Author

Benjamin Ng

Date of Award

Spring 2021

Document Type

Open Access Dissertation

Department

Chemical Engineering

First Advisor

William E. Mustain

Abstract

The work embodied in this thesis bridges the gap between modern and next-generation Li-ion batteries (LIBs). First, a detailed study of the complex interfacial interactions (e.g. electrode/electrolyte, particle/particle, particle/solid-electrolyte interphase) in existing LIBs (graphite anode, Li(Ni_0.5Mn_0.3Co_0.2)O_2 cathode) are investigated and then used to develop strategies for safer, more energy-dense, and more durable electrode materials (i.e. tailored hybrid materials). The first portion of this thesis focuses heavily on decoupling the complex thermodynamics, reaction kinetics, and mass transport properties in commercially available LIBs. This thesis includes a detailed investigation into the operating parameter space for LIBs, including temperature (-30^oC to +52^oC), state-of-charge (SOC, 0% to 100%), applied currents, and lifetime (>1000 cycles). Fundamental parameters are extracted from experimental data and implemented into two different computational models (tau lumped model and pseudo-2D model) to provide system-level predictions and isolate the inherent loss mechanisms that hinder performance, such as electrical conduction, lithium diffusion, electrolyte diffusion, and charge transfer resistance.

The next section of the thesis applies a large suite of characterization tools – including microscopy, multiple-location liquid N2 Raman spectroscopy, gas chromatography/mass spectroscopy, and X-ray photoelectron spectroscopy – to probe complex reactions that lead to cell failure. Modern LIBs, particularly commercial cells with large electrodes, can experience severe gassing, Li-plating, and anisotropic lithiation/delithiation. These negative behaviors can trigger a cascade of complex reactions that lead to thermal runaway. Such reactions include high surface area plated Li with the organic electrolyte (ethylene carbonate, dimethyl carbonate, diethyl carbonate, lithium hexafluorophosphate) under charge/discharge vs. open circuit storage. Electrolyte decomposition reactions can also occur that result in the release of large volumes of CO_2, H_2, O_2, CO, CH_4, C_2H_4, and C_2H_6 gases, which causes drastic morphological and microstructural changes to the electrode. Also, the severe polarization of the electrode at low temperatures can cause significant Li0 residence at high-stress regions (i.e. high curvature, edges, electrode ripples).

A paradigm shift is needed to move past the limiting factors that plague current LIB systems (e.g. low-moderate energy densities and inherent safety risks realized in Chapter 2 - Chapter 4). A search for new materials is required to meet the demands of the future. Conversion-based materials – such as transition metal sulfides, fluorides, and oxides – that leverage bond-breaking reactions are promising candidates to provide higher gravimetric and volumetric energy densities in comparison to the incumbent intercalation-based materials. Conversion materials also have a higher redox potential (~1V vs Li/Li^+), which additionally provides protection from Li-plating, resulting in safer batteries. However, in the literature, conversion-based materials have suffered from poor reaction reversibility that can lead to short battery life. Chapter 5 applies electroanalytical techniques and electron transfer theory to probe the reaction mechanisms that form the solid electrolyte interphase (SEI) and the conversion reaction for one conversion-based anode material, NiO. First, a combination of physical and electro-analytical techniques were used to investigate the SEI formation, which is the predominant capacity-degrading process in LIBs. One of the most important methods was the current-pulse relaxation method via galvanostatic intermittent titration technique (GITT), which allows for both diffusion and kinetics to be quantified along the reaction pathway (0%-100% SOC). Also, the Butler-Volmer (BV) and Marcus-Hush-Chidsey (MHC) models are used to investigate the effective transfer coefficients and reaction reorganizational energies. This information is used to provide new mechanistic insight into the rate-determining step and the SEI formation reaction pathway at different SOC and to compare the SEI formation chemistry with modern materials.

In addition to SEI formation, conversion materials undergo other degradative processes as well, including metal (charge) trapping, transformation of the transition metal in the oxide (NiO) to higher oxidation states, and agglomeration-induced loss of electrochemically active sites. Taking that into consideration, Chapter 5 introduces a new concept that isolates the NiO from the electrolyte, effectively eliminating all of the above-mentioned degradation mechanisms. This concept uses nanoconfinement of the conversion-based anode inside of small diameter carbon nanotubes. The CNT host was found to provide a termination-length for the SEI by specifically isolating the active material from the bulk electrolyte. In addition, the CNT host provides long-range interparticle electronic conductivity and immobilizes the reactants/products to one semi-closed packet. The result is a very high-capacity material (ca. 700 mAh g^-1) with very high coulombic efficiency (> 99.9%) that also has the ability for long-term operation (> 2000 deep charge/discharge cycles between 0-100% SOC at 1C).

Chapters 7 and 8 of this thesis are meant to provide some perspective on the state of the technology and where it is going. More specifically, Chapter 7 is a summary of all the fundamental findings in this work. Chapter 8 proposes future work that can be done to achieve long-life, high energy density lithium-ion batteries in the near future.

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