Dou brought a cell to the interview. Sixty-five ampere hours, pouch format, 315 watt hours per kilogram.
What he wanted to describe was not the energy density but a test. Put two of those cells next to each other with nothing between them but a very thin insulating layer, no further heat management of any kind, and heat one until it goes to explosion. The other one stays intact.
No thermal propagation. He calls it the distinctive safety trick of the cell, and it is the claim the rest of the conversation is built on.
The same cell takes a 6C rate at 100% depth of discharge and charges from 10 to 80% in about 15 to 16 minutes. That is worth pausing on, because rate is the standing objection to solid state: ions crossing a solid phase do not move as fast as ions in a liquid. Dou concedes the point directly, and says it is exactly why sulfides attract so much interest, since lithium transport in sulfide is quick.
Fifty years old, and only now in demand
Solid state is not a recent idea. Polymer-based solid-state batteries were reported in scientific journals in the 1970s, and fifty years of improvement have followed. The inorganic route started more than thirty years ago, with Toyota beginning research on oxide and sulfide electrolytes in the late 1990s.
What changed was not the science but the demand for it. The energy transition sent people looking for storage with higher energy density and a better safety profile, and solid state offers both. Startups spun out of research institutions across the US and Europe. Large Chinese players moved into semi-solid-state cells.
Fifty years of work has produced very little commercial product, and Dou's explanation is the interface. In a solid-state cell, lithium ions travel through three solid phases, cathode, anode and electrolyte, which adds two solid-to-solid interfaces that a liquid cell does not have. He lists four failure modes that live there: space charge separation, element interdiffusion, interface reaction, and electrochemical mechanical deformation.
He takes the last one as his example. It behaves like thermal expansion in a solid, where volume follows temperature. In a cell, the electrolyte changes volume as cycling proceeds, and that opens voids and cracks between the electrode surface and the electrolyte surface. Dendrites grow in those voids.
Everything people have added to solid-state cells has been an attempt to mitigate that. Nanoparticles in the early days. Later, some liquid electrolyte, which is where the semi-solid and hybrid cells came from in the first place.
The chemistry that makes a lithium-ion battery burn
Dou's safety argument is not a comparison of test results. It is a mechanism, walked through step by step, and it starts on the very first cycle of a liquid lithium-ion cell.
Lithium atoms forming at the anode are highly reactive, and they react with the liquid electrolyte to form lithium hydride. The hydrogen in that hydride sits at minus one valence and will react with nearly anything containing a proton to release hydrogen gas. Dou's description is that the cell now holds a hydrogen reservoir.
Separately, forming the SEI layer decomposes the organic liquid electrolyte into carbon monoxide, methane, ethylene and acetylene. All of them explosive.
Then the temperature. Once a cell self-heats to around 200 degrees Celsius, the cathode material starts to release oxygen.
His summary of what the cell now contains is a list read aloud: hydrogen, carbon monoxide, methane, ethylene, acetylene, oxygen, and high temperature. That is why thermal runaway in a lithium-ion cell is an explosive reaction rather than a slow failure. It is not an unfortunate side effect of the design. It is what the design assembles.
An inorganic electrolyte, oxide or sulfide, contains no hydrogen at all, so the first step never happens and the reservoir is never built. In a polymer electrolyte the interface is solid against solid, which makes that side reaction very slow, and the polymer can be designed with less proton content or none. Dou is careful to file the proton-free version as a target for the future rather than something in hand.
Where the cost actually sits
There are two cost drivers, materials and the manufacturing process, and Dou separates the routes by both.
Oxides need high temperature to form their ceramic inorganic structure, which is energy intensive. Sulfides are sensitive to oxygen and moisture and need a clean room. Neither route has a supply chain built yet, and the manufacturing equipment for both would have to be built from scratch, which makes both capital intensive.
Polymers are organic. No rare metals, no expensive materials. His precedent for what happens next is the liquid electrolyte, whose cost has fallen 97% in the thirty years since commercialisation, on economy of scale alone. He repeats the figure to make sure it lands.
The manufacturing half of the argument is the more interesting one. Because the polymer is soft, it is largely compatible with conventional liquid lithium-ion production. In Hytzer's process, 80% of the manufacturing steps are the same as a liquid line. Around 20% is the special step that makes the polymer electrolyte, and the supply chain does not have to be rebuilt around it.
Asked what the 20% is, Dou gives an answer that is a single substitution. A conventional line fills the cell with liquid electrolyte. Hytzer injects a precursor of the polymer, together with additives and lithium complex salts, as a eutectic liquid. Warm it to an elevated temperature and it polymerises in place. Only that step is different.
He also draws a distinction that matters more than it sounds. People conflate a polymer with a gel. A gel is long polymer chains entangled into a three-dimensional network that traps solvent inside, and heating it or shifting its pH releases that liquid. A polymer is simply soft. Heat it and it stays solid, and there is no small molecule to come out of the system. The difference between the two is molecular weight: small molecules make a liquid, and the same chemistry at higher molecular weight is a solid.
A rigid skeleton holding a soft electrolyte
The technical claim underneath the product is about contact.
An inorganic electrolyte forms a crystalline structure full of voids, which Dou describes as a mesh: lithium ions are small enough to pass through. The problem is that the mesh is rigid and the electrode is rigid, and rigid meeting rigid leaves interface problems, which is why people add other materials to the electrolyte to mitigate them.
A polymer at its working temperature is soft, makes very good contact, and addresses that problem well. It creates a different one. Soft material lacks mechanical strength, so the cell surface is not stable, and a compressed cell can squeeze the electrolyte out from between the electrodes and short internally.
Hytzer's answer is to have both. A rigid skeleton designed into the molecular structure provides the stability of the cell, with the soft polymer part grown in place by in-situ polymerisation so that the electrolyte forms outward from the electrode surface. Contact is good from the start, and the skeleton keeps the cell stable under compression.
What has already been in the field
The cost of all this puts the cells at the high end first: flying taxis, where the safety standards and the energy density demands are both severe, and premium EVs. That was the target from the beginning. Mass market comes when scale builds and cost falls.
Earlier generations have been out doing work. One powered a lander at the bottom of a deep ocean trench, more than 10,000 metres down, running continuously for 26 days, which Dou says broke the world record. Another powered an autonomous underwater vehicle. On the ground, an EV research project finished more than 11,000 kilometres of on-road testing. The current focus is getting the 65 ampere hour cell into mass production.
The third generation uses a lithium metal anode and reaches 400 to 500 watt hours per kilogram. It flew a heavy-lift drone carrying a 25 kilogram payload for around an hour, against under 20 minutes on a standard battery.
Dou does not sell the lithium metal. Cycle life is a problem, and lithium metal is sensitive to moisture and oxygen, which forms lithium hydride again, and more lithium makes that more dangerous.
So the fourth generation is anode-free, and the exchange about what that means is the most careful part of the conversation. There is no lithium metal foil in the cell. On the anode side there is only the current collector. All the lithium comes from the cathode, travels across during charging, picks up electrons at the collector and is reduced to lithium atoms there. Pressed on whether that is lithium metal in the end, Dou concedes the point with a qualification: it does not form a large batch of metal, only a thin layer, formed in place.
This piece draws on the full conversation, which is available with a complete transcript on the episode page.