Research On LiFePO₄-based Composites Prepared By Solvothermal And Microwave-solvothermal Methods

Olivine-type LiFeP04 has been subject to extensive study as a promising cathode material owing to its particular features such as high theoretical capacity(170 mAh·g-1),low cost,nontoxicity,and high safety.However,the poor lithium ion and electronic conductivity of LiFeP04 limited its applications.In our work,the synthesis process and modification of LiFeP04 had been studied after reviewed the development of Li-ion battery.LiFeP04 was synthesized by solvothermal method and microwave-solvothermal method using triethanola mine(TEA)and ethylene glycol(EG)as solvents,respectively,and then a calcinations processing with sucrose as carbon source was carried out to obtain LiFePO4/C cathodes.The crystal structure and the electrochemical performance of the prepared samples were characterized by X-ray diffraction(XRD),scanning electron microscopy(SEM),transmission electron microscopy(TEM),galvanostatic charge-discharge testing and electrochemical impedance spectroscopy(EIS).The results indicated that LiFePO4 synthesized at a low-temperature(120 ?)for 20 h with triethanola mine as solvent at a suitable pH had an olivine-type crystal structure.Spindle-like morphologies of LiFeP04 were obtained by using H2SO4 and CH3COOH as adjusting reagents to adjust the pH of the solvent to 7.0,but the as-prepared samples as cathode active material only delivered the initial discharge specific capacities of 38.2 mAh·g-1 and 51.9 mAh·g-1(0.1 C),respectively.While H3PO4 was adopted to mix into triethanola mine,the morphology and electrochemical performance of LiFePO4 changed obviously.And,as pH=8.6,Fe3(PO4)2· H2O phase and Li3P04 phase appeared,and then thin slice morphology of LiFeP04 was obtained after a calcination,which delivered the initial discharge specific capacities of 71.5 mAh·g-1 at 0.1C.With pH at 8.0,7.5 and 7.0,respectively,the obtained LiFePO4 presented self-assembled rod-like morphology,and with gradually decreaseing pH,its size decreased,Li3P04 phase disappeared and electrochemical performance improved.Specially,at pH=7.0,the as-prepared LiFePO4/C presented the initial discharge specific capacities of 151.4 mAh·g-1 at 0.1C,and showed good cycle and rate performances.After 40 cycles at 0.1C,it still kept about 150.1 mAh·g-1.At pH=6.6,the morphology of LiFePO4 changed to flake with length of 2?m and width of 500nm with a preferred crystal orientation at a(200)texture,and the initial discharge specific capacity was 80.2 mAh·g-1 at 0.1C.During the experimental processing,a salt named 2,2',2"-Nitrilotriethanol Phosphate(C6H15NO3.H3PO4)was found due to reacting of triethanola mine and H3PO4,which played the role of a novel phosphorus source,as well as a cheating agent to coordinate with metal ion by the N atom at different pH of the solvent,which may effect the structure and morphology of LiFeP04,obviously.LiFePO4 has also been synthesized at a low-temperature(160?)for 10 min by microwave-solvothermal method.During experiment,the relating synthesis conditions,such as temperature,reacting time and surfactant,were studied in detail.Results shown that the sample synthesized at 220 ? for 10 min following a heat-treatment exhibited excellent electrochemical performances,and the initial discharge specific capacity reach about 150.1 mAh·g-1 at 0.1C,even after 40 cycles,still keeping about 145.3 mAh·g-1.As 12.66wt%ascorbic acid was introduced as morphology controllable agent,because of its hindering effect,the particle size of LiFePO4 was downsized,and the morphology changed from spindle-like to cube.After calcinations,obtained LiFeP04/C cathode delivered the initial discharge specific capacity of 139.5 mAh· g-1 at 0.1C,and 91.1mAh g-1 at 10C.