The Research Of Designing High Performance Lithium-sulfur Batteries Polymer Cathode And Its Electrochemical Characterization

With the rapid development of the new generation of electronic products,the traditional lithium-ion battery has been more and more difficult to meet the current demand for high specific energy.To achieve a leap of specific energy battery,a new battery system has made to the hot spot of the present research.Lithium sulfur?Li-S?battery is assembled by sulfur as the cathode and lithium metal as anode.Compared with the conventional LiCo_xMn_yNi_zO₂ and LiFePO₄ materials which have been widely used in all kinds of electronic products,Li-S battery has better energy storage capacity because its theoretical energy is 2600 Wh/kg.Despite the promise of sulfur cathode,there exist many problems,such as poor cycling performance,low utilization of active substances,poor C-rate performance and hidden dangers.In order to solve the above problems,it is necessary to improve the conductivity of the electrode materials,prevent the dissolving and diffusion of polysulfide compounds,and rationally design the electrode structure.Herein,we successfully designed a crosslinked structure of cathode material which has been achived the long cycle performance of Li-S battery.The Li-S battery was assembled using the above cathode material.We test the electrochemical performance of the new crosslinked cathode material and explored the relationship between dissolution of polysulfide compound and chemical limits method.Also research the internal mechanism between electrochemical performance of Li-S battery and cathode crosslinked structure.The main content included the following three parts:The crosslinking cathode materials were prepared by reverse thermal polymerization,which obtained the organic sulfur polymer with higher sulfur content and degree of polymerization.On the one hand,chemical confinement method could effectively prevent the dissolution and diffusion of polysulfide compounds.On the other hand,the crosslinked materials with mesh hole structure provides the diffusion path of lithium ion.The results show that crosslinking material improved the cycle stability of the electrode.Further,a layer conductive polymer PEDOT:PSS thin film was uniform coated on electrode surface,which could effectively prevention of the diffusion of polysulfide compounds by physical barrier method.Above obtained electrode exhibited excellent electrochemical performance,because physical and chemical methods were together restriction of dissolution and diffusion of Polysulfide.The first discharge capacity is 1362 mAh/g at 0.1 C.After500 cycles,the capacity retention rate is 76.6%at 1C.The trithiocyanuric acid was deposited on the surface of grapheme by the co precipitation mode.Then sulfur radicals and trithiocyanuric acid were crosslinking polymerization to form electrode material with cross-linked structure and high conductivity.The content of active sulfur in electrode and the area loading of active materials increase significantly.The high conductivity of grapheme embedded effectively improved electronic transmission capacity in electrode,enhanced the rate capability of the battery.The dissolution and diffusion of polysulfide were effective suppression by the large number of covalently cross-linked between trithiocyanuric acid and sulfur atom.The experimental results show that the content of organic sulfur in electrode reached 90%.The initial discharge capacity of 1341 mAh/g at the rate charge and discharge current of 0.1 C.After 500 long cycling the content of organic sulfur in electrode is still as high as 49.2%.And the capacity decay rate is only 0.0404%per cycling.The high conductance polyaniline with thiophenol functional group was obtained by oxidative polymerization.Then,the polyaniline and sulfur radical were cross-linked by reverse polymerization to form cross-linked structure with covalent bond.Such a remarkable electrochemical performance of the obtained cathode is attributed to the formation of highly conductive paths consisting of polyaniline nanoplates,while the excellent cycling stability is mainly ascribed to the potently chemical confinement of sulfur with a large number of stable covalent bonds between sulfur and thiol groups of polyaniline.Utilization of cross-linked material as a cathode material remarkably enhanced the rate performance and stability of Li-S batteries at an active sulfur loading of 1.5mg/cm2,which showed an initial capacity of 1240 mAh/g at 0.1 C,and a discharge capacity of 495mAh/g even after 1000 deep charge and discharge cycle at 2 C with a capacity-retention of 66.9%and a low capacity fading rate of only 0.040%per cycling.