Preparation of Ti<sub>3</sub>C<sub>2</sub> nanowires with cross-linked network structure and its application in lithium-sulfur batteries

Abstract

<p indent="0mm">Lithium-sulfur batteries are considered as one of the most promising next-generation energy storage devices due to their high energy density and low cost, but their development and application still face many challenges such as the shuttle effect of lithium polysulfide. Recently, a new type of two-dimensional material, MXene (e.g., Ti₃C₂), has attracted much attention. The high specific surface area, high electrical conductivity and polar surface of Ti₃C₂ make it an ideal sulfur host for lithium-sulfur batteries. However, two-dimensional Ti₃C₂ nanosheets (Ti₃C₂-NS) are easy to aggregate together, which reduces the contact area between them and sulfur, and results in the performance degradation of the sulfur electrode. In this work, Ti₃C₂ nanowires (Ti₃C₂-NW) with cross-linked structure are successfully synthesized by etching the exfoliated Ti₃C₂ nanosheets in NaOH solution. Subsequently, sulfur-loaded Ti₃C₂-NW (Ti₃C₂-NW@S) is prepared by the melt-impregnation method and used as the positive electrode material for lithium-sulfur batteries. Scanning electron microscope (SEM) and transmission electron microscope (TEM) images show that Ti₃C₂-NW is composed of cross-liked nanowires with a width of <sc>5–10 nm,</sc> and sulfur particles are successfully loaded in the cross-linked network structure of Ti₃C₂-NW. Moreover, the EDX element mapping images show that the elemental distribution of S is consistent with those of C, O and Ti, further confirming the high dispersion of sulfur particles in Ti₃C₂-NW. Subsequently, Ti₃C₂-NW is characterized by using nitrogen adsorption and desorption technique. The type IV isotherm indicates that Ti₃C₂-NW has a mesoporous structure and its specific surface area is <sc>124.6 m² g⁻¹.</sc> XPS analysis proves the interaction between Ti atoms on the surface of Ti₃C₂-NW and S atoms from sulfur particles because of the presence of Ti-S peak in the high-resolution Ti 2p and S 2p XPS spectra. The loading of sulfur in Ti₃C₂-NW@S (~71 wt%) is determined by thermogravimetric analysis (TGA). It is also found that the type of Ti₃C₂ precursor used for etching affects the structure of the product. Selecting the multilayer Ti₃C₂ particles as the Ti₃C₂ precursor for etching in NaOH solution can only obtain the composite of Ti₃C₂ nanowires and nanosheets (Ti₃C₂-NWS). For comparison, sulfur-loaded Ti₃C₂-NS (Ti₃C₂-NS@S) and sulfur-loaded Ti₃C₂-NWS (Ti₃C₂-NWS@S) are prepared and used as the positive electrode material for lithium-sulfur batteries. The cyclic voltammogram of Ti₃C₂-NW@S shows two reduction peaks and an oxidation peak, corresponding to lithiation of sulfur. Note that the potential difference of redox peak of Ti₃C₂-NW@S is smaller than that of Ti₃C₂-NWS@S, implying the smaller electrode polarization and faster redox reactions of sulfur within Ti₃C₂-NW. The electrochemical impedance spectroscopy (EIS) test results show that Ti₃C₂-NW@S has lower charge transfer resistance (<italic>R</italic>_ct = 7.35 Ω) than Ti₃C₂-NWS@S (<italic>R</italic>_ct = 28.38 Ω), also revealing the rapid reaction of sulfur with Li⁺ on Ti₃C₂-NW. Accordingly, compared to Ti₃C₂-NS@S and Ti₃C₂-NWS@S, Ti₃C₂-NW@S exhibits better electrochemical performance, such as a reversible capacity of <sc>658 mA h g⁻¹</sc> at <sc>0.2 C</sc> after 100 cycles, and a capacity of <sc>436 mA h g⁻¹</sc> at <sc>1 C</sc> after 300 cycles. The superior electrochemical performance of Ti₃C₂-NW@S is attributed to the large specific surface area and mesoporous structure of Ti₃C₂-NW@S, which effectively provide the space for high loading of sulfur and accommodate the volume expansion of sulfur during the lithiation process. Moreover, the large surface area of Ti₃C₂ nanowires is conducive to capturing the lithium polysulfide and thereby suppressing the shuttle effect of lithium polysulfides. Meanwhile, the cross-linked structure is also favorable to the diffusion of lithium ions. This study shows that modifying the morphology and structure of Ti₃C₂ can effectively improve its electrochemical performance as the sulfur host for lithium-sulfur batteries and expand its application in other energy fields.