Metal Nitride,Metal Borides As Novel Sulfur Hosts For High-performance Lithium-sulfur Batteries

Lithium-sulfur battery is one of the most promising rechargeble energy storage system owing to the high energy density of 2600 Wh kg-1 low cost of sulfur and environmental friendliness.During the past decades,great effors have been dedicated by the scientists for lithium-sulfur batteries.However,they are still suffered from the fast capacity degradation,low sulfur utilization and low sufur mass loading,unsatisfied rate capability as well as poor Coulombic efficiency.These major problems are mainly derived from the insulating nature of sulfur,volume expansion of 80%upon cycling and the polysulfide shuttling.To overcome the bottleneck,this dissertation propose a rational design of cathode host materials based on the various of inorganic metal compounds,and propose different strategies by chemical binding or promoting polysulfide reaction redox to improve the Li-S battery performances.DFT calculations reveal the binding strength between polysulfides and metal compounds and provide the instinct interaction evolution at a molecular level.The main research contents of the dissertation are summarized as follows:(1)Titanium nitride hollow nanospheres with strong lithium polysulfide chemisorption as sulfur hosts for advanced lithium-sulfur batteries.Uniform TiN hollow nanospheres with an average diameter of?160 nm have been applied as highly efficient lithium polysulfide reservoirs for high-performance lithium-sulfur batteries.Combining the advantages of high conductivity and chemical trapping of lithium polysulfides,the obtained S/TiN cathode of 70 wt%sulfur content in the composite delivered an excellent long-life cycling performance at 0.5 and 1.0 C over 300 cycles.It is also worth to note that a stable capacity of 710.4 mAh·g-1 could be maintained even after 100 cycles at 0.2 C with a high sulfur loading of 3.6 mg·cm-1.The nature of the interactions between TiN and lithium polysulfide species was investigated by X-ray photoelectron spectroscopy studies.Theoretical calculations were also carried out to prove a strong binding between TiN and the lithium polysulfide species.It is expected that this kind of conductive and polar materials would pave a new way for the high-energy lithium-sulfur batteries in the future.(2)Conductive and polar titanium boride as a sulfur host for advanced lithium-sulfur batteries.Metallic and polar TiB2 nanomaterials are applied for the first time as sulfur hosts.The 70S/TiB2 composite exhibits a long-term cycling stability up to 500 cycles at the current density of 1 C with an ultra low fading rate of 0.058%per cycle.It is worth noting that even when the sulfur areal mass loading is up to 3.9 mg cm-2,a stable capacity of 837 mAh g-1 can be still maintained after 100 cycles.The outstanding electrochemical performance can be attributed to the strong anchoring effect of TiB2 to lithium polysulfides,which is confirmed by the X-ray photoelectron spectroscopy analyses and theoretical calculations with a favorable surface-passivated chemistry.The study presented here will shed a new light for metal borides as hosts to improve the cycling life of lithium-sulfur batteries and provide a deep comprehension of the instinct interaction evolution at a molecular level,which is invaluable in the material rational fabrication for future high-performance Li-S batteries.(3)Ultrathin Co-Bi/rGO composite enabled high-performance lithium-sulfur batteries via binding polysulfides and promoting the polysulfide conversion.Ultrathin Co-Bi in situ grown on reduced graphene oxide composite as efficient sulfur hosts for Li-S batteries has been prepared through a mild solution method.The Co-Bi could not only chemically trap polysulfides but also propel the acceleration of polysulfide conversion,while the conductive graphene with hierarchical porous texture affords accessible reaction interfaces for sulfur electrochemical redox.The DFT calculations confirm a strong binding strength between Co-Bi and lithium(poly)sulfides with the binding energies of 2.0-4.5 eV as well as a low lithium diffusion barrier of 0.447 eV.Therefore,the cathode based on Co-Bi/rGO-S composite delivers a high capacity of 972 mAh g-1 at 0.2 C after 100 cycles,impressive rate performance of 500 mAh g-1 at 8 C,outstanding cycling stability over 500 cycles at 1 C with the ultralow fading rate of 0.051%per cycle and remarkable peak capacity of 8.3 mAh cm-2 even with a high sulfur mass loading of 9.5 mg cm-2.(4)Electrocatalytic Ni-B for enhanced lithium-sulfur batteries.The Ni-B composite was prepared through a mild solution method.This composite is composed of nanoparticles covered by the membrane.Therefore,Ni-B modified KB-S cathode delivers an excellent electrochemical performance.The sulfur content is 63%in the total cathode.The cathode shows a high initial capacity of 1650 mAh g-1 and impressive rate capacity of 10 C.It is worthy noting that even for the high sulfur loading of 13 mg cm-2 or in a lean electrolyte condition of E/S:6,the cathode still can deliver a satisfied electrochemical performance even when the sulfur loading was as high as 13 mg cm-2 or in a lean electrolyte condition of E/S:6.