A lot of people may remember Richard Hammond forgetting to turn at the top of a hill climb in the Swiss Alps whilst behind the wheel of the fully electric Rimac C1 hypercar, resulting in a very expensive insurance claim. Other than the ridicule thrown at Hammond by his fellow presenters, they also laid bare a concerning downside of battery electric vehicles (BEVs). After the crash the lithium-ion (Li-ion) batteries caught fire, they then continued to burn/reignite for 5 days. This type of reaction after an impact is not ideal, and although EV advocates would argue that petrol is not exactly inert, it doesn't make it any less of a risk.
Burnout wreckage once the fire had finally been extinguished - The Grand Tour
The fire was not unique to Rimac's car, of course, and it is not unique to battery EV's. Any liquid lithium-ion battery has the potential to ignite in this way if the cells are compromised. It's why consumers are asked not to through batteries or battery powered devices into the normal household waste bins. Many of you will also remember the period when Samsung Note 7's would spontaneously decide to end their life in pyrotechnic fashion, the root cause of this was determined to be a design flaw in the battery which lead to short circuiting.
Ultimately it is the gases released when the battery cell is compromised which result in the fire and/or explosion
So why do lithium ion batteries catch fire? Well the underlying root cause is that they are contain flammable materials - I know that sounds facetious, but that is the starting point. Li-ion batteries work by transferring electric charge from a positive electrode (cathode) to a negative electrode (anode) via the movement of lithium ions (charged atoms), this requires a medium known an electrolyte through which metallic ions can travel. The cathode and anode are separated by a...separator, which is typically made of polymer.
Schematic of an Li-ion battery cell [1]
In the case of Li-ion batteries, the electrolyte is an organic (hydrocarbon based) solvent, containing lithium salt particles - and it's the solvent which is the main cause of fire and explosions. The solvents are volatile, and even at room temperature will release flammable gases such as hydrogen and methane, too add to this the cathode contains oxygen which can also be released in the event of over heating or electrical short. Ultimately it is the gases released when the battery cell is compromised which result in the fire and/or explosion [2][3].
The use of volatile liquids is not in itself an unreasonable risk - we use them frequently in our daily lives - because typically there are mitigation steps in place to prevent ignition. Most of these rely on a combination of product design and consumer education, e.g. I would hope most of you would know it's not recommended to chuck your deodorant can in a fire. The issue with Li-ion batteries though is that the battery itself is one of it's own ignition sources - as demonstrated in the Samsung Note 7. If the cell partitions are damaged, the electrolyte escapes and (being a conductive liquid) causes the circuit to shot. The short can then (and invariably does) result in a spark which is enough to ignite flammable gases.
Aside from safety, there are many other downsides to the liquid Li-ion batteries
Overheating is another cause of explosion. In this case, the heating of the battery contents increases the pressure within the cell until it fails, resulting in an ignition and subsequent explosion as the gas ignites and expands.
In a small battery like that in a phone, this is potentially harmful to anyone in the immediate vicinity, but the solvent will burn out relatively quickly and any explosion which may result will also be small. The batteries needed to power an EV are of course significantly larger, and contain thousands of cells and hundreds of kg of flammable material. So, when the a cell in an EV battery assembly is compromised, the resulting fire and explosions will be much larger. Not only that, because the batteries are assemblies of many cells, the failure of one cell can start a chain reaction which slowly works its way through the entire assembly. This is why EV fires can last for days, and are difficult to extinguish.
'Can't the cells be designed with thick walls to prevent this from happening?'... they could, but we all want thin and lightweight phones and laptops, and we don't want our electric cars to weigh the same as a Challenger tank. So the structural components within the batteries are typically very thin to minimise weight and size of the battery.
'Oh my god, this is terrible, I'll never get in an BEV again!'... hold off the boycott. Fortunately, Li-ion battery fires/explosions are pretty rare, particularly in EV's. This is because the manufactures of the cars and battery assemblies are perfectly aware of the hazards and so design/engineer the assemblies and chassis in a way to minimise the risk of a cell structure failure. It therefore takes a pretty significant effort (Richard Hammond) to compromise the batteries - but high impact collisions are an unfortunate reality in every day use of vehicles and so the risk can never be fully eliminated, only reduced.
Aside from safety, there are many other downsides to the liquid Li-ion batteries.
When the battery reaches it's end of life, or the car is damaged beyond repair, the battery will need to be recycled. This is an no easy task with thousands of pressurised volatile liquid containing cells to breakdown. [do some more research]
Then there is the weight, something which I've already mentioned. Anyone familiar with BEVs will be aware that the battery assemblies are very heavy, often accounting for 50% or more of the weight of the vehicle. They're also pretty bulky, a combination of compromising for safety and the shear number of cells needed to generate the power required for a modern day EV. The result is two fold. A lot of EV car models are tall, because the batteries need to be mounted low in the car to maintain a lower center of gravity and keep the car stable in corners, but the size of the batteries means they eat into what would be passenger space, so the cabin has to sit above the batteries resulting in a taller body and higher driving position. This means there are few 'sporty' EVs currently on the market and the majority behave much like their ICE SUV counterparts in the corners. There are some models which are exceptions to this at present, and future models do appear to be returning to more conventional body types as battery-chassis combinations are developed, but the high curb weight will remain an issue.
The other issue with weight is performance and that oh so critical range. If you want more range from a battery, easiest solution is to make it bigger. But bigger batteries are heavier batteries. A heavier battery requires more energy to accelerate. So you need more power from the battery and or additional motors to transfer more of that power to the wheels, ah yes, then you need a bigger battery otherwise you'll sacrifice your range... and so on. The result is a somewhat frustrating sliding scale of compromise, if you want high performance (straight line speed and acceleration) you compromise on handling, or you through lots of clever chassis technology to counter the stability issues, but that comes at significant cost. Result, the current 'best performing' EVs start at around $100k and go up to $2m.
There is of course lots of development ongoing and a lot of the issues highlighted are being addressed, but there is something on the horizon which is going to result in a step change in EV viability and leave no argument left for the ICE. Solid state batteries.
Solid state batteries may offer double the range, or same range for half the size, over the Li-ion equivalent
Solid state batteries (SSBs), are as they sound, batteries which have solid state constituent components, specifically they use a solid electrolyte which also acts as the separator. There are number of specific technologies being developed under the SSB umbrella, but broadly they can be split between those using the conventional carbon base anodes (of Li-ion battery tech) and those using a Lithium metal anodes. Within those groups there a variations on separator material and catholyte, with some combining the two. In a lithium metal SSB, the electrolyte and polymer separator is replaced with a solid state separator and the carbon/silicon anode is replaced with a lithium metal anode (hence the name). Lithium metal anodes have a higher energy density than the carbon anodes, meaning there is the potential for lithium metal SSB cells to have greater energy capacity. The elimination of the liquid electrolyte also means cells do not have to be separated, allowing for closer packing and reduced volume - but wait, there's more.
Structure of Li-ion battery(left) and solid-state battery(right) - All rights SAMSUNG SDI CO.,LTD
Solid state batteries have five key advantages over Li-ion:
No volatile liquid meaning elimination of that primary fire/explosion hazard
Higher energy density cells, allowing for reduced battery size for the same energy capacity (kWh)
Higher specific energy, or gravimetric energy density (Wh/kg) cells, allowing for reduced mass for the same energy capacity
Not reliance on slow diffusion of lithium into the carbon particles, allowing for reduced charging time
No capacity fade which results from chemical reactions with electrolyte, allowing for increased life
This then results in a waterfall of benefits:
The combination of increased energy density and specific energy means that SSBs will be able to deliver near double the energy for the same volume and mass, and therefore double the range, or same range for half the size, over the Li-ion equivalent.
Lighter cars with same energy capacity will have greater energy efficiency (kW/mile)
Smaller unit size also means more battery packs can be more easily incorporated into the chassis, leading to improved optimisation of mass distribution
Faster charging would mean reduced 'range stress', and go some way to reducing the necessity for large capacity batteries*
Increased life would improve general economics of owning and EV through reduced depreciation, longer term ownership and reduced batter recycling costs
*Car manufacturers are talking about developing BEVs with up to 1000 mile ranges, but this is effectively a solution to address the issues of slow charging times and insufficient charging infrastructure. Therefore one could argue its a solution to a problem that may not exist in 10 years time.
A downsized solid-state battery(right) with the same capacity as the Li-ion battery(left) - All rights SAMSUNG SDI CO.,LTD
So the result of all the above will be lighter, more efficient EVs, with viable real world ranges, which handle like their ICE counterparts and can be recharged in the time it takes to buy a disappointing breakfast in a service station. Oh, and if you do have an accident in one, you don't have to pay someone** to take it away and watch it burn for 4 days.
**this probably depends on what country you're in
Full floor battery packing may become a thing of the past - All rights Car Magazine
'So why aren't we hearing about this on Top Gear?' Well, the key phrase is 'in development'. Many of the companies working on this technology have only developed working single layer cells in the last 2-3 years. There is still a lot of development work to do before the theoretical benefits become a reality. It is likely we won't EV scale prototypes for another 2-3 years and then production probably a further 5 years down the line - if we're to assume similar scale up times to present day EV technology. But there is some encouraging activity in the market on this. A group including BMW and Volkswagen recently invested in a company developing Lithium-metal SSBs, called QuantumScape, and Toyota are developing SSBs in house, as too are Samsung.
Also worth mentioning that EV doesn't just mean automotive. Companies like Vertical Aerospace, Airbus and Joby Aviation are all developing BEV aircraft, where of course weight and space saving are so critical to performance and safety. I am sure those companies are also keeping a close eye on SSB development.
So the race to develop this game changing technology is well and truly on. Who will be first to market? Doesn't really matter, as long as the competition drives the development of the technology we will all be winners - well, except the petrochemical companies... but I'm sure they'll be fine.
References
Roy, P., Srivastava, S.K., 2015. Nanostructured anode materials for lithium ion batteries. Journal of Materials Chemistry A 3, 2454–2484.
Henriksen M, Vaagsaether K, Lundberg J, Forseth S, Bjerketvedt D. Explosion characteristics for Li-ion battery electrolytes at elevated temperatures. Journal of Hazardous Materials 2019;371:1–7. https://doi.org/10.1016/j.jhazmat.2019.02.108.
Henriksen M, Vaagsaether K, A.V: Gaataug, Lundberg J, Forseth S, Bjerketvedt D.Laminar burning velocity of the dimethyl carbonate-air mixture formed by the Li-ion electrolyte solvent. Combustion, Explosion, and Shock Waves (Accepted for publication, 2020).
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