What is the phase separation in the positive electrode of high voltage spinel lithium ion battery

　　High-voltage positive lithium-ion batteries represented by spinel lithium nickel manganese oxide have the advantages of high capacity, low cost, low environmental hazard, and strong safety, and are therefore recognized by the battery industry. ·

　　The high-voltage positive lithium-ion battery represented by spinel lithium nickel manganese oxide has the advantages of high capacity, low cost, low environmental hazard and strong safety, and has been recognized by the battery industry. From the perspective of basic theory, a deep understanding of phase separation in solid-state electrodes is of great significance to fundamentally solve the inherent stability defects of this type of material. From the perspective of practicality, studying the behavior of phase separation in actual porous composite electrodes and matching it with the size effect, crystal face adjustment and surface passivation film of lithium nickel manganese oxide is a combination of basic research and actual The ideal way to combine applications. However, this idea can only be realized with advanced characterization methods.

　　Dr. Zhou Jigang of the Canadian Light Source Energy Storage Group, Dr. Wang Jian of the Chemical Imaging Line Station, and Associate Professor Lu Mi of Xiamen University of Technology and Professor Fang Haitao of Harbin Institute of Technology have worked closely together to innovatively have element and orbit selectivity, chemical and electronic structure sensitivity Transmitted X-ray scanning microscopy (STXM) is used to study the behavior of phase separation in porous electrodes. For the first time through this work, the researchers realized the nano-level visualization of the correlation of multiple phase separation phenomena after a complex composite electrode was cycled and stored for a long time. The electrode after phase separation showed unevenness beyond prediction. This inhomogeneity is closely related to spinel size, crystal plane structure, and surface passivation.

　　This study found for the first time that the inhomogeneity of phase separation, which is traditionally believed to exist only during rapid charging and discharging, can be obtained under approximately steady-state reaction conditions. This discovery is of great significance for further deepening the understanding of the important electrode process of phase separation. This method can be extended to other electrode systems to study the reaction mechanism and attenuation mechanism.

　　"Phase separation imaging" uses a single-phase spectral decomposition fitting for the absorption spectrum of each pixel unit. This work is based on PrincipalComponent Analysis (PCA) to obtain each single phase (Ni4+ corresponds to the fully charged phase, Ni3+ corresponds to the partially reduced phase, and Ni2+ corresponds to the fully reduced phase). The use of PCA can avoid the human error introduced by the external standard spectrum, so the obtained phase separation imaging is more accurate and reliable. Figure c is the three corresponding Ni absorption spectra obtained using PCA. The high degree of unevenness of phase separation is well reflected in Figure b. It can be seen that the phase separation is firstly uneven in the electrode thickness direction. Secondly, this kind of phase separation exists in a single electrode particle, and the distribution between different electrode particles is different, and the morphology and size of the particles have an effect on the phase separation.

　　The author found that most of the completely reduced phase (Ni2+) exists on the surface of the electrode particles, and only one complete electrode particle has the Ni2+ phase, but its spectrum is different from the Ni2+ phase formed on the surface of the electrode particles. This result indicates that the Ni2+ on the surface should be associated with the surface passivation layer generated during the charge and discharge cycle of the high-voltage positive electrode. Through further observation, they found that the surface passivation layer of the large particle electrode can perfectly protect the fully charged phase inside the electrode particle. It needs to be emphasized again that this inhomogeneity of phase separation is obtained under slow experimental conditions, so the thermodynamic behavior of the reaction should be related to the intrinsic properties of the electrode particles, including surface defects and element segregation. Related work was published on Chemical Communications.