In Situ Formation of Li⁺ Ion Conductive Layers as the Solid Electrolyte Interphase (SEI) in Lithium Metal Batteries

The desired solid electrolyte interphase (SEI) on metallic Li plays a crucial role in preventing continuous electrolyte decomposition and Li dendrite growth. Promising SEI components such as lithium fluoride (LiF), lithium nitride (Li₃N), lithium oxide (Li₂O), and lithium carbonate (Li₂CO₃) exhibit reasonable conductivity and mechanical robustness. In contrast, carbonaceous components are soluble and loosely cover the Li surface, leading to ongoing electrolyte decomposition. Typically, these inorganic and organic species are randomly mixed and form passivating films along with dead Li⁰. Recent advancements have introduced localized high-concentration electrolytes or weakly coordinating solvents to establish inorganic-rich SEI composites. However, there is a lack of comprehensive studies on Li deposition and stripping processes over the SEI for long-term cycling, leaving the question of the cell-degradation process unanswered.<br/>Herein, we demonstrate the formation of two passivating films by controlling 1,2-dimethoxyethane (DME)/1,3-dioxolane (DOL) ratios, while maintaining fixed concentrations of the lithium bis(fluorosulfonyl)imide (LiFSI) and LiNO₃ in Li|Li symmetric cells. The electrolyte solutions with a higher proportion of DME formed porous films after 30 cycles. Remarkably, continuous Li deposition was observed above these porous films, indicating their mixed ionic and electronic conduction properties. A passivating film grew as cycling proceeded, reaching a thickness of ~40 μm after 100 cycles. In contrast, the electrolytes with a higher proportion of DOL formed dense and particulate-shaped films. Interestingly, Li deposition occurred beneath these films, maintaining a thickness of &lt; 15 μm. Thus, it is plausible that the dense film acted as a Li⁺ ion conductor. X-ray photoelectron spectroscopy and titration gas chromatography revealed the presence of predominant dead Li, Li₂O, and Li₂CO₃ in both passivating films. However, the dense film exhibited a higher Li content and a lower carbon atomic ratio (less than 5 atomic %) than the porous film. We attributed the predominant DOL-based electrolyte with weak coordination properties to contribute to forming anion-derived inorganic components. Cell degradation started over 400 h, accompanied by increased carbon contents on the film surface and reduced Li contents. In the presentation, I will discuss the role of electrolytes in the formation of distinct passivating films and their impact on the overall performance of LiFePO₄-based full-cell configurations.