Temperature Influence on the Staging Process in Carbon Materials during Anionic Intercalation
Authors: Gorbunov Mikhail V. , Mikhailova Daria 
Published: February 2024

A typical set-up of a dual-carbon battery consists of a pair of carbon electrodes and an organic-based electrolyte with a broad window of electrochemical stability. When such a cell is being charged to a reasonably high voltage, two processes occur: intercalation of cations at the electrode with low potential, and intercalation of anions at the one with high potential [1-2]. Although there are significant limitations for practical application of such systems, mostly related to electrolyte decomposition and exfoliation of carbon, the concept appears to be rather inspiring for investigations, as the materials required for dual-carbon batteries would be significantly cheaper compared to those for state-of-the-art electrochemical energy storage solutions.

The general idea of using carbon as a cathode material is known for many decades, however, most of the studies dedicated to it pursue the improvement of specific electrochemical characteristics through variation of the cell materials, mostly by trial and error approach [3-4]. Clearly, the results of such research are highly important, but the topic itself lacks the knowledge of electrochemical kinetics for the process of various anions being inserted into graphitic materials, which are typically used as dual-carbon battery cathodes.

In our work, we attempted to overcome the limiting factor of electrolyte decomposition by selecting a relatively unusual organic sulfones as solvents. Addition of a salt reduces the melting point of the sulfone, allowing to consider the system as a classic liquid electrolyte. Such cells are known to sustain the high voltages long enough to conduct the studies on structural evolutions which occur in the graphite cathode [4]. We selected two commercial graphitic materials and subjected them to anionic intercalation in various alkali-based electrolytes. Galvanostatic cycling with intermittent titration was conducted to evaluate the diffusion coefficients of anion insertion. The best-performing system was also studied in operando X-ray diffraction cells using the synchrotron radiation facilities, so anionic diffusion was coupled to different stages of intercalation. The staging process itself appears to be completely different at evaluated temperature. Thus, the experiment with KPF6 electrolyte at 298 K showed similar results with the work [5], with subsequent formation of intercalated stages down to stage 2 (see the figure), whereas at 358 K, multiple stages are present together almost from the beginning of intercalation process.

The results of the current study show a drastic influence of temperature on the structural behavior of graphite during anionic intercalation, and by electrochemical methods, the kinetics of these changes is characterized, which is necessary for further development of dual-carbon battery topic.

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Figure 1