The main drawback of Sodium-beta”-alumina is its high interfacial resistance. Even though bulk (single crystal) conductivity is very high, the practical assembly of a cell using NBA means using small solid electrolyte particles with additional treatment to make a dense pellet (mechanical pressing or sintering at 1600degrees). Without satisfactory pellet density, assembly leads to point contacts in the battery instead of a dense pellet, as well as poor contact between the solid electrodes and solid electrolyte. Ultimately, this would result in a battery with poor electrolyte conductivity and poor interfacial contacts. Promising attempts to reduce the interfacial impedance have been made, avoiding the high energy cost of sintering at high temperature.
NBA has been applied in the Na-S battery for 300degrees applications with compounds liquid at that temperature to insure good contact. This battery contains molten sulphur, molten sodium and highly corrosive polysulfides, potentially forming toxic H2S gas. Naturally, a resistant container is needed for such an electrochemical system. Even though this concept was a reliable candidate for automotive applications for a long time, the inherent cracking possibility leading to cell failure as well as propagation of the corrosive materials cannot be permitted in automotive vehicles prone to shocks.
Ionic liquids (ILs) containing a fraction of salt have been proposed as a viable alternative to organic solvents for LIBs and SIBs, as they do not share their flammable and volatile properties, presenting an improvement concerning the safety factor. Room temperature ILs need to be highlighted, as they have been extensively studied for ambient temperature applications.
Aside from being non-flammable, ILS also show high thermal and electrochemical stability. Even though these compounds are a promising alternative to flammable organic solvents, research is still in an early stage and ILs present other drawbacks such as a high energy consumption and cost of synthesis, and often have synthesis waste that are not environmentally friendly. For example, BMImBF4 requires 30 steps to be synthesized as well as hydrogen fluoride wast. Depending on the application, ILs need to be either highly conductive or electrochemically stable. For batteries, both are needed. A good example is Triethylsulphonium bis(trifluoromethylsulfonyl)imide with a reported conductivity of 8.2mS/cm and an electrochemical stability window of 5.5V.
Ionic liquids can be used to dissolve salts in order to increase the conductivity and facilitate ionic transport. For RT applications, pyrrolidinium and imidazolium are used to dissolve NaFSI or TFSI, reporting conductivities up to 3.2mS/cm at 298K, with an anodic electrochemical stability window of 5.2V vs Na+/Na0. Further research showed the anion FSI having a higher oxidation potential limit. Reports have even shown that ionic liquids with dissolved salts can even outperform organic electrolyte solutions due to a stable SEI formation.
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