Anion Charge Dependent Lithium Ion Diffusion in Solids

Abstract

Utilizing solid-state electrolyte materials for lithium-ion batteries not only fundamentally solve the safety problem of the commercial lithium-ion batteries, but also greatly increase the energy density of batteries. Lithium solid-state electrolyte materials should have high conductivities at room temperature comparable to liquid electrolyte, good (electro)chemical stabilities, mechanical properties, and excellent compatibilities with electrodes. The proposed design principles for lithium superionic conductors mainly include the concepts of bcc anion framework, low lithium phonon density center, and high distortion lithium-anion polyhedron. Moreover, the smaller anionic charge is well considered to benefit for the rapid transport of lithium ion, e.g. negative monovalent anions are more favorable for cation diffusion than negative divalent anions.However, our recent studies of the chalcopyrite-structured sulfides and non-spinel-structured halides show that the lithium-ion migration energy barrier can be effectively reduced by comprehensively regulating the anion charge and lithium-ion coordination environments. For example, for lithium ion diffusion between two adjacent lithium-anion octahedrons through a tetrahedral transition state, the greater the anionic charge is, the lower the lithium ion diffusion energy barrier will be. While for lithium ion diffusion between two adjacent lithium-anion tetrahedrons through an octahedral transition state, the smaller the anion charge is, the lower the lithium ion diffusion barrier will be. According to this design principle and choosing the appropriate cations to isolate the electronic conductive channels, we obtained the quaternary Li2CuPS4 solid electrolyte material with tetrahedral lithium occupations. Li2CuPS4 not only has high thermodynamic stability, but also theoretically possess the high ionic conductivity of 84.9 mS/cm at room temperature. In addition, the non-spinel halide Li3LaI6 with octahedral lithium occupations has relatively high ionic conductivity of 1.23 mS/cm at room temperature, and its thermodynamic stability and electrochemical stability are far superior to sulfides. This new understanding of comprehensively regulating the anion charge and lithium-ion coordination environments to enhance lithium ion transport can be applied for the design and optimization of new superionic conductors.