Åvall, G.; Ferrero, G.A.; Janßen, K.A.; Exner, M.; Son, Y.; Adelhelm, P.: In Situ Pore Formation in Graphite Through Solvent Co-Intercalation: A New Model for The Formation of Ternary Graphite Intercalation Compounds Bridging Batteries and Supercapacitors. Advanced Energy Materials 13 (2023), p. 2301944/1-13
10.1002/aenm.202301944
Open Access Version
Abstract:
One of the major differences between Li-ion and Na-ion batteries is how graphite reacts as negative electrode. When charging LIBs, Li-ions are being intercalated between the graphene layers forming binary graphite intercalation compounds (b-GICs). The same reaction is unfavourable for Na, however, intercalation is possible through co-intercalation of solvent molecules from the electrolyte solution. This means that solvated Na-ions rather than bare Na-ions intercalate, which leads to ternary GICs (t-GICs) along with an expansion of the graphite interlayer spacing to about 1.2 nm. Such a large interlayer spacing represents a micropore with parallel slit geometry. Not much is known about the formation process of t-GICs, but it is commonly believed that throughout the entire reaction the ion is either accompanied by its entire solvation shell or only by parts of it. Here, we elucidate for the first time, using two independent methods – one relying on graphite mass changes and one on electrochemical impedance spectroscopy – and supplemented by operando microscopy, entropymetry and ab initio simulations, that the storage mechanism is far more complex than previously thought and changes drastically during the electrochemical reaction. We propose a new model for the electrochemical solvent co-intercalation process: At beginning of the sodiation process, and on the main potential plateau, the graphite is flooded with free solvent molecules, after which the free solvents are preferentially replaced by solvated ions. When the graphite is almost fully sodiated, almost no free solvents remain in the structure and additional structural rearrangement of the solvated ions take place to reach the full storage capacity. In this way, t-GICs represent a unique case of switchable microporous systems and hence appear as bridge between ion storage in the bulk phase and in micropores, i.e. between batteries and supercapacitors.