Structural Design And Performance Studies Of Anode Based On Carbon Nanomaterials For Alkali Metal-ion Batteries

At present,anode materials for alkali metal ion batteries have the disadvantages of low specific capacity,short cycle life,and poor low-temperature performance.In this thesis,to solve these problems,1D and 2D carbon nanomaterials（carbon nanotubes,graphene）with unique structures and excellent properties are used as the conductive support of the high-capacity anode active materials or the active materials themselves.Through structural design and regulation,we have realized the improvement of the specific capacity of the anode material,the enhancement of the cycle stability and low-temperature performance,and revealed the relationship between material structures and the corresponding electrochemical performances,which provides effective guidance for designing anode materials of the alkali metal ion batteries.To solve the above three problems,the main research content and results are as follows:（1）Taking single-walled carbon nanotube（SWCNT）bundles（one-dimensional carbon nanomaterials）as the research object,to increase the specific capacity,the pristine SWCNT bundles were purified and then oxidized and annealed.As a result,the purified SWCNT bundles and oxidized SWCNT bundles were obtained,respectively,and their structures were characterized.Their electrochemical performances as anode materials for Li/Na/K ion batteries were tested and compared,and the ion storage sites were studied.The results show the purified SWCNT bundles with open ends deliver a specific capacity of Li-storage(425m Ah g^-1)at a rate of 0.1 C and that is about twice that of the pristine SWCNT bundles with closed ends,while their specific capacities of Na-storage and K-storage are very close.It can be inferred that Li⁺ions can be stored on the inner wall of purified SWCNT,while Na⁺ions and K⁺ions can’t.The Li,Na,and K-storage capacities of oxidized SWCNT bundle at a rate of 0.1 C are 636,154,and 199 m Ah g^-1,respectively,which are much higher than the corresponding specific capacities of purified SWCNT(425,105,85 m Ah g^-1).This indicates that introducing defects can greatly increase the number of active sites of the SWCNT bundle,and thus improve its specific capacity.In addition,we have focused on the K⁺storage sites of the purified SWCNT bundles,and proved that K⁺ions can be stored in the tube gap of the purified SWCNT bundle through X-ray diffraction and other means of characterization.The storage sites of Li,Na,and K in the pristine SWCNT bundle and purified SWCNT bundle were summarized.（2）A SWCNTB@Sn O₂@C coaxial and adaptive composite material with a single-walled carbon nanotube bundle（SWCNTB）as the core,tin dioxide in the middle,and carbon shell on the outer layer was designed.The SWCNT bundle is elastic and deformable in its radial direction.Under the confinement of the outer stiff carbon shell,the SWCNT bundle can produce radial elastic deformation to accommodate the huge volume changes of tin dioxide generated when alkali metal ions are inserted and extracted.The self-adaptive electrode material can not only avoid the cracking of the outer carbon shell but also maintain excellent electrical contact between Sn O₂ and the entire conductive network.Besides,the SWCNT bundle itself shows outstanding electrical conductivity.As a result,SWCNTB@Sn O₂@C shows excellent cyclic stability and rate performance.In terms of Li storage,it shows a specific capacity of 960 m Ah g^-1 after 600 cycles at a current density of0.5 A g^-1,and the capacity retention rate compared to the second cycle is 100%;in terms of Na storage,its specific capacity is 309 m Ah g^-1 after 300 cycles at a current density of 0.2A g^-1,and the capacity retention rate is 84%,which is much better than that of the MWCNT@Sn O₂@C electrode material with the coaxial structure and no adaptivity.（3）Reduced graphene oxide（RGO）（two-dimensional carbon nanomaterials）was used as the model anode material,and its structure was tuned and its electrochemical performance was optimized.By comparison with other carbon anode materials,the structural characteristics required for carbon anode materials with excellent low-temperature performance were summarized.RGO has a large electrode/electrolyte contact area,an appropriate amount of surface defects,large interlayer spacing,and high conductivity simultaneously.These structural features allow most alkali metal ions to be stored on its surface,which reduces migration distance that ion storage needed and the difficulty of ion insertion between layers in RGO.As a result,the adverse effect of low temperature on its electrochemical performance is reduced.Compared with graphite,hard carbon,and graphene oxide（GO）,as an anode material for lithium/sodium/potassium ion batteries,RGO shows much higher specific capacities at-40°C and higher capacity retention rates compared to its capacities at room temperature.Besides,the types of alkali metal ions also have a great influence on low-temperature performances.The specific capacity of K-storage at-40°C of the optimized RGO is much higher than those of Li-storage and Na-storage,and compared to room temperature,its K-storage capacity retention rate is as high as 58%.This is due to the higher ion mobility of K⁺ions in an electrolyte,which reduces the battery polarization.Therefore,to achieve low-temperature performance as good as possible,it is necessary to combine anode materials dominated by surface ion storage（such as RGO）with K⁺ions with high mobility in an electrolyte,which means potassium ion batteries show great development prospects in the field of low-temperature batteries.