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

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

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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.

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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.

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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.

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

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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

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  1. Kent J. Griffith
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  2. Kamila M. Wiaderek
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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.

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Correspondence to Clare P. Grey.

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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.

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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.

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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

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