Resolving the Discrepancy in Tortuosity Factor Estimation for Li-Ion Battery Electrodes through Micro-Macro Modeling and Experiment

Abstract

Battery performance is strongly correlated with electrode microstructural properties. Of the relevant properties, the tortuosity factor of the electrolyte transport paths through microstructure pores is important as it limits battery maximum charge/discharge rate, particularly for energy-dense thick electrodes. Tortuosity factor however, is difficult to precisely measure, and thus its estimation has been debated frequently in the literature. Herein, three independent approaches have been applied to quantify the tortuosity factor of lithium-ion battery electrodes. The first approach is a microstructure model based on three-dimensional geometries from X-ray computed tomography (CT) and stochastic reconstructions enhanced with computationally generated carbon/binder domain (CBD), as CT is often unable to resolve the CBD. The second approach uses a macro-homogeneous model to fit electrochemical data at several rates, providing a separate estimation of the tortuosity factor. The third approach experimentally measures tortuosity factor via symmetric cells employing a blocking electrolyte. Comparisons have been made across the three approaches for 14 graphite and nickel-manganese-cobalt oxide electrodes. Analysis suggests that if the tortuosity factor were characterized based on the active material skeleton only, the actual tortuosities would be 1.35-1.81 times higher for calendered electrodes. Correlations are provided for varying porosity, CBD phase interfacial arrangement and solid particle morphology.

Developments in X-ray tomography characterization for electrochemical devices

“…A key example of the use of three-phase mapping has been demonstrated by Usseglio-Viretta et al [70] who produced a comprehensive study comparing tortuosity-factor estimation methods for graphite and NMC electrodes. However, the applications of three-phase mapping and quantification are not limited to LIBs: Tan et al [71,72] assessed the effective molecular diffusivity of the pore phase and electrical conductivity of the conductive carbon and binder phase via three-phase segmentation in order to conduct simulations of a lithium-sulfur cell.…”

“…Image-based modelling can provide a powerful tool in the prediction and optimisation of LIBs [83]; as previously mentioned, models based on X-ray CT data may be employed to generate structures or evaluate local transport properties [70], allowing 3D or 4D distributions of local current or potential to be mapped [97], possibly even across multiple length scales [98], even to the extent of cell failure [95]. Nevertheless, imaged-based modelling such as this is not limited to LIB and has recently seen interest from other battery chemistries [71,99].…”

The electrode tortuosity factor: why the conventional tortuosity factor is not well suited for quantifying transport in porous Li-ion battery electrodes and what to use instead

“…In contrast, XCT allows nondestructive imaging of the microstructure with a capability of analysing much larger volumes (although sample preparation often destroys the sample from an electrochemical perspective). However, the lack of capability in capturing either the fine features (due to the resolution) or different phases (the carbon-binder domain is rarely well captured with X-ray tomography based on attenuation contrast) in the tomographic data can be a source of errors that impacts the quantification of microstructural properties, in particular interfacial area 28,29 .…”

Enabling fast charging of lithium-ion batteries through secondary- /dual- pore network: Part I - Analytical diffusion model

“…The long-term stability of the iron-oxide nanoparticles is also to be assessed. Lastly, the tortuosity factor after alignment was still relatively high, ߬ = 3.8 at ߝ = 0.32 (supplementary information of [22]) compared with what can be achieved with spherical particles [1,6,26,27]. This could be explained by the iron-oxide forming aggregates [22] possibly hindering lithium-ion diffusion or by a still-convoluted pathway because of the complex particle morphology, even when aligned.…”