Survey of USABC Contractor Lithium-ion TechnologyOctober 20, 2007
The United States Advanced Battery Consortium is essentially a communal technology incubator for the domestic automakers, providing development funding for new technologies. At the moment, advanced lithium ion is in vogue, with several large and several small companies being contractors. However, when you take a look at the list, one rather striking feature is that the USABC chose a wide range of lithium ion approaches to fund. For example, each contractor has a very different chemistry and/or cell format. It stands to reason that the USABC isn’t putting all its eggs in one basket, and it chose to fund the most promising examples of each.
But how do the contractors stack up against one another, and what are the inherent compromises of each approach? Each company is understandably reticent to reveal their exact specifications, as it is competitive information. However, with a little clever research into published white papers and presentations, you can uncover their approximate progress relative to one another.
IMPORTANT NOTE: This is a work in progress that only uses publicly available information. Please submit rebuttals, corrections, etc, to ensure that this comparison is accurate and up to date. I have no commercial interest or financial connection to any of these companies, other than as an American consumer.
Observations for each contractor:
A123: Their M1 technology is widely available, routinely reconfigured by the RC scene, and has a reputation for very high power and good safety even under abuse. Thus, the fundamental properties of this technology are well understood. Their more advanced batteries are based on the M1, and will supposedly feature thickened electrodes that will sacrifice some power density but increase overall energy storage. Only official word so far is “greater volumetric energy density”, but no word about gravimetric. To give an idea of where this could lead, maximum theoretical energy storage for LiFePO4 is variably quoted at 130-140Wh/kg. This puts on a damper on A123’s ability to meet the USABC’s long term energy storage goal of 200Wh/kg, but they still have a very competitive technology at the moment. Their choice of a cylindrical cell will make pack design slightly complicated due to poor heat dissipation and space management (compared to stack formats), but cylindrical formats tend to have the very highest energy densities, so as long as the amount of cooling and packaging equipment isn’t excessive, the tradeoff will be worthwhile.
Update 11/28/07- Confirmed from multiple sources at this point – A123 is developing a “flat” cell specifically for General Motors.
Additionally, 32 series energy density (volumetric) is confirmed at 260Wh/L, versus M1 cell at ~220Wh/L.
CPI/LGChem: These lithium polymer cells have been under development for some time – three different prototype generations have been described. The white paper indicating 95 Wh/kg is from 2003 however, so it’s expected that this has been improved upon since then. One apocryphal reference is http://www.aevehicles.com, which built the electric Pike’s Peak hill climb dragster using LGChem batteries. They claim energy densities of 160 Wh/kg for the cells that they use. Regardless, one source of concern is that as thermal stability of prototypes has improved, the energy actually went DOWN from 118 Wh/kg to 95Wh/kg. Their 95% capacity at 200 100%DOD cycles also does not compare favorably with the other contractors, but this was for the first generation, and is actually somewhat typical of conventional lithium polymers. This has apparently been amended using “improved fabrication technique”, but by how much, we do not know. Thus, 95%@200 cycles is the very minimal cycle life we can assume.
EnerDel: Lithium titanate is a much newer technology that has a reputation for very good safety as well as very good lifespan. Indeed, the cells are claimed to achieve 1000+ cycles even at elevated temperatures. However, EnerDel has explicitly confessed that LTO has much lower voltage than graphite or even hard carbon electrodes, and this makes for a very heavy energy penalty.
Nonetheless, they claim to have met the USABC mid term criteria. This means that a minimum of 80Wh/kg is assured, as well as a power density of at least 150W/kg. Similar LTO technology made by AltairNano has only about 78Wh/kg and 1600W/kg, so EnerDel probably falls within this range. We need more information before passing further judgment.
Whether this will be used in HEVs or PHEVs really remains to be seen, but for the moment other technologies seem to have an edge as far as versatility. EnerDel also has a hard carbon anode chemistry that also claims good safety, but better capacity. Specifications on this are unknown at the moment, however.
For pack construction, EnerDel has claimed that air cooling is sufficient for their cells. However, it’s known that chilling lithium batteries to below ambient temperatures extends their calendar life, and as far as I know it’s not possible to chill packs below ambient using air cooling.
Update 11/28/07- The folks from EnerDel got ahold of this comparison through the grapevine, and stressed that their batteries’ good safety characteristics were a missing factor in the comparison. I don’t have access to safety metrics (if there are any), but when I do I’ll try to incorporate them.
JCI/Saft: Saft batteries are formidable because they are used in high-demanding military applications. The cell featured here is the VLE lithium ion, and has the highest energy density of the group as well as a very long cycle life – although it’s not published what %capacity remains at 1500 cycles. As far as safety and thermal stability, however, it’s worth noting that Saft provided the batteries for GM’s Sequel fuel cell concept, and this concept had to stop twice during its record-setting 300-mile test run because the battery pack had overheated. When assembled into OEM modules, the VLE packs hold about 110Wh/kg.
Not mentioned is 3M, which is not developing full size batteries, but is instead researching fundamental improvements in electrolyte chemistry and electrode formulation.
Where do things go from here?
Each contractor’s individual cells appear to have met the basic, mid-term USABC targets. Perhaps the biggest factor for success, however, is going to be how effectively the individual cells can be incorporated into large modules and packs that make good use of space, and can be efficiently heated, cooled, and protected. Some of this is going to be chemistry dependent – both the fundamental battery chemistry as well as the electrolyte- and some of it is also going to be dependent on cell construction. Cylindrical cells, for example, are physically robust and energy dense due to their tightly wound electrode material and strong metal can. But, they are less efficiently packaged than stack/laminate formats, and tend to build up heat very quickly. Laminate cells, on the other hand, are easier to assemble into large packs, and dissipate heat much more effectively. They are much more fragile, however, and do not store as much energy. One rather significant problem facing them is that lithium ion cells tend to have gas evolve during heavy charging; while cylindrical cells are able to vent this pressure quite easily, laminate cells are an inherently closed system, and must be designed to be able to flex and expand slightly. Proper, slow charging should not induce this gas formation – but in the rapid recharge/regen and discharge environment of an EV or PHEV, this may not be possible.
Cylindrical, stack, and prismatic formats are all undergoing continual refinement. One novel cell format, however, is a “button”-shaped cell from Illinois-based startup Inventek, which achieves good volumetric energy density through the use of wound, ribbon-like electrode material. “Rolled Ribbon” also advertises very high power (>2000W/kg) and good heat dissipation due to large, high surface area current collection and heat dissipation. As with all the other cell formats, putting many cells close together in a module essentially creates a coupled thermal mass, and thus proper spacing and active cooling are going to be necessary. Nonetheless, as the USABC has obviously recognized, the large diversity of technological solutions is a good thing – the more competition the better!
Specific company references: