Two electrolyte decomposition pathways at nickel-rich cathode surfaces in lithium-ion batteries

Area of Science:

• Electrochemistry
• Materials Science
• Battery Technology

Background:

• Electrolyte decomposition limits lithium-ion battery lifespan.
• Mechanisms and soluble products of electrolyte decomposition at positive electrodes are poorly understood.
• Identifying these products is key to preventing further reactions at the negative electrode.

Purpose of the Study:

• To investigate electrolyte decomposition mechanisms at NMC and LCO positive electrodes.
• To identify soluble and gaseous decomposition products and their formation pathways.
• To understand how these products affect battery performance and degradation.

Main Methods:

• Operando gas measurements and solution Nuclear Magnetic Resonance (NMR) spectroscopy.
• Utilized a LiFePO4 counter electrode to isolate positive electrode reactions.
• Employed 17O-labeled singlet oxygen (1O2) to trace water formation pathways.

Main Results:

• Two distinct electrolyte decomposition routes identified at NMC electrodes based on onset potentials.
• Low potentials (<80% SOC): Ethylene carbonate (EC) dehydrogenation to vinylene carbonate (VC).
• High potentials (>80% SOC): Singlet oxygen (1O2) oxidizes EC, forming CO2, CO, and H2O, which initiates secondary reactions.

Conclusions:

• Water formation via 1O2 oxidation of EC is a key pathway for electrolyte decomposition at high potentials.
• Graphite electrodes consume water, mitigating secondary decomposition and reducing degradation in NMC/graphite cells.
• Insights into electrolyte decomposition and product consumption aid in developing strategies to enhance NMC-based battery longevity.