Abstract
R2R processing is used to manufacture a wide range of products for various applications which span many industrial business sectors. The overall R2R methodology has been in use for decades and this continuous technique traditionally involves deposition of material(s) onto moving webs, carriers or other continuous belt-fed or conveyor-based processes that enable successive steps to build a final version which serves to support the deposited materials. Established methods that typify R2R include tape casting, silk-screen printing, reel-to-reel vacuum deposition/coating and R2R lithography. Products supported by R2R manufacturing include micro-electronics, electro-chromic window films, PVs, fuel cells for energy conversion, battery electrodes for energy storage, and barrier and membrane materials. Due to innovation in materials and process equipment, high-quality yet very low-cost multilayer technologies have the potential to be manufactured on a very cost-competitive basis. To move energy-related products from high-cost niche applications to the commercial sector, a means must be available to enable manufacture of these products in a cost-competitive manner that is affordable. Fortunately, products such as fuel cells, thin- and mid-film PVs, batteries, electrochromic and piezoelectric films, water separation membranes, and other energy saving technologies readily lend themselves to manufacture using R2R approaches. However, more early-stage research is needed to solve the challenge of linking the materials (particles, polymers, solvents, additives) used in ink and slurry formulations, though the ink processing, coating and drying processes, to the ultimate performance of the final R2R product, especially for a process that uses multiple layers of deposition to achieve the end product.
To solve the problems associated with this R2R “Grand Challenge”, the R2R AMM DOE Laboratory Collaboration is executing a research program with outcomes that will ultimately link modeling, processing, metrology and defect detection tools, thereby directly relating the properties of constituent particles and processing conditions to the performance of final devices. The Collaboration team and their research efforts with industry involvement are illustrated schematically in Figure 1. This collaborative approach was designed to foster identification and development of materials and processes related to R2R for clean-energy materials development. Using computational and experimental capabilities by acknowledged subject matter experts within the supported National Laboratory system, this project leverages the capabilities and expertise at each of five national laboratories to further the development of multilayer technologies that will enable high-volume, cost-competitive platforms.
Figure 1 The R2R AMM DOE Laboratory Collaboration team and major research efforts. Source: ORNL
A typical R2R process has three steps: 1) mixing of particles and various constituents in a slurry, 2) coating of the ink/slurry mixture on a substrate, and 3) drying/curing and processing of the coating. Final performance of devices made via R2R processes is dependent on the active materials (e.g. electrochemical particles in battery or fuel cell electrodes) and the device structure that stems from the governing component interactions within the various steps. However, a fundamental understanding of the underlying mechanisms and phenomena is still lacking, which is why industrial-scale R2R process development and manufacturing is still largely empirical in nature.
The FY 2019 through FY 2021 program addresses aspects of the following two targets from the AMO Multi-Year Program Plan:
• Target 8.1 Develop technologies to reduce the cost per manufactured throughput of continuous R2R manufacturing processes.
o Increasing throughput of R2R processes by 5 times for batteries (to 50 square feet per minute (50 ft2/min)) and capacitors and 10 times for printed electronics and the manufacture of other substrates and MEs used in support of these products.
o Developing resolution capabilities to enable registration and alignment that will detect, align, and co-deposit multiple layers of coatings and print < 1-micron (1 µm) features using continuous process scalable for commercial production.
o Developing scalable and reliable R2R processes for solution deposition of ultra-thin (<10 nm) films for active and passive materials.
o Develop in-line multilayer coating technology on thin films with yields greater than 95%.
• Target 8.2 Develop in-line instrumentation tools that will evaluate the quality of single and multilayer materials in-process.
o Developing in-line QC technologies and methodologies for real-time identification of defects and expected product properties “in-use/application” during continuous processing at all size-scales with a focus on the “micro” and “nano” scale traces, lines, and devices
o Developing technologies to increase the measurement frequency of surface rheology without significant cost increases with a goal of a 10-nanometer in-line profilometry at a production rate of 100,000 square millimeters per minute (100,000 mm2/min).
The category of electrochemical conversion devices that apply to fuel cells was selected to begin the FY 2019 efforts; however, research was kept general enough that other applications could include low-temperature water electrolysis, CO2 separation/reduction and water filtration/purification. Of specific interest was ES technology. ES is a technology platform that is capable of fabricating advanced nanofiber-based materials across a wide range of applications and industrial domains. It offers high surface-to-volume ratio which makes nanofibers the ideal candidate for various applications where high porosity and high surface areas are desirable. There are currently a few barriers to commercialization of ES. It is limited to filtration applications produced in single-step, batch processes; advanced materials used in electronics, fuel cells and batteries require continuous, post-ES processing; complex multi-step physiochemical processes create challenges transferring technology from the bench to commercial production; and material transfer between process steps is time consuming. However, integration of R2R processing can enable the adoption of electrospun advanced materials into a wider range of applications
The FY 2019 collaborative effort successfully completed all tasks to develop an ES method using a R2R manufacturing process and to provide continuum-scale modeling, simulation, processing, and manufacturing techniques and metrology that demonstrate the feasibility and potential for scale-up. A R2R ES capability was established at ANL, which demonstrated fabrication of polymer nanofiber membranes on a 0.5 meter wide moving web using 56 nozzles. Nanofibers were produced using the ES system at ANL for three applications: (1) Li7La3Zr2O12 (LLZO) solid electrolyte, (2) La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) oxide fuel cell electrode, and polyvinylidene fluoride (PVDF) water filtration membrane. In-situ small angle X-ray scattering (SAXS) was conducted at ANL’s Advanced Photon Source (APS) to accelerate ES process optimization. ANL and ORNL both fabricated cubic phase LLZO (c-LLZO) from the electrospun precursors through post-annealing at temperatures well below the powder LLZO sintering temperatures (1100 °C), showing the benefit of the ES technology.
ORNL produced uniform PEMFC cathode gas diffusion electrode (GDE) coatings of uniform thickness on their pilot slot-die coating line and a membrane electrode assembly without an overlayer on the cathode with reasonable performance. NREL developed an empirical slide die coating window model and full-quadratic ink models for catalyst ink surface tension using the Box-Behnken methodology and completed a study for a multi-region process window. LBNL developed a generic particle network settling model as part of its longer-term coating drying modeling efforts but recognizing the deep lack of fundamental knowledge about the concentrated dispersions of interest, proposed a set of novel experiments for probing the influence of component interactions on macroscopic dispersion behavior. LBNL proposed to design and execute these experiments in order to obtain information that will be incorporated into its dispersion mixing and multilayer coating drying models. SNL completed two-layer slide and slot coating models using Goma 6.0. The models are equipped for slot- and slide-deposition systems with accommodation for three or more simultaneous miscible layers. SNL also advanced a user-interface that will greatly ease analysis of slide and slot-die coating head design.
Technology transfer for these and other technologies applicable to R2R manufacturing was initiated through collaboration with industry partners and through the continuation of three CRADA projects with industry and completion of one of those projects.