Understanding the Role of Overpotentials in Lithium Ion Conversion Reactions: Visualizing the Interface

Area of Science:

• Materials Science
• Electrochemistry
• Battery Technology

Background:

• Oxide conversion materials offer higher specific capacities than intercalation materials for Li-ion batteries.
• A major challenge is the large overpotentials caused by interface formation between products during conversion reactions.

Purpose of the Study:

• To investigate the structural evolution and interfacial phenomena during NiO conversion using operando X-ray reflectivity.
• To understand the mechanisms behind bulk and interfacial lithiation in NiO electrodes.
• To develop a potential-dependent nucleation model for conversion reactions.

Main Methods:

• Utilized operando X-ray reflectivity to monitor ultrathin NiO electrodes during electrochemical cycling.
• Employed density functional theory (DFT) calculations to determine interfacial energies.
• Developed a nucleation model based on calculated interfacial energies and applied potentials.

Main Results:

• Observed two distinct interfacial reactions preceding bulk NiO conversion: lithium accumulation at the Ni/NiO interface (2.2 V) and interfacial lithiation (1.9 V).
• Identified a space charge layer of lithium at the interface as a key factor in reducing conversion energy barriers.
• The bulk conversion reaction was observed at 0.6 V.

Conclusions:

• The study reveals previously unknown interfacial reactions in NiO conversion that occur at higher potentials than bulk conversion.
• Understanding and potentially controlling these interfacial phenomena, particularly the lithium space charge layer, is crucial for mitigating overpotentials in conversion batteries.
• The developed nucleation model provides insights into the mechanisms governing NiO conversion, paving the way for improved electrode design.