Designer solid electrolytes for no-liquid lithium batteries

Such solid batteries could offer better battery safety, but inferior ion conductivity is hampering their development compared with, for example, conventional liquid electrolyte Li-ion cells.

Inferior conductivity can be intrinsic to the electrolyte, or due to, for example, poor wetting of the cathode resulting in ionic resistance across the interface – poor wetting prevents the electrolyte ‘reaching into’ thick cathodes.

“Many studies have shown that inorganic ionic conductors tend to show better ion conductivity after multi-element substitution, probably because of the flattened potential barrier of Li-ion migration, which is essential for better ion conductivity,” said project leader Professor Ryoji Kanno.

The team set out to establish design rule for synthesising high-entropy crystals of lithium ion conductors using element substitution.

It took inspiration from the chemical compositions of two existing Li-based solid electrolytes: argyrodite (Li₆PS₅Cl) and LGPS (Li₁₀GeP₂S₁₂) crystals.

By substituting elements in the LGPS-type material Li_9.54Si_1.74P_1.44S_11.7Cl_0.3, the researchers created crystals with composition Li_9.54[Si_1−δM_δ]_1.74P_1.44S_11.1Br_0.3O_0.6, where M is either germanium or tin, and δ can be different values between 0 and 1.

With germanium and δ=0.4, the new material was used as a catholyte in a solid-state lithium battery with 1mm or 0.8mm thick cathodes

Discharge capacity was 26.4mAh/cm² at 25°C for 1mm cathodes and 17.3 mAh/cm² at -10°C for 0.8 mm, “with the area-specific capacity 1.8 and 5.3 times larger than those reported for previous state-of the-art all solid-state lithium batteries, according to the university. “Theoretical calculations suggested that the enhanced conductivity of the solid electrolyte could be a result of the flattening of the energy barrier for ion migration, caused by a small degree of chemical substitution in the crystal.”

“In effect, the proposed design rule lays a solid groundwork for exploring new super-ionic conductors with superior charge-discharge performance, even at room temperature,” said Kanno.