An Analysis of the Effect of Absorbent Membranes on Electrochemical Stability in Aqueous Li-ion Cells

Batteries have come a long way in terms of safety, portability, and energy density. Lithium-ion batteries are highly prevalent today and touted for their high energy density and rechargeability. One of the major drawbacks of lithium-ion systems are that they tend to be flammable. Since they are usually operated outside of their stability limits, flammability leads to catastrophic failure. Currently, almost all commercially available lithium-ion batteries are based on these systems. Water is a good solvent to use to bypass flammability. Water poses its own issues with stability such as its limited electrochemical stability window of 1.23V. A recent study from Science observed highly concentrated LiTFSI salt in water increased the stability window drastically, up to ~3.0V. One of the reasons this worked so well is because this salt is highly soluble in water. Using the principles of water affinity, in this study, we hypothesize that highly hydrophilic membranes with a lower salt concentration would yield an increase in stability window in a similar system. We found that a high molecular weight super absorbent membrane soaked in electrolyte with a swelling ratio of 14 on average, is able to increase the stability window of 5m LiTFSI by ~1.2V. Similarly, membranes of moderately high swelling ratios (~4–7) were also able to significantly increase the stability window of 5m LiTFSI. It was also found that a small addition of LiF to the electrolyte used in parallel with a membrane of moderately high swelling ratio is able to improve the stability window to a level on par with a high molecular weight membrane alone