Electronic conductivity of battery electrodes and the interfacial resistance at the current collector are key metrics affecting cell performance. However, in many cases they have not been properly quantified because of the lack of a suitably accurate and convenient non-destructive measurement method. There are also indications that conductivity across deposited films is not uniformly distributed. To characterize these variations, a micro-four-line probe has been developed for local mesoscale measurement of electronic conductivity of thin-film electrodes. The micro-four-line probe, coupled with a previously discussed mathematical model, overcomes key limitations of traditional point probes. This new approach allows pressure-controlled surface measurements to determine electronic conductivity without removal of the current collector. In addition, the probe allows one to measure the local interfacial contact resistance between the electrode film and the current collector. The method was validated by comparing to other conductivity sampling methods for a conductive test film. Three commercial-quality Li-ion battery porous electrodes were also tested and conductivity maps were produced. The results show significant local conductivity variation in such electrodes on a millimeter length scale. This method is of value to battery manufacturers and researchers to better quantify sources of resistance and heterogeneity and to improve electrode quality. A common electrode design for secondary batteries is a porous thin film of active material particles, conductive carbon particles, and polymeric binder. The film is coated on a metallic current collector. For commercially produced cells based on lithium-ion intercalation chemistry, the active materials are commonly a transition metal oxide on aluminum for the cathode and graphite on copper for the anode.Among the key properties determining electrode performance are the volume-averaged (effective or bulk) electronic conductivity of the film and the interfacial resistance at the current collector.1-4 These two quantities are surprisingly difficult to measure accurately for common thin-film electrodes because of the relatively large contact resistance between the sample and external probes, mechanical fragility of the sample, and the presence of the attached current collector. Lack of experimental data makes it hard to meet a longstanding need to be able to predict these parameters from knowledge of the composition and structure of the constituent materials.Commercial Li-ion battery electrodes are fabricated by first by making a slurry of the active material, carbon additive, binder, and a carrier solvent. This slurry is spread onto a metal foil current collector in a continuous process using a blade or slit to control deposition thickness, and then is immediately dried. Even in commercial coating processes it is difficult to achieve a uniform distribution of particles and porosity, leading to variability in the electronic conductivity of the electrodes.5 While this variability...

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Lithium-ion battery electrodes are generally made up of porous, thin films that are structurally heterogeneous on multiple length scales due to the manufacturing process. This in turn causes spatial variability in the electrical properties of the film. This work reports development of a low-cost, flexible probe for measurement of film conductivity and contact resistance to the current collector. When mounted on a moveable stage, the probe can quickly produce maps of electrode electrical properties at sub-millimeter resolution. Bulk conductivity and contact resistance are determined by inverting a 2D model of the experiment. The method is validated using a conductive silicone rubber placed on top of plain and corroded steel with significant contact resistance variation. Measurements on commercial quality electrode films, two cathodes and one anode, show statistically significant macro-variations in film properties perpendicular to the rolling direction and micro-variations throughout the film, variations that will impact electrode performance. The flexible-surface probe can be a significant tool to determine and minimize sources of variation in electrode manufacturing.

In Fortsetzung einer friiheren Arbeit 1 werden die Beobachtungen an EIektronenlawinen in Sauerstoff, Stickstoff und Luft bei hohen pd-Werten (200 bls t000 cm Torr) sowie die in diesen Gasen auftretenden Sekund~irprozesse mitgeteilt. Insbesondere wurde neben der Ionendriftgeschwindigkeit die 13ildung negativer Ionen in Sauerstoff beobachtet. Ein bemerkenswertes Ergebnis ist die zeitlich verz6gerte Entwicklung yon Elektronenlawinen in Stickstoff und Luft, die sich in einer ver-gr6Ber'cen An stiegszeie der Elektronenkomponen~ce (etwa t b~sec start etwa t. 10 -s see) ~iuBert. Es werden Erklgrungsm6glichkeiten diskutiert (Pr~iionisation oder Bildung ins*cabiler negativer Ionen). Die verz6gerte Entwicklung der Eleklronenlawinen macht die Diskrepanz zwischen den gemessenen und berechneten Aufbauzeiten in Luft vers'cgndlich.

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Li-ion battery electrode electronic properties, including bulk conductivity and contact resistance, are critical parameters affecting cell performance and fast-charge capability. Contact resistance between the coating and current collector is often the largest electronic resistance in an electrode and is affected by chemical, microstructural, and interfacial variations. Direct measurements of contact resistance and bulk conductivity have proven to be challenging. In their absence, a mechanical electrode peel test is often used to compare adhesion and electrical contact resistance. However, using a micro-flexible-surface probe, contact resistance can be directly determined. This work compares contact resistance and mechanical peel strength of multiple commercial-grade HE5050 and NCM523 cathodes and graphite and silicon anodes. It was found that peel strength correlates well with contact resistance in a carefully curated data set (p < 0.05) and in some situations may be a good metric to estimate electrical properties. However, there were distinct outliers in the data set, indicating that peel strength may not accurately reflect electrical properties when there is significant variation in electrode composition. These results illustrate the value of the micro-flexible-surface probe in quantifying contact resistance and bulk conductivity to better understand how battery composition and processing steps affect microstructure and resulting cell performance.

Lithium-Ion battery success can be defined by a series of measureable characteristics. Two important characteristics that have been difficult to fully explore are the ionic [1] and electronic tortuosity. Micro-N-line probes were previously developed [2][3] in order to accurately measure electronic thin-film conductivity on a micro-scale. Multiple iterations of the probe have been produced, tested and verified. In order to meet the final goal of a probe that could be implemented in any laboratory or production line, changes to the substrate, probe, and sizing were necessary. The final design also required substantial changes to methodology as well as the conductivity inversion calculations. The new probe is produced on a flexible substrate for a fraction of the original production cost and a fraction of the original production time. Here we show that the new probe has been used to test battery films and measure battery film heterogeneity on a mm scale (See Figure 1). COMSOL modeling of the battery under testing conditions shows minimal influence by the battery current collector on measurements; this allows for non-destructive measurements of standard battery films. The probe shows key advances in robustness and flexibility which allows for implementation on a rolling production line. New affordable stages have been developed to produce reliable measurements. High and low conductivity locations are still able to be found which is consistent with previous results. Acknowledgements This research is supported by the US Department of Energy through the Advanced Battery Material Research (BMR) Program. References [1] S.W. Peterson and D.R. Wheeler, "Direct Measurements of effective electronic transport in porous li-ion electrodes", Journal of the Electrochemical Society, vol. 161, no. 14, A2175-A2181, 2014 [2] B.J. Lanterman, A.A. Riet, N.S. Gates, J.D. Flygare, A.D. Cutler, J.E. Vogel, D.R. Wheeler, and B.A. Mazzeo, "Micro-four-line probe to measure electronic conductivity and contact resistance of thin-film battery electrodes", Journal of the Electrochemical Society, vol. 162, no. 10, A2145-A2151, 2015 [3] M.M. Forouzan, C.-W. Chao, D. Bustamante, B.A. Mazzeo, and D.R. Wheeler, "Experiment and simulation of the fabrication process of lithium-ion battery cathodes for determining microstructure and mechanical properties", Journal of Power Sources, vol. 312, pp. 172-183, 2016 Figure 1: Figure (a) showing the new probe, (b) the COMSOL model of the new probe, and (c) the results of a simple electrode conductivity mapping experiment run on a standard NMC TODA523 electrode. Figure 1

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