Understanding the conversion mechanism and performance of monodisperse FeF2 nanocrystal cathodes

Abstract

The application of transition metal fluorides as energy-dense cathode materials for lithium ion batteries has been hindered by inadequate understanding of their electrochemical capabilities and limitations. Here, we present an ideal system for mechanistic study through the colloidal synthesis of single-crystalline, monodisperse iron(ii) fluoride nanorods. Near theoretical capacity (570 mA h g−1) and extraordinary cycling stability (>90% capacity retention after 50 cycles at C/20) is achieved solely through the use of an ionic liquid electrolyte (1 m LiFSI/Pyr1,3FSI), which forms a stable solid electrolyte interphase and prevents the fusing of particles. This stability extends over 200 cycles at much higher rates (C/2) and temperatures (50 °C). High-resolution analytical transmission electron microscopy reveals intricate morphological features, lattice orientation relationships and oxidation state changes that comprehensively describe the conversion mechanism. Phase evolution, diffusion kinetics and cell failure are critically influenced by surface-specific reactions. The reversibility of the conversion reaction is governed by topotactic cation diffusion through an invariant lattice of fluoride anions and the nucleation of metallic particles on semicoherent interfaces. This new understanding is used to showcase the inherently high discharge rate capability of FeF2.

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.

from$8.99

All prices are NET prices.

Fig. 1: Nanorod structure and composition.
Fig. 2: Electrochemical performance.
Fig. 3: Electrolyte effects.
Fig. 4: Ex situ XRD and HRTEM.
Fig. 5: Conversion mechanism.

Data availability

The authors declare that all data supporting the findings of this study are included within the paper and its Supplementary Information files. Source data are available from the corresponding author upon reasonable request.

References

  1. 1.

    Schäfer, A. W. et al. Technological, economic and environmental prospects of all-electric aircraft. Nat. Energy 4, 160–166 (2019).

    Google Scholar 

  2. 2.

    Janek, J. & Zeier, W. G. A solid future for battery development. Nat. Energy 1, 16141 (2016).

    Google Scholar 

  3. 3.

    Choi, J. W. & Aurbach, D. Promise and reality of post-lithium-ion batteries with high energy densities. Nat. Rev. Mater. 1, 16013 (2016).

    CAS  Google Scholar 

  4. 4.

    Li, H., Balaya, P. & Maier, J. Li-storage via heterogeneous reaction in selected binary metal fluorides and oxides. J. Electrochem. Soc. 151, A1878 (2004).

    CAS  Google Scholar 

  5. 5.

    Li, C., Chen, K., Zhou, X. & Maier, J. Electrochemically driven conversion reaction in fluoride electrodes for energy storage devices. npj Comput. Mater. 4, 22 (2018).

    Google Scholar 

  6. 6.

    Amatucci, G. G. & Pereira, N. Fluoride based electrode materials for advanced energy storage devices. J. Fluorine Chem. 128, 243–262 (2007).

    CAS  Google Scholar 

  7. 7.

    Badway, F., Cosandey, F., Pereira, N. & Amatucci, G. G. Carbon metal fluoride nanocomposites. J. Electrochem. Soc. 150, A1318 (2003).

    CAS  Google Scholar 

  8. 8.

    Zhang, N., Xiao, X. & Pang, H. Transition metal (Fe, Co, Ni) fluoride-based materials for electrochemical energy storage. Nanoscale Horizons 4, 99–116 (2019).

    CAS  Google Scholar 

  9. 9.

    Bruce, P. G., Scrosati, B. & Tarascon, J.-M. Nanomaterials for rechargeable lithium batteries. Angew. Chem. Int. Ed. 47, 2930–2946 (2008).

    CAS  Google Scholar 

  10. 10.

    Liu, C., Neale, Z. G. & Cao, G. Understanding electrochemical potentials of cathode materials in rechargeable batteries. Mater. Today 19, 109–123 (2016).

    CAS  Google Scholar 

  11. 11.

    Ma, Y. & Garofalini, S. H. Atomistic insights into the conversion reaction in iron fluoride: a dynamically adaptive force field approach. J. Am. Chem. Soc. 134, 8205–8211 (2012).

    CAS  Google Scholar 

  12. 12.

    Ma, Y. & Garofalini, S. H. Interplay between the ionic and electronic transport and its effects on the reaction pattern during the electrochemical conversion in an FeF2 nanoparticle. Phys. Chem. Chem. Phys. 16, 11690–11697 (2014).

    CAS  Google Scholar 

  13. 13.

    Wang, F. et al. Ionic and electronic transport in metal fluoride conversion electrodes. ECS Trans. 50, 19–25 (2013).

    Google Scholar 

  14. 14.

    Ko, J. K. et al. Transport, phase reactions, and hysteresis of iron fluoride and oxyfluoride conversion electrode materials for lithium batteries. ACS Appl. Mater. Interfaces 6, 10858–10869 (2014).

    CAS  Google Scholar 

  15. 15.

    Karki, K. et al. Revisiting conversion reaction mechanisms in lithium batteries: lithiation-driven topotactic transformation in FeF2. J. Am. Chem. Soc. 140, 17915–17922 (2018).

    CAS  Google Scholar 

  16. 16.

    Wang, F. et al. Tracking lithium transport and electrochemical reactions in nanoparticles. Nat. Commun. 3, 1201–1208 (2012).

    Google Scholar 

  17. 17.

    Chuan-zheng, Y., Jian-min, H. A. O. & Guang-wen, P. E. I. Contributed papers brief introduction of X-ray multiple diffraction. Rigaku J. 17, 46–57 (2000).

    Google Scholar 

  18. 18.

    Li, L. et al. Origins of large voltage hysteresis in high-energy-density metal fluoride lithium-ion battery conversion electrodes. J. Am. Chem. Soc. 138, 2838–2848 (2016).

    CAS  Google Scholar 

  19. 19.

    Wang, F. et al. Conversion reaction mechanisms in lithium ion batteries: study of the binary metal fluoride electrodes. J. Am. Chem. Soc. 133, 18828–18836 (2011).

    CAS  Google Scholar 

  20. 20.

    Yamakawa, N., Jiang, M., Key, B. & Grey, C. P. Identifying the local structures formed during lithiation of the conversion material, iron fluoride, in a Li ion battery: a solid-state NMR, X-ray diffraction, and pair distribution function analysis study. J. Am. Chem. Soc. 131, 10525–10536 (2009).

    CAS  Google Scholar 

  21. 21.

    Doe, R. E., Persson, K. A., Meng, Y. S. & Ceder, G. First-principles investigation of the Li-Fe-F phase diagram and equilibrium and nonequilibrium conversion reactions of iron fluorides with lithium. Chem. Mater. 20, 5274–5283 (2008).

    CAS  Google Scholar 

  22. 22.

    Murray, C. B., Kagan, C. R. & Bawendi, M. G. Synthesis and characterization of monodisperse nanocrystals and close-packed nanocrystal assemblies. Int. J. Adv. Eng. Technol. 2, 668–676 (2012).

    Google Scholar 

  23. 23.

    Cargnello, M., Doan-Nguyen, V. V. T. & Murray, C. B. Engineering uniform nanocrystals: mechanism of formation and self-assembly into bimetallic nanocrystal superlattices. AIChE J. 62, 392–398 (2016).

    CAS  Google Scholar 

  24. 24.

    Huang, Q. et al. Insights into the effects of electrolyte composition on the performance and stability of FeF2 conversion-type cathodes. Adv. Energy Mater. 9, 1–11 (2019).

    Google Scholar 

  25. 25.

    Gu, W., Magasinski, A., Zdyrko, B. & Yushin, G. Metal fluorides nanoconfined in carbon nanopores as reversible high capacity cathodes for Li and Li-ion rechargeable batteries: FeF2 as an example. Adv. Energy Mater. 5, 1–7 (2015).

    Google Scholar 

  26. 26.

    Gordon, D. et al. Mixed metal difluorides as high capacity conversion-type cathodes: impact of composition on stability and performance. Adv. Energy Mater. 8, 1800213 (2018).

    Google Scholar 

  27. 27.

    Chun, J. et al. Ammonium fluoride mediated synthesis of anhydrous metal fluoride–mesoporous carbon nanocomposites for high-performance lithium ion battery cathodes. ACS Appl. Mater. Interfaces 8, 35180–35190 (2016).

    CAS  Google Scholar 

  28. 28.

    Song, H., Cui, H. & Wang, C. Extremely high-rate capacity and stable cycling of a highly ordered nanostructured carbon–FeF2 battery cathode. J. Mater. Chem. A 3, 22377–22384 (2015).

    CAS  Google Scholar 

  29. 29.

    Reddy, M. A. et al. CFx derived carbon–FeF2 nanocomposites for reversible lithium storage. Adv. Energy Mater. 3, 308–313 (2013).

    CAS  Google Scholar 

  30. 30.

    Roth, H. G., Romero, N. A. & Nicewicz, D. A. Experimental and calculated electrochemical potentials of common organic molecules for applications to single-electron redox chemistry. Synlett 27, 714–723 (2016).

    CAS  Google Scholar 

  31. 31.

    Pereira, N., Badway, F., Wartelsky, M., Gunn, S. & Amatucci, G. G. Iron oxyfluorides as high capacity cathode materials for lithium batteries. J. Electrochem. Soc. 156, A407 (2009).

    CAS  Google Scholar 

  32. 32.

    Huang, Q. et al. Cycle stability of conversion-type iron fluoride lithium battery cathode at elevated temperatures in polymer electrolyte composites. Nat. Mater. 18, 1343–1349 (2019).

    CAS  Google Scholar 

  33. 33.

    Tasaki, K. et al. Solubility of lithium salts formed on the lithium-ion battery negative electrode surface in organic solvents. J. Electrochem. Soc. 156, A1019 (2009).

    CAS  Google Scholar 

  34. 34.

    Tasaki, K. & Harris, S. J. Computational study on the solubility of lithium salts formed on lithium ion battery negative electrode in organic solvents. J. Phys. Chem. C 114, 8076–8083 (2010).

    CAS  Google Scholar 

  35. 35.

    He, Z. & Alexandridis, P. Nanoparticles in ionic liquids: interactions and organization. Phys. Chem. Chem. Phys. 17, 18238–18261 (2015).

    CAS  Google Scholar 

  36. 36.

    Xie, Z.-h, Jiang, Z. & Zhang, X. Review—promises and challenges of in situ transmission electron microscopy electrochemical techniques in the studies of lithium ion batteries. J. Electrochem. Soc. 164, 2110–2123 (2017).

    Google Scholar 

  37. 37.

    Wang, C.-m In situ transmission electron microscopy and spectroscopy studies of rechargeable batteries under dynamic operating conditions: a retrospective and perspective view. J. Mater. Res. 30, 326–339 (2014).

    CAS  Google Scholar 

  38. 38.

    Newman, J. & Thomas-Alyea, K. E. Electrochemical Systems 3rd edn (Wiley, 2004).

  39. 39.

    Porter, D. A., Easterling, K. E. & Sherif, M. Y. Phase Transformations in Metals and Alloys 3rd edn (Taylor & Francis, 2009).

  40. 40.

    Haynes, W. M., Lide, D. R. & Bruno, T. J. (eds) CRC Handbook of Chemistry and Physics (CRC, 2016).

  41. 41.

    Jain, A. et al. Commentary: the materials project: a materials genome approach to accelerating materials innovation. APL Mater 1, 011002 (2013).

    Google Scholar 

  42. 42.

    Cosandey, F., Al-Sharab, J. F., Badway, F., Amatucci, G. G. & Stadelmann, P. EELS spectroscopy of iron fluorides and FeFx/C nanocomposite electrodes used in Li-ion batteries. Microsc. Microanal. 13, 87–95 (2007).

    CAS  Google Scholar 

  43. 43.

    Dahmen, U. Orientation relationships in precipitation systems. Acta Metall. 30, 63–73 (1981).

    Google Scholar 

  44. 44.

    Guntlin, C. P., Tanja, Z. & Kravchyk, K. V. Nanocrystalline FeF3 and MF2 (M = Fe, Co, and Mn) from metal trifluoroacetates and their Li(Na)-ion storage properties. J. Mater. Chem. A 5, 7383–7393 (2017).

    CAS  Google Scholar 

  45. 45.

    Mitchell, D. R. G. DiffTools: electron diffraction software tools for DigitalmicrographTM. Microsc. Res. Tech. 71, 588–593 (2008).

    CAS  Google Scholar 

Download references

Acknowledgements

This publication arises from research funded by the John Fell Oxford University Press Research Fund. We acknowledge financial support from the Henry Royce Institute (through UK Engineering and Physical Science Research Council grant no. EP/R010145/1) for capital equipment. I.C. acknowledges support from a Modentech studentship. T.-U.W and H.-W.L. acknowledge the 2019 Research Fund (1.190031.01) of Ulsan National Institute of Science and Technology and Individual Basic Science & Engineering Research Program (NRF-2019R1C1C1009324) through the National Research Foundation of Korea funded by the Ministry of Science and ICT. N.G. and A.R. acknowledge support from the Royal Society. We are grateful to the David Cockayne Center for Electron Microscopy for the use of their electron microscopes, and we thank P. Holdway for assistance in the collection of XPS data. A.W.X. thanks P. Quijano Velasco and K. Hurlbutt for helpful scientific discussion. We also thank S. Yin and E. Für DeBier.

Author information

Affiliations

Authors

Contributions

A.W.X. and M.P. conceived and designed the experiments. A.W.X. performed the experiments, analysed the data and wrote the manuscript with input from all authors. H.J.L. assisted in the processing and Rietveld refinement of powder XRD data and provided helpful discussions regarding the interpretation of crystallographic data. I.C. assisted in the fabrication of electrodes, test cells and provided expertise regarding electrochemical testing. A.R. performed the collection of dark-field STEM and EELS data. T.-U.W. and H.-W.L. performed the in situ TEM lithiation experiments and provided guidance on the interpretation in situ data. J.F. prepared the ionic liquid electrolytes and provided advice on the properties of ionic liquids and the analysis of impedance data. S.W. assisted in the collection of powder XRD patterns as well as editing and formatting of the manuscript. N.G. provided support for materials synthesis and experimental advice. M.P. supervised the design of the project, provided expertise on the interpretation of electrochemical data, and gave frequent, in-depth scientific input.

Corresponding author

Correspondence to Mauro Pasta.

Ethics declarations

Competing interests

The authors declare no competing interests.

Additional information

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data

Extended Data Fig. 1 Stage 2 (1.75 V, \({x}_{L{i}^{+}}\) = 0.3) EELS Data.

a,b, Dark field STEM and corresponding EELS spectra for an FeF2 nanorod from Stage 2 of discharge. Yellow arrows denote direction of EELS line scan and black dashed boxes indicate areas over which the displayed spectra are averaged. EELS spectra from the core of the nanrod exhibit a splitting of the Fe L3 peak, specifically broadening toward higher energy loss values indicative of Fe3+. The greater width of this peak is evident when compared to the EELS spectra acquired from the surface. This increase in Fe oxidation state indicates that a disproportionation to form the trirutile phase has occurred. This oxidation is not a result of air exposure as this would oxidize material at the surface first. Instead, the EELS spectra near the nanorod surface are still indicative of Fe0 or Fe2+. Scale bar, 40 nm (a).

Extended Data Fig. 2 Stage 3 (1.7 V, \({x}_{L{i}^{+}}\) = 1.0) EELS Data.

a,b, Dark field STEM and corresponding EELS spectra for an FeF2 nanorod from Stage 3 of discharge. Line scans taken at different positions along the length of the nanorod indicate the presence of two distinct regions. There is a clear distinction between a region containing a strong Fe3+ signal related to the trirutile phase and a region containing only Fe0 and Fe2+ species. This data suggests that the onset of conversion coincides with the reduction of the Fe3+ rutile formed during the initial disproportionation as well as the formation of metallic iron in the nanorod interior. Scale bar, 50 nm (a).

Extended Data Fig. 3 Stage 7 (4.0 V, \({x}_{L{i}^{+}}\) = 0.0) EELS Data.

a,b, Dark field STEM and corresponding EELS spectra for an FeF2 nanorod from Stage 3 of discharge.Upon charging to the full capacity at 4.0 V vs. Li+/Li, the widespread presence of Fe3+ is observed at the interior of the nanorod. This potential is higher than the standard reduction potential of Fe3+ + e → Fe2+ (3.8 V vs. Li+/Li), and this transition is facilitated by the kinetically limited reinsertion of Fe from the double layer shell which causes the core of the nanorod to reconvert with a stoichiometry closer to FeF3.40 This delayed reconversion of the double layer shell is evidenced by EELS spectra from the nanorod surface, which show the distinct presence of Fe2+. Fe3+ is generated at the beginning of discharge and reformed at the end of charge. Scale bar, 30 nm (a).

Extended Data Fig. 4 Nanorod structure after 15 cycles.

a,b, Bright field TEM images comparing single nanorods after 15 cycles in the IL and LP30 electrolytes. c,d, The corresponding FFTs colored to differentiate between multiple phases present. Greater shape preservation is observed in the IL electrolyte. Strong rocksalt reflections in (c) indicate largely incomplete reconversion. Scale bars, 20 nm.

Supplementary information

41563_2020_621_MOESM2_ESM.mov

In-situ TEM lithiation (discharge).

41563_2020_621_MOESM3_ESM.mov

In-situ TEM lithiation (discharge).

41563_2020_621_MOESM4_ESM.mov

In-situ TEM lithiation (discharge).

Supplementary Information

Supplementary Figs. 1–34 and Discussion.

Video 1

In-situ TEM lithiation (discharge).

Video 2

In-situ TEM lithiation (discharge).

Video 3

In-situ TEM lithiation (discharge).

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Xiao, A.W., Lee, H.J., Capone, I. et al. Understanding the conversion mechanism and performance of monodisperse FeF2 nanocrystal cathodes. Nat. Mater. 19, 644–654 (2020). https://doi.org/10.1038/s41563-020-0621-z

Download citation

Further reading