Co-modification of LiNi0.5Co0.2Mn0.3O2 cathode materials with zirconium substitution and surface polypyrrole coating: towards superior high voltage electrochemical performances for lithium ion batteries

The combination of zirconium doping and surface conductive polypyrrole coating is successfully demonstrated to enhance the high voltage electrochemical performances of LiNi0.5Co0.2Mn0.3O2 (NCM) cathode materials. The lattice parameters calculated from the X-ray diffraction (XRD) patterns by Rietveld refinement reveal that the cation mixing degree is restrained and the lithium slabs spacing is broadened after zirconium substitution (NCMZ). A compact and continuous polypyrrole thin film is satisfactorily coated onto the surface of zirconium-doped (NCMZ/PPy) sample, which is confirmed by fourier transform infrared spectrometry (FT-IR), scanning electron microscopy (SEM) and transmission electron microscope (TEM). Though the initial discharge capacity of NCMZ/PPy sample is slightly lower than pristine one, the cycling stability is dramatically improved. Notably, NCMZ/PPy sample shows capacity retention of 89.83% at 1C after 100 repeated cycles, while that of pristine one remains only 69.36%. When the electrodes cycled at 2C for 200 cycles, the capacity retention of NCMZ/PPy still reaches up to 78.83%, and that of pristine one is only 56.79%. The presence of polypyrrole network acts as a capsule shell, which can protect the active substance from erosion caused by the HF attacking. As a result, the cycling stability and rate performance of co-modified sample are both better than those of bare LiNi0.5Co0.2Mn0.3O2 sample and zirconium doped materials.