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Title:Self-reporting of Mn ion dissolution and self-stabilization of cathode-electrolyte interface in lithium ion batteries
Author(s):Zhao, Lihong
Director of Research:Moore, Jeffrey S
Doctoral Committee Chair(s):Sottos, Nancy R
Doctoral Committee Member(s):Braun, Paul V; Dillon, Shen J
Department / Program:Materials Science & Engineerng
Discipline:Materials Science & Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Li-ion battery
operando characterization
autonomous strategy
Abstract:Battery degradation limits the lifespan of energy storage devices. This dissertation focuses on developing self-reporting strategies that enable in situ or operando characterization of cathodic Mn dissolution, which is a major cause of the capacity fade for some cathodes, and release of battery additives triggered by electrolyte degradation to self-stabilize the cathode-electrolyte interface. Lithium manganese oxide (LMO) is a promising cathode material for lithium-ion batteries due to its high operating voltage, high thermal stability, environmental friendliness, and low cost. One of the most significant issues limiting its performance is the dissolution of Mn, which leads to the loss of active components from the cathode, as well as the plating of Mn-rich solid electrolyte interface (SEI) on the anode. We develop and implement a self-reporting system to characterize the Mn ion dissolution during the cycling process. A UV-vis spectroscopic detection system consisting of a Mn probe and proton absorber is employed for this purpose. The Mn ion concentration is proportional to the normalized absorption peak intensity at the characteristic peak wavelength of the probe-Mn complex. The detection system is electrochemically and spectroscopically stable at the operating potential of Li-ion batteries. A customized battery cell with optical windows is built to characterize Mn ion dissolution during the cycling process. Besides spectroscopic analysis, we also investigate the mass transport of Mn in the customized cell with image analysis. The diffusion coefficient of the probe-Mn complex in the electrolyte is calculated based on the Mn ion distribution profile at the open-circuit condition. The experimental parameters for spectroscopic study are optimized with image analysis to provide a uniform Mn distribution in the electrolyte. The onset potential of Mn dissolution is determined in a miniature cell with a small electrolyte thickness and at high magnification. We find Mn dissolution starts with the emergence of the current response from LMO. Correlations between Mn dissolution, electrochemical reaction, and LMO phase transition are revealed in the operando Mn dissolution study. Mn dissolution occurs most rapidly at potentials corresponding to the phase transition potentials of LMO. Structural instability and change in surface chemistry during the LMO phase transition are regarded to be the cause of Mn dissolution. Furthermore, Mn dissolution intensifies after LMO is overdischarged to 2.5 V. Jahn-Teller distortion on Mn(III) at overdischarge potential causes irreversible damage to LMO and leads to enhanced dissolution in successive Mn dissolution. Battery additives are widely used to alleviate the capacity fade induced by the degradation at the electrode-electrolyte interface. 3-hexylthiophene (3-HT) is a battery additive that electro-polymerizes on the cathode surface during cycling and forms a protective passivation layer. However, prolonged or excess presence of 3-HT in the electrolyte induce side reactions and deteriorate battery performance. We design and prepare stimuli-responsive microcapsules with 3-HT as the core material. The microcapsules are stable in the battery electrolyte but rapidly release the core material on contact with HF, one of the electrolyte degradation products. 3-HT microcapsules are released upon electrolyte degradation and self-stabilize the cathode-electrolyte interface. The release profile of microcapsules in carbonate solvents with trigger ions further confirms the decomposition mechanisms of the shell wall polymer. Microcapsules are spin-coated on the cathode surface to control the spatial distribution of released 3-HT. We demonstrate an improved battery capacity and charge transfer properties with the autonomous release of 3-HT at the cathode surface during cycling.
Issue Date:2020-08-13
Rights Information:Copyright 2020 Lihong Zhao
Date Available in IDEALS:2021-03-05
Date Deposited:2020-12

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