Is the Solid Electrolyte Interphase an Extra-Charge Reservoir in Li-Ion Batteries?

Area of Science:

• Materials Science
• Electrochemistry
• Surface Science

Background:

• Advanced metal oxide electrodes in Li-ion batteries often exhibit higher reversible capacities than theoretically predicted.
• The origin of this excess capacity remains largely unelucidated, despite numerous investigations.

Purpose of the Study:

• To investigate the surface evolution and chemical processes occurring in advanced metal oxide electrodes during lithium storage.
• To identify the source of the anomalous extra capacity in these battery electrodes.

Main Methods:

• Utilized high-resolution soft X-ray absorption spectroscopy with tunable probing depth.
• Employed photoemission spectroscopy to analyze surface composition and evolution.
• Studied a carbon-coated ZnFe2O4 electrode as a representative conversion-alloying material.

Main Results:

• Identified a partially reversible alkyl lithium carbonates layer (approximately 5-7 nm thick) forming at the SEI surface.
• Demonstrated that this SEI layer functions as a lithium reservoir.
• Correlated the formation of this layer with increased lithium storage levels.

Conclusions:

• The SEI layer plays a crucial role beyond passivation, actively participating in lithium storage.
• The newly identified SEI reservoir contributes significantly to the observed extra capacity in metal oxide electrodes.
• This finding expands the understanding of SEI functionality in high-performance Li-ion batteries.