Operando Observation Of Intercalation Reaction And Investigation Of Structure Evolution In Lithium/Sodium Ion Batteries

There are some problems in the practical application of electrochemical energy storage devices,such as capacity attenuation and poor cryogenic performance.This dissertation discusses about the operando observation of intercalation reaction and investigation of the structure evolution in lithium/sodium ion batteries.Here we mainly investigate the structure/electrochemical evolution of battery electrode materials at different temperatures and reaction rates.Subsequently,in-situ oxygen defect construction and pre-intercalation of alkali metal ions were used to study the effect of material intrinsic structure changes on the electrochemical process.And the results are as follow:The capacity attenuation of batteries at high temperature.The complex electrochemical activation process of the battery was first detected via the cyclic voltammetry?CV?and the stairecase potential impedance test?SPEIS?for Li[Ni_0.6Co_0.2Mn_0.2]O₂?NCM622?and Li[Ni_0.8Co_0.1Mn_0.1]O₂?NCM811?at 25 and 55^oC.Through Mott-Schottky analysis,compared with that at 25 ^oC,NCM622 owned a lower flat band potential at 55 ^oC.NCM811 had a high flat band potential at 55 ^oC.The structure evolution of NCM811 and NCM611 was studied by in-situ X-ray diffraction?XRD?at 25 and 55 ^oC.At 25 ^oC,NCM622 exhibited structural reversibility.When the temperature was raised to 55 ^oC,spinel phase was observed via in-situ XRD when the charging voltage was⁴ V.No spinel phase occurred in NCM811 at 25 ^oC,while presented at 55 ^oC.The appearance of electrochemical insert spinel phase is due to the thermodynamic instability and high reactivity of NCMs in high delithiation state.The causes of the instability of ternary materials at high temperature are revealed,which provides a reference for solving the stability of ternary materials in the later stage.Subsequently,due to poor temperature stability of batteries,in-situ observation and electrochemical analysis were performed on the intercalation reactions of Na₃V₂?PO₄?₃ and NaV₂?PO₄?₃ during high and low temperatures at different rates.The asymmetric reaction mechanism was discovered and the first electrochemical phase diagram was drawn.We tested the CVs at different temperatures and obtained the ion diffusion coefficient by fitting not change linearly with temperature²93 K.Then,we carried out differential calorimetry scanning tests on Na₃V₂?PO₄?₃ and NaV₂?PO₄?₃,and found that the peak position appeared²83 K.Meanwhile,XRD tests at different temperatures found that the superlattice transition occurred for Na₃V₂?PO₄?₃ and NaV₂?PO₄?₃.In order to study the reaction mechanism,we conducted in-situ XRD experiments at different temperatures and CV scanning rates,and found the asymmetric reaction during charging/discharging.According to the in-situ XRD test results,the first electrochemical phase diagram was drawn.This study is helpful to understand the reaction state of batteries at different temperatures.Then,to achieve high cycling performance of materials,we studied the mechanism of oxygen vacancy stabilized reversible insertion.We proposed the concept of oxygen vacancy enabling reversible electrochemical process.Firstly,we used plasma etching method in the preparation of the MoO_3-x nanomaterials.XRD test found that etching material has larger interlayer spacing.Ultraviolet spectrum tests found etching of the material owned a lower band gap.X-ray absorption fine structure spectroscopy?XAFS?results found etching chemical state was reduced accompanied by the disorder of Mo-O bonds.X-ray photoelectron spectroscopy and positron annihilation spectroscopy were used to test the material and the analysis showed that oxygen defects were formed on the surface.Finally,in order to investigate the reaction mechanism,in-situ XRD tests were performed and fond the oxygen defect stabilized cathodes presented electrochemical reversiblity.This study helps to understand the effect of oxygen vacancies on structural stability of materials.Finally,for structural performance relevance and structural optimization mechanisms,the structure degradation of vanadium oxide and pre-intercalation enabling reversible electrochemical process were studied.Firstly,we studied the structural degradation process of V₂O₅.In-situ XRD and Raman analysis were carried out for different lithium ion insertion depth.In the state of 0-0.5 Li/V,we found that the structure was stable and reversible during the electrochemical process.When Li/V at 1.0,the XRD and Raman results indicated that the structure was highly irreversible.The long cycle in-situ XRD,in-situ Raman and ex-situ XAFS tests were analyzed.The results fond instability of chemical bonds leads to structural instability.In order to solve the unstable problem of V₂O₅,we used the alkali metal?Li⁺,Na⁺,K⁺?pre-intercalation methods to solve the unstable problem.To further study its reversible mechanism,we conducted in-situ XRD tests.We found that K⁺pre-intercalated V₂O₅ showed continuous and discontinuous lattice transition,owned the smallest cell volume transformation during the electrochemical process.Afterwards,we carried out ex-situ XAFS test,and through in-depth analysis.We found that the V-O and V-V bond stabilized after pre-intercalation.This work shows the transition metal oxygen bond and transition metal transition metal bond,which plays an important role in structural stability.