Author's Note (January 13, 2021): In response to questions below, I have made the following clarifications to the text:
- "Overstated Successes -> Power": Clarified cycle references.
- "Other Significant Challenges -> Lithium Autoignition": Added references for autoignition of lithium metal.
- "Other Significant Challenges -> Vibration and Dendrites": Added a reference on garnet electrolytes that discusses dendrite formation at cracks.
- "Overstated Successes -> Low Temperature": corrected a mistake where I had misread the units on a QS graph. The conclusions are the same.
Given QuantumScape's (NASDAQ: NYSE:QS) recent IPO and the subsequent runup in their stock, it is interesting to discuss both their considerable successes as well as the significant challenges that remain in order for them to achieve their stated performance. In this article, I will discuss mostly the technical aspects related to the possibility of achieving a successful product. In later articles I may revisit the possibility of QuantumScape achieving a business success, and also under what circumstances an investor can achieve a financial return. These three outcomes (working product, business success, investment return) appear completely decoupled.
Let's start by saying that building a solid state battery that will function at the rates and temperatures needed for real world applications is hard - very, very hard. So hard, in fact, that nobody has done it. I've read many dozens of research papers where scientists have tried, and tout their ability to get one or more features to behave, but then apologize for the lack of a complete working battery, and lay out the significant challenges ahead. Much of the below is an interpretation of their technical presentation, which can be found here, and the webinar, which is stored on YouTube here. So far, they have:
- Electrolyte: a free standing, thin solid electrolyte that will sit between the anode and cathode. While we don't know much, it does deliver some relevant performance.
- Pouch Cells: a functioning single layer pouch cell, at 70 x 85 mm, 3.2 mAh/cm2, for a total capacity of 190 mAh and 0.7 Wh. For comparison, an iWatch battery is 205 mAh, and an iPhone 12 Pro battery is 3,768 mAh. It would take 20 of these cells to power your phone, and 100,000 to power a Tesla.
- Lithium Metal Anode: They are using a thin lithium metal anode, which will help them achieve high energy density...someday.
- Fast Charging: 80% capacity in 15 minutes, which is a considerable challenge since dendrites are known to form in solid state electrolytes at fast charging rates. More on this later.
Areas of Overstated Success
All of these areas below are described as successful, because they are much better than has been achieved with solid state batteries in the past. But they are completely unacceptable for real world field electric vehicle performance.
- Power: They have done 1200 circuits of a 90 second OEM specified track simulation, which pulled pulses of 6C. In this track, 9 circuits is full depth of discharge, after which the battery was heated to 45 degrees C (113 degrees F) and charged to 80% in 15 minutes. The cell lost about 10% of its capacity in this 130 full-depth-of-discharge (FDOD)cycle test, meaning the battery will only last for 260 FDOD cycles or about 75,000 miles of aggressive driving. There is a note on the slide that it occurs at 3.4 atm, which likely means at high pressure. I'll comment on this later.
- Range: In much gentler, 1C / 1C cycling at 30 degrees C, the cell makes it for 800 cycles, or 240,000 miles. Respectable, but not better than the vehicles on the road today.
- Low Temperature Operation: They show discharge curves at 0 to -30 degrees Celsius, achieving 90 - 130 mAh/g active specific capacity. Comparing to NMC811 active specific capacity of 200 Ah/kg, the available current is from 45 - 65% of the room temperature capacity, but with an accompanying significant voltage drop. Based on voltage drop, capacity loss and the low rate of this test, this author estimates between a 50 – 80% loss in range during cold months. Also, note that the temperature capability of solid state batteries is VERY temperature sensitive - thus the power and cycle tests at 30 and 45 degrees above would have been significantly worse if run even a few degrees lower.
- Low Temperature Life: They show 100 or so cycles at -10 degrees C. Respectable, except that these cycles are at C/5 charge and C/3 discharge. Thus, not 80% in 15 minutes, but rather 5% charge in 15 minutes.
- Energy Density: They talk about being able to get to an energy density of 400 Wh/kg, which would be great. However, they clearly have not yet, as all their graphs are normalized to 100%, not to an actual capacity. And Amprius is already making cells with 450 Wh/kg, and Tesla claimed on their Battery Day that they could achieve 350 Wh/kg. So, while nice, this energy density they hope to achieve in 2028 will not beat today's state of the art, and will not be state of the art when it is achieved.
Other Significant Challenges
There are other challenges they do not mention, which will have to be overcome before they can put the first car in the field. Remember that they have spent $300 million so far, so these are not challenges that they didn't have the resources to address, but rather ones they have not solved yet and so remain silent about. Many of these are related, and come from the fact that they are using a brittle, ceramic electrolyte. These include:
- Multi-layer cells: They have been unable to make multi-layer cells. My expectation is that it is because of the unstable interface between the cathode, which expands as much as 10% on discharge, and the solid state electrolyte, which will not expand at all. They likely do their cycling under high isostatic pressure (remember the 3.4 atm mentioned earlier?), which will not flow through to inner layers. The inner layers will also be more rigidly constrained, so suffer more from the interfacial decay with cycling. Needless to say, 100,000 of their tiny pouch cells will never make a practical vehicle. It's important to mention here that, if your technology works, making a multilayer pouch cell is an easy afternoon's work.
- Vibration and Dendrites: The electrolyte is very, very stiff. It is well documented that dendrites will not grow through solid, single crystal garnet electrolytes. However, they grow freely at grain boundaries and defects. In their pristine, temperature and pressure controlled and vibration-free labs, they can get the cells to cycle. But in a rugged SUV or on our terrible South Carolina roads, cracks and other defects will become plentiful and dendrites will grow. This will in the best case destroy cycle life, and in the worst cause the battery to explode.
- Lithium Metal Ignition: They tout using lithium metal to increase energy density. But they don't mention that lithium metal auto-ignites at 179 degrees Celsius, generating 200 - 300 kJ/mol, or 30 - 40 kJ/g, a massive amount of energy - about three times higher than ethylene carbonate, a common component of lithium ion electrolytes. Pure lithium is the second most energetic element behind beryllium, and could be used as a component of rocket fuel (with an oxidant). In essence, they have replaced a burning separator and electrolyte for a much more flammable and energetic burning anode. There is plenty enough energy in the battery to raise the lithium to its ignition temperature, and if exposed to oxygen or water, it will likely ignite itself. There is plenty of oxygen available in the cathode materials. Here are some references on lithium metal autoignition (Reference 1, Reference 2, Reference 3, Reference 4)
- Cost: They claim lower cost, but are actually eliminating only one of the least expensive components - graphite. While this is true, they will have the added cost of building up their thin ceramic electrolyte and sintering it at high temperatures. My guess is that early on, their yields will be just terrible, if they can achieve production scale at all.
Given their success so far and their access to capital, I do think QuantumScape will succeed in getting a battery to market. However:
- It will have lower energy density than Amprius has achieved today.
- It will likely first show up in watches and wearables, then maybe phones.
- It will take much longer and cost much more to scale than they think.
- It will not be able to withstand the aggressive automotive environment.
- It will be far more expensive than today's lithium ion batteries, and will likely never achieve lower cost than contemporary lithium ion batteries.
- Once a suitable cell size is made, it may not be any safer than today's lithium ion batteries.
What upside can you expect? Here are some achievable goals:
- The cells will be popular in portable electronics.
- The cells may enable urban energy storage, allowing our coastlines to stop their brown outs and weather their storms.