First of a two-part series.
Whether it’s electric cars or smoothing over the ever-shifting output from solar panels, saving and supplying electricity on demand is an important part of the technical solution to climate change.
But as the technology inches forward, smartphones still struggle to make it through a whole day without charging up, battery-powered cars are too expensive or too limited in range for most consumers, and grid operators are still scratching their heads over how to price energy storage alongside power plants.
Getting energy storage to the right performance and price targets would be a huge boon to many clean technologies, but progress has been frustratingly slow, especially for those hoping for humanity to invent its way out of global warming.
Nonetheless, scientists and engineers around the world are working furiously to get the numbers right on storing energy in hope of making renewable power generators competitive with fossil fuels and turning electric cars into a viable alternative to gasoline and diesel-powered internal combustion engines.
At the moment, batteries, particularly lithium-ion batteries, are some of the most widespread options for energy storage across consumer electronics, for vehicles and on the power grid. Transportation and the grid account for 70 percent of U.S. energy use, while personal electronics dominated by lithium-ion batteries account for 2 percent.
Since they entered the market in 1991, lithium-ion batteries have rapidly ramped up in energy density and become a $20 billion industry. "They have improved remarkably, between 5 and 10 percent per year, in performance," said George Crabtree, a senior scientist at Argonne National Laboratory. "That’s pretty phenomenal, and it’s still improving."
The problem is that these batteries are still running to catch up with faster-moving demands. Lithium-ion batteries can now provide more power for longer durations, but consumers want thinner handsets, powerful processors and bigger screens on their phones, so people are still scrounging for power outlets as they were five years ago.
In cars, drivers want faster acceleration while still comfortably hauling five adults. Electric cars have yet to match the price, performance and range of gasoline engines in this regard. On the power grid, it costs about five times as much to catch and release electricity in a lithium-ion battery as it does to generate fresh power from a gas turbine.
As a result, energy storage needs to leapfrog incremental gains if it stands to move the needle in the climate fight. "You need another breakthrough to make those [applications] economic," Crabtree said.
Solutions uncover new problems
To this end, the U.S. Department of Energy launched the Joint Center for Energy Storage Research (JCESR) in 2012. With $120 million in funding from DOE, the program aims to produce batteries that are five times better and five times cheaper in just five years (ClimateWire, Dec. 5, 2012).
Crabtree, who leads JCESR, said almost every battery component is a target for innovation.
Inside a large battery that powers a car or stores electricity on the grid, there are hundreds of individual cells. In a conventional lithium-ion cell, the positive electrode, the cathode, is made of lithium cobalt oxide. The anode, the negative electrode, is usually made of graphite. In between is an electrolyte — often a liquid — that allows the charge-carrying lithium ions to flow through.
When charging, an outside electric current drives the positively charged lithium ions out of the cathode and into the anode, where they are stored. During discharge, the process reverses and generates a current that powers an electric motor or a computer.
Engineers have tinkered with these components for years, but many of the tactics to improve performance make big trade-offs.
A lithium metal anode, for example, can cram in far more lithium ions than graphite, increasing the energy density of the cell. It also has a nasty tendency to form projections called dendrites that short-circuit the device (ClimateWire, March 9).
Silicon, an alternative option, has 10 times the capacity of graphite anodes, but the material expands and contracts by a few times its original size during a cell’s operation.
Another approach is to increase the voltage in the cell beyond its current peak of 3.7 volts, leading to faster charging and a higher power output. However, at higher voltages, the electrolyte starts to break down, limiting the battery’s operating life. "If you could get to 5 volts, that would be a substantial improvement," Crabtree said.
On the cathode side, the common lithium cobalt oxide material becomes unstable when heated, which poses safety risks. It has a high energy per unit of weight, but low power per weight and a shorter life span.
With nickel manganese cobalt cathodes, some engineers have found they can make cells that are safer and last longer, but have a lower energy density.
4 promising pathways emerge
Crabtree has greater ambitions for JCESR than the small improvements the battery industry makes each year. With the program now halfway through its charter, researchers at JCESR have narrowed their focus to four promising approaches that could set the floor for battery performance much higher.
The first involves multivalent ions, atoms that can hold double or triple the charge of a monovalent lithium ion, leading to double or triple the energy density.
The next track uses a lithium sulfur battery chemistry. Rather than inserting lithium ions into the cathode, the sulfur cathode undergoes a chemical reaction with the lithium and forms a covalent bond. "The energy you store in those chemical bonds is way higher than you would with intercalations," Crabtree said.
JCESR’s other two energy storage thrusts concern flow batteries, devices that function like reversible fuel cells. These systems hold great promise for storing energy on the grid because they can scale power separately from energy, but they currently rely on expensive materials like vanadium (ClimateWire, Sept. 25).
One strategy is to replace the active vanadium components with organic molecules that serve the same purpose. Though organic materials require an organic solvent, which is more expensive than water, for the solvent used with vanadium, scientists are working to design the organic flow battery so the overall system will be cheaper.
The other idea is to attach the reagents to a polymer backbone in the electrolyte fluid. This would let the battery use a cheaper membrane to keep the active ions separate while allowing the solvent to flow through.
To move the market, the challenge for the next generation of energy storage will be to advance on all fronts at the same time.
"These batteries have to be pentathletes," said Dean Frankel, an analyst at Lux Research, a market research firm.
"In vehicles, you need better and cheaper batteries at the same time in order to get massive scale adoption," he added. "Cost is really king. Hand in hand with cost comes energy density."
On the grid, there is rapid growth in energy storage, but as utilities find more uses for storing power, from frequency regulation to shifting peak demand, they are finding that the economics change and learning that regulations haven’t caught up with the technology. "There is no one battery on the grid that can do everything," Frankel said.
However, Frankel said he is optimistic that energy storage will cross the tipping points needed to take root on the grid and under the hood.
"The stuff that Argonne [and JCESR are] commercializing in higher energy density batteries will be most impactful," he said. "Batteries are definitely on the cusp. These things are coming to fruition."
Tomorrow: What the market demands.
