| Batteries with high energy density,high safety and long cycle life become new trend with the increasing demand for cruising range of new energy vehicles.Compared with traditional lithium-ion batteries,high-theoretical energy density(2600 Wh kg-1)and low-cost lithium-sulfur batteries（Li-S）are considered to be more promising in the next-generation of power batteries.However,several key challenges hinder its large-scale application,such as low conductivity of sulfur-based cathode,polysulfide dissolution and shuttle,lithium dendrite growth,and poor thermal stability of separator.In the view of these issues,this thesis focuses on the the modification of separators in Li-S batteries,and proposed several approaches to functionally modify commercial polypropylene（PP）diaphragms with Montmorillonite（MMT）for changing the lithium ion transmission path inside the battery,and contolling the heat distribution of separator surface,and inhibiting the dissolution and shuttle of polysulfide on the positive electrode surface,so as to imrpove the performance of Li-S batteries.The main contents are as follows:（1）Li-MMT/PP separators were prepared based on Li ion exchange in MMT material,the Li-MMT functional layer controls the directionality/flux of lithium ions and restrains polysulfide（Li PS）,and effectively inhibits the dendritic growth in lithium negative electrode as well as the the shuttle effect of Li PS.It is shown that:the prepared Li-MMT/PP separator exhibits an interlayer spacing of 1.39 nm,and the 5.4μm-thick separator shows a Li-ion conductivity of 3.37 m S cm-1,the liquid absorption rate of the electrolyte is as high as 432%;The Li-MMT layer provides a fast channel for Li ion transmission and forms a uniform flow of lithium ions,which enables a uniform deposition of lithium ions and effectively inhibits the growth of lithium dendrites,therefore realizes dendrite-free lithium deposition;The negatively charged oxygen atoms on MMT surface form Lewis acid-base pairings with Li PS,which can effectively suppress the dissolution and shuttle effect of Li PS.Based on the above advantages,the Li-MMT/PP separator-based Li||Li symmetric battery shows a stable cycling of 800 hours at a current density of 1 m A cm-2 with a capacity of 1 m Ah cm-2,the Li-MMT/PP separator-based Li||Cu battery displays a Coulombic efficiency of>98.2%after 200cycles,the Li-MMT/PP separator-based Li-S battery exhibits good cycling stability with a high specific capacity of 776 m Ah cm-2 even after 180 cycles which is 87%higher than that of the Li-S batteries using PP separators.（2）PPy/Li-MMT@PP separators were prepared based on high thermal conductivity,high electrical conductivity Polypyrrole（PPy）-coated Li-MMT,which improve the thermal stability and electrochemical performance of Li-S batteries.It is shown that:After the introduction of PPy,the composite separator still maintains a high liquid absorption rate（350%）,and its ionic conductivity is increased to 3.63 m S cm-1;the excellent thermal conductivity of Li-MMT effectively improves the thermal shrinkage properties and improves its thermal stability,which resulting in a high-temperature deformation resistance of 150℃.The remarkable electrical conductivity of PPy and the good adsorption performance of Li-MMT on Li PS effectively improve the electron transfer ability of the composite separators and meanwhile inhibit the dissolution and shuttle effect of Li PS.Based on the above advantages,the PPy/Li-MMT@PP separator-based Li-S battery exhibits remarkable electrochemical performance:the specific capacity of the battery is as high as 928 m Ah g-1at a sulfur load of 2.5 mg cm-2 and a current density of 0.5 m A cm-2.Even at the high current density of 8 m A cm-2,the device still releases a specific capacity of 450 m Ah g-1.（3）High-conductivity carbon layer-intercated Li-MMT functional layer（C/Li-MMT）were succusfully prepared based on carbon intercalation and MMT layer,and studied the electrochemical performance of the C/Li-MMT/PP-based Li-S battery.It is shown that:the C/Li-MMT layer obtained by inserting hexadecy ltrimethy lammonium bromide（CTAB）into Li-MMT with a high-temperature carbonizing exhibits high electrical conductivity and polysulfide adsorption;the highly conductive network constructed by C/Li-MMT is favor to electron transfer,the strong adsorption characteristics of C/Li-MMT on polysulfides and the low lithium ion migration barrier caused by carbon intercalation can effectively inhibit the shuttle effect of polysulfides,and meanwhile improve the the utilization rate of long-chain polysulfides to short-chain sulfides.Based on the above advantages,the C/Li-MMT@PP separator-based Li-S battery shows excellent electrochemical performance:the initial specific capacity of the battery reaches to 1308 m Ah g-1at a current density of 5 m A cm-2,and the specific capacity is still as high as 897 m Ah g-1 after 180 cycles.The research shows that the electrochemical performance of Li-S batteries can be significantly improved through the application of high-conductivity multifunctional composite separators. |
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