In an exciting development that could greatly enhance the performance of hybrid electric vehicles, Mitsubishi Electric Corporation has demonstrated prototypes of a hybrid energy storage cell that combines the long life and fast charge capabilities of an electric double layer capacitor (EDLC) with the greater energy storage capacity of a lithium-ion battery.
The device, which would be used to store regenerated power from a motor or to level the output of a photovoltaic system, has an average voltage of 3.2V with a lower limit of 2V and an upper limit of 4V.
Mitsubishi actually displayed two prototypes of the device – the first a 10Wh, 3x3cm unit; and the second a 14Wh, 6x9cm flat-wound unit with power and energy densities of 3kW/kg and 60Wh/kg. With an 8C rate, the 3×3 proof of principles device managed to achieve 2,000 cycles before degrading 20 per cent – that’s four times the life cycle of the lithium-ion batteries that were used as a baseline.
One of the key features of the devices is an anode shared by the capacitor and the battery, each of which have a cathode and separator. In the case of the larger, flat wound device, the first two cathodes are on the front and back of a collector foil. The shared anode is made with carbon material with the capacitor anode activated by carbon and the battery cathode made active with a lithium-ion phosphate material. The structure of the shared anode is said to be the key to the device.
