We measured a charge-voltage-curve and get the characteristic one for a single step charging reaction.
What could be a scientific way to determine the voltage boundaries for our material?
Up to now we use a threshold of the slope. If the slope gets above a certain limit, we define this as our voltage boundaries.
To get the largest battery capacity for a given material, the charging voltage should be as high as possible, and the discharge voltage as low as possible. The limits are given by the stability of the materials: voltage ranges that deteriorate your materials including the electrolyte must be avoided. Do you have any indication of processes others than the desired one in charge-voltage-curves with an extended voltage range? If so, try to determine the onset voltage of these processes from your curve. If possible, find out what these processes are (electrolyte decomposition, phase transition, dissolution … ?), and how serious they affect the performance of your battery. In some cases, points of inflection of the charge-voltage-curve might be used as potential limit, but there is no general rule here, because it strongly depends on how fast and how destructive the secondary processes are. In addition, it depends on the research goals. To demonstrate the capacity limits of a material (e.g. for comparison with the theoretical capacity), it can be acceptable to use a larger voltage range, accepting faster degradation and limited cyclability. A better approach than that is to consider procedures like constant current followed by constant potential to avoid destructive potentials and to get full charge and discharge anyway. To demonstrate cyclability, you need to optimize your potential limit, and the best way to demonstrate the best limits is often to run several identical cells with different limits.
To sum up: Good scientific ways to find the voltage boundaries include (amongst others): (a) To do test series: cycle identical batteries with different limits. (b) To identify the stability limits by identifying the degradation processes, their impact and their onset voltages.
The threshold of the slope can depend on the current used and seems kind of arbitrary to me, but maybe there are good reasons (previous experience and limited amount of material and time) to use it for your paticular kind of systems.
Thank you Dr. Kramer for your answer. You confirm our thoughts.
We don't want to run several cells to find the best conditions empirical, because this uses a lot of measurement time, which is for us limiting. Up to now we don't see any side reactions in our charge-voltage curve over the whole voltage window, so this approach does not fit for our material.
Our potential window exceeds the potential between two fresh electrodes by factor 2.3 (Potential between two fresh electrodes=0.8 V, window 0.15 V -> 2V)
So we need to investigate further the capacity fading mechanisms in our system.
To know the voltage boundaries, before going to test the charge discharge analysis,
you go to cyclic voltammetry first. From CV we can able to find the potential window of the sample. Next you can go for charge discharge analysis.
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