Elizabeth Castillo-Martínez

Metal–Organic Nanosheets Formed via Defect-Mediated Transformation of a Hafnium Metal–Organic Framework

We report a hafnium-containing MOF, hcp UiO-67(Hf), which is a ligand-deficient layered analogue of the face-centered cubic fcu UiO-67(Hf). hcp UiO-67 accommodates its lower ligand:metal ratio compared to fcu UiO-67 through a new structural mechanism: the formation of a condensed “double cluster” (Hf12O8(OH)14), analogous to the condensation of coordination polyhedra in oxide frameworks. In oxide frameworks, variable stoichiometry can lead to more complex defect structures, e.g., crystallographic shear planes or modules with differing compositions, which can be the source of further chemical reactivity; likewise, the layered hcp UiO-67 can react further to reversibly form a two-dimensional metal–organic framework, hxl UiO-67. Both three-dimensional hcp UiO-67 and two-dimensional hxl UiO-67 can be delaminated to form metal–organic nanosheets. Delamination of hcp UiO-67 occurs through the cleavage of strong hafnium-carboxylate bonds and is effected under mild conditions, suggesting that defect-ordered MOFs could be a productive route to porous two-dimensional materials.

Response to Comment on “Cycling Li-O2 batteries via LiOH formation and decomposition”

We described a lithium-oxygen (Li-O2) battery comprising a graphene electrode, a dimethoxyethane-based electrolyte, and H2O and lithium iodide (LiI) additives, lithium hydroxide (LiOH) being the predominant discharge product. We demonstrate, in contrast to the work of Shen et al., that the chemical reactivity between LiOH and the triiodide ion (I3–) to form IO3– indicates that LiOH can be removed on charging; the electrodes do not clog, even after multiple cycles, confirming that solid products are reversibly removed.

Identifying the Structural Basis for the Increased Stability of the Solid Electrolyte Interphase Formed on Silicon with the Additive Fluoroethylene Carbonate

To elucidate the role of fluoroethylene carbonate (FEC) as an additive in the standard carbonate-based electrolyte for Li-ion batteries, the solid electrolyte interphase (SEI) formed during electrochemical cycling on silicon anodes was analyzed with a combination of solution and solid-state NMR techniques, including dynamic nuclear polarization. To facilitate characterization via 1D and 2D NMR, we synthesized 13C-enriched FEC, ultimately allowing a detailed structural assignment of the organic SEI. We find that the soluble poly(ethylene oxide)-like linear oligomeric electrolyte breakdown products that are observed after cycling in the standard ethylene carbonate-based electrolyte are suppressed in the presence of 10 vol% FEC additive. FEC is first defluorinated to form soluble vinylene carbonate and vinoxyl species, which react to form both soluble and insoluble branched ethylene-oxide-based polymers. No evidence for branched polymers is observed in the absence of FEC.

Understanding Fluoroethylene Carbonate and Vinylene Carbonate Based Electrolytes for Si Anodes in Lithium Ion Batteries with NMR Spectroscopy

Fluoroethylene carbonate (FEC) and vinylene carbonate (VC) are widely used as electrolyte additives in lithium ion batteries. Here we analyze the solid electrolyte interphase (SEI) formed on binder-free silicon nanowire (SiNW) electrodes in pure FEC or VC electrolytes containing 1 M LiPF6 by solid-state NMR with and without dynamic nuclear polarization (DNP) enhancement. We find that the polymeric SEIs formed in pure FEC or VC electrolytes consist mainly of cross-linked poly(ethylene oxide) (PEO) and aliphatic chain functionalities along with additional carbonate and carboxylate species. The formation of branched fragments is further confirmed by 13C-13C correlation NMR experiments. The presence of cross-linked PEO-type polymers in FEC and VC correlates with good capacity retention and high Coulombic efficiencies of the SiNWs. Using 29Si DNP NMR, we are able to probe the interfacial region between SEI and the Si surface for the first time with NMR spectroscopy. Organosiloxanes form upon cycling, confirming that some of the organic SEI is covalently bonded to the Si surface. We suggest that both the polymeric structure of the SEI and the nature of its adhesion to the redox-active materials are important for electrochemical performance.

Polymeric Redox Active Electrodes for Sodium Ion Batteries

Polymer binding agents are critical for the good performance of the electrodes of Na- and Li-ion batteries during cycling as they hold the electroactive material together to form a cohesive assembly because of their mechanical and chemical stability as well as adhesion to the current collector. New redox-active polymer binders that insert Na+ ions and show adhesion properties were synthesized by adding polyether amine blocks (Jeffamine) based on mixed propylene oxide and ethylene oxide blocks to p-phenylenediamine and terephthalaldehyde units to form electroactive Schiff-base groups along the macromolecule. The synthetic parameters and the electrochemical properties of these terpolymers as Na-ion negative electrodes in half cells were studied. Reversible capacities of 300u2005mAhu2009g-1 (50u2005wtu2009% conducting carbon) and 200u2005mAhu2009g-1 (20u2005wtu2009% conducting carbon) were achieved in powder and Cu-supported electrodes, respectively, for a polySchiff-polyether terpolymer synthesized by using a poly(ethylene oxide) block of 600u2005gu2009mol-1 in place of one third of the aniline units. The new redox-active polymers were also used as a binding agent of another anode material (hard carbon), which led to an increase of the total capacity of the electrode compared to that prepared with other standard fluorinated polymer binders such as poly(vinylidene) fluoride.