The challenges facing the manufacturing process of battery electrode sheet

Slurry preparation process challenges

(1) Water-based positive slurry. NMP is the only solvent for positive PVDF adhesives. NMP is toxic and carcinogenic, uses more energy to evaporate than water, and requires solvent recovery systems in industrial applications. Therefore, it is urgent to develop positive terminal slurry of drainage system.

(2) continuous stirring. Coating is a continuous process, but the traditional stirring process is an intermittent process. Continuous mixing process can reduce process time and energy consumption, and improve process control, resulting in a more consistent slurry.

(3) Increase solid content. The solid content of negative slurry is usually ~ 50%, and that of positive NMP slurry is ~ 70%. If solid content can be increased while maintaining the processing properties of the coating process, material and energy costs will be reduced. The ultimate limit is the solvent-free coating process.

(4) Thicker electrodes. Thick electrodes reduce the amount of foil and seperator in the battery and increase the energy density. However, because of the longer electron and ion conduction pathways in the electrode, thick electrodes will reduce the ratio performance of the battery. In addition, it is difficult to produce thick coatings with good mechanical properties in industry.

(5) synchronous double-sided coating. The coated drying oven is usually horizontal, with the wet coating generally on the upper part of the foil. The current double-sided coating is done by drying one side before re-coating the other side, which adds additional manufacturing time, and the A-side coating goes through the oven twice. At the same time double-sided coating requires a pole piece floating drying oven.

(6) porosity gradient electrode. The two-layer electrode model calculated an optimal porosity of 10% near the fluid collector and 50% on the surface of the coating rather than a uniform porosity of 30%. In production, this requires a continuous double coating or stencil technique.

Rheological challenge of electrode paste

Using rheology as a predictive tool in manufacturing presents three major challenges that need to be addressed:

(1) Differences between laboratory scale and industrial processes can be large, and the relationship between these amplification process changes and key properties (such as rheology) is unclear. It is necessary to use industry-related formulations, weight percentages and equipment (mixers and coators) in the study, as well as to characterize the rheology of the electrode paste. Rheology can detect subtle but significant changes within a slurry batch during mixing and coating. For example, poor mixing may result in uneven distribution of free carbon black in the slurry, resulting in changes in viscosity and viscoelasticity throughout the batch.

(2) Reveal the interactions between the components in the electrode paste. What is their relationship to the formulation and mixing process? How do they affect flow characteristics?

(3) Quantitative understanding of optimal rheology in manufacturing and process control. Hydrodynamic modeling is a means of understanding coating flows and potential problems, such as unstable flows, stock buildup in dead zones, and defects such as pinholes, blisters, cracking, and delamination.

Electrode coating challenges

(1) Many of the limitations of current coating techniques are related to the use of liquid pastes. Usually in these pastes, 30% to 60% of the mass is solvent, which needs to be removed from the electrode and leaves excessive porosity in the dry electrode layer, and therefore requires calendering in order to densify. In addition, higher water surface tension results in higher capillary forces during drying and poor fluid collection wetting, leading to cracking and delamination of coatings, especially for thick (> 100 μm) electrode. Dry or low-solvent electrode processes can avoid these problems, but have their own challenges: ensuring adequate mixing of the dry powder, preparation of the dry powder mixture into a film of specified width and thickness, and ensuring that the electrode film adheres to the fluid collector.

(2) The electrode of the wet process has uniformly distributed components and pore structure. The optimal composition, thickness and porosity of the current electrode are obtained through repeated experiments under the constraints of the slurry coating process. Relaxing these restrictions and providing greater flexibility could lead to higher performance electrode designs.

(3) Optimization and control of electrode structure parameters. Electrode thickness is one of the key structural parameters affecting the energy density of lithium-ion batteries. Thicker slurry coated electrodes cause slow transmission of lithium ions due to long transmission path. Thick electrodes are also prone to cracking and delamination. Therefore, many challenges need to be overcome to increase electrode thickness. Porosity is another key parameter that can have both positive and negative effects on the performance of lithium-ion batteries. Currently, the porosity of electrodes is mainly controlled by calendering processes. Thicker electrodes require additional processes to control the necessary porosity to ensure good performance. Tortuosity, that is, the ratio of the actual lithium ion transmission path length to the straight-line distance between the starting point and the ending point, is a structural parameter describing the difficulty of lithium ion transmission within the electrode. For electrodes with higher quality loads, the challenge is the need to develop new electrode structure strategies to achieve low electrode tortuosity by controlling the shape, size, and distribution of electrode holes without sacrificing other electrode properties.

(4) The main challenge of the dry electrode process is to reduce the content of the inactive material to a level comparable to that of wet coating, while some processes involve degreasing steps and sintering at high temperatures, making the process more costly and potentially difficult to scale up.

The challenge of electrode drying process

(1) Create an effective drying model that correlates the dynamic measurements of drying conditions with the final characteristics of the electrode to achieve a more controlled drying process. Examples include computational fluid dynamics models at the continuous level, convection heat and mass transfer models at the air-porous material interface, theoretical models of drying of two-component colloidal suspensions, and one-dimensional models of granular coatings including Brownian diffusion, sedimentation, and evaporation.

(2) The drying kinetics of the wet electrode film is particularly complex. In order to better control the electrode structure and its corresponding electron and ion transport characteristics, we need to understand the formation process of the electrode structure. Drying is fundamental to the formation of electrode structures, so it is necessary to develop advanced metrological tools to understand the physical processes occurring during drying and to measure and analyze the effects of solvent evaporation on defects, such as crack formation. The adhesive distribution can be characterized by energy spectrum EDX, Raman spectrum, Fourier transform infrared spectroscopy (FTIR) and multi-speck diffusion wave spectroscopy (MSDWS). Solvent evaporation can be measured by thermogravimetric analysis and quartz crystal microbalance (QCM). The surface temperature and drying stress during solvent evaporation can be measured by infrared (IR) thermograph. Active materials can be characterized by SEM, X-ray CT, and fluorescent-based imaging/microscopy.

Challenges facing electrode calendering processes

During the calendering process, the particle structure of the active material (AM) and the carbon colloidal phase (CBD) are compressed and rearranged, and the intergranular porosity is reduced. In turn, the compaction of particle pore structure of electrode will affect the transport performance of electron ions and battery performance.

(1) In the manufacturing process, the mechanical properties of the electrode are affected by the material composition, process parameters (such as roller temperature, speed) and electrode thickness. Due to the complex effects of these characteristics and parameters on electrode calendering deformation, it is necessary to characterize the mechanical properties of the electrode, such as hardness, elastic deformation, and adhesion between the electrode and the collector. In order to further understand the mechanism, a large number of systematic studies between parameters and material properties are needed, such as numerical simulation and experimental study to obtain the dynamic mechanical response of calender to porous carbon gel phase.

(2) The plastic deformation and fracture of trapped particles at high calendering levels face challenges, and a deeper understanding of interparticle forces is still needed to establish nonlinear constitutive behavior and study the microstructure evolution within the electrode in high fidelity.