Tesla’s Four Million Mile Battery

Source: Medium, Will Lockett
Photo: Charlie Deets on Unsplash

Tesla’s research partner may have just developed the ultimate battery.

A recent study let slip a secret Tesla has been keeping quiet for four-and-a-half years. The truth is, behind closed doors they have been developing a battery that is lightyears ahead of anything that currently exists, with durability that can last a hundred years, or four million miles, without needing to be replaced! Not to mention the fact it could lead to significantly lighter battery packs. So how have they achieved this? And could you soon buy a four million mile Tesla?

Before diving into this battery, we first need to understand why battery degradation is a problem.

At face value EVs seem to have shrugged off the battery degradation issue. After all, Tesla offers a 150,000-mile 8-year battery warranty, but there are two problems. Firstly, plenty of cars go well over 150,000 miles during their lifetime, and to replace a Tesla battery costs up to $22,000! So with today’s tech you can’t buy a useable high-mileage electric car without forking out tonnes for a new battery. Secondly, EVs are now being used for other applications, such as trucks and vehicle-to-grid (V2G). Both of these applications required the battery pack to survive far longer. Trucks can easily do over a million miles during their lifetime and V2G, where your car powers your house, can double the number of charge cycles an EV goes through in a year.

In short, for EVs to truly replace our old tech, they need far more durable batteries.

This is where Professor Jeff Dahn Of Dalhousie University comes in. He and his team partnered up with Tesla in 2016 to create the Tesla Advanced Battery Research division. Their primary goal is to make batteries last longer, they also aim to increase energy density and decrease costs.

Prof Dahn recently published a study showing the world the fruits of their labour, a LiO² battery, with an NMC 532 single crystal cathode capable of lasting four million miles. But what is a LiO² battery or a single crystal cathode? And how can they tell it will last so long?

A LiO² battery is also known as a lithium-air battery (Li for lithium, O² for oxygen). Unlike the Li-ion or LFP batteries in EVs today, a lithium-air battery uses oxygen ions to store energy, not lithium ions. This difference means their internal structure and chemistry needs to be entirely different from established battery technology. Furthermore, this new structure could allow lithium-air batteries to be five times more energy-dense than any Li-ion battery. This dramatic difference has given engineers grief as they have struggled to build lithium-air batteries. But even early attempts showed that they might offer a long lifespan. However, until Dahn’s breakthrough, Lithium-air batteries couldn’t even survive two months of testing.

Dahn’s battery hasn’t chased down this magically high energy density, instead he kept it comparable to LFP batteries, which means his battery is quite large for lithium-air batteries. Dahn also kept the charge rate similar to LFPs, which are slightly slower to charge than the best Li-ion batteries. This ‘extra space’ and middle-of-the-road charge rate what reduces stress inside the battery (they aren’t explaining this lower than expected energy density), allowing for it to have such a long-lasting life.

But there is one other thing that gave this battery its longevity. It is it’s weird cathode.

The cathode is a fancy name for the hunk of metal at the positive end of a battery. This metal has to store lithium-ions (or oxygen ions or LiO² battery) during discharge as they migrate from the anode to rejoin with the electrons that have just traveled around the circuit. Now, a solid metal having ions and electrons flowing through it can easily degrade. Impurities, faults and misaligned metallic crystals can lead to the cathode eroding away, breaking up, or even forming a crusty outer shell during charging and discharging, all of which reduce a battery’s capacity.

Normal batteries, such as Tesla’s current 2170 cells, use an NMC 811 polycrystalline cathode. Translated from engineering speak, this means the cathode is made of 80% nickel, 10% manganese and 10% cobalt (NMC 811) and is in the form of many separate crystalline structures all smooshed together. The low manganese and cobalt levels make the battery cheaper. However, higher levels would result in a more efficient and longer-lasting battery. A polycrystalline cathode also has many faults and misaligned crystals where degradation can set in, reducing its lifespan.

So Dahn and his team solved this with a single crystal NMC 532 cathode. This has much higher levels of manganese (30%) and cobalt (20%), which makes it more efficient. Also, rather than being loads of crystals squashed together, they have been formed into one cohesive crystal, which means there are very few falts for degradation to take hold, which massively extends its lifespan.

Dahn has been testing this LiO² battery with a weird cathode since 2017. During that time it has been repeatedly charged and discharged over and over again, 24/7, for four-and-a-half years! Most Li-ion batteries would be pretty much dead by now, but Dahn’s LiO² battery had only lost 5% capacity. By extrapolating the data, Dahn recons it could last for the equivalent number of charges as driving a Tesla four million miles, equating to about 100 years of typical driving.

Such a battery could revolutionize EVs. Electric trucks could operate at full capacity and cover hundreds-of-thousands of miles a year without fear of extortionate battery replacement bills. Renewable energy storage can last for far longer making the adoption of solar and wind far cheaper and more eco-friendly. EVs can be used as energy storage devices for home solar without fear of ruining their battery. EVs will become even more eco-friendly as electronic waste drops, and even vehicle turnover will dramatically reduce as cars start lasting decades, not years.

But remember this is only the beginning of LiO² batteries. If Dahn can develop this tech and push it to its theoretical limits then batteries could become far lighter and smaller. Imagine having a Tesla Model 3 with a 100 kg battery pack (normally 480 kg) that can still do 300 miles a charge. With such weight savings commercial electric planes become feasible, EVs will become far more efficient and even ultra-lightweight, high-performance EVs can emerge.

So, the question stands, will we see ultra-durable lithium-air batteries in Teslas soon?

Probably not.

Firstly, this is a very new technology. It could be decades before it is developed enough to be put into a car. The high levels of cobalt could also be a huge issue. Not only is cobalt more than twice as expensive as any other battery metal, but its mining is environmentally and humanely dubious. Many companies, including Tesla, want to have cobalt-free batteries to reduce their environmental and human impact, whilst also offering cheaper batteries. So if Dahn’s battery can’t go cobalt-free then it may be a dead-end. Furthermore, lithium-based batteries already have a problem with catching fire and lithium-air batteries need to be air breathing in order to work, making them way more dangerous in a fire! To say they pose a significant safety challenge is putting it mildly.

So we likely won’t be seeing this battery out in the wild for a while. But, if anyone can overcome these engineering problems, it is Dahn and Tesla. We also don’t know what else is being hidden from us in Dahn’s lab. Maybe this battery is far more developed than they are letting on? So who knows, in 10 years’ time you may be able to buy a four million mile Tesla.

https://medium.com/predict/teslas-four-million-mile-battery-567245ce9561