Application Of In-situ Characterization Techniques In The Mechanism Analysis Of Lithium Sulfur Batteries

Lithium-sulfur battery（LSB）is considered as the most promising next-generation energy storage system due to its high theoretical energy density,low cost,and environmental compatibility.However,the low electronic conductivity,shuttle effect,and complicated 16-electron reaction associated with S8 cathode have severely restricted its commercial use.In the past ten years,researchers have been devoted to optimizing LSB by constructing host materials with physical limitations,chemical-bonding and catalytic-conversion functions,as well as designing organosulfide materials with tunable functional groups and convenient fabricated methods to address these problems.Unfortunately,due to the limitation of characterization conditions,we still lack an in-depth understanding of the actual reaction mechanism and its impact on electrochemical performance.Recently,various in-situ characterization techniques for monitoring the real-time changes of battery status have been gradually developed,including X-ray diffraction（XRD）,Raman spectroscopy,Fourier transform infrared spectroscopy（FTIR）,and ultraviolet-visible（UV）spectroscopy,building a bridge between macro-electrochemical performance and electrode microstructure.Accordingly,we designed two lithium-sulfur battery cathode materials and studied their electrochemical performance and the structural evolution during the redox process via in-situ characterization technologies.The main work content can be divided into the following two parts:（1）In this part,a yeast-derived hollow nitrogen-doped carbon spheres（NCS）encapsulating the naturally abundant sulfur-limonene-based polysulfides（SLP）material（SLP@NCS）was designed to address the problems of poor electronic conductivity,low specific capacity,and complicated reaction mechanism of organosulfide cathode materials.Benefiting from the enhanced conductivity by the NCS,high capacity,and suppressed shuttle effect by the self-protection mechanism of SLP,SLP@NCS cathode exhibits a high specific capacity(1305 m Ah g^-1 at 0.1 C in the 1^st cycle),excellent cycle stability(727.9 m Ah g^-1 at 1 C after 300 cycles),and superior rate capability(536.1m Ah g^-1 at 5 C).In addition,various in-situ characterization techniques have been conducted to study its structural evolution during electrochemical processes.In-situ X-ray diffraction（in-situ XRD）results confirmed that SLP undergoes an organic-inorganic phase transition process（SLP-Li2S-S8）.In-situ FTIR observed the companying reversible changes of C-H,C-S,and C=C bond.In-situ Raman spectroscopy（in-situ Raman）confirmed that polysulfides are well confined in the SLP matrix during the charge-discharge process.The collaborative in-situ characterization strategies are extended to other organosulfide materials to help understand their electrochemical mechanism and guide the design of improving their electrochemical performance.（2）MXene is a new class of two-dimensional materials.In this part,we designed a bifunctional cysteine（CA）modified MXene as a host material for lithium-sulfur batteries.Specifically,positively charged CA was embedded into negatively charged HF etched multilayer Ti₃C₂（d-MXene）through electrostatic action,and CA-modified Mxene（MCA）was obtained with the assistance of ultrasound.The composite（SMCA）was obtained by heating S8 and stimulating the ring opening to bind to the sulfhydryl group of CA.The structure morphology and spectral characterization showed that CA could effectively expand the layer spacing of MXene,and the structure could still maintain well during the heating process,which prevents the decrease of conductivity caused by the restacking of MXene.Meanwhile,the S-S bond fracture mechanism of S8 grafted by CA can effectively inhibit the generation of long-chain polysulfides in the process of charge and discharge.Benefiting from the high conductivity of structure-stable MXene and the unique polysulfide inhibition effect of CA,the SMCA electrode shows excellent electrochemical cycling performance,showing 1288 m Ah g^-1 discharge capacity and almost completely reversible charging capacity at 0.1 C.After 200 cycles at 0.5 C,the discharge capacity of the SMCA electrode was maintained at 946 m Ah g^-1.The discharge capacity of 776 m Ah g^-1 is maintained for 600 cycles at 1 C,the capacity retention rate is 83.2%,and the average capacity loss per turn is 0.03%.In addition,in-situ characterization was used to study the structural evolution of the SMCA electrode during the charge and discharge process.In-situ XRD observed that the SMCA electrode undergoes a phase transition process of（α-S8_）-Li2S-（γ-S8）.In-situ Raman spectroscopy demonstrated that the shuttle effect of long-chain polysulfide was effectively inhibited during the discharge process.