Einfluss der Kompaktierung auf die Elektrodenmikrostruktur und elektrochemische Performance bei Lithium-Ionen-Zellen

Abstract

Due to their technological advantages, lithium-ion-batteries have been established in stationary and mobile applications. In general, for electrochemical storage systems, there is a fundamental conflict between high energy density and temporal high power density for different applications. During the electrode production process, the material- and process-related parameters define the resulting cell properties. In the last step of the electrode manufacturing, the compaction process, a high volume fraction of active material is targeted at the expense of internal porosity. In contrast, porosity is essential for the cell functionality, especially for kinetic requirements. Thus, tailoring the microstructure of the electrode active material results in an effcient design for different applications. The present work aims at investigating and correlating the microstructural changes by active mass compaction with the resulting electrochemical properties. For technologically favorable electrode active mass compositions, a wide compaction down to porosities of less than 20% was considered and electrochemically cycled between C/20 and 5C to map different applications. Microstructural analyses were used to determine the influence of compaction on the active material particles within the active mass and on the binder/additive phase. In addition to the change in pore size distribution, the studies give insight on the freely accessible surface of the active material particles within the porous active mass, the number of potential near-surface pores and pore openings with access to deeper lying areas and interactions on active mass components due to the external force application depending on the degree of compaction. The electrochemical properties of variously compacted LiNi1/3Co1/3Mn1/3O2 (NCM) and graphite electrodes were determined in half-cells with a Li/Li+ counter electrode and in fullcell configuration with a graphite or NCM counter electrodes of constant porosity to finally correlate them with the active mass microstructure. From the performed capacities, the specific energy vs. specific power (gravimetric consideration) and the energy density vs. power density (volumetric consideration) were displayed in Ragone plots. By using this result, an optimal porosity of the electrodes and, at the same time, an effcient trade-off of the target variables energy and power was achieved. Electrochemical long-term cycling also provided insights into the limitations of the electrode active masses depending on the degree of compaction in different applications. Focusing on the manufacturing parameters and microstructure of the active mass coating of lithium-ion cells, the present results give guidelines for the design optimization of electrodes for energy cells and power cells as well.