A solid cell needs pressure to keep working
A solid state battery replaces the liquid material that carries charged particles between the 2 ends of a normal lithium ion cell with a solid material instead, called a solid electrolyte. Without liquid to fill the microscopic gaps between the layers inside the cell, those layers have to be pressed together by outside force just to stay in contact, or the battery stops working. A battery test equipment maker states that sulfide based solid electrolytes, a softer material that can deform, need 1 to 10 megapascals of clamping pressure, a unit for measuring force spread over an area. Oxide based ceramic electrolytes, a harder material that cannot deform the same way, need 10 to 250 megapascals. The maker states this pressure requirement complicates cell stack design, limits which cell shapes are possible, and raises system cost.
Figures are the design pressure range a battery test equipment maker states for each electrolyte family, not a measurement of one named product.
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| Sulfide based | 1 to 10 megapascals |
| Oxide based ceramic | 10 to 250 megapascals |
Real programs already run inside that range
Three named solid state programs report the pressure their own test fixtures apply, collected by an independent battery engineering site. SES runs its fixture at 1.2 megapascals. Factorial runs its fixture at 1.3 megapascals. QuantumScape reports its fixture runs at 0.34 megapascals, below the sulfide range above, a single company figure rather than a sign the general range is wrong. Solid Power states only that its fixture runs above 5 megapascals, with no exact figure given.
Figures are the test fixture pressure each named program reports for itself, collected by an independent battery engineering site. Solid Power is not shown because it states only that its fixture runs above 5 megapascals, with no exact figure given.
Show the numbers
| SES | 1.2 |
| Factorial | 1.3 |
| QuantumScape | 0.34 |
A structural pack already claims the same job, for a softer cell
A structural battery pack builds the battery into the frame of the car, so the pack carries part of the load the frame used to carry alone. Volvo holds a granted United States patent for a structural pack design that presses conventional battery cells together with 20 to 200 kilonewtons per square metre of force, using 2 structural end pieces that also connect the pack to the rest of the car. That converts exactly to 0.02 to 0.2 megapascals. The design is built for ordinary rectangular cells that use a liquid electrolyte, not a solid one. The top of that range still sits below 0.34 megapascals, the test fixture pressure reported for QuantumScape above, and far below the oxide ceramic range described above.
One review names the opportunity and the problem, and settles neither
A peer reviewed engineering review states that the extra stiffness of a ceramic solid electrolyte could let battery cells carry structural load directly, the same idea a structural pack already proves works for a much softer material. The same review then states, in the next sentence, that a bonded structural pack may not be able to sustain the steady clamping pressure a solid cell needs to keep working through years of heating and cooling, and that ceramic electrolytes are brittle under the kind of impact a car body has to survive. No source states a projected mass or volume figure for a pack that actually puts a solid electrolyte cell inside a structural role, because nobody has built or announced one.
Researchers are also pressing sideways, not just clamping down
A separate study out of MIT found that 150 to 200 megapascals applied sideways across a solid electrolyte, not the perpendicular clamping pressure described above, was enough to bend off course a growing lithium metal dendrite, a needle like strand of metal that can otherwise grow through the electrolyte and short circuit the cell. Study lead author Cole Fincher, a materials scientist and electrochemical engineer at MIT, said this about the finding.
Understanding why and how solid electrolytes fail points us towards new ways to design better solid-state batteries that have higher performance and are more reliable.
Cole Fincher, materials scientist and electrochemical engineer at MIT, study lead author. Source 5.