Safer EV batteries could enter the crush zone

  • 26-Nov-2013 09:01 EST

The "nanoelectrofuel" flow battery being researched by the Illinois Institute of Technology and Argonne National Laboratory is said to be safer because the high-energy electrolyte is stored in an external tank.

“There’s been a lot of talk about the limited driving range and high costs of electric vehicles [EVs], but at the end of the day, range is really a cost problem,” said Ping Liu, Program Manager at the U.S. Advanced Research Projects Agency-Energy (ARPA-e). “So if the metric is range per sticker price, it’s clear that today’s EV batteries are too expensive, store too little energy, and weigh too much.”

“A while back, my ARPA-e colleagues and I asked ourselves: ‘What innovative approaches might drive down the costs while improving performance?’ And we thought: ‘Why not use the right chemistry for the purpose?’”

They decided that a safer battery chemistry—one that did not require large, heavy battery packs to shelter lithium-ion cells and the support systems to babysit them—could save enough weight to extend EV range even if the new batteries did not produce quite as much energy as Li-ion. It would also let designers rethink vehicle plans: “If you don’t have to worry about protecting batteries, you’re no longer limited to the architecture of conventional cars,” Liu said.

Tougher, safer batteries

Such is the rationale for the 22 multi-year R&D projects now under way in American industry and academia to produce safer—more “robust” and “abuse-tolerant”—battery chemistries and cell configurations. They are all part of ARPA-e’s RANGE (Robust, Affordable, Next-Generation Energy storage systems) program, which Liu leads. The $37 million program is looking for ways to use batteries in door panels and body structures not only for load-bearing purposes (see related article here), but also for energy absorption in crashes. The latter would allow designers to integrate batteries into the vehicle crush zones that deform and crumple to protect passengers.

The U.S. Department of Energy wants a built-in battery that can power a somewhat light electric car for at least 240 combined-cycle miles (386 km) on a charge.

The approaches vary. Some groups aim to perfect new electrochemistries that use nonflammable electrodes or replace volatile electrolytes with water-based or less flammable polymer electrolytes. One strategy is to avoid liquids altogether, building solid electrolytes from such materials as thin-film ceramics.

“We decided to look for fundamentally robust battery chemistries with built-in abuse-tolerance,” Liu explained. “And even though this or that chemistry may not perform quite as well as lithium-ion batteries,” the substantial weight savings that derive from incorporating them into the structure means that they would still supply enough energy to enable significantly better EV range.

Water-based electrochemistries

One of the three categories of chemistry on which RANGE is focusing comprises aqueous types that employ a water-based electrolyte. Some are flow cells, which resemble electrochemical cells except that the electrodes are not bathed in electrolytes. Instead, the electrolyte is stored externally, ready to feed into the cell to generate electricity. The separation makes them safer.

“Conceptually, these alternative chemistries challenge the conventional wisdom that flow cells don’t have enough energy density for vehicles and so have to be very large for even stationary applications,” said Liu. “We received a fantastic proposal from GE and Lawrence Berkeley National Laboratory for an aqueous flow battery with a different chemistry—one that would have an energy density that’s high enough to be relevant for transportation applications.”

The GE/Berkeley Lab team received a $900,000 grant to demonstrate the feasibility of the concept and build a working prototype over the next year. The system uses water-based solutions of inorganic chemicals that can transfer more than a single electron to boost the energy density. Energy is extracted by the partial oxidation of an energy-dense organic liquid "fuel," which yields a stable, hydrogen-depleted compound. Special electrocatalysts would directly extract charge carriers at the anode without production of hydrogen gas. The ions would combine with oxygen at the cathode to generate voltage and water—the sole reaction product.

Said Liu: “There’s nothing that’s exotic, and everything looks like it has commodity-like costs, which makes it exciting.”

He next pointed to another "multiple-electron" aqueous system under development at the University of Maryland and the U.S. Army Research Laboratory. The team received $405,000 to perfect “a new chemistry that uses a ‘tandem’ mechanism with multiple reaction steps to achieve high specific power.” According to the university, this nanomaterial battery would use hybridized “twin” ions and intercalation chemistries to step up cell voltage (from 1.2 to 2.5-3.0 V) and double its capacity.

Non-aqueous electrolytes

Some of the RANGE projects fall into the non-aqueous category—chemistries that contain volatile compounds. Thus, they may need extra maintenance systems that could cut into efficiency, but they would offer similar overall benefits, the program manager said.

The Illinois Institute of Technology is collaborating with Argonne National Laboratory on a $3.4 million effort to produce a flow battery that employs a high-energy-density electrolyte—a liquid nano-electro-fuel that contains multiple nanoparticles that make it energy-dense while ensuring stability and low-resistance flow inside the battery. A $450,000 RANGE award is meanwhile funding work at Oak Ridge National Laboratory on a project that takes advantage of a phase-change phenomena—from liquid to solid—that occurs inside the battery if it experiences a mechanical impact.

Solid-state cells

The third focus is on solid-state batteries with nonliquid electrolytes, which have always suffered from doubts about scaled-up potential in large batteries,” Liu said. Solid electrolytes do not pass charge carriers as easily as fluid ones. “Researchers have made quite a bit of progress in this area recently,” he said, "but the technology remains challenging.”

Eric Wachsman, an expert innovator in solid-oxide fuel-cell technology at the University of Maryland, is using $575,000 to introduce his multilayer thin-film ceramic processing techniques into the fabrication of a new Li-ion solid-state electrolyte with high conductivity. High conductivity, garnet-type solid lithium-ion electrolytes and high-voltage cathodes are to be integrated into tailored micro-/nano-structures. Wachsman hopes to produce a solid-state battery pack with lower weight and longer life.

A start-up called Solid Power in Louisville, CO, a spin-off of the University of Colorado at Boulder, was awarded $3.5 million to develop a low-cost solid-state battery with high specific energy. It would use a “chemistry that can’t be used with a liquid electrolyte because the active material would dissolve into the liquid electrolyte.”

The fourth group of R&D projects emphasizes the multifunctional aspects of structural batteries. Liu highlighted a $3.5 million project at the University of California - San Diego, where they “are innovating at multiple scales—at the materials, systems, and architectural levels”—to integrate the new batteries into the vehicle-support structure.

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