Studies Of Polyimide And Fluride As Cathode Materials For Energy Storage

Since 1990, lithium-ion batteries have been widely used in our daily lives, and their application has been extended from electronics to electrical devices, and even energy storage. However, the continuously increasing demand on energy/power density imposed more requirements on the electrode material, and the current cathode material cannot meet the challenge due to the intrinsic crystal structure and thus-resulted limited Li+ accommodation. In addition, the large scale application of lithium-ion battery has caused some concerns on resources and environmental pollution. Therefore, finding new cathode material of high capacity, environmental friendly is a hot-spot in battery field.Organic materials have attracted great interest for its potential capacity, structural diversity, easy processing, sustainability and environmental benignancy. On the other side, metal fluoride earns much attention because of its novel conversion reaction mechanism and thus-resulted high capacity. However, these two materials both have a shortcoming of low conductivity. Graphene, an emerging material, has been successfully used in energy storage because of its unique character such as excellent electrical conductivity and high specific surface area. Based on these, our research is focused on the organic material and metal fluoride. Polyimide(PI), FeF3 and BiF3 have been investigated as cathode materials for lithium/sodium secondary batteries. Graphene has been used to further enhance their electrochemical performance. The main results are detailed in the following:1、Polyimide(PI) as lithium secondary battery cathode materialsPI powder was prepared by a one-step method and PI/graphene film was obtained by a typical two-step method that was widely reported in PI film fabrication. The PI powder shows good electrochemical properties as cathode material. The cycling test shows the PI can release 110mAhg-1 capacity through a 2-eletrons transfer redox process, and the charge/discharge efficiency can be maintained at above 99%. In addition, it presented stable cycling and excellent cycle performance. After preparation optimization, the PI/(graphene-Printex C) film had a capacity of 40 mAh g-1. Through the result is not as good as its powder analogue, the work provides a clue for PI thin film electrode fabrication.2、Self-supported polyimide(PI) as lithium secondary battery cathode materialsSelf-supported electrode was synthesized as the cathode material for lithium secondary battery, the e PI/(graphene-KJC-600) slurry was directly coated on carbon paper and the electrode was then fabricated by a further heating treatment, the electrode contained no binder and no traditional current collector. The Self-supported electrode shows high capacity about 100mAh g-1, which is very close to the capacity of PI power and exhibits a satisfactory rate and cycle performance.3% Iron fluride(FeF3) as lithium secondary battery cathode materialsCo-precipitation, low temperature method, high-energy ball milling-assisted low temperature methods are used to synthesis FeF3 powders. The results reveal that the synthetic method affects the properties of the resulting materials in term of particle size and morphology. The FeF3 powder prepared by high-energy ball milling-assisted low temperature method shows the best electrochemical performance among the three samples possibly due to its smallest particle size and good crystallinity. FeF3/graphene and FeF3/amorphous carbon composites are prepared to evaluate the effect of different carbon on the electrochemical properties of FeF3. The FeF3/graphene shows high capacity, good cycling stability and excellent rate performance. Between 2.0-4.5 V, the specific discharge capacity at 16 C current density is 128.3 mAh g-1, more than half of its theoretical capacity.4、Iron fluride(FeF3) as sodium secondary battery cathode materialsSodium secondary battery has recently gained recognition as an intriguing candidate for next-generation battery systems because of the natural abundance of Na resources. However, FeF3 has seldom been reported in Na storage. After the successful work of using FeF3 in lithium secondary battery, we studied its application in Na storage. The results show that FeF3/graphene composite has much better capacity and cycling performance than bare FeF3 in Na storage. It gives a capacity of 206.4 mAh g-1 at a low current density of 0.1 C and even at 1 C, the FeF3/graphene composite can still give a discharge capacity of 144 mAh g-1. The work supplies a strategy for using FeF3 as cathode material for Na secondary battery.5、Bismuth fluoride(BiF3) as lithium secondary battery cathode materialsTwo BiF3 samples with different particle size were synthesized, and BiF3/ graphene composite material was further prepared by an in-situ method as well. The electrochemical tests show the addition of graphene and reducing the particle size can effectively improve the capacity and cycle performance. With the current density of 0.1 C, the BiF3/graphene composite can give a discharge capacity of 297.5 mAh g-1, corresponding to 3-eletrons transfer, and after 30 cycles, the discharge capacity decreased to 101.7 mAh g-1. Thought the result is not as good as other fluoride analogues, the BiF3/graphene composite shows much better electrochemical performance than other BiF3 material reported previously. The relatively rapid capacity decay may be caused by electrolyte decomposition.