Large-Capacity Capacitors Close to Volume Production

Higher-energy density capacitors seem likely to find application in the automobile- and energy-related markets, providing they can prove themselves first in consumer products.

Capacitors are considered to have a low energy density, but if the energy density can be boosted to about 20Wh/kg, they could well be able to compete with Li-ion rechargeable batteries and similar power sources in large-capacity storage applications where high output power is required, such as vehicles (Fig 1).

The market for large-capacity storage devices is growing rapidly. According to Fuji-keizai Co, Ltd of Japan, the market for large-capacity storage devices is expected to increase from about Yen310 billion in 2004 to about Yen670 billion in 2010, representing a growth of double or more. Within that market (Fig 2), added the firm, "The market for hybrid vehicles like Toyota's Prius will grow significantly." Toyota Motor Corp of Japan, which is aggressively promoting its hybrid vehicles, expects to sell more than 300,000 of them in fiscal 2005. The firm has already stated that it will "establish a production stance capable of handling a million units a year, within a few years," and if other manufacturers follow suit there is little doubt that the storage device market for hybrid vehicles will grow.

NiMH rechargeable batteries are currently the most common power source for hybrid vehicle drive motors, but Li-ion rechargeable batteries are expected to start being used in that role in 2007 or 2008. A source at one capacitor manufacturer, meanwhile, estimates that capacitors for vehicles will enter practical use between 2008 and 2010. While commercial application is lagging, capacitor manufacturers and even some automobile manufacturers are pushing ahead with development, confident that capacitors will eventually prove themselves.

There are two reasons for their optimism. The first is that if capacitor energy can be raised to about 20Wh/kg, the result will be smaller than the size of current NiMH rechargeable battery modules. The second reason is that if the ability of capacitors to handle high current inputs and outputs can be effectively utilized, it would mean an increase in the regenerative energy recovery ratio.

As far as the size of the capacitor module is concerned, a technology announcement in August 2005 from Fuji Heavy Industries, Ltd of Japan said that the hybrid vehicles with motors of about a dozen kW are capable of holding a capacitor module of about 40 liters in size, with stacks of cells with energy densities of 13Wh/kg. According to Hideki Shibuya, manager, Advanced Propulsion R&D Group, Subaru Technical Research Center at Fuji Heavy industries, this is "...about the same volume as the NiMH rechargeable battery units in common use now." The hybrid vehicles from Honda Motor Co, Ltd of Japan, also with motors of about a dozen kW in capacity, use NiMH rechargeable battery modules 39 liters in volume. If capacity energy density can be boosted to 20Wh/kg, it would be possible to reduce the number of cells required, and the result could well be smaller than the NiMH rechargeable battery design.

The capacitor charge/discharge characteristics make the smaller size possible. Capacitors can be charged and discharged to very close to 100% of discharge depth. NiMH rechargeable batteries and the like generally limit the discharge depth to only 20 or 30%, to prevent deterioration. This is why they mount batteries with much larger capacity than actually needed. While the energy density of the capacitor is lower, only the actual needed capacity has to be mounted, shrinking overall volume.

If hybrid vehicles spread, another key point for performance evaluation will be the recovery ratio for regenerative energy from braking. Current hybrid vehicles are unable to completely recover large currents from the generator into NiMH rechargeable batteries when large braking forces are applied, discarding the energy as heat. Li-ion rechargeable batteries share the same problem. Capacitors, on the other hand, can efficiently recover this energy, and as one engineer at an automobile manufacturer said, "I wouldn't be surprised to see a hybrid vehicle with capacitors developed any day now."

Appearing in Vehicles

The automobile manufacturers themselves are actively developing capacitors. Hybrid trucks with capacitor storage systems were released to the market in 2002 by Nissan Diesel Motor Co, Ltd of Japan, and Honda began lease sale in 2003 of a fuel cell vehicle with capacitor storage, while many firms in the automotive field including Toyota and Isuzu Central Laboratories Co, Ltd, an affiliate of Isuzu Motors Ltd of Japan, have filed countless related patents. Many of these patents are not related to power sources for drive motors in hybrid vehicles or other applications, but a range of applications like reinforcing peak output during operation of power steering, power brakes or similar systems, or serving as a heat source for faster catalyst activation.

It will take at least another three years before sufficient reliability can be assured to permit entry into the automotive market, and it is common for manufacturers to demand prior usage in consumer or industrial equipment first. This means that in order to enter the automobile market in 2010, volume production will have to start by about 2006 for some sort of consumer product. Now is the time to take action.

In fact, capacitor manufacturers have been joined by firms from other industries, such as Power Systems Co, Ltd of Japan, Nishinbo Industries, Inc of Japan and Meidensha Corp of Japan, in the volume production of capacitors. Power Systems began volume production of a capacitor with an energy density of 6.5Wh/kg in June 2005, providing a production scale suitable for energy-saving applications in the office automation sector but aiming at the automotive field.

Even Larger Capacities

Capacitor manufacturers are developing new types of capacitors in the hope of selling to markets including automotive and energy. The conventional type is a double-layer electrical capacitor, with cells using organic solvents like polypropylene carbonate (PC) as electrolytes and activated carbon electrodes to boost cell energy density. It has proven difficult to achieve energy densities of 10Wh/kg or more with these materials, however. Capacitor energy density is expressed as 1/2CV2, so any increase in energy density demands an increase in either the electrostatic capacity (C) of the electrode or the cell voltage (V), as shown in Fig 3.

Involved developers are simultaneously working on boosting electrostatic capacity by improving the electrode and boosting cell voltage by improving the electrolyte. Power Systems and Nippon Chemi-Con are primarily engaged in developing new electrode materials to boost electrostatic capacity. FDK Corp of Japan and Fuji Heavy Industries are using electrolytes with the same Li ions as Li-ion rechargeable batteries, which have cell voltages of 4.2V, in an effort to increase energy density.

New Electrodes

Power Systems, which is focusing on developing new electrode materials, is working on increasing the electrostatic capacity of double-layer electrical capacitors by replacing activated carbon with other materials. A new design with new carbon-based cathode and anode materials has achieved an energy density of 15Wh/kg. Called the nano-storage capacitor, it is scheduled to sample-ship in the fall of 2005.

The carbon-based electrode material was jointly developed with emeritus professor Masaki Yoshio of Saga University, Japan. Electrode details have not been disclosed, but the material is said to be able to absorb five to 10 times more charge than conventional electrode materials. Evaluations of prototype cells in applications like large motorcycle cell motor starters and power supplies for motorized 4-wheelers for the elderly are under way now. Energy density has reached 30Wh/kg in the laboratory, but a source at Power Systems said that volume production will start with 15Wh/kg products.

Nippon Chemi-Con is trying to raise energy density using metal complex polymers in electrodes. The firm has developed the ASED capacity with an energy density of 40Wh/l, utilizing a metal complex polymer developed by GEN3 Partners, Inc of the US for the electrode. The electrode gains not only electrostatic capacitance from the electrical double-layer, but also generates pseudo-capacitance using the oxygen reduction reaction. Stored electrode capacitance can be boosted to five times that of conventional designs, said a source at the firm.

A prototype cell with a carbon-based anode and metal complex polymer cathode attained an energy density of 20Wh/kg. If the polymer can be used for both electrodes, said the firm, it should be possible to reach 40Wh/kg. The electrolyte is PC-based, but the voltage has been raised from the common 2.5V to 3.3V. Nippon Chemi-Con plans to sample-ship the new capacitor in spring 2006, first aiming at commercialization for compact equipment such as portable gear and consumer electronics, and moving into large-size sectors like vehicles and energy in the future.

Electrolytes, Li Ions

FDK and Fuji Heavy Industries are not only working on better electrodes, but also on capacitors using Li-ion bearing electrolytes. The "Dual Carbon Cell" capacitor being developed by FDK, for example, uses a high 4.2V-rated voltage to achieve a volumetric energy density of 12Wh/l. This is about double the rated voltage of the firm's prior electrical double-layer capacitor, and about four times the volumetric energy density.

The electrode is not activated carbon, but carbon with a layered crystalline structure. It stores charge using the principle of intercalation, where electrodes are held between the crystal layers instead of being absorbed by holes as with activated carbon.

The outstanding feature of the newly-developed capacitor is that it can be used to replace existing Li-ion rechargeable batteries. The charging voltage is the same 4.2V, which means no changes are needed for the charging system, and the discharge curve is designed to provide the same gradual drop as Li-ion batteries. It still retains the crucial characteristics of the double-electrical layer capacitor, though, such as high-current input/output and a long charge/discharge cycle life.