Synthesis Of 3D Pomegranate-Like Carbon-Coated Niobium-Based Host Materials And Application In Li-S Batteries

With high theoretical capacity(Li:3860 mAh g^-1 and S:1675 mAh g^-1),high energy density and low sulfur price,lithium-sulfur（Li-S）batteries have promising application for next-generation electronics and electric vehicles.However,the inherent stability problems of lithium anodes and sulphur cathodes limit their practical application.Achieving efficient protection for both the lithium metal anode and the sulphur cathode is key to the development of practical Li-S batteries.Currently,constructing host materials to inhibit dendrite growth on lithium anode and suppress the shuttle effect of polysulfides on sulfur cathode have been extensively investigated.However,designing suitable host materials to simultaneously protect the anode and cathode has rarely been reported.Therefore,in this thesis,we propose a strategy for coupling polar nitride nanoparticles into hollow microsphere clusters.3D pomegranate-like carbon-encapsulated niobium-based materials were rationally designed and prepared,which were used as of hosts for cathode and anode of Li-S batteries,respectively.The electrochemical properties of Li-S batteries and the mechanism of the host materials on the enhanced performance are systematically studied.The main study consists of the following two parts:（1）To address the problems of uncontrollable dendrite growth and low Coulombic efficiency of lithium anode,3D pomegranate-like composite materials（Nb₄N₅@rGO）self-assembled from graphene sphere clusters encapsulated with Nb₄N₅nanoparticles were designed and synthesized using a spray drying-topological nitriding strategy.The good lithium affinity of Nb₄N₅ regulated the homogeneous lithium deposition,while an embedded lithium（Li_xNb₄N₅）transition phase generated from the Li ions intercalated into the Nb₄N₅ during the initial charge/discharge process reduced the nucleation overpotential and induces the selective deposition of Li into the interior of the cavity.Furthermore,the in-situ formed Li₃N@Nb ion/electron conducting interface induced by the subsequent conversion reaction between the Nb₄N₅ and Li regulated the lithium deposition kinetics.Therefore,the lithium anode based on the Nb₄N₅@rGO host showed a low overpotential（22.6 mV）,a high Coulombic efficiency（99.8%）and an 2000 h excellent cycling stability.（2）Aiming at the problems of slow conversion kinetics and severe polysulfide shuttling in sulfur cathode,a novel 3D self-assembled Nb₄N₅-Nb₂O₅@rGO host was designed and fabricated as well.In such architecture,the rich porous structure within the cavities of the spherical clusters provided sites for the sulfur loading and the physically confinement of polysulfides in the reaction,while the nano-Nb₄N₅-Nb₂O₅ heterojunction effectively combined strong chemisorption and high electrocatalytic conversion activity towards polysulfides.Therefore,the sulphur cathode based on this carrier had good cycling stability and rate capability.On this basis,the Li-S full cell（S-Nb₄N₅-Nb₂O₅@rGO||Li-Nb₄N₅@rGO）assembled based on cathode and anode host materials exhibited excellent electrochemical performance.Moreover,good cycling stability(496mAh g^-1 after 300 cycles at 2 C)and excellent rete performance were still achieved when the capacity ratio of anode to cathode（N/P）was 2:1.