Understanding the Role of Solvent Co-intercalation in Graphite Anodes

In the world of lithium-ion batteries (LIBs), the performance and longevity of the battery heavily rely on the materials used in its construction. One critical component is the graphite anode, which plays a vital role in the battery's electrochemical processes. Recent studies have shown that while the solid electrolyte interphase (SEI) has been extensively researched, the impact of solvent co-intercalation has been somewhat overlooked.

Research conducted by Besenhard et al. has shed light on this phenomenon, particularly focusing on the behavior of highly ordered pyrolytic graphite (HOPG) in electrolyte solutions based on ethylene carbonate (EC). Their findings reveal that as the graphite matrix undergoes electrochemical reduction, it experiences a significant expansion—up to 150%—when subjected to potentials more negative than 1.0 V. This expansion is attributed to the co-intercalation of solvent molecules, which subsequently decompose and form a stable product between the graphene sheets, hindering further intercalation and exfoliation.

The research highlights two distinct processes involved in the formation of the SEI layer on graphite electrodes. Initially, solvated lithium ions intercalate into the graphite structure at potentials just below 1 V, followed by their decomposition between the graphene layers. Additionally, at lower potentials, solvent molecules undergo direct reductive decomposition on the electrode surface, creating particle-like precipitates that contribute to the SEI layer's development.

The resulting SEI layer serves two important functions: it prevents the co-intercalation of solvent molecules from the edge of the graphite and inhibits direct solvent decomposition across the entire surface of the electrode. Importantly, the properties of the SEI and the formation processes are significantly influenced by the type of solvent used in the electrolyte.

For example, propylene carbonate (PC) has been shown to exhibit poor compatibility with graphite anodes when utilized as a solvent. Researchers found that when graphite is polarized in PC-based solutions, it leads to continuous solvent decomposition and significant exfoliation of graphene sheets, ultimately failing to create an effective SEI. Conversely, the use of EC-based solutions remains prevalent in commercially available LIBs due to their more stable interactions with graphite. Interestingly, recent studies suggest that adding specific organic molecules to PC-based solutions can suppress these detrimental effects, allowing for better lithium ion intercalation.

In summary, the interplay between solvent co-intercalation and the development of the SEI layer is a crucial aspect of graphite anode performance in lithium-ion batteries. Ongoing research in this area could lead to significant advancements in battery technology, particularly in enhancing the efficiency and stability of LIBs for various applications.