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We conducted a design study to compare the manufacturing costs at a level of 100,000 hybrid vehicle batteries per year for flexible package (Flex) cells and for rigid aluminum container (Rigid) cells. Initially, the Rigid cells were considered to have welded closures and to be deep-drawn containers of about the same shape as the Flex cells. As the study progressed, the method of fabricating and sealing the Rigid cells was expanded to include lower cost options including double seaming and other mechanically fastened closures with polymer sealants. Both types of batteries were designed with positive electrodes containing Li(Ni{sub 1/3}Co{sub 1/3}Mn{sub 1/3})O{sub 2} and graphite negative electrodes. The use of a different combination of lithium-ion electrodes would have little effect on the difference in costs for the two types of cells. We found that 20-Ah cells could be designed with excellent performance and heat rejection capabilities for either type of cell. Many parts in the design of the Flex cells are identical or nearly identical to those of the Rigid Cell, so for these features there would be no difference in the cost of manufacturing the two types of batteries. We judged the performance, size and weight of the batteries to be sufficiently similar that the batteries would have the same value for their application. Some of the design features of the Flex cells were markedly different than those of the deep-drawn and welded Rigid cells and would result in significant cost savings. Fabrication and processing steps for which the Flex cells appear to have a cost advantage over these Rigid cells are (1) container fabrication and sealing, (2) terminal fabrication and sealing, and (3) intercell connections. The costs of providing cooling channels adjacent to the cells and for module and battery hardware appear to favor Rigid cell batteries slightly. Overall, Flex cell batteries appear to have an advantage of about $1.20-$3.70 per cell for a 25-kW Battery of 20 cells or about $24 to $74 per battery. Container experts assisted with this study, including a paid consultant and personnel at container manufacturing companies. Some of the companies are considering entering the business of manufacturing containers for hybrid vehicle battery manufacturers. For this reason they provided valuable guidance on overall approaches to reducing the costs of the cell containers. They have retained the description of some specific designs and procedures for future possible work with battery manufacturers, with whom they are now in contact. Through the guidance of these experts, we determined that a new type of container could be manufactured that would have the best features of performance and low cost of both the Rigid and Flex containers. For instance, the aluminum layer in a tri-layer sheet can be sufficiently thick to form a rigid container that can be fabricated in two halves, much like a Flex container, and mechanically joined at the edges for strength. In addition to the mechanical joint, this container can be sealed at the edges, much like a Flex container, by means of an inner polymer liner that can be heat-sealed or ultrasonically welded. The terminals can be flat strips of metal sealed into the top of the container as part of the edge sealing of the container, as for the Flex cell. Ridges can be stamped into one side of the container to provide cooling channels and the exterior layer of the container stock can be coated with a thin, electrically insulating, polymer layer. We expect this type of container will provide excellent sealing and durability and be less expensive than either the Flex or the Rigid container, which the study initially considered. A major cost for the original Rigid container is the welding required for sealing the container. However, the welding of the current collector tabs to the terminal piece may be even more complex and costly than welding the container. It is important, therefore, to develop an inexpensive procedure for attachment of the foils to the terminal pieces. A lower-cost procedure, such as ultrasonic welding or mechanical clipping, might replace laser welding of the foils to the terminal pieces. A conclusion from our discussions with the container experts is that the manufacturing rate required for the containers for hybrid vehicle batteries is fairly low, and thus favors procedures requiring low tooling costs and little development effort. These conditions favor flexible packaging, heat sealing, shallow stamping, double seaming and ultrasonic welding. It works against deep drawing and untested procedures for welding and joining

Comparative costs of flexible package cells and rigid cells for lithium-ionhybrid electric vehicle batteries.