Preparation And Performances Of Novel LiFePO₄Composite Cathode Materials

Olivine-type LiFePO4has been considered as the one of the most promising cathode materials in lithium-ion batteries (LIBs) due to its outstanding advantages of low cost, environmental friendliness, excellent thermal and chemical stabilities, high theoretical specific capacity (170mAh·g-1), high and flat working voltage (3.43V vs. Li/Li+), etc. Unfortunately, pure LiFePO4is difficult to satisfy the demands of high-power applications caused by its poor intrinsic electronic conductivity and low lithium ion diffusion coefficient, which brings obstacles for large-scale commercial applications especially in the power batteries field. Therefore, in this dissertation, several modified LiFePO4composites were prepared by different modified methods to overcome its two drawbacks, and thus to speed up the process of its commercialization.Firstly, PEG grafted MWCNTs (MWCNTs-g-PEG or simply MP) were successfully prepared by an esterification method, and then used as novel conductive additives in LiFePO4-based cathodes after doping with lithium salts. The effects of different molecular weights of PEG grafted on MWCNTs and different mass fractions of MP on the morphology, electrochemical properties, electronic conductivity and thermal conductivity of LiFePO4/MP composite cathodes were evaluated. The results showed that the grafted PEG can effectively improve the dispersion of MWCNTs in the active materials, which can easily connect LiFePO4particles to fabricate a continuous electron and heat conductive network in the cathode. Even with only5wt.%conductive additives, LiFePO4/MP still exhibited higher electrochemical performances than that of the LiFePO4/acetylene black electrode (20wt.%acetylene black addition). Moreover, the lithium ion diffusion coefficient of LiFePO4/MP is around2orders of magnitude higher than that of LiFePO4/acetylene black electrode, and the lower PEG molecular weights, the higher performances. Among the LiFePO4/MP cathodes modified by different MP-350(the molecular weight of the grafted PEG is350) weight fractions,10wt.%addition showed the most excellent rate capability, cycling performances and thermal conductivity, also with improved charge-discharge property at low temperature.Secondly, LiFePO4coated by conductive polymer PANI, which is also electrochemically active, was prepared by an in-situ polymerization coating method. We systematically investigated the impacts of different HCl concentrations and PANI contents on the structure and performances of the LiFePO4/PANI composites. The results showed that the LiFePO4/PANI prepared by0.1M HCl exhibited the satisfactory electrochemical performances because the excessive acidity can easily result in the dissolution of LiFePO4, and meanwhile low HCl concentrations are not enough for doping PANI. The LiFePO4/PANI (400μL) composite containing10.2wt.%PANI revealed the best charge/discharge performances due to the suitable coating structure, which can greatly increase the conductivities of the composite and hence reduce the polarization of the batteries. The reversible discharge specific capacity for the LiFePO4/PANI (400μL) composite was153mAh·g-1at0.1C after100cycles and there was only3.2%capacity fading compared with the first discharge capacity. Even at2C rate, the discharge specific capacity still remained at～122mAh·g-1, but the electrochemical performances at higher rate need to be improved.Thirdly, in order to improve the charge/discharge performances of the LiFePO4/PANI at the high rate, PANI-PEG copolymers with mixed electron and lithium ion conduction performance were used to modify LiFePO4by two methods. The obtained LiFePO4/PANI-PEG composite exhibited uniform coating structure by dispersion LiFePO4particles in the aniline terminated PEG and aniline solution which can in-situ polymerize on the surfaces of the particles. The discharge specific capacity was as high as165mAh·g-1for this LiFePO4/PANI-PEG composite at0.1C. The reversible capacity was still remained125mAh·g-1at5C, showing76%capacity retention compared with that at0.1C. It is worthy note that the DLi+for the LiFePO4/PANI-PEG composite is3.4×10-13cm2·s-1, which is one orders of magnitude higher than that of LiFePO4(3.2×10-14cm2·s-1), confirming the excellent electrochemical performances of the LiFePO4/PANI-PEG composite.Finally, we chose polydopamine (PDA) as a novel carbon source to coat LiFePO4particles because PDA can adhere to the surfaces of any materials. The carbon coated LiFePO4/C composite can easily be obtained by auto-polymerization of dopamine on the LiFePO4precursors’ surfaces and then by sintering. The carbon content and carbon coating thickness can be adjusted and controlled by changing the ratio of dopamine and LiFePO4precursors. The carbon coating layer and "carbon bridge" between the LiFePO4particles fabricate a3D nano-conductive network in the LiFePO4/C active materials, which can ensure the electrons transport in this composite. The discharge specific capacity at0.1C increased from84mAh-g"1for the carbon-free LiFePO4to135mAh·g-1for the LiFePO4/C with2.02wt.%carbon content. And especially, this LiFePO4/C composite delivered a discharge capacity of～70mAh·g-1at10C rate. The excellent electrochemical performances of the LiFePO4/C composite suggest that the carbon coating by PDA as a carbon source is likely to be used as a novel carbon coating method in preparation of LiFePO4/C composites.