UC Berkeley

Chapter 1: Anionic framework materials, including anionic metal–organic frameworks (MOFs) and anionic covalent organic frameworks (COFs), are promising candidates as solid-state electrolytes for next-generation lithium metal batteries. In contrast to charge-neutral framework materials, which serve only as porous supports for a liquid electrolyte, anionic framework materials also act as sources of lithium ions. Immobilization of anions onto the backbone of the framework makes lithium ion the predominant mobile species within the material, turning the material into a single ionic conductor. Strategies for developing anionic framework materials are summarized and technical details for proper characterization of the materials are discussed.

Chapter 2: A new anionic MOF, termed MOF-688, was synthesized by linking ditopic amino functionalized polyoxometalate [N(C4H9)4]3[MnMo6O18{(OCH2)3CNH2}2] with 4-connected tetrahedral tetrakis(4-formylphenyl)methane building units through imine condensation. The structure of MOF-688 was solved by single crystal X-ray diffraction. Lithium ions exchanged MOF-688 gives high ionic conductivity (3.4 × 10-4 S cm-1 at 20 °C), high lithium ion transference number (0.87), and low interfacial resistance (353 Ω) against metallic lithium. A prototype lithium metal battery constructed using MOF-688 as the solid electrolyte can be cycled at room temperature with a current density of 0.2 C.

Chapter 3: Porous MOF glasses can provide the opportunity of generating monoliths without grain boundaries, which is the mechanically fragile part of the pellet made from crystalline MOFs. MOF glasses with carboxylate linkage was synthesized by linking Ti16O16(OEt)32 and ditopic linker fumaric acid. The obtained MOF glass, termed Ti-Fum, showed a nitrogen BET surface area of 923 m2 g-1, nearly three times higher than Ti-BPA—the phenolate-linked MOF glass with the highest BET surface area reported previously. The formation of the linkage between Ti-oxo clusters and bridging carboxylate was verified by infrared. The structural integrity of discrete Ti-oxo clusters inside the MOF glass was evidenced by X-ray absorption near edge structure (XANES) and 17O magic angle spinning (MAS) NMR spectroscopy. Two additional carboxylic acid linkers were also used to demonstrate the modularity and versatility of this approach.

Chapter 4: Two strategies were employed to design and synthesize anionic COFs that adsorb water from air at low relative humidity. In the first strategy, the Povarov reaction between aryl imines and aryl alkynes was applied to COFs to convert the imine linkages into quinoline linkages while introducing carboxylates at the same time. In the second strategy, a sulfonate containing COF was synthesized directly by reacting 4,4',4'',4'''-(pyrene-1,3,6,8-tetrayl)tetrakis(2-hydroxybenzaldehyde) with ammonium 2,5-diaminobenzenesulfonate. The conversion was evaluated by infrared and elemental analysis, and the crystallinity of the COFs was confirmed by powder X-ray diffraction. Compared with the COFs without side chain, the COFs with anionic side chain showed significantly improved water uptake performance at low relative humidity.

Chapter 5: This chapter provides an outlook on anionic frameworks as solid-state electrolytes and water harvesters.