Skip to main content

Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

  • Article
  • Published:

Niobium tungsten oxides for high-rate lithium-ion energy storage

Nature volume 559, pages 556–563 (2018)Cite this article

Subjects

Abstract

The maximum power output and minimum charging time of a lithium-ion battery depend on both ionic and electronic transport. Ionic diffusion within the electrochemically active particles generally represents a fundamental limitation to the rate at which a battery can be charged and discharged. To compensate for the relatively slow solid-state ionic diffusion and to enable high power and rapid charging, the active particles are frequently reduced to nanometre dimensions, to the detriment of volumetric packing density, cost, stability and sustainability. As an alternative to nanoscaling, here we show that two complex niobium tungsten oxides—Nb16W5O55 and Nb18W16O93, which adopt crystallographic shear and bronze-like structures, respectively—can intercalate large quantities of lithium at high rates, even when the sizes of the niobium tungsten oxide particles are of the order of micrometres. Measurements of lithium-ion diffusion coefficients in both structures reveal room-temperature values that are several orders of magnitude higher than those in typical electrode materials such as Li4Ti5O12 and LiMn2O4. Multielectron redox, buffered volume expansion, topologically frustrated niobium/tungsten polyhedral arrangements and rapid solid-state lithium transport lead to extremely high volumetric capacities and rate performance. Unconventional materials and mechanisms that enable lithiation of micrometre-sized particles in minutes have implications for high-power applications, fast-charging devices, all-solid-state energy storage systems, electrode design and material discovery.

This is a preview of subscription content, access via your institution

Access options

Buy this article

  • Purchase on Springer Link
  • Instant access to full article PDF
Buy now

Prices may be subject to local taxes which are calculated during checkout

Fig. 1: Crystal structure and particle morphology of Nb16W5O55 and Nb18W16O93.
Fig. 2: Electrochemistry of Nb16W5O55 and Nb18W16O93.
Fig. 3: X-ray absorption spectroscopy of Nb16W5O55 and Nb18W16O93.
Fig. 4: Nonlinear structural evolution of Nb16W5O55 and Nb18W16O93 from operando synchrotron diffraction.
Fig. 5: Charge, energy, and power densities of micrometre-sized Nb16W5O55 and Nb18W16O93 compared to state-of-the-art high-rate nanoscale materials and formulations.

Similar content being viewed by others

References

  1. Zhao, K., Pharr, M., Vlassak, J. J. & Suo, Z. Fracture of electrodes in lithium-ion batteries caused by fast charging. J. Appl. Phys. 108, 073517 (2010).

    Article  ADS  CAS  Google Scholar 

  2. Vetter, J. et al. Ageing mechanisms in lithium-ion batteries. J. Power Sources 147, 269–281 (2005).

    Article  ADS  CAS  Google Scholar 

  3. Downie, L. E. et al. In situ detection of lithium plating on graphite electrodes by electrochemical calorimetry. J. Electrochem. Soc. 160, A588–A594 (2013).

    Article  CAS  Google Scholar 

  4. Kim, C., Norberg, N. S., Alexander, C. T., Kostecki, R. & Cabana, J. Mechanism of phase propagation during lithiation in carbon-free Li4Ti5O12 battery electrodes. Adv. Funct. Mater. 23, 1214–1222 (2013).

    Article  CAS  Google Scholar 

  5. Wang, C. et al. A robust strategy for crafting monodisperse Li4Ti5O12 nanospheres as superior rate anode for lithium ion batteries. Nano Energy 21, 133–144 (2016).

    Article  CAS  Google Scholar 

  6. Odziomek, M. et al. Hierarchically structured lithium titanate for ultrafast charging in long-life high capacity batteries. Nat. Commun. 8, 15636 (2017).

    Article  PubMed  PubMed Central  ADS  CAS  Google Scholar 

  7. Oszajca, M. F., Bodnarchuk, M. I. & Kovalenko, M. V. Precisely engineered colloidal nanoparticles and nanocrystals for Li-ion and Na-ion batteries: model systems or practical solutions? Chem. Mater. 26, 5422–5432 (2014).

    Article  CAS  Google Scholar 

  8. Palacín, M. R., Simon, P. & Tarascon, J. M. Nanomaterials for electrochemical energy storage: the good and the bad. Acta Chim. Slov. 63, 417–423 (2016).

    Article  PubMed  CAS  Google Scholar 

  9. Wu, H. et al. Stable cycling of double-walled silicon nanotube battery anodes through solid-electrolyte interphase control. Nat. Nanotechnol. 7, 310–315 (2012).

    Article  PubMed  ADS  CAS  Google Scholar 

  10. Kasnatscheew, J. et al. A tutorial into practical capacity and mass balancing of lithium ion batteries. J. Electrochem. Soc. 164, A2479–A2486 (2017).

    Article  CAS  Google Scholar 

  11. Griffith, K. J., Forse, A. C., Griffin, J. M. & Grey, C. P. High-rate intercalation without nanostructuring in metastable Nb2O5 bronze phases. J. Am. Chem. Soc. 138, 8888–8899 (2016).

    Article  PubMed  CAS  Google Scholar 

  12. Stramare, S., Thangadurai, V. & Weppner, W. Lithium lanthanum titanates: a review. Chem. Mater. 15, 3974–3990 (2003).

    Article  CAS  Google Scholar 

  13. Shen, L., Zhang, X., Uchaker, E., Yuan, C. & Cao, G. Li4Ti5O12 nanoparticles embedded in a mesoporous carbon matrix as a superior anode material for high rate lithium ion batteries. Adv. Energy Mater. 2, 691–698 (2012).

    Article  CAS  Google Scholar 

  14. Prakash, A. S. et al. Solution-combustion synthesized nanocrystalline Li4Ti5O12 as high-rate performance Li-ion battery anode. Chem. Mater. 22, 2857–2863 (2010).

    Article  CAS  Google Scholar 

  15. Xu, G. B. et al. Highly-crystalline ultrathin Li4Ti5O12 nanosheets decorated with silver nanocrystals as a high-performance anode material for lithium ion batteries. J. Power Sources 276, 247–254 (2015).

    Article  ADS  CAS  Google Scholar 

  16. Ren, Y. et al. Nanoparticulate TiO2(B): an anode for lithium-ion batteries. Angew. Chem. Int. Ed. 51, 2164–2167 (2012).

    Article  CAS  Google Scholar 

  17. Liu, H. et al. Mesoporous TiO2–B microspheres with superior rate performance for lithium ion batteries. Adv. Mater. 23, 3450–3454 (2011).

    Article  PubMed  CAS  Google Scholar 

  18. Beuvier, T. et al. TiO2(B) nanoribbons as negative electrode material for lithium ion batteries with high rate performance. Inorg. Chem. 49, 8457–8464 (2010).

    Article  PubMed  CAS  Google Scholar 

  19. Augustyn, V. et al. High-rate electrochemical energy storage through Li+ intercalation pseudocapacitance. Nat. Mater. 12, 518–522 (2013).

    Article  PubMed  ADS  CAS  Google Scholar 

  20. Liu, X. et al. Urchin-like hierarchical H-Nb2O5 microspheres: synthesis, formation mechanism and their applications in lithium ion batteries. Dalton Trans. 46, 10935–10940 (2017).

    Article  PubMed  CAS  Google Scholar 

  21. Lai, C.-H. et al. Designing pseudocapacitance for Nb2O5/carbide-derived carbon electrodes and hybrid devices. Langmuir 33, 9407–9415 (2017).

    Article  PubMed  CAS  Google Scholar 

  22. Roth, R. S. & Waring, J. L. Phase equilibria as related to crystal structure in the system niobium pentoxide−tungsten trioxide. J. Res. Natl Bur. Stand. 70A, 281–303 (1966).

    Article  Google Scholar 

  23. Roth, R. S. & Wadsley, A. D. Multiple phase formation in the binary system Nb2O5−WO3. II. The structure of the monoclinic phases WNb12O33 and W5Nb16O55. Acta Crystallogr. 19, 32–38 (1965).

    Article  CAS  Google Scholar 

  24. Roth, R. S. & Wadsley, A. D. Multiple phase formation in the binary system Nb2O5–WO3. I. Preparation and identification of phases. Acta Crystallogr. 19, 26–32 (1965).

    Article  CAS  Google Scholar 

  25. Stephenson, N. C. A structural investigation of some stable phases in the region Nb2O5·WO3–WO3. Acta Crystallogr. B 24, 637–653 (1968).

    Article  CAS  Google Scholar 

  26. Naoi, K., Ishimoto, S., Isobe, Y. & Aoyagi, S. High-rate nano-crystalline Li4Ti5O12 attached on carbon nano-fibers for hybrid supercapacitors. J. Power Sources 195, 6250–6254 (2010).

    Article  ADS  CAS  Google Scholar 

  27. Cava, R. J., Murphy, D. W. & Zahurak, S. M. Lithium insertion in Wadsley−Roth phases based on niobium oxide. J. Electrochem. Soc. 130, 2345–2351 (1983).

    Article  CAS  Google Scholar 

  28. Kumagai, N., Koishikawa, Y., Komaba, S. & Koshiba, N. Thermodynamics and kinetics of lithium intercalation into Nb2O5 electrodes for a 2 V rechargeable lithium battery. J. Electrochem. Soc. 146, 3203–3210 (1999).

    Article  CAS  Google Scholar 

  29. Patoux, S., Dolle, M., Rousse, G. & Masquelier, C. A Reversible lithium intercalation process in an ReO3 type structure PNb9O25. J. Electrochem. Soc. 149, A391–A400 (2002).

    Article  CAS  Google Scholar 

  30. Han, J.-T., Huang, Y.-H. & Goodenough, J. B. New anode framework for rechargeable lithium batteries. Chem. Mater. 23, 2027–2029 (2011).

    Article  CAS  Google Scholar 

  31. Saritha, D., Pralong, V., Varadaraju, U. V. & Raveau, B. Electrochemical Li insertion studies on WNb12O33—a shear ReO3 type structure. J. Solid State Chem. 183, 988–993 (2010).

    Article  ADS  CAS  Google Scholar 

  32. Griffith, K. J., Senyshyn, A. & Grey, C. P. Structural stability from crystallographic shear in TiO2–Nb2O5 phases: cation ordering and lithiation behavior of TiNb24O62. Inorg. Chem. 56, 4002–4010 (2017).

    Article  PubMed  CAS  Google Scholar 

  33. Roberts, M. R. et al. Direct observation of active material concentration gradients and crystallinity breakdown in LiFePO4 electrodes during charge/discharge cycling of lithium batteries. J. Phys. Chem. C 118, 6548–6557 (2014).

    Article  CAS  Google Scholar 

  34. Strobridge, F. C. et al. Unraveling the complex delithiation mechanisms of olivine-type cathode materials, LiFexCo1–xPO4. Chem. Mater. 28, 3676–3690 (2016).

    Article  CAS  Google Scholar 

  35. Mary, T. A., Evans, J. S. O., Vogt, T. & Sleight, A. W. Negative thermal expansion from 0.3 to 1050 Kelvin in ZrW2O8. Science 272, 90–92 (1996).

    Article  ADS  CAS  Google Scholar 

  36. Lin, K. et al. Ordered structure and thermal expansion in tungsten bronze Pb2K0.5Li0.5Nb5O15. Inorg. Chem. 53, 9174–9180 (2014).

    Article  PubMed  CAS  Google Scholar 

  37. Cairns, A. B. & Goodwin, A. L. Negative linear compressibility. Phys. Chem. Chem. Phys. 17, 20449–20465 (2015).

    Article  PubMed  CAS  Google Scholar 

  38. Liu, H. et al. Intergranular cracking as a major cause of long-term capacity fading of layered cathodes. Nano Lett. 17, 3452–3457 (2017).

    Article  PubMed  ADS  CAS  Google Scholar 

  39. Murphy, D. W., Greenblatt, M., Cava, R. J. & Zahurak, S. M. Topotactic lithium reactions with ReO3 related shear structures. Solid State Ion. 5, 327–329 (1981).

    Article  CAS  Google Scholar 

  40. Cava, R. J., Santoro, A., Murphy, D. W., Zahurak, S. M. & Roth, R. S. The structures of the lithium inserted metal oxides Li0.2ReO3 and Li0.36WO3. J. Solid State Chem. 50, 121–128 (1983).

    Article  ADS  CAS  Google Scholar 

  41. Gracia, L. et al. Composition dependence of the energy barrier for lithium diffusion in amorphous WO3. Electrochem. Solid State Lett. 8, J21–J23 (2005).

    Article  CAS  Google Scholar 

  42. Shan, Y. J., Inaguma, Y. & Itoh, M. The effect of electrostatic potentials on lithium insertion for perovskite oxides. Solid State Ion. 79, 245–251 (1995).

    Article  CAS  Google Scholar 

  43. Chen, C. & Du, J. Lithium ion diffusion mechanism in lithium lanthanum titanate solid-state electrolytes from atomistic simulations. J. Am. Ceram. Soc. 98, 534–542 (2015).

    Article  CAS  Google Scholar 

  44. Jay, E. E., Rushton, M. J. D., Chroneos, A., Grimes, R. W. & Kilner, J. A. Genetics of superionic conductivity in lithium lanthanum titanates. Phys. Chem. Chem. Phys. 17, 178–183 (2015).

    Article  PubMed  CAS  Google Scholar 

  45. Emery, J., Buzare, J. Y., Bohnke, O. & Fourquet, J. L. Lithium-7 NMR and ionic conductivity studies of lanthanum lithium titanate electrolytes. Solid State Ion. 99, 41–51 (1997).

    Article  CAS  Google Scholar 

  46. Giddy, A. P., Dove, M. T., Pawley, G. S. & Heine, V. The determination of rigid-unit modes as potential soft modes for displacive phase transitions in framework crystal structures. Acta Crystallogr. A 49, 697–703 (1993).

    Article  Google Scholar 

  47. Dove, M. T., Trachenko, K. O., Tucker, M. G. & Keen, D. A. Rigid unit modes in framework structures: theory, experiment and applications. Rev. Mineral. Geochem. 39, 1–33 (2000).

    Article  Google Scholar 

  48. Islam, M. S., Driscoll, D. J., Fisher, C. A. J. & Slater, P. R. Atomic-scale investigation of defects, dopants, and lithium transport in the LiFePO4 olivine-type battery material. Chem. Mater. 17, 5085–5092 (2005).

    Article  CAS  Google Scholar 

  49. Dathar, G. K. P., Sheppard, D., Stevenson, K. J. & Henkelman, G. Calculations of Li-ion diffusion in olivine phosphates. Chem. Mater. 23, 4032–4037 (2011).

    Article  CAS  Google Scholar 

  50. Liu, H. et al. Effects of antisite defects on Li diffusion in LiFePO4 revealed by Li isotope exchange. J. Phys. Chem. C 121, 12025–12036 (2017).

    Article  CAS  Google Scholar 

  51. Zhang, C. et al. Synthesis and charge storage properties of hierarchical niobium pentoxide/carbon/niobium carbide (MXene) hybrid materials. Chem. Mater. 28, 3937–3943 (2016).

    Article  CAS  Google Scholar 

  52. Sun, H. et al. Three-dimensional holey-graphene/niobia composite architectures for ultrahigh-rate energy storage. Science 356, 599–604 (2017).

    Article  PubMed  ADS  CAS  Google Scholar 

  53. Zhang, S. Chemomechanical modeling of lithiation-induced failure in high-volume-change electrode materials for lithium ion batteries. Npj Comput. Mater. 3, 7 (2017).

    Article  ADS  CAS  Google Scholar 

  54. Maxisch, T., Zhou, F. & Ceder, G. Ab initio study of the migration of small polarons in olivine LixFePO4 and their association with lithium ions and vacancies. Phys. Rev. B 73, 104301 (2006).

    Article  ADS  CAS  Google Scholar 

  55. Roth, R. S. Thermal stability of long range order in oxides. Prog. Solid State Chem. 13, 159–192 (1980).

    Article  ADS  CAS  Google Scholar 

  56. Eyring L. & O'Keefe M. (eds) The Chemistry of Extended Defects in Non-Metallic Solids (North-Holland, Amsterdam, 1970).

    Google Scholar 

  57. Toby, B. H. & Von Dreele, R. B. GSAS-II: the genesis of a modern open-source all purpose crystallography software package. J. Appl. Cryst. 46, 544–549 (2013).

    Article  CAS  Google Scholar 

  58. Allpress, J. G. & Roth, R. S. The effect of annealing on the concentration of Wadsley defects in the Nb2O5−WO3 system. J. Solid State Chem. 3, 209–216 (1971).

    Article  ADS  CAS  Google Scholar 

  59. Ragone, D. V. Review of battery systems for electrically powered vehicles. In Proc. Mid-Year Meeting of the Society of Automotive Engineers (SAE, 1968).

  60. Rohatgi, A. WebPlotDigitizer https://automeris.io/WebPlotDigitizer (2017).

  61. Borkiewicz, O. J. et al. The AMPIX electrochemical cell: a versatile apparatus for in situ X-ray scattering and spectroscopic measurements. J. Appl. Cryst. 45, 1261–1269 (2012).

    Article  CAS  Google Scholar 

  62. Caglioti, G., Paoletti, A., Ricci, F. P. Choice of collimators for a crystal spectrometer for neutron diffraction. Nucl. Instrum. 3, 223–228 (1958).

    Article  CAS  Google Scholar 

  63. Ravel, B. & Newville, M. ATHENA, ARTEMIS, HEPHAESTUS: data analysis for X-ray absorption spectroscopy using IFEFFIT. J. Synchrotron Radiat. 12, 537–541 (2005).

    Article  PubMed  CAS  Google Scholar 

  64. Newville, M. IFEFFIT: interactive XAFS analysis and FEFF fitting. J. Synchrotron Radiat. 8, 322–324 (2001).

    Article  PubMed  CAS  Google Scholar 

  65. Thompson, A. et al. X-ray Data Booklet (Lawrence Berkeley National Laboratory, Berkeley, 2009).

    Google Scholar 

  66. Yamamoto, T., Orita, A. & Tanaka, T. Structural analysis of tungsten–zirconium oxide catalyst by W K-edge and L1-edge XAFS. X-ray Spectrom. 37, 226–231 (2008).

    Article  ADS  CAS  Google Scholar 

  67. Tougerti, A. et al. XANES study of rhenium oxide compounds at the L1 and L3 absorption edges. Phys. Rev. B 85, 125136 (2012).

    Article  ADS  CAS  Google Scholar 

  68. Jayarathne, U. et al. X-ray absorption spectroscopy systematics at the tungsten L-edge. Inorg. Chem. 53, 8230–8241 (2014).

    Article  PubMed  PubMed Central  CAS  Google Scholar 

  69. Drube, W., Treusch, R., Sham, T. K., Bzowski, A. & Soldatov, A. V. Sublifetime-resolution Ag L3-edge XANES studies of Ag-Au alloys. Phys. Rev. B 58, 6871–6876 (1998).

    Article  ADS  CAS  Google Scholar 

  70. Bersuker, I. B. The Jahn–Teller Effect (Cambridge Univ. Press, Cambridge, 2006).

    Book  Google Scholar 

  71. Whittle, T. A., Schmid, S. & Howard, C. J. Octahedral tilting in the tungsten bronzes. Acta Crystallogr. B 71, 342–348 (2015).

    Article  CAS  Google Scholar 

  72. Yan, L. et al. Electrospun WNb12O33 nanowires: superior lithium storage capability and their working mechanism. J. Mater. Chem. A 5, 8972–8980 (2017).

    Article  CAS  Google Scholar 

  73. Stejskal, E. O. & Tanner, J. E. Spin diffusion measurements: spin echoes in the presence of a time-dependent field gradient. J. Chem. Phys. 42, 288–292 (1965).

    Article  ADS  CAS  Google Scholar 

  74. Weppner, W. & Huggins, R. A. Determination of the kinetic parameters of mixed-conducting electrodes and application to the system Li3Sb. J. Electrochem. Soc. 124, 1569–1578 (1977).

    Article  CAS  Google Scholar 

  75. Shu, G. J. & Chou, F. C. Sodium-ion diffusion and ordering in single-crystal P2-NaxCoO2. Phys. Rev. B 78, 052101 (2008).

    Article  ADS  CAS  Google Scholar 

  76. Reddy, M. V. et al. Studies on the lithium ion diffusion coefficients of electrospun Nb2O5 nanostructures using galvanostatic intermittent titration and electrochemical impedance spectroscopy. Electrochim. Acta 128, 198–202 (2014).

    Article  CAS  Google Scholar 

  77. Ruscher, C., Salje, E. & Hussain, A. The effect of the Nb-W distribution on polaronic transport in ternary Nb-W oxides: electrical and optical properties. J. Phys. C 21, 4465–4480 (1988).

    Article  ADS  Google Scholar 

  78. Cava, R. J. et al. Electrical and magnetic properties of Nb2O5-δ crystallographic shear structures. Phys. Rev. B 44, 6973–6981 (1991).

    Article  ADS  CAS  Google Scholar 

  79. Dickens, P. G. & Whittingham, M. S. The tungsten bronzes and related compounds. Q. Rev. Chem. Soc. 22, 30–44 (1968).

    Article  CAS  Google Scholar 

  80. Hull, S. Superionics: crystal structures and conduction processes. Rep. Prog. Phys. 67, 1233–1314 (2004).

    Article  ADS  CAS  Google Scholar 

  81. Kamaya, N. et al. A lithium superionic conductor. Nat. Mater. 10, 682–686 (2011).

    Article  PubMed  ADS  CAS  Google Scholar 

  82. Kuhn, A., Duppel, V. & Lotsch, B. V. Tetragonal Li10GeP2S12 and Li7GePS8 – exploring the Li ion dynamics in LGPS Li electrolytes. Energy Environ. Sci. 6, 3548–3552 (2013).

    Article  CAS  Google Scholar 

  83. Mo, Y., Ong, S. P. & Ceder, G. First principles study of the Li10GeP2S12 lithium super ionic conductor material. Chem. Mater. 24, 15–17 (2012).

    Article  CAS  Google Scholar 

  84. Phani Dathar, G. K., Balachandran, J., Kent, P. R. C., Rondinone, A. J. & Ganesh, P. Li-ion site disorder driven superionic conductivity in solid electrolytes: a first-principles investigation of β-Li3PS4. J. Mater. Chem. A 5, 1153–1159 (2017).

    Article  CAS  Google Scholar 

  85. Bevan, D. J. M. & Hagenmuller, D. Non-Stoichiometric Compounds: Tungsten Bronzes, Vanadium Bronzes and Related Compounds 1st edn (Pergamon Press, Exeter, 1973).

    Google Scholar 

  86. Pinus, I., Catti, M., Ruffo, R., Salamone, M. M. & Mari, C. M. Neutron diffraction and electrochemical study of FeNb11O29/Li11FeNb11O29 for lithium battery anode applications. Chem. Mater. 26, 2203–2209 (2014).

    Article  CAS  Google Scholar 

  87. Galy, J. & Andersson, S. Structure cristalline de MoNb15O40F. Acta Crystallogr. B 24, 1027–1031 (1968).

    Article  CAS  Google Scholar 

  88. Idrees, F. et al. Facile synthesis of novel Nb3O7F nanoflowers, their optical and photocatalytic properties. CrystEngComm 15, 8146–8152 (2013).

    Article  CAS  Google Scholar 

  89. Zaghib, K., Mauger, A., Groult, H., Goodenough, J. B. & Julien, C. M. Advanced electrodes for high power Li-ion batteries. Materials 6, 1028–1049 (2013).

    Article  PubMed  PubMed Central  ADS  CAS  Google Scholar 

  90. Wen, C. J., Boukamp, B. A., Huggins, R. A. & Weppner, W. Thermodynamic and mass transport properties of “LiAl”. J. Electrochem. Soc. 126, 2258–2266 (1979).

    Article  CAS  Google Scholar 

  91. He, Y.-B. et al. Gassing in Li4Ti5O12-based batteries and its remedy. Sci. Rep. 2, 913 (2012).

    Article  PubMed  PubMed Central  CAS  Google Scholar 

  92. Lv, W., Gu, J., Niu, Y., Wen, K. & He, W. Review—gassing mechanism and suppressing solutions in Li4Ti5O12-based lithium-ion batteries. J. Electrochem. Soc. 164, A2213–A2224 (2017).

    Article  CAS  Google Scholar 

  93. Vinod Chandran, C. & Heitjans, P. Solid-state NMR studies of lithium ion dynamics across materials classes. Ann. Rep. NMR Spectrosc. 89, 1–102 (2016).

    Article  CAS  Google Scholar 

  94. Wang, Z. et al. Lithium diffusion in lithium nitride by pulsed-field gradient NMR. Phys. Chem. Chem. Phys. 14, 13535–13538 (2012).

    Article  PubMed  CAS  Google Scholar 

  95. Kuhn, A. et al. A new ultrafast superionic Li-conductor: ion dynamics in Li11Si2PS12 and comparison with other tetragonal LGPS-type electrolytes. Phys. Chem. Chem. Phys. 16, 14669–14674 (2014).

    Article  PubMed  CAS  Google Scholar 

  96. Kaus, M. et al. Local structures and Li ion dynamics in a Li10SnP2S12-based composite observed by multinuclear solid-state NMR spectroscopy. J. Phys. Chem. C 121, 23370–23376 (2017).

    Article  CAS  Google Scholar 

  97. Hayamizu, K. & Aihara, Y. Lithium ion diffusion in solid electrolyte (Li2S)7(P2S5)3 measured by pulsed-gradient spin-echo 7Li NMR spectroscopy. Solid State Ion. 238, 7–14 (2013).

    Article  CAS  Google Scholar 

  98. Gobet, M., Greenbaum, S., Sahu, G. & Liang, C. Structural evolution and Li dynamics in nanophase Li3PS4 by solid-state and pulsed-field gradient NMR. Chem. Mater. 26, 3558–3564 (2014).

    Article  CAS  Google Scholar 

  99. Hayamizu, K. et al. NMR studies on lithium ion migration in sulfide-based conductors, amorphous and crystalline Li3PS4. Solid State Ion. 285, 51–58 (2016).

    Article  CAS  Google Scholar 

  100. Holzmann, T. et al. Li0.6[Li0.2Sn0.8S2] – a layered lithium superionic conductor. Energy Environ. Sci. 9, 2578–2585 (2016).

    Article  CAS  Google Scholar 

  101. Ishiyama, H. et al. Nanoscale diffusion tracing by radioactive 8Li tracer. Jpn. J. Appl. Phys. 53, 110303 (2014).

    Article  ADS  CAS  Google Scholar 

  102. Holz, M. & Weingartner, H. Calibration in accurate spin-echo self-diffusion measurements using 1H and less-common nuclei. J. Magn. Reson. 92, 115–125 (1991).

    ADS  Google Scholar 

  103. Hayamizu, K. Temperature dependence of self-diffusion coefficients of ions and solvents in ethylene carbonate, propylene carbonate, and diethyl carbonate single solutions and ethylene carbonate + diethyl carbonate binary solutions of LiPF6 studied by NMR. J. Chem. Eng. Data 57, 2012–2017 (2012).

    Article  CAS  Google Scholar 

  104. Chowdhury, M. T., Takekawa, R., Iwai, Y., Kuwata, N. & Kawamura, J. Lithium ion diffusion in Li β-alumina single crystals measured by pulsed field gradient NMR spectroscopy. J. Chem. Phys. 140, 124509 (2014).

    Article  PubMed  ADS  CAS  Google Scholar 

  105. Hayamizu, K. & Seki, S. Long-range Li ion diffusion in NASICON-type Li1.5Al0.5Ge1.5(PO4)3 (LAGP) studied by 7Li pulsed-gradient spin-echo NMR. Phys. Chem. Chem. Phys. 19, 23483–23491 (2017).

    Article  PubMed  CAS  Google Scholar 

  106. Arbi, K. et al. Ionic mobility in Nasicon-type LiMIV2(PO4)3 materials followed by 7Li NMR spectroscopy. MRS Proc. 1313 (2011).

  107. Hayamizu, K., Matsuda, Y., Matsui, M. & Imanishi, N. Lithium ion diffusion measurements on a garnet-type solid conductor Li6.6La3Zr1.6Ta0.4O12 by using a pulsed-gradient spin-echo NMR method. Solid State Nucl. Magn. Reson. 70, 21–27 (2015).

    Article  PubMed  CAS  Google Scholar 

  108. Kuhn, A. et al. Li self-diffusion in garnet-type Li7La3Zr2O12 as probed directly by diffusion-induced 7Li spin-lattice relaxation NMR spectroscopy. Phys. Rev. B 83, 094302 (2011).

    Article  ADS  CAS  Google Scholar 

  109. Langer, J., Epp, V., Heitjans, P., Mautner, F. A. & Wilkening, M. Lithium motion in the anode material LiC6 as seen via time-domain 7Li NMR. Phys. Rev. B 88, 094304 (2013).

    Article  ADS  CAS  Google Scholar 

  110. Mali, M., Roos, J., Sonderegger, M., Brinkmann, D. & Heitjans, P. 6Li and 7Li diffusion coefficients in solid lithium measured by the NMR pulsed field gradient technique. J. Phys. F Met. Phys. 18, 403–412 (1988).

    Article  ADS  CAS  Google Scholar 

  111. Sugiyama, J. et al. Li-ion diffusion in Li4Ti5O12 and LiTi2O4 battery materials detected by muon spin spectroscopy. Phys. Rev. B 92, 014417 (2015).

    Article  ADS  CAS  Google Scholar 

  112. Sugiyama, J. et al. Lithium diffusion in spinel Li4Ti5O12 and LiTi2O4 films detected with 8Li beta-NMR. Phys. Rev. B 96, 094402 (2017).

    Article  ADS  Google Scholar 

  113. Wilkening, M. et al. Microscopic Li self-diffusion parameters in the lithiated anode material Li4+xTi5O12 (0 ≤ x ≤ 3) measured by 7Li solid state NMR. Phys. Chem. Chem. Phys. 9, 6199–6202 (2007).

    Article  PubMed  CAS  Google Scholar 

  114. Ruprecht, B., Wilkening, M., Uecker, R. & Heitjans, P. Extremely slow Li ion dynamics in monoclinic Li2TiO3—probing macroscopic jump diffusion via 7Li NMR stimulated echoes. Phys. Chem. Chem. Phys. 14, 11974–11980 (2012).

    Article  PubMed  CAS  Google Scholar 

  115. Wagemaker, M., van de Krol, R., Kentgens, A. P. M., van Well, A. A. & Mulder, F. M. Two phase morphology limits lithium diffusion in TiO2 (anatase): a 7Li MAS NMR study. J. Am. Chem. Soc. 123, 11454–11461 (2001).

    Article  PubMed  CAS  Google Scholar 

  116. Wagemaker, M. et al. The influence of size on phase morphology and Li-ion mobility in nanosized lithiated anatase TiO2. Chem. Eur. J. 13, 2023–2028 (2007).

    Article  PubMed  CAS  Google Scholar 

  117. Verhoeven, V. W. J. et al. Lithium dynamics in LiMn2O4 probed directly by two-dimensional 7Li NMR. Phys. Rev. Lett. 86, 4314–4317 (2001).

    Article  PubMed  ADS  CAS  Google Scholar 

  118. Ishiyama, H. et al. Direct measurement of nanoscale lithium diffusion in solid battery materials using radioactive tracer of 8Li. Nucl. Instrum. Methods B 376, 379–381 (2016).

    Article  ADS  CAS  Google Scholar 

  119. Bork, D. & Heitjans, P. NMR relaxation study of ion dynamics in nanocrystalline and polycrystalline LiNbO3. J. Phys. Chem. B 102, 7303–7306 (1998).

    Article  CAS  Google Scholar 

  120. Ruprecht, B. & Heitjans, P. Ultraslow lithium diffusion in Li3NbO4 probed by 7Li stimulated echo NMR spectroscopy. Diffusion Fundamentals 12, 100–101 (2010).

    Google Scholar 

  121. Sale, M. & Avdeev, M. 3DBVSMAPPER: a program for automatically generating bond-valence sum landscapes. J. Appl. Crystallogr. 45, 1054–1056 (2012).

    Article  CAS  Google Scholar 

  122. Avdeev, M., Sale, M., Adams, S. & Rao, R. P. Screening of the alkali-metal ion containing materials from the Inorganic Crystal Structure Database (ICSD) for high ionic conductivity pathways using the bond valence method. Solid State Ion. 225, 43–46 (2012).

    Article  CAS  Google Scholar 

  123. Brown, I. D. The Chemical Bond in Inorganic Chemistry: The Bond Valence Model (Oxford Univ. Press, Oxford, 2002).

    Google Scholar 

Download references

Acknowledgements

K.J.G. acknowledges support from The Winston Churchill Foundation of the United States, a Herchel Smith Scholarship and a Science and Technology Facilities Council Futures Early Career Award. K.J.G. and C.P.G. thank the EPSRC for a LIBATT grant (EP/M009521/1). L.E.M. acknowledges support from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska–Curie grant agreement number 750294 and a Charles and Katharine Darwin Research Fellowship. We thank I. Seymour from the University of Cambridge and B. Dunn from the University of California, Los Angeles for fruitful discussions. We thank J. Skepper and H. Greer from the University of Cambridge for assistance with electron microscopy and M. Avdeev from the Bragg Institute for the bond valence sum mapping program. We thank O. Borkiewicz from the Advanced Photon Source at Argonne National Laboratory and A. Kasam from the University of Cambridge for diffraction data reduction scripts. We thank Diamond Light Source for access to beamline B18 (SP11433, SP14956, SP16387, SP17913), where we obtained results presented here. This research used resources of the Advanced Photon Source (GUP40466, GUP41657, GUP47967), a US Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under contract number DE-AC02-06CH11357.

Reviewer information

Nature thanks S. Greenbaum, P. Woodward and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Author information

Authors and Affiliations

  1. Department of Chemistry, University of Cambridge, Cambridge, UK

    Kent J. Griffith, Lauren E. Marbella & Clare P. Grey

  2. X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne, IL, USA

    Kamila M. Wiaderek

  3. Diamond Light Source, Harwell Science and Innovation Campus, Didcot, UK

    Giannantonio Cibin

Authors
  1. Kent J. Griffith
    View author publications

    You can also search for this author in PubMed Google Scholar

  2. Kamila M. Wiaderek
    View author publications

    You can also search for this author in PubMed Google Scholar

  3. Giannantonio Cibin
    View author publications

    You can also search for this author in PubMed Google Scholar

  4. Lauren E. Marbella
    View author publications

    You can also search for this author in PubMed Google Scholar

  5. Clare P. Grey
    View author publications

    You can also search for this author in PubMed Google Scholar

Contributions

K.J.G. and C.P.G. conceived the idea. K.J.G. synthesized and characterized the materials and performed the electrochemistry experiments. K.J.G. performed the synchrotron diffraction and absorption experiments and analysed the data with support from K.M.W. and G.C. L.E.M. and K.J.G. performed the PFG NMR measurements. K.J.G. and C.P.G. wrote the manuscript with input from all co-authors.

Corresponding author

Correspondence to Clare P. Grey.

Ethics declarations

Competing interests

K.J.G. and C.P.G., via Cambridge Enterprise, have filed a UK patent application (GB1809467.2) covering the materials and high-rate energy storage application described in this manuscript.

Additional information

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

Extended data figures and tables

Extended Data Fig. 1 Overpotential in Li||Li symmetric cells and GITT of niobium tungsten oxides.

a, Li||Li symmetric cells were configured identically to those used for metal-oxide testing, with the exception of a second Li disk replacing the composite electrode. Rate testing was carried out with current densities corresponding to the rates shown in Fig. 2a–e, with 5 cycles at 100 μA (C/5), 500 μA (1C), 1 mA (2C), 2.5 mA (5C) and 5 mA (10C) and 10 cycles at 10 mA (20C), 20 mA (40C), 30 mA (60C) and 50 mA (100C). The ‘rates’ in parentheses indicate the inverse of the time (in hours) that the current was applied, simulating the conditions (current densities, periods of applied current and thus total charge transferred) of the rate test experiments. An excerpt of the results is shown here from the fifth cycle at C/5 until the end of the test; the test was performed twice with the same overpotentials observed in both cells. At low current densities, below 1 mA (2C), the overpotential is below 100 mV; however, at 5 mA (10C) the overpotential rises to 200 mV and increases to about 700 mV at 100C. b, c, Relative changes in lithium diffusion as a function of open-circuit voltage (Voc; b) and open-circuit voltage versus closed-circuit voltage (Vcc; c) from the GITT measurements. The plot in c shows the ‘thermodynamic’ electrochemical profiles at C/20 with a 12-h rest period at each point, reaching a full discharge in approximately one month. In Nb16W5O55, the fastest diffusion is observed from the dilute limit to Li4.5(5)Nb16W5O55, dropping by two orders of magnitude in the low-voltage window, where more than 1 Li+/TM are incorporated. The GITT data indicate that the second electrochemical region of Nb16W5O55 is broader than typically observed in a two-phase reaction, but the observed discontinuity in the DLiL–2 values in this region suggests that Nb16W5O55 approaches two-phase behaviour. The average diffusion coefficient in Nb18W16O93 is similar to that of Nb16W5O55. The bronze also displays discontinuities in DLiL–2 at 2.1, 1.85 and 1.7 V. In both phases, the low-voltage region (below 1.25 V, well over 1 Li+/TM) is characterized by an increasing overpotential and suppressed kinetics.

Extended Data Fig. 2 Lithium diffusion from 7Li PFG NMR spectroscopy.

a, The lithium diffusion coefficients of LixNb16W5O55 (x = 6.3, 8.4) and LixNb18W16O93 (x = 3.4, 6.8, 10.2) were measured in the temperature range 333–453 K (Table 1). The filled (85% signal contribution) and empty (15% signal contribution) symbols for Li6.3Nb16W5O55 correspond to the observed two-component diffusion (see main text and Methods). In most cases, the error bars (2σ, obtained from the fit) are smaller than the sizes of the points. b, Stimulated-echo PFG sequence used to measure 7Li diffusivities, showing both radiofrequency (7Li) and magnetic-field-gradient (Gz) pulses. Here, the gradient pulse duration (tg) includes the up-ramping, time on and down-ramping of the opt composite gradient pulses. During Δ, a short spoiler gradient was used to remove residual transverse magnetization. c–g, Representative 7Li decay curves showing the normalized NMR signal intensity as a function of gradient strength for the bronze structures Li10.2Nb18W16O93 at 453 K (c), Li6.8Nb18W16O93 at 453 K (d) and Li3.4Nb18W16O93 at 453 K (e) and the block structures Li6.3Nb16W5O55 at 353 K (f) and Li8.4Nb16W5O55 at 383 K (g). Black circles represent experimental data points and red lines represent mono- (c–e, g) or biexponential (f) fits to the data by the Stejskal–Tanner equation. Biexponential fits to all samples except Li6.3Nb16W5O55 did not lead to improved fits over those obtained with monoexponential fits. The poor signal-to-noise ratio of Li6.3Nb16W5O55, even with more than 100 mg of electrode sample, did not allow us to explore alternative models beyond the mono- and biexponential models, such as models considering the effect of anisotropic diffusion, which manifests as subtle differences in echo attenuation94. h, A representative one-dimensional 7Li NMR spectrum (Li8.4Nb16W5O55, static, 403 K) showing the shift (δ 7Li) region and lineshape.

Extended Data Fig. 3 Nb16W5O55 X-ray absorption spectroscopy.

a–d, LixNb16W5O55 ex situ (see Supplementary Fig. 14) Nb K edge XANES spectra (a, b) and derivative spectra (c, d). e, f, Operando Nb K edge XANES spectra of Nb16W5O55 and 22 discharge spectra, with each successive spectrum at about +11 mA h g–1 (e), and operando derivative spectra (f; 23 spectra shown, 5 colours labelled). The pre-edge and main edge are at about 18,991 and 19,004 eV, respectively. Spectra in b and d are vertically offset by 0.2 and 0.02, respectively, for clarity. g–l, LixNb16W5O55 ex situ W Lii edge XANES spectra (g, h), derivative spectra (i, j) and second-derivative spectra (k, l). Spectra in h, j and l are vertically offset by 0.5, 0.2 and 0.4, respectively, for clarity. m–p, LixNb16W5O55 ex situ W Li edge XANES spectra (m, n) and derivative spectra (o, p) near the W Li absorption edge. The pre-edge is at about 12,104 eV and the main edge at about 12,117 eV. Spectra are vertically offset by 0.5 in n and by 0.02 in p for clarity.

Extended Data Fig. 4 Nb18W16O93 X-ray absorption spectroscopy.

a–d, LixNb18W16O93 ex situ Nb K edge XANES spectra (a, b) and derivative spectra (c, d). The pre-edge and main edge are at about 18,991 and 19,004 eV, respectively. Spectra in b and d are vertically offset by 0.2 and 0.02, respectively, for clarity. e–j, LixNb18W16O93 ex situ W Lii edge XANES spectra (e, f), derivative spectra (g, h) and second-derivative spectra (i, j). Spectra in f, h and j are vertically offset by 0.5, 0.2 and 0.4, respectively, for clarity. k–m, LixNb18W16O93 ex situ W Li edge XANES spectra (k, l) and derivative spectra (m, n) near the W Li absorption edge. The pre-edge is at about 12,104 eV and the main edge at about 12,117 eV. Spectra are vertically offset by 0.25 in l and by 0.02 in n for clarity.

Extended Data Fig. 5 Lattice evolution of Nb16W5O55 and Nb18W16O93 upon lithiation.

a, b, Absolute lattice parameter values resulting from Rietveld refinement of operando diffraction data, analogous to that of Fig. 4c, Nb16W5O55 (a) and Fig. 4d, Nb18W16O93 (b). Error bars show the standard deviation of each parameter, as estimated from the fits (approximately equal to the symbol height in b). Shading distinguishes the different structural regions. For Nb18W16O93, the second stage (two-phase region) contains two sets of lattice parameters. c, Structure evolution of Nb16W5O55 as a function of rate. At high rates, the lithiation reaction becomes inhomogeneous (Extended Data Fig. 6) and cannot be fitted with a single set of lattice parameters over the whole electrode. The shaded grey area for the 5C data corresponds to the range of each unit-cell parameter. d, Structure evolution of Nb18W16O93 as a function of rate. The mechanism of LixNb18W16O93 lattice evolution does not appear to be strongly rate-dependent. The reaction extends further at lower rates within the same voltage range, partly because of a smaller overpotential at lower current densities (Extended Data Fig. 1a).

Extended Data Fig. 6 Solid-solution structure evolution and reversibility of Nb16W5O55.

a–c, Profile evolution of selected reflections at C/2 and d, the corresponding electrochemical discharge profile. e–g, Profile evolution of selected reflections at 5C and h, the corresponding electrochemical discharge profile. a, e, (\(60\bar{4}\)). b, f, overlapping (110) and (\(11\bar{1}\)). c, g, (\(10\hspace{2.77626pt}0\bar{1}\)) (left) and (\(60\bar{9}\)) (right), with smaller overlapping reflections. The evolution in each case commences analogously; as lithiation increases, the structure evolution at high rate becomes inhomogeneous and the contraction of the ac block is not fully realized. i, The overlapping (110) and (\(11\bar{1}\)) reflections shift smoothly to larger d spacing (smaller 2θ) for the entire sample at C/2, whereas in j, there is substantial intensity across a range of 2θ values at high rate. This inter-peak intensity is shown more clearly in the inset of j. The intensity range represents a range of lattice parameters (Extended Data Fig. 5c) and an inhomogeneous solid-solution reaction probably resulting from inhomogeneous lithium transport and concentration gradients within the electrolyte33,34. Diffraction patterns are shown from 0 to about 145 mA h g−1 (ascending), which corresponds to the full 5C discharge and partial C/2 discharge. k, l, Structure reversibility over a full electrochemical lithiation/delithiation cycle of Nb16W5O55 at C/2. In k, the (\(14\hspace{2.77626pt}0\bar{9}\))/(407) (left) and (020) (right) reflections are displayed over a full operando synchrotron X-ray diffraction discharge–charge cycle from 3.0 to 1.0 V with multi-redox (de)lithiation. The symmetry of discharge and charge is apparent, although a small amount of lithium remains in the structure after charging (Extended Data Fig. 7).

Extended Data Fig. 7 Electrochemical profile evolution and early-cycle lithium retention of Nb16W5O55 and Nb18W16O93.

a–f, There is an activation process on the first cycle for Nb16W5O55 (a–c) and Nb18W16O93 (d–f), which extends to a much smaller extent in the next several cycles, leading to an increase in intercalation voltage at the first ‘plateau-like’ feature and a broadening of the dQ/dV peaks (b, c, e, f). The phenomenon is associated with a retention of lithium in the structure. The occurrence of this activation phenomenon is structure-independent, indicating that it may have an electronic origin. The electrochemical data in a–f were collected at C/5 but the phenomenon is also observed at other rates. g, Diffraction patterns of Nb16W5O55 before lithiation and after the first charge from operando measurements at C/2. h, Lattice parameters of the pristine and charged structure show changes that indicate that some lithium was retained in the structure after charging the electrode, commensurate with the changes from first- to second-cycle electrochemistry.

Extended Data Fig. 8 Operando X-ray diffraction patterns of Nb18W16O93 from 1C to 10C.

a–l, The profile evolution of selected reflections and the corresponding electrochemical discharge profile is shown at 1C (a–d), 5C (e–h) and 10C (i–l). a, e, i, (040); b, f, j, overlapped (230) and (160); c, g, k, (001) (left, initially) and overlapped (330) and (190) (right, initially). The evolution in each case is similar in mechanism and differs in the extent of reaction, consistent with the electrochemical profiles. m–x, The two-phase region of LixNb18W16O93 for x ≈ 6.6–10.2 as a function of rate. Selected regions of the diffraction pattern at shown at 1C (m–p), 5C (q–t) and 10C (u–x). m, q, u, (040); n, r, v, overlapped (230) and (160); o, s, w, (001) (left, initially) and overlapped (330) and (190) (right, initially). The arrows from the electrochemical discharge profiles in p, t and x correspond to the first and last diffraction patterns collected in the two-phase region (about 29–45 mA h g–1), as indicated. Metastable intermediates that can occur at high rates in two-phase systems such as LiFePO4 would be difficult to distinguish in this system owing to the small compositional and structural changes that are associated with the Li6.6Nb18W16O93 to Li10.2Nb18W16O93 two-phase reaction.

Extended Data Fig. 9 Prospective lithium positions and pathways in block-type and bronze-type ternary niobium tungsten oxides and electrochemical comparisons to binary niobium oxides.

a, b, Bond valence sum maps of Nb16W5O55 (a) and Nb18W16O93 (b) depict stable lithium positions and pathways according to bond valence energy landscape calculations performed in 3DBVSMAPPER121. Calculations were performed over a fine grid with 149 × 20 × 116 points computed for Nb16W5O55 and 61 × 184 × 20 points computed for Nb18W16O93 along their respective crystallographic axes. Isosurface levels are shown at ‘2.0 eV’, which is a parameter used to visualize ionic pathways and not a quantitative estimation. The bond valence sum and bond valence energy landscape provide an indication of lithium positions and diffusion pathways in complex or novel systems and have shown good agreement with experimental and computational investigations of structure and dynamics121,122,123. c, Possible intrablock lithium positions for Nb16W5O55 based on LixMO3 in the low-lithium regime, before it undergoes intercalant-induced distortion. d, e, Block phases Nb16W5O55 and H-Nb2O5 are compared on the basis of Li+/TM (d) and gravimetric capacity (e) on the third cycle at C/5. f, g, Bronze-like phases Nb18W16O93 and T-Nb2O5 are compared on the basis of Li+/TM (f) and gravimetric capacity (g) on the third cycle at C/5.

Extended Data Table 1 Lithium self-diffusion coefficients of lithium-ion battery electrode materials, solid electrolytes, liquid electrolytes and reference compounds
Full size table

Supplementary information

Supplementary Information

This file contains seventeen supplementary figures and three supplementary tables that support the niobium tungsten oxide synthesis, characterization, and mechanistic analysis.

Rights and permissions

Reprints and permissions

About this article

Check for updates. Verify currency and authenticity via CrossMark

Cite this article

Griffith, K.J., Wiaderek, K.M., Cibin, G. et al. Niobium tungsten oxides for high-rate lithium-ion energy storage. Nature 559, 556–563 (2018). https://doi.org/10.1038/s41586-018-0347-0

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI: https://doi.org/10.1038/s41586-018-0347-0

This article is cited by

Comments

By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.

Search

Advanced search

Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing