In Situ TEM Observation On The Electrochemical Behavior And Conversion Mechanism Of Anode For Lithium Ion Battery

Rechargeable Lithium-ion batteries (LIBs) with outstanding properties are now needed urgently as preferred energy storage devices for electric vehicles and portable electronic device, which promotes the rapid development of new-type anode materials for LIBs. Owing to their high theoretical specific capacity, good cycleability and safety, metal oxides (sulfides) have been studied as one of the most promising alternative anode materials in LIBs. However, the severe volume expansion of the metal oxides (sulfides) during discharging process always causes a drastic pulverization and heavily damage of microstructure, thus further results in the rapid capacity fading in the first discharging process. To solve these issues, considerable approaches have been carried out to improve the electrochemical properties of the metal oxides (sulfides) anode materials and more progresses have been made, such as synthesis of the nanocomposites, unique nanostructures, and the introduction of various carbon additives. Whereas, their real electrochemical processes, fundamental mechanism, microstructure evolution, and the capacity fading remain unclear.In this dissertation, the microstructure evolution of the metal oxides (sulfides) anode materials has been investigated in real time using the in situ TEM inside JEOL-2100F microscope. Then, the conversion mechanism has been studied and the capacity retention of the electrode materials has been discussed. Finally, the reason of the capacity fading in the first cycle has been illustrated in detail. The concrete innovative results are summarized as follows:1. The electrochemical lithiation and delithiation cycles of CeO2were studied for the first time. The electrochemical process of CeO2was monitored by simultaneous determination of the structure with HRTEM and ED, and of the chemistry with EELS. The results revealed that the dispersed CeO2nanoparticles transformed to Ce2O3in the lithiation process, and the morphology change was not obvious, and underwent a fully reversible phase transformation between fluorite CeO2and cubic Ce2O3during the electrochemical process, namely CeO2+2Li++2e-(?)Ce2O3+Li2O. The results provide direct evidence and a profound understanding of the low reversible capacity during the discharge process of CeO2in LIBs.2. The dynamic structure and phase evolution of ZnO nanoparticles has been revealed. A previously unknown link between the lithiation mode and the reaction rate for ZnO anode materials in LIBs was studied. Two kinds of lithiation mechanism were revealed. With the potential of-3V, the reaction is quite violent and many large Zn nanocrystals grow quickly during the process. With the potential of-1V, as expected, the lithiation rate was much slower than that of-3V, and the lithiation mechanism was also found to be quite different; there were many cracks formed in the lithiated ZnO nanoparticles, but the ZnO nanoparticles kept their original shapes.3. The dynamic structures and phase evolutions of Fe2O3and CO3O4nanoparticles during the first lithiation have been investigated. The results suggested that single-crystalline Fe2O3/Co3O4nanoparticles were transformed to multicrystalline nanoparticles consisting of many Fe/Co nanograins (1-3nm) embedded in Li2O matrix during the first lithiation process. Surprisingly, the delithiated product was not Fe2O3/Co3O4but FeO/CoO accounting for the irreversible electrochemical process and the large capacity fading of the anode material in the first cycle. In the second and the following cycling the electrochemical reaction is reversible phase transformation between Fe/Co and FeO/CoO. Simlar conversion mechanism was also found in CuO and Mn3O4.4. The dynamic structure and phase evolution of individual CoS2nanoparticles have been studied. The results suggested that the size expansion of pure CoS2is47.1%while the size expansion of CoS2anchored on graphene is only28.6%, which is caused by the lithiation of nanoparticles homogeneously due to the good Li+conductivity of graphene sheets. The cycling performances of CoS2and CoS2/graphene as anode were examined in coin-type half cell configuration. The reversible capacity of CoS2is57mAh g-1after30cycles, which is much lower than that of CoS2/graphene of644mAh g-1.5. The effect of structure of CNT on electrochemical behaviors of filled Co9S8has been revealed. During the first discharging process the CNT with open end showed a radial expansion, and the lithiated filler was gradually extruded out of the open end of the CNT. This was in contrast to that of CNT with closed ends in which a huge axial elongation was suppressed and a large radial expansion was observed. Besides, the electrochemical behaviors of CNTs in the first sodiation process is similar with that in lithiation process. The difference is that the filled CNT with closed ends is prone to fracture under large current, suggesting the electrode materials show different electrochemical behaviors in Na-ion batteries and LIBs.