Maximizing Ionic Mobility By Lattice Disorder in Anti-Perovskite Solid Electrolytes
Authors: Kim Kwangnam , Siegel Donald J 
Published: February 2020

Amongst these requirements for Li-ion batteries, ensuring safe operation is near the top of the list. Liquid electrolytes, widely used in commercial Li-ion batteries, have safety issues due to their volatility and flammability. Also, they may suffer from dendrite formation which can yield an internal short-circuit. In principle, solid electrolytes avoid these problems and furthermore open the possibility of using a metallic Li anode that highly increases the energy density. Recently, the anti-perovskites Li3OCl was suggested as one of promising solid electrolytes due to its high ionic conductivity.1

The correlation between high ionic conductivity and disorder has been discussed previously. Here, disorder can refer to an amorphous phase,2 disorder in the occupation of a sub-lattice,3 or in the rotational degrees of freedom of complex anions.4 The anti-perovskite system presents an ideal venue to further explore the connection between ionic mobility and disorder, in terms of the tolerance factor5 that describes the degree of disorder. The disorder can systematically be introduced by substitutions of chalcogen and halogen ions. Then, the system will lose the perfect cubic symmetry in the forms of octahedral tilting and distortion.5

In this study, anti-perovskite solid electrolytes M3XY (M = Li or Na, X = O, S, or Se and Y = F, Cl, Br or I) are thoroughly examined to compare barrier energies of pathways and investigate the correlation between lattice disorder and high ionic mobility. The density functional theory (DFT) was used to predict atomic structures and minimum energy pathways. The lattice deviates from perfect cubic symmetry as octahedral disorder is induced by substitutions of anions, and the degree of disorder increases with higher variation of tolerance factor. We calculated barrier energies of all possible individual migration paths for vacancies and interstitial ions, and found minimum energy pathway that can contribute to the macroscopic ionic transport.
We reveal a clear correlation between high ionic conductivity and the lattice disorder. Compounds with higher disorder exhibits lower barriers for the migration of vacancies and interstitials, thus the disorder plays a key role in reducing the huddle for ionic migration. Therefore, it suggests that tuning lattice structure in terms of disorder can maximize the ionic conductivity. Perfect cubic symmetry presents only one unique ionic migration path along twelve octahedron edges, whereas the disorder causes environmental differences between paths due to the broken symmetry. It leads to the segregation of individual paths into energetically favorable and unfavorable channels for ionic migration. Compounds with higher degree of disorder show wider segregation and lower barriers of favorable paths. Therefore, higher disorder provides the possibility to create migration pathways by combining individual paths with low barriers and achieve high ionic conductivity.
References
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[3] B. Kozinsky, S. A. Akhade, P. Hirel, A. Hashibon, C. Elsässer, P. Mehta, A. Logeat and U. Eisele, Phys. Rev. Lett., 116, 055901 (2016).
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[5] R. J. D. Tilley, Perovskites: Structure-Property Relationships, John Wiley & Sons, Chichester, West Sussex, 2016.