Electric vehicles represented by Tesla compete for NCA, NCM811 or NCM622 high-nickel ternary materials as cathode materials for lithium-ion batteries. However, this high-nickel layered cathode material has safety problems, and the Canadian Light Source Energy Storage Group Dr. Zhou Wei and Dr. Wang Jian from the Chemical Imaging Line Station and Associate Professor Lu Mi from Xiamen University of Technology, for the first time, imaged the phase distribution before and after the thermal runaway of complex composite electrodes, and separated the phases before and after thermal runaway. Correlation was performed at the nanometer level and it was found that thermal runaway may be closely related to the distribution of conductive agents and binders.
Before the thermal runaway, the conductive agent and the binder are uniformly mixed in a coexisting agglomeration mode, but the agglomeration is uneven on the surface of the lithium cobaltate particles and between the particles. The thermal decomposition of PVDF is obvious after thermal runaway, while the conductive carbon black is unevenly distributed on the surface of lithium cobaltate in the form of agglomeration. PEEM can achieve a spatial resolution of 100 nm and can image a 50 um electrode surface. High spatial resolution and large imaging intervals enable high resolution imaging of multiple particles. The morphology of the lithium cobaltate particles before and after thermal runaway can be used to study the thermal runaway behavior of the same electrode particles.
Figure 1. Schematic diagram of element distribution and correlation after (a, b) (c, d) before thermal runaway
Phase separation imaging is a spectral decomposition of the absorption spectrum of the cobalt element in each pixel unit using a single phase, including Co2+ (phase formed by thermal runaway release of oxygen), Co3+ (LCO) or Co3.5+ (normally charged LCO). Hehe. The height non-uniformity of phase separation is well represented in Figures c and d. If the phase separation diagram is mapped to the obtained elemental distribution map, it can be seen that this phase separation has a great correlation with the distribution of the conductive carbon black before and after the thermal runaway. Thermal runaway significantly reduces the size of the phase separation, which is different from the previous studies in the thermal runaway imaging study of chemically charged non-identical electrode particles. This work is performed on the thermal runaway of the same particle in the actual electrode. The effect of electrode particle morphology, size and crystal plane orientation on phase separation is much smaller than that of particle environment, especially the influence of conductive agent.
Figure 2. Phase separation imaging and imaging spectroscopy before (a, c) (b, d) before thermal runaway
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