The Structural Design Of Transition Metal Compounds/Carbon Composites Applied In Lithium Sulfur Batteries

In recent years,with the rapid development of smart power grid,transportation network and portable electronic equipment,the requirements for new energy storage devices are getting higher and higher.Because of its high theoretical specific capacity and theoretical energy density,lithium sulfur battery can effectively improve the range of new energy electric vehicles,which is favored by researchers.However,lithium-sulfur battery is constrained by inherent shortcomings containing the poor conductivity of cathode materials and the discharge products（Li₂S/Li₂S₂）,volume expansion during charging and discharging,and the notorious shuttle effect.In order to solve the above problems,the spontaneous polarization reaction of ferroelectric materials is used to adsorb polysulfide.Through structural design,the shuttle effect is inhibited by combining physical limitation and chemisorption.At the same time,a highly efficient and stable reaction interface is constructed to accelerate the catalytic conversion of soluble polysulfide,improve the utilization rate and electrochemical properties of sulfur,and build a high-performance lithium sulfur battery.In order to inhibit the shuttle of soluble polysulfide,the Bi₄Ti₃O₁₂/CNT composites as the host material for lithium sulfur batteries are synthesized by simple one-step hydrothermal method.The wrapped and interpenetration CNT constructs a good conductive network.At the same time,the flower-shaped Bi₄Ti₃O₁₂ ferroelectric material not only can provide a large exposed surface,but also can cause its own polarization and effectively absorb polar lithium sulfide,thus it can significantly inhibit the shuttle effect,reduce the loss of active materials and improve the cycling stability.It still delivers a discharge specific capacity of 489 m Ah g^-1 at 1 C after 1000cycles,and the capacity attenuation rate is only 0.037%per cycles.In order to inhibit the volume expansion of the cathode and accelerate the conversion of soluble polysulfide,the nitrogen-doped carbon clad Fe₂O₃ composites（Fe₂O₃@NC）with cube-shaped yolk shell structure as host material for lithium sulfur batteries are obtained by hydrothermal route coupled with chemical etching method.In the Fe₂O₃@NC composites,nitrogen-doped carbon as the conducting skeleton,can fast transport electron,and Fe₂O₃ as strong polarity and catalytic stability,can quickly convert soluble polysulfide into insoluble lithium sulfide,significantly improve the electrochemical performance.At the same time,the design of the yolk shell structure can effectively limit the volume expansion and ensure the integrity of the cathode.After 100 cycles of 0.2 C,it still delivers a discharge capacity of 791 m Ah g^-1,and the retention rate is 78.6%.In order to promote the catalytic transformation of soluble polysulfide,a composite material which Ni Fe P nanoparticles situ grown on reduced graphene oxide sheets（Ni Fe P/r GO）is synthesized by hydrothermal method associate with high-temperature phosphating method.Then,a stable and efficient interface of polysulfide reaction has been constructed by Ni Fe P/r GO materials coated on PP separator.Compared with the single metal phosphide（Ni₂P/r GO）,the mixed metal phosphide doped with iron ions can provide more chemical active sites for the dissolved polysulfide and provide better catalytic transformation for the redox of the polysulfide.So that,the electrochemical performance of metal phosphide doped with iron ions is obviously better than that of single metal phosphate.Typically,after 500cycles at 1 C,it still has the discharge capacity of 701 m Ah g^-1,and the capacity decay rate per cycle is only 0.034%.