Charge storage mechanism of α-MnO 2 in protic and aprotic ionic liquid electrolytes

• Published in: Journal of power sources Vol. 460
• Main Authors: Lindberg, S., Jeschke, S., Jankowski, P., Abdelhamid, M., Brousse, T., Le Bideau, J., Johansson, P., Matic, A.
• Format: Journal Article
• Language: English
• Published: Elsevier 2020
• Subjects: Condensed Matter; Materials Science; Physics; MnO2; Hybrid; Supercapacitor; Protic; Ionic liquid
• ISSN: 0378-7753; 1873-2755
• DOI: 10.1016/j.jpowsour.2020.228111

In this work we have investigated the charge storage mechanism of MnO 2 electrodes in ionic liquid electrolytes. We show that by using an ionic liquid with a cation that has the ability to form hydrogen bonds with the active material (MnO 2) on the surface of the electrode, a clear faradaic contribution is obtained. This situation is found for ionic liquids with cations that have a low pKa, i.e. protic ionic liquids. For a protic ionic liquid, the specific capacity at low scan rate rates can be explained by a densely packed layer of cations that are in a standing geometry, with a proton directly interacting through a hydrogen bond with the surface of the active material in the electrode. In contrast, for aprotic ionic liquids there is no interaction and only a double layer contribution to the charge storage is observed. However, by adding an alkali salt to the aprotic ionic liquid, a faradaic contribution is obtained from the insertion of Li þ into the surface of the MnO 2 electrode. No effect can be observed when Li þ is added to the protic IL, suggesting that a densely packed cation layer in this case prevent Li-ions from reaching the active material surface.