Correlating Li/O2 cell capacity and product morphology with discharge current

Area of Science:

• Electrochemistry
• Materials Science
• Battery Technology

Background:

• The discharge rate is a critical factor influencing lithium/oxygen (Li/O2) battery performance.
• It affects cell capacity, discharge-phase morphology, and the efficiency of the oxygen-evolution reaction during recharging.

Purpose of the Study:

• To statistically analyze the relationship between discharge rate and performance in Li/O2 cells.
• To investigate the impact of discharge current density on the morphology of electrodeposited lithium peroxide (Li2O2).

Main Methods:

• Statistical analysis of first-discharge data from multiple Li/O2 cells across four discharge rates.
• X-ray diffractometry (XRD) to identify the crystalline phases of discharge products.
• Development of an air-free sample-transfer technique for scanning electron microscopy (SEM) of Li2O2.

Main Results:

• Battery capacity follows a power law with a Peukert's exponent of 1.6 ± 0.1 in the 0.1 to 1 mA cm(-2) range.
• XRD confirmed crystalline Li2O2 as the dominant discharge product.
• SEM revealed that at higher discharge rates, Li2O2 particles become needle-like, with decreasing average volume and increasing surface-to-volume ratios.

Conclusions:

• Discharge rate influences Li2O2 morphology, transitioning from toroidal shapes to needle-like structures at higher rates.
• The observed morphological changes support a locally mass-transfer-limited nucleation and growth mechanism for Li2O2.
• Understanding these rate-dependent morphological changes is crucial for optimizing Li/O2 battery design and performance.