Preparation And Modification Of PVDF-HFP Based Microporous Polymer Lithium Borate Electrolyte Membrane

In recent years, with the improvement of living standards, people living in the production and the growing demand for lithium-ion batteries. Lithium ion batteries have many advantages such as high energy density, high output voltage, wide operating temperature, safe and little possibility of electrolyte leakage, no memory effect and flexibility packaging and shapes. This paper aims to study the preparation of new high performance polymer electrolyte to improve their ion conductivity and thermal properties. The main contents of the dissertation are as follows:Firstly, The technology of preparing PVDF-HFP-based polymer electrolyte by phase inversion method and modified by the addition of PEO, the polymer electrolyte which contains different mass ratio of PEO were prepared. The pore structure of the microporous membrane was characterized by SEM, the degree of crystallinity of the microporous membrane was characterized by X-ray diffraction, the thermodynamic properties of the microporous membrane was characterized by Differential Scanning Calorimetry. At room temperature, the resulting polymer electrolytes show a high ionic conductivity up to 5.74×10-3 S·cm-1 and highest liquid electrolyte uptaking amount (209%) when the PEO content of 40% by mass.Then the performance of PVDF-HFP/PEO-based polymer electrolyte membrane was studied when adding the nanoparticles SiO2. The SEM and XRD results showed that the adding of the inorganic filler did not affect the pore structure of the polymer electrolyte membrane, and also did not affect the crystal structure of the polymer electrolyte, which remained amorphous. When the SiO2 content of 4%, the polymer electrolyte liquid electrolyte uptaking amount up to 275%. At room temperature, ionic conductivity of the polymer electrolyte was 5.94 × 10-3 S·cm-1. Tests show that the performance of the polymer electrolyte prepared to get a good improvement.Finally, the polymer electrolyte soaking different liquid electrolyte were prepared. In the discharge process, the lithium salt is an important part of polymer electrolyte. Ion conduction takes place in the amorphous phase of polymers. However, The counter anions in the polymer electrolyte have weak interactions with the polymers, hence transport more easily. The major drawbacks of dual ion conduction in traditional polymer electrolyte are the low Li+ ion transference number, resulting in electrolyte salt concentration polarization occurs. Concentration polarization occurs and the applied electric field will produce the opposite polarization voltage. Weaken the migration of ions, reducing battery efficiency and life. It is necessary to design new lithium salt to guarantee the stability of anions to gain good performance for lithium ion batteries. In this paper, the boron lithium salt for polymer electrolyte was studied, the results showed that the molecular structure which usually containing a large II key complexes, composed boron ions and alkoxy, o-phenol,1,2-dicarboxylic acid. By quantum chemical calculation as a guide, the Lithium bis[1,2-benzenediolato(2)-O,O’]borate(LBBB) and Lithium bis[1,2-oxalicdiolato (2)-O,O’]borate(LiBOB) were prepared. The structure of the lithium salt was characterized by Infrared Spectroscopy. The four lithium-ion electrolyte which are LiClO4, LiPF4, LiBOB and LBBB were prepared. The polymer electrolyte conductivity at room temperature was studied after soaking, and conductivity results show that the relationship between the four electrolyte solution is LiClO4>LiPF4> LiBOB>LBBB. The tests show that ionic conductivity and temperature are complex Arrhenius equation.