Preparation Of Hollow Polar Nano-sized Materials For Advanced Lithium-sulfur Batteries

Lithium sulfur?Li-S?batteries are considered as one of the most promising energy storage systems for the next generation in view of their high theoretical capacity(1675mAh g^-1)and high theoretical energy density(2500 Wh Kg^-1).In addition,compared with the traditional lithium-ion batteries,they have the advantages of natural abundance,low price and nontoxicity.However,the commercial application of Li-S batteries remains hindered by the low capacity and short cycle life.Its low capacity is mainly attributed to the poor conductivity of sulfur and its discharge product Li₂S₂/Li₂S,and the inferior reaction kinetics.The poor lifespan is mainly due to the dissolution of polysulfide in the electrolyte which leads to the shuttle effect,thus resulting in the continuous loss of active materials.In addition,the volumetric expansion?⁸0%?of the active material during cycling may cause the collapse of the electrode structure,which leads to the poor stability.In order to solve these problems,researchers have tried to use various carbon materials as the host for sulfur to improve the electrical conductivity of the electrode and thus improving the capacity of the battery.However,the non-polar carbon materials could not effectively adsorb the polar soluble polysulfides.Thus,the polysulfides are still easily to dissolve in the electrolyte,leading to severe shuttle effect and the loss of active materials,which causes rapid attenuation of capacity.Recently,researchers found that some polar materials?such as TiO₂,MnO₂,Ti₄O₇,etc.?could form strong chemical bonds with polysulfides,thereby adsorbing polysulfides,and effectively alleviating the shuttle effect.However,most of the polar materials used for Li-S batteries are irregular nanoparticles.These irregular polar materials are hard to adsorb all the polysulfides when the electrode with a high sulfur loading because the interaction between polar materials and polysulfide occurs on the surface of polar materials.A feasible method is to design the polar materials into hollow structure.Compared with the irregular solid hosts,the host materials with hollow structures have many advantages:I)providing enough internal space for the high sulfur loading;II)alleviating the volumetric effect of sulfur;III)exposing more adsorption sites compared to the solid hosts;IV)promising more effective encapsulation of polysulfides by locking them in the internal void space.In this paper,three kinds of polar materials with hollow structure were fabricated as the host materials.The interactions between polar host materials and polysulfides have been studied systematically.The specific works are as follows:1.Uniform Ni?OH?₂ hollow spheres constructed from ultrathin nanosheets as efficient polysulfide mediator for long-term lithium-sulfur batteries.First of all,the hollow sulfur spheres were synthesized by a facile method.The hollow structured sulfur could alleviate the volumetric effect.In addition,the obtained nano-sized sulfur can shorten the path of lithium ions and electrons thus improving the conductivity of the electrode.Then the Ni?OH?₂ hollow nanospheres constructed from ultrathin nanosheets coated on the hollow sulfur nanospheres.The content of sulfur in the S@Ni?OH?₂composite is as high as 81%.The experiments show a strong interaction between sulfur and the polar Ni?OH?₂.When used as the cathode of Li-S battery,it delivers 1020 mAh g^-1 at 0.2 C and shows superior cycle stability over 1000 cycles at 1 C with a capacity decay of about 0.04%per cycle.2.Honeycomb-like spherical cathode host constructed from hollow metallic and polar Co₉S₈ tubules for advanced lithium-sulfur batteries.The honeycomb-like Co₉S₈spheres were synthesized by a hydrothermal method and subsequent sulfidation reaction.Then the sulfur is steamed into the Co₉S₈ nanotubes by means of a melting-diffusion process.The experimental and theoretical calculations show that Co₉S₈ can effectively bind polar polysulfides.In addition,for the first time,we found that Co₉S₈ can accelerate the reaction kinetics.The S@Co₉S₈ delivers excellent cycle stability over 600cycles at 1 C and the capacity decay is only 0.026%per cycle.When the current density increases to 2 C,it still delivers 806 mAh g^-1.3.Chinese knot-like hollow NiCo₂S₄ as a multifunctional sulfur host for lithium-sulfur batteries.Chinese knot-like hollow NiCo₂S₄ was fabricated by a hydrothermal method and subsequent sulfidation reaction.The S@NiCo₂S₄ composite was obtained by filling the sulfur into the hollow nanotube by the melting-diffusion method.The unique interconnected structure of NiCo₂S₄ is beneficial to the conduction of lithium ions and electrons.The experimental results show that the polar NiCo₂S₄ can effectively adsorb polysulfides and promote the reaction.When used as the cathode material for Li-S battery,it shows a capacity of 1348 mAh g^-1 at 0.1 C.It could stably cycle 1000 cycles at 1 C and the attenuation rate is only 0.02%for each cycle.It still delivers high specific capacity and good rate performance when the sulfur loading is 5mg cm^-2.In summary,well-designed hollow polar nanomaterials can effectively improve the specific capacity and cycle stability of Li-S batteries,thus provides new ideas for the development of high capacity and long-term Li-S batteries.