Both the aerospace and automotive industries depend increasingly on electrochemical energy storage. Reduction in mass, increase in energy, and increase in power can benefit both of these areas dramatically. Supercapacitors are currently under consideration for use in both hybrid electric vehicles (HEV) and electric vehicles (EV) to improve delivery of power (due to their high rate capability), improve the life of the lithium-ion batteries (due to their ability to buffer the detrimental effects of high current pulses or alternating currents on the battery), and implement more efficient capture of regenerative breaking energy (due to their excellent charge acceptance at high rates).
Lithium-ion batteries have become the de facto energy storage technology for spacecraft, satellites, and electric vehicles, providing excellent versatility and higher energy than previous rechargeable battery technologies. This high energy can enable longer-lasting and more power-hungry missions, or enable extended-range electric vehicles. However, lithium-ion technology is not always ideal under pulse conditions, where short bursts of high power are needed. The physical design principles of lithium-ion batteries for high energy and those for high power are fundamentally opposed to one another, and rapid discharging can damage cells or at least reduce operating time before recharge is needed.
Supercapacitors, in contrast to lithium-ion batteries, can support extremely rapid discharge rates, but their total energy storage capacity is low. When lithium-ion batteries and supercapacitors are placed in parallel and discharged, a buffering effect is observed — the batteries can provide their usual high energy over long periods at moderate to low rates of discharge (i.e., routine instrument operation or cruising), and when high-current pulses are drawn from the circuit (i.e. for brief high-power communications transmissions or acceleration), the supercapacitors are discharged preferentially, protecting the battery from this exposure.
One limitation of this system is the difference in common operating voltages of these two components. While lithium-ion batteries are routinely charged above 4V, state-of-the-art supercapacitors are typically rated for operation to a maximum voltage of 2.7V, or their lifetime is dramatically reduced. This reduction in lifetime is due to the decomposition of the electrolyte, which can occur when operated at or beyond the limits of the electrochemical window of the particular solvent/salt system. When this occurs, the equivalent series resistance (ESR) of the supercapacitor typically increases, impacting its ability to efficiently deliver power. To make an effective hybrid system, therefore, two supercapacitor cells in series must be paired in parallel with each lithium-ion battery to avoid exceeding the maximum operating voltage of current supercapacitor technology. In addition to the hybrid system, the power output of supercapacitors on their own can benefit greatly — the peak power output is proportional to the square of the charged voltage. To reduce the number of capacitors necessary in a hybrid system to parity with the number of batteries, a system must be designed to operate with improved stability at voltages in the 3.5 – 5V range. This would reduce the complexity and the overall mass of the system.
Lithium-ion batteries owe their high voltage to a number of factors specific to the technology. While many battery components are fundamentally unstable at the electrode potentials during operation, many decades of research have led to electrolyte formulations that form stable, passivating films on the electrodes, allowing passage of lithium ions, but preventing continuous electrolyte decomposition. Supercapacitors rely entirely on the formation of electrical double layers, i.e. the accumulation of a stable layer of electrically adsorbed ions on each electrode. Many of the commonly used electrolyte components in batteries are not well suited to capacitors. Lithium salts can participate in undesired insertion into the electrodes, rather than adsorption, and the solvents are too viscous to reach the high conductivity required. Many additives that form passive films on battery electrodes also have the effect of increasing the resistance, which is acceptable in batteries, but much more detrimental in capacitors.
A number of novel electrolyte formulations were developed to enable supercapacitors to operate at higher voltages compared to state-of-the-art (SOA) electrolytes. The commonly accepted SOA electrolyte is 1.0 M methyltriethylammonium tetrafluoroborate (MTEA-BF4) in acetonitrile (AN). Several approaches were taken in electrolyte design to try to meet the above objectives — namely, replacement of AN by other solvents, including fluorinated carbonates, esters, and ethers; replacement of MTEA-BF4 with alternative salts, in particular spiro-bipyrrolidinium tetrafluoroborate (SBP-BF4); and addition of additives in small quantities to AN-based blends. Fluorinated carbonates, esters, and ethers were of interest. These included monofluoroethylene carbonate (FEC), bis(trifluoroethyl) carbonate (BTFEC), ethyl 2,2,2-trifluoroethyl carbonate (ETFEC), 2,2,2-trifluoroethyl methyl carbonate (TFEMC), bis(trifluoroethyl) ether (BTFE), methyl heptafluorobutyrate (MHFB), methyl trifluoroacetate (MTFA), ethyl trifluoroacetate (ETFA), and 2,2,2-trifluoroethyl butyrate (TFEB). Two additional solvents, tris(isopropyl) borate (TIPB) and diethyl sulfite (DES), were chosen due to their very broad liquid range and low viscosity, which could contribute to the high conductivity necessary for supercapacitors.
Two solvents added in larger proportion (TFEMC and BTFE) and two additives added in very small quantities (tris-trifluoroethyl phosphite or TTFEPi, and lithium difluoro-oxalato borate or LiDFOB) were particularly promising. New blends incorporating them, or derivative combinations thereof, could enable longer-life, lower-resistance, or higher-capacitance supercapacitors operating at higher voltages than SOA electrolytes allow. Slower capacitance fade, better high-voltage stability, and lower resistance were demonstrated compared to the baseline electrolyte. Use of these blends suggests two potentially important results. First, they suggest methods for enabling supercapacitors to be operated closer to the limits of lithium-ion cells. Second, the success of small-scale additives suggests that electrode film formation in supercapacitors may not be as detrimental as previously thought. As noted, this can enable hybrid battery/supercapacitor systems to be deployed with half as many supercapacitor cells, thereby reducing the mass of the system (critical for space and automotive applications) as well as the overall cost (critical for the automotive industry). This technology could increase the adoption of supercapacitors in a wider range of applications.