Abstract
The tortuosity factor of porous battery electrodes is an important parameter used to correlate electrode microstructure with performance through numerical modeling. Therefore, having an appropriate method for the accurate determination of tortuosity factors is critical. This paper presents a numerical approach, based on simulations performed on numerically-generated microstructural images, which enables a comparison between two common experimental methods. Several key issues with the conventional “flow through” type tortuosity factor are highlighted, when used to characterise electrodes. As a result, a new concept called the “electrode tortuosity factor” is introduced, which captures the transport processes relevant to porous electrodes better than the “flow through” type tortuosity factor. The simulation results from this work demonstrate the importance of non-percolating (“dead-end”) pores in the performance of real electrodes. This is an important result for optimizing electrode design that should be considered by electrochemical modelers. This simulation tool is provided as an open-source MATLAB application and is freely available online as part of the TauFactor platform.
Highlights
Microstructure plays a crucial role in the performance of lithiumion battery (LIB) electrodes, affecting electronic and ionic effective transport properties, electrochemical kinetics via the interfacial area between phases, as well as the mechanical properties due to the nonuniform distribution of phases
Tortuosity factors calculated by the two approaches are denoted: τ for a tortuosity factor derived from the conventional eRDM approach and τe for “electrode tortuosity factor” determined using the eSCM
Pores that start from the separator side and reach the current collector side would count as through-pores
Summary
Microstructure plays a crucial role in the performance of lithiumion battery (LIB) electrodes, affecting electronic and ionic effective transport properties, electrochemical kinetics via the interfacial area between phases, as well as the mechanical properties due to the nonuniform distribution of phases. Porous electrode structures can massively increase the specific interfacial area between phases, which can be used to increase the accessible capacity of the active materials at high rates. Rather than simulating electrochemical processes directly in 3D microstructure, most battery models apply a macroscopic treatment in which only certain homogenized metrics are used to represent the geometry, and the exact geometric details are disregarded. In the porous electrode theory, the two phases are considered as the superposition of two continua at any point in space, which are ascribed macro-homogenous parameters such as porosity ε, specific active surface area (ASA) a, and a tortuosity factor τ.5,6. In the porous electrode theory, the two phases are considered as the superposition of two continua at any point in space, which are ascribed macro-homogenous parameters such as porosity ε, specific active surface area (ASA) a, and a tortuosity factor τ.5,6 As reported in previous works, those parameters may in reality be inhomogeneous, and the tortuosity factor may be anisotropic[3]
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