Internal resistance is the resistance to current flowing through the inside of the battery when the lithium battery is working. According to the test method, it can be divided into AC internal resistance and DC internal resistance. The battery’s internal resistance is an important parameter to identify the quality of the lithium-ion battery. If the internal resistance of the battery is large, a large amount of Joule heat will be generated and the temperature of the battery will rise. As a result, the battery discharge operating voltage is reduced, and the discharge time is shortened, which seriously impacts battery performance and life. In the test to verify the electrochemical performance of lithium batteries by various factors, internal resistance is also an important parameter to investigate. Combined with the materials and process of lithium batteries, I will share with you the influencing factors of the internal resistance of lithium batteries.
Generally, battery internal resistance is divided into ohmic internal resistance and polarization internal resistance. Ohmic internal resistance is composed of electrode material, electrolyte, diaphragm resistance and contact resistance of various parts. Polarization internal resistance refers to the resistance caused by polarization during electrochemical reaction, including electrochemical polarization internal resistance and concentration polarization internal resistance. The ohmic internal resistance of the battery is determined by the total conductivity of the battery, and the polarization internal resistance of the battery is determined by the solid phase diffusion coefficient of lithium ions in the electrode active material.
Ohmic internal resistance
Ohmic internal resistance is mainly divided into three parts, one is ionic impedance, the other is electronic impedance, and the third is contact impedance. We hope that the internal resistance of lithium batteries will become smaller and smaller, so we need to take specific measures to reduce the ohmic internal resistance for these three items.
ionic impedance
Lithium battery ionic impedance refers to the resistance of lithium ions to transfer inside the battery. Lithium-ion migration speed and electron conduction speed play an equally important role in lithium batteries, and ionic impedance is mainly affected by positive and negative electrode materials, separators and electrolytes. To reduce ionic impedance, the following points need to be done:
Ensure that the positive and negative electrode materials and electrolyte have good wettability
When designing the pole piece, it is necessary to select a suitable compaction density. If the compaction density is too large, the electrolyte is not easy to infiltrate, which will increase the ionic impedance. For the negative electrode sheet, if the SEI film formed on the surface of the active material is too thick during the first charge and discharge, it will also increase the ionic impedance. Currently, it is necessary to adjust the formation process of the battery to solve it.
The influence of electrolyte
Electrolyte should have suitable concentration, viscosity and conductivity. When the viscosity of the electrolyte is too high, it is not conducive to the infiltration between it and the positive and negative active materials. At the same time, the electrolyte also requires a lower concentration, and too high a concentration is also not conducive to its flow and infiltration. The conductivity of the electrolyte is the most important factor affecting the ionic impedance, which determines the migration of ions.
The Effect of Separator on Ionic Impedance
The main factors affecting the ionic impedance of the diaphragm are: electrolyte distribution in the diaphragm, diaphragm area, thickness, pore size, porosity, and tortuosity coefficient. For ceramic diaphragms, it is also necessary to prevent ceramic particles from clogging the pores of the diaphragm, which is not conducive to the passage of ions. While ensuring that the electrolyte solution fully infiltrates the diaphragm, there should not be a surplus of electrolyte solution remaining in it, reducing the use efficiency of the electrolyte solution.
Electronic impedance
There are many factors affecting electronic impedance, which can be improved from the aspects of materials and processes.
Positive and negative plates
The main factors affecting the electronic impedance of the positive and negative plates are: the contact between the active material and the current collector, the factors of the active material itself, and the parameters of the plate. The active material should be in full contact with the current collector surface, which can be considered from the copper foil and aluminum foil base material of the current collector, and the adhesion of the positive and negative electrode slurry. The porosity of the active material itself, by-products on the particle surface, uneven mixing with the conductive agent, etc. will all cause changes in electronic impedance. Plate parameters such as the density of the active material are too small, and the particle gap is large, which is not conducive to electron conduction.
Separator
The main factors affecting the electronic impedance of the separator are the thickness of the separator, the porosity, and the by-products during the charge and discharge process. The first two are easy to understand. After the battery is disassembled, it is often found that the separator is covered with a thick layer of brown matter, which includes the graphite negative electrode and its reaction by-products, which will cause the pores of the separator to be blocked and reduce the service life of the battery.
Current collector substrate
The material, thickness, and width of the current collector, as well as the degree of contact with the tab, all affect the electronic impedance. The current collector needs to choose a substrate that is not oxidized and passivated. Otherwise, it will affect the impedance. Poor welding between copper and aluminum foil and tabs will also affect electronic impedance.
Contact resistance
Contact resistance is formed between the contact between the copper and aluminum foil and the active material, and it is necessary to focus on the adhesion of the positive and negative pastes.
Polarization internal resistance
When the current passes through the electrode, the phenomenon that the electrode potential deviates from the equilibrium electrode potential is called the polarization of the electrode. Polarization includes ohmic polarization, electrochemical polarization and concentration polarization, as shown in Figure 1. Polarization resistance refers to the internal resistance caused by the polarization of the positive and negative electrodes of the battery during the electrochemical reaction. It can reflect the internal consistency of the battery, but due to the influence of operation and method, it is not suitable for production. The polarization internal resistance is not constant, and it changes with time during the charging and discharging process, because the composition of the active material, the concentration and the temperature of the electrolyte are constantly changing. The ohmic internal resistance obeys Ohm’s law, and the polarization internal resistance increases with the increase of the current density, but it is not a linear relationship. Often increases linearly with the logarithm of the current density.
The DC internal resistance of the battery is equal to the sum of the polarization internal resistance and the ohmic internal resistance. The determination of DC internal resistance is of great significance. There are many factors that affect the polarization internal resistance, such as charge and discharge rate, ambient temperature, SOC state, electrolyte concentration and so on. Here is an example of the temperature’s effect on the internal resistance of a lithium iron phosphate battery. If you need relevant literature, you can PM the editor, as shown in the figure below:
Current battery internal resistance measurement methods used in the industry
In industrial applications, the precise measurement of battery internal resistance is carried out by special equipment. At present, there are two main battery internal resistance measurement methods used in the industry:
DC discharge internal resistance measurement method
According to the physical formula R=U/I, the test equipment forces the battery to pass through a large constant DC current in a short period of time (generally 2 to 3 seconds) (currently a large current of 40A to 80A is generally used). Measure the voltage across the battery currently, and calculate the current internal resistance of the battery according to the formula.
The accuracy of this measurement method is high, and if it is properly controlled, the measurement accuracy error can be controlled within 0.1%. However, this method has obvious shortcomings:
- Only large-capacity batteries or storage batteries can be measured, and small-capacity batteries cannot load a large current of 40A to 80A within 2 to 3 seconds;
- When the battery passes a large current, the electrodes inside the battery will be polarized, resulting in polarization internal resistance. Therefore, the measurement time must be very short, otherwise the error of the measured internal resistance value will be large;
- A large current passing through the battery will damage the electrodes inside the battery to a certain extent.
AC voltage drop internal resistance measurement method
Because the battery is equivalent to an active resistance, we apply a fixed frequency and a fixed current to the battery (currently generally use 1kHz frequency, 50mA small current), and then sample its voltage. After a series of processing such as rectification and filtering, the internal resistance of the battery is calculated through the operational amplifier circuit. The battery measurement time of the AC voltage drop internal resistance measurement method is extremely short, generally around 100 milliseconds.
The accuracy of this measurement method is also good, and the measurement accuracy error is generally between 1% and 2%.
Advantages and disadvantages of this method:
- Almost all batteries, including small-capacity batteries, can be measured using the AC voltage drop internal resistance measurement method. This method is generally used to measure the internal resistance of notebook battery cells.
- The measurement accuracy of the AC voltage drop measurement method is likely to be affected by the ripple current, and there is also the possibility of harmonic current interference. This is a test for the anti-interference ability in the measuring instrument circuit
- Using this method to measure will not cause too much damage to the battery itself.
- The measurement accuracy of the AC voltage drop measurement method is not as good as that of the DC discharge internal resistance measurement method.