Two cells based on the novel solid electrolytes performed very well in trials in comparison with lithium-ion batteries. The cells remained stable and operated consistently at a range of temperatures between -30 and 100 °C. They exhibited high energy and high power densities, and very small internal resistance levels. Their properties would allow the cells to be stacked close together without interference.

Further, the cells exhibited ultrafast charging, retained their charge for lengthy periods, and appeared to have a long lifespan with excellent cycling ability (after over 500 cycles, the cells retained around 75% of their initial discharge capacity).

Most traditional batteries rely on the flow of ions through a liquid electrolyte between two electrodes. However, batteries incorporating a liquid electrolyte are prone to problems, including low charge retention and difficulties in operating at high and low temperature. Previous designs for solid electrolytes have shown promise, but have proven expensive and some have exhibited problems with electrochemical stability.

Compared with lithium-ion batteries with liquid electrolytes, all-solid-state batteries offer an attractive option owing to their potential in improving the safety and achieving both high power and high energy densities. Despite extensive research efforts, the development of all-solid-state batteries still falls short of expectation largely because of the lack of suitable candidate materials for the electrolyte required for practical applications.

—Kato et al.

Yuki Kato and his team synthesized two new lithium-based superionic materials based on a crystal structure previously discovered by the same team. They studied these crystal structures using Synchrotron X-ray diffractometer, BL02B2, at SPring-8 and neutron diffractometer iMATERIA(BL20) at J-PARC.

a) Ionic conductivity of new superionic conductor, Li9.54Si1.74P1.44S11.7Cl0.3, developed in this project (together with those of the materials with Li10GeP2S12 (LGPS) analogue structure). New Li9.54Si1.74P1.44S11.7Cl0.3 exhibits 25 mS cm-1 at room temperature. This value is two times higher than that of original LGPS. b,c) Crystal structure and lithium conduction pathway of Li9.54Si1.74P1.44S11.7Cl0.3. The 3D conduction pathway in the structure causes high ionic conduction characteristics of new superionic conductor. Kato et al. Click to enlarge.

Superionic materials are solid crystal structures through which ions can hop easily, essentially maintaining a flow of ions similar to that which occurs inside a liquid electrolyte.

Both superionic materials developed by the team showed extremely high ionic conductivity and high stability. The researchers used their two new solid electrolytes to create two battery cell types; one high-voltage cell and one cell designed to work under large currents. Both all-solid-state cell types exhibited superior performance compared with lithium ion batteries. Kato’s team found that the cells provided high power density, with ultrafast charging capabilities and a longer lifespan than existing battery types.

Although the technology requires further development before it is commercially available, these results indicate that all-solid-state batteries may soon provide a much-needed boost to applications requiring stable, long-life energy storage, such as electric vehicles.

The addition of high energy electrodes into the solid-state cells could enhance the power of the batteries still further. Also, processing technology to complement the batteries that would allow for battery stacking is required before such configurations could be fully tested. Kato and his team are hopeful that their new materials will pave the way for all-solid-state batteries for multiple applications, including long-distance electric vehicles, in future.

History of lithium superionic conductors. The low power characteristics of all-solid-state batteries, due to their higher solid electrolyte-resistivity than conventional liquid electrolyte, still remain unsolved. The search for materials suitable for creating solid electrolytes has already produced some prototypes. So far, these superionic materials, which allow ions to move quickly and freely through their crystal structure, have been developed using the expensive element germanium—researchers are therefore keen to find alternative superionic conductors that could provide the basis for all-solid-state batteries. Source: Tokyo Institue of Technology. Click to enlarge.

This research is partially supported by New Energy and Industrial Technology Development Organization, Japan (NEDO).

Resources

• Yuki Kato, Satoshi Hori, Toshiya Saito, Kota Suzuki, Masaaki Hirayama, Akio Mitsui, Masao Yonemura, Hideki Iba & Ryoji Kanno (2016) “High-power all-solid-state batteries using sulfide superionic conductors” Nature Energy Article number: 16030 doi: 10.1038/nenergy.2016.30