Solvent-less processing of battery electrodes

Current Li ion battery (LIB) electrode manufacture takes place by slurry casting that suspends electrochemically active particulates, carbon particulates and polymeric binder in a fugitive liquid binder. In the case of active cathode materials such as those based on widely used Li(Co,Ni,Mn)O2, the solvent is organic to avoid reaction with the active particles, and is also required to dissolve the polymeric binder (usually PVDF). Once this solvent/suspension mixture is processed into a slurry with the required rheological and other properties, it is deposited, or cast, as a thin layer (~ 100 microns) onto a metallic foil where it dries to form a porous electrode for incorporation into a LIB. The process has been successfully scaled to the "gigafactory" scale, operating at up to 2 m width and production speeds of 100 m/min. However, the organic solvent required to form the slurry plays no useful role in the final Li ion battery and is only present transitorily during manufacture to dissolve the binder and to suspend electrochemically active and other electrode materials. These solvents, especially for cathodes, are usually toxic, flammable and expensive to buy and handle. Further, the controlled drying of the solvent mandates electrode drying lines that are energy intensive and occupy significant factory area.

Solvent-less, or dry processing, refers to a family of new and emerging processes with the objective of dispersing the small fraction of binder entirely in the solid state, while ensuring it retains its critical electrode mechanical stabilisation and adhesion function. Solvent-less processing could play a very significant role in increasing the sustainability of the fast-growing gigafactory manufacture of batteries, but is immature from a practical standpoint and much of the underpinning scientific understanding has yet to be developed.

The research will build on our proof-of-concept work on solvent-free, deformation processing of Li ion battery electrodes. We will take a science-led approach to understanding the underlying principles of shear deformation of multi-material mixtures as a function of temperature, including the effect of particulate size, surface energy, binder chemistry, etc. A key target is a better quantitative description of the conditions that promote the critical binder fibrilisation step. Where fibrilisation occurs, the pre-cursor active particulate, carbon additive and binder mixture spontaneously forms an integral composite preform.

In a second step, the composite preform is then formed by warm calendaring into a LIB electrode of area up to 10 x 10 cm and thickness less than 300 microns. The evolution of the porous microstructure, adhesion with the current collector and ongoing fibrilisation as a function of temperature will be key areas of research focus. These aspects will be studied using a combination of numerical simulation, interrupted processing at key stages and microstructural analysis by various microscopies and X-ray micro-tomography.

The electrochemical and cycling behaviour of the electrodes will be studied in detail. Both anodes and cathodes will be studied, along with full cells whose performance will be compared with and rationalised in terms of conventionally processed equivalents. Key issues will be long term mechanical stability given the different binder morphologies and understanding what benefits the fundamentally different dry processed microstructures may offer. Drawing the generated understanding together, consideration will be given to the design and implementation of approaches large scale manufacture of dry processed electrodes.

This project falls within the EPSRC energy and decarbonisation research areas.

This is a 4-year Faraday Institution Studentship (part of the course fee paid from Oxford Materials funds)