| Considering that sustainable clean energy technology is difficult to compete with traditional energy conversion technology in terms of energy density,the development of secondary battery system with high capacity and high energy density has become the primary task of the research and development of clean energy technology.Lithium sulfur battery(Li-S battery)has attracted more and more attention because of its high theoretical specific capacity,high energy density,abundant sources of active materials and low-cost.However,some key problems which restrict the technical advancement of Li-S battery have not been solved.The poor conductivity of the active material,the huge volume expansion of cathode during cycling and the "shuttle effect" of polysulfide,leads to a series of problems of Li-S battery.Meanwhile,lithium dendrites which bring serious safety problems are easily produced in the process of cycling.Lithium sulfide battery(Li2S battery)is a kind of Li-S battery with graphite,tin,silicon or other kinds of materials as anode active materials and lithium sulfide as cathode active materials.Li2S battery has attracted more and more attention recently since the preparation of anode is simpler and safer than the preparation of lithium metal,and it can avoid the generation of lithium dendrites.However,Li2S battery almost inherits all the disadvantages of the traditional Li-S battery,and due to the low electrochemical activity and poor ionic conduction ability of Li2S cathode,an over potential exists when activating the electrode.In order to overcome the shortcomings of Li2S battery,our research starts from the traditional Li-S battery.On the basis of studying of the effect of one-dimensional carbon materials on suppressing the "shuttle effect" we focus on the research and development of Li2S-C material which in situ synthesized based on different carbon source materials.We also apply the material to full cell and explore the activation mechanism of it.The main results and conclusions are presented as below:1.To solve the problems of volume expansion of sulfur cathode,"shuttle effect"of polysulfide and poor conductivity of sulfur in Li-S battery,multi wall carbon nanotubes,MWCNT,was modified as sulfur host material to solve the problems mentioned above.MWCNT was oxidized by H2O2 to introduce oxygen-containing functional groups on the surface to improve the dispersion ability of M WCNTs.Then MWCNT was etched by KOH at high temperature to improve its specific surface area.Finally,the reduction treatment was used to overcome the decrease of conductivity caused by the previous treatment,then the modified MWCNT with higher specific surface area and better conductivity was obtained.The sulfur cathode based on modified MWCNT effectively improved the discharge capacity and cycling performance.At 0.2C,the initial discharge capacity was 1272.7 mAhg-1,and the capacity retention rate after 50 cycles was 76.45%.Experiments showed that the electrochemical performance of sulfur electrode was also affected by the diameter of MWCNT and the reduction technology.The performance of the battery based on MWCNT with larger diameter was improved more obviously after modification,and the conductivity of MWCNT treated with stronger reductant(NaBH4)was better compared with the one treated with glucose.2.As Li2S cathode can avoid the safety problems caused by the lithium anode and solve the problems caused by high volume expansion rate of sulfur cathode,we also focus on the research of Li2S battery.To solve the problems of electrochemical inertness of lithium sulfide,Li2S-C composite was synthesized in situ to improve the conduction of electron and ion of Li2S cathode.The precursor of composite and its subsequent carbonization process had great influence on the structure-activity of the composite.It was found that in the process of preparation of lithium sulfide composite material(Li2S@OC)based on o-phenylenediamine,the precursor obtained by freeze-drying was more homogeneous than that obtained by the way of solvent evaporation and spray drying,and the size of lithium sulfide particle was smaller and the distribution was more uniform after carbonization.The way of secondary carbon coating of Li2S suppressed the growth and agglomeration of lithium sulfide crystal in the process of in-situ reaction,and the size of lithium sulfide particles got by this way was smaller(500 nm-1 ?m).In order to further reduce the size of lithium sulfide particle and promote the ion conduction of Li2S cathode,lithium sulfide carbon composite(Li2S@GC)with two-dimensional carbon film was synthesized in situ by using glucose as carbon source.Different from the one-dimensional fiber-bonded carbon network based on o-phenylenediamine carbon source,the carbon film suppressed the growth of lithium sulfide crystal better,the size of lithium sulfide particle(about 100nm)was significantly smaller than that of lithium sulfide in Li2S@OC.The discharge capacity of first cycle of lithium sulfide cathode based on Li2S@GC material was 1072 mAhg-1 Li2s,and the stable discharge capacity after 50 cycles at 0.1C was 610.7 mAhg-1Li2S,which is 25.6%higher than that of lithium sulfide battery based on o-phenylenediamine carbon source.3.Based on the research results above,the activation mechanism and charge-discharge behavior of Li2S-graphite battery were studied by using Li2S@GC as cathode material and graphite as anode material.The activation and properties of graphite were subject to electrolyte and binder.Experiments showed that ether electrolyte with lithium nitrate additive improved the cycling stability of graphite electrode by promoting the formation of SEI film on the surface of graphite electrode.The cycling performance was improved by sodium alginate-SBR composite binder,which made graphite materials more suitable for Li2S battery system.Finally,various activation processes of Li2S-graphite battery were studied.Among them,the capacity retention rate of the full cell activated by the graphite anode pre-activation process was better,the capacity retention rate after 50 cycles was 73.4%of the first cycle;the initial discharge capacity of the full cell which activated by the constant-voltage charging process reached up to 609.6 mAhg-1Li2S.The research above showed the feasibility of commercial application of Li2S-graphite battery. |
PDF Full Text Request