Effect of surface fluorine substitution on high voltage electrochemical performances of layered LiNi0.5Co0.2Mn0.3O2 cathode materials

Abstract

A strategy of surface fluorine substitution to enhance the high voltage electrochemical performance of LiNi0.5Co0.2Mn0.3O2 material has been proposed. The inter-slab spacing distance is broaden by fluorine doping, which is deduced from the lattice parameters calculated by Rietveld refinement method. Scanning electron microscopy indicates the fluorine substitution stimulates the growth of the primary particles. Though the initial discharge capacities of LiNi0.5Co0.2Mn0.3O2-zFz (z=0.02, 0.04, 0.06) is somewhat reduced, the capacity retention under high oxidation state were improved compared to that of bare one. For the optimal composition of LiNi0.5Co0.2Mn0.3O1.98F0.02, it exhibits a capacity retention of 81.1% at 1C after 100 cycles and delivers a discharge capacity of 123.5mAhg−1at 10C, and that of bare sample are just 70.1% and 109.6mAhg−1, respectively. Cyclic voltammetry and electron impedance spectroscopy measurements demonstrate that the fluorine doping could significantly lower the cell polarization and retard the impedance rise. Transmission electron microscope analysis of cycled electrodes is also performed and it implies that fluorine substitution can effectively safeguard the electrode from HF erosion to maintain the bulk structural stable permanently.