Synthesis Of LiFePO4_/C Thin Films And Hierarchical LiFePO₄Mesocrystals And Study On Their Electrochemical Performance

As one of the most promising cathode materials for new generation lithium ion batteries (LIBs), lithium iron phosphate has been attracting a lot of concerns and interests. With the fast development of electric and medical equipments, the energy conversion and storage devices as well as electric vehicles and hybrid electric vehicles, the need for high power and high energy thin film LIBs and large scale LIBs grows to a high level. To address the intrinsic low conductivity problem of LFP, LiFePO4/C thin films with high power and high energy and LiFePO4mesocrystals with large surface area and high porosity are investigated.By using LiFePO4plate-like crystals obtained from liquid chemical methods, tunable LiFePO4/C thin films demonstrating high power were fabricated by drop-casting route. The thickness, carbon content, and morphology of the LiFePO4/C thin film cathode can be adjusted by simply changing the amount of drop-cast, sucrose solution concentration and LiFePO4raw material. Carbon pyrolized from sucrose has a great effect on the surface morphology, micro-structure and electrochemical properties of the LiFePO4/C thin film cathode. Carbon from sucrose pyrolysis served as both conducting additive and adhesion binder. The resultant binder-free films consisted of well-packed LiFePO4plates coated with nano-carbon, a result of sucrose pyrolysis, and were directly assembled into cells for testing, totally get rid of the conductive additive and binder mixing pre-treatment process in the conventional method.This drop-casting route showed advantages as follows:a) the sucrose solution can stop the re-aggregation of the LiFePO4thin plates during the formation of the precursor LiFePO4/C thin films; b) carbon from sucrose pyrolysis can be coated on the LiFePO4thin plates homogeneously to enhance the electrochemical properties of the thin films; c) extra carbon can form an uniform web connected the whole active material and provided a strong adhesion to maintain the composite film stable on the Ti substrate during longtime cyclic processes.The effects of carbon content on the microstructure and electrochemical properties of such LiFePO4/C thin films were investigated. For micro-sized LiFePO4plate materials, it was found that23wt%carbon content could effectively improve the conductivity of the LifePO4material and keep the thin films intact.The obtained LiFePO4/C thin films showed great lithium intercalation/deintercalation properties and excellent cyclic stability, suitable for high power and high energy Li ion thin film micro batteries. The outstanding electrochemical performance of the LiFePO4/C thin films should be attributed to the high porosity of the films and fine dispersion of the LiFePO4plates that guarantees large surface area to ensure fast phase penetration and well coated nano-carbon webs to improve the charge-transfer properties and phase transition.Three dimensional hierarchical lithium iron phosphate mesocrystals were successfully synthesized via a simple one-step, rapid solvothermal route without any surfactant or additive. Co-solvents with various kinds and different ratio were applied as reaction media during the solvothermal synthesis. LiFePO4mesocrystals with different morphology and prime unites can be obtained by changing the ratio of the co-solvent and the concentration of the reactants. The effect of co-solvent on the morphology and crystallinity of the products were carefully studied, and it was found that the co-solvent played an important role in the synthesis of the LiFePO4mesocrystals and had a great effect on the morphology and composition units of the LiFePO4mesocrystals.Time-dependent solvothermal synthesis experiments were carefully carried out to study the formation mechanism of the dumbbell-like and flower-like LiFePO4mesocrystals, and it was likely taking a two step process:1) nuclearation growth of Li3PO4crystals and the formation of LFP nano-crystals from precursor particles;2) phase transformation from Li3PO4to LiFePO4crystals combined with self-assembly to LFP mesocrystals.The electrochemical performance of LiFePO4mesocrystals with different morphology and composition units were also investigated. It was found that the size of the small units and the whole structure had a great influence on the electrochemical performance of the LiFePO4mesocrystals under low rate testing condition, while the influence became much less essential when the testing rate increased.The obtained LiFePO4mesocrystals can be in situ carbon coated, and the electrochemical performance of the LiFePO4/C mesocrystals were effectively improved after carbon coating. High lithium-ion intercalation capacity of140mAh·g-1and143mAh·g-1were exhibited by the dumbbell-like LiFePO4and LiFePO4/C mesocrystal respectively when tested under a discharge rate of17mA·g-1, while even higher lithium-ion intercalation capacity of147mAh·g-1and161mAh·g-1were exhibited by the flower-like LiFePO4and LiFePO4/C mesocrystal respectively under the same testing condition. The excellent electrochemical properties of the LiFePO4mesocrystals should be attributed to the fast intercalation reaction and easy mass and charge transfer offered by the large specific surface and high porosity, which leading to much shorter diffusion distance of lithium ions and greatly enhanced the property.