A new semi-quantitative method was developed to detect Li deposition on Si/graphite anodes. This method is based on Post-Mortem glow discharge optical emission spectroscopy (GD-OES) depth profiling. Based on the contents of Si, Li, and O in the GD-OES depth profiles, we define a corridor, in which the minimum amount of metallic Li on the anode is located. This method was applied to three types of commercial 18650 cells with Si/graphite anodes in the fresh state and with Li plating intentionally produced by cycling at low temperatures. Additional cells were cycling aged at 45 °C to 80% SOH. The main aging mechanisms at 45 °C were determined using differential voltage analysis (DVA), SEM, and half cell experiments. Subsequently, the cells aged at 45 °C were further cycled under the conditions that had led to Li deposition for the fresh cells. Furthermore, the anode coating thickness for 18 types of commercial Li-ion cells are correlated with the specific energy, while distinguishing between graphite anodes and Si/graphite anodes. Our extensive Post-Mortem study gives deep insights into the aging behavior of state-of-the-art Li-ion cells with Si/graphite anodes.

A method based on glow discharge optical emission spectroscopy (GD‐OES) depth profiling is developed to detect copper deposition on graphite electrodes for the first time. Commercial 18650 cells with graphite anodes were subject to Cu dissolution by over‐discharge to 0 V. On a first approach, the depth profiles for Cu show significant differences for over‐discharged cells compared to a baseline graphite electrode from cells discharged to the end‐of‐discharge voltage. An accumulation of Cu is found on the anode surface by GD‐OES, which is consistent with SEM and EDX. The trend of the total Cu amount is compared with ICP‐OES measurements.

Cu dissolution in Li-ion cells during over-discharge to 0 V was investigated by Post-Mortem analysis. Commercial 18650 type cells with graphite anode and NMC/LMO cathodes as well as pilot-scale pouch full cells with graphite/NMC chemistry with reference electrode were investigated. The effects of discharge time at 0 V in the range of 100 h to 1000 h for fresh cells as well as the effect of cells cycled under Li deposition conditions were considered. For comparison, electrodes from cells discharged to the end-of-discharge voltage (2.0 V) were examined. By extensive Post-mortem analysis using inductively coupled plasma (ICP-OES), scanning electron microscopy (SEM) with BSE and SE detectors, energy dispersive X-ray analysis (EDX), and glow discharge optical emission spectroscopy (GD-OES), we show that Cu compounds are present on the anode surface and on the cathode from cells, which were over-discharged. Cross-sections show that the Cu originates from pitting corrosion of the negative current collector. Combined electrochemical/ICP-OES measurements in commercial cells as well as reference electrode measurements in 3-electrode pouch full cells suggest that Cu is dissolved as Cu+ ions.

Mixing graphite with Si particles in anodes of Li-ion batteries provides increased specific energy. In addition, higher Si contents lead to thinner anode coatings at constant areal capacity. In the present study, we systematically investigated the influence of the Si content on the susceptibility of Li plating on Si/graphite anodes. Si/graphite anodes with Si contents from 0 to 20.8 wt% combined with NMC622 cathodes were manufactured on pilot-scale. After initial characterization in coin half cells and by SEM, pouch full cells with fixed N/P ratios were built. Rate capability at different temperatures, and Post-Mortem analysis were carried out. Results from voltage relaxation, Li stripping, SEM measurements, glow discharge optical emission spectroscopy (GD-OES) depth profiling, and optical microscopy were validated against each other. A decreasing susceptibility to Li plating with increasing Si content in the anodes could be clearly observed. A critical C-rate was defined, at which Li plating was detected for the first time. It was also found that at 0 °C the critical C-rate increases with increasing Si contents. At 23 °C the SOC at which Li dendrites were first observed on the anode also increased with higher Si content.

Nowadays, long charging times have become one of the main limitations to a greater worldwide spread of electric vehicles (EV). Enabling high C-rates charging is a promising approach to eliminate this problem and alleviate range anxiety. When a battery is charged at high currents, several factors have to be taken into account. Temperature is certainly a key parameter because when it is too high this can lead to degradation of components (binder, electrolyte, active material, etc), however, when it is too low intercalation kinetics becomes sluggish. Using 3D-printed testing plates (PP3D plates) with Li-reference electrode, we developed a tool for electrochemical investigations of pouch cells. These plates enabled to build a new well-designed 3-electrode pouch cell. This setup allows the identification of the best high C-rate cycling procedure to improve the performance and cycling life of the lithium ion cells. We explored the electrochemical behavior of NMC811 cathodes and graphite anodes, during high discharge C-rates test up to 7 C and charge C-rates up to 2 C. Moreover, the temperature influence on charging performance and longtime cycling stability is investigated. The cells cycled at 25 °C using optimized procedures reached an 80% state of health after more than 1000 cycles.