Tuning The Interface Structure Of High-sulfur-loading Cathode For High-performance Lithium-sulfur Batteries

Rechargeable lithium-sulfur batteries have outstanding advantages such as high theoretical energy density(2600 Wh kg-1)and abundant sulfur resources and are considered to be an important part of the national "14th Five-Year Plan" for the new energy industry.However,their commercial application in portable electronic devices and electric vehicles is still constrained by many problems,the most critical of which are the shuttle effect of polysulfide intermediates and the slow redox kinetics.Especially under practical conditions such as high sulfur loadings,these problems of sulfur cathodes can lead to rapid battery capacity degradation.To address these shortcomings,in this thesis,the material-scale and electrode-scale interface structures of sulfur cathode were rationally designed and optimized,which efficiently regulated electrochemical reaction of lithium polysulfide and according achieved an efficient and stable high loading(> 4 mg cm-2)sulfur cathode,greatly promoting the commercialization of lithium-sulfur batteries.The research results are shown as follow:1.At the material scale,a high loading(15.3 wt%)cobalt single-atom catalyst(Co SAN-C)catalyst,which was fabricated through a salt-template method,achieved effective modulation of the conversion and deposition behavior of lithium(poly)sulfide on host materials.Multiple experimental characterizations and first-principles calculations demonstrated that there were densely populated Co-N4 moieties on Co SA-N-C.These atomic sites could chemically anchor soluble polysulfides via Co-S and N-Li bonds and catalyze their bidirectional conversion to insoluble end products(S or Li2S),thereby eliminating the redox shuttling of polysulfides.In addition,the resultant Co SA-N-C achieved a homogeneous spatial distribution of Li2 S nanoparticles instead of the traditional passivation layer and subsequently catalyzed the oxidation of Li2 S,which is beneficial for high sulfur utilization.Consequently,the as-fabricated Co SA-N-C/S cathode with a sulfur loading of 4.9 mg cm-2 could achieve a reversible areal capacity of 4.24 m Ah cm-2 after 120 cycles at 0.2 C(1 C=1675 m A g-1).The present high-loading single atoms catalysts provide fresh perspectives for the development of materials for Li-S batteries and other energy systems based on dissolution/precipitation mechanisms.2.At the material scale,nano-and micro-scale Mott-Schottky heterojunctions were constructed to catalyze the kinetic transformation of polysulfides on sulfur-loaded materials.Cobalt nanoclusters inlaid N-doped carbon porous nanosheets were designed as Co@NC Mott-Schottky electrocatalysts.Spectroscopy techniques and theoretical simulations revealed the spontaneous charge redistribution and the construction of a built-in electric field at the interface of Co@NC heterojunction.This Mott-Schottky interfacial effect effectively boosted the conversion kinetics of Li-S chemistry by reducing the activation energy for S reduction and decomposition barrier for Li2 S oxidation.The as-designed Li-S batteries displayed remarkable electrochemical performance under harsh conditions,including high rate(4 C),elevated temperature(55 ?),high S loading(10.73 mg cm-2)and low electrolyte/sulfur ratio(5.9 ?L mg-1),and a system-level gravimetric energy density of307.8 Wh kg-1.This Mott-Schotty-based catalyst provides fresh perspectives for the development of materials for lithium-sulfur batteries and other multielectron reaction energy systems.3.Stabilizing the electrode scale interface between electrode and separator in lithiumsulfur batteries by constructing an ultrathin conductive polymer layer.An ultralight(0.13 mg cm-2)and ultrathin(60 nm)polypyrrole layer was in-situ polymerized on a commercial Celgard separator by a gas-phase polymerization method.The functional PPy layer not only facilitated homogenous Li+ flux to enable uniform plating and stripping of metallic lithium,but also chemically immobilized soluble polysulfides to suppress their migration at the cathode side.Using a PPy/Celgard separator,Li-S cell with a high-sulfur-loading sulfur cathode(5.73 mg cm-2)achieved an initial areal capacity of 4.7 m Ah cm-2 and 75.6%capacity retention after 150 cycles at 0.2 C.The use of a lightweight,ultrathin conducting polymer functional layer to modulate the interfacial electrochemistry of the sulfur cathode offers a new opportunity to realize high-performance Li-S batteries.4.By combining material and electrode scale interface design,the molybdenum-based bilayer semi-liquid composite sulfur cathode was designed to systematically regulate loading,adsorption,conversion,and deposition behaviors of sulfur species.Vacuum filtration,solution casting,and magnetron sputtering methods were used to build a bilayer structure,which included an active Li2S6 solution/carbon nanofiber(CNF)layer at the bottom and a molybdenum nanocluster decorated carbon nanotube(CNT/Mo)film at the top.This bilayer structure could accommodate a large amount of Li2S6 catholyte and physically and chemically effectively confined them inside the cathode structure by physical blocking and chemical adsorption.More importantly,the metallic Mo nanoclusters on the CNT could provide a large number of active sites for catalyzing the redox reaction of Li2S6 and promoting the deposition of Li2 S particles.Benefiting from the suppressed polysulfides shuttling and catalyzed conversion kinetics,the CNF/Li2S6/Mo/CNT cathode with a sulfur loading of 7.64 mg cm-2 maintained a reversible areal capacity of 4.75 m Ah cm-2 with a capacity retention of 80 % after 100 cycles at 0.2 C.This semi-liquid sulfur anode with both material and electrode interface design provides a feasible way to promote the practical application of high-energy-density Li-S batteries.