- Email: [email protected]
Calendar and cycling ageing of activated carbon supercapacitor for automotive application
Microelectronics Reliability 52 (2012) 2477–2481
Contents lists available at SciVerse ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Calendar and cycling ageing of activated carbon supercapacitor for automotive application H. Gualous a,⇑, R. Gallay b, M. Al Sakka c, A. Oukaour a, B. Tala-Ighil a, B. Boudart a a
LUSAC, université de Caen Basse Normandie, Rue Louis Aragon, BP 78, 50130 Cherbourg-Octeville, France Garmanage, Clos-Besson, 6 CH-1728 Farvagny-le-Petit, Switzerland c Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium b
a r t i c l e
i n f o
Article history: Received 3 June 2012 Accepted 22 June 2012 Available online 19 July 2012
a b s t r a c t This paper presents supercapacitor ageing as a function of voltage, temperature and charge/discharge cycling solicitations. To investigate their effects, a test bench of accelerated supercapacitor calendar and cycling ageing was carried out. Experimental tests are performed at constant temperature when the supercapacitors are polarized at the maximum voltage in the case of calendar ageing. In the case of cycling test, supercapacitors are charged and discharged at constant current. It can be noted that when the supercapacitors are charged and discharged at constant current the temperature of the supercapacitor increases and that the average voltage is not zero. To quantify the supercapacitor ageing, the equivalent series resistance (ESR) and the equivalent capacitance (C) are measured. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Supercapacitors are used in numerous applications. They are used to provide peak power demands or to store braking energy in automotive applications for example. In such applications, supercapacitors long lifetime is required. The first step in the sizing of an energy storage system is to choose the technology. For high power, high cycling or/and maintenance free applications, supercapacitors are the favorite. For high energy applications, batteries are preferred. The supercapacitor capacitance increases slightly with the temperature and in a more important proportion with the applied voltage. It decreases in the course of time when a solicitation, which may be either a continuous voltage or charges/discharges cycles, is applied. The series resistance decreases with the temperature and increases with the ageing time. The capacitance and the series resistance will also decrease as a function of the charge/discharge current, in other word as a function of the frequency or cycling speed. This paper deals with BCAP0310 supercapacitor ageing by using a cycle of charge/discharge at constant current. Experimental results are presented and analyzed. 2. Supercapacitor lifetime The lifetime of an energy storage component is the time required to fail. The failure is defined as the lack of ability of a com⇑ Corresponding author. E-mail address: [email protected] (H. Gualous). 0026-2714/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.microrel.2012.06.099
ponent to fulfill its specified function. One of its characteristics will be out of the specification. For example the capacitance will be below its specified limit value, or the series resistance will be above its specified value, the component will be leaking, or will be opened. The lifetime is a statistical value which gives the best estimate for the service life based on the Weibull theory. A detailed presentation of the Weibull failure statistic theory is available in [1]. The Survivor function F(t) is the number of elements of the statistical sample which have not failed or lost their function at time t and are still working.
FðtÞ ¼ exp½ðko tÞp
ð1Þ
The failure rate k(t) is given in FIT (Failure In Time) which is the number of failures occurring during 10E9 h of working of 1 object. k(t) and k0 must not be confused. The latter is a constant (independent of time, but dependent on the temperature and the voltage) which corresponds to the inverse of the time necessary for 63% of the samples to fail. k(t) is the inverse of the Mean Time Between Failure (MTBF). In the Wear Out region k(t) is increasing with the time. The manufacturer specifications give the maximum value of k(t) within the announced lifetime: 50 FIT and 10 years of lifetime expectancy for example. 2.1. Failure modes The supercapacitors are concerned with typical two Failure Modes which are the ‘‘Early Failures’’ and the ‘‘Wear Out Failures’’. They are reflected in the well known curve as the ‘‘Bathtub’’ curve.
2478
H. Gualous et al. / Microelectronics Reliability 52 (2012) 2477–2481
In the early failures mode, at the beginning of the component usage, the failure rate is rapidly decreasing with time. The main ‘‘early’’ failures are screened by routine tests at the manufacturer or customer sites to avoid non-quality costs. For this reason we do not spend any time to adjust a failure mode model to estimate the aging. They are due to design and process weaknesses which have not been detected by the design and process FMEA performed during the development. They may also be due to production process variations or to material quality changes. The process variations are due to tool wear, operator changes or lack of formation. Early life failures, random failures and end of life failures can all be modeled by Weibull functions, and they all are contributions to the bathtub curve [11]. In the wear out failure mode the supercapacitors main failure mechanism is the container opening due to an internal overpressure [2,3]. The voltage and the temperature accelerate the build up of a gas pressure due to the electrolyte decomposition inside the cell. This pressure build up increases with the working time. When the pressure reaches a determined limit, a mechanical weak point designed on the container (a mechanical fuse), generally a groove on the can wall or a pressure relief on the lead, may open softly, avoiding an explosion of the device. The electrical functions of the supercapacitor are still working. The capacitance loss speed increases a little bit. Some traces of the electrolyte salt are visible close to the aperture. Solvent vapors are released from the component. For this reason it is required that supercapacitor modules are ventilated. In case of opening the component must be replaced. In the case of electrolyte leakage [4], the consequences are almost the same as in the previous case: the piece must be replaced. The origin of this failure may be due to internal overpressure, bad seal position, weak welding, perforation of the wall during electrode contacting, etc. The detection of leakage is sometime difficult. It may occur that some residual electrolyte is trapped in some area difficult of access, during the component production, and becomes apparent in the application. The supercapacitors failure modes may be, for example: (a) Cell container opening due to an internal overpressure [2]. The voltage and the temperature generate a gas pressure inside the cell which increases with the working time. When the pressure reaches a determined limit, a mechanical fuse, generally a groove on the can wall or a pressure relief, may open softly, avoiding an explosion of the device. (b) More than 20% loss of capacitance. The accessible carbon surface and the ions availability are reduced during the electrochemical cycling. (c) More than 100% increase of the ESR. The electrode adhesion on the collector is weakening with time and temperature. The ion availability is reduced. In these examples, the given numerical values (20% for the capacitance and 100% for the series resistance) are subject to be adapted for other particular applications, for which requirements may be different. To present the observed statistic of the cell opening, a special ‘‘Weibull’’ scale (see Fig. 1) is used to verify a straight line in the case of Weibull failure process. The steeper the slope is with large value of p, the more the process is under control. In that case almost all the failures occur during a short time period just before the given lifetime expectancy limit. The measurements presented in Fig. 1 have been performed on 14 supercapacitors pieces BCAP0140 (140 F, 7 m X) at 70 °C, with an applied dc voltage of 2.85 V. In this example the failure of one piece represents a loss of 7.1% of the tested samples. The first point in the bottom of the curve is given by the opening of the first cell after 250 h.
Fig. 1. Supercapacitor Weibull overpressure failure statistic during a 2.85 Vdc voltage solicitation at 70 °C. The power factor p = 4.5 and the mean lifetime 1/ k0 = 440 h.
2.2. Temperature and voltage as an ageing acceleration factor For each of the failure mode a statistical model must be developed. This is usually achieved by performing accelerated tests at different temperatures and different voltages, at constant voltage or in charge/discharge cycling. The charge/discharge cycling experiments have the disadvantage to require a lot of power which consequently limits the number of samples which can be tested. To get an estimation of the time required to reach 20% of electrode capacitance loss, the coefficient k0 and p must be determined for all the operating temperatures and for all the operating voltages. They are measured for a set of discrete values (50 °C, 60 °C, 70 °C, 2.3 Vdc, 2.5 Vdc, 2.7 Vdc, and 2.9 Vdc for example), and then, the coefficient k0 and p for the other temperatures and voltages are calculated using the following solicitation ratio:
n t1 V2 Ea 1 1 ¼ exp t2 V1 k T 1abs T 2abs
ð2Þ
In this relation t1 is the lifetime at the temperature T1 and voltage V1, t2 is the lifetime at the temperature T2 and voltage V2, Ea is the activation energy determined by the experimental data, k is the Boltzmann constant and n is a constant determined experimentally. An extensive study has been performed to demonstrate a general approach to assess electrochemical capacitor reliability as a function of operating conditions on commercial capacitor cells [5,4]. An Arrhenius law is used for the temperature dependency, while an inverse power law is used for the voltage dependency. Some electronic apparatus concepts are already available to estimate in situ the double-layer capacitor residual life by monitoring the temperature and voltage constraints of the application [6]. Supercapacitor capacitance lifetime expectancies (calendar ageing) are displayed in Fig. 2 as a function of the temperature for different values of the applied dc voltage. It is noteworthy that when the voltage is close to the electrochemical decomposition voltage there is an acceleration of the degradation phenomena, especially in the higher temperature domain. The challenge in most industrial applications is to size the storage system as small as possible, both for cost-saving and available volume reasons. To optimize the dimensions of the components and setting the reserve to a minimum value all the available field data, which allow fitting precisely the actual application requirements, must be considered. Practically the duration, temperature, and voltage solicitation stresses must be individually estimated. An equivalent stress weight is calculated for each contribution, based on the ‘‘derating’’ Eq. (2) given above. The lifetime expectation is evaluated by summing all the estimated contributions. It is also interesting to determine not only the time necessary to get a capacitance drop of 20%, but also the curve shape of capacitance drop and series resistance increase as a function of the time.
H. Gualous et al. / Microelectronics Reliability 52 (2012) 2477–2481
2479
case of a supercapacitor, this type of measurement is disturbed by the self-discharge of the cells which makes the result interpretation difficult because of the voltage variation during the experiment.
3. DC voltage test (calendar ageing)
Fig. 2. Supercapacitor capacitance lifetime expectation as a function of the temperature for different voltages. The supercapacitor is considered as failed when its capacitance drops below 80% of its nominal value.
The capacitance drop is not linear. The experimental work has shown that 3 phases may be distinguished on the capacitance drop curve. (1) A first phase which lasts between 1 and 12 h, depending on temperature and voltage, is in fact a measurement effect. It is caused by the charging of the electrode areas which have no counter electrode in their neighborhood. The ions need a long time to reach these distant areas. This leads to a discharge of the charges initially located on the paired electrode towards the unpaired electrode areas. (2) The second capacitance fading process is exponential. In the voltage and temperature conditions, the time constant is in the range of 2000 h. This part of the process may be attributed to a potential window shift combined with the ageing due to the construction asymmetry. (3) The third one occurs after an approximate drop of the capacitance of 15–20%. It appears as linear, but may also be exponential with a long time constant. It is not possible to find out in the limited experimental time. The attribution of the second and third process to a physical phenomenon is not well established. An attempt to correlate the exponential decay with leakage current measurement has been presented in the literature [7]. The increase of the equivalent series resistance is linear with the time. The values measured during the ageing experiment at 50 °C and 2.5 V are aligned on a line which slope is equal to 7.5 104 [%/h]. Charging and discharging create mechanical stresses in the electrode. It has been shown that the application of a voltage induces a reversible expansion of the electrode [8,9]. This mechanical motion, especially in the case of ionic insertion in the electrode, is known to be one of the origins of ageing in the battery domain. 2.2.1. Testing The supercapacitor reliability is estimated by means of different electrical tests which gives complementary information. The manufacturers are using two test types: ‘‘dc voltage test’’ and the ‘‘voltage cycling test’’. Calendar life testing is often mentioned in the literature [10]. This is a test which has its origin in the battery domain. The cells are prepared in different states of discharge (SOD) and stressed with different temperatures. The cell functionalities as the resistance and the capacitance are measured periodically with well defined charge/discharge conditions. Between the measurements the cells are floating: they are not connected to a power supply. In the
To carry out the accelerated calendar ageing, the supercapacitors were placed in a climatic chamber. They were polarized at different voltage value during the ageing process. According to the boiling point temperature of the electrolyte which is about 81.6 °C (at atmospheric pressure), the temperature values were selected in order to make it possible to observe the ageing rather quickly. Supercapacitors ageing were carried out following several phases. The measurements done to characterize ageing are sensitive to the polarization history. In particular the first measurement, used as the reference initial value, is obtained starting from a non-polarized state. In contrast, all the other subsequent values are obtained starting the measurement from a nominal voltage polarization state. At the beginning of the polarization the electrode areas which are difficult to access for the ions, like unpaired electrode area, are not charged because of the long time constant due to the high series resistance of the long ion path. During the capacitance measurement current is flowing from charged area towards uncharged area. The incidence on the measurement interpretation is a capacitance increase if the measurement is performed during the charge, a capacitance decrease if the measurement is performed during the discharge as the standards are requiring, and a series resistance increase. After about 24–48 h the supercapacitor may be considered in a steady state and the measured evolution start to be monotone. The capacitance and ESR of a BCAP0310_P supercapacitor manufactured by Maxwell Technologies have been measured periodically during a DC life test. The capacitor is expected to stand 100,000 h at 40 °C and 2.7 Vdc. In our test the supercapacitor was in a climatic room at the temperature of 65 °C with a continuous applied voltage of 2.7 Vdc. The supercapacitor has been polarized previously during 24 h at 65 °C before to start the experiment. The initial capacitance was 322 F and the initial ESR was 2.1 m X. The capacitance and the ESR are then measured once a week during a charge and during a discharge applied to perform the measurement. The ESR is measured at the beginning of the charge at 1.35 Vdc and at the beginning of the discharge at 2.7 Vdc. All the measurements are performed with a constant current of 10 A. The obtained results are presented in Fig. 3. If the test is interrupted during a long time, an increase of measured capacitance is observed. The increase is reversely exponential with the interruption time. When resuming the test, the
Fig. 3. Capacitance and ESR during a DC life test on a BCAP0310_P at 2.7 Vdc and 65 °C.
2480
H. Gualous et al. / Microelectronics Reliability 52 (2012) 2477–2481
Fig. 4. BCAP0350 lifetime experiment. The capacitance is recorded as a function of the duration of a constant voltage polarization of 2.8 Vdc and a constant temperature of 55, 65 and 70 °C. Fig. 6. BCAP0310_P capacitance and ESR evolution when the supercapacitor is continuously charged and discharged with a current of 20 A.
Fig. 5. BCAP0350 lifetime experiment. The capacitance is recorded as a function of the duration of a constant temperature 65 °C and constant dc voltage polarization of 2.8, 2.9 and 3 V.
capacitance decreases very fast to reach the value before the interruption. A calendar ageing of a BCAP0350 supercapacitor is carried out. The supercapacitor is aged at 2.8 V and a constant temperature of 55; 65 and 70 °C. Fig. 4 shows the supercapacitors equivalent capacitance variations according to the time during ageing process. These results show that the supercapacitor equivalent capacitance decreases according to the ageing time. This effect is accelerated when the temperature increases. The BCAP0350 supercapacitors were also tested at 65 °C and for 2.8, 2.9 and 3 Vdc voltage. Fig. 5 represents the experimental results of the equivalent capacitance variations as a function of ageing duration. These results show the effect of the dc voltage on the supercapacitor ageing. 4. Voltage cycling test Voltage cycling requires more powerful and expensive equipments. This limits the number of samples which may be studied. This method presents the advantage that the capacitance and the ESR may be measured in continuous during the experiment. One disadvantage is that it is difficult to have a control over the component temperature because it depends on both the current and the component thermal resistance. For small current the ageing during voltage cycling is equivalent to the ageing during a dc voltage solicitation if: (1) the internal component temperatures are the same, and (2) the ‘‘mean’’ voltage obtained by pondering the voltage classes visited during the cycling with the power law presented above (Eq. (2)) are the same.
The voltage cycling consists to charge and discharge the supercapacitor at constant current. In this experiment type, it is necessary to take into account the temperature increase due to the Joule losses of the current in the supercapacitor. From a practical point of view the increase of current intensity increases the number of cycles performed in a given time. It increases also the component internal temperature. The increase of rest time between the charge and discharge increases the time spent at high voltage and reduces the number of cycles performed in a given time. The experimental data have been measured on a BCAP0310_P manufactured by Maxwell Technologies in Rossens Switzerland at room temperature between 20 and 22 °C. The supercapacitor has been cycled between 1.25 and 2.5 Vdc without rest time with a current of 62 A. The current intensity has been chosen in order to get a component temperature of 65 °C. The nominal capacitance (310 F) and the nominal ESR (1.7 m X are specified at room temperature and at 1 Hz. The capacitor is expected to stand 1,000,000 cycles at 10 A. The capacitance and the ESR have been reported in Fig. 6 as a function of the voltage cycles number. They are measured with an impedance spectrometer at 10 mHz. The initial capacitance value was equal to 319 F while the initial ESR value was 2.9 m X. During the experiment the ESR has increased by a factor of 20%. The capacitance has decreased of 15% of the initial value. With a cycle time of 25 s the experiment has lasted about 1000 h. A simple calculation shows that for cycling the voltage cell between 1.25 V and 2.5 V without rest time, the equivalent continuous voltage would be 2.25 Vdc. With large current the voltage drops (0.120 V with 2 m X and 62 A) which occur during the current interruption are important, so that the cell almost never see the high voltage limit. An additional ageing due to the current intensity is expected to explain the relative important ageing found in the measurement. 5. Conclusion This paper presents supercapacitors ageing at constant temperature and constant voltage and charge/discharge cycling. The experimental results show that the supercapacitors calendar ageing is accelerated by the temperature and the voltage. The supercapacitors ageing were quantified during the tests in terms of series resistance and capacitance. References [1] Gallay R, Gualous H. Industrial production of double-layer capacitors. In: François Béguin, Elzbieta Frackowiak, editors. Carbons for electrochemical
H. Gualous et al. / Microelectronics Reliability 52 (2012) 2477–2481 energy storage and conversion systems. CRC Press. p. 430–68 [chapter 11]. [2] Kötz R, Ruch PW, Cericola D. Ageing and failure mode of electrochemical double layer capacitors during accelerated constant load tests. J Power Sources 2010;195:923–8. [3] Hahn M, Kötz R, Gallay R, Siggel A. Pressure evolution in propylene carbonate based electrochemical double layer capacitors. Electrochim Acta 2006;52:1709–12. [4] Miller JR, Klementov A, Butler S. Electrochemical capacitor reliability in heavy hybrid vehicles. In: Proceedings of 16th International Seminar On Double Layer Capacitors and Similar Energy Storage Devices, Deerfield Beach, USA; 2006. p. 218. [5] Goltser I, Butler S, Miller JR. Reliability assessment of electrochemical capacitors: method demonstration using 1-F commercial components. In: Proceedings of 15th International Seminar On Double Layer Capacitors and Similar Energy Storage Devices, Deerfield Beach, USA; 2005. p. 215. [6] Yurgil J. General motors corporation. Ultracapacitor useful life prediction. US patent /2006/012378.
2481
[7] Kötz R, Hahn M, Gallay R. Temperature behaviour and impedance fundamentals of supercapacitors. J Power Sources 2006;154:550–5. [8] Hardwick L, Hahn M, Ruch P, Holzapfel M, Scheifele W, Buqa H, et al. An in situ Raman study of the intercalation of supercapacitor-type electrolyte into microcrystalline graphite. Electrochim Acta 2006;52:675–80. [9] Hahn M, Barbieri O, Gallay R, Kötz R. A dilatometric study of the voltage limitation of carbonaceous electrodes in aprotic EDLC type electrolytes by charge-induced strain. Carbon 2006;44:2523–33. [10] Mizutani Y, Okamoto T, Taguchi T, Nakajima K, Tanaka K. Life expectancy and degradation behavior of Electric double layer capacitor. In: Proceedings of 7th International Conference on properties and applications of dielectric materials, Nagoya; June 2003. [11] How to define the adequate reliability requirement for a power electronic system? Power Electr Europe. 2012; No. 1.
Contents lists available at SciVerse ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Calendar and cycling ageing of activated carbon supercapacitor for automotive application H. Gualous a,⇑, R. Gallay b, M. Al Sakka c, A. Oukaour a, B. Tala-Ighil a, B. Boudart a a
LUSAC, université de Caen Basse Normandie, Rue Louis Aragon, BP 78, 50130 Cherbourg-Octeville, France Garmanage, Clos-Besson, 6 CH-1728 Farvagny-le-Petit, Switzerland c Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium b
a r t i c l e
i n f o
Article history: Received 3 June 2012 Accepted 22 June 2012 Available online 19 July 2012
a b s t r a c t This paper presents supercapacitor ageing as a function of voltage, temperature and charge/discharge cycling solicitations. To investigate their effects, a test bench of accelerated supercapacitor calendar and cycling ageing was carried out. Experimental tests are performed at constant temperature when the supercapacitors are polarized at the maximum voltage in the case of calendar ageing. In the case of cycling test, supercapacitors are charged and discharged at constant current. It can be noted that when the supercapacitors are charged and discharged at constant current the temperature of the supercapacitor increases and that the average voltage is not zero. To quantify the supercapacitor ageing, the equivalent series resistance (ESR) and the equivalent capacitance (C) are measured. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Supercapacitors are used in numerous applications. They are used to provide peak power demands or to store braking energy in automotive applications for example. In such applications, supercapacitors long lifetime is required. The first step in the sizing of an energy storage system is to choose the technology. For high power, high cycling or/and maintenance free applications, supercapacitors are the favorite. For high energy applications, batteries are preferred. The supercapacitor capacitance increases slightly with the temperature and in a more important proportion with the applied voltage. It decreases in the course of time when a solicitation, which may be either a continuous voltage or charges/discharges cycles, is applied. The series resistance decreases with the temperature and increases with the ageing time. The capacitance and the series resistance will also decrease as a function of the charge/discharge current, in other word as a function of the frequency or cycling speed. This paper deals with BCAP0310 supercapacitor ageing by using a cycle of charge/discharge at constant current. Experimental results are presented and analyzed. 2. Supercapacitor lifetime The lifetime of an energy storage component is the time required to fail. The failure is defined as the lack of ability of a com⇑ Corresponding author. E-mail address: [email protected] (H. Gualous). 0026-2714/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.microrel.2012.06.099
ponent to fulfill its specified function. One of its characteristics will be out of the specification. For example the capacitance will be below its specified limit value, or the series resistance will be above its specified value, the component will be leaking, or will be opened. The lifetime is a statistical value which gives the best estimate for the service life based on the Weibull theory. A detailed presentation of the Weibull failure statistic theory is available in [1]. The Survivor function F(t) is the number of elements of the statistical sample which have not failed or lost their function at time t and are still working.
FðtÞ ¼ exp½ðko tÞp
ð1Þ
The failure rate k(t) is given in FIT (Failure In Time) which is the number of failures occurring during 10E9 h of working of 1 object. k(t) and k0 must not be confused. The latter is a constant (independent of time, but dependent on the temperature and the voltage) which corresponds to the inverse of the time necessary for 63% of the samples to fail. k(t) is the inverse of the Mean Time Between Failure (MTBF). In the Wear Out region k(t) is increasing with the time. The manufacturer specifications give the maximum value of k(t) within the announced lifetime: 50 FIT and 10 years of lifetime expectancy for example. 2.1. Failure modes The supercapacitors are concerned with typical two Failure Modes which are the ‘‘Early Failures’’ and the ‘‘Wear Out Failures’’. They are reflected in the well known curve as the ‘‘Bathtub’’ curve.
2478
H. Gualous et al. / Microelectronics Reliability 52 (2012) 2477–2481
In the early failures mode, at the beginning of the component usage, the failure rate is rapidly decreasing with time. The main ‘‘early’’ failures are screened by routine tests at the manufacturer or customer sites to avoid non-quality costs. For this reason we do not spend any time to adjust a failure mode model to estimate the aging. They are due to design and process weaknesses which have not been detected by the design and process FMEA performed during the development. They may also be due to production process variations or to material quality changes. The process variations are due to tool wear, operator changes or lack of formation. Early life failures, random failures and end of life failures can all be modeled by Weibull functions, and they all are contributions to the bathtub curve [11]. In the wear out failure mode the supercapacitors main failure mechanism is the container opening due to an internal overpressure [2,3]. The voltage and the temperature accelerate the build up of a gas pressure due to the electrolyte decomposition inside the cell. This pressure build up increases with the working time. When the pressure reaches a determined limit, a mechanical weak point designed on the container (a mechanical fuse), generally a groove on the can wall or a pressure relief on the lead, may open softly, avoiding an explosion of the device. The electrical functions of the supercapacitor are still working. The capacitance loss speed increases a little bit. Some traces of the electrolyte salt are visible close to the aperture. Solvent vapors are released from the component. For this reason it is required that supercapacitor modules are ventilated. In case of opening the component must be replaced. In the case of electrolyte leakage [4], the consequences are almost the same as in the previous case: the piece must be replaced. The origin of this failure may be due to internal overpressure, bad seal position, weak welding, perforation of the wall during electrode contacting, etc. The detection of leakage is sometime difficult. It may occur that some residual electrolyte is trapped in some area difficult of access, during the component production, and becomes apparent in the application. The supercapacitors failure modes may be, for example: (a) Cell container opening due to an internal overpressure [2]. The voltage and the temperature generate a gas pressure inside the cell which increases with the working time. When the pressure reaches a determined limit, a mechanical fuse, generally a groove on the can wall or a pressure relief, may open softly, avoiding an explosion of the device. (b) More than 20% loss of capacitance. The accessible carbon surface and the ions availability are reduced during the electrochemical cycling. (c) More than 100% increase of the ESR. The electrode adhesion on the collector is weakening with time and temperature. The ion availability is reduced. In these examples, the given numerical values (20% for the capacitance and 100% for the series resistance) are subject to be adapted for other particular applications, for which requirements may be different. To present the observed statistic of the cell opening, a special ‘‘Weibull’’ scale (see Fig. 1) is used to verify a straight line in the case of Weibull failure process. The steeper the slope is with large value of p, the more the process is under control. In that case almost all the failures occur during a short time period just before the given lifetime expectancy limit. The measurements presented in Fig. 1 have been performed on 14 supercapacitors pieces BCAP0140 (140 F, 7 m X) at 70 °C, with an applied dc voltage of 2.85 V. In this example the failure of one piece represents a loss of 7.1% of the tested samples. The first point in the bottom of the curve is given by the opening of the first cell after 250 h.
Fig. 1. Supercapacitor Weibull overpressure failure statistic during a 2.85 Vdc voltage solicitation at 70 °C. The power factor p = 4.5 and the mean lifetime 1/ k0 = 440 h.
2.2. Temperature and voltage as an ageing acceleration factor For each of the failure mode a statistical model must be developed. This is usually achieved by performing accelerated tests at different temperatures and different voltages, at constant voltage or in charge/discharge cycling. The charge/discharge cycling experiments have the disadvantage to require a lot of power which consequently limits the number of samples which can be tested. To get an estimation of the time required to reach 20% of electrode capacitance loss, the coefficient k0 and p must be determined for all the operating temperatures and for all the operating voltages. They are measured for a set of discrete values (50 °C, 60 °C, 70 °C, 2.3 Vdc, 2.5 Vdc, 2.7 Vdc, and 2.9 Vdc for example), and then, the coefficient k0 and p for the other temperatures and voltages are calculated using the following solicitation ratio:
n t1 V2 Ea 1 1 ¼ exp t2 V1 k T 1abs T 2abs
ð2Þ
In this relation t1 is the lifetime at the temperature T1 and voltage V1, t2 is the lifetime at the temperature T2 and voltage V2, Ea is the activation energy determined by the experimental data, k is the Boltzmann constant and n is a constant determined experimentally. An extensive study has been performed to demonstrate a general approach to assess electrochemical capacitor reliability as a function of operating conditions on commercial capacitor cells [5,4]. An Arrhenius law is used for the temperature dependency, while an inverse power law is used for the voltage dependency. Some electronic apparatus concepts are already available to estimate in situ the double-layer capacitor residual life by monitoring the temperature and voltage constraints of the application [6]. Supercapacitor capacitance lifetime expectancies (calendar ageing) are displayed in Fig. 2 as a function of the temperature for different values of the applied dc voltage. It is noteworthy that when the voltage is close to the electrochemical decomposition voltage there is an acceleration of the degradation phenomena, especially in the higher temperature domain. The challenge in most industrial applications is to size the storage system as small as possible, both for cost-saving and available volume reasons. To optimize the dimensions of the components and setting the reserve to a minimum value all the available field data, which allow fitting precisely the actual application requirements, must be considered. Practically the duration, temperature, and voltage solicitation stresses must be individually estimated. An equivalent stress weight is calculated for each contribution, based on the ‘‘derating’’ Eq. (2) given above. The lifetime expectation is evaluated by summing all the estimated contributions. It is also interesting to determine not only the time necessary to get a capacitance drop of 20%, but also the curve shape of capacitance drop and series resistance increase as a function of the time.
H. Gualous et al. / Microelectronics Reliability 52 (2012) 2477–2481
2479
case of a supercapacitor, this type of measurement is disturbed by the self-discharge of the cells which makes the result interpretation difficult because of the voltage variation during the experiment.
3. DC voltage test (calendar ageing)
Fig. 2. Supercapacitor capacitance lifetime expectation as a function of the temperature for different voltages. The supercapacitor is considered as failed when its capacitance drops below 80% of its nominal value.
The capacitance drop is not linear. The experimental work has shown that 3 phases may be distinguished on the capacitance drop curve. (1) A first phase which lasts between 1 and 12 h, depending on temperature and voltage, is in fact a measurement effect. It is caused by the charging of the electrode areas which have no counter electrode in their neighborhood. The ions need a long time to reach these distant areas. This leads to a discharge of the charges initially located on the paired electrode towards the unpaired electrode areas. (2) The second capacitance fading process is exponential. In the voltage and temperature conditions, the time constant is in the range of 2000 h. This part of the process may be attributed to a potential window shift combined with the ageing due to the construction asymmetry. (3) The third one occurs after an approximate drop of the capacitance of 15–20%. It appears as linear, but may also be exponential with a long time constant. It is not possible to find out in the limited experimental time. The attribution of the second and third process to a physical phenomenon is not well established. An attempt to correlate the exponential decay with leakage current measurement has been presented in the literature [7]. The increase of the equivalent series resistance is linear with the time. The values measured during the ageing experiment at 50 °C and 2.5 V are aligned on a line which slope is equal to 7.5 104 [%/h]. Charging and discharging create mechanical stresses in the electrode. It has been shown that the application of a voltage induces a reversible expansion of the electrode [8,9]. This mechanical motion, especially in the case of ionic insertion in the electrode, is known to be one of the origins of ageing in the battery domain. 2.2.1. Testing The supercapacitor reliability is estimated by means of different electrical tests which gives complementary information. The manufacturers are using two test types: ‘‘dc voltage test’’ and the ‘‘voltage cycling test’’. Calendar life testing is often mentioned in the literature [10]. This is a test which has its origin in the battery domain. The cells are prepared in different states of discharge (SOD) and stressed with different temperatures. The cell functionalities as the resistance and the capacitance are measured periodically with well defined charge/discharge conditions. Between the measurements the cells are floating: they are not connected to a power supply. In the
To carry out the accelerated calendar ageing, the supercapacitors were placed in a climatic chamber. They were polarized at different voltage value during the ageing process. According to the boiling point temperature of the electrolyte which is about 81.6 °C (at atmospheric pressure), the temperature values were selected in order to make it possible to observe the ageing rather quickly. Supercapacitors ageing were carried out following several phases. The measurements done to characterize ageing are sensitive to the polarization history. In particular the first measurement, used as the reference initial value, is obtained starting from a non-polarized state. In contrast, all the other subsequent values are obtained starting the measurement from a nominal voltage polarization state. At the beginning of the polarization the electrode areas which are difficult to access for the ions, like unpaired electrode area, are not charged because of the long time constant due to the high series resistance of the long ion path. During the capacitance measurement current is flowing from charged area towards uncharged area. The incidence on the measurement interpretation is a capacitance increase if the measurement is performed during the charge, a capacitance decrease if the measurement is performed during the discharge as the standards are requiring, and a series resistance increase. After about 24–48 h the supercapacitor may be considered in a steady state and the measured evolution start to be monotone. The capacitance and ESR of a BCAP0310_P supercapacitor manufactured by Maxwell Technologies have been measured periodically during a DC life test. The capacitor is expected to stand 100,000 h at 40 °C and 2.7 Vdc. In our test the supercapacitor was in a climatic room at the temperature of 65 °C with a continuous applied voltage of 2.7 Vdc. The supercapacitor has been polarized previously during 24 h at 65 °C before to start the experiment. The initial capacitance was 322 F and the initial ESR was 2.1 m X. The capacitance and the ESR are then measured once a week during a charge and during a discharge applied to perform the measurement. The ESR is measured at the beginning of the charge at 1.35 Vdc and at the beginning of the discharge at 2.7 Vdc. All the measurements are performed with a constant current of 10 A. The obtained results are presented in Fig. 3. If the test is interrupted during a long time, an increase of measured capacitance is observed. The increase is reversely exponential with the interruption time. When resuming the test, the
Fig. 3. Capacitance and ESR during a DC life test on a BCAP0310_P at 2.7 Vdc and 65 °C.
2480
H. Gualous et al. / Microelectronics Reliability 52 (2012) 2477–2481
Fig. 4. BCAP0350 lifetime experiment. The capacitance is recorded as a function of the duration of a constant voltage polarization of 2.8 Vdc and a constant temperature of 55, 65 and 70 °C. Fig. 6. BCAP0310_P capacitance and ESR evolution when the supercapacitor is continuously charged and discharged with a current of 20 A.
Fig. 5. BCAP0350 lifetime experiment. The capacitance is recorded as a function of the duration of a constant temperature 65 °C and constant dc voltage polarization of 2.8, 2.9 and 3 V.
capacitance decreases very fast to reach the value before the interruption. A calendar ageing of a BCAP0350 supercapacitor is carried out. The supercapacitor is aged at 2.8 V and a constant temperature of 55; 65 and 70 °C. Fig. 4 shows the supercapacitors equivalent capacitance variations according to the time during ageing process. These results show that the supercapacitor equivalent capacitance decreases according to the ageing time. This effect is accelerated when the temperature increases. The BCAP0350 supercapacitors were also tested at 65 °C and for 2.8, 2.9 and 3 Vdc voltage. Fig. 5 represents the experimental results of the equivalent capacitance variations as a function of ageing duration. These results show the effect of the dc voltage on the supercapacitor ageing. 4. Voltage cycling test Voltage cycling requires more powerful and expensive equipments. This limits the number of samples which may be studied. This method presents the advantage that the capacitance and the ESR may be measured in continuous during the experiment. One disadvantage is that it is difficult to have a control over the component temperature because it depends on both the current and the component thermal resistance. For small current the ageing during voltage cycling is equivalent to the ageing during a dc voltage solicitation if: (1) the internal component temperatures are the same, and (2) the ‘‘mean’’ voltage obtained by pondering the voltage classes visited during the cycling with the power law presented above (Eq. (2)) are the same.
The voltage cycling consists to charge and discharge the supercapacitor at constant current. In this experiment type, it is necessary to take into account the temperature increase due to the Joule losses of the current in the supercapacitor. From a practical point of view the increase of current intensity increases the number of cycles performed in a given time. It increases also the component internal temperature. The increase of rest time between the charge and discharge increases the time spent at high voltage and reduces the number of cycles performed in a given time. The experimental data have been measured on a BCAP0310_P manufactured by Maxwell Technologies in Rossens Switzerland at room temperature between 20 and 22 °C. The supercapacitor has been cycled between 1.25 and 2.5 Vdc without rest time with a current of 62 A. The current intensity has been chosen in order to get a component temperature of 65 °C. The nominal capacitance (310 F) and the nominal ESR (1.7 m X are specified at room temperature and at 1 Hz. The capacitor is expected to stand 1,000,000 cycles at 10 A. The capacitance and the ESR have been reported in Fig. 6 as a function of the voltage cycles number. They are measured with an impedance spectrometer at 10 mHz. The initial capacitance value was equal to 319 F while the initial ESR value was 2.9 m X. During the experiment the ESR has increased by a factor of 20%. The capacitance has decreased of 15% of the initial value. With a cycle time of 25 s the experiment has lasted about 1000 h. A simple calculation shows that for cycling the voltage cell between 1.25 V and 2.5 V without rest time, the equivalent continuous voltage would be 2.25 Vdc. With large current the voltage drops (0.120 V with 2 m X and 62 A) which occur during the current interruption are important, so that the cell almost never see the high voltage limit. An additional ageing due to the current intensity is expected to explain the relative important ageing found in the measurement. 5. Conclusion This paper presents supercapacitors ageing at constant temperature and constant voltage and charge/discharge cycling. The experimental results show that the supercapacitors calendar ageing is accelerated by the temperature and the voltage. The supercapacitors ageing were quantified during the tests in terms of series resistance and capacitance. References [1] Gallay R, Gualous H. Industrial production of double-layer capacitors. In: François Béguin, Elzbieta Frackowiak, editors. Carbons for electrochemical
H. Gualous et al. / Microelectronics Reliability 52 (2012) 2477–2481 energy storage and conversion systems. CRC Press. p. 430–68 [chapter 11]. [2] Kötz R, Ruch PW, Cericola D. Ageing and failure mode of electrochemical double layer capacitors during accelerated constant load tests. J Power Sources 2010;195:923–8. [3] Hahn M, Kötz R, Gallay R, Siggel A. Pressure evolution in propylene carbonate based electrochemical double layer capacitors. Electrochim Acta 2006;52:1709–12. [4] Miller JR, Klementov A, Butler S. Electrochemical capacitor reliability in heavy hybrid vehicles. In: Proceedings of 16th International Seminar On Double Layer Capacitors and Similar Energy Storage Devices, Deerfield Beach, USA; 2006. p. 218. [5] Goltser I, Butler S, Miller JR. Reliability assessment of electrochemical capacitors: method demonstration using 1-F commercial components. In: Proceedings of 15th International Seminar On Double Layer Capacitors and Similar Energy Storage Devices, Deerfield Beach, USA; 2005. p. 215. [6] Yurgil J. General motors corporation. Ultracapacitor useful life prediction. US patent /2006/012378.
2481
[7] Kötz R, Hahn M, Gallay R. Temperature behaviour and impedance fundamentals of supercapacitors. J Power Sources 2006;154:550–5. [8] Hardwick L, Hahn M, Ruch P, Holzapfel M, Scheifele W, Buqa H, et al. An in situ Raman study of the intercalation of supercapacitor-type electrolyte into microcrystalline graphite. Electrochim Acta 2006;52:675–80. [9] Hahn M, Barbieri O, Gallay R, Kötz R. A dilatometric study of the voltage limitation of carbonaceous electrodes in aprotic EDLC type electrolytes by charge-induced strain. Carbon 2006;44:2523–33. [10] Mizutani Y, Okamoto T, Taguchi T, Nakajima K, Tanaka K. Life expectancy and degradation behavior of Electric double layer capacitor. In: Proceedings of 7th International Conference on properties and applications of dielectric materials, Nagoya; June 2003. [11] How to define the adequate reliability requirement for a power electronic system? Power Electr Europe. 2012; No. 1.