Ⅰ. Definition of supercapacitors
Supercapacitors(SC), also known as electrochemical capacitors, electric double layer capacitors(EDLC), gold capacitors, and farad capacitors, were developed in the 1970s. It is an electrochemical element that stores energy by polarizing the electrolyte. Different from traditional chemical power sources, it is a power source with special properties between traditional capacitors and batteries. Supercapacitors mainly rely on electric double layers and redox pseudocapacitive charges to store electrical energy, and this energy storage process is reversible. In other words, supercapacitors can be repeatedly charged and discharged hundreds of thousands of times.
Ⅱ. Classification of supercapacitors
Under normal circumstances, according to different energy storage principles, we can divide supercapacitors into three types: quasi-capacitors, electric double-layer capacitors and hybrid capacitors.
The quasi-capacitor mainly refers to the underpotential deposition of electroactive substances on the surface or bulk phase of the electrode material in a two-dimensional or quasi-two-dimensional space, a highly reversible reduction reaction occurs, and a capacitance related to the charging potential of the electrode is generated. Since the reaction takes place throughout the bulk phase, the achievable capacitance of this system is relatively large.
(1) Charging process: We connect the electrodes with the external circuit. Under the action of an external electric field, a large number of anions and cations gather on the surface of the electrode or solution. Through redox reactions, these ions enter the bulk phase of active oxides on the electrode surface, thereby realizing charge storage.
(2) Discharge process: The ions entering the oxide return to the electrolyte through the reverse reaction of the above redox reaction, and at the same time release the stored charge through the external circuit.
The way for electric double layer capacitors to store energy is mainly through the interfacial double layer formed between the electrodes and the electrolyte. The so-called interfacial double layer means that in the electric double layer capacitor, the electrodes and the electrolyte are in contact with each other. Under the interaction of Coulomb force, molecular and interatomic forces, stable double-layer charges with opposite signs appear on the solid-liquid interface.
(1) Charging process: We apply an electric field on the two electrodes. Under the action of an electric field, the anions and cations in the electrolyte move to the positive and negative poles respectively, thus forming an electric double layer. After the electric field is removed, it uses the property that the same kind of charges repel each other and the different kinds of charges attract each other to realize the stability of the electric double layer and generate a stable potential difference.
(2) Discharge process: We connect the electrodes with the external circuit. Under the action of potential difference, electrons move directional to form external current. At this time, the anions and cations adsorbed on the electrode surface return to the body of the electrolyte, and the electric double layer disintegrates.
Electric double layer capacitors have stable cycling and high power density, but their energy density is relatively low. Although a pseudocapacitor has a large capacity, its cycling stability and energy density are both low. Only when the two complement each other and join forces can we obtain supercapacitors that meet higher requirements. According to the difference in configuration of hybrid supercapacitors, we divide them into three categories: battery-type supercapacitors, composite supercapacitors and asymmetric supercapacitors. Among them, battery-type supercapacitors are composed of supercapacitors and battery electrodes; composite supercapacitors are composed of carbon and capacitors. Asymmetric supercapacitors use two electrodes to produce different chemical reactions, one is a redox reaction, and the other is a non-faradaic reaction.
Ⅲ. Parameters of supercapacitors
Supercapacitors use symmetrical electrode designs, that is, they have similar structures. When the capacitor is first assembled, each electrode can be considered a positive or negative electrode. Once the capacitor is charged from 100% for the first time, the capacitor becomes polarized, and every supercapacitor has a negative sign or logo on the outer shell. Although they can be short-circuited to reduce the voltage to zero volts, the electrodes still retain a small fraction of the charge and reversing polarity is not recommended. The longer capacitors are charged in one direction, the more polar they become. If a capacitor is charged in one direction for a long time and then changes polarity, the life of the capacitor will be shortened.
Supercapacitors have a recommended working voltage or optimal working voltage, which is determined based on the longest working time of the capacitor at the highest set temperature. If the applied voltage is higher than the recommended voltage, the life of the capacitor will be shortened. If the overvoltage lasts for a long time, the electrolyte inside the capacitor will decompose and form gas. When the pressure of the gas gradually increases, the safety hole of the capacitor will rupture or burst. Short-term overvoltage is tolerated by the capacitor.
3. Time Constant
If an ultra-large capacitor can be modeled as a simple series combination of a capacitor and a resistor, then the product of the capacitor and the resistor is a time constant, whose unit is S. This is equivalent to the time required to charge the capacitor at a constant voltage to 63.2% of its full capacity.
4. Average Discharge Power
The product of the average discharge current and the average discharge voltage is the average discharge power.
5. Maximum Output Power
When a suitable external load is connected to the capacitor, the maximum output power it can achieve is calculated as P= U2/(4R). Here U is the initial voltage of the capacitor, and R is the equivalent series resistance of the capacitor.
6. Charge Methods
Supercapacitors have various charging modes, such as constant current, constant power, constant voltage, etc. Or it is in parallel with a power source, such as a battery, fuel cell, DC converter, etc. If a capacitor is connected in parallel with a battery, adding a resistor in series with the capacitor loop will reduce the charging current of the capacitor and increase the life of the battery. If a resistor is connected in series, we need to ensure that the voltage output of the capacitor does not pass through the resistor, but is directly connected to the load. Many battery systems do not allow instantaneous high current discharge, otherwise it will affect the battery life.
7. Discharge Characteristics
When a supercapacitor discharges, it discharges according to a slope curve. After determining the capacitance and internal resistance requirements of the capacitor, we must first understand the influence of the resistance and capacitance on the discharge characteristics. In pulse applications, resistance is the most important factor. In small current applications, capacity is an important factor.
8. Discharge Efficiency
It refers to the percentage of the energy released by the capacitor to the charged energy in a specific charge and discharge cycle.
9. Self Discharge and Leakage Current
Self discharge and leakage current have the same essence in the structure of supercapacitors, which is equivalent to a high-impedance current path between the positive and negative electrodes inside the capacitor. This means that when the capacitor is charging, there will be an additional additional current, which we can regard as leakage current. When the charging voltage is removed and the capacitor is not connected to the load, this current will cause the capacitor to be in a discharge state. At this time, we regard this current as a self-discharge current.
In order to accurately measure the leakage current or discharge current, we must ensure that the capacitor has been charged continuously for more than 72 hours. This is because the structure of a capacitor determines its charging and discharging characteristics. The supercapacitor model can be equivalent to several supercapacitors with different internal resistances connected in parallel. During the charging process, the supercapacitor with lower internal resistance charges faster, and the voltage rises rapidly to be equal to the charging voltage. When the charging voltage is removed, the low internal resistance supercapacitor will start to discharge into the parallel high internal resistance supercapacitor if the higher internal resistance supercapacitor is not fully charged. This causes the voltage across the capacitor to drop relatively quickly. It should be noted that the larger the capacitor capacity, the longer it will take for the capacitor to be fully charged.
10. Discharge Capacity
The full capacity that a capacitor can release during discharge. Its specific calculation method is to integrate the product of an instantaneous voltage and current during the discharge process to the discharge time.
11. Reverse Voltage Protection
When a supercapacitor used in series is rapidly charged, low capacity voltage may become reverse polarity, which is not allowed and will reduce the lifespan of the capacitor. A simple solution is to connect a diode in parallel at both ends of the capacitor. Under normal circumstances, they are not conductive due to back pressure. Replacing a standard diode with a suitable zener diode can simultaneously protect the capacitor from overvoltage. It should be noted that the diode must be able to withstand the peak current of the power supply.
12. Ripple Current
Although supercapacitors have relatively low internal resistance. Compared with electrolytic capacitors, its internal resistance is still relatively large. When it is used in pulsating current situations, it will easily cause internal heating of the capacitor, which will lead to the decomposition of the electrolyte inside the capacitor, increase in internal resistance, and shorten the life of the capacitor. In order to ensure the service life of the capacitor, it is best to ensure that the temperature rise on the surface of the capacitor does not exceed 5°C when it is applied to a pulsating occasion.
13. Active Voltage Balancing
Active balancing circuits can force the voltage at the series node to match a reference voltage. Active balancing circuits draw very low currents in steady state while maintaining precise voltage balance regardless of the voltage imbalance. Only when the voltage exceeds the balance range, will a larger current be generated. These characteristics make active balancing circuits very suitable for occasions that require frequent charging and discharging. Its excellent balance capability and low power consumption provide the device with excellent performance and long life.
14. Passive Voltage Balancing
The passive voltage balance circuit uses resistors connected in parallel with capacitors to divide the voltage. This allows current to flow from the capacitor with the higher voltage to the capacitor with the lower voltage, and in this way balances the voltage. Choosing the resistor value is very important. We usually want to make the current allowed by the resistor larger than the expected leakage current of the capacitor. It is important to remember that leakage current generally increases with increasing temperature. Passive balancing circuits should only be used in applications where the capacitors are charged and discharged infrequently while being able to tolerate the extra current drawn by the balancing resistors. It is recommended to select the resistance value of the balance resistor so that the current of the balance resistor is more than 50 times greater than the leakage current of the capacitor. Although most balancing circuits use relatively high balancing resistors, the protection is not adequate when the series capacitors are very mismatched.
15. Series Configurations of Super capacitors
The voltage of a single supercapacitor is generally 2.5V or 2.7V. In many applications, we can use the series connection method to increase the voltage of the capacitor. It must be noted that in series applications, the capacitance of each single cell cannot exceed its maximum withstand voltage. Once long-term overvoltage, it will lead to decomposition of capacitor electrolyte, gas generation, increase of internal resistance and shortening of capacitor life. When discharging or charging, the difference in capacitance capacity or the difference in leakage current in a steady state will lead to unbalanced voltage division of series capacitors. When charging, the capacitors connected in series will divide the voltage, so that the capacitors with high capacity will bear greater voltage pressure. For example, if two 1F capacitors are connected in series, one is +20% capacity deviation, and the other is -20% capacity deviation.
16. Ambient Temperature
The normal operating temperature of supercapacitors is -40 ℃ to 70 ℃, and the combination of temperature and voltage is an important factor affecting the life of supercapacitors. Normally, for every 10°C increase in temperature of a supercapacitor, the life of the capacitor will be reduced by 30% to 50%. In other words, when possible, we should lower the operating temperature of the supercapacitor as much as possible to reduce the capacitance. The attenuation and the increase of internal resistance. If it is not possible to reduce the operating temperature, then we can reduce the voltage to offset the negative impact of high temperature on the capacitance. For example, if the operating voltage of the capacitor is reduced to 1.8V, then the capacitor can work at a high temperature of 65°C.
When using supercapacitors in low-temperature environments, we can increase the operating voltage above the specified voltage without affecting the life of the supercapacitor. Doing so can effectively offset the increase in the internal resistance of the supercapacitor at low temperatures. At high temperature, the increase in the internal resistance of the capacitor is permanent and irreversible because the electrolyte has decomposed. At low temperatures, the increase in the internal resistance of the capacitor is only a temporary phenomenon, because the increase in the viscosity of the electrolyte reduces the movement speed of ions.
17. Equivalent Series Resistance
The actual periodic variation of the capacitor's current versus voltage does not depend on frequency, but rather beyond the 90 degree phase angle relationship. This is because the impedance is a combination of capacitance and resistance, which increases the phase angle below 90 degrees and is frequency dependent. Its resistance component is expressed as equivalent series resistance, including electrolyte resistance and contact resistance.
Equivalent series resistance is an electrochemical parameter measured by the AC impedance technique (EIS), which uses a small amplitude sine wave voltage or current as a perturbation signal and measures its response to the current or voltage signal. This electrochemical measurement method is called electrochemical transient technology and is widely used to study electrode process dynamics and mass transfer laws.
Ⅳ. Characteristics of supercapacitors
1. High reliability
Supercapacitors have no moving parts and minimal maintenance during operation, so the reliability of supercapacitors is very high.
2. Low specific energy
Low specific energy is a significant drawback of current supercapacitors, which limits the driving range of electric vehicles to a certain extent.
3. High power density
The internal resistance of a supercapacitor is very small, and both the electrode liquid interface and the electrode material body can realize rapid storage and release of charges, so its output power density is as high as kw/kg, which is dozens of times that of ordinary batteries. Comparison of the curves of the relationship between the power density and energy density of batteries such as supercapacitors and electrochemical capacitors, rechargeable batteries and fuel cells, its power density is higher.
4. Long charge and discharge cycle life
Supercapacitors do not undergo electrochemical reactions during the charge and discharge process, and their cycle life can reach more than 10 times. The charge-discharge cycle life of today's batteries is only hundreds of times, which is only a few tenths of that of supercapacitors.
5. Short charging time
Judging from the charging test results that have been done so far, it only takes 10 to 12 minutes to fully charge the supercapacitor. The battery cannot be fully charged in such a short period of time.
6. Long storage life
Although the supercapacitor also has a small leakage current during the storage process, the migration of ions or protons inside the capacitor is generated under the action of an electric field, and there is no chemical or electrochemical reaction, nor does it produce new energy. substance. Moreover, the electrode materials used are also stable in the corresponding electrolyte, so the storage life of the supercapacitor is theoretically unlimited.