Characterizing the Tortuosity of Aligned Pores and Its Effects on Electrode Kinetics in Directionally Freeze-Cast Cathodes
Authors: Delattre Benjamin , Amin Ruhul , Tomsia Antoni P , Chiang Yet-Ming 
Published: February 2020

The prevailing electrode fabrication method for lithium-ion battery electrodes includes calendering at high pressures to densify the electrode and promote adhesion to the metal current collector. However, this process also increases the tortuosity of the pore network in the primary transport direction. In practice, to meet operational C-rates desired, and production throughput objectives, the thicknesses of commercial electrodes are restricted to less than ~100 µm, and the areal capacity to less than ~3.5 mAh/cm2.[
1
] With the aim of increasing cell energy density and decreasing cost by building thicker electrodes, several strategies to reduce pore tortuosity in the direction normal to the current collector have been proposed.[
2-4
] While improvements in the electrode transport and usable area capacity have been demonstrated when the pore tortuosity is reduced, specific correlations to pore tortuosity have been lacking.

Here, we use freeze-casting, a shaping technique able to produce low-tortuosity structures by using ice crystals as a pore-forming agent, to fabricate LiNi0.8Co0.15Al0.05O2(NCA) porous cathodes with a wide range of pore topologies. Two complementary approaches were then used to evaluate the tortuosity of these electrodes. In the first approach, the electrodes were scanned using synchrotron X-ray micro-tomography, and a heat transfer simulation based on a voxel algorithm was directly applied to the 2D binarized image sequence. This algorithm mimics mass transport and diffusion behavior by computing and solving Fourier’s law equations directly across the voxel domain of the porous phase. The second approach used direct current (DC) measurements of polarization and depolarization kinetics to determine the lithium ion diffusivity of the electrolyte-infused porous electrodes. Finally, the electrochemical performance of the electrodes were evaluated in half-cells.

The results allow comparison across a wide range of microstructures, and highlight the large impact of a relatively small numerical change in tortuosity on electrode kinetics. Under galvanostatic discharge, the lowest-tortuosity microstructures showed a three- to fourfold increase in area-specific capacity (15 mAh/cm2 at C/10 rate and >5 mAh/cm2 at 1C rate) compared to typical Li-ion composite electrodes. Hybrid pulse power characterization (HPPC) demonstrated improved power capability (150 mW/cm2 to 95 mW/cm2), while dynamic stress tests (DST) showed that an area-specific area capacity of 11.5 mAh/cm² could be reached with a lower discharge voltage limit of 2.5V, corresponding to 91% of the NCA theoretical capacity.

This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231, Subcontract No. 7056592 under the Advanced Battery Materials Research (ABMR) Program.

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