InOperando X-ray Studies of High-Performance Lithium-Ion Storage in Keplerate-Type Polyoxometalate Anodes.

Abstract
Polyoxometalates (POMs) have emerged as potential anode materials for lithium-ion batteries (LIBs) owing to their ability to transfer multiple electrons. Although POM anode materials exhibit notable results in LIBs, their energy-storage mechanisms have not been well-investigated. Here, we utilize various in operando and ex situ techniques to verify the charge-storage mechanisms of a Keplerate-type POM Na2K23{[(MoVI)MoVI5O21(H2O)3(KSO4)]12 [(VIVO)30(H2O)20(SO4)0.5]}·ca200H2O ({Mo72V30}) anode in LIBs. The {Mo72V30} anode provides a high reversible capacity of up to ∼1300 mA h g-1 without capacity fading for up to 100 cycles. The lithium-ion storage mechanism was studied systematically through in operando synchrotron X-ray absorption near-edge structure, ex situ X-ray diffraction, ex situ extended X-ray absorption fine structure, ex situ transmission electron microscopy, in operando synchrotron transmission X-ray microscopy, and in operando Raman spectroscopy. Based on the abovementioned results, we propose that the open hollow-ball structure of the {Mo72V30} molecular cluster serves as an electron/ion sponge that can store a large number of lithium ions and electrons reversibly via multiple and reversible redox reactions (Mo6+ ↔ Mo1+ and V5+/V4+↔ V1+) with fast lithium diffusion kinetics (DLi+: 10-9-10-10 cm2 s-1). No obvious volumetric expansion of the microsized {Mo72V30} particle is observed during the lithiation/delithiation process, which leads to high cycling stability. This study provides comprehensive analytical methods for understanding the lithium-ion storage mechanism of such complicated POMs, which is important for further studies of POM electrodes in energy-storage applications.

Citation
Lin, C.-C., Hsu, C.-T., Liu, W., Huang, S.-C., Lin, M.-H., Kortz, U., … Chen, H.-Y. (2020). In Operando X-ray Studies of High-Performance Lithium-Ion Storage in Keplerate-Type Polyoxometalate Anodes. ACS Applied Materials & Interfaces. doi:10.1021/acsami.0c09344

Acknowledgements
The authors thank the Instrumentation Center at National Tsing Hua University for providing assistance in electron spectroscopy for chemical analysis, XRD measurement for structural analysis, and TEM for morphological and microstructural analyses. The authors also thank Dr. Chih-Wen Pao and Dr. Jeng-Lung Chen for their help on the in operando synchrotron XAS (beamline TPS 44A1) in National Synchrotron Radiation Research Center (NSRRC), Hsinchu, Taiwan.

Publisher
American Chemical Society (ACS)

Journal
ACS applied materials & interfaces

DOI
10.1021/acsami.0c09344

PubMed ID
32841558

Additional Links
https://pubs.acs.org/doi/10.1021/acsami.0c09344

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