One characteristic flaw of lithium-ion batteries, anode plating, comes when a recharging cell dumps lithium ions faster than the anode can absorb them. This problem can be caused either by low temperatures, which slow the rate of diffusion, or by overcharging, which slows the rate of absorption. One of the jobs of the battery management system is to keep overcharging from ever happening.
Plating is bad for a number of reasons, particularly because it further reduces absorption, increasing the concentration of carbon ions until they begin to react with the oxygen in the electrolyte. The oxidation - equivalent to that in a burning lump of coal - creates a lot of heat, which in turn increases the rate of deposition.
A123 says its carbon anode combines the high rate of charging provided by graphitic carbon with the long life of nongraphitic types. It won’t give details of its proprietary formulation, saying only that it fine-tunes the size and structure of the particles.
Altair Nanotechnologies of Reno, Nev., wards off plating by coupling standard cobalt oxide cathodes with anodes made of lithium titanate spinel rather than graphite. The spinel won’t react with oxygen, and it also charges fast and lasts long. However, the energy density - at the current, early stage of development - is only half that of standard cobalt cells, and it is little better than that of nickel-metal-hydride cells.
The second-toughest problem after thermal runaway is limited life span, as measured by both the calendar and the number of charge-discharge cycles. A123’s Fulop says the cycle-life goals are easy to meet, but the calendar-life ones will be harder.
Cobalt-based cells for portable electronics lose as much as 20 percent of their capacity each year, starting from the day of manufacture. That may be tolerable for cellphones and other portables that are replaced every three or four years, but not for a car, which is expected to last 15 years.
The California Air Resources Board requires a vehicle’s power train to last for 10 years or 150 000 miles (240 000 km) with the original components. GM has said, meanwhile, that it expects battery packs for its Volt concept car to last for at least 4000 full-discharge cycles. That’s good but might not be good enough. At one charge-discharge cycle per day, the pack would last for 11 years - though it’s the rare car that runs 365 days a year for a decade.
Worse yet, auto and battery makers don’t have the luxury of spending 10 years testing lithium-ion packs. ”Ideally,” says Mark Verbrugge, director of GM’s materials and processes laboratory, ”we’d have half the life span to test it. But we don’t, so there’s no clean answer.” Meanwhile, automakers are ”oversizing” their battery packs to ensure they’ll power the car even after projected degradation. Of course, that strategy adds cost and weight."