Ti3C2Tx/Graphene Oxide Free-Standing Membranes as Modified Separators for Lithium–Sulfur Batteries with Enhanced Rate Performance

Lithium–sulfur batteries are considered as the most promising candidate for next-generation energy storage devices. However, they are subjected to the "shuttle effect" of soluble lithium polysulfides (LiPSs). Herein, a free-standing membrane composed of two-dimensional MXene material (Ti3C2Tx) and graphene oxide (GO) is synthesized by a simple vacuum-filtration method. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy are carried out to determine structure, morphology, and composition of the Ti3C2Tx/GO composite membrane, respectively. As a functional layer of trapping LiPS species, the Ti3C2Tx/GO composite membrane and commercial polypropylene (PP) are successfully assembled to be a hybrid separator, Ti3C2Tx/GO@PP, to suppress the shuttle effect of LiPSs. The porous and rough surface of the Ti3C2Tx/GO composite membrane is beneficial to improve the wettability of the commercial separator in an ether-based electrolyte. The cells with the Ti3C2Tx/GO@PP hybrid separator exhibit a low polarization potential of 0.26 V in the conversion from Li2S4 to Li2S2/Li2S and deliver a discharge capacity of 640.0 mA h g–1 for 5 C rate, indicating that the hybrid separator benefits the rate performance. According to the results of electrochemical impedance spectroscopy, increased discharge capacity is attributed to the reduced internal resistance and intensified Li+ diffusion. The results of X-ray photoelectron spectroscopy focusing on the surfaces of both sides of the hybrid separator indicate that the shuttle effect of LiPSs is suppressed through a coefficient of the terminated groups' catalytic conversion on long-chain LiPSs and the titanium-reactive centers' Lewis acid–base pairs on short-chain LiPSs. Combining with digital photographs of the H-type electrolytic cell, the results of UV–visible absorption spectroscopy suggest that the concentration of long-chain polysulfides declines instantly under the redox effect of the terminated groups on Ti3C2Tx surfaces and then infiltrate through the hybrid separator by virtue of concentration difference impetus. Generally, a Ti3C2Tx/GO@PP hybrid separator restrains LiPS diffusion and improves the rate performance of Li–S batteries.