Supercapacitors are devices that store a dense electrical charge in an electrical field that provides electronics or a power grid with a quick jolt of power on demand. They have a capacitance value far higher than typical capacitors but at the cost of lower voltage limits. Unlike typical capacitors, supercapacitors don’t use conventional solid dielectric (insulator) — they utilize electrostatic double-layer capacitance (typically made of carbon) and electromechanical pseudo-capacitance (metal oxide or conducting polymer). Both contribute to the capacitor’s total capacitance and are designed for many rapid charge/discharge cycles over long-term energy storage. Hybrid supercapacitors boost that capacitance, energy density, and operating voltage (3.8 V maximum) up to 10X over symmetric supercapacitors.
It’s important to note at this point that there are three types of capacitors, with the most basic being the electrostatic capacitor outfitted with a dry separator. This type of capacitor features very low capacitance and is mainly used to tune radio frequencies and filtering, and ranges from a few pico-farads (pf) to low microfarad (μF) in size.
The second capacitor offers electrolytic qualities and provides a higher capacitance than the electrostatic capacitor and is rated in microfarads, which is significantly larger than a pico-farad. These capacitors deploy a moist separator and are used for filtering, buffering, and signal coupling. As with batteries, the electrostatic capacity has a positive and negative that must be observed.
The third type, a supercapacitor, is rated in farads, which is thousands of times higher than the electrolytic capacitor. This type is widely utilized for energy storage undergoing frequent charge/ discharge cycles at high current with short durations.
Electrical Double-Layer Capacitors (EDLCs) are the most common capacitors in use today. While they offer high capacitance, stellar power density, and impressive efficiency, they have some drawbacks; most notably, “self-discharge,” meaning the stored energy will bleed if it hasn’t discharged in a short period, which becomes exacerbated at higher temperatures. That problem is mitigated by coupling the supercapacitor with a lithium-ion battery, providing increased density with minimal self-discharge. While including a battery with the capacitor is a great stopgap solution, it too comes with its problems including increased wear during the charge/discharge cycles and thermal runaway, which can be catastrophic.
Hybrid or Li-ion capacitors are designed to offer the best features of EDLCs and Li-ion batteries in a single package, essentially merging both technologies for increased performance and safety. Whereas EDLCs store energy using an electrostatic charge and Li-ion batteries use an electrochemical method, hybrid capacitors use one electrostatic electrode and one electrochemical. The result is an energy storage solution that provides a higher density over EDLCs but without the leakage, thermal runaway, overcharging/ short-circuiting, and other safety issues that plague rechargeable batteries.
Hybrid supercapacitors also have much lower self-discharge and standby current. In contrast, traditional supercapacitors have higher power capability due to lower ESR (Equivalent Series Resistance) and the operating temperature range is more extensive. Traditional supercapacitors can be discharged to zero volts for safety. Comparatively, a hybrid supercapacitor cannot be discharged fully, which could cause harm to those who touch it unprotected.
Hybrid Capacitor Design
Hybrid capacitors are typically designed using a positive electrode made of activated carbon immersed in a liquid electrolyte similar to the salt solution found in Li-ion batteries. A negative electrode, made of a carbon-based material doped with lithium ions, is also suspended in the electrolyte, along with a separator that prevents direct contact between the electrodes. Pre-doping the negative electrode with lithium ions stifles the capacitors’ electrical potential, allowing a higher output voltage than can be achieved without a high potential on the positive electrode, at a maximum of 3.8V. Like Li-ion batteries, some hybrid capacitors contain electrolytes that are toxic and highly flammable.
Considering the energy density is proportional to the square of the voltage, hybrid capacitors are several times more energy dense than EDLCs with a similar power density, meaning the same amount of energy can be stored in a more compact design over traditional EDLCs. Pre-doped electrodes also stabilize their potential, mitigating any potential leakage in a sustained stored state. Rather than intercalating or de-intercalating (insert or extract) the ions into a carbon lattice, like those in Li-ion batteries, hybrid capacitors absorb/desorb those ions on the electrode surface with no crystalline change taking place during the process. This increases the number of charge/discharge cycles hybrid capacitors can undergo without losing voltage for each cycle.
With the onset of the new technology, more companies and manufacturers are turning to hybrid capacitors for their increased densities and efficiencies including Li-ion capacitor components from Taiyo Yuden that feature an internal resistance (DCR) as low as 60 mΩ and come in cylindrical, compact metal enclosures with the smallest measuring just 30 × 10 mm. They also have an operating temperature range from -25 to +70 °C, which can be extended to +85 °C by reducing the voltage from 3.8 to 3.5 V if needed.
As touched on earlier, the properties of each type of capacitor denote their ideal applications. Li-ion or hybrid capacitors, for example, are best suited for applications that require high energy and power density, with low leakage and increased longevity, durability, and safety. They also excel in high operating temperatures beyond typical EDLCs including auxiliary, pulse, and hybrid power systems.
Another key application area for Li-ion (hybrid) capacitors includes power supplies for energy harvesting devices, such as solar panels, as they can take advantage of new power conversion technologies with minimal energy bleed. They can also store the energy generated and release it on demand. Moreover, hybrid capacitors have a long life expectancy, which reduces maintenance costs while maintaining efficiency. We can expect to see many manufacturers creating new energy harvesting applications utilizing this innovative technology in the coming months and years.
This article was written by Martin Keenan, Technical Director, Avnet Abacus, Waltham, United Kingdom. For more information, visit here .