Anion Charge and Lattice Volume Maps for Searching Lithium Superionic Conductors

Abstract

The effects of anion charge and lattice volume (lithium− anion bond length) on lithium ion migration have been investigated by utilizing the density functional theory calculations combined with the anion sublattice models. It is found that anion charge and lattice volume have great impacts on the activation energy barrier of lithium ion migration, which is validated by some reported sulfides. For the tetrahedrally occupied lithium, the less negative the anion charge is, the lower the migration energy barrier is likely to be. While for the octahedrally occupied lithium, the more negative anion charge is, the lower migration energy barrier is. Based on the full understandings of the anion sublattice model, general design strategies for developing lithium superionic conductors were proposed. Adjusting the electronegativity difference between the anion element and the nonmobile cation element by selecting the most suitable nonmobile cation element without changing the crystal structure framework can achieve low activation energy barriers for lithium ion migration. For the desired lithium superionic conductors with tetrahedrally occupied lithium, the fine nonmobile cation elements should give preference to those elements located at the right top of the periodic table of elements with large electronegativities. For the lithium superionic conductors with octahedrally occupied lithium, the fine nonmobile cation elements should give preferences to the elements located at the left bottom of the periodic table with small electronegativities.

Two-Dimensional Substitution: Toward a Better Understanding of the Structure–Transport Correlations in the Li-Superionic Thio-LISICONs

“…For a further improvement of lithium-ion batteries, the employment of a solid material as the electrolyte may enhance the electrochemical performance and improve the device safety of the current cell architectures. − To guarantee an optimal cell operation, the solid electrolyte, among other requirements, must allow fast Li + ionic motion among the electrodes. − While many highly conducting phases have been recently synthesized, − the list of materials that exhibit high ionic conductivity at room temperature is still extremely short due to the strict structural conditions that need to be satisfied to allow fast ionic motion. − High carrier concentration and a disordered lithium substructure are of paramount importance to achieve a high ionic conductivity. , Moreover, polyhedral connectivity and different lithium coordination strongly influence the energy landscape for the ionic diffusion, where larger changes of the coordination environment lead to higher activation barriers . Fast ionic jumps are possible via face-sharing tetrahedra, while ionic motion through edge-sharing polyhedra is unfavorable at typical lattice volumes of sulfides and oxides. , The anionic framework should also provide wide channels for the ionic motion, leading to the conception that larger lattice volumes are beneficial for the ionic diffusion. − However, while this condition seems to be generally valid, a number of studies report on an optimal channel size for the ionic conductivity. − After exceeding an optimal value, transport properties either decrease or reach a plateau for larger volumes. − In addition to these static structural parameters, larger anion polarizability is also required to lower the activation energy by means of a weaker cation–anion interaction. − All these aspects are highly convoluted as, for instance, more polarizable anionic frameworks often possess larger volumes. Therefore, it is challenging to discern the precise effect that each of these features has on the transport properties.…”

Paradigms of frustration in superionic solid electrolytes

“…This effect can be systematically explored in simulations by explicitly changing the volume. As shown in figure 4 a and discussed at length in previous studies by our group and others [ 6 , 11 , 20 , 21 , 60 ], volume has a drastic influence on both the site occupation and resulting diffusivity of the mobile cation species.

Figure 4 Structural frustration via ordering and site preference.

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“…Superionic behaviour has also been observed in materials with a variety of different crystallographic symmetries, as well as in glassy and interface-dominated composite materials [7]. Much is known regarding the mechanisms of ion conduction in specific materials, and several universal descriptors for superionic activity have recently been proposed or employed [1,3,5,6,[8][9][10][11]. Nevertheless, exceptions to these rules are numerous, suggesting much remains to be understood.…”

“…In each case, the connection to fast ion mobility is discussed in detail. Our intention is to elaborate upon some of the universal motivations for unusually low activation barriers for diffusion in superionic conductors, many of which are also echoed in a series of excellent reviews and descriptor-focused investigations over the past two decades [ 1 , 3 , 5 – 11 , 14 , 19 , 23 – 29 ]. We further suggest that the categories of frustration presented here may be used as a universal framework for classifying superionic solids and emerging design strategies.…”

Resilient Women and the Resiliency of Science

“…With this in mind, Niveen Khashab and Sara Skrabalak interviewed recent authors in the journal, the coauthors of this editorial, about how they define resilience and the times in which they were resilient. Their responses are below and this editorial accompanies a virtual collection of papers published during the COVID-19 pandemic in Chemistry of Materials by women corresponding authors. − In compiling this collection, we noted that really any manuscript published during this time could be included and is a reflection of the resiliency of individuals, teams, and science itself.…”