Periodic Reporting for period 1 - UltraThick Las (Development of Ultrathick Laser Ablation for Ultrathick Electrode Processing)
Okres sprawozdawczy: 2023-09-01 do 2025-08-31
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.
Electrodes for the study were made using 2 standard active materials namely graphite and NMC-811. Ultrathick electrodes with different thicknesses were successfully fabricated using standard recipes that have been developed by the lab for more than 8 years. Laser Structuring and electrochemical techniques were also employed in both half cell and full cell configuration and at different cycling rates . Graphite and NMR electrodes were cycled at different thicknesses and their power performance were evaluated. It was shown that the laser laser ablation does not improve significantly the power performance at higher C-rates for thicknesses higher than 200 microns. The ablation also does not significantly improve power performance for cells of higher salt concentration in the electrolytes. These observations suggests that the both the channel depth and electrolyte concentrations are key factors in the electrochemical performance of ablated electrodes. We also were able to directly recycle the ablated graphite material (filtered material) without further chemical treatment which was not the case for NMC-811. We also observed the same Li plating behaviors at different electrolyte concentrations as found in literature where increased concentrations decreases or eliminates lithium plating behavior.
We were able to measure the self-diffusion values of the liquid along the channels using the classic diffusion NMR (pulsed field gradient spin echo) technique. Not only this demonstrates that the liquid in the channels are more labile than in the pore matrix but also the values are not similar to the pure bulk liquid electrolyte even with the addition of a separator. This also demonstrates the limitation of the classic diffusion NMR technique (T2 based) as we weren't able to obtain the values that correspond to the species that exchange with the pores in the electrodes. We then tried to develop a method which could circumvent these limitations which are based on T1. The investigators based these techniques around Exchange NMR (EXSY). Two of these techniques are namely the following:
1.) Chemical Exchange Saturation Transfer NMR experiment (CEST)
2.) Selective Transverse Exchange NMR
CEST was first explored due to its proven use in literature to monitor changes in concentration in systems due to exchange between on environment to another. T1 values were successfully measured and we used the relevant data as base for determination of diffusion coefficients using the Torrey Bloch Relation. Initial results show that the fits can describe a porous environment but the fits can only be attained with very slow initial diffusion coefficient. It was then surmised that CEST might be blocking a portion of signal that properly describing the exchanging species. Selective exchange NMR was then explored which surprisingly revealed existing interactions between lithium and the graphite surface. This is complementary to zeta measurements in literature which have cited existing interactions between the lithium ion and graphite surfaces.
XPS and Pelletron experiments were also performed which experienced delays and the results are not reported here. Any breakthrough results will be written and published in a journal article.
We were able to measure the self-diffusion values of the liquid along the channels using the classic diffusion NMR (pulsed field gradient spin echo) technique. Not only this demonstrates that the liquid in the channels are more labile than in the pore matrix but also the values are not similar to the pure bulk liquid electrolyte even with the addition of a separator. This also demonstrates the limitation of the classic diffusion NMR technique (T2 based) as we weren't able to obtain the values that correspond to the species that exchange with the pores in the electrodes. We then tried to develop a method which could circumvent these limitations which are based on T1. The investigators based these techniques around Exchange NMR (EXSY). Two of these techniques are namely the following:
1.) Chemical Exchange Saturation Transfer NMR experiment (CEST)
2.) Selective Transverse Exchange NMR
CEST was first explored due to its proven use in literature to monitor changes in concentration in systems due to exchange between on environment to another. T1 values were successfully measured and we used the relevant data as base for determination of diffusion coefficients using the Torrey Bloch Relation. Initial results show that the fits can describe a porous environment but the fits can only be attained with very slow initial diffusion coefficient. It was then surmised that CEST might be blocking a portion of signal that properly describing the exchanging species. Selective exchange NMR was then explored which surprisingly revealed existing interactions between lithium and the graphite surface. This is complementary to zeta measurements in literature which have cited existing interactions between the lithium ion and graphite surfaces.
XPS and Pelletron experiments were also performed which experienced delays and the results are not reported here. Any breakthrough results will be written and published in a journal article.
The first key results are from the electrochemical measurements. It has been demonstrated in literature that laser structuring improves electrochemical power performance in various electrodes but at electrode thicknesses higher than >200 microns, this beneficial effect is not as significant. This effect is also more significantly beneficial at electrolyte concentrations <1.4M concentration. This greatly suggests that electrolyte diffusion and depth penetration is still a key factor in structured electrodes at greater channel depth.
A second key result is that we also demonstrated that we were able to directly recycle the ablated graphite (filtered material) and use it electrochemically without any further treatment regardless of laser radiation source (in comparison to what was found in literature).
The third key result comes from our observations developing the Transverse Relaxation Exchange NMR technique. Through the 1D spectra, it was demonstrated that lithium does not mobilize after trying many diffusion times. This was not an intended result of the experiment and this demonstrates that mobility phenomenon can be described with just the 1D spectra. Moreover, this result re-asks the question of surface attraction between the lihtium ion and the active material particles present in the electrode. This attraction is always overlooked even with evidences from zeta measurements and measurements.
A second key result is that we also demonstrated that we were able to directly recycle the ablated graphite (filtered material) and use it electrochemically without any further treatment regardless of laser radiation source (in comparison to what was found in literature).
The third key result comes from our observations developing the Transverse Relaxation Exchange NMR technique. Through the 1D spectra, it was demonstrated that lithium does not mobilize after trying many diffusion times. This was not an intended result of the experiment and this demonstrates that mobility phenomenon can be described with just the 1D spectra. Moreover, this result re-asks the question of surface attraction between the lihtium ion and the active material particles present in the electrode. This attraction is always overlooked even with evidences from zeta measurements and measurements.