Construction Of High Performance Carbon Based Nanocomposites And Their Energy Storage Characteristics

Lithium ion batteries have attracted considerable interest as some of the most promising energy storage devices for high power electric vehicles and portable electronic devices because of their attractive high energy density,cycling stability,and excellent rate capability.Despite these considerable advantages,active materials,such as sulfur in Lithium sulfur batteries and Mn₃O₄ in lithium ion batteries,cannot be solely used as the cathode/anode material due to their low electrical conductivity and drastic volume changes over extended cycling.To solve these disadvantages,enormous efforts have been made.Among them,an efficient strategy is constructing nanocomposites with carbon materials.The carbon hosts,like graphene sheets and mesoporous carbon,which act as both a volume buffer and a conductive network to absorb the internal stress and to increase ion and electron transport in the electrode,are believed to effectively suppress the volume change and enhangce the conductivity.Moreover,the traditional prepared method usually involves complex manufacturing processes and cannot achieve strong connection of active materials and carbon.Thus,it remains a great challenge to develop a facile and controllable active materials loading method for the fabrication of high-performance carbon based nanocomposite.By investigating the lithiation process and its impact to the microstructure,the relationship between materials microstructure and electrochemistry performances can be well understood.The main contents and results are as follows:?1?A novel synthesis of graphene-sulfur nanocomposites by electrolytic exfoliation of graphite rod coupled with in situ sulfur electrodeposition has been discovered.A high purity graphite rod is employed as the work electrode and the source of graphene for electrochemical exfoliation experiments.A Pt flake is chosen as the counter electrode.The electrolyte is obtained by mixing sulfuric acid solution and sulfourea.During the electrolytic process,SO₄^2-and OH^-ions are intercalated into the grain boundary of graphite,where the graphite is expanded by gaseous species such as oxygen releasedfrom the oxidation of intercalants.At the same time,sulfourea molecules could be infiltrated sufficiently into the expandable graphite layers,and in situ converted into sulfur particles.The electrochemical exfoliation of the graphite electrode and the electrodeposition of sulfur occur simultaneously,resulting in the graphene-sulfur nanocomposites being in situ formed.The nanocomposites deliver a high initial discharge capacity of 1080 mA h g-1 at 0.1 A g-1 and the capacity retains above 900 mA h g-1 over 60 cycles,which is showing promising characteristics as a high-performance cathode materials for Li-S batteries.This strategy can provide strong sulfur chemical interactions with the graphene host via electrochemical exfoliation and deposition synchronous reactions,achieving advanced cathode materials with long cycling life.?2?A controllable synthesis of mesoporous carbon-sulfur nanocomposites by electrodeposition of sulfur into the ordered mesopores with a self-limiting process has been discovered.The experiment was performed in a three-electrode system using a Pt flake?2cm × 2cm?as the work electrode,a Pt wire as the auxiliary electrode and a saturated calomel electrode?SCE?as the reference electrode,respectively.Powder mesoporous carbon is dispersed in the electrolyte under magnetic stirring.Sulfourea can be electro-oxidized to?SCN₂H₃?₂ in acidic solution.?SCN₂H₃?₂ is an unstable product in acidic solution,and irreversibly decomposes to elemental sulfur and cyanamide.During electrodeposition process,sulfourea molecule can infiltrate sufficiently into the channels of mesoporous carbon,and in situ converted into sulfur particles once mesoporous carbon contacted with Pt flake current collector.Uniform distribution and strong C-S bonds with a merits of high sulfur content?77%?achieved by this method.Moreover,the as-prepared materials demonstrate excellent electrochemical performance as cathode materials for lithium-sulfur battery.The sample delivers high specific capacities at various C-rates?about 1160,1070,860,750 and 590 mA h g-1 at current densities of 0.1,0.2,0.5,1.0 and 2.0 A g-1,respectively?and remarkable capacity retention,remaining above 857 mAh g-1 after 200 cycles at a high rate of 0.5 A g-1.This electrochemical strategy can be also applicable to numerous other sulfur composite cathodes for advanced for Li-S batteries.?3?A facile synthesis of the Mn₃O₄-reduced graphene oxide?Mn₃O₄-r-GO?nanocomposite based on a microexplosion mechanism and reduction treatment has been discovered.Mn²⁺ could be infiltrated sufficiently into graphite oxide?GO?sheets to form GO-Mn²⁺ composites,which is owing to the attraction of negatively charged oxygen-containing functional groups.After reaction of Mn²⁺ with MnO4-and H₂ O,the intercalation compound of MnO₂ could be formed within the layers of GO.When H₂O₂ is added under ultrasonic conditions,a microexplosion reaction occurred in the interlayer space of GO/MnO₂,resulting in the edges of GO/MnO₂ layers being exfoliated.The final Mn₃O₄-r-GO composite is obtained by hydrazine hydrate reduction.The well-dispersed ultrafine Mn₃O₄ particles?with a size of about 20 nm?are closely anchored onto the surface of r-GO sheets can be found in this research.Compared to pure Mn₃O₄,the Mn₃O₄-r-GO nanocomposite delivers higher specific capacity and superior rate capability,which act as a promising anode material of Li-ion batteries.Attributed to the confining,buffering and conducting effects of the r-GO sheets,as well as the uniform and small particle size of Mn₃O₄,the electrochemical performance of the Mn₃O₄-r-GO nanocomposite is enhanced.The sample delivers a high reversible specific capacity of more than 1100 mA h g-1 at 0.1 A g-1.Even at a high current density of 1.2 A g-1,this material can still achieve an acceptable capacity of 330 mA h g-1.?4?A simple synthesis of LiNi_1/3Co_1/3Mn_1/3O₂-graphene quantum dots nanocomposites by electrolytic exfoliation of graphite rod coupled with graphene quantum dots electrodeposition has been discovered.Two high purity graphite rods are employed as both the work electrode and the counter electrode.The electrolyte was obtained by mixing ethanol and NaOH.Powder LiNi_1/3Co_1/3Mn_1/3O₂ is dispersed in the electrolyte under magnetic stirring.During the electrolytic process,OH^-ions are intercalated into the grain boundary of graphite,where the graphite is expanded by gaseous species such as oxygen released from the oxidation of intercalants.Owing to the electrostatic adsorption,the negatively charged graphene quantum dots prepared by electrochemical exfoliation method,are deposited on the positively charged surface of LiNi_1/3Co_1/3Mn_1/3O₂ to form the LiNi_1/3Co_1/3Mn_1/3O₂-graphene quantum dots nanocomposites.The graphene quantum dots with a size of 2 nm can enhance the conductivity of composites.The LiNi_1/3Co_1/3Mn_1/3O₂-graphene quantum dotsnanocomposites asc as a cathod material of Li-ion batteries and delivers higher electrochemical performance.At a 0.1 C rate,the initial discharge capacity is 211 mA h g-1.Even at 2 C rate,the reversible capacity is still as high as 150 mA h g-1,implying the good rate capacity.