Failure Analysis And Electrode Interface Characteristics On The Electrode Materials For Lithium-Ion Power Batteries

As the development and popularization of new energy automobile, there are more and more requires on the performance of batteries. Rechargeable lithium-ion batteries are regarded as one of the important developing directions for electric vehicles (EVs) and hybrid electric vehicles (HEVs) due to their high energy density and high power density. Nowadays, the commercial electrode materials more or less show some problems which may cause the fading of capacity or the safety problems during the actual use. To solve these problems, this dissertation is focused on the studies about the failure analysis of the electrode materials for lithium-ion power batteries, electrode/electrolyte interface performance and the electrode interface characteristics on Li-excess Mn-based cathode, respectively. The main research contents and conclusions are as follows:Firstly, cyclic voltammetry (CV) and Electrochemical impedance spectroscopy (EIS) tests are used to investigate the performance of different cell systems. For two electrode button type cell, as the lack of reference electrode and its large contact resistance, it does not have the conditions on study the electrode/electrolyte interface performance effectively. Although the three electrode button type cell shows some improvement on potential detection of the system which can partially eliminate the adverse effects caused by electrode polarization and reduce the contact resistance, there are some limitation on the investigation of the resistance of solid electrolyte interphase (SEI) film and charge transfer process. However, it can show the variation of kinetic parameters well in the three electrode glass type cell, which is the premise on studying the reaction mechanism, failure mechanism and electrode interface characteristics of electrode materials.Graphite which is one of the most widely used anode materials for lithium-ion battery has been systematically investigated. EIS is used as the main method in order to better understand the failure mechanisms of graphite electrode in ethylene carbonate (EC)-based liquid electrolytes and poly (methyl methacrylate) Gel Polymer Electrolytes (PMMA-GPE). Then the impact of various concentrations of fluoroethylene carbonate (FEC) and vinylene carbonate (VC) on film forming performance and modification mechanism of graphite in EC-based liquid electrolytes and PMMA-GPE, respectively. It is found that the failure of graphite electrode in both EC-based liquid electrolytes and PMMA-GPE is mainly occurred in electrode/electrolyte interface. During the lithiation/delithiation process, the unstable SEI film on the surface of graphite particle can bring increase of the resistance of electrode/electrolyte interface, delay the transformation of Li-ions in the interface, decrease the reversibility of charge transfer process and increase the internal resistance, finally cause the failure of the cells. The film forming additives can improve the stability of electrode/electrolyte interface. However, it is found that excessive for FEC and VC both can cause the deterioration of the electrode/electrolyte interface performance. EIS combining with Scanning electron microscopy (SEM), Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) spectroscopy are used to deeply investigate the film forming mechanism of FEC and VC. It is found that this kind of additives can form relatively electrochemical and structure stable SEI film before the formation of SEI film by the decomposition of EC or other composition in the electrolyte. The transport efficiency of Li-ions in SEI film and the ability of graphite particle adapting to volume expansion are improved which is helpful on the improvement of electrochemical performance.Systematic EIS test combining with CV and charge/discharge test have been used to investigate the reaction and capacity fading mechanism of LiNio.5Co0.2Mn0.3O2 electrode with high cutoff voltage (3.0-4.5 V) and high working temperature (55℃). It was found that the phase transformation and Li-Ni site exchange in lattice at high voltage (>4.3 V) acted on charge transfer process were the main reasons of capacity fading with the cutoff voltage of 3.0-4.5 V. The degradation of LiNi0.5Co0.2Mn0.3O2 cathode with high-temperature was mainly associated with the strong catalytic activity of Ni4+ which can cause the side reaction between electrode and electrolyte to form unstable SEI film as well as the acceleration of phase transformation on the surface of electrode with high electrode polarization potential. Furthermore, it was found that improving the performance of SEI film should be one of the most important methods for improving the cycling performance of LiNi0.5Co0.2Mn0.3O2 cathode under high-temperature and we have proved it by systematic discussing the EIS results of the LiNi0.5Co0.2Mn0.3O2 cathode in electrolyte with fluoro-ether as an additive.Li-excess Mn-based cathode materials are regarded as one of the most promising cathode materials for next generation traction lithium-ion batteries as their high energy density and high power density. However, their lithiation/delithiation mechanism can not get consensus yet, so systematic electrochemical performances especially the electrode/electrolyte interface performance of commercial Li-excess Mn-based cathode materials are investigate to help us understanding the the reaction mechanism under room temperature and capacity fading mechanism under high temperature (55℃). It is found that the structure transformation caused by the second step of lithiation/delithiation process under high voltage (>4.5 V) and the failure of electrode/electrolyte interface caused by the decomposition of compositions in the electrolyte are the main reasons for the deterioration of commercial Li-excess Mn-based cathode materials under high temperature. Then 0.5Li2Mn030.5Li(Nio.44Mno.44Coo.12)02 Li-excess Mn-based cathode materials with smaller particle size and higher crystallinity are prepared by coprecipitation method. After a series of electrochemical characterization and compared with commercial Li-excess Mn-based cathode materials, it is found that although 0.5Li2Mn030.5Li(Nio.44Mno.44Coo.12)02 Li-excess Mn-based cathode materials has a bigger irreversible capacity in the first cycle, but the capacity retention rate and the rate performance are both improved. More importantly, the cycle stability of 0.5Li2Mn030.5Li(Nio.44Mno.44Coo.12)02 Li-excess Mn-based cathode under high temperature is much better than that of commercial Li-excess Mn-based cathode due to the improvement of the electrode/electrolyte interface performance.At last, combined with the formation mechanism of inductive reactance for LiCoO2 and LiMn2O4 cathode, systematic EIS tests are used to investigate the inductive reactance for LiNio.5Coo.2Mno.3O2 cathode and commercial Li-excess Mn-based cathode. It is found that the main reason of the inductive reactance formation mechanism for different electrode materials is almost the inhomogeneity in the electrode. Inductive reactance would more easily form under low temperature as the lower electron and Li-ion conductivity. As shown in the variation of the inductive reactance with the electrode polarization potential, inductive reactance of different electrode materials have different dependence with electrode polarization potential. Different from the LiCoO2 and LiMn2O4 cathode, whose inductive reactance are related to the inhomogeneity of lithiation/delithiation only, the inductive reactance is also related to the conditions of the electrode/electrolyte interface, and the strong dependence on the variations of the inductive reactance and SEI resistance of commercial Li-excess Mn-based cathode can reflect the point better.