Operando Fourier Transform Infrared Characterization And Mechanism Of Reaction Interface In Li/Mn-rich Cathode

As the decisive part of the energy density and power density of the entire battery,the cathode material of lithium ion batteries is currently a hot spot for modification.However,with the development of commercial cathodes and new cathode materials such as high-nickel cathodes,lithium/manganese-rich cathodes,the research on the failure mechanism and improvement mechanism of cathode materials is relatively lacking.As we all know,the anode will form a layer of organic and inorganic mixture on the surface during the cycling,known as solid electrolyte interface film.Similarly,it is found that a similar solid film will also be formed on the surface of the positive electrode during cycling,called the cathode electrolyte interface film.The nature of this interface film will directly affect the electrochemical performance of the battery,so the knowledge of interface film can provide an understanding of the electrochemical reaction process.However,it is difficult to characterize this interface film because of its high sensitivity to O₂ and CO₂ in the air by ordinary test methods.Therefore,a new characterization method that can isolate the air and achieve the real-time dynamic monitoring is required,that is,in situ/operando characterization.Among kinds of in situ characterizations,operando FTIR based on the organic components that is sensitive to infrared light to is very suitable for characterizing carbonate-based electrolyte systems.In this paper,the layered lithium/manganese-rich material Li_1.2Ni_0.2Mn_0.6O₂ is taken as the research object,with operando FTIR real-time monitoring the changes of organic compositions on the surface of the cathode to figure out the interface reactions at high-voltage region,the optimization mechanism of the electrolyte additives and the mechanism of coating modification on the cathode.To analysis the interface reactions and cathode electrolyte interface film forming mechanism of the cathode during the cycling can fill the gaps in the researches of the failure and modification mechanism about new cathode materials,and provide a theoretical basis for their further modification.This work is mainly divided into the following parts:?1?The relationship between the formation and stability of cathode electrolyte interface film on the cathode surface and electrolyte is studied,and the electrolyte additive TMSB is introduced and characterized for its effect on the formation mechanism of cathode electrolyte interface film.The operando FTIR is used to speculate the interface reaction and the cathode electrolyte interface film formation process on the cathode surface in electrolytes before and after additive added in real time.The results show that the additive can combine with the anions F^-and PF₆^-decomposed by the lithium salt in the electrolyte and produce polyanionic groups?TMSB-F?^-and?TMSB-PF₆?^-,so that the electrostatic force between the anion and the ethylene carbonate molecule is reduced when charging to the high voltage.That will prevent the ethylene carbonate molecule migrating to the cathode surface with the anion during charging,so the ethylene carbonate decomposition can be reduced at some extent.In addition,XPS and HRTEM results indicate that the additive will decompose before the solvent molecules and lithium salts under high pressure,forming a B-containing cathode electrolyte interface film with a thickness of only?8 nm,which is much smaller than that in conventional electrolytes?72 nm?.The high-quality cathode electrolyte interface film can effectively protect the active material from corrosion by the electrolyte,so that the battery system after the additive optimization shows more excellent rate performance and long cycle performance.It can keep a capacity of 49m A hg^–1 at 5 C and the capacity retention rate of 250 cycles at 1 C cycle is 18.4%higher than the unoptimized system.?2?The influence of nano-scale coating modification on the formation and stability of cathode electrolyte interface film is studied.Through the atomic layer deposition,an Al₂O₃ film is pre-built on the cathode surface as an artificial cathode electrolyte interface film,which can effectively reduce adverse interface reactions,inhibit interface phase transformation to a certain extent,and improve interface stability.In this work,TMA and O₃ are used as aluminum and oxygen sources,respectively.By depositing different process cycles,coating layers with different nanometer thicknesses are constructed on the cathode.Among them,the samples with 40 process cycles?LNMO@40Al?shows the most excellent electrochemical performances.Through the in situ FTIR,it is found that the presence of the passivation film can hinder the dehydrogenation reaction of the ethylene carbonate molecules to a certain extent,which will lead to the irreversible formation of soluble vinylene carbonate and continuous consumption of solvent molecules;in addition,the passivation film can reduce the surface activity of TM-O bond on the pristine material and cut off the catalytic decomposition path of solvent molecules and lithium salts,forming a more stable interface environment.The discharge capacity of the LNMO@40Al at 1 C is 224.7m Ah g^-1 in the first cycle,which is much higher than that of the pristine material(179.7m Ah g^-1),and the coulomb efficiency of the LNMO@40Al remains stably over 99%even after 500 cycles.?3?The formation mechanism of cathode electrolyte interface film on Li/Mn-rich cathode material under the two modification methods of electrolyte and nano passivation film coating is deeply analyzed.The former mainly achieves the formation of a high-quality cathode electrolyte interface film while is also mainly constituted by organic compositions.At the same time,based on the nature of space macromolecules and B atoms lacking electrons,the ethylene carbonate is prevented from migrating to the cathode surface,thereby forming a certain protection for the cathode,and reducing part of the ethylene carbonate decomposing.Because the focus of the modification is not on the material,it can only play the role of maximizing the performance of cathode,but cannot inhibit the unstable interface reaction itself.The ALD-coated aluminum oxide treatment can isolate cathode activity to block the side reaction of the solvent caused by the active oxygen and the transition metal during the redox process.At the same time,the existence of the passivation film changes the solid-liquid interface environment and interface reactions,so it can induce the formation of ethylene carbonate different from the pristine material composition,which can make the interface more stable and thus play a role in improving the electrochemical performance.