Solvent Co-Intercalation As a Strategy for the Exploration of Novel Electrode Reactions for Rechargeable Metal-Ion Batteries
Authors: Åvall Gustav , Alvarez Ferrero Guillermo , Son Youhyun , Janßen Knut , Adelhelm Philipp 
Published: February 2024

Electrochemical solvent co-intercalation into graphite, i.e., where instead of a bare ion a solvated ion is intercalated, is broadly seen as detrimental as it often leads to extreme volume expansion and delamination of the active material. It is also, however, accepted as the initial step in solid electrolyte interphase (SEI) formation.[1] Moreover, in 2014 it was shown that, surprisingly, the reaction can be extremely reversible and fast when glyme based electrolytes are used.[2] In addition, the solvent co-intercalation reaction allowed for storage of sodium in graphite, previously not thought possible due to a lack of thermodynamically stable high capacity sodium-graphite binary compounds, showing that solvent co-intercalation opens up for novel electrolyte-electrode combinations. This sparked additional interest in the phenomenon, where many electrolyte solutions have now been explored for several metal-ions (Li+, Na+, K+ as well as Mg2+ and Ca2+) showing that when electrochemical solvent co-intercalation occurs the redox reaction, both capacity (stoichiometry) and potential, becomes dependent on the exact electrolyte formulation. Hence systems with solvent co-intercalation offers an extremely diverse chemistry.[3,4] Thus, the process of electrochemical solvent co-intercalation seems ubiquitous when graphite anodes are used – leading to either successful SEI formation, destruction of the active material, or in rare cases to reversible solvent co-intercalation – Yet, the underlying mechanism is poorly understood and has received very little attention.
Here we show that the simply model of reversible solvent co-intercalation often shown schematically in the literature is flawed and inconsistent with several experimental facts. Based on electrode weight change measurements, impedance spectroscopy and operando microscopy we present a new consistent model – showing that electrochemical solvent co-intercalation does not happen through a steady continuous reaction but is driven by i) flooding events where large amounts of solvated ions and free solvents enter the graphite and ii) preferentially replacement of free solvents by solvated ions. Armed with this new model of the underlying mechanism of solvent co-intercalation we show how solvents which have previously been thought to not electrochemically co-intercalate can be made to reversibly co-intercalate. Thus, with these new insights, we greatly expanded the possible electrolytes where the reaction occurs reversibly. We also discuss typical properties of systems where reversible co-intercalation occurs (the redox reaction being tunable by the electrolyte composition, extremely fast kinetics) and how to detect solvents co-intercalation as it is sometimes overlooked.[5] Finally, we discuss the use of solvent co-intercalation in full cells, along with our own data on full cells and on a system where both anode and cathode operate via a solvent co-intercalation mechanism – the solvent co-intercalation battery (CoIB).[5]

References

[1] K. Xu, Chemical reviews
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[2] B. Jache, P. Adelhelm, Angewandte Chemie,
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[3] J. Park, Z-L. Xu, K. Kang, Frontiers in Chemistry, 8 (2020), 2296-2646
[4] B. Jache, J. O. Binder, T. Abe, P. Adelhelm, Phys. Chem. Chem. Phys., 18 (2016) 14299-14316
[5] G. A. Ferrero, G. Åvall, K. A. Mazzio, Y. Son, K. Janßen, S. Risse, P. Adelhelm, Adv. Energy Mater.
12 (2022)2202377