Study On The Application Of Electrode Characterization Methods In Lithium-Ion Battery

As the application fields of lithium-ion battery (LIBs) quickly increase, the demand of volumetric energy density and power density for LIBs become more urgent, especially in the HEV, PHEV and EV areas. It needs to keep high energy density meanwhile power density must be higher. LIBs include electrodes (anode and cathode), electrolyte and separator, the major energy generation and transmission is in the electrode regions, therefore, the electrode performance directly determines the battery performance. To optimize the electrode structure for improving the battery energy/power density has become a popular issue.Unlike former flat electrode, the electrode of LIBs which contains active material particles, other fillers (such as the conductive agent, the binder) and current collectors shows the porous morphology, because every kind of active particle has own special morphology, and so it increases the complexity of the porous structure and reaction interface about the porous electrode. For a comprehensive understanding of the electrode structure and influence mechanism on the battery performance, this paper will both study the physical and the electrochemical properties of the electrode system, and a lot of new characterization methods have been introduced. The study of the physical properties includes the pore structure, adhesive force, wettability, homogeneity; the study of the electrochemical characteristics includes electronic and ionic conductivity of the solid-phase, electronic and ionic conductivity of the liquid-phase, reaction areas and reactive state of electrode during the charging. Finally we creates a simple electrochemical model for LIBs, and all the measured characterization parameters will be applied to the model, we also make experimental cells to test and verify based on the optimal design of simulation output, the performance of the battery capacity and discharge rate have shown excellent results. The main findings are as following:1. By adopting the mercury porosimetry, the pore structure of the electrode including the porosity, pore size distribution and tortuosity has been characterized. In view of the characteristic of electrode, the optimum strategy about the pretreatment of the electrode, pressure of mercury intrusion and breakthrough pressure point has been determined, and the electrode with the different particle size, morphology, particle size distribution has been characterized, the results show the electrode prepared from spherical particles owning the large size and narrow distribution has the superior pore structure which is beneficial to ion diffusion in the electrolyte; the advantage of image method is visible and can be located, as an auxiliary method of the mercury porosimetry, it can accurately describe the pore structure information at any position of the electrode, so it can help researchers to quickly find the key points of improvement about the pore structure. Advanced image method includes the even cross-section preparation of electrode, digitized treatment of the image, in-situ and high-precision FIB-SEM, three-dimensional CT method; constant pressure method to test impedance is not only simple, fast, little error, but also has the closer correlation with the battery, because the longitudinal impedance it measures includes contact resistances between the collector and the particles. Aimed at the wettability of electrode, we developed the surface tension method and liquid drop method, and the surface tension method which is more accurate and flexible to adjust absorption mass and time can help us to fully understand the wettability of the electrode; Determination about gloss and roughness of the electrode can help us to assess the homogeneity of the pole piece.the electrode which has the higher gloss and the lower roughness will show the better uniformity, and two methods could be used together.2. Based on the basic principles of the blocking electrode and the counter electrode, the AC impedance method could be used to measure the electronic and ionic conductivity of the solid/liquid phase, the result of liquid conductivity is in good agreement with the data of the mercury porosimetry; the electrochemical effective area can be measured by the similar AC method, Taking the calibration capacitance element as reference, the optimal frequency and voltage amplitude of effective capacitance are determined, experiments confirmed that the reactive activity of the electrode has a close positive correlation with the applied temperature; by means of making simple modification about the microscope, we have successfully achieved observation of lithium deposition during the reaction procedure, and then evaluate the effect of the current, cycle number and SOC on the electrode state and capacity during the charge procedure.3. The battery model of charging and discharging about LIBs is built based on the dynamics equations, all the measured characterization parameters will be put into the model to calculate the effect of all the parameters on rate discharge performance. Simulation results confirm that the fine pore structure and good electrical conductivity of the solid-liquid phase with the electrode can significantly improve the platform height and slope of voltage curve, and therefore result in the more excellent rate performance. Select the most excellent experimental design based on the experimental conditions to make experimental cells, and evaluation results show that the rate performance and low-temperature performance are improved by 46% and 30% respectively.