Lithium ion dynamics in LiZr2(PO4)3 and Li1.4Ca0.2Zr1.8(PO4)3

Abstract

High ionic conductivity, electrochemical stability and small interfacial resistances against Li metal anodes are the main requirements to be fulfilled in powerful, next-generation all-solid-state batteries. Understanding ion transport in materials with sufficiently high chemical and electrochemical stability, such as rhombohedral LiZr₂(PO₄)₃, is important to further improve their properties with respect to translational Li ion dynamics. Here, we used broadband impedance spectroscopy to analyze the electrical responses of LiZr₂(PO₄)₃ and Ca-stabilized Li_1.4Ca_0.2Zr_1.8(PO₄)₃ that were prepared following a solid-state synthesis route. We investigated the influence of the starting materials, either ZrO₂ and Zr(CH₃COO)₄, on the final properties of the products and studied Li ion dynamics in the crystalline grains and across grain boundary (g.b.) regions. The Ca²⁺ content has only little effect on bulk properties (4.2 × 10⁻⁵ S cm⁻¹ at 298 K, 0.41 eV), but, fortunately, the g.b. resistance decreased by 2 orders of magnitude. Whereas, ⁷Li spin-alignment echo nuclear magnetic resonance (NMR) confirmed long-range ion transport as seen by conductivity spectroscopy, ⁷Li NMR spin–lattice relaxation revealed much smaller activation energies (0.18 eV) and points to rapid localized Li jump processes. The diffusion-induced rate peak, appearing at T = 282 K, shows Li⁺ exchange processes with rates of ca. 10⁹ s⁻¹ corresponding, formally, to ionic conductivities in the order of 10⁻³ S cm⁻¹ to 10⁻² S cm⁻¹.