Guiying Zhao

Electrochemical properties of carbon-coated LiFePO4 and LiFe0.98Mn0.02PO4 cathode materials synthesized by solid-state reaction

Olivine structured LiFePO4/C (lithium iron phosphate) and Mn2+-doped LiFe0.98Mn0.02PO4/C powders were synthesized by the solid-state reaction. The effects of manganese partial substitution and different carbon content coating on the surface of LiFePO4 were considered. The structures and electrochemical properties of the samples were measured by X-ray diffraction (XRD), cyclic voltammetry (CV), charge/discharge tests at different current densities, and electrochemical impedance spectroscopy (EIS). The electrochemical properties of LiFePO4 cathodes with x wt.% carbon coating (x= 3, 7, 11, 15) at γ =0.2C, 2C (1C= 170 mAh·g−1) between 2.5 and 4.3 V were investigated. The measured results mean that the LiFePO4 with 7 wt.% carbon coating shows the best rate performance. The discharge capacity of LiFe0.98Mn0.02PO4/C composite is found to be 165 mAh·g−1 at a discharge rate, γ = 0.2C, and 105 mAh·g−1 at γ =2C, respectively. After 10 cycles, the discharge capacity has rarely fallen, while that of the pristine LiFePO4/C cathode is 150 mAh·g−1 and 98 mAh·g−1 at γ=0.2 and 2C, respectively. Compared to the discharge capacities of both electrodes above, the evident improvement of the electrochemical performance is observed, which is ascribed to the enhancement of the electronic conductivity and diffusion kinetics by carbon coating and Mn2+-substitution.

Sol-gel Synthesis and Electrochemical Properties of LiMn1.8Co0.2O4 Cathode Material

Spinel LiMn 1.8 Co 0.2 O 4 and LiMn 1.2 Co 0.2 Cr 0.6 O 4 compounds were prepared through sol-gel method using citric acid as a chelating agent. The structural and electrochemical properties of cathode compounds were investigated by mean of X-ray diffraction, cyclic voltammetry, galvanostatic charge-discharge test and AC impedance spectra systematically. It is found that the high rate discharge capability of electrodes is distinctly improved by chromium doping, indicating that Cr oxidation stabilized the spinel structure.

Enhanced Rate Performance of Carbon Nanotube-Coated LiNi0.45Al0.05Mn1.5O4 Cathode Material

Carbon nanotubes (CNTs) has been coated on the surface of LiNi0.45Al0.05Mn1.5O4 cathode material and the effect of carbon nanotube coating on the electrochemical performances is investigated through the characterization of X-ray diffraction (XRD), cyclic voltammetry (CV), galvanostatic charge-discharge test and electrochemical impedance spectroscopy (EIS), respectively. The results reveal that the CNTs-coating would remarkably enhance the discharge capacity and rate capability, especially at high rate. The improvement of high-rate performance via carbon nanotube coating might be due to the faster kinetic of both the Li-ion diffusion and the charge transfer reaction.

Influence of Al Substitution on the Electrochemical Performance of Spinel LiMi0.5-xMn1.5AlxO4 Cathode

Al-doped LiMi_0.5-xMn_1.5Al_xO₄ cathodes materials were synthesized by a simple sol-gel method. The structure and electrochemical properties have been investigated systematically by X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectra, cyclic voltamogram (CV), charge-discharge tests and Ac impedance spectroscopy. XRD patterns reveal that trace of Li_xNi_1-xO-like impurity phase exists in the pristine LiNi_0.5Mn_1.5O₄ and is suppressed by substituting of Ni²⁺ with Al³⁺. With Al³⁺-substitution, the electrochemical performance of spinel cathodes are enhanced, which is ascribed to the improvement of structural stability and electronic conductivity as well as the suppression of impurity phase. However, the discharge capacity deteriorates in the case of LiNi_0.40Mn_1.5Al_0.10O₄ spinel, which might result from the blocking of migration pathway of electrons in octahedral site.