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Lithium dynamics in LiMn2O4 probed directly by two-dimensional (7)Li NMR
V W Verhoeven1, I M de Schepper, G Nachtegaal
1Interfaculty Reactor Institute, Delft University of Technology, Mekelweg 15, 2629 JB Delft, The Netherlands.
Physical Review Letters
|May 1, 2001
Summary
Magic-angle-spinning nuclear magnetic resonance (MAS NMR) revealed lithium dynamics in LiMn2O4. Lithium ions exhibit charge ordering and millisecond timescale hopping between crystallographic sites starting at 285 K.
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.
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