Designed Fabrication And Lithium/sodium-storage Properties Of Alloying Anodes

The development of high-capacity electrode materials is urgently needed to meet the growing demands for high-energy-density batteries.Alloying anodes possess high capacities that are several times higher than commercial carbon-based anodes,which are believed to be candidates for next-generation high-energy-density batteries.However,the large volume change of alloying anodes upon cycling always leads to a structural deterioration of materials/electrodes and the unstable solid electrolyte interfacial（SEI）film on the surface of materials,resulting in severe capacity degradation.Therefore,one of the most important research projects is to construct robust material/electrode structures for alloying anodes to enhance the cycling stability and rate performance,as well as the initial Coulombic efficiency（ICE）.In this thesis,several strategies such as adjustment of lattice chemistry,morphological and structural design,modification of interface,and binder engineering,were employed to solve the problems of alloying anodes induced by volumetric change at the level of material/electrode structures.Moreover,the effects of structural strategies on the electrochemical behaviors of alloying materials were also investigated.The following are the main contents and findings of this theis:（1）One-dimensional Bi-Sb/C nanofibers with different Bi/Sb ratios were prepared by an electrospinning method to address the structural deterioration of active materials.The relationship between mechanical properties and pulverization behaviors was investigated.Theoretical analyses and in-situ characterizations demonstrate that tuning the lattice chemistry can lead to a reduction in the elastic modulus of Bi–Sb alloys and an enhancement of its toughness,which is beneficial to release the stress during（de-）sodiation processes.The as-prepared Bi_0.25Sb_0.75/C nanofibers exhibit high anti-pulverization capability and outstanding cycling stability with a capacity retention of 73.1%after 2500 cycles at 2 A g^-1.Based on the lattice softening effect,the strategy to enhance the anti-pulverization capability for active materials sheds some light on the development of high-performance alloying anodes.（2）A presodiation method is developed by immersing electrodes in a sodium-naphthalene-dimethoxyethane（Na-Naph-DME）solution to address the problem of poor ICE of alloying anodes.The treated Bi_0.25Sb_0.75/C anode can be partially sodiated owing to the strong reducing activity of the Na-Naph-DME solution,and a SEI layer is formed on the surface of active materials after contacting with electrolytes.The controlled ICEs from 65%to 163%for the Bi_0.25Sb_0.75/C anode can be achieved by modulating the concentration of the Na-Naph-DME solution and the immersion time.Besides,the effect of different presodiation degrees on the electrochemical performance of full cells assembled by the presodiated Bi_0.25Sb_0.75/C anode and Na₃V₂（PO₄）₃@C cathode was explored in detail.Benefiting from the compensation for Na loss,full cells with a suitable presodiation degree deliver high specific capacity and superior cycling performance.This work paves a new avenue for the design of full cells based on alloying anodes.（3）Poly（3,4-ethylenedioxythiophene）/poly-（styrene-4-sulfonate）（PEDOT:PSS）is composited with the sodium alginate（SA）binder to boost the electrochemical performance of alloying anodes at the high mass loading.The composite binder with an appropriate PEDOT:PSS content possesses improved electrical conductivity and electrolyte wettability while maintaining good adhesion,which is in favor of the electronic and ionic transport of electrode structures.The Bi_0.25Sb_0.75/C anode prepared by the conductive composite binder shows a superior Na-storage capability.Even at a high mass loading of 4.1 mg cm^-2,the Bi_0.25Sb_0.75/C anode exhibits a high areal capacity of 0.78 m Ah cm^-2 after 100 cycles.The conductive composite binder may also be extended to other electrode materials,which can enhance the electrochemical performance of Si anodes for lithium-ion batteries.（4）Konjac glucomannan（KGM）is developed as a novel binder for Si-based anodes to enhance the interfacial interaction between active materials and binders.Contributing to the abundant hydroxyl functional groups,the KGM binder features high adhesion and robust mechanical properties,which can provide plenty of sites to contact active materials.Moreover,Si nanoparticles are modified with an amorphous Si O₂ protective layer（Si@Si O₂）to further improve the interfacial adhesion.Molecular mechanics simulations and experimental results suggest that a robust electrode can be tightly formed due to the enhanced interaction between the KGM binder and Si@Si O₂ active materials.Furthermore,apart from bridging KGM molecules,the Si O₂ protective layer of active materials may also act as a buffer layer for Si cores to relieve stresses induced by volumetric change.The as-prepared KGM/Si@Si O₂ anode displays excellent structural stability upon cycling.After1000 cycles at 2 A g^-1,the KGM/Si@Si O₂ anode maintains a highly reversible capacity of1278 m Ah g^-1,with a small capacity decay of 0.056%per cycle.