Testing the resistance of lithium-ion batteries commonly involves three methods: DCIR, ACIR, and EIS. Each method has distinct testing principles and physical significance, offering unique insights into the battery’s performance. To understand the differences among these methods, we first need to clarify various terms that describe resistance to current flow.
01. What is Resistance?
Resistance does not necessarily refer to a resistor component; rather, it describes the opposition a device or material presents to the flow of current, fundamentally converting electrical energy into other forms of energy irreversibly. Resistance is a specific case of impedance when the reactance part is zero.
02. Terms Describing Current Resistance
The internal resistance of a lithium-ion battery includes ohmic resistance, interface impedance, charge transfer impedance, diffusion impedance, ohmic polarization resistance, electrochemical polarization resistance, and concentration polarization resistance.
- Ohmic Resistance: Primarily composed of the resistance of electrode materials, electrolytes, separators, and contact resistance of various parts. It is related to the battery’s size, structure, and assembly.
- Polarization Resistance: Refers to the deviation of the electrode potential from the equilibrium potential when current passes through the electrode. It is caused by electrochemical reactions and is not constant; it changes during charging and discharging due to variations in the composition of active materials, electrolyte concentration, and temperature. Ohmic resistance follows Ohm’s Law, while polarization resistance increases with current density but not linearly, often increasing linearly with the logarithm of the current density.
Different types of batteries have different internal resistances. Even within the same type, the internal resistance can vary due to inconsistencies in chemical characteristics. Internal resistance is measured in milliohms and is a crucial technical indicator of battery performance. Typically, a lower internal resistance indicates a stronger ability for high-current discharge, while higher internal resistance suggests weaker discharge capabilities.
03. What is DCIR?
DCIR (Direct Current Internal Resistance) measures the total resistance within the battery, including ohmic resistance, interface impedance, charge transfer impedance, diffusion impedance, ohmic polarization resistance, electrochemical polarization resistance, and concentration polarization resistance. Since Li ions undergo significant spatial changes during testing, it is referred to as dynamic resistance.
DCIR is tested by charging and discharging the battery with a specific current rate (I) for a certain duration (t), and recording the voltage before (U1) and after (U2) the process:
R = (U2 – U1)/I
The purpose of testing DCIR is to obtain the battery’s impedance as it would be in actual operating conditions.
04. What is ACIR?
ACIR (Alternating Current Internal Resistance) measures battery resistance using alternating current to eliminate the effects of polarization and directly measure the material’s resistance to current. With a sufficiently high frequency (f = 1/T), the current cycle is short enough that Li ions do not move significantly from their original positions, merely oscillating back and forth.
At high frequencies, the following assumptions are made:
1. No charge movement occurs, so there is no charge accumulation, and no polarization occurs.
2. The capacitance remains unchanged since the charge distribution does not change.
3. There is no diffusion impedance because the position of Li ions does not change.
Typically, a frequency of 1000Hz is used, at which point the ACIR value is considered equivalent to the ohmic resistance, assuming the battery behaves as a pure resistor.
05. What is EIC?
Electrochemical impedance spectroscopy (EIS): It is a non-destructive parameter determination and effective method for determining battery kinetic behavior. A small-amplitude sinusoidal voltage signal with a frequency of w1 is applied to the battery system, and the system generates a sinusoidal current response with a frequency of w2. The change in the ratio of the excitation voltage to the response current is the impedance spectrum of the electrochemical system.
It can be found here that the ACIR and EIS test methods are the same, both using alternating current for testing. However, ACIR is only a test at a certain frequency, while EIS is a test within a frequency range, and the purposes of the two are different.
EIS uses alternating current within a certain frequency range to test. Different components respond differently to currents of different frequencies to split the circuit into different parts, and then artificially stipulate that each part corresponds to a certain component. In fact, in the EIS test, the entire circuit participates at 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 roughly divide them and make specific analysis of a certain component.
Post time: Jul-12-2024
