Self-supported Co_1-xS-CNFs Composite For Negative Electrode Of Lithium Ion Batteries

Lithium-ion batteries(LIBs)have drawed much attention owing to their large energy density,excellent rate and cycle performance,are regarded as energy storage devices.The low specific capacity of commercial graphite anodes limits its development in the field of energy storage.Therefore,it is necessary to further develop anode materials with higher energy density and higher power density.The transition metal cobalt sulfide(Co_1-xS)has attracted extensive research interest due to its high theoretical specific capacity(870 m Ah g^-1),good cycle performance and relatively simple preparation method.However,during the reaction of the Co_1-xS,due to repeated insertion and deintercalation of lithium ions(Li⁺),the Co_1-xS electrode will undergo volume expansion and even structural damage.In view of the above problems,there are currently two effective methods:one is to design and prepare the Co_1-xS electrode materials of different shapes and structures to improve battery performance;the other strategy is to introduce carbon-containing materials to alleviate the problem of volume expansion during the reaction of the Co_1-xS electrode.In addition,nano-structured Co_1-xS is compounded with carbon materials to improve its conductivity.This paper combines the above two strategies to prepare a composite electrode of self-supporting carbon nanofibers(CNFs)and the Co_1-xS,using the strong reducibility of carbon at high temperatures and the catalytic effect of metal cobalt to convert amorphous carbon to graphitic carbon,And further synthesized the onion carbon coated the Co_1-xS particle composite material,used as the negative electrode of LIBs to alleviate the volume expansion problem,and then enhance the rate function and cycle performance of the Co_1-xS-CNFs electrode.The main examination details of the paper are as follows:(1)Oxygen plasma treatment of the CNFs introduces oxygen-containing polar groups to improve the hydrophilicity of the surface of the CNFs and supports the Co_1-xS nanoparticles,and the prepared the Co_1-xS-CNFs electrode is used as the negative electrode of the LIBs.Oxygen plasma is used to treat the CNFs to introduce oxygen-containing functional groups,improve the surface hydrophilicity of the CNFs and promote the contact of the active material Co_1-xS with the electrolyte.The generated defects and nucleation sites promote the growth of the active material Co_1-xS.The introduction of the CNFs improves the conductivity of the Co_1-xS-CNFs composite electrode and can also serve as a framework for supporting material structures.By changing the plasma treatment atmosphere,time,the molar mass ratio of cobalt salt and thiourea,and different solvothermal reaction temperatures,the impact of preparation factors on the rate behavior,cycle performance and electrochemical performance of the Co_1-xS-CNFs composite electrode was explored.The research results show that the comparison of the characterization results of the samples under the plasma treatment time of 30 s,60 s,90 s and 120 s in the oxygen atmosphere shows that the Co_1-xS-CNFs electrode is in the plasma treatment for 90 s in the oxygen atmosphere.The Co_1-xS-CNFs electrode has a reversible capacitance of 839 m Ah g^-1 at a current density of 100 m A g^-1,and maintains a capacity of 232 m Ah g^-1 at a current density of2000 m A g^-1.In addition,at this current density,the containment retention rate after500 cycles was close to 100%,and the capacity retention rate after 950 cycles was40%.Compared with pure CNFs,the Co_1-xS-CNFs composite electrode treated with oxygen plasma shows a higher specific discharge capacity.(2)Use the excellent catalytic performance of transition metal cobalt to construct onion carbon-coated the Co_1-xS particle self-supporting composite material,and use the prepared Co_1-xS-CNFs electrode for the battery negative electrode.In the previous part of the work,the Co_1-xS particles are grown on the surface of the CNFs,and there is still a high probability of falling off during the electrochemical procedure.In this part of the work,the paper introduces the cobalt source into the CNFs while preparing the CNFs,and uses metallic Co to catalyze the conversion of amorphous carbon to graphitic carbon at high temperatures,improving the degree of graphitization and mechanical properties of the Co-CNFs electrodes,and the Co_1-xS-CNFs self-supporting electrode was obtained by solvothermal vulcanization treatment,and finally the Co_1-xS-CNFs composite electrode with onion carbon coated the Co_1-xS particles was obtained.The results of the study show that the comparison of the various characterization results of samples doped with different cobalt salts at0.1 g,0.2 g,0.4 g,0.6 g and 0.8 g shows that when the doped cobalt salt content is 0.2g,the Co_1-xS-CNFs electrode exhibits excellent electrochemical performance,with a reversible capacity of 620 m Ah g^-1 at a current density of 100 m A g^-1,and the capacity of 252 m Ah g^-1 is maintained at a current density of 2000 m A g^-1 and there is no significant decrease in capacity after 840 cycles at this current density.The reason is that the CNFs can enhance the overall conductivity and can also serve as a carbon skeleton to support the active material Co_1-xS.In addition,the formed onion carbon coating layer can be used as a buffer layer to alleviate the volume expansion caused by the repeated insertion and extraction of Li⁺.These results show that this special onion-like carbon structure is an effective strategy to improve battery performance.