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The Impacts of Formation Process on Lithium Battery Performance and the Research of Polyaniline Modified the Anode

Author: ChenDanDan
Tutor: JiaoZheng;ZhaoBing
School: Shanghai University
Course: Chemical processes
Keywords: Lithium ion battery Pre-charge Formation gas Solid electrolyteinterface (SEI) film Polyaniline
CLC: TM912
Type: Master's thesis
Year: 2013
Downloads: 1
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Abstract

During the initial charge and discharge of the lithium-ion battery, the electrolytedecomposes on anode materials then forms a passivation film. The film is anelectrically and chemically insulating layer while allowing Li-ion transport. This layeris generally termed the solid electrolyte interface （SEI）.The formation of the SEI film has extremely important influence on theperformance of the battery. First of all, the SEI film is insoluble in organic solventand can exist in organic electrolyte solution stably. The solvent molecules ofelectrolyte can’t pass through the passivation film, so the solvent molecules areprevented to embed effectively. While it can avoid the solvent molecules destructingthe electrode material, and prominently improve the cycle performance and life-timesof the electrode.Although the SEI is critical for battery operation, the SEI layer in commercialbatteries has three significant adverse effects on battery performance.（1） Because the SEI film formation in cells results in a10-25%loss in energydensity. This is referred to as the irreversible first cycle loss, and is due to Li-ionbecoming irreversibly trapped in the SEI layer.（2） The SEI film is non-uniform layer,including organic and inorganic-rich regions, the inability of inorganic-rich regions toaccount for the10-20%volume expansion of anode materials during charge/dischargecycle generates fractures in the SEI. Sequential SEI cracking/reforming cycles lead toSEI thickening, which is the primary cause of capacity fading with cycling--a majorfactor limiting battery lifetime.（3） SEI integrity is essential for preventing theformation of metallic Li dendrites, which grow on exposed anode surface. Dendriticgrowth from the anode to the cathode, termed dendritic shorting, is a primary cause ofcatastrophic Li-ion battery failure.Pre-charging is a critical process of SEI film formation. This paper studies theeffect of varied pre-charging temperature on the performance of the steel case LiFePO₄battery. By comparing discharge capacities, discharge voltage plateau, ACresistance, cycle performances and surface morphology of the electrodes pre-chargedunder20°C,30°C and40°C, it can be concluded that the30°C is the best pre-chargingtemperature. Furthermore, the generation of gas during the pre-charging process isstudied. It shows that the trend of gas generation continuously increased since2.5Vand it reaches a maximum at the voltage of3.3V. Then the gas is absorbed insidebattery obviously in the deep formed LIBs, because the deep formed negativeelectrode can react with gas. In the pre-sealed process, the batteries are charged todifferent states at3.3V,3.65V,3.8V, exhausted gas and sealed. Then the ratedischarge performances, capacity retention at high temperature, cycle life-times andsafety performances are compared, it shows that the cell sealed at3.3V reaches thebest performance.The batteries suffer great irreversible capacity losses in the process of SEI filmformation. And the volume change of anode electrodes during Li⁺intercalation/de-intercalation process is much large, which will make the SEI film cracking andreforming to consume Li⁺, leading to battery capacity fading.A conductive polymer layer which plays a role similar to the SEI film issynthesized on anode electrode to enhance the performance of the battery. Firstly thepolyaniline （PANi） is electrodeposited on the graphite electrode as a protective layer.Then the initial coulombic efficiency, charge/discharge specific capacity and the cycleperformance of the two electrodes are compared. It shows that electrodepositing PANifilm on the graphite electrode can effectively decrease initial charge/dischargeirreversible capacity loss and improve the cycle performance of the batteries.

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