Study On The Alkali Metal Ions Storage Mechanism Of FeTiO₃ Material

With the rapid development of electric vehicles and portable electronic devices,there comes higher requirement for the energy density of energy storage devices.At present,the energy storage technology of rechargeable secondary batteries is becoming more and more mature,and carbon-based anode materials represented by graphite have occupied the leading position in the market due to their good stability and low cost.However,the lower theoretical capacity of graphite(372 mAh g^-1)greatly restricts the overall energy density of the battery.Therefore,it is a current research hotspot to seek for alternative materials with high energy density for carbon-based anodes.Among them,ferrous titanate material（FeTiO₃）has great application potential attributing to its good chemical stability,high theoretical capacity,and low cost,which is favored by researchers.However,there are still many unresolved scientific problems in FeTiO₃ materials,such as the origin of the difference in lithium/sodium storage capacity is unclear,and the electrochemical energy storage mechanism is controversial or yet to be revealed.In view of the above key scientific issues,this paper will take FeTiO₃ material as the research object,and use various experimental characterization methods to conduct in-depth research on its alkali metal ion storage mechanism,clarify the root cause of the capacity difference phenomenon,and provide future construction of high energy density energy storage devices with theoretical guidance.The specific research contents of this paper are as follows:1.Mechanism study on the origin of the difference in lithium/sodium storage performance of FeTiO₃:In this paper,ethylene glycol was used as a soft template,and FeTiO₃ materials with uniform monodisperse nanowire morphology were prepared by a simple solvothermal method combined with calcination treatment.Despite that it is considered that FeTiO₃ electrodes should theoretically have the same electrochemical energy storage mechanism for lithium/sodium storage,the electrochemical test results show that the sodium storage capacity(425 mAh g^-1)of FeTiO₃ nanowires is significantly lower than its lithium storage capacity(1260 mAh g^-1).In order to explore its origin,studies on the charge storage principle of FeTiO₃ with aid of ex-situ X-ray photoelectron spectroscopy and transmission electron microscopy were performed,which is further combined with advanced in-situ magnetic monitoring technology.In order to explore the origin of the performance mismatch,this paper studies the charge storage principles of FeTiO₃ by ex-situ X-ray photoelectron spectroscopy and transmission electron microscopy,and combined with advanced in-situ real-time magnetic monitoring technology.The results show that the lithium storage mechanism of FeTiO₃ is not the same as the sodium storage mechanism.FeTiO₃ material would participate in the electrochemical conversion reaction with a low depth during the sodium storage process,accompanying by the oxidation of oxygen species,which leads to different amounts of Fe⁰ particles in the discharge products.A large number of spin-polarized electrons can store in Fe⁰ particles at low potential,resulting in the spin-polarized capacitance,which reasonably explains the fundamental reason for the difference in the performance of lithium storage and sodium storage in FeTiO₃ materials2.Research and application of potassium storage mechanism of FeTiO₃:The lithium/sodium storage mechanism of FeTiO₃ has been systematically explored in the previous article,but the potassium storage mechanism of FeTiO₃ has not been reported yet.The previous studies have shown that traditional testing methods combined with in-situ magnetic testing have great advantages for exploring the energy storage mechanism of FeTiO₃ materials.In this study,we applied the in-situ magnetic measurement technology to the field of potassium ion storage for the first time,in order to gain a comprehensive understanding of the potassium storage mechanism of FeTiO₃ materials,and further master the rules of alkali metal ion storage behaviors of FeTiO₃ materials,thus providing the subsequent mechanism researches and the corresponding modification with theoretical guidance.In this paper,we focus on a single-phased hollow FeTiO3（SPH-FTO）hexagonal prism synthesized through a complexing-reagent assisted approach and find that,the K-ion storage in this compound occurs predominantly with an intercalation mechanism and partially a conversion mechanism,which is fundamentally different from its lithium and sodium storage mechanisms.Furthermore,using advanced in situ magnetometry,we can demonstrate the existence of a spin polarized surface capacitance in SPH-FTO hexagonal prism K-ion batteries for the first time and quantify the ratio（ca.3.8%）of the part where conversion reaction occurs to the entire active material by the variation in magnetization.In addition,theoretical calculations demonstrate that large-radius potassium ion insertion leads to greater rehybridization around the cation,which reasonably explains the different mechanisms between potassium and lithium/sodium storage behaviors of FeTiO₃ materials.We also demonstrate a K-ion hybrid capacitor assembled with the prepared SPH-FTO hexagonal prisms anode and activated carbon cathode,delivering a high energy-density(217 Wh kg^-1)and high power-density(5000 W kg^-1)as well as extraordinary stability(>92%capacity retention after 10000 cycles at 1000 mA g^-1).This new understanding is used to showcase the inherently high K-ion storage properties from the earth-abundant FeTiO₃,and is instructive for the search for electrodes for high-performance potassium-based energy storage devices.