Li ion dynamics in TiO2 anode materials with an ordered hierarchical pore structure – insights from ex situ NMR

Abstract

Ex situ Nuclear Magnetic Resonance (NMR) measurements were carried out to study lithium ion dynamics in lithium intercalated mesoporous anatase (Li_xTiO₂) serving as an anode material for rechargeable lithium-ion batteries. As has been shown recently, hierarchically ordered TiO₂ shows excellent cycling performance and ensures a high lithium storage capacity. ⁷Li spin–lattice relaxation NMR and stimulated echo NMR serve as a powerful combination to shed light on the Li hopping processes from an atomic-scale point of view. To determine atomic Li jump rates and microscopic activation energies temperature-variable SLR NMR measurements, in both the laboratory and rotating frame of reference, as well as mixing-time dependent spin-alignment echo NMR measurements were carried out. The results point to moderate Li diffusivities; however, in a lithium-ion cell this is compensated for by taking advantage of nm-structured materials with greatly reduced diffusion lengths. Importantly, although a phase transition from tetragonal symmetry to orthorhombic symmetry takes place at increased states of charge, the diffusion parameters and activation energies probed (0.4 to 0.5 eV) do depend weaker on Li content x than expected. Thus, despite the increased value of x, the evolution of the orthorhombic phase seems to support Li diffusivity rather than to affect the transport properties in a negative way. This interesting feature might be highly beneficial for the excellent cycling behavior observed recently.