Solvent Co-Intercalation for Electrochemical Energy Storage
Authors: Åvall Gustav , Alvarez Ferrero Guillermo , Son Youhyun , Janßen Knut , Adelhelm Philipp 
Published: September 2023

One of the major differences between sodium- and lithium-ion batteries (SIBs and LIBs) is how the ions electrochemically react with graphite, where large amounts of lithium can be stored in the graphite galleries, but sodium cannot form thermodynamically stable, high capacity binary graphite intercalation compounds (b-GICs).[1] In 2014, however, it was discovered that graphite had a much greater capacity to reversibly store sodium-ions than previously thought – but only when using ether based electrolytes.[2] The increased storage capabilities in ether based electrolytes comes about by the formation of a ternary graphite intercalation compound (t-GIC), whereby the sodium-ion is intercalated along with it solvation shell. Thus, graphite can store solvated sodium-ions, but not bare ones. This unconventional mechanism is already known to occur for certain solvents, such as propylene carbonate, but was always thought to be accompanied by complete destruction of the graphite.[3]
Since then, several studies have been conducted on electrochemical solvent co-intercalation for energy storage,[4] but the topic is still unexplored. Several fundamental questions have received quite conflicting answers – Especially with regards to the SEI and the reaction mechanism - the system displaying both clear staging in graphite, with at least one well defined plateau for stage I formation, but also large pseudocapacitive features. Following development on electrochemical double-layers and confined electrolytes,[5] it appears in systems where reversible electrochemical co-intercalation occurs the system will display both clear faradaic and non-faradaic properties. This is indeed what is seen, as the system offers a moderate amount of energy density, similar to batteries, while still being highly reversible and extremely fast, even at low temperatures, similar to capacitors.[6,7] Moreover, the reaction is tunable by altering the electrolyte composition - opening up for a great deal of exploration.[8] These properties, along with using abundant materials, makes the system applicable to problems where both the properties of capacitors and batteries are needed, such as in grid storage.
This talk purport to summarize our research activity on electrochemical solvent co-intercalation phenomena, ranging from fundamental questions such as how to detect solvent co-intercalation, to the nature of the electrochemical reaction. We also present our work on using solvent co-intercalation in full cells, as well as a new type of battery where both the anode and cathode operate by a solvent co-intercalation mechanism – the solvent co-intercalation battery (CoIB).[9]

References

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