Template assisted lithium superoxide growth for lithium-oxygen batteries.

Developing batteries with energy densities comparable to internal combustion technology is essential for a worldwide transition to electrified transportation. Li-O2 batteries are seen as the 'holy grail' of battery technologies since they have the highest theoretical energy density of all battery technologies. Current lithium-oxygen (Li-O2) batteries suffer from large charge overpotentials related to the electronic resistivity of the insulating lithium peroxide (Li2O2) discharge product. One potential solution is the formation and stabilization of a lithium superoxide (LiO2) discharge intermediate that exhibits good electronic conductivity. However, LiO2 is reported to be unstable at ambient temperature despite its favorable formation energy at -1.0 eV per atom. In this paper - based on our recent work on the development of cathode materials for aprotic lithium oxygen batteries including two intermetallic compounds, LiIr3 and LiIr, that are found to form good template interfaces with LiO2 - a simple goodness of fit R factor to gauge how well a template surface structure can support LiO2 growth, is developed. The R factor is a quantitative measurement to calculate the geometric difference in the unit cells of specific Miller Index 2D planes of the template surface and LiO2. Using this as a guide, the R factors for LiIr3, LiIr, and La2NiO4+δ, are found to be good. This guide is attested by simple extension to other noble metal intermetallics with electrochemical cycling data including LiRh3, LiRh, and Li2Pd. Finally, the template concept is extended to main group elements and the R factors for LiO2 (111) and Li2Ca suggest that Li2Ca is a possible candidate for the template assisted LiO2 growth strategy.

A lithium-oxygen battery based on lithium superoxide.

Batteries based on sodium superoxide and on potassium superoxide have recently been reported. However, there have been no reports of a battery based on lithium superoxide (LiO2), despite much research into the lithium-oxygen (Li-O2) battery because of its potential high energy density. Several studies of Li-O2 batteries have found evidence of LiO2 being formed as one component of the discharge product along with lithium peroxide (Li2O2). In addition, theoretical calculations have indicated that some forms of LiO2 may have a long lifetime. These studies also suggest that it might be possible to form LiO2 alone for use in a battery. However, solid LiO2 has been difficult to synthesize in pure form because it is thermodynamically unstable with respect to disproportionation, giving Li2O2 (refs 19, 20). Here we show that crystalline LiO2 can be stabilized in a Li-O2 battery by using a suitable graphene-based cathode. Various characterization techniques reveal no evidence for the presence of Li2O2. A novel templating growth mechanism involving the use of iridium nanoparticles on the cathode surface may be responsible for the growth of crystalline LiO2. Our results demonstrate that the LiO2 formed in the Li-O2 battery is stable enough for the battery to be repeatedly charged and discharged with a very low charge potential (about 3.2 volts). We anticipate that this discovery will lead to methods of synthesizing and stabilizing LiO2, which could open the way to high-energy-density batteries based on LiO2 as well as to other possible uses of this compound, such as oxygen storage.

In Situ Formed Ir3Li Nanoparticles as Active Cathode Material in Li-Oxygen Batteries.

Lithium-oxygen (Li-O2) batteries are a promising class of rechargeable Li batteries with a potentially very high achievable energy density. One of the major challenges for Li-O2 batteries is the high charge overpotential, which results in a low energy efficiency. In this work size-selected subnanometer Ir clusters are used to investigate cathode materials that can help control lithium superoxide formation during discharge, which has good electronic conductivity needed for low charge potentials. It is found that Ir particles can lead to lithium superoxide formation as the discharge product with Ir particle sizes of ∼1.5 nm giving the lowest charge potentials. During discharge these 1.5 nm Ir nanoparticles surprisingly evolve to larger ones while incorporating Li to form core-shell structures with Ir3Li shells, which probably act as templates for growth of lithium superoxide during discharge. Various characterization techniques including DEMS, Raman, titration, and HRTEM are used to characterize the LiO2 discharge product and the evolution of the Ir nanoparticles. Density functional calculations are used to provide insight into the mechanism for formation of the core-shell Ir3Li particles. The in situ formed Ir3Li core-shell nanoparticles discovered here provide a new direction for active cathode materials that can reduce charge overpotentials in Li-O2 batteries.

Toward Highly Efficient Electrocatalyst for Li-O2 Batteries Using Biphasic N-Doping Cobalt@Graphene Multiple-Capsule Heterostructures.

For the promotion of lithium-oxygen batteries available for practical applications, the development of advanced cathode catalysts with low-cost, high activity, and stable structural properties is demanded. Such development is rooted on certain intelligent catalyst-electrode design that fundamentally facilitates electronic and ionic transport and improves oxygen diffusivity in a porous environment. Here we design a biphasic nitrogen-doped cobalt@graphene multiple-capsule heterostructure, combined with a flexible, stable porous electrode architecture, and apply it as promising cathodes for lithium-oxygen cells. The biphasic nitrogen-doping feature improves the electric conductivity and catalytic activity; the multiple-nanocapsule configuration makes high/uniform electroactive zones possible; furthermore, the colander-like porous electrode facilitates the oxygen diffusion, catalytic reaction, and stable deposition of discharge products. As a result, the electrode exhibits much improved electrocatalytic properties associated with unique morphologies of electrochemically grown lithium peroxides.

Interfacial effects on lithium superoxide disproportionation in Li-O2 batteries.

During the cycling of Li-O2 batteries the discharge process gives rise to dynamically evolving agglomerates composed of lithium-oxygen nanostructures; however, little is known about their composition. In this paper, we present results for a Li-O2 battery based on an activated carbon cathode that indicate interfacial effects can suppress disproportionation of a LiO2 component in the discharge product. High-intensity X-ray diffraction and transmission electron microscopy measurements are first used to show that there is a LiO2 component along with Li2O2 in the discharge product. The stability of the discharge product was then probed by investigating the dependence of the charge potential and Raman intensity of the superoxide peak with time. The results indicate that the LiO2 component can be stable for possibly up to days when an electrolyte is left on the surface of the discharged cathode. Density functional calculations on amorphous LiO2 reveal that the disproportionation process will be slower at an electrolyte/LiO2 interface compared to a vacuum/LiO2 interface. The combined experimental and theoretical results provide new insight into how interfacial effects can stabilize LiO2 and suggest that these interfacial effects may play an important role in the charge and discharge chemistries of a Li-O2 battery.

Size- and support-dependent evolution of the oxidation state and structure by oxidation of subnanometer cobalt clusters.

Size-selected subnanometer cobalt clusters with 4, 7, and 27 cobalt atoms supported on amorphous alumina and ultrananocrystalline diamond (UNCD) surfaces were oxidized after exposure to ambient air. Grazing incidence X-ray absorption near-edge spectroscopy (GIXANES) and near-edge X-ray absorption fine structure (NEXAFS) were used to characterize the clusters revealed a strong dependency of the oxidation state and structure of the clusters on the surface. A dominant Co(2+) phase was identified in all samples. However, XANES analysis of cobalt clusters on UNCD showed that ∼10% fraction of a Co(0) phase was identified for all three cluster sizes and about 30 and 12% fraction of a Co(3+) phase in 4, 7, and 27 atom clusters, respectively. In the alumina-supported clusters, the dominating Co(2+) component was attributed to a cobalt aluminate, indicative of a very strong binding to the support. NEXAFS showed that in addition to strong binding of the clusters to alumina, their structure to a great extent follows the tetrahedral morphology of the support. All supported clusters were found to be resistant to agglomeration when exposed to reactive gases at elevated temperatures and atmospheric pressure.

Disproportionation in Li-O2 batteries based on a large surface area carbon cathode.

In this paper we report on a kinetics study of the discharge process and its relationship to the charge overpotential in a Li-O2 cell for large surface area cathode material. The kinetics study reveals evidence for a first-order disproportionation reaction during discharge from an oxygen-rich Li2O2 component with superoxide-like character to a Li2O2 component. The oxygen-rich superoxide-like component has a much smaller potential during charge (3.2-3.5 V) than the Li2O2 component (∼4.2 V). The formation of the superoxide-like component is likely due to the porosity of the activated carbon used in the Li-O2 cell cathode that provides a good environment for growth during discharge. The discharge product containing these two components is characterized by toroids, which are assemblies of nanoparticles. The morphologic growth and decomposition process of the toroids during the reversible discharge/charge process was observed by scanning electron microscopy and is consistent with the presence of the two components in the discharge product. The results of this study provide new insight into how growth conditions control the nature of discharge product, which can be used to achieve improved performance in Li-O2 cell.

Evidence for lithium superoxide-like species in the discharge product of a Li-O2 battery.

We report on the use of a petroleum coke-based activated carbon (AC) with very high surface area for a Li-O(2) battery cathode without the use of any additional metal catalysts. Electrochemical measurement in a tetra(ethylene) glycol dimethyl ether-lithium triflate (TEGDME-LiCF(3)SO(3)) electrolyte results in two voltage plateaus during charging at 3.2-3.5 and 4.2-4.3 V versus Li(+)/Li. Herein we present evidence from Raman and magnetic measurements that the lower plateau corresponds to a form of lithium peroxide with superoxide-like properties characterized by a low temperature magnetic phase transition and a high O-O stretching frequency (1125 cm(-1)). The magnetic phase transition and the high O-O stretching frequency disappear when charged to above 3.7 V. Theoretical calculations indicate that a surface superoxide structure on lithium peroxide clusters and some lithium peroxide surfaces have an unpaired electron and a high O-O stretching frequency that help explain the observations. These results provide evidence that the form of the lithium peroxide discharge product is important to obtaining a low charge overpotential, and thus improving the round-trip efficiency between discharge and charge.

A room temperature rechargeable Li2O-based lithium-air battery enabled by a solid electrolyte.

A lithium-air battery based on lithium oxide (Li2O) formation can theoretically deliver an energy density that is comparable to that of gasoline. Lithium oxide formation involves a four-electron reaction that is more difficult to achieve than the one- and two-electron reaction processes that result in lithium superoxide (LiO2) and lithium peroxide (Li2O2), respectively. By using a composite polymer electrolyte based on Li10GeP2S12 nanoparticles embedded in a modified polyethylene oxide polymer matrix, we found that Li2O is the main product in a room temperature solid-state lithium-air battery. The battery is rechargeable for 1000 cycles with a low polarization gap and can operate at high rates. The four-electron reaction is enabled by a mixed ion-electron-conducting discharge product and its interface with air.

Structural diversity by mixing chalcogen atoms in the chalcophosphate system K/In/P/Q (Q = S, Se).

The new thiophosphate salt K(4)In(2)(PS(4))(2)(P(2)S(6)) (1), the selenophosphate salts K(5)In(3)(mu(3)-Se)(P(2)Se(6))(3) (2), K(4)In(4)(mu-Se)(2)(P(2)Se(6))(3) (3), and the mixed seleno-/thiophosphate salt K(4)In(4)(mu-Se)(P(2)S(2.36)Se(3.64))(3) (4) are described. For the first time, a structurally different outcome of a chalcophosphate reaction was observed when sulfur and selenium are mixed, for example, by the use of K(2)S/P(2)Se(5)/S/In instead of K(2)Se/P(2)Se(5)/Se/In or K(2)S/P(2)S(5)/S/In. In compounds 1-4 indium atoms exist in a variety coordination environments. While in 1, indium is octahedrally coordinated, in 2-4 tetrahedral, trigonal-bipyramidal, and octahedral coordination environments are found for indium atoms. This remarkable structural diversity possibly is a reason, why particularly indium chalcophosphate flux reactions often produce a large variety of compounds at intermediate temperatures. In the mixed seleno-/thiophosphate salt K(4)In(4)(mu-Se)(P(2)S(2.36)Se(3.64))(3) (4) most of the chalcogen sites around the tetrahedrally coordinated P atoms show mixed S/Se occupancy. There is, however, a preference for Se binding to In ions and S binding to potassium ions.

Soft-hard acid-base interactions: probing coordination preferences of sulfur and selenium in mixed chalcophosphates in the family APbPS(4-x)Se(x) (A = K, Rb, Cs; x = 0-4).

The synthesis and structures of the three new compounds, KPbPS(1.84)Se(2.16) (1), RbPbPS(1.56)Se(2.43) (2), and CsPbPS(3.46)Se(0.54) (3), are reported. The solid state structures of 1-3 consist of two-dimensional layers of [PbP(S/Se)(4)] separated by alkali metal ions. The structure of 1 was solved in the orthorhombic space group Pna2(1). Compounds 2 and 3 possess the CsSmGeS(4) structure type, crystallizing in the orthorhombic space group P2(1)2(1)2(1). All compounds were refined as racemic twins. All chalcogen sites around the tetrahedrally coordinated P atoms show mixed S/Se occupancy; however, there is a preference for Se binding to Pb ions and S binding to alkali ions. A (31)P magic angle spinning NMR study on 1 suggests that, in mixed seleno-/thiophosphates, all of the anions [PS(x)Se(4-x)](3-) (x = 0, 1, 2, 3, 4) are present. The different amount of sulfur and selenium present in KPbPS(1.84)Se(2.16) (1), RbPbPS(1.56)Se(2.43) (2), and CsPbPS(3.46)Se(0.54) (3) is reflected in the solid state absorption spectra from which bandgaps of 2.2 eV were determined for 1 and 2, and a blue-shift to 2.5 eV was observed because of the higher sulfur-content in 3. Thermogravimetric analysis experiments indicated that, upon heating, compound 1 decomposes forming PbSe and sulfur together with other unidentified products. A Raman spectrum of compound 1 showed more bands than are usually observed in seleno- or thiophosphate salts and is another indicator of the mixed seleno-/thiophosphate anions found in 1.

Effect of atomic layer deposition coatings on the surface structure of anodic aluminum oxide membranes.

Anodic aluminum oxide (AAO) membranes were characterized by UV Raman and FT-IR spectroscopies before and after coating the entire surface (including the interior pore walls) of the AAO membranes by atomic layer deposition (ALD). UV Raman reveals the presence of aluminum oxalate in bulk AAO, both before and after ALD coating with Al2O3, because of acid anion incorporation during the anodization process used to produce AAO membranes. The aluminum oxalate in AAO exhibits remarkable thermal stability, not totally decomposing in air until exposed to a temperature >900 degrees C. ALD was used to cover the surface of AAO with either Al2O3 or TiO2. Uncoated AAO have FT-IR spectra with two separate types of OH stretches that can be assigned to isolated OH groups and hydrogen-bonded surface OH groups, respectively. In contrast, AAO surfaces coated by ALD with Al2O3 display a single, broad band of hydrogen-bonded OH groups. AAO substrates coated with TiO2 show a more complicated behavior. UV Raman results show that very thin TiO2 coatings (1 nm) are not stable upon annealing to 500 degrees C. In contrast, thicker coatings can totally cover the contaminated alumina surface and are stable at temperatures in excess of 500 degrees C.

The Effect of Potassium Impurities Deliberately Introduced into Activated Carbon Cathodes on the Performance of Lithium-Oxygen Batteries.

Rechargeable lithium-air (Li-O2) batteries have drawn much interest owing to their high energy density. We report on the effect of deliberately introducing potassium impurities into the cathode material on the electrochemical performance of a Li-O2 battery. Small amounts of potassium introduced into the activated carbon (AC) cathode material in the synthesis process are found to have a dramatic effect on the performance of the Li-O2 cell. An increased amount of potassium significantly increases capacity, cycle life, and round-trip efficiency. This improved performance is probably due to a larger amount of LiO2 in the discharge product, which is a mixture of LiO2 and Li2O2, resulting from the increase in the amount of potassium present. No substantial correlation with porosity or surface area in an AC cathode is found. Experimental and computational studies indicate that potassium can act as an oxygen reduction catalyst, which can account for the dependence of performance on the amount of potassium.

Structure-induced enhancement of thermal conductivities in electrospun polymer nanofibers.

Polymers that are thermally insulating in bulk forms have been found to exhibit higher thermal conductivities when stretched under tension. This enhanced heat transport performance is believed to arise from the orientational alignment of the polymer chains induced by tensile stretching. In this work, a novel high-sensitivity micro-device platform was employed to determine the axial thermal conductivity of individual Nylon-11 polymer nanofibers fabricated by electrospinning and post-stretching. Their thermal conductivity showed a correlation with the crystalline morphology measured by high-resolution wide-angle X-ray scattering. The relationship between the nanofiber internal structures and thermal conductivities could provide insights into the understanding of phonon transport mechanisms in polymeric systems and also guide future development of the fabrication and control of polymer nanofibers with extraordinary thermal performance and other desired properties.

Raman Evidence for Late Stage Disproportionation in a Li-O2 Battery.

Raman spectroscopy is used to characterize the composition of toroids formed in an aprotic Li-O2 cell based on an activated carbon cathode. The trends in the Raman data as a function of discharge current density and charging cutoff voltage provide evidence that the toroids are made up of outer LiO2-like and inner Li2O2 regions, consistent with a disproportionation reaction occurring in the solid phase. The LiO2-like component is found to be associated with a new Raman peak identified in the carbon stretching region at ∼1505 cm(-1), which appears only when the LiO2 peak at 1123 cm(-1) is present. The new peak is assigned to distortion of the graphitic ring stretching due to coupling with the LiO2-like component based on density functional calculations. These new results on the LiO2-like component from Raman spectroscopy provide evidence that a late stage disproportionation mechanism can occur during discharge and add new understanding to the complexities of possible processes occurring in Li-O2 batteries.

Effect of the size-selective silver clusters on lithium peroxide morphology in lithium-oxygen batteries.

Lithium-oxygen batteries have the potential needed for long-range electric vehicles, but the charge and discharge chemistries are complex and not well understood. The active sites on cathode surfaces and their role in electrochemical reactions in aprotic lithium-oxygen cells are difficult to ascertain because the exact nature of the sites is unknown. Here we report the deposition of subnanometre silver clusters of exact size and number of atoms on passivated carbon to study the discharge process in lithium-oxygen cells. The results reveal dramatically different morphologies of the electrochemically grown lithium peroxide dependent on the size of the clusters. This dependence is found to be due to the influence of the cluster size on the formation mechanism, which also affects the charge process. The results of this study suggest that precise control of subnanometre surface structure on cathodes can be used as a means to improve the performance of lithium-oxygen cells.

Magnetism in lithium-oxygen discharge product.

Nonaqueous lithium-oxygen batteries have a much superior theoretical gravimetric energy density compared to conventional lithium-ion batteries, and thus could render long-range electric vehicles a reality. A molecular-level understanding of the reversible formation of lithium peroxide in these batteries, the properties of major/minor discharge products, and the stability of the nonaqueous electrolytes is required to achieve successful lithium-oxygen batteries. We demonstrate that the major discharge product formed in the lithium-oxygen cell, lithium peroxide, exhibits a magnetic moment. These results are based on dc-magnetization measurements and a lithium-oxygen cell containing an ether-based electrolyte. The results are unexpected because bulk lithium peroxide has a significant band gap. Density functional calculations predict that superoxide-type surface oxygen groups with unpaired electrons exist on stoichiometric lithium peroxide crystalline surfaces and on nanoparticle surfaces; these computational results are consistent with the magnetic measurement of the discharged lithium peroxide product as well as EPR measurements on commercial lithium peroxide. The presence of superoxide-type surface oxygen groups with spin can play a role in the reversible formation and decomposition of lithium peroxide as well as the reversible formation and decomposition of electrolyte molecules.

Networks of ultrasmall Pd/Cr nanowires as high performance hydrogen sensors.

The newly developed hydrogen sensor, based on a network of ultrasmall pure palladium nanowires sputter-deposited on a filtration membrane, takes advantage of single palladium nanowires' characteristics of high speed and sensitivity while eliminating their nanofabrication obstacles. However, this new type of sensor, like the single palladium nanowires, cannot distinguish hydrogen concentrations above 3%, thus limiting the potential applications of the sensor. This study reports hydrogen sensors based on a network of ultrasmall Cr-buffered Pd (Pd/Cr) nanowires on a filtration membrane. These sensors not only are able to outperform their pure Pd counterparts in speed and durability but also allow hydrogen detection at concentrations up to 100%. The new networks consist of a thin layer of palladium deposited on top of a Cr adhesion layer 1-3 nm thick. Although the Cr layer is insensitive to hydrogen, it enables the formation of a network of continuous Pd/Cr nanowires with thicknesses of the Pd layer as thin as 2 nm. The improved performance of the Pd/Cr sensors can be attributed to the increased surface area to volume ratio and to the confinement-induced suppression of the phase transition from Pd/H solid solution (α-phase) to Pd hydride (ß-phase).

Tuning the architecture of mesostructures by electrodeposition.

When the dimension of materials decreases to mesoscale, their properties can change dramatically, depending on the boundary conditions imposed by the sample architecture including geometry, morphology, and hierarchical structures. Here we show that electrodeposition, a method for reducing materials from a solution onto a substrate, can provide a versatile pathway to tailor the architecture of mesostructures. Novel lead (Pb) structures ranging from nanowires, mesoparticles with octahedral, decahedral, and icosahedral shapes to porous nanowires, multipods, nanobrushes, and even snowflake-shaped structures were synthesized through systematically exploring electrodeposition parameters including reduction potentials, solution concentration, starting materials, supporting electrolytes, and surfactants.