Analyse der Degradation von Lithiumhochvoltkathodenmaterialien durch kombinierte elektrochemische und schwingungsspektroskopische Operando-Untersuchungen

Jehnichen, Philipp Andreas; Korte, Carsten (Thesis advisor); Martin, Manfred (Thesis advisor)

Aachen (2020)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2020

Abstract

The future of mobility will follow new visions and rules. The key contribution to the electric mobility establishment is the further development of batteries with higher energy and power density. Furthermore, a production costs reduction is necessary to appeal to a broader group of buyers. Both conditions could be fulfilled by the high-voltage cathode material lithium-nickel-manganese oxide (LNMO). The advantage of LNMO, besides the elimination of cobalt, is the high energy density of 700 Wh/kg with an oxidation potential of 4.7 vs. Li/Li+. The material has an advantageous charge and discharge rate due to its spinel structure. However, so far this material has not been commercially successful. The reasons for this are aging mechanisms, which greatly reduce the cycle stability. The reasons for the aging of LNMO have not yet been comprehensively proven in the literature. This is where the doctoral thesis comes in providing proof of the immobilization of the electrochemically active lithium using Operando-Raman spectroscopy. First experiments with the help of the ECC-Opto-Std cell on half cells with LNMO as cathode and lithium as anode material were carried out to set up the experiment and optimize the cell. The immobilization of the electrochemically active lithium was demonstrated in differently balanced full cells with lithium titanate as the anode material. Operando-Raman spectroscopy has shown that the cathode active material LNMO can be oxidized and reduced during the cyclization as long as electrochemically active lithium is available. If there is a complete loss of capacity in the half cells, the cathode material remains in the oxidized state of Ni4+. This proves that the main reason for the limited number of cycles is the immobilization of the electrochemically active lithium. In addition to the immobilization, decomposition reactions of the cathode material and electrolyte take place. This could be verified with the help of scanning electron microscopy and secondary-ion mass spectrometry. Analyzes identified a mechanism for aging in the high-voltage cathode cell and suggested an approach for future work to increase cycle stability.

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