After a decade of intense focus on lithium-ion, material scientists are beginning to look to new battery chemistries for future electric cars.
The new chemistries could require a long wait -- 10 or 20 years in some cases -- but they might change the future of electric transportation. "We're getting to the end of the road as far as being able to extract more energy out of lithium-ion," John Kopera, vice president of commercial operations for Sion Power, a maker of lithium-sulfur batteries, told us. "But we can see a time when batteries could have higher energy, weigh less than half as much, and still be less costly than lithium-ion."
Sion Power isn't alone in believing better battery chemistries are on the horizon. The Advanced Research Projects Agency (ARPA-e) has tabbed a variety of chemistries for government-sponsored development through its Batteries for Electrical Energy Storage in Transportation (BEEST) program, including lithium-sulfur, lithium-air, zinc-air, and magnesium. Other materials scientists are targeting aluminum-ion and even lead-acid as possible EV solutions. Using lead-acid chemistry with enhanced power density, Energy Power Systems hopes to replace the nickel-metal hydride batteries in hybrid cars with batteries that cost less than half as much.
The road to EV battery nirvana is fraught with difficulties. It has taken more than a century to reach today's state. Moreover, the EV battery industry has been notorious for overpromising and underdelivering on its technology. "People who develop batteries often don't appreciate all the problems that are involved," Elton Cairns, a professor of chemical engineering at the University of California-Berkeley, who designed fuel cells for the Gemini space programs, told us. "Each new idea looks really great until you get farther along and discover all the problems." Still, battery chemistries such as nickel-metal hydride and lithium-ion have provided great gains for the auto industry over the last 20 years. Experts say more are on the way, especially if automakers and consumers appreciate the fact that battery development takes painstaking work and patience.
The wait for lithium-ion was a long one. John Goodenough, a professor of mechanical engineering and material science at the University of Texas, began exploring lithium-iron-phosphate chemistries in the late 1970s. The technology hit the automotive radar in the early 1990s, and it took two more decades for it to reach its current level.
Some experts say lithium-ion may be reaching its limits. Though development continues on lithium-ion batteries that are said to offer specific energies of more than 300Wh/kg, many materials scientists say lithium-ion won't exceed its current level of 150-200Wh/kg. "Mature battery technologies typically reach about 40 percent of their theoretical energy," Kopera said. "Lithium-ion is already at 40 percent."
The reason is simple. In practice, battery designers must add dead-weight components (such as electrolytes, terminals, and housings) that boost the battery's mass and thereby reduce its specific energy. Scientists say lithium-ion is reaching the point where mass reduction is getting more difficult.
That's why more battery developers are focusing on lithium-sulfur chemistry, which offers much higher theoretical energy than lithium-ion at a lower cost. "Lithium-ion uses materials that are expensive, like cobalt and nickel," Cairns said. "But if you take out the expensive materials and replace them with something like sulfur, which is literally dirt cheap, you can reduce the cost by a third or so."
Dealing with challenges
The problem with lithium-sulfur is that it has been plagued by short cycle life. Over the past 20 years, many scientists have developed lithium-sulfur batteries with high energy content, but most of the batteries have worn out after about 50 cycles. Since automakers tend to seek more than a thousand cycles, lithium-sulfur hasn't been seriously considered by the auto industry until now. "There are applications that need low numbers of cycles, but automotive is not one of them," Kopera said. "Automakers want the batteries to cycle, not hundreds of times, but thousands of times. Some are saying they want 2,000 cycles, because they want some margin for warranty."
Combining lithium and sulfur, materials scientists hope to boost energy by a factor of four
or more over today's lithium-ion batteries.
The problem is that lithium tends to react with virtually everything on the periodic table, including most of the materials inside a battery. As a result, it often produces byproducts that are detrimental to battery life.
Sion Power, which received a $5 million ARPA-e award to develop lithium-sulfur, is improving battery life by employing new materials in virtually every part of the battery. "All components are up for investigation," Kopera said. "The only materials that aren't are the lithium and the sulfur."
According to Sion engineers, the key is protecting the battery's lithium. By alloying it with the proper combination of materials, engineers can create a lithium-metal anode that doesn't react with other elements and therefore has less dendritic growth, which could otherwise cause short-circuiting, Kopera said.
Cairns, who is also developing a lithium-sulfur battery, said the chemistry could easily enable EVs to hit a 300-mile range. That should free up automakers to loosen their requirements for high cycle life. "If you think about it, when you have a battery that goes 300 miles on a charge, you no longer need 1,000 cycles," he said. "After a thousand cycles, the car would have 300,000 miles on it, and other components on it would have already worn out." Five hundred cycles would be a more realistic number, because it translates to about 150,000 miles.
Long-range solutions In its effort to encourage the development of alternate batteries, ARPA-e is also supporting the creation of a high-energy lithium chemistry that essentially breathes air.
Lithium-air batteries use an air cathode to draw oxygen from the atmosphere; the oxygen then reacts with the lithium to produce electricity. ARPA-e has granted $1.2 million to the Missouri University of Science and Technology and nearly $5 million to Polyplus Battery Co. to speed up development of the technology.
Experts say making lithium-air a reality won't be easy. Engineers must find a way to gain access to the oxygen in the air without allowing in any moisture, which would hurt the battery. The solution is to create a selective membrane that resides between the lithium-based negative electrode and the reaction chamber. This would let the battery breathe oxygen without allowing water or nitrogen to seep through. Polyplus says it has created a manufacturable version of such a membrane, which would "would enable an EV to travel from New York City to Raleigh, NC (500 miles) on a single charge, for less than $10 on average."
Lithium-based batteries aren't the only ones that hold promise. Donald Sadoway, an MIT professor who has teamed up with Bill Gates on a storage battery for grid applications, is also working on an aluminum-ion chemistry for EVs. The aluminum battery, which employs a solid polymer electrolyte, is probably the most novel idea on the battery horizon -- and possibly the riskiest. But Sadoway told us he sees tremendous potential: 500Wh/kg at $100/kWh.
"Aluminum is the third most abundant element in the earth's crust, 8,000 times more abundant than lithium, and available in the US," he said. "This means cheap raw materials and a secure supply chain."
Back to the future
Not all solutions are targeted at the distant future. Energy Power Systems (EPS) is reaching back to the age-old lead-acid chemistry in its effort to change the future of the EV. Instead of developing a high-energy battery and then reducing the cost, the startup is starting with low-cost lead-acid chemistry and then adding power density and life.
Subhash Dhar, who has notably served in executive roles with Energy Conversion Devices and Ener1, founded EPS with the idea of developing a battery with the same power density as nickel-metal hydride at about a third of the cost. Right now, his battery appears to be one of the strongest short-term solutions. It reportedly offers about six times the power density and almost 10 times the life of traditional lead-acid, and it does so for about $100/kWh -- a figure that's little more than a dream for most EV battery companies.
As Dhar told us last month:
The concept has always been to start with a chemistry that gives you high energy density, and then hope you can reduce the cost. But the industry has never made much progress in terms of cost. So we turned it upside down -- we started with low cost and improved the technology, so we can get the performance without disturbing the cost structure.
In truth, the EPS battery doesn't offer traditional high performance. Its energy density is 40Wh/kg -- a tiny fraction of what lithium-based batteries offer. But Dhar said he isn't worried. He anticipates the battery being used in full hybrids, mild hybrids, and start-stop applications, where power density matters far more than energy density. He also foresees it employed in plug-in cars, in tandem with lithium-ion batteries. Using Dhar's setup, the lithium-ion battery would supply the range, while his lead-acid design would produce the power. The bottom line, he said, is automakers might be able to build hybrids and plug-in cars for less money.
Most people aren't ready to make the transition away from lithium-ion. Many say lithium-ion costs will drop sharply in the coming decade, possibly hitting $250/kWh for EV battery packs and $150/kWh for cells by 2025.
Still, boosting energy density may be a more difficult task. Despite claims by US Energy Secretary Steven Chu that energy density will double or triple while battery costs drop 50 percent in the next three to four years, many in the battery community are doubtful.
Most experts say that the wait for $5,000 batteries and 300-mile ranges will be a long one. History has shown that battery development often takes 20 years or more of serious, well-funded effort, they say. "It can take a very, very long time to bring research in the lab to a strong commercial position," Cairns said.
That's why most material scientists are openly rooting for lithium-ion to succeed, even if lithium-ion is a competitor. "We want to see lithium-ion be successful," said Kopera of Sion Power. "It paves the way for our higher-energy batteries down the road.