Lithium ion batteries (LIBs) have become omnipresent in our society. The growing demand for them has led to increasing interest in more environmentally friendly ways of production.
Today, the dominant process for the manufacturing of electrodes for LIBs is the solvent-based casting process. This process, however, comes with several drawbacks. The most common employed solvent used in this process is the toxic and environmentally hazardous solvent N methyl pyrrolidone (NMP). The drying process is very energy-intensive and expensive equipment is needed for the solvent recovery.
To attain high energy densities, a high mass loading is advantageous since it decreases the share of passive materials like current collectors or separator. Yet in the solvent based electrode production process, the conditions during the mixing and dispersing processes in combination with capillary forces arising during the drying process have a strong influence on the electrode’s microstructure and can lead to low mechanical quality and poor electrochemical performance. This is especially critical for electrodes with high mass loadings.
Solvent-free extrusion provides an alternative approach to produce high-energy electrodes for LIBs in a continuous process that circumvents the use of NMP. Herein, we present the investigation of a dry extrusion process utilizing NCM 811 as active material, carbon black and graphite as conductive additives, a PVDF copolymer as binder and ethylene carbonate as processing agent. The influence of the formulation on the rheological properties of the extrudate was examined with a capillary rheometer. The feasibility in an industrial relevant process was explored by conveying the pastes directly from the extruder through a slot die of adjustable height and applying them on an aluminum collector foil in a roll-to-roll process. Subsequently, the influence of the formulation and processing parameters on the rate capability was investigated.
With our solvent-free approach, we produced cathodes with an active material content as high as 90 % in the composite, which is close to state of the art for conventional electrodes and high yet useful areal capacities of 5 mAh/cm2 are demonstrated.