FDK has developed lithium-ion capacitors with high output power and excellent charge-discharge cycle characteristics. It has been used in the fields of high voltage sag compensation devices and load averaging of solar power generation. In addition, its application in the field of automobiles such as hybrid vehicles that require high output power has also progressed. This article will introduce the characteristics of lithium-ion capacitors and the measures taken for hybrid vehicles by FDK.
In recent years, various measures have been taken to cope with the depletion of fossil fuels and prevent global warming. In response to fossil fuel problems, natural energy such as solar power and wind power has been actively introduced. In terms of preventing global warming, we began to implement measures to reduce emissions such as electrification and motor-assisted driving for cars with high CO2 emissions.
However, these countermeasures have caused new issues such as power system instability and increased power consumption to surface. To solve these problems, power storage components are essential.
Prior to this, power storage components have been developed with lithium ion rechargeable batteries (LIB) as the center. However, due to different applications, LIB's output characteristics and charge and discharge cycle life (hereinafter referred to as life) have limits. We have developed "EneCapTen", a high-output, long-life lithium-ion capacitor (LIC) for applications that are difficult to support with LIB. This article will introduce LIC's application plan for the future market that is expected to grow-the hybrid vehicle market.
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 compounds (salt: LiPF6, solvent: PCEC) for the electrolyte. The positive electrode stores electricity through the effect of the electric double layer. Like the LIB, the negative electrode stores electricity by the oxidation-reduction reaction of lithium ions.
By adding lithium ions, LIC not only increases the voltage to about 4V, but also increases the electrostatic capacity of the negative electrode storage. The overall electrostatic capacity of the unit can be increased to about twice that of the original electric double layer capacitor (EDLC). Therefore, LIC has the advantage of high voltage and large capacity compared to EDLC (Table 1).
For example, the energy density per unit volume is 10 to 50 Wh / L, which is much larger than the 2 to 8 Wh / L capacity of EDLC.
Although the energy density is lower than LIB, the output density of LIC is high and the life is long. In addition, it has two characteristics of excellent high temperature characteristics and smaller self-discharge than EDLC.
Different positive electrode, higher safety
At present, there are three main requirements for power storage applications: â‘ safety, â‘¡ long life, â‘¢ low price. The safety of â‘ is the most important element. Power storage components are used to store energy. If the storage cannot be stabilized, the components will become very dangerous as the energy density increases.
At present, in order to improve safety, various measures such as coating insulators for the diaphragm are adopted for LIB, but in essence, the safety of the electricity storage principle itself is the most ideal.
The difference between LIB and LIC is the positive electrode. The positive electrode of LIB uses lithium oxide, while LIC uses activated carbon. Lithium oxide contains not only a large amount of lithium but also an important factor that can catch fire-oxygen.
Therefore, if a short circuit occurs inside the unit for some reason, the heat generated by the short circuit will decompose the lithium oxide, and may further develop into thermal decomposition of the entire unit, resulting in severe heat generation.
The positive electrode of LIC uses activated carbon, although it will react with the negative electrode when an internal short circuit occurs, but then the positive electrode will not react with the electrolyte, which 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 the LIC, the positive electrode and the electrolyte will not react. The positive electrode of LIB will react with the electrolyte, resulting in thermal decomposition of the constituent materials, resulting in severe heat generation.
Excellent high temperature durability
Regarding â‘¡Long life, due to the relatively high price of power storage components, the longer the use time, the lower the life cycle cost of the product. Moreover, if the service life is long, the frequency of replacement and waste can be reduced, and the load on the environment is small.
LIB narrows the range of charge and discharge (depth of charge and discharge) in order to reduce deterioration and achieve a long life, but the capacity that can be actually used is reduced. The original hope was to increase the depth of charge and discharge to achieve a long life.
The principle of charge and discharge of EDLC is to simply adsorb or desorb the ions in the electrolyte and have a long life, but it is difficult to extend the life under actual use conditions alone.
The weakness of electricity storage components is that the temperature will rise. When repeatedly charging and discharging, the internal resistance will cause the temperature to rise, which will greatly affect its life. Therefore, high temperature durability is a necessary condition.
The deterioration caused by high temperature is mainly caused by the oxidative decomposition of the positive electrode electrolyte. The higher the potential of the positive electrode, or the higher the ambient temperature, the easier the oxidative decomposition occurs. Therefore, when used in a place with a high ambient temperature, it is necessary to reduce the potential of the positive electrode. However, if the EDLC lowers the positive electrode potential, the voltage of the cell will also decrease, so the capacity cannot be ensured.
However, even if the LIC lowers the positive electrode potential, the voltage of the cell itself will not drop significantly, so the capacity can be ensured. Moreover, since it can be used at a position where the positive electrode potential is far from the oxidative decomposition region, the high-temperature durability is excellent (Figure 2).
Figure 2: Positive potential of LIC that is not prone to oxidative decomposition
LIC can reduce the positive electrode potential, so it can prevent the oxidative decomposition of electrolyte
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