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Characterization of the structure and chemistry of the solid–electrolyte interface by cryo-EM leads to high-performance solid-state Li-metal batteries

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Abstract

Solid-state lithium-metal (Li0) batteries are gaining traction for electric vehicle applications because they replace flammable liquid electrolytes with a safer, solid-form electrolyte that also offers higher energy density and better resistance against Li dendrite formation. Solid polymer electrolytes (SPEs) are highly promising candidates because of their tuneable mechanical properties and easy manufacturability; however, their electrochemical instability against lithium-metal (Li0), mediocre conductivity and poorly understood Li0/SPE interphases have prevented extensive application in real batteries. In particular, the origin of the low Coulombic efficiency (CE) associated with SPEs remains elusive, as the debate continues as to whether it originates from unfavoured interfacial reactions or lithium dendritic growth and dead lithium formation. In this work, we use state-of-the-art cryo-EM imaging and spectroscopic techniques to characterize the structure and chemistry of the interface between Li0 and a polyacrylate-based SPE. Contradicting the conventional knowledge, we find that no protective interphase forms, owing to the sustained reactions between deposited Li dendrites and polyacrylic backbones and succinonitrile plasticizer. Due to the reaction-induced volume change, large amounts of cracks form inside the Li dendrites with a stress–corrosion–cracking behaviour, indicating that Li0 cannot be passivated in this SPE system. On the basis of this observation, we then introduce additive engineering, leveraging from knowledge of liquid electrolytes, and demonstrate that the Li0 surface can be effectively protected against corrosion using fluoroethylene carbonate, leading to densely packed Li0 domes with conformal and stable solid–electrolyte interphase films. Owing to the high room-temperature ionic conductivity of 1.01 mS cm−1, the high transference number of 0.57 and the stabilized lithium–electrolyte interface, this improved SPE delivers an excellent lithium plating/stripping CE of 99% and 1,800 hours of stable cycling in Li||Li symmetric cells (0.2 mA cm−2, 1 mAh cm−2). This improved cathodic stability, along with the high anodic stability, enables a record high cycle life of >2,000 cycles for Li||LiFePO4 and >400 cycles for Li||LiCoO2 full cells.

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Fig. 1: 3D morphology and chemistry of the Li-containing dendrites plated using the baseline SN-SPE.
Fig. 2: Structure and chemistry of the densely packed Li0 domes plated using FEC-SPE.
Fig. 3: Chemical composition of the FEC-SPE-derived SEI and the electrochemical performance.
Fig. 4: Li0 deposit morphology and electrochemical behaviour of prepared SPEs under large-area capacity conditions.
Fig. 5: Room-temperature performance of FEC-SPE-based full cells employing different cathode materials, area capacities and N/P ratios.

Data availability

The data that support the findings of this study are available within the paper and its Supplementary Information. Any other data are available from the corresponding author on request.

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Acknowledgements

This research was primarily supported by the Early Career Research Program, Materials Science and Engineering Divisions, Office of Basic Energy Sciences of the US Department of Energy, under award DE-SC0021204, and additional support for synthesis was received from start-up funding to H.L.X. provided by UC Irvine. R.L., E.H. and X.-Q.Y. were supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Vehicle Technology Office of the US Department of Energy through the Advanced Battery Materials Research (BMR) Program, under contract DE-SC0012704. This research used resources of the Center for Functional Nanomaterials (CFN), which is a US Department of Energy Office of Science User Facility, at Brookhaven National Laboratory under contract no. DE-SC0012704. This work made use of facilities and instrumentation at the UC Irvine Materials Research Institute (IMRI), which is supported in part by the National Science Foundation through the UC Irvine Materials Research Science and Engineering Center (DMR-2011967). XPS work was performed using instrumentation funded in part by the National Science Foundation Major Research Instrumentation Program under grant CHE-1338173.

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R.L., X.-Q.Y., K.X. and H.L.X. conceived the idea. R.L., E.H. and Y.H. contributed to the experimental design. R.L. and C.W. performed the cryo-EM study and related electrochemical cycling. Y.H. performed optimization and performance tests of the electrolytes and electrodes. P.Z. performed SEM and XPS studies. R.L. and Y.H. wrote the manuscript with input from all authors.

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Correspondence to Ruoqian Lin, Xiao-Qing Yang, Kang Xu or Huolin L. Xin.

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Nature Nanotechnology thanks Guanglei Cui, Xinyong Tao and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Supplementary Table 1 and Figs. 1−30.

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Lin, R., He, Y., Wang, C. et al. Characterization of the structure and chemistry of the solid–electrolyte interface by cryo-EM leads to high-performance solid-state Li-metal batteries. Nat. Nanotechnol. 17, 768–776 (2022). https://doi.org/10.1038/s41565-022-01148-7

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