(Europe Section Alessandro Volta Medal Award) Electrolytes and Metallic Anodes: About an Old Argument That Marks the Fundamental Understanding of Modern Metal Anodes in Advanced Battery Systems

Abstract

Simple, yet meaningful, experiments, already carried out in the 18th century by Galvani and Volta, are at the very base of modern batteries that shape our society. From the very first muscle twitch to batteries found nowadays in nearly every part of modern life, with the best known example of a modern battery represented by the lithium ion battery (LIB), the same basic principles are present. The comparably fast and seminal development of LIBs through assiduous collective efforts was granted the 2019 Nobel Prize in Chemistry.[1] Besides all modern perspectives, fundamental principles are still being derived from the early works of Volta and Galvani, which have not been always in agreement.Because the LIB technology is more and more approaching its ceiling in terms of specific energy (Wh kg−1) and energy density (Wh L−1) imposed by the nature of intercalation chemistry, metal-based anodes, such as Li, Na, Mg, Ca, Zn and Al, are currently considered for the next evolutional step of battery technology.[2,3,4] Independent of the chosen materials, the chemistry of the anodes (graphite, silicon or metal anodes) with the electrolyte plays a crucial role in the development of the batteries. In LIBs, the formation of a solid electrolyte interphase (SEI) on graphite in organic media induced by an external current/voltage and involving the reduction of the electrolyte components was key for the stable operation of such anodes.[5] Similar processes for metal-based anodes do not necessarily require electrode polarization conditions and can occur spontaneously. For example, supplementary to SEI formation on the Li surface, induced by the extremely low redox potential of metallic lithium (−3.04 V vs. SHE), other processes of Li deterioration, namely galvanic corrosion (worth also considering here: dendrite growth, volume changes of the Li anode, low Coulombic efficiency), have to be scrutinized. Recently, battery researchers revisited the corrosion phenomena in lithium metal batteries (LMBs).[6,7] A systematic study focused on the corrosion-related processes in model Li anodes (using Li powder) revealed the great significance that such processes exercise on the performance of LMBs.[7]From a different perspective, flammable non-aqueous electrolytes, widely used in the LIB/LMB systems, are causing increased safety concerns. As an alternative, Zn has been regarded as a potential anode material for aqueous batteries because of its high theoretical volumetric capacity and suitable redox potential (−0.76 V vs. SHE), along with an intrinsic safety from the aqueous electrolyte nature. Similar to the Li anode in nonaqueous electrolytes, the Zn anode has also long-standing challenges in conventional alkaline electrolytes, such as chemical instability (e.g. corrosion) and electrochemical irreversibility (e.g. non-uniform electrodeposition/-dissolution, dendrite growth).[8]In this perspective, the two worlds of Li-based and Zn-based battery systems are united and attributed to the lessons by Volta and Galvani. Metals from the Volta pile (Cu and Zn) and modern metals (Li) are discussed side by side and in correlation to each other. Solutions to address their challenges, and thus overcome the barriers of their application, in rechargeable systems will be highlighted. It is emphasized that the combination of metals (e.g. Li) with metal ions (e.g. Zn2+) can result in formation of intermetallic phases, able to improve stability of the electrodes under cell operation conditions. This can further result in an increase in safety and cycle life of the batteries utilizing metals as negative electrodes.[9] Special attention will be also given to the obstacles caused by the alkaline electrolytes with focus on the fundamental understanding of Zn reaction mechanisms. Approaches to overcome the challenges are presented and evaluated for their potential to facilitate further research and development of electrolytes in these systems.