Electric cars and plug-in hybrids are nothing without the highly-sophisticated lithium-ion battery packs which power them, yet they’re also largely under-appreciated by those who own or drive a plug-in vehicle.
But how are electric car battery packs made — and just how much time, energy and effort goes into ensuring that your electric car’s battery pack is reliable and healthy for mile after mile?
Earlier this week, we were among the first journalists in the world to tour Nissan’s high-tech lithium-ion processing plant in Sunderland, England, where Nissan makes some 60,000 lithium-ion cells every year for use in Nissan’s popular LEAF hatchback and e-NV200 electric minivan.
Yesterday, we explained how Nissan built its lithium-ion battery processing facility, and detailed the great lengths it goes to in order to keep the cell production facility super-clean. Today, we’re going to explain how Nissan takes the raw cathode, anode and separator materials to make the individual cell pounces that are used to build each and every battery module. Tomorrow, we’ll show you how those modules are assembled into a battery pack.
Meeting the Mother Roll
When they first arrive at the Nissan battery facility from Japan, the sealed rolls of anode and cathode material look nothing like the lithium-ion battery cells they will eventually become. Shipped in ‘mother rolls,’ cathode and anode materials are kept in hermetically sealed clean bags until they enter into the clean room environment via a special air lock. Like the manufacturing machines which are used in the clean room, each roll is carefully inspected to ensure it is truly clean before it is transported into the inner sanctum of the clean room.
In terms of the amount of materials it consumes every day, month or year, Nissan was careful to avoid letting any trade secrets slip. In the words of Matt Loader, Electric Vehicle and Zero Emission Mobility Communications at Nissan Technical Centre Europe, “We’re going to show you how we make the cake, but we’re not going to tell you all of the ingredients.”
What we can tell you however, is that Nissan’s electrode material has a finite shelf life, and Nissan employs the same ‘just-in-time’ stock philosophy that the rest of its automotive plants follow, meaning that any electrode material on site is destined for relatively immediate use. During our tour of the semi-clean storeroom where sealed bags of electrode material is stored prior to its entry into the clean room, we counted at least 100 mother rolls each of cathode and anode.
Each roll contains enough material for about 1,300 lithium-ion cells, and contains some 2 kilometres of material if rolled out end to end. On entry into the clean room via the air lock, each bag containing respective cathode, anode and separator materials are opened and left in the clean room atmosphere for around 24 hours, ensuring that there’s no moisture contained in each roll which could interfere with the cell making process.
Once thoroughly dehumidified, each mother roll of cathode and anode material is fed through a special machine where insulation tape is applied at regular intervals. Once the insulation tape has been applied, each mother roll is split into three segments across its width, and four segments along its length, yielding twelve ‘slit rolls’ for every mother roll.
It’s worth noting too that at this point in the process, cathode, anode and separator is kept in its own specific area to avoid cross-contamination.
At this early stage in the manufacturing process — and throughout Nissan’s cell construction and module assembly — hundreds of individual tests are made to both raw materials and cells every day, ensuring they meet the tough requirements necessary for correct battery operation. In addition to the constant measuring, inspections and computerised testing, a special team of material engineers working in clean-room conditions strip, analyse and scrutinise every part of the process down to the microscopic level.
That traceability and continual testing was something reinforced to us several times on our tour. From the point the mother rolls enter the factory, each and every step traces and records pertinent data about the raw materials and final cells. At any point in the process, it’s possible to trace a particular cell to a particular machine, a particular employee, and even a particular mother roll. And at the point the finished cells are packaged into modules and stacked together to form a final battery pack, the data associated with the cell manufacture is associated with the vehicle the battery pack ends up in.
It’s a mind-boggling cradle-to-grave philosophy that enables Nissan to trace faulty units right back to the start of their lives when needed, and also ensure the upmost quality is retained at all times.
Layering the Cell
Back to the production process. Like any battery cell, Nissan’s lithium-ion battery cells are made by producing a sandwich-like construction of anode, separator and cathode. Before that can happen however, each electrode has to be cut to size using super-accurate cutting dies. And when we say super accurate, we mean it: Nissan said the staff responsible for maintaining the cutting dies were sent to Japan for 8-months of constant training before they were ready to operate and maintain the equipment used in the Lithium-ion cell facility.
Once cut, the electrodes are brought together for the first time in a large automated machine, with anode entering on one side and cathode on the other. With sophisticated camera equipment to inspect and correctly position each sheet, the machine creates a sandwich of electrodes and separators which ultimately will become a battery cell.
Nissan declined to comment on the total number of layers in each cell, but once the correct number of layers have been reached, the stack of cathode, separator and anode material that will become the cell pass to the next machine, where an automated process welds the correct cathodes and anodes together to provide electrical connection between each part of the ‘sandwich,’ and tabs are welded on to the outermost electrodes for connection to the rest of the cells in each battery module.
Once welding has been completed, the cells are then laminated together for strength, and three of the four sides of each cell are sealed together to form a battery cell pouch.
From here, the cells are sent into an even cleaner ‘inner’ clean room, where the life-blood of each cell — the electrolyte — is injected. But before that can happen, each dry, open-ended cell is tested by a specialised machine which passes a high voltage through the dry cell to check for any manufacturing defects in the cell layering, welding and lamination process. Any cell failing at this point is taken apart by Nissan’s crack team of battery specialists to pinpoint and remedy the problems which caused the cell to fail the high-voltage test.
Cells which pass the high-voltage ‘leak’ test are sent through an airlock into the inner clean room, where they are taken out and weighed by sophisticated computer equipment before and after electrolyte injection to ensure that each cell gets the exact amount of electrolyte it needs.
While Nissan allowed us to tour the rest of the clean room area, we were told that the ultra-clean electrolyte injection area is so sensitive to moisture and outside contaminants that even allowing one too many cleanroom workers in the area can have catastrophic effects. As a consequence, we were treated at this point to a video presentation of this step taking place.
The electrolyte injection takes place in side a vacuum-sealed area, to ensure that no gas remains in the cell and any excess moisture in the cell is correctly removed. Once ‘gassed,’ the cells pass into a final vacuum chamber, where the final open end of the cell is sealed.
Aged like a fine wine
Once the cells have been measured for the correct dimensions and seal thickness, the cells are transported through a clean room airlock in carrier racks in groups of 36 cells to a rack.
From this point, the sealed cells have left the fully-suited clean room area, but are still in semi-clean room surroundings, where all staff must wear appropriate hand and foot covering to ensure contamination is kept to a minimum.
At this point the cells — much like a fine wine — must be ‘aged’ in an area known as the ‘ageing room,’ where they are left to electrically activate. This process essentially allows the newly-injected electrolyte and the electrode material to interact, preparing the cell for its first charge.
Of ageing, Nissan said not every cell manufacture ages their cells, but its own ageing process has been carefully designed to ensure cells reach optimum charge-holding capabilities before they receive their first proper charge.
We’ve reached the point in our tour where clean-room suits are no-longer needed, a fact that we were only too happy to find out. After just half an hour in our clean-room suits, we were finding them to be less than comfortable. Given the staff in the clean room areas work eight hours a day, five days a week, we’re in awe of of the men and women who work behind masks to ensure that the LEAF’s battery pack goes as far as it’s meant to. And we think you should be too.
We’ll be back tomorrow following the correctly aged cells as they make their way from the ageing room into the module production facility and ultimately, into fully-assembled battery packs.
If you missed yesterday’s story, you’ll find it here.
Nissan provided train fare and hospitality to enable Transport Evolved to bring you this first-person report. Due to restrictions on bringing external ‘dirty’ equipment into the clean room area, all photographs in this article were taken by official Nissan photographers.
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