America's fabled love affair with their cars is known around the world. In fact, for the first time in history the number of cars in America is greater than the number of drivers. Cars represent independence and mobility, and in many situations are the only options available.
Due to the cost of oil and rising environmental concerns, Americans are rethinking their love affair with cars, or at least those powered by oil. In the U.S., 99 percent of cars and trucks run on petroleum, using two-thirds of the country's supply. A shift toward alternative transportation, such as battery or electric vehicles, is essential for reducing oil dependency. A new type of lithium-ion battery, which features an extremely long lifespan, looks to make a big dent in solving the problem.
What does the battery need to do?
True success in the transportation market for hybrid electric vehicles (HEVs), full electric vehicles (EVs) and plug-in hybrid vehicles (PHEVs) depends upon a significant improvement in battery technology. Safety, recharge time, power delivery, extreme temperature performance, environmental friendliness, and life (cycle and calendar) are all concerns with rechargeable battery technologies available for electric vehicles today—where 10 percent of all cars sold globally may likely be HEVs or EVs by 2010.
Presently, hybrid vehicle makers tend towards incorporating nickel metal-hydride (NiMH) and lithium-ion (Li-ion) batteries into their products. In short, traditional batteries in those technologies don't make the grade when it comes to the aforementioned areas. Toyota Motor Corp's Prius, Camry, and Highlander, for instance, use a sealed NiMH battery pack to supply power to the car's electric motor. When compared to the lithium-ion battery, the NiMH's power level is lower and the self-discharge rate is higher. With a shelf life of just three years, NiMH is not the ideal solution for EVs.
Traditional lithium-ion batteries offer high specific energy and low weight. However, due to their high cost, intolerance of temperature extremes, and safety—with safety being the single most significant hurdle in adopting Li-ion batteries today—these batteries are not the ideal solution, either. In fact, Toyota recently decided to delay (by one to two years) the launch of new high-mileage hybrids with Li-ion batteries due to their concerns over safety. Traditional Li-ion batteries offer a 3 to 5 year lifespan with 1,000 cycles. Another issue is the size of the Li-ion battery necessary to guarantee the desired range in a vehicle for an acceptable battery lifespan. While we accept shorter than advertised run-times on our laptops and cell phones (as we can simply plug them in while in use), we do not have this luxury with a moving vehicle.
The nLTO technology for EVs
Research at Altairnano has created a new Li-ion battery with multiple benefits, including a battery that lives longer than the average car. In fact, these batteries could offer a more than 20 year lifespan with 25,000 full recharge cycles (essentially 250,000 miles driving range for the life of the battery pack). These nano-titanate based batteries have one-third the weight and four times the power for the same sized NiMH battery. The technology is based on a nano-size lithium titanate oxide (nLTO) battery electrode material where nLTO is substituted for graphite, the standard negative electrode material employed in common Li-ion rechargeable batteries.
Figure1: Relative mechanical processes in graphite and nLTO crystals during battery operation
Stress, Strain, and Lifespan
During charge of conventional lithium-ion batteries, lithium atoms (the reduced lithium ions) deposit inside the anode and are then released on discharge. Graphite possesses a two-dimensional crystal structure. When the lithium ions enter or leave the anode, the graphite planes of the anode shift and strain to a 10 percent greater separation of the planes to accommodate the lithium ion's size. On discharge, these processes are reversed.
Over the life of the battery, this repeated shifting and straining fatigues the graphite planes, which causes the graphite structures to fracture. These fractures cause a loss in electrical contact between the particles, thereby reducing the battery's capability and capacity, and ultimately, its life.
The use of nano-size lithium titanate oxide to replace graphite makes nLTO a "zero strain" material. Thus, the material essentially does not change shape upon the entry and exit of a lithium ion into and from a particle. The nLTO material has a three-dimensional crystal structure. The structure contains sites for lithium atom inclusion that are roughly the same size as the atom. Therefore there is virtually no stress or strain involved with the charge and discharge process. Thus the battery can be charged and discharged significantly more often than conventional lithium-ion batteries because of the absence of particle fatigue, which plagues materials such as graphite. Conventional lithium-ion batteries can be typically charged about 1,000 times before they are no longer useful (about 3 years), whereas, cells using nLTO materials can achieve over 25,000 charge and discharge cycles (more than 20 years).
About the authors
V. Evan House is the Director of Advanced Materials & Power Systems (e-mail: ehouse@altairnano.com), and Fayth Ross is the Marketing Manager at Altairnano (e-mail: fross@altairnano.com).
