Lithium Storage Performance Of Key Materials For Lithium Secondary Batteries With High Capacity

Now the lithium-ion batteries (LIBs) have been the dominant power source for advanced portable electronic devices and are also considered as the most competitive system to supply power for future electric vehicles (EVs). For LIBs, in order to meet the large-scale power requirements, searching for new electrode materials with higher energy density is still a challenging task. Sulfur cathode and tin-based anode, both show some outstanding advantages such as high theoretical specific capacity, low cost and environmental friendliness, which has been considered as one of the most promising cathode candidates for next-generation batteries. However, the practical application of these two kinds of electrodes suffers from low sulfur/tin utilization and poor cycle life. The problems can be ascribed to the poor electrical conductivity of sulfur and some tin-based composites, and to the large volume changes during the repeated charge-discharge cycling. This Ph.D work was aimed at developing some sulfur/tin based composites applicable for rechargeable lithium batteries. The main results are summarized as follows:The mixture of styrene butadiene rubber (SBR) and sodium carboxyl methylcellulose (CMC), a water-soluble binder system, have been successfully introduced to the sulfur cathode system. Compared with conventional poly (vinylidene fluoride)(PVDF) binder, the SBR-CMC binder significantly improves cycling performance of the sulfur cathode. The SBR-CMC mixture is not only a high adhesion agent but also a strong dispersion medium, which favors the uniform distribution between insulating sulfur and conductive carbon black (CB) and ensures a good electrical contact, leading to a high sulfur utilization. Furthermore, the improvement in cyclability is ascribed to structural stability of the sulfur cathode promoted by the SBR-CMC binder during charge/discharge cycles due to the combined effects of homogeneous distribution of the S and CB particles in the composite cathode, the low electrolyte uptake, and the suppressed agglomeration of L12S.A sulfur composite based on a porous multi-walled carbon nanotubes (PCNTs) with multiple mesopores structure was synthesized and studied as a cathode material for Li-S battery. The PCNTs was prepared via a simple activation of multi-walled carbon nanotubes (MWCNTs) by potassium hydroxide. The potassium hydroxide activation process results in a significantly enhanced specific surface area with numerous small pores. The as-obtained PCNTs are employed as the conductive matrix for sulfur in the sulfur cathode, SBR-CMC was used as binder. Compared with the composite sulfur cathode based on the original MWCNTs, the sulfur-PCNTs cathode shows a significantly improved cycle performance and columbic efficiency. The improvement in the electrochemical performance for S-PCNT is mainly attributed to the enlarged surface area and the porous structure of the unique mesopores carbon nanotube host, which cannot only facilitate transport of electrons and Li+ions, but also trap polysulfides, stabilize the electrode structure during charge/discharge process.A nanocluster composite assembled by interconnected ultrafine SnO2-C core-shell (SnO2@C) nanospheres is successfully synthesized via a simple one-pot hydrothermal method and subsequent carbonization. As an anode material for lithium-ion batteries, the thus-obtained nano-construction can provide a three-dimensional transport access for fast transfer of electrons and lithium ions. With the mixture of styrene butadiene rubber and sodium carboxyl methyl cellulose as a binder, the SnO2@C nanocluster anode exhibits superior cycling stability and rate capability due to a stable electrode structure. Discharge capacity reaches as high as1215mA h g-1after200cycles at a current density of100mA g-1. Even at1600mA g-1, the capacity is still520mA h g-1and can be recovered up to1232mA h g-1if the current density is turned back to100mA g-1. The superior performance can be ascribed to the unique core-shell structure. The ultrafine SnO2core gives a high reactive activity and accommodates volume change during cycling; while the thin carbon shell improves electronic conductivity, suppresses particle aggregation, supplies a continuous interface for electrochemical reaction and alleviates mechanical stress from repeated lithiation of SnO2.Acetylene black incorporated porous3-dimensional (3D) SnS2nanoflowers have been successfully synthesized via a simple solvothermal route. The composites are composed of acetylene black adorned SnS2secondary microspheres which are assembled from a number of nanosheets. The nanocomposites possess a large specific surface area of129.9m2g-1and a high conductivity of0.345S cm-1. As anode materials for lithium ion batteries, the nanocomposites show high cyclability and rate capability and deliver an average reversible capacity as high as525mAh g-1at a current density of400mA g-1over70cycles. The high electrochemical performance can be attributed to the synergistic effect of acetylene black and the unique microstructure of SnS2. The acetylene black serves as not only a conductive agent to accelerate the transfer of electrons in the composites, but also as a buffer matrix to restrain the volume change and stabilize the electrode structure during the alloying/dealloying process. The porous structure of SnS2also helps to stabilize the electrode structure and facilitates the transport for lithium ions.