Li-ion batteries with high energy and power densities use organic liquid electrolytes; these, however, require relatively stringent safety precautions, making large-scale systems more complicated and expensive. The use of solid electrolytes—which would address many of those issues associated with the liquid electrolytes—is currently limited by their conductivities.

In 2011, Toyota and its partners published a paper in the journal Nature Materials describing the development of the germanium-based lithium superionic conductor Li₁₀GeP₂S₁₂ that featured a new three-dimensional framework structure. Kamaya et al. reported that the new material exhibited an extremely high lithium ionic conductivity of 12 mS cm⁻¹ at room temperature, representing the highest conductivity achieved in a solid electrolyte at that time, exceeding even those of liquid organic electrolytes.

In 2011, the new solid lithium electrolyte Li₁₀GeP₂S₁₂ (LGPS) was reported, featuring liquid-like Li ion conduction in a crystalline solid matrix. The ultrafast room temperature transport of tetragonal LGPS with a conductivity of several mS/cm came as a surprise as it exceeds the values of the best crystalline Li conductors by one order of magnitude. Beside the fundamental aspect of unexpectedly high Li mobility the authors reported a large effective electrochemical window rendering this material interesting for Li based batteries. The high cost of germanium hampers such application. Therefore, there has been a strong upsurge of interest recently in synthesizing LGPS-type materials based on the homologous elements Si and Sn. A theoretical study published by Ceder and coworkers highlights the potential of such hypothetical tetragonal LGPS-type Li ion conductors. It was predicted that the beneficial properties of tetragonal LGPS are retained when Ge is replaced by Si or Sn.

Here, we report the synthesis of Ge-free LGPS-type electrolytes, namely tetragonal Li₁₀SnP₂S₁₂ (LSnPS) and Li₁₁Si₂PS₁₂ (LSiPS). Both show extremely high Li⁺ diffusivities with the values of the Si compound exceeding those of LGPS, the present record holder. The materials were comprehensively characterized both with respect to their structure and Li ion dynamics. These results are compared with those recently obtained for Li₁₀GeP₂S₁₂ and Li₇GePS₈, two members of the tetragonal LGPS solid solution which both showed very similar Li diffusivities. … LSiPS shows an even higher Li diffusivity than LGPS while LSnPS has a slightly lower Li diffusivity. We further demonstrate that the synthesis of tetragonal LiSiPS is only possible at high pressure, hence limiting scalability, while the synthesis of LSnPS can easily be scaled up to larger volumes.

—Kuhn et al.

(A hat-tip to Olof!)

Resources

• A. Kuhn, Oliver Gerbig, Changbao Zhu, Frank Falkenberg, Joachim Maier, and Bettina V. Lotsch (2014) “Ultrafast Li Electrolytes Based on Abundant Elements: Li₁₀SnP₂S₁₂ and Li₁₁Si₂PS₁₂” arXiv:1402.4586 [cond-mat.mtrl-sci]

• Noriaki Kamaya, Kenji Homma, Yuichiro Yamakawa, Masaaki Hirayama, Ryoji Kanno, Masao Yonemura, Takashi Kamiyama, Yuki Kato, Shigenori Hama, Koji Kawamoto and Akio Mitsui (2011) A lithium superionic conductor. Nat Mat. 10, 682–686 (2011) DOI: 10.1038/nmat3066

• Yifei Mo, Shyue Ping Ong, and Gerbrand Ceder (2012) “First Principles Study of the Li₁₀GeP₂S₁₂ Lithium Super Ionic Conductor Material,” Chemistry of Materials 24 (1), 15-17 doi: 10.1021/cm203303y