Applications of Zwitterions and Zwitterionic Polymers for Li‐Ion Batteries

Abstract

A zwitterion is a neutral compound that has both a cation and an anion in the same molecule. Quaternary ammonium cations are frequently used for zwitterions. Zwitterions with quaternary ammonium cations are also common in biological molecules, such as phospholipids, which are the main components of cell membranes. Chemically, they have broad applicability because they are dielectric, non‐volatile, and highly polar compounds with a large dipole moment. In addition, after salt addition, ion exchange does not occur in the presence of zwitterions. Owing to these characteristics, zwitterions have been applied as novel electrolyte materials targeting high ionic conductivity. In this review, application of zwitterions and their polymers for Li‐ion batteries is addressed.

Unexpected Energy Applications of Ionic Liquids

“…Thus, while there are not many neat ZIs that are liquid at room temperature-that requires very delocalized positive and negative charges-there are many possible ZI/Li salt combinations that melt below room temperature. [142,144] Furthermore, in the same way that the advantageous transport properties of other high salt content electrolytes has been recognized in recent years, [145] these high salt content ZI mixtures exhibit promising conductivities and transference numbers. Thus, there is substantial scope for the development of ZI-based electrolytes, through new cationic and anionic groups, exploration of different salt combinations (for example, using sodium salts), and their incorporation into gel or polymer electrolytes.…”

“…Of course, the concept of zwitterions (ZIs or "inner salts") is not new, and indeed zwitterionic liquids have been pioneered by the Yoshizawa-Fujita and Ohno group for over two decades. [142] Nevertheless, on the back of advances in zwitterion-based electrolytes for lithium and sodium batteries-in both liquid and solid form-it is worth highlighting this intriguing class of material. Much of the impetus behind the development of ZI-based electrolytes is that as the cationic and anionic species are covalently tethered, the ZI will not migrate in an electric field (i.e., during battery operation) and thus should not compete with the transport of the target ions, such as Li + and Na + , resulting in a high transference number.…”

“…Zwitterions are highly polar and thus have mostly been used as additives to enhance target ion dissociation or electrochemical stability in polymer electrolytes, [142] and more recently polymeric zwitterions with excellent properties have been reported. [143] Nevertheless, as for ILs, there is great scope in the design of ZI structures through the type and size of the cationic and anionic species and the linker length, and thus the physical and thermal properties (e.g., T m ) are tunable.…”

Ionic liquids and their derivatives for lithium batteries: role, design strategy, and perspectives

“…Through the development of these peculiar structures, ILs and their derivatives attained new functions while maintaining their previously mentioned advantages. For example, by fixing a pair of cation and anion or several cation structures with covalent bonds, new groups of materials such as ZILs [39][40][41] and PILs [42][43][44][45] are obtained. As charges are fixed via covalent bonds, ZILs and PILs tend to have T m and glass transition temperature (T g ) higher than those of ILs [39][40][41][42][43]45] .…”

Solvent-free transformation of protic ionic liquids into zwitterions

“…Zwitterions (ZIs) are versatile compounds containing positively and negatively charged functional groups. [1][2][3][4] Although they share similarities with ionic liquids (ILs), such as high thermal stability and negligible vapor pressure, they typically have high melting points and low ionic conductivity. Extensive research has been conducted on their applications in electrochemistry, 4,5 cellulose dissolution, 6 and biological uses.…”