Cationic Redox Reactions Research Articles

Advances in soft X-ray RIXS for studying redox reaction states in batteries.

Redox (reduction and oxidation) chemistry provides the fundamental basis for numerous energy-related electrochemical devices. Detecting the electrochemical redox chemistry is pivotal but challenging because it requires independent probes of the cationic and anionic redox states at different electrochemical states. The synchrotron-based soft X-ray mapping of resonant inelastic X-ray scattering (mRIXS) has recently emerged as a powerful tool for exploring such states in electrochemical devices, especially batteries. High-efficiency mRIXS covers the energy range of the absorption edge with the extra dimension of information on the emitted photon energies. In this frontier article, we review recent representative demonstrations of utilizing soft X-ray mRIXS for detecting the novel chemical state during electrochemical operation and for quantifying the cationic redox reactions through inverse partial fluorescence yield analysis (mRIXS-iPFY). More importantly, the non-divalent states of oxygen in electrodes involving oxygen redox reactions could be reliably captured by mRIXS, with its reversibility quantified by the intensity variation of the characteristic mRIXS feature through a super-partial fluorescence yield analysis (mRIXS-sPFY). These recent demonstrations inspire future perspectives on using mRIXS for studying the complex phenomena in energy materials, with both technical and scientific challenges in RIXS theory, in situ/operando experiments, and spatially resolved RIXS imaging.

Elucidation of Anionic and Cationic Redox Reactions in a Prototype Sodium-Layered Oxide Cathode.

The ever-increasing demand for large-scale energy storage has driven the prosperous investigation of sodium-ion batteries (NIBs). As a promising cathode candidate for NIBs, P2-type Na2/3Ni1/3Mn2/3O2 (NaNMO), a prototype sodium-layered oxide, has attracted extensive attention because of its high operating voltage and high capacity density. Although its electrochemical properties have been extensively investigated, the fundamental charge compensation mechanism, that is, the cationic and anionic redox reactions, is still elusive. In this report, we have systematically investigated the transition metal and oxygen redox reactions of NaNMO nanoflakes using bulk-sensitive soft X-ray absorption spectroscopy and full-range mapping of resonant inelastic X-ray scattering from an atomic-level view. We show that the bulk Mn3+/Mn4+ redox couple emerges from the first discharge process with the increment of inactive Mn3+ upon cycling, which may have a negative effect on the cyclability. In contrast, the bulk Ni redox mainly stems from the Ni2+/Ni3+ redox couple, in contrast to the conventional wisdom of the Ni2+/Ni4+ redox couple. The quantitative analysis provides unambiguous evidence for the continuous reduction of the average valence state of Mn and Ni over extended cycles, leading to the voltage fading. In addition, we reveal that the oxygen anions also participate in the charge compensation process mainly through irreversible oxygen release rather than reversible lattice oxygen redox. Such understanding is vital for the precise design and optimization of NaNMO electrodes for rechargeable NIBs with outstanding performance.

Aqueous Electrolyte Secondary Battery Using Fast Redox Reactions in Single-Walled Carbon Nanotubes

We would like to propose a new concept aqueous electrolyte secondary battery using simultaneous redox reactions of cations and anions in single-walled carbon nanotubes (SWCNTs) that will solve three major problems of Li ion batteries (LIBs): safety, cost and slow-charging problems. Recently we often hear fire and explosion accidents involving LIBs. The accidents were mainly caused by the use of flammable organic electrolytes. The use of organic electrolytes also leads to another problem: the slow-charging problem. Since ion mobility in organic electrolytes is much lower than that in aqueous electrolytes, it is very hard to obtain fast-charging for LIBs. The time-consuming charging process of LIBs hinders the market growth of electric vehicles (EVs). The high cost of LIBs due to the use of rare metals is another problem for the wider distribution of EVs. Therefore, we have to solve the problems of safety, cost and slow-charging. The new concept battery we proposed uses mono-valent or di-valent metal halide (e.g. LiI, NaI, ZnI2) aqueous electrolytes instead of the flammable organic electrolytes used in LIBs. The use of aqueous electrolytes is not only much safer than organic electrolytes, but also advantageous to cost and fast-charging, because alkali metal halides such as LiI, NaI, ZnI2 consist only of abundant elements, and the ion mobilities of both metal ions and halogen ions in water are much faster than those in organic electrolytes. We use SWCNTs as negative and positive electrodes. At positive electrode, iodide ions are oxidized inside the empty SWCNTs and caught as a form of diatomic molecule inside SWCNTs. On the other hand, alkali metal ions are caught by quinone organic molecules encapsulated in SWCNTs and di-valent ions are reduced on the surface of SWCNTs. We used a SWCNT sample purchased from Meijo NanoCarbon Co. The mean tube diameter of the SWCNT sample was determined to be 2.5 nm by XRD analysis. Reagent-grade 9,10-phenanthrenequinone (PhQ) powder sample was also purchased from Wako Pure Chemical Industries. SWCNT and PhQ powder samples were sealed in a vacuum glass tube, and heated at 200ºC for 10 h. The obtained PhQ@SWCNT sample was analyzed by TEM, SEM, Raman and XRD measurements. The electrochemical measurements were performed in a three electrode cell. PhQ@SWCNT sample (anode) was paired with an empty SWCNT electrode (cathode). For CV measurements, we used Ag/AgCl reference electrode. Aqueous electrolytes of 1 mol/L LiI or NaI were used. TEM, FT-IR, XRD, XPS measurements confirmed the encapsulation of PhQ molecules in SWCNTs. The content of encapsulated PhQ molecules per SWCNT weight was determined to be 38.0 wt% by TG measurements. Both in the cases of LiI and NaI aqueous electrolyte cases, PhQ@SWCNT could work as electrode active materials for Li and Na ion charge/discharge. The observed discharge potential for Na ion is about 0.1 V lower than that for Li ion. The cathode reaction of the proposed battery is fixed to the redox reaction of iodide ions, namely to the redox potential of I-/I2 (about +0.45 V vs. Ag/AgCl). Therefore, a lower redox potential of the quinone anode is better to obtain higher EMF from the proposed battery. So, NaI electrolyte is better than LiI electrolyte in terms of the potential. The discharge potential difference between LiI and NaI indicates that PhQ molecule attached to Na ion (PhQ-Na) is chemically more active than PhQ-Li. On the other hand, iodine electrode (cathode) properties have been reported in our previous papers. The molecular structures and phase transformations of the encapsulated iodine molecules were also fully investigated. Therefore, we were convinced that the iodine electrode should work as cathode in the new concept battery. The full cell performance of the proposed cell was also measured. The cathode potential was kept at iodine redox potential as we expected. So, It was also found that PhQ@SWCNT can work as anode electrode. Therefore, basically we have succeeded in demonstrating that the new concept battery, namely, a safe, low-cost and fast charging secondary battery using aqueous electrolyte can work very nicely. However, the amount of the observed charge capacity is quite different from that of the discharge capacity. We should investigate the reason why such a big irreversible capacity was observed in order to improve the proposed new concept aqueous secondary battery. We also would like to talk about di-valent metal cases in the conference.

Improving Reversibility of Anionic Redox Reaction of NaVO3 By Metal Doping for Sodium-Ion Batteries

Traditionally, positive electrode materials in lithium/sodium-ion batteries (LIBs/NIBs) undergo charge and discharge through redox reaction of the transition metal (TM) within the structure. As a result, the number of electron transfer is limited by the available valence states of the TM. Recently, there is an increased interest in solid-state anionic redox reaction (ARR) in positive electrode materials that compensates the charge transfer by the oxygen atoms. ARR can potentially increase the capacity of the active material with the additional charge-transfer site. It can also open the door to more transition-metal compounds that are originally thought to be inactive. Our previous work has shown that NaVO3 (NVO) can undergo anionic redox reaction while the oxidation state of V remains 5+ between 1.5 and 4.9 V vs. Na/Na+.[1] It can give a capacity of more than 100 mAh g-1, though its capacity fades with cycling. Here, we aim at improving the stability and capacity of the material by incorporating other transition metals into the structure to utilize both the anionic and cationic redox reactions. As a start, we synthesized a single phase Na1.3Co0.1V0.9O3 (NCVO) with Co2+ doping by a typical solid-state method. Both NVO and NCVO materials were first discharged to 1.2 V to investigate the amount of V5+/4+ redox reaction that is available in the material and then charged to 4.7 V. Figure 1a shows the first and second charge-discharge curve of the two materials. Interestingly, the incorporation of Co into the structure reduces the activity of V5+/4+ during initial discharge, with a smaller capacity of 38 mAh g-1 for NCVO as opposed to 94 mAh g-1 for NVO to 1.2 V. Though, the amount of charge and discharge capacities during 2nd cycle are the same, suggesting changes in reaction mechanism of the material with Co2+ addition. The available capacity of about 180 mAh g-1 is higher than that can be obtained from Co and V redox reactions alone, so part of the capacity comes from oxygen redox reaction. Figure 1b shows the cycle performance of NCVO and it gives good stability. In addition, NCVO can delivers better rate performance with a discharge capacity of 130 mAh g-1 at 50 mA g-1. Further works with techniques such as ex-situ XRD, XPS and GITT to characterize the material and understand the reaction mechanism are underway. More results on the effect of the metal dopant will be presented at the meeting. [1] J. Zhang, B. Su, A. Kitajou, M. Fujita, Y. Cui, M. Oda, W. Zhou, P. H.-L. Sit, D. Y. W. Yu, J. Power Sources 400, 377 (2018). Figure 1. (a) Initial discharge and second charge-discharge curves of NVO and NCVO electrodes at 10 mA g−1 between 1.2 and 4.7 V vs. Na/Na+; (b) Cycle performance of NCVO between 1.2 and 4.7 V. Figure 1

High Reversibility of Lattice Oxygen Redox Quantified by Direct Bulk Probes of Both Anionic and Cationic Redox Reactions

The reversibility and cyclability of anionic redox in battery electrodes hold the key to its practical employments. Here, through mapping of resonant inelastic X-ray scattering (mRIXS), we have independently quantified the evolving redox states of both cations and anions in Na2/3Mg1/3Mn2/3O2. The bulk-Mn redox emerges from initial discharge and is quantified by inverse-partial fluorescence yield (iPFY) from Mn-L mRIXS. Bulk and surface Mn activities likely lead to the voltage fade. O-K super-partial fluorescence yield (sPFY) analysis of mRIXS shows 79% lattice oxygen-redox reversibility during initial cycle, with 87% capacity sustained after 100 cycles. In Li1.17Ni0.21Co0.08Mn0.54O2, lattice-oxygen redox is 76% initial-cycle reversible but with only 44% capacity retention after 500 cycles. These results unambiguously show the high reversibility of lattice-oxygen redox in both Li-ion and Na-ion systems. The contrast between Na2/3Mg1/3Mn2/3O2 and Li1.17Ni0.21Co0.08Mn0.54O2 systems suggests the importance of distinguishing lattice-oxygen redox from other oxygen activities for clarifying its intrinsic properties.

(Invited) Materials Design of High-Energy Cation/Anion Redox in Li-Rich Cathode Materials: Li5FeO4, Li4Mn2O5, and Li2MO3

The energy density of many lithium metal oxide battery cathodes is limited by redox reactions associated with the transition metal alone. Anionic redox reactions are affording new opportunities to increase this energy density. Here, we present results from first-principles density functional theory (DFT) calculations aimed at understanding the combined cation/anion redox of Li-rich materials, and designing new materials that will enable high-capacity, reversible cycling with minimal oxygen evolution. We illustrate this approach on three Li-rich compounds: Li5FeO4, Li4Mn2O5, and Li2MnO3. Li5FeO4: Initial removal of Li is accompanied by Fe migration to form a disordered rocksalt structure. A local Li6-O coordination of oxygen, identified by DFT calculations, raises the O 2p band and enables reversible O-/O2- redox behavior, previously unknown in this material. This insight leads to the prediction and subsequent experimental demonstration of anionic and cationic redox reactions with good reversibility and without any obvious O2 gas release. Li4Mn2O5: We study the recently-reported, high-capacity, disordered rocksalt-type Li4Mn2O5 compound and also determine the ground state ordered structure of Li4Mn2O5 via a DFT-based enumeration method. DFT calculations show that the delithiation process occurs via a three-step reaction pathway involving the complex interplay of cation and anion redox reactions: i) Mn3+ ➝ Mn4+ (LixMn2O5, 4 > x > 2), ii) O2− ➝ O1− (2 > x > 1), and iii) Mn4+ ➝ Mn5+ (1 > x > 0) concomitant with Mn migration from the original octahedral site to the adjacent tetrahedral site. Finally, we predict that alloying with M = V and Cr in Li4(Mn,M)2O5 would produce new stable compounds with substantially improved electrochemical properties. Li2 MO3: We catalog the family of Li2 MO3 compounds as active cathodes or inactive stabilizing agents using high-throughput density functional theory (HT-DFT). With an exhaustive search based on design rules that include phase stability, cell potential, resistance to oxygen evolution, and metal migration, we predict a number of new Li2MIO3–Li2MIIO3 active/inactive electrode pairs, in which MI and MII are transition- or post-transition metal ions, that can be tested experimentally for high-energy-density lithium-ion batteries.

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.

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.

Anomalous Pseudocapacitive Behavior of a Nanostructured, Mixed-Valent Manganese Oxide Film for Electrical Energy Storage

While pseudocapacitors represent a promising option for electrical energy storage, the performance of the existing ones must be dramatically enhanced to meet today's ever-increasing demands for many emerging applications. Here we report a nanostructured, mixed-valent manganese oxide film that exhibits anomalously high specific capacitance (∼2530 F/g of manganese oxide, measured at 0.61 A/g in a two-electrode configuration with loading of active materials ∼0.16 mg/cm(2)) while maintaining excellent power density and cycling life. The dramatic performance enhancement is attributed to its unique mixed-valence state with porous nanoarchitecture, which may facilitate rapid mass transport and enhance surface double-layer capacitance, while promoting facile redox reactions associated with charge storage by both Mn and O sites, as suggested by in situ X-ray absorption spectroscopy (XAS) and density functional theory calculations. The new charge storage mechanisms (in addition to redox reactions of cations) may offer critical insights to rational design of a new-generation energy storage devices.