| specific energy | 11–14 W·h/kg |
|---|---|
| energy density | 19–25 W·h/L |
| specific power | 160–2800 W/kg |
| Charge/discharge efficiency | 95% |
| Self-discharge rate | < 5%/month (temperature dependent) |
| Cycle durability | >10,000 |
| Nominal cell voltage | 2.2–3.8 V |
A lithium-ion capacitor (LIC) is a hybrid type of capacitor. Activated carbon is used as cathode. The anode of the LIC consists of carbon material which is pre-doped with lithium ion. This pre-doping process lowers the potential of the anode and allows a high output voltage.
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The positive electrode (cathode) employs activated carbon material at which charges are stored in an electric double layer which is developed at the interface between the carbon and the electrolyte similar to electric double-layer capacitors (EDLC).
The negative electrode (anode) is made of a properly designed carbon material, which is pre-doped with lithium ions. This pre-doping process lowers the anode potential and results in a high cell output voltage. Typically, output voltages for LICs are in the range of 3.8–4.0V. As a consequence, LICs have a high energy density. Furthermore, the capacity of the anode is several orders of magnitude larger than the capacity of the cathode. As a result, the change of the anode potential during charge and discharge is much smaller than the change in the cathode potential.
The electrolyte used in an LIC is a lithium-ion salt solution.
In order to avoid direct electrical contact between anode and cathode, a separator material is used.
Typical properties of an LIC are:
Batteries, EDLC and LICs all have their own properties, which make them suitable for specific applications. The lithium-ion capacitors have a higher power density as compared to batteries, and LIC’s are safer in use than LIBs, in which thermal runaway reactions may occur. Compared to the electric double-layer capacitor (EDLC), the LIC has a higher output voltage. They have similar power densities, but energy density of an LIC is much higher.
The Ragone plot (figure 1), shows that the lithium-ion capacitor combines the high energy of LIBs with the high power density of EDLC’s.
Cycle life performance of LICs is much better than batteries and is similar to EDLCs.
Lithium-ion capacitors are quite suitable for applications which require a high energy density, high power densities and excellent durability. Since they combine high energy density with high power density, there is no need for additional electrical storage devices in various kinds of applications, resulting in reduced cost of ownership.
Potential applications for lithium-ion capacitors are, for example, in the fields of wind power generation systems, uninterruptible power source systems (UPS), voltage sag compensation, photovoltaic power generation, energy recovery systems in industrial machinery, and transportation systems.