Phonon–Ion Interactions: Designing Ion Mobility Based on Lattice Dynamics

Abstract

been found in numerous materials covering virtually all chemical compositions and crystal structures. [1][2][3] These materials are also critical in many applications, enabling a number of emerging technologies ranging from memristors to smart windows and fuel cells to name a few. [4][5][6][7][8] In particular, solid-state lithium (Li) electrolytes are believed to help with enabling lithium metal anodes, which will increase the energy density of a battery system and usher a new era of electromobility. [9][10][11] Research in solid-state electrolytes has been largely driven by the designs and discoveries of new materials with higher ionic conductivity. [10,12,13] The progress is particular evident in the research of Li-ion [14][15][16] and Na-ion conductors [17] where recent breakthroughs have led to marked increases in room-temperature ionic conductivity rivalling that of liquid electrolytes. [18] These advancements catalyzed research for new ion conductors that can be further record setting in ion conductivity, while also meeting other device-level requirements such as (electro)chemical stability and good mechanical properties, [19][20][21][22][23] both of which prevent contact loss or other morphological instabilities during cycling. [24][25][26][27][28][29][30][31] New descriptors have been proposed to accomplish rational design of new ion conductors and have been used in several high-throughput computational screenings, [13,[32][33][34] which has led to discovery of new promising Li-ion conductors [13] including those in the chloride family [13,[35][36][37] that display not only high ionic conductivity but also good electrochemical stability. [32,33,38,39] The design of new descriptors has also greatly benefited from the renewed interest in the fundamental of ionic conductivity in solids. [40] Here, concepts such as lattice dynamics or bond frustrations have been revisited in particular in light of new advances in computational capabilities to seek fundamental atomistic mechanisms that promote room-temperature superionic conductivity. [41][42][43][44][45] Clearly, in certain materials such as RbAg 4 I 5 , an impressive Ag + conductivity ≈0.25 S cm -1 can be reached, [46] and the questions is how to the reach the same level of ionic conductivity in other polycrystalline ionic conductors such as solid Li + and Na + electrolytes whose conductivity has so far plateaued at ≈24 mS cm -1 [10] and 41 mS cm -1 , [17] respectively.In this progress report, we will highlight one of the approaches to understand fundamental processes responsible for ion conductivity in solid ionic conductors namely the This review is focused on the influence of lattice dynamics on the ionic mobility in superionic conductors in particular solid-state Li-ion conductors. After a succinct review of the static view of ionic conduction, the role of polarizability as the underlying cause of lattice softness is discussed in connection with the anharmonicity and the roles of lattice dynamics on ionic conductivity as proposed in early theories in th...

Anharmonic lattice dynamics of superionic lithium nitride

“…Phonon or vibrational density-of-states data have also been used to construct a range of cheap-to-compute descriptors for various properties of fast-ion conductors, including for ionic conductivity, activation energy, and electrochemical stability window [20,21]. Computationally cheap descriptors of specific properties are increasingly used in high-throughput materials discovery, where data from electronic-structure methods are combined with data-mining techniques to screen large numbers of materials to identify candidates with, hopefully, desirable properties [22,23,24,25].…”

“…18 Phonon or vibrational density-of-states data have also been used to construct a range of cheap-to-compute descriptors for various properties of fast-ion conductors, including for ionic conductivity, activation energy, and electrochemical stability window. 20,21 Computationally cheap descriptors of specic properties are increasingly used in high-throughput materials discovery, where data from electronic-structure methods are combined with data-mining techniques to screen large numbers of materials to identify candidates with, hopefully, desirable properties. [22][23][24][25] The Debye frequency, obtained by speed-of-sound measurements, and the mobile-ion phonon band-centre, u av , calculated from density functional theory (DFT) or from inelastic neutron scattering data, have previously been proposed as descriptors for ionic conductivity and activation energy, respectively, within individual solid electrolyte families.…”

Diffuson-mediated thermal and ionic transport in superionic conductors

“…37 Nevertheless, an inverse slope of ~37 meV is in agreement with those reported for other superionic conductors. 39 In comparison to the energy range of the calculated phonon density of states it is unclear what this value really represents, except that the vibrational states of the Ag + ions do not change significantly through the substitution series, and that the overall vibrational properties are not significantly affected by the composition. By novel considerations of the phonon entropy (Supplemental Note 6), we…”

“…15,27,36,39 With that, reducing the slope of the Meyer-Neldel plot becomes a main goal. 39 By deriving the multi-excitation entropy using phonon-fluctuation considerations, we show that the Meyer-Neldel slope can be related to the prominent vibrational modes of ionic conduction and a characteristic number of phonons found to be in the range of equilibrium phonon occupations (Supplemental Note 6). 7 This leads to a novel concept and design principle in solid-electrolyte research: determining and manipulating phonon occupations (e.g.…”

Understanding fast-ion conduction in solid electrolytes

“…It is therefore useful to understand why certain materials exhibit particularly high ionic conductivities, while others, including materials that appear structurally or chemically similar, do not. Understanding the factors that promote fast-ion conduction in specific families of solid electrolytes can help direct the development of generalized 'design principles' that may then be used to design and synthesize new materials with improved ionic conductivities [3][4][5][6][7][8] or to identify completely new families of potential fast-ion conductors, through, for example, high-throughput computational screening [9][10][11][12]. The development of quantitative models of ion transport that can describe, and ideally also explain, the exceptional ionic conductivities of fastion conducting materials presents an additional intriguing challenge.…”