ABSTRACT
High-energy-density and low-cost lithium-ion batteries are sought to meet increasing demand for portable electronics. In this study, a cobalt-free Li(Li0.17 Ni0.17 Fe0.17 Mn0.49 )O2 (LNFMO) cathode material is chosen, owing to the reversible anionic redox couple O2- /O- . The aim is to elucidate the Fe-substitution function and oxygen redox mechanism of experimentally synthesized Li(Li0.16 Ni0.19 Fe0.18 Mn0.46 )O2 by DFT. The redox processes of cobalt-containing Li(Li0.17 Ni0.17 Co0.17 Mn0.49 )O2 (LNCMO) are compared with those of LNFMO. Redox couples including Ni2+ /Ni3+ /Ni4+ , Fe3+ /Fe4+ or Co3+ /Co4+ , and O2- /O- are found, confirmed by a X-ray photoelectron spectroscopy, and explained by redox competition between O and transition metals. In LNFMO and LNCMO, O ions with an Li-O-Li configuration readily participate in oxidation, and the most active O ions are coordinated to Mn4+ and Li+ . Oxidation of O in LNCMO is triggered earlier, along with that of Co. Fe substitution activates O ions, contributes additional oxygen redox charge compensation of 0.44 e per formula unit, avoids concentrated accumulation of oxygen oxidation, and improves structural stability. This work provides new scope for designing cobalt-free, low-cost, and higher-energy-density cathode materials for Li-ion batteries.
ABSTRACT
Lithium-rich, Mn-based layered oxides Li2 MnO3 -LiMO2 (M=Ni, Co) have been considered as promising cathode candidates owing to their high capacity. However, the resources shortage and high price of cobalt make it imperious to substitute cobalt with other high-abundance elements. Here, we synthesized a low-cost, cobalt-free, Fe-substituted oxide material, Li(Li0.16 Ni0.19 Fe0.18 Mn0.46 )O2 . It exhibited a high reversible capacity of 169.2â mAh g-1 after 100 cycles and maintained an extraordinarily high discharge potential during cycling. X-ray photoelectron spectroscopy and DFT calculations revealed that super iron FeIV exists in the delithiated state, and oxygen participates in the redox reaction in addition to the Ni2+ /Ni4+ and Fe3+ /Fe4+ redox couples. The anionic oxidation preferentially occurred on oxygen with a Li-O-Li configuration and with oxidized Fe and Ni coordination.
