Application And Mechanism Of Transition Metal Sulfides For Cathode Materials In High Capacity And Long Life Lithium-sulfur Batteries

With the miniaturization of electronics as well as wide applications of electric vehicles and large-scale energy storage stations,traditional lithium-ion(Li-ion)secondary battery can no longer meet vast market demands for energy storage devices with high energy density,which calls for the development of a new lithium battery system based on new electrochemical principles and electrode materials with high energy density.The Lithium-sulfur(Li-S)battery has been known as one of the most promising energy storage devices due to its unique advantages such as high theoretical capacity(1675 mA h g-1)and energy density(2500 kW kg-1).In addition,sulphur(S),the cathode active material,is inexpensive,environmentally-friendly and abundant in nature.Based on the application of transition metal sulfides(MS2)and active material of sulfur composites in Li-S batteries,this thesis mainly studies the mechanism of how different MS2 become adsorbed by lithium polysulfides(LiPSs),MS2 loaded on three-dimensional metal framework as support of active material of sulfur and Polar MS2 and Non-polar Carbon-based composite Reinforced Electrode Materials and Adsorption of LiPSs the synergic effects of MS2 and carbon-based materials to obtain high-performance cathode materials with long cycle life,high rate performance and low cost for commercialized Li-S batteries.The detailed contents and conclusion are as follows:(1)Studies of the adsorption mechanism of different metal sulfides to LiPSs.We synthesized polar nickel sulfide(Ni3S2)and vanadium sulfide(VS2)through hydrothermal method and took them as the carrier of sulfur to explore the adsorption mechanism among them.As a result,VS2 can adsorb the LiPSs effectively,which increases the battery capacity.Its high conductivity also contributes to the promotion of rate performance.Under the charge-discharge density of 0.1,0.2,0.5,1 and 2 C(1 C= 1000 mA g-1),the initial charging capacity of VS2-S electrode is 1214.6,875.8,742.3,649.7 and 537.3 mA h g-1,respectively.Besides,the loading volume of S can increase to 70.9%using VS2 as a carrier,which can increase the energy density.This chemisorption method can also be applied to other MS2 to control the diffusion effects of LiPSs.(2)The structural design and electrochemical test of cathode materials by employing MS2 directly loaded on 3D metal framework.To make CuS particles grow on the Cu foam(CF),we applied the liquid phase method and used copper acetate and cupric nitrate as different copper source and finally obtained CF-Cu-C-S and CF-Cu-N-S,respectively.S was then loaded into the 3D framework during traditional wet processes.The many advantages of taking CF-CuS electrode directly as the carrier of S include:avoiding seperation of conductive agent and binder from S during the volumn expansion of the active materials;facilitating the tranportation of ion and electron due to porous surface and high conductivity of CF;decreasing the charging barrier and advancing the oxidation process from Li2S to Li2S,(2 ? x<8)according to the charge-discharge curves.As a result,the capacity retention rate of CF-CuS-C-S electrode is 95.4,98.6,101.8,95.7 and 102.2%after 10 cycles under the charge-discharge density of 0.1,0.2.,0.5,1 and 2 C,respectively,indicating an excellent cycle performace.(3)The design and electrochemical tests of high-rate-performance cathode composites with carbon spheres loaded on polar metal sulfides.By applying the solution method,we loaded conductive FeS into hollow carbon spheres(BP2000)with a high specific surface area,then we obtained a stable BP2000-FeS-S by soaking sulfur into the composite during wet processes.BP2000 has a relatively high conductivity and specific surface area,which can increase the utilization of S/Li2S and S loadings.Also,the porous structure of BP2000 can restrain the diffusion of LiPSs,and with chemical bonding,FeS deposited on BP2000 can adsorb LiPSs.The FeS can also function as fillers to control the pore size of BP2000 to avoid the loss of S/LiPSs.As a result,the discharge capacity of BP2000-FeS-S under the current density of 0.1 C can reach 1206.2 mA h g-1,and after 500 cycles under the current density of 0.5 C,the capacity retention rate is 69.7%.Meanwhile,even under the current density of 2 C,the discharge capacity can reach 659.2 mA h g-1,indicating a high power density and cycle life.