Supercapacitor Technology: Targets and Limits

Y. Maletin, N. Stryzhakova, S. Zelinsky, S. Chernukhin, D. Tretyakov

YUNASKO Abstract:

• Supercapacitor (SC) inner resistance, in particular, that for Yunasko devices is close to low limit and can further be reduced by a factor of 2 at the most.

• Still the lowest resistance values are important since they imply high efficiency (hence, low heat generation and improved safety) and low RC-constant (hence, grid frequency regulation).

• Higher operating temperature (about 100 °C) and higher rated voltage (about 3 V) are good challenges, and they are in Yunasko R&D portfolio today.

• Since the SC energy density is limited by 5-8 W.h/kg, hybrid devices can become a good choice when the larger energy density is critical while a shorter cycle life can be accepted (up to 37 Wh/kg and 10’000 full charge-discharge cycles in Yunasko technology today).

• Along with the discussion of possible SC performance limits, the most recent Yunasko SC modules, their performance and possible applications will also be presented.

1. Inner resistance. In our studies we use different approaches to develop carbon-carbon SC devices of low inner resistance. Inherently, SC’s cannot compete with batteries if the large energy density is required since the electrode surface only takes part in charge accumulation. As a result, SC devices can mostly be used for short-term power applications, and therefore, our efforts are mostly aimed at reducing their inner resistance. Deep electrochemical and spectral studies of various nanoporous carbons to be used in either positive or negative electrode of SC device have been carried out [1], the study including the cyclic voltammetry (CV) measurements in a three-electrode cell, porous rotating disc electrode, impedance spectroscopy (EIS) and NMR spin-echo measurements. It is worth noting that so-called carbon-carbon SC’s can demonstrate much better performance if positive and negative electrodes are made of different carbons. Some steps of this approach were described recently [1,2] in more detail, in particular, two independent methods were developed to measure the effective in-pore electrolyte mobility and thus to select the most promising carbons for positive and negative electrodes. As a result SC devices of the lowest resistance and highest power density were developed – e.g., in Fig. 1 and Table 1 see the comparative test results of Dr. D. Corrigan, copied out his report to Wayne State University in winter 2014. Also, in Table 2 the test results of Dr. J.R. Miller et al. [3] are listed to confirm the lowest RC-constant (hence, the lowest inner resistance) of our SC’s. Thus, the Yunasko SC devices do demonstrate substantially higher power capability and efficiency than the best competitors, so what can be the target and where might be the limit? It should be noted that in today’s Yunasko technology we face a significant contribution of aluminum current collectors and current leads to the total inner resistance value – up to 20-25%. This is the reason why we use those thick and wide current leads instead of conventional tabs in our cell assembly technology. And the evaluations show that further reduction in the inner resistance value will be limited by a factor of ca. 2, when the contribution from aluminum parts reaches 50% and a further increase in their cross-section value would result in significant increase in mass and, correspondingly, loss of energy and power densities. So, the minimum RC-constant value, if one

does not sacrifice the energy density, can be anticipated to be about 50 ms with the maximum power density1 about 200 kW/kg.

Table 1 Comparison of carbon-carbon SC devices (as tested by Dr. Dennis Corrigan, Wayne State University, Detroit)

Company Mass, kg

Capacitance, F

Voltage, V

ESR, mΩ

Eff. % @ CP 10 s discharge

Eff. % @ CP 5 s discharge

Maxwell Technologies 0.26 1200 2.7 0.50 95.6 92.0

Ioxus 0.29 1200 2.7 0.40 96.7 93.9 Yunasko 0.25 1200 2.7 0.09 99.2 98.3 Yunasko 2.5 200 16 1.07 98.5 97.2

Fig. 1. Comparative test results obtained by Dr. Dennis Corrigan (Wayne State University, Detroit, 2014)

Table 2 Initial properties of some selected SC cells evaluated by Dr. John R. Miller et al. [3]

Company Capacitance, F

DC resistance, µΩ

Stored energy*, W.h/kg

RC-constant, s

Maxwell Technologies 3140 230 4.5 0.7

Ioxus 3100 230 4.3 0.7 Nippon Chemi-con 1090 860 2.8 1.1 Batscap 1290 330 4.0 0.4 Yunasko 1230 160 4.0 0.2

* Calculated assuming discharge to half rated voltage                                                                                                                1    Can be evaluated as: Pmax = U2/(4Rin×m), where Rin is the inner resistance, and m is the mass.  

2. High operating temperature and high voltage. Both acetonitrile and carbonate based electrolytes have an obvious drawback – rather low operating temperature limit of about 70 °C. Besides, near this limit the operating voltage should normally be reduced to keep the life cycle. On the other hand, the temperature under the hood and near engine can be about 100 °C, and it would be a good challenge to develop the electrolyte chemically and electrochemically stable at this temperature and at the same time withstanding the voltages of at least 2.7 V. Bearing in mind also cost issues we have not included ionic liquids in our current studies, though have tested some of them to keep an eye on this technology. On the other hand, there are some patent applications recently appeared (e.g., see [4,5]) that disclose the use of sulfolane and other commercially available sulfones in electrolytes for SC’s at elevated temperatures (the solvent boiling point is typically above 200 °C) and voltages. These and some other high temperature electrolytes have attracted our attention but, unfortunately, as can be seen from Fig. 2, an increase in solvent boiling point is normally accompanied by an increase in its viscosity2 and, correspondingly, by a decrease in the electrolyte conductivity.

Fig. 2. Close to exponential plot of viscosity vs. the boiling point for 50 aprotic solvents that can potentially be used in SC technology.

So, most of the electrochemical systems like those disclosed in [4,5] demonstrate rather high resistance and, correspondingly, low power capability and low efficiency. Nevertheless, we have finally developed an electrochemical system capable of providing the sustainable operation within the temperature range between -20 and +100 °C and fairly low resistance [6]. The performance of SC prototypes based on this new electrochemical system is comparable with those listed in Table 2, namely, the RC-constant at room temperature is 0.5-0.6 s and that at 100 °C is 0.15-0.20 s. 3. Hybrid device – a way to increase the energy density. The general state-of-art of hybrid devices can be characterized by the energy density of about 10-15 W.h/kg and the power density of about 0.6-1.0 kW/kg at 95% efficiency. Most of the currently available devices comprise one electrode copied out SC technology and another copied out battery technology, the two electrodes being connected in series through the separator impregnated with electrolyte. Our researches in                                                                                                                2  This can be accounted for the same Van-der-Waals interactions between the solvent molecules that increase both boiling point and viscosity.

hybrids are aimed at increasing their energy density up to 40-50 W.h/kg keeping at the same time the high power capability typical for SC devices, and our approach is as follows:

• Both positive and negative electrodes should comprise the components of Li-ion and SC technology, e.g., a negative electrode is a mixture of Li4Ti5O12 and activated carbon powders, while a positive electrode is a mixture of various lithiated metal oxides and/or phosphates and activated carbon powders. Thus, a so-called internal parallel connection of battery and SC electrodes can be realized.

• Mass ratio of various electrode components should be properly balanced in order to match their charge-discharge characteristics and to provide smooth charge-discharge curves of positive and negative electrodes.

• Various organic electrolytes can be used, and acetonitrile, though being not typical for Li-ion technology, can preferably be used as a solvent to keep the high conductivity and in-pore mobility resulting in the low inner resistance of a hybrid device.

Performance of hybrid prototypes is illustrated in Fig. 3, wherein Ragone plots based on the measurements carried out in the Institute of Transportation Studies (UC Davis, CA) and also as presented on the JM Energy website are shown. As can be seen from Fig. 3, Yunasko hybrid devices give a chance to substantially increase the energy density keeping at the same time the high power capability typical for most of the SC devices available in the market.

Fig. 3. Ragone plots for two hybrid devices: Yunasko and JM Energy.

As another side of the coin, an increase in energy density can be (and typically is) accompanied by a shorter cycle life of various power supply units provided that their chemistry and charge-discharge processes are similar or follow each other. An example is illustrated in Fig. 4, wherein the number of full charge-discharge cycles for a typical Li-ion battery, JM Energy LIC device, Yunasko hybrid and SC device are plotted vs. their energy density in logarithmic scale. A fairly good correlation confirms that an increased share of Li-intercalation processes in the total energy accumulation, which can be presented as a sum of “chemical” in-volume intercalation and “physical” on-surface double electric layer charging processes, shortens the cycle life of the system. This gives us a chance to adjust the energy/cycling ratio to the application requirements.

Fig. 4. Plot of cycle number vs. energy density (in logarithmic scale) for some related technologies: Li-ion battery, Yunasko hybrid, JM Energy hybrid and SC.

Targets and Limits of SC Technology:

• As we see it now, the power capability and efficiency of carbon-carbon SC devices can further be increased, and a feasible limit of 0.05 s for the RC-constant and 200 kW/kg for the maximum power density can soon be achieved.

• The energy density of carbon-carbon SC devices can slightly be increased from current 4-5 W.h/kg to 6-7 W.h/kg due to possible increase in the rated voltage.

• An increase in the rated voltage and also an increase in the operating temperature limit up to ~3.0 V and 100 °C, respectively, can be achieved due to the use of electrochemically stable aprotic organic electrolytes, in particular, those based on sulfolane.

• The energy density can substantially be increased (with a target value of 50-60 W.h/kg) due to “parallel” hybridization of both electrodes in the electrochemical system, in particular, if the cycle life of the order of 104 is acceptable.

References:

1. Y. Maletin, V. Strelko, N. Stryzhakova, S. Zelinsky, A.B. Rozhenko, D. Gromadsky, V. Volkov, S. Tychina, O. Gozhenko, D. Drobny. Carbon Based Electrochemical Double Layer Capacitors of Low Internal Resistance. Energ. Environ. Res. 2013 Dec, 3(2), pp. 156-165.

2. Y. Maletin, N. Stryzhakova, S. Zelinsky, S. Chernukhin, D. Tretyakov, S. Tychina, D. Drobny. Electrochemical Double Layer Capacitors and Hybrid Devices for Green Energy Applications. Green. 2014, vol. 4 (1-6), pp. 9–17.

3. J. R. Miller, S. M. Butler, S. McNeal. Life Performance of Large Electrochemical Capacitors. Proceedings 46th Power Sources Conference, Paper # 27.1, June 9-12, 2014, Orlando, FL, USA, pp. 380-381.

4. Y. Ito, H. Yanagisawa. Electrolyte Solution for Electric Double Layer Capacitors, and Electric Double Layer Capacitor. PCT pat. appl. WO2013128824 (A1), publ. Sep.6, 2013.

5. T. Hommo, R. Sato, T. Oikawa, T. Tamachi, I. Shinoda, S. Watanabe. Electrolytic Solution for Electric Double Layer Capacitor, Electric Double Layer Capacitor Using the Same, and Manufacturing Method therefor. US pat. appl. 20120044614 A1, publ. Feb.23, 2012.

6. Patent pending.