Characterising the structural properties of polymer separators for lithium-ion batteries in 3D using phase contrast X-ray microscopy

Abstract

Separators are an integral component for optimising performance and safety of lithium-ion batteries; therefore, a clear understanding of how their microstructure affects cell performance and safety is crucial. Phase contrast X-ray microscopy is used here to capture the microstructures of commercial monolayer, tri-layer, and ceramic-coated lithium-ion battery polymer separators. Spatial variations in key structural parameters, including porosity, tortuosity factor and pore size distribution, are determined through the application of 3D quantification techniques and stereology. The architectures of individual layers in multi-layer membranes are characterised, revealing anisotropy in porosity, tortuosity factor and mean pore size of the three types of separator. Detailed structural properties of the individual layers of multi-layered membranes are then related with their expected effect on safety and rate capability of cells.

Contradictory concepts in tortuosity determination in porous media in electrochemical devices

“…at high concentration losses Brus et al, 2014). One reason for this may be that image-based techniques typically do not consider all of the transport phenomena during diffusive mass transport such as Knudsen effects and are inherently limited by the imaging resolution (Finegan et al, 2016).…”

“…The 3D porosity of the overall sample and the 2D porosity of each image slice were determined by counting the pixels of the respective phases. The 2D pore diameter was calculated by dividing the pore volume fraction by the interface area between the two phases (Russ and DeHoff, 2000;Exner, 2004;Finegan et al, 2016;Taiwo et al, 2016a) for each image plane along all three axes of each sample:…”

“…The tortuosity is achieved by relating the heat flux through the porous phase of the sample to the heat flux through a dense sample volume of equal dimensions (Cooper et al, 2014;Trogadas et al, 2014;Brown et al, 2016a;Tjaden et al, 2016b). 2) The MATLAB application TauFactor uses an iterative over-relaxation method to solve the diffusion equation and, again, compares the flux through the porous phase to the flux through the dense volume of equal dimensions Brown et al, 2016b;Finegan et al, 2016). 3) The pore centroid method calculates the path length through a porous structure by following the centre of mass of pores of each 2D image slice along the in-plane direction of the volume and dividing it by the thickness of the volume (Gostovic et al, 2007;Smith et al, 2009;Shearing et al, 2012;Cooper et al, 2013;Cooper et al, 2014).…”

“…For example, multi-layered battery separators can exhibit distinctive microstructural characteristics along their thickness. The image-based calculation methods presented here can be applied to evaluate the effect of such changes on the tortuosity of the overall porous layer as presented in (Finegan et al, 2016). …”

Multi-scale 3D investigations of a commercial 18650 Li-ion battery with correlative electron- and X-ray microscopy

“…The authors and others have led work over the past 5 years in the application of X-ray tomography to explore these materials both ex-situ [14,[17][18][19][20][21] and in-situ [14,15,22,23]. Additional work using tomography and radiography to characterize cell architecture during failure [22] and post mortem [24,25] as well as to understand the role of safety features [26][27][28] help to build a comprehensive understanding of the role of each component in driving device degradation and failure. This work is complemented by extensive investigations using SEM [29,30] Collectively, these studies highlight the importance of understanding the multi-scale nature of Li-ion batteries from the pack to the particle levels [25,36]; it is essential that researchers identify appropriate length scale(s) for understanding key mechanisms that affect the performance and reliability of cells [36].…”