Design Of Heterostructure Anode Materials And Electrochemical Performance For Sodium-Ion Batteries

With the progress of science and technology development,energy plays an increasingly important role as cornerstone of social development.With the gradual depletion of petroleum-based fossil energy sources,developing renewable energy sources and low-cost energy storage devices has become particularly urgent.As the most commercially successful secondary battery system,lithium-ion batteries are widely used in many fields such as smart devices,power vehicles,and base stations.However,the limited and uneven distribution of lithium resources has resulted in high prices for lithium-ion batteries,making it difficult to apply them to large-scale energy storage and low-cost vehicle batteries.In contrast,sodium,as a crustal and surface-enriched element,is widely distributed and has significant cost advantages.The redox potential of sodium is-2.71 V vs.the standard hydrogen potential（SHE）,which is close to that of lithium（-3.04 V vs.SHE）.In addition,Li and Na are in the same group adjacent to each other in the periodic table,and the physicochemical properties of sodium and lithium are close to each other,so the experience of developing lithium-ion batteries can be easily migrated to sodium-ion batteries.Sodium-ion battery is expected to become one of the most promising energy storage systems in the"post-lithium ion battery".Anode is an important component of batteries,however,graphite anode used in commercial lithium-ion batteries shows poor sodium storage performance.There is still much room for the development of high-performance sodium-ion anode materials.Single-component electrode materials are constrained by their own limitations and the problem of slow kinetics due to the large radius of sodium ions,which makes it difficult to meet the requirements of high-performance electrodes and needs to be improved urgently.Among the various improvement methods,heterostructure construction has been given high expectations due to its advantages of operability and low cost.In the present study,the heterostructure construction method was used to effectively improve electrode performance from the perspective of structural design overcoming volume expansion and slow kinetics of sodium storage.Moreover,the synergistic effect in heterostructure was clearly explored,providing insights for heterostructures design.Firstly,an inorganic-organic heterogeneous material Sb@NGA-CMP with Sb nanoparticles with a size distribution of 3-5 nm coated in covalent organic framework material（NGA-CMP）was designed and constructed,and Sb@NGA-CMP was used as a sodium-ion anode,which could reach a reversible capacity of 223 m Ah g^-1 when the current density was 5 A g^-1,and still maintain excellent stability of 344 m Ah g^-1 after5,000 cycles at 1 A g^-1 current density.It still maintains an excellent performance when assembled into a full cell.Mechanistic studies revealed that both components,Sb and NGA-CMP,exhibited enhanced sodium storage activity and played a role of 1+1 over2 in the heterostructured Sb@NGA-CMP:（1）Sb³⁺was introduced as the catalyst for the synthesis of NGA-CMP and subsequently reduced in situ,and the obtained Sb nanoparticles acted as modifiers to effectively reduce the degree of layer stacking of NGA-CMP,exposing more active sites;（2）Thanks to the spatially confined domain of NGA-CMP and the in situ synthesis strategy in the one-pot method,the Sb nanoparticles with a range of 3-5 nm were uniformly covered in the NGA-CMP framework.The NGA-CMP framework could enhance the electrode activity and effectively mitigate the large volume expansion during the sodium storage process,avoiding the coarsening of Sb as well;（3）The strong electronic coupling effect of Sb and pyrazine N on the NGA-CMP framework synergizes with theπ-electronic conjugate structure of the NGA-CMP to significantly enhance the electrical conductivity of the material and improve the overall electrode reactivity.Secondly,a conversion reaction cell model was designed and constructed to investigate the sodium storage behavior of Sn O₂ under atomic loading of eight different elements（V,Cr,Mn,Fe,Co,Ni,Cu or Zn）to systematically probe the role of heterogeneous elements in the conversion reaction.The single-atom modification,as a unique heterostructure,fundamentally eliminates the second phase in the heterostructure,allowing isolated testing of the host material activity and facilitating the identification of active sites.A comprehensive analysis of the electrochemical performance data of this model cell has led to the proposal and confirmation of a significant thermodynamic-dependent catalytic effect in conversion-type heterostructure electrodes.Among them,Ni single atom loaded Sn O₂(Ni _SA-Sn O₂)as the most electrochemically active composite,exhibited a high capacity of 332 m Ah g^-1at a surface density of 3.4 mg cm^-2 and a high capacity retention of 80.1%after 500cycles,which is the best performance ever reported in a Sn O₂-based heterostructured composite without hybridization with carbon.The Ni-O-Sn active sites were revealed through detailed characterization and theoretical analysis,and the deep mechanism behind the catalytic effect was resolved for the transition metal active d-orbitals weakening the surface covalent bonds and lowering the reaction energy barriers,which in turn improves the electrode capacity and energy efficiency.In addition,the active Ni-O-Sn structure was further extended into conventional heterostructures and bimetallic oxides,and excellent electrochemical performance was achieved.Thirdly,a carbon-coated Bi₂Se₃-Zn Se multilevel heterostructures（Bi₂Se₃-Zn Se/N-C@C）as sodium-ion fast-charging anode exhibiting a high specific capacity of 360 m Ah g^-1 at a current density of 20 A g^-1,an excellent fast-charging capability of charging from 0%SOC to 100%SOC in only 58 seconds.Furthermore,Bi₂Se₃-Zn Se/N-C@C still maintains a capacity of 318.4 m Ah g^-1 after 3,000 cycles,with a capacity decay of only 0.028%per cycle.The sodium storage mechanism was explored through a series of electrochemical tests and characterizations,and the key role of the heterostructure on ion transfer and charge transfer was further elucidated:（1）The uniformly distributed metallic Bi₂Se₃ nanosheets in the electrode synergized with built-in electric field between the Bi₂Se₃-Zn Se facilitated the charge transfer,promoted the homogeneous distribution of the charge,and enhanced the system electrical conductivity;（2）The richness of the Bi₂Se₃-Zn Se heterogeneously polarized interface and nitrogen-doped carbon together improve the system adsorption sites and reduce the migration barrier of sodium ions in the electrode bulk phase;（3）The heterogeneous structure of the shell layer of multilayered carbon-covered nitrogen-doped carbon plays a key role in the system long term stability.The above results provide important theoretical additions and inspiration for high-performance heterostructured anode electrode materials,offering new insights into sodium-ion storage.