Synthesis and Modification of Olivine-type LiFePO4and High Voltage Spinel-type LiNi0.5Mn1.5O4as Cathode Materials for Lithium Batteries
|School||Northeast Normal University|
|Course||Physical and chemical|
|Keywords||lithium-ion battery cathode LiFePO4 LiNi0.5Mn1.5O4|
In recent years, in order to solve the problems of energy crisis and environmentalpollution, electric cars as a distinguished representative of clean energy cars, replacing thetraditional motor vehicles has become the inevitable trend of the development of the industry.Lithium-ion batteries appear as the preferred power supply for vehicles due to itsoverwhelming advantages of its high energy density and good cycle performance. Thedevelopment of cathode materials is gaining particular interest and a hot spot, because it is akey to improving the performance of lithium ion battery and decreasing costs.Olivine-type LiFePO4is a promising candidate cathode material because of its highenergy density, low cost, environmental compatibility and safety. LiFePO4has a suitabletheoretical capacity of170mAh g-1, a flat discharge potential of3.4V （vs. Li+/Li） as well asexcellent thermal stability and cycling performance. However, it has inherent poor electronicconductivity （109S cm-1） and slow lithium ion diffusivity, which are disadvantageous forits application in secondary batteries. In view of these issues, we presented our proposals onthe basis of previous studies. It is demonstrated that LiFePO4material with excellentelectrochemical performance has been synthesized. In addition, in order to meet the growingdemand of large energy storage systems, much attention has been devoted to the investigationon the cathode material with high voltage to increase the energy density. SpinelLiNi0.5Mn1.5O4display a flat voltage profile at4.7V with the theoretical capacity of147mAhg-1, therefore contributing to high energy density650Wh kg-1, higher than the LiFePO4（500Wh kg-1）, is a very promising cathode materials. Main contents are listed as follows:1. We synthesized three different crystal structures of iron phosphate, includingamorphous structure, monoclinic structure, as well as of α-quartz structure. Then LiFePO4/Cis prepared with iron phosphate as iron source. Tests on the electrochemical properties ofLiFePO4/C show that amorphous structure of iron phosphate is the most suitable crystalstructure as iron source to synthesize LiFePO4.2. Iron phosphate was prepared by two routes from FeSO4·7H2O. One is the formationof Fe3（PO4）2precipitate in the first step and subsequent oxidation to FePO4precipitate. Theother is the oxidation of ferrous to ferric ion firstly, and then to form FePO4precipitatedirectly. LiFePO4cathode material is synthesized by a simple solid-state reaction methodwith the two kinds of FePO4·2H2O as iron source and citric acid as carbon source. This studyexamines the effects of different oxidation routes to prepare FePO4·2H2O on theelectrochemical performance of as-synthesized LiFePO4. The results indicate iron phosphate obtained through one step precipitation has a smaller particle size, more uniform particledistribution, higher conductivity and faster diffusion rate of lithium ion, which isdemonstrated to be more applicable as the iron source to synthesize LiFePO4/C. As-preparedLiFePO4/C shows an excellent rate capability and cycle performance. The initial dischargecapacities of160.6mAh g-1and107mAh g-1are achieved at0.1C and10C, respectively.The good capacity retention of97%after300cycles is maintained at the rate of5C.3. Nanoscale LiFePO4/C particles are synthesized through a combination ofelectrospinning （ES） and annealing. An obvious advantage of electrospinning method lies ingetting the separated nanofibers precursor easily, which change the arrangement manneramong particles and impede the growth and agglomeration of the LiFePO4particles,contributing to the formation of nanosized LiFePO4particles. Transporting in nanoparticlesystems typically encompasses shorter transport lengths for Li+transport, and speeds up thediffusion of lithium ions in LiFePO4. Here, Polyvinylpyrrolidone （PVP） is used as thefiberforming agent of electrospinning, which is also needed as a reducing agent and carbonsource. In situ carbon-coated LiFePO4particles are achieved by the pyrolysis of PVP duringthe thermal treatment. The LiFePO4particles coated and connected by interlaced carbon areuniformly distributed in the range of5080nm. It is found that as-prepared nanoscaleLiFePO4/C composite has a desirable electrochemical performance. It offers a dischargecapacity of163.5mAh g-1and110.7mAh g-1at the rate of0.1C and10C, respectively. Inaddition, this cathode provides an excellent cyclability with capacity loss of less than3%at0.1C and5%at5C after500cycles. The excellent electrochemical performance can beattributed to the conductive carbon network and nanosized LiFePO4particle together.4. Sub-micrometric LiNi0.5Mn1.5O4cathode material is prepared using a combination ofelectrospinning and annealing. Well-separated nanofiber precursors through electrospinningimpede the growth and agglomeration of LiNi0.5Mn1.5O4particles. The sample with Fd3mspace group possesses a well-grown, uniform and small octahedral crystals with the sizeabout150200nm. Sub-micrometric size is facile to reduce the transport path lengths oflithium ions and electrons and increase the electrode-electrolyte interface, which contributesto the favorable electrochemical performance. At0.1C rate, it shows an initial dischargecapacity of132.7mAh g-1with capacity retention rate over96%after100cycles. Under therate of5C, it can still deliver a discharge capacity of116.7mAh g-1.