Electric vehicles, hybrids, and renewable energy have at least one thing in common–if they’re ever going to be more widely used, representing the majority of cars on the road or a large share of electricity supply, batteries need to get significantly better. Batteries will need to store more energy, deliver it faster and more reliably, and ultimately, cost far less. The specific ways batteries need to improve vary by the application, but in all these areas, researchers have been making significant headway.
Last week, MIT researchers led by Yang-Shao Horn , a professor of materials science and engineering and mechanical engineering, and Paula Hammond, a professor of chemical engineering, announced a new approach to high-power lithium-ion batteries, the type that’s useful for hybrid vehicles or for stabilizing the electricity grid. High-power batteries accept and deliver charge rapidly. In hybrids, the goal is to supplement the gasoline engine, allowing it to run at its most efficient. The battery drives the car at low speeds for short distances and boosts acceleration, lowering demand on the engine. It also captures energy from braking that would otherwise be lost as heat. For the electricity grid, such batteries could buffer changes in supply and demand of electricity–something that’s becoming more important as more variable sources of electricity are introduced, such as wind and solar power.
The MIT researchers demonstrated a new battery electrode, based on specially treated carbon nanotubes, that last for thousands of cycles without any loss in performance. Batteries made from these electrodes could deliver enough power to propel large delivery vans or garbage trucks, for example, without the batteries being too heavy to be practical. (The researchers need to increase the thickness of the electrodes for them to be practical in these applications.) Companies such as A123 Systems, based in Watertown, MA, have also developed very high-power lithium-ion batteries, and other academic groups and startups are developing carbon nanotube-based ultracapacitors, which store energy using a different mechanism than batteries that’s particularly useful for high power and long life.
While the new electrodes could eventually be useful for hybrids, and for stabilizing the grid, they aren’t particularly good for other applications such as all-electric vehicles. For electric vehicles, the total amount of energy that batteries store is more important than how fast that energy can be delivered, since it’s the total amount that determines how far these cars can travel between charges. The MIT researchers who developed the new carbon nanotube electrodes are also developing a different type of battery to store large amounts of energy. Called a lithum-air battery, where one of a battery’s two electrodes is replaced by an interface with the air, the technology has recently attracted large amounts of government funding and interest from companies such as IBM. In theory, such batteries could store three times as much energy as conventional lithium-ion batteries. But the design has a number of problems that make it hard to commercialize, among the vulnerability of its active materials to moisture (the lithium metal it uses can catch fire if it gets wet) and the batteries’ tendency to stop working after being recharged just a few times.