The purpose of this study was to understand the electrochemical behavior of the interface between porous electrodes and electrolytes of lithium-ion batteries. We propose a new analytical approach that is a combination of the transmission line model theory for cylindrical pores and electrochemical impedance spectroscopy using symmetric cells. Mathematical model and experimental impedance behavior results agree with each other. By mathematically fitting the experimental impedance plots, the individual internal resistance components of the actual porous electrode/electrolyte interface could be described as the following four parameters: electric resistance (R e ), electrolyte bulk resistance, (R sol ), ionic resistance in pores (R ion ), and charge-transfer resistance for lithium intercalation (R ct ). In actual electrodes, the R ion obtained in this study is a characteristic parameter of the porous electrode/electrolyte interface that is important to consider for thick electrodes.

To understand the relationship between the specific energy and power of lithium (Li)-ion batteries, the dependence of the internal resistance of porous electrodes with high loading weight on thickness was systematically investigated. The ionic resistance in pores (Rion) and chargetransfer resistance for Li intercalation (Rct) normalized per unit electrode geometric area were assessed using a combination of electrochemical impedance spectroscopy with symmetric cells and the transmission line model for cylindrical pores. The changes of Rion and Rct and their magnitude show opposite trends with respect to electrode thickness. For thin electrodes, Rion is lower than Rct. The specific power decreases slightly as the electrodes become thicker because the total internal resistance is predominantly affected by the charge-transfer resistance, and there is no delay of the response in the depth direction. In contrast, for thick electrodes, Rion is higher than or approximately equal to Rct, so there is a delay of the reaction in the depth direction. As a result, the power of the battery is dramatically reduced because the total internal resistance is strongly influenced by both Rion and Rct.

We used a suite of transmission electron microscopy (TEM) and associated electron spectroscopy methods to examine the local structure and changes in the electronic structure of LinormalNi0.8normalCo0.15normalAl0.05normalO2 positive electrode material. We found a scattered rock-salt phase near grain surfaces and grain boundaries, where Ni3+ turned to Ni2+ , deduced from relative intensity ratios and fine structures of the L2,3 white-line peaks of the transition metals. The spatial distribution of the degraded phase throughout the secondary particle was found using a scanning TEM-electron energy loss spectroscopy spectral imaging technique and multivariate analysis. The degradation process and its relationship to the surface reactions with electrolytes is discussed based on the spatial-distribution map of the degraded phases.

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Factors affecting the cycling life of cylindrical lithium-ion batteries of LiNi0.8Co0.15Al0.05O2(NCA) with graphite were examined in terms of the rechargeable capacity and polarization of NCA derivatives of LizNi0.8Co0.15Al0.05O2−δ(0.8 ≤z≤ 1.05).