Energy density: Improving the lithium-ion battery

The cost and weight of batteries is the Achilles heel for electric vehicles. Today’s lithium-ion batteries used in cars such as the GM Volt are serviceable but expensive, up to 60% of the cost of the car. This has provided a major incentive for science and industry to chase large-scale battery improvement for decades. The lithium-ion battery itself was one result. Now a company using a technology licensed from the Argonne National Laboratory (USA) is bringing into production a version of lithium-ion battery that promises cost reductions up to 45%. Envia Systems (California, USA), funded in part of the General Motors Corporation and a grant from the U.S. Department of Energy, has announced the beginning of market development for the new battery technology. ‘Market development’ means that commercial production will begin as soon as the company works out manufacturing specifications with potential buyers. This apparently is in the offing, as the CEO of Envia, Atul Kapadia, indicates the time-frame will be about eighteen months.

The key to the Envia technology is a solution to the efficiency (or inefficiency) of the battery’s cathode and anode components. In simple terms, batteries work by the generation of electrical ions, in this case lithium ions, which move from the anode (negative) side of the battery to the cathode (positive) side, providing a stream of electricity. The cathode and the materials used to make it were one part of the problem. Earlier research found that incorporating manganese into the materials of the cathode increased the efficiency – called energy density – of the battery. The Envia team uses that improvement and turned their attention to the anode and the Argonne Lab research. Here they found that by blending silicon into the typical graphite material of the anode increased the efficiency, but there was a problem – silicon in battery fluid ‘swells.’ The result was a battery that could only be recycled (recharged) at most ten times. The solution was to coat the silicon in carbon and mix that with carbon fibers. This protects the electrical properties of the silicon while increasing the recharging cycles up to 400 times (so far).

The result of these changes, according to the company and testing by the U.S. Naval Surface Warfare Center (Crane, Indiana), is an energy density of 400 watt-hours per kilogram at a cost of about $150 per kilowatt-hour. Most current lithium-ion battery packs used in electric cars run about 120 watt-hours per kilogram. The higher energy density allows for smaller and less heavy battery packs, which also makes cooling more efficient. (Lithium-ion batteries are notorious for ‘thermal runaway’ – bursting into flame.)

This all sounds very promising. It certainly adds some spice to the competition. The fact that a major car manufacturer and the U.S. government have a hand in seeing this innovation to market lends credibility. As ever, however, the technology is still several years of testing and manufacturing process development away from appearing in new model cars. It’s appropriate to be enthusiastic about this technological fix, but not ecstatic.

Research Spectrum

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