Rethinking the Design of Ionic Conductors Using Meyer–Neldel–Conductivity Plot

Abstract

DOI: 10.1002/aenm.202100325 1 mS cm−1.[6–8] Recently, the sulfide fast ionic conductor Li9.54Si1.74P1.44S11.7Cl0.3 reached the room-temperature ionic conductivity of 25 mS cm−1, which even exceeds that of liquid electrolytes.[1] However, these ionic conductors were mostly discovered through chemical intuition and trial and error. Therefore, to guide the selection and identification of the most promising candidates and accelerate the design process, it is urgent to rationalize a general design principle. In parallel with experimental efforts, computation has greatly advanced our understanding of the relation between structural and physical factors and ion transport.[9–12] Low activation energy had generally been thought to lead to high ionic conductivity. The local structure of the migrating ion along the diffusion path has been shown to strongly affect the activation energy of the ionic conduction.[7,13–15] Ion transport is also considered to correlate to the lattice dynamics. Links between low activation energy and low-energy optical phonons,[16] a soft lattice with highly polarizable anions,[17,18] and a low lithium phonon band center[19,20] have all been proposed. In addition, the low activation energy of fast ionic conductors can be rationalized on the basis of correlated ion transport, where hops of different ions are correlated, partially compensating for the high activation energy required for single-ion hopping.[21,22] The accepted common ground linking low activation energy to high ionic conductivity is the Arrhenius equation: