Li- and Mn-rich (LMR) cathode materials that utilize both cation and anion redox can yield substantial increases in battery energy density1,2,3. However, although voltage decay issues cause continuous energy loss and impede commercialization, the prerequisite driving force for this phenomenon remains a mystery3,4,5,6 Here, with in situ nanoscale sensitive coherent X-ray diffraction imaging techniques, we reveal that nanostrain and lattice displacement accumulate continuously during operation of the cell. Evidence shows that this effect is the driving force for both structure degradation and oxygen loss, which trigger the well-known rapid voltage decay in LMR cathodes. By carrying out micro- to macro-length characterizations that span atomic structure, the primary particle, multiparticle and electrode levels, we demonstrate that the heterogeneous nature of LMR cathodes inevitably causes pernicious phase displacement/strain, which cannot be eliminated by conventional doping or coating methods. We therefore propose mesostructural design as a strategy to mitigate lattice displacement and inhomogeneous electrochemical/structural evolutions, thereby achieving stable voltage and capacity profiles. These findings highlight the significance of lattice strain/displacement in causing voltage decay and will inspire a wave of efforts to unlock the potential of the broad-scale commercialization of LMR cathode materials.
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We gratefully acknowledge support from the US Department of Energy (DOE), Office of Energy Efficiency and Renewable Energy, Vehicle Technologies Office. This work was also supported by the Clean Vehicles, US–China Clean Energy Research Centre (CERC-CVC2) under US DOE EERE Vehicle Technologies Office. Argonne National Laboratory is operated for DOE Office of Science by UChicago Argonne, LLC, under contract number DE-AC02-06CH11357. This research was also supported by the National Key R&D Program of China (2016YFB0700600), Soft Science Research Project of Guangdong Province (no. 2017B030301013) and the Shenzhen Science and Technology Research Grants (no. ZDSYS201707281026184). This research used resources of the Advanced Photon Source (11-ID-C, 9BM and 34-ID-C), a US DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under contract no. DE-AC02-06CH11357. Work at Brookhaven National Laboratory was supported by the DOE, Office of Science, Office of Basic Energy Sciences, under contract no. DE-SC0012704. Electron microscopy was carried out at the Center for Nanoscale Materials, an Office of Science user facility, supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under contract no. DE-AC02-06CH11357.
The authors declare no competing interests.
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Extended data figures and tables
Extended Data Fig. 1 The morphology, particle distribution, surface area and composition of the LMR cathode.
a, b The SEM image of the LMR powder. The particle size ranges from 200–600 nm. c The particle size distribution of the as-prepared LMR cathode. The mean particle size of LMR cathode is around 583 nm. d The nitrogen adsorption/desorption isotherm of LMR. e The corresponding BET plot of LMR. The nitrogen adsorption/desorption isotherm and the corresponding BET plot of the as-prepared LMR cathode indicate that its specific surface area is calculated to be 2.804 m2 g−1. f SEM-EDS results of the pristine LMR cathode. The actual chemical composition complies well with the design (Mn : Co : Ni = 4 : 1 : 1).
a, b The first charge/discharge curve and corresponding dQ/dV curve of LMR cathode. The first charge profile exhibits two distinct electrochemical stages at different voltage ranges. Stage 1 corresponds to the Li+ extraction (de-lithiation) from LiTMO2 domains with concomitant oxidation of Ni2+ and Co3+. Stage 2 corresponds to the activation of Li2MnO3 domains, further Li+ extraction and at this stage oxygen is oxidized (at high potentials) to per-oxo species: (2O2− ↔ [O2]2− + 2e−). c, d The galvanostatic intermittent titration technique (GITT) test of the first charge. The Li-ion diffusion coefficient keeps stable in stage 1 but dramatically decreases after the activation of Li2MnO3 domains (stage 2).
Extended Data Fig. 3 The morphology, particle distribution, surface area and in situ DEMS of the Li2MnO3 cathode.
a The SEM image of the as-prepared Li2MnO3 powder. The as-prepared Li2MnO3 exhibits a single-particle morphology with average particle size of 100–200 nm. b The nitrogen adsorption/desorption isotherm of the as-prepared Li2MnO3 powder. c The corresponding BET plot of the as-prepared Li2MnO3 powder. The specific surface area of Li2MnO3 is calculated to be 3.1106 m2 g−1. d The particle size distribution of the as-prepared Li2MnO3 powder. e In situ DEMS for the first charge of Li2MnO3. The signal of O2 evolution is not detected until 20% delithiation of Li2MnO3.
a The ex situ XRD patterns of the first charge/discharge for the LMR cathode. The superlattice evolution of the LMR cathode can be observed in the 2 theta range of 1.5–1.8°. The obvious lattice parameter changes can be observed from ex situ XRD pattern, particularly in the 2 theta range of 3.0–6.0°. b The in situ XRD patterns of the as-prepared LMR cathode during the first charge/discharge in the voltage range of 2.0–4.8 V using a current rate of C/10 (1C = 250 mA g-1). The obvious lattice parameter changes can be observed from in situ XRD patterns, particularly in the 2 theta range of 1.3–1.5°. Generally, the structure evolution observed in in situ XRD is completely consistent with that in ex situ XRD (Figure 3c). c-e Ex situ Mn K-edge EXAFS spectra of the samples at pristine, 4.5V and 4.8V and the corresponding fitting results. Detailed fitting results are shown in Extended Data Table 2.
Extended Data Fig. 5 Visible lattice displacement observations using TEM of the LMR charged to 4.47 V.
High magnification TEM image of the LMR charged to 4.47 V. Although the layered structure is maintained, lattices in the marked areas are deformed significantly.
a High magnification TEM image of the LMR charged to 4.5 V. b The enlarged image of the selected area in Extended Data Fig. 6a. c The corresponding Fourier pattern of the select area of Extended Data Fig. 6a. d, e, f and g The simulated patterns of standard electron diffractions for layer , spinel , and Li2MnO3 /. h, i and j TEM images of the LMR cathode charged to 4.5 V. The lattice displacements are highlighted with yellow marks. k, l and m The corresponding FFT images of Extended Data Fig. 6h, i and j. Obvious lattice displacements are observed in the different particles and the corresponding FFT images confirm the existence of spinel phase at 4.5 V, which is highly consistent with Fig. 4g results.
a, b The reciprocal lattice viewed along the a* axis of the LMR cathode at 4.5 V. c The selected area electron diffraction (SAED) image of the sample charged to 4.5 V. In addition to the typical layered structure and weak Li2MnO3 reflection, the reflection that corresponds to the spinel lattice can be also observed. d, e, f and g The simulated patterns of standard electron diffractions for Layer , spinel , and Li2MnO3 /.
Extended Data Fig. 9 Visible structural observations and chemical state changes of the LMR charged to 4.8 V.
a, b Low and High magnification TEM image of the LMR charged to 4.8 V. A clear reconstruction surface layer with the spinel phase can be visualized. c Electron energy loss spectroscopy line scans of the O K-edge, Mn L-edge, Co & Ni L-edge for the LMR charged to 4.8 V along the direction from surface to bulk. The intensity of O-K edge pre-peaks substantially reduces from the interior to the exterior and almost disappears near the surface. Concurrently, Mn L-edge shows left shift near the surface. d Electron energy loss spectroscopy line scans of the O K-edge, Mn L-edge, Co & Ni L-edge for the LMR charged to 4.8 V along the surface fringe. The O-K line-scan parallel to the surface confirms that the oxygen release uniformly occurs in the entire particle surface as the disappeared O pre-peak.
a The XRD pattern of as-prepared LixNi0.13Mn0.54Co0.13O2. The diffraction peaks belonging to an O2 phase with P63mc symmetry are indexed by blue marks, while some tiny peaks belonging to O3 phase with R-3m symmetry are indexed by red marks. b High-resolution TEM image showing the atomic arrangements of O2 phase LMR. The Li2MnO3-like domain is rarely observed in the O2-type LMR, which suggests that the Li@Mn6 structural motifs in the O2 type LMR cathode are dispersed in the TM layer instead of being aggregated to form a Li2MnO3-like domain. c The charge/discharge profiles of O2 phase-based LMR cathode. The cells were activated at C/10 within first 3 cycles and then cycled at C/3. The smooth charging behaviour with no apparently differentiated voltage plateaus indicates effectively suppressed differential electrochemical activities. d The voltage stability of the O2 type LMR cathode during cycles presents in the plot of average (mean) voltage profiles vs cycle number.
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Liu, T., Liu, J., Li, L. et al. Origin of structural degradation in Li-rich layered oxide cathode. Nature 606, 305–312 (2022). https://doi.org/10.1038/s41586-022-04689-y