Investigations of Design Factors on Power-Oriented Cells and Effects of Additives on Overcharge Safety for Lithium-Ion Batteries

• Main Authors: Yi-Shiun Chen, 陳奕勳
• Other Authors: Yuan-Yao Li
• Format: Others
• Language: en_US
• Published: 2010
• Online Access: http://ndltd.ncl.edu.tw/handle/89017825807427761695

博士 === 國立中正大學 === 化學工程所 === 98 === With the popularization of cell-phones, laptops, and 3C products, lithium ion batteries (LIBs) have widely introduced into humans’ lives and been recognized as the most promising batteries in these applications because of their high energy density, good performance, environment friendly and other merits over other battery chemistries. Though the current LIB system, LiMxOy (M = metal) cathode and carbon/graphite anode with organic electrolyte, had been developed from 1991 by Sony, and was made over billions in these ten years, LIBs still have some challenges need to be solved and wait for studying. Chapter 2 successfully demonstrates how a small change in cathode preparation affects the high-rate discharging characteristics of LIB power cells due to a small variation in the cathode impedance. Since power-oriented LIBs have different requirements than the traditional energy-oriented ones and their design concept is also different than energy-type cells, some new problems not found in energy-oriented LIBs must be carefully considered. This study illustrates the importance that cathode impedance, a contributor to total cell impedance that can be ignored in the traditional energy-type LIBs, is very important in power cells. This study uses LiMn2O4 cathode-graphite anode 18650 cylindrical cells as model power LIBs and also investigates the charge-discharge performance of these model batteries made from cathodes with the same recipe but dried at different oven temperatures. The high impedance cathode produced under a high drying temperature causes the cell to fail during high-power applications. Cell heating during extreme high rate discharging periods not only causes cathode to peel from the substrate, but also pore closure in the porous separator. These phenomena, increasing the cell resistance and reducing the transfer rate of charged species, are believed to be the main causes for the poor cycle life of model batteries in high rate discharge tests. The influence of tab position and quantity as well as multi-segment electrodes in cell design, also an important factor for the performance of power-oriented LIBs, have been investigated in chapter 3. The resistances of cells with traditional and center-tab designs are simulated by a simplified model. It shows that tab position significantly influences the cell resistance even when other components in the cell are fixed. The performances of both center-tab and traditional designs are compared by the cell direct-current resistance (DCR), body temperature, and 15 A cycling test to demonstrate the impact of cell design. The multi-tab design would provide lower internal resistance than the single-tab design for power cells; however there are diminishing returns for cells with three or more tabs as the additional tabs do not significantly reduce the resistance for an 18650-size cell. The two-tab design with 4-equivalent-segment is concluded to be the best choice for an 18650 power cell considering the process ability and overall high rate performance. The difference in the waste power between the center-tab and the traditional designs, which is transformed into heat, is the main reason for the poor high-rate cycling performance as expected. Such modeling provides a quick design reference for a power cell with new size to get the most suitable electrode design. In chapter 4, we discussed the overcharge safety of a 103450 prismatic Li-ion cell. Due to the different mechanical designs between cylindrical and prismatic cells, the required functions of the electrolyte additives are quite different. These overcharge additives, such as BP, CHB and TAB, were used well in the cylindrical cells that can trigger the protective device by gas evolution over 4.5 V. However, rate of heat generation in the overcharge period for LiCoO2-graphite prismatic Li-ion cells has been found to be more important than the gas evolution in this work. The rate of heat generation from the polymerization of 3 wt.% CHB (cyclohexyl benzene) is high enough to cause the explosion or thermal runaway of a battery in a 2C, 4.55 V overcharge test; which is not found for an LIB containing 2 wt.% CHB + 1 wt.% TAB (tert-amyl benzene). In the 12 V overcharge test by 1 C charge rate, the thermal fuse was broken by the high skin temperature (ca. 80 oC) due to the polymerization of 3 wt.% CHB, which was also the case for LIBs containing 2 wt.% CHB + 1 wt.% TAB. The disconnection of the thermal fuse, however, did not interrupt the thermal runaway of LIBs without any additives because the battery voltage was too high (ca. 4.9 V). The influence of specific surface area of active materials in the anode on the polymerization kinetics of additives has to be carefully considered in order to add correct amount of overcharge protection agents.