The preparation and electrochemical performance of solid solutions LiCoO_2-Li_2MnO_3 as cathode materials for lithium ion batteries

摘要

In the paper, we have investigated the preparation and electrochemical performance of the solid solution Li_2MnO_3-LiCoO_2 (or written as Li[Li_(x/3)Co_(1-x)Mn_(2x/3)]O_2). The chemical composition determined by ICP-AES indicates that the chemical formula of our prepared samples is very closely to that of the ideal Li_2MnO_3-LiCoO_2 solid solution in chemical stoichiometry. It was found that the oxidation states of Co and Mn are +3 and +4, respectively by redox titration and X-ray photoelectron spectroscopy (XPS). XRD shows that most peaks of the samples prepared at 950 deg C could be simply indexed as a layered alpha-NaFeO_2 structure (space group R_(3m), no. 166) except some additional peaks between 21deg and 25deg , which is caused by the appearance of the super lattice structure. The monoclinic phase has been formed for Li[Li_(x/3)Co_(1-x)Mn_(2x/3)]O_2 (x=0.5) at 750 deg C, which is reversibly transformed into a hexagonal phase with the increasing of sintering temperature from 750 to 950 deg C. It is found that the charge capacity of the samples Li[Li_(x/3)Co_(1-x)Mn_(2x/3)]O_2 (0.5 < x < 0.7) increases and the discharge capacity has no obvious change with the increasing of the sintering temperature due to the appearance of a flat charge voltage plateau at 4.45 V. Moreover, the discharge capacity of the samples gradually reduces with the increasing Li_2MnO_3 content. The sample has the excellent electrochemical cycling performance with the charge-end voltage of 4.5 V. XRD patterns of the charged product indicate that the layered structure has been kept during the charge process, which may be the reason for its good cycling performance.