Research on Correlation Between Cathode Powder Resistance and Electrode Resistance

1. Preface

With the rapid development of the lithium battery industry, lithium-ion batteries have been widely used in mobile phones, computers, automobiles, energy storage, and other fields. Users have higher and higher requirements for fast charging, and the corresponding requirements for battery rate performance are also increasing. Rate performance is closely related to battery resistance, which includes ionic resistance and electronic resistance. Because the electronic portion can be evaluated before slurry mixing and coating, accurate powder resistance measurement offers a fast route to pre-screen materials for cathode powder battery testing and to predict finished electrode resistance, accelerating formulation choices and process optimization.

In the full battery electronic resistance, positive electrode resistance usually accounts for a relatively large share. Positive electrode sheet resistance is affected by a variety of materials, including active materials, adhesives, and conductive carbon; the active material content ratio is as high as 95% or more. When the ratio of other materials is fixed, powder resistance and electrode sheet resistance are theoretically expected to show a certain degree of correlation. If this correlation can be determined, evaluation of cell resistance can be advanced from the electrode end to the material end — saving R&D and production costs while accelerating R&D progress.

There are currently two methods for testing cathode active material powder resistance: the four-probe method and the two-probe method. The four-probe method characterizes the lateral resistance of the surface of the powder tablet, while the two-probe method tests the longitudinal resistance of the powder tablet. Electrode resistance is usually tested using the two-probe method; for details, see “New method for monitoring stability and uniformity of battery electrode.”

2. Experimental Equipment and Test Methods

In order to ensure the comparability of the test results, the four-wire method based on the two-probe method and the double-disc electrode with controllable pressure were selected for the powder resistance and electrode resistance tests. The powder resistance tester reached a maximum applied pressure of 200 MPa, making the compacted state of the powder closer to the compacted state of the powder in the electrode.

2.1 Experimental Equipment

2.1.1 Powder resistance & compaction density test

Model PRCD1100 (IEST), electrode diameter 16mm, test pressure range 10~200MPa, holding time 10s. The equipment is shown in Figure 1(a) and 1(b).

Figure 1. (a) PRCD1100 appearance; (b) PRCD1100 structure — used for cathode active material powder resistance testing

2.1.2 Electrode resistance & compaction density test

Model BER1300 (IEST), electrode diameter 14mm, applied pressure 25MPa, holding time 25s. The equipment is shown in Figure 2(a) and 2(b).

2.2 Test Method

2.2.1 Powder Resistance & Compaction Density Test

Take 1~2g of powder sample, weigh it and pour it into the fixture (PR-Device-16), pre-vibrate the powder on the pre-vibration instrument (Pre-V1) to make the accumulation state of the powder consistent before pressing. Put the fixture into the powder resistance meter PRCD1100, start the PRCDMS software to set the test pressure and pressure holding time parameters, and the software automatically reads the data of powder sample thickness, compacted density, resistance, resistivity, conductivity and other data.

2.2.2 Electrode Resistance & Compaction Density Test

Cut the rolled electrode into a rectangular size of about 5cm × 10cm, place it between the two electrodes of the electrode resistance meter, start the MRMS software, and set parameters such as test pressure, holding time, and active substance quality. The software automatically reads electrode thickness, compacted density, resistance, resistivity, conductivity, and other data.

3. Case Study 1: Lithium Cobalt Oxide (LCO) System

Lithium cobalt oxide material is currently the most widely used cathode material for consumer batteries. Due to its large powder resistance and poor conductivity, LCO material often needs to be modified, such as by doping or coating. Figure 3(a) shows the powder resistance test of LCO material before and after modification, with the relationship curve between powder conductivity and powder compaction density automatically obtained.

From the figure, at compaction density less than 3.87 g/cm³ (pressure applied at 75 MPa), the conductivity of the modified powder is lower than before modification; when compaction density is greater than 3.87 g/cm³, the conductivity of the modified powder begins to exceed that of the unmodified powder, and increases rapidly with further compaction density increase. Comparing Figure 3(b), the conductivity of electrodes (electrode compaction density 4.0 g/cm³) prepared from the two powders with the same formula and process shows that the modified LCO electrode conductivity is significantly better than before modification — confirming the modification was effective. These results show that for this material, when the pressure state of the powder is consistent with the pressure state of the powder in the electrode, powder resistance and electrode resistance show a consistent trend.

Figure 3. (a) Powder conductivity vs. compaction density for LCO before/after modification; (b) corresponding electrode conductivity — confirming powder resistance and electrode resistance track together

 4. Case Study 2: Nickel Cobalt Manganese (NCM) System

Nickel-cobalt-manganese ternary materials are the first choice of cathode materials for most power batteries due to their higher gram capacity. Gram capacity is mainly affected by Ni content — the higher the Ni content, the higher the reversible capacity, but increasing Ni content also affects cycling, rate, and thermal stability to a certain degree. Figure 4(a) shows the relationship between powder conductivity and powder compaction density for three different Ni content levels. As Ni content increases, powder conductivity also shows an increasing trend. Comparing electrode conductivity (electrode compaction density 3.2 g/cm³) prepared with the same formula and process in Figure 4(b), the same trend of electrode conductivity increasing with Ni content is observed. These results indicate that ternary powder resistance and electrode resistance have a corresponding relationship.

5. Case Study 3: Lithium Iron Phosphate (LFP) System

With the market’s strong requirements for power battery safety, lithium iron phosphate materials are gradually becoming the preferred cathode material for many power battery companies due to their high structural stability. However, LFP material itself has poor conductivity and needs to be modified in advance before use in power batteries requiring high rate performance. Common modification methods are doping, carbon coating, and nano-sizing. Figure 5(a) compares powder conductivity and compaction density of LFP materials with three different modification methods. Under the same compaction density conditions, the conductivity trend of the three materials is: LFP-1 > LFP-2 > LFP-3. Comparing electrode conductivity (electrode compaction density 2.4 g/cm³) prepared with the same formula and process, the same trend — LFP-1 > LFP-2 > LFP-3 — is observed, so powder conductivity can be used to quickly evaluate the effect of modification.

Figure 5 (a) Comparison of the relationship between the conductivity of the LFP powder before and after modification and the compacted density; (b) Comparison of the conductivity of the electrode prepared by the LFP material before and after modification

6. Conclusion

In this paper, the four-probe method and the controllable-voltage double-disc electrode method were used to test the powder resistance and electrode resistance of lithium cobalt oxide, ternary, and lithium iron phosphate materials, with compaction density obtained simultaneously for both. When the compacted state of the powder is close to the compressed state of the powder in the electrode, powder resistance shows the same trend as electrode resistance across all three systems, so evaluation of the cell’s electronic resistance can be advanced from the electrode end to the powder end. Integrating this powder resistance screening into the development workflow enables faster material selection and optimization, significantly reducing costs and time-to-market for new battery designs.

7. References

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