Press Release

Development of a Superionic Conductor Exhibiting the Highest Known Lithium-Ion Conductivity

—Shed Light on the Practical Application of All-Solid-State Lithium Batteries with High Safety—

Key points of this announcement

• ・Discovered a superionic conductor exhibiting the highest known lithium-ion conductivity
• ・Elucidated a superionic conduction pathway in the three-dimensional framework of the superionic conductor
• ・A superionic conductor is applicable to all-solid-state ceramic batteries with noncombustibility and high safety

Professor Ryoji Kanno and Lecturer Masaaki Hirayama of the Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology and colleagues from Tokyo Institute of Technology, Toyota Motor Corporation, and the High Energy Accelerator Research Organization have discovered a superionic conductor^*1 exhibiting the highest lithium-ion conductivity reported to date. The lithium-ion conductivity of this superionic conductor (Li₁₀GeP₂S₁₂) was found to be 12 mS cm⁻¹ at room temperature (27℃), a value twice as high as that of an existing lithium ion conductor, Li₃N (6 mS cm⁻¹) and also exceeding even the ionic conductivity of organic electrolytes^*2, which are used in conventional lithium-ion secondary batteries. Additionally, the crystal structure of Li₁₀GeP₂S₁₂ was elucidated using the high resolution, high intensity neutron diffractometer (SuperHRPD) at the Japan Proton Accelerator Research Complex (J-PARC)^*3, and a superionic conduction pathway^*4 in the three-dimensional framework structure of Li₁₀GeP₂S₁₂ was clarified. This discovery indicates that all-solid-state ceramic batteries^*5 with properties of noncombustibility and high safety are promising candidates for next generation high energy density batteries. Development of such batteries is a fiercely competitive area of research as they represent key devices for the success of electric and hybrid vehicles and smart grids, thereby providing a new guiding principle in battery development. Results of the present study were published in Nature Materials on July 31, 2011(UK time).

Results
In the present study, this research team has discovered a new sulfide material, Li₁₀GeP₂S₁₂, which is a superionic conductor with the following properties: 1) noticeably high lithium-ion conductivity, 12 mS cm^-1 at room temperature (27℃); 2) decomposition voltage exceeding 5 V (Fig. 1); and 3) functionality as an electrolyte material of all-solid-state batteries. In particular, this new material has superior ionic conductivity far exceeding that of organic electrolytes (Fig. 2). Moreover, precision neutron crystallographic analyses performed using the high resolution bank of the high resolution, high intensity neutron diffractometer, SuperHRPD (BL08), at the Japan Proton Accelerator Research Complex (J-PARC) revealed that Li₁₀GeP₂S₁₂ has a unique three-dimensional structure (Fig. 3) in which lithium forms a series of chain-like units in its framework, yielding high lithium-ion conductivity to this material.

Background
Batteries are key electricity-storing devices that are critical to enabling the widespread introduction of electric vehicles, plug-in hybrid vehicles, and smart grids into society. Developing the next generation of batteries that is superior to the existing lithium-ion batteries in terms of capacity, cost, and safety has become an urgent task. Electrolytes represent the key to developing batteries with these desirable properties. Existing lithium-ion batteries require safety devices because they involve the use of combustible organic electrolytes. Batteries composed of ceramic material alone are regarded as devices that would have ultimately superior stability; such batteries would be safe, reliable, superior, and long-lasting devices with higher capacity and higher output. However, properties of solid electrolytes prevent the practical utilization of ceramic batteries. In particular, the ionic conductivity of existing solid electrolytes is approximately 0.1 to 1 mS cm⁻¹, a value that is one or more order of magnitude lower than that of organic electrolytes.

Materials and Methods
Examination of sulfide systems that were expected to have ionic conductivity as high as a superionic conductor led to the discovery of the superionic conductor Li₁₀GeP₂S₁₂, which exhibited high ionic conductivity in the course of searching for new materials. The crystal structure of Li₁₀GeP₂S₁₂ was determined by neutron diffractometry using the high resolution, high intensity neutron diffractometer SuperHRPD (BL08) at J-PARC. Moreover, the team revealed that batteries equipped with LiCoO₂, which is commonly used as a cathode of lithium-ion batteries, exhibit superior properties, indicating the applicability of Li₁₀GeP₂S₁₂ as a material.

Future Prospects
The solid electrolyte material (Li₁₀GeP₂S₁₂) that was discovered in the present study is applicable to making lithium batteries composed of solid-state material only, allowing the development of high capacity batteries with improved safety on the path to developing all-solid-state batteries as well as ultra-compact ceramic batteries. The present discovery will further accelerate the efforts in view of a prospect that “next generation batteries are aiming at all-solid-state^*6.” By making existing lithium-ion batteries entirely solid state, new batteries superior in safety and stability with a long life can be developed, contributing to achieving even higher capacity. This research team is also pursuing the development of safer, more stable, and longer lasting batteries. Moreover, achieving higher safety, stability, and longer life is the greatest challenge in the development of next generation batteries (innovative batteries^*7) that far exceed the capacity of existing batteries. An important step in this process is to replace combustible electrolytes with noncombustible or flame-retarding electrolytes. Successful development of noncombustible inorganic solid electrolytes exhibiting high ionic conductivity comparable or superior to that of organic electrolytes can significantly contribute to the realization of large-scale and high capacity batteries. This research group plans to pursue research toward high energy density batteries by further improving the conductivity and stability of the discovered material.

Glossary

*1　Superionic conductor:
Solid material in which ions move around as if they are in liquid. It has been thought that the maximum ionic conductivity of silver- or copper-ion conductors is on the order of 1 S cm⁻¹ and that of lithium ion conductor is on the order of 1 mS cm⁻¹. In particular, the development of materials that have both ionic conductivity and stability has been anticipated in lithium superionic conductors that would be utilized for high energy density batteries. Material development has beenconducted in polymer, inorganic material  (crystal and amorphous) since the 1960s (Figure 4 shows the history of the development of superionic conductors and achieved ionic conductivities).

*2 Organic electrolytes:
Electrolytes that are currently used in lithium-ion batteries. They are combustible, and thus safety devices are indispensable. Replacing organic electrolytes with noncombustible solid materials would facilitate the downsizing and cost reduction of batteries.

*3 Japan Proton Accelerator Research Complex (J-PARC):
A collective term for the complex of shared-use facilities that were constructed in Tokai, Ibaraki prefecture by the High Energy Accelerator Research Organization and the Japan Proton Accelerator Research Complex as a joint project. Using secondary particles such as neutrons, muons, mesons, and neutrinos produced by colliding accelerated protons with nuclear targets, leading-edge science research and industrial applications in materials and life science as well as nuclear and particle physics are being conducted.

*4 Superionic conduction pathway:
Continuous space required for lithium ions to move in the crystal structure of solid materials. Actual conductivity depends on the size of the space and on the interactions of lithium ions with other surrounding atoms.

*5 All-solid-state ceramic batteries:
Batteries whose components: cathodes, anodes, and electrolytes, are all composed of ceramic material. They are in principle noncombustible and thus completely safe. However, their drawback is that power is limited mainly because the ionic conductivity of electrolyte materials is low. The key to solving this issue is assumed to be an increase in the ionic conductivity of electrolyte materials.

*6 “Next generation batteries are aiming at all-solid-state”:
Title of a feature article published in the May 2010 issue of Nikkei Electronics. The article concluded that research on applications of solid electrolytes to batteries in which 5V cathode materials are utilized as well as to lithium–sulfur and lithium–air batteries that have attracted attention as post lithium-ion batteries will be pursued in order to achieve safety, stability, and long-life in batteries. In particular, the article mentioned that “securing of safety is the first priority in lithium-ion batteries for electric vehicles and large-scale stationary lithium-ion batteries” by indicating that the “final destination is solid electrolyte.” Moreover, battery users express their anticipation as follows: “we want to use solid electrolytes for lithium-ion batteries,” reflecting the growing demand for batteries having a longer life. On the other hand, the article forecasts that “research on solid electrolytes in a mobile phone market will likely advance in accordance with the development of post lithium-ion batteries that exceed an energy density of 300 Wh/kg.” The development of a solid electrolyte that has ionic conductivity exceeding organic electrolytes was finally achieved in the present study.

*7 To develop electric vehicles that have an equivalent mileage to gasoline-powered vehicles, a battery capacity five to seven times larger than that of existing batteries is required (reference: “Recommendations for the Future of Next-Generation Vehicle Batteries,” The Ministry of Economy, Trade and Industry of Japan, August 2006). To achieve this goal, many researchers and organizations, including the New Energy and Industrial Technology Development Organization (NEDO) of Japan, are pursuing the development of innovative batteries.

Contact Information
<About the details of the present study>
Professor Ryoji Kanno
Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology
Email: kanno@echem.titech.ac.jp
TEL: 81-45-924-5401
FAX: 81-45-924-5401

<About J-PARC>
Kunihiro Suzuki, Section Leader
Public Relations Section, J-PARC Center
TEL: 81-29-284-3587
FAX: 81-29-282-5996

<Public Relations>
Yoko Hirai, Chief
Evaluation and Public Relations Division, Tokyo Institute of Technology
Email: hyo.koh.sya@jim.titech.ac.jp
TEL: 81-3-5734-2975
FAX: 81-3-5734-3661

Yohei Morita, Public Relations Officer
High Energy Accelerator Research Organization　
Email:press@kek.jp
TEL: 81-29-879-6047
FAX: 81-29-879-6049

Figure 1. Temperature dependence of ionic conductivity of the newly discovered superionic conductor (Li₁₀GeP₂S₁₂) in the present study. Its ionic conductivity is 12 m Scm^-1 at room temperature (27℃) and 0.41 m Scm⁻¹ even at −30℃. These values are among the highest in lithium superionic conductors.　　

Figure 2. Comparison of the ionic conductivity of the newly discovered superionic conductor (Li₁₀GeP₂S₁₂) and that of various other superionic conductors. Temperature dependence of the ionic conductivity of various ionic conductors such as dry polymers and inorganic amorphous is shown in addition to that of organic electrolytes and gel-polymer electrolytes used in lithium-ion batteries. In particular, Li₁₀GeP₂S₁₂ exhibits the highest ionic conductivity among these materials from room to low temperature.

Figure 3. Crystal structure and ionic conduction pathway of the newly discovered superionic conductor (Li₁₀GeP₂S₁₂). This structure was elucidated with the high resolution, high intensity neutron diffractometer at the Japan Proton Accelerator Research Complex (J-PARC). a, Whole structure. b, Three-dimensional framework structure. c, One-dimensional lithium-ion conduction pathway. Thermal vibration of lithium ions is shown in the upper side. Lithium ions are thermally vibrating up and down extensively, indicating that these ions are directly involded in superionic conductivity.

Figure 4. History of research on developing superionic conductors. The ionic conductivity and era of discovery of each material are shown. First-generation materials were found in the course of studying phenomena where ions are rapidly moving around in a solid. In developing second-generation materials, their applications as actual materials were also considered. The newly discovered superionic conductor has an ionic conductivity exceeding 10 mS cm^-1, suggesting it can be a next-generation lithium superionic conductor.