Maureen Tang1 Samantha Morelly1 Nicolas Alvarez1

1, Drexel University, Philadelphia, Pennsylvania, United States

Despite the societal and economic importance of advanced batteries, the complicated relationships between electrode processing, structure, and performance are still poorly understood. In this work, we combine fundamental rheological and electrochemical studies to investigate the relationships between slurry microstructure, electrode morphology, and battery rate capability at industrially-relevant compositions of inactive material. In one example of our approach, dry-mixing LiNi0.33Mn0.33Co0.33O2 (NMC) with carbon black decreases the free carbon concentration and consequently the ability of the slurry to form a gel network, as revealed by small-angle oscillatory shear measurements. Less free carbon weakens the strength of the slurry’s viscous and elastic moduli and is also reflected by a decrease in electronic conductivity of the dried electrode. Despite a clear dependence of slurry moduli and electronic conductivity on free carbon concentration, there is no obvious relationship between in-plane electronic conductivity and battery rate capability, demonstrating that short-range electronic contacts are more important than either ion transport or long-range electronic conductivity to cathode rate capability [1]. We further explain this finding by measuring the critical gelation concentration of carbon black and showing that it is independent of the NMC volume fraction, indicating that active material resides in the interstitials of the percolating carbon network [2].

Our identification of short-range electronic contacts as the key parameter for electrode performance motivates the development of new methods to observe and quantify these contacts. We use SEM-EDS to quantify the carbon black heterogeneity and show its sensitivity to a variety of manufacturing parameters, including the impact of polymer chain scission during mixing. In order to adapt this approach for carbonaceous active materials such as graphite, we substitute carbon black with commercial carbon-coated Fe nanoparticles as a contrast-enhancing agent that permits spectroscopic distinction between active material, conductive additive, and binder [3]. The Fe nanoparticles further enable nano x-ray computed tomography (XCT) to obtain three-dimensional images of the active material and carbon-binder-domains with 126 nm voxel resolution. Future work will discuss the relationships between the observed microstructure and the battery performance in more detail, as well as our efforts towards alternative electrode designs with optimal carbon connectivity.

[1] Morelly et al, J. Power Sources 387, 49–56 (2018).
[2] Morelly et al,. Polymers. 9, 461 (2017).
[3] Morelly at al, in revision, (2018).