Why Separator Pore Space Connectivity Matters for Battery Operation

摘要

Traditionally, when discussing lithium ion battery (LIB) performance, diffusion of lithium ions in the electrolyte-filled pore space is given by the diffusion of lithium ions in the electrolyte scaled by the effective transport coefficient of the microstructure, which is the ratio of porosity and tortuosity along the through-plane (TP) direction between the electrodes. [1] While electrolyte-surface interactions also play a role in transport, the geometry of structure plays a key role in transport as we have shown in a previous study. [2] Microporous polyolefin membranes have been used as separators in LIBs for several decades, and have been manufactured with a variety of thicknesses, pore structures, and surface chemistries. [3] Recently, we have shown that it is possible to obtain quantitative reconstructions of LIB separators using focus-ion-beam scanning electron microscope (FIB-SEM) tomography. Polyethylene (PE) and polypropylene (PP) separators exhibit distinct morphologies that stem from the different processes used to manufacture them. The PE separator microstructure is isotropic, while that of PP is anisotropic. However, the respective porosities, TP tortuosities, and thus the effective transport coefficients, of the PE and PP microstructures are similar. Here we show that these microstructural characteristics are not sufficient to predict separator performance. We use the imaged PE and PP structures as a case study to determine whether a more complete analysis of structure can help us gain insights into what type of separator structure is optimal. We perform topological and nodal analysis and carry out numerical diffusion simulations on tomographic reconstructions of commercial battery separators and computationally generated separator structures of comparable porosity, TP tortuosity, and effective transport coefficient. High connectivity of the pores, as found in PE separators, enables ion gradients to be smoothed out within a fraction of the separator thickness. A structure with multiple straight cylindrical channels, though offering excellent TP tortuosity and effective transport, has zero connectivity density and, due to the likely presence of defects, is more susceptible to Li plating if integrated into a lithium ion battery. We propose that connectivity should be considered along with effective transport in separators, in order to understand and optimize separator performance, and that topological and network theory should be applied regularly to characterize the porous structures in energy applications. This work highlights the importance of the connectivity of the separator pore space. In particular, we show that simply reducing the tortuosity of the separator pore space at the expense of connectivity between pores, is not favourable for today's commercial batteries as it prevents smoothing out of in-plane ion gradients that emerge due to the microscale porosity of battery electrodes. This new understanding enables us to design separator microstructures that are safer and accommodate fast charge and discharge.