|Comparison of cycle life, 8C. Click to enlarge.|
Mitsubishi Electric Corp has demonstrated prototypes of a hybrid energy storage cell that combines the fast charge-discharge power capabilities and long life of an EDLC (electric double layer capacitor) and the greater energy storage capacity of a Li-ion battery in a single cell. The new device is expected to be used for storing regenerated power from a motor (i.e., through regenerative braking) or leveling the output of a photovoltaic system.
The company showed two prototypes of the device: a 10 Wh, 3 x 3 cm unit developed as a proof-of-principles device; and a 14 Wh, 6 x 9 cm flat-wound unit with power and energy densities of 3 kW/kg and 60 Wh/kg, respectively. Under an 8C rate, the 3 x 3 proof-of-principles unit achieved 2,000 cycles before degrading 20%—some four times the cycle life of the Li-ion batteries used as a baseline.
|The 14 Wh prototype. Click to enlarge.|
Average voltage is 3.2V, lower limit voltage is 2V and the upper limit is 4V.
The devices feature an anode shared by the capacitor and the battery, each of which have a cathode and a separator. The shared anode in the 3 x 3cm type is sandwiched between the two cathodes. In the larger, flat wound device, the first and second cathodes are formed on the front and back of a collector foil.
The key to realize the new device is the structure of the shared anode, Mitsubishi Electric said. The anode is made with carbon material. The capacitor cathode was made with activated carbon, and the battery cathode was made with a lithium-iron phosphate material.