Energy Storage, 2nd Edition

Chapter 10Electrolytes, Separators, and Membranes

Electrolytes, separators, and membranes (ESMs) constitute a class of components that serve important functions in electrochemical energy storage devices, and yet, their roles are often underestimated. Although they may not be directly involved in the main reduction and oxidation reactions of the chemical species that supply the electrochemical energy, the properties of the ESMs could significantly impact the performance and lifetime of the device.

ESMs are comprised of materials that are ion conductive but electron insulating. In other words, they allow the passage of ions such as Li⁺ and H⁺, but they must block the transport of the electrons. ESMs are generally placed between the two main electrodes (i.e., anode and cathode) inside the electrochemical cell. Although they have much in common, there are also subtle differences between “electrolytes”, “separators” and “membranes”. This is often influenced by the usage of the terms in the scientific literature, academia, and industry. The main function of an “electrolyte” in energy storage is considered to be ion transport and salt dissociation, and thus, an electrolyte generally can be a liquid or solid. On the other hand, “separators” and “membranes” physically separate the electrodes to prevent their contact (and electrical shorting), in addition to their basic function of ionic transport and permeation of species. Therefore, “separators” and “membranes” are generally made of ...