Chaotropic Anion-Based "Water-in-Salt" Electrolytes for Beyond Li-Ion Energy Storage Systems

Abstract

Electrochemical energy storage systems (EESS) with high energy density, safety, low cost, and low carbon footprint have become indispensable in the modern era of ubiquitous electronics, electric vehicles, and grid storage. Current EESS lack these characteristics due to the utilization of critical materials that are precious, flammable, difficult to recycle, and pose a threat to the environment. Whereas aqueous electrolytes offer higher safety and lower cost. Nevertheless, their biggest bottleneck is the narrow electrochemical stability window (ESW) that is preventing them from attaining higher energy and power densities in most of the aqueous EESS. For instance, multivalent metal-ion-based aqueous rechargeable batteries promise high energy density due to the multivalent redox chemistry of metal ions (Al3+, Zn2+, etc.) but they exhibit much lower energy density in real experiments due to the limited ESW of the aqueous electrolytes. The water-in-salt (WIS) electrolytes can potentially eliminate this barrier by offering a larger ESW. However, an anion’s nature, i.e., kosmotropic (water structure-maker) or chaotropic (water structure breaker) plays an important role in the expansion of the ESW in WiS-electrolytes. In general, the kosmotropic anions have a negative impact on the ESW by disturbing the solvation shell of the counter cations. On the other hand, chaotropic anions increase the ESW by limiting the fraction of free water in the electrolytes by pushing water molecules into the solvation shells of the cations.We demonstrate WIS-electrolytes of metal-perchlorates (metal: Al, Zn, etc.) containing strong chaotropic anion (ClO4 -). As a result, the WiS-electrolytes of Zn- and Al-perchlorates demonstrate expanded ESW of 2.80 V and 4 V, respectively. The electrochemical tests of the electrolytes are performed using carbon-based redox-active electrode materials. We employ the new electrochemical systems in aqueous rechargeable multivalent dual-ion batteries, revealing superior performance to standard aqueous electrolytes. Our findings provide new possibilities for widening the ESW and enhancing the energy and power density in aqueous EESS, especially for grid storage applications. A. Zafar, G. Abbas, K. Knizek, M. Silhavik, P. Kumar, P. Jiricek, J. Houdkova, O. Frank, J. Cervenka, Journal of Materials Chemistry A, 2022, 10, 2064-2074.A. Zafar, G. Abbas, M. Silhavik, K. Knizek, O. Kaman, F. J. Sonia, P. Kumar, P. Jiricek, J. Houdková, O. Frank, J. Cervenka, Electrochimica Acta, 2022 404, 139754.