Interfacial melting mechanism of nanocrystals determined by interfacial energy and interfacial stress.

To clarify the interface melting mechanism, a unified analytical expression was developed to describe the depression and superheating of Tm(D) functions for metallic nanoparticles, nanostructures, and nanoparticles embedded in a coherent or incoherent interface. Tm(D) functions are determined by the sign of γss, fss (or γsl and fsl), and D0 as caused by the change of interface environments. We found that there is TCIm(D) > TNSm(D) > TIIm(D) > TNPsm(D) for Ag nanocrystals within different interfaces. Moreover, for a given size, Tm(D)/Tm(∞) decreases with the reduction of γss/fss for nanoparticles, nanostructures and nanoparticles embedded in incoherent interfaces, while an opposite trend occurs for the coherent interfaces. In addition, we also found that there is TNPsm(D)/Tm(∞) < TIIm(D)/Tm(∞) < TNSm(D)/Tm(∞), which is in agreement with the relation of γNPssl/fNPssl < γIIss/fIIss < γNSss/fNSss. By analyzing the γss(D) (or γsl(D)), fss(D) (or fsl(D)) and γss(D)/fss(D) (or γsl(D)/fsl(D)) functions of Ag nanocrystals and comparing with their Tm(D) functions, it is found that there is a high consistency between the variation of γss(D)/fss(D) (γsl(D)/fsl(D)) and Tm(D)/Tm, which reveals that the size dependence of Tm(D)/Tm is determined by γss(D)/fss(D) (or γsl(D)/fsl(D)). Our predictions show a good agreement with the available theoretical and experimental results.

Conducting polymer-coated MIL-101/S composite with scale-like shell structure for improving Li-S batteries.

Lithium-sulfur batteries are regarded as a promising energy storage system. However, they are plagued by rapid capacity decay, low coulombic efficiency, a severe shuttle effect and low sulfur loading in cathodes. To address these problems, effective carriers are highly demanded to encapsulate sulfur in order to extend the cycle life. Herein, we introduced a doped-PEDOT:PSS-coated MIL-101/S multi-core-shell structured composite. The unique structure of MIL-101, large specific area and conductive shell ensure high dispersion of sulfur in the composite and minimize the loss of polysulfides to the electrolyte. The doped-PEDOT:PSS-coated sulfur electrodes exhibited an increase in initial capacity and an improvement in rate characteristics. After 192 cycles at the current density of 0.1C, a doped-PEDOT:PSS-coated MIL-101/S electrode maintained a capacity of 606.62 mA h g-1, while the MIL-101/S@PEDOT:PSS electrode delivered a capacity of 456.69 mA h g-1. The EIS measurement revealed that the surface modification with the conducting polymer provided a lower resistance to the sulfur electrode, which resulted in better electrochemical behaviors in Li-S battery applications. Test results indicate that the MIL-101/S@doped-PEDOT:PSS is a promising host material for the sulfur cathode in the lithium-sulfur battery applications.