Main performance index of super capacitor

The main performance indicators of supercapacitors include specific capacitance, energy density and power density, internal resistance and cycle stability.

1. Specific capacitance

In a supercapacitor , each electrode is a capacitor. Therefore, for supercapacitors , the total capacitance is obtained by connecting the positive and negative capacitors in series. If the positive capacitance is represented by Cp and the negative capacitance is represented by Cn, the total capacitance is:

In the test, researchers are more concerned about the specific capacitance of supercapacitors, which is divided into two types, one is mass specific capacitance, that is, the capacitance value per unit mass; the other is volume specific capacitance, that is, the capacitance value per unit volume.

2. Energy density and power density

Energy density and power density are the main indicators to characterize the performance of supercapacitor. , the theoretical energy density E and power density P of an ideal supercapacitor can be calculated by the following two formulas:

In the formula: C is the specific capacitance; V is the potential window, that is, the difference between the oxygen evolution potential of the positive electrode and the hydrogen evolution potential of the negative electrode; R s is the equivalent series resistance of the capacitor; Q is the total charge stored in the capacitor. It can be seen from equation (3) that the energy density of the supercapacitor is proportional to the specific capacitance and the square of the potential window, the specific capacitance is related to the electrode material, and the potential window is related to the electrolyte. The energy density and power density of supercapacitors are proportional to the square of the potential window, so it is more effective to expand the potential window to improve the performance of supercapacitors than to increase the specific capacitance value.

3. Internal resistance

The internal resistance of a supercapacitor refers to the series resistance between the positive electrode capacitor and the negative electrode capacitor, which is closely related to the electrode material, electrolyte, diaphragm and assembly method. Generally speaking, smaller internal resistance is beneficial for the improvement of supercapacitor performance. The larger the electrode thickness, the higher the internal resistance, and the general electrode thickness is less than 150μm. Moreover, the pore size of the material should not be too small, generally above 1.5nm, so as to facilitate the entry of ions in the electrolyte and fully infiltrate them to form an electric double layer and avoid increasing the internal resistance.

The resistance of the electrolyte is also the main factor affecting the internal resistance of the supercapacitor. For the aqueous electrolyte , the diameter of the ion is small, the mobility is high, and it can easily pass through the diaphragm and into the pores of the electrode, so the internal resistance is very high. For organic electrolytes, the solutes are generally organic polymers with large diameters, so the probability of being hindered during the migration process is much greater, and it is difficult to enter the pores of the electrode, and the internal resistance is relatively large.

4. Cyclic stability

Cycling stability refers to the ability of supercapacitors to maintain the degree of electrical properties after multiple charging and discharging, which is mainly reflected in whether the attenuation of the capacitance value is too large after multiple charging and discharging. The cycle stability is calibrated by the attenuation degree of the capacitance value after the supercapacitor is charged and discharged thousands of times. The capacitance value measured in one cycle is the initial value, and the capacitance value of the device after thousands of charge and discharge cycles is the final value. The capacitance value of the supercapacitor can be plotted by recording the capacitance value every hundreds of times between the values. The capacitance retention of the supercapacitor can be obtained by comparing the initial value with the final Z value. Since the electrolyte has a certain erosion on the electrode material, especially the Faraday pseudocapacitor, the capacitance value of the supercapacitor has a certain attenuation. Nevertheless, the cycle life of supercapacitors is still far ahead of batteries, which is also a major advantage of supercapacitors.