Strain-free structural transition during Li-ion rocking chair battery operation based on LTO anodes

LTO (Li4Ti5O12) spinel to cubic transformation [1] induced by Li insertion in octahedral sites and concomitant migration of pristine lithium from tetrahedral sites, is in principle strain-free; correspondly can guarantee a longer life-time and cyclability, in spite of a lower energy density, for higher frequency applications. The transformation to a cubic structure though apparently does not affect the volume, creates at the level of the surface a strong rearrengement of the electrode changing morphology and chemical composition [2], that can exceed the theoretical capacitance [3]. The structure of the device is controlled, during batteries charge and discharge cycles, at the atomic level as a function of time by using XAS [4], a chemical sensitive and short range probe, and by selectively tuning the detection depth by collecting electrons, total and partial yield, and photon fluorescence yield. X-ray absorption experiments have been conceived and realized to study the modification of the signals related to the various atomic species in LTO electrodes selected at different states of charge during the first Li insertion process and thickness. In particular the comparison with multiple scattering models including inverted spinel transformation and Li2TiO3 reversible growth on the electrodes was able to pin critical transition and surface layer modification that can be additional sources of capacity of the device. Furthermore the instability of the highly reactive oxide since the initial pristine material conditions was critically discussed. References 1. Panero, S. Phys. Chem. Chem. Phys. 2001, 3, 845−847 2. Birrozzi A., et al. Journal of The Electrochemical Society, 162 (12) A2331-A2338 (2015) 3. Kitta M. et al. Langmuir 28, 12384 (2012) 4. Di Cicco et al., Adv. Enegy Mat. 1500642 (2015)