All-solid-state systems with solid electrolytes (SEs) are potential candidates for next-generation batteries and are expected to provide a high power and energy density with reliable and improved safety characteristics. Sulfide-based lithium-ion conductors have the advantages of high conductivities together with suitable electrochemical windows and mechanical properties; thus, they are intensively studied as promising SEs. Li₁₀GeP₂S₁₂ (LGPS) is a new member in the crystalline sulfide electrolyte family and exhibits an ionic conductivity of 1.2 × 10⁻² S cm⁻¹, which is comparable to that of organic liquid electrolytes. The all-solid-state battery, LiCoO₂/LGPS/In−Li, showed excellent charge−discharge characteristics using the LGPS electrolyte. However, Ge is a relatively expensive element and could limit the widespread use of LGPS materials.

The type of crystal structure is an important component in the design of ionic conductors. Materials with similar types of structures and high ionic conducting solids might provide high conducting characteristics. The LGPS-type structure is suitable for high ionic diffusion along the one-dimensional tunnels and/ or two-dimensional planes that participate in high ionic diffusion. Si- and Sn-based analogous with Ge-free materials might be promising as SEs for practical applications.

—Sun et al.

The atomic arrangement of the new material named LSSPS. Two representations of the new germanium-free material with the structure Li10.35[Sn0.27Si1.08]P1.65S12 (Li3.45[Sn0.09Si0.36]P0.55S4.

Compared to conventional lithium-ion batteries that contain lithium ion conducting liquids, all-solid-state batteries of the future promise a suite of advantages: improved safety and reliability, higher energy storage and longer life cycles.

The discovery of superionic conductors—solid crystals that enable fast movement of ions—is spurring the development of such batteries, but promising designs have so far relied on the use of rare metals such as germanium, making them too expensive for large-scale applications.

Due to its high chemical stability and ease of fabrication, Kanno says that the new material widens the possibilities of fine-tuning solid electrolytes to meet diverse industry and consumer needs.

In 2011, Kanno and his team, working in collaboration with Toyota Motor Corporation and Japan's High Energy Accelerator Research Organization (KEK), published a landmark paper in Nature Materials that introduced a solid electrolyte with the structure Li₁₀GeP₂S₁₂ (LGPS). This material became an important forerunner in the race to develop viable all-solid-state batteries. It exhibited an ionic conductivity of 1.2 x 10^-2 S cm^-1 at room temperature, a level comparable with—and even exceeding some—liquid electrolytes used in existing batteries.

The team went on to design other solid electrolytes based on the same LGPS crystal structure, with promising results.

In their latest study, the researchers kept the same framework structure of LGPS, and finely adjusted the ratio and positioning of the tin, silicon and other constituent atoms. The resulting material LSSPS (composition: Li_10.35[Sn_0.27Si_1.08]P_1.65S₁₂ (Li_3.45[Sn_0.09Si_0.36]P_0.55S₄)) achieved an ionic conductivity of 1.1 x 10-² S cm^-1 at room temperature, almost reaching that of the original LGPS structure.

Although further work will be required to optimize performance for different usage purposes, the new material raises hopes for low-cost production without sacrificing performance.

Cyclic voltammogram and charge-discharge curves. The material exhibits high stability and cycling ability, with good capacity retention during 20 cycles. Click to enlarge.

Resources

• Yulong Sun, Kota Suzuki, Satoshi Hori, Masaaki Hirayama, and Ryoji Kanno (2017) “Superionic Conductors: Li_10+δ[Sn_ySi_1–y]_1+δP_2−δS₁₂ with a Li₁₀GeP₂S₁₂-type Structure in the Li₃PS₄–Li₄SnS₄–Li₄SiS₄ Quasi-ternary System” Chem. Mater. doi: 10.1021/acs.chemmater.7b00886