Design And Electrochemical Performance Of The Key Electrode Materials For Lithium Sulfur Battery

The ever-increasing global demand for energy has become the chief concern of the world today.The growing depletion of the traditional fossil energy already can not satisfy the demand of sustainable development of human society.Therefore,it is imperative to seek new ways for harvesting and storing energy.Secondary batteries occupy important status in the new energy system.However,the traditional secondary battery systems can not meet the huge demands of the important emerging fields,such as electrical vehicles and large-scale energy storage,in terms of energy density and resource reserves.Due to the low-cost,high-energy-density and environmental friendliness of the lithium sulfur batteries,they are the potential candidates for next-generation batteries.Unfortunately,the commercialization of lithium sulfur batteries is still facing some urgent challenges,such as the safety issues caused by the dendritic growth,the poor electrochemical accessbility due to the low conductivity of sulfur,and the shuttle effects caused by the dissolution of intermediates,which affect the utilization of active materials and the cycling stability.In response to these problems,this paper focuses on the design of key electrode materials for lithium sulfur batteries and explores ways to effectively improve the overall performance of batteries.The main research contents are as follows:(1)Aimed at the dendritic growth of lithium metal anode,a 3D carbon skeleton with lotus root structure was designed to realize the selective deposition of lithium ions inside these channels,thus effectively suppressing the dendritic growth and improving the stability of the Li metal anode.A series of hollow carbonfibers with different aperture ratios were prepared.The experimental and simulation results show that when the aperture ratios are?10,lithium ions can only deposit inside these inside channels.The conclusion is also well verified by the results of transmission electron microscopy(TEM)and scanning electron microscopy(SEM).The 3D carbon skeleton can remain stable after cycling even more than3500 hours,which proves that the structural design can effectively improve the stability and safety of the composite lithium anode.When modified by Nafion-SEI,the Coulombic efficiency of the composite anode reaches 99.0%.When paired with Li Fe PO₄ as the full cell(the capacity ratio of Li to Li Fe PO₄is 1.8:1),there is no obvious capacity fading even after120 cycels at 0.5 C.The research results propose a new strategy to effectively control the selective deposition of lithium ions through geometrical regulation,which provides a new idea for improving the stability and safety of lithium metal anode.(2)To facilitate the ununiform deposition of Li ions,via Au modification on one side of the carbon fibers matrix(CFs@Au),we succeed in achieving selective deposition of Li ions on the back side of current collector which is away from separator,and hence improving the safety effectively.As a result,this composite anode can remain stable without short circuit after cycling even more than 700 hours;the Coulombic efficiency of the CFs@Au-Li anode remains 99.2%throughout 400 cycles.What is more,the Li-S full cell with the composite anode exhibits outstanding performance,even with limited Li amount:the capacity remains at 700 m Ah g^-1 after 100 cycles.This backside-deposition strategy provides new insight into the safe Li metal anode design for high energy density battery systems such as Li-S and Li-O₂.(3)A new strategy is proposed by introducing a multifunctional Li-ion pump to improve the homogenous distribution of Li-ion.Via coating a?-phase of poly(vinylidene fluoride)(?-PVDF)film on the Cu foil(Cu@?-PVDF),a piezoelectric potential across this film is established near the electrode surface because of its piezoelectric property,which can serve as a driving force to regulate the migration of Li-ion across the film.As a result,this protected Li anode can remain stable without short circuit after cycling even more than600 hours and the Coulombic efficiency(CE)of Cu@?-PVDF remains around 99.0%throughout 600 cycles.Meanwhile,the lithium-sulfur(Li-S)full cell paired with the Li-Cu@?-PVDF anode demonstrates excellent electrochemical property.After 100 cycles,the capacity keeps?1000 m Ah g^-1.This facile strategy via regulating the migration of Li-ion provides a new perspective for the safe and reliable Li metal anode.(4)Due to the poor conductivity of the polyacrylonitrile-sulfur(PANS)composite,we prepared a coaxial composite material with blending PANS and carbon nanotubes which owns improved conductivity and rate performance.The CNT skeleton provides channels for electron conduction,thus effectively improving the electrical conductivity of the composite.The composite cathode can achieve a high reversible capacity of?900 m Ah g^-1 at even 5 C.At the same time,the special electrochemical behavior of PANS was tested and analysised.According to the results,it is considered that the irreversible insertion of lithium ions into the structure of PANS results in the irreversible capacity of the PANS at the first discharge,thus resulting the low initial coulombic efficiency.The results of this study provide a new insight for understanding the electrochemical reaction mechanism of PANS.