Lithium Battery Electrode Coating Uniformity Evaluation: In-situ AB Surface Resistance Test Method

1. Preface

The performance and consistency of lithium-ion batteries are heavily influenced by the quality and uniformity of the electrode coating process. As the industry increasingly adopts double-sided coating techniques to improve production efficiency, ensuring consistent conductive networks on both the A-side and B-side of electrodes — for both cathode and anode coating layers — has become critical. Non-uniform battery electrode coating can lead to imbalanced current distribution, localized lithium plating, and reduced cycle life.

This paper introduces an in-situ AB surface resistance test method developed by IEST to evaluate the coating uniformity of cathode and anode materials. Using the BER2500 electrode resistance tester, this approach simultaneously measures R_total, R_A, and R_B in a single measurement, providing a rapid and effective way to monitor battery electrode coating quality and support electrode coating process optimization.

2. Why AB Surface Coating Uniformity Matters

Electrode coating uniformity refers to the consistency of coating thickness and surface density distribution across the coated area. The better the thickness uniformity and conductive-agent distribution consistency, the higher the coating uniformity — and vice versa. Currently, there is no unified measurement index for coating uniformity across the industry. Battery manufacturers typically use one of the following metrics:

• Deviation or percentage deviation of coating thickness/surface density at each point relative to the area average
• Difference between maximum and minimum coating thickness or surface density within a defined area

Coating thickness is typically expressed in µm. However, the accuracy of thickness gauges is often insufficient to resolve subtle conductive network heterogeneity — particularly in double-sided coating where A-side and B-side processing conditions differ. Battery electrode coating manufacturers therefore need an effective, rapid method that goes beyond thickness measurement to distinguish AB surface uniformity differences.

Note: Typical wet thickness ranges reflect industry-standard electrode coating process parameters. Actual values depend on target areal capacity, active material density, and formulation solid content. Resistance-based uniformity testing is sensitive to conductive network variations within these thickness ranges that gauge-based measurement misses.

2.1 Possible Reasons for AB Surface Coating Differences

The electrode coating process involves many interacting parameters that can introduce AB surface differences. Key sources of non-uniformity include:

• Incoming slurry state: Electrode slurry consists of micron-sized active particles and nano-sized conductive agent particles suspended in a binder solution. Gravity, Brownian motion, and buoyancy cause particle settling, agglomeration, and redistribution over time — altering conductive agent and active particle distribution before and during coating.
• Conductive agent and binder migration during drying: During drying of the wet electrode coating, binder and conductive agent particles dissolved in solvent can migrate toward the electrode surface as solvent evaporates. This causes binder float and uneven conductive agent distribution — particularly pronounced in double-sided coating, where the A-side coating is dried first.
• B-side drying asymmetry: When B-side drying parameters match A-side parameters exactly, the presence of the cured A-side coating alters the thermal and solvent-transport environment on the B-side. Even under identical set conditions, the B-side wet coating dries at a different rate and in a different thermal gradient — producing systematic differences in binder and conductive agent distribution between the two sides.

2.2 Impact of AB Coating Differences on Battery Performance

Non-uniform battery electrode coating between the A-side and B-side of a double-sided electrode creates compounding performance issues that worsen with cycling:

• Uneven N/P ratio and current distribution: Inconsistent surface density or conductivity between AB sides creates a locally imbalanced N/P ratio or active-material utilization difference. Under high-rate charge/discharge or overcharge, this produces uneven current distribution and may trigger lithium plating on the underloaded side.
• Differential lithiation state and long-term mechanical failure: Different conductivities between AB sides of an electrode cause the two sides to reach different lithiation depths at the same applied current. Over long cycling, this accumulates different mechanical stresses on each side — eventually causing electrode coating cracking, delamination, and cell failure.

3. Experimental Methods: BER2500 AB Surface Resistance Test

The BER2500 in-situ AB surface resistance test provides a single-step, electrically relevant assessment of electrode coating uniformity — measuring the conductive network quality on both sides of a double-sided electrode in one measurement under controlled contact pressure.

Equipment: BER2500 (IEST) — 14 mm sample diameter; controllable contact pressure; automated data logging. The BER2500’s AB test returns three values per contact point in a single push-down measurement:

• \(R_{total}\) — total through-resistance of the double-sided electrode
• \(R_A\) — A-surface resistance (top coating layer)
• \(R_B\) — B-surface resistance (bottom coating layer)

Figure 1 (a) BER2500 appearance; (b) BER2500 structure diagram

Procedure:

1. Randomly sample multiple measurement sites across the electrode foil — 10 locations per sheet were used in this study.

2. At each site, perform the single-push AB test under fixed contact pressure (25 MPa) to obtain \(R_{total}\), \(R_A\), and \(R_B\).

3. Compute statistics per sheet and per side: mean (\(\bar{R}\)), standard deviation (\(\sigma\)), and coefficient of variation (\(COV = \sigma/\bar{R} \times 100\%\)). A larger COV indicates poorer electrode coating uniformity.

Why resistance, not thickness? Unlike thickness-only measurement methods, AB-surface resistance directly measures the electrically relevant outcome of coating heterogeneity — conductive network continuity and contact resistance to the current collector. This makes resistance-based anode coating measurement and cathode coating measurement significantly more sensitive to formulation and process differences that impact actual cell performance.

\[\mathrm{COV} = \sqrt{\frac{\sum_{i=1}^n (R_i – \overline{R})^2}{(n-1)\overline{R}}} \times 100\%\]

Note: Where n represents the number of tests, Ṝ represents the average value of all test resistances.

4. Data Analysis: Cathode and Anode Coating Uniformity Results

In-situ AB surface resistance testing at 25 MPa across multiple cathode electrode sheets reveals consistent, quantifiable differences in A-side and B-side coating quality — information inaccessible to thickness-only methods. Figure 2 shows the in-situ AB surface test results for three different double-sided cathode electrode sheets at 25 MPa. Figures 2(a) and 2(b) are two repeatability tests for cathode electrode 1; Figures 2(c) and 2(d) are the results for cathode electrodes 2 and 3 respectively. Each group uses 10 measurement sites. Red dots indicate mean values; green triangles indicate the COV for each test group.

Key findings from the electrode coating uniformity data:

• Cathode electrode 1 (Figures 2a, 2b): \(R_A\) values are slightly larger than \(R_B\) values across both test repetitions — indicating a systematic AB difference in conductive network quality. The relationship \(R_A + R_B \approx R_{total}\) holds consistently, validating the measurement method. Both test repetitions show \(COV\) below 5\%, confirming good electrode coating uniformity and test reproducibility.

• Cathode electrodes 2 and 3 (Figures 2c, 2d): Both show higher resistance values than electrode 1. Electrode 2 demonstrates better AB surface coating uniformity than electrode 3, as indicated by lower \(COV\). The \(R_A + R_B \approx R_{total}\) relationship holds for both electrodes.

Figure 3(a) and 3(b) compare two cathode electrodes with different process formulas. Cathode electrode 1 shows better AB surface resistance uniformity than cathode electrode 2, demonstrating that formulation changes are detectable by resistance-based electrode coating measurement even when thickness measurements may show no significant difference. Figures 3(c) and 3(d) show the AB surface resistance results for carbon-coated and primer-coated aluminum foil current collectors. In both primer-coated foil samples, B-surface resistance is consistently greater than A-surface resistance — a systematic difference attributable to the asymmetric drying history of the electrode coating process.

Figure 3 (a) and (b) are respectively the in-situ AB surface test results of cathode electrode 1 and cathode electrode piece 2, (c) and (d) are respectively the in-situ AB surface test results of carbon-coated aluminum foil 1 and primer-coated aluminum foil 1.

5. Conclusion

The BER2500 in-situ AB surface resistance test delivers a fast, sensitive, and manufacturing-relevant metric for battery electrode coating uniformity evaluation. By resolving A-side and B-side resistances independently and using COV as a uniformity index, battery manufacturers can:

• Detect subtle process or formulation issues that thickness gauges cannot resolve

• Identify systematic AB surface asymmetry arising from double-sided electrode coating process conditions

• Optimize cathode and anode coating processes — including drying parameters, slurry formulation, and coating sequence — based on directly relevant electrical data

• Improve cell consistency and reliability at the electrode manufacturing stage, before assembly

6. References

[1] Lang Peng, Ren Jian. Thinking about Development of Chinese Lithium-ionPower Battery Critical Process Equipment [J]. Special Equipment for the Electronic Industry, 2009, 38(11): 23-26.

[2] Song Lan, Xiong Ruoyu, Song Huaxiong, et al. Multiscale nonuniformity of lithium-ion batteries [J]. Energy Storage Science and Technology, 2022, 11(02): 2095-4239.2021.0409.

7. FAQs