Preparation And Electrochemical Properties Of Composite Material As Cathode For Lithium-Sulfur Cells

Lithium–sulfur（Li–S）battery is considered to be one of the most promising battery systems due to its high specific capacity(1675 m Ah g^-1),high energy density(2600 Wh kg^-1)and low cost.However,some knotty problems in the discharging/charging process of Li–S battery,such as shuttle effect of lithium polysulfides（Li PS）,volume expansion and lithium dendrites limit its wide application.In view of the above problems,novel sulfur host matrix was designed to prepare composite electrode for Li–S battery.In addition,the inhibition of lithium polysulfide shuttles effect ofγ-polyglutamic acid and other traditional binders on Li–S battery was studied.The detailed works are as follows:（1）Carbon paper/sulfur composite electrode was prepared by using carbon paper as sulfur host and Li₂S₆as sulfur source.The results show that the composite electrode endows good electrochemical performance with reversible capacity of 486m Ah g^-1after 200 cycles at 0.2 C when the sulfur loading is 1.22 mg cm^-2.In order to suppress the shuttle effect of Li PS during the charging/discharging process of sulfur cathode,manganese dioxide（Mn O₂）is grown on carbon paper to prepare carbon paper/manganese dioxide matrix.The influence of Mn O₂on the shuttle effect and the electrochemical performance of Li–S battery is analyzed.The results show that the capacity of the sulfur electrode with carbon paper/Mn O₂is 664m Ah g^-1after 200 cycles at 0.2 C when the sulfur loading is controlled at 1.22 mg cm^-2.These results indicate that Mn O₂loaded on carbon paper can inhibit the shuttle effect of Li PS and improve the electrochemical performance of Li–S battery.（2）A novel type of carbon foam was synthesized by carbonizing the commercial sponge as the sulfur host to improve the electrochemical performance of Li–S battery,and the cyclic stability of sulfur electrode was further enhanced by loading Mn O₂on carbon foam.The results show that the reversible capacity of the carbon foam/Mn O₂/sulfur electrode reaches to 360 m Ah g^-1after 200 cycles at 0.1 C when the sulfur loading is 3 mg cm^-2,and the capacity retention rate is 135%compared with the initial capacity(267 m Ah g^-1).Carbon foam/CNT/Mn O₂composite was prepared by adding CNT to fabricate the high sulfur loading Li–S battery.The effect of CNT loaded on the carbon foam/CNT/Mn O₂composite and the electrochemical performance of sulfur cathode was carefully analyzed.The results indicate that the reversible capacity of carbon foam/CNT/Mn O₂/sulfur electrode is 632 m Ah g^-1after 200 cycles at 0.1 C,and the capacity retention rate is 78%.Those demonstrate that CNT loading can improve the electrochemical performance of Li–S battery at high sulfur loading.（3）γ-polyglutamic acid was used as the binder of Li–S battery,and the detailed mechanism on Li–S batteries was study.Compared with polyvinylidene fluoride and other traditional binders,γ-polyglutamic acid is rich in amino and carboxyl groups,which can improve the adsorption capability of Li PS and achieve the inhibition of lithium polysulfide shuttles.In addition,γ-polyglutamic acid possesses lots of hydrogen bonds due to amino group and carboxyl group.The existing hydrogen bonds can improve the flexibility and cohesiveness of sulfur electrode,and enhance the structural stability of sulfur cathode during cycling.The results show that the initial discharge capacity of sulfur cathode utilizingγ-polyglutamic acid binder is 991 m Ah g^-1,and the capacity maintains at 620 m Ah g^-1after 100 cycles.The corresponding capacity retention rate is 62.5%,which is higher than those of polyvinylidene fluoride（53.9%）,polyvinyl pyrrolidone（52.8%）,sodium carboxymethyl cellulose（40.7%）and polyacrylonitrile（51.5%）.The results reveal thatγ-polyglutamic acid binder is helpful to facilitate the electrochemical reaction,enhance the adsorption of Li PS,and thus improve the electrochemical performance of Li–S battery.