The Preparation Of Amorphous Carbon And Amorphous Carbon Decorated Nanomaterails For Sodium Based Energy Strorage Systems

The exhaustion of fossil fuels and the severe hazes continuely urge people to develop new energy resources.New energy sources,mainly include solar,wind and tidal energy,have disadvantages such as uneven spatial distribution,and intermittent time due to the uncertain natural conditions.Therefore,the development and optimization of large-scale energy storage technologies is an important prerequisite for replacing traditional fossil energy with new energy.Among them,the use of electrochemical energy storage in various electrochemical devices（including lithium-ion batteries,sodium-ion batteries,multivalent ion batteries,lithium-sulfur,sodium-sulfur batteries,lead-acid batteries,flow batteries,and super capacitors）is currently the most effective large-scale energy storage technology,and the lithium-ion battery is currently the most widely used portable mobile power supply with sufficient energy density and sufficient cyc le life.However,due to limited lithium resource reserves,as well as the increasing demand and continuous consumption of lithum,the price of lithium continues to rise.Compared to lithium-ion batteries,sodium-ion batteries have the same electrochemical behavior as lithium-ion batteries.However,the reserves of sodium metal resources are much richer than lithium ion resources,and they are evenly distributed worldwide.There is no depletion phenomenon for sodium metal resources with the intensive use of sodium.However,because the radius and atomic weight of sodium ions are larger than that of lithium ions,and the standard electrode potential of sodium is higher than lithium,the energy density of sodium ion batteries is lower than that of lithium ion batteries.Therefore,the development of sodium electrode materials with high specific capacity is one of the most effective ways to increase the energy density of sodium ion batteries.However,this type of electrode material always perform very poor cycle stability due to the structural instability of the electrode material during the sodium intercalation/deintercalation process.Therefore,how to improve the structural stability of electrode materials with high-capacity during cycling process is a current research hotspot in the field of electrochemical energy storage.In this paper,we have effectively improved the cycle stability of sodium based energy strorage systems by decorating these electrode materials with amorphous carbon.Besides,the electrochemical performance of pure amorphous carbon as electrode was also investigated.The details as follows:（1）In Chapter 2,a（MoS₂/carbon fiber（CF））@MoS₂@C hybrid has been prepared by an efficient method combining the electrospinning,hydrothermal and annealing techniques.Coupled with graphite as cathode,the hybrid enables the full cell（i.e.,dual ion batteries）to deliver a high initial discharge capacity of 112.3 m A h g^-1 at 0.2 A g^-1 and maintain a high reversible capacity of 90.5 m A h g^-1 at 0.5 A g^-1after 500 cycles.These remarkable sodium storage properties of（MoS₂/CF）@MoS₂@C are mainly attributed to its three-dimensional framework with its highly conductive,cross-linked structure,which effectively increases the conductivity of the hybrid and the mass loading of MoS₂.Moreover,the MoS₂nanoplates embedded inside CF,as well as being sandwiched between the CF/MoS ₂and a thin carbon layer,also can effectively buffer the pulverization and aggregation of the electrode,and thus maintain the struct ural integrity of（MoS₂/CF）@MoS₂@C during the charge-discharge process.（2）Beside MoS₂,Antimony（Sb）based materials have been also considered as one of promising anodes for sodium ion batteries（SIBs）owing to their high theoretical capacities and appropriate sodium inserting potentials.However,its volume expansion is even more seriouse than MoS ₂ during the cycling process.For confirm the protective function of amorphous carbon for Sb,in Chapter 3,we develop a novel Sb/C hybrid encapsulating the Sb nanorods into highly conductive N&S co-doped carbon（Sb@（N,S-C））frameworks.As an anode for SIBs,the Sb@（N,S-C）hybrid maintains high reversible capacities of 621.1 m A h g^-1 at 100 m A g^-1 after150 cycles,and 390.8 m A h g^-1 at 1 A g^-1after 1000 cycles.At higher current densities of 2,5 and 10 A g^-1,the Sb@（N,S-C）hybrid also can display high reversible capacities of 534.4,430.8 and 374.7 m A h g^-1,respectively.Such impressive sodium storage properties are mainly attributed to the unique cross-linked carbon networks providing highly conductive frameworks for fast transfer of ions and electrons,alleviating the volume expansion and preventing the agglomeration of Sb nanorods during the cycling.（3）MoO₂ is a promising electrode material but has not been well investigated.For further confirming the protective function of amorphous carbon for this recently emerging electrode,in Chaper 4,MoO₂@C nanoflowers were synthesized through a grinding method,followed by an annealing process.In addition,irregular MoO₂@C nanoparticles could be synthesized,when the amount of ammonium molybdate was increased by twice while other condition kept the same.With excellent dispersity and structural integrity,the as-obtained MoO₂@C nanoflowers deliver a reversible capacity of172 m A h g^-1 at 0.1 A g^-1 and a capacity of 166 m A h g^-1 after 1000 cycles at 1.0A g^-1.（4）The above three chaperts are mainly focus on invstigating the protective function of amorphous carbon as anode electrodes.In Chapter 5,th e pure amorphous carbon is directly used as cathode material.The N and S codoped hollow carbon nanobelts（N/S-HCNs）are synthesized by a self-templated method.The as-synthesized carbon nanobelts exhibit excellent performance in pseudocapacitance and electric double layer anions adsorption.After pairing the N/S-HCNs cathode with a tin foil anode in a carbonate electrolyte,the obtained SIC achieves a high specifc capacity of 400 m A h g^-1 at 1 A g^-1（based on the mass of cathode material）and energy density of 250.35 Wh kg^-1 at 676 W kg^-1（based on the total mass of cathode and anode materials）.Besides,the presented SIC also demonstrates high cycling stability with almost 100%capacity retention after 10000 cycles,which is among the best results of the reported SICs,suggesting the potential for high-performance energy storage applications.