With consumers used to the convenience of refueling their vehicle at the gas station in a few minutes, one of the biggest disadvantages of electric vehicles is the time it takes to recharge their batteries. Now, by separating the energy storage and energy discharging functions of the battery into separate physical structures, researchers at MIT have achieved a breakthrough that could allow EVs to be recharged in the same time it takes to refuel a conventional car. The technology could also provide an inexpensive alternative for energy storage for intermittent, renewable energy sources such as wind and solar.

The new battery employs an architecture known as a "semi-solid flow cell," which sees the battery's positive and negative electrodes (cathodes and anodes) made up of solid particles suspended in a liquid electrolyte. These oppositely charged particles are pumped through systems separated by a filter, such as a thin porous membrane.

The MIT researchers say that by separating the energy storage and energy discharge functions of the battery into separate physical structures allows them to design more efficient batteries. This should allow for a complete battery system - including all its structural support and connectors - that is half the size and cost of existing rechargeable batteries.

"Recharge" in a jiffy

While such size and cost reductions alone would make them attractive for use in EVs, it is the possibility of "refueling" the battery by pumping out the discharged liquid and pumping in a fully charged replacement that could be the real game changer and make EVs more competitive with conventional gas- or diesel-powered vehicles. The technology also allows for the liquid to be recharged in the usual way when time isn't a factor.

Although flow batteries have been around for a while, they have used liquids with very low energy density. This has resulted in batteries that are much bigger than fuel cells and require rapid pumping of the fluid, which also reduced their efficiency.

The liquids in the new semi-solid flow batteries developed under the leadership of MIT materials science professors W. Craig Carter and Yet-Ming Chiang, provide a 10-fold improvement in energy density over present liquid flow batteries. This means the liquid suspensions - which have been dubbed 'Cambridge crude' as they look and flow like thick black goo and could take the role of petroleum in transportation - don't have to be pumped rapidly to deliver their power.

Grid storage

The researchers say the new battery could also be scaled up to very large sizes at low cost, making it suitable for large-scale energy storage for utilities faced with the often difficult task of matching electricity production to demand. The technology could also help overcome the problem of storing energy from intermittent renewable energy sources such as wind and solar.

At the moment the battery is built around the proven chemistry of lithium-ion batteries and better anode and cathode materials and electrolytes are needed before a practical, commercial version of the battery is viable. But Chiang says because the design architecture of the new battery "is not linked to any particular chemistry," it can be used with different chemical combinations. He and his colleagues are now exploring different possibilities and as better materials are discovered, they can be adapted to the battery's architecture.

Under a three-year ARPA-E grant awarded in September 2010, the team's target is to have a "fully-functioning, reduced-scale prototype system" that is ready to be engineered for production as a replacement for existing EV batteries by the end of 2013.

Chang and Carter have also founded a company called 24M, which has licensed the technology and has raised over US$16 million in venture capital and federal research funding.

Source: MIT News