Anionic Redox Activity Research Articles

Scalable Room-Temperature Synthesis of Multi-shelled Na3(VOPO4)2F Microsphere Cathodes

Summary The large-scale application of rechargeable batteries in electric vehicles and energy-storage systems is still hindered by their high cost, particularly from the active materials. Here we proposed a concept of integration of extraction-separation and materials-preparation to further reduce the materials' cost. The power of this concept was demonstrated by the large-scale room-temperature synthesis (150 g per batch) of multi-shelled Na3(VOPO4)2F microspheres based on in situ generated bubbles as soft templates. The as-prepared Na3(VOPO4)2F shows superior rate capability with discharge capacity of 81 mAh g−1 at 15 C current rate and capacity retention of 70% after 3,000 cycles. Ex situ UV-vis was first employed to characterize the variation of oxidation state for vanadium during the charging and discharging process. This is the first report of a rapid and facile large-scale room-temperature controllable synthesis of Na3(VOPO4)2F microspheres, and this approach could be extended to synthesize other similar compounds.

(Invited) The Paradox of High-Energy Density Materials for Li-Ion Batteries

Increasing the energy density of cathode materials is a central goal of the ongoing research in Li-ion batteries. This implies improving simultaneously the capacity and the potential of the materials used at the positive electrode of the electrochemical device. While the electrochemical potential is well controlled through an appropriate choice of the redox centre involved in the Li-driven electrochemical reactions, the increase of capacity appears more difficult to achieve without penalizing the material structural stability.[1-3] So far, materials showing the highest energy densities are the Li-rich layered transition metal oxides (LLOs) for which a cumulative cationic and anionic redox activity has been demonstrated. Ever since, numerous materials have been synthesized with various combinations of 3d, 4d, 5d or non transition metals [4-6] and different mechanisms have been proposed to rationalize their electrochemical activity vs. Li or Na with no real consensus about the directions to follow to overcome the structural instability that often comes along with the anionic process. In this paper, we review the different interpretations proposed in the literature to address the fundamental questions of increasing both the potential and the capacity of current positive electrodes with the hope that a common language will help in clarifying the relationship between the material electronic structure, the potential, the (extra)-capacity and the consequences of high-energy-density on the material structural stability. This unified picture highlights that a trade-off needs to be found for the development of high-energy-density materials for alkali-ion batteries.

Insight into the Relationship between Transition Metal and Anionic Redox As Found in Simple Li-Rich CuO-Li2O Composite Cathode Materials

Conventional lithium ion battery cathode materials derive their capacity exclusively from transition metal cation redox. Many recent studies on novel Li-rich cathode materials have found substantial reversible anionic redox that shows promise to greatly enhance battery capacity. Unfortunately, this anionic (oxygen) redox contribution to the charge compensation of lithium extraction and insertion typically results in cathode degradation and oxygen gas release. While previous studies have emphasized methods such as the doping of 4d or 5d transition metals to stabilize the formation of peroxo-like species, the mechanism of anionic redox in relation to the transition metals present is still largely unknown. This study employs simple Li-rich cathode materials as a model for further understanding of the mechanism of anionic redox. Here we have synthesized a number of materials including simple 3d transition metal (e.g., Cu) doped lithium oxide to more closely examine the relationship between oxygen redox and the transition metal present in the cathode material. The materials studied contain a high ratio of lithium to transition metal to probe the maximum anionic redox activity. We will discuss our results in which galvanostatic measurements are combined with quantitative gas evolution analysis, in addition to ex-situ resonant inelastic X-ray scattering and X-ray photoelectron spectroscopy on cycled electrodes, to continue to clarify the required conditions for anionic redox in lithium ion battery cathode materials.

Activation of Surface Oxygen Sites in a Cobalt-Based Perovskite Model Catalyst for CO Oxidation.

Anionic redox chemistry is becoming increasingly important in explaining the intristic catalytic behavior in transition-metal oxides and improving catalytic activity. However, it is a great challenge to activate lattice oxygen in noble-metal-free perovskites for obtaining active peroxide species. Here, we take La0.4Sr0.6CoO3-δ as a model catalyst and develop an anionic redox activity regulation method to activate lattice oxygen by tuning charge transfer between Co4+ and O2-. Advanced XAS and XPS demonstrate that our method can effectively decrease electron density of surface oxygen sites (O2-) to form more reactive oxygen species (O2- x), which reduces the activation energy barriers of molecular O2 and leads to a very high CO catalytic activity. The revealing of the activation mechanism for surface oxygen sites in perovskites in this work opens up a new avenue to design efficient solid catalysts. Furthermore, we also establish a correlation between anionic redox chemistry and CO catalytic activity.

Na2Mn3+0.3Mn4+2.7O6.85: A cathode with simultaneous cationic and anionic redox in Na-ion battery

Here we report a high-capacity cathode, Na2Mn3+0.3Mn4+2.7O6.85, based on simultaneous cationic (Mn4+/Mn3+) and anionic (O2-/(O2)n-) redox process in Na-Mn-O system for Na-ion batteries (NIBs). An obvious voltage plateau at ~ 4.2 V vs. Na+/Na is reversibly presented during Na+ deinsertion/insertion demonstrated by XPS analysis of the participation of oxygen redox. This cathode delivers ~ 213 mA h g-1 in the voltage range of 1.5–4.5 V composed of two voltage plateaus from both cationic and anionic redox reaction. The two voltage plateaus exhibit the reversible phase transition upon Na+ insertion/deinsertion corresponding to a solid-solution region (high plateau) and a two-phase region (low plateau), respectively. The results indicate that anionic redox activity is available to develop high-capacity cathodes for NIBs.

Fundamental understanding and practical challenges of anionic redox activity in Li-ion batteries

Our increasing dependence on lithium-ion batteries for energy storage calls for continual improvements in the performance of their positive electrodes, which have so far relied solely on cationic redox of transition-metal ions for driving the electrochemical reactions. Great hopes have recently been placed on the emergence of anionic redox—a transformational approach for designing positive electrodes as it leads to a near-doubling of capacity. But questions have been raised about the fundamental origins of anionic redox and whether its full potential can be realized in applications. In this Review, we discuss the underlying science that triggers a reversible and stable anionic redox activity. Furthermore, we highlight its practical limitations and outline possible approaches for improving such materials and designing new ones. We also summarize their chances for market implementation in the face of the competing nickel-based layered cathodes that are prevalent today. The discovery of anionic redox chemistry in Li-rich cathode materials provides much hope for enhancing battery performance. Tarascon and Assat analyse the underlying science behind anionic redox and discuss its practical limitations as well as the routes to overcome the application barriers.

Electrostatic Interactions versus Second Order Jahn–Teller Distortion as the Source of Structural Diversity in Li3MO4 Compounds (M = Ru, Nb, Sb and Ta)

With the advent of layered rocksalt oxides showing anionic redox activity toward Li, there has been an increased focus on designing new rocksalt structures and, more particularly, compounds pertaining to the Li3MO4 family. The structural richness of this family is nested in its ability to host many different cations, leading to the formation of superstructure patterns whose predictability is still limited. Thus, there is a need to understand the formation of such superstructures, as cationic arrangements have a crucial effect on their physical properties. Herein we propose a combined experimental and theoretical approach to understand the interactions governing cation ordering in binary systems of general composition given by Li3MyM′1–yO4 (M and M′ being Ru, Nb, Sb, and Ta). Through complementary X-ray diffraction and X-ray absorption spectroscopy techniques, we reveal a solid-solution behavior for the Li3RuySb1–yO4 system, as opposed to Li3SbyNb1–yO4 that enlists four rocksalt structures with different cation orderings. We use DFT calculations to rationalize such a structural diversity and find that it is controlled by a delicate balance between electrostatic interactions and charge transfer due to a second order Jahn–Teller distortion. This insight provides a new viewpoint for understanding cationic arrangements in rocksalt structures and guidelines to design novel phases for applications such as Li-ion batteries or ionic conductors.

Ab initio identification of the Li-rich phase in LiFePO4.

A recent discovery of anionic redox activity in Li-rich layered compounds opens a new direction for the design of high-capacity cathode materials for lithium-ion batteries. Here using extensive ab initio calculations, the thermodynamic existence of the Li-rich phase in LiFePO4 to form Li1+xFe1-xPO4 with x not exceeding 12.5% has been proved. Anionic redox activity and structural stability during delithiation are further investigated. Interestingly, it is found that Li1+xFe1-xPO4 cannot be delithiated completely and thus cannot achieve extra capacity by anionic redox activity, because the local oxygen-ion redox will cause the fracture of the rigid framework formed by phosphate tetrahedral polyanions. Although an extra capacity cannot be realized, the excess Li-ions at Fe sites can enhance the Li-ion diffusivity along the adjacent [010] channel and contribute to the shift from 1D to 2D/3D diffusion. This study provides a fresh perspective on olivine-type LiFePO4 and offers some important clues on designing Li-rich cathode materials with high energy density.

Reversible anionic redox activity in Na3RuO4 cathodes: a prototype Na-rich layered oxide

We present a Na-rich layered cathode, Na3RuO4 (Ru5+), with solo oxygen-related anionic redox activity, which can be utilized as a prototype for electrode design in sodium-ion battery.

Phosphorus anionic redox activity revealed by operando P K-edge X-ray absorption spectroscopy on diphosphonate-based conversion materials in Li-ion batteries.

When cycling diphosphonate-based organic-inorganic hybrid materials as negative battery electrodes, specific charges exceeding the maximum for a metal redox reaction are recorded. Classical explanations are electrolyte reduction and solid electrolyte interphase (SEI) oxidation. Using operando X-ray absorption spectroscopy (XAS) at the P K-edge experiment, we demonstrate an additional contribution of reversible ligand co-cycling based on the P-atoms of the diphosphonate ligands, upon delithiation occurring during the first potential plateau, which matches perfectly to previous investigations using Fe K-edge XAS. We thus investigate experimentally an anionic redox process in situ. Thereby we could demonstrate a possible methodology to measure P K-edge XANES on Li-ion battery active materials under operando conditions, which could be applied to verify the predicted P co-cycling of other conversion-type negative electrode materials.

Enabling the high capacity of lithium-rich anti-fluorite lithium iron oxide by simultaneous anionic and cationic redox

Anionic redox reactions in cathodes of lithium-ion batteries are allowing opportunities to double or even triple the energy density. However, it is still challenging to develop a cathode, especially with Earth-abundant elements, that enables anionic redox activity for real-world applications, primarily due to limited strategies to intercept the oxygenates from further irreversible oxidation to O2 gas. Here we report simultaneous iron and oxygen redox activity in a Li-rich anti-fluorite Li5FeO4 electrode. During the removal of the first two Li ions, the oxidation potential of O2− is lowered to approximately 3.5 V versus Li+/Li0, at which potential the cationic oxidation occurs concurrently. These anionic and cationic redox reactions show high reversibility without any obvious O2 gas release. Moreover, this study provides an insightful guide to designing high-capacity cathodes with reversible oxygen redox activity by simply introducing oxygen ions that are exclusively coordinated by Li+. It is challenging to exploit anionic redox activity to boost performance of battery electrodes, especially for anti-fluorite structures. Here the authors report simultaneous anionic and cationic redox in Li5FeO4, which enables its high capacity and eliminates the undesired oxygen gas release.

Simultaneous anionic and cationic redox

It is challenging to unlock anionic redox activity, accompanied by full utilization of available cationic redox process, to boost capacity of battery cathodes. Now, material design by tuning the metal–oxygen interaction is shown to be a promising solution.

Structure-Induced Reversible Anionic Redox Activity in Na Layered Oxide Cathode

Summary Anionic redox reaction (ARR) in lithium- and sodium-ion batteries is under hot discussion, mainly regarding how oxygen anion participates and to what extent oxygen can be reversibly oxidized and reduced. Here, a P3-type Na 0.6 [Li 0.2 Mn 0.8 ]O 2 with reversible capacity from pure ARR was studied. The interlayer O-O distance (peroxo-like O-O dimer, 2.506(3) A), associated with oxidization of oxygen anions, was directly detected by using a neutron total scattering technique. Different from Li 2 RuO 3 or Li 2 IrO 3 with strong metal-oxygen (M-O) bonding, for P3-type Na 0.6 [Li 0.2 Mn 0.8 ]O 2 with relatively weak Mn-O covalent bonding, crystal structure factors might play an even more important role in stabilizing the oxidized species, as both Li and Mn ions are immobile in the structure and thus may inhibit the irreversible transformation of the oxidized species to O 2 gas.

Strategies for Developing High-Energy Density Lithium-Ion Battery Cathodes

Rechargeable lithium-ion batteries (LIBs) have the potential to meet DOE’s energy and cost goals for transportation applications but significant barriers to commercialization remain. Performance limitations in cathode materials are currently considered as the bottleneck even after decades of intense developmental effort in this area. In order to increase energy density of the state-of-the-art lithium transition-metal oxide (Li-TM oxide) cathodes, strategies for either increasing the operating voltage window or the charge density/capacity of the oxides are needed. It is well-known that when cycled at high voltages (> 4.3 V), Li-TM oxides suffer from structural instability, extensive side reactions with the electrolyte, poor cyclability and severe thermal runaway reactions [1, 2]. Utilizing a unique diagnostic approach combining carefully prepared cathode model samples and advanced analytical techniques with high spatial resolution and chemical specificity, we recently obtained important insights on the relationships among oxide’s specific physical properties, reaction mechanisms and its reactivities. The results suggest that majority of the high-voltage stability issues are associated with the physical and chemical properties of the oxide surface, both pristine and changes associated with the cycling [3, 4]. In this presentation, we will discuss these relationships and explore surface modification approaches in addressing the instabilities. Furthermore, recent reports suggested that drastic increase in oxide capacity is possible by utilizing both cationic and anionic redox activities in Li-excess oxides [5, 6]. Insights on the physical and chemical processes occurring during the charge and discharge of these unique oxides will be presented.

The Li3RuyNb1–yO4 (0 ≤ y ≤ 1) System: Structural Diversity and Li Insertion and Extraction Capabilities

Searching for novel high-capacity electrode materials combining cationic and anionic redox processes is an ever-growing activity within the field of Li-ion batteries. In this respect, we report on the exploration of the Li3RuyNb1–yO4 (0 ≤ y ≤ 1) system with an O/M ratio of 4 to maximize the number of oxygen lone pairs, responsible for the anionic redox. We show that this system presents a very rich crystal chemistry with the existence of four structural types, which derive from the rocksalt structure but differ in their cationic arrangement, creating either zigzag, helical, jagged chains or clusters. From an electrochemical standpoint, these compounds are active on reduction via a classical cationic insertion process. The oxidation process is more complex, because of the instability of the delithiated phase. Our results promote the use of the rich Li3MO4 family as a viable platform for a better understanding of the relationships between structure and anionic redox activity.

Dynamic, Reversible Oxygen Redox As a Mediator of Voltage Hysteresis in Lithium-Rich Layered Oxide Electrodes

Anionic redox activity has recently been shown to be an important process that governs the electrochemical behavior of lithium-rich layered oxide electrodes. In 3d transition metal (TM) systems, it is typically believed that during the first charge this redox activity occurs via reversible depopulation of localized O2p electronic states. However, the electrochemistry of lithium-rich layered oxides with 3d TMs changes drastically after the first charge, suggesting that the redox mechanisms likely change. In this work, we investigate the nature of this change and reveal the crucial role of oxygen redox in mediating the unusual electrochemistry in these materials. By coupling a variety of structural probes with Scanning Transmission X-ray Microscopy (STXM) throughout the entire cycle life of a Li- and Mn- rich Ni/Mn/Co layered oxide (LMR-NMC), we correlate spatially localized transmission X-ray Absorption Spectra (XAS) at the TM L edges and O K edge with the evolving LMR-NMC atomic structure. Through our results we develop a unifying picture of how the structure-redox processes in LMR-NMC evolve as the material degrades, considering anion redox, transition metal migration, oxygen evolution, and vacancy formation. Specifically, we find that anion redox plays a crucial role in both the charge/discharge hysteresis during all cycles as well as the fading voltage over extended cycling. Our results paint a holistic picture of the electrochemical mechanism of LMR-NMC, linking understanding of the structural evolution, redox mechanisms, and electrochemical behaviors.

Requirements for Reversible Extra-Capacity in Li-Rich Layered Oxides for Li-Ionbatteries

The structural stability and redox mechanism of Li-rich transition metal oxides (LLO) are two very important aspects for high energy density. The former is related to the irreversible loss of lattice oxygen and capacity fading during cycling, while the latter determines the overall capacity of the materials. This paper aims at clarifying the factors governing the structural stability, the extra capacity and the redox mechanism of LLOs upon Li-removal. The results show that the structural stability against oxygen vacancy formation is improved with increasing M-O covalency, while it decreases with increasing thed-shell electron number and with electrochemical extraction of lithium from the lattice. The redox mechanism of Li2-xMO3 electrodes formed by 3d metals or by heavier metals with ad0electronic configuration is related to the electron depletion from the oxygen lone-pairs (localized and non-bonding O(2p) states) leading to an irreversible anionic redox ending with the reductive elimination of O2 upon cycling. For these phases, long-term cycling is predicted to be very unlikely due to the irreversible loss of lattice oxygen upon charging. For the electrodes formed by 4d and 5d metals with intermediate dnelectronic configurations (n ≥2), reversible cationic and anionic redox activities are predicted, therefore enabling reversible extra-capacities. The very dierent redox mechanisms exhibited by Li2-xMO3electrodes are then linked to the delicate balance between the Coulomb repulsions (Uterm) and the M-O bond covalency (Δ term) through the general description of Charge-Transfer vs. Mott-Hubbard insulators. The present findings will provide a uniform guideline for tuning the band structures of Li2MO3phases and thus activating desired redox mechanisms, being beneficial for the design of high-energy density electrode materials for Li-ion battery applications.

Evidence for anionic redox activity in a tridimensional-ordered Li-rich positive electrodeβ-Li2IrO3.

Lithium-ion battery cathode materials have relied on cationic redox reactions until the recent discovery of anionic redox activity in Li-rich layered compounds which enables capacities as high as 300 mAh g-1. In the quest for new high-capacity electrodes with anionic redox, a still unanswered question was remaining regarding the importance of the structural dimensionality. The present manuscript provides an answer. We herein report on a β-Li2IrO3 phase which, in spite of having the Ir arranged in a tridimensional (3D) framework instead of the typical two-dimensional (2D) layers seen in other Li-rich oxides, can reversibly exchange 2.5 e- per Ir, the highest value ever reported for any insertion reaction involving d-metals. We show that such a large activity results from joint reversible cationic (Mn+) and anionic (O2)n- redox processes, the latter being visualized via complementary transmission electron microscopy and neutron diffraction experiments, and confirmed by density functional theory calculations. Moreover, β-Li2IrO3 presents a good cycling behaviour while showing neither cationic migration nor shearing of atomic layers as seen in 2D-layered Li-rich materials. Remarkably, the anionic redox process occurs jointly with the oxidation of Ir4+ at potentials as low as 3.4 V versus Li+/Li0, as equivalently observed in the layered α-Li2IrO3 polymorph. Theoretical calculations elucidate the electrochemical similarities and differences of the 3D versus 2D polymorphs in terms of structural, electronic and mechanical descriptors. Our findings free the structural dimensionality constraint and broaden the possibilities in designing high-energy-density electrodes for the next generation of Li-ion batteries.

Solid-State NMR of the Family of Positive Electrode Materials Li2Ru1–ySnyO3 for Lithium-Ion Batteries

The possibilities offered by ex situ and in situ operando 7Li solid-state nuclear magnetic resonance (NMR) are explored for the Li2Ru1–ySnyO3 family (0 < y < 1), shown previously to display cationic and anionic redox activity when used as a positive electrode for Li ion batteries. Ex situ NMR spectroscopic studies indicate a nonrandom Sn/Ru substitution in the family. In the first charge, an increased metallicity at 4 V is deduced from the NMR spectra. Surprisingly, no striking difference is observed at 4.6 V compared to the pristine electrode, although the electronic structure is expected to be very different and the local cation environment to be distorted. For in situ operando measurements, we designed a new electrochemical cell that is compatible with NMR spectroscopy and one-dimensional magnetic resonance imaging (MRI). These operando measurements validate the ex situ observations and indicate that the environment formed at 4 V is specific of the initial charge and that there is little, if no, electrolyte decomposition, even at 4.6 V. This is another attractive feature of these compounds.

Lignosulfonate‐Modified Electrode for Electrocatalytic Reduction of Acidic Nitrite

AbstractSodium lignosulfonate (LS) undergoes oxidative electropolymerization on a glassy carbon (GC) electrode from sulfuric acid solution to form a chemically modified electrode exhibiting anionic character and redox activity. Cyclic voltammetry reveals the existence of two redox systems at E°′ values of +0.29 and +0.53 V, respectively. Peak currents are proportional to the scan rate as expected for surface confined systems. The GC|poly‐LS electrode shows electrocatalytic activity toward the reduction of acidic nitrite. When operating in a constant potential amperometric mode (at 0.0 V, vs. Ag/AgCl), a linear relationship between nitrite concentration and reduction current is observed over the range of 1 to 250 μM. The detection limit reaches 0.3 μM (S/N=3). The electrode may be practically applied for nitrite determination in human saliva.