Uporaba elektrokemijske impedančne spektroskopije za razločitev impedančnih prispevkov reakcije prenosa naboja in desolvatacije za Li kovinsko anodo

Urbanija, Nejc (author), Dominko, Robert (mentor)

Kovinski litij se že več kot 40 let uporablja kot anoda v litijevih baterijah. Njegova stabilnost v organskih elektrolitih je povezana s spontano tvorbo pasivne površinske plasti, ki učinkovito preprečuje nadaljnjo elektronsko interakcijo z elektrolitom. Po drugi strani ta plast prevaja litijeve ione, zato je splošno znana kot trdna elektrolitska medplast (Solid Electrolyte Interphase, SEI). Podrobna sestava in morfologija spontano nastale SEI sta močno odvisni od uporabljenega elektrolita. Model delovanja SEI predvideva, da je struktura plasti vsaj dvojna. Bližje kovinski anodi nastane tanek (nekaj nm) kompakten sloj SEI, ki je elektronsko neprevoden, omogoča pa prevajanje Li+ ionov. Za tem kompaktnim slojem sledi debelejši (nekaj 100 nm) porozni sloj SEI. V porah tega dela SEI se zadržuje elektrolit, preko katerega poteka transport aktivnih zvrsti do elektrode. Ta model je osnovan na meritvah impedančnih spektrov, ki so bile opravljene v kombinaciji s sistematičnim spreminjanjem parametrov celice, kar je omogočilo bolj natančno razumevanje procesov. Trenutno v raziskovalni skupnosti še ni vzpostavljenega konsenza o prisotnosti in velikosti impedančnega prispevka za reakcijo prenosa naboja, ki poteka na fazni meji med kovinskim litijem in kompaktnim SEI, ter prispevka za desolvatacijo Li+ iona, ki poteka na meji med poroznim in kompaktnim SEI. V sklopu diplomskega dela smo z meritvami impedančne spektroskopije na simetričnih celicah z litij-kovinskimi elektrodami poskušali določiti in razločiti med prispevkom reakcije prenosa naboja in desolvatacije litijevega iona. V ta namen smo litijeve elektrode predhodno pasivirali v različnih mešanicah elektrolitov in topil. Tako pripravljene elektrode smo potem uporabili za sestavljanje simetričnih celic z različnimi koncentracijami soli v elektrolitu. Celicam smo pomerili impedančne spektre pri potencialu odprtega tokokroga. Ti spektri so bili analizirani z namenom določitve sprememb zaradi nižje koncentracije elektrolita pri enaki predhodni sestavi SEI. Sklepali smo, da spreminjanje koncentracije elektrolita znatno spremeni impedančni prispevek desolvatacije, ne pa tudi prispevek redoks reakcije prenosa naboja, saj se ta dogaja na fazni meji, do katere elektrolit nima dostopa.

• Language: Slovenian
• Keywords: impedančna spektroskopija, kovinska litijeva anoda, SEI
• Work type: Bachelor thesis/paper
• Typology: 2.11 - Undergraduate Thesis
• Organization: FKKT - Faculty of Chemistry and Chemical Technology
• Year: 2022
• PID: 20.500.12556/RUL-139476
• COBISS.SI-ID: 125670659
• Publication date in RUL: 02.09.2022

Secondary language

• Language: English
• Title: Use of electrochemical impedance spectroscopy to discern between impedance contributions due to charge transfer reaction and desolvation for Li metal anode

Abstract

Lithium metal has been used as an anode in lithium batteries for more than 40 years. Its stability in organic electrolytes is linked to the spontaneous formation of a passive surface layer, which effectively prevents further electronic interaction with the electrolyte. On the other hand, this layer conducts lithium ions and is therefore commonly known as the Solid Electrolyte Interphase (SEI). The detailed composition and morphology of the spontaneously formed SEI is strongly dependent on the electrolyte used. The SEI working model assumes that the structure of the layer is at least double. Closer to the metal anode, a thin (few nm) compact SEI layer is formed, which is electron-impermeable but allows the conduction of Li+ ions. This compact layer is followed by a thicker (some 100 nm) porous SEI layer. The pores of this part of the SEI hold the electrolyte through which the active species are transported to the electrode. This model is based on measurements of impedance spectra taken in combination with a systematic variation of the cell parameters, which allowed a more accurate understanding of the processes. Currently, there is no established consensus in the research community on the presence and magnitude of the impedance contribution for the charge transfer reaction, which takes place at the phase boundary between lithium metal and compact SEI, and the contribution for Li+ ion desolvation, which takes place at the boundary between porous and compact SEI. As part of the thesis work, we have attempted to determine and distinguish between the contribution of the charge transfer reaction and the desolvation of the lithium ion by impedance spectroscopy measurements on symmetric cells with lithium-metal electrodes. For this purpose, lithium electrodes were pre-passivated in different mixtures of electrolytes and solvents. The electrodes prepared in this way were then used to assemble symmetric cells with different salt concentrations in the electrolyte. The impedance spectra of the cells were measured at open circuit potential. These spectra were analysed to determine the variation due to a lower electrolyte concentration at the same SEI pre-composition. We hypothesised that changing the electrolyte concentration significantly changes the impedance contribution of desolvation, but not the contribution of the redox charge transfer reaction, since this occurs at the phase boundary to which the electrolyte has no access.

• Keywords: impedance spectroscopy, Li metal anode, SEI