High Performance Reported for Polyoxometalate Li-S Batteries

By Bhavna KavetiMay 31 2022Reviewed by Susha Cheriyedath, M.Sc.

An article recently published in the journal Materials Today Nano discusses the development of modified Keggin and Dawson type Polyoxometalates (POMs) to improve the performance of  Li-S batteries.

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​​​​​​​Study: Effective polysulfide adsorption and catalysis by polyoxometalate contributing to high performance Li-S batteries. Image Credit: RESTOCK images/Shutterstock.com

Although lithium-sulfur (Li-S) batteries are promising energy storage systems, they still suffer from a few drawbacks, including the migration of soluble lithium polysulfides (LiPS) intermediates, severe volume change, and low conductivity of sulfur.

Li-S Batteries as Energy Storage Systems

Increasing energy demand calls for high energy storage technology. Li-S batteries have high energy density and high specific capacity making them highly valued energy storage systems. Moreover, the cost-efficiency and low toxicity of elemental sulfur make it an environment-friendly cathode material, making Li-S batteries a next-generation energy storage system. However, soluble lithium polysulfides (LiPSs; Li₂S_x, x = 4-8) causes the “shuttle effect,” leading to low Coulombic efficiency and poor cycle stability. To this end, building the functional interlayer is necessary to improve Li-S battery performance.

POMs are anionic metal oxide clusters with good stability, redox property, and diversity. Hence POMs are extensively used in electrochemistry, catalysis, and energy systems. K₃[H₃Ag^IPW₁₁O₃₉].12H₂O (silver-substituted Keggin) served as Li-S batterie’s Lewis acid and base catalyst. Silver (Ag(I)) ions in silver-substituted Keggin are used as Lewis acid centers to strengthen the attachment of sulfur (S)-moieties.

(NH₄)₆V₁₀O₂₈ (NVO) clusters immobilize LiPS by attraction of oxygen (O) atoms of NVO by Li cations of LiPS or by the interaction of vanadium atoms of NVO with sulfur anions of LiPS. However, the application of POMs as Li-S battery interlayer for shuttle inhibition is rarely reported, and the mechanism behind the inhibition remains unclear.

Interlayers for Li-S Batteries

In the present study, the team designed three types of Li-S batteries with Keggin or Dawson-type POMs as interlayers. They observed that the cells with H₃[PW₁₂O₄₀]. xH₂O (PW₁₂) interlayer had higher LiPS binding energy and represented better cycle performance than K₆[P₂W₁₈O₆₂].14H₂O (P₂W₁₈) interlayer. The adsorption ability of (NH₄)₆[P₂Mo₁₈O₆₂].11H₂O (P₂Mo₁₈) towards LiPS is higher than P₂W₁₈. The cells containing P₂Mo₁₈ as interlayer had a better electrochemical performance. PW₁₂ exhibited catalytic function on LiPS, thus promoting Li₂S/LiPS conversion.

Research Findings

Scanning electron microscope (SEM) images of PW₁₂ particles reveal the uniform surface of this interlayer without any agglomeration, suggesting that PW₁₂ has sufficient dispersion on the separator. The interlayer did not show any visible cracks during the folding process, suggesting its ruggedness and structural flexibility, and the interlayer had a thickness of 5 micrometers.

The elemental mapping of the PW₁₂ interlayer showed an even distribution of phosphorous (P), W, O, and carbon (C) elements, and Fourier transform infrared spectroscopy (FTIR) of bare PW₁₂ showed absorption peaks of P–O_a, W–O_c–W, W=O_d, W–O_d–W at 1081.8, 983.5. 889.0 and 804.1 centimeter inverse, respectively.

X-ray photoelectron spectroscopy (XPS) revealed the chemical bonding and elemental composition of the prepared PW₁₂ interlayer. The peak position of 134.6 electronvolts in XPS spectra aligns with the binding energy of P 2p. Moreover, the peaks at 36.2 and 38.3 electronvolts were assigned to W 4f_7/2 and W 4f_5/2, respectively, and those at 532.6 and 531.3 electronvolt in O 1s spectra correspond to the surface adsorbed O-O species of Keggin structure.

Electrolyte dripped on polypropylene (PP) separator and PW₁₂ interlayer helped in contact angle tests, and results showed that the contact angle of PW₁₂ was less than the PP separator. This test confirmed that rich voids of PW₁₂ interlayer increase the electrolyte’s wettability, which facilitates lithium-ion mobility.

Subsequently, the evaluation of electrochemical performance for Li-S batteries containing POMs interlayers and C-zinc oxide (ZnO)/S cathode revealed that the PW₁₂ interlayer had a higher capacity of 1607.8 milliampere hour per gram and the lowest polarization voltage (ΔE) of 165.9 millivolts than P₂W₁₈ and P₂Mo₁₈. These experimental results indicated the enhanced catalytic activity and rapid reaction kinetics of PW₁₂.

The cyclic voltameter curves of PW₁₂-containing cell at cyclic voltammetry of 0.1 millivolts per second showed two reduction peaks at 2.28 and 2.05 volts, indicating the reduction of S to Li₂S_x (4 ≤ x ≤ 8), followed by conversion into solid-state Li₂S₂/Li₂S electrochemically. Moreover, the peak at 2.39 volts indicated the oxidation of Li₂S₂/Li₂S to Li₂S_x and S.

Conclusion

In summary, the researchers designed PW₁₂, P₂W₁₈, and P₂Mo₁₈ interlayers containing Li-S batteries, and PW₁₂ exhibited strong chemical interactions, effective catalytic activity for LiPSs, and low redox potentials, presenting the best electrochemical performance. Moreover, PW₁₂ had an outstanding reversible capacity of 1032.7 milliampere hour per gram after 100 cycles.

The adsorption capacity of P₂Mo₁₈ to LiPSs is greater than P₂W₁₈. However, the cell with a P₂W₁₈ interlayer displayed superior electrochemical performance. This work demonstrated the efficiency of POMs as interlayer materials for Li-S batteries.

Reference

Song, J., Jiang, Y., Yizhong Lu, Wang, M., Linlin Fan, Y. C., Liu, H., and Gao, G. (2022). Effective polysulfide adsorption and catalysis by polyoxometalate contributing to high-performance Li-S Batteries. Materials Today Nano. https://www.sciencedirect.com/science/article/pii/S2588842022000591

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