Interphase engineering of reactive metal surfaces using ionic liquids and deep eutectic solvents—from corrosion control to next-generation batteries

Abstract

Ionic liquids are unique solvents composed entirely of ions and have recently been considered for applications ranging from synthesis, separations, electrochemical devices, tribology and corrosion. In this perspective, we summarise the literature, and look at the future prospects, surrounding the use of ionic liquids in the engineering of interphases to control charge transport thereby leading to improved performance of high-energy density batteries, including Mg, Li and Na metal as well as corrosion protection of reactive engineering alloys, such as aluminium, magnesium and steel alloys. The ability to create task-specific ionic liquids by controlling the chemistry of either the anion or the cation means that interphases can be engineered for specific substrates and applications. Thus far, fluorine containing anions, such as bis(trifluoromethane) sulfonamide and its analogues, have been favoured for controlling the conductive solid-electrolyte interphase layer on Li and Na, while ionic liquids containing organophosphate anions have been used to form nanometre thick protective interphases on Mg alloys. Recently, ionic liquids based on carboxylate anions have also been shown to provide excellent corrosion inhibition for steel. In the search for cost-effective solutions, a relatively new class of ionic liquids, termed deep eutectic solvents, have also been explored as potential media for controlling surface films on reactive metals. The deep eutectic solvents class of ionic liquid materials offers many possible combinations of chemistry that can be targeted to produce desired properties in this context.

Ionic Liquids and Organic Ionic Plastic Crystals: Advanced Electrolytes for Safer High Performance Sodium Energy Storage Technologies

“…This can also be made possible through the use of additives (i.e., H 2 O, vinylene carbonate) which have been used routinely in conventional battery electrolytes in the past . In addition to the intrinsic bulk properties of the electrolyte, an important role of the IL is the promotion of a beneficial solid–electrolyte interphase (SEI) . This SEI often dictates the electrochemical stability, ion/charge transport at the electrode surface and may be the critical factor in the successful cycling of an electrode .…”

“…There has been much interest in the characterization of the SEI which forms upon the alkali anode since the first report of an ionic liquid enabling plating/stripping of Li metal . More recently there have been reports of studies toward understanding the SEI and interfacial structure within sodium based electrolytes, but very few in depth studies with ionic liquid electrolytes .…”

“…Using scanning electron microscopy (SEM) and energy dispersive X‐ray spectroscopy the study revealed that higher concentrations of salt minimize electrolyte breakdown at the surface of Na metal and promote better SEI formation without tarnishing the anode surface. Due to the number of studies reported, the breakdown of the [FSI] − anion is well understood through a variety of different approaches in Li systems . Another in depth analysis of the SEI by Qian et al also in a superconcentrated electrolyte, albeit for LiFSI solvated in DME, show how these electrolytes outperform less concentrated analogues and prepare a highly conductive SEI layer …”

Water distribution at the electrified interface of deep eutectic solvents

“…The presence of water in DESs is almost inevitable and impossible to eliminate. 6,7,24,49,50 The presence of latent water in DESs has a signicant impact on the physico-chemical properties of DESs including the structure, dynamics, electrochemical window and melting point. 7,8 Because of this, a number of experimental and computational studies have been dedicated to the investigation of the inuence of water on the physico-chemical properties of DESs, with special emphasis on Reline.…”

Solid Electrolyte Interphases on Sodium Metal Anodes

“…By controlling the chemistry of anions or cations, taskspecific ILs can be produced to modulate interfaces for specific substrates and applications. [121] For instance, ILs containing different anions, such as FSI − and TFSI − , demonstrate distinguishing stabilities against Na metal anodes, which is relevant to their unique interfacial structures (Figure 7a). [122] Hosokawa et al experimentally investigated the properties of FSI − and TFSI − -based ILs for Na rechargeable batteries.…”