Diffusion-induced 7Li NMR spin-lattice relaxation of fully lithiated, mixed-conducting Li7Ti5O12

• Li+ self-diffusion coefficients for Li7Ti5O12 and Li4Ti5O12 were directly determined from the maxima of the diffusion-induced NMR spin-locking rate peaks
• NMR points to extremely slow Li ion hopping in Li4Ti5O12 but enhanced diffusivity in the fully lithiated compound Li7Ti5O12 that is, however, much lower than in Li4 + xTi5O12 with 0 < x < 1.
• Together with results from literature a full picture of activation barriers as seen via NMR is provided for Li4+xTi5O12 with x ranging from 0 to 3.

Li4Ti5O12 (LTO) belongs to one of the most promising anode materials for lithium-ion batteries. Its superior cycling performance and negligible aging make it a potential candidate to be used in, e.g., stationary applications. Besides this application-oriented interest it serves as an excellent model system to study Li ion transport in a 3D mixed conducting host crystallizing with spinel structure. Whereas Li ion diffusion in Li4Ti5O12 was the subject of several studies that appeared over the past years; Li ion transport in mixed conducting Li7Ti5O12 crystallizing with rock-salt type structure is, however, much less frequently investigated. Li7Ti5O12 is the compound that is formed after an LTO-type battery has been fully charged. In the present study we used spin-lock NMR relaxometry to quantify Li ion diffusion in terms of jump rates, activation energies and microscopic Li ion self-diffusion coefficients. Extending the measurements to higher temperatures enabled us to record the diffusion induced rate maxima from which Li+ self-diffusion coefficients were obtained directly. Compared to the non-lithiated source material Li4Ti5O12, showing poor Li ion diffusivity, Li ion diffusivity in Li7Ti5O12 is clearly enhanced but by far as high as recently reported for spinel-type Li4+xTi5O12, with x being significantly smaller than 1. Obviously, the small number of vacant 16c sites in Li7Ti5O12 as well as repulsive the 8a–16c interactions are responsible for the low Li ion diffusivity found.

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