| Lithium ion battery is widely used in today’s society.Improving the energy density,power density and cycle stability of batteries is the future direction of the continued development of lithium ion battery.To design high-performance batteries,understanding the operating mechanism of each component in the battery device(such as the de-intercalation process of the positive/negative material;the mass transfer process of the electrolyte)and the interactions between the parts(such as the formation of a passivation layer at the electrode/electrolyte solid-liquid interface;the energy level alignments of the battery interfaces)are all critical.Traditional electrochemical characterization methods can gain insight into the internal reaction mechanism of the battery.For example:using various electrochemical testing techniques to obtain parameters such as deintercalation potential,energy density,cycle retention,and rate performance;or use impedance test parameters study the lithium ion diffusion inside the battery,electrochemical behaviors at the interface,and lithium ion deintercalation mechanism.These methods are essential for understanding the electrochemical processes that occur inside the batteries,and have been widely used in battery research and development,but they only provide very limited indirect electrochemical information.In recent years,a variety of electrochemical in situ characterization techniques have shown great characterization potential in the mechanism research of lithium ion battery.In situ characterization methods,combining the traditional electrochemical measurements with other analysis techniques(such as spectroscopy imaging,electron microscope imaging,or scanning probe imaging),can collect multi-dimensional data to analyze the battery electrochemical process,which is critical for understanding the microscopic mechanism of electrochemical properties.Therefore,this thesis focuses on the study of materials property evolutions in the binder free-thin film electrodes prepared by sputtering through in situ Scanning Probe Microscopy and in situ X-ray diffraction,covering positive/negative materials.Firstly,the first experiment chapter focus on the nano-observations of the transformation of stoichiometric LiCoO2(LCO)with an O3-I structure to its lithium defective O3-II phase,which are difficult to be distinguished because of their similar crystal symmetry.Interestingly,moreover,the O3-II phase shows metallic conductivity,whereas the O3-I phase is an electronic insulator.How to effectively reveal the intrinsic mechanism of the conductivity difference and nonequilibrium phase transition induced by the lithium deintercalation is of vital importance for its practical application and development.In the first section,based on the developed technology of in situ peak force tunneling Atomic Force Microscopy(PF-TUNA)in liquids,the phase transition from O3-I to O3-II and consequent insulator-to-metal transition of LCO thin-film electrodes with preferred(003)orientation prepared via magnetron sputtering were observed under an organic electrolyte for the first time.Then,studying the post-mortem LCO thin-film electrode by using ex situ time-relative XRD and conductive Atomic Force Microscopy,we find the phase relaxation of LCO electrodes after the nonequilibrium deintercalation.Moreover,X-ray absorption spectroscopy results indicate that the oxidation of Co ions and the increasing of O 2p-Co 3d hybridization in the O3-II phase lead to electrical conductivity improvement in Li1-x Co O2.Simultaneously,it is found that the nonequilibrium deintercalation at a high charging rate can result in phase-transition hysteresis and the O3-I/O3-II coexistence at the charging end,which is explained well by an ionic blockade model with an antiphase boundary.Secondly,in situ characterization methods were used to study the physical and chemical property changes at the electrode electrolyte interface with different crystal orientations of the cathode material in the second experiment chapter.Surface properties of cathode materials play important roles in the transport of lithium ions/electrons and the formation of the surface passivation layer.Optimizing the exposed crystal facets of cathode materials can promote the diffusion of lithium-ions and enhance cathode surface stability,which may ultimately dominate cathode’s performance and stability in lithium-ion batteries.In this section,polycrystalline LiCoO2(LCO)thin films with(0003)and{10(?)1}preferred orientations were prepared as the well-defined model electrodes.In situ Current-Sensing Atomic Force Microscopy(CSAFM)was employed to investigate the lithium de-intercalation and electronic conductivity evolution of the(0003)and{10(?)1}facts in the organic electrolyte at the nanoscale.It was found that the lithium deintercalation following a“Li-rich core model”in the LCO grains,and the LCO grains with(0003)crystal face show less conductivity than those with{10(?)1}faces.In addition,electrochemical measurements confirm that the thin film electrodes with{10 (?)1}preferred orientation not only show smaller electrode polarization,and therefore better rate performance due to its straighter ion diffusion channels.However,it also more readily forms a stable surface passivation layer compared with the(0003)preferred orientation due to its higher surface work function and abundant metal adsorption sites.Additionally,in the third part,the classic in situ electrochemical atomic force microscope(EC-AFM)was used to observe the crack initiation and expansion on the surface of the Cu O film anode with a special nano-array structure during the charge and discharge process,and the formation and ablation of the surface SEI film.Since the volume expansion/shrinkage of the anode material during charging and discharging will cause damage to the electrode integrity,loss of active material,and decrease in cycle capacity retention.In the third section,the Cu O nanorod array was produced via glancing angle deposition and directly used as anode materials for thin film electrodes of LIBs.The obtained Cu O nanorod array anodes show good LIB performance with a capacity of220μAh/cm~2/μm tested at 100μA/cm~2 after 80 cycles and excellent rate performance.The obtained properties for Cu O nanorod array anodes were much better than thin film anodes without nanorod array structure.It is found that Cu O nanorods within the electrodes may serve as the hosts for Li~+,and ease intercalation by shortening Li~+ion diffusion pathways,resulting in the remarkable cycling stability and rate performance.On the other hand,the breathing of Cu O nanorod array electrode clearly observed by in situ EC-AFM with the appearance and disappearance of cracks,indicating that the nanorod-array may act as a buffering to alleviate the giant volume variations during the cycling,which increasing the stability of the Cu O nanorod array electrodes.The last part of this thesis extends the research object to the characterization of solid-state electrochemical systems of lithium-based multilayers device.The structure of the multilayer film lithium-based device discussed in this section is LiCoO2/Si O2/Si.The applied bias voltage can reversibly tune the longitudinal resistance of this device,so the device can also be regarded as resistive switching based on the electrochemical reaction.The resistance change process is accompanied by the deintercalation of lithium ions in the LiCoO2 layer driven by the electric field,and the lithiation and delithiation processes in the Si O2/Si layer.This is similar to all-solid-state thin-film lithium ion batteries in their structure and solid-state electrochemical reaction mechanism.Therefore,the observation of the changes in the material properties induced by lithium ion diffusion in such solid-state electrochemical structures is critical for understanding the intrinsic properties of the battery materials during the electrochemical process,and has important significance for the optimal design of all-solid-state lithium ion batteries. |
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