In Situ Magnetic Testing Technology For Research On Anode Materials For Transition Metal-based Lithium Batteries

Lithium-ion batteries(LIBs)have been widely used in various fields due to its advantages of high energy density,long cycling life and environmental friendliness.With the rapid development of human society,people have put forward higher requirements for energy supply.So far,new anode materials such as 3d transition metals compounds have drawn more and more attention because of their rich crust resources,higher specific capacity,and higher operating voltages.However,there is limited progress in the exploration of the basic principles of the physical and chemical processes of the material itself and the interface during the operation of the electrode material.Although conventional test methods have solved many mechanism problems,due to the complexity of battery internal environment,there are still many controversies(such as the phenomenon of extra capacity in metal compounds),which has brought huge challenges to technological breakthroughs in the research field.In order to solve this challenge,researchers should analyze and study the internal reaction mechanism of anode materials from multiple perspectives based on the physical properties of the anode materials.The 3d transition metal elements have high adjustability in structural characteristics,and unique electronic structure(magnetic moment caused by uncompensated charge caused by 3d orbital splitting)and other characteristics.Additionally,the Nobel Prize winner in 2019,Goodenough and Whittingham had been committed to using magnetic methods to analyze and study the reaction mechanism inside lithium-ion batteries.Based on the close correlation between the magnetization and the valence,lattice structure,band structure and particle size of 3d transition metal elements,we used PPMS-SQUID and electrochemistry test tools to independently build the in stiu magnetic measurement device.In order to characterize the variation of element valence and lattice structure caused by the oxidation-reduction reaction of the transition metal electrode materials,in situ real-time magnetic monitoring method can provide the most direct experimental characterization method for the relevant electrode materials in this paper.Based on the unique characteristics of 3d transition metals(magnetic moments caused by uncompensated charges caused by 3d orbital splitting),in situ real-time magnetic monitoring method was used to analyze transition metal anode materials(Fe₃O₄,Sn-Co,Sn-Fe)conducted a systematic study.The specific work content mainly includes the following three parts:1.Extra storage capacity in transition metal oxide lithium-ion batteries revealed by in situ magnetometry.In this work,based on the controversy of the extra capacity in the metal compound,we synthesized monodispersed hollow Fe₃O₄nanospheres by using a traditional hydrothermal method,which is used as the anode material of the lithium ion battery.With advanced in situ magnetic monitoring,we investigated the evolution of the internal electronic structure in Fe/Li₂O nanocomposites to reveal the origin of extra storage capacity in this type of LIB.The in situ magnetometry clearly demonstrated that in the Fe₃O₄/Li model battery systems during low-voltage discharge,the electrochemically reduced Fe nanoparticles can store a large number of spin-polarized electrons,which results in a large excess capacity and a pronounced change in the interface magnetization.The results are consistent with the"space charge"mechanism proposed by Maier.J et al.We further quantified the surface capacitance of metal nanoparticles through the variation in magnetization,and found this capacitance to be in close agreement with the experimentally determined capacitance.These findings demonstrate the presence of spin-polarized surface capacitance of metal nanoparticles in LIBs.In addition,we have shown that the in situ real-time magnetic monitoring method is a versatile tool to study the evolution of materials related to transition metals in otherwise inaccessible device configurations.2.Operando magnetometry study on the electrochemical behavior of transition metals of Sn-Co alloy lithium ion batteries.In this work,to study the electrochemical mechanism of Sn-Co intermetallic,an operando magnetometry was used to investigate the magnetic evolution during the electrochemical cycling.Depending on the stoichiometry,Sn-Co intermetallic films deposited by magnetron sputtering(Sn₇Co₃and Sn₃Co₇)show different evolution of magnetic signatures.The magnetic responses of stannum richer phases Sn₇Co₃show that the liberated Co nanoparticles can fully recombine with Sn during the delithiation.An increase of Co nanoparticle size during cycling can be revealed by a steady increase of the magnetic moment.As for the cobalt richer Sn₃Co₇,the magnetic moment cannot drop to the initial value by charging,indicting a partial reversibility of cobalt.Importantly,the excess Co would form a barrier layer that inhibits the further reaction of Li with the remaining intermetallic material,leading to lower capacities.Decreasing the grain size of intermetallic could weaken the effect of the inactive skin.Our results eliminate the controversies over the charge storage mechanism and provide valuable insights toward the design of high-performance Sn based alloy anodes.3.In-situ magnetic monitoring on the reaction mechanism of Sn-Fe alloy.In this work,Sn₈Fe₂alloy thin films were prepared by PVD co-sputtering.With advanced in situ magnetic monitoring,the reaction mechanism of Sn-Fe alloy was systematically studied.During the charging process,the liberated Fe particles will partially combine with Sn,showing the partially reversible behavior.In addition,the excess Fe particles will form a dense barrier layer to prevent Li⁺from further reacting with Sn in the electrode.These results are similar to the electrochemical behavior of Sn₃Co₇alloy.However,the magnetic response of Sn-Fe alloy exhibits the effect of spin-polarized surface capacitance,in contrast,no magnetic variation of Sn-Co alloy at the low voltage range,which may be related to the formation of gel-like polymer.This work demonstrated the charge storage mechanism of Sn₈Fe₂alloy thin films in lithium-ion batteries,and provides effective insights for the next generation of high-performance Sn-based batteries.