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Analysis of thermal runaway of lithium battery anode material

Lithium ion batteries with high nickel laminar positive electrode represented by NCA, NCM811 or NCM622 have the advantages of high capacity, low cost and small environmental harm. At present, electric cars represented by tesla are in competition with each other. However, the positive electrode with a high nickel layer has safety problems, especially the decomposition and release of oxygen from the low-layer material at high temperature will cause thermal runaway, which will lead to the combustion explosion of the battery. From the perspective of basic theory, it is of great significance to understand the phase separation of solid electrode under thermal runaway to fundamentally solve the intrinsic stability defect of this kind of material.
From the perspective of practicality, it is an ideal method to combine basic research with practical application to study the behavior of phase separation in the actual porous composite electrode and to correlate it with the size effect of positive electrode material, crystal surface regulation and surface passivation film. However, this assumption can only be realized by means of advanced characterization.
Canadian light source energy storage group Zhou Ji Gang Dr Dr Wang and chemical imaging line stand and associate professor at xiamen university of science and technology road close close cooperation, will be innovative elements and selective orbit, chemical and electronic structure sensitivity of transmission X-ray scanning microscopy (PEEM) used to study the thermal runaway of cobalt acid lithium layer electrode particles in the phase separation behavior in the porous electrode. The work highlights the research in the form of Chemical Communications back cover.
Through in-situ study, the phase distribution of complex composite electrode before and after thermal runaway was imaged at the particle level of a single electrode for the first time, and the correlation of various phase separation phenomena before and after thermal runaway was visualized at the nanometer level. The phase separation before and after thermal runaway presents an unpredicted heterogeneity at the particle level of a single electrode. The inhomogeneity is not obviously related to grain size and grain surface structure, but it is closely related to the distribution of conductive agents and adhesives.
This is the first time to realize nano-visualization of phase separation of the same particle before and after thermal runaway, and to correlate it with its electrode environment. This method is of great significance for further understanding the thermal runaway behavior of layered materials and is suitable for other electrode systems to study the reaction mechanism and attenuation mechanism under thermal runaway.
Firstly, the element sensitivity of PEEM was used for nano-level imaging of electrode components, including lithium cobalt oxide, PVdF and the distribution of conductive carbon black.
Before the heat loss control, the conductive agent and binder were mixed uniformly in a co-existing agglomeration pattern, but the distribution of such agglomeration on the surface of lithium cobalt oxide particles and among particles was uneven. The thermal decomposition of PVdF was obvious after thermal runaway, while the conductive carbon black was still distributed unevenly on the surface of lithium cobalt oxide in the form of agglomeration. PEEM can achieve a spatial resolution of 100nm and image an electrode surface of 50um. High spatial resolution and large imaging range realize high resolution imaging of multiple particles. The imaging of the morphology size of lithium cobalt oxide particles before and after thermal runaway can be used to study the thermal runaway behavior of the same electrode particles.
Phase separation imaging USES a single phase for the absorption spectrum of cobalt elements in each pixel unit, including the spectral decomposition fitting of Co2+(the phase formed by runaway heat release of oxygen), Co3+(LCO) or Co3.5+(normally fully charged LCO). The high inhomogeneity of phase separation is well illustrated in figure c and d. If the phase separation diagram is corresponding to the obtained element distribution diagram, it can be seen that this phase separation has a great correlation with the distribution of conductive carbon black before and after thermal runaway. Thermal runaway significantly reduced the size of the phase separation, and in literature on the same electrode particles is obtained by imaging studies chemical charge thermal runaway of the conclusion, this work for real electrode electrochemical charging the same particle thermal runaway, shows that the electrode shape, particle size and crystal orientation are far less particle environment's influence on the phase separation, especially the influence of conductive agent.


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