Design,Preparation And Electrochemical Application Of Carbon-based Multifunctional Layer On Separator For Lithium Sulfur Battery

The lithium sulfur(Li-S)battery possesses advantages of ultra-high theoretical energy density,low-cost and environmental-friendly,which makes it to be a promising energy storage system.However,its commercialization is still frustrated by some shortcomings,such as low practical energy density,short cycle life and poor security.The main reason is that the intermediate polysulfides are easily dissolved in electrolyte and then diffused to lithium anode.Subsequent side reaction between polysulfides and lithium may lead to shuttling effect and irreversible loss of active materials.Because pores of polyolefin separators including polyethylene and polypropylene are oversized,polysulfides can pass through them without any difficult,leading to the serious shuttling effect.The commercial separator has tremendous potentiality for further modification,which can meet the needs of Li-S battery with high specific energy and long service life.In this paper,the author prepared various carbon nanomaterials modified layers on the surface of commercial separator.For achieving multifunction of modified layer,some methods including addition of nano adsorbent,design of coating structure and improvement of coating technology.The carbon layer has the basic physical adsorption and blocking effect for polysulfides.Nano adsorbents such as LiMn₂O₄,Ti₄O₇ and Ni can chemically adsorb polysulfides and promote the conversion of active substances.Synergistic effect of carbon layer and nano adsorbent can effectively restrain the shuttling effect of polysulfides.In addition,the cycle life and safety performance of battery was significantly enhanced by using the BaSO₄ ceramic separator as basement and phase conversion method of PVDF.The research content mainly includes the following five parts:(1)The composite of Super P conductive carbon and LiMn₂O₄ nanoparticles was coated on polyethylene(PE)separator to form modified layer.The Super P layer has dual functions of physical barrier and recycling for polysulfides and LiMn₂O₄ has strong chemical adsorption for polysulfides.The combination of Super P and LiMn₂O₄can effectively inhibit the shuttling effect.In addition,the spinel structure LiMn₂O₄ has unique three-dimensional channels,which can promote the lithium ions passing through separator.The cell with modified separator delivered the first discharge capacity of 888 m Ah g^-1 at 0.5C and the remaining capacity was 619 m Ah g^-1 after 300cycles.(2)The nanosized Ti₄O₇ supported on carbon nanotubes composite was coated on PE separator to form o CNTs-Ti₄O₇ layer.The carbon nanotubes were interlaced to form a conductive network and Ti₄O₇ as a conductive metal oxide can directly adsorb and promote conversion of polysulfides on its surface,which had a catalytic effect for redox reaction of active substance.The first discharge capacity of cell with modified separator was 888 m Ah g^-1 at 0.5C and remained 658 m Ah g^-1 after 250 cycles.(3)Ketjen black with ultra-high specific surface area and abundant pore structure was coated on the surface of BaSO₄ ceramic layer to form KB-BaSO₄ double-layer modified separator.The polar surface of BaSO₄ can solve the difficult problem of coating and peeling of Ketjen black layer.In comparison of conductive carbons with low specific surface area,Ketjen black can increase the contact area of polysulfides,leading to the enhancement of adsorption and obstruct.The first discharge specific capacity reaches 1428 m Ah g^-1 at 0.05C.In addition,the BaSO₄ layer faced with lithium electrode can restrain the growth of lithium dendrite and prevent the puncture of separator.(4)Nanosized Ketjen black was transformed into IKB-Ni composite by micro granulation and catalytic graphitization method and then coated on the surface of polypropylene(PP)separator.The IKB-Ni layer can significantly inhibit the shuttling effect of polysulfides and greatly improve the electrochemical performance.Ni nanoparticles have high catalytic activity and can accelerate the redox reaction rate of Li-S battery.The discharge capacities of battery are 1542 and 524 m Ah g^-1 at 0.05C and 2C,respectively.After 200 cycles at 0.5C,the remaining capacity is 720 m Ah g^-1.(5)PVDF/Super P modified layer was formed on the surface of PP separator by phase conversion method.The mesh-like PVDF framework was loaded with Super P carbon nanoparticles,which can inhibit the diffusion of polysulfides and realize the recovery and utilization of sulfur components.Moreover,the network structure formed by PVDF can significantly improve the coating quality,which results in preventing the cracking and peeling of carbon layer.It can extend the effective usage time of modified layer and greatly improve the cycle life of Li-S battery.