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Molecular crowding electrolytes for high-voltage aqueous batteries

Nature Materials volume 19, pages 1006–1011 (2020)Cite this article

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Abstract

Developing low-cost and eco-friendly aqueous electrolytes with a wide voltage window is critical to achieve safe, high-energy and sustainable Li-ion batteries. Emerging approaches using highly concentrated salts (21–55 m (mol kg–1)) create artificial solid–electrode interfaces and improve water stability; however, these approaches raise concerns about cost and toxicity. Molecular crowding is a common phenomenon in living cells where water activity is substantially suppressed by molecular crowding agents through altering the hydrogen-bonding structure. Here we demonstrate a ‘molecular crowding’ electrolyte using the water-miscible polymer poly(ethylene glycol) as the crowding agent to decrease water activity, thereby achieving a wide electrolyte operation window (3.2 V) with low salt concentration (2 m). Aqueous Li4Ti5O12/LiMn2O4 full cells with stable specific energies between 75 and 110 W h kg−1 were demonstrated over 300 cycles. Online electrochemical mass spectroscopy revealed that common side reactions in aqueous Li-ion batteries (hydrogen/oxygen evolution reactions) are virtually eliminated. This work provides a path for designing high-voltage aqueous electrolytes for low-cost and sustainable energy storage.

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Fig. 1: The electrochemical stability window and FTIR spectra of the molecular crowding electrolytes.
Fig. 2: Electrochemical behaviour and OEMS of L-LTO/LMO full cell in 2 m LiTFSI–94%PEG–6%H2O.
Fig. 3: HER stability of molecular crowding electrolyte.
Fig. 4: Comparison of the measured electrolyte stability window of various aqueous electrolytes and potential electrode materials.

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Data supporting the findings of this study are available from the corresponding author upon reasonable request.

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Acknowledgements

The work described in this paper was fully supported by a grant from the Research Grant Council of the Hong Kong Special Administrative Region, China (project no. CUHK14307318). We thank G. T. Cong for assisting with the DFT calculations, Y. C. Zhou for assisting with the electrode fabrications and W. W. Wang for recording videos.

Author information

Authors and Affiliations

  1. Electrochemical Energy and Interfaces Laboratory, Department of Mechanical and Automation Engineering, The Chinese University of Hong Kong, Hong Kong Special Administrative Region, China

    Jing Xie, Zhuojian Liang & Yi-Chun Lu

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  1. Jing Xie
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  2. Zhuojian Liang
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  3. Yi-Chun Lu
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Contributions

J.X. and Y.-C.L. conceived the project, analysed the data and wrote the manuscript. J.X. performed the DFT calculation and conducted the experiments with contributions from Z.L. (OEMS measurement).

Corresponding author

Correspondence to Yi-Chun Lu.

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Competing interests

J.X. and Y.-C.L. are inventors with a patent application (US application no. 16/798,136) on the molecular crowding electrolytes described herein. Z.L. declares no competing interests.

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Extended data

Extended Data Fig. 1 Fabrication of 2 m LiTFSI-xPEG-(1-x)H2O electrolytes.

Stoichiometric amounts of water, PEG 400 and LiTFSI were used to prepare 2 m LiTFSI-xPEG-(1-x)H2O (x=94% is shown as an example). All the solutions were fabricated in an argon-filled glove box at room temperature after bubbling water and PEG 400 with argon for 10 minutes to prevent oxygen contamination on electrodes.

Extended Data Fig. 2 Ionic conductivity of the constructed “molecular crowding” electrolytes.

The ionic conductivity of 2 m LiTFSI-xPEG-(1-x)H2O was measured at 25 oC, x= 71, 80, 90, 94%,100%.

Extended Data Fig. 3 Flammability testing.

a, An ignited cotton swab was immersed in the commercial 1 M LiPF6 in EC/DEC(vol%=1/1) and 2 m LiTFSI-94%PEG-6%H2O. 1 M LiPF6 in EC/DEC(vol%=1/1) electrolyte was ignited immediately, while the fire of cotton swab was extinguished in 2 m LiTFSI-94%PEG-6%H2O. b, The snapshot of Supplementary Video 3 compares the flammability of glass fibre soaked with 2 m LiTFSI-94%PEG-6%H2O, 2 m LiTFSI-PEG and the film of 2 m LiTFSI-PEO after treated with propane flame. The full process is shown in Supplementary Video 3.

Extended Data Fig. 4 1H-NMR spectra.

1H-NMR spectra of 2 m LiTFSI-xPEG-(1-x)H2O, x= 0%, 71%, 80%, 90%, 94%. The 1H shift of H2O decreases upon PEG addition.

Extended Data Fig. 5 Snapshots of 2 m LiTFSI-94%PEG-6%H2O and 2 m LiTFSI-H2O electrolyte during MD simulations.

a, The snapshot of 2 m LiTFSI-94%PEG-6%H2O electrolyte during 20 ps-120 ps of the simulation. b, The snapshot of 2 m LiTFSI-H2O electrolyte during 4 ps-10 ps of the simulation.

Extended Data Fig. 6 Cell design of online electrochemical mass spectrometry.

The cell design is adopted from our previous work35,36. To avoid the fast evaporation of water in the electrolyte with continuous gas flowing, the Ar gas was directed into a blank cell(right one) which consists of only electrolyte (without electrodes) for gas washing before flowing into the electrochemical cell(left one).

Extended Data Fig. 7 Comparison of electrochemical stability window obtained from different working electrodes.

a, Electrochemical stability windows of hydrate melt 19.4 m LiTFSI-8.3 m LiBETI determined by liner sweep voltammetry tests on AB coated Al foil and bare Al/Ti current collector (Al for cathodic scanning and Ti for anodic scanning, which were used in ref. 9). b, Electrochemical stability windows of 32 m KAc-8 m LiAc determined by liner sweep voltammetry tests on AB coated Al foil and bare Ti current collector (adopted in ref. 17).

Extended Data Fig. 8 Nyquist plots of electrochemical impedance spectroscopy(EIS) for the LTO/LMO full cell with 2 m LiTFSI-94%PEG-6%H2O and 2 m LiTFSI-PEO at 25 oC.

The Nyquist plots of electrochemical impedance spectroscopy(EIS) for the LTO/LMO full cell with 2 m LiTFSI-94%PEG-6%H2O and 2 m LiTFSI-PEO at 25 oC shows that the interfacial resistance of 2 m LiTFSI-94%PEG-6%H2O is much smaller than polymer electrolyte 2 m LiTFSI-PEO.

Extended Data Fig. 9 Voltage profile of L-LTO/LMO and gel pre-coated Li/LMO at 1 C in 2 m LiTFSI-94%PEG-6%H2O electrolyte during the 1st cycle.

The voltage profile of L-LTO/LMO and gel pre-coated Li/LMO at 1 C in 2 m LiTFSI-94%PEG-6%H2O electrolyte during the 1st cycle shows the potential application of our electrolyte for 4.0 V aqueous battery.

Supplementary information

Supplementary Information

Supplementary Figs. 1–4, Tables 1–3, Discussion I, captions for Videos 1–3 and refs. 1–15.

Supplementary Video 1

Flammability testing of the 2 m LiTFSI–94%PEG–6%H2O electrolyte. The fire of an ignited cotton swab was extinguished after being immersed in the 2 m LiTFSI–94%PEG–6%H2O electrolyte, suggesting that the electrolyte is non-flammable and safe.

Supplementary Video 2

Flammability testing of commercial LiPF6 in EC/DEC electrolyte. The commercial LiPF6 in the EC/DEC electrolyte started to burn after an ignited cotton swab was immersed in the electrolyte, indicating that the electrolyte is flammable.

Supplementary Video 3

The comparison of the flammability of 2 m LiTFSI–94%PEG–6%H2O, 2 m LiTFSI–PEG and 2 m LiTFSI–PEO. The 2 m LiTFSI–94%PEG–6%H2O-soaked glass fibre was not ignited, while those soaked in 2 m LiTFSI–PEG and polymer electrolyte 2 m LiTFSI–PEO (EO/Li+, 11:1) were both ignited, indicating that the 2 m LiTFSI–94%PEG–6%H2O exhibits superior safety compared with the 2 m LiTFSI–PEG and solid PEO polymer electrolyte.

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Xie, J., Liang, Z. & Lu, YC. Molecular crowding electrolytes for high-voltage aqueous batteries. Nat. Mater. 19, 1006–1011 (2020). https://doi.org/10.1038/s41563-020-0667-y

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