Design, Preparation And Structural Control Of High Performance Polypropylene Lithium Ion Battery

The commercial lithium ion battery separators are polyolefin microporous membranes, such as polyethylene (PE) and polypropylene (PP), have been widely applied due to the good chemical properties and low price. However, the lack of polar groups in polymer chain and high crystallinity result in a low affinity with polar electrolyte. This leads to low ionic conductivity and high liquid electrolyte leakage.In this paper, materials were firstly modified through graft copolymerization, and then the impacts of copolymer chain structure on membrane morphology, pore structure, aggregation structure and thermal properties were investigated. The relationships between these changes and electrochemical properties were also investigated. The ultimate aim is to prepare high-performance lithium-ion battery separator with high ionic conductivity, good liquid-retaining capacity.For graft copolymer system, chlorinated polypropylene (CPP) as macro-initiator in the presence of aluminium tricholoride (AlCl3), successfully initiated the atom transfer radical polymerization of methyl methacrylate (MMA) with the iron dichloride tetrahydrate (FeCl2-4H2O)/triphenyl phosphine (PPh3) as catalyst in N-methyl-2-pyrrolidone at 80 ℃. The results showed that MMA can polymerize with controlled characteristic. The polymerization is first order reaction with respect to monomer concentration from the kinetics plot. The polymerization displayed living character as evidence by a linear increase of molecular weight with conversion and a relatively narrow molecular weight distribution (MWD). The effect of AICl3 was discussed and the mechanism of atom transfer radical polymerization of MMA by CPP/AlCl3 was presumed. On this basis, the effect of different reaction conditions on the polymerization reaction was further studied.The membranes were fabricated by immersion precipitation phase inversion using N-methyl pyrrolidone as solvent and water as coagulation. It was found that the viscosity of casting solution decreased with the increase of molecular weight under the same solid content. The pore structure appeared from mesh structure to finger-like structure, and the mechanical properties of membranes decreased with the increase of degree of grafting.Finally, the polymer electrolytes were assembled into coin cells to test electrochemical properties, such as ionic conductivity, cell charge-discharge capacity, electrochemical stability window. It was found that the ionic conductivity, cell charge-discharge capacity, electrochemical stability window of the grafted products were improved compared with raw material. The ion conductivity at room temperature can reach 2.26×10"3S/cm. The electrochemical stability window reached 4.5V. The electrolyte leakage decreased even though the porosity increased which suggested that the PMMA had good affinity with electrolyte.The products with good affinity with water were successfully synthesized using the same polymerization process to initiate N-vinyl pyrrolidone polymerization. Water flux and rejection of the modified membranes compared with the original membrane were improved highly.