EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Li4+xTi5O12 (LTO) is a well-known anode active material with promising properties for its use in lithium-ion batteries. LTO can accommodate up to 3 excessive Li+ ions (and e− electrons) in its crystal structure with marginal volume changes and is, therefore, known as a so-called zero-strain material. During Li+ insertion, the Li-ions change their preference for the available crystallographic positions. This occupational disorder gives rise to a huge increase in both electronic and ionic conductivity. Rapid Li+ exchange between the sites 8a and 16c governs the main conduction mechanism in samples with x being larger than 0. Here, we focused on the conduction mechanisms present at the start of insertion, that is, in the non-lithiated Li4Ti5O12. Via impedance spectroscopy, we observed two different conduction processes, a fast short-range and a slow long-range process, respectively. While the slow process must be characterized by an activation energy Ea of 0.83 eV, the faster one reflects Li+ translational dynamics with Ea = 0.54 eV. Interestingly, cycling the sample between RT and 200 °C in a slightly reducing nitrogen atmosphere increases the conductivity of the short-range process. To identify the responsible mechanism, we mimicked this soft annealing by annealing the sample in a vacuum at 300 °C. This treatment further increased the conductivity of the short-range process while that of the long-range process becomes slightly reduced. Making use of electron paramagnetic resonance (EPR) spectroscopy, we found that in vacuo annealing generates a strong resonance signal with a Landé-factor of 2.003 pointing to free electrons that are most likely trapped in oxygen vacancies. Most importantly, EPR gave no evidence for Ti3+, which could give rise to polaronic conductivity. Therefore, we assume that the oxygen vacancies induce increased short-range ionic conductivity.