What factors affect the cycle performance of lithium-ion batteries?
1. Types of 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 process is, 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.
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 and rapid electrolyte consumption, thus affecting cycle performance. In cell design, if a material with poor cycle performance is selected for one electrode, it is unnecessary to select a material with better cycle performance for the other electrode.
2. Compaction of positive and negative electrodes
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 speaking, the greater the compaction, the greater the damage to the material structure, which is the basis for ensuring that lithium-ion batteries can be cycled. In addition, battery cells with high positive and negative electrode compaction are difficult to guarantee a high liquid retention, which is the basis for the battery cell to complete normal cycles or more cycles.
3. Excessive negative electrode
In addition to considering the impact of the 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 system, it is common for the negative electrode graphite to become the "weak link" in the cycle process. If the negative electrode is not sufficiently excessive, the cell may not have lithium deposition before cycling, but after hundreds of cycles, the positive electrode structure changes very little, but the negative electrode structure is severely damaged and cannot fully receive lithium ions provided by the positive electrode, thus causing lithium deposition and premature capacity decline.
4. Electrolyte volume
There are three main reasons why insufficient electrolyte level affects cycle life: first, insufficient electrolyte injection; second, even with sufficient electrolyte injection, inadequate aging time or insufficient immersion due to excessive compaction of the positive and negative electrodes; and third, the electrolyte inside the cell being depleted during cycling. Whether for cells that cycle hundreds or thousands of times or those that fail after only a few dozen cycles, if the electrolyte was sufficient before cycling but depleted afterward, increasing the electrolyte level can likely improve cycle performance to some extent.
5. Coating film density
Considering the impact of membrane density on cycle life as a single variable is almost an impossible task. Inconsistent membrane density will either lead to differences in capacity or differences in the number of winding or stacked layers of 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 corresponding increase in membrane density can absorb more electrolyte to ensure cycle life.
6. Moisture
Excessive moisture can cause side reactions with the positive and negative electrode active materials, damage their structure, and thus affect the cycle. At the same time, excessive moisture is also not conducive to the formation of the SEI film. However, while trace amounts of moisture are difficult to remove, they can also ensure the performance of the battery cell to a certain extent.
7. Objective conditions of the test
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. In addition, different materials have different sensitivities to the above objective factors. A unified testing standard and an understanding of the commonalities and characteristics of important materials should be sufficient for daily work.
