One of the most commonly used cathode active materials (CAM) are lithium nickel manganese cobalt oxides often referred to NMC with the general formula of Li[NixMnyCoz]O2. In this work, polycrystalline and single-crystalline NMC811 active materials are used to investigate the influence of the particle morphology on the evolution of SOC heterogeneities. Therefore, LIB pouch cells were cycled from 3.0 to 4.4V at 0.5C with artificial graphite as the anode active material and one of the specified NMC materials as the CAM. After electrochemical aging, the cells are opened followed by separation of the electrodes and subsequent delamination of the CAM. This enables CAM particle analysis using single-particle inductively coupled plasma optical emission spectroscopy (SP-ICP-OES) to measure the Li/Ni intensity ratio of individual particles that are introduced into the plasma. Based on the measured Li/Ni intensity ratios, the SOC of the particles can be estimated using an external matrix-matched calibration.
In doing so, it could be demonstrated that the polycrystalline NMC811 material exhibits a higher degree of heterogeneity in the SOC compared to the single-crystalline active material after cycling. These results may be attributed to the different characteristics of the active materials as polycrystalline materials are prone to particle cracking of the secondary particles due to volume changes during cycling. In contrast to this, single-crystalline NMC materials provide better surface stability due to higher calcination temperatures and heavily reduced particle cracking.