Studies On Reaction Kinetics In Lithium-sulfur Batteries

As an efficient energy storage system,lithium-ion batteries have profoundly changed human society since their commercialization.From various portable electronic devices to electric vehicles that have become hot in recent years,to the large-scale energy storage of the power grid,we are all the time.No longer enjoying the convenience brought to us by lithium-ion batteries.However,the current commercial layered transition metal oxide cathodes are gradually approaching the limit of their theoretical capacity,and it is difficult to meet the future demand for high energy density and low manufacturing cost.Focusing on the longer-term future,we need to explore and develop new types of materials.Rechargeable Battery.Lithium-sulfur battery has attracted much attention as a potential high-energy density secondary battery.It has ultra-high theoretical specific capacity and low material cost.However,the intermediate product polysulfide dissolves and diffuses,and the charge-discharge product has electronic insulation properties.And the slow kinetic reaction restricts its commercial application.At present,researchers mainly use the method of adding polar compounds to sulfur/carbon cathodes and constructing structured electrodes to inhibit the diffusion of polysulfides and accelerate the conversion of polysulfides.Transition metal compounds are widely used in the study of catalysts in lithium-sulfur batteries,and most of their performance improvements are simply attributed to conductivity and polar adsorption characteristics,which is not conducive to our understanding of the catalytic properties of lithium-sulfur batteries from the mechanism.This makes the research of catalysts full of accidental factors.This paper focuses on the regulation of electronic structure,from the introduction of nitrogen vacancy defects,the doping of cations to the construction of heterojunctions,while improving the catalytic performance,it focuses on the analysis of the related factors of the changes in the electronic structure and the improvement of catalytic performance,thereby providing lithium The design and research of sulfur catalysts provide an effective and controllable strategy.The main research contents of this paper are as follows:1.We proposed N vacancy-engineered Ni₃N_0.85 for efficiently catalyzing the conversion of polysul-fides.The overall strategy is to adjust the d-band of Ni₃N rationally by introducing N vacancies.The upshift of d-band centers renders surface Ni bridging sites strongly adsorbing the terminal S atoms and weakening the S-S bonds.The temperature-dependent experiments demonstrated that Ni₃N_0.85 has much lower activation energy than Ni₃N and C.After assembled into Li-S batteries,Ni₃N_0.85significantly promoted the polysulfide conversion and improved the electrochemical properties of batteries.Because of the enhanced kinetics,the overall cycling properties were also improved.A Ni₃N_0.85 cell can deliver a high capacity of 1200.4 mAh g^-1 with a loading of 5.2 mg cm^-2.Even after 1000 cycles,the Ni₃N_0.85 cell can still show 61%capacity retentions.We believe that Ni₃N_0.85 materials and the d-band tuning strategy for accelerated catalysis will advance the development of high-performance Li-S batteries.2.We prepared a novel Ni₂Co₄P₃ electrocatalyst for polysulfides conversion and revealed the molecular or atomic level picture of catalysis.Codopants in the metal sites of Ni₂P lifts the d-band of Ni₂Co₄P₃,leading to a strong adsorption of polysulfides as compared to Ni₂P.Using the Ni₂Co₄P₃ catalyst,we further proposed a microreactor cathode strategy for ultrahigh S loading.The MLSC contains Ni₂Co₄P₃ nanowires catalysts,which are electrically wired by the 3D continuous PNS.An ion-selective filtration shell consisting of N-CMs and SPEEK seals the microreactor to limit polysulfides inside and allow Li-ions in and out of S cathodes.The good catalytic activity and conductivity of Ni₂Co₄P₃ accelerate the polysulfides conversion and enhance the rate capability of MLSC cells.Generally,the microreactor design provides a new strategy to construct S cathodes with high performance and high loads.3.We constructed a CoSe₂/MoS₂ heterostructure on conductive CC.edge sites of MoS₂ has high catalytic activity whereas basal surface of MoS₂ is almost inactive.Owing to the two-dimensional crystallographic structure,MoS₂ shows more basal surface than edges.To use the high catalytic activity of MoS₂,we grew nano-sized CoSe₂ onto the basal surfaces to form heterostructures.CoSe₂ has the strong adsorption capability to-wards polysulfides and but relative lower catalytic activity than MoS₂.The CoSe₂/MoS₂ heterostructure can couple polysulfide adsorption and catalysis.The synergic effects of CoSe₂ and MoS₂ significantly improve the electrochemical properties of Li-S bat-teries.With the assistance of conductive CC scaffold,CC@CoSe₂/MoS₂ can deliver a capacity of 714.5 mAh g^-1 with an extremely low decaying rate of 0.022%per cycle.This work provides a integrated and monolithic design of synergic adsorption and catalysis for high-performance Li-S batteries.