The Research On The Preparation And The Electrochemical Performance Of LifepO₄/MWCNTs

Among the well-known Li-inserted cathode materials for lithium ion secondary battery, the olivine-type LiFePO4is considered as one of the most promising cathode material for lithium-ion power battery because of its low cost, good cycling stability, better thermal stability, excellent security and environment-friendly performance. However, LiFePO4has a main drawback of low electronic and ionic conductivity that hinders it to be commercialized. Currently, some traditional methods as CV, PITT, GITT or EIS had been used to studied lithium-ion conduction mechanism in LiFePO4. But, the feasibility of these methods used to study the Li-inserted electrodes, especially the LiFePO4electrode with a process of two-phase transformation, had not been systematically studied. In this article, the lithium-ion conduction mechanism was systematically studied based on the Li-inserted process of LiFePO4. In addition, the electronic and ionic conductivity of LiFePO4had been improved largely. But the performance of LiFePO4coated with carbon nanotube was not studied comprehensively. In this paper, comprehensive research to LiFePO4coated with carbon nanotubes had been done. The main resuits and conclusions were shown as following.Study on the electrode kinetics of LiFePO4. Potential scanning curves got by cyclic voltammetry could be used to judge the value of the Exchange current density to determine the reversible properties of the material.Research a variety of traditional techniques used to make a determination of lithium ion diffusion coefficient, and point out the shortcomings. After in-depth study, we found that the lithium ions transferring into cathode material were similar as the nonequilibrium carrier in semiconductor. The inpouring Li-ions can be called nonequilibrium Li-ions. Than the diffusion equation should be revised. The electrochemical diffusion coefficient, named as D, can be worked out approximately in terms of the relationship between the nonequilibrium Li-ions and the relevant potential difference. The diffusion coefficient got by this approach better reflects the kinetics of LiFePO4cathode materials. The conclusion of this study can provide guides in theory for the preparation, modification and testing methods of LiFePO4. With the new method the D was determined as9.21×10-ncm2/s for LiFePO4doped with10%MWCNTs. The order of this value was almost consistent with the one got by CV, PITT and GITT. But it was kept invariable VS. x in LixFePO4.The LiFePO4/MWCNTs composite was synthesized by ball milling. The XRD spectrum demonstrated that MWCNTs didn’t change the olivine structure of LiFePO4and the SEM showed that MWCNTs decentralized into the grains of LiFePO4. The result of electrochemical test indicated that the composite with the MWCNTs in size of60-100nm and length of1-2um exhibited the best electrochemical performance. The first charge-discharge specific capacities of this composite were136mAh/g and129mAh/g respectively at0.1C ate in room temperature. The first capacity lost was5.2%. The difference between charge-discharge platforms of this composite was the least compared to the others. This result showed that the material had the largest electrochemical diffusion coefficient of lithium. At the same time, the capacity of the composite only lost4.0%after10cycles.Study on optimization of milling time. The LiFePO4/MWCNTs milled12h with the same carbon nanotubes exhibited the best electrochemical performance. The first charge-discharge specific capacities of this composite were132.8mAh/g and126.3mAh/g respectively at0.1C rate in room temperature. The first capacity lost was4.9%. This shows that the extension of ball milling time can be more effectively dispersed carbon nanotubes. Due to destruction of carbon nanotubes, the electrochemical properties of16h milling material rather than was decline. The breaking part of the carbon nanotube could be combined with lithium ion. The irreversible combination could come down the electrochemical performance.Study on supersonic dispersion method for LiFePO4with10%carbon nanotubes in size of60-100nm, length of1-2um, and purity of>95wt%. The SEM test showed that the greater supersonic power the better dispersion effect of carbon nanotubes. The LiFePO4/MWCNTs dispersed60min at100%power in20℃exhibited the best electrochemical performance. The first charge-discharge specific capacities of this composite were138.1mAh/g and132.2mAh/g respectively at0.1C rate in room temperature. The first capacity lost was4.3%.Study on optimization of ultrasonic ambient temperature. The SEM tests showed that the greater the ambient temperature the better dispersion effect of carbon nanotubes. Electrochemical tests showed that the reversible performance of material increaseed within the range of20℃to60℃and kept constant from60℃to80℃. The capacity was diminished when ambient temperature was greater than60℃. This result could be explained by the increased activity of carbon. Some of the active carbon atoms could be combined with lithium ion. The irreversible combination could come down the capacity. The LiFePO4/MWCNTs dispersed60min at100%power in60℃exhibited the best electrochemical performance. The first charge-discharge specific capacities of this composite were145.4mAh/g and137.9mAh/g respectively at0.1C rate in room temperature. The first capacity lost was5.2%.The effect of the LiFePO4mixed with10%acidized MWCNTs by supersonic dispersed method were investigated. The MWCNTs in size of60-100nm, length of5-15μm, purity of>95wt%were treated by different concentration acid mixed with sulfuric acid and nitrate. The SEM tests of carbon nanotubes showed that the swarm of carbon nanotube was separated as the concentrations increasing. The impurities in carbon nanotube were got rid of after acid treatment. The LiFePO4mixed with the MWCNTs acidized by mixture of sulfuric acid(7.2mol/L) and nitrate(6mol/L) exhibited the best electrochemical performance. The first charge-discharge specific capacities of this composite were144.4mAh/g and138.3mAh/g respectively at0.1C rate in room temperature. The first capacity lost was4.1%.Study on optimization of acid temperature. The MWCNTs in size of60-100nm, length of5-15℃m, purity of>95wt%were treated by the mixture of sulfuric acid(7.2mol/L) and nitric acid(6mol/L) using reflux device. The SEM test of carbon nanotube showed that the carbon nanotubes had been purified and separated entirely after heated acid-treated. The LiFePO4mixed with the MWCNTs acidized at40℃ambient temperature exhibited the best electrochemical performance. The first charge-discharge specific capacities of this composite were150.2mAh/g and143.8mAh/g respectively at0.1C rate in room temperature. The first capacity lost was4.3%. Removing the MWCNTs the first charge-discharge specific capacities of LiFePO4were166.8mAh/g and159.8mAh/g,98%and94%of the theory value respectively. After50cycles, discharge specific capacity still remained in132.8mAh/g at0.5C rate. The charge-discharge specific capacities at1C rate were130.1mAh/g and119.8mAh/g respectively. Then a conclusion would be made that the high temperature acid treatment diminished the electrochemical performance of LiFePO4. The reason was that nitric could oxidize carbon nanotube to hydroxyl and carboxyl functional groups in surface. Such groups containing oxygen would improve the dispersion property of carbon nanotubes to a certain extent, and then improve the conductivity of LiFePO4. However, the oxygen-containing functional groups would be combined with lithium ion.The effect of LiFePO4/MWCNTs synthesized by microwave was investigated. The MWCNTs in size of60-100nm, length of1-2um, were doped by ball milled in raw material mixed with FeC2O4·2H2O, Li2CO3and NH4H2PO4. The ratio of the MWCNTs to the final composite was10%. The SEM test has shown that, with the time of microwave radiation being prolonged, the size of the material particles increased. And the carbon nanotubes were embedded into the material particles. The XRD test results showed that while the time of microwave radiation at100%power was greater than9min, Fe2P began to emerge. The LiFePO4/MWCNTs heated9min at100%power exhibited the best electrochemical performance. The first charge-discharge specific capacities of this composite were126.0mAh/g and110.3mAh/g respectively at0.1C rate in room temperature. The first capacity lost was12.4%.Study on optimization of the settings of microwave oven’s time and power. The rule of settings kept the product of time and power in equal. The SEM tests show that the material heated12min by80%power and9min by100%power contained finer particles.The LiFeP/VMWCNTs heated12min at80%power exhibited the best electrochemical performance. The first charge-discharge specific capacities of this composite were141.9mAh/g and133.9mAh/g respectively at0.1C rate in room temperature. The first capacity lost was5.6%. Removing the MWCNTs the first charge-discharge specific capacities of LiFePO4were157.7mAh/g and148.7mAh/g,92.7%and87.4%of the theory value respectively. After50cycles, discharge specific capacity still remained in116.0mAh/g at0.5C rate. The charge-discharge specific capacities at1C rate were114.4mAh/g and106.9mAh/g respectively. Therefore, microwave radiation at80%power was large enough to reach the reaction temperature. When reaction temperature is reached, the longer to be radiated, there was more resultant.