(Best Student Presentation) Uncovering the Electrochemical Sodium-Solvent Co-Intercalation Reaction with Graphite

Abstract

Sodium-ion batteries (SIBs) is on the verge of commercialisation, with several companies announcing large scale production during 2023. SIBs have many similarities with lithium-ion batteries (LIBs), and offers comparable cycle life, as well as specific energies and power as LIBs while often being made with more abundant materials.[1] There is, however, a clear difference between LIBs and SIBs when it comes to the ions interaction with graphite – the standard anode in LIBs. Sodium-ions are unable to form thermodynamically stable structures inside graphite with high specific capacities,[2] often showing less than 20 mAh/g. But, in 2014 it was shown that when a diglyme (G2) based electrolyte was used suddenly 110 mAh/g of sodium-ion could be stored inside graphite through a solvent co-intercalation mechanism, i.e., solvated sodium-ions can be stored in much larger quantities in graphite than bare ions.[3] Surprisingly, even if the reaction is accompanied by a large expansion of the graphite to accommodate the solvated ions, the reaction is highly reversible and very fast, with thousands of cycles having been reported and current densities of several A/g seem to have no large impact on the storage capability.[4] The system is also able to operate at low temperatures, and shows a great deal of tunability through small changes in the electrolyte composition.[5-7] These properties opens up for applications in grid energy storage, as the system is able to deliver a moderate amount of energy, but at high power over a wide temperature range and over many cycles, while also being made by cheap and available resources.Even if several articles have been published on the subject, many fundamental questions about the reaction remain debated, such as the existence of and possible properties of an SEI that allows solvated ions through, or the stoichiometry of the reaction itself. It is often argued that either the entire solvation shell, consisting of 2 G2 molecules, is intercalated along with Na+, or partial desolvation occurs and only a part of the solvation shell enters the graphite, Figure 1. Yet, there is very little evidence for either case. Here, we report on two independent methods, one relying on ex-situ analysis of the graphite weight at different potentials and one on in-situ electrochemical impedance spectroscopy, that shows that the reaction when using a G2 or monoglyme (G1) based electrolyte is far more complicated than previously thought and changes drastically during the sodiation process. We show that at high voltages the system is opened up by solvated ions and is subsequently flooded with solvent molecules, and these flooding events occur again during parts of the sodiation process. After the system has successfully formed a stage I compound the system enters a non-faradaic process where the free glyme molecules inside the graphite is preferentially replaced by solvated ions until the graphite reach its full storage capacity of solvated ions.The mass measurements and impedance measurements are supplemented with ab initio molecular dynamics of the electrolytes, and density functional theory simulations of solvated ions inside of graphite. The proposed electrochemical process is also compared with and corroborated by material characterisation techniques, such as electrochemical dilatometry and operando X-ray techniques, as well as NMR measurements.