Scientific Assessment of Electrode Flexibility: Analysis of Evaluation Methods for Flexibility of Anode and Cathode Electrodes

1. Introduction

In battery research and development (R&D), electrode flexibility is a critical parameter influencing battery performance. Analogous to the “skeletal flexibility” of a battery, it requires sufficient strength to support volumetric changes during charge/discharge cycles while possessing adequate ductility to withstand repeated mechanical stress. Numerous methods exist for evaluating electrode flexibility. Different manufacturers may employ distinct approaches based on their product characteristics and testing requirements, such as manual folding over rollers to observe light transmission, winding around needles to assess coating detachment, or the three-point bending method. However, these manual methods suffer from significant testing errors, poor reproducibility, and yield limited information.

IEST Battery Electrode Flexibility Testing System (BEF1000) differs from conventional methods. This instrument clamps the electrode at a specific bend angle within the test fixture and then applies displacement to induce deformation. It measures the force-displacement curve at varying degrees of deformation. Analysis of the force-displacement relationship enables precise comparison of flexibility between different samples. Significant differences exist between cathode and anode material flexibility, necessitating distinct testing and analysis methodologies:

• Cathode: Evaluated for bendability via single-cycle compression.

• Anode: Evaluated for recoverability via cyclic testing.

Figure 1. IEST Battery Electrode Flexibility Testing System (BEF1000) Equipment

2. Cathode Electrode Flexibility Testing: Challenging the “Critical Point” in Single-Cycle Tests

Cathode electrodes are generally more brittle, exhibiting coating cracking during testing. Therefore, the single-cycle constant-speed mode is typically employed. Analysis of the force-displacement curve determines the bendability (flexibility) of the cathode electrode. Figure 2 presents test data for two LFP (Lithium Iron Phosphate) cathodes with different compaction densities. Both curves exhibit an abrupt change point, indicating stress release during testing. This stress release primarily originates from cracking of the electrode coating. Generally, a shorter displacement at fracture indicates poorer bendability, i.e., inferior flexibility.

Figure 2. Force-Displacement Curves for Cathodes with Different Compaction Densities

How can differences be further discerned when the fracture displacements are comparable? The fracture force or curve slope can be analyzed. As shown in Figure 3, at the same displacement, a higher force indicates greater resistance to bending deformation, meaning the electrode is “stiffer”. Does a stiffer electrode inherently imply worse flexibility? Stiffness (hardness) reflects a material’s resistance to localized deformation, while flexibility reflects its deformability prior to fracture. These properties are governed by different factors; they may vary in tandem or inversely. Typically, electrode stiffness and flexibility exhibit a negative correlation. Higher stiffness often coincides with lower flexibility (increased brittleness). A stiffer electrode (e.g., high active material content, high compaction density, low binder content, or intrinsically stiff binder) is generally more prone to fracture or coating detachment during bending, winding, or impact, signifying poor flexibility. However, the strength and direction of this correlation are influenced and constrained by multiple factors including binder type/content, active material, conductive additive, compaction density, porosity, and microstructure. It is not a simple, strong positive or negative correlation.

Figure 3. Force-Displacement Curves for Sample A and Sample B

3. Anode Electrode Flexibility Testing: Targeting “Fatigue” in Cyclic Tests

Compared to cathodes, anode electrodes are generally softer and do not exhibit significant coating cracking during compression testing. Therefore, analysis can focus on another dimension: Recoverability (the ability of a material to return to its original shape after the removal of an applied force). The Yuaneng BEF1000 can perform reciprocating tests, evaluating anode recoverability via the “residual stress” measured upon returning to the origin point. The origin of residual stress is as follows: Force is zeroed before testing commences. During reciprocation, the electrode experiences some energy loss, or irreversible deformation. Consequently, upon returning to the initial position, the force exerted by the electrode deviates from the initial zero value. This deviation force is termed “residual stress”. A larger absolute value of residual stress indicates greater energy loss during the reciprocating test, signifying poorer recoverability (and thus poorer flexibility).

Figure 4. Schematic of Residual Stress Analysis

Figure 5 shows data for two anode electrodes containing different binders. Reciprocating tests yielded their residual stresses. The results indicate that Sample 1 exhibited a residual stress closer to the initial value (zero) after one reciprocating cycle. This suggests less irreversible damage occurred during testing, indicating superior recoverability and, consequently, better flexibility for Sample 1. Furthermore, the IEST BEF1000 can perform multi-cycle reciprocating tests to observe electrode fatigue results. See the article: Electrode Fatigue Testing | Evaluation of Anode Electrode Recoverability.

Figure 5. Reciprocating Test Results for Two Anode Electrodes

4. Summary

Based on the force-displacement curve characteristics and the physical state of the electrode during testing, the optimal testing mode and analysis method can be selected. Although electrode flexibility resides hidden within the battery, it constitutes a critical “invisible battlefield” determining product competitiveness. Through scientific testing methodologies and insightful data analysis, we empower each electrode with enhanced resilience – enabling batteries not only to “travel farther” but also to “endure longer”.