Lithium-ion batteries (LIB) are currently one of the most important systems for energy storage in mobile and stationary devices. One of the main challenges in LIB research is to increase both, the cells energy and power density without sacrificing the cells cyclic stability.
The overall cell performance is strongly determined by the microstructure of the electrodes. Although Li-ion electrodes are a common subject of research, there is still a lack of knowledge about how the cell performance quantitatively depends on the electrodes’ microstructure. Porosity, which is mainly determined by the calendaring process, is one main parameter that has to be taken into account. Low porosities are sought to increase the energy density of the cells but have a negative effect on their rate capability. The aim of this study was twofold: First, we investigated the correlation between the densification of active mass, the resulting microstructure (porosity), and its electrochemical properties such as rate capability and ageing of assembled lithium-ion cells. Secondly, we did an experimental study on how the current load sequence affects performance and ageing of the cells.
The active mass coating consisted of LiNi1/3Co1/3Mn1/3O2 (NCM 111), PVdF and conductive carbon. One-side coated Cathode foils were uniaxial compacted with up to 1000 MPa to achieve various porosities and layer thicknesses. Samples of these foils were assembled to 2-electrode Swagelok®-type cells for electrochemical analyses while the graphite anode remained constant. Electrochemical cycling tests were run between 4.2 V and 2.5 V for up to 83 cycles featuring a variety of C-rates from 1C to 5C. To investigate the current load sequence and its effect on ageing we altered the C-rates during cycling between 1C and 5C – and we also altered the sequence of C-rates for different test series.
Some of the main findings of the study are: The ideal porosity for a high capacity density can be found between 18% and 35% depending on the C-rate. The rate capability significantly decreases with an increasing C-rate and densification. The current load sequence (1C-2C-5C vs. 5C-2C-1C) strongly influences the ageing behavior of the cells.