Experimental investigation into battery electrode surfaces: The distribution of liquid at the surface and the emptying of pores during drying

Abstract

Hypothesis: Drying constitutes a key step in the production of thin, particulate films as the complex microstructure develops and functional additives redistribute throughout the film, thus shaping the film properties. The onset of pore emptying constitutes a paramount marker in the film formation process as capillary liquid transport emerges.

Experiments: Film shrinkage, development of surface liquid content and liquid depletion in surface pores of thin films are studied by means of a novel experimental approach, using the example of lithium-ion battery anodes. An optical brightener is incorporated into the films, which are produced in a convective slot-nozzle dryer. After various drying times, images of the film, which emit light fluorescently during exposure to UV-A radiation, are captured and analyzed by image processing. Film shrinkage is observed by means of a laser displacement sensor.

Findings: As soon as the first pores empty, pore fluid is transported convectively. By exerting an external force, i.e. a pressure surge, on the film, the transition from a fully saturated to a partially emptied film is revealed. The significance of drying rate and particle shape are studied. We show that liquid depletion in surface pores can occur both prior and concurrent with the end of film shrinkage.

Keywords: Capillarity; Drying; Film formation; Film shrinkage; Lithium-ion battery; Porous media; Thin film.