Structural Factors and Electrochemical Behaviors of 3D Architected Carbon Electrodes
Authors: Narita Kai , Greer Julia R 
Published: December 2020

Cycling of lithium-ion batteries composed of intercalation materials and electrolyte involves interfacial reactions and transports, governed by differences in ion concentrations, currents, and potentials. Those electrochemical behaviors have been investigated by characterizing simplified electrode structure such as thin film and single particle, and bridging the knowledge toward commercial batteries with modeling and simulations. However, the stochastic structure of slurry electrodes commonly used in commercial batteries makes it hard to investigate local environments in the battery system and understand relations between electrochemical factors and resulting phenomena such as solid electrolyte interface formation and lithium dendrite formations at local sites. Here, we address this challenge by developing deterministic 3D architected carbon electrode with micron-to-centimeter form factors and mechanical integrity, which enables us to prescribe the electrode architecture with the consideration of distribution of ion concentrations, currents and potentials and investigate electrochemical responses in battery operation and then conduct post-characterization of the electrode.

The 3D architected carbon electrodes were fabricated by the combination of direct light processing (DLP) 3D printing and pyrolysis process. The uniform shrinkage of 3D printed UV-cured polymer during pyrolysis led to the fabrication of 3D architected carbon with the prescribed structure. The electrode is fully controllable in micron-to-centimeter length scales to prescribe porous structure, electrode thickness and relative position to the counter electrode.

To demonstrate the 3D architected carbon as an electrode of lithium ion batteries, we constructed a half-cell with beam-based 3D architected carbon (Figure 1) using a 2032 coin cell. To compare effects of structural factors, we chose to vary two structural factors independently, either sample thickness or beam diameter, which directly relate to diffusion length in electrolyte or electrode and locked all other parameters. Those structural factors (i.e. beam diameter and thickness) were varied in the estimated electrode diffusion-limit regime. The galvanostatic cycling at different current densities showed improved rate performance as decreasing electrode diffusion length, which agreed with the estimation that electrode diffusion was the rate-limiting process. In the regime, furthermore, slurry electrodes made by pulverizing 3D carbon electrode into particles showed similar rate performance to the 3D architected carbon electrode for the same mass loadings. These results also indicate that non-tortuous porous structure of the 3D architected carbon did not take advantage of effective lithium ion transport trajectories in electrolyte because of the electrode diffusion-limiting regime.

In addition, we used the 3D architected carbon electrode to investigate solid electrolyte interface (SEI) variation in a cell scale. The deterministic 3D architecture can prescribe local ion concentrations in the porous structure and its mechanical integrity enables us to investigate SEI on the 3D architected carbon electrode at different locations as post-characterization in nanoscale. This multi-scale tunable 3D architected electrode with mechanical integrity enables to elucidate relations between structural factors of battery electrodes and electrochemical behaviors, which may lead to designing rational battery electrode in multi-scales.