Research Of The Low-temperature Plasma Preparation Of Li-S Battery And Its Electrochemical Properties

Lithium-sulfur battery,has attracted widespread attentions,due to its high energy density?2600 Wh/Kg?,high capacity?1675 mhA/g?,abundance in nature,low cost and non-pollution.However,during the charging/discharging process,the formed polysulfide is easy to dissolve into the electrolyte and travel to the anode,resulting in the loss of sulfur active substances.Moreover,the poor conductivity of sulfur and lithium sulfide leads to a poor rate performance of Li-S battery.In addition,the growth of the dendrites of lithium anode will lead to the unstable interface between the electrode and the electrolyte in the cycling process,which destroys the formed solid SEI membrane and constantly consumes the electrolyte,resulting in the irreversible deposition of lithium metal,the decrease of the coulomb efficiency of the battery.The impaling of the SEI membrane even causes the short circuit of the battery.In view of above problems,this thesis proposes a strategy based on the low-temperature plasma to fabricate/modify the S-C composite cathode and Li metal anode,and studies the electrochemical performance of the as-fabricated Li-S batteries:?1?Graphite was modified by N₂+H₂ mixture plasma.During plasma process,nitrogen-related various bonds was formed and graphite was etched by hydrogen plasma.The formatted of surface micro/nano structure generated by plasma etching increased the specific surface area of graphite and sulfur load.At the same time,with the aid of plasma spectral analysis,it was found that the formed NH group in plasma process helped in further decomposition of nitrogen and improvement of N-doping efficiency.The pyridinic N and pyrrolic N formed in the process of plasma improved the electrical conductivity of the material and enhanced the adsorption of polysulfide formed during charging and discharging.The capacity of first discharge before and after modification of the Li-S batteries reached 530 mAh/g and 701 mAh/g at 0.1 C?1C=1675 mA/g?,and the capacity kept 361 mAh/g and 433 mAh/g after 50 cycles at0.2 C,respectively.The excellent electrochemical properties of modified materials had been demonstrated.?2?In order to further improve the electrochemical performance of lithium-sulfur batteries,the sputtered ZnO layer decorated nickel foam was applied to host lithium metal in the side of anode.The lithiophilic property of nickel foam and the non-uniform charge distribution during the deposition of lithium metal had effectively been solved.The overpotential of symmetrical lithium metal battery with modified nickle foam skeleton,even kept working 800 h,was always¹0 mV at the current density of 1 mA/cm² and capacity of 1 mAh/cm².In comparison,the battery assembled by commercial lithium got short-circuited after 300 h working due to the formation of dendrite.On the basis of the study?1?,it was found that capacity had been further enhanced when the modified graphite and lithium metal anode were adopted.Thus,the capacity of first discharge of the Li-S batteries reached 715 mAh/g at 0.1 C?1 C=1675 mA/g?,and the capacity kept 486 mAh/g after 50 cycles at 0.2 C.?3?The S-C composite cathode was prepared by a one-step plasma process of dissociation of CS₂ vapor,where any additional heating?at room temperature?and a long time?just 30 min?were not involved.Two excitation modes of the plasma,i.e.,the capacitance and inductance plasma,were compared in terms of electrochemical performance of the obtained S-C composite cathode.It was found that the inductively coupled plasma?ICP?fabricated cathode had better electrochemical performance possibly due to the higher plasma density and the higher dissociation ratio in ICP.When nitrogen was introduced into the reactive gases of CS₂+Ar,a S/C composite cathode with a hierarchical structure and N-doped carbon,was fabricated in a one-step ICP process.The analysis of electrochemical performance,chemical compositions and bond configuration shown that the N-doping increased the conductivity and diffusion rate of Li⁺of the cathode and effectively alleviate the polysulfide shuttle via the physical/chemical adsorption.Taking the total weight of carbon sulphur as the mass of active material,the initial discharge capacity of the battery was 923 mAh/g at 100 mA/g,and the capacity was 600 mAh/g after 100cycles,besides,the capacity can kept 231 mAh/g even after 500 cycles at 1 A/g.