Ion dynamics in solid-state batteries - A 7Li NMR study

• Main Author: Haaks, Michael
• Format: Others
• Language: en
• Published: 2018
• Online Access: https://tuprints.ulb.tu-darmstadt.de/7363/7/Dissertation_Haaks_Version_19.04.2018.pdf; Haaks, Michael <http://tuprints.ulb.tu-darmstadt.de/view/person/Haaks=3AMichael=3A=3A.html> (2018): Ion dynamics in solid-state batteries - A 7Li NMR study.Darmstadt, Technische Universität, [Ph.D. Thesis]

Rising energy demand, in combination with a mobile society, makes it more and more important to store energy when available and release it upon demand. Rechargeable lithium ion batteries are widely used today for powering small devices, e.g., smartphones and cameras. However, further improvements are required, if lithium ion batteries are supposed to power electric vehicles. One requirement to achieve this task is the development of an all-solid state battery. Here, the flammable liquid electrolyte is replaced by a solid material. A few years ago, the main drawback of solid materials was the relatively low conductivity in comparison to that of their liquid counterparts. While the conductivity of solid electrolytes could be enhanced to the level of liquid electrolytes used in modern lithium ion batteries, the mechanism of lithium ion dynamics in those amorphous solids is still not understood. The aim of this work is to analyse the dynamics of lithium ions in highly conducting solid materials on broad time and length scales. For this purpose, a combination of well established and newly developed 7Li NMR methods are used. The new conjunction of 7Li diffusion measurements and 7Li field cycling relaxometry achieved in this work turns out to be an ideal combination. While the former probes the long-range transport on a mesoscopic length scale, the latter gives insight into local lithium ion jump dynamics on a microscopic length scale in the same temperature range. Additionally, field-cycling experiments allow the determination of the true activation energy by measuring temperature dependent susceptibility or relaxation-rate maxima. The present research study reveals heat induced changes of the investigated samples when approaching their glass-transition temperatures Tg with partly opposite effects. While the 0.7Li2S–0.3P2S5 system is known for an enhanced dynamics due to this kind of ceramization, the heat treatment of the 0.7Li2S–0.3B2S3 samples leads to an occurance of a second lithium species with reduced dynamics. Additionally, comparison with literature reveals that the results strongly depend on details of the methods used to prepare the samples. Furthermore, it is shown in this work that a broad Gaussian distribution of activation energies exists in all studied solid lithium ion conductors, which is responsible not only for the local lithium ion jump dynamics, but also for the long-range diffusion. This indicates that the difference of activation energies usually obtained from distinct methods is a mere consequence of probing various averages of the resulting logarithmic distribution of correlation times. Hence, no specific model is necessary to describe the activation energy discrepancy often observed when comparing results of DC conductivity and NMR experiments. These findings are comfirmed by re-analyzing data given in the literature.