Realizing high-performance silicon-based lithium-ion batteries

Aupperle, Felix; Figgemeier, Egbert (Thesis advisor); Rensmo, Håkan (Thesis advisor)

Aachen : ISEA (2021, 2022)
Book, Dissertation / PhD Thesis

In: Aachener Beiträge des ISEA 161
Page(s)/Article-Nr.: 1 Online-Ressource : Illustrationen, Diagramme

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2021

Abstract

Within the quest of finding a next-generation high-capacity lithium-ion battery (LIB) the anode material plays a key-role. Especially materials which alloy together with lithium (Li) are very promising and in the focus of industry and academia. Here, silicon (Si) stands out because of its higher theoretical capacity compared to the state-of-the-art electrode materials such as graphite (Gr). Despite the advantages like low operating potential, high abundance on earth, low costs, and non-toxicity its large-scale commercialization is still delayed. The main problem is that Si undergoes a huge volume change during the (de-)alloying process which is causing a low cycling performance and low Coulombic efficiency (CE) originated from an unstable solid electrolyte interphase (SEI) layer. To overcome these problems it turned out that the addition of electrolyte additives to the main electrolyte system is the most economical, effective, and scalable approach. In this context, the work of this thesis deals with the investigation of high-performance and high-capacity LIBs by coupling Si-based LIBs with novel electrolyte formulations. The first part of the thesis sets the focus on the electrochemical performance of full pouch cells made of developmental electrode materials consisting of two different Si-based as well as one Gr-based anode material. Thereby, the electrolyte system, cut-off voltage, and temperature were varied and the most outstanding cells underwent a post-mortem analysis. Among the electric cycling data also laser microscope pictures were used to compare and evaluate the different behaviors. In the second part these cells were reconstructed in coin cells and the harvested anode and cathode electrodes were analyzed in detail via X-ray photoelectron spectroscopy (XPS). This analysis helped to explain the influence of the electrolyte additives on the chemical nature of the electrode/electrolyte interface (EEI) and gave an explanation to the electrochemical performance differences. In particular, it could be shown that the addition of (2-cyanoethyl)triethoxysilane (TEOSCN) to the electrolyte system forms a chemical and mechanical stable passivation layer on both electrodes. The third part provides additional insides of the interfacial chemistry and thermal reactivity of different SEI layers on Si-based anodes via XPS, differential scanning calorimetry (DSC), and Density Functional Theory (DFT) investigations; and again it could be shown that the electrolyte additive TEOSCN is able to form a more chemical, mechanical, and thermal stable SEI layer on top of a Si-based anode. Overall this work shows how the realization of high-performance Si-based LIBs via designer electrolyte additives can be addressed. Thereby, the work gives a direct inside view into the cell chemistry as well as the direct practical realization in industry related full pouch cells.