In Situ Dynamic Characterization Of Microstructure Evolution And Investigation Of Electrochemical Sodiation/Desodiation Mechanism Of Electrode Materials For Sodium-ion Batteries

With the rapid development of microelectronics and electric vehicles,sodium-ion battery（SIB）has been widely regarded as the most potential energy storage and conversion device with grid-level energy storage ability,due to its abundant sodium source and low cost.Compared with the developed lithium-ion battery（LIBs）technology,the development of sodium ion battery is still in its infancy.Thus the electrochemistry in SIBs,especially the interaction mechanism between anode materials and sodium ions,is not well understood.Moreover,considering that sodium ions have larger ion radius and mass,as well as more negative working potential compared with lithium ions,researchers cannot explain the electrochemical behaviors of anodes in SIBs by simply using the as-established electrochemical reaction model or theory in LIBs.Therefore,it is urgent to investigate the microstructural and phase evolutions during sodiation/desodiation and deeply understand the electrochemical reaction mechanism between the anodes and sodium ions,ultimately providing theoretical guidance for designing high-performance anode materials for SIBs.In this dissertation,the advanced in situ transmission electron microscopy（TEM）technology has been utilized to dynamically study the microstructure and phase evolutions of anode materials for SIBs,and investigate the interfacial ion transport behaviors between electrodes.Finally,their electrochemical storage mechanisms have been revealed.The main innovative results are as follows:（1）In situ TEM identifying the conversion mechanism of NiCo₂O₄ for SIBs.The morphology and structure evolutions of NiCo₂O₄ electrode during sodiation/desodiation have been characterized in real time by using electron diffraction and high-resolution TEM（HRTEM）imaging.A two-step conversion reaction mechanism upon the first sodiation and the asymmetric phase transformation upon the first desodiation has been revealed.NiCo₂O₄ is first converted into intermediate phases of CoO and NiO that are then further reduced to Co and Ni phases;later,Co and Ni cannot be recovered to original NiCo₂O₄ phase,and divalent metal oxides of CoO and NiO are identified as desodiated products.Such asymmetric reactions account for the huge capacity loss during the first charging–discharging cycle of NiCo₂O₄-based SIBs.After that,a reversible and symmetric phase transformation between CoO/Co and NiO/Ni phases is established during subsequent sodiation–desodiation cycles.（2）In situ TEM study of the structure and phase transition of ultrathin bismuthene nanosheet for SIBs.Atomically-thin bismuthene nanosheets are successfully prepared by liquid ultrasonic exfoliation method,which can overcome the issues of large volume expansion and structural collapse of bulk bismuth electrode,and can accommodate the reversible insertion and extraction of sodium ions.In virtue of the in situ TEM observation,a two-step intercalation-alloying reaction mechanism of ultrathin bismuthene nanosheets is revealed and the non-equilibrium sodiation is demonstrated bacause the alloying reaction starts to take place before the intercalation reactions completely finishes.In addition,multistep phase transitions from Bi→NaBi→c-Na₃Bi（cubic）→h-Na₃Bi（hexagonal）have been clearly identified during alloying reactions.It is found that the formation of cubic-Na₃Bi effectively alleviates the drastic volume change of bismuth electrode during charging/discharging.Further,both in situ TEM experiments and the electrochemical measurements have demonstrated the excellent cycling performances of bismuthene nanosheets in SIBs.,（3）In situ TEM visualization of interfacial sodium transport and electrochemistry between few-layer phosphorene.Liquid shear exfoliation has been utilized to prepare the monolayer and few-layer phosphorene nanosheets.Later,an unusual sample configuration is deliberately established inside TEM to enable dynamic observation of sodium ionic transport within a single-crystal nanostructure and between different few-layer phosphorene nanosheets.An unhindered and reversible ionic shuttling between few-layer phosphorene is for the first time directly observed in real time.In addition,we find that sodium transport kinetics is closely related to the interface orientation.Furthermore,by fast high-resolution TEM imaging with⁴0 ms resolution,the unique stripe-like sodium transport pathways in phosphorene have been dynamically tracked,and the multiple phase evolutions of phosphorene have been successfully revealed with atomic spatial resolution:P→NaP₅→Na₃P₁₁→NaP→Na₃P.