What a solid electrolyte trades against
A battery electrolyte is the material lithium ions travel through between the 2 electrodes as a battery charges and discharges. The electric vehicle batteries sold today use a liquid electrolyte, which is flammable. A solid electrolyte replaces the liquid with a solid material. Researchers group the solid electrolytes tested for batteries into 3 families, sulfide, oxide and polymer. How fast lithium ions move through a material is called its ionic conductivity, measured in siemens per centimeter. The higher the number, the faster a battery can charge and discharge.
Sulfide conducts fastest, but needs a dry room
Sulfide electrolytes conduct lithium ions fastest. LGPS, a sulfide material in a peer reviewed comparison table, reaches 1.2 times 10 to the negative 2 siemens per centimeter at room temperature, matching a liquid electrolyte. That speed comes from a material that reacts with ordinary air. Producing it requires a moisture controlled space called a dry room, held below 1 part per million of moisture. An independent manufacturing study states that requirement adds 30% to 50% to the capital a plant needs to spend. Solid Power, a company building a sulfide electrolyte at automotive scale, names BMW, Ford and SK Innovation as partners.
Oxide survives air and heat, but needs extreme heat to make
Oxide electrolytes trade speed for stability. LLZO, an oxide material in the same table, is chemically stable and resists heat. It reaches 3.0 times 10 to the negative 4 siemens per centimeter at room temperature, about 40 times slower than LGPS, both figures from the same table. QuantumScape, a company building an oxide separator, states its ceramic is non combustible and safer than a polymer separator, because a hydrocarbon can burn. Turning the powder into a usable ceramic requires sintering, heating it until it fuses into a dense solid, above 1100 degrees Celsius. A separate study states that heat rules oxide ceramics out of continuous roll to roll manufacturing, a production line a liquid electrolyte or a polymer can use.
Figures are processing or sintering temperatures for each named material, from 1 peer reviewed comparison table. 2 materials, the Li2S P2S5 glass and LGPS, are stated as a single figure rather than a range, and are drawn here with the low and the high set to the same number.
Show the numbers
| Li2S P2S5 glass, sulfide | 280 degrees Celsius |
| Li6PS5Cl, sulfide | 300 to 400 degrees Celsius |
| LGPS, sulfide | 550 degrees Celsius |
| LATP, oxide | 800 to 900 degrees Celsius |
| LLZO, oxide | 1100 to 1200 degrees Celsius |
Polymer is easiest to shape, but needs heat to conduct
Polymer electrolytes are the easiest to process, and can be rolled into thin sheets on a production line the oxide ceramic cannot use. The tradeoff is conductivity. PEO, the most studied polymer material in the comparison table, reaches only 10 to the negative 5 siemens per centimeter at 25 degrees Celsius. Heated to 60 to 80 degrees Celsius, it reaches a workable 10 to the negative 3. Blue Solutions, a French company that states it has worked on solid state batteries since 1997, is developing its fourth generation of lithium metal batteries, targeting a start of production year of 2030. It states the headline goal for that generation is a polymer that works at room temperature, something its earlier generations did not do.
No material wins on all 3
Each family also differs in how many volts it can handle before it breaks down chemically, called its electrochemical stability window, which limits which cathode chemistries it can pair with.
The window is how many volts each material can handle before it breaks down chemically. LGPS, the fastest conducting material in this article, has the narrowest window here. Figures are named materials from 1 peer reviewed comparison table, not a general claim about either family, since the same table lists a different sulfide, the Li2S P2S5 glass, with a wider window than most of the oxides shown.
Show the numbers
| LLZO, oxide | 0 to 6 volts |
| Li2S P2S5 glass, sulfide | 0 to 5 volts |
| LATP, oxide | 2.8 to 4.2 volts |
| Li6PS5Cl, sulfide | 1.0 to 2.5 volts |
| LGPS, sulfide | 1.7 to 2.1 volts |
LGPS, the fastest conducting material named here, has the narrowest window, 1.7 to 2.1 volts. LLZO, the oxide, has the widest, 0 to 6 volts. No solid electrolyte in the 5 sources wins on speed, heat resistance and easy manufacturing at once.