Synthesis And Properties Improvement Of Novel Cathode Material LiFePO₄/C For Lithium-ion Battery

Lithium-ion battery is developed to the directions of high energy density, high safety and excellent electrochemical properties. The researches are focused on the electrode materials, electrolyte, separator and interrelated techniques. LiFePO₄ is one of the most promising cathode materials for the lithium-ion battery because it is abundant, environmentally benign, stable, safe and has a theoretical specific capacity of 170mAh/g. The main problem of LiFePO₄ is low electronic conductivity and low lithium ion diffusivity. The volumetric energy density is also low, which is resulted from the low gravimetric density of the material. In order to overcome these defaults, the first way is to synthesize small fine particles of LiFePO₄ through improved methods, the second way is to coat LiFePO₄ with electronically conductive materials such as carbon or metal powder, the final way is to substitute Li or Fe by other metal ions to enhance the inherent electronic conductivity.In this paper, LiFePO₄ coated with carbon was synthesized through one-step solid-state reaction from the idea of coating. The carbon came from sucrose. The LiFePO₄/C was characterized by TG/DTA, XRD, EA, SEM, CV analysis. We also studied the electrochemical properties of the product at different conditions. The results indicated that the sample sintered at 600℃ for 24h showed the best electrochemical properties. The first discharge capacity was 125mAh/g with a current density of 10mA/g. When the density increased to 100mA/g, the fist discharge capacity decreased to 100mAh/g.Rheological phase reaction has been used to synthesize electrode materials of lithium-ion battery successfully, such as LiMn₂O₄, LiMnO₂, LiNiO₂, ZnCo₂O₄ and so on". Here, the rheological phase reaction was introduced to synthesize LiFePO₄/C material, and the carbon on the particles came from citric acid, which acted as a new carbon source. The material showed an excellent electrochemical property, especially the high rate property. The first discharge capacity is 121mAh/g with a current density of 10mA/g, and increased to 135mAh/g after 10 cycles. When the density increased to 100mA/g, the fist discharge capacity decreased to 110mAh/g, and retained over 100mAh/g after 100 cycles, whichindicated that the coat of carbon can improve the electronic conductivity efficiently. We also studied the electrochemical properties of the LiFePO4/C under 55 "C. These results indicated that the LiFePO4/C can be synthesized by rheological phase reaction using lower temperature and fewer sintering time comparing to the solid state method.There are two regions in the charge-discharge process of LiFei.xMnxPO4/C. One is the potential of the Mn3+/Mn2+vs. Li/Li+, which can be used as the 4V cathode material. The other is the potential of the Fe3+/Fe2+ vs. Li/Li+. Here, a series of LiFei.xMnxP04 were obtained by rheological phase reaction. The results showed that the LiFeo.4Mno.6PO4 has an electrochemical activity. Our future work is to optimize the synthesized conditions to improve the electrochemical properties of the material.Finally, we discussed the thermal stability of three common cathode materials using differential scanning calorimetry (DSC) method, including LiMn2O4, LiNio.85Coo.15O2 andLiFePO4. The results indicated that the order of the thermal stability is LiFePO4 > LiMn2O4> LiNio.85Coo.15O2, which approved the superiority of LiFePO4 used as the cathode material for the lithium-ion battery.