Exploring the Molecular-Scale Structures at Solid/Liquid Interfaces of Li-Ion Battery Materials: A Force Spectroscopy...

Area of Science:

• Materials Science
• Electrochemistry
• Surface Science

Background:

• Understanding the solid-liquid interface is crucial for optimizing lithium-ion battery performance.
• Propylene carbonate (PC) is a common electrolyte solvent, and LiCoO2 (LCO) is a widely used cathode material.
• The molecular structure at the LCO/PC interface influences interfacial resistance and battery degradation.

Purpose of the Study:

• To elucidate the molecular-level structure of propylene carbonate (PC) at the LiCoO2 (LCO) interface.
• To identify and characterize solvation forces and molecular orientations at this critical battery interface.
• To correlate interfacial structure with observed force spectroscopy data.

Main Methods:

• Frequency modulation atomic force microscopy (FM-AFM) was employed to measure force spectroscopy data at the LCO/PC interface.
• Sparse modeling-based modal analysis was applied to decompose FM-AFM force curves into distinct components (solvation force, background, noise).
• First-principles calculations were integrated with force curve analysis to simulate the solid/liquid interface and interpret vibrational modes.

Main Results:

• The study successfully decomposed FM-AFM force curves, automatically separating oscillatory solvation forces from background and noise.
• Distinct damped vibrational modes were identified at the LCO/PC interface, characterized by periods of approximately 0.57 nm and shorter.
• These vibrational modes were attributed to solvation forces arising from bulk-like PC molecules and PC molecules oriented parallel ('lying down') to the LCO surface.

Conclusions:

• The molecular structure and solvation dynamics at the LCO/PC interface have been clarified on a molecular scale.
• The findings reveal specific molecular arrangements of PC at the cathode interface, impacting battery performance.
• This work provides fundamental insights into electrolyte-electrode interactions, essential for designing next-generation lithium-ion batteries.