New structurally integrated layered-spinel lithium-cobalt-manganese-oxide composite cathode materials for lithium-ion batteries

Abstract

The layered-spinel Li-Co-Mn-O composites {x[0.5Li(CoMn)O4]·(1-x)[0.5Li2MnO3·0.5LiCoO2] (x = 0, 0.25, 0.50, 0.75 and 1)} which consist of layered Li1.2Co0.4Mn0.4O2 and spinel LiCoMnO4 are designed, for Li1.2Co0.4Mn0.4O2 may deliver a high capacity and LiCoMnO4 may provide structural stability. The composites are prepared via a sol-gel method and characterized by X-ray powder diffraction, scanning electron microscopy, Raman spectra, high-resolution transmission electron microscope and X-ray photoelectron spectrometer. With the increase of spinel content, x[0.5Li(CoMn)O4]·(1-x)[0.5Li2MnO3·0.5LiCoO2] composites change from a layered structure (x = 0) to mixed layered-spinel structures (x = 0.25, 0.50 and 0.75), and then to a spinel structure (x = 1). All samples have uniform particles with smooth surfaces, and primary particle sizes become larger with the increase of spinel content. Electrochemical tests show that the layered-spinel composites present improved cycling performances and rate capabilities compared with those of Li-rich materials. The composite of x = 0.25 exhibits the highest discharge capacity and the best rate capability between 2.00 and 4.95 V. Electrochemical impedance spectra show that the composite of x = 0.25 has the minimum charge transfer resistance in the initial several cycles. The sample of x = 0.25 possesses the superior electrochemical performance than others, and the reason can be attributed to the lower charge transfer resistance and more stable structure.