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Mechanistic insights of Li⁺ diffusion within doped LiFePO₄ from Muon Spectroscopy.

ABSTRACT: The Li⁺ ion diffusion characteristics of V- and Nb-doped LiFePO₄ were examined with respect to undoped LiFePO₄ using muon spectroscopy (µSR) as a local probe. As little difference in diffusion coefficient between the pure and doped samples was observed, offering D_Li values in the range 1.8-2.3?×?10^-10?cm² s^-1, this implied the improvement in electrochemical performance observed within doped LiFePO₄ was not a result of increased local Li⁺ diffusion. This unexpected observation was made possible with the µSR technique, which can measure Li⁺ self-diffusion within LiFePO₄, and therefore negated the effect of the LiFePO₄ two-phase delithiation mechanism, which has previously prevented accurate Li⁺ diffusion comparison between the doped and undoped materials. Therefore, the authors suggest that µSR is an excellent technique for analysing materials on a local scale to elucidate the effects of dopants on solid-state diffusion behaviour.

SUBMITTER: Johnson ID

PROVIDER: S-EPMC5841297 | biostudies-literature | 2018 Mar

REPOSITORIES: biostudies-literature

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Mechanistic insights of Li+ diffusion within doped LiFePO4 from Muon Spectroscopy.

Johnson Ian D ID Ashton Thomas E TE Blagovidova Ekaterina E Smales Glen J GJ Lübke Mechthild M Baker Peter J PJ Corr Serena A SA Darr Jawwad A JA

Scientific reports 20180307 1

The Li+ ion diffusion characteristics of V- and Nb-doped LiFePO4 were examined with respect to undoped LiFePO4 using muon spectroscopy (µSR) as a local probe. As little difference in diffusion coefficient between the pure and doped samples was observed, offering DLi values in the range 1.8-2.3 × 10-10 cm2 s-1, this implied the improvement in electrochemical performance observed within doped LiFePO4 was not a ...[more]

PMID: 29515155

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Project description:The rational development of fast-ion-conducting solid electrolytes for all-solid-state lithium-ion batteries requires understanding the key structural and chemical principles that give some materials their exceptional ionic conductivities. For the lithium argyrodites Li6PS5X (X = Cl, Br, or I), the choice of the halide, X, strongly affects the ionic conductivity, giving room-temperature ionic conductivities for X = {Cl,Br} that are ×103 higher than for X = I. This variation has been attributed to differing degrees of S/X anion disorder. For X = {Cl,Br}, the S/X anions are substitutionally disordered, while for X = I, the anion substructure is fully ordered. To better understand the role of substitutional anion disorder in enabling fast lithium-ion transport, we have performed a first-principles molecular dynamics study of Li6PS5I and Li6PS5Cl with varying amounts of S/X anion-site disorder. By considering the S/X anions as a tetrahedrally close-packed substructure, we identify three partially occupied lithium sites that define a contiguous three-dimensional network of face-sharing tetrahedra. The active lithium-ion diffusion pathways within this network are found to depend on the S/X anion configuration. For anion-disordered systems, the active site-site pathways give a percolating three-dimensional diffusion network; whereas for anion-ordered systems, critical site-site pathways are inactive, giving a disconnected diffusion network with lithium motion restricted to local orbits around S positions. Analysis of the lithium substructure and dynamics in terms of the lithium coordination around each sulfur site highlights a mechanistic link between substitutional anion disorder and lithium disorder. In anion-ordered systems, the lithium ions are pseudo-ordered, with preferential 6-fold coordination of sulfur sites. Long-ranged lithium diffusion would disrupt this SLi6 pseudo-ordering, and is, therefore, disfavored. In anion-disordered systems, the pseudo-ordered 6-fold S-Li coordination is frustrated because of Li-Li Coulombic repulsion. Lithium positions become disordered, giving a range of S-Li coordination environments. Long-ranged lithium diffusion is now possible with no net change in S-Li coordination numbers. This gives rise to superionic lithium transport in the anion-disordered systems, effected by a concerted string-like diffusion mechanism.

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Mechanistic insights of Li⁺ diffusion within doped LiFePO₄ from Muon Spectroscopy.

Publications

Mechanistic insights of Li<sup>+</sup> diffusion within doped LiFePO<sub>4</sub> from Muon Spectroscopy.

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