows that Li4FeMoO6 crystallizes in monoclinic structure (C2/m space group) similar to Li2MnO3 and hence in the layered structure notion the formula can be written as Li[Li0.33Fe0.33Mo0.33]O2. XRD analysis reveals that Li and Fe are disordered over 4h and 2d sites, whereas 2a sites are only filled with Mo atoms. HRTEM and electron diffraction studies indicate presence of structural disorder and planar lattice defects such as stacking faults (SFs) or twinning. During electrochemical cycling in the potential range of 1.5 ̶ 4.6 V, Li4FeMoO6 exhibits initial charge capacity of 313 mAh g−1. It shows 90% reversible capacity for the first cycle and after 100 cycles the capacity retention is about 75.5%. Above 4.6 V, Mo dissolution is observed which brings down the capacity retention to 44% in 100 cycles. Interestingly, Li4FeMoO6 show high rate performance which can be attributed to good conductivity arising from the presence of mixed valent Mo5+/Mo6+ and Fe2+/Fe3+.