A More Detailed Look at a Prismatic Li-ion Cell and Pack Manufacturing Process
23 February 2010
by Bill Cooke
In January, EnerDel hosted a group of journalists at its Indiana facilities and provided a plant tour showcasing its cell manufacturing and pack assembly processes. EnerDel believes its Hague Road plant is the first, and as yet at this time only, plant capable of achieving high volume production of transportation-grade, lithium ion cells within the US.
EnerDel is scheduled to ship its first cells and packs for the Think City EV on 1 March 2010. The lithium-ion version of the THINK vehicle will have 384 cells in its 25 kWh battery pack and a range of more than 100 miles. While specific elements will vary by sub-chemistry and company, the general steps behind making a prismatic cell are expected to remain consistent for the next several years.
Green Car Congress appreciates EnerDel’s cooperation in sharing their process with our readers. EnerDel’s plant is immaculate, we ate lunch on the shop floor, and the cell production steps occur in isolated clean rooms within the plant. The two battery killers are contamination and moisture.
Li-ion Battery Basics
|Click to enlarge.|
Schematic Overview. (This is a schematic for a prismatic cell. Cells are also packaged in cylinders.) The center portion represents a single functioning circuit: anode, cathode, separator with electrolyte.
The anode current collector (copper color) and anode coating (green) form the anode electrode. The current collector is coated on both sides, enabling one electrode to function in two circuits. The same logic applies to the cathode.
A cell is the smallest finished product and one cell will have 15-25 anode / cathode pairs. The bookend collectors are coated on only one side.
The current collectors extend beyond the circuit via tabs and are welded together to make an electrical connection. All of the cathode collectors come off one side of the cell and the anode collectors on the opposite side. If they were to touch they would create an electrical short.
This schematic is not to scale, does not show the cell packaging material and is more colorful than reality. The current collectors are very thin (100 microns = copier paper), the separator is thinner (trash bag) while the thickness of the anode and cathode coating will vary by chemistry and application but is between 0.05-0.3 millimeter thick.
Functional overview. During discharge, lithium ions and electrons travel from the anode to the cathode via the electrolyte. The separator is permeable to lithium ions and the electrolyte but provides a mechanical barrier to prevent shorting between the cathode and anode. The goal is to minimize the gap between the anode and the cathode to maximize ionic conductivity and limit the circuit’s impedance.
The flow of electrons enables work to be done. (Remember from HS physics, electrons flow from negative to positive but historical custom, which pre-dates the discovery of electrons, has current traveling from positive to negative). During charging the process is reversed with ions and electrons traveling from the cathode to anode. Most people use discharge as the default condition and the cathode is frequently called the positive electrode.
The blanket term “lithium-ion” covers a number of different anode / cathode / electrolyte sub-chemistries with much of the attention focused on the cathode chemistry although people are dedicating their careers to improving the anode, electrolyte and separators as well. Each chemistry has its characteristics for power density, energy density, safety, cost and long-term reliability.
During discharge, two “half reactions” are occurring simultaneously—one at the anode and one at the cathode. The two half reactions for a Lithium Manganese Oxide (LMO) cell (with forward being discharge):
Anode (hard carbon): Anode (Hard Carbon): LixC6 ⇔ x Li+ + xe- + 6C
Cathode (LMO): Li+ + xe- + Mn02 ⇔ LiMn02
The cell voltage is determined by the difference in voltage between the two half reactions and the cells frequently operate in the 3-4.5V range depending upon chemistry and conditions.
|Figure 1. Key components in creating an electrode.|
I. Electrode coating process. The anode and cathode materials are a mixture of active ingredients, solvent and binder. EnerDel purchases their raw materials from suppliers in Japan and Korea but is working on localizing production in the US. The active materials are defined by the cell chemistry. According to a recent study by the Boston Consulting Group, the active materials and purchased parts for a cell can cost $200-$270/kWh and many people believe the entire battery pack systems (cells, packs, electronic controls) are currently selling for $750 to $950/kWh in large applications.
|Figure 2. Negative coating (anode). Click to enlarge.|
The cathode uses an aluminum foil as the current collector, the anode uses a copper foil. There can be one or two coating lanes (figure 1) to create a negative or positive coating for a given roll of current collector. Most rolls are coated on both sides. The coating is applied wet and the solvent is removed during the curing process and EnerDel's coating line can run up to 30 m/hour (figure 2).
Dave Hahn, EnerDel’s director of component manufacturing, said the process is very similar to making audio tapes although the coating is thicker for a battery (note: audio tapes were a common way of storing music in the 20th century). The coating equipment is one of the plant’s major capital items.
One of EnerDel’s sister companies is investigating dry coatings that don’t need to be cured. Although they look similar—i.e., jet black—the anode and cathode coatings have different active ingredients.
|Figure 3. Anode and cathode pair.|
II. Electrode Stamping Through Final Assembly. After coating, the roll is calendarized, compressed via rollers, and the electrodes are punched out. The coating covers 90%+ of the electrode with a little tab of current collector sticking out. These tabs allow the electrode to be connected to a functional circuit.
Figure 3 shows the collector tabs for the anode and cathode coming off separate sides; Figure 3 is also an oversimplification. The actual stack (figure 5) has a separator threaded through the anode /cathode pair to prevent shorting. Process: place anode, weave in separator, place cathode, weave in separator. Repeat 15-25 times. The separator is continuous for the entire stack.Once all of the pairs have been added, the electrode’s current collector tabs are welded together: anode on one side, cathode on the opposite, creating a stack.
Like many elements of the cell construction, the design and manufacture of the stack (figure 4) involves proprietary ideas. One of the more interesting ones is how you can get an adequate distribution of electrolyte between the anode and cathode pair when the electrolyte is added when the stack is in the pouch. (It’s like adding the dressing between every layer of meat with the sandwich already in the bag and knowing either too little or too much dressing leads to scrapping the sandwich.) The answer lies in an artful blend of twisting, turning, shaking and vacuum.
|Figure 4. Finished stack. The white covering is the separator.||Figure 5. Cross section of stack. Click to enlarge.|
III. Charge and Test. The stack, electrolyte and pouch create the finished cell (figure 6). The cell is the smallest functional unit in a lithium-ion battery system. It is sealed and cannot be repaired.
|Figure 6. Finished cell.|
The cell is charged in an end-of-line machine that looks like a 1960s computer. Each “channel” can charge / monitor one cell. EnerDel has 1,000 channels with 11,000 more channels due by the end of April. It takes 21 days to make a cell, with 14 of those days devoted to seeing if there is any infant mortality among the cells after charging.
Enerdel uses prismatic cells because they can get a higher packaging efficiency and better heat transfer per unit of volume from them than from a cylindrical cell. Thermal management is crucial for lithium-ion cells and engineers try to focus as much attention on the cell’s environment as the occupant’s environment.
The cells operate between -10 °C and 50 °C with available power being limited in the cold threshold and cell life being limited in the heat. Many other automotive components are expected to survive -40 °C to 85 °C, so external heating or cooling of the cell may be required depending upon vehicle design and intended geographical market.
The heatsink is a sheet of aluminum with a fin on top (figure 7, above thumb) which provides structural integrity and promotes heat transfer. Each heatsink holds two cells, one on each side, and electrically they are in parallel forming an element (figure 8). The heatsink has a plastic picture frame to prevent anyone from being shocked during handling—although the voltage is only 3.6-3.7 volts at this point. Each cell has a unique (2D) bar code and another bar code is assigned to each element. EnerDel assigns additional barcodes throughout the process for complete traceability. The element is compliant enough to allow for compression when installed into a module.
|Figure 7. The heat sink.||Figure 8. The element.|
Twelve elements go into a module (figure 9) and each module weighs a little more than 25 lbs (12 kg). By limiting each module to twelve elements, the ergonomics are more manageable and the voltage stays below 50V, an OSHA threshold. The circuitry required to handle the current required for propulsion is internal to the module.
|Figure 9. The module.|
The flat wire harness (orange) provides the circuitry to handle the diagnostic information, voltage and temperature for each element. EnerDel can track performance to the individual cell to anticipate failures and insure long life.
Being proactive in detecting cell failure is important since a single cell failure, and the required system reaction, will shut down half of the vehicle’s capacity. There are 384 cells in the vehicle.
Two modules go into each sub pack and the sub-pack is the line replaceable unit—the entity that is replaced in the field.
|Figure 10. Battery module and sub-pack. John Corbett, EnerDel Quality Manager, is demonstrating how the module lightly compresses the elements to improve heat transfer and ionic conductivity.|
Each sub-pack contains its own electronic control unit, a gray plastic cover (which normally covers the electronics as well, and ducting to support forced heat transfer. The “mail slot” in front is part of the sub-pack’s ventilation system.
Field service involves swapping out sub-packs and sending the unit back to EnerDel for analysis. EnerDel expects a properly trained and equipped mechanic / technician to be able to do the job in an hour. EnerDel will analyze and repair the sub-pack within their facility. EnerDel has invested a lot of energy into system diagnostics, reliability and serviceability. They can replace any component down to the cell.
Although the sub-pack isn’t capable of powering a vehicle by itself, it can provide a useful surrogate for understanding battery performance and economics using industry generic values for price and performance. Each sub-pack has approximately 3 kWh gross capacity which translates into a vehicle range of approximately 6 miles for a GM Volt to more than 12 miles for a TH!NK. Vehicle range varies based upon vehicle size, performance, operating conditions, how much discharge the manufacturer is willing to tolerate and how conservative the manufacturer is in protecting for battery performance degradation over time.
|Assumptions behind cost calculations|
Using generic pricing (see assumptions at right), each sub-pack is probably worth $2,000 to $2,500 with each cell is probably worth $20-$30. Since EnerDel’s goal is to have a system life > 10 years, and one cell can shut down the system, being able to replace individual cells is crucial to getting the full value out of the system. Many analysts believe that for electric cars to be “subsidy-free competitive” with liquid-fueled cars, the costs still need to come down by a factor of two or three and oil needs to be at $100/barrel. As Prabahakar Patel of CPI pointed out last year “Lithium-ion battery costs have already come down by a factor of 15 over the past 18 years”.
Another way to lower the effective cost of the battery pack to the consumer is capture the pack’s residual value. EnerDel expects the pack to depreciate more slowly than the rest of the vehicle and to have a secondary life in fork lift trucks or residential power storage once the car has been recycled. Charles Gassenheimer, EnerDel’s chairman and who used to run a hedge fund, is convinced that capturing the pack’s residual value in its second, post automotive, life is crucial to making electric vehicles a reality—rather than “beating the battery suppliers down to lower their cost”.
Similar Technology: Two different solutions
The TH!NK vehicle pack (figure 11) has 8 sub-packs (only 4 of which are loaded in this picture) for a total capacity of approximately 24 kWh and a vehicle range greater than 100 miles. The pack is wired into two parallel systems each supplying 350 volts. Either system can be shut down due to an internal failure and the vehicle will fully function, it will just have half the range.
Exact vehicle timing has not been released but the TH!NK vehicle (figure 12) is expected to begin production in Elkhart County, Indiana with the EnerDel battery by early 2011.
|Figure 11. TH!NK battery pack.||Figure 12. TH!NK vehicle.|
Satisfying a customer with different constraints, EnerDel has created a two-pack solution for an all electric Volvo C30 which has a range of 120-150 km (75-90 miles). This range would support 90% of the world’s commuters with a single daily charge.
The bottom pack fits into the space taken by the gas tank and the top pack integrates into the center console and extends through the rear seat. Each pack weighs approximately 150 kg. Combined, both packs have 24 kWh of nominal energy of which 22.7 kWh is used to power the car.
As an all electric vehicle, the traditional engine, transmission and gas tank are replaced with an electric drive system. The battery packs fit within the existing vehicle package and meet Volvo’s safety requirements.
The Volvo C30 eDrive is undergoing evaluation during 2010 and the vehicle modeling in the picture is slated for crash testing.
|Figure 11. Volvo C30 battery pack.||Figure 12. Volvo C30 eDrive. Picture: Michael Nemeth.|
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