Preparation And Properties Of LiFePO₄/PANI Composites For The Cathode Material Of Lithium Ion Battery

LiFePO₄ was synthesized by a low-temperature solvothermal method and then a heat treatment with sucrose as carbon source was carried out to give carbon-coated LiFePO₄?C-LiFePO₄?cathodes.The crystal structure and electrochemical performance of the as-prepared samples were characterized by X-ray diffraction?XRD?,scanning electron microscopy?SEM?,transmission electron microscopy?TEM?,galvanostatic charge-discharge and cyclic voltammetry?CV?tests.The results indicated that the LiFePO₄ synthesized at relative low-temperature?120??with glycerol as a solvent had a uniform olivine-type crystal structure and a spindle-shaped morphology.The particle size was affected seriously by the reactant concentration,e.g.became smaller with the increasing of reactant concentration,but the particles easily agglomerated at much higher concentration.Comparatively,C-LiFePO₄ synthesized at 0.5 M(C-LFP_0.5)showed better charge-discharge capabilities than others synthesized at 0.1 M and 1.0 M of reactant concentration.Galvanostatic charge-discharge tests at various rates at room temperature showed that the as-prepared C-LFP_0.5 sample presented the initial discharge specific capacities of 153.9?147.0?135.4?123.0?113.3 and 84.7 mAh·g-1 at the rate of 0.1?0.2?0.5?1.2 and 5 C,respectively.Also,the discharge capacity could recover to the original level when the rate restored to 0.1 C.In addition,cyclic voltammetry?CV?test indicated that C-LFP_0.5 also had a better reversible than other samples when Li+ insertion/extraction in it.The doped polyaniline?PANI?were prepared by the chemical oxidative polymerization.Hydrochloric acid?HCl?,dodecylbenzene sulfonic acid?DBSA?and camphor sulfonic acid?CSA?were used as dopant,respectively.The morphology and structure were characterized by scanning electron microscopy?SEM?,infrared?IR?spectroscopy and ultraviolet visible?UV?spectroscopy.The charge-discharge performance was investigated as the cathode material in lithium ion batteries by galvanostatic charge-discharge test.The results showed that aggregation structure particles were observed in the undoped and doped PANI.The size of particles changed with different acids as dopant.With DBSA and CSA as the organic doping acids,the conductivity of PANI was higher and the transition of charge was easier.Galvanostatic charge-discharge test indicated that doped PANI showed high specific capacities as cathode material in lithium ion batteries.CSA doped PANI?PANI-CSA?had a better charge-discharge performance.The initial charge and discharge specific capacities of PANI-CSA were 125.3 and 118.2 mAh·g-1,respectively,at a rate of 17 mA·h-1·g-1 and after 50 cycles it still kept at about 82.5 mAh·g-1.C-LiFePO₄/PANI-CSA composites for cathode material were prepared by solution blending PANI-CSA with various C-LiFePO₄ samples.The structure and electrochemical performance of the as-prepared samples were characterized by scanning electron microscopy?SEM?,thermogravimetric analysis?TG-DTA?,galvanostatic charge-discharge and electrochemical impedance spectroscope?EIS?tests.The results showed that PANI could coat effectively on the surface of C-LiFePO₄ particles by solution-blending method.Electrochemical performance of C-LiFePO₄ could be improved by coating with a certain amount of PANI.Especially for C-LFP_0.5,C-LFP_0.5/10%PANI-CSA?coating with 10%?composite had an initial discharge specific capacity of 165.3 mAh·g-1 at 0.1 C and the discharge specific capacities at 0.2,0.5,1,2 and 5 C could reach about 154?141.9?133.7?124.8? 108.7 mAh·g-1,respectively.In addition,AC impedance analysis indicated that C-LiFePO₄/PANI-CSA composites had a lower charge transfer resistance at the interface than the parent C-LiFePO₄ particles.