Earlier this week, General Motors invited members of the media to their Technical Center in Warren, MI, for a briefing to talk about batteries, and show off their battery development lab. The purpose of the meeting was to provide some background on the design of lithium ion batteries, the current status of battery developments, and the hurdles that remain before bringing lithium ion batteries to market.
The session began with an introduction from GM's VP of Environment and Energy Beth Lowery. She repeated the recent mantra about the need for energy diversity, which we've been hearing from the likes of Rick Wagoner, Bob Lutz and other GM executives for the past few months. She then reviewed the time-line for General Motors hybrid vehicles, from the initial two-mode hybrid buses that have on the road since 2003 through the current mild hybrids to the light duty two-mode hybrids including the two-mode Vue hybrid later in 2008. She wrapped up with fuel-cells and E-Flex before handing off to Denise Gray.
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Denise Gray was appointed as Director of Hybrid Energy Storage Systems at GM last November and is responsible for everything to do with batteries for hybrid vehicles. Her department currently has thirty chemists, scientists and engineers working on development and testing of batteries and she threw out a recruiting call. If you have any battery expertise, Denise Gray is looking for you! Since taking over, her group has been focused on consolidating the battery efforts at various locations in the US and Germany in order to minimize redundancy and maximize the output. The critical factors that have to be overcome to succeed are optimizing the battery chemistry to get the best balance of power output and energy capacity, working out the mass manufacturing of cells and packs, developing the electronics and controls, system integration and testing. All in all a pretty trivial problem.
General Motors currently has development contracts with Johnson Controls-Saft and A123-Cobasys to work on lithium ion battery packs, initially for the plug-in hybrid Vue program. The lessons learned from that will also be applied to the Volt program although the balance of the battery chemistry will be different for that program since it's primarily battery driven and requires greater energy than the parallel hybrid Vue. Joe LoGrasso is the Engineering Manager for Hybrid Energy Storage Systems and he gave an overview of the differing battery requirements for the two-mode, plug-in and series hybrids, ranging from short-range low-speed electric to 40+ miles. The E-Flex needs substantially higher power output than the parallel hybrids and much more energy density. Since the series hybrid has to get all of it's motive power from the battery, and can't rely on an engine for power, the battery has to do much more.
Meeting those differing requirements means differences in the chemistry and construction of the individual cells. The battery for the two-mode needs very thin electrodes and thicker current collectors, to provide shorter bursts of power when needed, but only limited energy capacity. Moving to the plug-in and series systems, the cells require thicker electrodes in order to store more energy for longer range. In reviewing the current different types, Joe mentioned some of the new materials being developed for lithium batteries including lithium titanate anodes. So far, the only company using lithium titanate is AltairNano which up until now hadn't been mentioned in connection with General Motors. A graph showing the range and power capabilities of various current battery types with the E-Flex requirements overlaid indicated that the needs of the series hybrid cover the scope of all of the best of what exists today.
Lithium batteries have the energy density and power output, and tend hold the power when sitting better than other types. The biggest problems that need to be conquered are lifespan, cold temperature performance and most importantly, robustness and abuse tolerance. Since the battery pack is built up from hundreds or thousands of individual cells, the interconnects that tie all the individual cells together have to be very carefully constructed in order to ensure that the pack survives the life of the vehicle. The process goes from developing individual cells to meet the performance requirements, integrating those cells into a pack and then finally integrating that pack into the vehicle system. While lithium batteries have been used in consumer electronics devices for many years, combining them into large format packs for use in the automotive environment is relatively new. Another problem that has to be dealt with is scaling up the construction of potentially tens of millions of larger format cells, in a consistent manner to ensure that they perform consistently and reliably in automotive applications. After all, no car-maker wants to be facing a large-scale battery recall like laptop manufacturers did in early 2006.
The GM people were joined by Mary Ann Wright, the new CEO of Johnson Controls-Saft, Ed Bednarcik, VP and General Manager of A123 systems and Scott Lindholm, Vice President-Systems Engineering of Cobasys. Mary Ann Wright provided and overview of the construction of lithium ion cells. The basic principal is similar to lead acid batteries with separated positive and negative electrodes sitting in an electrolyte material. The primary difference lies in the chemistry of the coating on the electrode plates and layout. The lithium ion cells being developed are cylindrical with alternating, concentric positive and negative electrodes. The positive electrode (cathode) is coated with the lithium, while the negative electrode (anode) is graphite with a copper foil conductor. The electrolyte is an organic solution with added lithium salt, and the precise mixture, can be tuned to optimize the life and performance of the battery.
Ed Bednarcik provided an overview of the A123 manufacturing process for lithium ion cells. Until now, A123 has produced cells for use in power tools, primarily from Black & Decker/DeWalt, and has only recently moved into the automotive realm. Batteries for consumer electronics typically would have at most only a few cells, and even performance of the cells has not been as critical. Because of the large number of individual cells that must be combined in an automotive scale battery pack, consistent cell-to-cell performance is more important. In order to help achieve this, A123 produces the electrode coatings in large batches so that large numbers of cells can be produced from the same material. A123 also uses nano-materials in their chemistry which means that they also had to develop unique mixing and handling technologies. The electrodes are also assembled in clean room environments to help achieve the required consistency. Of course all that adds to the cost, but it's a necessary evil in order to meet the reliability requirements. Achieving the necessary power output of the battery means that they need thin coatings on the electrodes, and to help improve the consistency, the entire process includes high levels of automation. Finally, assembling the cells includes laser welded cans, with 100 percent X-ray inspection and testing of the completed cells.
As a relatively young company, A123 has no experience in assembling large-scale automotive battery packs. Conversely, Cobasys has a lot of experience in integrating cells into battery packs and packs into vehicles, having supplied the lead acid and NiMH battery packs for the EV1 and the NiMH packs for the current GM mild hybrid system. When it became clear that lithium ion was going to be required to meet the performance requirements of the plug-in and series hybrid vehicles, Cobasys decided that there were enough companies already developing lithium ion chemistry and they should instead focus on their integration expertise. They examined the companies working in the lithium cell arena and ultimately struck up a partnership with A123. A123 will focus on the cell development and Cobasys will handle the pack integration and assembly.
Scott Lindholm of Cobasys provided background on the system integration that they are doing. They are designing and building the mechanical packaging and controls for the battery packs. Currently, Cobasys produces all of their NiMH packs with air cooling. The plan is to do the same with the lithium ion packs even though the cells are now cylindrical instead of rectangular. Tesla Motors is using liquid cooling on their battery pack, but that adds a significant amount of weight and complexity to the system. Because, GM is not planning on selling a $100,000 sports car, that's something they are trying to avoid.
Because of the heat that can be generated by lithium ion packs, it's important for the cell manufacturer to do whatever they can to minimize internal resistance and the pack integrator has to do the same with the interconnects in the pack. At the same time the heat that will be generated has to be dissipated and the temperature of cells needs to be kept even across the pack. Uneven temperatures can lead to degraded performance and potential "thermal events." Additionally, exposure to high voltages for customers has to be eliminated. The last thing anyone wants is a customer getting electrocuted by their battery. The fans used for thermal management also need to be reliable and quiet. Finally the pack has to be robustly constructed to withstand the inevitable vibrations of daily use, provide protection to the cells in the event of a crash, and withstand any kind of environment it may be exposed to especially if it's mounted under the vehicle.
Overall, GM provided a very informative briefing. They understand the scope of the issues that must be overcome to get a vehicle like the Volt to production in a timely manner. They are working diligently in cooperation with battery suppliers to make it happen, and in discussions with various people, the impression is that it will happen sooner rather than later.