The Effect Of Bacterial Cellulose Separator On Polysulfide Shuttle And Lithium Dendrite Growth

Lithium-sulfur（Li-S）battery is considered to be promising next-generation energy storage systems due to their high theoretical specific capacity,high energy density,and low cost.However,their application is severely hindered by its practical performance,which is limited by the shuttle effect and inhomogeneous formation of lithium dendrite.Separator is an important component of the battery,and its characteristics directly affect the charge-discharge performance and safety performance of the battery.Commercially available separator normally has a low polarity and poor selectivity for ion transmission.Therefore,it is difficult to meet the requirements of high-performance lithium-sulfur batteries.Bacterial cellulose（BC）material has abundant oxygen containing functional groups and unique porous feature,which make it an ideal separator material for Li-S battery application.Gaining insight into the effect of BC separator on the polysulfide shuttle and formation of lithium dendrite in Li-S battery is the premise for its successful application in Li-S battery.In this thesis,BC separator is fabricated and its working mechanism in Li-S battery is investigated through comparative studies.Based on the understanding in the working mechanism of BC separator in Li-S battery,BC separator is further grafted with polyethyleneimine（PEI）to improve its battery performance.The main contents of the thesis are summarized as follows:（1）BC separator is fabricated by vacuum freeze drying.Polyethylene separator（PE）,polypropylene separator（PP）and glass fiber separator（GF）are selected as reference separator for comparisons.The interaction between the BC separator and the polysulfides,and the effect of the BC separator on the stability of the lithium anode are revealed by physical characterizations and electrochemical studies of the separators.The results show that compared to other separators,the BC separator has a larger specific surface area and porosity,higher ionic conductivity and ion transference number,and improved barrier and adsorption effect on polysulfides.Due to these favorable features,Li-S battery assembled with BC separator exhibit excellent cycling and rate performance.It has an initial discharge specific capacity of 1476 m Ah g^-1 at0.1C rate.The rich oxygen-containing functional groups could participate in the formation of the SEI film,which alleviates the decomposition of the electrolyte and improves the interfacial stability of the lithium anode.Its lithium metal electrode remained stable after 800 h of cycling in the symmetrical battery test without obvious lithium dendrite growth.（2）The PEI@BC separator is prepared by immobilizing polyethyleneimine（PEI）on the surface of BC separator by chemical grafting to further modulate the growth behavior of lithium dendrite in the anode.The Li/Li symmetrical cell assembled with PEI@BC separator exhibits excellent cyclic stability,which can continuously plate/stripe for more than 800 hours with an overpotential of～40 m V at 2 m A cm^-2.Further characterizations show that the hydroxyl groups and amino groups in PEI@BC separator can participate in the formation of Li₂O and Li₃N,respectively,contributing to the formation of robust SEI film with high ionic conductivity.Therefore,the lithium deposition is well regulated.Meanwhile,the polar amino group can restrain the polysulfide migration via chemisorption.As a consequence of these merits,good charge-discharge performance is demonstrated.A discharge capacity of 472.1 m Ah g^-1is retained for Li-S full cell after 200 cycles at 0.2C.