Electrochemical and structural characterization of carbon coated Li _1.2Mn_0.56Ni_0.16Co_0.08O₂ and Li_1.2Mn_0.6Ni_0.2O₂ as cathode materials for Li-ion batteries

Carbon coated Li_1.2Mn_0.56Ni_0.16Co _0.08O₂ and Li_1.2Mn_0.6Ni _0.2O₂ were synthesized as cathode materials for Li-ion batteries by a self-combustion reaction, and were characterized by XRD, SEM, HRTEM and Raman spectroscopy in conjunction with electrochemical measurements. Initial discharge specific capacities of 270 mAh g^-1 and 230 mAh g^-1 are obtained for Li_1.2Mn_0.56Ni _0.16Co_0.08O₂ and Li_1.2Mn _0.6Ni_0.2O₂, respectively at slow rates (e.g. C/10) in galvanostatic charge-discharge cycling. The cathode material Li _1.2Mn_0.56Ni_0.16Co_0.08O₂ can provide a discharge capacity of 110 mAh g^-1 at 4 C rate. The importance of the presence of Co in the structure for obtaining high rate capabilities was proven by comparison with Li_1.2Mn _0.6Ni_0.2O₂ electrodes, which can exhibit only 80 mAh g^-1 at 2 C rate. The electrochemical impedance spectra of Li_1.2Mn_0.56Ni_0.16Co_0.08O ₂ recorded at various potentials during charging indicate that there is a substantial increase in the charge-transfer resistance at voltages higher than 4.4 V, indicating that the kinetics of Li⁺ ions insertion into this material is controlled by charge-transfer rather than by Li⁺ ions diffusion, at high potentials.