Electrochemical Properties Of Olivine-type Phosphate LiFe_xMn_yCo_1-x-yPO₄

The accurate determination of the state of charge （SOC） is very important, when thelithium ion battery is used as the power supply in electric bicycles or in electric vehicles.Usually, the cost of the electronic devices to determine the SOC is10²0%of the total cost.The flat operation voltage of single phase olivine-type phosphate is not good to determinethe SOC. In this work multi-phosphate was proposed to use as the lithium ion batterypositive electrode material. The existence of the multi-operation-voltage makes the SOCdetermination more simple and makes the cost lower. Having a multi-operation-voltage,the material should have good electrochemical properties as possible.In the present work, the research was focused on the lithium ion battery positiveelectrode material, olivine-type phosphate LiFe_xMn_yCo_1-x-yPO₄. The influence of thecomposition and surface coating on the electrochemical performance was studied. It isexpected to synthesize olivine-type phosphate LiFexMnyCo1-x-yPO₄exhibiting highcomprehensive electrochemical properties and having multi-operation-voltage plateaus.（1） Firstly, the single-phase LiFePO₄and LiCoPO₄were studied. LiFePO₄andLiCoPO₄that contain different carbon content were synthesized via high-temperature solidstate reaction method. The influence of the heating temperature and the carbon content ontheir electrode properties was investigated, and the preparation condition was optimized.At last the optimum condition has been found. On basis of the above-mentioned work, wesynthesize LiFe_0.3Mn_0.6Co_0.1PO₄and measure the purity and the particle size of theproducts with help of the XRD, SEM etc techniques, and investigate charging-dischargingbehavior and cycling stability by electrochemical techniques. The results showed that theoptimum preparation condition is nitrogen atmosphere,700℃to heat and10wt.%sucroseadditive. The discharge capacity is120mAh/g, and it is89mAh/g after50cycles.（2） The electrochemical properties of high-Mn-content LiFe_xMn_yCo_1-x-yPO₄werestudied. The purpose to select high-Mn content material is to increase the energy density,because the operation voltage of Mn³⁺/Mn²⁺couple is4.1V, while the operation voltage of Fe³⁺/Fe²⁺couple is3.5V. however, the results showed that the higher Fe-content favoriteelectrode properties. For the sample with x=0.4, the discharge capacity is112mAh/g at0.1C rate at the1stcycle and is100mAh/g at the50^thcycle. The capacity retention is89.3%.the discharge capacity is100mAh/g at1C rate.（3） The influence of Ag-coating on the LiFe_0.4Mn_0.6PO₄electrode was also studied.The results showed that the optimum Ag-content is3wt.%. the sample has a109.9mAh/gcapacity, and has capacity retention of97.4after50cycles. The discharge capacity canreach106.9mAh/g at1C rate. From the cyclic voltammetric measurements it can be foundthat the difference between oxidation and reduction peak potentials becomes smaller.Electrochemical impedance spectroscopy measurements explored that the Ag-coating candecrease the charge transfer resistance. But, the in-depth investigation explored thatAg-coating has a negative effect on the battery safety. Our experiment has confirmed thatAg-coating is not available if the positive electrode operates at more than3.7V.