Surface Modifications Of Carbon Fluoride And The Applications In Li-CF_x Battery

Li-graphite fluoride battery with the theoretical of 2180 Wh kg-1, owns the highest specific energy in the exsting primary batteries. Also, Li-graphite fluoride battery has flat discharge potential, wide operating temperature（-60 to 180 ℃）, low self-discharge and long storage life. It is considered to be one of the most development ptential batteries. However, as the cathode materials of Li-graphite fluoride battery, graphite fluoride has a low electronic conductivity and low surface energy, which leads to electrode polarization and reduces the rate discharge performance of batteries. That inhibits the application of CF_x in high power devices. What is more, the price of graphite fluoride is very high, it can reach to several thousands RMB per kilogram. Hence, Li-graphite fluoride battery is limited to use in high technology fields. The technologies of devices in aerospace, military, medical treatment and so on are innovated so fast that the matching power sources with high energy density, high power density and low price are emergently required.In this paper, the conducting materials have been coated on the commercial graphite fluoride, in order to modify the electronic conductivity and the surface affinity of CF_x, and finally enhance the electrochemical performances of Li-CF_x cells. Metal, non-metal substance, conducting polymer and metallic oxide have been coated on CF_x. Concretely, Ni-P alloy, carbon, polypyrrole and maganese dioxide are selected as coating materials. SEM, TEM, XRD, TGA and BET measurements are used to characterise the morphologies, structures and components of the materials. Galvanostatic discharge, rates discharge and alternating current impedance are employed to analyze the electrochemical performances of modified CF_x cathodes. In allusion to the high price of CF_x, the hybrid cathodes of CF_x combined with MnO₂ and modified CF_x combined with MnO₂ have been researched. The main contents and conclusions are as follows:Ni-P has been coated on the surface of CF_x by electroless deposition and the Ni-P-CF_x composites with different Ni contents are manufactured. The Ni-P on CF_x shows a dendritic crystal. The specific surface areas and specific capacities of Ni-P-CF_x composites decrease as the Ni content increase for the high specific gravity of Ni. When exluding the Ni content to calculate the specific capacity, it is found that the electrochemical performances of CF_x are improved in the Ni-P-CF_x composites. When the content of Ni-P is 43.7 wt%, the discharged capacity of CF_x is 865 m Ah g-1 at 0.1 C. The maximum discharge rate can reach up to 3.55 C. From the discharged Ni-P-CF_x cathodes, we can find that the Ni-P-CF_x particles are tightly interweaved with conductive agent. The electronic conductivity and surface energy are improved by the Ni-P coating. As a result, the electrochemical performances are enhanced.Carbon coated CF_x have been prepared based on microencapsulation. Firstly, polyurea is coated on CF_x via interfacial polymerization. And then, sulfuric acid carbonization and high temperature carbonization are used respectively to form two different CF_x@C. The high temperature CF_x@C（T-CF_x@C） shows an irregularity particle shape and a low specific surface area. The discharge plateau of T-CF_x@C is higher and the resistance is lower compared to CF_x. However, the discharge capacity of T-CF_x@C is very low. EDS and FTIR analyses indicate that the carbon content of T-CF_x@C is high and there is some CF_x decomposed during the carbonization. sulfuric acid carbonized CF_x@C（S-CF_x@C） shows a spherical shape. The thickness of carbon coating is about 800 nm, which is consist of countless carbon microspheres. The electrochemical performances of S-CF_x@C are greatly improved by the carbon coating with a maximum discharge rate of 5 C. FTIR analysis demonstates that there is some polyurea left in S-CF_x@C, which can dissolve during the making electrode process and act as binder to enhance the particles combination.The PPy has been coated on the CF_x via in situ chemical oxidative polymerization. The morphologies, structures, and electrochemical performances of CF_x@PPy with various PPy contents have been researched. When the PPy content is low, the PPy can coat on the CF_x uniformly. Adding the PPy content, the thickness of PPy coating will increase and the redundant PPy will aggregate to form dendritic particles. The electrochemical performances of CF_x@PPy are enhanced as the PPy content decrease. The CF_x@PPy with a PPy coating of 80 nm can deliver steadly to 6 C rates, corresponding to the average power density is 7091 W kg-1. In addition, the CF_x@PPy shows execllent performances when at high or low temperatures discharge. At 100 ℃, the CF_x@PPy can deliver capacities about 4 times as the raw CF_x at 0.1 C rate. At-20 ℃, the CF_x@PPy can deliver steadly to 1 C rates, rather than 0.1 C with the raw CF_x. PPy coating can enhance the facial conductivity and inprove the adsorption capacity of CF_x, thus increase the discharge performances. However, too much PPy will extend the diffusion layer of Li ion and pare down the holistic capacity.Manganese dioxide with a nanofiber stacking structure has been coated on CF_x by hydrothermal process. The AC impedance shows that the CF_x@MnO₂ have lower resistances than CF_x. The CF_x@MnO₂ present good rates capabilities. When the CF_x is 3.4 wt%, the CF_x@MnO₂ can discharge stably till 5 C rates. The surface energy and electronic conductivity are modified by the MnO₂ coating. As a result, the electrolyte infiltrating in cathode particles and reacting matter exchanges are accelerated.The hybrid cathodes of CF_x combined with MnO₂ have been researched. The rate capabilities of MnO₂ are inherited by the hybrid cathodes, which the maximum discharge rate is 2 C. when the contents of CF_x are 10²0 wt%, the utilization of active materials are relatively high at different rates. The hybrid cathodes of CF_x@PPy combined with MnO₂ show excellent electrochemical performances, which can discharge with a maximum rate of 5 C.The paper studied the combination effects of conducting materials coating on CF_x. The hybrid cathodes of CF_x or modified CF_x combined with MnO₂ are researched to enhance the electrochemical performances and lower the costs. The work provides the experimental basis and important research directions for the commercial Li/CF_x batteries in future.