In order to test the resistance of lithium-ion batteries, we often use three methods, namely DCIR, ACIR, and EIS. So what are the test principles of these three methods? What is the physical significance? What is the difference between the three? What’s the connection? In order to understand the differences between these three charging methods, we first need to figure out various “words” to describe current resistance.
1. What is resistance?
The resistance does not necessarily refer to the resistance device but describes the obstruction of a device or material to the current flowing through it. Its essence is to irreversibly convert electrical energy into other forms of energy. Resistance is a special case when the impedance is 0 in the reactance part.
2. What is Impedance?
Impedance is a basic concept that describes a circuit or device. With a specific voltage, the current will look like. Impedance is defined as instantaneous voltage divided by current. Impedance includes resistance, tolerance and sensibility. The essence of resistivity is dissipation of electrical energy. The essence of capacitance is to store electrical energy in the form of an electric field in space or dielectric, and the essence of inductivity is to store electrical energy in a magnetic field in space or magnetic dielectric. Both capacitive and inductive energy are stored electrical energy, which can be released at other times, instead of converting electrical energy into thermal energy consumption like resistance. However, capacitance and insensibility have a great impact on the voltage-current ratio at a certain moment in the circuit. Therefore, the impedance is defined as a synthetic parameter that combines resistance, tolerance and sensibility.
The expression of impedance is complex:
Z= R+ jX
The real part of the complex number represents the resistance of dissipated electrical energy, and the imaginary part represents the Reactance of stored electrical energy. The resistance represents the attenuation of the signal amplitude, and the reactance represents the change of the signal phase. Because the reactance (capacitance, inductance) is the conversion of the energy form of the reversible electric magnetic field, and according to the electromagnetic field theory, this conversion process is related to the change rate of the electric or magnetic field, which is reflected in the signal is the frequency of the signal. That is, the reactance part in the impedance is related to the frequency. The following is the impedance expression of the resistance part, the capacitance part and the inductance part:
ZR=R ZL=jwL ZC=1/(jwC)
3. What kinds of “words” describe current resistance in the battery?
The internal resistance of lithium-ion batteries we often refer to include: {Ohm internal resistance, interface impedance, charge transfer impedance, diffusion impedance} and {Ohm polarization internal resistance, electrochemical polarization internal resistance and concentration polarization internal resistance}. Resistance to current by the properties of the substance itself: {Ohm internal resistance, interface impedance, charge transfer impedance, diffusion impedance}.
Polarization internal resistance = (resistance reflected by the material when the current is applied) – (resistance of the substance itself to the current), polarization internal resistance includes: ohmic polarization internal resistance, electrochemical polarization internal resistance and concentration polarization internal resistance.
4. What is the ohm resistance, interface impedance, charge transfer impedance and diffusion impedance of the battery?
The ohm resistance should be called ohmic resistance, because the ohmic resistance only contains the resistance part and does not take into account tolerance and sensibility. Ohm internal resistance is mainly divided into three parts, one is ion resistance, the other is electronic resistance, and the third is contact resistance.
Ion resistance: The resistance to the transmission of lithium ions inside the battery is mainly affected by positive and negative electrode materials, diaphragms and electrolytes.
Electron resistance: the resistance to electron transmission. It is mainly affected by the contact between the active substance and the collecting fluid, the active substance itself, the plate parameters, and the collecting fluid substrate.
Contact resistance: The contact resistance is mainly formed by the contact between the particles of the active substance and between the active substance and the collector. The contact resistance is mainly affected by the adhesion of the positive and negative slurry.
Interface impedance: the resistance of Li ions through the SEI membrane.
Charge transfer impedance: It is reflected in the adsorption ability of the interface to ions and the ability of charge transfer to cause ions to undergo electrochemical reactions on the interface
Diffusion impedance: resistance to the diffusion of Li ions in active substances and electrolytes.
5. What is ohm polarization, electrochemical polarization and concentration polarization of batteries?
In the case of external current passing through, because the ohm resistance causes local charges to accumulate, so that the current is greater than the resistance provided by the ohmic resistance (R ohm). This phenomenon is called ohm polarization, ohm polarization internal resistance = (R test ohm) – (R ohm). Electrochemical polarization and concentration polarization are also the same. For a more specific explanation, please refer to the article (lithium electricity – why is it difficult for lithium-ion batteries to charge at low temperatures? Why does a lithium-ion battery have an irreversible capacity loss after cycling at low temperature?) The above definition of internal resistance of the battery is people’s simplified description of lithium-ion batteries. In fact, the battery is a com‐ plex electrochemical system. The total impedance of this process can be abstracted into three electrical components, namely:
- Internal resistance RΩ: internal resistance of electrolyte and electrode.
- Double-layer capacitor Cdl: derived from the inactive ions in the electrolyte, no chemical reaction occurs, only the charge distribution is changed.
- Faraday impedance Zf: derived from the active ions in the electrolyte, with redox re‐ actions and charge transfer.
Charge transfer and mass transfer
These two processes can be abstracted as: charge transfer resistance (Rct) and War‐ burg impedance (ZW) respectively.
Therefore, every particle that undergoes an electrochemical reaction can be represented by the following circuit diagram:
Then, the ideal state model of the entire lithium-ion battery is shown in the following figure:
(In the ideal state, there is no side reaction, only the de-embedding and embedding of Li particles)
After understanding the meaning of various “words” and the ideal model of lithium-ion batteries, let’s analyze DCIR, ACIR, EIS.
I. What is DCIR?
DCIR (Direct Current Internal Resistance) DC internal resistance test.
The measured DC internal resistance includes all the resistance in the battery: {Ohm internal resistance, interface impedance, charge transfer impedance, diffusion impedance} and {Ohm polarization internal resistance, electrochemical polarization internal resistance and concentration polarization internal resistance}. Because there is a significant change in the spatial position of Li ions during the test, it is called dynamic resistance. DCIR is to use a specific magnification current (I) to charge and discharge for a certain period of time (t), to record the battery voltage (U1) before charge and discharge and the voltage after charge and discharge (U2), including: R=（U2-U1）/I We believe that the value of the DC internal resistance test is the impedance shown by the lithium-ion battery in the working process.
The purpose of testing the DC resistance is to obtain the impedance of the battery in actual working conditions.
II. What is ACIR?
ACIR (Alternating Current Internal Resistance) AC internal resistance test.
The reason why alternating current is used to test the resistance of the battery is that we want to eliminate the impact of polarization and directly measure the resistance of the properties of the substance to the current.
When using alternating current,
f=1/T
When the frequency is large enough, the period of the current is smaller. In a short period of time, the Li ion has no time to move for a long distance but moves back and forth from the original position.
Therefore, when the frequency is large enough, it is assumed that:
1. The charge does not move, so there will be no accumulation of charge, and the charge distribution does not change, so it is believed that there will be no polarization.
2. Similarly, because the charge distribution does not change, the capacitance will not change.
3. The position of the Li ion has not changed, so there will be no diffusion behavior, so there is no diffusion impedance in the circuit.
III. What is EIS?
Electrochemical Impedance Spectrum (EIS): is a non-destructive parameter determination and effective battery kinetic behavior measurement method. A sine wave voltage signal with a frequency of w1 and a small amplitude is ap‐ plied to the battery system, and the system generates a sine wave current response with a frequency of w2. The ratio between the excitation voltage and the response current is the impedance spectrum of the electrochemical system.
It can be found that ACIR and EIS are tested in the same way, using AC pow‐er for testing. However, ACIR is only tested at a certain frequency, while EIS is tested at a frequency range. The two purposes are different. The purpose of ACIR is to test the ohmic resistance of the battery, so what is the purpose of EIS? The EIS includes Nyquist diagram and Bode diagram.
The common Nyquist diagram in the battery is as follows:
For the above Nyquist diagram, we may ask why it is divided by frequency. What is the meaning of each division? For the ultra-high frequency part, f=100K Hz, the period of alternating cur‐ rent is particularly small, which can be understood that the Li ions vibrate in situ and generate inductance. The intersection of the curve and the horizontal coordinates is R ohm, which is considered to be the ohm resistance of the battery. For the high-frequency part, due to the limitation of the alternating current cycle, the moving distance of Li ions is limited, and it can only move for a very small period of time. At this time, we will discuss the Li ions in different positions separately:
1. The Li ions in the electrolyte, in this small moving position, are subjected to little resistance;
2. The Li ions in the positive and negative materials may not have been separated from the original lattice site in this small moving position, so the resistance is also very small;
3. The Li ion located at the interface; the moving distance is just the part of the inter‐ face. The Li ion is constantly entering and exiting,and is subjected to greater resistance.
To sum up, in the high-frequency part, Li ions are resisted from all aspects. But among them, the contribution of the interface is the largest, so we think that high frequency represents the interface impedance. For the intermediate frequency part, the cycle of the alternating current is enough to support the Li ion to move for a distance. So, the Li ions in the electrolyte have moved away. Who will fill the gap? Of course, it is the Li ions in the active material, but if the Li ions in the active material want to be de-embedded, an electrochemical reaction is required, so it is believed that the charge transfer impedance contribution is very large at this time, so the intermediate frequency is assigned to the charge transfer impedance. For the low-frequency part, f=0.01 Hz, the Li ion at this time has enough time to enter and exit the interface, and can also complete the electro chemical process. Then the fully assembled Li ions are already on the way to diffusion. At this time, the Li ions are subjected to more impedance from diffusion, so the low-frequency part is used to characterize the diffusion impedance.
In summary, EIS is tested using alternating current within a certain frequency range. Through the different degrees of response of different components to different frequency currents, each part of the circuit is split, and then artificially stipulates that each segment corresponds to a certain component. Infact, in the EIS test, the overall circuit participates in each frequency, and each component contributes. Therefore, the purpose of EIS is to amplify the performance of specific components through different frequencies, so as to make a general division and achieve specific analysis of a certain component.
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