(39a) Tuning the Mechanical and Electrochemical Properties of Free-Standing Dry Electrode Films Via the Incorporation of Non-Fluorinated Binders to Dry Fibrillation Processing

Solvent-free or dry electrode manufacturing is a growing area of interest which directly addresses many of the current limitations associated with wet electrode (slurry-cast) processing^{1 2}. For the dry electrode process, the active material, conductive agent and binder are combined in the absence of a solvent, eliminating the need for costly/energy demanding drying stages, potential toxic emissions and strict solvent recovery regulations from popular choices of organic solvents such as NMP^{3 4}. Dry electrode manufacturing also inherently achieves a more homogenous distribution of electrode components, circumventing a limitation of slurry-cast processes in which the binder components tend to separate in the drying process of thicker electrodes⁵. This binder migration process in wet electrode processing can lead to cell performance issues such as capacity fading and electrode delamination⁶. Issues in wet processing techniques achieving thick electrodes can limit their use in high energy density cells for energy demanding applications⁷ e.g. electric vehicles.

Perhaps the most well-documented dry electrode process, is the Maxwell-type polymer fibrillation technique through which a polymer is plastically deformed under applied heat and/or shear mixing to form thin, interconnecting fibrils which act as the binder material⁸. Critically, this technique shows industrial promise due to its compatibility with existing roll-to-roll manufacturing equipment used in electrode production¹. At present, this technique is heavily reliant on PTFE as a binder due to its ability to fibrillate with very few true fibrillating alternatives present⁹. Although PTFE is the dominant fibrillation binder of choice for dry electrode manufacturing, it is not without issues for electrode manufacturing. For instance, poor adhesion between PTFE and the current collector necessitates the need for a primer coating on the current collector and the inherent hydrophobicity of PTFE can create issues with electrolyte wettability^{2 10}. Additionally, fluoropolymers (such as PTFE and PVDF) are becoming increasing unpopular choices as binders due to environmental concerns of PFAS pollution and greenhouse gas emissions through their manufacture and application, with government regulations related to PFAS chemicals anticipated to scale back their use¹¹. These manufacturing and environmental concerns highlight a need to propose alternative binders for this type of dry electrode manufacturing.

Here, we investigate alternative binders for use in Maxwell-type dry electrode manufacturing of NMC₆₂₂ electrodes. The unique properties of PTFE fibrillisation have established it as a ubiquitous binder in this dry electrode manufacturing process. We explore the feasibility of lowering the overall PTFE content used as a binder for dry electrode processing through complexing it with several other promising non-fluorinated binder alternatives, assessing the electrochemical and mechanical properties of these composites. Additionally, we explore the beneficial role of plasticizer additives in these alternative binder formulations for selective tuning of the mechanical properties of the free-standing cathode sheets.

Tensile strength measurements evaluating ultimate stress and strain of free-standing electrode films fabricated from composite binder formulations demonstrate the potential benefits to the structural properties of the electrode film. Careful examination of SEM images and elemental mapping will explore the role and uptake of these non-fluorinated binder alternatives in the microstructure of the electrode sheets. Comparisons between the electrode sheet resistance and electrochemical cycling of the cathodes will explore the electrochemical performance of these dry electrode binder formulations. Current literature research commonly reports overall PTFE binder content utilised in quantities of 2 wt%^{12 13 14}. We attempt to quantify the lower limit of PTFE necessary for constructing electrodes from composite binder mixtures while still maintaining structural integrity of the composite free-standing film and delivering high performance as an electrode. Importantly, the results presented here constitute major steps towards lowering the reliance of dry electrode fibrillization on PTFE, a significant advancement for optimising electrode performance and mitigating potential environmental impact of dry electrode manufacturing.

References

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