Direct observations of structure-electrochemical activity relationships continue to be a key challenge in secondary battery research. (6)Li magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy is the only structural probe currently available that can quantitatively characterize local lithium environments on the subnanometer scale that dominates the free energy for site occupation in lithium-ion (Li-ion) intercalation materials. In the present study, we use this local probe to gain new insights into the complex electrochemical behavior of activated 0.5(6)Li2MnO3·0.5(6)LiMn(0.5)Ni(0.5)O2, lithium- and manganese-rich transition-metal (TM) oxide intercalation electrodes. We show direct evidence of path-dependent lithium site occupation, correlated to structural reorganization of the metal oxide and the electrochemical hysteresis, during lithium insertion and extraction. We report new (6)Li resonances centered at ∼1600 ppm that are assigned to LiMn6-TM(tet) sites, specifically, a hyperfine shift related to a small fraction of re-entrant tetrahedral TMs (Mn(tet)), located above or below lithium layers, coordinated to LiMn6 units. The intensity of the TM layer lithium sites correlated with tetrahedral TMs loses intensity after cycling, indicating limited reversibility of TM migrations upon cycling. These findings reveal that defect sites, even in dilute concentrations, can have a profound effect on the overall electrochemical behavior.

The successful development of Li-air batteries would significantly increase the possibility of extending the range of electric vehicles. There is much evidence that typical organic carbonate based electrolytes used in lithium ion batteries form lithium carbonates from reaction with oxygen reduction products during discharge in lithium-air cells so more stable electrolytes need to be found. This combined experimental and computational study of an electrolyte based on a tri(ethylene glycol)-substituted trimethylsilane () provides evidence that the ethers are more stable toward oxygen reduction discharge species. X-ray photoelectron spectroscopy (XPS) and FTIR experiments show that only lithium oxides and no carbonates are formed when electrolyte is used. In contrast XPS shows that propylene carbonate (PC) in the same cell configuration decomposes to form lithium carbonates during discharge. Density functional calculations of probable decomposition reaction pathways involving solvated oxygen reduction species confirm that oligoether substituted silanes, as well as other ethers, are more stable to the oxygen reduction products than propylene carbonate. These results indicate that the choice of electrolyte plays a key role in the performance of Li-air batteries.

The potential- and coverage-dependent infrared absorption spectroscopy (IRAS) of linearly bound CO on single-phase polycrystalline arc-melted Pt, PtRu(1/1), PtRu(8/2), PtOs(8/2), PtRuOs(8/1/1), PtRuOs(65/25/10), and Ru electrodes in 0.5 M H2SO4 are correlated with the potential-dependent X-ray photoelectron spectroscopy (XPS) of the PtRu(1/1), PtOs(8/2), and PtRuOs(65/25/10) substrates. The CO stretching frequencies decrease as the mole fraction of Pt in the alloy is decreased. The CO oxidation onset on pure Pt at 100.0% CO coverage is 0.5 V vs a reversible hydrogen electrode and shifts negatively as the alloy mole fraction of Pt is reduced. At CO dosing conditions that yield 100% coverage on pure Pt, the CO bandwidths increase with decreasing Pt mole fraction:  on pure Pt the bandwidths increase as the CO coverage is reduced. The effects of CO coverage and bulk alloy composition on the Stark tuning rates (STRs) have been systematically examined on Pt, and a series of binary and ternary alloy surfaces. The XPS data confirm a potential-dependent surface distribution of oxides and no significant surface segregation of the alloying components. The systematic displacement, to lower frequencies, of the linear STRs as the mole fraction of Pt is reduced suggests no significant island formation on the arc-melted alloy surfaces. The XPS data also suggest that the alloying metals, rather than Pt, are responsible for activation of the water required for methanol oxidation in the direct methanol fuel cell potential window.

Surface alumina coatings have been shown to be an effective way to improve the stability and cyclability of cathode materials. However, a detailed understanding of the relationship between the surface coatings and the bulk layered oxides is needed to better define the critical cathode-electrolyte interface. In this paper, we systematically studied the effect of the composition of Ni-rich LiNiMnCoO (NMC) on the surface alumina coatings. Changing cathode composition from LiNiMnCoO (NMC532) to LiNiMnCoO (NMC622) and LiNiMnCoO (NMC811) was found to facilitate the diffusion of surface alumina into the bulk after high-temperature annealing. By use of a variety of spectroscopic techniques, Al was seen to have a high bulk compatibility with higher Ni/Co content, and low bulk compatibility was associated with Mn in the transition metal layer. It was also noted that the cathode composition affected the observed morphology and surface chemistry of the coated material, which has an effect on electrochemical cycling. The presence of a high surface Li concentration and strong alumina diffusion into the bulk led to a smoother surface coating on NMC811 with no excess alumina aggregated on the surface. Structural characterization of pristine NMC particles also suggests surface Co segregation, which may act to mediate the diffusion of the Al from the surface to the bulk. The diffusion of Al into the bulk was found to be detrimental to the protection function of surface coatings leading to poor overall cyclability, indicating the importance of compatibility between surface coatings and bulk oxides on the electrochemical performance of coated cathode materials. These results are important in developing a better coating method for synthesis of next-generation cathode materials for lithium-ion batteries.

We produced millions of morphologically identical platinum catalyst nanoparticles in the form of ordered arrays epitaxially grown on (111), (100), and (110) strontium titanate substrates using electron beam lithography. The ability to design, produce, and characterize the catalyst nanoparticles allowed us to relate microscopic morphologies with macroscopic catalytic reactivities. We evaluated the activity of three different arrays containing different ratios of (111) and (100) facets for an oxygen-reduction reaction, the most important reaction for fuel cells. Increased catalytic activity of the arrays points to a possible cooperative interplay between facets with different affinities to oxygen. We suggest that the surface area of (100) facets is one of the key factors governing catalyst performance in the electrochemical reduction of oxygen molecules.

Layered Li(Ni 1−x−y Mn x Co y )O 2 (NMC) oxides are promising cathode materials capable of addressing some of the challenges associated with next-generation energy storage devices. In particular, improved energy densities, longer cycle-life, and improved safety characteristics with respect to current technologies are needed. However, sufficient knowledge on the atomic-scale processes governing these metrics in working cells is still lacking. Herein, density functional theory (DFT) is employed to predict the stability of several low-index surfaces of Li(Ni 1/3 Mn 1/3 Co 1/3 )O 2 (NMC111) as a function of Li and O chemical potentials. Predicted particle shapes are compared with those of single crystal NMCs synthesized under different conditions. The most stable surfaces for stoichiometric NMC111 are predicted to be the nonpolar (104), the polar (012) and (001), and the reconstructed, polar (110) surfaces. Results indicate that intermediate spin Co 3+ ions lower the (104) surface energy. Furthermore, it was found that removing oxygen from the (012) surface was easier than from the (104) surface, suggesting a facet dependence on surface-oxygen vacancy formation. These results give important insights into design criteria for the rational control of synthesis parameters as well as establish a foundation on which future mechanistic studies of NMC surface instabilities can be developed.

Density functional theory calculations are performed within the generalized gradient approximation (GGA+U) to determine stable terminations of both low- and high-index spinel LiMn2O4 (LMO) surfaces. A grand canonical thermodynamic approach is employed, permitting a direct comparison of off-stoichiometric surfaces with previously reported stoichiometric surface terminations at various environmental conditions. Within this formalism, we have identified trends in the structure of the low-index surfaces as a function of the Li and O chemical potentials. The results suggest that, under a range of chemical potentials for which bulk LMO is stable, Li/O and Li-rich (111) surface terminations are favored, neither of which adopts an inverse spinel structure in the subsurface region. This thermodynamic analysis is extended to identify stable structures for certain high-index surfaces, including (311), (331), (511), and (531), which constitute simple models for steps or defects that may be present on real LMO particles. The low- and high-index results are combined to determine the relative stability of each surface facet under a range of environmental conditions. The relative surface energies are further employed to predict LMO particle shapes through a Wulff construction approach, which suggests that LMO particles will adopt either an octahedron or a truncated octahedron shape at conditions in which LMO is thermodynamically stable. These results are in agreement with the experimental observations of LMO particle shapes.

To reduce fuel cell cost, durable and inexpensive electrode catalysts need to be developed to replace precious metal materials, particularly for the electrocatalytic oxygen reduction at cathodes. In this study, we explored the structure and the energetics of Fe-xN (x ) 2,4) incorporated into carbon nanotubes and graphene using density functional theory to show that these structures are more stable than iron atoms on nanotubes and that pyridinic structures of Fe-4N are more favorable than pyrrolic structures. EXAFS spectra simulated from the optimized structures show good agreement with results of measurements obtained on arrays of aligned nanotubes doped with iron and nitrogen, which have demonstrated activity toward oxygen-reduction reactions.