A 3.90 V iron-based fluorosulphate material for lithium-ion batteries crystallizing in the triplite structure

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Li-ion batteries have empowered consumer electronics and are now seen as the best choice to propel forward the development of eco-friendly (hybrid) electric vehicles. To enhance the energy density, an intensive search has been made for new polyanionic compounds that have a higher potential for the Fe2+/Fe3+ redox couple. Herein we push this potential to 3.90 V in a new polyanionic material that crystallizes in the triplite structure by substituting as little as 5 atomic per cent of Mn for Fe in Li(Fe1−δMnδ)SO4F. Not only is this the highest voltage reported so far for the Fe2+/Fe3+ redox couple, exceeding that of LiFePO4 by 450 mV, but this new triplite phase is capable of reversibly releasing and reinserting 0.7–0.8 Li ions with a volume change of 0.6% (compared with 7 and 10% for LiFePO4 and LiFeSO4F respectively), to give a capacity of ~125 mA h g−1.

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Figure 1: Crystal structure and diffraction patterns of the tavorite and triplite phases.
Figure 2: Structural changes on Mn substitution.
Figure 3: Voltage–composition curves for the tavorite and triplite phases.
Figure 4: Changes in electrochemistry with Mn substitution.
Figure 5: Synchrotron XRD Rietveld refinement of the chemically oxidized Li0.25(Fe0.8Mn0.2)SO4F sample.
Figure 6: Structural relationship between the tavorite and triplite phases.


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Many discussions with M. Armand, N. Recham, C. Delacourt, C. Masquelier, D. Larcher, G. Férey, Y. Chabre, C. Frayret and D.W. Murphy are gratefully acknowledged. We thank C. Davoisne for the TEM images and ALISTORE-ERI for sponsoring this research. Use of the Advanced Photon Source at Argonne National Laboratory was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. The EXAFS measurements were carried out with the support of the Diamond Light Source and we gratefully acknowledge G. Cibin for help with running the X-ray absorption spectroscopy experiments as well as E. J. Schofield and A. V. Chadwick for discussions in analysing the XANES data.

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P.B., M.A. and J-M.T. carried out the synthesis, the electrochemical work and designed the research approach; B.C.M., G.R. and J-N.C. analysed the crystal structure and diffraction patterns; M.T.S. and J-C.J. collected the Mössbauer measurements; B.C.M. and S.A.C. collected and analysed the EXAFS measurements; M-L.D. conducted the DFT calculations and developed the theoretical framework; B.C.M., G.R. and J-M.T. wrote the manuscript and all authors discussed the experiments and final manuscript.

Correspondence to J-M. Tarascon.

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