Xiangyi Luo

A nanostructured cathode architecture for low charge overpotential in lithium-oxygen batteries

The lithium-oxygen battery, of much interest because of its very high-energy density, presents many challenges, one of which is a high-charge overpotential that results in large inefficiencies. Here we report a cathode architecture based on nanoscale components that results in a dramatic reduction in charge overpotential to ~0.2 V. The cathode utilizes atomic layer deposition of palladium nanoparticles on a carbon surface with an alumina coating for passivation of carbon defect sites. The low charge potential is enabled by the combination of palladium nanoparticles attached to the carbon cathode surface, a nanocrystalline form of lithium peroxide with grain boundaries, and the alumina coating preventing electrolyte decomposition on carbon. High-resolution transmission electron microscopy provides evidence for the nanocrystalline form of lithium peroxide. The new cathode material architecture provides the basis for future development of lithium-oxygen cathode materials that can be used to improve the efficiency and to extend cycle life.

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.

The Effect of Oxygen Crossover on the Anode of a Li–O2 Battery using an Ether-Based Solvent: Insights from Experimental and Computational Studies

Crosstown traffic: Further development of Li-O(2) batteries may eventually lead to their use in transportation applications. One problem that needs to be addressed is electrolyte decomposition, which has been partially mitigated by using ether- rather than carbonate-based solvents. The influence of oxygen crossover from the cathode to the anode on electrolyte, and lithium anode, decomposition in ether-based Li-O(2) batteries is investigated.

Synthesis of Porous Carbon Supported Palladium Nanoparticle Catalysts By Atomic Layer Deposition: Application for Rechargeable Lithium-O2 Battery

In this study, atomic layer deposition (ALD) was used to deposit nanostructured palladium on porous carbon as the cathode material for Li-O2 cells. Scanning transmission electron microscopy showed discrete crystalline nanoparticles decorating the surface of the porous carbon support, where the size could be controlled in the range of 2-8 nm and depended on the number of Pd ALD cycles performed. X-ray absorption spectroscopy at the Pd K-edge revealed that the carbon supported Pd existed in a mixed phase of metallic palladium and palladium oxide. The conformality of ALD allowed us to uniformly disperse the Pd catalyst onto the carbon support while preserving the initial porous structure. As a result, the charging and discharging performance of the oxygen cathode in a Li-O2 cell was improved. Our results suggest that ALD is a promising technique for tailoring the surface composition and structure of nanoporous supports in energy storage devices.

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.

Binder-Free V2O5 Cathode for Greener Rechargeable Aluminum Battery

This letter reports on the investigation of a binder-free cathode material to be used in rechargeable aluminum batteries. This cathode is synthesized by directly depositing V2O5 on a Ni foam current collector. Rechargeable aluminum coin cells fabricated using the as-synthesized binder-free cathode delivered an initial discharge capacity of 239 mAh/g, which is much higher than that of batteries fabricated using a cathode composed of V2O5 nanowires and binder. An obvious discharge voltage plateau appeared at 0.6 V in the discharge curves of the Ni-V2O5 cathode, which is slightly higher than that of the V2O5 nanowire cathodes with common binders. This improvement is attributed to reduced electrochemical polarization.

Study on the Catalytic Activity of Noble Metal Nanoparticles on Reduced Graphene Oxide for Oxygen Evolution Reactions in Lithium-Air Batteries.

Among many challenges present in Li-air batteries, one of the main reasons of low efficiency is the high charge overpotential due to the slow oxygen evolution reaction (OER). Here, we present systematic evaluation of Pt, Pd, and Ru nanoparticles supported on rGO as OER electrocatalysts in Li-air cell cathodes with LiCF3SO3-tetra(ethylene glycol) dimethyl ether (TEGDME) salt-electrolyte system. All of the noble metals explored could lower the charge overpotentials, and among them, Ru-rGO hybrids exhibited the most stable cycling performance and the lowest charge overpotentials. Role of Ru nanoparticles in boosting oxidation kinetics of the discharge products were investigated. Apparent behavior of Ru nanoparticles was different from the conventional electrocatalysts that lower activation barrier through electron transfer, because the major contribution of Ru nanoparticles in lowering charge overpotential is to control the nature of the discharge products. Ru nanoparticles facilitated thin film-like or nanoparticulate Li2O2 formation during oxygen reduction reaction (ORR), which decomposes at lower potentials during charge, although the conventional role as electrocatalysts during OER cannot be ruled out. Pt-and Pd-rGO hybrids showed fluctuating potential profiles during the cycling. Although Pt- and Pd-rGO decomposed the electrolyte after electrochemical cycling, no electrolyte instability was observed with Ru-rGO hybrids. This study provides the possibility of screening selective electrocatalysts for Li-air cells while maintaining electrolyte stability.

Compatibility of lithium salts with solvent of the non-aqueous electrolyte in Li–O2 batteries

The stability of lithium salts, especially in the presence of reduced oxygen species, O2 and H2O (even in a small amount), plays an important role in the cyclability and capacity of Li-O2 cells. This combined experimental and computational study provides evidence that the stability of the electrolyte used in Li-O2 cells strongly depends on the compatibility of lithium salts with solvent. In the case of the LiPF6-1NM3 electrolyte, the decomposition of LiPF6 occurs in the cell as evidenced by in situ XRD, FT-IR and XPS analysis, which triggers the decomposition of 1NM3 solvent due to formation of HF from the decomposition of LiPF6. These reactions lead to degradation of the electrolyte and cause poor cyclability of the cell. The same reactions are not observed when LiTFSI and LiCF3SO3 are used as the lithium salts in 1NM3 solvent, or LiPF6 is used in TEGDME solvent.

Holey two-dimensional transition metal oxide nanosheets for efficient energy storage

Transition metal oxide nanomaterials are promising electrodes for alkali-ion batteries owing to their distinct reaction mechanism, abundant active sites and shortened ion diffusion distance. However, detailed conversion reaction processes in terms of the oxidation state evolution and chemical/mechanical stability of the electrodes are still poorly understood. Herein we explore a general synthetic strategy for versatile synthesis of various holey transition metal oxide nanosheets with adjustable hole sizes that enable greatly enhanced alkali-ion storage properties. We employ in-situ transmission electron microscopy and operando X-ray absorption structures to study the mechanical properties, morphology evolution and oxidation state changes during electrochemical processes. We find that these holey oxide nanosheets exhibit strong mechanical stability inherited from graphene oxide, displaying minimal structural changes during lithiation/delithiation processes. These holey oxide nanosheets represent a promising material platform for in-situ probing the electrochemical processes, and could open up opportunities in many energy storage and conversion systems.

Synthesis and characterization of uniformly dispersed Fe3O4/Fe nanocomposite on porous carbon: application for rechargeable Li–O2 batteries

Uniformly dispersed core-shelled Fe/Fe3O4 nanocomposite on porous carbon was synthesized via a wet-chemistry approach, which was tested as a cathode material in rechargeable Li–O2 battery, showing highly active catalytic effect towards the electrochemical reactions, of particular, oxygen reduction reaction. XPS data showed the oxygen reduction reaction took place on the surface of the catalyst during discharge of the cell. Both XRD and XPS data demonstrated that lithium peroxide partook in the reversible reactions in the Li–O2 cell with a TEGDME-based electrolyte.