Materials: The choice of materials is the primary factor affecting the performance of lithium-ion batteries. If materials with poor cycle performance are chosen, no matter how reasonable the process or how perfect the manufacturing, the cycle performance of the cell will inevitably be compromised. If better materials are chosen, even if there are some problems in the subsequent manufacturing process, the cycle performance may not be too bad (a lithium cobalt oxide cell with a single charge of only about 135.5 mAh/g and lithium plating, although it drops by more than 100 cycles at 1C, still has more than 90% of the capacity at 0.5C and 500 cycles; a cell with black graphite particles on the negative electrode after disassembly has normal cycle performance).
From a materials perspective, the cycle performance of a full battery is determined by the poorer of two factors: the cycle performance of the positive electrode paired with the electrolyte, and the cycle performance of the negative electrode paired with the electrolyte. Poor cycle performance may be due to two reasons: firstly, the crystal structure changes too rapidly during cycling, preventing the completion of lithium insertion and extraction; secondly, the active material and the corresponding electrolyte may fail to form a dense and uniform SEI film, causing premature side reactions between the active material and the electrolyte, leading to rapid electrolyte consumption and thus affecting cycle performance.
When designing a battery cell, if one electrode is confirmed to use a material with poor cycle performance, then the other electrode does not need to use a material with better cycle performance to avoid waste.
Electrode compaction: While excessive compaction of the positive and negative electrodes can increase the energy density of the battery cell, it can also reduce the cycle performance of the material to some extent. Theoretically, greater compaction is equivalent to greater damage to the material structure, which is the foundation for ensuring the cyclicability of lithium-ion batteries. In addition, cells with high electrode compaction are unlikely to maintain a high liquid retention capacity, which is essential for the cell to complete normal cycles or more cycles.
Moisture: Excessive moisture can cause side reactions with the positive and negative electrode active materials, damage their structure, and thus affect cycle life. At the same time, excessive moisture is also detrimental to the formation of the SEI film. However, while trace amounts of moisture are difficult to remove, they can also, to some extent, ensure the performance of the battery cell.
Coating membrane density: Considering the impact of membrane density on cycle life as a single variable is virtually impossible. Inconsistent membrane density leads to either capacity differences or differences in the number of winding or stacked layers in the cell. For cells of the same type, capacity, and material, reducing membrane density is equivalent to adding one or more winding or stacked layers. The increased membrane density allows it to absorb more electrolyte, ensuring better cycle life.
Excessive negative electrode: Besides considering the impact of initial irreversible capacity and coating density deviation, the effect of excessive negative electrode on cycle performance is also a factor to consider. For lithium cobalt oxide plus graphite systems, the graphite negative electrode often becomes the "weak link" in the cycling process. If the negative electrode excess is insufficient, the cell may not have lithium deposition before cycling, but after several hundred cycles, the positive electrode structure changes very little, while the negative electrode structure is severely damaged and cannot fully accept lithium ions provided by the positive electrode, resulting in lithium deposition and premature capacity decline.
Electrolyte quantity: There are three main reasons why insufficient electrolyte quantity affects the cycle: first, insufficient electrolyte injection; second, although the electrolyte injection quantity is sufficient, the aging time is not enough or the positive and negative electrodes are not fully immersed due to excessive compaction; and third, the electrolyte inside the cell is consumed as the cycle progresses.
Objective conditions during testing: External factors such as charge/discharge rate, cutoff voltage, charging cutoff current, overcharge/over-discharge during testing, test chamber temperature, sudden interruption during testing, and contact resistance between the test point and the cell can all affect the cycle performance test results to varying degrees. Furthermore, different materials exhibit different sensitivities to these objective factors. Therefore, a unified testing standard and an understanding of the commonalities and characteristics of key materials should be sufficient for routine work.
In summary, much like the "weakest link" principle, among the many factors affecting the cycle performance of battery cells, the ultimate determining factor is the weakest link. Furthermore, these influencing factors also interact with each other. Given the same materials and manufacturing capabilities, higher cycle life often means lower energy density. Finding the perfect balance that meets customer needs and ensuring consistency in cell manufacturing is the most important task.
