What a normal battery moves lithium through
A standard lithium ion battery moves lithium ions between 2 electrodes, the parts that send and receive electric charge, through a liquid electrolyte, a lithium salt dissolved in a solvent, soaked into a porous separator that keeps the electrodes apart, according to Battery University, an independent technical reference site.
A solid state battery replaces that liquid with a solid material instead. A materials science review groups those solid materials into families, including polymers, oxides, sulfides, halides and garnets, and states sulfide and halide types carry current best. A solid polymer electrolyte has reached an ionic conductivity, a measure of how well a material carries current, of 0.114 milliSiemens per centimeter, lower than a liquid electrolyte but still enough to work in a battery, the review states.
The safety case, and where it still breaks
Removing the liquid removes a fuel source for thermal runaway, a heat chain reaction that can end in a fire, the review states. A solid electrolyte can also act as a barrier that blocks a dendrite, a needle like filament of lithium metal that can grow through a battery and cause a short circuit. A solid barrier does not remove the danger on its own.
Haegyeom Kim, a materials staff scientist at Lawrence Berkeley National Laboratory, found in a peer reviewed 2025 study that where a small void opens inside a solid electrolyte, lithium metal fills it and a dendrite can still grow through and short circuit the cell. Kim and University of California Berkeley collaborators fixed it with an added tin and carbon buffer layer. Tin attracts lithium, a property called lithophilic, and the position of that layer decides where new lithium metal forms.
We realized that the tin acts as a lithophilic layer like silver, so the tin positioning is important, as that's where the plating happens,
Haegyeom Kim, a materials staff scientist at Lawrence Berkeley National Laboratory. Source 5.
The buffered cell completed 450 charge and discharge cycles, 1 full charge and 1 discharge counted as 1 cycle, without a dendrite related failure, where an unbuffered cell in the same test short circuited.
These are 3 different cells under 3 different test conditions, not a single improvement curve. Battery University reports a general claim that early solid state prototypes reach about 100 cycles, source 2. A Lawrence Berkeley National Laboratory and University of California Berkeley test with an added buffer layer completed 450 cycles without a dendrite short circuit, source 5. QuantumScape reports more than 1,000 cycle equivalents for its own 24 layer prototype cell, source 4, plotted here at 1,000.
Show the numbers
| Early prototypes | 100 |
| Berkeley Lab buffer test | 450 |
| QuantumScape 24 layer cell | 1,000 |
How much headroom the chemistry has in theory
The numbers above describe cells that were built and tested. The same review also states theoretical ceilings for 3 different battery chemistries, the highest energy density each could reach under ideal conditions, far above what any shipping cell reaches today.
All 3 figures come from 1 review paper, in the same unit. The best lithium ion cells available now are stated as a range of 340 to 580 watt hours per kilogram, and this chart plots the top of that range. Lithium sulfur is a different battery chemistry from the lithium metal design described elsewhere in this article, and its ceiling of up to 2,600 describes that chemistry alone, never the general solid state design this article is about. The aircraft figure is a stated industry requirement of greater than 2,000, not a measured battery result, plotted here at 2,000.
Show the numbers
| Best lithium ion cells available now | 580 |
| Lithium sulfur with a solid electrolyte | 2,600 |
| What electric aircraft need | 2,000 |
What the furthest along real design keeps
QuantumScape, the company with the most detailed public solid state design, names 4 parts, a solid ceramic separator, a lithium metal anode, the negative electrode, a cathode, the positive electrode, and a liquid electrolyte for the cathode, which it calls a catholyte. The company couples that ceramic separator with the liquid catholyte, so its own design still carries 1 liquid even after replacing the separator and the anode with solid materials.
The company states its ceramic separator does not burn, unlike a standard polymer separator, which can burn. QuantumScape targets a commercial energy density of 800 to 1,000 watt hours per liter, how much energy a cell packs into a given volume, as of December 2023. Its own 24 layer prototype cell has completed more than 1,000 charge and discharge cycles with more than 95% energy retention, a company reported result.
Dates the industry has already missed
Bosch research laboratories once predicted a solid state battery would reach the market by 2020 and be used in cars by 2025. Battery University own page, last updated in 2024, records that prediction as unmet. The International Energy Agency states in a 2024 report that solid state batteries are on track to be commercially available beyond 2030.
Almost every number in this article describes a laboratory result, a company target or an industry prediction still to be met, not a battery in a car a reader can buy today.