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Abstract
Ionic transport in solid electrolytes can often be approximated as ions performing a sequence of hops between distinct lattice sites. If these hops are uncorrelated, quantitative relationships can be derived that connect microscopic hopping rates to macroscopic transport coefficients; i.e.\ tracer diffusion coefficients and ionic conductivities. In real materials, hops are uncorrelated only in the dilute limit. At nondilute concentrations the relationships between hopping frequency, diffusion coefficient, and ionic conductivity deviate from the random walk case, with this deviation quantified by singleparticle and collective correlation factors, f and f_{I}. These factors vary between materials, and depend on the concentration of mobile particles, the nature of the interactions, and the host lattice geometry.
Here we study these correlation effects for the garnet lattice using latticegas Monte Carlo simulations. We find that for noninteracting particles (volume exclusion only) singleparticle correlation effects are more significant than for any previously studied threedimensional lattice. This is attributed to the presence of twocoordinate lattice sites, which causes correlation effects intermediate between typical threedimensional and onedimensional lattices. Including nearestneighbour repulsion and onsite energies produces more complex singleparticle correlations and introduces collective correlations. We predict particularly strong correlation effects at x_{Li}=3 (from site energies) and x_{Li}=6 (from nearestneighbour repulsion), where x_{Li} = 9 corresponds to a fully occupied lithium sublattice. Both effects are consequences of ordering of the mobile particles. Using these simulation data, we consider tuning the mobile ion stoichiometry to maximise the ionic conductivity, and show that the "optimal" composition is highly sensitive to the precise nature and strength of the microscopic interactions.
Finally, we discuss the practical implications of these results in the context of lithium garnets and other solid electrolytes.
Here we study these correlation effects for the garnet lattice using latticegas Monte Carlo simulations. We find that for noninteracting particles (volume exclusion only) singleparticle correlation effects are more significant than for any previously studied threedimensional lattice. This is attributed to the presence of twocoordinate lattice sites, which causes correlation effects intermediate between typical threedimensional and onedimensional lattices. Including nearestneighbour repulsion and onsite energies produces more complex singleparticle correlations and introduces collective correlations. We predict particularly strong correlation effects at x_{Li}=3 (from site energies) and x_{Li}=6 (from nearestneighbour repulsion), where x_{Li} = 9 corresponds to a fully occupied lithium sublattice. Both effects are consequences of ordering of the mobile particles. Using these simulation data, we consider tuning the mobile ion stoichiometry to maximise the ionic conductivity, and show that the "optimal" composition is highly sensitive to the precise nature and strength of the microscopic interactions.
Finally, we discuss the practical implications of these results in the context of lithium garnets and other solid electrolytes.
Original language  English 

Article number  170824 
Journal  Royal Society Open Science 
Volume  4 
Issue number  11 
DOIs  
Publication status  Published  1 Nov 2017 
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 1 Finished

Dr B Morgan URF  Modelling Collective LithiumIon Dynamics in Battery Materials
1/10/14 → 30/09/19
Project: Research council
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Dataset for "LatticeGeometry Effects in Garnet Solid Electrolytes: A LatticeGas Monte Carlo Simulation Study"
Morgan, B. (Creator), Zenodo, 2 Jul 2017
Dataset