Development of Lithium-ion Capacitors for Hybrid Vehicles (I)

FDK has developed a lithium-ion capacitor with high output and excellent charge-discharge cycle characteristics. It is now being used in high voltage sag compensation devices and load leveling for solar power generation. In addition, its application in the automotive field that requires high output power, such as hybrid vehicles, has also progressed. In this article, FDK will introduce the characteristics of lithium-ion capacitors and the measures taken for hybrid vehicles.

In recent years, various measures have been taken to deal with the depletion of fossil fuels and prevent global warming. In response to the fossil fuel problem, natural energy such as solar power generation and wind power generation have been actively introduced. In terms of preventing global warming, emission reduction measures such as electrification and motor-assisted driving have begun to be implemented for cars with high CO2 emissions. _However

, these measures have led to new issues such as unstable power systems and increased power consumption. To solve these issues, power storage components are indispensable.

Previously, the development of power storage components has been centered on lithium-ion rechargeable batteries (LIBs), but due to different uses, there are limits to the output characteristics and charge-discharge cycle life (hereinafter referred to as life) of LIBs. We have developed the high-output, long-life lithium-ion capacitor (LIC) "EneCapTen" for applications that LIB cannot support. This article will introduce the application of LIC in the hybrid vehicle market, a market that is expected to grow in the future.

High-voltage, high-capacity LIC

LIC is a capacitor that uses activated carbon for the positive electrode, carbon material for the negative electrode, and lithium-ion organic matter (salt: LiPF ₆ , solvent: PCEC) for the electrolyte. The positive electrode stores electricity through the effect of the double electric layer. The negative electrode stores electricity through the redox reaction of lithium ions, just like LIB.

By adding lithium ions, the voltage of LIC is not only increased to about 4V, but also the electrostatic capacity stored in the negative electrode is increased, and the electrostatic capacity of the entire unit can be increased to about twice that of the original double-layer capacitor (EDLC). Therefore, LIC has the advantages of high voltage and large capacity compared to EDLC (Table 1).

For example, the energy density per unit volume is 10-50Wh/L, which is much larger than the 2-8Wh/L capacity of EDLC.

Although the energy density is lower than that of LIB, LIC has high output density and long life. In addition, it has excellent high temperature characteristics and less self-discharge than EDLC.

Different positive electrode, higher safety

At present, the main requirements for power storage applications are three points: ① safety, ② long life, and ③ low price. Among them, ① safety is the most important factor. Power storage components are used to store energy. If they cannot store energy stably, the components will become very dangerous as the energy density increases.

At present, various measures are taken to improve the safety of LIB, such as coating the separator with insulating materials, but in essence, the safety of the power storage principle itself is the most ideal.

The difference between LIB and LIC lies in the positive electrode. The positive electrode of LIB uses lithium oxide, while LIC uses activated carbon. Lithium oxide not only contains a large amount of lithium, but also contains oxygen, an important factor that can cause fire.

Therefore, if a short circuit occurs inside the unit for some reason, the heat caused by the short circuit will decompose the lithium oxide, and it can further develop into thermal decomposition of the entire unit, causing severe heat. On the other hand,

the positive electrode of LIC uses activated carbon. Although it will react with the negative electrode when an internal short circuit occurs, the positive electrode and the electrolyte will not react afterwards, so it can be said to be safe in principle (Figure 1).

Figure 1: LIC where the positive electrode does not react with the electrolyte Even if an internal short circuit occurs in LIC, the positive electrode and the electrolyte will not react. However, the positive electrode of LIB will react with the electrolyte, causing thermal decomposition of the constituent materials, resulting in severe heating.

Excellent high-temperature durability

Regarding ②long life, since storage components are relatively expensive, the longer they are used, the lower the product life cycle cost. In addition, if the life is long, the replacement frequency can be reduced, waste can be reduced, and the environmental load is small.

LIB narrows the charge and discharge range (charge and discharge depth) to reduce degradation and achieve long life, but this actually reduces the available capacity. The original hope is to expand the charge and discharge depth to achieve long life.

The charge and discharge principle of EDLC is to have a long life simply by adsorbing or desorbing ions in the electrolyte, but it is difficult to extend the life under actual use conditions.

The weakness of storage components is that the temperature will rise. When repeatedly charged and discharged, the internal resistance will cause the temperature to rise, which will greatly affect its life. Therefore, high-temperature durability is a necessary condition.

Deterioration caused by high temperature is mainly caused by oxidative decomposition of the positive electrode electrolyte. The higher the potential of the positive electrode or the higher the ambient temperature, the more likely it is to cause oxidative decomposition. Therefore, when used in a place with a high ambient temperature, the potential of the positive electrode needs to be lowered. However, if the positive electrode potential of EDLC is lowered, the voltage of the unit will

also drop, so the capacity cannot be guaranteed. Even if the positive electrode potential of LIC is lowered, the voltage of the unit itself will not drop significantly, so the capacity can be guaranteed. In addition, because it can be used in a position where the positive electrode potential is far away from the oxidation decomposition area, the high-temperature durability is very good (Figure 2). (To be continued Special Contributor: Yasushi Suzuki, Director of FDK Capacitor Business Promotion Office)

Figure 2: Cathode potential of LIC that is not prone to oxidative decomposition LIC can reduce the cathode potential and thus prevent oxidative decomposition of the electrolyte.