Research On Bidirectional Protection Of Positive And Negative Electrodes Of Lithium-Sulfur Full Battery By "Two-in-One" Host

Lithium-Sulfur batteries are considered to be the most promising next-generation secondary battery systems due to their high theoretical energy density(2600 Wh kg-1),low cost,and environmental friendliness.However,the shuttle effect of polysulfides in the cathode and the uncontrollable dendrite growth in the anode severely restrict the commercial application of Li-S batteries.Therefore,realizing positive and negative electrode protection is the key to develop practical Li-S batteries.At present,a lot of research has been done on the shuttle effect of sulfur cathodes and the dendrite growth of lithium anodes.However,bidirectional protection based on positive and negative electrodes is rarely reported.In view of the above problems,this paper rationally designed and synthesized a bilayer nitrogen-doped carbon framework embedded with a heterostructure ZnSe-CoSe2@NC as a "two-in-one" host for bidirectional protection of sulfur cathode and lithium anode.During this process,the properties of the material were studied,and the electrochemical performance of its individual application in the positive electrode and the negative electrode,as well as the combination of the two into a full battery,was studied.The main contents of this paper are as follows:Firstly,the precursor ZIF-8-ZIF-67 was synthesized by a two-step hydrothermal reaction,and then calcined in a tube furnace to obtain the ZnSe-CoSe2@NC heterostructure.Theoretical and experimental results show that ZnSe-CoSe2@NC can combine with the electrons of polysulfide to form Zn-S bond and Co-S bond,which has strong adsorption to polysulfide and can catalyze the mutual conversion of polysulfide,thus inhibiting the shuttle effect of polysulfide.As a result,S/ZnSeCoSe2@NC has an initial discharge capacity of 1427 mAh g-1 at a current density of 0.1 C.The long-cycle data show that the reversible capacity of 620 mAh g-1 can be maintained stably after 400 cycles at a high current of 1 C.Besides,under argon atmosphere,the Li/ZnSe-CoSe2@NC composite negative electrode was synthesized by melting ZnSe-CoSe2@NC and Li by molten lithium filling.The experimental results show that ZnSe-CoSe2@NC is transformed into Li2Se and metal Co and Zn by highly reductive Li during the initial Li deposition.Abundant metal Zn and Co can also be used as the preferred nucleation sites of Li,effectively regulating the subsequent Li deposition and inhibiting the growth of lithium dendrites.The theoretical calculation shows that the diffusion barrier of Li on the surface of ZnSeCoSe2@NC is approximately equal to that on the surface of lithium metal,and ZnSeCoSe2@NC is an ideal host of lithium metal.As a result,the Li/ZnSe-CoSe2@NC symmetrical battery operated stably for 1000 hours at 0.5,1,and 3 mA cm-2 currents,and did not short-circuit even at high currents of 7 mA cm-2.At last,the Li/ZnSe-CoSe2@NC anode was assembled with the S/ZnSeCoSe2@NC cathode to form a Li-S full battery.The assembled Li-S full battery exhibits an initial discharge specific capacity of 909.7 mAh g-1 at 1 C,and successfully maintains a reversible capacity of 731.8 mAh g-1 after 400 cycles,even at a high current of 2 C,the full cell operates stably for 1000 cycles.The Li-S full cell can still achieve a high areal capacity of 4.16 mAh cm-2 at 0.2 C when the areal sulfur loading is increased to 6.08 mg cm-2.The assembled pouch Li-S full battery can also run stably for 100 cycles at 0.2 C,with an energy density of about 437 Wh kg-1 based on the total mass.The above results indicate that ZnSe-CoSe2@NC is an ideal "two-in-one" host for Li-S full batteries.