Lithium battery companies do not rely on button cells (such as CR2032, CR2025, etc.) for cycle testing in large-scale product development and quality control, mainly due to the following technical and management reasons, which together determine that the laboratory data of button cells cannot directly reflect the performance and life of actual battery products:
Differences in structure and design lead to unreliable data
Half-cell vs. full-cell structure:
- Laboratory button cells/coin cell battery usually use metal lithium sheets as negative electrodes (half-cell structure), while the actual commercial lithium battery negative electrode materials are graphite, silicon-based materials, etc. (full cells are required to be tested). Metallic lithium has extremely high activity, and its electrochemical behavior (such as lithium dendrite growth and interface side reactions) is completely different from that of commercial negative electrodes, resulting in the inability to transfer the cycle stability test results to real products.
- For example, the attenuation mechanism of the positive electrode material in the half-cell test may be masked by the irreversible consumption of the lithium negative electrode, and cannot reflect the actual attenuation after the positive and negative electrodes are coupled in the full battery (such as electrolyte decomposition, SEI film thickening, etc.).
Size and process differences
Button cells have small electrode areas (usually ≤20mm[CR20 Series] in diameter) and thin coatings (manual or small-scale preparation), while mass-produced cells use automated coating and roller pressing, with higher electrode thickness and compaction density. This leads to:
- Uneven current distribution: The edge effect of button cells is significant, the local current density is too large, and the decay is accelerated.
- Mechanical stress difference: Mass-produced batteries are subjected to greater stress from electrode expansion/contraction during the cycle, and the electrode fixing method of button cells cannot simulate this scenario.
Test conditions do not match actual applications
The amount of electrolyte does not match:
Button cells have excessive electrolyte (usually immersing the entire cell), while mass-produced batteries use precise injection (strictly optimized electrolyte/active material ratio). Excess electrolyte will mask side reactions (such as solvent decomposition and gas production), resulting in falsely high cycle life test results
Packaging and pressure conditions:
The steel shell hard packaging of button cells cannot simulate the flexible constraints of pouch cell or Prismatic cell batteries. Mass-produced batteries require specific external pressure to maintain interface contact, but the internal spring design of button cells cannot provide an equivalent mechanical environment, affecting lithium ion transmission and interface stability.
Data errors are large and lack statistical significance
Manual operation introduces deviations
Coin cell battery assembly relies on manual operation (such as coating, cutting, and lamination), and variables such as electrode uniformity, alignment accuracy, and electrolyte wettability are difficult to control. Studies have shown that the capacity error of button cells assembled by different operators can reach 10%~15%, and the cycle life deviation exceeds 20%.
Small samples cannot represent mass production consistency
Companies need to test hundreds to thousands of batteries to evaluate mass production consistency (such as capacity distribution and cycle life standard deviation). Button cell battery experiments can only test a small number of samples (usually <10), and the data lacks statistical significance.
Coin Cell battery charge and discharge curve:
The real function of button batteries:
Although button batteries are not used for cycle testing, they are still very useful. Both material development companies and battery cell manufacturers cannot do without the production and testing of button batteries.
Applicable scenarios of button batteries: supplementary role in the early stage of R&D
- Material screening and mechanism research
Because the corresponding electrode of button batteries is an excess of lithium sheets, and the coating surface density is low, it is generally tested with a small current to calculate the specific capacity of the material to the greatest extent.
For example:
- Quickly verify the specific capacity and voltage platform of new materials (such as new positive electrodes);
- Verify the specific capacity and efficiency of positive and negative electrode materials as a reference for the design capacity of soft packs and square batteries;
- As the first threshold for material screening, or benchmark with supplier data;