Lithium-ion batteries represent the dominant rechargeable battery on the market today. They can be found in many applications including consumer electronics, electric vehicles and industrial equipment. Due to the tremendous adoption of lithium-ion batteries in recent years, battery technology is the focus of a diverse set of research areas aiming to improve battery lifetime, performance and safety.   

One critical area of research is improvements to electrode processing and manufacturing. Specifically, researchers have been focused on optimizing the electrode manufacturing process, which involves mixing active solid cathode or anode particles with binders, additives, and solvents that result in a slurry with complex, non-Newtonian flow properties. Efficient processing of these slurries is highly dependent on understanding the slurry’s flow properties, also called their rheological profiles.   

A detailed understanding of the slurry’s rheology can provide better control of the slurry’s storage stability against sedimentation, pumpability and transportability, and the ability form uniform, defect-free films with higher coat weight. These measurements help streamline process optimization efforts and enable researchers to derive meaningful structure-property relationships as they develop novel materials that push the boundaries of the battery performance. The following studies highlight the work of leading researchers as they utilize rheology to improve battery material development, handling and processing.  

All lithium-ion batteries require cathode coating with a slurry. Modern industrial battery manufacturing uses slot-die coating on cathodes, in which a slot die head evenly coats the cathode with a smooth, uniform slurry. As the demand for faster, more efficient Li-ion battery manufacturing grows, researchers Hawley and Li at Oak Ridge National Laboratory and the University of Tennessee sought to speed up slot-die coating by raising its temperature to reduce the cathode slurry’s viscosity.¹ Reduced slurry viscosity results in maximum application velocity and minimal imperfections such as air entrainment and thickness variations. The team used a TA Instruments Discovery Hybrid Rheometer (DHR) and found that increasing the slurry temperature from 25 °C to 75 °C reduced the viscosity by 23% at 60 °C, resulting in a potential increase in the coating speed by roughly 14% which translates to faster electrode production rates while maintaining “comparable capacity retention during long-term cycling and high-rate discharge testing.” The slurries’ yield stress and equilibrium storage modulus increased monotonically between 25 °C and 60 °C, “providing the additional benefit of higher sedimentation resistance of the active materials.”

All solid-state batteries are a new configuration with solid electrodes and a solid electrolyte instead of traditional liquid or polymer gel electrodes and electrolytes. Researchers from Vanderbilt’s mechanical engineering, material science, and chemical and biomolecular engineering departments investigated ink formulation for the composite electrodes in all solid-state batteries (ASSBs).² Scalable manufacturing of composite electrodes relies upon creating an ink that combines the electrode’s solid material, binder, and solvent. Ink engineering involves optimizing the ink’s rheology, aggregation behavior, and stability for a desired coating process, resulting in improved performance of composite electrodes in ASSBs.

The Vanderbilt team of Shen, Dixit, Zaman, Hortance, Rogers and Hatzell used a TA Instruments Discovery Hybrid Rheometer to evaluate different combinations of solvents and binders. They found that terpineol solvent and polyvinyl butyral (PVB) binder, a less common combination in the battery industry, offer “improved wettability and adhesion at the solid-solid interface” as well as “improved dynamic surface tension and rheology leading to improved electrode and capacity performance.” Rheological measurements helped them identify this ideal combination. Rheology is crucial for designing electrode inks for manufacturing and determining the allowable process conditions.

Researchers Khakani, Verdier, Lepage, Rochefort, Prébé, Aymé-Perrot, and Dollé from the University of Montreal Department of Chemistry, Hutchinson, and Total SA took a different approach to streamline manufacturing of composite lithium-ion battery electrodes by designing a solvent-free process that is more environmentally sustainable and cost effective.³ Their dry process uses a polymer processing aid (PPA) and circumvents conventional issues with solvent-based wet electrode processing. Their dry coating needed a homogenous blend with enough viscosity to evenly coat the electrode. A TA Instruments Discovery Hybrid Rheometer (DHR) helped researchers optimize their blend and define the range of shear forces required to apply the mixture. Their resulting blend had ideal viscoelastic properties and was proven successful in full cell tests, paving the way for more environmentally friendly and low-cost battery manufacturing.

Although most commercial batteries use liquid organic electrolytes, these organic solvents are flammable and not suitable for all applications. Solid polymer electrolytes (SPEs) are being researched as a safer alternative with reduced flammability and improved mechanical properties that can help suppress dendrite formation.

Brian Jing and Christopher Evans at the University of Illinois at Urbana-Champaign developed SPEs from polymer networks with dynamic covalent cross-links that improve the safety and performance of the electrolyte in a sustainable and recyclable manner.⁴ They developed Poly(ethylene oxide) (PEO) networks and studied the effect of LiTFSI salt on their transition temperature.

One of the critical aspects of successfully designing materials for batteries is understanding the material’s behavior at different temperatures. This is even more important for PEO-based material where the modulus can drop substantially during heating. Jing and Evans used a TA Instruments Discovery Hybrid Rheometer to determine the changes in modulus of their PEO-based SPE with temperature. They observed that while the material became softer and more flowable at higher temperatures, the shear modulus of the resultant material was greater than 1 MPa. This is a significant achievement as a high modulus of these materials could potentially help suppress dendrite formation at the high end-use temperatures of the batteries while the network chemistry ensures a high conductivity.

The use of boronic esters for formation of dynamic covalent crosslinks also allowed them to dissolve the electrolyte in pure water in 30 minutes and recover the raw monomers. The electrolytes also exhibit self-healing upon mechanical damage while retaining >95% of their conductive and mechanical properties, further augmenting the current efforts of the industry to move towards recyclable and reprocessable materials for more sustainable batteries.

As these research examples demonstrate, the high demand for lithium-ion batteries is pushing the limits of manufacturing globally, making it even more critical to think about process optimization at the material development stage. As innovation moves at a break-neck pace, labs around the world work to develop batteries with the right balance of performance and safety. These examples illustrate how rheology is a critical technique in the scientist’s toolkit to design and efficiently produce safer, better-performing batteries. The driving factors behind their research – faster manufacturing, improved safety, superior end use qualities – will not disappear. Battery scientists can confidently learn from others’ breakthroughs and adopt their techniques as we continue improving battery production and products.   

TA Instruments is proud to support these efforts with our industry-leading rheometers that offer unparalleled measurement accuracy while maintaining a high level of versatility with numerous accessories and providing a seamless user experience. Visit our battery material testing page to learn more about the instruments powering tomorrow’s battery discoveries. Download our battery testing brochure for even more information about top testing techniques and contact our experts to learn about the best instrumentation for your lab.