Brian J. Ingram

Low temperature stabilization of cubic (Li7−xAlx/3)La3Zr2O12: role of aluminum during formation

The lithium lanthanum zirconium oxide garnet, Li7La3Zr2O12 (LLZ), has received significant attention in recent years due to its high room temperature lithium ion conductivity and its stability against lithium metal. Together these features make it a promising electrolyte candidate for a high energy all solid-state battery. Previous studies have shown that incorporation of aluminum cations during the synthesis stabilizes the higher conductivity cubic phase of LLZ; however the incorporation process and its effect on the phase transition are still unclear. In the present study, we have combined powder X-ray diffraction (XRD), 27Al and 7Li MAS NMR and high-resolution X-ray diffraction (HRXRD) to determine the disposition of Al cations during the formation of low temperature cubic LLZ. At temperatures as low as 700 °C, the aluminum is incorporated into amorphous or nanocrystalline grain boundary phases. Above 700 °C, the Al cations are associated with a poorly crystalline anti-fluorite phase Li5AlO4, composed of molecular [AlO4]5− anions. This phase then reacts with tetragonal LLZ to form cubic LLZ over a 25 hour period at 850 °C. Although the reaction appears complete by powder X-ray diffraction, 27Al NMR spectra showed overlapping resonances suggesting multiple Al environments due to uneven substitution of the 24d Li(1) site. This was confirmed by high-resolution XRD and was consistent with a series of closely related cubic LLZ phases with slightly different Al concentrations, indicating the slower Al(III) diffusion within the lattice has not reached equilibrium in the time allotted. The disorder over the two crystallographic tetrahedral sites by lithium and aluminum cations at this temperature contributes to the observed lattice enlargement associated with the low temperature cubic phase.

The Effect of Chromium Oxyhydroxide on Solid Oxide Fuel Cells

Hexavalent chromium species like the oxyhydroxide, CrO{sub 2}(OH){sub 2}, or hexoxide, CrO{sub 3}, are electrochemically reduced to Cr{sub 2}O{sub 3} in solid oxide fuel cells and adversely affect the cell operating potentials. Using a narrowly focused beam from the Advanced Photon Source, such chromium oxide deposits were unequivocally identified in the active region of the cathode by X-ray diffraction, suggesting that the triple phase boundaries were partially blocked. Under fuel cell operating conditions, the reaction has an equilibrium potential of about 0.9 V and the rate of chromium oxide deposition is therefore dependent on the operating potential of the cell. It becomes diffusion limited after several hours of steady operation. At low operating potentials, lanthanum manganite cathodes begin to be reduced to MnO, which reacts with the chromium oxide to form the MnCr{sub 2}O{sub 4} spinel.

Potassium-Assisted Chromium Transport in Solid Oxide Fuel Cells

Chromium transport from stainless steel current collectors to the cathode can be a route for rapid performance degradation in solid oxide fuel cells. Typical volatile chromium species, such as CrC 2 (OH) 2 , are manageable; however, volatile species with higher vapor pressures may play a significant role in chromium transport and deposition. One such species, K 2 CrO 4 , is readily formed by the chemical reaction between KOH or K 2 O and Cr 2 O 3 and electrochemically reduced along triple-phase boundaries. The vapor pressure and expected chromium deposition rate of potassium chromate and chromium oxyhydroxide phases are compared to the long-term electrical performance of anode-supported SOFCs. Experimental evidence supports the findings that potassium and chromium, in the presence of an applied electric field, greatly enhances the degradation of cell performance (-85% in 500 h).

In situ synchrotron X-ray diffraction studies of lithium oxygen batteries

Lithium oxygen batteries were studied in situ by synchrotron X-ray diffraction. Crystalline Li2O2 was shown to form reversibly on a plain carbon cathode under normal cycling conditions. However, if the cell was polarized to induce electrolyte decomposition before the oxygen reduction reaction was initiated, LiOH was found to cycle reversibly on the cathode instead. A mechanism linking the LiOH production to the electrolyte decomposition was proposed.

Phase-Controlled Electrochemical Activity of Epitaxial Mg-Spinel Thin Films.

We report an approach to control the reversible electrochemical activity (i.e., extraction/insertion) of Mg(2+) in a cathode host through the use of phase-pure epitaxially stabilized thin film structures. The epitaxially stabilized MgMn2O4 (MMO) thin films in the distinct tetragonal and cubic phases are shown to exhibit dramatically different properties (in a nonaqueous electrolyte, Mg(TFSI)2 in propylene carbonate): tetragonal MMO shows negligible activity while the cubic MMO (normally found as polymorph at high temperature or high pressure) exhibits reversible Mg(2+) activity with associated changes in film structure and Mn oxidation state. These results demonstrate a novel strategy for identifying the factors that control multivalent cation mobility in next-generation battery materials.

Microstructural Degradation of ( La , Sr ) MnO3 ∕ YSZ Cathodes in Solid Oxide Fuel Cells with Uncoated E-Brite Interconnects

Transmission electron microscopy was used to investigate the local chemistry and microstructure of the active cathodes in solid oxide fuel cells operated for 500 h (at 700 and 800 degrees C) under different electrochemical conditions while exposed to a chromia-forming stainless steel. Several distinct microstructural changes were observed owing to chromium interactions with the (La,Sr)MnO3 (LSM)-yttria stabilized zirconia (YSZ) cathode; the nature and magnitude of which depended on the temperature, electrochemical load, and physical location. Nanosized particles of (Cr,Mn)(3)O-4 and Cr2O3 were observed on the surface of the YSZ, regardless of the overall extent of degradation; the quantity increased with decreasing temperature and increasing current. The results indicate that Mn species facilitate the formation of a stable Cr-Mn-O nuclei on the YSZ, on which further growth occurs, including growth of Cr2O3. In cases of severe performance degradation, LSM decomposes completely, which does not appear to be strongly correlated with the nanoparticles on the YSZ, but results from a more destructive mechanism. Following this decomposition, severe pore filling of Cr-containing species occurs. The amount of microstructural degradation was largest near the cathode/electrolyte interface directly beneath the interconnect-cathode contact channels. These results indicate that two distinct mechanisms of degradation occur, with the electrochemical decomposition of (La,Sr)MnO3 as the primary cause for severe performance degradation

In situ synchrotron x-ray studies of dense thin-film strontium-doped lanthanum manganite solid oxide fuel cell cathodes.

Using a model cathode-electrolyte system composed of epitaxial thin-films of La{sub 1-x}Sr{sub x}MnO{sub 3-{delta}} (LSM) on single crystal yttria-stabilized zirconia (YSZ), we investigated changes in the cation concentration profile in the LSM during heating and under applied potential using grazing incidence x-rays. Pulsed laser deposition (PLD) was used to grow epitaxial LSM(011) on YSZ(111). At room temperature, we find that Sr segregates to form Sr enriched nanoparticles and upon heating the sample to 700 C, Sr is slowly reincorporated into the film. We also find different amounts of Sr segregation as the X-ray beam is moved across the sample. The variation in the amount of Sr segregation is greater on the sample that has been subject to 72 hours of applied potential, suggesting that the electrochemistry plays a role in the Sr segregation.

Enhanced Stability of the Carba-closo-dodecaborate Anion for High-Voltage Battery Electrolytes through Rational Design

Future energy applications rely on our ability to tune liquid intermolecular interactions and achieve designer electrolytes with highly optimized properties. In this work, we demonstrate rational, combined experimental-computational design of a new carba- closo-dodecaborate-based salt with enhanced anodic stability for Mg energy storage applications. We first establish, through a careful examination using a range of solvents, the anodic oxidation of a parent anion, the carba- closo-dodecaborate anion at 4.6 V vs Mg0/2+ (2.0 vs Fc0/+), a value lower than that projected for this anion in organic solvent-based electrolytes and lower than weakly associating bis(trifluoromethylsulfonyl)imide and tetrafluoroborate anions. Solvents such as acetonitrile, 3-methylsulfolane, and 1,1,1,3,3,3-hexafluoroisopropanol are shown to enable the direct measurement of carba- closo-dodecaborate oxidation, where the resultant neutral radical drives passive film formation on the electrode. Second, we employ computational screening to evaluate the impact of functionalization of the parent anion on its stability and find that replacement of the carbon-vertex proton with a more electronegative fluorine or trifluoromethyl ligand increases the oxidative stability and decreases the contact-ion pair formation energy while maintaining reductive stability. This predicted expansion of the electrochemical window for fluorocarba- closo-dodecaborate is experimentally validated. Future work includes evaluation of the viability of these derivative anions as efficient and stable carriers for energy storage as a function of the ionic transport through the resulting surface films formed on candidate cathodes.