Moses Ender

Studies on LiFePO4 as Cathode Material in Li-Ion Batteries

Lithium iron phosphate is a promising cathode material for the use in lithium-ion batteries meeting the demands of good stability during cycling and safe operation due to reduced risk of thermal runaway. However, slow solid state diffusion and poor electrical conductivity reduce power capability. For further improvement, the identification of the rate determining processes is necessary. Electrochemical impedance spectroscopy (EIS) has proven to be a powerful tool for the characterization of electrochemical systems. In this contribution a deconvolution of the impedance with the distribution of relaxation times (DRT) is used to obtain a better resolution in frequency domain. Therewith, the cathodic and anodic polarization processes are identified and an impedance model for the cell is proposed. Introduction The demand for high energy density rechargeable batteries for electric and hybrid electrical vehicle systems has rised particular interest in lithium-ion batteries in recent times. Many kinds of material are in consideration as the cathode material to meet the requirements of a satisfying cathode performance (1). In 1997, Lithium iron phosphate (LiFePO4) was proposed by John B. Goodenough’s group for the first time (2). It has been investigated as a promising candidate for cathode material in lithium-ion batteries due to its high theoretical capacity (170 mAh/g) compared with other iron-based compounds (3), its good thermal stability leading to high safety (3;4), and its cycling stability (5). One major drawback of lithium iron phosphate is its poor conductivity (ionic and electronic), which generally causes capacity losses at higher charge–discharge currents (6;7). Recently, many studies on lithium iron phosphate have been published, mainly focused on the understanding and improvement of lithium-ion conduction in the active material (8;9;10). However, there are other loss processes which influence the performance of LiFePO4-cells (11). Their identification and physical interpretation according to literature is not consistent. EIS has proven to be a powerful tool for the characterization of electrochemical systems (3;11;12). In general, physio-chemical processes with different time constants can be distinguished in the EIS spectra. However, the identification of these processes remains ambiguous as they overlap in the frequency domain. ECS Transactions, 28 (30) 3-17 (2010) 10.1149/1.3505456

Transient 3D FEM Impedance-Model for Mixed Conducting Cathodes

The performance of solid oxide fuel cells (SOFCs) is often determined by the polarization resistance of the electrodes. Electrochemical impedance spectroscopy (EIS) enables a deconvolution of individual electrochemical processes. In case of mixed ionic-electronic conducting (MIEC-) cathodes the impedance spectra result from the coupling of gas diffusion, surface exchange and bulk diffusion of oxygen ions. In this paper we present a three-dimensional (3D) finite element method (FEM) model which allows the transient simulation of the underlying processes in a porous cathode structure. The developed model is validated with a well established homogenized 1D model by comparing the area specific resistance and the corresponding impedance spectra. In case of a homogeneous 3D microstructure the FEM simulation results show an excellent agreement with the homogenized 1D model. Furthermore, the 3D FEM model is applied for impedance simulations of a technical MIEC cathode which microstructure was reconstructed from FIB tomography.