Small Change—Great Effect: Steep Increase of Li Ion Dynamics in Li₄Ti₅O₁₂ at the Early Stages of Chemical Li Insertion

Abstract

Lithium titanate (LTO) is one of the most promising anode materials for large-scale stationary electrochemical storage of energy produced from renewable sources. Besides many other aspects, such as negligible formation of passivation layers and no volume expansion during lithiation, the success of LTO is mainly based on its ability to easily accommodate and release Li ions in a fully reversible way. This feature is tightly connected with Li self-diffusion. As yet, little information is available about microscopic Li diffusion properties and elementary steps of Li hopping at low intercalation levels, i.e., at values of x being significantly smaller than 1. Here, we used 7Li spin-locking NMR relaxometry to probe absolute hopping rates of LTO (homogeneous) solid solutions in quasi-thermodynamic equilibrium. As a result, the largest increase of Li diffusivity is observed when small amounts of Li are inserted. Strong Coulomb repulsions caused by the simultaneous occupation of neighboring 8a and 16c sites serve as an explanation for the enhanced Li diffusivity found. At even larger values of x, Li mobility slows down but is still much faster than in the host material with x = 0. Our results experimentally corroborate the outcome of recently published calculations on the DFT level focusing on both dynamic and structural aspects. The findings favor the formation of LTO solid solutions upon chemical lithiation; the steep increase in Li diffusivity found might also help with understanding the flat insertion potential observed.

Challenges of Spinel Li4Ti5O12 for Lithium‐Ion Battery Industrial Applications

“…Na + and K + prefer to occupy the 8a site in lithium titanate spinel. Schmidt et al [120] proposed that, based on the 7 Li spin-locking NMR relaxometry results, small amounts of inserted Li (i.e., partial Li transfer) could improve Li ion diffusivities greatly. Because the ionic radius of Na + (1.02 Å) and K + (1.38 Å) are larger than that of Li + (0.76 Å), the lattices of Li 4 Ti 5 O 12 are effectively enlarged.…”

“…[109] However, whether the extra Li storage will damage the "zero strain" structure of Li 4 Ti 5 O 12 , and whether the rate and cycle performance of Li 4 Ti 5 O 12 will be deterioration still remain largely unknown. Schmidt et al [120] quantified the diffusion coefficients directly via spinlock NMR relaxometry. Therefore, the crystal lattice changes of Li 4 Ti 5 O 12 during the discharge to 0 V need to be clarified.…”

Fast Na ion transport triggered by rapid ion exchange on local length scales

“…In this study, we combined time-domain nuclear magnetic resonance (NMR) spectroscopy with broadband conductivity spectroscopy to quantify short-range as well as long-range Na + ion diffusivities. Using only NMR spin-lattice relaxometry working at atomic scale 36,37 we managed to get a sneak peek at the very rapid exchange processes the Na ions are performing at temperatures very well below ambient. The data collected provide a first experimental rationale for the very high ionic conductivity observed in NZSP-type compounds.…”

Li Ion Dynamics in Nanocrystalline and Structurally Disordered Li2TiO3

“…The results obtained for layer-structured β-Li 2 TiO 3 are briefly compared with those collected for another Li titanate viz. spinel-type Li 4 Ti 5 O 12 (LTO), which is a well-known zero-strain anode material for lithium-ion batteries [18][19][20][21].…”