| The Lithium Ion Battery (LIB) has longer cycle life and higher energy density. The battery is lighter than others. Moreover, it is safe and reliable. Fast charging and discharging are also its advantages. Nowadays traditional lead-acid storage battery and nickel-cadmium storage battery are replaced by high-performance LIB little by little. The LIB has become the main choice of power battery.As the core part of LIB, separator is an electron insulator. It can also allow positive ion and negative ion to pass freely in the electrolyte, so it can make ions pass normally. When the battery is too hot, separator can also close its hole to obstruct the current transport. Then the LIB will stop working. As a result, the LIB can protect itself. The property of separator is important to the LIB capacity, cycle life and safety performance.According to the difference of physical and chemical property, commercial separator of LIB include microporous membrane, composite membrane, weaving membrane, separator paper, non-woven fabrics, rolling membrane and so on. Relatively stable chemical properties and great mechanic performance of polyolefin material make it suitable as thermoplastic material. When the temperature is above melting temperature, multi-hole polyolefin can shrink the bores and close them by itself. Impedance of the battery will rise observably. Electricity will stop transmission. As a result, it can prevent the high temperature of the battery and moreover its explosion. So it’s safe and reliable membrane material of LIB. They are reasons why we should use polyethylene, polypropylene or other polyolefin microporous separator in the LIB.At present, the main difficulties are the choice of basic material and technology of making bores. The technology of making bores includes preparing and making bores in the membrane and the stability of the product and production equipment. Basis materials are mainly polypropylene, polyethylene or other polyolefin and many kinds of annexing agent. The main difficulties of making bores include uneven thickness of membrane, weak hardness, not sufficient porosity.In this thesis, 1μm-100μm polyethylene and polypropylene films were chose. We can make hexagonal structure optical template by assembling optical fiber. These PE and PP films were heated on the melting temperature, and were pressed by needle template at different time. Take it out and cool it at room temperature. The size of bores will change as time goes on.The membrane has many advantages. For example, the way of making is simple. Moreover, bore diameter structure, bore diameter size, porosity, air permeability are adjustable. The membrane has high quality.The diameter of the membrane is monodispersed. It can be used in lithium ion secondary battery, fuel cell, water treatment and many other fields.In addition to that, we can also use transfer approach. We use Nano-Al2O3 coating to make ceramic polyethylene recombination membrane. By using Al2O3 coating, we can improve the performance of porosity, liquid absorption rate, contact angle, heat stability, electric conductivity, electrochemical stability and battery circulation. According to the experiment result, when there is 30% Al2O3, there will be the most obvious performance improvement. At last, we can make monodisperse Nano-Fe2O3 by heating with different calcination temperature. Then, we make LiFePO4 out of Nano-Fe2O3 by solid phase sintering.The diameter of monodisperse Nano-Fe2O3 particles will be bigger with the increasing temperature. With low temperature, the spherical shape of particles are better.We can use low range to improve the discharge capacity of LiFePO4 made with lower calcination temperature. |
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