For environmental, cost, security and efficiency reasons, we’re moving further away from fossil fuels and closer to electricity, fundamentally changing the way we use energy. But electricity is easier to generate than it is to store, and with little progress having been made in battery technology, we face the challenge to find a cleaner, higher-power, energy-storage device. Supercaps are stepping up to that challenge by increasing energy density with new nanomaterials that enlarge electrode surface area and thus boost their ability to hold an electric charge. These advances in materials science/production methods and a thinner form factor for an optimal physical fit are equipping supercaps to be players in peak power management and energy storage at all levels from embedded electronics in portable devices to grid-level storage where they can complement batteries for better efficiency.
The beauty of a supercap as a power source is that, as it transfers and stores a charge, an electrostatic or physical effect takes place rather than a chemical reaction as occurs in a battery. Because that physical effect is reversible, a supercap can charge and discharge quickly, over and over again. But energy storage is directly proportional to a supercap’s capacitance, which, in turn, is proportional to its plate or electrode surface area to which charging particles cling. Electrode surface area also determines current-carrying capability.
Nanomaterials extend charge-carrying surface area
Because a supercap’s ability to hold a charge per unit weight has been small, it’s been a niche player until now. But new materials are adding energy density to a supercap’s technical advantages — high power density, fast charge and discharge, long lifetime, tolerance to a wide temperature range, reliability and being maintenance-free. Traditional supercaps or EDLCs (electrochemical double layer capacitors) use activated carbon, a porous, amorphous material. Nanomaterials such as carbon nanotubes, graphene-based electrodes and carbide-derived carbon are adding heft to this emerging class of supercaps.
Vertically aligned, single-walled carbon nanotubes, hexagons bound and tied into a tube, have a crystal structure and physical properties that lead to an enlarged electrode surface area vs. activated carbon. Each carbon nanotube is a single nanometer in diameter. Graphene has a similar atomic structure to carbon nanotubes but differs in being a flat sheet of carbon atoms, connected into hexagons like a honeycomb lattice. In being flat, it’s similar to silicon so that engineers can process graphene with some of the familiar techniques they use for silicon.
Coating carbon materials with 2- to 5-nm particles of silicon dioxide, also known as silica or nano sand, is another method being used to beef up capacitance. The nanoparticles self-assemble and can be less costly than potentially expensive carbon nanotubes and graphene. Also being developed is a hemp-based alternative to graphene. It can’t match graphene in performance but is said to match energy at a fraction of the price. Exploration of higher temperature and voltage performance using new electrolyte materials such as ionic liquid is also under way.
A second major approach, also counting on new electrode materials to boost a supercap’s energy density, is the production of asymmetric or hybrid supercaps that have one battery electrode and one supercap electrode. One combination is nanoporous nickel hydroxide and activated carbon, which together increase energy density and can translate to smaller size. Another is the use of laser-scribed graphene for its conductivity and manganese dioxide, used in alkaline batteries. Also being embraced is a method that forms one electrode using nanoporous metal oxide with a liquid crystal templating technology. Ruthenium oxide doped supercaps have been produced for specialized high energy and peak power delivery applications for a number of years. Hybrid supercaps with lithium doped to a carbon-based material of the negative electrode also target a significant increase in energy density and are known as lithium-ion capacitors (LICs).
In LICs, one electrode is like a battery electrode — hard carbon with intercalated, pre-doped lithium (Figure 1). The other electrode is a supercapacitor activated-carbon double-layer electrode. The lithiated anode gives a higher energy density by raising the cell operating voltage, but the charge/discharge characteristic resembles that of a supercapacitor.
Figure 1. Hybrid supercapacitor block diagram
LICs aren't subject to the same problems as Li-ion batteries including dendrite formation and shorting as there is no lithium metal deposition taking place during charge and discharge. During the charge and discharge, the hybrid supercapacitor acts as a capacitor with ions moving in and out of the field created at the electrodes, with no redox reactions like in a battery. This is seen clearly by the rise and fall in voltage as the device charges and discharges, rather than remaining at steady voltage like batteries do. Hence, there is little risk of high amperages or high voltages causing lithium metal to deposit. In batteries with oxidation/redox reactions, lithium metal deposits internally when charged or discharged much faster than the safe operating window, leading to greater potential for internal shorts and thermal runaway that leads to fires and explosions.
Flat form factor translates to functional fit, reduced cost
Size is key because a small, flat form factor provides a fit where traditional cylindrical EDLCs or EDLCs with extraneous packaging present real-estate challenges. To date there have been great strides made in supercaps with the development of prismatic, pouch-style hybrid supercaps or LICs improving packaging, volumetric energy density, and reducing costs. Producing a flat cell that can be built on battery manufacturing lines has the potential to reduce manufacturing costs. Hybrid supercaps with activated carbon as the cathode and carbon material pre-doped with lithium ion as the anode, adsorb and desorb ions at their cathodes and store and release lithium ions at their anodes to charge and discharge, thus achieving five to seven times the energy density of a traditional supercap (see Table). That increase in energy density makes it easy to fit them into a smaller size.
Table. Comparative characteristics
Indeed, in a 3-W application, a single flat 360-F 4.0 Volt hybrid cell replaces six cylindrical 100-F EDLC cells configured 2s3p, yielding an increase in storage capacity in a smaller volumetric footprint and a cost advantage to the OEM (Figure 2). These hybrid supercaps operate over a voltage range of 4-2.5 V, are not susceptible to thermal runaway and typically are used without the need for power management or cell balancing.
Figure 2. A single flat hybrid cell can replace six cylindrical EDLC cells. The hybrid cell is lying beneath the cylindrical cells above.
For backup power in cloud-computing server farms, space and cost are factors as the number of servers has to keep up with the ubiquitous use of mobile devices and the Internet. But current 20-J/cc cylindrical EDLC supercaps are form-factor inefficient, present no on-board solution, and offer a single-cell voltage of just 2.7 V, with higher voltages requiring power management. High leakage current and low energy density also present drawbacks. In contrast, these new flat hybrid supercaps deliver a single-cell voltage of 4V, low leakage current and 150 J/cc for high-density DDR4 backup.
Portable electronics is an area of great interest where supercapacitors have seen very limited use so far as button cells for real time clock or memory holdup functions when the main battery is not available or being swapped out. However, as power decouplers and battery replacements, there are many more opportunities for creating strong value propositions for supercaps in smaller embedded and portable electronics, which are seen as the largest and fastest growing market segment over the next 10 years. In form factors as thin as 0.5mm, new EDLC supercap solutions enable placement anywhere. As existing cylindrical supercap solutions are taking more room on the board, their value diminishes as smaller and thinner electronic devices come to market. New applications for supercaps could emerge from these ultrathin devices.
Heftier supercaps energize diverse systems
A supercap’s materials represent a large fraction of their cost, with the electrode being particularly costly. But as these innovations drive demand, and volume production takes place, costs will come down. As they become mainstream, the market for supercaps will become increasingly diverse.
The new higher energy density in both symmetric and asymmetric supercaps enable them to empower renewable systems to store wind power and photovoltaic energy, thus energizing off-grid storage for accessible power upon demand. A supercap’s long cycle life, high reliability and wide range of temperature tolerance are of particular significance to off-shore windmills that are out of reach and need to be maintenance-free. Buoys that keep track of ocean energy, conveying knowledge of ocean waves for shipping purposes are also candidates for supercaps.
Their increased energy density will enable designers to size battery packs in hybrid vehicles for continuous power instead of peak power since supercaps can take over that task and locally harvest and power distributed electric motors. Sizing an engine for average rather than peak power results in a more efficient system with a size/cost advantage. In heavy vehicles that run for short distances, supercaps capture energy ordinarily wasted during braking and release it during acceleration.
Supercaps with more heft can power mobile industrial robotics systems that now rely on both batteries and off-board power provided through a tether, which limits the range of movement to the tether’s length. If supercaps were used to supply peak power while batteries ensured continuous power, designers could eliminate off-board power and increase mobility.
The trend toward distributed energy storage in electric and hybrid vehicles also lends itself to supercaps that can energize small wireless sensor networks to carry vital data and/or power to subsystems still connected to the primary wired network backbone. Supercaps can store energy scavenged by thermal and vibration sources and power electric window openers, door locking and infotainment systems. They can also energize power steering, seat lighting, security and telematics.
Smaller supercaps can supply energy to embedded systems and bring distributed power everywhere in automotive, industrial and even to wearables. Increased energy density and size/cost advantage will bring the benefits of these emerging supercaps to an ever broader array of applications as we embrace them as mainstream energy-storage devices for sustainable energy.
Gene Armstrong is Director of Applications of The Paper Battery Company. Paper Battery Company is developing ultrathin supercapacitors to enable revolutionary, system-level power and energy solutions. Its supercapacitors and integrated solutions enable power and energy management in a smaller size, with greater energy efficiency, and longer runtimes.