Electrochemical and spectroscopic studies on carbon-coated and iodine-doped LiFeBO3 as a cathode material for lithium-ion batteries

Abstract

In this study, monoclinic LiFeBO3 was carbon-coated and iodine doped via a solid-state reaction to improve the electrochemical performance of pristine LiFeBO3 as cathode material in lithium-ion batteries. In order to enhance the electrical conductivity of LiFeBO3, the highly electronegative iodide anion was doped in a limited amount (x = 0.005) at the oxygen site of the borate to produce LiFeBO3−xI2x. A thin carbon layer was then deposited in situ on the LiFeBO2.995I0.01 particles (to produce “LFBI/C”) to protect them from air and moisture. Field-emission scanning electron microscopy (FE-SEM) data indicated the aggregated morphology of the synthesized samples. Powder X-ray diffraction (XRD) and 7Li magic angle spinning (MAS) nuclear magnetic resonance (NMR) measurements revealed that both the doped and undoped LiFeBO3-based samples had mainly monoclinic structures, although a small amount of a vonsenite-type phase was formed upon iodine doping. X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray spectroscopy (EDX) data confirmed the successful insertion of iodine in the cathode material. The specific discharge capacity of LFBI/C at 0.05 C rate (144.68 mAh g−1) was higher than that of carbon-coated LiFeBO3 (122.46 mAh g−1). The increased capacity of LFBI/C was also evident in long charge–discharge cycles conducted at 1 C rate and in the overall rate performance. Interestingly, the iodine-doped sample exhibited significantly high specific discharge capacity even at 10 C rate.