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Fictitious phase separation in Li layered oxides driven by electro-autocatalysis

Nature Materials volume 20, pages 991–999 (2021)Cite this article

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Abstract

Layered oxides widely used as lithium-ion battery electrodes are designed to be cycled under conditions that avoid phase transitions. Although the desired single-phase composition ranges are well established near equilibrium, operando diffraction studies on many-particle porous electrodes have suggested phase separation during delithiation. Notably, the separation is not always observed, and never during lithiation. These anomalies have been attributed to irreversible processes during the first delithiation or reversible concentration-dependent diffusion. However, these explanations are not consistent with all experimental observations such as rate and path dependencies and particle-by-particle lithium concentration changes. Here, we show that the apparent phase separation is a dynamical artefact occurring in a many-particle system driven by autocatalytic electrochemical reactions, that is, an interfacial exchange current that increases with the extent of delithiation. We experimentally validate this population-dynamics model using the single-phase material Lix(Ni1/3Mn1/3Co1/3)O2 (0.5 < x < 1) and demonstrate generality with other transition-metal compositions. Operando diffraction and nanoscale oxidation-state mapping unambiguously prove that this fictitious phase separation is a repeatable non-equilibrium effect. We quantitatively confirm the theory with multiple-datastream-driven model extraction. More generally, our study experimentally demonstrates the control of ensemble stability by electro-autocatalysis, highlighting the importance of population dynamics in battery electrodes (even non-phase-separating ones).

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Fig. 1: Phase evolution of Lix(Ni1/3Mn1/3Co1/3)O2 during the second cycle at different cycling currents, probed with operando XRD.
Fig. 2: Lithium composition maps of individual platelet particles quenched at different overall states-of-charge during the second cycle, imaged with scanning transmission X-ray microscopy.
Fig. 3: Autocatalytic and autoinhibitory reactions in electrochemically driven systems.
Fig. 4: Inter-particle inhomogeneity from interface limitation and intra-particle inhomogeneity from solid-diffusion limitation.
Fig. 5: Multiple datastream workflow for quantitative model extraction and prediction.

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Data availability

Data represented in Fig. 1 can be accessed at: https://doi.org/10.7910/DVN/EMJFMU. Data represented in Fig. 2 can be accessed at: https://doi.org/10.7910/DVN/AVP2I5. Additional data for this study are available from the authors upon reasonable request.

Code availability

Code for solving the Fokker–Planck equation can be found at: https://github.com/hbozhao/e-autocat. Additional code is available from the authors upon reasonable request.

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Acknowledgements

This work was supported by the Toyota Research Institute through the Accelerated Materials Design and Discovery programme. The characterization aspect of the work was supported by the Assistant Secretary for Energy Efficiency, Vehicle Technologies Office of the US Department of Energy under the Advanced Battery Materials Research Program. J.H. was funded through the Korea Institute of Science and Technology (2E30993; 2V08350). We would like to acknowledge support from the following people: S. Kalirai from the Lawrence Berkeley National Laboratory for assistance in STXM experiments; C. J. Takacs from SLAC for assistance in the operando XRD holder design and instrument operation; C.-N. Yeh from Stanford University for help in atomic force microscopy; X. Xu from Stanford University for taking SEM images; S. Ahn, J.-H. Park and S.-K. Doo from Samsung Advanced Institute of Technology for providing the NMC111 and Li-Rich NMC materials; J. Haag and H. Sommer from BASF for providing the NMC 83:5:12 material; Microvast for providing single-crystalline NMC test materials; Y. Zhang and M. Scott for providing the air-free transfer holder for STXM; P. Csernica and E. Carlson for finding errors in the manuscript. The authors used resources of the following facilities: the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, supported by the US DOE, Office of Science, Office of Basic Energy Sciences (DE-AC02-76SF00515); the Advanced Light Source, a US DOE Office of Science User Facility (DE-AC02-05CH11231); the Stanford Nano Shared Facilities, supported by the National Science Foundation (ECCS-2026822); and the 1D XRS KIST-PAL beamline at the Pohang Accelerator Laboratory.

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Author notes

  1. These authors contributed equally: Jungjin Park, Hongbo Zhao, Stephen Dongmin Kang.

Authors and Affiliations

  1. Department of Materials Science and Engineering, Stanford University, Stanford, CA, USA

    Jungjin Park, Stephen Dongmin Kang, Kipil Lim, Chia-Chin Chen, Jihyun Hong & William C. Chueh

  2. Stanford Institute for Materials & Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, CA, USA

    Jungjin Park, Kipil Lim, Jihyun Hong & William C. Chueh

  3. Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA, USA

    Jungjin Park, Young-Sang Yu & David A. Shapiro

  4. Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA

    Hongbo Zhao, Richard D. Braatz & Martin Z. Bazant

  5. Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, CA, USA

    Kipil Lim, Jihyun Hong & Michael F. Toney

  6. Applied Energy Division, SLAC National Accelerator Laboratory, Menlo Park, CA, USA

    Kipil Lim, Jihyun Hong, Michael F. Toney & William C. Chueh

  7. Center for Energy Materials Research, Korea Institute of Science and Technology, Seoul, Korea

    Jihyun Hong

  8. Department of Mathematics, Massachusetts Institute of Technology, Cambridge, MA, USA

    Martin Z. Bazant

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Contributions

J.H. obtained the first preliminary data and initiated the project with his ideas. J.P. synthesized the materials, prepared the samples and led the experiments. J.P., K.L. and S.D.K. performed the XRD measurements. K.L. designed the operando XRD holder. J.P., S.D.K. and C.-C.C. carried out the STXM measurements with assistance from Y.-S.Y. Analysis of the XRD and STXM data was conducted by S.D.K. The model was constructed by H.Z. and S.D.K. The simulation was conducted by H.Z. who also performed the inverse problem solving. H.Z., S.D.K. and C.-C.C. interpreted the modelling results. S.D.K. drafted the manuscript. All authors reviewed and commented on the manuscript. M.F.T. supervised the XRD experiments. Y.-S.Y. and D.A.S. supervised the STXM experiments. M.Z.B. and R.D.B. supervised the simulation and modelling. W.C.C. supervised the overall project.

Corresponding authors

Correspondence to Jihyun Hong, Martin Z. Bazant or William C. Chueh.

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Extended data

Extended Data Fig. 1 Autocatalysis behavior in Lix(Ni0.5Mn0.3Co0.2)O2 agglomerate particles.

Fast cycling (2C) during the a second and b 3rd cycle. Prior to the second cycle, NMC532 was lithiated to a charge-counted Li fraction of 0.985 with a voltage hold at 2.5 V. After the 2nd cycle measurement, the cell was fully lithiated with a voltage hold at 2.0 V (during which diffraction measurements were not available at the beamline). Using the identical sample, the 3rd cycle was measured.

Extended Data Fig. 2 Autocatalysis behavior in Lix(Ni0.83Mn0.05Co0.12)O2 single crystalline particles.

Prior to the 2nd cycle, NMC 83:5:12 was lithiated to a charge-counted Li fraction of 1 with a voltage hold at 2.5 V.

Extended Data Fig. 3 Autocatalysis behavior in Li-rich layered oxide agglomerate particles.

The initial composition of the material was Li1.17(Ni0.21Mn0.54Co0.08)O2. Prior to the 2nd cycle, the material was charged up to a specific capacity of 108.3 mAh/g and discharged by the same amount (voltage hold at 2.5 V at the end of discharge). This capacity, which is within the cation-redox region, was selected to conduct the experiment while avoiding the irreversible and complex structural changes associated with anion-redox. The contrast between the fast delithiation case (1112 mA/g; upper panels in a) and all other conditions (lower panels in a and all panels in b) indicate autocatalysis behavior similar to other NMC compositions. The autocatalytic effect manifests at a much earlier stage of delithiation, compared with other NMC compositions. c The (003) peak positions monitored during the operando XRD experiment, confirming the reversibility in the early capacity region of cation-redox.

Extended Data Fig. 4 Compilation of mass-specific exchange current curves obtained from EIS.

All curves were obtained using particles in electrodes made with the same recipe as used for the operando XRD experiments. Agglomerate particles were used for NMC111, NMC532, and Li/Mn-rich NMC. Single crystalline particles were used for NMC 83:5:12.

Supplementary information

Supplementary Information

Supplementary Sections 1–13 and Figs. 1–28.

Supplementary Video 1

Particle ensemble simulation of autocatalysis in reaction- and diffusion-limited cases (thin electrode).

Supplementary Video 2

Porous electrode simulation of autocatalysis in reaction- and diffusion-limited cases (thick electrode with electrolyte transport effect).

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Park, J., Zhao, H., Kang, S.D. et al. Fictitious phase separation in Li layered oxides driven by electro-autocatalysis. Nat. Mater. 20, 991–999 (2021). https://doi.org/10.1038/s41563-021-00936-1

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