Synthesis Of MnO-Graphene Composite Materials And Their Applications For Electrochemical Energy Storage

With the continuous consumption of fossil fuels,electrochemical energy storage devices that can achieve the efficient storage and output of electric energy are increasingly widely used,so they have become a research hotspot.Currently,the main energy storage equipment includes fuel cells,secondary batteries and capacitors.Graphene has a good application prospect in electrochemical energy storage devices due to its high specific surface area(2600 m² g^-1),excellent conductivity and good mechanical properties.Meanwhile,manganese oxide has become an important electrochemical energy storage material due to it has the advantages of low cost,no pollution,abundant resources,high theoretical capacity(756 m Ah g^-1),low discharge voltage platform(<0.2v)and so on.However,the poor conductivity,low inital coulombic efficiency,the larger volume expansion and poor cycling performance of MnO limits its practical application.The combination of in-situ synthesized manganese oxide/graphene composite and nanocomposite structure are expected to make full use the both of the advantages to obtain excellent electrochemical energy storage materials.In this paper,a series of electrode materials with controllable morphology,size and components have been prepared by simple,low-cost and geen hydrothermal method.At the same time,hollow carbon capsules have been synthesized by a direct physical mixing method following a one-step calcination,and the sponge like graphene cubes have been prepared by a CVD method.They have been used as electrode materials for lithium ion batteries,lithium ion capacitors and lithium sulfur batteries.Their electrochemical energy storage performances have been systematically studied,mainly as follows:(1)MnO/graphene cubes with core-shell structure have been prepared and served as the anode electrode of lithium batteries.The homogeneous Mn CO₃-C cubes were prepared by an in-situ hydrothermal method:using manganese dioxide(MnO₂)as the manganese source,glucose as the carbon source,urea as the PH regulator and sodium dodecylbenzene sulfonate(SDBS)as the surfactantand,and its formation mechanism was also investigated.The different core-shell structures of MnO-graphene cubes(MnO@G-700,MnO@G-1200)have been prepared by the calination treatment of Mn CO₃-C cubes at two different temperatures.MnO@G composite have been successfully used as the anode electrode of lithium ion battery and tested its half-battery performance.It was paired with the cathode electrode material lithium iron phosphate to assemble the full battery and measured its full battery performance.The results show that the initial coulombic efficiency of MnO@G-1200 is as high as 85%,much higher than the value of literature(56-75%)after the calcinatination at 1200?.At 50 m A g^-1,the discharge capacity of MnO@G-1200 is 1092 m Ah g^-1.Even at a high current density of 1 A g^-1,the capacity is still as high as 586 m Ah g^-1,which is 61.5%higher than that of commercial MnO.After 500 cycling at 500 m A g^-1,the specific capacity is not any attenuation,presents an upward trend and finally reaches to 870 m Ah g^-1.The excellent electrochemical performance of MnO@G electrode is mainly attributed to the following reasons:the specific surface area of MnO@G-1200 is significantly reduced after high temperature calcinating and the low specific surface area is conducive to the reduction of its irreversible capacity,thus significantly improving its initial coulombic efficiency.At the same time,the MnO@G-1200 material endows excellent conductivity and stable cubic core-shell structure,which will greatly improve the rate performance and cycling stability.(2)MnO/graphene nanowire composites have been prepared and served as the anode electrode of lithium batteries.Using graphene oxide(GO)as the carbon source,MnO₂ as the manganese source,urea as the precipitant and SDBS as the structure guide agent to prepare MnOOH nanowires coated with graphene(MnOOH@G)by a in-situ hydrothermal method.And a series of MnO@G composites with different RGO contents were prepared by high-temperature calination treatment.MnO@G composites were used as the anode materials of lithium-ion battery and their electrochemical performance was tested.The results show that the obtained MnO@0.5G have the best electrochemical lithium storage performance when the additive amount of GO was 0.5 g under hydrothermal conditions.Its initial coulombic efficiency is 86.3%,much higher than that of pure MnO nanowires(60.8%).At the current density of 50 and 500 m A g^-1,the discharge capacity is up to 1185 and 884 m Ah g^-1.Even at a high current density of 3 A g^-1,the capacity is still as high as 508 m Ah g^-1,showing an excellent rate performance.the specific capacity of MnO@0.5G electrode has not any undiminished and finally up to886 m Ah g^-1 after 500 cycles at 1 A g^-1,showing an excellent cyclic stability.Research results show that the reduced graphene oxide can significantly enhance the conductivity of the MnO and its pore structure also provide more storage space for lithium ion.Meanwhile,GO with rich oxygen-containing groups can form rivet structure(Mn CO₃)with Mn source in the hydrothermal process,which greatly improve the structural stability of MnO@0.5G,thus improving the initial coulombic efficiency,specific capacity,rate performance and cyclic stability of lithium ion batteries.(3)MnO/graphene nano-peapods composites have been prepared and used as the anode electode of lithium-ion battery.Nanowire and nanorod MnOOH-C composites were prepared by hydrothermal synthesis method that using benzoic acid as carbon source,potassium permanganate as manganese source,urea as alkaline regulator and SDBS as structural guide agent.Nonapepods and naorod graphene coated MnO composites(NP1200,NR1200)were respectively prepared through high temperature calcination.The electrochemical properties of NP1200 and NR1200 as anode materials were tested.The initial coulombic efficiency of NP1200 electrode is high up to 85.9%and the specific capacity are 1168 and 945 m Ah g^-1 at 50 and 500 m A g^-1,respectively.Even at the high current density of 1 A g^-1 and 3 A g^-1,its specific capacity still reache to 728 and 505 m Ah g^-1,respectively.The specific capacity has no attenuation and finally reach to 883 m Ah g^-1 after1000 cycle at 1 A g^-1.The electrochemistry performance of NP1200 is higher than NR1200,and much higher than that of commercial MnO and MnO nanowire.Furthermore,NP1200 have been served as the anode electode and LFP as the cathode electrode to assembled a full battery NP1200/LFP.The initial coulombic efficiency of the full battery is 90.56%and the specific capacity are 165 and 121 m Ah g^-1 at 0.2 C and 5C,respectively.The specific capacity retention is as high as 86%after 100 cycles at 0.5C.The results indicate that NP1200 have sufficient internal buffer space,excellent electrical conductivity and stable core-shell structure,which can effectively increase the structural stability of the electrode,improve transfer reaction of charge and the electron and increase the wettability lead to NP1200 and NP1200/LFP showing excellent electrochemical performance of lithium storage.(4)S,N co-doped hollow porous carbon capsules have been prepared and applied in lithium-sulfur battery.S,N double doped hollow porous carbon capsules(SNHCC)were obtained through direct physical mixture and co-pyrolysis assembly method:using trisodium citrate as the carbon source and activator,thioureas as the dopant and the ratio of raw materials was reasonably controled.SNHCC has a high specific surface area(1762m² g^-1)and a large pore volume(1.88 cm³ g^-1).SNHCC has been used as the catode material of lithium-ion capacitor.Its specific capacities are 305 and 210 m Ah g^-1 at 1 and6 A g^-1.Even under 10 A g^-1,the capacity is as high as 165 m Ah g^-1,showing a high specific capacity and excellent rate performance.In addition,the specific capacity of SNHCC electrode has a slightly rising trend after 5,000 cycles at 3 A g^-1,showing a stable cycling performance.NP1200 has been served as the anode material,SNHCC as the cathode material to assemble a full lithium-ion capacitor NP1200//SNHCC.The specific capacity of NP1200//SNHCC are 57 and 35.75 F/g at 1 and 10 A g^-1,showing high specific capacity and excellent rate performance.Furthermore,the energy density of NP1200//SNHCC is 126.71 Wh/kg at low power density for 671 W/kg.Even at high power density of 5722 W/kg,the energy density of NP1200//SNHCC is still as high as79.5 Wh/kg.The capacity of the full capacitor is unchanged after 10000 cycles,showing an excellent cycling performance.It is worth noting that SNHCC material has a unique hollow capsule structure with a wall thickness of 50 nm and a microporous structure,which allows the passage or storage of gas phase S or small molecule S and hinders the passage or storage of large molecule S,which are contribute to buffering the shuttle-through effect of polysulfide.Therefore,SNHCC has been used as the host material for elemental sulfur to synthesize SNHCC/S composites with high S load and applied in lithium-sulfur battery.SNHCC can effectively inhibit the volume expansion effect of elemental sulfur,the transition effect of polysulfide and showing excellent electrochemical performance due to its high specific surface area,large pore volume,stable hollow capsules structure and abundant polar S and N doped elements.The specific capacities of SNHCC/0.7S electrode are 1240 and 1227 m Ah g^-1 at 1 C and 2 C.Even at high current density of 5 C and 10 C,the specific capacity is still as high as 905 and 759m Ah g^-1,showing an excellent rate performance.The capacity retention rate of SNHCC/0.7S are 96.2%after 100 cycles at 3 C.The capacity retention rate is still high up to 85.4%after 500 cycles at 6 C,showing an excellent cyclic performance.Besides,the SNHCC/0.7S electrode after 500 cycles can light up 38 LEDs,indicating that the electrode material has a great application potential in lithium-sulfur battery.(5)Sponge-like graphene cubes have been prepared and applied in lithium-sulfur battery.The precursor Mg CO₃ cubes were prepared by a hydrothermal method that using magnesium sulfate hephydrate,ammonium bicarbonate and SDBS as raw materials.The porous MgO cubes were obtained through the calcination of the precursor.The porous MgO cubes serve as template via vapor deposition method to prepare sponge-like graphene cubes(SGCs)with micron size(?1.5 m).SGCs has high specific surface area,abundant micro/mesoporous structure,excellent conductivity and stable 3D porous structure.SGC/S composites have been obtained by compounding with different quality of elemental sulfur and apllied in lithium-sulfur battery.Especially,the SGC/0.6 S electrode has high average specific capacity(the capacity is 1185 m Ah g^–1 at 1 C),excellent rate performance(the specific capacity are 860 and 735 m Ah g^-1,respectively at 5 C and 10 C)and good cyclic stability(capacity retention of 95.5%at 1 C for 100cycles;capacity retention of 81.9%at 3 C for 500 cycles).The SGC/0.6S cell after 500cycles can light up 38 LEDs,indicating that the electrode material has excellent electrochemical performance and has a good application prospect.