How Voltage Drops are Manifested by Lithium Ion Configurations at Interfaces and in Thin Films on Battery Electrodes

TL;DR

This study uses Density Functional Theory to analyze how voltage manifests at the atomic level in lithium-ion battery interfaces, revealing the role of surface dipoles and charge distributions in electrode behavior.

Contribution

It introduces a detailed atomic-scale analysis of voltage effects at battery electrode interfaces, emphasizing the importance of surface dipoles and distinguishing between different voltage definitions.

Findings

Surface dipoles influence voltage at interfaces.

Organic solvents reduce predicted voltages in vacuum.

Distinction between electron voltage and Li content voltage is crucial.

Abstract

Battery electrode surfaces are generally coated with electronically insulating solid films of thickness 1-50 nm. Both electrons and Li+ can move at the electrode-surface film interface in response to the voltage, which adds complexity to the "electric double layer" (EDL). We apply Density Functional Theory (DFT) to investigate how the applied voltage is manifested as changes in the EDL at atomic lengthscales, including charge separation and interfacial dipole moments. Illustrating examples include Li(3)PO(4), Li(2)CO(3), and Li(x)Mn(2)O(4) thin-films on Au(111) surfaces under ultrahigh vacuum conditions. Adsorbed organic solvent molecules can strongly reduce voltages predicted in vacuum. We propose that manipulating surface dipoles, seldom discussed in battery studies, may be a viable strategy to improve electrode passivation. We also distinguish the computed potential governing electrons, which is the actual or instantaneous voltage, and the "lithium cohesive energy" based voltage governing Li content widely reported in DFT calculations, which is a slower-responding self-consistency criterion at interfaces. This distinction is critical for a comprehensive description of electrochemical activities on electrode surfaces, including Li+ insertion dynamics, parasitic electrolyte decomposition, and electrodeposition at overpotentials.