Later this month, two new electric cars—the GM Volt and the Nissan Leaf—will start appearing in eco-conscious driveways across the United States.
The Nissan Leaf promises 73 miles per electric charge, while the GM Volt gets 35 miles per charge, although it also has a backup gasoline engine for longer trips. But GM and Nissan are taking different approaches to ensuring that the batteries in these cars last and remain safe. The way these batteries perform over the next few years will suggest which approach is better, and could shape the design of future electric cars and plug-in hybrids, which all major automakers have promised. Some critics say that Nissan’s battery-pack design, which uses a relatively simple cooling system, could allow the batteries to overheat, decreasing the life of the battery and posing a safety concern.
Both GM and Nissan use lithium-ion batteries (a technology that’s long been used in laptops and mobile phones) as opposed to nickel-metal hydride batteries, which have proved reliable in gas-electric hybrids such as the Toyota Prius, but which are bulky and heavy.
In choosing lithium-ion over nickel-metal hydride, GM and Nissan are taking a risk because such batteries haven’t yet proved reliable in the demanding role of powering a car. Car batteries must endure temperature extremes, harsh jolts, and continuous vibrations from the road, and have to perform well for about a decade. In a few rare cases, lithium-ion batteries can overheat and catch fire, a problem that has required massive recalls of some laptop batteries. The batteries needed for electric vehicles must also store far more energy—so a fire caused by auto batteries could be particularly dangerous.
Another drawback of using lithium-ion batteries is that they quickly lose their ability to hold charge. After a couple of years of use, it’s not unusual for them to store half as much power as they did when they were new. Automakers want car batteries that will last for the life of a vehicle—about eight to 15 years.
To address these issues, GM and Nissan have made significant changes to the lithium-ion batteries they’re using. Instead of using lithium cobalt oxide —the material preferred in laptop batteries because of its high energy density—as the electrode, they’re using lithium manganese oxide, which stores a relatively large amount of energy, but is more stable, in part due to the arrangement of its atoms. In a manganese-oxide electrode, atoms form a three-dimensional structure that maintains its shape even as lithium ions move in and out of the electrode as the battery is charged and discharged. The less stable structure of conventional battery electrode materials can be damaged as lithium ions move in and out, which shortens the useful lifetime of a battery.