UltraThick Las · Development of Ultrathick Laser Ablation for Ultrathick Electrode Processing

Development of Ultrathick Laser Ablation for Ultrathick Electrode Processing

One of the great challenges that hamper Li-ion battery’s ubiquity in electric automobiles and/or high-power applications stems from the balance between energy (mileage and capacity) and power (torque and charging times). Increasing the electrode thickness increases the nominal cell energy density at around 17% (from 246 Wh/kg to 300 Wh/kg) in a cost efficient manner during fabrication due to the increase in active material deposited. Unfortunately, the increase in electrode thickness, is detrimental to the electrode’s power output. These ultra-thickfilm electrodes (> 100 μm) can only deliver ≈ 30% of active material (AM) specific capacity at current densities higher than 1C (1hr or less of charge/discharge time) vs. ≈ 85% AM specific capacity from thin-film electrode (< 50 μm) counterparts. At high current densities, the charging rate is limited by lithium diffusion from the electrolyte bulk to the electrode pore reservoirs resulting in early reaction termination . The ion replenishment process is affected by the increased length of the ion path in the pores for thicker electrodes. Thus, shortening the ion path length in ultra-thick electrode is essential to improve their electrochemical power performances. One way to decrease ion path distance is by increasing locally the porosity through the use of laser-assisted structuring (laser ablation) by drilling holes and/or trenches as deep as the whole electrode thickness and/or drawing free standing structures. This was demonstrated through various works (Pfleging et.al.) which showed an increase in capacity delivery (upto 20%). The technique is planned to be raised to technology readines level 4 or 5 but hurdles are still present. One of these hurdles is the balancing between the mass loss and capacity delivered vis a vis the ablation pattern. One can determine this by cycling all pattern imaginable which could lead to using too much resources to develop. In this study, we conceptualize the use of the diffusion coefficient of the lithium ion as a paramenter to predict the electrochemical performanc of patterned electrodes. An existing technique in Nuclear Magnetic Resonance (NMR) called Diffusion NMR has been widely used to determine self diffusion coefficients of species in various liquid media ranging from pure liquids to electrolyte solutions. However, limitations of the technique, which stems from relaxation time T2 hampers its use to determine diffusion coefficients of the electrolyte salt in battery electrodes. We aim to devlop an exchange experiment where the relaxation time T1 of various electrolyte species are instead correlated with the diffusion coefficient. This can be theoretically determined through the Torrey-Bloch equation relation and upon successful development, A diagnostic tool could be developed to determine the best pattern for Li-ion diffusion determination. The project also envisions the development of patterns that could deliver better power performance in comparison to the state of the art of basic lines. Finally, we also aim to develop a a diagnostic tool to predict electrochemical performance through just measuring the diffusion coefficient upon successful measurement via NMR.

Data: CORDIS, © European Union