March 16, 2009 Researchers have developed a new advanced Lithium Ion battery that will allow mobile phone and laptop computers to be fully charged in seconds. Electric car batteries may be charged in as little as five minutes, removing one of the main barriers to wider uptake of EVs. Solar and wind power generation could also benefit as better batteries could be used to store surplus energy.
MIT researchers Byoungwoo Kang & Gerbrand Ceder have discovered a way to make a lithium iron phosphate (LiFePO4) battery charge and discharge about as fast as a supercapacitor. In a typical lithium ion cell when a current is applied to charge the cell, lithium ions move away from the cathode compound and are trapped at the anode storage medium. When the battery discharges producing current, those ions travel back to the cathode medium and in so doing produce current flow.
Speed of charging in typical lithium-ion cells is slowed by virtue of the fact that it takes time for the lithium ion to move off the cathode material. Various techniques have been tried to increase that speed including the nanoparticle doping strategy that A123 Systems uses.
The scientists noted that lithium iron phosphate forms a lattice that creates small tunnels through which the lithium ions flow, but that although the cathode seemed ideal it still took some time for those ions to travel. The novel solution they devised was to create a lithium phosphate glassy surface to coat these tunnels. This glassy surface acts as a speedway that rapidly transports the lithium ions on and off the cathode.
Extremely high rates can be achieved, at a 200C rate (corresponding to an 18 second total discharge) more than 100mAh g can be achieved, and a capacity of 60mAh g is obtained at a 400C rate (9 sec to full discharge). Such discharge rates are two orders of magnitude larger than those used in today’s lithium ion batteries. Typical power rates for lithium ion battery materials are in the range of 0.5 to 2 kW/kg. The specific power observed for the modified LiFePO4 (170kWkg at a 400C rate and 90kWkg at a 200C rate) is two orders of magnitude higher. At this point the researchers have only tested the cells to 50 cycles but have noted no degradation. They have made a small prototype cell which can be fully charged in 10 to 20 seconds, compared with six minutes for cells made in the standard way.
This new ability to charge and discharge lithium-ion batteries within seconds blurs the distinction between batteries and ultracapacitors. Besides being able to charge one’s cellphone in seconds, this will have a major impact on electric cars. If electric grid power was available, an electric car with a 15kWh battery could be charged in five minutes. This would require the delivery of 180 kw of energy in that time frame.
Two companies have already licensed the technology one of which includes A123 Systems. Because it involves a new approach to manufacturing lithium-ion battery materials, rather than a new material, it could be ready within two to three years.
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