What Are the Core Factors That Determine the Cycle Performance of Lithium-ion Batteries?

The cycle performance of lithium-ion batteries is extremely important. It not only affects the battery's service life, but from a macro perspective, a longer cycle life also means more efficient resource utilization.

Whether you're working in the lithium battery industry or are a consumer interested in battery technology, understanding the factors that influence the cycle performance of lithium-ion batteries is essential. The following is a detailed analysis of these key factors.

1. Material Type

Material selection is the primary factor determining the cycle performance of lithium-ion batteries. If the material itself has poor cycle performance, even the most optimal process design and meticulous manufacturing processes will not guarantee the cycle life of the battery cell. Conversely, if excellent materials are selected, even if there are certain defects in the manufacturing process, the cycle performance will generally not be significantly reduced.

From a material perspective, the cycle performance of the entire battery is primarily determined by the performance of the positive electrode and electrolyte combination, or the performance of the negative electrode and electrolyte combination, whichever is worse.

If a material's cycling performance is unsatisfactory, the following reasons may be the cause: First, the crystal structure changes too rapidly during cycling, hindering lithium ion insertion and extraction; second, the active material and electrolyte cannot form a uniform, dense SEI film, causing premature side reactions in the active material and rapid electrolyte consumption, thus shortening the cycle life.

During the battery cell design phase, if the material selected for one electrode has poor cycling performance, there is limited room for improvement in the other electrode's high cycling performance, which can lead to a waste of resources.

2. Positive and Negative Electrode Compaction

While the compaction of the positive and negative electrodes can increase the energy density of the battery cell, excessive compaction often negatively impacts cycling performance. Theoretically, greater compaction applies greater stress to the material structure, and a stable material structure is essential for the sustainable cycling of lithium-ion batteries. Furthermore, high compaction can reduce the electrode's ability to retain liquid, a critical requirement for ensuring the battery cell successfully completes multiple cycles.

3. Moisture Content

Excessive moisture in the battery cell can cause side reactions with the positive and negative electrode active materials, damaging the material structure and thus reducing cycling performance. At the same time, high moisture levels are also detrimental to the formation of a stable SEI film. However, for trace amounts of moisture that are difficult to completely remove, it can actually help maintain the overall performance balance of the battery cell.

4. Coating Area Density

It is almost impossible to analyze the impact of coating area density on cycling performance alone, as it simultaneously affects multiple battery cell properties. Uneven area density can lead to capacity variations or affect the number of cell windings/laminates.

For cells of the same model, capacity, and material, reducing the area density is equivalent to increasing the number of windings or laminates, thereby increasing the number of separators. This helps absorb more electrolyte and ensures cycling stability.

Furthermore, a lower area density can improve cell rate performance and facilitate the baking and dehydration process of the electrode sheets and bare cells. However, an excessively low area density can make precise coating control difficult, and large active material particles can also negatively impact coating and rolling.

Increasing the number of layers can improve cycling, but it requires more foil and separators, resulting in higher costs and lower energy density. Therefore, during design and evaluation, a comprehensive balance must be struck between cycle performance, rate capability, process controllability, and cost.

5. Anode Overload

Anode overload not only affects the initial irreversible capacity and coating deviation, but also directly impacts cycle performance. In lithium cobalt oxide/graphite systems, the anode graphite often becomes the "bottleneck" of cycling.

If the anode underload is insufficient, while lithium deposition is less likely to occur in the initial stages of the cell, after multiple cycles, the positive electrode structure will remain largely unchanged, while the anode may be severely damaged and unable to fully absorb the lithium ions provided by the positive electrode, resulting in premature capacity loss.

6. Electrolyte Quantity

Insufficient electrolyte can affect cell cycle performance. The main causes include: insufficient electrolyte injection, inadequate electrode immersion (e.g., excessive compaction or insufficient aging time), and electrolyte consumption during cycling.

An incomplete SEI film can lead to side reactions between the anode and electrolyte. Furthermore, defective areas are constantly repaired during cycling, consuming reversible lithium and electrolyte. Therefore, whether it's a high-durability cell capable of hundreds of cycles or one whose performance degrades after a few cycles, ensuring sufficient electrolyte can improve cycling performance to a certain extent.

7. Objective Testing Conditions

Cell cycling performance test results are affected by a variety of external conditions, such as charge and discharge rate, cutoff voltage and current, overcharge and overdischarge conditions, test temperature, test interruptions, and the contact resistance between the cell and the test point. Furthermore, different materials have varying sensitivities to these conditions.

Therefore, establishing unified testing standards and understanding the properties of commonly used materials can generally meet routine testing needs.

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Like the "barrel principle," cell cycling performance is ultimately determined by the weakest link, and these factors often influence each other. With the same materials and manufacturing level, higher cycling performance generally means lower energy density. Therefore, the key is to find a balance between meeting customer needs and ensuring consistent cell manufacturing. This is the core task of improving cycling performance.

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