Development of lithium-ion capacitors: high voltage, large capacity, and high safety

FDK has developed a lithium-ion capacitor with high output power and excellent charge-discharge cycle characteristics. It has begun to be used in fields such as high voltage sag compensation devices and load balancing of solar power generation. In addition, its application in the automotive field requiring 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 countermeasures have led to new issues such as unstable power systems and increased power consumption. To solve these issues, power storage devices are indispensable.

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

High voltage and large 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: LiPF6, solvent: PCEC) for the electrolyte. The positive electrode stores electricity through the double electric layer effect. 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 2 times 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 to 50Wh/L, which is much larger than the 2 to 8Wh/L capacity of EDLC.

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

Different positive pole, higher safety

At present, there are three main requirements for the use of power storage: 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 be stored stably, the components will become very dangerous as the energy density increases.

At present, in order to improve the safety of LIB, various measures are taken, such as coating the diaphragm with insulating materials. However, 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 cell for some reason, the heat caused by the short circuit will cause the lithium oxide to decompose, and may further develop into thermal decomposition of the entire cell, resulting in severe heating.

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 with no reaction between the positive electrode and 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 reduced.

In order to reduce degradation and achieve a long life, LIB has narrowed the charge and discharge range (charge and discharge depth), but this actually reduces the available capacity. The original hope was to expand the charge and discharge depth to achieve a long life.

The charging and discharging 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 by this alone.

The weakness of energy storage components is that they will heat up. 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 undergo oxidative decomposition. Therefore, when using in places with high ambient temperatures, the potential of the positive electrode needs to be lowered. However, if the positive electrode potential of EDLC is lowered, the voltage of the cell will also drop, and the capacity cannot be ensured.

LIC, on the other hand, does not significantly reduce the voltage of the cell itself even if the positive electrode potential is lowered, so the capacity can be maintained. In addition, since it can be used in a position where the positive electrode potential is far from the oxidation decomposition region, it has excellent high-temperature durability (Figure 2).

Figure 2: Cathode potential of LIC that is not prone to oxidative decomposition

LIC can reduce the positive electrode potential, thereby preventing the oxidative decomposition of the electrolyte.