Instability Of Cathode Research Articles

Investigation of Interfacial Phenomena of High-Voltage Li-Ion Battery with Li-Rich Layered Oxide Cathode

Enabling the high energy density Li-ion batteries requires the use of high-capacity cathode material such as Li-rich layered oxide cathode (xLi2MnO3·(1-x)LiMO2 (M=Mn,Ni,Co)) that can be charged to high-voltage above 4.5 V vs. Li/Li+. A critical limiting factor for high-voltage battery operation is however the anodic and thermal instability of conventional electrolyte, which causes rapid performance fade and safety issue, in particular, at elevated temperatures. Our recent research results have shown that the use of functional additives enables the cell operation in such harsh conditions. Spectroscopic surface analysis data indicated that structural degradation of both Li-rich layered oxide cathode and graphite anode materials and instability of surface film at cathode during cycling in the conventional electrolyte were the main reasons for the rise of interfacial resistance and performance fade. Nonetheless, those problems can be resolved through interfacial stabilization using appropriate electrolyte additives. In this presentation, we report the interfacial phenomena and high-energy performance of Li-rich layered oxide cathode under a harsh condition of high charge cut-off voltage above 4.6 V vs. Li/Li+. The mechanistic studies of surface film/solid electrolyte interphase (SEI) layer formation and composition of cathode and anode using ex situ ATR FTIR spectroscopy combined with X-ray photoelectron spectroscopy and scanning electron microscopy, and structural changes using Raman spectroscopy, and the correlation between interfacial phenomena and high-voltage cycling performance would be discussed.

Insight into the Redox Chemistry of Layered LiNiO2-YSy Cathode Material: A First-Principles Study

With the rapid development of Li-ion batteries, their application to the electric vehicles (EVs) and hybrid electric vehicles (HEVs) would be expected. In recent years, Ni-based layered oxide cathode materials, such as LiNi1/3Co1/3Mn1/3O2 (NCM) and LiNi0.8Co0.15Al0.05O2 (NCA), garner considerable attention in lithium ion batteries due to their relative high energy density [1-2]. However, the safety concern of the batteries associated with the oxygen release, which is a large problem resulting from the instability of cathode and decomposition of electrolyte[3-4], should be further resolved for practical application. The layered oxides LiTMO2 (TM=Ni, Co) become unstable in a highly delithiated state. On one hand, the extraction of more and more lithium ions gives rise to the overlapping of TM-3d band with O-2p band, which will increase the oxygen participation in the redox chemistry and causes destabilization of the lattice. On the other hand, upon oxidation (i.e. delithiation), increasing amount of unstable Ni4+(Co4+) will be formed, followed by the reduction to Ni3+(Co3+) at elevated temperatures with concurrent loss of oxygen. Meanwhile, the high voltage required for deep charging would oxidize electrolyte, thereby resulting in oxygen release and capacity fade. Therefore, the deep charging of layered oxide cathodes involving the anion oxidation would accelerate oxygen release, which seriously affects the safety of batteries. Some research efforts have been performed to enhance the safety by doping with anions[5-6]. However, the mechanisms of redox chemistry in systems where multiple anions coexist remain unclear so far. The stability and safety of layered-oxysulfide cathode by tuning the redox chemistry were studied based on first-principles. Since the Ni-based cathodes have more favorable prospect in the future, we chose LiNiO2, the representative of the layered-oxide cathode, as a model in calculations. LiNiO2-ySy, wherein oxygen anions are partially substituted by sulfur anions, was also studied. In order to understand the role of S2- in enhancing the stability and safety, the redox chemistry that occurs in S-substituted systems was discussed in detail. For Li1-xNiO1.89S0.11, at the initial charging stage (0≤x≤0.33), the sulfur anions alongside nickel and oxygen atoms are all involved in the charge compensation, although sulfur anions contribute less to the process. In the next charging stage (x＞0.33), only nickel and oxygen atoms become oxidized owing to the donation of electrons, while sulfur anions are not involved. The partial oxidation of sulfur provides a novel line of thought in terms of designing the Li-rich layered cathode materials with charge compensation by multiple anions.

Interfacial Phenomena of 4.7V Li-Ion Battery of Li-Rich Layered Oxide/Graphite at Elevated Temperature

High-energy performance and interfacial phenomena of 55 ○C full-cells with Li-rich layered oxide cathode and graphite anode under high charge cut-off voltage of 4.7V are reported. Anodic and thermal instabilities of conventional electrolyte are the critical issues to limit high-voltage and high-temperature cell operation. Our recent research results have shown that the use of functional additives enables the operation of Li-ion cells in such harsh condition. Surface analysis data indicated that structural degradation of both cathode and anode materials and instability of surface film at cathode were main reasons for the rise of interfacial resistance in the conventional electrolyte. Nonetheless, those problems can be resolved through interfacial stabilization using appropriate electrolyte additives. The surface film/ solid electrolyte interphase (SEI) layer formation and composition, and their correlation to high-voltage cycling performance are discussed. Cathode active material of Li1.13Mn0.463Ni0.203Co0.203O2 (LMNC) was synthesized by solid state method at 900 ○C using the precursor of (Mn0.463Ni0.203Co0.203)CO3 coprecipitate and LiOH.H2O in the air. Lithium 2016 coin full-cells, consisting of Li1.13Mn0.463Ni0.203Co0.203O2 cathode, graphite anode, the electrolyte of 1M LiPF6/EC:EMC as the base electrolyte and with blended additives of 5 wt% di-(2,2,2 trifluoroethyl)carbonate (DFDEC) and 3 wt% vinylene carbonate (VC) were assembled in an argon-filled glove box. The full-cells were tested between 2.5 and 4.7 V (4.75 V vs. Li/Li+) at the rate of C/5 at 55 ○C in the constant temperature chamber. For the characterization of surface/SEI composition, attenuated total reflection FTIR combined with X-ray photoelectron spectroscopic (XPS) analyses were conducted. Fig. 1 compares the cycling ability of Li1.13Mn0.463Ni0.203Co0.203O2//graphite full-cells at 55 ○C in the base electrolyte only and with blended additives of DFDEC-VC between 2.5 and 4.7 V at the rate of C/5. In the base electrolyte, the full-cell exhibits initial charge and discharge capacities of 320 and 214 mAhg−1, respectively, with initial coulombic efficiency of 67 %. Discharge capacity decreases to 80 mAhg−1 after 50 cycles, yielding very low capacity retention of 37 % and poor coulombic efficiency. On the contrary, with blended additives, significantly improved cycling performance is achieved; initial charge and discharge capacities are 323 and 227 mAhg−1, respectively, corresponding to initial coulombic efficiency of 71 %, and improved capacity retention of 77 % at the 50th cycle, delivering discharge capacities of 227−174 mAhg−1. The detailed studies of surface composition and formation mechanism, and their relation to high-voltage and high-temperature cycling performance would be presented in the meeting. Acknowledgements This research was supported by the Korean Ministry of Trade, Industry & Energy (A0022-00725 & R0004645), Chungnam National University, and Nano Material Technology Development Program through the National Research Foundation funded by the Ministry of Science, ICT and Future Planning (2009-0082580). Figure 1

The Mechanistic Role of Lithium Salts in Aprotic Li-O2 Batteries

Lithium oxygen (Li-O2) research generates much interest and many expectations among electrochemists. The high theoretical specific energy, the simplicity of preparation and operation of electrochemical cells, the sizable commercial possibilities, and the ever-increasing number of new publications, has directed many groups to Li-O2 battery research. Over the past decade, significant progress in the study of possible Li-O2 battery systems has also prompted interest from the chemical and automotive industries. Nevertheless, practical Li-O2 batteries are far from realization. Many scientific challenges are related to the oxygen reduction reactions (ORR). In aprotic Li-O2 cells the general assumption is that during discharge, oxygen is reduced on the cathode to form insoluble lithium oxide compounds. Experimental evidence support that the major final product is lithium peroxide (Li2O2). Nevertheless, the electrolyte solution and the carbon cathode instability toward reduced oxygen species lead to intrinsic instability and irreversible formation of many side products. Electrochemical formation of Li2O2 can proceed in two ways: (i) One electron transfer to molecular oxygen to form LiO2, which continues to react by receiving another electron and Li ion to form Li2O2. (ii) Two electron transfer pathway that skip over the formation of LiO2, and forms directly Li2O2. Besides the electrochemical routes, Li2O2 can be formed chemically by disproportionation of Li-superoxide to form molecular oxygen as the second product. In fact, electrochemically formed LiO2 is unstable, its further reaction to form Li2O2 is fast1. The possibility of isolating lithium superoxide and to assess its lifetime in different aprotic media are still under investigation. The rout which Li2O2 is being formed can affect the growth mechanism. Li2O2 is insoluble in aprotic solvents, thus during ORR, crystalline Li2O2 is deposit on the cathode surface. When fully covered by ORR products, the continuation of the process depends on electrons crossing the product layer to reach fresh oxygen molecules. Due to poor electronic conductivity of the oxygen reduction product layer, the cathode is deactivated and the ORR is terminated after the deposition of a few layers of Li-peroxide. The stabilization of superoxide intermediate can change the growth mechanism of the Li2O2 layer. In contrast to lithium peroxide, superoxide radical can be soluble in aprotic solvents that contain lithium cations. During discharge, oxygen is reduce to superoxide through a free cathode site. Then, the charged radical can diffuse in the solution and transfer charge or to disproportionate in all dimensions, leading to a thick deposits of Li2O2. This mechanism can be considered as a top down growth, as superoxide can disproportionate on top of a Li2O2 particle. Using aprotic solvents with high Guttman donor number such as DMSO, DMF, and DMA in Li-oxygen cells can help to extend the superoxide lifetime. The presence of high DN solvent reduces the electrophilicity (Lewis acidity) of the Li cations and thereby helping to extend the stability of O2 -· before disproportionating to Li2O2. In practice using high donor number solvents in Li-O2 cells is challenging due to their reactivity toward the lithium metal anode and reduced oxygen species. Recently it was reported that superoxide radicals can be stabilized indirectly by the electrolytes’ counter anions. In highly associated electrolytes the counter anions are coordinated to the solvated lithium cations, these interactions practically reduce the lithium cations Lewis acidity and thereby temporarily preventing them to bind to the reduced oxygen moieties. As a result, soluble species such as superoxide can take part in the growth mechanism of Li-peroxide deposits on the cathode surface. In this manner we can stabilize superoxide moieties, lead to massive Li2O2 deposition by in a top-down mechanism (i.e. high ORR capacity) although using relatively low DN number solvents such as glymes - CH3O(CH2CH2O)nCH3. Glymes are suggested to be relatively stable towered lithium metal, lithiated carbons and reduced oxygen species.

Recent development on perovskite‐type cathode materials based on SrCoO3 − δ parent oxide for intermediate‐temperature solid oxide fuel cells

AbstractThe slow oxygen reduction reaction kinetics as well as structural and chemical instability of cathode are the main barriers hindering the development of the solid oxide fuel cell operated at intermediate temperature. Therefore, immense efforts have been devoted to address these two demanding issues. Perovskite cathodes based on SrCoO3 − δ show promising electroactivity on oxygen reduction at intermediate temperature but are susceptible to contaminates in air. In this short review, we mainly cover the recent advances in SrCoO3 − δ‐based perovskite cathode development, with emphasis on doping strategies in electroactivity enhancement and cathode tolerance to CO2. Some future research directions are also recommended at the end of this review. © 2016 Curtin University of Technology and John Wiley & Sons, Ltd.

Performance of Li-Air Battery Cell with Folded Structure

The Li-Air batteries which convert the electrochemical reactions of lithium and O2 into electrical energy have high theoretical gravimetric capacity (3861 mAh/gLi) that is ten times higher than that of lithium ion batteries (372 mAh/gc). Its potential to generate high energy density makes it a promising candidate to replace the lithium ion batteries which practically reached the gravimetric energy density limit in the electric vehicle applications. To meet the requirements of the electric vehicle applications, the Li-Air batteries need to overcome the technical barriers that limit the cycle life such as electrolyte and cathode instability. Although recent developments have been focused on improving the cycle life (1) and safety (2) of Li-Air battery in efforts to extend its application into the electric vehicle, it is also important to design a cell structure that can achieve the highest possible energy density. The Li-Air battery uses lithium metal, porous carbon layer, protective layer and gas diffusion layer (GDL). The properties of lithium metal anode and porous carbon cathode layer are well known, and the cell design will mainly be affected by the mechanical property of the electrolyte, protective layer and GDL. Figure 1 shows the schematic of a Li-Air fold cell where the Li metal anode, protective layer, and cathode are folded. The oxygen can be distributed to the cathode through the GDL which is inserted between the two cathode layers that are formed from folding. The cell design enables the two layers of the cathode to share one common GDL layer and thus increasing energy density of the cell. The capacity of the Li-Air fold cell can be controlled by changing the length (L) and the number of folds of the cell. The width (W) of the cell affects the O2 distribution in cathode, thus the choice of W is limited. Figure 2 shows the areal capacity of three Li-Air fold cells which contained polyethylene oxide with LiTFSI as electrolyte in the cathode and protective layer. The cells were tested in 1 atm O2 at 60 C. The capacities of the cells were adjusted by changing the number of folds. For the cells in Figure 2, the three cells had WxL dimension of 1x5 cm and were folded 1, 10 and 50 times which resulted in the cells with capacity values of 0.02, 0.2 and 1 Ah, respectively. Figure 2 indicates that the performance of the Li-Air fold cell is reproducible as the number of folds increase. In the presentation, the effects of cell dimension on cell performance, the technical difficulties of manufacturing Li-Air cells with high energy density as well as the issues that need to be overcome to improve the cycle life will be discussed.

High performance lithium-sulfur batteries with a facile and effective dual functional separator

Lithium-sulfur (Li-S) batteries stand as an important candidate for next-generation high-energy secondary batteries due to its high specific capacity, low cost and environmental friendliness. However, practical application of Li-S batteries suffers from low rechargeability, poor rate capability and cycling instability of sulfur cathode, which can be mainly ascribed to the poor conductivity of sulfur and the dissolution of the intermediate polysulfides generated during discharge-charge cycles. In this work, a Nafion/super P-modified dual functional separator is designed to improve the long-term cycle stability and rate capability of the pure sulfur cathode. The electrostatic repulsion between the SO3− groups and the dissolved negative Sn2− ions, and the trap and reutilizing effect of super P for polysulfides, provide double insurance to confine the polysulfides within the cathode side, leading to great improvement in both reversible capacity and cycling stability of the sulfur cathode as compared to the battery with pristine Celgard separator. With such dual functional separator, a simple elemental sulfur cathode with 70% S content delivers a high initial discharge capacity of 1087mAhg−1 at 0.1C and a long-term cyclability with only 0.22% capacity fade per cycle over 250 cycles at 0.5C.

Oxygen character in the density of states as an indicator of the stability of Li-ion battery cathode materials

Oxygen gas evolution from lithium battery cathode materials during charging to high voltages is a known mechanism leading to cathode instability and ultimately Li-ion batteries fires. We present the simple thesis that the tendency toward oxygen destabilization in metal-oxide materials used as Li-ion battery cathodes can be correlated to the amount of oxygen-derived weight in the partial density of states (PDOS) near the Fermi level (EF), which is easily calculable with density functional theory (DFT). If the oxygen PDOS dominates the total DOS at EF at any point during delithiation (charging), electrons are withdrawn from the O rather than the M states, and the O2− ions are oxidized to peroxide (O22−), and then often to O2 gas. A demonstration of the utility of this perspective is given using DFT to calculate the PDOS for compounds with a broad range of structures, transition metals and degrees of lithiation: LiFePO4, Li2CuO2, LiCoO2, LiNiO2, and Li2RuO3. The resulting computational spectra correlate well to the experimental stability in the modeled compounds. We conclude that the oxygen stability of cathode materials derives from their fundamental electronic structure as function of changing voltage (and lithiation).

ChemInform Abstract: Mechanistic Insights for the Development of Li—O2 Battery Materials: Addressing Li2O2 Conductivity Limitations and Electrolyte and Cathode Instabilities

AbstractReview: 139 refs.

Recent Progress on Stability Enhancement for Cathode in Rechargeable Non‐Aqueous Lithium‐Oxygen Battery

The pressing demand on the electronic vehicles with long driving range on a single charge has necessitated the development of next‐generation high‐energy‐density batteries. Non‐aqueous Li‐O2 batteries have received rapidly growing attention due to their higher theoretical energy densities compared to those of state‐of‐the‐art Li‐ion batteries.To make them practical for commercial applications, many critical issues must be overcome, including low round‐trip efficiency and poor cycling stability, which are intimately connected to the problems resulting from cathode degradation during cycling. Encouragingly, during the past years, much effort has been devoted to enhancing the stability of the cathode using a variety of strategies and these have effectively surmounted the challenges derived from cathode deteriorations,thus endowing Li‐O2 batteries with significantly improved electrochemical performances. Here, a brief overview of the general development of Li‐O2 battery is presented. Then, critical issues relevant to the cathode instability are discussed and remarkable achievements in enhancing the cathode stability are highlighted. Finally, perspectives towards the development of next generation highly stable cathode are also discussed.

Mechanistic insights for the development of Li-O2 battery materials: addressing Li2O2 conductivity limitations and electrolyte and cathode instabilities.

The Li-air battery has received significant attention over the past decade given its high theoretical specific energy compared to competing energy storage technologies. Yet, numerous scientific challenges remain unsolved in the pursuit of attaining a battery with modest Coulombic efficiency and high capacity. In this Feature Article, we provide our current perspective on challenges facing the development of nonaqueous Li-O2 battery cathodes. We initially present a review on our understanding of electrochemical processes occurring at the nonaqueous Li-O2 cathode. Electrolyte and cathode instabilities and Li2O2 conductivity limitations are then discussed, and suggestions for future materials research development to alleviate these issues are provided.

Combining Accurate O2 and Li2O2 Assays to Separate Discharge and Charge Stability Limitations in Nonaqueous Li-O2 Batteries.

Li-air batteries have generated enormous interest as potential high specific energy alternatives to existing energy storage devices. However, Li-air batteries suffer from poor rechargeability caused by the instability of organic electrolytes and carbon cathodes. To understand and address this poor rechargeability, it is essential to elucidate the efficiency in which O2 is converted to Li2O2 (the desired discharge product) during discharge and the efficiency in which Li2O2 is oxidized back to O2 during charge. In this Letter, we combine many quantitative techniques, including a newly developed peroxide titration, to assign and quantify decomposition pathways occurring in cells employing a variety of solvents and cathodes. We find that Li2O2-induced electrolyte solvent and salt instabilities account for nearly all efficiency losses upon discharge, whereas both cathode and electrolyte instabilities are observed upon charge at high potentials.

WE-C-103-03: Design of a Novel 3D Field Emission Electron Source for High Power X-Ray Tube

Purpose: Field emission electron sources today have shown great potentials in various medical imaging applications, such as computed tomography (CT), which demands extremely high emission current. High emission current can be achieved by raising the gate voltage at the expense of increased cathode instability and reduced cathode life. This paper presents the design of a new 3D field emission electron source that has the potential to overcome the limits of conventional field emission cathode in emission current and cathode life. Methods: The 3D cathode design is based on a semi-enclosed rectangular cavity structure, with one face open to the anode for electron beam emission. All the inner faces of the cavity including four sides and one bottom are set to be the emission surfaces. The 3D cathode has a depth (cavity height) of 5mm along the source axial direction, along with a 10mm×10mm electron beam aperture that is 10mm away from the anode. Results: The performance of both the 3D and conventional 2D cathode was evaluated using the commercial software package CST Studio Suite, under the same operating condition. We have demonstrated that the total emission current of the 3D cathode is 2.4 times the one of the 2D cathode, from a total 3 times emission surface area. The slightly reduced electron transmission rate in the 3D cathode is due to the electron collision inside the cavity structure. Conclusion: We present a novel field emission electron source design based on a 3D semi-enclosed cavity structure, by utilizing the multiple inner surfaces of the cavity for electrons emission. The 3D cathode has transverse dimensions the same as a conventional single surface 2D cathode, but larger emission area due to its extended space. The 3D cathode design was demonstrated to have the capability of providing much higher cathode current density.

Confinement jumps in a non-neutral plasma

Measurements of confinement jumps in pure electron plasmas confined on magnetic surfaces are presented and discussed. The experiments were performed in the Columbia non-neutral torus stellarator [T. S. Pedersen, J. P. Kremer, R. G. Lefrancois, Q. Marksteiner, N. Pomphrey, W. Reiersen, F. Dahlgren, and X. Sarasola, Fusion Sci. Technol. 50, 372 (2006)]. The jumps exhibit hysteresis and are associated with a negative differential resistance. The jumps occur at particular emission currents of the biased emissive filaments that create and sustain the electron plasmas independent of the methods used to affect the emission current. This observation, as well as other experimental evidence, supports that the jumps are caused by a cathode instability. The jumps can also be triggered by the application of a bias potential on a nearby mesh. In most circumstances, the jumps occur between two stable but measurably different equilibrium states. These different equilibrium states have substantially different confinement times. The cathode physics is important for the jumps because the cathode instability provides the perturbation that triggers the jump of the whole plasma into the other equilibrium state, but as mentioned, an external electrostatic perturbation is also capable of triggering such a jump.

Behaviour of Ni, NiO and LixNi1−xO in molten alkali carbonates

Ni, NiO and LixNi1-xO are the cathodic materials commonly used in molten carbonate fuel cells (MCFCs). Since the instability of the cathode is recognized as a major hindrance to MCFC development, in this report the behaviour of these nickel species in molten alkali carbonates is reviewed, analyzing step by step the processes of lithiation, dissolution and sinterization at cell operating temperatures.

Lithium beam generation and focusing with a radial diode on PBFAII

The performance of a 15-cm-radius applied-magnetic-field ion diode was investigated on the PBFA II accelerator at a power of 23 TW. The power coupling between the accelerator and diode was measured and compared with numerical simulations that show the effects of the electron flow in the MITL. The power coupled to the cathode of the diode was 18 MW. Measurements of the lithium beam generated from an electric-field-emission LiF anode showed a lithium beam power of 9 TW. The lithium beam was ballistically focused in a gas cell filled with 2 torr argon. The resultant focused power density was ∼1.8 TW/cm2 equivalent on a cylindrical target at the centerline of the diode. The focused power was limited by the 20- to 30-mR divergence of the beam caused by the LiF source used and by virtual cathode instabilities in the anode–cathode gap. The ion mode instability in the virtual cathode was studied extensively by measurement of waves in the ion emission pattern from the anode and of the E-P0 correlation between variations in the beam energy and transverse momentum. The instability Played a dominant role in the limitation of the focused lithium power.

Instability and reliability of silicon field emission array

The instability and reliability of silicon field emission cathode are investigated. The total instability is divided into four components which are extracted from the emitting current curve I(t). The appearance of the I(t) curve, the meaning and causes of the four components are described and discussed, respectively. Reliability is defined and a set of failure modes are reported. To make a normalized measurement of reliability, a computerized system is proposed.

Striation Formation during Steady-State Glow Discharge

The formation of striations in a normal glow discharge has been correlated with a relaxation-type phenomenon occurring at the cathode dark space-negative-glow interface and caused by cathode instabilities. A beamlike stream of electrons, in excess of that number necessary to maintain the normal glow brought about by cathode instabilities, is postulated to cross the cathode-fall region and to interact with neutrals in the negative glow causing a surplus of positive ions at the interface between the cathode dark space and the negative glow. This group of positive ions constitutes a positive striation which in turn is accelerated toward the cathode. The experimentally observed times for these striations to cross the cathode-fall region give a velocity of approximately 104 cm/sec, which is in good agreement with other observations of positiveions striation velocities.

Iodine positive-column oscillation associated with cathode instability

A positive-column oscillation in low-pressure iodine vapour with frequency about 100 kc/s has been found to be associated with oscillations of small corona-type protuberances of the cathode glow. Two ways are described for suppressing the oscillations.

Cathode instability in argon atmospheres

EXTENSIVE experimental and analytical studies have provided a basic understanding of the manifestation of arc cathode instability relative to fluctuations of arc voltage as influenced by electrical, chemical, and geometrical parameters. These studies have contributed significantly to the development of controlled cathode stabilization systems of the argon-shielded consumable electrode arcs as applied to the welding of mild steel.