Shear and Flow Properties of an NMC Based Dry Cathode Powder

Optimization of electrode manufacturing for lithium-ion batteries has been increasingly focused upon due to inefficiencies and environmental concerns surrounding traditional slurry electrode processing methods. To eliminate the pitfalls of utilizing a slurry there has been an effort to use dry powder coating techniques. While this eliminates the cost, time, and environmental concerns associated with using a solvent, dry powder coating does present additional challenges in powder handling and processing. In this application note the new TA Instruments™ Powder Accessory is utilized to characterize powder bulk shear and flowability properties of a lithium nickel manganese cobalt (NMC) cathode powder with different ratios of carbon black and polymer binder. The powder cohesion, unconfined yield strength, flow function and total flow energy were characterized to provide formulation and scale-up insight. Thermogravimetric analysis (TGA) is utilized as a facile technique to identify binder content in the powder. Scanning electron microscopy (SEM) in conjunction with energy dispersive spectroscopy (EDS) ties the bulk properties of the powder to the underlying microstructure that affects the specific powder shear and flow behavior. The results show that altering the polymeric binder content influences the interparticle interaction and subsequent bulk shear and flow properties.

As lithium-ion batteries (LIBs) have emerged as the most prominent energy storage solution, the manufacturing processes behind their design have come into focus [1]. A costly and timeconsuming segment of battery manufacturing is the fabrication of the electrodes. The components of the electrode are mixed with a solvent to form a slurry, which is then coated and dried onto a metal foil current collector. This solution-based process allows for adequate mixing to produce a uniform electrode coating in a large-scale roll-to-roll process, but the use of solvent adds time and cost to the overall electrode manufacturing. The drying and solvent recovery accounts for over 40% of the overall energy consumption and a significant portion of the cost. Second, the drying steps required to remove the solvent create additional time added to the manufacturing process [2]. Additionally, solvent, especially for cathode fabrication, is generally an environmentally toxic organic solvent, such as N-Methyl-2-pyrrolidone (NMP) [3].

Based upon the above considerations, there have been efforts to implement dry powder coating processes to fabricate LIB electrodes. As a result, powder properties are critical to the manufacturing process. Flowability is important for transporting powder in scale-up and production and the cohesion of the powder impacts coating uniformity. The dry powder coating process involves mixing the active electrode components with a polymeric binder in the dry powder state. The mixed electrode powder is then applied to a metal foil and a combination of temperature and pressure is utilized to adhere the powder electrode to the substrate. During this process, the powder will be under compression and shear. This dry electrode process reduces the time and cost associated with using solvent and is a more cost-efficient process compared to using a slurry precursor [3].

One of the most widely used cathode materials is lithium nickel manganese cobalt (NMC). This is largely due to its high energy density and long cycle life compared to other cathode materials [4]. The amount of NMC loading in the cathode material is critical for the battery energy density. In this note, the powder rheological properties of a fixed content of NMC based cathode powder will be analyzed. The NMC is mixed with differing ratios of carbon black (CB) and polyvinylidene fluoride (PVDF) binder. These components were varied as the CB contributes a conductive network to the system while the PVDF provides a physical network for adhesion to the substrate. The interplay between these two network functions must be understood to optimize the electrode design. With PVDF’s insulating properties, high PVDF content can result in an insufficient conducting network where battery performance will suffer. On the other hand, lack of PVDF binder can have detrimental effects on the electrode’s coating properties during processing. Optimizing the amount of PVDF binder and carbon black with the active NMC cathode material is essential for final performance of the electrode.

In this note, powder NMC samples mixed with varying amounts of CB and PVDF are investigated to explore impacts on powder flowability and provide insight into powder processing conditions. The powder shear and flow properties are explored using TA Instruments Powder Rheology Accessory. Scanning electron microscopy (SEM) and thermogravimetric analysis (TGA) are also used to correlate the powder properties with the microstructure and PVDF content, respectively.

The mixed cathode powder samples were procured from SpectraPower, LLC (Livermore, CA) and composed of NMC811, carbon black, and PVDF binder. Three powders were utilized for testing with weight ratio compositions of 96:3:1, 96:2:2, and 96:1:3 for NMC:CB:PVDF.

Powder rheology testing was performed on the TA Instruments HR-30 performance rheometer using the powder shear and flowability accessories. Shear experiments were performed per ASTM D7891, with a 9 kPa pre-shear consolidating stress with subsequent shear steps at 7, 6, 5, 4, and 3 kPa consolidating stress [5]. The powder flowability experiments were conducted at a rotor tip speed of 100 mm/s. Analysis was performed in TRIOS™ Software using the automated powder analysis software feature. Additional details of powder testing and analysis can be found in TA Instruments application notes RH123 and RH126 [6] [7]. The PVDF content in the cathode powder was confirmed using the TA Instruments Discovery™ TGA 5500 where a temperature ramp from ambient to 1000 °C was performed at a heating rate of 20 °C/min. Particle size, shape, and surface morphology of the powder was investigated by SEM using a Phenom™ XL benchtop SEM by Thermo Fisher. The powder was prepared on double-sided carbon tape and imaged at a 5 kV accelerating voltage.

With the increasing demand for cost saving in manufacturing processes of LIBs, the use of powder coating technologies to fabricate electrodes is being increasingly explored. Understanding the bulk properties of the powders is necessary for optimal formulation and design of the mixing and coating process. In this work, NMC-based cathode powders were analyzed for insight into optimizing the electrode formulation and processing conditions. Discovery TGA provides a quality check on the binder content from the formulation and determines how much binder is present in the sample. With the TA Instruments Powder Accessory, the shear and flowability properties were characterized, showing that flowability and flow energy are impacted by increasing PVDF binder content. Using SEM imaging, it was shown that higher PVDF content acts to coat the surface of the particles and act as physical binder of the cathode particles, acting to reduce interparticle friction and increase flowability. As a result, the shear powder analysis shows a higher flow function and lower cohesion for greater PVDF content. The binding effect of the particles leads to an overall higher total flow energy when tested in the powder flow configuration with a smooth surfaced cup and impeller rotor. These results can be used to optimize the formulation and the powder mixing process and ensure homogenous cathode powders that will have repeatable and consistent electrochemical performance.