LiMnPO₄（LiCoPO₄） Cathode Materials For Li-ion Battery

With the large-scale application of lithium-ion batteries in electric vehicles and energystorage batteries, this put forward higher requirements on the safety performance and theenergy density of the battery. LiFePO4lithium transition-metal phosphate system material hasbecome the first choice of the large capacity power battery due to their good cyclicperformance, excellent chemical and thermal stability, low toxicity, and low cost. LiMnPO4and LiCoPO4have similar olivine structure with high potential, high energy density. Both arepromising lithium ion battery cathode materials.LiMnPO4cathode material has attracted attention due to high voltage, stable structure,and high energy density. However LiMnPO4has poor ionic and electronic conductivity, carbon coating and ion doping are the main effective methods to improve its performance.First, the influences of different synthesis method on the electrochemical properties of theLiMnPO4materials were investigated. LiMnPO4/C materials were synthesized by solid-statereaction. The pure LiMnPO4phases can be obtained under different sintering temperatures,carbon content, and carbon sources. But the electrochemical performence of the LiMnPO4materials is not ideal. The reason may be due to that the particle of LiMnPO4/C materialprepared by solid-state method is too large. So it can not improve the poor electronicconductivity. The LiMnPO4/C materials is also synthesized by rheological phase reaction, butimpurity phase (Mn3(PO4)2) was detected in the XRD pattern. The LiMnPO4/C has beenprepared by hydrothermal method, XRD results show that the samples synthesized at160oCless than8h have impurity phase. When prolonging the reaction time or increasing thetemperature, impurity phase disappears. The LiMnPO4/C material synthesized at190oC andincorporated in40%carbon source has the relatively well electrochemical performance.Carbon content does not affect the structure but the morphology and electrochemicalproperties of the material.In order to overcome the poor conductivity of LiMnPO4, we choose the Fe2+ions thathave the similar ionic radius with Mn2+ions to improve its electrochemical properties. TheLiMn1-xFexPO4/C cathode material was synthesized by hydrothermal method. The effect ofreaction time, carbon content, carbon source and iron content on their physical properties andelectrochemical performance were studied. The results showed that when using sucrose ascarbon source, the LiMn0.5Fe0.5PO4/C synthesized by hydrothermal reaction180oC for12hand15%carbon source has the best surface morphology and electrochemical properties. Thesample delivers the discharge capacity of142.6mAh g-1as well as having a good cycleperformance. Compared with four different carbon sources, the LiMn0.5Fe0.5PO4/Csynthesized with glucose has the best electrochemical performance. Fe doping also has a greatinfluence on the performance of the material, when amount of Fe is x=0.5, the LiMn1-xFexPO4/C has the highest initial discharge capacity and low polarization degree.We also adopt LiMnPO4-Li3V2(PO4)3composite to improve the performance of thematerial. Li3V2(PO4)3belongs to the fast ion conductor and possess stable structure, goodcycle stability and high capacity. Li3V2(PO4)3-LiMnPO4composite can effectively overcomeLiMnPO4shortcomings. The Li3V2(PO4)3-LiMnPO4/C composite material prepared bysolid-state method, the effect of different calcination temperature and carbon content onelectrochemical performance is further studied. The material synthesized at650oC,10%carbon content, has better electrochemical properties.In addition, LiCoPO4has the advantages of high voltage, high energy density, high safety,good cyclic performance, and excellent stability, however low electronic conductivity andlithium ion diffusion coefficient limits the application. The Li3V2(PO4)3-LiCoPO4/C materialsare synthesized by solid-state method for the first time. XRD results show that both the peaksof orthorhombic Li3V2(PO4)3and monoclinic Li3V2(PO4)3appear in the XRD patterns of thecompsite materials and no impurity phase appears. At the same time, compared with otherproportion, Li3V2(PO4)3-LiCoPO4/C with1:1mole ratio has the best electrochemicalproperties.