Since their commercialization in 1990, the electrodes of the lithium-ion battery have remained fundamentally the same. While energy density improvements have come from reducing the cell packaging, higher capacity electrodes are needed to continue this trend. A lithium metal anode, where the negative electrode half reaction is the plating and stripping of metallic lithium, is explored as an alternative to current graphite anodes. The specific capacity of the lithium metal anode is over ten times that of the graphite anode, making it a serious candidate to further improve the energy density of lithium batteries. Electrodeposited lithium metal forms dendrites, sharp needles that can grow across the separator and short circuit the battery. Thus, a chief goal is to alter lithium’s plating morphology. This was achieved in two separate ionic liquid electrolytes by co-depositing lithium with sodium. The co-deposited sodium is thought to block dendritic sites, leading to a granular deposit. A nucleation study confirmed that metal deposits from the ionic liquid electrolyte containing sodium, prevented dendritic growth from nucleation on, and not after dendrites had already grown. A model based on the geometry of the nuclei was used to gain insight into the effect of the solid electrolyte interface (SEI) that forms on freshly deposited lithium metal. In addition to sodium, the effect of alkaline earth metals on the lithium deposit morphology was also explored. While these metals did not deposit from the ionic liquid electrolyte, their addition also resulted in granular, dendrite free, deposits. The alkaline earth additives generally increased the overpotential for nucleating on the substrate and lowered the current density achievable. Strontium and barium showed the least of these negative effects while still providing a dendrite free deposit. A second hurdle for lithium metal anodes is the instability between the electrolyte and lithium metal. A protective SEI layer that prevents undesired side reactions is difficult to form because of the large volume change associated with cycling. Formation of a better SEI on lithium metal was attempted through the addition vinylene carbonate, which greatly improved the coulombic efficiency of lithium metal plating and stripping. The effect of gases, such as oxygen, nitrogen and carbon dioxide, on the SEI layer was also investigated. It was found that the presence of nitrogen and oxygen improved the coulombic efficiency by facilitating a thinner SEI layer. This work presents attempts at improving the lithium metal anode both by increasing the coulombic efficiency of the redox process and by eliminating dendrite growth. The coulombic efficiency was improved through the bubbling of gases and addition of organic additives but work remains to increase this value further. Dendritic growth, which poses a safety hazard, was completely eliminated by two methods: 1) co-deposition and 2) adsorption of a foreign metal. Both methods could potentially be applied to different electrolytes, making them promising methods for preventing dendritic growth in future lithium metal anodes.

The shortage of energy sources and the global climate change crisis have become critical issues. Solving these problems with clean and sustainable energy sources (solar, wind, tidal, and so on) is a promising solution. In this regard, energy storage techniques need to be implemented to tackle with the intermittent nature of the sustainable energies. Among the next-generation energy storage systems, lithium sulfur batteries has gained prominence due to the low cost, high theoretical specific-capacity of sulfur. Extensive research has been conducted on this battery system. Nevertheless, several issues including the “shuttle effect” and the growth of lithium dendrites still exist, which could cause rapid capacity loss and safety hazards. Several methods are proposed to tackle the challenges in this dissertation, including cathode engineering, interlayer design, and lithium metal anode protection. An asymmetric cathode structure is first developed by a non-solvent induced phase separation (NIPS) method. The asymmetric cathode comprises a nanoporous matrix and ultrathin and dense top layer. The top-layer is a desired barrier to block polysulfides transport, while the sublayer threaded with cationic networks facilitate Li-ions transport and sulfur conversions. In addition, a conformal and ultrathin microporous membrane is electrodeposited on the whole surface of the cathode by an electropolymerization method. This strategy creates a close system, which greatly blocks the LiPS leakage and improves the sulfur utilization. A polycarbazole-type interlayer is deposited on the polypropylene (PP) separator via an electropolymerization method. This interlayer is ultrathin, continuous, and microporous, which defines the critical properties of an ideal interlayer that is required for advanced Li–S batteries. Meanwhile, a self-assembled 2D MXene based interlayer was prepared to offer abundant porosity, dual absorption sites, and desirable electrical conductivity for Li-ions transport and polysulfides conversions. A new 2D COF-on-MXene heterostructures is prepared as the lithium anode host. The 2D heterostructures has hierarchical porosity, conductive frameworks, and lithiophilic sites. When utilized as a lithium host, the MXene@COF host can efficiently regulate the Li+ diffusion, and reduce the nucleation and deposition overpotential, which results in a dendrite-free and safer Li–S battery.

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Thesis advisor: Dunwei Wang / Thesis advisor: Matthias Waegele / Lithium metal anode (LMA) holds great promise as alternative anode material for next-generation high energy density batteries. Efficiency and safety are two most critical concerns that impede practical application of LMA due to unstable interface between the electrode and the electrolyte. Solid electrolyte interphase (SEI), a passivation layer formed from electrolyte decompositions on the LMA surface, dictates the chemical and mechanical evolution of the electrode/electrolyte interface, and therefore directly affect the cycle life of lithium metal batteries. Although significant progress has been achieved to improve battery performance, a thorough understanding of SEI functions and properties is still inadequate. Both compositional and structural complexity severely hinder the efforts to uncover the SEI formation and evolution mechanism. To achieve stable lithium plating and stripping over cycling, it is necessary to lay a foundation of composition-structure-property relationships that can guide rational design of ideal SEI.First, to solve the safety and efficiency issues simultaneously, a facile and effective way to enable LMA in nonflammable electrolyte was identified by simply introducing oxygen into the battery. Reversible lithium plating and stripping was realized in a flame retardant triethyl phosphate solvent otherwise incompatible to LMA. A unique electrochemically induced electrolyte decomposition pathway was proposed and studied computationally and experimentally. The SEI formation mechanism enriches the knowledge of on the complex reactions toward an ideal SEI. The operation of Li-O2 batteries and Li-ion batteries were also demonstrated in a nonflammable phosphate electrolyte system. To understand the unique role of different SEI compositions, in the second part of this thesis, we designed and synthesized two-component artificial SEI model structures for comparison study. Our central hypothesis is that tailoring LiF and Li3PO4 compositions in the SEI layer can achieve a balanced and improved electrode/electrolyte stability. A magnetron sputtering method was developed to prepare LiF and Li3PO4 mixture films on Cu substrate. Preliminary results from battery cycling tests shows that mixture SEI structure is correlated to improved Coulumbic efficiency. Next, to understand detailed Li+ ion transport properties of the SEI. We presented an outline the current understanding of Li+ ion transport mechanisms and their dependence on the SEI. We also built on this fundamental knowledge to discuss practical effects in experimental systems. Lastly, we shared our perspectives on critical remaining questions in this field. In parallel to study on electrochemical energy system, developing electrochemical methods for integrated catalysis constitutes another part of thesis. We demonstrated that reactivity of an immobilized iron catalyst could be altered by application of an electrochemical potential to a surface to enable polymerization of different classes of monomers. A method was developed to pattern functional surfaces by using electrochemical potential to activate and deactivate polymerization reactions. The orthogonal reactivity of switchable polymerization catalysts was utilized to create patterned surfaces functionalized with two different polymers initiated from mixtures of monomers. / Thesis (PhD) — Boston College, 2022. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

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