Preparation Of Transition Metal/MXene Composites And Their Catalytic Performance For Lithium-sulfur Conversion Reaction

Lithium-sulfur battery（LSB）is expected to be the next generation of energy storage systems due to its high theoretical energy density.However,the sulfur reduction reaction（SRR）is sluggish in kinetics,leading to the accumulation of soluble lithium polysulfides（Li PSs）in the electrolyte,aggravating the"shuttle effect"and reducing the Coulombic efficiency of the battery.While the solid-solid conversion of Li₂S₂ to Li₂S is the rate-determined step of the overall electrochemical reaction,but its reaction energy barrier is high and it is very slow kinetically,making the actual capacity of the battery decreased and the rate performance worsen.To address the above issues,this thesis developed two types SRR catalysts applied in lithium-sulfur batteries to accelerate the redox kinetics of S₈ to Li₂S conversion for high-performance lithium-sulfur batteries.The main contents are as follows.1.Composite materials（TMNPs@MXene,TM=Co,Fe,Ni）of transition metal nanoparticles and MXene（Ti₃C₂T_x）were prepared as an adsorption material for Li PSs and SRR catalyst by a two-step hydrothermal/high-temperature thermal reduction method.MXene with high electrical conductivity and high affinity for lithium polysulfides（Li PSs）can effectively activate the catalytic activity of TMNPs.And TMNPs are uniformly dispersed on MXene nanosheets,providing abundant active sites for the conversion of polysulfides.The electrochemical results show that the Co NPs@MXene composites exhibit better SRR catalytic performance than Fe NPs@MXene and Ni NPs@MXene.Co NPs@MXene composites with a faster diffusion of Li PSs and lower activation energy of conversion reaction,promoting the nucleation of Li₂S.Meanwhile,the RDE test results indicated that Co NPs@MXene composites possessed a higher half-wave potential(E_1/2),a higher exchange current density（J₀）and a lower Tafel slope（η）.And DFT calculations proved that the high SRR electrocatalytic activity of Co NPs@MXene composites originated from their moderate adsorption strength on Li PSs and the low Gibbs energy barrier of Li₂S₂ to Li₂S solid-phase conversion.Using MXene@Co NPs/S as the cathode material,the corresponding lithium-sulfur batteries exhibited more excellent capacity and rate performance.2.The composites of transition metal phthalocyanine complexes（MPc,M=Co;Fe;Ni）and MXene were synthesized as an adsorption material for Li PSs and SRR catalysts by the liquid-phase ultrasonication method.The MXene substrate activates the catalyst,while the transition metal phthalocyanine complex is a typical single-atom catalyst that is uniformly loaded on the MXene nanosheet.And the single metal site can exhibit the highest activity and the largest effective catalytic specific surface area.The electrochemical test results indicated that Co Pc@MXene exhibited the highest catalytic activity in the MPc@MXene（M=Co;Fe;Ni）composites.In addition,the effect of modified groups（-NH₂ and-SO₃H）of cobalt phthalocyanine ligand ring on its SRR catalytic activity was further investigated in this study.The electrochemical results suggested that Co Pc-NH₂@MXene exhibited better SRR catalytic activity,while Co Pc-SO₃H@MXene had a"toxic"effect on both catalytic activity and the performance of the battery.Appling Co Pc-NH₂@MXene/S as the cathode material for lithium-sulfur batteries,the batteries exhibited more excellent rate and cycling performance.