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Monovalent versus Divalent Cation Diffusion in Thiospinel Ti2S4
Patrick Bonnick1, Xiaoqi Sun1, Ka-Cheong Lau2
1Department of Chemistry and the Waterloo Institute of Nanotechnology, University of Waterloo , Waterloo, Ontario N2L 3G1, Canada.
The Journal of Physical Chemistry Letters
|April 21, 2017
Summary
This study measured lithium-ion (Li+) and magnesium-ion (Mg2+) diffusion in Ti2S4, finding Mg2+ diffusion is significantly slower and has higher activation energy, limiting MgxTi2S4 performance.
Area of Science:
- Materials Science
- Electrochemistry
- Solid-state Chemistry
Background:
- Understanding ion diffusion kinetics is crucial for developing advanced battery materials.
- Titanium disulfide (Ti2S4) is explored as a potential host material for multivalent ion batteries.
- Divalent ions like Mg2+ offer higher volumetric capacity than monovalent Li+ but often exhibit slower diffusion.
Purpose of the Study:
- To quantify and compare the diffusion coefficients (D) of Li+ and Mg2+ in Ti2S4.
- To investigate the influence of ion concentration (x) and temperature on ion diffusion.
- To determine the activation energies (Ea) for Li+ and Mg2+ diffusion in Ti2S4.
Main Methods:
- Galvanic intermittent titration technique (GITT) was employed to measure diffusion coefficients.
- Experiments were conducted at 60 °C and across a range of ion concentrations.
- Temperature-dependent GITT measurements were used to calculate diffusion activation energies.
Main Results:
- Li+ diffusion coefficient (D_Li) decreased gradually from 2 × 10^-8 to 2 × 10^-9 cm²/s with increasing concentration.
- Mg2+ diffusion coefficient (D_Mg) decreased sharply from 2 × 10^-8 to 1 × 10^-12 cm²/s at low concentrations.
- Mg2+ exhibited a significantly higher activation energy (Ea,Mg = 540 ± 80 meV) compared to Li+ (Ea,Li = 260 ± 50 meV).
Conclusions:
- The rapid decrease in Mg2+ diffusion kinetics limits the practical discharge capacity of MgxTi2S4.
- Higher Mg2+ activation energy explains the poorer room-temperature electrochemical performance of Mg-based compounds.
- Divalent cation diffusion is more complex, potentially influenced by site occupancy and cation-cation interactions.
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