EC2.1: Interfaces and Impedance
Monday AM, November 28, 2016
Sheraton, 2nd Floor, Back Bay B
9:15 AM - *EC2.1.01
Simulating Diffusional Impedance Behavior of Polycrystalline Electrode Particles Using the Smoothed Boundary Method
Min-Ju Choe 1 , Hui-Chia Yu 1 , Ping-Chun Tsai 2 , Bohua Wen 2 , Yet-Ming Chiang 2 , Katsuyo Thornton 1
1 University of Michigan Ann Arbor United States, 2 Massachusetts Institute of Technology Cambridge United StatesShow Abstract
Microstructures strongly affect transport phenomena in electrode materials and their performance. However, explicit consideration of the complex structures such as the grain boundary network poses a challenge in simulations. In this work, we develop an innovative method for incorporating the surface and interface diffusion with the bulk diffusion based on the smoothed boundary method. Complex grain structures are described using multiple domain parameters, where the value of domain parameter is uniformly one inside the corresponding grain, and zero outside. As such, the grain boundaries are implicitly defined by the transition regions of domain parameters, and the diffusion equations can be straightforwardly solved on a standard Cartesian grid system. This method is applied to simulate the diffusional impedance of polycrystalline electrode materials. The simulations show that both grain boundary diffusivity and grain size affect the diffusional impedance. With high grain boundary diffusivities, the concentration of ions along the grain boundaries is similar to that at the electrode particle surfaces, such that radial diffusion within each primary particle dominates the impedance behavior. Conversely, with low grain boundary diffusivities, the overall radial diffusion in the secondary particle determines the impedance behavior. In coordination with the modeling effort, electrochemical impedance spectroscopy measurements of polycrystalline single secondary cathode particles are conducted. The simulations and experiments are combined to reveal the impact of interfacial transport on electrode materials’ performance. In turn, the insight gained through this work opens a new avenue for extracting microstructural characteristics (e.g., the grain size) from its impedance behavior.
9:45 AM - EC2.1.02
Inhomogeneous Conductivities in Li7La3Zr2O12 Ceramics Investigated by Spatially Resolved Impedance Spectroscopy and Elemental Analytics
Andreas Wachter-Welzl 1 , Julia Kirowitz 1 , Reinhard Wagner 2 , Stefan Smetaczek 1 , Maximilian Bonta 1 , Daniel Rettenwander 3 , Stefanie Taibl 1 , Georg Amthauer 2 , Andreas Limbeck 1 , Jurgen Fleig 1
1 Vienna University of Technology Vienna Austria, 2 Chemistry and Physics of Materials University of Salzburg Salzburg Austria, 3 Massachusetts Institute of Technology Cambridge United StatesShow Abstract
Current Li-ion batteries suffer from problems caused by the chemical instability of their organic electrolyte. Therefore a lot of research focuses on replacing the organic electrolyte by inorganic solid ion conductors. Cubic Li7La3Zr2O12 (LLZO) garnets and its variants are among the most promising candidates for next generation all solid state Li-ion batteries [1,2]. They provide a high Li-ion conductivity and combine chemical and electrochemical stability. One crucial aspect is the doping of the material, in order to stabilize its cubic phase but also in terms of diffusion paths and mobile defects. Different dopants have been investigated, but the specific effect of each dopant, the importance of the exact Li-ion stoichiometry, as well as degradation phenomena are still not completely understood. For nominally identical dopant content, for example, rather different conductivities were reported.
In this contribution, we present a combined study of electrochemical impedance spectroscopy (EIS) and elemental analysis (inductively coupled plasma mass spectrometry, ICP-MS) has been used to determine the effects of varying lithium and dopant content on the Li-ion conductivity. The roles of sintering temperature and preparation procedure, but also effects of the sample dimension are considered and reasons behind severe variations of effective Li-ion conductivities are discussed. Besides overall Li-ion conductivity measurements using blocking electrodes and analysis of the bulk composition, measurements on microelectrodes of different sizes (20 – 300 µm) were performed to obtain information on local Li-ion conductivities. Within one and the same sample, conductivity variations up to almost one order of magnitude were found. Laser ablation (LA) ICP-MS was employed to obtain local information on the exact composition and thus of stoichiometric variations in the samples. For example, a correlation between spatially varying Al content and local conductivities was found, with higher conductivities primarily in the outer regions of the ceramics. These studies provide a deeper understanding for the variation of Li-ion conductivity in publications on nominally similar samples. Furthermore they help in finding optimal dopant contents, Li contents and preparation procedures of LLZO.
 Murugan, R.; et al.; Angew. Chem. 2007, 119, 7925-7928
 Thangadurai, V.; et al.; Chem. Soc. Rev.