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Copper-coordinated cellulose ion conductors for solid-state batteries

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Abstract

Although solid-state lithium (Li)-metal batteries promise both high energy density and safety, existing solid ion conductors fail to satisfy the rigorous requirements of battery operations. Inorganic ion conductors allow fast ion transport, but their rigid and brittle nature prevents good interfacial contact with electrodes. Conversely, polymer ion conductors that are Li-metal-stable usually provide better interfacial compatibility and mechanical tolerance, but typically suffer from inferior ionic conductivity owing to the coupling of the ion transport with the motion of the polymer chains1,2,3. Here we report a general strategy for achieving high-performance solid polymer ion conductors by engineering of molecular channels. Through the coordination of copper ions (Cu2+) with one-dimensional cellulose nanofibrils, we show that the opening of molecular channels within the normally ion-insulating cellulose enables rapid transport of Li+ ions along the polymer chains. In addition to high Li+ conductivity (1.5 × 10−3 siemens per centimetre at room temperature along the molecular chain direction), the Cu2+-coordinated cellulose ion conductor also exhibits a high transference number (0.78, compared with 0.2–0.5 in other polymers2) and a wide window of electrochemical stability (0–4.5 volts) that can accommodate both the Li-metal anode and high-voltage cathodes. This one-dimensional ion conductor also allows ion percolation in thick LiFePO4 solid-state cathodes for application in batteries with a high energy density. Furthermore, we have verified the universality of this molecular-channel engineering approach with other polymers and cations, achieving similarly high conductivities, with implications that could go beyond safe, high-performance solid-state batteries.

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Fig. 1: Structure and ion-transport performance of the Li–Cu–CNF solid-state ion conductor.
Fig. 2: Structural evolution during the synthesis of Li–Cu–CNF.
Fig. 3: Li+ conductivity and transport mechanism in Li–Cu–CNF.
Fig. 4: Demonstration of solid-state Li metal batteries using the Li–Cu–CNF ion conductor.

Data availability

The data that support the findings of this study are available within this article and its Supplementary Information. Additional data are available from the corresponding authors upon reasonable request. Source data are provided with this paper.

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Acknowledgements

L.H. and C.Y. acknowledge support from the University of Maryland A. James Clark School of Engineering. Q.W. and Y.Q. acknowledge support from the National Science Foundation (NSF) under grant DMR-2054438. Y.-Y.H. acknowledges support from the NSF under grant DMR-1847038. Solid-state NMR experiments were carried out at the National High Magnetic Field Laboratory, which is supported by the NSF through NSF/DMR-1644779 and the State of Florida. The NMR diffusion work at Hunter College was supported in part by the US Office of Naval Research, grant N00014-20-1-2186. This research used resources at the 8-ID Beamline of the National Synchrotron Light Source II, a US Department of Energy Office of Science User Facility operated by Brookhaven National Laboratory under contract no. DE-SC0012704. Access to the High Flux Backscattering Spectrometer was provided by the Center for High Resolution Neutron Scattering, a partnership between the National Institute of Standards and Technology (NIST) and the NSF under agreement DMR-1508249. Certain commercial equipment, instruments and materials are identified in this paper in order to specify the experimental procedure adequately. Such identification is not intended to imply recommendation or endorsement by the NIST. K.X. acknowledges support from the Joint Centre for Energy Security Research (JCESR), an energy hub funded by the US Department of Energy, Office of Science, Basic Energy Science under IAA SN2020957. We thank F. Lin, D. Hou and Z. Yang from Virginia Tech for providing the NMC811 cathode material.

Author information

Authors and Affiliations

Authors

Contributions

L.H. conceived the SPE concept and supervised the project. C.Y. and W.X. designed the experiments and conducted the material synthesis, characterization and electrochemical tests. S.H. and Y.G. also contributed to the material synthesis. Y.Q. and Q.W. carried out the DFT and MD simulations and XANES calculation. X.Z. and R.B. contributed to the structural analysis. M.N.G. and S.G. carried out the NMR analysis and determination of the ion diffusion coefficient. J.Z., P.W. and Y.-Y.H. carried out the 6Li NMR analysis. Y.M. and M.T. carried out the neutron-scattering characterization. B.H.K. and F.J. contributed to the XAS experiment. C.W. provided LiMn2O4 material and assisted with testing and evaluating the electrochemical stability window. L.H., C.Y., Y.Q., Q.W., A.B., C.W., A.I., P.A., K.X. and M.W. drafted the paper. All authors contributed to the final manuscript.

Corresponding authors

Correspondence to Yue Qi or Liangbing Hu.

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The authors declare no competing interests.

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Peer review information Nature thanks Lynden Archer and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Fig. 1 Structural characterization during the synthesis of Li–Cu–CNF.

ad, Fibre XRD patterns of the CNFs (in the format of densified wood for high-resolution diffraction patterns) after the following treatment steps. a, Cu–CNF–NaOH obtained from Cu2+-saturated NaOH aqueous solution. Peaks are indexed on the basis of the literature23. The (003) reflection is observed with a spacing of 0.51 nm, while the (001) and (002) reflections are absent, indicating that the Cu–CNF–NaOH features a threefold symmetric structure along the direction of the cellulose molecular chain. b, Cu–CNF washed with water to remove NaOH, demonstrating an amorphous structure. c, Cu–CNF after removing water by DMF exchange and evaporating DMF. The fibre XRD pattern shows a mostly amorphous structure with a small angle peak at roughly 2 nm corresponding to cellulose II22, possibly because of a small number of cellulose chains without coordinated Cu that form cellulose II after NaOH is removed. A high q peak at roughly 0.4 nm in the equatorial direction indicates the average molecular chain-to-chain distance of cellulose II. The green arrow in the meridian direction shows a peak corresponding to 0.47 nm in real space. The yellow arrows are pointing to peaks indicating the repeating unit of the Cu–CNF is roughly 1 nm along the cellulose chain. The 0.47 nm and 1 nm repeating distances are absent in all known cellulose structures, and therefore we attribute them to the unique structure of the Cu–CNF. d, Li–Cu–CNF after inserting Li+ in Cu–CNF and evaporating the solvent. The amorphous cellulose structure is maintained with some weak diffraction peaks of cellulose II. The yellow and green arrows indicate the same peaks as in c. eh, XAS analysis of the Cu–CNF and Li–Cu–CNF samples. e, f, Cu K-edge X-ray absorption near edge structure (XANES) spectra of: e, Cu–CNF, Li–Cu–CNF and a CuO standard; f, Cu2O and Cu standard samples. The green dashed line shows the calculated XANES spectrum of Li–Cu–CNF, in good agreement with the experimentally measured spectrum. Cu–CNF and Li–Cu–CNF show similar yet broadened pre-edge peaks to CuO at 8,986 eV (1s → 3d transition), without the characteristic peaks of Cu2O or Cu metal, indicating that the Cu ions in Cu–CNF and Li–Cu–CNF are of +2 valency. g, h, Fourier-transformed Cu K-edge extended X-ray absorption fine structure (EXAFS) spectra of: g, Cu–CNF; h, Li–Cu–CNF. On the basis of the EXAFS spectra, in Cu–CNF and Li–Cu–CNF, the Cu2+ are bonded with O atoms with an average bonding distance of 1.97 Å, consistent with that in reported Cu–organic complexes48, indicating that the Cu2+ is coordinated with the hydroxyl groups of the cellulose molecules.

Extended Data Fig. 2 DFT calculations and MD simulations of the Li–Cu–CNF structure.

a, Assigned COMPASS II force-field types and atomic charges in typical cellulose units for MD simulations. b, Optimized atomic structures of the representative systems of Li–Cu–CNF used to evaluate interactions between Li+ and different oxygen-containing functional groups and water molecules, and corresponding energy–distance relationships for different Li+-bonding environments given by molecular mechanics calculations using COMPASS II FF and DFT calculations. The difference between the total system energy at r = 10.0 Å and the minimum energy is taken as the Li+-dissociation energy. The Li+ is strongly bonded with both anionic COO and RO groups with dissociation energies of more than 5.0 eV. The dissociation energy of the Li+ is roughly 3.0 eV for ROH and EO groups, and 1.5 eV for H2O molecules. The strong interactions between the Li+ and one or two oxygen species in cellulose suggest slow Li+ movements in the absence of multiple Li–O coordination. In the H2O molecule, the O atom has an atom type of o2* and charge of −0.82 e, while the H atom has an atom type of h1o and a charge of +0.41 e for force-field calculations. c, To simulate Cu2+ coordination in cellulose, we optimized the atomic structure of two AGUs connected by one Cu2+ (Cu–(AGU)2 system) to serve as a structural building block. Two H atoms are deprotonated by the Cu. The average optimized Cu–O bond length (1.96 Å) is close to that observed in the experiment (1.97 Å), and the calculated XANES of the Cu–(AGU)2 system is also in good agreement with the experimental measurement (Extended Data Fig. 1e), showing that our computational model for the structure of the Cu–O complex is reasonable. Atom types and atomic charges in force-field calculations are given for Cu and its connected O atoms, which are categorized as ROH for statistics (Fig. 3d in the main text). d, Top view of a 2 × 2 supercell of the periodic Cu-coordinated CNF structure as a starting structure for the simulation, built with the most reasonable model that we proposed on the basis of the fibre XRD pattern (Fig. 2c). Every two nearby cellulose chains are connected by one Cu atom through the hydroxyl oxygen atoms. The unit cell is denoted by dashed blue lines. e, Top and side views of the Cu–CNF–NaOH. f, Top and side views of the amorphous Cu–CNF obtained by removing NaOH aqueous solution from Cu–CNF–NaOH and then equilibrating the system with NPT dynamics simulations. In Cu–CNF, we reserved 144 H2O molecules to keep an H2O:AGU ratio of 1:1. g, Schematic of the computational approach used to obtain the atomic structure of the final amorphous Li–Cu–CNF model (top and side views).

Extended Data Fig. 3 Bound water analysis of Li–Cu–CNF.

a, 1H MAS NMR spectra of Li–Cu–CNF with peak deconvolution. b, 1H MAS NMR spectra of Cu–CNF (dried at 30 °C under vacuum for three days to remove water). c, FTIR of the pristine CNFs and solid-state Li–Cu–CNF. Both the pristine CNFs and Li–Cu–CNF show a broad –OH stretching peak at roughly 3,300 cm−1. d, The −OH stretching peak of Li–Cu–CNF deconvoluted into three bands at 3,464 cm−1, 3,235 cm−1 and 2,886 cm−1, which can be assigned to bound water molecules in different hydrogen-bonding states49,50. e, The atomic mean square displacement (MSD) change in CNF and Li–Cu–CNF as a function of temperature, as measured by QENS. The Cu–CNF sample after DMF solvent exchange with some residual DMF (Cu–CNF–DMF) is also shown for comparison. f, Elastic neutron-scattering intensity of free water plotted against temperature (60 μl H2O on Cu foil) upon cooling; data derived from ref. 51. g, DSC curve of Li–Cu–CNF in a cooling process from 30 °C to −30 °C. h, Plots of H2O–H2O radial distribution function (RDF) (solid lines) and coordination number (dashed lines) in liquid bulk water (red lines) and Li–Cu–CNF (blue lines). The first minimum of the RDF plot for the liquid bulk water system at 3.4 Å (indicated by the black dashed line) was applied to calculate the coordination numbers. The distance is defined as the distance between the O atoms of the H2O molecules. i, Stress–strain curve of Li–Cu–CNF along the direction of the CNF fibre. For more analysis, see Supplementary Discussion 11.

Extended Data Fig. 4 Electrochemical stability of Li–Cu–CNF.

a, b, The electrochemical stability window of Li–Cu–CNF was measured by both anodic and cathodic LSV scans at 0.1 mV s−1. a, The first three anodic scans from OCV to 5.4 V. b, The first three cathodic scans from OCV to 0 V. c, Top, reduction and oxidation potentials (versus Li+/Li) obtained from DFT calculations for (bottom) different structures representative of the cellulose and Li–Cu–CNF systems, including: (1) glucose; (2) AGU dimer; (3) AGU–COOLi; (4, 5) two isomers of (AGU)2–COOLi; (6) AGU–CH2OLi; (7, 8) two isomers of (AGU)2–CH2OLi; (9) Cu– (AGU)2; (10) H2O dimer; and (11) (AGU)2–(H2O)2. C, H, O, Li and Cu atoms are represented by grey, white, red, purple and blue spheres, respectively. Water molecules are depicted with stick models. The experimental oxidation potential for Li–Cu–CNF (black) and the redox potentials for EC are denoted with dashed lines (blue for reduction and red for oxidation) for reference. See Supplementary Discussion 12 for more detailed analysis.

Extended Data Fig. 5 Ionic conductivities and transference numbers of Li–Cu–CNF and Li–CNF.

a, Voltage profile of the galvanostatic Li plating and stripping between two ends of the Li–Cu–CNF with aligned cellulose fibres (length 1 cm) at 0.01 mA. b, EIS Nyquist plots of aligned Li–Cu–CNF materials of different lengths, ranging from 1 cm to 3 cm, for measuring the intrinsic conductivity of Li–Cu–CNF along the direction of the cellulose molecular chain. c, Resistance corresponding to the high-frequency semi-circle in b of the aligned Li–Cu–CNF with different lengths. d, EIS Nyquist plots of the aligned Li–Cu–CNF with a length of 3 cm and cross-sectional area of 0.03 cm2 at different temperatures, ranging from 10 °C to 60 °C. e, f, EIS Nyquist plots of the Li–Cu–CNF paper electrolyte (through-plane) at different temperatures (e, 60 °C to 0 °C; f, −2 °C to −20 °C); g, the corresponding temperature-dependent through-plane ionic conductivity of the Li–Cu–CNF paper electrolyte. h, d.c. polarization curve of the Cu2+ in the Li–Cu–CNF electrolyte in a Cu//Cu-CNF//Cu cell, showing that the Cu2+ conductivity is 1.0 × 10−8 S cm−1, much lower than the Li+ conductivity in Li–Cu–CNF. i, Simulated structure of Li–CNF by MD. The Li–CNF system consists of 16 cellulose chains surrounded by Li+ and water molecules. Different chains are denoted by different colours. Li+ ions are indicated by purple spheres and water molecules as stick models. The size of the Li–CNF system is given roughly. The simulations show that, without the participation of Cu2+, the Li+ and water molecules adsorb only on the surface of the cellulose structures. j, d.c. polarization curve, and k, EIS Nyquist plots before and after polarization of the Li//Li–Cu–CNF//Li cell. l, d.c. polarization curve, and m, EIS Nyquist plots before and after polarization of the Li//Li–CNF//Li cell. n, Table showing the parameters measured by d.c. polarization and EIS for calculating the Li+-transference number.

Extended Data Fig. 6 NMR analysis of Li-coordination environments and diffusion pathways.

a, b, 6Li NMR spectra and simulations for: a, Li–Cu–CNF; b, Li–CNF. c, d, 6Li NMR spectra for: c, CH2COOLi∙2H2O, and d, LiPF6, as references for the COO∙∙∙Li and LiPF6 peak assignments in Li–Cu–CNF. e, f, 6Li NMR spectra and simulations for: e, Li–Cu–CNF, and f, Li–CNF after 6Li→7Li tracer exchange, which was performed by cycling either the Li–Cu–CNF or the Li–CNF electrolyte (natural abundance: 92.4% 7Li and 7.6% 6Li) between two 6Li-enriched metal electrodes (that is, symmetric 6Li//Li–Cu–CNF//6Li cells). g, Table showing the amount of Li+ in the different chemical environments of Li–Cu–CNF and Li–CNF before and after 6Li→7Li tracer exchange, derived from the relative spectral areal integrals of the 6Li resonances in the NMR spectra shown in a, b, e, f. The normalized peak area for each sample (Li–Cu–CNF and Li–CNF, before and after 6Li→7Li tracer exchange) can be quantitatively compared between different samples as the normalized peak area is proportional to the amount of 6Li in each individual product. We took the total number of 6Li in the pristine Li–Cu–CNF (before 6Li→7Li tracer exchange) to be 100%, and calculated the ‘relative 6Li number’ of each component by comparing the fitted peak area (Extended Data Fig. 6a, b, e, f) with the total area of 6Li in the pristine Li–Cu–CNF (Extended Data Fig. 6a).

Extended Data Fig. 7 Numerical analyses of MD simulations for Li+ transport in Li–Cu–CNF.

a, Displacement plots for six Li+ ions that have displacements of more than 15.0 Å in the simulated Li–Cu–CNF system (Fig. 2i) with an H2O:AGU ratio of 1:1, and indexes of COO/RO atoms that are bonded to the six moving Li+ ions (Li–O distance less than 2.5 Å). The different colours of the COO/RO atoms indicate they are from different cellulose chains. b, Coordination numbers of Li+ ions coordinating with all available oxygen atoms (Li–O, including the oxygen atoms in cellulose and bound water molecules) and with just water molecules (Li–H2O) for the six fastest and six slowest Li+ ions in the Li–Cu–CNF system with an H2O:AGU ratio of 1:1. c, MSD plots for Li–Cu–CNF systems with different number of water molecules, and for the Li–CNF system with water molecules on the surface of the CNFs. d, Radial distribution functions (RDFs) for Li–Li and COO–RO pairs in Li–Cu–CNF with an H2O:AGU ratio of 1:1. The locations of the first peak of the Li–Li pair and the second peak of the COO-RO pair indicate the Li+ hopping distance (roughly 3.0 Å) between the residence sites. The first peak of the COO–RO pair indicates the distance between the two O atoms within the same COO group. e, MSD plots for Li+, COO and RO groups and Cu2+ in the simulated Li–Cu–CNF system with an H2O:AGU ratio of 1:1. The average MSD plots show that Li+ moves fast while COO, RO, and Cu2+ in the Li–Cu–CNF backbone move much more slowly. For further analysis, see Supplementary Discussion 13.

Extended Data Fig. 8 The Li–Cu–CNF paper electrolyte and its electrochemical performance.

a, Top-view SEM image of the Li–Cu–CNF paper electrolyte. b, Digital photos (top and back) of a permeability test of the Li–Cu–CNF paper electrolyte to demonstrate the denseness. c, Li plating/stripping cycling performance of the Li–Cu–CNF paper electrolyte at 0.5 mA cm−2, with 2 h for each plating/stripping half cycle, for a total of 300 h at room temperature. d, SEM image and e, corresponding EDX spectrum of the Li-metal anode after long-term cycling with the Li–Cu–CNF paper electrolyte. The SEM image of the cycled Li anode shows a fairly smooth surface without Cu particles deposited on the surface. The EDX shows no detectable Cu element on the Li surface, and instead only C, O, F and P, indicating the formation of a solid electrolyte interphase (SEI) on the Li-metal anode.

Extended Data Fig. 9 Demonstration using Li–Cu–CNF as a paper electrolyte and ion-conducting binder for solid-state LiFePO4 batteries.

a, Fabrication steps for incorporating the cathode material (LiFePO4 here) with the Li–Cu–CNF ion-conducting binder via the traditional slurry-casting method. The Cu–CNF suspension is first mixed with the cathode material, CNT additive and sodium alginate binder in an aqueous solution to obtain the cathode slurry. The slurry is then cast on aluminium foil using a doctor blade and vacuum dried at 35 °C. The cathode electrodes are then soaked in Li+ electrolyte to achieve the insertion of Li+ into the Cu–CNF, followed by vacuum drying to obtain solid electrodes containing the Li–Cu–CNF binder. b, c, EIS of the solid-state batteries using thick LiFePO4 cathodes (roughly 120 μm), made by filtration-pressing with the addition of: b, Li–Cu–CNF; c, Li–CNF. d, A pouch solid-state battery made using a Li anode, the Li–Cu–CNF paper SPE, and a LiFePO4 solid-state cathode containing the Li–Cu–CNF ion-conducting binder, which shows good flexibility while still powering an LED light.

Extended Data Fig. 10 Electrochemical performances of high-voltage cathodes with the solid-state Li–Cu–CNF electrolyte.

a, b, Typical galvanostatic charge/discharge voltage profile of a solid-state NMC811 cathode with the Li–Cu–CNF electrolyte cycled at 100 mA g−1 and room temperature (a); and its discharge capacities during cycling (b). c, d, Typical galvanostatic charge/discharge voltage profile of the solid-state LiMn2O4 cathode with the Li–Cu–CNF electrolyte cycled at 50 mA g−1 and room temperature (c); and its discharge capacities during cycling (d).

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This file contains Supplementary Discussion, including Supplementary Figs. 1–21 and Supplementary Tables 1–8, and additional references.

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Yang, C., Wu, Q., Xie, W. et al. Copper-coordinated cellulose ion conductors for solid-state batteries. Nature 598, 590–596 (2021). https://doi.org/10.1038/s41586-021-03885-6

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