Battery challenges: cost and performance

As the world becomes ever more dependent on batteries to power modern life, challenges from fire risk in portable devices to grid-level storage for solar and wind farms require increasingly diverse approaches. Researchers from both MIT and industry offered a variety of different solutions at the Materials Day Symposium on Oct. 18.

Among significant progress noted in developing and deploying new battery technologies:

• General Electric has developed a model to assess the economics of energy storage for wind and solar energy projects, guiding how to optimize selection of storage type, size, and market;
• MIT Professor Yet-Ming Chiang is developing an air-breathing, water-based sulfur flow battery that may compete on cost with pumped water and compressed air storage technologies;
• 3M has developed Novec fluid that provides fire protection for battery cells; and
• Bosch is working to replace flammable electrolytes in lithium ion batteries with solid lithium-ion electrolytes.

In remarks opening the “Materials for Electrochemical Energy Storage” symposium, Materials Processing Center Director Carl V. Thompson noted the wide range of demands for batteries differing in scale from small ones integrated with piezoelectric energy harvesting for remote autonomous sensors to large scale batteries for cars, buses, aircraft and energy storage at power generation sites.

“They all require innovation in materials and the ways to make materials for the electrodes, for the electrolytes, especially,” Thompson says. “These innovations are required to reduce cost and also to improve performance.” The right measures for tracking progress in battery research and development, he suggests, are “cost and performance.” About 140 attended the sessions in Kresge Auditorium.

40,000 wind turbines

Glen D. Merfeld, product science leader at GE Global Research, points to almost 40,000 wind turbines installed globally. “We have data from wind turbines around the world where we can simulate the potential impact of storage,” he says. GE’s database comparing physical capabilities of storage technologies can also tell operators how to size storage and its cost. With a market potentially reaching hundreds of millions of dollars, trade-offs are needed at large scale, he says. “In some markets, to extract the most value from storage, you might want to make [the] smallest investment and take a look in a year’s time to see if it’s still a good investment,” he suggests.

As a result of greenhouse-gas-reduction goals set by the U.S. and other nations under the 2015 Paris Climate accords, Jessika Trancik, associate professor of energy studies with MIT’s Institute for Data, Systems, and Society, estimates that solar energy installations globally could grow by five times and wind energy installations by three times. This will enable solar and wind to provide an estimated 4 percent and 9 percent of global energy production in 2030. “Energy storage can a play critical role in supporting the growth of renewables,” she says. But storage has to increase operators’ profits for them to justify installing it at those intermittent solar and wind generators, she adds, and storage costs will have to fall along with solar and wind energy costs to remain competitive.

Developing storage over the next decade is also very important “to maintain the momentum that we’ve seen coming out of the Paris Agreement,” Trancik says. Trancik’s research developing a formula to compare the value of different storage technologies — such as lead-acid batteries, pumped hydro, and compressed-air storage — compared solar and wind performance at sites in Barnstable, Massachusetts; McCamey, Texas; and Palm Springs, California.

“One of the takeaways from this is that it’s really important to focus on developing storage now,” Trancik says. Her studies found that the optimal cost structure of a storage technology is the same across locations because, despite differences in the frequency and amplitude of energy price spikes, their durations are similar across sites. An important finding for materials researchers is that “It’s really important to develop the electrochemical batteries because the pumped hydro storage and compressed air energy technologies are only valuable in certain locations, whereas a battery is something that we can install anywhere,” Trancik says.

MIT Department of Materials Science and Engineering Professor Yet-Ming Chiang notes that 75 percent of grid-level energy storage today is in lithium ion batteries, so that technology still has a long life. A company he co-founded, 24M, is attempting to develop the lowest cost lithium ion battery. However, he believes his latest research can match the cost of pumped water and compressed air storage technologies with a new air-breathing, water-based sulfur flow battery intended for long-duration grid storage.

Sulfur is an abundant byproduct of natural gas and oil refining, and sulfur offers the lowest cost — except for water and air — in terms of dollars per amp-hour storage, Chiang says. The cost of chemicals for this setup can be as low as less than half a dollar per kilowatt hour, and the energy density of the fluids can be as high as 150 watt hours per liter.

Chiang describes his flow battery as a system in which lithium ions, or sodium ions, in a water-based solution flow across a LISICON or NASICON membrane between a lithium sulfide anode and a cathode that absorbs or releases oxygen. The sulfur, however, does not react with the oxygen. The equilibrium voltage depends on whether the catholyte is acidic (1.7 volts) or basic (0.85 volts). “We have a membrane-limited voltage efficiency, which we understand; we demonstrate good cycling over long durations,” he says. Compared to existing battery technologies such as vanadium redox flow batteries and lithium ion batteries, “We really do believe this is the lowest dollar per stored energy of any rechargeable battery chemistry that exists today, and the longer the storage duration, the better one is able to capitalize on that lower energy cost,” Chiang says. “As a fully-developed system, this flow battery could provide storage at a cost comparable to pumped hydroelectric storage but without the locational constraints,” he says.

New anode material

Kevin Eberman PhD ’98, a product development manager in 3M’s Electronic Materials Solutions Division, observes that after 20 years of improvement, lithium-ion batteries are the dominant energy storage technology for portable, mobile applications ranging from electronic devices to electric car batteries. “We’re out to 1,000 cycles easily with these high energy densities,” he notes. Expected cycle life can reach 10,000 for grid-level energy storage. Although electric-powered vehicles deliver efficiency of about 90 percent compared to 20 to 30 percent efficiency for gasoline-powered ones, energy density improvement is leveling off to about 3 percent per year enhancement. “The low hanging fruit has been picked,” he says.

One alternative is replacing the graphite [carbon] anode in lithium-ion batteries with silicon materials that store more lithium ions, Eberman says. A potential problem is that silicon material expands, potentially damaging the battery, but it doesn’t have to be solved all at once, he says. “We think the 3M silicon alloy is probably the leading option for replacing graphite in the anode. If you put in just 20 percent, you can get some 6 percent improvement in energy density. If you put in more of the alloy, you can get more improvement.” Increasing energy density is one way to cut cost. 3M’s internal tests show up to 700 cycles in batteries using its silicon alloy material. “So, we are on the cusp of really going big with this,” Eberman says.

In a separate collaboration, 3M works with New Hampshire-based Nanoscale Components Inc. on a process to add a small amount of lithium to the silicon alloy anode to extend its life. “We can see 500 cycles in a high energy density cell with high silicon alloy loading,” he says. Eberman also discussed 3M’s battery technologies for cathodes, aluminum foils and electrolyte additives. 3M also developed Novec fluid that provides fire protection by squelching small fires that start from short circuits in battery cells before they can spread outside the battery. Eberman showed a movie demonstrating the fire suppression effect. “That’s another exciting area that 3M is going to contribute to around safety of batteries and potentially have a big impact on the safety and the overall risk of installing big energy storage systems built on lithium ion,” he says.

Martin Z. Bazant, MIT professor of chemical engineering and applied mathematics, addressed a different concern with lithium ion batteries: phase transformations that interfere with the essential function of the battery, which is movement of lithium ions back and forth between the anode and electrode when the battery charges or discharges. In rechargeable lithium metal batteries, in particular, filaments grow in either mossy or dendritic shapes, which can lead to a short circuit in the cell. These dendrites form fractal patterns similar to tree leaves, lightning or snowflakes. Bazant’s recent work shows the dendrites are a secondary growth pattern on top of mossy lithium solids, which form first. Significantly, his research points to a way of blocking mossy growth by using nanoporous ceramic separators.

Bazant’s research shows that in lithium iron-phosphate batteries, lithium ion concentrations in crystal electrodes form separate areas, or phases, of low lithium content and of high lithium content. This phase separation, which happens at nanoparticle scale, can slow the rate of charging and discharging in the battery as well as lead to cracking or other problems in the electrode. But at high current, Bazant’s research shows, lithium ion concentrations become more uniform across the electrode crystal as evidenced in experiments carried out at Stanford and supported by his own theoretical calculations. Understanding this complex behavior is a key to designing better batteries, he suggests. “The most surprising prediction from fundamental physics here is that there is a critical current above which the system is homogeneous,” he says. “If you want high rates, what you need to do is suppress phase separation so the whole material turns on uniformly over its surface, and everywhere it’s exposing kind of intermediate concentrations so you can add and remove lithium freely.”

Replacing flammable electrolytes

Boris Kozinsky ’00, PhD ’07, principal scientist at Bosch Research and Technology Center in Cambridge, Massachusetts, discussed the quest to replace flammable electrolytes in lithium ion batteries with lithium-based, solid-state electrolytes. Bosch’s approach is to run computer simulations of molecular dynamics to see how reactions propagate and identify materials with good conductivity and stability. Lithium phosphorous sulfur is one material studied, while another, lithium garnet ceramic oxide, shows good stability and good conductivity, he reports.

Yang Shao-Horn, the W.M. Keck Professor of Energy in MIT’s departments of Mechanical Engineering and Materials Science and Engineering, wants to increase battery power by using ligand redox such as oxygen or sulfur to potentially double or triple the amount of stored energy. “We can move beyond lithium ion batteries,” she says. Her research focuses on determining relationships between bulk electronic structure and functionality of metal and oxygen sites for charge/energy storage and catalyzing reactions relevant to clean energy and environmental relevance.

Pollution counter

Trancik recently launched the Carbon Counter web-based calculation tool, which lets users compare costs and greenhouse gas impacts of popular cars, SUVs, and pickup trucks. It also points out where more environmentally friendly power trains are available for the same model type. Her studies show, for example, the all-electric Nissan Leaf could cover the vehicle daily energy needs of almost 90 percent of personal vehicles in the U.S. However, “It’s very difficult to get 100 percent,” she says, pointing to a need to combine electric cars with alternatives such as car sharing.

3M’s Eberman calculates that electric car battery pack production will increase by 2.5 times by 2020 with a cost of about $21,000 for a 200-mile pack. “That really is kind of a tipping point, a point where you could go from having less than 1 percent of cars being electric vehicles to being a significant portion of the market,” Eberman says.

The Materials Day symposium was followed by a student poster session.

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