| In this paper, the development of cathode materials for lithium ion batteries was reviewed in detail. The aims of the present study were to focus on the preparation processes, the structural characterization, the modification of materials, the electrochemical properties, and the kinetic behaviors of the olivine LiMnPO4 cathode material for lithium ion batteries. Differential thermoanalysis thermogravimetry (TG/DTA), X-ray diffraction(XRD), scanning electron microscopy(SEM), charge-discharge test, AC impedance test, and Chronoamperometry were used to characterize the synthesized materials.Olivine LiMnPO4 was synthesized in the range of 500℃ to 600℃ by the method of solid-state reaction combining with the addition of carbon black and ball-milling of reagents and precursors. Sample prepared at 600℃ with 24h' sintering presents pure phase with small and almost identical particles size (about 100200nm). The initial discharge capacity of this sample is approximately 100 mAh·g-1.The electrochemical reaction resistance increases with the deintercalation of lithium ions according to AC impedance results, which indicates that electrochemical reaction occurred at the interface is enhanced with less lithium ions occupying in the materials. The diffusion coefficient ranges from 10-17cm2·s-1 to 10-13cm2·s-1 through diffusion coefficient test. Diffusion coefficient in the fully-charged state and the fully-discharged state of LiMnPO4 prepared at 600℃ with 24h' sintering are 5.65 × 10-13cm2·s-1 and 3.69×10-15cm2·s-1 respectively. It is suggested that the lower the concentration of lithium ions in the lattice is, the easier lithium ions deintercalated out of materials.Iron doping was used to improve the conductivity of the material and the synthesized LiMn1-xFexPO4(x=0.10.3) were well dispersed with the particle size from 100 to 200nm. Besides, it presents high capacity and excellent rate capability and cycling performance. LiMno.7Feo.3PO4 synthesized at 600℃ for 24h displays a high capacity of 133mAh·g-1 at 1C rate. Capacity keeps at 145mAh·g-1 after thirty cycles at 0.1C rate. It is obvious that iron doping improved the electrochemical performance ofthe material greatly.The electrochemical reaction resistance is much lower in the partly discharged state than in the fully charged or fully discharged state by the measurement of AC impedance for carbon-containing LiMno.7Feo.3PO4. The reaction resistance in the partly discharged state almost keeps identical. It is indicated that the mixed-valence is beneficial to the transfer of electron which happens in the interface, while the single valance goes against the electrochemical reaction kinetics.Diffusion coefficient decreases with the going-on of discharge according to the diffusion coefficient test result of LiMrio.7Feo.3PO4, which indicates that the lower the concentration of lithium ions in the material is, the faster lithium ions diffuse. The diffusion coefficient in LiMni.xFexPO4 ranges from lO'^cm^s"1 to lO^'cm^s"1, which is nearly 100 times of that in LiMnPO4. It indicates that iron doping improved the diffusion velocity of lithium ions in the material. |
PDF Full Text Request