Lithium Peroxide Research Articles

Analyzing Microstructural and Electrolyte Phase Limitations in Li-Air Battery Performance

Lithium-air batteries, despite having a superior theoretical performance as compared to other lithium chemistries, demonstrates poor discharge performance. The discharge product in organic electrolyte Li-air cells is lithium peroxide (Li2O2), which precipitates as a solid phase during discharge and gives rise to pore network evolution for the porous electrode. To make matters worse, lithium peroxide is electronically insulating and in turn evolves the electrochemically active interface. On the other hand, as dissolved oxygen is consumed during electrochemical discharge, additional oxygen from air is to be transported from ambient to reaction zone. Finite solubility and finite rate oxygen transport may lead to oxygen starvation for faster operation (related with electrolyte phase transport description). The relative severity of these microstructural and electrolyte phase limitations jointly determines the cell performance. Consistent and comprehensive descriptions for microstructural evolution and electrolyte phase transport are developed here, and in turn physicochemical evolution in Li-air cells investigated. Depending on the severity of these resistive modes, different strategies for cell performance improvement are highlighted.

Improving Cathode and Electrolyte Stability in Lithium-Oxygen Batteries

Lithium-Oxygen and Lithium-Air batteries have been of great interest for energy storage due to their high theoretical energy density, which have the potential to enable other technologies such as electric and hybrid aircraft. However, due to the reactivity of the discharge product in nonaqueous electrolytes cells, cycling performance is severely inhibited which occurs due to side reactions at both sides of the deposited lithium peroxide (Li2O2), and consumes both the carbon from the supporting cathode structure as well as the organic solvent from the electrolyte. In this study, a synergistic approach utilizing computational modeling and experimental screening was applied to investigate novel cathode alternatives and coatings in addition to alternative electrolyte solvents to increase material stability and avoid both forms of side reaction driven decomposition.

Kinetics of lithium peroxide oxidation by redox mediators and consequences for the lithium\u2013oxygen cell

Lithium–oxygen cells, in which lithium peroxide forms in solution rather than on the electrode surface, can sustain relatively high cycling rates but require redox mediators to charge. The mediators are oxidised at the electrode surface and then oxidise lithium peroxide stored in the cathode. The kinetics of lithium peroxide oxidation has received almost no attention and yet is crucial for the operation of the lithium–oxygen cell. It is essential that the molecules oxidise lithium peroxide sufficiently rapidly to sustain fast charging. Here, we investigate the kinetics of lithium peroxide oxidation by several different classes of redox mediators. We show that the reaction is not a simple outer-sphere electron transfer and that the steric structure of the mediator molecule plays an important role. The fastest mediator studied could sustain a charging current of up to 1.9 A cm–2, based on a model for a porous electrode described here.

Molecular Sieve Induced Solution Growth of Li2O2 in the Li–O2 Battery with Largely Enhanced Discharge Capacity

The formation of the insulated film-like discharge products (Li2O2) on the surface of the carbon cathode gradually hinders the oxygen reduction reaction (ORR) process, which usually leads to the premature death of the Li-O2 battery. In this work, by introducing the molecular sieve powder into the ether electrolyte, the Li-O2 battery exhibits a largely improved discharge capacity (63 times) compared with the one in the absence of this inorganic oxide additive. Meanwhile, XRD and SEM results qualitatively demonstrate the generation of the toroid Li2O2 as the dominated discharge products, and the chemical titration quantifies a higher yield of the Li2O2 with the presence of the molecular sieve additive. The addition of the molecular sieve controls the amount of the free water in the electrolyte, which distinguishes the effect of the molecular sieve and the free water on the discharge process. Hence, a possible mechanism has been proposed that the adsorption of the molecular sieves toward the soluble lithium superoxides improves the disproportionation of the lithium superoxides and consequently enhances the solution-growth of the lithium peroxides in the low donor number ether electrolyte. In general, the application of the molecular sieve triggers further studies concerning the improvement of the discharge performance in the Li-O2 battery by adding the inorganic additives.

Nanostructuring one-dimensional and amorphous lithium peroxide for high round-trip efficiency in lithium-oxygen batteries

The major challenge facing lithium–oxygen batteries is the insulating and bulk lithium peroxide discharge product, which causes sluggish decomposition and increasing overpotential during recharge. Here, we demonstrate an improved round-trip efficiency of ~80% by means of a mesoporous carbon electrode, which directs the growth of one-dimensional and amorphous lithium peroxide. Morphologically, the one-dimensional nanostructures with small volume and high surface show improved charge transport and promote delithiation (lithium ion dissolution) during recharge and thus plays a critical role in the facile decomposition of lithium peroxide. Thermodynamically, density functional calculations reveal that disordered geometric arrangements of the surface atoms in the amorphous structure lead to weaker binding of the key reaction intermediate lithium superoxide, yielding smaller oxygen reduction and evolution overpotentials compared to the crystalline surface. This study suggests a strategy to enhance the decomposition rate of lithium peroxide by exploiting the size and shape of one-dimensional nanostructured lithium peroxide.

Electron irradiation effects on lithium peroxide

In this study, electron irradiation effects on lithium peroxide (Li2O2), which is an important discharge product of Li–air (or Li–O2) batteries, were investigated using selected-area electron diffraction (SAED) and high-energy resolution electron energy-loss spectroscopy (EELS). The results obtained show that Li2O2 to Li2O transformation occurs with 80 and 300 keV incident electrons under high electron dose rates at 20 and −183 °C. The Li2O2 to Li2O transformation rate for 300 keV was 1/5 of that for 80 keV with the irradiation taking place at −183 °C. We also present a series of the EELS spectra that can be used as a criterion to judge the molar ratio of Li2O to Li2O2 in the general systems where Li2O2 and Li2O coexist.

Exploring the LiOH Formation Reaction Mechanism in Lithium–Air Batteries

Identifying the electronic factors that enhance or hinder the formation of primary discharge/charge products or secondary parasitic species is crucial for defining the fundamental chemical reactions that may take place within the complex electrolytic media of a lithium–air battery. For example, several reaction mechanisms have been proposed to explain the formation of the LiOH discharge product in the presence of LiI and water; however, none of these have been demonstrated or fully understood. A similar situation exists for the decomposition reaction, which leads toward molecular oxygen evolution. Herein we present a mechanistic theoretical study of the reactions taking place in the electrolytic media of a Li–O2 battery with dimethoxyethane (DME) in the presence of both water and the LiI additive. The results reveal that water is the most energetically favorable source of protons yielding LiOH. The effect of iodide in lithium peroxide bond scission can be ascribed to a halogen-bond interaction. The reacti...

Theoretical aspects in structural distortion and the electronic properties of lithium peroxide under high pressure.

The structural phase transition and electronic properties of Li2O2 under pressures up to 500 GPa have been investigated using first-principles calculations. Two new structural phase transitions have been proposed at pressures around 75 GPa from the P63/mmc structure to the P21 structure and around 136 GPa from the P21 structure to the P21/c structure. The calculated phonon spectra have confirmed the dynamical stability of these structures. The pressure dependence of the lattice dynamics, O-O bond length, and band gaps in Li2O2 have also been reported. The band gaps of the P63/mmc, P21, and P21/c structures calculated by PBE and HSE06 have shown increasing trends with increasing pressure. Interestingly, the P63/mmc band gap and c/a ratio have significantly decreased with the increasing O-O bond length and ELF value around 11 and 40 GPa. At these pressures, the phonon frequency of the O-O stretching modes has softened. This finding reveals the effects of structural distortion in three phases of Li2O2. Our study provides structural understanding and the electronic properties of Li2O2 under high pressure, which might be useful for investigating the charge transport through Li2O2 in lithium-air batteries and CO2 capture.

A new thin layer cell for battery related DEMS-experiments: the activity of redox mediators in the Li-O2 cell.

The reversibility of current Li-O2 batteries suffers from high charging overpotentials. To address this problem, the use of redox mediators has been proposed, which are supposed to improve the sluggish reaction kinetics of the oxygen evolution reaction via a solution mediated oxidation of lithium peroxide. In this study, we present a new thin layer cell for battery related differential electrochemical mass spectrometry (DEMS) experiments, which exhibits a high electrode surface area to electrolyte volume ratio which is closer to the situation in batteries other approaches/cells with their usually large electrolyte excess. The confined volume also allows a better distinction between the mediating activity of a redox system and a near continuous electrochemical reaction of this species. One further benefit of the new thin layer cell is that experiments can easily be performed under different O2-partial pressures. This new set-up allows the highly sensitive detection of volatile species formed during the OER. Therefore, small changes in the number of electrons transferred per oxygen molecule are observable. These changes help to identify side reactions and possible decomposition of the reaction products. During our experiments, we investigated the impact of TTF, TMPD, Fc and TEMPO on the oxidation of Li2O2. Within our experiments, we are able to precisely determine the potential at which the catalytic activity of the redox mediation starts. A comparison between the potential at which we observe the activity of the redox mediator to the half wave potential of the redox system could be explained with an outer sphere electron transfer for the oxidation of Li2O2 by a redox mediator. This observation is confirmed by a theoretical treatment of the redox mediation mechanism. Moreover, insights into the number of transferred electrons per oxygen molecule during the activity of the different redox mediators reveal the presence of side reactions. This finding is also underlined by an unexpected shift of the CO2 evolution onset for the redox mediator containing electrolytes. Our experiments also reveal that a Li-O2 cell, which contains a redox mediator, undergoes less fluctuation in its reversibility compared to a cell without a redox mediator.

Comparative Study of Special Features of the Oxygen Reaction (Molecular Oxygen Ionization and Evolution) in Aqueous and Nonaqueous Electrolyte Solutions (a Review)

Studies of the oxygen reaction, including the oxygen ionization and evolution processes occurring at typical electrode materials in aqueous and nonaqueous electrolytes, are analyzed. A connection between the problematics of the oxygen electrode reaction in nonaqueous media and the developing of novel batteries, in the first place, Li–O2 batteries, is emphasized. Unlike aqueous solutions, the oxygen reduction in aprotic electrolytes was shown to occur without breaking of the O–O bond; it is accompanied by formation of poorly soluble product of two-electron reaction (Li2O2) in the pores of positive electrode. The effect of the solvent donor number and the anion composition on the oxygen reduction mechanism and the lithium peroxide deposit structure is described. A marked reduction of the Li2O2 oxidation overvoltage when passing from carbonaceous materials to platinum-containing catalysts in the positive electrode is elucidated; in the latter case, the effect of electrocatalyst type upon the Li2O2 formation reaction is somewhat reduced. The elucidation of the contribution of processes occurring at the free and lithium-peroxide-covered electrode surface during the oxygen reaction for wide variety of active materials is formulated as the main basic problem of the future research.

Clarification of Solvent Effects on Discharge Products in Li-O2 Batteries through a Titration Method.

As a substitute for the current lithium-ion batteries, rechargeable lithium oxygen batteries have attracted much attention because of their theoretically high energy density, but many challenges continue to exist. For the development of these batteries, understanding and controlling the main discharge product Li2O2 (lithium peroxide) are of paramount importance. Here, we comparatively analyzed the amount of Li2O2 in the cathodes discharged at various discharge capacities and current densities in dimethyl sulfoxide (DMSO) and tetraethylene glycol dimethyl ether (TEGDME) solvents. The precise assessment entailed revisiting and revising the UV-vis titration analysis. The amount of Li2O2 electrochemically formed in DMSO was less than that formed in TEGDME at the same capacity and even at a much higher full discharge capacity in DMSO than in TEGDME. On the basis of our analytical experimental results, this unexpected result was ascribed to the presence of soluble LiO2-like intermediates that remained in the DMSO solvent and the chemical transformation of Li2O2 to LiOH, both of which originated from the inherent properties of the DMSO solvent.

Cobalt porphyrin and Salcomine as novel redox shuttle species to enhance the oxygen evolution reaction in Li[sbnd]O2 batteries

LiO2 secondary batteries outclass other battery systems in terms of energy density, at least on a theoretical point of view. However, these systems suffer issues from both the metallic lithium electrode and the oxygen electrode. For example, the cathode discharge product (lithium peroxide) is a semiconductor of large bandgap (5 eV), which yields severe cyclability issues, as the re-charge is limited. Using electrolyte additives may be one option to mitigate (if not solve) this issue. Herein, a new redox mediator compound (N,N′-Bis(salicylidene)ethylenediaminocobalt (II), noted Co-salen) is studied with regards to its impact on the oxygen reduction and evolution reactions in non-aqueous medium. Its performances are compared to those of a Cobalt porphyrin (noted Co(II)-Po). Electrochemical experiments in three-electrode setup, Differential Electrochemical Mass Spectrometry (DEMS) and UV–Visible spectroscopy, demonstrate that Co-salen can also greatly improve the oxygen reduction kinetics (the ORR onset potential is positively-shifted by 230 mV in the presence of Co-salen), thanks to the formation of a complex with oxygen in solution. Secondly, Co-salen enables to efficiently transport electrons in the solution, and help to recover the electrode surface (the re-charge of large Li2O2 particles becomes possible). Therefore, enhancing both the oxygen reduction and the oxygen evolution reactions (ORR and OER) with only one additive in the electrolyte is feasible, and places Co-salen among the best compounds to be used as redox shuttles for non-aqueous LiO2 batteries.

(Invited) Metal-Organic Framework-Derived Electrocatalysts for Li‒O2 Batteries

Recently, metal-organic framework (MOF) materials have been studied as cathode materials for Li‒O2 batteries due to their high catalytic activity, uniform structure, and simple preparation process. However, further optimization of materials and structures should be undertaken to improve their electrochemical performance for the practical application.1-3 Here we report a uniform MOF-derived multiple-capsule heterostructure, nitrogen-doped cobalt@graphene, as the cathode catalysts for Li‒O2 cells. This N-Co@graphene multiple-nanocapsule structure improves the electrical properties and makes the high/uniform electroactive zones possible. In addition, the rational design of flexible foam-based electrodes possessing colander-like porous structures facilitates the oxygen diffusion, catalytic reaction, and deposition of discharge products. The electrode thus exhibits much improved electrochemical performance associated with unique morphologies of electrochemically grown lithium peroxides. Figs. 1a-c display the microstructure and morphology of N-Co@graphene nanocomposite. The composite has a lotus shower-like shape, where the crystalline Co nanoparticles, sized of 10‒20 nm, display highly ordered lattice fringes, being wrapped by few-layer graphenes, thus forming core-shell nanostructures. Note that there are few layers of specific lattice fringes with 0.25 nm lattice spacing existing on the surface of Co particles, which are indexed to be cobalt nitride that are generated from the nitrogen dopant into surficial lattices of Co crystalline. Fig. 1d show its EELS profile, which confirms the presence of C, N, and Co elements in the composite. Figs. 1e, f show its electrochemical performance in Li‒O2 cells. The composite electrodes exhibit reduced charge-discharge overpotential and good cycling stability. This finding is mainly attributed to the improved electrical properties, electrocatalytic activity, and structural stability of N-Co@graphene nanostructures. Fig. 1 Structural and electrochemical characterization of MOF-derived N-Co@graphene catalysts. (a-c) TEM images, and (d) EELS profile of N-Co@graphene nanocomposite. (e,f) Voltage profiles versus selected cycles of N-Co@graphene composite based on carbon fiber (e) and nickel foam (f) electrodes.

Improvement of Charge/Discharge Capacity of Lithium-Air Battery Using Porous Carbon Nanotube Electrode

Recently, renewable energy such as solar and geothermal energy has been attracting great attentions for sustainable society. To minimize the risk of insufficient supply, secondary batteries are promising device to store the part of energy. Recent increase of the amount of renewable energy, increase of the energy density of the secondary batteries is highly required. Lithium air battery is one of the most promising secondary battery due to their larger theoretical energy density (11700 Wh/kg) than the other secondary batteries1, 2. During the discharge reaction of lithium air battery, Li in anode oxidizes to Li+ ions and diffuses through the electrolyte to react with oxygen at the cathode, resulting in the generation of lithium peroxide such as Li2O2. The reverse reaction is proceeded as a charge reaction3. As the cathode materials, carbon-based materials such as carbon blacks have been studied because of its high electrical conductivity, light weight and low cost4. However, it was pointed out that, diffusion of Li+ ions and oxygen is disturbed owing to pore clogging of Li2O2 during discharge reaction, greatly limiting charge-discharge capacity of the cell4. To solve the problem, single-walled carbon nanotubes (SWNTs) are emerged as the candidate for cathode material since SWNTs form continuous porous structure, which is preferable for the diffusion of oxygen and Li+ ions5, 6. However, it was found that the pore clogging by Li2O2 was occurred and the capacity of the battery was still limited. In this study, we introduce the internal space in the SWNT network using polystyrene beads (PS beads) as a template (3D network electrode) to avoid the clogging of Li2O2. The advantage of this approach is the good controllability of the pore size by taking advantage of the good size controllability of PS beads. 3D network electrode was prepared by removing PS beads from SWNT/PS beads composite film by heating at 450 ºC for 1 h. The removal of PS beads was confirmed by thermogravimetric analysis and scanning electron microscope observations. Mercury intrusion porosimetry analysis revealed the size of the pore was almost comparable to the size of the PS beads used as the template. In the charging and discharging experiment of the cell, lager capacity were observed (4807 mAh/mg) compared with that of the electrode without using PS beads (2143 mAh/mg). The results is explained by the improved diffusion of Li+ ions and oxygen during the discharge reaction and wider accumulation space for Li2O2 at the cathode. 1)Zhou, H.; He, P.; Wang, Y.; Li, D. Wuli, 2012, 41, 86. 2)Alan, C. L.; Bryan, D.; McCloskey, Chem. Rev. 2014, 114, 11721. 3)Brian, D. A.; Claudio, R.; Robert, B.; Mickel, L. T.; Karim, Z.; Linda, F. N. Energy Environ. Sci. 2013, 6, 1772. 4)Mahesh, D. B.; Hugh, G.; Michael, N.; Colm, O. Phys. Chem. Chem. Phys. 2014 , 16, 12093. 5)Yong, C.; Fujun, L.; Dai-Ming, T.; Zelang, J.; Chang, L.; Dmitri, G.; Atsuo, Y.; Haoshen, Z.; J. Mater. Chem. A, 2013, 1, 13076. 6)Hee-Dae, L.; Kyu-Young, P.; Hyelynn, S.; Eui, Y. J.; Hyeokjo, Gwon.; Jinsoo, K.; Yong, H. K.; Marcio, D. L.; Raquel, O. R.; Xavier, L.; Ray, H. B.; Kisuk, K. Adv, Mater, 2013, 25, 1348.

Influence of Ammonium Salts on Discharge and Charge of Li–O2 Batteries

Li–air (O2) batteries are promising because of their high theoretical energy density. However, these batteries are plagued with numerous challenges, one of which involves modulating the battery discharge process between a solution or surface-driven formation of the desired lithium peroxide (Li2O2) discharge product, and the oxidation of Li2O2 below 4 V (vs Li/Li+). In this work, we show that tetrabutylammonium (TBA) salts dissolved in ether or dimethyl sulfoxide (with no lithium salt present) can be used as a Li–O2 electrolyte with a lithium metal anode to support Li2O2 formation, lead to >500 mV reduction in charging overpotentials at low current rates as compared to that with lithium salt, and support the oxidation of Li2O2 during charge. Furthermore, on the basis of results from several spectroscopic techniques, we propose a mechanism that involves electrochemical-induced transformation of TBA to tributylamine at ∼3.55 V, and the formation of a tributylamine oxide intermediate in the presence of O2 or ...

Organic hydrogen peroxide-driven low charge potentials for high-performance lithium-oxygen batteries with carbon cathodes

Reducing the high charge potential is a crucial concern in advancing the performance of lithium-oxygen batteries. Here, for water-containing lithium-oxygen batteries with lithium hydroxide products, we find that a hydrogen peroxide aqueous solution added in the electrolyte can effectively promote the decomposition of lithium hydroxide compounds at the ultralow charge potential on a catalyst-free Ketjen Black-based cathode. Furthermore, for non-aqueous lithium-oxygen batteries with lithium peroxide products, we introduce a urea hydrogen peroxide, chelating hydrogen peroxide without any water in the organic, as an electrolyte additive in lithium-oxygen batteries with a lithium metal anode and succeed in the realization of the low charge potential of ∼3.26 V, which is among the best levels reported. In addition, the undesired water generally accompanying hydrogen peroxide solutions is circumvented to protect the lithium metal anode and ensure good battery cycling stability. Our results should provide illuminating insights into approaches to enhancing lithium-oxygen batteries.

Mass and charge transport relevant to the formation of toroidal lithium peroxide nanoparticles in an aprotic lithium-oxygen battery: An experimental and theoretical modeling study

The discharge and charge mechanisms of rechargeable Li-O2 batteries have been the subject of extensive investigation recently. However, they are not fully understood yet. Here we report a systematic study of the morphological transition of Li2O2 from a single crystalline structure to a toroid like particle during the discharge–charge cycle, with the help of a theoretical model to explain the evolution of the Li2O2 at different stages of this process. The model suggests that the transition starts in the first monolayer of Li2O2, and is subsequently followed by a transition from particle growth to film growth if the applied current exceeds the exchange current for the oxygen reduction reaction in a Li-O2 cell. Furthermore, a sustainable mass transport of the diffusive active species (e.g., O2 and Li+) and evolution of the underlying interfaces are critical to dictate desirable oxygen reduction (discharge) and evolution (charge) reactions in the porous carbon electrode of a Li-O2 cell.

A Chronoamperometric Differential Electrochemical Mass Spectroscopy Study on the Growth of Li2O2 and Its Effect on the Mechanism of Oxygen Reduction in Dimethylsulfoxide Based Electrolytes

We report on potential step experiments into the oxygen reduction region in combination with differential mass spectroscopy (DEMS) and the rotating ring disc electrode technique (RRDE). We can thus distinguish between the one electron reduction path leading to superoxide and the 2 electron process leading to lithium peroxide. The evolution of the mass spectroscopic response in the DEMS experiments and evolution of the current at the ring electrode discloses that the deposition of lithiumperoxide hampers the capacity of the electrode for the consecutive transition of two electrons. The latter is required to form Li2O2 on a direct electrochemical pathway. Instead, superoxide becomes the main product of oxygen reduction. A proportionality between coverage with Li2O2 and the share of oxygen reduced to superoxide is observed at platinum. The effect of the Li2O2-deposit is less severe at large overpotentials. However, completion of a single monolayer of Li2O2 on platinum and two monolayers on gold inhibit further oxygen reduction. We suggest that growth of larger Li2O2 structures observed by other authors is caused by a chemical rout to Li2O2 via superoxide disproportionation. Experimental evidence for the latter reaction is provided.In addition, we observe a non-monotonic faradaic current transient typical for nucleation after steps to certain potentials at the gold electrode and ascribe this to the formation of 2D Li2O2. This implies that nuclei of Li2O2 have to form before the solid phase can grow uninhibited. Evidently the involved surface energies cause a crystallisation overpotential and therefore impair the formation of solid Li2O2.

Lithium Superoxide Hydrolysis and Relevance to Li–O2 Batteries

Fundamental understanding of reactions of lithium peroxides and superoxides is essential for the development of Li–O2 batteries. In this context, an investigation is reported of the hydrolysis of lithium superoxide, which has recently been synthesized in a Li–O2 battery. Surprisingly, the hydrolysis of solid LiO2 is significantly different from that of NaO2 and KO2. Unlike KO2 and NaO2, the hydrolysis of LiO2 does not produce H2O2. Similarly, the reactivity of Li2O2 toward water differs from LiO2, in that Li2O2 results in H2O2 as a product. The difference in the LiO2 reactivity with water is due to the more exothermic nature of the formation of LiOH and O2 compared with the corresponding reactions of NaO2 and KO2. We also show that a titration method used in this study, based on reaction of the discharge product with a Ti(IV)OSO4 solution, provides a useful diagnostic technique to provide information on the composition of a discharge product in a Li–O2 battery.

Enhanced energy capacity of lithium-oxygen batteries with ionic liquid electrolytes by addition of ammonium ions

Lithium-oxygen (Li–O2) batteries with ionic liquid-based electrolytes have attracted much attention because of their superior battery performance and high safety. However, the practical energy capacities achieved to date are markedly lower than those of Li-ion batteries. A dominant factor that limits the energy capacities of Li–O2 batteries is the insulating characteristics and insolubility of lithium peroxide (Li2O2), which gradually accumulates on the positive electrode as a discharge product. Herein, we report that ammonium ions function as a promoter for the solution-route formation of Li2O2, which results in a significant improvement of the energy capacity of Li–O2 cells. Scanning electron microscopy analyses revealed that the structure of Li2O2 changed from spherical particles to toroidal particles with an increase in the ammonium ions concentration. Ionic additives that have been reported to function as promoters the solution-route formation of Li2O2 in ether-based electrolyte systems do not exhibit a promoting effect in ionic liquid-based electrolytes.