Broadband impedance spectroscopy of Li₄Ti₅O₁₂ : from nearly constant loss effects to long-range ion dynamics

Li₄Ti₅O₁₂ (LTO) is known as one of the most robust and long-lasting anode materials in lithium-ion batteries. As yet, the Li-ion transport properties of LTO are, however, not completely understood. Here, we used broadband impedance spectroscopy spanning a wide temperature range to investigate the full electrical response of LTO over a wide frequency range. It turned out that the isotherms recorded entail information about two relaxation processes. While at high temperatures the isotherms show a frequency independent plateau that corresponds to poor long-range ion transport (<10⁻¹¹ S cm⁻¹ (298 K), 0.79 eV), they reveal a second region, seen at lower temperatures and higher frequencies, which we attribute to short-range ion dynamics (10⁻⁸ S cm⁻¹) with a significantly reduced activation energy of ca. 0.51 eV. At even lower temperatures, the isotherms are fully governed by nearly constant loss behavior, which has frequently been explained by cage-like dynamics. The present results agree with those earlier presented by ⁷Li NMR spin-lattice relaxation measurements being sensitive to dynamic processes taking place on quite different length scales. Our findings unveil complex Li⁺ ion dynamics in LTO and help understand its superior electrochemical properties