Design And Their Electrochemical Performance Of Highly Active Lithium Sulfide Cathode Materials

Lithium sulfide（Li₂S）has a theoretical specific capacity as high as 1166 mAh/g.When it replaces sulfur as a lithium-sulfur battery cathode,it can avoid volume expansion during discharge when sulfur is used as cathode and lithium dendrite growth when metal lithium is used as anode,so Li₂S has more practical application prospects.However,due to the poor air stability,relatively high preparation cost,and low initial utilization rate of Li₂S,the practical application of Li₂S cathode is blocked.To this end,this paper studies the preparation of nano-Li₂S cathode materials by using dilithium trisulfide（Li₂S₃）and lithium sulfate（Li₂SO₄）as precursors,respectively,and the use of polyacrylic acid（PAA）as a modification additive for Li₂S cathodes,which greatly improves the electrochemical performance of Li₂S cathode.The main findings of this paper are as follows:（1）Li₂S₃ was mixed with polyvinylpyrrolidone（PVP）and carbon nanotubes（CNTs）and then heat-treated to prepare a nano-lithium sulfide/carbon（n-Li₂S/C）composite cathode material.The electrochemical characterization results show that the first discharge specific capacity of the n-Li₂S/C cathode can reach 910 mAh/g,and the battery can maintain 484 mAh/g after 150 cycles at a constant current charge-discharge rate of 1C.The material structure characterization and electrochemical impedance analysis show that the good rate performance stems from the excellent dual design of electron and ion channels.（2）Polyethylenimine（PEI）was used as surfactant to prepare Li₂SO₄ nanoparticles（less than 80 nm）with narrow particle size distribution and controllable size by antisolvent crystallization method at low temperature.The effects of PEI content and temperature on the size of Li₂SO₄ particles were studied.The freestanding Li₂S/CNTs electrodes were prepared by mixing the highly dispersed Li₂SO₄ and CNTs,followed by vacuum filtration and carbothermal reduction.SEM results revealed that the Li₂S nanoparticles obtained by carbonthermal reduction had a good dispersibility within the CNTs.The charge-discharge voltage curve shows that the charge cut-off voltage for the first activation of the freestanding Li₂S/CNTs electrode can be set to 3.20 V,thus avoiding the decomposition of used ether-based electrolyte and achieving an initial discharge specific capacity as high as 900 mAh/g;the constant current charge-discharge cycle shows that the battery can still deliver 623 mAh/g after 80 cycles at 0.5 C.The results show that the PEI-improved Li₂SO₄ as a precursor is beneficial to obtain small-sized,well-dispersed Li₂S nanoparticles and corresponding high-activity Li₂S cathodes,and to a certain extent reduces the preparation cost of Li₂S cathodes.（3）The use of PAA to modify the Li₂S cathode reduces the initial activation barrier and charge cut-off voltage during the first charging of Li₂S cathode.The mechanism of PAA affecting the initial activation of Li₂S and the optimal content of PAA were investigated.Under the action of PAA,the initial charging cut-off voltage of the Li₂Sbased battery drops to 3.00 V at 0.1 C,and the discharge specific capacity reaches 791 mAh/g.These results show that PAA can act as an activator of the Li₂S cathode,reduce the initial activation barrier and improve the utilization rate of active materials,which is beneficial to promote the practical application of Li₂S cathodes.