Measurement of Electrolyte Self-Diffusion in Laser Structured Electrodes

It is generally stated that a limiting factor in fast charging and high-power discharging of lithium-ion batteries (LiB) stems from the diffusion kinetics of Li⁺ in the electrode pore network. Numerous approaches in enabling fast charging of LiBs include active material development, electrolytes with high ionic conductivity and the management of the charging and discharging temperature. Another promising method is laser micro structuring to modify the electrode architecture regarding an enhanced lithium-ion diffusion kinetics. An increased high-rate capability and boost in cell lifetime have been demonstrated with such 3D electrodes [[i], [ii]] but the related mechanisms leading to a substantial impact to diffusion kinetics are still poorly understood. Furthermore, it is imperative to find an optimal ablation pattern with regard to the desired application scenario that minimizes active mass loss in order to create the economic basis for efficient upscaling of the process.<br/> <br/>Chemical Exchange Saturation Transfer Nuclear Magnetic Resonance (CEST-NMR) is a technique that exploits spin magnetization saturation for Magnetic Resonance Imaging (MRI) [[iii]]. The generated images allow the best contrast for detecting chemical biomarkers. The image contrasts are a result of saturation exchange between the biomarker and water and is detectable through changes in longitudinal relaxation times (T₁). Through the Torrey-Bloch Relaxation, T₁ can be related with the magnetization profile to extract the effective self-diffusion coefficient. In this work, a modified CEST experiment, is conceptualized to study the impact of laser generated 3D patterns in pore self-diffusion of electrolyte species.<br/> <br/>Exchange NMR and T₁ measurements were realized at various soaking times and electrolyte amounts for the following electrolyte species: Li⁺, PF₆^-, ethyl carbonate (EC), and ethyl methyl carbonate (EMC), using laser structured, graphite-based electrodes casted on non-metallic substrates (to reduce RF interference). Magnetization profiles show the presence of confined species as well as the approximation of the diffusion properties. Using isotope exchange experiments with ⁶Li enriched electrolyte, concentration maps revealed the rate of ⁶LiPF₆ intrusion into ⁷LiPF₆ rich pre-soaked electrodes. The impact of laser generated 3D patterns in pore diffusion will be discussed in detail.<br/><br/><br/>[i]. Zheng, Y. et al. 3D silicon/graphite composite electrodes for high-energy lithium-ion batteries. Electrochim. Acta <b>317</b>, 502–508 (2019).<br/><br/>[ii]. Smyrek, P., Pröll, J., Rakebrandt, J.-H., Seifert, H. J. & Pfleging, W. Manufacturing of advanced Li(NiMnCo)O₂ electrodes for lithium-ion batteries . Laser-based Micro- Nanoprocessing IX <b>9351</b>, 93511D (2015).<br/><br/>[iii]. Wu, B. et al. An overview of CEST MRI for non-MR physicists. EJNMMI Phys. <b>3</b>, (2016).