Vacuum Filtration Solutions for Lithium-Ion EV Battery Manufacturing

 

Introduction 

 

The global market for rechargeable (secondary) lithium-ion battery manufacturing continues to grow due to the explosive demand for electric vehicles (EV's) driven by government policies and changing consumer behavior.  Additionally, energy storage from renewable energy sources (solar and wind) is the next frontier for lithium-ion batteries. 

Three common li-ion cell designs are used: prismatic cell batteries, pouch cell batteries, and cylindrical cell batteries for EV battery manufacturing, energy storage, and consumer electronics (phones, laptops, etc..).

 

Pouch Cells

The packaging of foil pouch cell batteries is lightweight, compact, and allows for the best pound for pound power delivery and efficiency.  However, the foil pouch is more susceptible to damage than other lithium-ion battery styles.with solid packaging.  Despite this, the combination of size, performance, and efficiency make pouch cells the latest standard for EV’s.

 

Prismatic Cells

Solid aluminum or steel external cases have been the traditional standard for consumer electronics and EV’s.  While prismatic cell batteries have a more robust construction than the pouch, the rigid packaging and larger size are comparative disadvantages.

 

Cylindrical Cells

With a fixed shape and solid metal casing, cylindrical cell batteries are the least expensive and were primarily used for early generation EV’s.  The fixed shape, size, and weight of cylindrical cell batteries are disadvantages in performance when compared to the pouch and prismatic cell batteries.

 

Vacuum is integral to most battery manufacturing processes including: pneumatically conveying raw materials, mixing the electrode slurry, electrolyte filling, degassing and the final sealing of the complete EV battery pack. At each stage, vacuum helps to maintain material purity, precision, and consistency which results in higher battery power, energy density, cycle life, and charging speed.  Concurrently, each process creates contamination (particles, liquids, and vapors) that carry over into the vacuum pump.  Purpose-built filtration solutions will capture contaminants, protect the vacuum pump, reduce maintenance costs, and promote uptime in this 24/7 industry.   

In our blog, we will focus on three primary manufacturing processes (Electrode Manufacturing, Cell Assembly, and Cell Finishing), how vacuum is used within each, and the recommended filtration solutions. 

 

Electrode Manufacturing

 

 

Prior to manufacturing, raw materials are mined and processed into a refined powder form.  Vacuum pumps and blowers are used for pneumatically conveying materials such as Lithium, Graphite, Cobalt, and Manganese.  Once the materials are available, the first step is to manufacture the electrodes (Cathode and Anode), which are critical to the exchange of lithium ions.  Slurry mixing combines dry materials: Active materials (see below)*, conductive additives (carbon black), binders, and solvents (de-ionized water for the Anode and N-Methyl pyrrolidone (NMP) for the cathode.)  Vacuum is used to remove air bubbles and promote a homogeneous slurry, which is critical to electrode efficiency and performance. 

 

*Active Materials 

Graphite Anode  

• Graphite is primarily used. 
• Silicone is the next generation due to higher capacity and energy density. 

Cathode  

• Lithium Cobalt Oxide (LiCoO2) 
• Lithium Manganese Oxide (LMnO2) 
• Lithium Nickel Oxide (LiNiO2) 

 

The final slurry is used to coat conductive metal strips of aluminum or copper.  The strips are dried via heat to extract and recover residual NMP solvent prior to being compressed/smoothed and cleaned in the calendering process, cut/slit, and processed into rolls.  The rolls undergo a final vacuum drying process to remove any residual solvents and moisture.  The electrode material is now stored in a dry room for local cell assembly or vacuum packaged for use by other cell manufacturers.

Both oil lubricated and dry vacuum pump technology are regularly deployed for mixing and drying processes.  For both types, inlet filtration is vital for protection from contamination and prevention of reduced vacuum performance and breakdowns.

 

Air Filtration Solutions for Raw Material Pneumatic Conveying

 

Inlet Filters, Filter Silencers and Vent Filters: 

• Due to purity requirements for raw materials, the air needed for conveying via pressure or vacuum must be filtered.  Inlet filters and filter silencers such as Solberg’s F, FS and 2G series will capture ambient contamination while reducing the intake air noise. 
• Also, since these systems are installed within clean room environments, dust from the hopper vents must be captured to protect the surrounding plant areas and operators. 
• The micron rating and efficiency of the internal filter elements must be tailored to meet the specific system requirements: 99+% efficient for 5- or 1-micron particles, high-efficiency H14/ULPA Classification (99.995% efficient for 0.1 micron particles). 

Inlet Vacuum Filters: 

• A vacuum pump or blower requires high-efficiency filtration to capture the fine dust particles that bypass upstream dust collectors. 
• Inline vacuum filters such as Solberg’s CSL, CSS, and ST Series serve as the last line of defense for the vacuum pump or blower. 

 

Vacuum Filtration Solutions for Electrode Manufacturing

 

Heavy Duty Inlet Vacuum Filter:

• When applying vacuum during the mixing and drying processes, particles will carry over toward the vacuum pump, so particulate filters such as Solberg’s CSL, ST, and WL Series are necessary to prevent pump contamination.   
• The internal filter element, micron rating and efficiency are selected based on the specific operating conditions of the Li-Ion battery manufacturing process.  5-micron polyester (99+% efficient) performs well in most cases; however, different filter media, micron ratings, and efficiencies are available depending on the specific process conditions.  
• Various materials of construction (carbon steel, stainless steel, etc..) and connection types (DN Flanges, ANSI Flanges, and ISO K Flanges) are available depending on the process piping and vacuum pump connections. 
• Routine service includes cleaning and eventually replacing the internal filter element(s). 

 

Vapor Condensing Filters and Heat Exchangers for Vacuum Pump Applications: 

• Vapor condensing under vacuum conditions is a challenge due to the vapor pressures of NMP and water.  This is of considerable importance for oil-lubricated pumps used for electrode manufacturing, as the oil can emulsify when vapors condense in the oil.
• A multi-stage condensing filter and heat exchanger, such as the Solberg JRS Series or JCT Series, operates in conjunction with a plant-supplied chilled fluid source (glycol, water) to create a cold surface for condensing vapors.  The required fluid temperature is dependent on the specific process conditions.   
• The internal cooling surface of the JRS or JCT is a proprietary stainless steel pleated pack designed to maximize surface area, surpassing the performance of traditional cooling coil designs.  With appropriate sizing, dwell time on the cooling surface is maximized which enhances the condensing performance.   
• Multiple filtration stages will boost filtration efficiency, therefore, stainless steel demister pads are a common second stage to separate condensed droplets, and final activated carbon adsorptive filter elements are ideal for capturing residual vapors. 
• Various materials of construction (carbon steel, stainless steel, etc. and connection types (DN Flanges, ANSI Flanges, ISOK Flanges) must be considered to handle the process conditions and fit within the available space on or near the vacuum pumps. 
• Filtration experts will specify a filter size, cooling surface area, and chilled fluid requirements depending on the process flow, operating vacuum level, operating temperature, and chemistry. 

By protecting the vacuum equipment with an engineered inlet filtration solution during the electrode manufacturing process, the Operator will achieve longer pump service intervals and reduced maintenance costs.   

 

Cell Assembly 

 

 

The dried electrode material is cut and then stacked in layers along with separator material to create the core of the cell.  The separator provides a necessary barrier between the anode and cathode while allowing the continuous exchange of lithium ions during the discharge and charging stages.  The layers alternate between Anode-Separator-Cathode-Separator, etc.., and the cell structure is then ultrasonically welded to the terminals that will eventually carry the electrical current.  For today’s EV’s, the pouch-style cell is becoming standard due to a lower profile shape, compact size, lighter weight, flexibility, and strength.  The cell core is placed inside the aluminum-coated polymer foil and sealed on three sides, and one opening remains for future electrolyte filling.   

For prismatic cell battery and cylindrical cell designs, the foil layers are wound into coils and then packaged into a robust metal shell.  During this stage, terminals are ultrasonically welded, and then the shell is sealed via laser or ultrasonic welding.  At this point, the cells are prepared for the electrolyte filling process.   

Electrolyte filling occurs when a mixture of lithium salt in an organic solution (hexafluorophosphate LiPF6) is dosed/injected into the cell structure.  The electrolyte promotes the flow of lithium ions from the anode to the cathode during battery discharge and cathode to anode while the battery is charging.  When filling prismatic and cylindrical cells, the dosing needle is inserted through a valve/fitting.  The filling occurs in under vacuum conditions for two primary reasons: 

• Vacuum evacuates the cell to remove air and impurities.  
• Vacuum promotes the capillary effect (wetting) and uniform distribution of the electrolyte mixture throughout the cell.
• Once filling is complete, the packaging is sealed via welding or beading prior to the finishing stage. 

For a pouch cell battery, the remaining opening allows for dosing needle access for filling.  Vacuum filling occurs to promote the capillary effect (wetting) and occurs in multiple stages of partial filling and evacuation.  Once complete, the pouch is vacuum sealed prior to the finishing stage. 

 

Vacuum Filtration Solutions for Cell Assembly

 

Liquid Separators:

• When filling the cell under vacuum, some electrolyte solution will carry over toward the pump, so a liquid separator is ideal to prevent pump contamination.   
• A multi-stage liquid separator like the Solberg LRS Series creates a low-velocity and high-efficiency separator to capture the electrolyte. 
  • Internal baffle for initial separation 
  • Low-velocity bottom chamber for liquid collection 
• Final filter element to capture any residual liquid droplets and suspended particles.

Various materials of construction (carbon steel, stainless steel, etc. and connection types (DN Flanges, ANSI Flanges, ISOK Flanges) are considered to withstand the process conditions and fit within the available space on or near the vacuum pumps.

 

Vapor Condensing Filters and Heat Exchangers for Vacuum Pump Applications:

 

• Vapor condensing under vacuum conditions is challenging due to the vapor pressure of the electrolyte.  This is especially important for oil-lubricated pumps, as the oil can emulsify when vapors condense in the oil. 
• A multi-stage condensing filter, such as the Solberg JRS Series or JCT Series, operates in conjunction with a plant-supplied chilled fluid source (glycol, water) to create a cold surface for condensing vapors.  The required fluid temperature is dependent on the specific process conditions. 
• The internal cooling surface of the JRS or JCT is a proprietary stainless steel pleated pack designed to maximize surface area, surpassing the performance of traditional cooling coil designs.  With appropriate sizing, dwell time on the cooling surface is maximized which enhances the condensing performance.   
• Multiple filtration stages will boost filtration efficiency, so stainless steel demister pads are a common second stage to separate condensed droplets, and final adsorptive filter elements are ideal for capturing residual vapors. 
• Various materials of construction (carbon steel, stainless steel, etc. and connection types (DN Flanges, ANSI Flanges, ISOK Flanges) must be considered to handle the process conditions and fit within the available space on or near the vacuum pumps. 

For electrolyte filling applications, Solberg’s Engineering Team will specify the filter size, cooling surface area, and chilled fluid requirements depending on the process flow, operating vacuum level, operating temperature, and electrolyte chemistry.

 

 

Cell Finishing 

 

 

Approximately one-third of the manufacturing time is dedicated to the finishing stage, which includes three primary processes:  Formation, Aging, Testing.  During the formation stage, the cell receives its first charge and is subsequently discharged and charged based on specific parameters.  Lithium ions are embedded into the graphite crystals of the anode, which creates a protective layer between the electrode and the electrolyte called the Solid Electrolyte Interface (SEI).  This layer is critical for preventing self-discharge over the lifespan of a battery.  The fast-charging results in reactions between the electrolyte and electrodes.  The resulting toxic gases (C2H4, CO2, CO, H2) are pushed out of a pouch cell and into an adjacent dead space/gas bag. While inside a vacuum chamber, the bag is pierced, the gases are evacuated, and the pouch is then vacuum sealed.  With a prismatic cell, the cell itself is pierced, and the gases are evacuated via vacuum.  The cell is then          re-sealed to prevent any ingress of impurities into the cell and prevent safety issues, reduced cell life, and performance losses. 

Once degassing is complete, the cells are aged over the course of days and weeks, and performance is monitored.  If there is no degradation, the cells are then ready for a final leak test before being installed in a battery pack.  Typical testing of hard case cells is done in a vacuum chamber, and instrumentation is used to detect any electrolyte leaks.  Pouch cells can also be tested for electrolyte leaks; however, these can easily expand and become damaged under vacuum, so a special procedure and chamber are used to prevent this.

 

Vacuum Filtration Solutions for Cell Finishing

Heavy Duty Inlet Vacuum Filter:

• Particulate filters such as Solberg’s CSL, ST, and WL Series are purpose-built for removing impurities and sealing during the cell finishing stage. 
• The internal filter element, micron rating and efficiency are selected based on the specific operating conditions.  5-micron polyester (99+% efficient) performs well in most processes; however, different filter media, micron ratings, and efficiencies are available depending on the specific process conditions. 
• For degassing, the CSL or WL style filter can be equipped with an adsorptive filter element to capture vapors created during the formation stage.   
• Various materials of construction (carbon steel, stainless steel, etc..) and connection types (DN Flanges, ANSI Flanges, and ISO K Flanges) are considered depending on the process piping and vacuum pump connections. 
• Routine service includes cleaning and eventually replacing the internal filter element(s). 

 

Conclusion

 

The explosive global demand for lithium-ion batteries will continue to drive investment in ancillary equipment required for Electrode Manufacturing, Cell Assembly, and Cell Finishing.  Conveying, slurry mixing, electrolyte filling, degassing, and other processes all require vacuum.  Given the sophistication and precision of today’s vacuum pumps, protection is needed to capture contamination in particle, liquid, and vapor form.  Given the 24/7 nature of this industry and the fact that most battery plants are operating at or near 100% capacity, so there is no margin for downtime and lost production.  Purpose-built vacuum filtration solutions provide pump protection and allow them to operate continuously and efficiently, which enables maximum battery production and performance.

 

Solberg Manufacturing’s global industry partnerships in lithium-ion battery manufacturing, current customers, and project experiences provide us with a clear understanding of the various vacuum process conditions.  We offer technical advice to leading equipment manufacturers, vacuum pump OEM’s, service companies, and strategic end-users to demonstrate how high-efficiency filtration maximizes vacuum equipment uptime and production output while reducing costly overhauls and service.