Mehdi Soleimanzade This sometimes happens. I explain three scenarios. First, side reactions. You may realize that battery cyclers are estimating capacity based on Columb counting e.g. integrate current vs. time until the cut-off potential is reached, not actual storage capacity of the electrode. So, if there are any side reactions that consume current without affecting the voltage (e.g. charge is not actually intercalating to a site in the electrode, if you use intercalation-based chemistry), then this current will also be integrated adding to the capacity. If this occurs in the discharge step, then you will get >100% CE. Second: the way you started your cycling test and the default configuration of the cycler. If the cycler calculated CE as discharge/charge (as it should be in almost all cycles), you should make sure that your first step in the cycling procedure is to charge, then the cycler reports to you the correct value as CE100%. As you were In high C-rate, there is a high concentration gradient in the electrolyte. When you do the transition step going to low C-rate, that gradient may still be there in the first cycle after transition and this will make the battery reach charge cut-off potential relatively quickly. When you do the discharge step of this cycle, you already have spent some time on the low C-rate so the concentration gradient is relaxed a bit. Therefore, in discharge, it can take longer time to reach the cut-off voltage. The overall effect of this is CE>100% for this transition cycle, but afterwards, it will return to normal that CE

Let's assume that this is the case for half-cell. I think the Coulombic efficiency over 100% is related to some cases: (1) Irregular amount of Li ions transport during charge-discharge. If the Li-ion is less intercalated due to some structural interference during the discharge process and the maximum amount of Li-ion is released during the charge process , the Coulombic efficiency may exceed 100%. This phenomenon can cause the structural failure of active material. (2) Side reactions during charge process. The side reaction can make it appear that more capacity is generated than the amount of Li-ions released from the active material. (3) measuring error.

I think cases (1) and (2) are more likely to happen, and if this happens repeatedly, it may adversely affect the battery.

Well explained. Thank you @jaewon

Mehdi Soleimanzade This sometimes happens. I explain three scenarios. First, side reactions. You may realize that battery cyclers are estimating capacity based on Columb counting e.g. integrate current vs. time until the cut-off potential is reached, not actual storage capacity of the electrode. So, if there are any side reactions that consume current without affecting the voltage (e.g. charge is not actually intercalating to a site in the electrode, if you use intercalation-based chemistry), then this current will also be integrated adding to the capacity. If this occurs in the discharge step, then you will get >100% CE. Second: the way you started your cycling test and the default configuration of the cycler. If the cycler calculated CE as discharge/charge (as it should be in almost all cycles), you should make sure that your first step in the cycling procedure is to charge, then the cycler reports to you the correct value as CE100%. As you were In high C-rate, there is a high concentration gradient in the electrolyte. When you do the transition step going to low C-rate, that gradient may still be there in the first cycle after transition and this will make the battery reach charge cut-off potential relatively quickly. When you do the discharge step of this cycle, you already have spent some time on the low C-rate so the concentration gradient is relaxed a bit. Therefore, in discharge, it can take longer time to reach the cut-off voltage. The overall effect of this is CE>100% for this transition cycle, but afterwards, it will return to normal that CE