Lithium dynamics in LiMn2O4 probed directly by two-dimensional (7)Li NMR.

Area of Science:

• Solid-state chemistry
• Materials science
• Nuclear magnetic resonance spectroscopy

Background:

• LiMn2O4 is a promising cathode material for lithium-ion batteries.
• Understanding lithium ion dynamics is crucial for optimizing battery performance.
• Previous studies suggested complex lithium behavior within the LiMn2O4 structure.

Purpose of the Study:

• To investigate the crystallographic sites and dynamics of lithium ions in LiMn2O4.
• To directly observe and quantify lithium ion diffusion using advanced NMR techniques.
• To correlate microscopic lithium dynamics with macroscopic material properties.

Main Methods:

• One-dimensional (1D) magic-angle-spinning nuclear magnetic resonance (MAS NMR) spectroscopy.
• Rotor-synchronized two-dimensional (2D) exchange NMR spectroscopy at variable temperatures.
• Analysis of NMR spectra to identify lithium resonances and exchange pathways.

Main Results:

• 1D MAS NMR identified three distinct lithium resonances corresponding to different crystallographic sites.
• Evidence of charge ordering of Mn3+ and Mn4+ ions was observed at low temperatures.
• 2D exchange NMR directly revealed millisecond timescale lithium ion exchange between the 8a and 16c sites starting around 285 K.
• At higher temperatures (380 K), lithium ions were observed to hop between multiple sites.

Conclusions:

• The study provides direct microscopic evidence of lithium ion mobility and charge ordering in LiMn2O4.
• The observed lithium dynamics, including site exchange and hopping, are consistent with macroscopic measurements.
• MAS NMR is a powerful tool for elucidating ion transport mechanisms in battery materials.