Severe safety concerns are impeding the large-scale employment of lithium/sodium batteries. Conventional electrolytes are highly flammable and volatile, which may cause catastrophic fires or explosions. Efforts to introduce flame-retardant solvents into the electrolytes have generally resulted in compromised battery performance because those solvents do not suitably passivate carbonaceous anodes. Here we report a salt-concentrated electrolyte design to resolve this dilemma via the spontaneous formation of a robust inorganic passivation film on the anode. We demonstrate that a concentrated electrolyte using a salt and a popular flame-retardant solvent (trimethyl phosphate), without any additives or soft binders, allows stable charge–discharge cycling of both hard-carbon and graphite anodes for more than 1,000 cycles (over one year) with negligible degradation; this performance is comparable or superior to that of conventional flammable carbonate electrolytes. The unusual passivation character of the concentrated electrolyte coupled with its fire-extinguishing property contributes to developing safe and long-lasting batteries, unlocking the limit toward development of much higher energy-density batteries.

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This work was supported by the Elements Strategy Initiative for Catalysts & Batteries (ESICB) of the Ministry of Education, Culture, Sports, Science and Technology (MEXT) and JSPS KAKENHI Grant-in-Aid for Specially Promoted Research (Grant Number JP15H05701). This work was partially supported by Nippon Shokubai Co. and the ‘Priority Issue (No. 5) on Post K computer’ project of MEXT. The calculations were performed at the super-computers of the National Institute for Materials Science and The University of Tokyo as well as the K computer at RIKEN, partly through the HPCI System Projects (Project ID: hp160075, hp160174, hp160225, hp160080). J.W. is grateful to the Japan Society for the Promotion of Sciences for a JSPS Fellowship at The University of Tokyo (no. 16F16051). Y.Y. is grateful to JSPS Grant-in-Aid for Young Scientists (A) (no. 26708030). The authors are deeply grateful to R. Hagiwara, T. Nohira and K. Matsumoto (Kyoto University) for the fruitful discussion on the electrolyte design. The authors acknowledge J. Ma for synthesizing the Na3V2(PO4)3 material, S.-i. Nishimura and S.-C. Chung for their valuable suggestions on both the SEI identification and the manuscript, and L. Lander for the Rietveld refinement work.

Author information


  1. Department of Chemical System Engineering, The University of Tokyo, Bunkyo-ku, Tokyo, 113-8656, Japan

    • Jianhui Wang
    • , Yuki Yamada
    • , Eriko Watanabe
    • , Koji Takada
    •  & Atsuo Yamada
  2. Elements Strategy Initiative for Catalysts & Batteries (ESICB), Kyoto University, Nishikyo-ku, Kyoto, 615-8245, Japan

    • Yuki Yamada
    • , Keitaro Sodeyama
    • , Yoshitaka Tateyama
    •  & Atsuo Yamada
  3. Center for Green Research on Energy and Environmental Materials (GREEN) and Research and Services Division of Materials Data and Integrated System (MaDIS), National Institute for Materials Science (NIMS), Tsukuba, Ibaraki, 305-0044, Japan

    • Keitaro Sodeyama
    •  & Yoshitaka Tateyama
  4. PRESTO, Japan Science and Technology Agency (JST), Kawaguchi, Saitama, 333-0012, Japan

    • Keitaro Sodeyama


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A.Y. conceived and directed the project. Y.Y. and J.W. proposed the concept and designed the experiments. J.W. and K.T. performed the experiments and analysed the data. K.S., E.W. and Y.T. designed and conducted the theoretical calculations. All authors contributed to the discussion. J.W., Y.Y. and A.Y. wrote the manuscript.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Atsuo Yamada.

Electronic supplementary material

  1. Supplementary Figures, Tables and References

    Supplementary Figures 1–18, Supplementary Tables 1–2 and Supplementary References

  2. Supplementary Video 1

    Flame test of the electrolyte vapour of 1.0 M LiPF6/DMC:TMP (1:1 by volume)

  3. Supplementary Video 2

    Flame test of the electrolyte vapour of 3.3 M NaFSA/TMP

  4. Supplementary Video 3

    Flame-test comparison between 1.0 M NaPF6/EC:DEC (1:1 by volume) and 3.3 M NaFSA/TMP

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