Transport and Kinetic Phenomena Linked to Power Performance of Lithium-Ion Batteries.

[摘要] While energy density of a Li-ion cell depends on the choice of Li-ion active materials, power density depends on cell design and a set of well-balanced transport and kinetic material properties. Furthermore, the Li-ion cell rate capability improvement often comes at the expense of underutilizing available energy due to safety and cycle-life constraints. Hence, in this study, various transport and kinetic phenomena occurring inside a cell are examined to optimize the cell power performance.Lithium-ion battery active materials are polycrystalline consisting of crystallites of varying size and orientation separated by grain boundaries. To investigate the grain boundary influence on battery performance, a single polycrystalline particle Li-ion cell model is developed. A Voronoi grain size distribution is employed in generating polycrystalline particles. Under galvanostatic and potentiodynamic cycling conditions, intercalation-induced stress, effective Li+ diffusivity, and capacity utilization are examined. It is found that the effective Li+ diffusivity is highly correlated with the grain boundary density while the maximum intercalation-induced stress depends on both the grain boundary density and the network structure. In addition, the particle capacity utilization improves with increasing grain boundary density, especially at high C-rates.During cycles, many Li-ion active materials undergo a volumetric strain that may cause the material to fracture. On the other hand, the stress field has a benefit of enhancing Li+ diffusivity inside active materials. To estimate the intercalation-induced stress level, an in-situ AFM system is utilized in measuring particle morphological changes during cycles. Furthermore, a numerical method is used to quantify the Li+ diffusivity enhancement caused by the intercalation-induced stress field.The rate capability of a Li-ion cell depends on multiple transport and kinetic phenomena occurring inside the cell, and the rates at which such phenomena occur depend on cell material properties. To understand how a cell electrochemical dynamic response changes with material properties, a sensitivity analysis of transport and kinetic parameters on cell performance is performed. It is found that different types of material properties have a significant influence on specific parts of a cell operating potential profile. Moreover, given a set of material properties, associated overpotentials are quantified.

[发布机构] University of Michigan

[效力级别] Mechanical Engineering

[关键词] Li-ion Battery Rate Capability;Mechanical Engineering;Engineering;Mechanical Engineering