Modification Study Of LiFePO₄ Used As Cathode Materials For Lithium Ion Battery

Presently Lithium Iron Phosphate has become more and more concerned for its so many obvious characters such as few toxic, low raw material cost, high specific capacity, excellent cycle performance and safety, and it is considered to be the most promising cathode material for dynamical battery. However, the low electronic conductivity and tap density must be conquered with the practicability. For the industrial practicability, LiFePO₄/C composite materials were synthesized by adopting cheap carbon sources to improve the electrochemical characters. The crystalline structure, morphology of composite particles and the charge-discharge performances of the samples were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and galvanostatic charging-discharge experiments. The dipping method was used to enhance the tap density.The high temperature solid-state reaction method with milling was used to synthesize the LiFePO₄/C composite cathode materials by doping different carbon sources (sucrose, coal tar asphaltum and petroleum asphaltum). The results showed that the LiFePO₄/C samples, which possessed good reversibility and cycle performance, were pure olive-type structure and spherical particles whose diameter was nm level. The materials synthesized by doping petroleum asphaltum owned the most excellent electrochemical performance. The specific capacitiy was 165mAh/g at the rate of 0.1C and wasn't reduced after 30 cycles. The specific capacities were 160mAh/g and 153mAh/g at the rate of 0.5C and 1C, respectively.The LiFePO₄/C composite cathode materials with different amount of carbon were synthesized by doping different amount of petroleum asphaltum. The results showed that the LiFePO₄/C samples, which possessed good reversibility and cycle performance, were pure olive-type structure. The diameter of the particles and the tap density were reduced as the increase of the amount of petroleum asphaltum. When the amount of petroleum asphaltum was changed between 5mass% and 16mass%, the specific capacities of the LiFePO₄/C composite cathode materials were enhanced as the increase of the amount of petroleum asphaltum. But the specific capacities were reduced as excess petroleum asphaltum doped when the amount of petroleum asphaltum overran 16mass%. That is to say that the most suitable amount of petroleum asphaltum was 16mass%.(NH₄)₂Fe(SO₄)₂·6H₂O, LiNO₃ and H₃PO₄ were adopted to confect the dipping solution and the dipping method was used to improve the tap density of the LiFePO₄/C composite cathode materials. The results showed that LiFePO₄/C composites after dipping were still pure olive-type structure and the tap density was enhanced from 1.40g/cm³ before dipping to 1.64g/cm³. But the reversibility and cycle performance were poorer for the weakened crystallization, reduced amount of carbon, impurity and cracks engendered in the LiFePO₄/C particles. It needs deep study to improve the tap density of LiFePO₄/C materials.