We build a lithium-ion car battery

The electrochemically active ingredient in the anode is graphite – it’s the component that actually stores lithium ions. It 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 latex-based 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 very 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.

2: ANODE MIXING

This key step is carried out with equipment designed for the food industry to mix bread and chocolate. Production-quality anode material is mixed in volumes of hundreds of litres. It needs hours of mixing to ensure the powders, solvent and binder are uniformly distributed in the slurry.

3: ANODE COATING

The slurry is attached to an electrical conductor to allow the lithium ions to flow in/out of the graphite, so it is spread thinly over a copper sheet, about half the thickness of a sheet of paper. Copper is used for the anode and aluminium for the cathode because the conditions in the cell would otherwise mean 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 that is critical to how efficiently the anode stores charge.

4: 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.

5: DRY ROOM

Lithium will react if exposed to moisture in the air and degrade the performance of the cell, so the final production processes are carried out in a dry room where relative humidity is just 0.5% – one tenth of the Atacama desert. Workers in here have to take frequent rehydration breaks.

6: DIE-CUTTING THE LAYERS

The ribbon of copper and its graphite coating are cut to the A7 shape to make individual 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 precise die-cutter to ensure each sheet locates precisely between the separator/ insulator and has no rough edges – otherwise the cell could short-circuit and possibly catch fire.

7: CELL ASSEMBLY

Each of the anode layers has to be interleaved with a separator and the cathode sheets. It’s a fiddly process that’s handled automatically by a Z-fold stacking machine. Our cell will end up with about one metre of separator between the 15 layers.

8: WELDING THE CURRENT COLLECTOR

Ultrasonic welding joins the layers of the anode together into a robust tab – the connection point through which electricity flows in/out of the cell.

9: PACKAGING THE CELL

The anode layers are housed inside an air-tight pouch made from aluminised polymer – a more robust version of the material used to package food such as coffee beans. In the cell, it has to remain air tight for 15 years; for coffee, it’s 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% of our A7 cell’s approximate 200g weight is actually lithium.