Effects of binder and solvent on cathode manufacturing for Li-ion batteries

Lithium-ion batteries (LIBs) have played a significant role in consumer electronics and electric vehicles. The major focus of research in Li-ion batteries has been to maximize performance and minimize cost. However, as the availability of raw materials becomes more difficult and political, there is a need to consider the recyclability and safety of Li-ion electrodes. One hindrance to realizing recyclability is the use of polyvinylidene fluoride (PVDF), a fluorinated polymer, as a binder in cathode electrodes. Another hindrance is the use of a toxic solvent NMP, used to solubilize PVDF. This work investigates the use of thermoset binders on mechanical and electrical performance of cathode electrodes to eliminate fluorinated polymers and allow for the use of alternative solvents. The first step in this work was to examine the importance of manufacturing steps on the performance of PVDF based electrodes for comparison to non-fluorine based binders. Our results clearly demonstrate that the calendering process has a significant, if not dominant role, in the final microstructure and performance of PVDF based electrodes. More specifically, we show that the calendering process significantly improves the short- and long-range contacts between electrons and ion active sites regardless of coating and drying conditions. Thus, in this work a standard protocol with calendering was followed in the manufacturing of electrodes for performance testing. This choice has profound implications for the choice of non-fluorinated polymer binders in electrode manufacturing. Non-fluorine based electrodes were developed and tested as potential replacements for PVDF in cathode binders. (Meth)acrylate resins were investigated as alternative binders in NMC111 cathodes: a flexible urethane-based monomer, and a stiff Bis-GMA based monomer. We determined the effects of thermal initiator content, polymer concentration, and curing parameters on the mechanical and electrode performance of cathode electrodes. The results show that there is an optimum thermoset formulation that leads to similar performance to PVDF calendered electrodes. Furthermore, the flexible urethane formulation exhibits higher rate capability compared to the stiff polymer formulation. This is demonstrated to be due to the high deformation during calendering, which causes the brittle resin to break down before and during electrode manufacturing. Lastly, the two resin formulations can be optimized to exhibit comparable performance to PVDF electrodes under calendering. One of the significant advantages of the selected thermoset resins is their solubility in a wide range of solvents. Therefore, less-toxic and inexpensive solvents were considered to replace NMP in the cathode manufacturing process. We restricted ourselves to green solvents to increase the impact of solvent replacement. We used NMP and three green solvents (PC, TEP, and DMSO) to fabricate NMC111 electrodes. The solvent effects on electrodes were quantified by measuring the mechanical properties of electrodes, rheological properties of slurries, electrode microstructure, and electrode performance. The results clearly show that replacing NMP in cathode electrode manufacturing is more difficult than just changing the binder. Even though evaporation rate, dielectric constant, and dipole moment of green solvents were chosen to be similar to NMP, the results show that NMP serves an important purpose in the distribution of carbon binder in the final electrode. Unfortunately, none of the green solvent alternatives performed as well as NMP based electrodes. Therefore, we conclude that a suitable solvent alternative to NMP for thermoset-based electrodes must result in good carbon binder domain (CBD) distribution, otherwise non-fluorine based resins will not result in good mechanical stability or performance. These results provide important implications for processing, designing, and selecting less toxic alternative binders and solvents to manufacture recyclable cathode electrodes.