# Capacitors and supercapacitors: Science and technology

Why storing charge physically means almost endless life

Source page: https://www.materialinsights.org/energy/e_supercaps/sg-sci/

What this tab covers The physics and chemistry: how the energy is stored, which materials it needs (scarcity, toxicity, recyclability) and what makes it age.

Supercapacitors hold charge physically at the surface of porous carbon rather than in a chemical reaction, which is why they last for about a million cycles but hold only a few per cent of a lithium-ion battery's energy per kilogram. Lithium-ion capacitors and pseudocapacitors sit in between.

#### Mechanism
Mixed

A classic electric double-layer capacitor (EDLC) stores energy as ions in the electrolyte line up against the huge internal surface of activated carbon electrodes; nothing is chemically converted, so charging and discharging take seconds. Pseudocapacitors add fast surface redox reactions on metal oxides or polymers, and lithium-ion capacitors pair a carbon electrode with a lithium-doped battery-type electrode, raising cell voltage and energy. A Skeleton 3.0 V cell stores about 6.3 Wh in 0.57 kg, roughly 11 Wh/kg on its datasheet, against about 250 Wh/kg for a lithium-ion cell (Skeleton's own comparison).^[1](https://www.skeletontech.com/hubfs/02-DS-2026-04-02-Skeleton-SCF3400%2c%20SCX5000-D60%20supercapacitor-1M.pdf?hsLang=en)^[2](https://energy-saxony.net/wp-content/uploads/2024/05/SkeletonTechnologies_CompanyIntro_2026.pdf)^[4](https://www.bestmag.co.uk/hybrid-supercapacitors-solution-to-the-power-energy-tradeoff/)
 EDLC cell specific energy (Skeleton SCX5000) 11.1 Wh/kg^[1](https://www.skeletontech.com/hubfs/02-DS-2026-04-02-Skeleton-SCF3400%2c%20SCX5000-D60%20supercapacitor-1M.pdf?hsLang=en) Lithium-ion cell specific energy (company comparison, Skeleton) 250 Wh/kg^[2](https://energy-saxony.net/wp-content/uploads/2024/05/SkeletonTechnologies_CompanyIntro_2026.pdf) Supercapacitor specific power (company claim, Skeleton) up to 60 kW/kg^[2](https://energy-saxony.net/wp-content/uploads/2024/05/SkeletonTechnologies_CompanyIntro_2026.pdf) Lithium-ion capacitor cell voltage (Musashi, as reported in trade press) 3.8 V^[4](https://www.bestmag.co.uk/hybrid-supercapacitors-solution-to-the-power-energy-tradeoff/)

#### Material scarcity and toxicity
Mixed

The key material is activated carbon, made mostly from coconut shells in Japan and South-East Asia. DOE says materials are 71% of EDLC manufacturing cost, and carbon must fall from about USD 15/kg to below USD 10/kg for grid use to pay. EDLCs need no lithium, cobalt or nickel. The real hazard is the electrolyte. Most commercial cells use a salt (TEABF4) dissolved at about 1 mole per litre in acetonitrile or propylene carbonate. Acetonitrile is preferred because it conducts better in the cold. It is also highly flammable (flash point about 6 C, calculated from 42 F). When heated it breaks down into hydrogen cyanide gas, and it is dangerous to life and health at 137 ppm. Propylene carbonate is safer but conducts less well.^[3](https://www.energy.gov/sites/default/files/2024-02/10_Technology Strategy Assessment -%2310 Supercapacitors_508_rev.pdf)^[8](https://www.mdpi.com/2313-0105/10/2/54)^[9](https://cameochemicals.noaa.gov/chemical/11)
 Materials share of EDLC manufacturing cost 71%^[3](https://www.energy.gov/sites/default/files/2024-02/10_Technology Strategy Assessment -%2310 Supercapacitors_508_rev.pdf) Activated carbon price, now vs needed USD 15/kg vs below USD 10/kg^[3](https://www.energy.gov/sites/default/files/2024-02/10_Technology Strategy Assessment -%2310 Supercapacitors_508_rev.pdf) Typical commercial electrolyte about 1 M TEABF4 in acetonitrile or propylene carbonate^[8](https://www.mdpi.com/2313-0105/10/2/54) Acetonitrile flash point; IDLH 42 F (about 6 C, calculated); 137 ppm^[9](https://cameochemicals.noaa.gov/chemical/11)

#### Degradation and ageing
Favourable

Because no chemical phase change occurs, EDLC cells are rated for about a million full cycles, with end of life defined as capacitance falling to 80% or internal resistance doubling. Ageing is driven mainly by heat and voltage, which slowly decompose the electrolyte. DOE puts pseudocapacitors at up to 200,000 cycles and hybrids at up to about 100,000 (some lithium-ion capacitors are sold with higher ratings), because their electrodes swell and shrink. The weak point is self-discharge: a supercapacitor can fall from full to half charge in a month, so it is useless for holding energy for long.^[1](https://www.skeletontech.com/hubfs/02-DS-2026-04-02-Skeleton-SCF3400%2c%20SCX5000-D60%20supercapacitor-1M.pdf?hsLang=en)^[3](https://www.energy.gov/sites/default/files/2024-02/10_Technology Strategy Assessment -%2310 Supercapacitors_508_rev.pdf)
 EDLC cycle life 1,000,000 cycles^[1](https://www.skeletontech.com/hubfs/02-DS-2026-04-02-Skeleton-SCF3400%2c%20SCX5000-D60%20supercapacitor-1M.pdf?hsLang=en)^[3](https://www.energy.gov/sites/default/files/2024-02/10_Technology Strategy Assessment -%2310 Supercapacitors_508_rev.pdf) Pseudocapacitor / hybrid cycle life up to 200,000 / up to 100,000 cycles^[3](https://www.energy.gov/sites/default/files/2024-02/10_Technology Strategy Assessment -%2310 Supercapacitors_508_rev.pdf) Calendar life assumed by DOE 16 years^[3](https://www.energy.gov/sites/default/files/2024-02/10_Technology Strategy Assessment -%2310 Supercapacitors_508_rev.pdf) Self-discharge 100% to 50% charge in about a month^[3](https://www.energy.gov/sites/default/files/2024-02/10_Technology Strategy Assessment -%2310 Supercapacitors_508_rev.pdf)

#### Variants compared^[1](https://www.skeletontech.com/hubfs/02-DS-2026-04-02-Skeleton-SCF3400%2c%20SCX5000-D60%20supercapacitor-1M.pdf?hsLang=en)^[2](https://energy-saxony.net/wp-content/uploads/2024/05/SkeletonTechnologies_CompanyIntro_2026.pdf)^[3](https://www.energy.gov/sites/default/files/2024-02/10_Technology Strategy Assessment -%2310 Supercapacitors_508_rev.pdf)^[4](https://www.bestmag.co.uk/hybrid-supercapacitors-solution-to-the-power-energy-tradeoff/)

| Variant | Specific energy | Cycle life | Notes |
|---|---|---|---|
| **EDLC supercapacitor** | below 8 Wh/kg (DOE) to 11 Wh/kg (Skeleton cell); up to 16 claimed | up to 1,000,000 | Carbon only; fastest; highest self-discharge |
| **Pseudocapacitor** | above 10 Wh/kg | up to 200,000 | Metal oxide or polymer electrodes; swelling issues |
| **Lithium-ion capacitor (hybrid)** | 10 to 20 Wh/kg (pulse types); 20 Wh/L (Musashi prismatic, company figure) | 50,000 to over 1,000,000 | 3.8 to 4.0 V cells; low self-discharge |
| **Skeleton SuperBattery (hybrid, company claims)** | 65 Wh/kg | 50,000 | Charges in under 60 s |

#### Sources

1. [SCF3400 and SCX5000 D60 supercapacitor cell datasheet](https://www.skeletontech.com/hubfs/02-DS-2026-04-02-Skeleton-SCF3400%2c%20SCX5000-D60%20supercapacitor-1M.pdf?hsLang=en) Skeleton Technologies, 2026
2. [Skeleton Technologies company introduction 2026 (company presentation, hosted by Energy Saxony)](https://energy-saxony.net/wp-content/uploads/2024/05/SkeletonTechnologies_CompanyIntro_2026.pdf) Skeleton Technologies, 2026
3. [Technology Strategy Assessment: Supercapacitors (Energy Storage Grand Challenge, #10)](https://www.energy.gov/sites/default/files/2024-02/10_Technology Strategy Assessment -%2310 Supercapacitors_508_rev.pdf) US Department of Energy, Office of Electricity, 2023
4. [Hybrid supercapacitors: solution to the power-energy trade-off (trade press; manufacturer figures as reported)](https://www.bestmag.co.uk/hybrid-supercapacitors-solution-to-the-power-energy-tradeoff/) Best Magazine, 2026
5. [Tuning of ionic liquid-solvent electrolytes for high-voltage electrochemical double layer capacitors: a review (Wang et al.)](https://www.mdpi.com/2313-0105/10/2/54) Batteries (MDPI), vol. 10, art. 54, 2024
6. [CAMEO Chemicals datasheet: Acetonitrile](https://cameochemicals.noaa.gov/chemical/11) US National Oceanic and Atmospheric Administration (NOAA), 2024

## Other sections

- [Summary](https://www.materialinsights.org/energy/e_supercaps/index.md)
- [Overview](https://www.materialinsights.org/energy/e_supercaps/overview/index.md)
- [Correlation](https://www.materialinsights.org/energy/e_supercaps/correlation/index.md)
- [Engineering and performance](https://www.materialinsights.org/energy/e_supercaps/sg-eng/index.md)
- [Economics](https://www.materialinsights.org/energy/e_supercaps/sg-econ/index.md)
- [Impact and additionality](https://www.materialinsights.org/energy/e_supercaps/sg-impact/index.md)
- [What to track](https://www.materialinsights.org/energy/e_supercaps/tracking/index.md)
- [Relationships](https://www.materialinsights.org/energy/e_supercaps/links/index.md)