In the quest for better batteries, the Electric Double Layer (EDL) has emerged as one of the most critical frontiers. While bulk electrolyte properties often dominate discussions, the real action unfolds within just a few nanometers where the liquid electrolyte meets the solid electrode. The structure of this interfacial region governs interfacial resistance, capacitance, and ultimately the composition and stability of the Solid Electrolyte Interphase (SEI).
At Compular, we use confined molecular dynamics simulations under constant potential (CPMD) to directly observe how the EDL organizes itself at operating voltages. This approach allows us to quantify how solvent molecules, anions, Li⁺, and electrolyte additives populate and evolve within the EDL and, providing insight into how their collective interactions govern interfacial chemistry and, ultimately, SEI formation.
As a representative example, we investigated an electrolyte composed of 1 M LiPF₆ in an approximately 1:2:4 mixture of FEC:EC:EMC at a constant potential of 4 V. Several key insights emerge:
- Anion exclusion at the anode: PF₆⁻ anions are largely excluded from forming ion pairs with surface Li⁺ cations at the anode.
- Desolvation-driven surface coordination: For a Li⁺ ion to coordinate directly with the electrode surface, it must shed one of its solvating molecules. Accordingly, all Li⁺ ions observed exhibit a reduced coordination number of 3, compared to the bulk value of 4.2.
- Recovery of bulk solvation: Moving away from the anode surface, the Li⁺ coordination number gradually returns to its bulk average of approximately 4.2.
- Cluster-level insight: Using our CHAMPION cluster analysis, we map the relative contributions of distinct solvation and ion-pairing motifs to the Li⁺ EDL structure.
- Role of FEC additives: FEC appears in approximately 15% of surface-adsorbed Li⁺ clusters, indicating a meaningful interfacial population of adsorbed clusters containing FEC that can contribute to SEI chemistry.
Preferred reduction pathways: Redox calculations performed on the identified clusters provide insight into the most likely interfacial reduction mechanisms.
Conclusion
CPMD simulations, when combined with CHAMPION cluster analysis and targeted redox calculations on EDL-derived molecular clusters, provide a powerful and quantitative framework for evaluating how electrolyte solvents and additives influence EDL structure and SEI formation. This methodology enables rational electrolyte design rooted directly in interfacial chemistry.
