Study On Electrochemical Performance Of Ti₃C₂T_X-Based Electrodes For Lithium-Sulfur Battery

With the continuous growth of the world's population,rapid economic development,and forwarding advances in technology,the demand for energy in human society has increased explosively.While developing clean energy to replace traditional non-renewable fossil fuel energy,there is an urgent need to develop a corresponding high-performance,safe,and long-life storage system.Lithium metal has a very high theoretical specific capacity(3860 m Ah·g^-1),the lowest electrode potential(-3.04 V vs.standard hydrogen electrode),and the lightest mass(0.534 g·cm^-3).Lithium-sulfur batteries with metal lithium as the anode and sulfur as the cathode have extremely high theoretical specific capacity(1672 m Ah·g^-1)and energy density(2567 Wh·kg^-1 and2780 Wh·L^-1),environmentally friendly and low price and other advantages are expected to replace the lithium-ion battery that has reached its theoretical energy density and become the next-generation energy storage system.However,the practical application of lithium-sulfur batteries is still restricted by many problems,the most serious of which are:1)the growth of lithium dendrites on lithium metal anodes during the charging and discharging process,and 2)the shuttle effect of lithium polysulfides,which are intermediate products of sulfur electrochemical reaction,which cause safety issues and poor cycle performance of lithium-sulfur batteries.In view of the extensive research of MXenes materials in the field of lithium batteries,based on Ti₃C₂T_x material,this paper designed and prepared dendrite free Li/LTO/Ti₃C₂/Cu lithium metal anode,Ti₃C₂@CF-S cathode which can chemically anchored lithium polysulfides,and TiO₂/Ti₃C₂ S host cathode material that effectively inhibit lithium polysulfides shuttling,focusing on exploring the influence of the structure and composition of the above three electrode materials on the electrochemical performance of lithium-sulfur batteries,and studying the inhibition of lithium dendritic growth of lithium metal anode and the effect of lithium polysulfides adsorption and anchoring mechanism.The three lithium-sulfur battery anode or cathode materials proposed in this paper have certain enlightening significance for the practical application of high-performance lithium-sulfur batteries,and the following research results have been obtained:(1)Li has been deposited into the lithiophilic LTO/Ti₃C₂/Cu composite electrode,in order to obtain a lithium metal anode without lithium dendrite growth.Ti₃C₂T_x and Li₄Ti₅O₁₂(LTO)materials have been used in lithium ion battery anode materials because of their essential advantages.Ti₃C₂T_x has a suitable layered structure and high conductivity,which can provide space for insertion and extraction of lithium ions for stable lithium storage.LTO has the advantage of zero strain in the process of lithium ion insertion and extraction,so that the electrode based on LTO has good reversibility and structural stability.In order to effectively utilize the essential advantages of Ti₃C₂T_x and LTO,an LTO/Ti₃C₂/Cu composite electrode composed of lithiophilic LTO and layered structure Ti₃C₂ is used as the electrochemical deposition host of lithium metal.In the process of lithium deposition,Ti₃C₂ provides layered space for the deposition of Li ion.Due to the characteristics of lithophile,LTO grown on the surface of Ti₃C₂ can induce uniform lithium deposition.Moreover,the conductive framework with higher specific surface area strongly adsorbs Li ion and can form a uniform microscopic electric field.These advantages can fundamentally inhibit the growth of lithium dendrites and the formation of dead lithium,and improve the utilization rate of lithium,thereby obtaining a stable and long-life lithium metal anode.The lithium symmetrical battery assembled by LTO/Ti₃C₂/Cu has been stably cycled for 2000 h with coulombic efficiency maintained around 100%.The Li Fe PO₄ battery with Li/LTO/Ti₃C₂/Cu as the anode has153.6 m Ah·g^-1 initial specific capacity at 0.2 C,after 200 cycles the battery still has 71%capacity;the battery has been stably cycled for 500 weeks at 5 C,and its discharge capacity decay rate is as low as 0.08%.The coulombic efficiency of a lithium-sulfur battery with Li/LTO/Ti₃C₂/Cu as the anode can still maintain 99.46%after stably cycled at 0.5 C for 1000 cycles.(2)Wrap 29.4 wt%two-dimensional Ti₃C₂ nanosheets on carbon cloth fiber(CF)with a single fiber diameter of about 10?m to obtain a Ti₃C₂@CF electrode with a specific surface area of 580.4 m²·g^-1,which successfully avoided two-dimensional Ti₃C₂nanosheets re-stacking and increases the specific surface area of the electrode material.The polar two-dimensional Ti₃C₂ nanosheets can strongly chemically adsorb lithium polysulfides to form Ti-S bonds and inhibit lithium polysulfides dissolved in the electrolyte and shuttle effec.The three-dimensional framework of CF provides abundant tens of micrometers macropores,which can provide buffer space for the huge volume expansion caused by lithiation of sulfur to lithium sulfide.And the packaging of flexible two-dimensional Ti₃C₂ nanosheets enhances the ductility of CF,so that it can withstand the deformation stress caused by the volume expansion of lithium sulfide,thus successfully avoiding the electrode structure from pulverization and collapse during charge and discharge process.The high conductivity of the Ti₃C₂@CF electrode provides a high-speed channel for the transmission of electrons.Finally,a lithium-sulfur battery cathode with high sulfur load,high current stable circulation and long life is obtained.The sulfur loading of Ti₃C₂@CF-S cathode reaches 4 mg·cm^-2,the initial specific capacity of 0.5 C is as high as 1175.2 m Ah·g^-1,and its capacity is 626.0m Ah·g^-1 after 1000 charge-discharge cycles at 1 C.Increasing the sulfur loading of Ti₃C₂@CF-S cathode to 12 mg·cm^-2 can obtain an area specific capacity as high as 14.7m Ah·cm^-2.It provides an advanced and reasonable design route for lithium-sulfur battery cathode materials to realize high-performance lithium-sulfur batteries.(3)In order to overcome the technical obstacles of introducing carbon fiber,a sulfur cathode host material that can effectively inhibit the shuttle effect of lithium polysulfides has been designed and prepared,and its electrochemical performance was studied and the mechanism of inhibiting the shuttle effect of lithium polysulfide was discussed.After Ti₃Al C₂ is etched into multiple layers of Ti₃C₂T_x,the surface of the sheet is oxidized to obtain TiO₂/Ti₃C₂,and materials with different oxidation degrees are prepared.As the content of TiO₂ increases,the specific surface area of TiO₂/Ti₃C₂increases,and the internal resistance of the assembled TiO₂/Ti₃C₂-S cathode increases,and the initial specific capacity decreases,but the cycle performance tends to be more stable.The TiO₂/Ti₃C₂-3-S cathode has an initial specific capacity of 1277.4 m Ah·g^-1 at0.5 C.After 500 cycles of cycling,the discharge specific capacity can still maintain42.36%,and its decay rate is 0.12%.