Let's see if I can get some help here.
I'm working in a rechargeable battery system where the ionic liquid (1 ethyl 3 methyl imidazolium chloride/ AlCl3 with molar ratio AlCl3/EMIm higher than 1, acidic conditions) is in contact with a liquid metal Galinstan (Ga, In and Sn).
During the charging/discharging process, a dark layer is formed between the electrolyte and the anode and the performance of battery decreases. Several additives are being used to protect the anode but the list of additives is unending. Can somebody give me an idea of how to evaluate the stability of the passivation layer without the battery assembly and cycling?
I need to point out that the ionic liquid is highly sensitive to moisture.
Best regards,
Jorge A. Soto P.
Hi Jorge, I suggest you collect "dark layer" and analyze its elemental composition.
Two hypothesis running in the head : 1) IL components decomposition - should contain carbon, 2) electrolysis products of Galinstan minor components (because technical grade Ga, In and Sn are used).
Hi Andrei, thanks you for the answer. We have already analyzed the dark precipitate and it contains: 48 %atom C, 10 % N, 6 % Al, 18 %Cl (related to the ionic liquid), only 0.5 % of In (from liquid metal) and the difference is oxygen from solvent to remove the ionic liquid. Moreover, we observed the same precipitate when pure Gallium is used instead of Galinstan. The cut off voltage is not higher than 2.7V and the IL is supposed to be stable under these conditions.
So, we think that a passivation layer can protect the anode. We would like to find a conductive and stable additve that will protect the electrode but does not decrease the performance of battery.
You have 48/10 for C/N ratio by weight, dividing by atom masses we have 5.6 /1 for C/N atoms ratio. MeImEt products should have 6/2. In other words, dark precipitate originate from impurities of IL.
AlCl3 quality also can affect on stability - FeCl3 contamination accelerate tarring of organic reagents in reaction where AlCl3 is used
Once the EMImCl and AlCl3 are mixed at a given molar ratio, the IL is dried (vaccum at 80 °C) and electrolyzed (2 electrodes of Al at 5 mA for several days to remove the protonic impurities). The IL is filtered and dried again before the battery assembly.
Assay for EMImCl is 98% and AlCl3 99%.
Do you know any other way to remove impurities from the IL? I s recrystallization of EMImCl necessary (without high loss) before mix it with AlCl3?
Purification of AlCl3 (for electrochemistry) - sublimation over Al (Na) powder http://www.sciencedirect.com/science/article/pii/0020165070801933
Purification of EMIC (EMImCl) for electrochemistry - see attachment. Subsequent electrolysis should remove traces of electroactive compounds.
Solvents for purification should be with low level of non-volataile impurities (distilled)
Commercial EMIC maybe purified by activated carbon - it produces hard to removable microparticles. Activated carbon is carbon with -OH and -COOH groups on surface (should be collected on anode).
Thank you Andrei,
I think that I will purify the chemicals before mix them and dry the obtained IL just before the battery assembly.
My guess is it is decomposition of the EMIM with formation of carbenes followed by polymerisation. Purity especially of the EMIM Cl is essential, but when the coditions get too harsh....at what temperature are you running?
I'm working at room temperature and the voltage range for the battery is 0 - 2.7 V
That might actually be enough to start decomposing the cation. What purity and color was the EMIM?
EMIm was purchased with 98% purity (green color), once the IL (EMIm/AlCl3) is obtained and dried an electrolysis is done (~ 5 mA, Al electrodes, under Ar, ~ 48h) to remove protic impurities. Then, centrifugation is used to get the clear IL.
well, anything but 99% and white below 500ppm of water will not do for this application I am afraid. Green indicates Nickel impurity from the steel reactor it was made in.
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