In Situ Magnetic Monitoring Of Lithium Battery Energy Storage Mechanisms

With the rapid development of new energy cars and electronic portable devices,there is sharp increase in the market demand for lithium-ion batteries.At present,the intercalation/deintercalation type electrode materials such as graphite dominate the entire lithium-ion batteries(LIBs)market with their unique stability and low-cost.In order to further optimize the electrochemical properties of lithium-ion batteries,considerable efforts have been conducted on transition metal compound electrodes.The capacity of this type of electrodes can reach a very high value(700-1200 m Ah g^-1),which is about three times that of commercial graphite,and is a very promising electrode material for energy storage.This manuscript takes transition metal compounds as the research object,and conducts in-depth research on its own lithium storage mechanism,promoting the construction of high energy density energy storage devices based on transition metal electrodes in the future.The main research results are as follows:1.Cobalt oxide(CoO)is a promising electrode for high energy density Li-ion batteries(LIBs),where the charge storage is believed to take place solely during the electrochemical oxidation/reduction processes.However,this simple picture has been increasingly challenged by reported anomalously large storage capacities,indicating the existence of undiscovered extra charge reservoirs inside the system.Here,an advanced operando magnetometry technology is employed to monitor the magnetization variation of the CoO LIBs in real time and in this particular system,it is clearly demonstrated that the anomalous capacity is associated with both the reversible formation of a spin capacitor and the growth of a polymeric film at low voltages.Furthermore,operando magnetometry provides direct evidence of the catalytic role of metallic Co in assisting the polymeric film formation.These critical findings help pave the way for better understanding the charge storage mechanisms of transition metal oxides and further utilizing them to design novel electrode materials.2.Transition metal fluorides(TMFs)have been regarded as one of the promising cathode material candidates for high energy density lithium-ion batteries(LIBs),in which a conversion reaction with lithium could accommodate multiple lithium ions per metal cation.However,inadequate understanding of the internal electrochemical reaction mechanism has impeded their application.Here,we design a novel iron fluoride(Fe F₂)aggregates assembled with nano-cylinder-like clusters as cathode material to build Fe F₂LIBs,and employ advanced in situ magnetometry to detect their intrinsic electronic structure during cycling in real time.The experiments,together with X-ray photoelectron spectroscopy analysis,show that Fe F₂can not involve in complete conversion reactions when the Fe F₂ LIBs operate between 1.0-4.0 V.Importantly,the corresponding conversion ratio of Fe F₂ in this conventional voltage window can be further quantified by the variation in magnetization.In addition,we firstly demonstrate that that the spin-polarized surface capacitance exists in the Fe F₂ cathode through monitoring the magnetic responses over various voltage ranges.The research presents an original and insightful method to examine the conversion mechanism of TMFs,and significantly provides an important reference for future artificial design of energy system based on spin-polarized surface capacitance.