LITHIUM ION CAPACITORS (LIC)

Hybrid ultracapacitors are capacitors with one electrode of battery, merging the advantages and characteristics of ultracapacitors and battery. Most common type of hybrid is Lithium ion capacitor (LIC), combining the characteristics of Lithium Ion battery with EDLC.  Anode is doped with lithium ions, while cathode uses a carbon variety- activated carbon, CNT or Graphene.

construction of Lithium Ion capacitor

Lithium-ion capacitors (LICs) are constructed using a hybrid design that combines features of lithium-ion batteries and supercapacitors. The structure enables LICs to achieve high energy density and longevity compared to traditional batteries and supercapacitors.

The figure above brings out difference between EDLC and Lithium ion capacitor (LIC). Positive electrode of LIC is EDLC electrode- activated carbon, CNT or graphene. Negative electrode is pre-doped with lithium ions during manufacture, as in a battery. Electrolyte contains lithium salt, and is a battery electrolyte. The negative electrode undergoes doping during charge, when ions from electrolyte enter and occupy spaces in the electrode structure (made of activated carbon). During discharge, the ions retract and are drawn back into electrolyte. LIC thus employs EDLC carbon electrode at cathode, and battery electrode at anode. Since ions enter the structure of anode (intercalation), large number of ions can enter and effectively make contact with huge surface area. Coupled with higher dielectric constant of surface barrier with battery electrode, a very high capacitance results at anode.  Separator is chemically inert material, which prevents direct contact between the anode and cathode while allowing ion passage.

The anode is thus an electrode with quick discharge and long cycle life, with energy density like battery. This design allows LICs to achieve high energy density of 45 Wh/Kg onward, even above 100 Wh/Kg, while also offering lower self-discharge rates and longer cycle life compared to traditional batteries. Use of carbon nanotubes (CNT) or Graphene enables much larger surface area and capacitance at electrodes, and is therefore becoming the trend.

Features of Li-ion Capacitors

• High working voltage
• High energy density
• High power density
• Wide temperature range
• Life much longer than battery, going into tens of of thousand charge discharge cycles.

EDLC capacitor electrodes are identical in nature and have voltage rating not over 2.8 V typically. Voltage of both electrodes of EDLC goes up and down in similar fashion during charge discharge, but in opposite direction, resulting in rise and fall of anode voltage from zero to maximum cell voltage as shown in figure below.

However, in LIC, anode has most part of capacitance, which means most change in voltage during charge-discharge occurs at cathode, while anode stays more or less at steady voltage. This allows a large variation in voltage at cathode, and overall cell voltage of up to 3.6-3,8 V can be achieved.

Lithium-ion capacitors (LICs) significantly outperform traditional lithium-ion batteries in terms of lifespan. LICs can endure over 50,000 charge/discharge cycles, while lithium-ion batteries typically last around 2,000 to 5,000 cycles before significant degradation occurs. This extended lifespan is due to the electrostatic energy storage mechanism in LICs, which minimizes mechanical and chemical wear compared to the chemical reactions in batteries that contribute to their aging. Additionally, LICs maintain performance across a wider temperature range, enhancing their durability in various conditions.

Lithium ion capacitor can be discharged from its full charged voltage of 4.0 V to half the level of 2.0 V, while EDLC can be discharged fully. However, the available or usable discharge energy is much higher.

Energy density of LIC is lower than that of lithium ion battery, while its power density is much higher. The Ragone’ chart below brings shows hybrid capacitor energy and power densities in comparison with supercapacitors (EDLC) and batteries at this stage of technology. Efforts are being made to improve the energy densities as also power delivery levels of hybrid capacitors to still higher levels.

Self-discharge rate of LIC is of the order of 5% over 3 months, and drop in capacitance is lower than 10% after 100,000 charge-discharge cycles.

Temperature limits for lithium-ion capacitors

Lithium-ion capacitors (LICs) have a specific operating temperature range of -20°C to 70°C. They can maintain approximately 50% capacity at -10°C under high discharge rates, which is superior to traditional lithium-ion batteries that drop to around 50% capacity at 5°C. This makes LICs suitable for applications in environments with significant temperature fluctuations, while they also exhibit stable performance without the risk of thermal runaway associated with batteries.

EDLCs can function effectively in a broader range, from -40°C to +85°C, with no risk of thermal runaway due to their electrostatic charge storage mechanism. This makes supercapacitors more suitable for extreme temperature applications, outperforming LICs in both low and high-temperature environments.

Applications of Lithium Ion Capacitors

High energy density and power density make LICs suitable for large number of applications where size and power density is important. There is often no need of battery or other energy storage devices, which saves on costs. Some of the applications are

• Wind turbines for pitch control
• Solar power generation
• Voltage sag compensation in grid system
• Electric vehicles
• Stationary energy storage systems

Reference Books

• Passive Components: R. P. Deshpande
• Ultracapacitors- Future of Energy Storage: R. P. Deshpande