The Dynamic Mechanical Response of Solvent-Free Li Ion Battery Electrodes and the Implications for Large-Scale Manufacture
Authors: Matthews Guillaume , Meyer Benjamin , Grant Patrick S. 
Published: December 2024

A recent report by the Faraday Institution, the UK’s national institution for battery research, has estimated that 40 gigafactories will be commissioned in Europe alone by 2030 for a projected Li ion battery (LIB) capacity reaching 1,100 GWh per annum. This represents an unprecedented effort to tackle the net-zero goals set by the European Union and will require colossal investments from private and public sectors.

Today, almost all the commercial electrodes produced for LIBs are manufactured by slurry-casting, a process that has dominated battery manufacturing for the last 30 years. Slurry casting lines have become larger and faster, but the electrode manufacturing process itself has remained mostly the same. Despite its ubiquity, slurry casting has several inherent restrictions such as high energy consumption (and associated high embedded carbon), extensive use of flammable and highly toxic solvents (especially for the positive electrode where aqueous binder systems have yet to be satisfactorily developed), and large capital and operational costs that are mostly due to large-scale solvent drying and recovery steps. Recently, solvent-free processing methods for LIB electrodes have gained attention from both industry and research laboratories, with publications describing potential advantages over slurry casting in terms of sustainability, cost and safety due to the complete elimination of solvents from the electrode manufacturing process. Moreover, some studies have also suggested that solvent-free electrodes can show better electrochemical performance than conventional slurry cast electrodes.

However, despite the growing interest in solvent-free processing, compelling exemplification at the laboratory scale, and rapid take-up by some parts of the LIB industry, significant manufacturing challenges are emerging, which may hinder the continued roll-out of solvent-free electrodes for commercial Li-ion batteries. Amongst these issues are the dynamic mechanical properties of solvent-free electrodes, which differ significantly from their slurry cast counterparts. Moreover, the difference in the manufacturing set-up for solvent-free electrodes (shearing, current collector integration, calendaring, winding, etc.) makes different, but poorly understood and quantified, mechanical demands on solvent-electrodes to those required for slurry cast electrodes. Therefore, understanding how solvent-free electrodes can be manufactured commercially to high tolerance will help accelerate solvent-free electrode development, which would in turn make a major contribution to a more sustainable LIB industry.

We investigate the crucial role of binder on the mechanical properties and processability of solvent-free LIB electrodes. We report for the first time a detailed analysis of electrode mechanical response during compression using a combination of static and dynamic loading conditions and high-resolution direct image correlation. We reveal that the electrodes show strong viscoelastic character. To complement the ex-situ viscoelastic characterization experiments, we record real-time on-line calendaring data during solvent-free electrode densification under a wide range of electrode formulations and process parameters. Despite the differences in strain rate of laboratory and calendaring experiments, we show how together the data correlates strongly and can be fused to yield a comprehensive constitutive model of the dynamic response of solvent-free electrodes. To rationalize this constitutive behavior at a structural level, we perform detailed characterization of electrodes using a range of advanced characterization techniques including high resolution analytical scanning electron microscopy, transmission electron microscopy, X-ray computed tomography and X-ray diffraction. We reveal in detail the fine-scale fibril-nature of the binder and its critical and complex role, even at very low fractions, in controlling the macroscopic mechanical response of the electrodes. The microscopy shows how the fibrils respond to various process conditions, with a combination of stretching, unspooling, tangling, etc. Finally, we discuss how the fundamental mechanical properties of solvent-free electrodes properties, under certain combinations of electrode formulation and process parameters, can lead to particular manufacturing challenges. We discuss how challenges these might be overcome to allow solvent-free electrode manufacture that can operate to high tolerance and at high speed.