The massive 300-550 kg battery packs that go into electric cars are probably the most important component by far, just like the importance of an internal combustion engine to a traditional car. However, the journey that these lithium-ion batteries make when being produced is a very interesting one: from multiple (sometimes unsafe) mines in far-off countries to being packaged into a powerful, high capacity battery which can drive a car forward at very high speeds.
So how exactly are these lithium-ion batteries for electric cars made? The short answer is that a number of rare metals need to be dug out of the earth from various mines. These are then packaged into small individual battery cells (alongside other materials such as plastic, aluminum, and steel), before themselves being packed into battery modules. The end result is a battery pack which is made up of multiple battery modules, a cooling system/mechanism and a small electrical power management system. Let’s explore some of this in more detail below!
Table of Contents
- Battery Structure And Necessary Raw Materials
- How Rare Metals Are Mined
- Making The Battery Cell
- Creating A Battery Module
- Producing The Battery Management (Power) System
- Battery Cooling System Construction
- Putting It All Together: The Battery Pack
Battery Structure And Necessary Raw Materials
Before we can go into exactly how electric car batteries are produced, it is worth talking about the battery structure and the materials that go into them. Okay, so pretty much all modern electric cars use lithium-ion batteries, which are rechargeable and contain lots of lithium atoms which can be electrically charged and discharged (known as an ion). A fully charged battery will have the ions at the negative electrode (the cathode), which will transfer to the positive electrode (the anode) when they have been discharged (i.e. used up). When you plug your EV in to charge back up, the ions move back to the negative electrode, restoring the car’s battery capacity and therefore driving range.
This particular movement of ions occurs inside an individual battery cell, similar to the battery inside your cell phone. However an electric car naturally has much more power than a single cell could provide, so multiple battery cells are grouped together into a battery module (where the cells are arranged in a frame to protect from vibrations and heat build-up). Finally, multiple modules are organised into the overall battery pack which is installed in electric cars. Every car manufacturer uses different numbers of cells and modules within the pack, but as an example the BMW i3 has a total of 96 battery cells across 8 modules (12 cells per module, and 8 modules per pack):
Materials Within A Battery Cell
In general, a battery cell is made up of an anode, cathode, separator and electrolyte which are packaged into an aluminium case.
The positive anode tends to be made up of graphite which is then coated in copper foil giving the distinctive reddish-brown color.
The negative cathode has sometimes used aluminium in the past, but nowadays is a mix of cobalt, nickel and manganese leading to a mixed-metal oxide material.
The electrolyte is a lithium salt in a dissolved solution known as a solvent (which is usually ethylene carbonate). Finally, the separator (which - as its name suggests - physically separates the anode and cathode) is a micro-porous compound which is usually plastic-based such as polypropylene or polyethylene.
Materials Within A Battery Module
As mentioned earlier, the module houses the individual cells and protects them from vibration and heat. The main container typically uses a mix of aluminium or steel, and also plastic.
The individual battery cells within the module need protection from heat and vibration, so a number of resins are used to provide mechanical reinforcement to the cells within the module:
Materials Within A Battery Pack
The battery pack’s housing container will use a mix of aluminium or steel, and also plastic (just like the modules). The battery pack also includes a battery management (power) system which is a simple but effective electrical item, meaning it will have a circuit board (made of silicon), wires to/from it (made of copper wire and PVC plastic for the insulation), and resistors/capacitors which use a mix of materials:
- Metals like copper, nickel, lead, tin and aluminium
- Plastics for the housing like PVC, PET and polystyrene.
Phew, we have really drilled down into the raw materials at play here! One of the things you may have realized is that plastic such as PVC/PET and metals such as steel/copper/aluminium are very common and thus these are not a massive issue for car manufacturers to source: their supply is plentiful both locally and internationally.
However some of the rarer metals that make up a battery cell (like lithium and cobalt) are not as easy to source, and the rapid rise of electric cars means that greater pressure is placed on rare earth miners than in the past. The next chapter examine how some of these rare metals are mined, so that the battery cells can be manufactured.
How Rare Metals Are Mined
The process of mining the rare metals varies depending on the mine, however our ‘Electric Cars Aren’t Green?’ sums up how some of the mines operate:
At a mine in Jiangxi, China, workers use ammonium sulfate poured into big holes to dissolve the clay.
- What’s left is hauled out of the ever-expanding hole, before being run through multiple acid baths to dissolve other unwanted compounds.
- The resulting compounds are baked in a kiln, finally revealing the rare metals required in electric car batteries.
- Just 0.2% of the result is the rare metals; the other 99.8% is waste.
- This 99.8% waste earth (and other compounds) - which is now contaminated with toxic material - is dumped back into the originally-created holes.
Many of these rare earth mining processes also unleashes plumes of sulphur dioxide into the atmosphere, and can harm aquatic life in nearby rivers and streams too. Finally, 50-60% of cobalt comes from the Congo, which unfortunately has a poor human rights record with 40,000 children working in cobalt mines for $1-2 per day.
The reason we are mentioning this in a “making of…” article is because car manufacturers are rightly considered about the chemical composition of their EV batteries, and they are working on tweaking the metal composition to reduce the dependence on some of the worse metals such as cobalt and nickel. Hence the material list that we mentioned towards the start of this article might (or hopefully!) start to change throughout 2020-2030.
Making The Battery Cell
The first thing to point out is that a battery cell which goes into an electric car is not a round, circular battery like we use in our home electrics (and not like the one in our diagram earlier!). A round battery would not be a very efficient use of space (since the car might have hundreds of them in its battery pack) - so instead a flat-cell battery cell is produced. This is made up of multiple sheets of cathode, anode and separator - which can then be stacked on top of each other like the filling in a Subway! This end result is a cubic shape which is then packaged into an aluminium and plastic frame.
With that said, let’s back up a bit and drill down into the detail. The first thing is to create the sheets of cathode and anode, which arrive in the factory as a big metallic calender roll (just like a big master roll of carpet):
(thanks to the handy video from MotoManTV)
This then goes through a set of processes:
- Mixing: creating the lithium slurry compound via giant mixers.
- Coating: the lithium compound is applied to the metallic roll.
- Drying: the resulting electrode roll is then dried in very high temperatures (and very low humidity conditions), which reduces cracking of the surface material.
- Pressing: a roll pressing machine is then used to repeatedly apply pressure to the roll, which increases its density and thus gives it higher energy/power capacity per volume, along with improve electric conductivity and material adhesiveness.
- Slitting/Cutting: the final worked sheets are cut down to the required length with highly precise cutting tools to avoid frayed edges, and they will be cut small enough to fit into the final battery cell.
At this point we end up with a flat ‘electrode sheet’ (known as a cell layer). These individual cell layers are then stacked on top of each other in the form of anode, separator and then cathode to end with up:
(thanks to the handy video from Lithium Battery Company)
Each layer is connected with a tab, which is what you can see sticking out on the image above. These tabs are stuck or welded together, and the layers are laminated with aluminium foil. Finally, the liquid electrolyte solution is injected in-between the layers which allow the ions to travel between the two electrodes.
At this point, the flat battery cell is finished. We have a fairly durable item which can charge and discharge as required. However they cannot go straight into a battery module and the final product: like a good cheese or wine [or both together: aged cheese and wine go great together… but back to the article!], the cell needs to be aged over time. They are also charged and discharged to activate the cell regularly and ensure its electrical properties are as expected.
Creating A Battery Module
Just like cell layers were stacked on top of each other to create a battery cell, the finalised battery cells are then stacked on top of each other within a metal (aluminium/steel) or plastic frame which is purpose built.
As mentioned earlier, the module protects the individual cells from both heat and vibrations. This is achieved by applying resins and/or thermal interface material between each cell layer (like we have cheese or sauce between the main ingredients of a sandwich!). There might also be more active cooling mechanisms applied, hence the necessary materials (as we discuss later) will be fitted in-between the cells as well.
Every car manufacturer has a different number of battery cells per battery module: some might have just four cells per module, leading to a module which looks like:
(thanks to the handy video from MotoManTV)
Other car manufacturers may have more cells per module, or they might have much larger battery cells leading to much larger battery modules (and then fewer modules in the battery pack):
(thanks to this handy video - ignore the packing tape along the top edges though; this is because the battery is being packed for return, not installation!)
The exact design of the battery (and the composition of cells and modules in the pack) is constantly evolving, but the basic idea is that the battery module helps to protect the all-important cells from heat and vibrations/movements.
Producing The Battery Management (Power) System
At this point we have lots of battery modules, packed with all the power capacity that will be needed to move the car forward. However it would not be safe purely to hook this up to the motor controller and hope for the best: we need to ensure that the battery is safe in all conditions, and does not give out too much or too little power in any given situation (otherwise the battery and other car components could get damaged).
The CEO of Daimler (who produce Mercedez-Benz and SMART cars) famously said in 2017 that “The intelligence of the battery does not lie in the cell but in the complex battery [management] system,”. In other words, we need a smart electrical component that can manage the huge amount of power that the EV’s battery is storing away.
The actual BMS module will be a standard electrical component, with a PCB using silicon, and it will have a range of resistors and transistors (made from electrically conductive metal and plastic housing). Naturally every car manufacturer will use their own BMS system, although an example to look at if you want more detailed information is the 12-Cell Lithium BMS Module from Zera (which is aimed more at EV ‘DIY’ conversions, but the same principles apply to mainstream EVs). A general BMS module will look something like:
The battery/power management system is wired up to each battery module, allowing for the state of charge and state of capacity to be measured, along with a host of other safety and management measures. If you are interesting in electrics, then Renesas have a very handy guide on the specifics of a BMS.
Battery Cooling System Construction
Have you ever felt how hot your smart phone gets when it is installing loads of updates, or it is being charged? Well that is with a single battery cell: imagine what happens when you have a hundred or more battery cells, all working to move a car along a road really fast! As you can guess, this generates a lot of heat and the battery cells need to be protected from this.
There are a few ways of doing this. We mentioned earlier on about resins and/or TIM being applied between battery cells. This is one approach, although some car manufacturers apply additional methods when building the battery module (and ultimately the pack):
- Metal cooling plate: this is where a stainless steel heat exchanger unit is applied to the underside of the battery modules (hence the underside of the cells), and the heat can be moved along the plate - away from the cells.
- Thermal fins: if you have seen inside a computer, you will have seen lots of metal fins starting at the CPU (the brains of the computer) and moving away from it, usually with a fan attached to this unit. This is an example of thermal fins, which are thin pieces of metal which help to pick up heat from a heat source (like a battery cell or a CPU) and transfer it across its fins, away from that heat source.
- Water cooling: the Tesla Model S has coils of water pipes throughout the battery pack, and this contains a water-glycol solution which removes heat from the underside of the cells and provides cooling to them instead. This is a similar approach to high-end computer systems which are water-coolant cooled via a network of pipes and a pump.
Putting It All Together: The Battery Pack
Finally, we have the battery pack: where the battery cells (contained in the modules) are all housed together, cooled via the cooling system and managed by the power management system (and the motor controller, a separate component to the battery). The overall frame will be made from a mix of metal (usually steel or aluminium) and also plastic. The various components will be welded and/or bolted into place. The end result might look like the following Tesla batteries:
Or like the following mock-up of a Nissan Leaf (with the plastic housing partially cut away):