Evidence for a Solid-Electrolyte Inductive Effect in Superionic Conductors

Abstract

<p>Identifying and optimizing highly-conducting lithium-ion solid electrolytes is a critical step towards the realization of commercial all–solid-state lithium-ion batteries. Strategies to enhance ionic conductivities in solid electrolytes typically focus on the effects of modifying their crystal structures or of tuning mobile-ion stoichiometries. A less-explored approach is to modulate the chemical-bonding interactions within a material to promote fast lithium-ion diffusion. Recently, the idea of a solid-electrolyte inductive effect was proposed, whereby changes in bonding within the solid-electrolyte host-framework modify the potential-energy landscape for the mobile ions, resulting in an enhanced ionic conductivity. This concept has since been invoked to explain anomalous conductivity trends in a number of solid electrolytes. Direct evidence for a solid-electrolyte inductive effect, however, is lacking—in part because of the challenge of quantifying changes in local bonding interactions within a solid-electrolyte host-framework. <a></a><a>Here, we consider the evidence for a solid-electrolyte inductive effect in the archetypal superionic lithium-ion conductor Li<sub>10</sub>Ge<sub>1−<i>x</i></sub>Sn<i><sub>x</sub></i>P<sub>2</sub>S<sub>12</sub>, using Rietveld refinements against high-resolution temperature-dependent neutron-diffraction data, Raman spectroscopy, and density functional theory calculations.</a> Substituting Ge for Sn weakens the {Ge,Sn}–S bonding interactions and increases the charge-density associated with the S<sup>2-</sup> ions. This charge redistribution modifies the Li<sup>+</sup> substructure causing Li<sup>+</sup> ions to bind more strongly to the host-framework S anions; which in turn modulates the Li-ion potential-energy surface, increasing local barriers for Li-ion diffusion. Each of these effects is consistent with the predictions of the solid-electrolyte inductive effect model. Density functional theory calculations further predict that this inductive effect occurs even in the absence of changes to the host-framework geometry due to Ge → Sn substitution. These results provide direct evidence in support of a measurable solid-electrolyte inductive effect and demonstrate its application as a practical strategy for tuning ionic conductivities in superionic lithium-ion conductors.</p>

Review on the lithium transport mechanism in solid‐state battery materials

“…(1) Electronegativity of non‐Li cations. A higher ionic conductivity was reported in Li 10 GeP 2 S 12 than that in Li 10 SnP 2 S 12 , which is interpreted by the difference in electronegativity between Sn (1.7) and Ge (2.0) 207 . The Sn atoms donate more electrons to the neighboring S atoms because of their lower electronegativity, leading to stronger Coulombic attractions between Li and S ions.…”

“…Such rule has been demonstrated in Li 3 PX 4 (X = O and S), 235,285 LiTi 2 (PX 4 ) 3 (X = O and S), 195,286 Li 6 PX 5 I (X = S and Se), 287 and Li 3 ErX 6 (X = Cl and I) 288 . However, a few exceptions have also been reported, in which other factors more significantly affect the activation energy, for example, the inductive effect in Li 10 XP 2 S 12 (X = Ge and Sn) 207 and the anion disorder in Li 6 PS 5 X (X = Br and I) 289 . The effect of cationic doping on Li transports mainly affects the Li concentration in lattices and further the distribution of Li sublattices, which is attributed to the changed elemental composition of SEs induced by cationic doping, different from the direct interactions between anions and Li ions.…”

Two-Dimensional Substitution Series Na3P1–xSbxS4–ySey: Beyond Static Description of Structural Bottlenecks for Na+ Transport

“…Following the paradigm of “the softer, the better”, the introduction of ions with higher polarizability leads to a weakening of bonding interactions, softer lattice vibrations, and consequently flatter energy landscapes, which ultimately result in lower activation barriers. Indeed, for many sulfide-based materials, lower activation barriers than those of the harder oxides can be found, ,, and overall, the strength of the bonding interaction between the mobile ion and its framework seems to be paramount. , …”

Visualizing the Chemical Incompatibility of Halide and Sulfide‐Based Electrolytes in Solid‐State Batteries

“…[ 4–6 ] A solid electrolyte class that fulfills the requirement of high ionic conductivity are sulfide‐based materials of argyrodite type such as Li 6 PS 5 X (X = Cl, Br, I), [ 7–9 ] Li 3 PS 4 [ 10,11 ] or Li 10 GeP 2 S 12 . [ 12,13 ] Based on their high ionic conductivity, stability against In/LiIn anode and advantageous mechanical properties (e.g., malleability), these sulfides are prominently used as electrolytes in solid‐state battery studies. [ 2,14–18 ] Unfortunately, these compounds have a narrow thermodynamic stability window, leading to limitations in the cathode due to decomposition reactions, [ 2,16,19 ] for example the electrolyte Li 6 PS 5 Cl with LiNi 0.6 Co 0.2 Mn 0.2 O 2 lead to PO x and SO x .…”