# Bulk fatigue induced by surface reconstruction in layered Ni-rich cathodes for Li-ion batteries

## Abstract

Ni-rich layered cathode materials are among the most promising candidates for high-energy-density Li-ion batteries, yet their degradation mechanisms are still poorly understood. We report a structure-driven degradation mechanism for NMC811 (LiNi0.8Mn0.1Co0.1O2), in which a proportion of the material exhibits a lowered accessible state of charge at the end of charging after repetitive cycling and becomes fatigued. Operando synchrotron long-duration X-ray diffraction enabled by a laser-thinned coin cell shows the emergence and growth in the concentration of this fatigued phase with cycle number. This degradation is structure driven and is not solely due to kinetic limitations or intergranular cracking: no bulk phase transformations, no increase in Li/Ni antisite mixing and no notable changes in the local structure or Li-ion mobility of the bulk are seen in aged NMCs. Instead, we propose that this degradation stems from the high interfacial lattice strain between the reconstructed surface and the bulk layered structure that develops when the latter is at states of charge above a distinct threshold of approximately 75%. This mechanism is expected to be universal in Ni-rich layered cathodes. Our findings provide fundamental insights into strategies to help mitigate this degradation process.

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## Data availability

The data that support the findings of this study are available from the corresponding author upon request.

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## Acknowledgements

We thank K. Kleiner, K. J. Griffith, O. Pecher and W. Meng for their support with the long-duration XRD experiments, and J. Morzy and J. Lu for assisting in the TEM experiments. C.X. thanks M. Jones and H. Liu for discussions on the XRD data analysis. This work is supported by the Faraday Institution under grant no. FIRG001. C.X. acknowledges support from a Science and Technology Facilities Council (STFC) Experimental Design Award from the STFC Batteries Network (grant no. ST/R006873/1). S.P.E. was funded via an EPSRC iCASE (award no. 1834544) and via the Royal Society (grant no. RP\R1\180147). M.F.G. thanks the German Research Foundation (DFG, Research Fellowship GR nos 5342/1-1 and 5342/2-1) for funding.

## Author information

Authors

### Contributions

C.X. and C.P.G. conceived the idea of the study. C.X. performed the electrochemistry and SEM experiments and analysed the results. C.X. conducted the PXRD experiments and analysed the data with support from S.J.D., M.F.G., S.P.E. and C.C.T. in the LDE experiments. C.X., K.M. and P.J.R. performed the ssNMR experiments and analysed the data. J.L., A.M., C.D. and B.L.M. conducted the (S)TEM experiments and analysed the results. C.X., K.M. and C.P.G. wrote the manuscript with input from all coauthors.

### Corresponding author

Correspondence to Clare P. Grey.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

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

## Extended data

### Extended Data Fig. 1 Electrochemistry of the operando long-duration NMC/Gr full-cell.

a, Capacity retention of the LDE cell and standard coin cells made in-house. The spikes in discharge capacity are attributed to the slow diagnostic cycles (C/20 rate) performed once every 50 ageing cycles. The error bars represent the standard deviations for the data obtained for two cells. b, Voltage profiles of the LDE NMC811/graphite full-cell at cycle numbers of 100, 200, 300, 400 and 500. These cycles are all performed at the same rate of C/2.

### Extended Data Fig. 2 Diffraction results of two operando long-duration cells in initial cycles.

a–f, Voltage profiles and the corresponding diffraction patterns of two operando long-duration full-cells: (ac) in the 6th cycle for the cell with a upper cutoff voltage (UCV) of 4.2 V, and (d–f) in the 4th cycle for the cell with a UCV of 4.3 V. g, h, Evolutions of the lattice parameters of the NMC cathode in the cell with a UCV of 4.2 V (g) and 4.3 V (h), respectively. The electrolyte used in both of these two cells was 1 M LiPF6 in EC:EMC 3:7 with 2 wt% vinylene carbonate, and both cells were cycled with a lower cutoff voltage of 2.5 V and at a rate of C/2 using CCCV charge – CC discharge protocol. Regime 1 highlighted in (g) represents the SoC range where the c parameter increases as the SoC increases and Regime 2 represents the SoC range where c decreases as the SoC increases.

### Extended Data Fig. 3 Ex-situ SR-PXRD pattern of an electrochemically aged NMC at a charged state (4.3 V versus Li/Li+).

a, SR-PXRD pattern of an electrochemically aged NMC at a charged state (4.3 V versus Li/Li+) after 300 cycles along with fit performed using three rhombohedral $$R\bar 3m$$ phases with different lattice parameters (termed active, intermediate and fatigued). The final SoC was reached by charging the aged NMC in a half-cell at a rate of C/20 with a voltage hold until the current dropped below C/1000. b. Expansion of the region containing the (003) reflection in (a).

### Extended Data Fig. 4 Laboratory PXRD results of the fatigued and fresh NMC.

a, X-ray diffraction pattern of the fresh NMC811 at 4.2 V, and aged NMC811 at 4.2 V and 4.3 V. b, Expanded (003) region. c, Fitting of the Rietveld refinement against the bottom diffraction pattern in a, corresponds to the aged NMC811 at 4.3 V after 1200 cycles. Owing to the poor quality of the pattern which was obtained from a laboratory diffractometer as compared to that from synchrotron, we were unable to resolve the intermediate phase and the Rietveld refinement was carried out using two rhombohedral phases. The obtained refinement results are: (1) active phase: phase fraction = 53(1) %, a = b = 2.8158(2) Å, c = 14.080(2) Å, SoC ≈ 84 %; (2) fatigued phase: phase fraction = 47(1) %, a = b = 2.8186(2) Å, c = 14.386(2) Å, SoC ≈ 76 %.

### Extended Data Fig. 5 Experimental and calculated 7Li ssNMR spectra at variable temperatures.

Comparisons of modelled (red dashed line) and experimental (black solid line, identical to Fig. 3b, c) 7Li variable temperature (VT) NMR spectra for “fresh” NMC811 at 4.2 V (top) and aged NMC811 held at 4.2 V (bottom). In all cases a motionally averaged sharp component corresponding to Li ions with high mobility (blue dotted lines) and a non-averaged component corresponding to Li ions with lower mobility (green dotted lines) were required to produce good agreement between experiment and calculation. The Li hopping rates of the high mobility component as well as its fraction of the total signal integral are plotted in Extended Data Fig. 6. More detailed discussions associated with the results are in the Supplementary Information.

### Extended Data Fig. 6 Calculated Li hopping rates and fraction of the high mobility component from VT NMR results.

a, b, Calculated hopping rates for the high mobility signal component for fresh (a) and aged (b) NMC811 samples as a function of temperature. Different v1/2,noexch values were used for calculating the hopping rates (see legend in plot). Blue and red shading have been added to the plot for illustrational purposes. c, Proportion of the high mobility component for fresh (blue) and aged (red) NMC samples as a function of temperature. More detailed discussions associated with these results are given in the Supplementary Information.

### Extended Data Fig. 7 7Li VT ssNMR spectra of an aged NMC cathode after 1200 cycles at 4.3 V versus Li compared to fresh samples.

7Li VT ssNMR spectra of an aged NMC cathode after 1200 cycles at 4.3 V versus Li (top panel), a fresh NMC at 4.2 V (middle panel) and 4.4 V (bottom panel). The final SoC of the aged sample was achieved by charging the cathode in a half cell at a rate of C/20 and with a voltage hold at 4.3 V until the current drops below C/1000, and the corresponding diffraction analysis is shown in Extended Data Fig. 4c. Spectra were acquired at variable temperatures at 4.7 T magnetic field strength and 60 kHz MAS frequency. The same scaling factor was applied for the spectra of each sample, and the spectra for different samples are shown with arbitrary scaling. See more discussion in the Supplementary Information.

### Extended Data Fig. 8 DF-STEM images of the surface of an aged NMC811 particle.

The surface of the particle is uniformly covered by a rock salt layer. The NMC sample for STEM imaging was aged for 300 cycles, and the final SoC was at 4.3 V versus Li. A single crystalline particle was examined by STEM. We observed that the particle was uniformly covered by a rock salt layer along the surface as highlighted in red. No rock salt structure was observed inside the particle. Several images are shown in the inset as examples. All images were taken along the [100] zone axis of the layered NMC structure.

### Extended Data Fig. 9 Single-crystal NMC: SR-PXRD and electrochemical performance.

a, SR-PXRD pattern of the single-crystal NMC. b, Discharge capacity retention of a single-crystal NMC/graphite full-cell at a rate of C/2 between 2.5 V and 4.2 V. The electrolyte used was LP57 (1 M LiPF6, EC/EMC 3/7) with 2 wt% VC. The apparent “spikes” in discharge capacity arise from the slow diagnostic cycles (C/20 rate) performed once every 50 ageing cycles. c, Voltage profiles of a single-crystal NMC/graphite full-cell at ageing cycles of 98, 198, 298, 398 and 498. These cycles are all performed at the same rate of C/2.

## Supplementary information

### Supplementary Information

Supplementary Figs. 1–7, Tables 1–4 and discussion.

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Xu, C., Märker, K., Lee, J. et al. Bulk fatigue induced by surface reconstruction in layered Ni-rich cathodes for Li-ion batteries. Nat. Mater. (2020). https://doi.org/10.1038/s41563-020-0767-8