Understanding the functionality of electrode-electrolyte interphases in rechargeable batteries using photoelectron spectroscopy and electrochemical impedance spectroscopy

Križan, Alenka (author), Gaberšček, Miran (mentor)

Electrochemical systems and rechargeable battery systems, in particular, are essential to the transition from fossil fuels to renewable energy sources. The overall performance of a battery system is critically dependent on the processes that occur at the electrolyte interface of the electrode. Still, in-depth understanding of the interfacial processes remains limited due to experimental challenges associated with interface studies and due to the high complexity of battery systems. This work focuses on studies of three distinct electrode/electrolyte interphases which differ in their chemical composition, complexity, and whether a solid phase is present between the electrode and the electrolyte or not. The core findings are obtained using two principal methods, electrochemical impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS). In the first study, a model system consisting of a thin film gold electrode and a carbonate electrolyte is probed using dip-and-pull ambient pressure XPS (APXPS). By monitoring the drop in electrostatic potential at the electrode|electrolyte interface with operando dip-and-pull APXPS, the meniscus electrochemistry is shown to be significantly affected by a severe iR drop along the meniscus. The meniscus resistance is estimated to be 1000-times higher than the resistance of the bulk electrolyte and is shown to be particularly problematic for conducting operando studies of faradaic processes. Strategies to minimize measurement artefacts due to the iR drop are proposed. The second system of interest is the solid electrolyte interphase (SEI) that forms on composite silicon graphite electrodes (Si/g electrodes) when three distinct combinations of additives are added to the electrolyte. Based on the analysis of XPS spectra collected from composite Si/g electrodes, the structural fragments present in the SEI are proposed. By complementing the information obtained from XPS with EIS measurements, we correlate the SEI chemical composition and thickness to the electrochemical properties of Si/g. Moreover, we demonstrate the importance of performing electrochemical measurements in a way that allows the individual contributions of each electrode to be distinguished. In the third study, the interphase between Li metal and carbonate electrolyte is modified by applying a coating onto Li metal electrode surface. The applied coating consists of a biopolymer and LiNO$_3$ and has previously been shown to suppress dendrite growth. A series of EIS measurements are conducted to determine the mechanism of mass transport through the coating. Transmission line model (TLM) of the studied system is constructed and verified by comparing TLM-based simulations with experimentally observed trends. Finally, the usefulness of the constructed TLM is demonstrated by employing the TLM to determine the transport parameters of a new system, namely, a system in which the liquid electrolyte has been replaced by a polymer electrolyte.

• Language: English
• Keywords: electrode electrolyte interphase, X-ray photoelectron spectroscopy, electrochemical impedance spectroscopy, mass transport, electrochemical processes
• Work type: Doctoral dissertation
• Typology: 2.08 - Doctoral Dissertation
• Organization: FKKT - Faculty of Chemistry and Chemical Technology
• Year: 2025
• PID: 20.500.12556/RUL-176636
• COBISS.SI-ID: 268215555
• Publication date in RUL: 05.12.2025

Secondary language

• Language: Slovenian
• Title: Razumevanje funkcionalnosti medfaze elektroda-elektrolit v akumulatorjih z uporabo fotoelektronske in elektrokemijske impedančne spektroskopije

Abstract

Elektrokemijski sistemi, zlasti akumulatorji, so ključnega pomena za uspešen prehod s fosilnih goriv k obnovljivim virom energije. Zmogljivost akumulatorjev je v veliki meri odvisna od procesov, ki potekajo na faznem stiku med elektrodo in elektrolitom. Razumevanje teh procesov je omejeno, vzroka za to pa sta predvsem zahtevnost eksperimentov, ki bi omogočili vpogled v te procese, ter kompleksnosti akumulatorjev. V tej disertaciji smo se osredotočili na preiskovanje treh medfaz elektrode in elektrolita (ang. electrode electrolyte interphase, EEI), ki so sestavni del treh različnih elektrokemijskih sistemov. Obravnavani EEI se razlikujejo po kemijski sestavi, stopnji kompleksnosti ter prisotnosti oziroma odsotnosti trdne elektrolitske faze (ang. solid electrolyte interphase, SEI). Glavne ugotovitve temeljijo na dveh ključnih metodah: elektrokemijski impedančni spektroskopiji (EIS) in rentgenski fotoelektronski spektroskopiji (ang. X-ray photoelectron spectroscopy, XPS). Prva študija preučuje modelni sistem, ki je sestavljen iz tankoplastne zlate elektrode in karbonatnega elektrolita. Za spremljanje procesov na faznem stiku med elektrodo in elektrolitom smo uporabili metodo tankega meniska in različico XPS, ki omogoča meritve pri sobnem tlaku (ang. ambient pressure XPS, APXPS). Z operando meritvami smo spremljali padec elektrostatskega potenciala na faznem stiku med elektrodo in elektrolitom. S pomočjo eksperimentov in simulacij smo določili, da je upornost meniska približno tisočkrat večja od upornosti bulk elektrolita. Visoka upornost meniska povzroči velik ohmski padec vzdolž meniska. Slednje predstavlja velik problem predvsem za operando študije faradejskih procesov, kar je v študiji sistematično prikazano. Druga študija je preiskovala trdno elektrolitsko medfazo (SEI), ki nastane na silicij–grafitnih (Si/g) elektrodah pri uporabi treh različnih kombinacij elektrolitskih aditivov. Na podlagi analize XPS spektrov smo predlagali strukturne fragmente, ki so prisotni v SEI. Razlike v kemijski sestavi in debelini SEI, do katerih pride pri uporabi različnih kombinacij elektrolitskih aditivov, smo povezali z elektrokemijskimi lastnostmi s pomočjo meritev z EIS. Na ta način smo določili nekatere izmed degradacijskih mehanizmov. V raziskavi smo med drugim jasno pokazali, kako pomembno je, da so elektrokemijske meritve zasnovane tako, da je lahko razločimo prispevke posameznih elektrod. Tretji preiskovani sistem je bila kovinska litijeva elektroda v stiku s karbonatnim elektrolitom. Površina litijeve elektrode je bila modificirana s tanko prevleko, sestavljeno iz biopolimera in LiNO$_3$. S serijo meritev EIS smo določili vpliv tanke prevleke na elektrokemijski odziv litijeve elektrode. Na podlagi pridobljenega razumevanja smo razvili model transmisijske linije (TLM). Veljavnost modela smo potrdili s primerjavo simulacij in eksperimentalnih trendov. S pomočjo eksperimentov in simulacij smo doloˇcili tudi mehanizem masnega prenosa skozi prevleko. Splošnost razvitega modela TLM smo prikazali tako, da smo model uporabili za določitev transportnih parametrov v novem elektrokemijskem sistemu, natančneje, v sistemu, v katerem smo tekoči elektrolit nadomestili s polimernim elektrolitom.

• Keywords: faza med elektrodo in elektrolitom, rentgenska fotoelektronska spektroskopija, elektrokemijska impedančna spektroskopija, masni transport, elektrokemijski procesi