Mitsubishi Electric Corp prototyped a storage battery device by combining a lithium-ion (Li-ion) capacitor and a Li-ion secondary battery in a cell.
The device features the high-speed charge-discharge characteristics of a capacitor and the long charge-discharge time of a Li-ion battery. It is expected to be used for regenerating power from a large-size motor and leveling the output of a photovoltaic system.
Though Mitsubishi Electric plans to commercialize the device, it does not have a specific schedule for it.
This time, the company prototyped two types of the device. One is for the verification of principles and has a cell size of 3 x 3cm while the other is for the verification of practicality and has a cell size of about 9 x 6cm.
As for the cycle life of the 3 x 3cm type, its decay rate reaches 20%, which is the indicator of cycle life, at about 2,000 cycles.
The 9 x 6cm type is called "flat winding type" and has an output of 14Wh. Its output density and energy density are approximately 3kW/kg and 60Wh/kg, respectively. The average voltage, lower limit voltage and upper limit voltage are 3.2V, 2V and 4V.
Both of the two types have a negative electrode shared by the capacitor and the battery. A positive electrode and a separator are prepared for each of the capacitor and the battery.
The shared negative electrode of the 3 x 3cm type is sandwiched between the "first positive electrode" of the capacitor part and the "second positive electrode" of the battery part. As for the 9 x 6cm type, the first and second positive electrodes formed on the front and back of a collector foil, the separator and the shared negative electrode are flatly wrapped.
The key to realize the new device is the structure of the shared negative electrode, Mitsubishi Electric said.
The shared electrodes of the prototypes were made with carbon material. The first positive electrode of the capacitor part and the second positive electrode of the battery part were made with activated carbon and material including iron phosphate (LiFePO4), respectively.
