Engineering the solid electrolyte interphase as a solid-state electrolyte

Area of Science:

• Materials Science
• Electrochemistry
• Chemical Engineering

Background:

• The solid electrolyte interphase (SEI) is critical for lithium metal batteries but often described qualitatively.
• A conceptual shift views the SEI as a functional solid-state electrolyte (SSE), enabling quantitative analysis of its properties.
• Engineering the SEI as an SSE is key to advancing lithium metal battery performance.

Purpose of the Study:

• To engineer the SEI as a functional solid-state electrolyte (SSE) by addressing three key axes: transport chemistry, interfacial nucleation thermodynamics, and mechanical robustness.
• To demonstrate the impact of electrolyte salt concentration, current collector surface modification, and ceramic nanofiller reinforcement on SEI performance.
• To establish a multidimensional design framework for the SEI as an engineerable component in battery technology.

Main Methods:

• Utilized separator-free Cu|SEI|Li cells to evaluate SEI performance under varying conditions.
• Manipulated electrolyte salt concentration (e.g., LiFSI in DME) to tune SEI transport chemistry.
• Modified current collector surfaces (e.g., Cu vs. Au) to influence interfacial nucleation thermodynamics.
• Incorporated ceramic nanoparticles (SiO2, TiO2, Al2O3) to create mechanically robust, nanofiller-reinforced SEI layers.

Main Results:

• Increasing LiFSI concentration from 4 M to 6 M improved first-cycle coulombic efficiency (CE) from 32% to 85% at higher current densities.
• Harvested residual SEI (rSEI) demonstrated separator-free plating/stripping capability.
• Switching current collectors from Cu to Au-coated substrates increased average CE from ~15.8% to ~70%.
• Nanofiller-reinforced SEI achieved a first-cycle CE of 82.7% at practical current densities, supporting ~50x higher current densities than baseline SEI-SSE.

Conclusions:

• The SEI can be engineered as a self-assembled composite SSE through control of transport, nucleation, and mechanical properties.
• This engineering approach significantly enhances lithium metal battery performance, enabling separator-free operation.
• SEI interphase engineering offers a pathway to bridge the gap between liquid-electrolyte and solid-state batteries by achieving SSE-like functionality in situ.