Solid electrolytes: garnet LLZO, argyrodite Li₆PS₅Cl and LGPS

In a lithium-ion cell a liquid organic electrolyte carries Li⁺ between the electrodes. Lithium metal, with the lowest reduction potential (−3.04 V vs SHE), would be a far better anode than graphite (372 mAh g⁻¹), but in liquids it grows dendrites that can cause fires. Replacing the liquid with a solid electrolyte that also acts as the separator is meant to suppress dendrites and make all-solid-state lithium metal batteries safe (Lee H. et al. 2024).

A useful solid electrolyte needs high ionic conductivity at room temperature, a wide electrochemical window and chemical compatibility with lithium metal and the cathode. The candidates fall into three families: oxides, sulfides and polymers (Lee H. et al. 2024). This page sits in the battery materials hub.

Garnet Li₇La₃Zr₂O₁₂ was reported as a fast lithium-ion conductor by Murugan, Thangadurai and Weppner in 2007 (Murugan et al. 2007). It is attractive for its stability against lithium metal, wide electrochemical window and easy handling (Lee H. et al. 2024; Fukuda et al. 2025).

Undoped LLZO is tetragonal at room temperature: its lithium ions are ordered and the conductivity is poor. Near 650 °C it turns cubic (Ia3̅d), with disordered lithium and a much higher conductivity (Kang et al. 2025; Tong et al. 2015). To keep the cubic phase at room temperature, aliovalent dopants are added that create lithium vacancies and disrupt the ordering (Kang et al. 2025):

• Ga sits on lithium sites; above 0.20 Ga per formula unit the garnet is fully cubic, and Li₇₋₃ₓGaₓLa₃Zr₂O₁₂ reached 1.46 mS cm⁻¹ at 25 °C (Wu et al. 2017).
• Ta substitutes for Zr; Al-free Li₆.₄La₃Zr₁.₄Ta₀.₆O₁₂ gave 0.72 mS cm⁻¹ bulk conductivity at 25 °C with an activation energy of 0.26 eV (Tong et al. 2015). Ta-doped LLZO kept its cubic phase in contact with Li–Si at 500 °C, where Ga-doped LLZO reverted to tetragonal (Kang et al. 2025).
• Al-doped cubic LLZO densified by hot pressing exceeded 0.45 mS cm⁻¹ (Fukuda et al. 2025).

In S cm⁻¹ these doped garnets span roughly 2 × 10⁻⁴ to 1.5 × 10⁻³ S cm⁻¹ (calculated from the reported mS cm⁻¹ values).

Sulfide electrolytes conduct lithium about as well as liquids. Li₁₀GeP₂S₁₂ (LGPS), with a new three-dimensional framework, reached 12 mS cm⁻¹ at room temperature, the highest of any solid electrolyte at the time and above liquid organic electrolytes (Kamaya et al. 2011).

Argyrodite Li₆PS₅Cl combines a conductivity of about 10 mS cm⁻¹ with mechanical softness, which makes it the flagship electrolyte for all-solid-state batteries (Kim et al. 2026). Liquid-phase synthesis gives particles below 100 nm with 2.4 mS cm⁻¹ that improve contact in composite cathodes (Zhang et al. 2026).

The drawbacks: even the trace moisture of an industrial dry room degrades the surface into an oxygen-rich layer and cut the conductivity by 36% within three days (Kim et al. 2026); against lithium metal Li₆PS₅Cl keeps decomposing into Li₂S and Li₃P (Lee S. et al. 2026); and sulfides have a narrow electrochemical window (Lee H. et al. 2024).

Poly(ethylene oxide) dissolves lithium salts through its ether oxygens and is the most studied dry polymer electrolyte host; it is flexible and can be made into self-supporting films. But ions move only in its amorphous regions, PEO is mainly crystalline below 65 °C, so room-temperature conductivity is low, and the Li⁺ transference number is generally below 0.5, which causes concentration polarisation (Lee H. et al. 2024). Thin PEO layers are also used as soft buffers between LLZO and lithium (Lee H. et al. 2024).

• Point contacts. A rigid ceramic touches lithium only at points, so the local current density is high (Lee H. et al. 2024). Garnets also pick up Li₂CO₃ from air, which raises the interfacial resistance; polishing, polymer buffers and ALD coatings bring it down (Lee H. et al. 2024).
• Dendrites through ceramics. In LLZO, lithium grows along voids, cracks and grain boundaries (Hoinkis et al. 2023). Al, Ta and La segregate to grain boundaries during sintering, deplete lithium there and raise the electronic conductivity, which promotes lithium nucleation and dendrites (Yao et al. 2026). Porosity, electron leakage at grain boundaries and poor solid contact cause short circuits in LLZO cells (Cheng et al. 2026). A glass-ceramic Ta-LLZO with an amorphous grain-boundary phase reached a critical current density of 1.15 mA cm⁻² (Hoinkis et al. 2023).
• Dendrites through sulfides. Lithium deposits on existing cracks and defects; even Li–In alloy dendrites grow along pores and grain boundaries in Li₆PS₅Cl layers (Lee H. et al. 2024).

Silicon is an alternative anode for solid-state cells; see silicon anode.

For a paper, slide or poster, draw the cell as a stack: cathode composite (cathode particles mixed with electrolyte particles), a dense solid electrolyte layer, and a lithium metal anode, with a zoom on the anode interface showing point contacts, a dendrite running along a grain boundary and an interlayer that fixes it. In LabCanvas, the figure editor's Library has battery objects, including a solid-state cell you can edit and label, and the 3D Studio has battery models for a cell-level view; export the figure as a high-resolution image.