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In-situ TEM Study Of Electrochemical Intercalation Processes In Metal Oxides

Posted on:2021-05-16Degree:DoctorType:Dissertation
Country:ChinaCandidate:G G XuFull Text:PDF
GTID:1361330605479019Subject:Physical chemistry
Abstract/Summary:PDF Full Text Request
In-situ transmission electron microscopy(in-situ TEM),with ultra-high spatial and temporal resolution,can be used to study the dynamic process of electrochemical reactions.It has been widely used to study the evolution of electrode materials,electrolytes and solid electrolyte interfaces during battery operation.It is noted that special in-situ TEM cells are needed for in-situ electrochemical(EC)TEM experiments and conventional in-situ TEM cells have some shortcomings,such as:(1)It is difficult to carry out quantitative electrochemical analysis inside TEM chamber because of the low active material load;(2)The electrolyte and electrode materials are normally in point-contact,which deviates from a real battery structure and may change the ions diffusion mode in electrode materials;(3)It cannot use the conventional organic electrolyte,which misses the true interface between the electrolyte and the electrode material;(4)The imaging quality is reduced due to the SiNx sealing layer.In view of this,we build an in-situ EC-TEM cell,which improves the load of active electrode.The in-situ EC-TEM cell can operate not only under constan-voltage charge/discharge mode,but also under constant-current(galvanostatic)charge/discharge mode.Using the in-situ TEM cell,the lithiation and sodiation mechanisms in Mn3O4 and Nb2O5 were characterized by in-situlquasi-in-situ TEM methods,respectively.The discharge products of each electrochemical discharge state were studied by TEM,electron diffraction,high resolution TEM,and EDS analysis.The results are summarized as follows:(1)In-situ EC-TEM cells,with a sandwich structure(cathode/electrolyte+separator/anode)similar to a coin cell,were constructed for better reproducing the real battery reaction environment.The in-situ EC-TEM cells allow a much higher active material loading(up to μg level).Silicon(Si)nanoparticles and Mn3O4 nanorods were loaded onto the in-situ EC-TEM cells to build a Si/lithium metal battery and a Mn3O4/lithium metal battery,respectively.Conventional battery tests can be performed on these cells,including constant current charge/discharge test,electrochemical impedance spectroscopy test and cyclic voltammetry test.The test results were consistent with those of coin cells.In addition,the design of the TEM cell ensures the high imaging quality and enables the high-resolution observation during charging and discharging.(2)The electrochemical lithiation mechanism of Mn3O4 nanorods was revealed by in-situ/quasi-in-situ TEM.It is well known that Mn3O4 is one of the most promising candidates for next-generation anode materials due to its low cost,nontoxicity,low operating potential,and high theoretical specific capacity(936 mAh g-1),However,a longstanding question regarding how lithium ions are inserted into Mn3O4 still exists.Herein,we use in-situ TEM to investigate phase transitions of Mn3O4 electrodes under the galvanostatic charge/discharge mode and constant-voltage discharge mode.In galvanostatic mode,the lithiation of Mn3O4 undergoes multi-step phase transitions following a reaction pathway of Mn3O4+Li+→LiMn3O4+Li+→MnO+Li2O→Mn+Li2O.It is also found that lithium ions prefer to enter Mn3O4 along the{101} planes to form LiMn3O4 with the help of transitional boundary phase of LixMn3O4.These results are in sharp contrast to those obtained under a constant-voltage discharge mode,where only a single-step lithiation process of Mn3O4+Li+→Mn+Li2O is observed.The results indicated that the conversion reaction pathway is strongly influenced by the charge/discharge conditions.(3)The electrochemical sodiation mechanism of T-Nb2O5 nanosheets was revealed by in-situ TEM.Besides,T-Nb2O5 is found to be a good candidate as the anode material for sodium ion batteries owing to its high theoretical specific capacity(200 mAh/g)and high ionic diffusion coefficient.It is generally believed that the sodium ions diffuse rapidly in the(001)planes but hard aross the(001)planes.In this study,to clarify the sodiation mechanism,the electrochemical sodiation behaviors along the(001)lattice planes and perpendicular to the(001)plane of the T-Nb2O5 nanosheets are studied by in-situ TEM.The results indicate that there are large number of dislocations and domain boundaries in T-Nb2O5 nanosheets,and sodium ions can diffuse across the(001)lattice planes through these defects,and then diffuse rapidly in the(001)planes.Meanwhile,we find modulation structure in the direction of[001]of the original nanosheet,where alternating compressive and tensile strains exist.These strain distributions can be regulated by the insertion of sodium ions,whereas the modulation structure maintained after sodiation.The results suggested that the T-Nb2O5 nanosheets with defective structure is more conducive to the diffusion of sodium ions in sodium ion batteries.
Keywords/Search Tags:In-situ TEM, lithium ion battery, sodium ion battery, galvanostatic charge/discharge, electrochemical reaction mechanism
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