The ability to measure the electronic conductivity of battery film coatings is a pressing need in the battery industry; however, these measurements can be difficult, and prior methods have not been sufficiently accurate and robust. New surface probes were developed that can accurately measure electronic conductivity of battery film coatings.
The invention consists of multiple lines of conductive material that have been deposited/patterned on a flexible, compliant substrate (electrical traces on a flexible printed circuit board). A critical feature is that the lines are spaced on the order of tens of microns, or the order of the thickness of the materials that are to be interrogated.
A computer-controlled fixture allows the probe to be scanned across the surface of the electrode sample, allowing a local conductivity map to be created. The method also allows simultaneous measurement of bulk film conductivity and contact resistance between the film and the current collector. The measurements are made as part of a roll-to-roll process that could be adapted to a battery manufacturing line.
From a general point of view, the major problem that is being solved is to enable the ability to perform a conductivity measurement of a very thin (microns thick) conductive material that is attached to a highly conductive material. Traditional conductivity measurements fail under these circumstances and cannot measure the conductivity of a layered material such as those found in battery films, where a very thin active material (such as graphite in the case of an anode, or lithium cobalt oxide in the case of a cathode) is attached to a thin metal film (such as aluminum or copper).
The use of a flexible substrate instead of a rigid substrate solves many important problems and allows for much easier connection to the probe. Previous probes had to be connected either on the ends (which were on the side facing the material to be interrogated that contained the patterned lines) or through vias that would have needed to be introduced through the rigid substrate material.
The flexible substrate also allows for probing of materials without the materials needing to be cut to the dimensions below that of the prior rigid probe. This allows for a measurement that can be performed on much larger materials than previous probes allowed. Multiple lines allow for redundancy of the measurement. The new probes have at least six lines, so if one line breaks, the other lines can still accomplish the measurement (with appropriate scaling and numerical inversion). In fact, this also allows for rudimentary estimation of some degrees of anisotropy of electrical properties.
Lines across the entire probe allow for continuity checks that help to determine the status of the lines; for example, whether they are broken. Flexibility of the substrate allows for conformal attachment to both the holder/apparatus and to different sample geometries. New probing geometries are possible due to this approach, which takes advantage of the ability to not only have planar probing geometries.
Eventually, manufacture of these devices can take place in more traditional flexible printed circuit board processes instead of the cleanroom processes that are currently used. They could even be manufactured in a roll-to-roll process and then cut. Reliability of connections to the substrate seems to improve significantly because the new flexible probes can more substantially conform to the material it is interrogating.