| Olivene structured LiMnPO4 is one of the most promising cathode materials for lithium ion batteries due to various advantages such as high energy density, low cost, environmental friendliness and high cycling stability. However, the intrisincly low electronic and ionic conductivity of LiMnPO4 have largely limited its electrochemical performance, especially at higher current densities, which has became a major barrier for its practical application. For the purpose of improving the electrochemical performances of LiMnPO4, solid state reaction and reflux method were used to prepare carbon coated nano structured LiMnP04. Besides, cations doping or substitution was used to further improve the electrochemical performances of carbon coated LiMnPO4Oleic acid was used as surfactant and carbon source to prepare nano LiMnPO4 during solid state reaction, the impacts of oleic acid and cations doping on the electrochemical performances of LiMnPO4 were carefully studied. Due to the hydrophilic and lipophilic feature of oleic acid, the particle size of precursor prepared by ball-milling can be controlled to about 100 nm by surface adsorption. Furthermore, the carbon layer which came from the decomposition of oleic acid during hot treatment can prevent LiMnPO4 particles from agglomeration and abnormal growth, resulting in well dispersed and nano sized LiMnPO4. Electrochemical tests show that LiMnPO4 prepared with oleic acid displays good capacity and cycling performance. Cations doping shows that Mg2+ and Ni2+ doping are harmful for the electrochemical performance of LiMnPO4, while Fe2+ and Co2+ doping can largely improve its electrochemical performance. For instance, Co2+ doped sample shows a capacity of 144 mAh/g at a current rate of 0.05 C. It was found that the Li ion diffusion coefficient in LiMnPO4 can be improved while the charge transfer resistance can be decreased by Co2+ doping, leading to improved electrochemical performances.LiMnPO4 cathode material with nano rod structure was prepared by one step reflux method for the first time, the formation mechanism of its morphology was carefully studied. Experiment results revealed that LiMnPO4 nano rods were prepared at suitable PH value by using PEG 400 as surfactant. LiMnPO4 prepared by reflux method only shows 58 mAh/g at 0.1 C. Therefore, LiFexMn1-xPO4(x=0.2,0.4) were prepared to study the impacts of Fe2+ substitution on the electrochemical performances of LiMnP04. Experiment results show that Fe2+ substitution had no obvious impact on the morphology of the reflux products. However, galvanostatic cycling shows that the capacity of LiMnP04 can be largely increased by Fe2+ substitution. At a current rate of 0.1 C. the capacity of samples x=0.2 and 0.4 were 110 and 141 mAh/g, respectively. CV result shows that the Fe2+ substitution can decrease the polarization during charging/discharging, accelerating the electrochemical process. EIS result shows that the Fe2+ substitution can decrease the charge transfer resistance, as well as increase the Li-ion diffusion coefficient in the bulk material, resulting in improved electrochemical performance.Carbon coated LiFe1/3Mn1/3Co1/3PO4 was prepared by solid state reaction with stearic acid as carbon source, the impacts of heating temperatures on the morphology and electrochemical performance of final products were carefully studied. The particle size did not increase obviously with the temperature growth, which can be attributed to the well formed carbon coating provided by stearic acid. At the same time, due to well coated carbon layer and fine crystallinity for all samples, the heating temperature shows little impact on the electrochemical performance of LiFe1/3Mn1/3Co1/3PO4, whose charge/discharge curves at different current rates nearly overlapped with each other. LiFe1/3Mn1/3Co1/3PO4 prepared at different temperatures all show a reversible capacity of 120 mAh/g at 0.05 C, and a good cycling performance can be seen for each other. |
PDF Full Text Request