Metrics for Fast Supercapacitors as Energy Storage Devices

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Metrics for Fast Supercapacitors as Energy Storage Devices Ali Eftekhari ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04532 • Publication Date (Web): 10 Sep 2018 Downloaded from http://pubs.acs.org on September 10, 2018

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ACS Sustainable Chemistry & Engineering

Metrics for Fast Supercapacitors as Energy Storage Devices

Ali Eftekhari Belfast Academy, 2 Queens Road, Belfast BT3 9FG, United Kingdom Email: [email protected]

Abstract Supercapacitors are investigated as energy storage devices, alternatives to batteries, but their electrochemical performance is usually inspected with the metrics of classic capacitors. The resulting inconsistency in the literature has caused confusion about the potentials and limitations of supercapacitors. First, the average power density of a supercapacitor cannot be directly compared with the relatively constant power density of counterpart batteries. Second, specific capacitance is the capability of capacitors for charge separation by the potential perturbation and does not represent the capacity for energy storage when the delivered charge has a nonlinear dependency on the potential. Third, many new supercapacitors are not even faster than their counterpart batteries to justify a practical development, but the problem is buried under the shield of inappropriate metrics. This paper clarifies that employing the appropriate metrics for energy storage can lead us in designing faster supercapacitors for the practical applications. Keywords: Supercapacitors; Specific capacitance; Power density; Fast charging; Rate capability; Batteries

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Introduction The increasing demand for new types of energy storage devices has emerged a confusing overlap between supercapacitors and batteries, though this boundary has been of interest for a long time.1-2 Batteries store energy by storing the charge slowly through an electrochemical reaction, whereas capacitors separate the charge through double layer charging, which is quite fast through rearrangement of the ions at the atomic scale. The latter process is several orders of magnitude faster than the slow solid-state diffusion in batteries. However, in practice, supercapacitors are entangled with sluggish processes because it is not the pure capacitive response as is the case in the classic capacitors.3 In fact, the performance of both double layer capacitors and pseudocapacitors is closer to that of batteries rather than classic capacitors (including electrolytic capacitors). Nevertheless, supercapacitors are judged by the classic metrics of capacitors while being utilised as an alternative to batteries.

Historically, supercapacitors aimed at specific applications where limited energy was required such as uninterrupted power sources (UPSs), but the current trend is to use them as energy storage devices, e.g., in electric vehicles (a good example of commercial development is the so-called Capabus, supercapacitor-powered bus in China). Therefore, it is of vital importance to examine supercapacitors against their capacity for energy storage rather than capacitive performance. The terminology of supercapacitors has been described elsewhere.4 Although Conway coined the term for both double layer and pseudocapacitive performance, it has been used as a commercial name at the same time. In any case, terminology debate is meaningless, as the term supercapacitor is commonly used by the researchers for both double layer and pseudocapacitive materials.

In the case of pseudocapacitive supercapacitors, it has been recently clarified that the underlying electrochemical system is similar to batteries, and the critical difference is that the energy of redox sites are distributed over a broad range of potential in pseudocapacitive materials.4 Since the nearby redox sites react at different potentials, there is less competition between them, and thus, the diffusion process is much faster. In the case of battery systems, the diffusion coefficient at the redox potential is a few orders of magnitude smaller than the other potentials. In other words, the prime advantage of supercapacitors is the capability for faster charging/discharging. Otherwise, the decaying potential is a disadvantage, which should be addressed by regulating the Page 2 of 13 ACS Paragon Plus Environment

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output voltage in the final product.

Cyclic Voltammetry The rate capability of batteries is reasonably judged by the C-rate, which is indeed an industrial method of presentation, but commonly followed in the fundamental research too. 1C equals the current required to achieve the theoretical capacity within an hour. Since the theoretical capacity is not known for supercapacitors, the applied current is directly reported. Owing to different values of the specific capacitance reported in the literature, one cannot simply estimate the practical power of a supercapacitor. The general impression is based on the cyclic voltammetry, which uses the same unit for charging/discharging. Roughly speaking (based on a quick overview of the literature), one can realise typical values of the potential scan rate is 1 mV s–1 for batteries, 10 mV s–1 for pseudocapacitors, and 100 mV s–1 for double layer capacitors. In other words, if briefly browsing the literature, the cyclic voltammograms of battery materials are usually recorded between with scan rates in the range of 0.1 - 1 mV s–1, pseudocapacitors in the range of 1 - 10 mV s–1, and double layer capacitors in the range of 10 - 100 mV s–1. Nevertheless, this might be misleading, as the charge/discharge rate depends on the peak width. In Figures 1a and 1b, if hypothetically assuming that the overall capacities of the battery and supercapacitor are equal, the battery needs only 10s for the passage of the total number of electrons; whereas, the supercapacitor needs the entire experiment time of 100s to do so at a typical scan rate of 10 mV s–1. Therefore, the potential scan rate in cyclic voltammetry is not an appropriate metric for comparing batteries and supercapacitors. Note that this is just a schematic model highlighting that the scan rate in cyclic voltammetry does not represent the same timescale of charge/discharge for supercapacitors and batteries.

The shape of the peaks in cyclic voltammetry can provide useful information about both kinetics and thermodynamics of battery materials. Even in the case of supercapacitors, deviation from featureless CV (rectangular shape) is quite informative. The point emphasised here is not about the usefulness of cyclic voltammetry but the inappropriate interpretation of the potential scan rate. It is mistakenly believed that if a supercapacitor is well performed at the potential scan rate of 10 mV s–1, this means the energy storage is much faster of a battery material. The above illustration demonstrates that the rate capability of this supercapacitor is still at the level of a battery material, which is performed under 1 mV s–1. It should be kept in mind that it does not Page 3 of 13 ACS Paragon Plus Environment


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matter that the cyclic voltammetry is performed with three-electrode cells; the point of comparison here is the total time required for charging the half- or full-cell. This discussion is not limited to supercapacitors and also covers conversion-based battery materials, which have a pseudocapacitive nature.5

Galvanostatic Charge/Discharge Profiles One may argue that the primary metric for judging the rate capability is the power density, which is the same for both batteries and supercapacitors. The power of a battery is calculated by the standard equation of P = VI, where I is the applied current and V the cell voltage, which is constant. The power of a supercapacitor is proportional to its decaying voltage. The power densities reported in the literature are indeed the average values estimated from the point of 50% discharge. In the ideal supercapacitor performance, the power supplied decreases when discharging (Figure 1c). As an energy storage device, a constant power should be supplied, and thus, the power supply should be electronically regulated (technically, regulating the voltage by consuming a higher current). This means that the cell should be discharged faster to compensate its decaying power to compete with the counterpart battery. So much the worse, deviation from the ideal behaviour, which is inevitable, causes a rapid decline in the power density in supercapacitors (Figure 1c). Moreover, it is obvious that reporting the maximum power density Pmax is not useful for a supercapacitor. As a rough estimation, in a supercapacitor with a Pmax twice than that of its counterpart battery, the power density delivered is less than that of the battery during most of the discharge time (Figure 1c). Furthermore, this deviation significantly reduces the energy efficiency, which is of vital importance in energy storage devices.6

Figure 1. a and b compares the actual time required for charging/discharging of a battery and a supercapacitor, respectively, at a typical scan rate of 10 mV s–1. c illustrates the changes in power density during discharging and possible deviation from the theoretical performance. The theoretical and practical power density curves are shown by dashed and solid lines. d demonstrates that the


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potential window has no impact on the specific capacitance of an ideal supercapacitor (or theoretically), but an enormous impact on that of real supercapacitors (e). This impact is much stronger for batteries (f). If calculating the specific capacitance for a battery, the specific capacitance becomes extremely large (inset of f). The units are for the sake of comparison only, and the discharge current is assumed 1 A g–1. Note that these numbers are typical values for the sake of comparison rather than actual values. It can be seen as a sort of normalisation.

Specific Capacitance Another misleading parameter is the specific capacitance, which is indeed the first metric reported in the literature. In 2017, 50% of the papers on supercapacitors reported the specific capacitance as a key metric right in the abstract (this stands for over 2,000 papers). Capacitance is the electrode capability in separating the charge in response to the potential difference, which is of importance in the functionality of capacitors in electronic circuits. Now that the prime purpose of supercapacitors is energy storage, this does not provide any meaningful information, at least in comparative studies. High capacitance means that a capacitor is capable of separating a significant amount of charge by a tiny change in the applied potential. If calculating the specific capacitance for batteries (i.e., not common), the value is much higher than that of supercapacitors. Table 1 compares typical metrics for supercapacitors and batteries (these are typical values for the sake of comparison, but the corresponding experimental data can be found in the literature, see, for example, comprehensive references given in5,7-14). These are simply typical materials in each category for a comparative outlook of the categories, not materials. For instance, it is highlighted that charging new anode materials (such as Ge, Si, etc.) is faster than available double layer capacitors. It is irrelevant here that these anode materials suffer from volume expansion, which should be tackled for the practical development. The point is that the emerging battery materials are faster with higher power densities as compared with the available supercapacitors. This pictures the competition ahead for supercapacitor if the target application is energy storage.

Whilst the specific capacitance of an ideal capacitor is potential-independent (Figure 1d), the nonlinear dependence in supercapacitors (particularly pseudocapacitors) makes the potential cutoffs critically important (Figure 1e). Depending on the width of the potential window, the specific capacitance can be massively changed. The point is that specific capacitance does not provide useful information for energy storage. Brousse et al. similarly stressed that it is misleading to interpret the battery performance of some electroactive materials such as Ni(OH)2 as pseudocapacitive materials.15 As illustrated in the inset of Figure 1f, limiting the potential window Page 5 of 13 ACS Paragon Plus Environment


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for some battery materials can result in an enormously high specific capacitance while exhibiting an ideal capacitive response. In fact, for ideal battery performance displaying a flat plateau (e.g., at low discharge rates or in the case of reconstitution reaction16), the specific capacitance tends to ∞. Moreover, the actual charging/discharging Cmes (i.e., the practical measure for the commercial products) are in the same range, while the power density would suggest significantly higher values for supercapacitors. It is also evident that power density is not the appropriate measure to compare the rate capability of supercapacitors and batteries due to their different power supply scheme.

Impedance Spectroscopy Impedance spectroscopy is a powerfulness technique in the realm of electrochemistry, which can also be used for estimating the rate capability of energy storage systems, as the frequency is representative of the charge/discharge rate (though via an AC signal).17 Although impedance spectroscopy is the prime technique for measuring the rate capability of ultrafast supercapacitors which are primarily aimed for AC line filtering7,17-18, this opportunity is ignored for examining the rate capability of conventional supercapacitors or batteries. A recent model has described the similarities of these systems and their corresponding impedance responses.3,17

Concluding Remarks Overall, the characteristic advantage of batteries is high capacity (i.e., high energy), and that of supercapacitors is high rate capability (i.e., high power). It should be taken into account that the rate capability of batteries is much higher if the full capacity is not targeted because severe factors limiting the charging rate occurs at higher degrees of charge status. A commercial example is Tesla car model S3, whose battery can be charged to the level of 80% within 30min (with supercharging outlet, of course), but an extra hour is required for the remaining 20%. This means a supercapacitor with 80% of its battery counterpart (i.e., very optimistic), the supercapacitor must be at least 3 times faster to compete with the battery. This is due to the practical limit for the voltage cutoff to preserve the cell stability.

In conclusion, there is no distinct boundary between batteries and supercapacitors. The most promising energy storage materials have a mixed mechanism. Similar materials are examined by different metrics owing to different labels. High-energy supercapacitors aimed for energy storage Page 6 of 13 ACS Paragon Plus Environment

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has no similarity with capacitors, and unfortunately, the advantage of fast performance has been sacrificed in favour of higher energy storage capacity. Although the energy density of supercapacitors is approaching that of batteries, still, batteries gain the considerable advantage of constant power. Therefore, if supercapacitors should be significantly faster to practically compete with batteries. Achieving a higher specific capacitance or energy density without considering their real impact on energy storage19-20 does not help the practical development of supercapacitors. Instead, the key advantage of supercapacitors, i.e., fast performance, should be targeted. In designing supercapacitors for AC line filtering, it has been reported that supercapacitors can be as fast as electrolytic capacitors7,17-18, but unfortunately, this strategy of research has not yet been followed for energy storage.

Inappropriate metrics have caused supercapacitors to lag behind batteries since majorities of the current research focus is on wrong (or more precisely less useful) cases. Albeit, this failure has affected the prospect of the alternatives to lithium-ion batteries too.21 As described in4, sacrificing the ordered lattice structure of battery materials results in diminishing the dominance of solidstate diffusion and thus faster performance. New battery materials, particularly anode materials, gain this feature and have pseudocapacitive nature to some extent.11 Notwithstanding, relying on the inappropriate metrics discussed above, a majority of supercapacitor research is limited to materials with narrow potential windows (i.e., results in high specific capacitance but low energy storage capacity) or slow supercapacitors which cannot compete even with the available batteries. This does not mean supercapacitors have less potential for energy storage, but the obsession with the superficial metric of specific capacitance or wrong interpretation of power density has overshadowed the prime requirement of fast performance.

The problem is that inappropriate metrics give a false impression that an electroactive material has a potential for developing a practical supercapacitor. If aligning the graphene nanosheets vertically, the resulting supercapacitor will be super fast, which can be charged/discharged with scan rates of up to 500 V s–1 in cyclic voltammetry.7,17-18 However, since the energy density of this architecture is not high enough, these supercapacitors are mainly used for AC line filtering. The energy density is definitely an important factor for energy storage devices, but if sacrificing the key advantage of supercapacitors, i.e., high rate capability, they cannot compete with the battery counterparts in the practical applications. Therefore, it is of vital importance to use appropriate Page 7 of 13 ACS Paragon Plus Environment


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metrics to project which of the available choices have the practical potentials for fast energy storage by supercapacitors. The commercialisation of supercapacitor materials mainly depends on their rate capability to compete with the available batteries because it is unlikely to beat the energy density of batteries. In this direction, the metrics should make sense when applying to the final product.


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References [1] Conway, B. E. Transition from “Supercapacitor” to “Battery” Behavior in Electrochemical Energy Storage. J. Electrochem. Soc., 1991, 138, DOI 10.1149/1.2085829 [2] Simon, P.; Gogotsi, Y.; Dunn, B. Where Do Batteries End and Supercapacitors Begin?. Science, 2014, 343, DOI 10.1126/science.1249625 [3] Eftekhari, A. Surface Diffusion and Adsorption in Supercapacitors. ACS Sustainable Chem. Eng., 2018, 6, DOI 10.1021/acssuschemeng.8b01075 [4] Eftekhari, A.; Mohamedi, M. Tailoring Pseudocapactive Materials from a Mechanistic Perspective. Mater. Today Energy, 2017, 6, DOI 10.1016/j.mtener.2017.10.009 [5] Zhang, Y.; Yu, S.; Lou, G.; Shen, Y.; Chen, H.; Shen, Z.; Zhao, S.; Zhang, J.; Chai, S.; Zou, Q. Review of Macroporous Materials As Electrochemical Supercapacitor Electrodes. J. Mater. Sci., 2017, 52, DOI 10.1007/s10853-017-0955-3 [6] Eftekhari, A. Energy Efficiency: a Critically Important but Neglected Factor in Battery Research. Sustainable Energy Fuels, 2017, 1, DOI 10.1039/c7se00350a [7] Miller, J. R.; Outlaw, R. A.; Holloway, B. C. Graphene Double-Layer Capacitor with AC Line-Filtering Performance. Science, 2010, 329, DOI 10.1126/science.1194372 [8] Eftekhari, A. Supercapacitors Utilizing Ionic Liquids. Energy Storage Mater., 2017, 9, DOI 10.1016/j.ensm.2017.06.009 [9] Kumar, K. S.; Choudhary, N.; Jung, Y.; Thomas, J. Recent Advances in Two-Dimensional Nanomaterials for Supercapacitor Electrode Applications. ACS Energy Lett., 2018, 3, DOI 10.1021/acsenergylett.7b01169 [10] Eftekhari, A.; Li, L.; Yang, Y. Polyaniline Supercapacitors. J. Power Sources, 2017, 347, DOI 10.1016/j.jpowsour.2017.02.054 [11] Eftekhari, A. Low Voltage Anode Materials for Lithium-Ion Batteries. Energy Storage Mater., 2017, 7, DOI 10.1016/j.ensm.2017.01.009 [12] Eftekhari, A. LiFePO4/C Nanocomposites for Lithium-Ion Batteries. J. Power Sources, 2017, 343, DOI 10.1016/j.jpowsour.2017.01.080 [13] Eftekhari, A. Lithium-Ion Batteries with High Rate Capabilities. ACS Sustainable Chem. Eng., 2017, 5, DOI 10.1021/acssuschemeng.7b00046



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[14] Zhao, B.; Ran, R.; Liu, M.; Shao, Z. A Comprehensive Review of Li4Ti5O12-Based Electrodes for Lithium-Ion Batteries: The Latest Advancements and Future Perspectives. Mater. Sci. Eng. R Rep., 2015, 98, DOI 10.1016/j.mser.2015.10.001 [15] Brousse, T.; Bélanger, D.; Long, J. W. To Be or Not To Be Pseudocapacitive?. J. Electrochem. Soc., 2015, 162, DOI 10.1149/2.0201505jes [16] Huggins, R. A. Pulse applications of electrochemical cells - materials aspects. Ionics, 1995, 1, DOI 10.1007/BF02426003 [17] Eftekhari, A. The Mechanism of Ultrafast Supercapacitors. J. Mater. Chem. A, 2018, 6, DOI 10.1039/c7ta10013b [18] Fan, Z.; Islam, N.; Bayne, S. B. Towards Kilohertz Electrochemical Capacitors for Filtering and Pulse Energy Harvesting. Nano Energy, 2017, 39, DOI 10.1016/j.nanoen.2017.06.048 [19] Gogotsi, Y.; Simon, P. True Performance Metrics in Electrochemical Energy Storage. Science, 2011, 334, DOI 10.1126/science.1213003 [20] Balducci, A.; Bélanger, D.; Brousse, T.; Long, J. W.; Sugimoto, W. Perspective—A Guideline for Reporting Performance Metrics with Electrochemical Capacitors: From Electrode Materials to Full Devices. J. Electrochem. Soc., 2017, 164, DOI 10.1149/2.0851707jes [21] Eftekhari, A. On the Theoretical Capacity of Lithium Batteries and Their Counterparts. ACS Sustainable Chem. Eng., 2018, 6, DOI 10.1021/acssuschemeng.7b04330


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Figure Captions

Figure 1. a and b compares the actual time required for charging/discharging of a battery and a supercapacitor, respectively, at a typical scan rate of 10 mV s–1. c illustrates the changes in power density during discharging and possible deviation from the theoretical performance. The theoretical and practical power density curves are shown by dashed and solid lines. d demonstrates that the potential window has no impact on the specific capacitance of an ideal supercapacitor (or theoretically), but an enormous impact on that of real supercapacitors (e). This impact is much stronger for batteries (f). If calculating the specific capacitance for a battery, the specific capacitance becomes extremely large (inset of f). The units are for the sake of comparison only, and the discharge current is assumed 1 A g–1. Note that these numbers are typical values for the sake of comparison rather than actual values. It can be seen as a sort of normalisation.



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Table 1. Comparing the energy storage performance of supercapacitor and battery materials with various metrics. The values are typical averages for each case derived from the literature. The cited reviews and comprehensive tables therein can be used for this purpose. Note that it is just a schematic comparison to stress how the actual rate of charging/discharging might be different from the values given by the common metrics.

Material

Aluminum

System

Applied Specific Scan rate in Specific Charging current in Potential Capacity / Capacitance voltammetry galvanostatic Duration / Range / V mAh g–1 s / F g–1 / mV s–1 / A g–1

Electrolytic Capacitor 0–20

Microporous Double layer 0–1.0 Carbon capacitor (Aqueous)

Ref.

0.001

5

55

200

100

20

10

6

Porous Carbon

Double layer capacitor (NonAqueous)

0–3.0

165

200

100

20

30

6-8

Polyaniline

Pseudocapacitor

0–0.8

110

500

10

2

80

9

Ruthenium Oxide

Pseudocapacitor

0–1.0

220

800

10

4

60

6

Germanium

Lithium-ion Battery (Anode)

0–1.5

500

1200

400

500

3.6

10

LiFePO4

Lithium-ion Battery (Cathode)

3.4–3.5

150

5400

1

2

100

11

Li4Ti4O12

Lithium-ion Battery (Anode)

1.5–1.55 150

10800

1

1

50

10,12, 13


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For Table of Contents Use Only. Graphical Abstract Specific capacitance has no use if supercapacitors are developed for energy storage. Also, power density should be normalised.



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Graphical Abstract

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Metrics for Fast Supercapacitors as Energy Storage Devices

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