High-Viscosity Material and Processing Strategies to Enable 3D Printed Lithium-ion Batteries

Abstract

In lithium-ion batteries (LIBs), a single-layer cell consists of a cathode and anode electrode, each attached to metallic current collectors. A microporous polymer separator is positioned between the electrodes to prevent short circuiting while allowing lithium-ion (Li-ion) transport via the liquid electrolyte. The electrodes are flat composite films that are typically manufactured using slot-die coating or tape casting; these processes require slurries with low viscosities of 101–102 Pa·s, shear-thinning behavior, and negligible yield stresses to produce uniform films. Meanwhile, the separators are often made with polyolefins such as polyethylene (PE) and polypropylene (PP), and they are manufactured by extruding a hot melt into a freestanding sheet, then stretching it to create a porous microstructure. Today’s LIB material formulations are incompatible with many of the manufacturing approaches used to produce microbatteries, batteries with customized form factors, and batteries with three-dimensional (3D) electrodes. 3D electrodes go beyond conventional uniform films by engineering electrode architectures and macro-scale porosity to enhance Li-ion transport. By doing so, 3D electrodes can mitigate the energy and power trade-off that constrains conventional electrodes. 3D batteries can be made with additive manufacturing (AM) methods, but doing so often requires slurries with high viscosities of 102–105 Pa·s, finite yield stresses between 101–103 Pa, and self-supporting behavior to enable shape retention, all of which impact the electrochemical behavior of the resulting separators and electrodes. This dissertation investigates how high-viscosity, self-supporting electrode and separator slurries can be formulated using readily available, conventional battery materials to enable AM for 3D LIBs.First, an AM-compatible separator was investigated. A high-viscosity, self-supporting separator was formulated using poly(vinylidene fluoride) (PVDF) and silica (SiO2) by examining the impacts of non-solvents, inorganic additives, and mass ratios on slurry rheology and pore formation during dry phase inversion processing. The resulting formulation was compatible with a direct-on-electrode fabrication technique, in which the separator slurry was deposited directly onto the anode and dried in place, forming a uniform 20 μm-thick film without infiltrating the anode’s pores. The separator had a 51% porosity and an ionic conductivity of 0.286 mS cm-1. Additionally, it did not deform after 60 minutes of thermal annealing at 200 °C, indicating high thermal integrity for improved safety at elevated temperatures. When tested in LFP|LTO coin cells, the separator demonstrated a 58% increase in specific discharge capacity at 2C relative to a tri-layer PP/PE/PP separator, and it had a 304% higher specific discharge capacity when compared to a ceramic-coated PE separator. This separator formulation offers a pathway to reenvision LIB separator fabrication using manufacturing approaches that rely on shape retention during material deposition rather than hot melt extrusion processes. Next, AM-compatible electrode slurries were investigated using polycrystal and single-crystal LiNi0.8Mn0.1Co0.1O2 (NMC-811), LiFePO4 (LFP), and Li4Ti5O12 (LTO) active materials in combination with carbon black (CB) or acetylene black (AB) conductive additives, and PVDF binders. While prior studies have increased slurry viscosity using carbon nanotubes (CNTs) or reduced graphene oxide (rGO), the approach used in this thesis offers a pathway to enable AM of SEs and IDEs with conventional battery materials used in today’s commercialized electrodes. The high-viscosity electrode slurries were developed by examining the impact of mixing procedure, conductive carbon surface area, solvent fraction, and solids mass ratios on slurry rheology and specific discharge capacity. The LTO, LFP, polycrystal NMC-811 (PC811), and single crystal NMC-811 (SC811) slurries developed in this dissertation had viscosities ranging from 100.3 to 889.2 Pa·s at a 1 s-1 shear rate and yield stresses between 38.5 and 668.3 Pa. Based on rheological behavior, these electrodes slurries can enable LIB fabrication using AM and other advanced manufacturing approaches such as dual-sided slot-die coating and multi-layer extrusion. To examine the fabrication capabilities of the high-viscosity electrode and separator formulations, this dissertation investigated two types of 3D LIBs as case studies: interdigitated electrodes (IDEs) and structured electrodes (SEs). IDEs consist of interlocking 3D cathode and anode architectures that significantly shorten ionic transport distances, while SEs have pore channels engineered into the thickness of the electrode to reduce tortuosity for rapid ion transport. One challenge in IDE fabrication is developing a separator formulation that can accommodate 3D electrode topographies. To investigate the performance of IDEs made with a 3D printed separator, LFP and LTO were used as zero-strain test materials. The resulting IDE had average electrode feature widths of 98.2 μm and an average separator thickness of 16.3 μm. Other researchers have shown that these dimensions are within the range needed to reduce ion transport distances for improved rate performance. Aligning with these results, the experimental IDE cell demonstrated a specific discharge capacity that was 78.2% higher than that of a planar baseline cell made with the same materials at a high 10C discharge rate. This case study demonstrates the capabilities of the electrode and separator slurries developed in this thesis and offers a pathway to manufacture IDE batteries using more practical materials and slurry compositions. In the second case study, high-viscosity polycrystalline NMC-811 (PC811) and single-crystal NMC-811 (SC811) slurry formulations were evaluated for their feasibility in fabricating line-patterned SEs. NMC-811 is a cathode material of interest due to its reduced cobalt content, as well as its higher specific capacity and upper cutoff voltage (≥ 4.2 V) relative to LFP. Prior research studies have fabricated NMC-811 SEs using laser ablation; however, this approach limits the range of designs that can be explored due to challenges in managing laser-induced thermal effects on the cathode microstructure. A slurry-based 3D printing process provides a pathway to investigate NMC-811 SEs before significant time and effort are expended on developing scalable manufacturing processes. Two types of line-patterned SE architectures were fabricated using the PC811 and SC811 formulations developed in this thesis and then electrochemically tested. The resulting SEs had feature widths ranging from 115–199 μm and active material loadings from 5.85 to 20.49 mg/cm2, demonstrating the potential to control effective electrolyte volume fractions in SE designs. The results of this thesis show that high-viscosity, self-supporting electrode and separator slurries can be formulated to fabricate SEs and IDEs with more readily available, conventional materials used in today’s LIBs. The separator and electrode formulations developed in this dissertation have rheological behavior that is compatible with 3D printing, dual-sided slot-die coating, multi-layer extrusion, and other processing techniques that require slurries with high shape retention during material deposition. Based on their rheology and material composition, these formulations may provide a basis for exploring more advanced cell designs in the future.