The Study On Electrochemical Properties Of Sulfur Composites And The Analysis On Energy Storage Mechanism Of Metal Sulfide

Two-dimensional transition metal dichalcogenides(TMDs)adopt layered structures,where metal atoms are sandwiched between chalcogen atoms,with neighboring layers interacting through weak van der Waals forces.Two-dimensional TMDs show great promise for their potential applications as electrode materials in scecondary batteries because of the weak interlayer van der Waals interactions,which allow the reversible accommodation and extraction of alkali metal ions(lithium ions and sodium ions).Nanostructured SnS₂ and TiS₂have attracted broad attentions due to their high theoretical capacity.A comprehensive understanding of the reaction mechanism was needed to further improve their cycle performance and rate performance.However,it is still not clear about the internal microscopic reaction mechanism,especially the reaction pathway and the phase transition process in the whole reaction.The stability of the internal structure of electrode material is an important factor affecting the electrochemical cycling performance.Therefore,it is very important to study the reaction mechanism in real time.In situ transmission electron microscopy(TEM)technique offers a powerful tool for directly visualizing the electrochemical reaction processes of electrode at the atomic scale in real time.In our study,in situ electron microscopy was applied to study the structural and morphological evolution of TiS₂ and SnS₂ during the sodiation reaction,and the relationship with reaction kinetics.This study is of great significance for further understanding the reaction mechanism of sulfide and directing the optimization of electrochemical performance.In addition,different preparation methods were used to prepare(sulfur/carbon electrode)to improve the electrochemical performance,including electrochemical deposition method,spray-drying method and in situ vapor-phase polymerization coating method.The electrochemical performance of lithium sulfur battery has been significantly improved.The main contents in this thesis are as following:(1)We studied the sodiation reaction dynamics of TiS₂ by employing in situ transmission electron microscopy and first-principles calculations.During the sodium-ion intercalation process,we observed multiple intermediate phases(phase ?,phase Ib,and phase Ia),with varied sodium occupation sites and interlayer stacking sequences,different from its lithiation counterpart.Further insertion of Na ions prompted a multistep extrusion reaction,which led to the phase separation of Ti metal from the Na₂S matrix,with its 2D morphology expanded to a3D morphology.First-principles calculations revealed that the as-identified phases were thermodynamically preferred at corresponding intercalation/extrusion stages,compared to other possible phases.(2)We investigated the reaction pathway of SnS₂ by employing in situ TEM.The intercalation reaction occurred after the sodium ion inserted the SnS₂ interlayer.An intermediate rocksalt phase with the disordering cations(Na and Sn)was observed after intercalation.During the following extrusion reaction,SnS and Sn were generated,and our experimental observation and first-principles calculation results indicated that the Na ions prefer to diffuse along the zigzag[010]direction in intermediate SnS phase.Subsequently,Sn and Na underwent alloying reaction,and the final product was a mixture of Na₁₅Sn₄ and Na₂S.After sodiation reaction,in situ observation and ex situ analysis of the desodiation process,revealed a disordered rocksalt phase(Na_ySnS₂)formed,indicating the alloying product(Na₁₅Sn₄ and Na₂S)could eventually transform back to the Na_ySnS₂(rocksalt)phase,and the2D structure of pristine SnS₂ could not be recovered in subsequent electrochemical cycling,which explained the irreversibility of the de-intercalation reaction.(3)Two unique cetyltrimethyl ammonium bromide(CTAB)modified sulfur/carbon(S/CNT,S/RGO)nanostructured composite electrodes were prepared on 3D Ni foams by a facileelectrodeposition method.S nanodots and carbon nanocomposites were directly deposited onto the Ni foams by the electrodeposition methode,which obviously increase the electron/ion conductivity of the whole electrodes.In this structure,S nanodots were coated and immobilized by CTAB-modified carbon materials which could inhibit the shuttle effect of polysulfide and provide effective accommodation of the volume change of S during the cycling processes.Ascribed to the unique structure,the hybrid electrodes exhibited balanced high performance,including:large reversible capacity,stable cyclability and high Coulomibic effciency.Among them,the S/RGO/Ni foam nanostructure electrode exhibited a high reversible capacity of 1450mAh g^-1 at 0.1C.(4)The practical application of high-energy-density lithium-sulfur is hindered by low S loading and polysulfide,we designed a composite material of porous spherical ketjen black-carbon nanotubes(PSKC)with a three-dimensional structure via a spray-drying method.The PSKC material possesses a large pore volume and high specific surface area and is an excellent carbon skeleton for loading high content S.After loading sulfur,the S/PSKC cathode with a high S loading of 80 wt%displayed an initial capacity of approximately 1226 mA h g^-1 with a rapidly decreasing coulombic efficiency.To solve the polysulfide shuttling issue,the S/PSKC material was encapsulated by a polymer protective layer via in situ vapor-phase polymerization.The elemental mappings of a S/PSKC@polypyrrole microsphere cross-section revealed that the polypyrrole(PPy)coating layer was simultaneously formed outside and inside the microsphere during the vapor-phase polymerization process.The S/PSKC@PPy cathode showed a specific capacity of 1109.2 mA h g^-1 at 0.15 C,and the reversible capacity remained 931.6 mA h g^-1after 100 cycles.The S/PSKC@PPy cathode exhibited superior cyclic stability and improved coulombic efficiency,demonstrating the PPy layer effectively suppresses polysulfide shuttling.