1. Working process of lithium-ion batteries
A physical model of the charging and discharging process of lithium-ion batteries. The blue arrow indicates charging, and the red arrow indicates discharging. The blue green lattice structure is the positive electrode material, and the black layered material is the negative electrode material. At present, mainstream lithium-ion batteries are generally named according to the type of positive electrode material, with lithium iron phosphate, lithium manganese oxide, etc. being the types of positive electrode materials; The negative electrode is made of graphite material; The positive current collector is aluminum foil, and the negative current collector is copper foil.
Taking discharge as an example, describe the physical process of lithium-ion battery discharge.
After the external load is connected, a current path is formed outside the battery body. Due to the potential difference between the positive and negative electrodes, electrons near the negative electrode first move towards the positive electrode through the collecting fluid and external wires; The concentration of lithium ions around the negative electrode increases. The electrons that pass through an external circuit from the negative electrode to the positive electrode combine with the lithium ions near the positive electrode, embed them into the positive electrode material, and the concentration of lithium ions near the positive electrode decreases. The lithium ion concentration difference between the positive and negative electrodes is formed. In this way, the first push of the battery discharge process is completed.
As lithium ions are pushed away from the negative electrode by the difference in ion concentration, vacancies appear near the negative electrode, and lithium ions in the negative electrode material are deintercaled from the negative electrode and enter the electrolyte; A large amount of lithium ions pass through the separator from the electrolyte and move from the negative electrode to the positive electrode. Meanwhile, electrons that originally existed in a bound form with lithium ions are sent to the positive electrode through an external circuit. The battery has started the discharge process according to the load requirements.
Charging is the reverse process of discharge, with the same stages of detachment, movement, and insertion. However, the driving force behind the development of the process comes from the charger, while the direction of ion movement is from the positive electrode to the negative electrode. I won't go into detail here.
2. Internal resistance composition of lithium-ion batteries
By understanding the working process of lithium-ion batteries, the hindering factors during the process form the internal resistance of lithium-ion batteries.
The internal resistance of a battery includes ohmic resistance and polarization resistance. Under constant temperature conditions, the Ohmic resistance remains relatively stable, while the polarization resistance varies with factors that affect the polarization level.
Ohmic resistance is composed of the contact resistance of various parts such as electrode material, electrolyte, diaphragm resistance, and the connection of current collector and pole ear, which is related to the size, structure, and connection method of the battery.
Polarization resistance, the resistance that only appears at the moment of loading current, is the sum of various trends inside the battery that hinder charged ions from reaching their destination. Polarization resistance can be divided into two parts: electrochemical polarization and concentration polarization. Electrochemical polarization is caused by the inability of electrochemical reactions in the electrolyte to reach the speed of electron movement; Concentration polarization is caused by the insertion and removal of lithium ions into and out of the positive and negative electrode materials, and their movement speed in the material being slower than the speed at which lithium ions gather towards the electrode.
3. Factors affecting the internal resistance of lithium-ion batteries
From the above process, the influencing factors of battery internal resistance can be inferred.
3.1 External factors
Temperature and ambient temperature are important factors affecting various resistances. Specifically, in lithium-ion batteries, temperature affects the activity of electrochemical materials, directly determining the speed of electrochemical reactions and ion movement.
The demand for current or load is directly related to the magnitude of current and polarization resistance. The general trend is that the larger the current, the greater the polarization resistance. On the other hand, the thermal effect of current affects the activity of electrochemical materials.
3.2 Factors related to the battery itself
The difficulty of lithium ion insertion and removal in positive and negative electrode materials determines the magnitude of the material's internal resistance, which is a part of the concentration polarization resistance.
The movement rate of lithium ions in electrolyte is influenced by the conductivity of the electrolyte and is an important component of electrochemical polarization resistance.
The diaphragm, with its own resistance, directly forms a part of the Ohmic internal resistance. At the same time, its hindrance to the movement rate of lithium ions also forms a part of the electrochemical polarization resistance.
The current collector resistance, component connection resistance, is an important component of the ohmic internal resistance of batteries.
The level of craftsmanship, the manufacturing process of the electrode, whether the coating is uniform, and the compaction density, as well as the level of craftsmanship during the processing of these battery cells, will also have a direct impact on the polarization resistance.
4. Measurement of internal resistance of lithium-ion batteries
There are generally two methods for measuring the internal resistance of lithium-ion batteries: direct current measurement method and alternating current measurement method.
4.1 DC Internal Resistance Measurement Method
Using a current source, apply a short pulse to the battery and measure the difference between its terminal voltage and the open circuit voltage. Divide this difference by the test current to determine the DC internal resistance of the battery.
The polarization resistance of lithium-ion batteries is affected by the magnitude of the loading current. In order to avoid this factor as much as possible, the DC measurement method for internal resistance has a shorter power on time and a larger loading current.
In theory, the smaller the measured current, the less likely it will cause polarization reaction, reducing the interference of polarization resistance. However, due to the small internal resistance of the battery itself, which is in the milliohm range, the current is too small, and the voltage detection instrument is limited by measurement accuracy, which cannot exclude the interference of measurement errors on the results. Therefore, people weigh the impact of instrument accuracy and polarization resistance to find a measurement current value that balances the relationship between the two.
For ordinary battery cells, the measured current is generally around 5C-10C, which is very high. As the capacity of the battery cells increases or multiple cells are connected in parallel, their internal resistance decreases. Therefore, without the improvement of instrument accuracy, it is difficult to reduce the measured current.
4.2 AC Internal Resistance Measurement Method
Load the battery with a small amplitude AC input as an excitation and monitor the response of its terminal voltage. Use specific programs to analyze data and determine the AC internal resistance of the battery. The resistance value obtained from analysis is only related to the characteristics of the battery itself, and is independent of the size of the excitation signal used.
Due to the capacitance characteristics of batteries, the measured resistance values vary depending on the frequency of the excitation signal. The results of software analysis can be represented by a set of complex numbers, with the horizontal axis representing the real part and the vertical axis representing the imaginary part. In this way, a graph is formed, known as the AC impedance spectrum, as shown in the above figure.
Through further data analysis, people can obtain the Ohmic resistance of this battery, the diffusion resistance of the SEI film, the capacitance value of the SEI film, the equivalent capacitance value of charge transfer in the electrolyte, and the diffusion resistance value of charge in the electrolyte from the AC impedance spectrum. Then, an equivalent model of the battery can be drawn to further study the battery performance.
5. The Application of Internal Resistance in Engineering Practice
Internal resistance, as one of the key characteristics of lithium-ion batteries, can be applied in various fields such as engineering and manufacturing through its research results.
Internal resistance is closely related to battery charge, therefore it is applied to SOC estimation in battery management systems;
The internal resistance directly reflects the degree of battery aging, and some people use the internal resistance of the battery cell as the basis for evaluating the health state of the battery (SOH);
The consistency of single cell resistance directly affects the module capacity and lifespan after grouping, and is therefore widely used as a static indicator for cell sorting and grouping;
Internal resistance is an important indicator of battery failure, which is studied and used in the fault diagnosis system of power lithium battery packs;
Internal resistance combined with capacity loss and other indicators can also determine whether there is lithium evolution in the battery, which is applied in the field of phased utilization of retired batteries.