Study On The Preparation Of Metal Sulfide/carbon Composite And Its Alkali Metal Ion Energy Storage

The energy crisis and environmental pollution have become the two most serious issues.Thus,they have drawn much attention to explore innovative technologies and seek sustainable energy as well as energy storage devices.As one of the most promising energy storage technologies,lithium-ion batteries(LIBs)have been widely utilized as power sources for portable electronic devices,electric vehicles and implantable medical devices due to their high energy and power density.Although LIBs have achieved great success,the high cost and scarcity of lithium resources are severe barriers for their further development,especially in large-scale devices.Recently,owing to the cheap and ubiquitous sodium resources,sodium-ion batteries(SIBs)are attracting more and more attention which are undoubtedly the most potential alternative to Li-ion technology.However,commercial graphite possesses a relatively low theoretical capacity for lithium storage(only 372 mAh g-1)and is unsuitable for sodium storage.It is still a great challenge to develop novel anode materials with high reversible capacity,suitable lithiation/sodiation potential and long cycle life.Recently,metal sulfides have drawn much attention and have been extensively studied as promising anodes for LIBs and SIBs because of their high theoretical capacity and good conductivity.In this thesis,we have prepared the FL-SnS2/RGO,CP@MoS2,and Fe7S8@CNF composites and systematically investigated their lithium and sodium storage properties.Meanwhile,we design and prepare a Se@carbon(Se@C)composite by incorporating Se into a heteroatoms(N and O)dual-doped honeycomb porous carbon microbubble(HDHPCM)matrix and the lithium/sodium/potassium storage properties were systematically studied.The main contents of this paper are as follows:Firstly,we study the sodiation mechanism of SnS2 using ex situ selected area electron diffraction(SAED)and high-resolution transmission electron microscopy(HRTEM)characterizations.A three-step sodiation mechanism of intercalation and conversion as well as alloying is revealed.A large anisotropic volumetric variation along the c axis is demonstrated during the sodiation process,which results in a rapid capacity decline upon cycling.To realize both high capacity and excellent cycling operation,we have especially designed a few-layer SnS2/reduced graphene oxide(FL-SnS2/RGO)hybrid with plenty of SnS2 nanosheets sandwiched between RGO nanosheets.The features of our structure design are triple:(1)the RGO nanosheets serve as a flexible cushion layer to buffer the anisotropic volumetric expansion during sodiation;(2)the few-layer SnS2 nanosheets provide a shortened diffusion distance for sodium ions;(3)the RGO layers work as high-speed electronic pathways,thus the resulting hybrid material is endowed with a high electrochemical activity for sodium-ion storage.Secondly,a hierarchical composite nanoarchitecture of MoS2 ultrathin nanosheets grown on nitrogen-doped carbon polyhedra(CP@MoS2)has been successfully fabricated.The unique CP@MoS2 nanospheres demonstrate improved alkali-metal ion(Li+ and Na+)storage performances,which can be correlated with their unusual structural features.In particular,the nitrogen-doped carbon polyhedra are able to significantly boost the electrical conductivity of the hybrid architecture and largely mitigate the agglomeration of MoS2 nanostructure.The porous architecture of CP and ultrathin MoS2 nanosheets can significantly assuage the internal strain aroused by the huge volume expansion during the charge and discharge processes.The ultrathin MoS2 building blocks have a large electrolyte/electrode interphase and the migration distance for e-and Li+/Na+could be considerably diminished.The sheet-like MoS2 nanostructure can render a great deal of storage sites toward lithium and sodium.All these advantages would definitely boost the Li+ and Na+ storage capabilities of the as-obtained CP@MoS2 nanospheres,bringing about large specific capacity,excellent rate property,and stable cyclability.When measured as a negative electrode for Li storage,these CP@MoS2 nanospheres manifest a large charge capacity of approximately 549 mAh g-1 and superior cycle life of 900 cycles and excellent rate property.Furthermore,they also demonstrate improved electrochemical activity for Na+ ion storage.Thirdly,we successfully synthesized the Fe7S8/C/RGO composite with a 3D iron sulfide-carbon interlocked graphene structure by a facile method.Utilized for lithium/sodium-ion storage,the 3D networks facilitated the transportation of electrons/ions,and promoted the structural stability of electrodes,thus improving the cycling performance and rate capability.When estimated as anodes for LIBs and SIBs,it shows excellent rate capabilities,outstanding cycling stability and high specific capacities.As anode materials for LIBs,Fe7S8/C/RGO composites demonstrate excellent Lf storage with a high rate reversible capacity of 1091 mAh g-1 after 500 cycles at a high current density of 2 A g-1.For SIB tests,Fe7S8/C/RGO composites retain stable discharge capacities of 269 mAh g-1 after 500 cycles even at a high current density of 5 A g-1.Such an excellent lithium/sodium storage performance indicates that the Li&Na/Fe7Ss/C/RGO system may be a very promising energy storage system and the structural design inspires advances in LIBs and SIBs.Finally,we design and prepare a Se@Carbon(Se@C)composite by incorporating Se into a heteroatoms(N and O)dual-doped honeycomb porous carbon microbubble(HDHPCM)matrix.The obtained HDHPCM with large surface area and high pore volume possesses a unique structure with interconnected macro/meso/micropores.Such unique structure not only exhibits large electronic conductivity and offer abundant electrolyte pathways,but also facilitates the interaction between carbon with Se and acts as micro reaction chambers to effectively suppress the volume change of the electrode and trap the polyselenides intermediates during cycling.Additionally,the heteroatom dopants(N and O)help to enhance the interaction between carbon and Se/Li2Se/Na2Se/K2Se.Therefore,the synergic effect of heteroatom(N and O)dual-doping and porous structure can effectively improve the electrochemical properties.As expected,using Se@HDHPCM composite as the cathode material for both lithium-selenium(Li-Se),sodium-selenium(Na-Se),and potassium-selenium(K-Se)batteries,a high-rate performance and high cycling stability have been achieved in low-cost carbonate electrolyte.For Li-Se battery,at a current density of 0.2 C(1 C=678 mA g-1),the Se@HDHPCM cathode delivers an initial charge capacity of 675 mAh g-1 and a reversible capacity of 576 mAh g-1 with coulombic efficiency nearly 100%and a capacity decay as low as 0.14%per cycle after 100 cycles.For Na-Se battery,the Se@HDHPCM composite cathode exhibits a high initial charge capacity of 688 mAh g-1 and retains as high as 244 mAh g-1 after 300 cycles at a current rate of 1.0 C.