A Bay Area start-up firm with 35 employees has developed a new automotive-grade lithium-ion battery that delivers about twice as much energy per unit of mass as current EV batteries. The new rechargeable batteries from Newark-based Envia Systems were recently shown to store 400 W·h/kg, which compares favorably to the 120 to 250 W·h/kg provided by existing commercial counterparts. Technicians at the U.S. Navy's Naval Surface Warfare Center in Crane, IN, conducted the performance tests on the novel electrochemical device.
One of the principal barriers to widespread adoption of electric cars is "range anxiety"— potential customers’ fears that the maximum ranges of today’s EVs (only 80 to 100 mi/130 to 160 km) will leave them stranded with no power. Because energy density is the main determinant of EV range, Envia’s new batteries could enable a doubling or tripling in range. And at a projected cost of only $125/kW·h when commercialized, the new batteries could cost less than half that of current batteries, cutting the price of an EV battery pack—now as much as $10,000—by half or more.
To bring the new technology to market, however, the new battery’s operational lifetime will have to be doubled. Tests have confirmed that the Envia cells can be recharged some 400 times and still retain 80% of their original storage capability, but to last for the entire lifetime of a vehicle they must withstand at least a thousand recharging cycles.
Targeting better EV batteries
“We founded the company in 2007 using venture capital funding from Silicon Valley sources with the mission to develop practical high-energy-density batteries for electric cars,” said Sujeet Kumar, Co-founder, President, and CTO. “Right now, EV battery costs are too high and EV range too low for them to compete in the market.”
The company has received a $4 million grant from the U.S. Department of Energy's Advanced Research Projects Agency-Energy (ARPA-e), as well as a $7 million investment from General Motors.
Kumar pointed out that the untested firm gained credibility with potential investors and customers by building 5-gram "coin cells" of 5 g (0.2 oz) mass that made it possible for others to test Envia’s technology for themselves.
“At the beginning,” Kumar recalled, “we focused on improving the cathode because it is the most expensive component in a cell—some 40% of the total cost.” The cathode is the electrode to which the lithium ions travel.
Envia researchers modified a novel cathode material that the company licensed from the DOE’s Argonne National Laboratory, where researchers several years ago had identified a material with a unique microscopic structure that could handle high charge fluxes. Argonne’s patented "layered-layered" microstructures integrate an electrochemically inactive lithium-based compound with an active lithium-based component to provide improved structural and electrochemical stability at high potentials.
Starting with the Argonne material, company researchers systematically tried out some 300 chemical compositions during a total of 25 million test channel hours to find a formulation with a higher voltage capability that resisted the tendency of one of the components, manganese, from escaping the cathode and dissolving into the electrolyte—a deleterious process that cuts storage capacity.
To achieve these goals, the specialists fine-tuned the original chemical composition of the layered-layered crystal (which contains nickel, cobalt, manganese, and lithium manganese oxide) by adding several trace elements and by altering the morphology—size, shape, distribution, density, and porosity—of its component particles. A surface modification—a proprietary nanocoating process—helps to encase the inactive component within the layered active structure to stabilize the electrode and reduce oxygen activity at the surface of charged particles.
Better anodes, too
“Next we addressed the anode," Kumar continued. “Most battery makers use graphite, but we wanted to use silicon, which is widely touted as the next-generation electrode material” because of its high charge capacity per unit mass. But silicon electrodes have poor cyclability because of the large volume expansion and structural failure that occurs when lithium ions react with silicon. “Silicon electrodes typically last only 10 cycles,” he noted.
The research team tackled this challenge by employing a porous form of silicon that is better able to handle repeated expansion and contraction. They then combined the porous silicon with three types of carbon stabilizers: carbon fiber, graphite, and a graphene-like substance. The carbon supplies pathways for electrons to move through the silicon-carbon nanocomposite alloy even when the material is damaged from multiple charging cycles.
“Finally, we developed an electrolyte that is stable at high voltages,” said Kumar. “We took an existing electrolyte formulation that was compatible with our electrodes and blended with it certain additives to make it more resistant to high voltages.”
A major consideration in choosing the specific materials was that they had to be compatible with conventional production equipment so the batteries could be commercialized relatively easily, according to Kumar. For the same reason, the team chose a traditional large-format pouch cell type, which would be familiar to potential users. This work took place at Envia’s cell prototyping and manufacturing plant in Jiaxing, China.
Researchers also had to deal with another key user concern: the possibility of thermal runaway whereby a Li-ion battery warms excessively from the exothermic heat associated with structural changes of the electrodes and the electrolyte, causing it to catch fire. Kumar said that Envia’s pouch cells are substantially thinner than many similar cells, which promotes heat dissipation and lower temperatures. He added that the devices have also passed nail-puncture tests, a standard test of battery safety.
“We’re happy with the overall safety of our materials and of our design. All that remains is to await the results of third-party qualification tests to confirm our beliefs,” he said.
The solution to extending the cycle life of the cells will probably require substantial improvements to the electrodes, but Kumar is unfazed. “We feel that we have a lot more room to squeeze out improvements,” he said.
The firm’s management does not plan to produce batteries itself, said Chairman and CEO Atul Kapadia, but instead license the technology to battery manufacturers or enter into joint ventures. “We don’t want to go the vertical route that some other battery start-ups have taken,” he said. “It’s a difficult path.”
“Envia’s new battery technology represents exactly the kind of innovation and breakthroughs that ARPA-e is looking for from the American research and development community,” said ARPA-E Director Arun Majumdar at a recent energy innovation conference. “We hope that this low-cost and high-density battery technology enables widespread adoption of electric vehicles across the country and around the world.”