Design And Preparation Of Fe-Based And Hard Carbon Anode Materials And Their Lithium/Sodium Storage Properties

At present,the development of high-performance lithium-ion batteries（LIBs）and sodium-ion batteries（SIBs）has attracted much attention.For LIBs,the low specific capacity and poor rate performance of commercial graphite anodes are the main reasons for limiting their energy density and power density.Therefore,the development of anode materials with high capacity and high rate performance is the key to improving the endurance and charging speed of LIBs.For SIBs,Na⁺cannot be inserted into graphite anodes in traditional ester electrolytes.Therefore,the anode materials of SIBs are still in the basic research stage.The iron-based materials based on the conversion mechanism have the characteristics of high specific capacity,low cost,variety of morphology,and are an attractive class of anode materials for LIBs and SIBs.On the other hand,SIBs may be mainly used in the field of fixed energy storage in the future,and their electrode materials are more focused on low cost and long cycling stability.Therefore,in addition to iron-based materials,hard carbon materials are also a kind of attractive anode materials for SIBs.This thesis mainly focuses on the design,preparation and lithium/sodium storage performance of iron-based materials and hard carbon materials.The specific research contents are as follows:（1）Among iron-based materials,iron phosphide（FeP）has attracted wide attention due to its high theoretical specific capacity and low potential vs.Li⁺/Li.However,the conductivity of FeP is poor,and its significant volume change during the lithiation/delithiation process causes the electrode structure to collapse easily and lose its activity.Thus,in this paper,a method for vertically growing carbon-coated FeP nanorod arrays（FeP@C-NA）on different dimensional carbon networks is developed.Firstly,FeP@C-NA were successfully grown on the surface of reduced graphene oxide（G）by this method（G⊥FeP@C-NA）.In this structure,the ultra-small FeP nanorods can greatly reduce the internal stress generated during the lithiation/delithiation process,and can also shorten the diffusion distance of lithium ions;the pores between FeP@C-NA can not only accommodate the volume expansion,but also accelerate the diffusion of the electrolyte;in addition,the unique dual carbon network consisting of internal G and outer carbon layer can greatly improve the conductivity of FeP and stabilize the overall structure of the electrode.Thanks to its unique structure,the conductivity and structural stability of G⊥FeP@C-NA have been greatly improved,G⊥FeP@C-NA show a rapid kinetic process dominated by pseudocapacitive behaviors.When used as a anode material for LIBs,G⊥FeP@C-NA exhibits a very high specific capacity,excellent rate performance and cycling stability.Further,a lithium-ion full cell was successfully assembled with a cathode material of the commercial LiFePO₄,which proved the practicability of G⊥FeP@C-NA.（2）In order to further prove the universality of the synthetic method in（1）,we continue to try to grow FeP@C-NA on the surface of carbon nanotubes（CNTs）and have achieved success.The prepared CNTs⊥FeP@C-NA also showed excellent lithium storage performance similar to G⊥FeP@C-NA,which proves that our developed method of growing FeP@C-NA is universally applicable on different carbon matrixes,and this type of nanocomposite structure is very effective for improving the lithium storage performance of FeP materials.（3）Among the iron-based materials,the theoretical specific capacity of ferrous sulfide（FeS）is relatively low,and the volume expansion during sodiation/desodiation is relatively small,so it is more suitable as the anode material for SIBs.However,FeS also has the problem of poor conductivity and volume expansion.Thus,in this paper,a new pie-like FeS@Cnanocomposite（P-FeS@C）was successfully prepared.In this material,the ultra-small FeS nanocrystals are completely embedded in a disordered carbon network rich in mesopores and sealed by a protective carbon shell.The results show that the unique pie-like structure can effectively stabilize the internal FeS nanocrystalline particles and significantly accelerate the reaction kinetics of FeS（dominated by pseudocapacitive behaviors）,which makes the P-FeS@C exhibit excellent cycling stability and rate performance.It is very rare that P-FeS@C shows a good sodium storage activity even at 80 A g^-1.Further,a sodium-ion full cell was successfully assembled with a cathode material of Na₃V₂（PO₄）₂O₂F,which also showed excellent sodium storage performance and proved the practicability of P-FeS@C.（4）Through preliminary exploration,we found that FeS has high sodium storage activity,but the poor cycling stability,while the commercial Fe3O4 has low sodium storage activity,but the excellent cycling stability.Considering the above situations as well as Fe₃O₄ can be easily converted to FeS,a novel core-shell Fe₃O₄@FeS composite with the morphology of regular octahedra has been prepared via a facile and scalable strategy by using commercially available Fe₃O₄（C-Fe₃O₄）as the low-cost precursor.When is used as the anode material for SIBs,the main sodium storage activity of Fe₃O₄@FeS comes from the FeS shell,while the internal Fe₃O₄core does not react and mainly to stabilize the surface FeS.Thus,Fe3O4@FeS shows excellent cycling stabiliy and rate performance in sodium ion half-cell and full-cell,which successfully combines the advantages of C-Fe3O4 and FeS.It provides a new idea for the development of high-performance carbon-free FeS-based SIBs anode materials.（5）Hard carbon is the most promising anode material for SIBs because of the advantages of low cost,good cycling stability and low electrode potential vs.Na⁺/Na.However,the low initial Coulombic efficiency（ICE）and poor rate performance seriously hinders its practical application.It is difficult to solve these two problems at the same time by the current modification methods.Thus,in this paper,a flexible self-supporting hard carbon paper（HCP）is rationally developed after two steps of calcination by using commercially available tissue.Combined with self-supporting structure and ether electrolyte,HCP simultaneously shows the ultra-high ICE（91.2%）,excellent rate performance,long cycling stability and outstanding low temperature performance.More importantly,we have also deeply studied the sodium storage mechanism,capacity decay mechanism and kinetics of hard carbon in ester/ether electrolytes,clarifying the main reasons for the improvement of ICE and rate performance.This work has opened up a new ideas for promoting the commercial application of hard carbon.