Skoltech researchers have proposed an explanation for a long-standing conundrum in lithium-ion battery science. Their study provides a new insight into the role of ethylene carbonate — dubbed the “magic” electrolyte component — in lithium-ion batteries and expounds why that material and the electrochemically similar propylene carbonate behave so differently toward battery anodes made of graphite. Published in the Journal of Materials Chemistry A, the findings will guide the design of electrolytes for safer and more efficient lithium-ion batteries.

At the very beginning of the era of commercial lithium-ion batteries, researchers encountered the issue of graphite anode corrosion. Propylene carbonate-based electrolytes, which are quite friendly to metallic lithium, turned out to be highly corrosive to graphite.

This issue hindered the use of graphite electrodes until ethylene carbonate was introduced as an alternative to propylene carbonate. While the two materials’ molecules are very similar from the electrochemical perspective, they behave quite differently toward graphite anodes. This phenomenon — known as the EC-PC disparity, after the abbreviated names of the compounds — and the role of the “magic solvent” ethylene carbonate have been extensively investigated and discussed in the battery community for decades, with numerous hypotheses put forward. However, there is still no consensus.

This matter is of more than theoretical importance, because of its implications for battery design beyond the choice of EC over PC as the solvent basis for the electrolyte.

In their new paper, Senior Research Scientist Sergey Luchkin and Principal Industrial Engineer Egor Pazhetnov from Skoltech Energy proposed that the presence of ethylene carbonate in the electrolyte leads to the formation of a thin layer of very viscous liquid on the surface of graphite. That layer protects graphite by preventing too many electrolyte molecules from penetrating between its layers (excessive intercalation) and eventually peeling off layers of graphite and thus damaging the anode (corrosive exfoliation). Experiments carried out to check this hypothesis confirmed that this layer does indeed appear in EC-based electrolytes but not in PC-based ones.

Notably, the viscous liquid layer appears before and therefore influences the formation of the solid electrolyte interphase. SEI is a crucial component of Li-ion batteries. It is a thin film of solid electrolyte that forms on the anode surface during the battery’s initial cycling at the factory. That layer protects the graphite anode from fast degradation and prevents the liquid electrolyte from continuous electrochemical reduction.

This new insight into the interfacial processes in lithium-ion batteries provides a new perspective on the interplay between electrolyte composition and the dynamics at the anode-electrolyte interface, which is critical for the development of more stable and efficient batteries via the intelligent design of the solid electrolyte interphase.

The approach suggested in the study extends beyond lithium-ion batteries, offering valuable insights for the emerging complementary technologies of sodium- and potassium-ion batteries. These face similar challenges in solid electrolyte interphase formation. The research advances our understanding of how the physical properties of the electrolyte components contribute to interfacial dynamics, potentially accelerating innovation in the field of energy storage.

The study reported in this story was supported by the Russian Science Foundation under grant No. 23-23-00041.