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Dynamical investigation of tunable magnetism in Au@Ni-carbide nanocrystals by a combined soft and hard x-ray absorption spectroscopy

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Nano Research 2022年第5期15卷 4320-4326页

作者： Weifeng Huang Jianxin Kang Tingwen Chen Dawei Pang Lihua Wang Hang Wei Changchun Yang Dongfeng Zhang Lin Guo College of Chemistry and Chemical Engineering Inner Mongolia UniversityHohhot 010021China School of Electronic Engineering Jiujiang UniversityJiujiang 332005China School of Chemistry Beihang UniversityBeijing 100191China Institute of Microstructure and Property of Advanced Materials Beijing University of TechnologyBeijing 100124China

Nickel based magnetic nanocrystals have been widely applied in magnetic and catalytic facilities.Tunable magnetic properties of nickel can be easily obtained via non-magnetic doping or phase transformation.However,phase transformation from face centered cubic(fcc)to hexagonal close packed(hcp)induced magnetism adjustment of Ni are always confused with nickel carbide(Ni_(3)C),due to the similar atomic structures of hcp-Ni and Ni3C.Here,we present series of Au@Ni-carbide magnetic materials achieved from the controlled carbonation of Au@Ni core-shell structures,whose magnetism is tunable by adjusting the amount of carbon in the Ni layer.Ex-situ hard x-ray absorption spectroscopy(xAS)at the metal K edge and soft xAS at both metal L edge and carbon K edge provide solid evidence for the carbonation process from fcc-Ni to Ni_(x)C,rather than phase transformation to hcp-Ni.Further investigation reveals that the magnetism of the hybrids is mainly contributed from the residual fcc-Ni.The result represents an accurate and effective way to distinguish hexagonal Ni_(3)C from hcp-Ni,and provides the pathway to control magnetism of Ni-based materials for applications.

关键词： x-ray absorption spectroscopy soft x-ray absorption spectroscopy carbide nickel carbonation process magnetism

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Transition-metal redox evolution and its effect on thermal stability of LiNixCoyMnzO_(2) based on synchrotron soft x-ray absorption spectroscopy

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Journal of Energy Chemistry 2021年第8期30卷 446-454,I0010页

作者： Chen Liang Wenhua Zhang Zesen Wei Zhaoyu Wang Qingsong Wang Jinhua Sun State Key Laboratory of Fire Science University of Science and Technology of ChinaHefei 230026China National Synchrontron Radiation Laboratory University of Science and Technology of ChinaHefei 230026China

Based on the synchrotron soft x-ray absorption spectroscopy experiments,the fundamental electronic structures of layered Li NixCoyMnzO_(2)(NCM)are investigated systematically and the data of transitionmetal(TM)L-and O K-edges spectra are collected.Distribution of Ni ions under different oxidation states is evaluated according to linear combination fit.It is found that the ratio of Ni^(4+)expands with the increase of Ni since it dominates in charge compensation during charging,and that the existence of Ni^(3+)is nearly negligible in delithiated NCM.The valence state of Co also strongly depends on Ni content,the perceptible position shift of Co L_(3)-edge absorption peak towards higher energy in Ni-rich material agrees well with the small voltage plateau at around 4.2 V.The stability of Mn is verified as no obvious spectral change with the Mn L-edge is observed.Moreover,as Ni content rises,the O 2p holes near the Femi level increases with higher oxidation state of Ni,indicating the enhanced hybridization of O 2p-TM 3 d.Delithiated NCMs with higher Ni content are prior to lose electron existing in highly hybridized Ni3 dO 2 p bands upon heating,which accounts for the pronounced O_(2)release in phase transitions and the deterioration in thermal stability.These detailed observation of the electronic structure evolution is one of the key ingredients to improving the electrochemical and thermal performance of NCM.

关键词： Lithium ion battery safety Cathode material soft x-ray absorption spectroscopy Thermal stability

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Insight into the structural evolution and thermal behavior of LiNi_(0.8)Co_(0.1)Mn_(0.1)O_(2) cathode under deep charge

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Journal of Energy Chemistry 2022年第2期31卷 424-432页

作者： Chen Liang Lihua Jiang Zesen Wei Wenhua Zhang Qingsong Wang Jinhua Sun State Key Laboratory of Fire Science University of Science and Technology of Chin1Hefei 230026AnhuiChina National Synchrotron Radiation Laboratory University of Science and Technology of Chin1Hefei 230026AnhuiChina

By virtue of the crucial effect of the crystal structure and transition metal(TM)redox evolution on the performance of LiNi_(x)Co_(y)Mn_(z)O_(2)(NCM)cathode,systematical investigation is carried out to better understand the charge mechanism upon deep charging.Based on the results of x-ray diffraction and highresolution transmission electron microscope,phase transformations existing on particle surface are promoted by high potential because of the deeper lithium vacancies,accompanied by more substantial structure instability.soft x-ray absorption spectroscopy indicates that Ni acts as the major contributor to charge compensation while Co displays a remarkable redox activity over the deep charge range.The elevated integrated intensity of pre-edge in O K-edge spectra reveals the extensive amount of holes formed in O 2 p orbitals and the enhanced hybridization of TM 3 d-O 2 p orbitals.Considering the close relationship between thermal behavior and structural evolution,the tendency of phase transitions and O_(2) release upon heating is accelerated by voltage rise,demonstrating the aggravated instability due to deeper Li utilization.Remaining Li contents in NCM are employed to estimate the amount of oxygen released in structural transformation and its detrimental effect on stability declares Li contentdependent characteristics.Furthermore,the extended Li vacancies,higher proportion of Ni4+and stronger orbital hybridization are considered as three factors impeding the thermal stability of the highlydelithiated NCM.

关键词： Lithium ion battery safety Overcharge soft x-ray absorption spectroscopy Thermal property Cathode material

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