Venkataraman Thangadurai

Anhydrous proton conduction at 150 °C in a crystalline metal–organic framework

Metal organic frameworks (MOFs) are particularly exciting materials that couple porosity, diversity and crystallinity. But although they have been investigated for a wide range of applications, MOF chemistry focuses almost exclusively on properties intrinsic to the empty frameworks; the use of guest molecules to control functions has been essentially unexamined. Here we report Na(3)(2,4,6-trihydroxy-1,3,5-benzenetrisulfonate) (named β-PCMOF2), a MOF that conducts protons in regular one-dimensional pores lined with sulfonate groups. Proton conduction in β-PCMOF2 was modulated by the controlled loading of 1H-1,2,4-triazole (Tz) guests within the pores and reached 5 × 10(-4) S cm(-1) at 150 °C in anhydrous H(2), as confirmed by electrical measurements in H(2) and D(2), and by solid-state NMR spectroscopy. To confirm its potential as a gas separator membrane, the partially loaded MOF (β-PCMOF2(Tz)(0.45)) was also incorporated into a H(2)/air membrane electrode assembly. The resulting membrane proved to be gas tight, and gave an open circuit voltage of 1.18 V at 100 °C.

Negating interfacial impedance in garnet-based solid-state Li metal batteries

Garnet-type solid-state electrolytes have attracted extensive attention due to their high ionic conductivity, approaching 1 mS cm-1, excellent environmental stability, and wide electrochemical stability window, from lithium metal to ∼6 V. However, to date, there has been little success in the development of high-performance solid-state batteries using these exceptional materials, the major challenge being the high solid-solid interfacial impedance between the garnet electrolyte and electrode materials. In this work, we effectively address the large interfacial impedance between a lithium metal anode and the garnet electrolyte using ultrathin aluminium oxide (Al2O3) by atomic layer deposition. Li7La2.75Ca0.25Zr1.75Nb0.25O12 (LLCZN) is the garnet composition of choice in this work due to its reduced sintering temperature and increased lithium ion conductivity. A significant decrease of interfacial impedance, from 1,710 Ω cm2 to 1 Ω cm2, was observed at room temperature, effectively negating the lithium metal/garnet interfacial impedance. Experimental and computational results reveal that the oxide coating enables wetting of metallic lithium in contact with the garnet electrolyte surface and the lithiated-alumina interface allows effective lithium ion transport between the lithium metal anode and garnet electrolyte. We also demonstrate a working cell with a lithium metal anode, garnet electrolyte and a high-voltage cathode by applying the newly developed interface chemistry.

Bio-inspired phosphole-lipids: from highly fluorescent organogels to mechanically responsive FRET.

Sensitive gels: the amphiphilic features of phosphole-lipids lead to intriguing self-assembly properties and the formation of highly fluorescent organogels. Moreover, the dynamic structural features of the system make it possible to amplify the mechanochromic emission shifts (100 nm) in a donor-acceptor system through thermally and mechanically responsive fluorescence resonance energy transfer (FRET).

Fast Solid-State Li Ion Conducting Garnet-Type Structure Metal Oxides for Energy Storage

Lithium ion batteries are the most promising energy storage system on the market today; however, safety issues associated with the use of flammable organic polymer-based electrolytes with poor electrochemical and chemical stabilities prevent this technology from reaching maturity. Solid lithium ion electrolytes (SLIEs) are being considered as potential replacements for the organic electrolytes to develop all-solid-state Li ion batteries. Out of the recently discovered SLIEs, the garnet-related structured Li-stuffed metal oxides are the most promising electrolytes due to their high total (bulk + grain boundary) Li ion conductivity, high electrochemical stability window (∼6 V versus Li(+)/Li at room temperature), and chemical stability against reaction with an elemental Li anode and high-voltage metal oxide Li cathodes. This Perspective discusses the structural-chemical composition-ionic conductivity relationship of Li-stuffed garnets, followed by a discussion on the Li ion conduction mechanism, as well as the electrochemical and chemical stability of these materials. The performance of a number of all-solid-state batteries employing garnet-type Li ion electrolytes is also discussed.

NMR relaxometry as a versatile tool to study Li ion dynamics in potential battery materials.

NMR spin relaxometry is known to be a powerful tool for the investigation of Li(+) dynamics in (non-paramagnetic) crystalline and amorphous solids. As long as significant structural changes are absent in a relatively wide temperature range, with NMR spin-lattice (as well as spin-spin) relaxation measurements information on Li self-diffusion parameters such as jump rates and activation energies are accessible. Diffusion-induced NMR relaxation rates are governed by a motional correlation function describing the ion dynamics present. Besides the mean correlation rate of the dynamic process, the motional correlation function (i) reflects deviations from random motion (so-called correlation effects) and (ii) gives insights into the dimensionality of the hopping process. In favorable cases, i.e., when temperature- and frequency-dependent NMR relaxation rates are available over a large dynamic range, NMR spin relaxometry is able to provide a comprehensive picture of the relevant Li dynamic processes. In the present contribution, we exemplarily present two recent variable-temperature (7)Li NMR spin-lattice relaxation studies focussing on Li(+) dynamics in crystalline ion conductors which are of relevance for battery applications, viz. Li(7) La(3)Zr(2)O(12) and Li(12)Si(7).

Chemically Stable Proton Conducting Doped BaCeO3 -No More Fear to SOFC Wastes

Development of chemically stable proton conductors for solid oxide fuel cells (SOFCs) will solve several issues, including cost associated with expensive inter-connectors, and long-term durability. Best known Y-doped BaCeO3 (YBC) proton conductors-based SOFCs suffer from chemical stability under SOFC by-products including CO2 and H2O. Here, for the first time, we report novel perovskite-type Ba0.5Sr0.5Ce0.6Zr0.2Gd0.1Y0.1O3−δ by substituting Sr for Ba and co-substituting Gd + Zr for Ce in YBC that showed excellent chemical stability under SOFC by-products (e.g., CO2 and H2O) and retained a high proton conductivity, key properties which were lacking since the discovery of YBCs. In situ and ex- situ powder X-ray diffraction and thermo-gravimetric analysis demonstrate superior structural stability of investigated perovskite under SOFC by-products. The electrical measurements reveal pure proton conductivity, as confirmed by an open circuit potential of 1.15 V for H2-air cell at 700°C, and merits as electrolyte for H-SOFCs.

Lattice Parameter and Sintering Temperature Dependence of Bulk and Grain-Boundary Conduction of Garnet-like Solid Li-Electrolytes

The dependence of the bulk and grain-boundary lithium-ion conduction on the lattice parameter by substitution of trivalent La by divalent Mg, Ca, Sr, Sr 0.5 Ba 0.5 , or Ba and monovalent Li in garnet-like Li 5 La 3 Ta 2 O 12 , and the effect of sintering temperature were investigated. The ionic bulk conductivity increases with increasing ionic radius of the divalent alkaline earth ion and corresponding increased lattice parameter. An exception is Mg, which is too small for replacing La and forms a second phase. The lattice parameter is also found to increase with increased sintering temperature, except for the mixed Sr 0.5 Ba 0.5 substituted sample. The Ca, Sr, Sr 0.5 Ba 0.5 , and Ba compounds show mainly bulk resistances with minor boundary contribution at room temperature, which decreases with increasing size of the alkaline earth ion. In contrast, the multiphase Mg-substituted sample exhibits an appreciable grain-boundary contribution to the total resistance. Microstructural investigations indicate the dependence of the grain-boundary resistance on the grain size, sinterability, and formation of transient or steady-state phase boundary compositions, which are caused by different chemical diffusion coefficients of the components. This is related to the higher conductivities of Li 6 MgLa 2 Ta 2 O 12 and Li 6 BaLa 2 Ta 2 O 12 annealed at 900°C compared to samples annealed at 950°C.

Macroscopic and microscopic Li+ transport parameters in cubic garnet-type “Li6.5La2.5Ba0.5ZrTaO12” as probed by impedance spectroscopy and NMR

The garnet-type “Li6.5La2.5Ba0.5ZrTaO12”, crystallizing with cubic symmetry was prepared according to a conventional solid state synthesis method using metal oxides and salt precursors of high purity. The formation of the “single-phase” garnet-type structure was studied by powder X-ray diffraction (PXRD). Electron microprobe analysis (EMPA) coupled with a wavelength-dispersive spectrometer (WDS) showed a rather homogeneous distribution of Ta ions and Zr ions compared to that of Ba ions and La ions in “Li6.5La2.5Ba0.5ZrTaO12”. Li ion dynamics were complementarily studied using variable-temperature AC-impedance spectroscopy and 7Li NMR measurements. The bulk (ion) conductivities probed are in very good agreement with results reported earlier, illustrating the excellent reproducibility of the Li transport properties of “Li6.5La2.5Ba0.5ZrTaO12”. In particular, AC impedance and NMR results indicate that the Li transport process studied is of long-range nature. Finally, the chemical compatibility of the electrolyte “Li6.5La2.5Ba0.5ZrTaO12” was tested with Li2FeMn3O8, being a high-voltage cathode material. As shown by variable-temperature PXRD measurements, the garnet-type structure (bulk) was found to be stable up to 673 K.

First Total H+/Li+ Ion Exchange in Garnet-Type Li5La3Nb2O12 Using Organic Acids and Studies on the Effect of Li Stuffing

Garnet-type Li(5+x)Ba(x)La(3-x)Nb(2)O(12) (x = 0, 0.5, 1) was prepared using a ceramic method, and H(+)/Li(+) ion exchange was performed at room temperature using organic acids, such as CH(3)COOH and C(6)H(5)COOH, as proton sources. Thermogravimetric analysis showed that H(+)/Li(+) ion exchange was nearly (100%) completed using the x = 0 member with CH(3)COOH, while it proceeded to about 40% for x = 0.5 and 13% for x = 1. In C(6)H(5)COOH, proton exchange proceeded to about 82% for x = 0, ∼40% for x = 0.5, and ∼25% for x = 1. Similar proton-exchange trends were reported in H(2)O, where ion exchange occurs more readily for garnets with lower Li content in Li(5+x)Ba(x)La(3-x)Nb(2)O(12), that is, when excess Li ions preferentially reside in the tetrahedral sites of the garnet structure.

Facile proton conduction in H+/Li+ ion-exchanged garnet-type fast Li-ion conducting Li5La3Nb2O12

Garnet-type Li5La3Nb2O12, among several other Li-stuffed garnet compounds, is known to be unstable under moisture and undergoes H+/Li+ ion-exchange at room temperature. While there have been reports on proton-exchange in water, the transport of the protons in the garnet structure is yet to be characterized by electrical methods. In this work, proton-exchange was performed on the pellet form of Li5La3Nb2O12 for 4, 7, and 10 days, while the pellet generally showed less proton-exchange due to less surface area exposed to water, longer treatment times did allow the reaction to proceed further. Neutron diffraction (ND) studies were utilized to locate the H-site in the garnet-type structure and to confirm which Li ion sites were exchanged. For H exchanged Li5La3Nb2O12, the neutron diffraction studies indicated that H exchanged preferentially for Li in the distorted octahedral sites of the Iad space group (no. 230). AC impedance measurements were done on as-prepared Li5La3Nb2O12 under 3% H2O + N2 and 3% D2O + N2, which suggested proton conduction under humid conditions at 23–300 °C. Li5La3Nb2O12 shows a proton conductivity of 10−5 S cm−1, of the same order of magnitude as Li ion conduction, at room temperature in humidity where protons seem to be incorporated from the solid and gas phase equilibrium reaction. Proton reversible Nafion electrode experiments indicate facile proton conduction in Li garnets under humidity.