New insights into the electrode/electrolyte interface on positive electrodes in Li-Ion batteries

International audienceUnderstanding and controlling the reactivity at the electrode/electrolyte interface (EEI) is one of the key issues for the development of high capacity and efficient lithium-ion batteries. The heterogeneous and partially catalytic reaction of the electrode with the electrolyte triggers the formation of surface films on the electrode surface which can cause degradation of the cell performance. Whereas the EEI layer properties are quite well known for negative electrodes such as lithium metal and graphite [1,2], the EEI layer on positive electrode materials is still puzzling. Especially the interface layers on high voltage and high capacity positive electrodes, whose potentials approach the limit of electrolyte stability against oxidation [3], is quite unexplored. One of the challenges in understanding the reactions at the surface of the electrode is the complicated composition of the positive electrodes, containing not only the active material but also conductive agents and polymeric binders, that can modify the EEI layers on the electrode. To bypass these ambiguities, there is a need for study model electrodes such as thin films or pure active material electrodes, which allow for investigating solely the reactivity of the electrolyte at the active material surface. Here, combining X-ray Photoelectron Spectroscopy (XPS and X-ray Absorption and Emission Spectroscopy (XAS/XES), of model electrodes, we will show how the species formed at the electrode/electrolyte interface are affected by change in charging potential and the structure and nature of the transition metal in the material. XES and XAS will be used to shed light on the change of electronic structure upon delithiation

Rate-Dependent Nucleation and Growth of NaO2 in Na-O2 Batteries

Understanding the oxygen reduction reaction kinetics in the presence of Na ions and the formation mechanism of discharge product(s) is key to enhancing Na–O2 battery performance. Here we show NaO2 as the only discharge product from Na–O2 cells with carbon nanotubes in 1,2-dimethoxyethane from X-ray diffraction and Raman spectroscopy. Sodium peroxide dihydrate was not detected in the discharged electrode with up to 6000 ppm of H2O added to the electrolyte, but it was detected with ambient air exposure. In addition, we show that the sizes and distributions of NaO2 can be highly dependent on the discharge rate, and we discuss the formation mechanisms responsible for this rate dependence. Micron-sized (∼500 nm) and nanometer-scale (∼50 nm) cubes were found on the top and bottom of a carbon nanotube (CNT) carpet electrode and along CNT sidewalls at 10 mA/g, while only micron-scale cubes (∼2 μm) were found on the top and bottom of the CNT carpet at 1000 mA/g, respectively.Seventh Framework Programme (European Commission) (Marie Curie International Outgoing Fellowship, 2007-2013))National Science Foundation (U.S.) (MRSEC Program, award number DMR-0819762)Robert Bosch GmbH (Bosch Energy Research Network (BERN) Grant)China Clean Energy Research Center-Clean Vehicles Consortium (CERC-CVC) (award number DE-PI0000012)Skolkovo Institute of Science and Technology (Skoltech-MIT Center for Electochemical Energy Storage

Systematic selection of solvent mixtures for non-aqueous redox flow batteries – vanadium acetylacetonate as a model system

Employing solvent mixtures in the electrolyte of non-aqueous redox flow batteries can increase energy density and efficiency. In this paper, active species solubility, electrolyte conductivity, and redox reaction rates were examined systematically among a number of binary and ternary mixtures, consisting of acetonitrile and 5 polar aprotic co-solvents to identify mixtures with enhanced active species solubility and redox reaction rates. Although we used vanadium acetylacetonate as a model, the methodologies presented here are applicable when evaluating solvent mixtures for other active species. Our approach is distinctive in that it elucidated the trade-offs in desirable properties that are necessary when solvent mixtures are used for non-aqueous redox flow batteries. We found that in a vanadium acetylacetonate–based non-aqueous redox flow battery, the use of an 84/16 vol% acetonitrile/1,3-dioxolane binary solvent mixture resulted in an increase in the positive and negative sides reaction rates compared to that observed with pure acetonitrile. This binary mixture resulted in an improvement in reaction rate with no decrease in energy density and is a promising solvent system for other active species used for non-aqueous flow batteries. Keywords: Vanadium acetylacetonate; Non-aqueous electrolyte; Redox flow battery; Organic electrochemistry; Energy Storag

Multinuclear Magnetic Resonance Spectroscopy and Density Function Theory Calculations for the Identification of the Equilibrium Species in THF Solutions of Organometallic Complexes Suitable As Electrolyte Solutions for Rechargeable Mg Batteries

We present a multinuclear nuclear magnetic resonance (NMR) and density functional theory (DFT) study of electrolyte solutions based on organometallic complexes with aromatic ligands. These solutions constitute a unique electrolyte family possessing a wide electrochemical window, making them suitable for rechargeable magnesium batteries. In our previous study we identified equilibrium species in the solutions based on a combination of Raman spectroscopy and single-crystal XRD analyses, and herein we extend our studies to include multinuclear NMR analyses. These solutions are comprised of the metathesis reaction products of MgCl_2–<i>x</i>Ph_<i>x</i> and AlCl_3–<i>y</i>Ph_<i>y</i> in various proportions, in THF. In principle, these reactions involve the exchange of ligands between the magnesium and the aluminum based compounds, forming ionic species and neutral molecules, such as Mg₂Cl₃⁺·6THF, MgCl₂·4THF and AlCl_4–<i>y</i>Ph_<i>y</i>^– (<i>y</i> = 0–4). The identification of the solution phase species from the spectroscopic results is supported by spectral analyses of specially synthesized reference compounds and DFT quantum-mechanical calculations. The current approach reveals new aspects about the NMR shift of the organometallic complexes and, in particular, facilitates differentiation between ionic and neutral species

Controlling Solution-Mediated Reaction Mechanisms of Oxygen Reduction Using Potential and Solvent for Aprotic Lithium–Oxygen Batteries

Fundamental understanding of growth mechanisms of Li₂O₂ in Li–O₂ cells is critical for implementing batteries with high gravimetric energies. Li₂O₂ growth can occur first by 1e^– transfer to O₂, forming Li⁺–O₂^– and then either chemical disproportionation of Li⁺–O₂^–, or a second electron transfer to Li⁺–O₂^–. We demonstrate that Li₂O₂ growth is governed primarily by disproportionation of Li⁺–O₂^– at low overpotential, and surface-mediated electron transfer at high overpotential. We obtain evidence supporting this trend using the rotating ring disk electrode (RRDE) technique, which shows that the fraction of oxygen reduction reaction charge attributable to soluble Li⁺–O₂^–-based intermediates increases as the discharge overpotential reduces. Electrochemical quartz crystal microbalance (EQCM) measurements of oxygen reduction support this picture, and show that the dependence of the reaction mechanism on the applied potential explains the difference in Li₂O₂ morphologies observed at different discharge overpotentials: formation of large (∼250 nm–1 μm) toroids, and conformal coatings (<50 nm) at higher overpotentials. These results highlight that RRDE and EQCM can be used as complementary tools to gain new insights into the role of soluble and solid reaction intermediates in the growth of reaction products in metal–O₂ batteries

Influence of Edge- and Basal-Plane Sites on the Vanadium Redox Kinetics for Flow Batteries

The reaction kinetics of V^II/V^III and V^IVO²⁺/V^VO₂⁺ redox on carbon electrodes in sulfuric acid limit the development of vanadium redox flow batteries (VRFB) with high power and efficiency characteristics. Cyclic voltammetry and symmetric flow cell measurements on selectively masked graphite foil and highly oriented pyrolytic graphite electrodes revealed that edge carbon sites provide faster kinetics for V^II/V^III and V^IVO²⁺/V^VO₂⁺ redox than basal carbon, especially at low vanadium concentrations. The understanding was used to explain the marked enhanced kinetics of carbon paper electrodes with heat-treatments in air relative to that without, which was supported by X-ray photoelectron spectroscopy measurements that showed much higher amounts of surface functional groups on the heat-treated carbon upon exposure to the V^V species in the electrolyte. Of particular significance to note is that markedly enhanced kinetics for the V^II/V^III redox for the heat-treated carbon were found at both low and high vanadium concentrations, while similar enhancement was found for the V^IVO²⁺/V^VO₂⁺ redox for low vanadium concentrations but much smaller increased kinetics were noted for high vanadium concentrations required for practical flow batteries. This result was further confirmed by symmetric flow cell measurements that show much higher currents for the V^II/V^III electrolyte using heat-treated carbon in comparison to the as-received, while comparable currents were found for V^IVO²⁺/V^VO₂⁺ electrolyte, indicating that the redox kinetics of V^II/V^III can be limiting for VRFBs using as-received carbon (low edge carbon and oxygen functional groups). These findings provide new insights and strategies for carbon electrode designs for high-power VRFBs

Inorganic Solid-State Electrolytes for Lithium Batteries: Mechanisms and Properties Governing Ion Conduction

This Review is focused on ion-transport mechanisms and fundamental properties of solid-state electrolytes to be used in electrochemical energy-storage systems. Properties of the migrating species significantly affecting diffusion, including the valency and ionic radius, are discussed. The natures of the ligand and metal composing the skeleton of the host framework are analyzed and shown to have large impacts on the performance of solid-state electrolytes. A comprehensive identification of the candidate migrating species and structures is carried out. Not only the bulk properties of the conductors are explored, but the concept of tuning the conductivity through interfacial effects—specifically controlling grain boundaries and strain at the interfaces—is introduced. High-frequency dielectric constants and frequencies of low-energy optical phonons are shown as examples of properties that correlate with activation energy across many classes of ionic conductors. Experimental studies and theoretical results are discussed in parallel to give a pathway for further improvement of solid-state electrolytes. Through this discussion, the present Review aims to provide insight into the physical parameters affecting the diffusion process, to allow for more efficient and target-oriented research on improving solid-state ion conductors.National Science Foundation (U.S.) (Graduate Research Fellowship (1122374)Taiwan. Ministry of Science and Technology (102-2917-I-564-006-A1)BMW GroupSkoltech-MIT Cente

Electrode–Electrolyte Interface in Li-Ion Batteries: Current Understanding and New Insights

Understanding reactions at the electrode/electrolyte interface (EEI) is essential to developing strategies to enhance cycle life and safety of lithium batteries. Despite research in the past four decades, there is still limited understanding by what means different components are formed at the EEI and how they influence EEI layer properties. We review findings used to establish the well-known mosaic structure model for the EEI (often referred to as solid electrolyte interphase or SEI) on negative electrodes including lithium, graphite, tin, and silicon. Much less understanding exists for EEI layers for positive electrodes. High-capacity Li-rich layered oxides yLi[subscript 2–x]MnO[subscript 3]·(1–y)Li[subscript 1–x]MO[subscript 2], which can generate highly reactive species toward the electrolyte via oxygen anion redox, highlight the critical need to understand reactions with the electrolyte and EEI layers for advanced positive electrodes. Recent advances in in situ characterization of well-defined electrode surfaces can provide mechanistic insights and strategies to tailor EEI layer composition and properties.BMW GroupMIT/Battelle postdoctoral associate programTaiwan. Ministry of Science and Technology (02-2917-I-564-006-A1)National Defense Science and Engineering Graduate (NDSEG) FellowshipUnited States. Department of Defense (32 CFR 168a DoD)United States. Air Force. Office of Scientific ResearchUnited States. Department of Energy. Office of Science (Contract No. DE-AC02-05CH11231

Rate-Dependent Nucleation and Growth of NaO₂ in Na–O₂ Batteries

Understanding the oxygen reduction reaction kinetics in the presence of Na ions and the formation mechanism of discharge product(s) is key to enhancing Na–O₂ battery performance. Here we show NaO₂ as the only discharge product from Na–O₂ cells with carbon nanotubes in 1,2-dimethoxyethane from X-ray diffraction and Raman spectroscopy. Sodium peroxide dihydrate was not detected in the discharged electrode with up to 6000 ppm of H₂O added to the electrolyte, but it was detected with ambient air exposure. In addition, we show that the sizes and distributions of NaO₂ can be highly dependent on the discharge rate, and we discuss the formation mechanisms responsible for this rate dependence. Micron-sized (∼500 nm) and nanometer-scale (∼50 nm) cubes were found on the top and bottom of a carbon nanotube (CNT) carpet electrode and along CNT sidewalls at 10 mA/g, while only micron-scale cubes (∼2 μm) were found on the top and bottom of the CNT carpet at 1000 mA/g, respectively

Inorganic Solid-State Electrolytes for Lithium Batteries: Mechanisms and Properties Governing Ion Conduction

This Review is focused on ion-transport mechanisms and fundamental properties of solid-state electrolytes to be used in electrochemical energy-storage systems. Properties of the migrating species significantly affecting diffusion, including the valency and ionic radius, are discussed. The natures of the ligand and metal composing the skeleton of the host framework are analyzed and shown to have large impacts on the performance of solid-state electrolytes. A comprehensive identification of the candidate migrating species and structures is carried out. Not only the bulk properties of the conductors are explored, but the concept of tuning the conductivity through interfacial effectsspecifically controlling grain boundaries and strain at the interfacesis introduced. High-frequency dielectric constants and frequencies of low-energy optical phonons are shown as examples of properties that correlate with activation energy across many classes of ionic conductors. Experimental studies and theoretical results are discussed in parallel to give a pathway for further improvement of solid-state electrolytes. Through this discussion, the present Review aims to provide insight into the physical parameters affecting the diffusion process, to allow for more efficient and target-oriented research on improving solid-state ion conductors