In this article, Frank Lev shares his experience with supercapacitors, focusing on lithium-type hybrid supercapacitors (HSC).
Many leading supercapacitor manufacturers have developed and commercialised HSCs, which are increasingly used in applications traditionally reserved for batteries or conventional capacitors. It is worth mentioning that the HSCs are not hybrid systems combining batteries and supercapacitors. They are supercapacitors with increased energy density.
The following terminology helps clarify the context.
- Capacitance: The maximum amount of charge stored by a capacitor, measured in farads (F)
- Cycle life: The maximum number of charge/discharge cycles a supercapacitor can achieve. Typically, HSCs can deliver more than 50,000 charge/discharge cycles
- Volumetric energy density: the charge per unit of volume, expressed in (Wh/L).
- Gravimetric energy density: the charge per unit of weight, expressed in (Wh/kg).
Please note that in the article, the term “energy density” is used in a general sense and is accompanied by (Wh/L) or (Wh/kg) for more clarity - Working voltage in (V)
- Self-discharge: The rate of discharge in (V/h) when not in use or during storage.
Supercapacitors
Supercapacitors are energy-storing devices that bridge the gap between capacitors and batteries. The history of charge storage dates back to the invention of the Leyden jar in the 18th century, followed by Helmholtz laying the foundation for the theory of electric double-layer capacitance in the 19th century.
The most prevalent current design comprises two dielectrically separated electrodes and an organic electrolyte. They store electric charge statically via an interface between the hard and liquid phases, called the double layer, and are known as electric double-layer capacitors (EDLCs).
Owing to their electrochemical design, EDLCs have much larger capacitance and energy density than conventional capacitors. However, they are substantially inferior to lithium-ion batteries (LIB) in this critical metric. An electrochemical battery uses redox reactions called Faradaic to store electrical charge. In contrast, the electrostatic EDLC processes are called non-Faradaic. EDLCs can endure millions of cycles due to the absence of the Faradaic phase, which typically degrades electrodes through chemical reactions.
Both batteries and EDLCs based on aqueous electrolytes do not exceed 2V cell voltages. While the pinnacle of contemporary design – LIBs use organic electrolytes, enabling cell voltages around 4.0V. Commercial LIBs with transient metal cathodes comprising Ni, Mn, and Co have an energy density of 260Wh/kg, whereas the best commercial EDLC cells have about 7Wh/kg.
Energy and Power Tradeoffs
A Ragone plot (Fig 1) shows the typical tradeoffs between power and energy of various electrochemical storage technologies, including EDLCs and LIBs:
- EDLCs typically have much higher power density than batteries, reaching several kW/kg
- EDLCs can be charged and discharged in seconds due to their capacitor-like electrostatic kinetics
- EDLCs have substantially lower energy density than that of Tesla-type LIBs.
An automotive kinetic energy recuperation system (KERS) is a good example of EDLCs demonstrating their superiority over batteries in rapid energy exchanges. The KERS stores a portion of otherwise wasted deceleration energy and returns it when maximum power is needed for acceleration. One of the leading supercapacitor OEMs, Skeleton Technologies, provided its high-performance EDLCs for the KERS installed in Honda Indy 200 race cars.
Power depends on internal resistance
Although crucial for storage, energy density is not the most important metric when the application requires rapid energy release, as in pulse power. A common formula for power is P = V2/4R, where V is the cell voltage, and R is its internal resistance. Hence, the internal resistance of any electrochemical device is the critical contributor to its power density and efficiency.
The low resistance of EDLCs enables them to perform well in applications where batteries are less efficient or lack sufficient cycle endurance. The internal resistance of EDLCs is expressed as the equivalent series resistance (ESR) in milliohms. ESR causes a voltage drop and power loss during discharge.
Oftentimes, a smaller supercapacitor with lower ESR can deliver more energy than a larger one with higher ESR, because the latter’s energy is wasted as internal heating rather than delivered to the external user. All supercapacitor OEMs are focused on reducing ESR; Skeleton Technology produces supercapacitors with the lowest ESR in the industry.
Apart from ESR, leakage current is a critical parameter because it determines the rate of capacity loss when the device is not in use. EDLCs’ self-discharge is typically much higher than that of batteries.
Why a hybrid supercapacitor
Supercapacitors are classified into three types based on their energy storage mechanisms: the previously mentioned EDLCs, HSCs, and pseudocapacitors.
Pseudocapacitors store energy through reversible charge transport at the electrode-electrolyte interfaces, accompanied by redox reactions. Pseudocapacitors can have higher capacitance than similar EDLCs; however, their redox reactions shorten the useful life of electrodes and electrolytes, leading to faster capacitance decay.
EDLCs store charges via physical electrostatic ion adsorption, resulting in rapid charging and discharging. In the everlasting quest for energy, power, efficiency, and endurance, a new breed of energy storage devices has matured recently to rival both supercapacitors and batteries. These products are HSCs.
These devices’ architecture takes advantage of both technologies they are derived from: one electrode is akin to EDLC, whereas the other is similar to LIB. Thus, HSCs use both mechanisms, non-Faradaic EDLC capacitance and Faradaic LIB capacitance, simultaneously. The LIB-type electrodes have a large energy density, while the EDLC electrode has high power density and excellent cycling stability. Thus, the HSCs possess an energy density compatible with some batteries and a cyclic endurance and power similar to those of EDLCs. Fig 2 schematically illustrates the principle of HSC.
Diving deeper into HSC technology
As mentioned, HSC combines the advantages of EDLCs and LIBs, including high energy and power density and a long cycle life. Among numerous HSC technologies developed to date, the LIB-based HSC is the most promising.
HSCs’ capacitive performance, similar to that of EDLCs, allows the same formulas to be used for their design calculations. Thus, capacitance (𝐶) can be expressed as the ratio of the change in stored charge (Δ𝑄) to the variation in applied voltage (Δ𝑉) as the voltage of a capacitor is swept at a constant voltage scan rate (𝜈=𝑑𝑉𝑑𝑡) in the CV diagram. Because the current (𝑖) flowing through a capacitor is proportional to 𝜈, this proportionality is also equal to C, as described in the following Equation: 𝐶=Δ𝑄Δ𝑉=𝑑𝑄/𝑑𝑡𝑑𝑉/𝑑𝑡. Once C is determined, the energy is derived using the formula: E = ½CV².
A single HSC typically operates at 3.8-4.0 V nominal voltage. HSCs have a working cell voltage 25% higher and a capacitance up to 9x higher than commercial EDLCs with organic electrolyte. They also have about 10% lower leakage current than similar EDLCs.
Alternatively, EDLCs have higher power capability due to lower ESR and a broader temperature range. They can be discharged to zero volts, whereas HSCs cannot be fully discharged. Although HSC design is similar to that of EDLCs, one of its electrodes, as mentioned, is made of lithium-intercalated graphite instead of activated carbon. Electrolytes are also different.
During HSC discharge, lithium ions from the anode migrate through the electrolyte while electrons flow through the external circuit. The cathode simultaneously releases stored charge from its electric double layer. The synergy between the two processes delivers high power output and a relatively high energy density due to higher cell voltages of 3.8-4.0V compared to symmetric EDLCs 2.7-3.0V.
The charging process reverses this flow. Lithium ions return to the anode structure while the cathode rebuilds its charge layer, which happens much faster than in conventional LIBs because the activated carbon cathode doesn’t require slow solid-state diffusion. Thus, a full charge cycle can be completed in seconds rather than hours.
Temperature affects performance, but not as severely as in electrochemical batteries. The electrostatic storage mechanism at the cathode remains functional even at extreme temperatures. Lithium intercalation at the anode slows in cold conditions but doesn’t stop completely, giving HSCs a usable range from -40°C to +85°C. Table 1 summarises the design and performance metrics of a typical HSC.
Summary of HSC capabilities
- Much higher energy density than EDLCs.
- Much higher power density than LIBs.
- Longer cycle life
- Better cold performance.
- Faster charging
Can HSC outperform batteries?
Commercial HSCs are based on LIB technology, but typically have lower energy density (Wh/kg) than same-weight LIBs. Some experimental HSCs have demonstrated energy densities comparable to those of LFP batteries. For example, Wang et al. reported an HSC with the energy density of 168Wh/kg at 501W/kg. Still, HSCs tend to complement LIBs rather than replace them. HSCs such as Skeleton’s SuperBattery are by far more energy-dense than the best lead-acid batteries. In many cyclic applications, they could outlast and outperform any VRLA battery. The SuperBattery (Fig 3) has the following metrics:
- Specific energy density of 65Wh/kg.
- Endurance of more than 50,000 lifecycles
- Charging time not exceeding a minute.
- Competitive cost compared to other HSCs.
- Safer than LIBs in confirmed overcharging, nail penetration and overheating tests.
LIBs continue improving their energy density. LIBs with 393Wh/kg are now available, according to Shmuel De-Leon. However, having HSCs compete with LIBs on energy density makes less sense than optimising them for power density.
Optimised for power
In contrast with Skeleton SuperBattery, most HSCs are optimised for power pulses and are often called hybrid pulse capacitors. These devices typically have an energy density of 10-20Wh/kg but can outperform LIBs in power density (W/kg) and (W/L), cycle life, and fast charging. They deliver a high power output of up to 10,000W/kg, a cycle life of over one million cycles, and operate over a wide temperature range from -40°C to +85°C.
In applications requiring frequent high-current pulses, these HSCs reduce maintenance costs and improve system uptime compared to batteries. HSCs are optimal for the following applications:
- Start-stop automotive systems
- Regenerative braking buffers
- UPS ride-through
- Grid smoothing
- Fast power bursts
- Extreme cycle-life use cases
Long Sing Technology Group, based in Hong Kong, specialises in manufacturing HSCs for pulse-power applications, augmenting low-power batteries or other energy sources. For instance, smart meters intermittently require high-power pulses to transmit data.
An HSC is typically paired with a solar panel or a small-capacity LIB to provide a low-power, continuous energy supply. Such a synergetic design enables the meter to operate 15-20 years without a power source replacement.
Power actuators in various industrial applications require high-power pulses, which may shorten battery lifespan and increase maintenance costs, especially in remote locations. An HSC provides a viable solution for such applications, enduring numerous cycles without degrading.
Medical backup power systems must operate uninterrupted during power transitions. HSCs provide an instant response and eliminate the power drop that may occur when batteries kick in.
The high pulse capability of SPCs ensures a reliable start of electric motors even under heavy load. Long Sing Technology summarises the necessary data for an engineering selection of an applicable SPC in Table 2.
All electric vehicles use regenerative braking. As previously mentioned, KERS captures a vehicle’s kinetic energy and converts it back to electricity during deceleration. In contrast to conventional vehicles, EVs are known for having longer-lasting brake pads.
Despite some regenerative action, EV batteries cannot efficiently accept the inrush of braking energy because it arrives in relatively short-duration pulses. The HSCs can do this job much more efficiently. Still, due to cost and complexity constraints, HSCs are seldom used in mainstream EVs. However, when they are, the EVs’ efficiency is improved by 15-25%.
Inherent safety of HSC technology
Internal short circuits are among the most prevalent culprits responsible for thermal runaways and the ensuing fires in LIBs. One of the most dangerous issues associated with LIB fires is the tendency of their cathode active materials to generate oxygen that feeds the fire, making extinguishing extremely challenging.
In contrast, the HSC’s activated-carbon-based cathode does not generate oxygen, making the consequences of an internal short circuit much less severe. The SPEL diagram (Fig 4) shows the crucial difference between LIB with a typical NMC cathode and HSC with an activated carbon cathode.
Many supercapacitor OEMs produce HSC
Panasonic and Musashi of Japan, Long Sing of China, Skeleton of Estonia, Abracon and Eaton of the USA, and SPEL of India are among the leading producers of HSCs that combine the benefits of EDLCs and LIBs. All the mentioned manufacturers use similar HSC technologies based on lithium-doped graphite and activated carbon electrodes. They universally claim much higher energy densities than EDLCs and much longer lifecycles than LIBs. Their HSCs are also much safer than LIBs.
Musashi prismatic HSC baseline specifications
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Fig 5: Musashi prismatic hybrid supercapacitors. Prismatic HSCs are optimal for energy storage (Fig 5)
- High operating voltage (3.8V)
- High output (1200A charge/discharge)
- High energy density (20Wh/L)
- Rapid charging/discharging due to high C-rates
- High durability (over 1 million 100% discharge/recharge cycles)
- Wide operating temperature range without performance degradation (-30℃ to 70℃)
- High safety (no thermal runaway)
- Minimal self-discharge (voltage drop at 10,000 hours is less than 5%)
- Certified to UL 810A
High cell voltage of Abracon cylindrical HSCs
The nominal cell voltage of Abracon HSC is 4.0V, much higher than that of its competitors. The cycle life is not less than 50,000 cycles. The capacitance is from 10F to 1200F.
SPEL pouch HSC
SPEL is the developer and manufacturer of HSCs that have 3X the energy density of any EDLC (Fig 6). For instance, the SPEL G-Series LIC 3000F has 23Wh/kg, compared to the Skeleton 3000 F EDLC with 7Wh/kg. SPEL G-Series products are optimised for high power density of 7500 W/kg, a maximum charging current of 150 A, and a lifetime of over 100,000 cycles. Their cell voltage is 3.8V.
HSCs are ideal for data centres
The growing energy demands of data centres (DCs) require improved energy storage solutions to ensure reliable, stable operation.
A plausible solution is a strategy to reduce DC maximum energy consumption during peak periods, called peak-load shaving. Peak shaving reduces energy costs and prevents DC’s energy overload. HSC may provide power load levelling, ensuring a stable power supply during peak loads or outages. The effectiveness of HSC in addressing the DC’s energy requirements is summarised as follows:
- Peak Shaving: HSCs provide energy during peak demand periods, reducing grid power consumption
- Smoothing Power Fluctuations: HSCs level off power surges caused by fluctuating workloads
- High Efficiency and Long Lifespan: HSCs have high charge-discharge efficiency and can last over 20 years, providing a reliable, low-maintenance solution for handling peak energy demands
- Reduced Need for Backup Generators: HSCs provide immediate power, reducing reliance on less-efficient battery-based power supplies.
HSCs enable DCs to achieve real-time efficiency and faster data processing, driven by the proliferation of AI.
New developments
Advances in the development of LIBs and EDLCs have significantly improved the performance of HSCs. Recently developed HSCs based on sodium-ion battery technologies demonstrate compatibility with LFP batteries.
In addition to traditional activated carbon, graphite, and acetylene black, a plethora of new materials have been recently researched and implemented. Upgraded electrolytes, binders, current collectors, and packaging methods considerably improved HSC performance.
The OEMs never stop researching better technologies to enhance the performance and reduce the cost of their HSCs. For example, a nanocrystalline Li4Ti5O12 integrated with carbon nanofibres increased the conductivity and diffusivity of Li-based electrodes, enabling an experimental HSC to achieve an energy density of 40Wh/L and a power density of 7,500W/L.
In another development, an HSC based on graphene doped with Li4Ti5O12 and sucrose-derived carbon electrodes demonstrated an energy density of 95Wh/kg and a power density of 450W/kg. The described HSC was optimised for maximum energy density, resulting in a relatively low power density due to the power-to-energy tradeoff.
Lou et al. HSCs based on MxCo3-xS4 (where M = Ni, Mn, Zn) with high-surface-area hollow tubular structures have demonstrated excellent electrochemical performance. They achieved a capacitance of 1094F/g at 10A/g with 96% retention after 20,000 cycles. They further enhanced performance using NiCo multi-shelled particles with graphene, achieving 52.6Wh/kg and 1604W/kg.
Zhou et al substantially improved HSC’s electrochemical kinetics by using graphene with homogeneously dispersed MnO and N-doped carbon as the electrode materials, achieving 127Wh/kg and 2,500W/kg, as well as an 8-sec charging time. This development demonstrates the ability of HSC technologies to compete with some power-optimised LIBs.
A transparent and flexible HSC was developed by Niederberger et al using NixFeyOz with graphene oxide. This HSC retained its electrochemical stability after more than 1000 bending cycles.
Conclusion
HSCs bridge the performance gap between LIBs and EDLCs. They deliver high power density, exceptional cycle life, and wide temperature operation. These characteristics make them ideal for utility meters, industrial sensors, backup power systems, and other applications requiring pulse power delivery. HSCs are suitable for “Stop-and-Go” engine-starting applications and outperform the currently used lead-acid batteries.
Properly engineered HSC applications enable maintenance-free operation for 15-20 years in demanding environments. The technology continues to mature, with improved performance and decreasing costs.
HSCs are available as cylindrical, prismatic and pouch cells. Each configuration has its pros and cons, influenced by space, cooling, and cost considerations.
Verified Market Research (VMR) predicts “hybridisation,” in which the distinction between a “battery” and a “supercapacitor” will begin to blur. According to VMR, HSC (Lithium-Ion Capacitors) will account for 26.6% of all new installations in 2026.
