Active electrode materials (i.e. the coating layers on top of current collectors) in lithium-ion batteries are porous. They are normally composed of a solid frame with interconnected pores/channels, in which mass transport follows a tortuous pathway. Therefore, porosity and tortuosity (defined as the ratio of the average tortuous pathway to the projected straight path) are particularly important parameters which indicate the diffusivity and conductivity of the porous materials and, eventually, the battery performances. Existing quantification methods of them, however, are generally cumbersome and hardly applicable to in-production measurements, as they are mostly based on impedance and polarisation measurements from specially-fabricated cells or numerical diffusion simulations using the microstructural tomographic images of porous materials.<br/>Here we report the development of a novel quantification method based on air-coupled ultrasonic waves. We utilise the fundamental fact that in a porous material with a rigid frame (i.e. the electrode sheets in air before wetting with electrolyte), there exists a slow Biot wave mode which only travels in the gaseous phase, through the same tortuous pathway that ions will diffuse through in later battery operation. This makes it possible to quantitatively extract porosity and tortuosity from simple, real-time ultrasonic testing of the electrode layers during manufacturing. To enable this, we first develop a wave physics model that describes the propagation of the ultrasonic Biot waves in porous multilayers. This model takes full account of the boundary conditions across solid-porous interfaces and is based on the transfer matrix method. Then, the model is employed to perform a thorough, multivariant search for the optimal experimental configurations, in the solution spaces of e.g. frequency, incident angles and test modes (transmission or reflection). The sensitivities of porosity, tortuosity and permeability delivered by this investigation allows the optimization of experimental ultrasonic tests for quantitative inversion of these parameters. We believe that the proposed non-contact measurement method has the potential to become a next-generation quantification tool for quality control during porous material production and better understanding of battery performance.

Quantifying porous layers in lithium-ion batteries using ultrasound

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