Development And Application Of In-situ Electrochemical Raman Spectroscopy For Lithium Battery Systems

Lithium battery is one of the most important energy storage devices due to its high energy storage density and good rechargeable performance.Thanks to the increasing development of the society,the lithium batteries of higher energy storage density and longer charge/discharge cycles are in need.However,the performance improvement of lithium battery is severely limited by the instability of material structure and electrode/electrolyte interface in electrochemical reaction.Moreover,the decay mechanism of material structure and electrode/electrolyte interface is still unclear,so it is difficult to improve the stability material structure and electrode/electrolyte interface.Therefore,it is very important to develop in-situ electrochemical spectroscopic characterization methods to help understand the evolution mechanism of material structure and electrode/electrolyte interface and provide guidance for the performance improvement of lithium battery.Raman spectroscopy is sensitive to the local structure(M-O coordination,defects,etc.)and can detect interfacial species of trace amount and weak signal by the surface-enhanced Raman spectroscopy(SERS)effect.Therefore,it will be helpful to understand the evolution mechanism of material structure and electrode/electrolyte interface in electrochemical reaction by developing in-situ electrochemical Raman spectroscopy.The development status of the in-situ electrochemical Raman spectroscopy for lithium battery can be briefly concluded as follows:most of the relative work focused on material structural research;interface was difficult to study and seldom focused on due to its very complex components and structures;the testing conditions and the reliability of the research methods are still needed to improve.The advantage achievement of Raman spectroscopy relies on the further development of in-situ electrochemical Raman spectroscopy based on the features of lithium battery.Raman spectroscopy cannot provide sufficient information of long-range periodic structure and the comprehensive evolution of the material structure.X-ray diffraction is a useful tool to characterize the periodic structure,so it will be meaningful to combine the in-situ electrochemical Raman spectroscopy with in-situ electrochemical X-ray diffraction to comprehensively understand the material structural evolution from both the local structure and the periodic structure.This doctoral dissertation aims to solve out the above problems from the instrumental methods and the systems research.The key points in this dissertation can be concluded as follows:(1)Developed the in-situ electrochemical Raman cell of excellent electrochemical and spectroscopic testing performance by solving out the airtightness,conduction and signal-collecting efficiency problems in previous reported work;the performance of practical battery is comprehensively related to the properties of cathode,anode,separator and electrolyte,so the cross-sectional cell to simultaneously observe cathode,anode,separator and electrolyte in operation is developed.(2)Studied the potential decay-structural evolution for NiMn Li-rich material by in-situ electrochemical Raman spectroscopic study of the Mn-O coordination evolution.The activated Li2MnO3 was found to involve in the reaction from 3.0 V to 3.7 V(potential decay interval)and indicated the instability of the actived Li2MnO3 may cause the potential decay from 3.0 V to 3.7 V;studied the D and G band change process of the single layer graphene(SLG)in the electrochemical reaction to find the interaction between Li+ and SLG.The SLG was found to form defects at 1.6 V and it could recover when the potential went positively;studied the polysulfides conversion mechanism to understand the promotion effects of Ni NPs on Li-S battery performance.The conversion rate of short chain polysulfides to Li2S2 and Li2S was found to be accelerated by in-situ electrochemical Raman spectroscopy and this helped understand the effects of Ni NPs on the capacity increase in Li-S battery.(3)Probed the effective methods of electrode fabrication and SERS substrate construction to promote the SERS effects to detect the interfacial species of weak signal on the electrode.Studied the interfacial problems relative to Si and Sn anode materials and discussed the reliability problems.Proposed the further solutions to the reliability problems.(4)Developed the in-situ electrochemical XRD cell with arc-shaped window by solving out the airtightness,conduction and XRD window background interference problems in previous reported work.Developed the methods combination of the in-situ electrochemical Raman spectroscopy and the in-situ electrochemical XRD to help comprehensively understand the phase transition and the reversibility of the structural evolution of TiNb207 from both the local structure and the periodic structure.The local coordination showed obvious structural inversibility in electrochemical cycling.Discussed the current problems of the methods combination and proposed the assumption about the future development of methods combination.