The company then turned its attention to the opposite electrode, which is usually made of graphite. Researchers decided to use silicon, which can store far more energy but typically works for only a short number of charges, since it swells and cracks. Envia addressed these issues by using a porous form of silicon, which is better able to tolerate expansion and contraction, and by mixing the silicon with various forms of carbon, including carbon fiber and graphite. The carbon is meant to provide a path for electrons to take through the material, bridging gaps that form as the silicon cracks. The researchers also had to modify the electrolyte to keep it from breaking down at the high voltage levels seen in the battery cell.
To develop the materials, Envia took the unusual approach of testing new electrode materials in complete batteries, with both electrodes and an electrolyte. Usually researchers test electrode materials in isolation to identify those with promising properties, such as high energy capacity. But sometimes materials that look great on their own are incompatible with electrolytes or other electrodes. On the other hand, some materials that don’t look great on their own may do well when paired with the right electrolyte. So Envia tested batches of 1,500 battery cells—each with a different combination of electrodes and electrolyte—to find the best combinations. (Envia prepares the electrode and electrolyte materials by hand. Wildcat Discovery Technologies, one of Technology Review’s TR50 most innovative companies, uses a robotic system to speed up a similar process.)
After testing small coin-sized cells, Envia built cells large enough for use in electric cars. Each weighs one kilogram and stores 400 watt-hours. Commercial lithium-ion batteries store about 120 to 250 watt-hours per kilogram.
Lower-energy batteries often have safety features that make them attractive for use in cars. Sujeet Kumar, Envia’s president and CTO, says the company’s batteries have passed nail puncture tests, one key test of battery safety.
Because the materials can be made with conventional equipment, they could be relatively easy to commercialize. Kumar says Envia doesn’t plan to manufacture batteries itself, but to license its technology to battery manufacturers or create joint ventures.
But the cells aren’t yet ready for use in electric cars. To last the life of a vehicle, they need to be able to recharge over 1,000 times and still maintain 80 percent of their original storage capacity. The company is still testing the new batteries, but after only 400 charges, they have dropped to 72 percent of capacity, Kumar says. Solving the problem could require substantial improvements to the electrodes. The cells also have to be put through several other tests of performance and safety before they’re qualified for use in vehicles.