Application Of In-situ Magnetic Testing In Magneto-electrochemistr

Energy and information technologies are two important driving forces for the development of human society,among which electrochemical energy storage and spintronic devices are considered as the key to achieve continuous breakthroughs in the above two fields respectively.Electrochemical energy storage is a field of manipulating and storing charges through electrochemical process.Spintronics aims at studying the dual properties of electrons（charge and spin）to achieve mutual modulation between electron transport and magnetic correlation effects.Thus energy storage and spintronics devices both are essentially manipulating electrons.The magnetoelectric coupling in spintronics provides an idea for the characterization of electrochemical energy storage mechanism,which enables us to analyze electrochemical behavior directly from the unique point of view of charge through magnetometry.Meanwhile,the method of charge manipulation in electrochemical energy storage opens up a new way for magnetoelectric control in spintronics.The intersection of spintronics and electrochemistry was clearly more beneficial to each other’s development,and“magneto-electrochemistry”was born under the influence of this idea.Due to the unique electronic structure and magnetic properties,3d transition metal-based materials are widely used in electrochemical energy storage and spintronic devices,which lays a foundation for the application of magnetometry in magneto-electrochemistry.The operando magnetometry system we built independently can truly explore the dynamic process of charge transfer in complex environments,help us analyze the electrochemical mechanism from a more essential perspective,and further provide ideas for the design of spintronic devices.In this paper,based on the concept of magneto-electrochemistry,some key issues in electrochemical energy storage and magnetoelectric control are systematically studied by operando magnetometry.The specific research contents include the following four parts:1.Revealing the electrochemical conversion mechanism of FeS₂ sodium-ion batteries by operando magnetometry.In spite of the excellent electrochemical performance in lithiumion batteries（LIBs）,transition-metal compounds usually show inferior capacity and cyclability in sodium-ion batteries（SIBs）,implying different reaction schemes between these two types of systems.Herein,coupling operando magnetometry with electrochemical measurement,we peformed a comprehensive investigation on the intrinsic relationship between the ion-embedding mechanisms and the electrochemical properties of the typical Fe S₂/Na（Li）cells.Operando magnetometry together with ex-situ transmission electron microscopy（TEM）measurement reveal that only part of Fe S₂ is involved in the conversion reaction process,while the unreactive parts form“inactive cores”that lead to the low capacity.Through quantification with Langevin fitting,we further show that the size of the iron grains produced by the conversion reaction are much smaller in SIBs than that in LIBs,which may lead to more serious pulverization,thereby resulting in worse cycle performance.The underlying reason for the above two above phenomena in SIBs is the sluggish kinetics caused by the larger Na-ion radius.Our work paves a new way for the investigation of novel SIB materials with high capacity and long durability.2.“Space charge”hydrogen storage mechanism proved by operando magnetometry.A“space charge”hydrogen storage mechanism is proposed and experimentally investigated in Ni/Ti O₂ multilayers in which H⁺is accommodated on the Ti O₂ side,while e-is stored on the side of Ni.The process of electron transfer on Ni surface during hydrogen loading/unloading was investigated by operando magnetometry,and the results are consistent with those of near-atmospheric X-ray photoelectron spectroscopy.Thermal desorption-mass spectroscopy results show that after loading with H₂,Ni/Ti O₂ exhibits an extra desorption peak,which is in contrast to Ni nanoparticles or Ti O₂ alone,indicating a synergistic hydrogen storage effect due to the presence of both phases.These crucial results open up a new way for the safe storage of hydrogen,and provide a powerful tool for researching the electron transfer behavior of H at heterogeneous interfaces in the hydrogen evolution catalytic process.3.Magnetoelectric coupling in ferromagnetic metals Fe,Co and Ni based on space charge mechanism.Ferromagnetic metals show great prospects in ultralow-power-consumption spintronic devices,due to their high Curie temperature and robust magnetization.However,there is still a lack of reliable solutions for giant and reversible voltage control of magnetism in ferromagnetic metal films.Here,we propose a novel space-charge approach which allows us to achieve a non-volatile modulation of 30.3 emu g-1 under 1.3 V in Co/Ti O₂ multilayer granular films.The robust endurance with more than 5,000 cycles was demonstrated.Similar phenomena exist in Ni/Ti O₂ and Fe/Ti O₂ multilayer granular films,which shows its universality.The magnetic change of 107%in Ni/Ti O₂ underlines its potential in voltage-driven ON-OFF magnetism.Such giant and reversible voltage control of magnetism can be ascribed to space-charge effect at the ferromagnetic metals/Ti O₂ interfaces,in which spin-polarized electrons are injected into the ferromagnetic metal layer with the adsorption of lithium-ions on the TiO₂ surface.These results open the door for a promising method to modulate the magnetization in ferromagnetic metals,paving the way towards the development of ionic-magnetic-electric coupled applications.4.Electrical control of ON-OFF magnetism in Sn-Co alloying via reversible ionic motion.Owing to electric-field screening,the effects of magneto-electric in metallic systems is almost limited to a few surface/interface atoms.Here,to go beyond the interfacial effects,we report that the magnetism of the bulk(Sn_0.5nm/Co_0.2nm)₁₀₀ multilayered alloy metallic could be modulated substantially through electrochemically-driven exchange of Li-ion by applying voltages at only～1.5 V.With advanced operando magnetometry,a fully reversible ON and OFF switching of magnetism in(Sn_0.5nm/Co_0.2nm)₁₀₀multilayered alloy metallic was demonstrated at room temperature.When the electrodes were fully discharged to 0 V,the Sn/Co was transformed to Co（superparamagnetism:ON）by alloying reactions,and in the fully charged state at 1.5 V,Co was recombine with Sn to form Sn/Co（paramagnetism:OFF）,which was confirmed by means of ex-situ TEM and X-ray spectroscopy.These results together provide a step forward low-consumption,high-endurance electrical switching of metallic magnets beyond the electric-field screening.