The global energy transition, involving the widespread adoption of electric vehicles and grid-scale energy storage, demands energy storage devices made up of abundant, inexpensive minerals. For this to be achieved, the large Co content in conventional Li-ion battery cathodes (e.g., LiCoO₂) must be replaced while also maintaining or improving the energy density of the battery. Alternative low-Co and Co-free materials (e.g., layered LiNixMnyCozO₂, spinel LiNi₀.₅Mn₁.₅O₄, and olivine LiFePO₄) are promising alternatives due to their theoretically higher energy densities or improved safety properties from the industry standards. However, in practice, these materials exhibit both bulk and interfacial instabilities that limit their practical energy density and cycle lifetime. It is well known that reactions between the delithiated (charged) cathode surface with the electrolyte generates electrolyte decomposition species that form an interphase layer called the cathode electrolyte interphase (CEI), where such reactions are concomitant with a crystallographic reconstruction of the surface of the bulk material. The CEI is air sensitive, disordered, nanometers thick and evolves as a function of state of charge and cycle number, making it difficult to fully understand its composition and effect on device performance.
The dynamic nature of the CEI necessitates development of chemical characterization tools that can analyze surface reactivity during battery operation. Commercial cathode films are also composites including not just the electrochemically active material but also conductive carbon additive and polymer binder, meaning we need spatially resolved tools to study CEI composition across the film to isolate reactivity by film component. In this thesis, we have developed and applied spatially resolved and operando characterization tools to study the CEI of low-Co and Co-free cathode materials and use these data to pinpoint the degradation reactions at play during battery operation. In the first chapter, we introduce the three most prevalent types of cathode materials (layered, spinels, and olivines) used in Li-ion batteries. We then highlight recent progress in the analytical characterization tools that have been developed to elucidate CEI composition, spatial arrangement, and formation pathways during battery operation while discussing the difference in surface reactivity between each cathode active material as revealed by these techniques. Major findings from my own thesis work, detailed in following chapters, are discussed in parallel within this broader context. Finally, equipped with a deeper understanding of the CEI and the processes that lead to its formation, we discuss what remains to be discovered and enabled by optimizing these complex interfaces.
The second chapter investigates the composition of the CEI formed by the Li-rich layered cathode material, Li₂RuO₃, to better understand performance decline in this class of materials. To bridge this gap in understanding, we use solid-state NMR (SSNMR) and surface-sensitive dynamic nuclear polarization (DNP) NMR to achieve high resolution compositional assignment of the CEI. We show that the CEI that forms on Li₂RuO₃, when cycled in carbonate-containing electrolytes, is similar to the solid electrolyte interphase (SEI) that has been observed on anode materials, containing components such as polyethylene oxide (PEO) structures, Li acetate, carbonates, and LiF. The CEI composition deposited on the cathode surface on charge is chemically distinct from that observed upon discharge, supporting the notion of crosstalk between the SEI and the CEI, with Li+-coordinating species leaving the CEI during delithiation. We use electrochemical impedance spectroscopy (EIS) to assess the impedance of the CEI on Li₂RuO₃ as a function of state of charge in connection with the migration of CEI species as identified with NMR. Migration of the outer CEI combined with the accumulation of poor ionic conducting components on the static inner CEI may contribute to the loss of performance over time in Li-excess cathode materials. This work demonstrates the utility of SSNMR for studying electrolyte decomposition at the cathode-electrolyte interface which is then applied in the following chapter to more commercially relevant materials.
In the third chapter, we study the CEI and surface reactivity of the Ni-rich layered material LiNi₀.₈Mn₀.₁Co₀.₁O₂ (NMC811). The high specific capacities of Ni-rich transition-metal oxides have garnered immense interest for improving the energy density of Li-ion batteries. However, Ni-rich cathodes suffer from interfacial instabilities that lead to formation of electrochemically inactive phases at the cathode particle surface as well as the formation of a CEI layer on the composite surface during electrochemical cycling. We use a combination of ex situ SSNMR spectroscopy and X-ray photoemission electron microscopy (XPEEM) to provide chemical and spatial information, on the nanometer length scale, on the CEI deposited on NMC811 composite cathode films. XPEEM elemental maps offer insight into the lateral arrangement of the electrolyte decomposition products that comprise the CEI and paramagnetic interactions (assessed with electron paramagnetic resonance (EPR) and relaxation measurements) in 13C SSNMR provide information on the radial arrangement of the CEI from the NMC811 particles outward. Using this approach, we find that LiF, Li₂CO₃, and carboxy-containing structures are directly appended to NMC811 active particles, whereas soluble species detected during in situ 1H and 19F solution NMR experiments (e.g., alkyl carbonates, HF, and vinyl compounds) are randomly deposited on the composite surface. We show that the combined approach of ex situ SSNMR and XPEEM, in conjunction with in situ solution NMR, allows spatially resolved, molecular-level characterization of paramagnetic surfaces and new insights into electrolyte oxidation mechanisms in porous electrode films. The in situ solution NMR cell developed here is one of the first of its kind developed specifically for studying electrolyte decomposition products during or directly after battery operation, which is further developed in the next chapter.
The fourth chapter focuses on studying the surface reactivity of the high-voltage LiNi₀.₅Mn₁.₅O₄ (LNMO) spinel cathode material. Unfortunately, LNMO-containing batteries suffer from poor cycling performance because of the intrinsically coupled processes of electrolyte oxidation and transition metal dissolution that occurs at high voltage. In this work, we use operando EPR and NMR spectroscopies to study these high voltage reactions, applying the in situ cell design from the previous chapter to operando conditions (characterization during battery charging). We demonstrate that transition metal dissolution in LNMO is tightly coupled to HF formation (and thus, electrolyte oxidation reactions as detected with operando and in situ solution NMR), indicative of an acid-driven disproportionation reaction that occurs during delithiation (battery charging). Leveraging the temporal resolution (s-min) of magnetic resonance, we find that the LNMO particles accelerate the rate of LiPF6 decomposition and subsequent Mn²⁺ dissolution, possibly due to the acidic nature of terminal Mn-OH groups and protic species generated upon oxidizing the solvents. X-ray photoemission electron microscopy (XPEEM) provides surface-sensitive and localized X-ray absorption spectroscopy (XAS) measurements, in addition to X-ray photoelectron spectroscopy (XPS), that indicate disproportionation is enabled by surface reconstruction upon charging, which leads to surface Mn³⁺ sites on the LNMO particle surface that can disproportionate into Mn²⁺(dissolved) and Mn⁴⁺(s). During discharge of the battery, we observe high quantities of metal fluorides (in particular, MnF₂) in the cathode electrolyte interphase (CEI) on LNMO as well as the conductive carbon additives in the composite. Electronic conductivity measurements indicate that the MnF₂ decreases film conductivity by threefold compared to LiF, suggesting that this CEI component may impede both the ionic and electronic properties of the cathode. Ultimately, to prevent transition metal dissolution and the associated side reactions in spinel-type cathodes (particularly those that operate at high voltages like LNMO), the use of electrolytes that offer improved anodic stability and prevent acid byproducts will likely be necessary.
In the fifth chapter, we conduct an in situ X-ray spectroscopy, electron microscopy, and electron diffraction experiment to study the oxidation of the surface of Li metal, which is of critical importance for next generation Li metal batteries. Elemental Li is one of the most promising anode materials for high energy density Li batteries if it can replace graphite because it increases the specific capacity by an order of magnitude. However, Li metal is extremely reactive and is easily oxidized by air and moisture, even under inert conditions (e.g., in argon-filled gloveboxes, ultrahigh vacuum chambers). The industrial production of Li metal anodes, their surface evolution upon contact with the electrolyte, and electrodeposition behavior upon battery cycling all rely on the initial oxidative processes that take place prior to cell assembly. To better understand Li metal oxidation, we deposit pure Li on a Cu substrate and dose the Li deposit with various amounts of oxygen gas. During this experiment, we monitor the surface composition in situ using low-energy electron microscopy (LEEM), low-energy electron diffraction (LEED), and XPS measurements. We show that by evaporating Li onto Cu substrates, we can bypass long sputtering times needed to study commercial Li foils that usually exhibit alkali metal impurities and thick contamination layers from their external environment. Combined insights from LEED, LEEM and DFT calculations indicate that upon oxygen dosing of this ultrapure Li film, oxygen adsorbs to Li, forming a disordered layer, followed by (111) oriented polycrystalline Li₂O growth. DFT was particularly instrumental in elucidating the precise work function of the surface for the intermediate oxide phases (timescale of seconds) to correlate with trends observed via in situ LEEM imaging experiments.
To conclude, we reflect on the overarching insight on cathode degradation that we have learned from these studies and discuss remaining knowledge gaps in the field. We highlight promising future avenues to study for stabilizing the cathode-electrolyte interface of these materials, such as adapting the characterization methods developed here for more high throughput study of next generation electrolyte formulations.
| Identifer | oai:union.ndltd.org:columbia.edu/oai:academiccommons.columbia.edu:10.7916/xnmk-an74 |
| Date | January 2024 |
| Creators | Hestenes, Julia Carmen |
| Source Sets | Columbia University |
| Language | English |
| Detected Language | English |
| Type | Theses |
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