Study On Ion-Electron Channel Construction In Anode Materials And The Associated Effect For Lithium/Sodium Storage Performances

As technology improves by leaps and bounds,which drives people’s thirst for energy.It is necessary to promote the structural transformation of the energy so that overcome the deterioration of environment and energy crisis.The electrochemical energy storage technology is an important section of the structural transformation of the energy,which plays a key role in promoting the sustainable development strategy.Lithium-ion batteries have many merits,such as high energy density,high power density,and long-life span,and they shine in the field of portable electronic devices and electric vehicles.However,the most widely used intercalation-type electrode materials are inhibited by their low theoretical specific capacity and have been gradually hard to meet the ever-growing needs of market.Sodium-ion batteries,which are considered the most promising substitute for lithium-ion batteries,are facing the same challenges in area of search electrode materials.The energy density of electrochemical energy storage devices is associated with the specific capacity and operating voltage,therefore,designing high-capacity electrode materials is an important means to boost their performance.Although alloy-type and conversion-type electrodes have a high theoretical specific capacity,they usually suffer from poor conductivity,sluggish electrochemical kinetics,and unstable structure during the charge and discharge process.Moreover,there are still many details of their electrochemical process and reaction mechanism to be revealed,providing more theoretical basis for the development of high-performance electrodes.As for the main problems of charge transfer and stability of ion/electron transport channel for various electrode materials,this work applied designing composites,introducing defects,and constructing heterostructure technology to enhance the electrochemical performances of electrodes,and further studied the effect of charge transport channel modulation mechanism and microstructure on electrochemical kinetics.The main results are as follows:1.To alleviate the inferior electrochemical reversibility,poor conductivity,dissolution of intermediate product,and the instable ion/electron channels of transition metal sulfides anode,Ni S_1.23Se_0.77 nanosheets/mesoporous carbon spheres composites（NSSNs@HMCS）were prepared efficiently via sacrificial template method,chemical deposition,and anion exchange method.The morphology of electrode transformed into the uniform nanometer-scaled sulfides/selenides heterostructures after the first charge/discharge process,therefore,the ions/electrons transfer channels were successfully optimized.Firstly,the boundary-riched heterostructure can accelerate the Na-ions diffusion,and suppress the dissolution of intermediates;secondly,the heterostructures kepet close contact with mesoporous carbon shell,improving the conductivity and decresing the electrochemical polarization;thirdly,mesoporous characterization and hollow morphology can further modulate the volume expansion and boost structural stability.Benefit from the above advantages,the obtained electrode exhibited excellent electrochemical performances.For example,the discharge specific capacities of NSSNs@HMCS anode are respectively 815 and 837 m Ah g^-1 for the first cycle under a current rate of 0.1 A g^-1,corresponding initial coulombic efficiency is approximately 97.4%;a discharge capacity of 353 m Ah g^-1 can be obtained even under a high current density of 5.0 A g^-1;the electrode remained a high discharge capacity of 405 m Ah g^-1 under a current density of 2.0 A g^-1,corresponding capacity retention and attenuation are respectively around 95.6%and 0.003%.2.To solve the problems of low conductivity,large volume change,and agglomeration of red phosphorus anode,a nitrogen-rich carbon nanotube/bubble cross-linked matrix was synthesized by pyrolysis of dual-metal MOFs（N-CBCNT）,which composited with red phosphorus via vaporization-condensation method（noted as N-CBCNT@r P）.There are several merits:N-CBCNT possess a three-dimensional conductive network,inner CNTs can offer extra conductivity and structural stability;hierarchical pore structure of N-CBCNT can alleviate the collapse and avoid agglomeration;nitrogen-riched characterization of N-CBCNT significantly enhanced the interaction between red phosphorus and carrier,and promoted the lithium ions and electrons transfer.As results,the initial discharge and charge capacities of electrode are respectively 1995 and 1746 m Ah g^-1 at a current density of 0.1 A g^-1,corresponding initial coulombic efficiency is around 87.5%;the large loading electrode(around 3.0 mg cm^-2)exhibited a capacity of around 524 m Ah g^-1 at 5.0 A g^-1 after 1500 cycles,corresponding capacity retention and attenuation are respectively around 83.4%and 0.011%;the assembled full cell with Li Fe PO₄counter electrode also showed an impressive cycling performance,the discharge capacity remained around 1590 m Ah g^-1 after 50 cycles at a current density of 0.2 A g^-1,shown a capacity retention of 98.7%.3.Based on the effect of heterostructure construction and atom doping strategies on energy storage mechanism,in order to tackle the low electronic conductivity and voltage hysteresis of Nb₂O₅-based anode,the sulfur-doped Nb₂O₅nanosheets/reduced graphene oxide composite（S-Nb₂O₅/SG）was obtained through pyrolysis of Nb S₂ nanosheets on graphene oxide surface.According to analysis,doped sulfur atoms can not only reduce the lattice energy of Nb₂O₅ crystal,but also enhance the coupling between Nb₂O₅ and graphene,which faciliated the deep reduction and improve the discharge capacity;the stronge interaction between Nb-containing species and graphene caused to the Nb₂O₅ tend to spread on graphene surface,and in situ transform into micrometer-sized layered heterostructures that possess continue Li-ions/electrons pathways;the micrometer-sized layered heterostructures have high stability and low charge transfer resistance.As results,S-Nb₂O₅/SG exhibited significantly higher electrochemical performance than reported Nb₂O₅-based anode.The discharge capacity is 598 m Ah g^-1 under a current density of0.1 A g^-1;electrode remained a discharge capacity of 313 m Ah g^-1 with a high capacity retention of close to 100%under a high current rate of 5.0 A g^-1 after 1000charge/discharge cycles.