I detta arbete har elektrokemisk impedansspektroskopi använts för att producera reproducerbara impedansdata för katodmaterialet NMC811. Data som skulle kunna användas som basen för parametrisering och konstruktionen av en fysik-baserad modell. Dessutom har effekten av laddningstillstånd, temperatur, och historian av cellen på impedansen undersökts. Baserat på resultaten av preliminära test så har experiment konstruerats i vilka katodens impedans i en NMC811//Grafit cell har undersökts vid olika temperaturer och laddningstillstånd, både efter laddning och urladdning av cellen. Reproducerbara resultat kunde erhållas och det visades hur laddningstillstånd och temperatur har en stor påverkan på impedansen. Tyvärr så kunde inget sägas om hystereseffekten på grund av dålig stabilitet i lågfrekvensområdet av impedansmätningarna. / In this work, electrochemical impedance spectroscopy has been used to try and produce reproducible impedance data for the cathode material NMC811. Data that could serve as the basis of parameter extraction for the construction of a physics-based model. Furthermore, the effect of state of charge, temperature, and history of the cell on the impedance has been analysed. Based on the results of preliminary tests, an experimental cycle was constructed in which the cathode impedance of a NMC811//Graphite cell was measured at varying temperatures and state of charge, both following charge and discharge of the cell. Reproducible results were achieved, and it was shown how the state of charge and temperature of the cell had a major effect on the measured impedance. Unfortunately, no conclusions could be made about the history effect on impedance due to poor stability in the low frequency regions of the impedance measurements.

Understanding the degradation mechanisms of Li-ion batteries is essential to gain insights into battery aging. The primary research area of this thesis is the positive electrode, NMC811. The purpose of the thesis is to understand how low and elevated temperatures affect the aging of NMC811, by considering the effects on resistance.  The aim of the thesis is to investigate the degradation mechanisms of NMC811. Here, three-electrode Li-ion pouch cells are assembled with LiNi8Mn1Co1O2 (NMC811) as the positive electrode, graphite as the negative, gold wire as the reference electrode, and LiPF6 as the electrolyte. The positive electrode impedance is recorded at temperatures –10, 22, and 40 ºC. Also, symmetric and half cells are built for validation measurements. The Nyquist diagrams are fitted through equivalent circuits to determine the cells’ impedance at voltages 3.8 and 3.0 V vs Li+/Li. The resistances observed and analyzed in this project are the high-frequency resistance, the contact resistance, the charge transfer resistance, and the resistance due to the electrode–electrolyte interphase. By comparing these resistances, it is observed that the charge transfer resistance has the highest dependence on the ambient temperature. The increase in charge transfer resistance at –10 ºC is suggested to depend on the Ni-rich electrode, which tends to contribute to volume changes in the electrode, affecting the intercalation and de-intercalation of Li-ions. The resistance reduces significantly at 40 ºC, due to the loss of lithium inventory in the active material. This thesis has thus shown that temperature has a significant effect on cell internal resistance, especially on the electrode–electrolyte interface, which describes the charge transfer reactions.

State-of-the-art Li-ion cathode materials are based on LiMO2 (M=Ni, Mn, Co) layered transition metal oxides (denoted NMC) with Ni-rich composition because of their high specific capacity. Yet, these materials suffer from poor capacity retention due to crack formation during de-/lithiation cycling. Particle cracking leads to exposure of new electrode surface which leads to Li-inventory loss, increased side reactions, and electric disconnection. Quantification of the extent of cracking is therefore desirable, especially during in situ whilst cycling of the Li-ion cell. Herein, we evaluate and improve an analytical methodology based on electrochemical impedance spectroscopy (EIS) in order to estimate the changes in electrochemically active surface area of both poly- and single-crystalline Ni0.8Mn0.1Co0.1(NMC811) active materials. A transmission-line model (TLM) applied to both non-blocking and blocking electrode condition was utilized in order to deconvolute and interpret the acquired experimental data. Fits of the complex TLM equivalent-circuits to the impedance spectra was facilitated by developing a global stochastic iterative function based on local multivariate optimization. Impedance analysis during short- term cycling showed that the single-crystalline NMC811 suffered from less particle cracking and side reactions compared to polycrystalline NMC811, which was also confirmed from post-mortem gas adsorption analysis. A novel approach to estimate the extent of particle cracking in commercial Li-ion cells by utilizing an empirically strong positive correlation between the charge-transfer capacitance and resistance was proposed. The work presented herein demonstrates the unique prospects of the EIS methodology in the development and research of future rechargeable batteries