Preparation And Characterization Of Porous Inorganic Separator And Their Performance In Lithium Ion Batteries

Separator is an important part in lithiumion battery, and the safety performanceof the lithium ion battery largely depends on the property of the separator. Up to now,polymer separator are widely used in the commercialized lithium-ion batteries, butsuch separatoror is easy to punctured by lithium dendrites or undergo obviousdimensional changes at elevated temperatures, causing internal short-circuit of thebattery and resulting in the battery damage even firing. Moreover, owning to theintrinsically hydrophobic properties of the polymer separator, it is hard for thepolymer separator to infiltrate and hold the non-aqueous electrolyte, which limits therate performance of the battery, thus adversely affecting the application of thelithium-ion battery for the electric vehicle and hybrid-electric vehicle. Theorganic-inorganic composite separator has been developed, but there are still issues ofthe poor thermal stability, poor ionic conductivity.Because of its insulating property, good thermal stability and excellentelectrolyte retention capacity, the porous inorganic membrane has been taken intoaccount as lithium-ion battery separator for the first time. This dissertation is focuseson lithium-ion battery system, and aim at improving the safety performance and ratecapability of the battery. The porous Al₂O₃, SiO₂and AAO separators have beenprepared. The technique of SEM is adopted to characterize the microstructure andmorphology of the as-prepared membranes. The electrochemical methods includinggalvanostatically charged and discharged test, cyclic voltammetry, and impedancemeasurement were used to systematically investigate their electrochemicalperformances.Firstly, porous Al₂O₃separator has been prepared through twicehigh-temperature sintering of nano-Al₂O₃, micron-sized Al₂O₃and the pore-formingagent of EDTA. The microstructure, porosity and electrolyte-infiltration performancesof porous Al₂O₃separator have been studied. The results demonstrate that: comparingwith the polymer separator, Al₂O₃separator possesses higher porosity and excellentelectrolyte retention performance; after infiltrated with electrolyte of1M LiPF6/EC+DEC （1:1, w/w）, the Al₂O₃separator exhibits an excellent ion conductivity. TheLiFePO₄/graphite cell using the inorganic separator shows higher discharge capacity,rate capability, and better low-temperature performance than that using thecommercialized polymer separator. The LiFePO₄/graphite battery possesses better cycle performance, rate performance, and low temperature performance. Thus, theporous Al₂O₃can be used as a lithium ion battery separator. All the evidences indicatethat the inorganic separator is very promising to be applied in the large-sizedlithium-ion batteries, especially for the long-term energy storage systems.Secondly, the porous SiO₂separator with good mechanical strength has beenprepared by sintering the commercialized raw materials of SiO₂which is cheap andeasy to get. The electrolyte-infltrated SiO₂separator exhibits excellent ionicconductivity even at as low as-20°C and much better electrolyte retentionperformance than polymer at50°C. The LiMn₂O₄/Li cell using the SiO₂separatorshows higher discharge capacity, rate capability and better low-temperatureperformance than that using the commercial polymer separator. Furthermore, the SiO₂separator can alleviate LiMn₂O₄/Li capacity fading at high temperature of55°C. Theexcellent electrochemical performance of the SiO₂separator can be attributed to thefollowing reasons:（1） excellent electrolyte infiltration and retention performance ofhydrophilic SiO₂;（2） capillary force of the porous pores in the SiO₂separator;（3）SiO₂can capture the trace amounts of moisture and acidic impurity in the electrolyte.All these results indicate that the SiO₂separator is very promising to be applied in thelithium-ion batteries, especially for the long-term energy storage systems.Thirdly, through-hole anodic aluminum oxide （AAO） film has been quickly andefficiently prepared within a simple homemade device. The AAO film has a highporosity of72%and good mechanical strength even as thin as60μm. The AAO filmhas excellent performances in electrolyte uptake and retention, and the wettability ofthe electrolyte to the hydrophilic AAO film is much better than that of thecommercialized polymer separator. Compared with the polymer separator, theLiFePO₄/graphite cell using the AAO separator shows better cycling capacity, ratecapability, and low-temperature performance. The effect of the AAO separator onperformance of LiFePO₄/graphite cell has also been investigated by the EIS. It isdemonstrated that the AAO separator is very promising to be applied in thelithium-ion batteries.Finally, an integrative cell with a porous Al₂O₃membrane as both a support anda separator has been fabricated by coating the electrode materiasl on each side of theseparator, while the electrolyte was infltrated inside. The LiFePO₄/graphiteintegrative cells are evaluated in coin-type cells and exhibited good cycle capacity. The self-standing integrative cell is a simple and promising technology to assemblethe battery stacks and meanwhile had an obvious advantage of forming a frmstructure, which could avoid inner short circuit during being moved or crashed. Suchself-standing integrative cell with Al₂O₃porous separator could provide a competitivecandidate to the currently rolling battery assembly, especially for large-sized energystorage device.