In Situ Transmission Electron Microscopy Study On The Morphological Evolution And The Energy Storage Mechanism Of Anode Materials In Sodium-ion Batteries

With the growing demands for clean energy in social development,especially in portable electronic devices and electric vehicles,the search for a novel electrochemical energy storage device has been attracted great attention.Sodium-ion batteries（SIBs）are widely considered the most promising grid-level energy storage and conversion device in the future due to the abundant natural reservoir of sodium resources and the similar physicochemical properties to lithium.However,as compared to traditional lithium-ion batteries（LIBs）,the detailed studies onSIBs are still in their infancy.Significantly,some critical issues of different electrodes inSIBs still lack a comprehensive and in-depth discussion,such as microstructure and phase evolution behaviors,the dynamic process of ions transports,and the interfacial interactions.These issues are closely related to the differences in physicochemical properties between sodium and lithium ions,such as ionic radius,atomic mass,and redox potential.Therefore,it is imperative to conduct a real-time observation to reveal the physical and chemical changes of various electrodes during sodiation/desodiation cycling.In this thesis,nano-scale SIBs were constructed inside TEM using the advanced in situ TEM techniques.In addition,the corresponding electrode evolution behaviors during the sodiation/desodiation process,including microstructure and phase evolution processes,ion transport kinetics,and electrode failure mechanisms,were tracked in real-time.It is an essential prerequisite for designing a novel electrode inSIBs to understand the physicochemical evolution of various electrodes during the sodiation/desodiation process.This thesis aims to provide some comprehensive and detailed insights into sodium storage mechanisms through in situ TEM observation,which would,in turn,help guide and design next-generation electrode materials forSIBs.The main innovative results in this thesis are as follows:（1）In situ TEM study on the conversion-alloying reaction mechanisms ofSnS/N-G electrode forSIBs.As for the rapid capacity fading of alloy-type electrode materials due to the huge volume expansion,the nanoelectrode of ultra-smallSnS nanocrystals anchored on graphene was designed and prepared,and the in situ TEM technique was introduced to investigate the nanostructures and phase evolution during electrochemical charging/discharging processes.The microscopic morphology evolution and phase transformation processes of theSnS/N-G electrode during the sodiation-desodiation process were tracked in real-time by electron diffraction,high-resolution imaging,and electrochemical analyses.The results show that theSnS/N-G electrode undergoes a stepwise conversion-alloying reaction during the sodiation and with the eventual generation of Na₁₅Sn₄ and Na₂S phases.The phase transformation during the desodiation is significantly reversible.In addition,for the microscopic morphology evolution during the sodiation-desodiation,the flexible graphene substrate significantly alleviates the pulverization ofSnS nanocrystals due to the volume expansion.It not only provides the two-dimensional support for buffering volume expansion,but also takes the merits of graphene to improve ion/electron transport kinetics,which is further confirmed by the electrochemical analyses.This work provides a comprehensive understanding of the important role of the above nanostructure in improving electrode stability.（2）In situ TEM study on the conversion reaction mechanism of FeS₂ nanotube electrode forSIBs.The significant volume expansion and slow reaction kinetics of the conversion-type FeS₂during the sodiation-desodiation process severely hinder its large-scale practical application inSIBs.As for these issues,the poly-crystalline FeS₂ nanotubes（NTs）consisting of tiny FeS₂ crystallites were prepared by morphological engineering strategies to improve electrochemical performance.In situ TEM observations reveal that one-dimensional shapes can afford straight pathways for Na+transport to expedite reaction kinetics,and the poly-crystalline structure can buffer significant volume expansion and structural strain.We have identified an intercalation-conversion reaction mechanism from the FeS₂ phase to the Na₂S+Fe phases via the intermediate Na FeS₂ phase upon initial sodiation.Moreover,a reversible and symmetric conversion reaction between Na FeS₂ and Na₂S+Fe phases is established during subsequent sodiation-desodiation cycles.The in situ sodiation-desodiation cycles and the electrochemical long-cycle have proved the significant role of the structural design in optimizing the conversion-type electrode material.（3）In situ TEM study on the microstructure and phase evolutions of NH₄V₄O₁₀ nanobelts when used as anode electrode inSIBs.Aiming at the low working voltage and poor electrochemical performance when the NH₄V₄O₁₀ electrode is used as the cathode of a sodium-ion battery,the one-dimensional nanostructure of NH₄V₄O₁₀ nanobelts were prepared by a simple hydrothermal method.In addition,in situ TEM techniques and electrochemical analyses were performed to track the sodium storage mechanism and phase evolution behavior of NH₄V₄O₁₀nanobelts when they were used as anode electrodes in sodium-ion batteries.The results demonstrate that a stepwise Na-storage reaction mechanism is revealed,initiating with the interlaminar intercalation of Na ions accompanied with the appearance of Na_xNH₄V₄O₁₀ phase and ending with the conversion reaction with the final formation of V₂O₃ and Na₂O phase.While upon desodiation,the V₂O₃ phase can only be oxidized to the VO₂ phase rather than the original NH₄V₄O₁₀ phase.Afterward,a reversible conversion reaction between VO₂ and V₂O₃ phases is established upon the subsequent（de）sodiation cycles.Moreover,the in situ observation also witnesses the emergence of nanopores in nanobelts that may alleviate significant structural strain and contribute to the long-term cycling stability during the following（de）sodiation cycles.For the first time,this work has validated the practicability of NH₄V₄O₁₀ as an anode material in sodium-ion batteries and afforded a paradigm of revisiting existing cathodes to explore their possible anode utilization.