Free Standing Electrodes: From Roll Product to 41-Layer Pouch Cells

Abstract

Processes to produce free standing electrodes for Lithium-Ion Batteries (LIB), using lithium iron phosphate (LFP) as active material for the cathode and artificial graphite for the anode, are being developed. The production of these electrodes is realized by using a slot die to produce a liquid film which solidifies in a precipitation bath and is wound up after moving through washing baths. This procedure consumes significantly less energy compared to conventional coating processes, as the drying needs are reduced, and the electrodes have a broader spectrum of use cases due to their high flexibility.In this work, the development process of the manufacturing procedures of pouch cells using free standing electrodes will be presented. This includes the characterization of the electrodes, usability trials with research pilot line equipment, and cycling tests of built pouch cells of up to more than 7 Ah per device.The screening of electrodes through cycling in coin cells showed a specific discharge capacity of > 120 mAhg-1 for 50 cycles at 0.2C.This innovative and sustainable electrode production process required adjustments throughout the whole production process of a LIB, starting with the slot die geometry to reduce thick edges of the released film. The electrodes’ surfaces were investigated regarding homogeneity and roughness using a digital microscope. Gas adsorptioneasurements showed the specific surface of cathodes at around 3.6 m²g-1. Challenges during calendaring of the free-standing electrodes included thickness deviations of the pristine electrodes and uneven tension throughout the width of the samples.After laminating the free-standing electrodes with current collectors, they were electrochemically and mechanically analysed. Therefore, additional adjustments had to be made to attach tabs on multiple layers of these flexible electrodes through a single ultrasonic welding spot. All electrodes were successfully processed inside a dry room (dew point < -40 °C) using industrially relevant pilot scale equipment (punching, stacking, electrolyte filling, sealing).The manufactured pouch cells all showed an initial OCV of around 400 mV and were formed prior to cycling. During three formation cycles at 0.1C, followed by a constant Voltage step, the two-layered pouch cells showed a specific discharge capacity of . These cells reached roughly 90 mAhg-1 of discharge capacity after 20 cycles at 0.2C, but higher C-rates reduced the capacity and cell life significantly. Measuring the cells’ resistance after cycling at 3.3 V using 10 measurement points at 1 kHz revealed the high inner resistance of > 2 Ohm. Post-mortem analysis revealed lithium deposition throughout the whole surface of the anode. In the following upscaling process, the number of layers was increased to 11 layers (2 Ah cells) and 41 layers (7 Ah cells). These pouch cells showed similar performances, specific discharge capacities of around 110 mAhg-1 during formation, but with decreased cycling capabilities at C-rates higher than 0.1C. The main cause for this behaviour is the high inner resistance of these cells.In this work we demonstrate that free-standing electrodes can be handled at pilot scale in a comparable manner as conventionally coated electrodes. The produced cells were all successfully cycled but showed higher inner resistances and lower capacities at high C-rates (> 0.5C). The proof-of-concept was therefore achieved, further improvement of the cell performance will be focused on in the future. Figure 1