Cell Manufacturing Process
◈ Slurry Mixing
A typical slurry consists of the cathode (or anode) active material, conductive additive, and binder. A commonly used conductive additive is carbon black. An organic binder is usually a dielectric polymer like polyvinylidene fluoride (PVDF) with a wide good electrochemical stability window. As a binder solvent, N-methyl-2-pyrrolidone (NMP) is used to dissolve PVDF. Water-based binders such as carboxymethylcellulose (CMC) and styrene butadiene rubber (SBR) are considered as alternatives for cost-efficient and non-toxic processing.
There are hydrodynamic shear mixing, ball-milling, and ultrasonic homogenization. Shear mixers (e.g. planetary mixers) are the most common for the industrial use. For mixing, sequence and additive properties (e.g. adhesion) are important.
Since organic solvent drying is time-consuming and costly and commonly used NMP is toxic, water-based binders such as cellulose are proposed despite water not being compatible with many cathodes. Hence, increasing slurry concentration and even dry mixing are considered for improved cost, time, and energy efficiency later for coating/drying.
Rheology of the slurry (e.g. viscosity, flow yield stress, wettability) is to be considered for slurry optimization, especially for later coating different thickness. Slurry viscosity can be determined by factors such as total solid loading, active material size and relative amount, binder molecular weight, etc.
There are hydrodynamic shear mixing, ball-milling, and ultrasonic homogenization. Shear mixers (e.g. planetary mixers) are the most common for the industrial use. For mixing, sequence and additive properties (e.g. adhesion) are important.
Since organic solvent drying is time-consuming and costly and commonly used NMP is toxic, water-based binders such as cellulose are proposed despite water not being compatible with many cathodes. Hence, increasing slurry concentration and even dry mixing are considered for improved cost, time, and energy efficiency later for coating/drying.
Rheology of the slurry (e.g. viscosity, flow yield stress, wettability) is to be considered for slurry optimization, especially for later coating different thickness. Slurry viscosity can be determined by factors such as total solid loading, active material size and relative amount, binder molecular weight, etc.
◈ Coating
- Cathode materials are coated onto an aluminum current collector while the anode materials are coated onto a copper electrode.
- It is desirable that the surface tension of the mixed slurry matches with the surface energy of the current collector for proper wetting and coating.
- Solvent-based coating techqniues are the following:
* Doctor blade coating: Moving the doctor blade along the surface to even out the thickness and ensure that the active material is evenly distributed. It is most commonly used at the lab-scale for research.
* Slot die coating: Dispersing the slurry pushed through a narrow and adjustable slot-die onto the current collector. It is most widely used at the industrial level. Changing flow rate requires a change in pumping with good pressure drop control, which can be done by adjusting the slot gap using a thicker shim. Features are:
a. Advanced approach for enhanced thickness control, reproducibility, and flexibility b. Capable of coating multiple layers of different materials simultaneously and deposition of gradient or patterned coatings c. Key parameters: Slurry flow rate, pressure, temperature, shear, substrate speed, and distance from slot
- It is desirable that the surface tension of the mixed slurry matches with the surface energy of the current collector for proper wetting and coating.
- Solvent-based coating techqniues are the following:
* Doctor blade coating: Moving the doctor blade along the surface to even out the thickness and ensure that the active material is evenly distributed. It is most commonly used at the lab-scale for research.
* Slot die coating: Dispersing the slurry pushed through a narrow and adjustable slot-die onto the current collector. It is most widely used at the industrial level. Changing flow rate requires a change in pumping with good pressure drop control, which can be done by adjusting the slot gap using a thicker shim. Features are:
a. Advanced approach for enhanced thickness control, reproducibility, and flexibility b. Capable of coating multiple layers of different materials simultaneously and deposition of gradient or patterned coatings c. Key parameters: Slurry flow rate, pressure, temperature, shear, substrate speed, and distance from slot
◈ Drying
- A wet electrode then dried for solvent evaporation. NMP solvent is recovered by a condenser and distillation.
- The wet slurry drying goes through the following - Slurry (Aggregation), Semi-slurry (Firm consolidation), Solid with solvent residue (Film shrinkage and formation of a condensed layer), and Solid (Pore empyting, segregation and bonding, formation of a compacted solid film coating) before full drying.
- A slurry mixed with water-based binder may take a longer time to dry than that with traditional NMP-based slurry despite being cost and time efficient for not having to go through the solvent recovery step.
- The wet slurry drying goes through the following - Slurry (Aggregation), Semi-slurry (Firm consolidation), Solid with solvent residue (Film shrinkage and formation of a condensed layer), and Solid (Pore empyting, segregation and bonding, formation of a compacted solid film coating) before full drying.
- A slurry mixed with water-based binder may take a longer time to dry than that with traditional NMP-based slurry despite being cost and time efficient for not having to go through the solvent recovery step.
◈ Calendering
- Process of smoothing and compressing a material during production by passing a single continuous sheet in between a pair of rolls.
- Reduces electrode porosity and alters pore microstructure with increased tortuosity. Increases the bonding strength between the electrode and the current collector. Improved particle contacts lead to energy and power density enhancement.
- Reduces electrode porosity and alters pore microstructure with increased tortuosity. Increases the bonding strength between the electrode and the current collector. Improved particle contacts lead to energy and power density enhancement.
◈ Slitting
- Conventional process of cutting the large, coated film is by using a blade or chisel.
- Laser cutting with good power and speed control is widely applied for improved shape and edge quality of the electrode.
- Laser cutting with good power and speed control is widely applied for improved shape and edge quality of the electrode.
◈ Stacking/Winding/Rolling
- All layers including electrodes and separators are wound into a spiral or jelly-rolled to be inserted in a cylindrical cell. In pouch cells, all layers are stacked layer by layer or via Z-stacking.
◈ Welding
- Contact resistance determines the quality of welding between tabs and current collectors. Ultrasonic welding uses high frequency to induce heat under applied pressure for welding. Resistance spot welding uses materials’ resistance for heat generation to join materials. Laser welding happens through the adsorption of laser beams on the materials at the certain point to heat, melt, and create joints.
◈ Enclosing
-Enclosing includes packaging and electrolyte filling. The filling process is done in vacuum through the desired pressure level using a high-precision injection device and is followed by wetting.
-Electrolyte wetting is influenced by electrode surface energy and morphology (e.g. pore structures), electrolyte viscosity and surface tension, etc.
-Electrolyte wetting is influenced by electrode surface energy and morphology (e.g. pore structures), electrolyte viscosity and surface tension, etc.
◈ Formation/Aging
-Formation of the dense and robust SEI layer is needed for good battery performance without over-consumption of the electrolyte, especially for the graphite anode with intercalation potential lower than the reduction potential of electrolyte. The formation and aging process can be delicate and time-consuming as the amount of electrolyte relative to the materials that it covers; temperature, and pressure need to be controlled.
-Gases generated from formation cycles are released before final sealing or re-sealing.
-Gases generated from formation cycles are released before final sealing or re-sealing.
◈ References
Total energy consumption for cell production (kWhcons / kWhcell)
The pie charts above are plotted based on the source data provided by the following reference:
◈ Dry electrode technology
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The promising technology involves the process of dry mixing, electrode fabrication, calendaring, laminating, and slitting, thereby removing the electrode drying part from the process to reduce the electrode manufacturing time and cost. For the case of 4680 batteries (e.g., Tesla), cost reduction is expected of up to 50% with adoption of drying electrode technology. The technology enables thicker electrode coating, which is desirable for achieving higher energy density.
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Despite apparent advantages, however, achieving even dispersion and uniform distribution of active materials, conductive agents, and binders through dry mixing can be challenging as it depends strongly on active particle size and rheology and binder type and properties. When using PTFE as binder, more problems such as low adhesion between a dry mix and a current collector and poor wettability of electrolyte need to be addressed.
- Hot-pressing: Hot-pressing on the as-mixed electrode material is performed for thermal activation of the binder followed by roll-pressing. High dependence on the polymeric binder and the cost can be an obstacle to realize a thick electrode with high active material fraction.
- Extrusion: Flow injection of polymeric binder into the dry-mixed powder under mechanical pressure to induce blending of different materials. A mixed batch is then followed by calendering and laminating with the current collector. The limited polymer use and low production rate can be an issue.
- Electrostatic spraying: Direct spray coating of electrode material on a current collector followed by hot rolling to enhance adhesion between. The method can be applied to thick film fabrication despite that the uniform coating is not guaranteed.
- Roll milling: Fully utilizing the characteristics of different roll configuration with different rotation speeds, gaps for pressing and calendaring to control the shear force during milling for fabricating the smooth film. The technique may include fibrilization of PTFE. Despite its potential to be the mainstream technology, parameter control depending on the particle morphology and binder choice besides PTFE needs to be optimized.