Relevance between the Bulk Density and Li⁺-Ion Conductivity in a Porous Electrolyte: The Case of Li[Li_1/3Ti_5/3]O₄

The Li+-ion conductivity (σLi) in an electrolyte is an important parameter with respect to the performance of all-solid-state lithium-ion batteries (LIBs). However, little is known about how σLi in a porous electrolyte differs from that in a highly dense electrolyte. In this study, the relationship between the bulk density (dbulk) and apparent σLi (σLiapp) in a porous electrolyte of Li[Li1/3Ti5/3]O4 (LTO) was examined by theoretical and experimental approaches. The theoretical calculations demonstrated that dbulk and σLi have a simple relationship irrespective of the radius of the spherical pores in the electrolyte; i.e., σLi increases almost linearly with increasing ζ,where ζ is the ratio of d bulk to the theoretical density. In fact, the observed σLiapp of LTO, which was determined by four-probe alternating-current impedance measurements, increased with increasing ζ. Hence, with this relationship, σLiapp can be estimated by ζ and intrinsic σLi (σLiint) and vice versa; such estimations provide critical information for determining the optimum compositions of composite electrodes for all-solid-state LIBs. The temperature dependence of σLiapp in LTO and differences between the calculated and experimental results are also discussed.