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
| Identifer | oai:union.ndltd.org:UPSALLA1/oai:DiVA.org:uu-449968 |
| Date | January 2021 |
| Creators | Sjödin, Mattias |
| Publisher | Uppsala universitet, Strukturkemi |
| Source Sets | DiVA Archive at Upsalla University |
| Language | English |
| Detected Language | English |
| Type | Student thesis, info:eu-repo/semantics/bachelorThesis, text |
| Format | application/pdf |
| Rights | info:eu-repo/semantics/openAccess |
| Relation | UPTEC K, 1650-8297 ; 21015 |
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