
Anyone involved in the production of coin cells may have encountered a common issue: a batch of assembled cells experiencing widespread difficulties in reaching the desired voltage during charging. In other words, during the charging process, the cell voltage reaches a certain value, starts to level off, or even decreases, and consistently fails to reach the set voltage. This phenomenon can hinder research and testing of battery performance.
There are various reasons for this occurrence, and the following are some possible explanations:
1. Check Fixture Contacts and Channels: Firstly, the simplest and easiest factor to rule out is checking whether the fixture has poor contact or if the testing channel is damaged. Testing the cells with a different fixture and channel can help verify this. If the cells charge and discharge normally with a different fixture or channel, it indicates that the issue lies with the testing hardware, not the cells.
2. Internal Contamination of Cells: If most cells from the same batch exhibit this behavior, it's advisable to check if the cell casings were improperly stored, resulting in stains or contaminants inside the casing. These contaminants may dissolve, react, and consequently affect the battery's performance.
3. Dendrites Piercing the Separator Causing Micro Short-Circuits: Disassembling the cells and examining the negative electrode under a microscope can reveal if dendrites have formed. If present, dendrites may pierce the separator, causing a short circuit. Alternatively, specialized micro-short-circuit testing equipment can be used directly on the cells.
4. Improper Selection of Separator: If the separator has too large a gap, it can create a direct connection between the positive and negative electrodes, leading to a short circuit.
5. Delamination of Electrode Material: Disassembling the cells and inspecting whether the positive electrode material shows signs of delamination is crucial. Delamination can result in poor contact between the electrode material and the current collector, significantly reducing electronic conductivity.
6. Excessive Impurities in Electrode Material: If a significant portion of newly assembled cells still experiences overcharging, it may be due to excessive impurities in the prepared materials. Consider synthesizing a new batch of materials.
In addressing these factors, systematic troubleshooting and testing can help identify and resolve the root cause of the voltage charging issue in assembled coin cells.