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Study on specific effects of high frequency ripple currents and temperature on supercapacitors ageing
MR-11575; No of Pages 5 Microelectronics Reliability xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/mr
Study on specific effects of high frequency ripple currents and temperature on supercapacitors ageing R. German a,⁎, A. Sari a, P. Venet a, O. Briat b, J.-M. Vinassa b a b
Université de Lyon, Université Lyon 1, Laboratoire Ampère, UMR CNRS 5005, Villeurbanne F-69622, France Université de Bordeaux, IMS, UMR CNRS 5218, Talence F-33405, France
a r t i c l e
i n f o
Article history: Received 25 May 2015 Received in revised form 19 June 2015 Accepted 20 June 2015 Available online xxxx Keywords: Power devices Energy storage systems Supercapacitors Reliability Ageing
a b s t r a c t Although supercapacitors can store less energy than batteries, they are particularly appealing for hybrid vehicle applications (for example, for braking energy recovery, or stop and start systems supply…). As power networks in hybrid vehicle experience high frequency ripple currents due to power electronics components such as static converters, experiments are done to study the impact of high frequency ripple currents on supercapacitor ageing. The results suggest that high frequency ripple currents do not create new ageing mechanisms. A complementary ageing test is also carried out to study the effect of temperature on supercapacitor ageing at very low voltage. It clearly shows that a significant part of increase in supercapacitor resistance is only linked to temperature, instead of the combined effect of temperature and voltage as assumed by literature for supercapacitor ageing. Whereas tests demonstrate that capacitance loss through time is linked (for the most part) to the combined effect of voltage and temperature. Hence, this paper shows that resistance increase is only partially linked to capacitance loss. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Supercapacitors (SCs) [1], also called Electrochemical Double Layer Capacitors [2] or Ultracapacitors [3,4] are energy storage systems (ESSs) capable of delivering higher power than batteries over a wider range of temperature [5], and with a longer lifetime, than batteries [6]. These characteristics make SCs desirable for high cyclability/high power applications in railway [7] and electrical traction systems, such as personal hybrid vehicles (HVs) [8], or as a supplement for batteries [9]. Power electronics in HVs power network (such as static converters) generate high frequency ripple currents. To understand their effect on SCs ageing, tests are performed for representative frequencies existing in different applications. Selected for testing are commercial 3000Farad SCs from 3 different manufacturers built with the most common technology (carbon/carbon electrodes and organic acetonitrile/TEABF4 electrolyte). For most of the studied frequencies, (except for 1 kHz test which has been perturbed by voltage regulation problems), HF ripple current does not influence SC ageing. To demonstrate that 1 kHz should not be an exception to the observation above, a second test is performed, but at reduced voltage. In this test, HF 1 kHz ripple currents have no effect on SC ageing. This
⁎ Corresponding author. E-mail address: [email protected] (R. German).
test also allows us to draw new conclusions. SC resistance increase appears to be only linked to temperature, which is different from capacitance loss mechanisms that need both voltage and temperature. 2. SC operational mode and ageing SCs use electrostatic effect between ions and electric charges (double layer effect) to store energy (go to Fig. 7). The electrodes are porous to maximize the storage surface (thus the capacitance), and to minimize the equivalent series resistance (ESR). As the SCs use ions and porous electrodes for energy storage, they are only able to store energy at low frequency (LF) as shown in Fig. 1. SC capacitance (C100mHz) is calculated in Eq. (1) for a 100 mHz frequency and represents the energy stored in LF (at 100 mHz the capacitance is near the maximum as shown in Fig. 1). The 100 mHz ESR (R100mHz), representing the cumulative effect of connections, conduction and porosity on current circulation, is calculated in Eq. (2). −1 2 π f ImðZSC Þ f¼100mHz
ð1Þ
R100mHz ¼ ReðZSC ð2 π f ÞÞj f¼100mHz
ð2Þ
C100mHz ¼
Where ZSC is the SC impedance and f the frequency of the electrical signal. Ageing phenomena for SCs lead to a decrease in capacitance and an increase in ESR. They are the result of reactions between parasitic chemical groups at the electrode surface [10] and electrolyte [11].
http://dx.doi.org/10.1016/j.microrel.2015.06.026 0026-2714/© 2015 Elsevier Ltd. All rights reserved.
Please cite this article as: German R, et al, Study on specific effects of high frequency ripple currents and temperature on supercapacitors ageing, Microelectronics Reliability (2015), http://dx.doi.org/10.1016/j.microrel.2015.06.026
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R. German et al. / Microelectronics Reliability xxx (2015) xxx–xxx
Fig. 3. Thermocouple location on the SC package.
At regular intervals, SCs are disconnected from the DC and AC supplies and an impedance spectroscopy is carried out on each component from 10 kHz to 10 mHz.
Fig. 1. Evolution of SC capacitance with frequency.
Literature considers that those reactions are accelerated by temperature, voltage and current rate (in the case of cycle ageing) [12].
3. Experiment setup Fig. 2 presents the test bench developed for SC ageing tests. The floating (i.e. constant) constraints (USC, T) are applied by independent DC power supplies and a temperature chamber. The ripple current (IAC) is applied to the stacked SCs by a programmable AC current supply and the DC supplies are protected from ripple current by coils (L1…Ln). The temperature of SCs during ageing is monitored by a thermocouple placed on the middle of the component packaging (see Fig. 3). We tested 33 SCs from three manufacturers (A, B, C) to get a representative sample of commercial high capacitance ACN/TEABF4 SCs [13]. The tested SCs are equivalent in terms of technology and performances as shown in Table 1. The tested SCs are high capacitance (3000 F), high power (ESR b0.29 mΩ), and low voltage elements (b 2.8 V to minimize the solvent decomposition rate), which are typically designed for hybrid vehicle use. As presented in Table 2, three components of each manufacturer are aged at 2.8 V, 60 °C floating constraints (i.e. constant voltage and temperature). For a second batch of SCs (from manufacturers A, B and C), sinewave ripple currents are added to the floating constraints on two SCs by manufacturer and by investigated ripple current (see Table 2). Three particular frequencies are investigated for 2.8 V, 60 °C floating constraints with a 12 ARMS value for ripple current. The RMS value has been estimated by the industrial and academic partners of Supercal project (see Acknowledgements) based on the usual ripple rates of DC–DC converters used in hybrid vehicles. The 10 kHz frequency is in the same order of magnitude of DC–DC converters recorded in HVs power network. The 1 kHz frequency approximately corresponds to the minimum value of ZSC for the tested SCs. The 100 Hz frequency corresponds to the limit of capacitive zone of tested SCs [14] (see Fig. 1) or a full wave rectified 50 Hz signal (for example when SCs are charging). After observing the result from the above test, a second ageing test is performed for 1 kHz ripple current at 0.1 V.
4. Results 4.1. Effect of ripple current frequency [14,15] Fig. 4 presents the evolution of C100mHz and R100mHz as a function of ageing time. The ageing tests take up to 6600 h (9 months) and lead to a minimum capacitance decrease of 30%. As SCs are considered not usable after 20% for capacitance drop [13], the results are very representative of SCs lifetime. For 100 Hz and 10 kHz, the impact of ripple current on SCs is not evidently different from the (2.8 V, 60 °C) floating ageing results. The frequency also seems not to be a significant ageing acceleration factor, as the results are the same for 100 Hz and 10 kHz ripple current. The ripple current does not contribute to an increase of SC temperature as shown in Fig. 5. This is because the SCs are high current components (see Table 1) designed for a 15 °C joule effect self-heating when a 130 ARMS current is applied [16]. As the self-heating is proportional to the square of the current, the self-heating from 12 ARMS current should be negligible. Fig. 5 presents the evolution of SCs temperature during ageing tests. For 100 Hz and 10 kHz ripple current, the temperature does not increase at all before 5000 h. The slight increase of temperature (b 1.5 °C) observed for manufacturers A and B between 5000 h and 7000 h (for 10 kHz ageing test) can be related to the change in slope of the SC R100mHz parameter after 5000 h for 10 kHz ageing test (see Fig. 4) possibly indicating an effect of overageing on connections (at this stage the SCs are in extreme capacitance loss state (N−20%)). This negligible rise of temperature is a confirmation of the results presented in
Table 1 Electric characteristics of different manufacturer acetonitrile/active carbon commercial 3000 F SC. Electrode
Activated carbon
Electrolyte
Acetonitrile
Capacitance
3000 F
Manufacturer Equivalent series resistance ESR (mΩ) Nominal voltage UN (V) Limit impulse current b1 s (A) Max operating temperature Tmax (°C) Cell weight M (g) Mass energy WM (kJ/kg) Peak mass power PM b 1 s (kW/kg)
A, B, C 0.20 b ESR b 0.29 2.7 ≤ UN ≤ 2.8 ≈2000 A 60 b Tmax b 65 510 b M b 650 17.8 b WM b 21.5 8.60 b PM b 10.6
Table 2 Experiments setup.
Fig. 2. Scheme of the experimental test bench.
Floating constraints
USC = 2.8 V T = 60 °C
RMS current Ripple (IAC) Number of tested elements A, B, C
0A 3, 3, 3
12 A 100 Hz 2, 2, 2
USC = 0.1 V T = 60 °C 12 A 1 kHz 2, 2, 2
12 A 10 kHz 2, 2, 2
12 A 1 kHz 2, 2, 2
Please cite this article as: German R, et al, Study on specific effects of high frequency ripple currents and temperature on supercapacitors ageing, Microelectronics Reliability (2015), http://dx.doi.org/10.1016/j.microrel.2015.06.026
R. German et al. / Microelectronics Reliability xxx (2015) xxx–xxx
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Fig. 4. Evolution of C100mHz and R100mHz with ageing time.
Fig. 4 (100 Hz and 10 kHz). It tends to show that the HF current ripples are not a factor causing ageing acceleration for SCs. In comparison, the results obtained for 1 kHz current ripple are slightly different from the floating ageing results (for capacitance C100mHz) and even more different for R100mHz [15]. Those results could be due to a particular effect of 1 kHz frequency on gas production during ageing such as suggested in [17] or to the ageing test disturbance caused by two voltage breakdowns (during the 1 kHz test the voltage regulation system broke down twice as shown in Fig. 4). During the breakdowns voltage was uncontrolled and unrecorded leading to a succession of overvoltage and undervoltage for several hours. Before the first breakdown the SCs aged at 1 kHz are strictly ageing in the same way as floating ageing test. Manufacturer C elements were stopped because they started to leak. The second breakdown is accompanied by a sudden increase in R100mHz. The temperature also increases significantly by 3 °C. The increase of R100mHz cannot explain the temperature increase by joule effect (see manufacturer data on selfheating [16]). That seems to indicate that the voltage breakdowns lead to a modification of electrolyte and creates a few exoenergetic reactions in the electrolyte. To be sure that the differences observed between the 1 kHz and the floating ageing tests are caused by the two breakdowns but not the particular effect of 1 kHz current ripple, we decide to observe the effect, of the 1 kHz current ripple again, but this time at very low voltage to minimize ageing related to floating constraints. 4.2. Effect of temperature on SC ageing (at low voltage) To test the effect of ripple current at 1 kHz, a second ageing test of one year (8700 h) is carried out. The SCs of the three manufacturers
are placed at 60 °C in a thermal oven and maintained at very low voltage (+ 0.1 V to avoid negative voltage). As it is commonly noted by manufacturer, and verified in literature [8], an increase of the SC voltage by 200 mV results in an increase of ageing kinetic by a factor 2. Thus, at 100 mV, the ageing kinetic due to voltage is negligible (104 factors smaller compared to a 2.8 V ageing at the same temperature condition). Thus, the capacitance loss and the ESR gain must be very small. The test consists of two phases as shown in Fig. 6. For phase 1, 1 kHz/12 ARMS ripple current is applied to the SCs. Then, for phase 2 (just before 6000 h), the ripple current is stopped. Then, during phase 2, SCs are only subjected to 0.1 V, 60 °C floating conditions. Fig. 6 presents the evolution of C100mHz and R100mHz for this test. We can notice that the capacitance and the resistance evolution are not affected when the ripple current stops for phase 2. This clearly shows, that ripple currents have no particular effect, on the tested SCs ageing (as long as their RMS value small enough to not create SC major self-heating). C100mHz is hardly (b 2%) affected by the 1 year (8700 h) ageing test at 0.1 V. Thus, storing short-circuited SCs, even at high temperature, will have a very small impact on their capacitance. The evolution of R100mHz is quite different. At 6000 h, by comparing with the R100mHz increase with 2.8 V, 60 °C floating ageing results, we notice that the components of the manufacturer A and B gain respectively +40% and + 10% for R100mHz (@ 6000 h for 0.1 V, 60 °C), which represents 50% and 25% of the increase of R100mHz (@6000 h for 2.8 V, 60 °C). From that, we can conclude that temperature plays an important part in causing resistance increase (even when voltage is negligible).
Please cite this article as: German R, et al, Study on specific effects of high frequency ripple currents and temperature on supercapacitors ageing, Microelectronics Reliability (2015), http://dx.doi.org/10.1016/j.microrel.2015.06.026
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R. German et al. / Microelectronics Reliability xxx (2015) xxx–xxx
Fig. 7. Movement of charges for HF AC current.
Fig. 5. Evolution of temperature for 100 Hz, 1 kHz and 10 kHz current ripple.
This phenomenon is independent of the 1 kHz ripple current, as an important increase of R100mHz happens with, and without, ripple current (see Fig. 6). This significant increase cannot be explained by the acceleration factor observed on the capacitance loss. Consequently, we can deduce that the increase of R100mHz, is caused by several mechanisms: A part of the increase of R100mHz is influenced by the chemical reactions linked with the capacitance decrease (i.e. gas creation caused by reaction between parasitic surface group and surface group and electrolyte). Another part of the R100mHz increase is linked with totally different phenomenon linked only to temperature. As this part of resistance increase is not linked to voltage,
which is required for capacitance ageing, we can conclude that the supplementary effects causing R100mHz increase are not linked with capacitance ageing (thus they probably take place outside the electrodes porosity). We can also notice that the supplementary mechanisms for R100mHz ageing are not necessarily present for all SCs manufacturers. For example, for manufacturer C, R100mHz stays constant for the low voltage ageing test. It indicates that the increase of resistance is mainly linked to the capacitance loss for manufacturer C. Thus, the supplementary causes of R100mHz increase, are linked with porosity engineering factors, which tend to reduce the charge flow, without any relation to electrode storage surface. The first possible cause can be the degradation of the binding agent that sticks active carbon electrode and collector together. This makes the contact area smaller. The second possible cause could be that some parasitic chemicals inside the electrolyte dislocate without other reactant (zero order reaction), or react together with temperature effect leading to a degradation of electrolyte conductivity. Finally, the separator itself can also become less conductive for ions due to ageing, because of fouling and/or slow decomposition mechanisms (the SCs separator is just simply built of paper). 4.3. Interpretation of HF ripple current transparency A physical interpretation is given in Fig. 7. As described in part 2, SCs are relatively low frequency ESSs. At the ageing test frequencies (between 100 Hz and 10 kHz), we can deduce that ions are not fast enough to compensate the supplementary electric charges from the AC current (the capacitance is close to 0 F for this frequency range as shown in Fig. 1). Supplementary electric charges from one electrode negate with the supplementary charges from the other electrode. Thus, there is no supplementary formation of double layer and thus no supplementary ageing reactions between electrolyte and electrodes. This explains why SCs are not affected by high frequency current ripples. 5. Conclusion
Fig. 6. Evolution of C100 mHz and R100 mHz.
This article presents the possible effect of HF current ripple and the particular effect of temperature on SC ageing kinetic. 33 SCs from 3 different manufacturers are aged up to 12 months with or without ripple current. Three frequencies (100 Hz, 1 kHz and 10 kHz) are investigated. As these SCs are designed for around 120 ARMS current use and as the maximum ripple induced by DC–DC converters is estimated to be 10% of the mean current, the current ripple for one component is fixed to 12 ARMS.
Please cite this article as: German R, et al, Study on specific effects of high frequency ripple currents and temperature on supercapacitors ageing, Microelectronics Reliability (2015), http://dx.doi.org/10.1016/j.microrel.2015.06.026
R. German et al. / Microelectronics Reliability xxx (2015) xxx–xxx
The evolution of the electric parameters of SCs (C100mHz and R100mHz) is monitored with regular impedance spectroscopies. The temperature of SCs is also monitored during ageing. The tests reveal that HF current ripples do not have any significant effect on SC ageing. Consequently, we can conclude that the supplementary electric charges from AC current do not participate in double layer formation, because the frequency is too high for ions, to compensate the supplementary charges brought by ripple current. Thus, HF ripple currents do not impact SCs, because no new zone of double layer ageing is created. These conclusions have a great appeal to SC integrators, as they won't have to use costly coils to protect SCs from HF ripple currents (from domestic appliance (~100 Hz) to high frequency static converters (~10 kHz)). The complementary test shows that SC capacitance is hardly affected under low voltage/high temperature ageing test whereas their resistance can increase a lot. That shows that the ageing mechanisms affecting SC capacitance and SC ESR are not identical. Acknowledgement French national research agency (ANR) is supporting these research works which are part of the Supercal project. Supercal is a project held by IMS Bordeaux which combines three academic laboratories (Ampère (Lyon), IMS (Bordeaux), French institute of sciences and technology for transport, development and networks (IFSTTAR)) and manufacturers from transportation and energy storage devices sectors (PSA Peugeot Citroën, Valeo, Batscap). References [1] Ayadi M, Briat O, Lallemand R, Eddahech A, German R, Coquery G, Vinassa JM. Description of supercapacitor performance degradation rate during thermal cycling under constant voltage ageing test. Microelectron Reliab Sept.–Oct. 2014;54(9–10): 1944–8.
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[2] Kang J, Wen J, Jayaram SH, Yu A, Wang X. Development of an equivalent circuit model for electrochemical double layer capacitors (EDLCs) with distinct electrolytes. Electrochim Acta Jan. 2014;115:587–98. [3] Brouji HE, Briat O, Vinassa J-M, Bertrand N, Woirgard E. Comparison between changes of ultracapacitors model parameters during calendar life and power cycling ageing tests. Microelectron Reliab 2008;48(8):1473–8. [4] Zhang L, Wang Z, Hu X, Sun F, Dorrell DG. A comparative study of equivalent circuit models of ultracapacitors for electric vehicles. J Power Sources Jan. 2015;274:899–906. [5] Kang J, Jayaram SH, Rawlins J, Wen J. Characterization of thermal behaviors of electrochemical double layer capacitors (EDLCs) with aqueous and organic electrolytes. Electrochim Acta Oct. 2014;144:200–10. [6] Ongaro F, Saggini S, Mattavelli P. Li-ion battery-supercapacitor hybrid storage system for a long lifetime, photovoltaic-based wireless sensor network. IEEE Trans Power Electron Sep. 2012;27(9):3944–52. [7] Wang W, Cheng M, Wang Y, Zhang B, Zhu Y, Ding S, Chen W. A novel energy management strategy of onboard supercapacitor for subway applications with permanent-magnet traction system. IEEE Trans Veh Technol Jul. 2014;63(6): 2578–88. [8] German R, Sari A, Venet P, Ayadi M, Briat O, Vinassa J-M. Prediction of supercapacitors floating ageing with surface electrode interface based ageing law. Microelectron Reliab Sept.–Oct. 2014;54(9–10):1813–7. [9] Lahyani A, Venet P, Guermazi A, Troudi A. Battery/supercapacitors combination in uninterruptible power supply (UPS). IEEE Trans Power Electron 2013;28(4): 1509–22. [10] Toles CA, Marshall WE, Johns MM. Surface functional groups on acid-activated nutshell carbons. Carbon 1999;37(8):1207–14. [11] Azais P, Duclaux L, Florian P, Massiot D. Causes of supercapacitors ageing in organic electrolyte. J Power Sources 2007;171:1046–53. [12] Chaari R, Briat O, Deletage JY, Woirgard E, Vinassa J-M. How supercapacitors reach end of life criteria during calendar life and power cycling tests. Microelectron Reliab 2011;51(911):1976–9. [13] M. technologies. Product Guide, Maxwell Technologies BOOSTCAP Ultracapacitors; 2009. [14] German R, Sari A, Venet P, Briat O, Vinassa J-M. Study of Static Converters related Ripple Currents Effects on Supercapacitors Ageing within DC Networks. Presented at the Industrial Electronics (ISIE), 2014 IEEE 24rd International Symposium on, Rio de Janeiro; 2015. [15] German R, Briat O, Sari A, Venet P, Ayadi M, Zitouni Y, Vinassa J-M. Impact of high frequency current ripple on supercapacitors ageing through floating ageing tests. Microelectron Reliab 2013;53(9–11):1643–7. [16] Maxwell technology, K2 series ultracapacitors datasheet, Document number: 1015370.3.2009. [17] Meyer, Method for the production of fuel gas, 4,936,931, 1990.
Please cite this article as: German R, et al, Study on specific effects of high frequency ripple currents and temperature on supercapacitors ageing, Microelectronics Reliability (2015), http://dx.doi.org/10.1016/j.microrel.2015.06.026
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/mr
Study on specific effects of high frequency ripple currents and temperature on supercapacitors ageing R. German a,⁎, A. Sari a, P. Venet a, O. Briat b, J.-M. Vinassa b a b
Université de Lyon, Université Lyon 1, Laboratoire Ampère, UMR CNRS 5005, Villeurbanne F-69622, France Université de Bordeaux, IMS, UMR CNRS 5218, Talence F-33405, France
a r t i c l e
i n f o
Article history: Received 25 May 2015 Received in revised form 19 June 2015 Accepted 20 June 2015 Available online xxxx Keywords: Power devices Energy storage systems Supercapacitors Reliability Ageing
a b s t r a c t Although supercapacitors can store less energy than batteries, they are particularly appealing for hybrid vehicle applications (for example, for braking energy recovery, or stop and start systems supply…). As power networks in hybrid vehicle experience high frequency ripple currents due to power electronics components such as static converters, experiments are done to study the impact of high frequency ripple currents on supercapacitor ageing. The results suggest that high frequency ripple currents do not create new ageing mechanisms. A complementary ageing test is also carried out to study the effect of temperature on supercapacitor ageing at very low voltage. It clearly shows that a significant part of increase in supercapacitor resistance is only linked to temperature, instead of the combined effect of temperature and voltage as assumed by literature for supercapacitor ageing. Whereas tests demonstrate that capacitance loss through time is linked (for the most part) to the combined effect of voltage and temperature. Hence, this paper shows that resistance increase is only partially linked to capacitance loss. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Supercapacitors (SCs) [1], also called Electrochemical Double Layer Capacitors [2] or Ultracapacitors [3,4] are energy storage systems (ESSs) capable of delivering higher power than batteries over a wider range of temperature [5], and with a longer lifetime, than batteries [6]. These characteristics make SCs desirable for high cyclability/high power applications in railway [7] and electrical traction systems, such as personal hybrid vehicles (HVs) [8], or as a supplement for batteries [9]. Power electronics in HVs power network (such as static converters) generate high frequency ripple currents. To understand their effect on SCs ageing, tests are performed for representative frequencies existing in different applications. Selected for testing are commercial 3000Farad SCs from 3 different manufacturers built with the most common technology (carbon/carbon electrodes and organic acetonitrile/TEABF4 electrolyte). For most of the studied frequencies, (except for 1 kHz test which has been perturbed by voltage regulation problems), HF ripple current does not influence SC ageing. To demonstrate that 1 kHz should not be an exception to the observation above, a second test is performed, but at reduced voltage. In this test, HF 1 kHz ripple currents have no effect on SC ageing. This
⁎ Corresponding author. E-mail address: [email protected] (R. German).
test also allows us to draw new conclusions. SC resistance increase appears to be only linked to temperature, which is different from capacitance loss mechanisms that need both voltage and temperature. 2. SC operational mode and ageing SCs use electrostatic effect between ions and electric charges (double layer effect) to store energy (go to Fig. 7). The electrodes are porous to maximize the storage surface (thus the capacitance), and to minimize the equivalent series resistance (ESR). As the SCs use ions and porous electrodes for energy storage, they are only able to store energy at low frequency (LF) as shown in Fig. 1. SC capacitance (C100mHz) is calculated in Eq. (1) for a 100 mHz frequency and represents the energy stored in LF (at 100 mHz the capacitance is near the maximum as shown in Fig. 1). The 100 mHz ESR (R100mHz), representing the cumulative effect of connections, conduction and porosity on current circulation, is calculated in Eq. (2). −1 2 π f ImðZSC Þ f¼100mHz
ð1Þ
R100mHz ¼ ReðZSC ð2 π f ÞÞj f¼100mHz
ð2Þ
C100mHz ¼
Where ZSC is the SC impedance and f the frequency of the electrical signal. Ageing phenomena for SCs lead to a decrease in capacitance and an increase in ESR. They are the result of reactions between parasitic chemical groups at the electrode surface [10] and electrolyte [11].
http://dx.doi.org/10.1016/j.microrel.2015.06.026 0026-2714/© 2015 Elsevier Ltd. All rights reserved.
Please cite this article as: German R, et al, Study on specific effects of high frequency ripple currents and temperature on supercapacitors ageing, Microelectronics Reliability (2015), http://dx.doi.org/10.1016/j.microrel.2015.06.026
2
R. German et al. / Microelectronics Reliability xxx (2015) xxx–xxx
Fig. 3. Thermocouple location on the SC package.
At regular intervals, SCs are disconnected from the DC and AC supplies and an impedance spectroscopy is carried out on each component from 10 kHz to 10 mHz.
Fig. 1. Evolution of SC capacitance with frequency.
Literature considers that those reactions are accelerated by temperature, voltage and current rate (in the case of cycle ageing) [12].
3. Experiment setup Fig. 2 presents the test bench developed for SC ageing tests. The floating (i.e. constant) constraints (USC, T) are applied by independent DC power supplies and a temperature chamber. The ripple current (IAC) is applied to the stacked SCs by a programmable AC current supply and the DC supplies are protected from ripple current by coils (L1…Ln). The temperature of SCs during ageing is monitored by a thermocouple placed on the middle of the component packaging (see Fig. 3). We tested 33 SCs from three manufacturers (A, B, C) to get a representative sample of commercial high capacitance ACN/TEABF4 SCs [13]. The tested SCs are equivalent in terms of technology and performances as shown in Table 1. The tested SCs are high capacitance (3000 F), high power (ESR b0.29 mΩ), and low voltage elements (b 2.8 V to minimize the solvent decomposition rate), which are typically designed for hybrid vehicle use. As presented in Table 2, three components of each manufacturer are aged at 2.8 V, 60 °C floating constraints (i.e. constant voltage and temperature). For a second batch of SCs (from manufacturers A, B and C), sinewave ripple currents are added to the floating constraints on two SCs by manufacturer and by investigated ripple current (see Table 2). Three particular frequencies are investigated for 2.8 V, 60 °C floating constraints with a 12 ARMS value for ripple current. The RMS value has been estimated by the industrial and academic partners of Supercal project (see Acknowledgements) based on the usual ripple rates of DC–DC converters used in hybrid vehicles. The 10 kHz frequency is in the same order of magnitude of DC–DC converters recorded in HVs power network. The 1 kHz frequency approximately corresponds to the minimum value of ZSC for the tested SCs. The 100 Hz frequency corresponds to the limit of capacitive zone of tested SCs [14] (see Fig. 1) or a full wave rectified 50 Hz signal (for example when SCs are charging). After observing the result from the above test, a second ageing test is performed for 1 kHz ripple current at 0.1 V.
4. Results 4.1. Effect of ripple current frequency [14,15] Fig. 4 presents the evolution of C100mHz and R100mHz as a function of ageing time. The ageing tests take up to 6600 h (9 months) and lead to a minimum capacitance decrease of 30%. As SCs are considered not usable after 20% for capacitance drop [13], the results are very representative of SCs lifetime. For 100 Hz and 10 kHz, the impact of ripple current on SCs is not evidently different from the (2.8 V, 60 °C) floating ageing results. The frequency also seems not to be a significant ageing acceleration factor, as the results are the same for 100 Hz and 10 kHz ripple current. The ripple current does not contribute to an increase of SC temperature as shown in Fig. 5. This is because the SCs are high current components (see Table 1) designed for a 15 °C joule effect self-heating when a 130 ARMS current is applied [16]. As the self-heating is proportional to the square of the current, the self-heating from 12 ARMS current should be negligible. Fig. 5 presents the evolution of SCs temperature during ageing tests. For 100 Hz and 10 kHz ripple current, the temperature does not increase at all before 5000 h. The slight increase of temperature (b 1.5 °C) observed for manufacturers A and B between 5000 h and 7000 h (for 10 kHz ageing test) can be related to the change in slope of the SC R100mHz parameter after 5000 h for 10 kHz ageing test (see Fig. 4) possibly indicating an effect of overageing on connections (at this stage the SCs are in extreme capacitance loss state (N−20%)). This negligible rise of temperature is a confirmation of the results presented in
Table 1 Electric characteristics of different manufacturer acetonitrile/active carbon commercial 3000 F SC. Electrode
Activated carbon
Electrolyte
Acetonitrile
Capacitance
3000 F
Manufacturer Equivalent series resistance ESR (mΩ) Nominal voltage UN (V) Limit impulse current b1 s (A) Max operating temperature Tmax (°C) Cell weight M (g) Mass energy WM (kJ/kg) Peak mass power PM b 1 s (kW/kg)
A, B, C 0.20 b ESR b 0.29 2.7 ≤ UN ≤ 2.8 ≈2000 A 60 b Tmax b 65 510 b M b 650 17.8 b WM b 21.5 8.60 b PM b 10.6
Table 2 Experiments setup.
Fig. 2. Scheme of the experimental test bench.
Floating constraints
USC = 2.8 V T = 60 °C
RMS current Ripple (IAC) Number of tested elements A, B, C
0A 3, 3, 3
12 A 100 Hz 2, 2, 2
USC = 0.1 V T = 60 °C 12 A 1 kHz 2, 2, 2
12 A 10 kHz 2, 2, 2
12 A 1 kHz 2, 2, 2
Please cite this article as: German R, et al, Study on specific effects of high frequency ripple currents and temperature on supercapacitors ageing, Microelectronics Reliability (2015), http://dx.doi.org/10.1016/j.microrel.2015.06.026
R. German et al. / Microelectronics Reliability xxx (2015) xxx–xxx
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Fig. 4. Evolution of C100mHz and R100mHz with ageing time.
Fig. 4 (100 Hz and 10 kHz). It tends to show that the HF current ripples are not a factor causing ageing acceleration for SCs. In comparison, the results obtained for 1 kHz current ripple are slightly different from the floating ageing results (for capacitance C100mHz) and even more different for R100mHz [15]. Those results could be due to a particular effect of 1 kHz frequency on gas production during ageing such as suggested in [17] or to the ageing test disturbance caused by two voltage breakdowns (during the 1 kHz test the voltage regulation system broke down twice as shown in Fig. 4). During the breakdowns voltage was uncontrolled and unrecorded leading to a succession of overvoltage and undervoltage for several hours. Before the first breakdown the SCs aged at 1 kHz are strictly ageing in the same way as floating ageing test. Manufacturer C elements were stopped because they started to leak. The second breakdown is accompanied by a sudden increase in R100mHz. The temperature also increases significantly by 3 °C. The increase of R100mHz cannot explain the temperature increase by joule effect (see manufacturer data on selfheating [16]). That seems to indicate that the voltage breakdowns lead to a modification of electrolyte and creates a few exoenergetic reactions in the electrolyte. To be sure that the differences observed between the 1 kHz and the floating ageing tests are caused by the two breakdowns but not the particular effect of 1 kHz current ripple, we decide to observe the effect, of the 1 kHz current ripple again, but this time at very low voltage to minimize ageing related to floating constraints. 4.2. Effect of temperature on SC ageing (at low voltage) To test the effect of ripple current at 1 kHz, a second ageing test of one year (8700 h) is carried out. The SCs of the three manufacturers
are placed at 60 °C in a thermal oven and maintained at very low voltage (+ 0.1 V to avoid negative voltage). As it is commonly noted by manufacturer, and verified in literature [8], an increase of the SC voltage by 200 mV results in an increase of ageing kinetic by a factor 2. Thus, at 100 mV, the ageing kinetic due to voltage is negligible (104 factors smaller compared to a 2.8 V ageing at the same temperature condition). Thus, the capacitance loss and the ESR gain must be very small. The test consists of two phases as shown in Fig. 6. For phase 1, 1 kHz/12 ARMS ripple current is applied to the SCs. Then, for phase 2 (just before 6000 h), the ripple current is stopped. Then, during phase 2, SCs are only subjected to 0.1 V, 60 °C floating conditions. Fig. 6 presents the evolution of C100mHz and R100mHz for this test. We can notice that the capacitance and the resistance evolution are not affected when the ripple current stops for phase 2. This clearly shows, that ripple currents have no particular effect, on the tested SCs ageing (as long as their RMS value small enough to not create SC major self-heating). C100mHz is hardly (b 2%) affected by the 1 year (8700 h) ageing test at 0.1 V. Thus, storing short-circuited SCs, even at high temperature, will have a very small impact on their capacitance. The evolution of R100mHz is quite different. At 6000 h, by comparing with the R100mHz increase with 2.8 V, 60 °C floating ageing results, we notice that the components of the manufacturer A and B gain respectively +40% and + 10% for R100mHz (@ 6000 h for 0.1 V, 60 °C), which represents 50% and 25% of the increase of R100mHz (@6000 h for 2.8 V, 60 °C). From that, we can conclude that temperature plays an important part in causing resistance increase (even when voltage is negligible).
Please cite this article as: German R, et al, Study on specific effects of high frequency ripple currents and temperature on supercapacitors ageing, Microelectronics Reliability (2015), http://dx.doi.org/10.1016/j.microrel.2015.06.026
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R. German et al. / Microelectronics Reliability xxx (2015) xxx–xxx
Fig. 7. Movement of charges for HF AC current.
Fig. 5. Evolution of temperature for 100 Hz, 1 kHz and 10 kHz current ripple.
This phenomenon is independent of the 1 kHz ripple current, as an important increase of R100mHz happens with, and without, ripple current (see Fig. 6). This significant increase cannot be explained by the acceleration factor observed on the capacitance loss. Consequently, we can deduce that the increase of R100mHz, is caused by several mechanisms: A part of the increase of R100mHz is influenced by the chemical reactions linked with the capacitance decrease (i.e. gas creation caused by reaction between parasitic surface group and surface group and electrolyte). Another part of the R100mHz increase is linked with totally different phenomenon linked only to temperature. As this part of resistance increase is not linked to voltage,
which is required for capacitance ageing, we can conclude that the supplementary effects causing R100mHz increase are not linked with capacitance ageing (thus they probably take place outside the electrodes porosity). We can also notice that the supplementary mechanisms for R100mHz ageing are not necessarily present for all SCs manufacturers. For example, for manufacturer C, R100mHz stays constant for the low voltage ageing test. It indicates that the increase of resistance is mainly linked to the capacitance loss for manufacturer C. Thus, the supplementary causes of R100mHz increase, are linked with porosity engineering factors, which tend to reduce the charge flow, without any relation to electrode storage surface. The first possible cause can be the degradation of the binding agent that sticks active carbon electrode and collector together. This makes the contact area smaller. The second possible cause could be that some parasitic chemicals inside the electrolyte dislocate without other reactant (zero order reaction), or react together with temperature effect leading to a degradation of electrolyte conductivity. Finally, the separator itself can also become less conductive for ions due to ageing, because of fouling and/or slow decomposition mechanisms (the SCs separator is just simply built of paper). 4.3. Interpretation of HF ripple current transparency A physical interpretation is given in Fig. 7. As described in part 2, SCs are relatively low frequency ESSs. At the ageing test frequencies (between 100 Hz and 10 kHz), we can deduce that ions are not fast enough to compensate the supplementary electric charges from the AC current (the capacitance is close to 0 F for this frequency range as shown in Fig. 1). Supplementary electric charges from one electrode negate with the supplementary charges from the other electrode. Thus, there is no supplementary formation of double layer and thus no supplementary ageing reactions between electrolyte and electrodes. This explains why SCs are not affected by high frequency current ripples. 5. Conclusion
Fig. 6. Evolution of C100 mHz and R100 mHz.
This article presents the possible effect of HF current ripple and the particular effect of temperature on SC ageing kinetic. 33 SCs from 3 different manufacturers are aged up to 12 months with or without ripple current. Three frequencies (100 Hz, 1 kHz and 10 kHz) are investigated. As these SCs are designed for around 120 ARMS current use and as the maximum ripple induced by DC–DC converters is estimated to be 10% of the mean current, the current ripple for one component is fixed to 12 ARMS.
Please cite this article as: German R, et al, Study on specific effects of high frequency ripple currents and temperature on supercapacitors ageing, Microelectronics Reliability (2015), http://dx.doi.org/10.1016/j.microrel.2015.06.026
R. German et al. / Microelectronics Reliability xxx (2015) xxx–xxx
The evolution of the electric parameters of SCs (C100mHz and R100mHz) is monitored with regular impedance spectroscopies. The temperature of SCs is also monitored during ageing. The tests reveal that HF current ripples do not have any significant effect on SC ageing. Consequently, we can conclude that the supplementary electric charges from AC current do not participate in double layer formation, because the frequency is too high for ions, to compensate the supplementary charges brought by ripple current. Thus, HF ripple currents do not impact SCs, because no new zone of double layer ageing is created. These conclusions have a great appeal to SC integrators, as they won't have to use costly coils to protect SCs from HF ripple currents (from domestic appliance (~100 Hz) to high frequency static converters (~10 kHz)). The complementary test shows that SC capacitance is hardly affected under low voltage/high temperature ageing test whereas their resistance can increase a lot. That shows that the ageing mechanisms affecting SC capacitance and SC ESR are not identical. Acknowledgement French national research agency (ANR) is supporting these research works which are part of the Supercal project. Supercal is a project held by IMS Bordeaux which combines three academic laboratories (Ampère (Lyon), IMS (Bordeaux), French institute of sciences and technology for transport, development and networks (IFSTTAR)) and manufacturers from transportation and energy storage devices sectors (PSA Peugeot Citroën, Valeo, Batscap). References [1] Ayadi M, Briat O, Lallemand R, Eddahech A, German R, Coquery G, Vinassa JM. Description of supercapacitor performance degradation rate during thermal cycling under constant voltage ageing test. Microelectron Reliab Sept.–Oct. 2014;54(9–10): 1944–8.
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Please cite this article as: German R, et al, Study on specific effects of high frequency ripple currents and temperature on supercapacitors ageing, Microelectronics Reliability (2015), http://dx.doi.org/10.1016/j.microrel.2015.06.026