Micro-Four-Line Probe to Measure Electronic Conductivity and Contact Resistance of Thin-Film Battery Electrodes

Abstract

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...

Quantifying Tortuosity of Porous Li-Ion Battery Electrodes: Comparing Polarization-Interrupt and Blocking-Electrolyte Methods

“…Landesfeind et al simplified the blocking electrolyte TLM by neglecting electronic resistance because of its typical small magnitude in comparison to ionic resistance (a fact confirmed by our own experiments). 27,28 Knowing R ion based on this model, the effective conductivity can be determined based on the cross-sectional area of the cell (A), the thickness of the sample (l), and the intrinsic conductivity of the electrolyte (k int ):…”

Characterization of mechanical properties of thin-film Li-ion battery electrodes from laser excitation and measurements of zero group velocity resonances

“…The long-term goal of this research is to develop an acoustic probe to detect small variations in a battery film's local material properties such as density, thickness and elasticity. This proposed probe can be used as an aid to a conductivity probe developed at BYU [9] to better understand the by cathode and received by anode [7]. The cells can be connected in parallel or series to provide for different needs with battery casing.…”

“…On top of the current collector is the coating layer composed of large active material particles, carbon additive for better conductivity, polymeric binder, and pores [10]. The distribution of this mixture of materials can vary significantly in structure due to manufacturing differences, thus causing the heterogeneity on the micron as well as on the millimeter-scale [9], [11].…”

Mathematical Model of Four-Line Probe to Determine Conductive Properties of Thin-Film Battery Electrodes

“…Particular attention is paid to developing a method that is robust and sufficiently sensitive, given typical noise levels, so that local values of conductivity can be determined for thin-film battery electrodes. In an accompanying paper 2 we show experimental results for a micro-four-line probe, constructed using semiconductor fabrication techniques, and test fixture that have been used to probe spatial variations in electronic conductivity and contact resistance for commercial-grade lithium-ion battery electrodes. The model presented in this paper is the first step in that effort.…”

“…Furthermore, the lines for our device are embedded in a flat insulating substrate, enabling pressure on the sample to be controlled and measured during the electrical measurement. 2 Conduction models.-A mathematical model is needed to extract a film conductivity from a four-sensor measurement. As shown in Fig.…”