Synthesis And Lithium Storage Performance Of Carbon/Manganese-Based Compounds With Multi-Cavity Nanostructures

To achieve the goals of "Dual Carbon Strategy"(carbon peaking at 2030s and carbon neutralization at 2060s),the booming markets of green powered batteri es and large grids to replace the fossil fuels,have urgently called for advancing lithium-ion batteries(LIBs)with higher energy and power density.However,the anode materials of LIBs have not been effectively evolved since the first launch of commercial LIBs using graphite anode,while significant progresses have been made in the development of cathode materials with several generations updated.Among the existing anode materials,manganese-based chalcogen compounds(MCCs,such as oxides,sulfides,manganates,etc.)with typical conversion mechanism,have been proposed as a promising anode candidate for next-generation LIBs,due to their high theoretical specific capacity,natural abundance,low cost and environmental friendliness.Nonetheless,the further application of MCCs anode is severely impeded by two main intrinsic shortcomings,namely,the poor electronic and ionic conductivity leading to poor rate capability,and the huge volume change during cycling resulting in poor cycling stability by serious particle pulverization.In this thesis,an effective nanoscale multi-cavity confinement strategy has been developed to encapsulate fine MCCs nanoparticles(NPs)forming yolk-shell pomegranate nanostructures with void buffer to address above issues.Such a rational design affords multiple attractive features,mainly including:(1)the three-dimensional heteroatom-doped mesoporous carbonaceous network functions as electron highways for fast electron transport;(2)the numerous well-defined nanocavities as template contribute to in situ growth of fine MCCs NPs under spatial confinement,thereby significantly shortening the Li+diffusion distance;(3)the interconnected mesoporous nanostructures enable fast kinetics of mass transfer;(4)single-or double-layer buffering structure can effectively accommodate the volume expansion.Based on these,this thesis is consisted by five parts of research works as the following:1.Synthesis and lithium storage performance of nitrogen-doped porous carbon spheres/Mn3O4 with multi-cavity nanostructuresA yolk-shell nanostructured composite with Mn3O4 NPs confined in multi-cavities was developed through in situ growth of fine Mn3O4 NPs within the numerous cages of nitrogen-doped mesoporous carbon nanospheres by hydrothermal process and calcination.Of which,the obtained Mn3O4 NPs yield a narrow size distribution of～10 nm,effectively reducing the Li+diffusion path length.Meanwhile,the high specific surface area and open porous structure could promote the mass transfer process upon electrochemical reactions.Nitrogen-doped carbon networks greatly improve the electronic conductivity of composite.And the void space remained between Mn3O4 NPs and mesoporous cavity shell can effectively buffer the further volume expansion,which could be tuned by varying the loading amounts of Mn3O4.Consequently,the optimal anode of Mn3O4(M)@NMCN(22)with a medium size of Mn3O4 NPs and 22 nm of cavity diameter of carbonaceous matrix,exhibits the best lithium storage performance among the composites with different buffering space,where a high discharge capacity of 883.8 mAh g-1 is delivered at a current density of 100 mA g-1,as well as a good rate capability(317.5 mAh g-1 maintained at 5 Ag-1).2.Synthesis and lithium storage performance of nitrogen-doped porous carbon spheres/MnO with multi-cavity nanostructuresTo improve the cycling stability and capacity achievement of manganese oxides anode,a phase engineering approach was employed to obtained fine MnO NPs embedded multi-cavity yolk-shell composite,via a facile carbothermal reduction method by increasing the calcination temperature.Notably,compared to Mn3+ state,Mn2+ affords lower coordination number,smaller ion radius,and stronger crystal field strength in the octahedral position,thereby is expected to deliver a better cycling stability.In this regard,N-doped mesoporous carbon nanospheres with cavities of 7 nm,22 nm and 40 nm were prepared,which acted as templates to in situ grow MnO NPs with different mass loading for size tuning of voids buffering.As a result,the optimized MnO(10)@NMCN(22)anode,with a KMnO4 precursor loading of 10 mg and a cavity size of 22 nm,shows the best lithium storage performance among the prepared composites,which achieves a high discharge specific capacity of 872.8 mAh g-1 at 100 mA g-1 and 210.5 mAh g-1 at 5 A g-1.Besides,MnO(5)@NMCN(7),MnO(10)@NMCN(22)and MnO(20)@NMCN(40)produced by employing the KMnO4 precursor loading of 5,10 and 20 mg with the cavity size of 7,22 and 40 nm,respectively,exhibit the best performance in their family by moderate buffer space.3.Synthesis and lithium storage performance of nitrogen,sulfur-codoped porous carbon spheres/MnS with multi-cavity nanostructuresConsidering that rock-salt α-MnS offers higher electrochemical activity and improved electrical conductivity than MnO due to the weaker Mn-S bond,and sulfur atoms introduced into the carbonaceous matrix could further improve the electrolyte wettability as well as electrical conductivity of the composite.Therefore,a sulfuration treatment was performed on the above MnO-based nanocomposite to form MnS-based multi-cavity yolk-shell nanostructure for further enhancing the hybrid performance.Note that the sulfuration process not only in situ transforms MnO into α-MnS,but also introduces S atoms doping onto the N-doped porous carbon spheres.Similarly,the buffer space of the composites was adjusted by tuning the loading of MnS and the cavity size of mesoporous carbonaceous matrix.The optimal MnS(10)@NSMCN(22)anode,with a KMnO4 precursor loading of 10 mg and a cavity size of 22 nm,demonstrates much more improved cycling performance with a discharge capacity of 879.5 mAh g-1 retained after 200 cycles at 400 mA g-1.Moreover,operando optical observation has directly verified the improved cycling stability benefiting from sufficient voids through visualizing the structural evolution.4.Synthesis and lithium storage performance of nitrogen-doped porous carbon spheres/ZnMn2O4 with multi-cavity nanostructuresInspired by the attractive synergistic effect between metal cations in bimetallic oxides,it is possible to further improve the electrochemical performance of Mn-based anodes.However,bimetallic oxides also suffer from poor electrical conductivity and large volume variation upon cycling,which could be effectively alleviated by above carbonaceous matrix confinement route.To this end,bimetallic ZnMn2O4 NPs were in situ grown onto the nanocavities of mesoporous Ndoped carbon matrix to form the multi-cavity yolk-shell nanostructure,where the optimum size of ZnMn2O4 NPs was calculated to be 17.39 nm.Therefore,ZnMn2O4 NPs with a typical diameter of 17 nm confined in the 22 nm of cavity were prepared.As a result,the ZnMn2O4(17)@NMCN anode shows the most outstanding battery performance among the prepared samples,which exhibits a high discharge capacity of 811.4 mAh g-1 after 200 cycles at 200 mA g-1,and maintains 335.0 mAh g-1 at a high current density of 3.2 A g-1.5.Synthesis and lithium storage performance of nitrogen-doped porous carbon spheres/ZnMn2O4 with high wettability and multi-cavity nanostructuresTo further enhance the mechanical strength and electrolyte wettability of the yolk-shell pomegranate-like composite,structural engineering based on wet-chemistry was employed on the mesoporous carbonaceous shell.To investigate the cavity size contribution,two types of porous carbon matrix with double-layer buffer structure were constructed,namely,one with smaller cavity of inner mesoporous layer and larger cavity of outer layer(SLNMCN),and the other with larger cavity of inner layer and smaller cavity of outer layer(LSNMCN).After that,ZnMn2O4 NPs were in situ grown within the cavities of inner layer of carbonaceous matrix.The obtained ZnMn2O4@SLNMCN anode shows the highest electrolyte wettability by the smallest contact angle,contributing to the notably improved battery performance with a high discharge specific capacity of 729.2 mAh g-1 maintain after 200 cycles at 200 mA g-1.