Optimization Of Carbon Coating Of LiFePO₄ And Pilot Plant Test

Olive type LiFePO4 as cathode material of Li-ion batteries has attracted the eyes from both academy and industry world because of its high specific capacity, long cycle life and calendar life, high safety and low cost. For the purpose of application, we try to optimize carbon coating during the synthesis of LiFePO4 using either solid-state or aqueous precipitation method, and discuss the influence on the electrochemical performance of LiFePO4 Besides, we conducted the pilot plant test of the LiFePO4/C composites on a 10 kg scale and mass production on a 100 kg scale, during which we discuss the mechanism of process conditions that influence the physical and electrochemical performance of LiFePO4 cathode material.Ferrocene has been chosen as the catalyst of graphitizing material during one step solid state synthesis of LiFePO4. Carbon coating was improved by lowering D/G(disordered/graphene) ratio and increasing sp2/sp3 ratio. As a result, the conductivity and the electrochemical performance was improved. Moreover, the catalization ability of ferrocene seems to be different for different kinds of carbon sources. As for polypropylene, the pyrolyze temperature was high and the pyrolyzed carbon was more easily catalyzed into sp2-coordinated structure, thus the conductivity was improved more significantly.It is common to relate ferrocene with Fe2O3 because they can both be reduced into nano Fe particles under reduction atmosphere. And hence we chose the precusor with Fe2O3 to catalyze carbon during carbon coating. Acetylene gas was used as carbon source and a CVD method was proposed to coating LiFePO4 with carbon. After some experiments, we discoverd that each system with Fe2O3 can catalyze acetylene into carbon nano fibers under certain conditions. And the process that carbon fiber grows is competing with that of LiFePO4. As for those precusor system composed of more active components, the growth temperature of carbon nano fibers should be lower. Thus for different kind of precusor, different CVD conditions shoule be adopted. Besides, the growth of carbon fibers comsumed some Fe2O3, which may reduce the content of active material thus the process should be well controled. In addition, we find that amorphous carbon is depositing along with carbon fibers, and forms a continous carbon film outside LiFePO4 particles, while its thickness increases with carbon content. This kind of film has a negative impact on the electrochemical performance by preventing Li ions transfering between electrolyte and cathode particles. Thus proper conditions should be controled to get balance between amouphous carbon and carbon nano-fibers, and increase the electronic conductivity without increasing carbon content, improve the specific capacity and energy density.In order to get a thin carbon film coating and good electronic conductivity, we prepared LiFePO4/graphene composites with co-precipitation method. The electrochemical performance of the composites was proved to be influenced by the intrinsic crystallinity and electronic conductivity of coating carbon. The intrinsic crystallinity can be improved by extending the reaction time of co-precipitation, and the electronic conductivity of coating carbon actually depends on the removing of oxygen containing groups, which can be improved by high temperature heat treatment. The specific capacity and rate performance of the composites can be improved by increasing the crystallinity of LiFePO4 and purity of graphene respectively.Also, we conducted the pilot plant test of LiFePO4 massive producing, during which we found the parameters that had influences on the morphology and electrochemical performance of LiFePO4. For example, wet ball milling can improve the homogeneity of the precusor, suitable temperature of spray dry can prevent the size of the secondary particles of the precusor from growing up. The pyrolyzing properties of the precusors should also be taken into accout when setting the temperature of the furnace. Expeling of moisture from the furnace in time made a significant improve of the atmosphere in the furnace and promise the purity of LiFePO4. Adding polymer as a second carbon source to improve the reduction atmosphere and a homogeneous carbon coating is also important. Ball mill of the product can improve the processing property of the cathode material. At last, we obtained products with good electrochemical performance on both 10 kg scale and 100 scale mass prodution.