A recent article in Materials Today Nano talks about the development of Keggin and Dawson-type polyoxometalates (POMs) modified to improve the performance of Li-S batteries.
Study: Adsorption and catalysis of effective polysulfides by polyoxometalates that contribute 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 some drawbacks, such as the migration of soluble lithium polysulfide (LiPS) intermediates, severe volume changes, and low conductivity of sulfur.
Li-S batteries as energy storage systems
Increasing energy demand requires high energy storage technology. Li-S batteries have a high energy density and a large specific capacity that make them highly valued energy storage systems. In addition, the cost-effectiveness and low toxicity of elemental sulfur make it an environmentally friendly cathode material, making Li-S batteries a next-generation energy storage system. However, soluble lithium polysulfides (LiPS; Li2Sx, x = 4-8) cause the “throwing effect”, which leads to low coulomb efficiency and poor cycle stability. To do this, it is necessary to build the functional intermediate layer to improve the performance of the Li-S battery.
POMs are clusters of anionic metal oxides with good stability, redox properties and diversity. Therefore, POMs are widely used in electrochemical, catalysis and energy systems. K3[H3AgIPW11O39].12H2O (Silver Replaced Keggin) served as the acid catalyst and Lewis base of the Li-S battery. Silver (Ag (I)) ions in silver-substituted Keggin are used as Lewis acid centers to strengthen the binding of sulfur (S) fragments.
The (NH4) 6V10O28 (NVO) groups immobilize LiPS by the attraction of NVO oxygen (O) atoms by LiPS Li cations or by the interaction of NVO vanadium atoms with LiPS sulfur anions. However, the application of POM as an intermediate layer of Li-S batteries for shuttle inhibition is rarely reported and the mechanism behind the inhibition is still unclear.
Interleaved layers for Li-S batteries
In the present study, the team designed three types of Li-S batteries with Keggin or Dawson-type POMs as interleaved layers. They observed that cells with H3[PW12O40]. The xH2O intermediate layer (PW12) had a higher LiPS binding energy and represented a better cycle performance than K6[P2W18O62].14H2O (P2W18) intermediate layer. The adsorption capacity of (NH4) 6[P2Mo18O62].11H2O (P2Mo18) to LiPS is higher than P2W18. Cells containing P2Mo18 as an intermediate layer had better electrochemical performance. PW12 showed catalytic function in LiPS, thus promoting Li2S / LiPS conversion.
Research results
PW12 particle scanning electron microscopy (SEM) images reveal the uniform surface of this interleaved layer without any agglomeration, suggesting that PW12 has sufficient dispersion in the separator. The intermediate layer showed no visible cracks during the folding process, suggesting its robustness and structural flexibility, and the interleaved layer was 5 micrometers thick.
Elemental mapping of the PW12 intermediate layer showed a uniform distribution of the elements phosphorus (P), W, O, and carbon (C), and naked PW12 Fourier transform infrared spectroscopy (FTIR) showed peaks of absorption of P-Oa, W-Oc – W, W = Od, W – Od – W at 1081.8, 983.5. 889.0 and 804.1 reverse centimeters, respectively.
X-ray photoelectron (XPS) spectroscopy revealed the chemical bond and elemental composition of the prepared PW12 intermediate layer. The maximum position of 134.6 electronvolts in the XPS spectrum aligns with the binding energy of P 2p. In addition, the peaks of 36, 2 and 38, 3 electron volts were assigned to W 4f7 / 2 and W 4f5 / 2, respectively, and those of 532, 6 and 531, 3 electron volts in the spectra O 1s correspond to the species OO adsorbed on the surface of the Keggin structure.
The dripping electrolyte on the polypropylene (PP) separator and the PW12 intermediate layer helped in the contact angle tests, and the results showed that the PW12 contact angle was less than the PP separator. This test confirmed that the rich voids in the PW12 intermediate layer increase the wettability of the electrolyte, which facilitates the mobility of lithium ions.
Subsequently, the evaluation of the electrochemical performance of the Li-S batteries containing intercalated layers of POM and zinc oxide cathode C (ZnO) / S revealed that the intermediate layer PW12 had a higher capacity of 1607.8 thousand Per hour per gram and lower polarization voltage (ΔE) of 165.9 millivolts than P2W18 and P2Mo18. These experimental results indicated improved catalytic activity and rapid reaction kinetics of PW12.
Cyclic voltameter curves of the cell containing PW12 at a cyclic voltammetry of 0.1 millivolts per second showed two reduction peaks at 2.28 and 2.05 volts, indicating the reduction from S to Li2Sx ( 4 ≤ x ≤ 8), followed by conversion to Li2S2 in solid state. / Li2S electrochemically. In addition, the peak at 2.39 volts indicated the oxidation of Li2S2 / Li2S to Li2Sx and S.
Conclusion
In summary, the researchers designed the PW12, P2W18, and P2Mo18 interleaved layers containing Li-S batteries, and PW12 showed strong chemical interactions, effective catalytic activity for LiPS, and low redox potentials, presenting the best electrochemical performance. In addition, PW12 had an exceptional reversible capacity of 1032.7 milliamperes per hour per gram after 100 cycles.
The adsorption capacity of P2Mo18 to LiPS is greater than P2W18. However, the cell with a P2W18 intermediate layer showed 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, YC, Liu, H. and Gao, G. (2022). Adsorption and catalysis of effective polysulfides by polyoxometalate that contribute to high-performance Li-S batteries. Materials Today Nano. https://www.sciencedirect.com/science/article/pii/S2588842022000591
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