In-situ Magnetic Testing Reveals The Energy Storage Mechanism Of Transition Metal-based Anode

As an important part of energy storage devices,alkali metal ion batteries（for instance,lithium-,sodium-and potassium-ion batteries）have attracted extensive attention on account of their high energy density and environmental protection.With the increasing demand for energy in human society,higher requirements and standards are put forward for energy storage devices and hence it is necessary to design energy storage devices with high-performance.In alkaline metal ion batteries,as the host material of alkaline metal ions（lithium ions,sodium ions and potassium ions）,the anode plays a vital role in the capacity and cycle performance of the battery.Among them,3d transition metal-based anode materials are favored by researchers in recent years due to their high capacity,low cost and simple preparation.In order to further improve the performance of 3d transition metal-based anode materials,it is necessary to understand the physical and chemical processes and structural evolution of electrode materials in electrochemical processes.At present,although other conventional testing techniques have solved some problems about electrochemical mechanism,there are still many controversies on the reaction mechanism due to the complexity of the internal system and battery reaction,especially the limited understanding of the reaction processes at the interface scale,which limits the further development of alkali metal ion batteries.Magnetic test provides a new idea for studying the energy storage mechanism of alkali metal ion batteries.Due to the unique electronic structure of 3d transition metals（net magnetic moment caused by 3d orbital splitting）,transition metal-based electrode materials usually undergo corresponding magnetic changes during charge and discharge（electron transfer）.The changes of magnetism are closely related to the changes of element valence,particle size and structural phase transition during the electron transfer process of alkali metal ion batteries.Therefore,we can combine electrochemical test and magnetic test to study the reaction mechanism of alkali metal ion batteries.At the same time,due to the high sensitivity of magnetic properties to electron distribution,magnetic tests can obtain information that cannot be obtained by other conventional testing methods.In this paper,we systematically investigate the reaction mechanism of some transition metal-related anodes by self-designed operando magnetometry based on a Physical Property Measurement System（PPMS）.The specific work mainly includes the following three parts:1.Interfacial space charge storage mechanism in FeSe₂ alkali metal ion batteries（AMIBs）revealed by operando magnetometry.In this work,we used advanced in situ magnetic testing technology to prove the space charge storage mechanism in FeSe₂lithium ion batteries（LIBs）,sodium ion batteries（SIBs）and potassium ion batteries（PIBs）,which clarified the energy storage mechanism of FeSe₂,including not only intercalation and conversion reactions,but also space charge storage process.We prepared FeSe₂ rod-like nanoparticles as anode materials for LIBs,SIBs and PIBs through hydrothermal reaction,and used the self-designed flexible packaging battery for in situ magnetic tests.The magnetic test results show that the increase and decrease of magnetization are caused by the reduction and oxidation of Fe at high voltage.For low voltage range,the magnetic changes are dominated by space charge storage.In the Fe/M₂Se（M=Li,Na,K）system formed during discharge,spin-polarized electrons are filled into Fe while Li⁺is stored on the side of M₂Se,forming a space charge zone.Moreover,magnetic and kinetic tests proved that the space charge effect weakened with the increase of the radius of Li,Na and K ions.This work deepens our understanding of space charge storage,and lays a solid foundation for studying the complex interfacial effect between electronic conductor and ionic conductor in electrochemical process of various energy storage devices.2.The catalytic effect of transition metals in CoO LIBs investigated by operando magnetometry.In this work,a series of cobalt oxide electrodes were prepared by magnetron sputtering,and the energy storage mechanism of cobalt oxide was studied by in situ magnetic tests.The results show that in addition to the conversion reaction and space charge storage mechanism,there is also a Co-catalyzed lithium storage mechanism.We successfully detected the magnetic responses caused by the reversible formation and decomposition of polymer/gel-like film（PGF）under the catalysis of Co.In addition,a series of CoO/Co films were prepared under different sputtering atmosphere and sputtering time.The influence of Co content and electrode thickness on the catalytic processes were deeply investigated.The results clearly proved that the increase of Co content could enhance the catalytic effect.Moreover,the decrease of film thickness can increase the proportion of Co-catalyzed lithium storage in electrochemical lithium storage,making the magnetic responses of catalytic process more obvious.This work emphasizes the importance of real-time magnetic testing in the field of catalysis,deepens the understanding of transition metal catalytic mechanism,and provides a key guiding role for the design of new energy storage devices based on catalytic energy storage.3.Operando magnetometry revealed the electrochemical reaction mechanism of Co_1-xS LIBs.In this work,Co_1-xS was obtained by high temperature calcination and vulcanization of ZIF-67 precursor,and the electrochemical mechanism of Co_1-xS LIBs was investigated by detecting the magnetization variations during the charge and discharge process.In the discharge process,Co was first generated by conversion reaction;subsequently,in the continuous discharge process,spin-polarized electrons filled into Co and space charge storage occurred;in the final stage of discharge,the electrons stored in Co transferred to PGF forming radical anions,which leads to catalytic lithium storage.In the charging process,electrons first transferred back from PGF to Co,and then the spin-polarized electrons were released from Co.Finally,Co was oxidized during conversion reaction.This work clarified the reaction mechanism of cobalt-based transition metal sulfides and provided a powerful technology for studying the electrochemical process of transition metal based materials.