Preparation Of Lithium Ion Battery And Research On Gel-type Polymer Electrolytes

Since the birth of lithium ion batteries in 1990s, they have been developed very rapidly due to their predominate advantages, such as high voltage, high energy density, better cycling performance, and friendliness to environment. So far, they have been widely used in a lot of fields, for example mobile communication devices, portable electronics, military equipments and medical treatment. Conseeuqntly, the research on the preparation of lithium ion batteries and capacity fading is of great importance. In order to improve the performance of lithium ion batteries and broaden their applications, it is necessary to use gel polymer electrolytes. In this dissertation, work has been done on these two aspects.1. In the case of lithium ion batteries with high performance, their behaviors are closely related to materials and technologies. Through design of technologies, cathode (LiCoO₂) and graphitic anodes for lithium ion batteries were prepared and prismatic cells (type 603450, Al gasket) with 1 Ah capacity were assembled. The cells displayed excellent cycling behavior. After 1500, the capacity retention ratio is above 60%. Through measurements such as XRD, XPS, SEM, EIS and CV, the changes of the cathodes and anodes after different cycles were investigated and further understanding on capacity fading of lithium ion batteries were achieved. During cycling, the structures of the LiCoO₂ cathode and graphitic anode materials were not markedly changed. Due to reactions of the surface of LiCoO₂ with the liquid electrolyte, the surface structure of the cathode was changed and the binder content at the surface decreased, leading to partial capacity fading. The graphitic anode continuously reacted with the liquid electrolyte. Though the reactions were not very evident, after long term cycling the thickness of the SEI (solid-electrolyte-interface) film and the AC resistance increased, leading to evident capacity fading. Different graphitic anode materials resulted in different capacity fading rate. These results show clearly that the capacity fading of lithium ion batteries with high performance is mainly ascribed to the reactions of the anode with the liquid electrolytes, which provides a good direction to achieve lithium ion batteries with excellent cycling behavior.2. Polymer electrolytes were invented in 1973. Since then, great progress on theretical research and practical applications of all solid-state polymer electrolytes have been achieved. However, the ionic conductivity of all solid-state polymer electrolytes cannot still meet the requirements of practical application of lithium ion batteries. As a result, compromise products, gel polymer electrolytes (GPEs), were given to birth in 1994. The GPEs have good mechanically processing performance of polymers and high ionic conductivity of liquid electrolytes. Consequently, it can not only be used as electrolytes but also substitute separators. Furthermore, since the polymers have a good thermoplasticity, polymer lithium ion batteries can be manufactured into different shapes such as platform, prismtatic and round with good applications.In this thesis, GPEs based on PMMA (poly methyl methacrylate) were at first synthesized, then the preparation, conducting mechanisms and interface properties of PMMA/PVDF (poly(vinyl difluoride)) blended system and PMMA/PEGDA/PVDF crosslinked system were investigated. Based on the blended PMMA/PVDF, three GPEs were prepared, i.e. stick-like, crosslinked porous and sandwiched structured ones. Finally, porous GPEs by adopting vesicant were first explored.Stick-like GPEs were first prepared based on PMMA from thermal polymerization, whose average molecular weight is about 47,000. By precipitating PMMA from its NMP solution via treatment with iced water, the precipitated PMMA presents a stick-like array. After adding PVDF, the sticks become thinner. However, when the amount of PVDF is above 20%, no array of sticks was obtained.The prepared stick-like polymer presents fine tractility along the parrallel direction of the array of the sticks and good tenacity along the vertical one. Adding liquid electrolyte in the weigh ratio of 1:1, the GPE exhibits good mechanical strength with ionic conductivity of 0.32 mS/cm. The polymer is almost amorphous with very small crystallization degree of 4.45%, which is very important to ensure good channels for the movement of ions in the GPEs. In the blend of PVDF and PMMA, there is an interaction between PVDF and PMMA, which ensures homogeneous blending. This blend has a good retention ability of liquid since the evaporation temperature of liquid electrolyte in the GPE increases dramatically, which is favorable for the improvement of safety and cycling life of lithium ion batteries.Results from electrochemical evaluation show that the electrochemical window of the GPE is above 4.7 V, which can meet the requirements of polymer lithium ion batteries. The fabricated polymer lithium ion battery adopting the GPE exhibits good capacity retention, indicating promising application of the blending GPEs in lithium ion batteries.The crosslinked porous GPEs are based on the blend of PVDF and PMMA. There are no pores in the simple blend membrane when PVDF is up to 70 wt%. In the case of vaporizing the solvent followed by crosslinking there are also no pores in the blend of PVDF and PMMA. While in the case of crosslinking followed by vaporizing the solvent, a crosslinked porous PVDF-PMMA-PEGDA was achieved. After gelled by adding liquid electrolytes, these pores were retained. When there are more pores in the polymer membrane, the ionic conductivity can be higher. The ionic conductivity of the crosslinked porous membrane can reach to 5.5 mS/cm. After 50 cycles, the capacity retention of the Li/GPE/LiCoO2 is 92%, which indicates good cycling behavior.The sandwiched structured PGEs were first prepared, which are based on PVDF and PMMA and consist of PVDF layer, PMMA layer and PVDF layer. In this novel sandwiched structure, the solubility of PMMA into liquid electrolyte is greatly alleviated, and the mechanical processing ability is improved and the ionic conductivity is increased. The outer PVDF layers present porous structure, which provides pathways for ions into and from the GPEs, and the middle PMMA layer is solid and has good capacity to absorb liquid electrolytes. The ionic conductivity is 2.4 mS/cm when the weight ratio between the polymer and the liquid electrolyte is 25:75. DTA results demonstrate that this sandwiched GPE exhibits good retention ability of liquid electrolyte. The evaporation temperature of liquid electrolyte in the blend of PMMA/PVDF is increased for 70°C compared with the PP/PE composite film. In the case of the sandwiched GPEs, it is 180℃, much higher than that of the blending GPE (120℃), which shows marked improvement of the retention ability of the liquid electrolytes and thermal stability. The fabricated LiCoO₂/sandwiched GPE/Li battery exhibits good cycling performance since there is no obvious capacity fading after 110 cycles.Porous PVDF GPEs were prepared first by adopting vesicant. The amount of the vesicant AC and the decomposition temperature present evident influence on the pore structure. The dispersion and the diameters of the pores are homogenenous, and the pore size is at the range of sub-micrometer, as equal as that of PP/PE film. The ionic conductivity is also effectively increased. Compared with the phase inverstion method of Bellcore Company, this method presents low cost and is easily for industrialization, which will produce great influence on the production of polymer lithium ion batteries.