WE BUILD A LITHIUM ION BATTERY
EV BATTERY PRODUCTION IS THE NEW FRONTIER OF THE AUTOMOTIVE INDUSTRY, BUT HOW ARE THEY MADE? WE OPEN THE CHEMISTRY SET TO FIND OUT
OPPORTUNITIES like this one don’t come around very often – the chance to build a lithium-ion electric car battery cell from scratch, to see the inner workings of a device that usually stays closed because its contents are flammable, and to hear an expert ‘how it works’ explanation from one of the world’s foremost experts on EV batteries.
There are only a couple of places in the United Kingdom where batteries are built from raw materials and one of them is the prototype manufacturing plant at Warwick Manufacturing Group (WMG), a department of the illustrious University of Warwick. Our mentor for the day is Professor David Greenwood, a brilliant communicator of the intricate flows of electrons and lithium ions inside these anonymous-looking grey packages. So free your mind of the notion of crankshafts, valves and bearings and introduce to it instead the anodes, cathodes, separators and LiPF6 electrolytes of our electrically driven future.
Our task is to make a small 3.5V A7-sized pouch cell – about the size of a credit card at 50mm by 70mm – using machinery on WMG’s prototype production line. In a production car, like the Nissan Leaf or Jaguar I-Pace, dozens of these pouches are built up into modules before the modules are wired together to make the battery. The internal parts and the principles of design and build are the same as for bigger pouch cells, like those fitted to the Leaf, but also the prismatic designs and cylinders used by Tesla. Tesla’s 18,650 cells may be a very different shape but they contain the same basic componentry, just rolled up inside a tube.
There are four main components – anode, cathode, separator and lithium electrolyte – and four main processes: make the anode, make the cathode, pack them into a pouch and fill with the liquid electrolyte that contains lithium atoms. In an uncharged state, these atoms are stored on the cathode. During charging, they lose an element and temporarily become lithium ions.
There are 11 manufacturing steps. Our session will take a couple of hours, but a volume plant, like Tesla’s Gigafactory, knocks out 20 cells a second. Time to don the protective green rubber gloves, white lab coat and go mix chemicals.
01 MATERIAL WEIGHING FOR ANODE
The electrochemically active ingredient in the anode is graphite – it’s the component that actually stores lithium ions that reach it during the charging process. The graphite comes as a fine powder, each particle of which is just 10 microns across – one fifth of a human hair. Here, the graphite is mixed in a water-based solvent with carbon black and a latexbased binder to make a slurry.
The carbon makes the graphite electrically conductive, and the binder is an adhesive so it will stick to a copper backing plate in the next stage.
A similar process is used to make the cathode, but the very fine nickel particles can only be handled while wearing pressurised breathing masks, so aren’t appropriate for our visit.
02 ANODE MIXING
This key step is carried out with equipment designed for the food industry to mix bread, or chocolate. Production-quality anode material is mixed in volumes of hundreds of litres for hours to ensure the powders, solvent and binder are uniformly distributed.
03 ANODE COATING
The slurry is spread thinly over a copper sheet about half the thickness of a sheet of paper which acts as an electrical conductor to allow the lithium ions to flow in/out of the graphite.
Copper is used for the anode and aluminium for the cathode because otherwise the conditions in the cell would lead to the cathode dissolving and the cell being destroyed.
In a production factory, the coating and drying machine that makes the anode can be up to 90 metres long. The material passes through it in just a minute to optimise the drying process.
04 CALENDERING
This maximises the charge storage capacity of the graphite in the anode by gently crushing the coating to increase its density. The process originated as a finishing technique for textiles.
05 DRY ROOM
Lithium will react if exposed to moisture in the air and degrade cell performance, so the final production processes are carried out in a dry room where relative humidity is just 0.5 percent – one tenth of the Atacama desert. Workers must take frequent rehydration breaks.
06 DIE-CUTTING THE LAYERS
The ribbon of copper and its graphite coating are cut to the A7 shape to make layers. To achieve performance of 3.5V and 6Ah, our cell needs 15 layers – seven anodes and eight cathodes. Each cut is made by a die-cutter to ensure sheets are located precisely in relation to a porous polymer layer – the separator – which will keep the anode and cathode apart to prevent short-circuiting and possible fire. Lithium ions pass through the separator to the anode. Elements go to the anode via the charger, as they cannot pass through the separator. They combine with the ions to recreate lithium atoms, When all lithium atoms have re-formed at the anode, the cell is fully charged. The process is reversed as the battery is in use.
07 CELL ASSEMBLY
The fiddly process of interleaving the anode layers with a separator and the cathode sheets is handled by a Z-fold stacking machine. Our cell will have about a metre of separator between the 15 layers.
08 WELDING THE CURRENT COLLECTOR
Ultrasonic welding joins the layers of the anode into a robust tab – the connection point through which electricity flows in/out of the cell.
09 THE PACKAGING CELL
The anode layers are housed inside an airtight pouch of aluminised polymer, a more robust version of the material used to package food such as coffee beans. In the cell, it has to remain airtight for 15 years; for coffee, just six months. The edges of the pouch are heat sealed but left oversized to allow room for a later process.
10 ADDING ELECTROLYTE
Just 2ml of liquid electrolyte containing lithium hexafluorophosphate (LiPF6) is pumped into the cell and the pouch sealed in an ultra-dry vacuum to keep moisture away from the reactive and flammable lithium. With all the other parts needed to make the cell function, only 4 percent of our A7 cell’s approximate 200g weight is actually lithium.
11 FOMATION AND AGEING
This is the Frankenstein moment, when the pouch becomes a functioning cell. A special rig repeatedly charges and discharges the cell while its behaviour is monitored for quality control. Initial reactions between the electrolyte and the anode create the solid electrolyte interface (SEI) – a protective layer – and in the process give off gas, which is why the pouch was left oversized. It will be purged and trimmed later.
Amazingly, the cell takes days to form. Typically the small cell in a Tesla might take three to four days, but some larger pouch cells can take an astonishing 28 days. Our smaller A7 cell will probably be one to two days in the making.