Discriminating the Mobile Ions from the Immobile Ones in Li_4+xTi₅O₁₂: ⁶Li NMR Reveals the Main Li⁺ Diffusion Pathway and Proposes a Refined Lithiation Mechanism

Abstract

One of the most widely known anode materials with excellent cycling performance in lithium-ion batteries is zero-strain lithium titanate Li4Ti5O12 (LTO). This is based on the highly reversible insertion and deinsertion of Li ions varying the composition of Li4+x Ti5O12 from x = 0 to x = 3. The facile incorporation of Li is tightly connected with bulk Li ion dynamics. Although macroscopic electrochemical properties have been studied in great detail, bulk Li ion diffusion pathways have rarely been probed experimentally from an atomic scale point of view. Here, we evaluated 1D and 2D 6Li magic angle spinning (MAS) NMR spectra recorded as a function of x to make the local magnetic changes during Li insertion visible. This enabled us to directly show that rapid Li exchange involves the 8a and 16c sites, forming a 3D network throughout the LTO spinel structure. In contrast, considering the time scale set by our NMR experiments, the Li ions residing on the mixed occupied octahedral positions 16d are quite immobile. Under the MAS conditions applied, they definitely do not participate in rapid Li self-diffusion as evidenced by rather long site-specific longitudinal relaxation times as well as the absence of any NMR coalescence effects. Based on the NMR spectra, we propose a refined Li insertion mechanism which includes the initial formation of a highly conducting quasi solid solution characterized by rapid Li ion exchange.

Abundant grain boundaries activate highly efficient lithium ion transportation in high rate Li4Ti5O12 compact microspheres

“…It is well-known that the R ct is affected by both D Li and the electronic conductivity, indicating that the more nanocrystals in compact LTO-60% ethanol sample constructs richer boundaries that can form more abundant 3D charge transport network for high efficient ionic and electronic transportation. The R ct of both the LTO-50% ethanol and LTO-60% ethanol samples decreases from 15% SOC to 82% SOC as well as the initial lithiation process, which can be attributed to that the lithiation of grain boundary can further decreases the boundary resistance of LTO 31,32 . The mobile lithium ions initially gather along the grain boundary where the lithium ion diffusivity increased dramatically as compared to the poor diffusivity of the Li 4 Ti 5 O 12 and Li 7 Ti 5 O 12 phase 12,39 .…”

“…Based on the electrochemical analysis, we proposed that the grain boundaries throughout the spheres may form 3D ionic transportation tunnel to achieve high electronic and ionic conductivity. However, the reaction mechanism is not yet clear, and there is lack of the real space observation of lithium ions transportation that is of great significance for understanding the detailed process of how grain boundaries transport the ions 32 . In addition, our prepared LTO sub-microsphere with diameter size of 500nm may be not sufficient to reveal the ionic conductivity of grain boundaries.…”

Mechanisms of the decrease in low-temperature electrochemical performance of Li4Ti5O12-based anode materials

“…However, the truth deviates from the ideal, particularly charging/discharging under the condition of high rates or low temperatures. Lithium ions cannot fully intercalate while discharging nor deintercalate while charging, Consequently, lithium ions accumulate continually in octahedral 16c positions and tetrahedral 8a positions (Lithium ions in Li 4 Ti 5 O 12 are believed to be along the [011] orientation diffusing from 8a-16c-8a during the charge/discharge process 28,29 .). The XRD peak intensities of (311) and (400) are related to the number of lithium ions at 8a (tetrahedron) and 16c (octahedron) sites, respectively 21 .…”

Enhanced rate performance of Li4Ti5O12 anodes with bridged grain boundaries

“…Both computational and experimental effort have been investigated to resolve the contradiction of phase separation vs. solid solution contradiction. [7,18] Experimental studies [19][20][21] Li occupancy is credited for the formation of apparent solid solution, and thus for the excellent rate performance of Li 4+x Ti 5 O 12 (0 < x < 3).…”