Preparation Of Polypyrrole Coated Transition Metal Oxide Composites And Their Application In Lithium Sulfur Battery

As the most promising electrochemical energy storage system today,the biggest advantage of lithium-sulfur batteries is the theoretical specific capacity of up to 1675 mAh g-1 and a theoretical energy of up to 2500 Wh kg-1.At the same time,As the active material of lithium-sulfur batteries,sulfur has the advantages of abundant reserves in nature and low prices.However,there are still many problems in the commercial application of lithium-sulfur batteries.For example,the low conductivity of sulfur,the volume expansion of sulfur during charging and discharging,and the shuttle effect of polysulfides during cycling,etc.In order to make up for the above defects,a common solution is to combine sulfur with conductive polymer or porous carbon to improve the conductivity,or introduce compounds with strong polarity,such as metal oxides,sulfides,hydroxides,etc.,to suppress the shuttle effect.A composite material of conductive polymer-coated metal oxides is prepared as a loading matrix of sulfur in the cathode materials.The function of the conductive polymer is to improve the conductivity of the positive electrode material.The presence of metal oxides can also adsorb polysulfides to inhibit the occurrence of the shuttle effect.The conclusions obtained are as follows.(1)Fe3O4 nanoparticles with uniform particle size were prepared by hydrothermal method,Fe3+ produced during the etching of Fe3O4 with hydrochloric acid was used to catalyze the polymerization of pyrrole on the surface of Fe3O4.The Fe3O4@PPy composite material was formed by controlling the concentration of hydrochloric acid and the etching time.The spherical structure of Fe3O4 is maintained and the porous structure is conducive to sulfur loading.The Fe3O4@PPy/S composite prepared has a sulfur content of 56%and maintains the original spherical structure.The electrochemical test shows that the initial capacity of the Fe3O4@PPy/S composite cathode at 1 C rate is 529.4 mAh g-1,the capacity after 300 cycles is 388.1 mAh g-1,the capacity retention rate is 73.4%,and the capacity attenuation per cycle is 0.089%.(2)As reported,compared with single metal oxides,bimetallic oxides generally exhibit better conductivity and have better adsorption effect on polysulfides,In order to improve the electrochemical performance of the composite positive electrode.CoFe2O4 nanoparticles with uniform particle size were prepared.Fe3+ generated during the etching of CoFe2O4 by hydrochloric acid was used to catalyze the polymerization of pyrrole on the surface of CoFe2O4.The CoFe2O4@PPy composite material was formed by controlling the concentration of hydrochloric acid and the etching time.The spherical structure of CoFe2O4 is maintained and the porous structure of is conducive to sulfur loading.The prepared CoFe2O4@PPy/S composite has a sulfur content of 56%and maintains the original spherical structure.The electrochemical impedance test shows that the CoFe2O4@PPy/S composite positive electrode has a lower electrochemical impedance than Fe3O4@PPy/S,and its electrochemical performance is also more excellent.At a current density of 1 C,the capacity of the first cycle is 566.3 mAh g-1,and it can still maintain 506.8 mAh g-1 after 300 cycles,the capacity retention rate is as high as 89.5%,and the capacity decay per cycle is 0.035%.