Structural and Electrochemical Analysis of Thick Laser Patterned Electrodes for Fast Charging Batteries
Authors: Dunlap Nathan A. 
Published: July 2022

For widespread adoption of electric vehicles, lithium-ion batteries need to achieve energy densities of around 350 Wh/kg, cost less than $100/Wh, and be able to charge quickly within 15 minutes. The production of most inactive materials within modern Li-ion cells (e.g. separator, current collector, packaging) have already undergone tremendous optimization but many opportunities remain for improving the performance of the electrodes and reducing the cost and time of wetting, formation and aging processes. By increasing the thickness of electrodes clear opportunities exist for enhancing the energy densities of conventional cell materials and reducing cost. For example, doubling the thickness of electrodes in full cells from 50 μm to 100 μm can increase the energy density of the cell by about 16% and reduce the cost of the cell by 50% (from $249/kWh to $172/kWh)1. These benefits arise from increasing the ratio of electrode active material to inactive material within cells; however, increased thickness is often accompanied by reduced rate performance and increasingly difficult electrolyte wetting and formation processes due to the challenge of infiltrating the electrode pore structure with electrolyte and displacing gases during formation2. One emerging electrode processing technology involves the laser-ablation of small micro pores or channels into Li-ion battery electrodes to enhance their electrolyte wetting and rate performance. Pulsed lasers are increasingly being used in advanced manufacturing processes due to their ability to weld, sinter, or remove materials quickly and with very high precision. When applied to high energy density thick electrodes, micro-structuring by laser patterning has been shown to improve the rate capabilities of cells while maintaining high capacity retention3. These laser ablated pores or channels provide low tortuosity paths for lithium ions to reach deeper regions in the thick electrodes more quickly than before, limiting concentration gradients and thus reducing unwanted lithium plating at elevated rates. Despite these benefits, the utilization of ultrafast lasing ablation systems as an in-line technique to selectively remove patterned regions of active material from lithium-ion battery electrodes during production has not yet been widely adopted by the industry. In this work, we identify and manufacture laser-ablated 3D electrode architectures for enhanced Li+ transport and electrochemical performance with the hope of inspiring confidence in the benefits, practicality and cost effectiveness of the laser patterning technique.

Preliminary modelling work at NREL has explored the limitations of planar electrodes for fast charge performance and the importance of microscale features for improving ion-transport4-8. With the correct pore/channel geometry, the energy density of ablated electrodes can be significantly improved compared to traditional architectures. We have combined simulation models and experimental data to optimize the 3-D laser patterning of our electrodes. Modelling elucidated the electrodes baseline transport limitations and quickly identified the most promising electrode architectures for favorable cost, energy- and power-density.

Next, we used our benchtop ultrafast femtosecond laser ablation system to pattern our thick (ca. 100 µm) electrodes. After processing, the electrodes were inspected using cross-section scanning electron microscopy (SEM) so that ablation parameters such as laser wavelength, power and repetition rate could be optimized and distributions of feature sizes could be identified giving insight into the limitations of the laser system. We observed that the anode and cathode materials responded significantly different to the laser ablation process and that this must be considered during electrode preparation. Next, scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) and x-ray diffraction (XRD) were used to probe for local chemical or structural changes in the electrodes’ with patterning to determine if the remaining active materials experienced permanent laser-induced damages. Once successful achievement of the target electrode architecture was confirmed, the benefits of the achieved 3-D patterning were investigated by cycling the electrodes in full cell coin cells and comparing their performance against standard cells with unmodified electrodes (benchmark).

Cycling data clearly shows improved rate performance and capacity retention derived from the patterning of the thick graphite and NMC electrodes. Direct structuring of the electrodes has improved their ion transport and reduced lithium concentration gradients (cell polarization) resulting in high specific energy- and high power-density. While clear evidence of rapid lithium plating was observed in the standard benchmark cells at rates exceeding 1C, this has been largely mitigated in the laser ablated electrodes. Lastly, we probed the wetting of these electrodes with electrochemical impedance spectroscopy to map in real time the improved rate and extent of wetting achieved through the adoption of our 3-D laser patterns. Achieving shorter wetting times will correspond to reductions in cell manufacturing costs and is expected to present a significant market advantage.

Figure 1