Functionality of Oxide Coating for Li[Li_0.05Ni_0.4Co_0.15Mn_0.4]O₂ as Positive Electrode Materials for Lithium-Ion Secondary Batteries

Surface-modified Li[Li0.05Ni0.4Co0.15Mn0.4]O2 oxides were studied. The oxide particles were coated by heteroelements such as Al2O3, Nb2O5, Ta2O5, ZrO2 and ZnO. Metal oxide-coated Li[Li0.05Ni0.4Co0.15Mn0.4]O2 did not show significant difference in X-ray diffraction patterns. Thickness of the formed coating layer was around 10 nm, as observed by transmission electron microscopy. Electrochemical properties of heteroelement-coated Li[Li0.05Ni0.4Co0.15Mn0.4]O2 were investigated using coin type Li-ion cells employing graphite as an anode at 60 °C. Metal oxide-coated Li[Li0.05Ni0.4Co0.15Mn0.4]O2 obviously showed higher capacity with good cyclability. Also, area-specific impedance was significantly lower for the metal oxide-coated Li[Li0.05Ni0.4Co0.15Mn0.4]O2 during cycling, compared with that for bare Li[Li0.05Ni0.4Co0.15Mn0.4]O2. Among them, Al2O3-coated Li[Li0.05Ni0.4Co0.15Mn0.4]O2 had the best electrochemical performances. The metal oxide coating layer transformed to metal fluoride layer during cycling, as proved by time-of-flight−secondary ion mass spectroscopy. The newly formed metal fluoride layer would be greatly effective against HF attack during cycling. Possible reasons for the effectiveness of the metal oxide coating are discussed.