4 Point Probe VS. 2 Point Probe Battery Resistance Test Methods

1. Preface

In this paper, the difference of the two test methods is tested by the method of 4 point probe and 2 point probe.Lithium ion battery resistance is one of the important indicators to measure the battery performance, the size of the lithium ion battery resistance directly affects the lithium ion battery capacity, cycle life and safety performance, affect the factors of lithium ion battery resistance has electrode materials, formula, electrolyte, coating homogenization process, electrode resistance, etc., the electrode resistance reflects the electrode material performance and formula. Through the electrode resistance test results, on the one hand, it can be used to improve the homogenate coating process and formula to realize the rapid evaluation of the material system; on the other hand, it can timely screen and classify and eliminate the electrode with large resistance value, so as not to flow into the single manufacturing process, and improve the quality of terminal products. The schematic diagram of the two test methods is as follows:

Figure 1. (a) Schematic of 2 point probe (b) Schematic of 4 point probe

During 2 point probe tests, the terminal is placed on the sample and input the AC voltage signal to collect the current for the sample resistance and obtain the sample resistivity through the conversion relationship; the 4 point probe are placed on the surface of the sample, and the voltage signal between the sample resistivity is obtained through the conversion relationship.

2. Experimental Equipment and Test Methods

2.1 Experimental Equipment

Battery Electrode Resistance Tester, model BER2500,electrode diameter of 14mm, can apply a pressure of 5~60MP a, the appearance of the equipment as shown in Figure 2.

Figure 2. (a) BER2500 appearance diagram (b) BER2500 structure diagram

2.2 Test Method

Set the parameters such as test pressure strength and pressure retention time on the MRMS software to start the test. The software automatically reads the electrode thickness, resistance, resistivity, conductivity and other data.

2.3 Sample Information

Select the positive electrode plate cathode-1, anode-1, the negative electrode sheet cathode-2, anode-2, pure membrane samples, aluminum foil and copper foil, and test the electrode plate resistance by 2 point probe and 4 point probe, respectively.

3. Data Analysis

3.1 Analysis Of Resistance Test Of Electrode Sheet With Foil Material

When the cathode electrode resistance is tested, it can be seen from Figure 3 ( a ) that for the small resistance electrode cathode -1 , the difference between the electrode resistivity tested by the four-probe method (2 .1 ^ 10 ^-6 Ω*cm) and the pure aluminum foil resistivity (2 .884 ^ 10 ^-5 Ω*cm) is about 1 0 times, but for the electrode cathode -2 which has a larger value of resistance, the electrode resistivity tested by the four-probe method is 1 .3316 Ω*cm, which is much larger than the pure aluminum foil resistivity. In Figure 3(b), the electrode resistivity of cathode -1 tested by two-probe method (1,444.94 Ω*cm) is much larger than that of aluminum foil (0 .370026 Ω*cm), and similarly for the large-resistance electrode cathode -2, the resistivity measured by the two-probe method is much larger than that of the aluminum foil resistivity; as can be seen from Figure 4 ( a) and ( b), the resistivities of the four- and two-probe When testing the resistivity of negative electrode, also for the small resistance electrode, the four-probe method can not distinguish the effect of the coating, and only the sample with large resistance can be distinguished, while the two-probe can obviously measure the resistance difference between different coatings and foils.

Figure 3. (a) Four-probe method positive electrode plate resistance test (b) Two-probe method positive electrode plate resistance test

Figure 4. (a) 4 point probe method negative electrode plate resistance test (b) 2 point probe method negative electrode plate

3.2 Analysis of Foil Pure Membrane Resistance

The resistance of dry-coated pure film samples without foils was tested by two methods, as can be seen in Figure 5: the resistivity of single-layer film tested by the 4 point probe method (0 .27 Ω * cm) is almost the same as that of the double-layer film tested (0 .26 Ω * cm) and is much larger than that of pure aluminum foil (2 .884 ^ 10 ^-5 Ω*cm) or pure copper foil (1.832 ^ 10 ^-5 Ω*cm), and that of 2 point probe method is also almost the same as that of double-layer film tested (1.23 Ω * cm), but the two-probe will be larger than that of pure aluminum foil. The resistivity of a single-layer film tested by the 2 point probe method (1 .27 Ω * cm) is also nearly the same as that of a double-layer film (1 .23 Ω * cm), but the 2 point probe resistivity will be greater than that of the 4 point probe method.

Figure 5. Pure membrane resistance test by 4 point probe and 2 point probe

3.3 Mechanism Analysis

The equivalent circuit diagram of the two test methods is shown in Figure 6, Figure a represents the circuit diagram of the 2 point probe electrode resistance, The figure shows that the current voltage is applied to the vertical ends of the sample, The result is the contact resistance of the polar sheet R, the R coating, the total resistance of the R collector fluid, Then the resistivity and the resistivity; Figure b is a schematic diagram of the circuit testing the electrode resistance by the 4 point probe method, The applied current is applied on the sample surface, So the current direction can be divided into paths 1,2, and 3, When the electrode sheet resistance is small, Electrons mostly pass through from path 2, Very arely through pathways 1 and 3, The measured electrode resistance is close to the foil resistance, When the sheet resistance is large, The probability of the electron passing through path 1 increases, The resistance value also increases, When testing the electrode sheet resistance by the 4 point probe method, Since the measured absolute value is small, Even getting up to the Ω level, The instrument accuracy, range and system pressure control and stability requirements are also high, It is therefore difficult to obtain stable data. In conclusion, it is appropriate to select the resistance of the composite layer.

Figure 6. (a) Schematic diagram of two probe circuit (b) Schematic diagram of four probe circuit

4. Summary

This paper compares the difference between the electrode resistance, the results show that test strip foil electrode sample, 4 point probe test resistance value is far less than the 2 point probe, and sometimes the resistance is almost close to the resistance of the foil, unable to distinguish the coating, and because of the small value, instrument precision, range and system pressure control stability requirements is high, so it is difficult to get stable data, only test the coating without foil, resistance value in Ohm level. The 2 point probe test is the total penetration resistance of the electrode plate, which can significantly distinguish the difference of the coating, and is suitable for testing lithium batteries and all types of supercapacitors.

5. References

[1] Hiroki Kondo et al. Influence of the Active Material on the Electronic Conductivity of the Positive Electrode in Lithium-Ion Batteries. Journal of The Electrochemical Society, 2019,166 (8) A1285-A1290

[2] B.G.Westphal et al. Influence of high intensive dry mixing and calendering on relative electrode resistivity determined via an advanced two point approach. Journal of Energy Storage 2017, 11, 76–85

[3] Nils Mainusch et  al. New Contact Probe and Method to Measure Electrical Resistances in Battery Electrodes. Energy Technology.2016, 4, 1550-1557