Solid Electrolyte Comparative Matrix

The Solid State Battery page sets out the electrolyte taxonomy:

inorganic sulfides
oxides/garnets
halides
solid polymer electrolytes.

However, a taxonomy doesn’t tell an engineer which family to pick for a given application. The four families trade off against each other on:

conductivity
voltage stability
mechanical behaviour
difficulty to actually manufacture

The right choice depends on which of those constraints matters most for the application. This page puts representative numbers against each family side by side.

Property	Sulfides (LGPS / argyrodite)	Oxide / garnet (LLZO)	Halides (Li₃InCl₆ / Li₃YCl₆)	Polymer (PEO)
Bulk ionic conductivity, 25°C	~1–12 mS/cm (10⁻³–1.2×10⁻² S/cm)	~0.1–1 mS/cm (10⁻⁴–10⁻³ S/cm)	~0.05–5 mS/cm, composition-dependent	<0.1 mS/cm at RT; >10 mS/cm once melted above ~60–70°C
Electrochemical stability window vs Li/Li⁺	Narrow “true” window: reduces ~1.7 V, oxidises ~2.1 V	Thermodynamically wide (~0–6 V) but reactive at direct metallic-Li contact	Oxidatively robust to ~4.2 V; reductively unstable against Li metal	Practical window ~4.5 V; oxidative degradation accelerates above ~3.8–4 V
Young’s / shear modulus	Soft: E ≈ 22–28 GPa, G ≈ 8–10 GPa	Rigid ceramic: E ≈ 140–146 GPa, G ≈ 56–61 GPa	Similar order to sulfides; cold-pressable	Compliant, viscoelastic, well under 1 GPa
Moisture sensitivity / processing	Hydrolyses to H₂S; industrial dry rooms need roughly −45 to −60°C dew point	Stable in ambient air short-term; slowly forms a Li₂CO₃ surface layer	Hydrolyses, but without H₂S evolution; still needs dry handling	Can be processed in ambient / low-humidity air — easiest of the four to handle

Conductivity is the headline number, but it’s less differentiating than it looks: sulfides such as Li₊GeP₂S₁₂ (LGPS) were the first solid electrolyte to match liquid-electrolyte conductivity, reaching 1.2×10⁻² S/cm at room temperature (Kato et al., Adv. Energy Mater. 2020, reviewing Kamaya et al.’s original 2011 result), and argyrodite Li₊PS₅Cl commonly reaches several mS/cm once optimised for synthesis route and density. Garnet LLZO and the halides sit roughly an order of magnitude lower in their un-doped forms, and PEO is another two to three orders of magnitude below that at room temperature. This is precisely why every commercial PEO cell (Blue Solutions/Bolloré’s bus batteries being the best-known example) run hot, typically 60–80°C, rather than at ambient.

Electrochemical stability window is where the “true vs. apparent” distinction from the parent page matters most. Sulfides look narrow on paper, reduced at ~1.7 V and oxidised at ~2.1 V vs Li/Li⁺ (first-principles decomposition studies). This would rule them out against both lithium metal and a 4V-class cathode. In practice, cells work anyway because the decomposition products (mixed Li₂S/Li₃P and polysulfide phases) form a passivating, self-limiting interphase rather than continuing to react (operando XPS studies). Halides invert the problem: oxidatively robust to over 4 V, which is why they’re increasingly used as a cathode-side buffer layer rather than a bulk separator, but reductively unstable against Li metal (Li₃YBrₓCl₆₋ₓ study).

Mechanical behaviour drives the pack-level pressure requirements covered on the parent page. Sulfides are genuinely soft for a ceramic, experimentally measured Young’s modulus for Li₊PS₅Cl is around 28 GPa (ACS Appl. Energy Mater. 2024), close to first-principles predictions of ~22–27 GPa (J. Phys. Chem. C 2025). This allows cold-pressing at modest stack pressure instead of high-temperature sintering. LLZO is roughly five times stiffer (Young’s modulus ~140–146 GPa, shear modulus ~56–61 GPa: Wolfenstine et al.), which is mechanically favourable for dendrite suppression but forces sintering above 1000°C and makes the resulting pellet brittle.

Moisture sensitivity is the most operationally expensive difference between the families. Sulfides hydrolyse to release toxic H₂S gas, and while academic gloveboxes run at −80°C dew point, industrial-scale dry rooms are more commonly cited at around −45 to −60°C depending on the specific compound and acceptable H₂S concentration (Randrema et al., Batteries & Supercaps 2024; quantification study, ACS Appl. Energy Mater.) Substantially more stringent, and more costly to build, than the ~−40°C dry rooms already standard for conventional lithium-ion manufacturing. Argyrodite compositions evolve roughly eight times less H₂S than Li₄P₃S₁₁ under equivalent exposure, which is part of why argyrodite rather than LGPS has become the more common commercial sulfide choice despite LGPS’s higher peak conductivity. Halides avoid H₂S entirely but still hydrolyse and need dry handling; oxides and polymers are the only families that tolerate real ambient-air processing.

Needs review: figures above are representative ranges drawn from the cited studies, not a single canonical source — actual conductivity, ESW, and modulus for any given commercial cell depend heavily on dopant, density, and synthesis route. Treat these as order-of-magnitude comparisons for architecture decisions, not datasheet-grade specs for a specific supplier’s material.