Abstract
In conventional intercalation cathodes, alkali metal ions can move in and out of a layered material with the charge being compensated for by reversible reduction and oxidation of the transition metal ions. If the cathode material used in a lithium-ion or sodium-ion battery is alkali-rich, this can increase the battery’s energy density by storing charge on the oxide and the transition metal ions, rather than on the transition metal alone1,2,3,4,5,6,7,8,9,10. There is a high voltage associated with oxidation of O2− during the first charge, but this is not recovered on discharge, resulting in reduced energy density11. Displacement of transition metal ions into the alkali metal layers has been proposed to explain the first-cycle voltage loss (hysteresis)9,12,13,14,15,16. By comparing two closely related intercalation cathodes, Na0.75[Li0.25Mn0.75]O2 and Na0.6[Li0.2Mn0.8]O2, here we show that the first-cycle voltage hysteresis is determined by the superstructure in the cathode, specifically the local ordering of lithium and transition metal ions in the transition metal layers. The honeycomb superstructure of Na0.75[Li0.25Mn0.75]O2, present in almost all oxygen-redox compounds, is lost on charging, driven in part by formation of molecular O2 inside the solid. The O2 molecules are cleaved on discharge, reforming O2−, but the manganese ions have migrated within the plane, changing the coordination around O2− and lowering the voltage on discharge. The ribbon superstructure in Na0.6[Li0.2Mn0.8]O2 inhibits manganese disorder and hence O2 formation, suppressing hysteresis and promoting stable electron holes on O2− that are revealed by X-ray absorption spectroscopy. The results show that voltage hysteresis can be avoided in oxygen-redox cathodes by forming materials with a ribbon superstructure in the transition metal layers that suppresses migration of the transition metal.
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Data availability
Supporting research data have been deposited in the Oxford Research Archive and will be available at https://ora.ox.ac.uk/objects/uuid:646b18a1-88b0-4575-8282-2bcdcbe20a7d.
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Acknowledgements
P.G.B. is indebted to the EPSRC, including the SUPERGEN programme, the Henry Royce Institute for Advanced Materials (EP/R00661X/1, EP/S019367/1, EP/R010145/1) and the Faraday Institution (FIRG007, FIRG008) for financial support. We thank H. Playford and R. Smith at ISIS, Harwell Campus for collecting neutron diffraction data. Support from the EPSRC (EP/K040375/1 ‘South of England Analytical Electron Microscope’) is also acknowledged. We acknowledge the use of the University of Oxford Advanced Research Computing (ARC) facility (https://doi.org/10.5281/zenodo.22558) in carrying out this work, and the resources provided by the Cambridge Tier-2 system operated by the University of Cambridge Research Computing Service (http://www.hpc.cam.ac.uk) funded by EPSRC Tier-2 capital grant EP/P020259/1, via the Advanced Materials for Alkali-ion Batteries (AMAiB) project. Synchrotron radiation experiments were performed at the i21 beamline, Diamond Light Source, Harwell, UK, with supporting data collected from the ADRESS beamline, Swiss Light Source, Villigen, Switzerland, and BL27SU, Spring8, Japan. We acknowledge technical and experimental support at the ADRESS beamline by T. Schmitt, D. McNally, X. Lu, L. Nue and M. Dantz and at the BL27SU beamline by K. Tsuruta. We thank N. Rees for help in collecting NMR spectra.
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R.A.H., U.M., M.R.R. and P.G.B. conceived the study. U.M. and R.A.H. carried out the materials synthesis, characterization and testing. R.A.H., U.M., J.W.S., M.R.R. and L.C.D. contributed to the measurement processing and interpretation of the spectroscopic data. M.A.P.-O. performed the DFT calculations. L.J. collected, processed and interpreted the NMR data. J.G.L. performed and interpreted the ADF-STEM measurements. R.A.H., A.N., A.W. and K.-J.Z. performed high-resolution RIXS and sXAS measurements. R.A.H. and P.G.B. wrote the manuscript with contributions and revisions from all authors.
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Extended data figures and tables
Extended Data Fig. 1 Further structural characterization of pristine materials.
a, b, Neutron powder diffraction data for (a) Na0.75[Li0.25Mn0.75]O2 and (b) Na0.6[Li0.2Mn0.8]O2 refined using the P63/mmc space group, which excludes superstructure ordering. Rietveld refinement was performed with GSAS II software. Refinement parameters are given in the tables in the lower part of the figure, including the goodness of fit (G.O.F). Inductively coupled plasma (ICP) optical emission spectroscopy was used to confirm the chemical compositions.
Extended Data Fig. 2 Diffraction peaks arising from ribbon superstructure ordering in Na0.6[Li0.2Mn0.8]O2.
a, PXRD data for pristine Na0.6[Li0.2Mn0.8]O2 indexed using the P63/mmc space group which does not account for superstructure peaks arising from in-plane ordering in the TM layer. b, Superstructure region of PXRD compared with computer generated diffraction patterns. Model crystal structures were prepared with different alignments of ribbon ordered layers. The only structure to successfully match all the peaks is the P21/c space group. Structures are all viewed along the [010] direction.
Extended Data Fig. 3 Operando gas evolution analysis.
OEMS collected on Na0.6[Li0.2Mn0.8]O2 at 10 mA g−1 between 2 V and 4.5 V. No direct O2 loss is observed. Only a very small quantity of CO2 is released at 3.5–4.2 V, characteristic of alkali carbonate decomposition; a small amount is released at 4.5 V due to direct electrolyte oxidation. Overall, 0.005 moles of CO2 per mole of active material were detected during charge compared with 0.4 moles of charge stored per mole of active material. Even if all of this CO2 arose from O loss from the lattice, it would constitute only a minimal contribution (0.02 moles of charge stored per formula unit, f.u., or about 5%), to the charge capacity observed.
Extended Data Fig. 4 Manganese L-edge spectra and low-resolution RIXS for Na0.6[Li0.2Mn0.8]O2.
a, Electrochemical load curve for first cycle of Na0.6[Li0.2Mn0.8]O2 showing state of charge points selected for ex situ analysis. b, Manganese L-edge data collected in inverse partial fluorescence yield mode show that Na0.6[Li0.2Mn0.8]O2 remains unchanging at Mn4+ throughout the charge and discharge cycle. Standards shown below are MnO (+2), Mn2O3 (+3) and Li2MnO3 (+4). c, Low-resolution RIXS spectra collected at BL27SU, Spring8 synchrotron, Japan, at 531 eV excitation energy show a new feature at an emission energy of approximately 523 eV corresponding to new hole states formed on O and an increase in the elastic peak intensity (labelled with arrows). These new features disappear on discharge indicating O reduction, and the spectra are almost superimposable with those collected for the pristine material. The intensity of both features appears much less pronounced than the O redox features measured on honeycomb-ordered O-redox materials.
Extended Data Fig. 5 Two-phase evolution in Li environment in Na0.6[Li0.2Mn0.8]O2.
Ex situ 6Li MAS NMR spectra for Na0.6[Li0.2Mn0.8]O2 collected at different states of charge illustrate the two-phase nature of the charge–discharge plateau. Negligible change in the lithium environment is observed below the end of the plateau on discharge in the single-phase region. Arrows indicate the unique isotropic chemical shifts for Li.
Extended Data Fig. 6 Further electron microscopy imaging showing retention of ribbon and loss of honeycomb ordering.
a, b, ADF-STEM images showing further spatial investigation of the charged and discharged samples of (a) Na0.6[Li0.2Mn0.8]O2 with ribbon ordering and (b) Na0.75[Li0.25Mn0.75]O2 with honeycomb ordering. A further image showing Na0.6[Li0.2Mn0.8]O2 after 10 charge–discharge cycles is also included.
Extended Data Fig. 7 Energetic stability afforded by O2 formation and computed discharge voltage.
a, Energetics of possible configurations of desodiated structural models for Na0.0Li0.25Mn0.75O2. The models considered are: P2-type stacking with Li in the TM layer (P2/LiTM); O2-type stacking with Li in the TM layer (O2/LiTM); O2-type stacking with Li in the AM layer (O2/LiAL); and O2-type stacking with Li in the AM layer and with in-plane Mn disorder (O2/LiAL/Mndis). In the last case, various Mn disorder configurations were investigated corresponding to the different crosses. The lowest-energy structure is pictured (ab-plane) and possesses clusters of vacancies and Mn-bound O2 with an O–O bond length of 1.2 Å corresponding to molecular O2. For simplicity, the energies of the optimized models are plotted relative to the energy of the model P2/LiTM, the energy of which was set to zero. The yellow curve is a guide to the eye to indicate the models with the lowest total energies. b, Calculated lattice parameters for pristine, charged, and discharged Na0.75Li0.25Mn0.75O2. They are compared with experimental data. The deviation between theory and experiment is also reported. c, d, Structural models used to compute the change in energy and average voltage for Na0.75Li0.25Mn0.75O2 (c) and Na0.6Li0.2Mn0.8O2 (d) respectively. Discharge reactions and calculated voltages are given in (iii). Purple, Mn; green, Li; yellow, Na.
Extended Data Fig. 8 Ribbon ordering identified in P3-type Na0.6[Li0.2Mn0.8]O2.
PXRD data for P3-type Na0.6[Li0.2Mn0.6]O2 reproduced with permission from ref. 14. Below are calculated diffraction patterns for the P3 structure with and without ribbon ordering of Li and Mn in the TM layer. Vertical dotted lines show positions of superstructure peaks. The structure used for the calculation of the ribbon-ordered P3-type Na0.6Li0.2Mn0.6O2 diffraction pattern is shown to the right and possesses an offset arrangement of ordered layers.
Extended Data Fig. 9 Evolution of electrochemical behaviour over resting and cycling.
a, Electrochemical load curves for Na0.6[Li0.2Mn0.8]O2 electrodes charged to 4.5 V at a rate of 10 mA g−1, then rested at open circuit voltage (OCV) for varying amounts of time before discharge at 10 mA g−1. b, c, Electrochemical load curves for ribbon-ordered Na0.6[Li0.2Mn0.8]O2 and honeycomb-ordered Na0.75[Li0.25Mn0.75]O2 electrodes respectively, cycled between 2.0 V and 4.5 V at a rate of 10 mA g−1. First cycle blue.
Extended Data Fig. 10 Gradual loss of ribbon superstructure ordering from diffraction.
Ex situ PXRD patterns for Na0.6[Li0.2Mn0.8]O2 in the discharged state (2.0 V) after 1, 10 and 20 charge–discharge cycles (between 2.0 V and 4.5 V at 10 mA g−1). Peaks arising from the periodicity uniquely along the c-axis remain sharp upon cycling (indexed as 002 and 004 according to the P63/mmc space group without superstructure), whereas all other peaks, especially the 010 and 110, which are unique to ordering within the ab plane, broaden and reduce in intensity as the ribbon superstructure is lost on cycling.
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House, R.A., Maitra, U., Pérez-Osorio, M.A. et al. Superstructure control of first-cycle voltage hysteresis in oxygen-redox cathodes. Nature 577, 502–508 (2020). https://doi.org/10.1038/s41586-019-1854-3
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DOI: https://doi.org/10.1038/s41586-019-1854-3
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