Building a Better Battery
The future of powering electric vehicles.
This is the first part in an eight-part series on the future of transportation. New articles published every Monday.
Futurists have often said technology obeys Moore's Law, which maintains the amount of transistors that can cheaply fit on a microchip doubles every two years. Thus, we end up with more processing power in progressively tinier packages.
But this exponential paradigm doesn't apply to the development of hybrid electric vehicles and electric vehicles. In this arena, technology advancements are limited by the battery—a product of chemistry, not electronics. As a BMW exec tasked with developing fuel-efficient technology told the Financial Times, what we don't know now about certain reactions, we may not know decades from now. Thus, it's hard to predict what the car battery of the future will be.
For those who didn’t pay attention in class: Batteries are typically comprised of three main parts: a cathode (positive electrode), an anode (negative electrode), and an electrolyte (an ion-rich liquid that separates the electrodes). The movement of metal ions between the cathode and the anode through the electrolyte (and back) releases electrons, generating electricity.
Lead-acid batteries, found in conventional automobiles, have a low ratio of energy to weight, which means it takes a lot of battery to provide just a little juice. Nickel-metal hydride batteries, the ones powering today's hybrids like the Toyota Prius, are significantly lighter, but offer only a slight improvement in efficiency. Neither can compete with gasoline-fueled internal combustion.
Several technologies are competing to fuel the next generation of EVs. All of them, however, have serious weaknesses that researchers are still attempting to address. "People are betting on different horses at this point in time," says Matt Keyser, a senior engineer in energy storage systems at the National Renewable Energy Laboratory in Golden, Colorado. "Which one is going to come out and win is anyone's guess."
Here's a look at some of the technologies vying to corner the EV market:
These batteries use lithium ions as the electrolyte. A battery pack made of these cells, while more powerful than lead-acid and nickel-metal hydride batteries, is still 10 times weaker than an internal combustion engine of the same weight. Versions of these batteries are already used in in both the Tesla Roadster and Chevy Volt, as well as many electronic devices, such as laptops and cell phones. The knock on current lithium ion technology: It dispenses its stored energy slowly, so acceleration may be slow, and the batteries take several hours to charge. Also, while lithium is plentiful, it's not extensively mined, so it’s expensive to obtain. It may take up to 10 years for supply to catch up to projected demand.
Ultracapacitors charge quickly and dispense their charge speedily (curing the slow acceleration problem that plagues some electric cars). They also last much longer than batteries—they can be recharged over and over again, whereas batteries eventually will not recharge. That's because ultracapacitors use electric fields, instead of slowly depleting chemicals, to get charges. They are already in use in short-run electric buses in Russia and garbage trucks in the United States. The downside: They only hold their charge for a limited time, so it's unlikely that ultracapacitors will become a viable option for powering a car alone. "I think ultracapacitors are a technology that's going to work with [battery] systems," says Savinell. However, one Texas-based company called EEStor says it has solved the storage problem, claiming its ultracapacitors will enable a small car to travel 250 miles on a single charge that only takes five minutes to complete.
Like batteries, fuel cells have cathodes and anodes and involve a chemical reaction, specifically making water and electrons (and thus electricity) by combining hydrogen with oxygen. The technology is simple enough, but the safety issues are the drag: The transport and onboard storage of highly explosive (remember the Hindenburg?) hydrogen gas could keep fuel cells from catching on. In addition, the catalysts needed to split hydrogen atoms into protons and electrons (like platinum, palladium, rhodium, nickel) are very expensive. "Fuel cells from a mobile standpoint are difficult," says NREL's Keyser. "Maybe in twenty five or thirty years down the road, we may be able to deal with all the storage issues, the transport issues, the infrastructure issues, the catalyst itself." Seemingly agreeing with Keyser's skepticism is the Obama administration, which cut $100 million from the federal hydrogen fuel cell program in 2009.
Similar to fuel cells, redox flow batteries would require filling stations rather than plug-in capability. In this case, a charged electrolyte flows through the battery, producing electrons. After a while, the electrolyte loses its charge and needs to be pumped out and replaced. The electrolyte is typically made with vanadium, which is the 22nd most abundant element in the world. It's also very safe. "If you were to spill this on the road and light a cigarette near it, it's not going to go off like hydrogen," says Keyser. "The big thing with [redox flow batteries] is: Are you going to get the energy density or power density that you need for the car itself?" Right now, even lithium ion cells are several times more powerful than redox flow cells. German researchers, however, claim they have a method to increase the distance redox flow batteries can power a car by four to five times, rendering them roughly equal to lithium ion batteries.
Savinell and Keyser both point to metal air batteries as the technology of the future. This battery uses the oxygen in the air as its cathode, which means it doesn't need as much material and gets more energy for its weight. Depending on what material is used for the anode, metal air batteries could be anywhere from three times more powerful than lithium ion batteries of the same weight to as powerful as an internal combustion engine. IBM intends to bring these to market in five years for smaller electronics. "For lithium air, I think that's more ten to fifteen years down the road [to power a car]," says Keyser. "We're just starting to really look at that and understand all the benefits and the costs associated with lithium air batteries." One major barrier remains: When the oxygen reacts with the electrolyte to form ions, it also creates a solid that can gunk up the air intake, blocking the battery's function. Researchers are searching for an electrolyte that will produce the necessary ions but avoid the formation of this solid.
Illustrations by Will Etling.