What is a Hybrid Supercapacitor
Lithium-Ion Battery (LIB)
A lithium-ion or Li-ion battery is a type of rechargeable battery which uses the reversible oxidation reduction (redox) chemical reaction of lithium ions in a lithium metal oxide to store energy. The anode (negative electrode) of a conventional lithium-ion cell is typically lithium doped graphite made from carbon. The cathode (positive electrode) is some form of metal oxide. The electrolyte is typically a lithium salt in an organic solvent (Figure 1)
The use of Lithium Metal Oxide in the battery cell cathode is where thermal runaway risk is concentrated. When excessively heated, often due to dendrite growth and resulting short circuit, the LIB can reach extremely high temperatures which release the oxygen in the metal oxide and provides an internal source of oxygen to feed the fire. This is referred to as thermal runaway.
Battery Aging and Capacity Loss
Lithium ions move between electrodes through a non-aqueous “watery” substance we call electrolyte, in order to produce energy. This creates the negative and positive charges we call electricity.
In theory, this could go on forever, but there are side-effects preventing this. Lithium battery ageing comes down to a phenomenon we call solid electrolyte interphase where lithium-ions become permanently trapped and no longer function as part of the energy transport. (Figure 2) Because SEI degradation occurs, LIBs have typical life-cycle limits of 2,000 to 3,000 80% DoD cycles.
In elementary terms, what happens is the lithium-ions gradually trap in the interphase. Thus, over time there are fewer and fewer of them to manufacture the positive and negative charges. This causes capacity loss, or a reduction in the energy the electrochemical battery can store.
If batteries don’t reach end-of-life through degradation of the SEI, it’s likely because dendrites growth results in premature catastrophic failure. Dendrites are tree-like structures that can form on the lithium plating in a battery (Figure 3). Dendrites typically begin as the result of microscopic impurities in the Anode (-) material. They can quickly penetrate a battery’s ceramic separator, causing the battery to short circuit. Dendrites can also increase unwanted reactions between electrolyte and lithium metal oxide in the cathode electrode, speeding up battery failure.
Lithium dendrites are metallic microstructures that form on the anode (-) electrode during the charging process. They can cause short circuits and lead to catastrophic failures and even fires when the metal oxide of an LIB cathode (+) electrode is overheated. This often results in thermal runaway.
Thermal runaway occurs due to mechanical, thermal, physical, or electro-chemical abuse that damages a battery cell.
• Mechanical – Internal structural failure, typically a manufacturing issue
• Thermal – Overheating of the battery due to rapid charge/discharge or continuous high ambient temperature operation
• Physical – Puncture or crush damage to the battery
• Electro-chemical – Dendrite growth, retention of full charge, or rapid charge/discharge heating
Damage causes an extremely rapid and elevated internal temperature in the battery that reaches temperatures high enough to induce rapid exothermic decomposition of the cell materials.
As decomposition occurs, the lithium-ion battery heat builds up more quickly inside the battery than it can be dissipated.
The result is ignition of the battery or even explosion.
The decomposition of one cell in a storage system can propagate the thermal runaway process to other nearby battery cells, modules or racks within the Energy Storage System.
This creates a domino effect of all the cells catching fire, hence the term “Thermal Runaway”. (Figure 4)
Electric Double Layer Capacitors (EDLCs)
Electrostatic double-layer capacitors, a type of supercapacitor, use carbon electrodes, or derivatives, with much higher electrostatic double-layer capacitance than electrochemical pseudo-capacitance of a typical LIB or the electrostatic capacitance of a ceramic or wound foil capacitor, achieving a greater separation of charge in a Helmholtz double layer at the interface between the surface of a conductive electrode and an electrolyte. A graphene (Helmholtz) layer applied to the electrodes increases surface area and therefore electrostatic capacitance (Figure 5)
Hybrid supercapacitors, sometimes referred to as Lithium-Ion Capacitors (LIC), combine the underlying structures of both batteries and supercapacitors in one physical unit. These hybrid components are not just a simple packaging of a distinct battery and supercapacitor pair in a common housing. Instead, they are energy sources that merge the chemistry of a battery with the physics of an EDLC supercapacitor in a single structure. As a result, these hybrid devices overcome the separate shortcomings of batteries and EDLC supercapacitors while providing clear benefits in life cycles, operating temperature range, safety, and power density.
Hybrid supercapacitors are asymmetric devices comprising a Li-doped graphite anode (-) and an activated carbon cathode (+). The use of an EDLC cathode eliminates the presence of metal oxides, the catalyst for thermal runaway. Although the charge movement is primarily done electrochemically, it is also electrostatic, operates a lower energy density (Wh/kg) when compared to the Li-ion battery, and without the presence of metal oxides does not generate excessive heat.
Among other attributes, this combination of technologies results in higher energy efficiency (>97%), decent energy density for stationary storage applications (120-160 Wh/kg), a very high cycle-life count (20,000 to 50,000 cycles @ 100% DoD and 25°C) and very fast responsiveness to high discharge rates (up to 5 C-rate) (Figure 6).
As an added benefit, no metal oxides are used and therefore these hybrid supercapacitors do not pose any risk of fire or thermal runaway (Figure 7).
As with all components and design approaches, each energy storage solution offers tradeoffs in performance and capabilities. Table 1 shows the positive (“+”) and negative (“-”) attributes of these relative to each other, for typical cases.
Safety of Hybrid Supercapacitors
Using a carbon-based material doped by Lithium Ions at the negative electrode may create concerns about the safety, similar to Lithium-Ion Batteries (LIB). However, the material composition of their positive electrodes (cathode) are very different: LIB uses metal oxide and Lithium-Ion Capacitor uses carbon-based materials such as activated carbon, which does not contain oxygen.
This differentiates their reactions when an internal short-circuit occurs. In LIBs, when an internal short-circuit occurs, the temperature of the internal cell rises by the short-circuit current. A following reaction between the negative electrode (anode) and the electrolytic solution causes an increase in the pressure of the internal cell, followed by a collapse of the crystal at the positive electrode (cathode) and a release of oxygen in oxidation products of the positive electrode. This causes another thermal runaway, and, in some cases, an ignition or an explosion might occur due to a further rise in pressure of the internal cell and vaporization of the electrolytic solution.
In contrast, the internal pressure of the cell also rises in Lithium-Ion Capacitors, but after that, thanks to the difference of the materials in the positive electrodes (cathode), the thermal runaway phenomenon will not occur and the reaction quietly finishes with the opening of the safety valves. Thus, Lithium-Ion Capacitors will not cause any serious accidents such as fires or explosions by thermal runaway even if an internal short-circuit or other accident occurs, thanks to the difference of the material of its positive electrode (cathode) compared to LIBs.
Lithium-Ion Capacitor can be said to be as logically safe an energy device as conventional non-aqueous solvent based EDLCs. Below (Figure 8) are the results of a nail penetration test to a cylinder type Lithium-Ion Capacitor of 200 Farads, assuming an actual internal short-circuit. These results show that the Lithium- Ion Capacitor is a safe device. Even if the temperature of an external wall of the cell increases to 100ºC after short-circuiting, the temperature gradually decreases and the cell does not cause serious problem such as major deformations or explosions.