Formation Of Lithium Peroxide Research Articles

Understanding the Reaction Mechanism and Kinetics of Mediated Li-O2 Batteries Using Flow Set-Ups and Cyclic Voltammetry

Li-O2 batteries are very promising candidates for electromobility because of their very high theoretical energy density. Moreover, they are composed of abundant materials, as opposed to Li-ion batteries, which would make them a greener and cheaper alternative. The principal reaction in Li-O2 batteries is the formation of lithium peroxide during discharge via the oxygen reduction reaction (ORR), that is then oxidised during the charge by the oxygen evolution reaction (OER). One of the major challenges of Li-O2 batteries is related to the insulating nature of their discharge product, that significantly limits the capacity and hinders performance. A strategy to overcome this is by using redox mediators (RMs), which shuttle electrons from the electrode surface to the solution and thus facilitate a solution-based mechanism for the formation of large crystals. Understanding the reaction kinetics of these RMs can therefore aid in increasing the battery performance.Quinones, specifically 2,5-Di-tert-butyl-1,4-benzoquinone (DBBQ),is a commonly used discharge RM in this type of batteries. However, their reaction kinetics and how these vary with different electrolyte conditions are not yet well understood. DBBQ undergoes two single electron reductions, whose potentials and kinetics vary with salt concentrations and solvent having a direct effect on its ability to mediate the ORR.In this work, we have developed a Li-O2 flow cell to study the reaction mechanism and kinetics of DBBQ, and performed cyclic voltammetry (CV) studies to better understand how these vary with changing solvent and salt conditions. In the Li-O2 flow set-up the battery is positioned outside the nuclear magnetic resonance (NMR) magnet and plastic tubing then runs through the NMR and electron paramagnetic resonance (EPR) spectrometers. In this way, the reduction of the DBBQ can be monitored while the battery is cycling. In the proton NMR, the neutral DBBQ signals disappear on discharge as the singly reduced DBBQ semiquinone monoanion is a radical. The fast electron exchange between the neutral and radical species broaden out the NMR signals on discharge as the EPR signal grows. On charge, the neutral DBBQ signal reappears and the EPR signal disappears. The Li-O2 flow set-up is very sensitive to air and moisture exposure as the tubing is not perfectly impermeable to them. Therefore, having the CVs as a complementary method to study the reaction mechanisms and kinetics is very useful.We have developed a theoretical framework of Li-coupled electron transfer reactions that can predict the electrochemical kinetics in the presence of high and low donor number solvents and used to quantify thermodynamic and kinetic parameters. We define E0 app and k0 app as the apparent thermodynamic and kinetic parameters that describe the overall system and can be derived from the Nernst equation and Butler Volmer kinetics. These overall descriptors depend on the pathway the reaction takes (Figure 1). The different pathways in different solvents/salts are due to changes in the Li+ solvation strength as well as the competing ionic association of the counter anion.Experimentally, we have shown that in three different solvents, DME, tetraglyme and DMSO the behaviour of DBBQ can be explained by this theoretical framework. In DME, the DBBQ behaviour is most reminiscent of pathway 1 where the kinetics slow down with increasing Li+ concentration. In DMSO the kinetics present a maximum at Li+ concentrations around 1M. Simulations and fitting of the CV experiments in DMSO for the first reduction are in good agreement with pathways 2 and 3, while in tetraglyme it is harder to explain the trends as they do not fit into just one of the pathways.In conclusion we have studied the mechanism and kinetics of DBBQ as a RM for Li-O2 batteries with an in-situ flow set up and CVs and build a theoretical framework to understand how these vary in different electrolyte conditions. This method can be extended to study both the second reduction of DBBQ and other discharge and charge RMs for Li-O2 batteries. Figure 1

Oxidation and combustion of stabilized lithium metal powder (SLMP)

Power systems based on combustion are of interest for space missions where the use of solar and nuclear energy is impractical. A novel design of such systems involves the so-called filtration combustion of metal powders with oxygen supplied by a chemical oxygen generator. In missions to Mars and Venus, the atmospheric CO2 could be added to the oxygen flow. A stabilized lithium metal powder (SLMP) is a promising fuel for this application, but its oxidation and combustion in O2 and CO2 has not been studied yet. In the present work, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) have been used to investigate high-temperature oxidation of SLMP in O2/Ar and CO2 environments. Further, combustion of SLMP in vertical quartz tubes with two open ends at natural infiltration of O2 and CO2 has been studied in a laser ignition facility. The TGA has revealed the formation of lithium peroxide (Li2O2) in addition to lithium monoxide (Li2O) at temperatures below 400 °C. Scanning electron microscopy has shown that the oxidized particles are hollow shells, which implies that the oxidation process includes growth of a solid oxide layer on the surface of the lithium droplet and simultaneous growth of a cavity inside the droplet. The laser ignition of SLMP in O2 results in vigorous combustion of the top layer followed by the downward propagation of a counterflow combustion wave. The TGA and DSC have shown that the reaction of SLMP with CO2 is a multi-stage process, which includes the formation of lithium oxide and its subsequent conversion into lithium carbonate (Li2CO3). Self-sustained combustion of SLMP was not achieved in a CO2 environment, apparently because the formed products hinder the transport of CO2.

Critical Void Dimension of Carbon Frameworks to Accommodate Insoluble Products of Lithium-Oxygen Batteries.

High-energy density lithium-oxygen batteries (LOBs) seriously suffer from poor rate capability and cyclability due to the slow oxygen-related electrochemistry and uncontrollable formation of lithium peroxide (Li2O2) as an insoluble discharge product. In this work, we accommodated the discharge product in macro-scale voids of a carbon-framed architecture with meso-dimensional channels on the carbon frame and open holes connecting the neighboring voids. More importantly, we found that a specific dimension of the voids guaranteed high capacity and cycling durability of LOBs. The best LOB performances were achieved by employing the carbon-framed architecture having voids of 0.8 μm size as the cathode of the LOB when compared with the cathodes having voids of 0.3 and 1.4 μm size. The optimized void size of 0.8 μm allowed only a monolithic integrity of lithium peroxide deposit within a void during discharging. The deposit was grown to be a yarn ball-looking sphere exactly fitting the shape and size of the void. The good electric contact allowed the discharge product to be completely decomposed during charging. On the other hand, the void space was not fully utilized due to the mass transfer pathway blockage at the sub-optimized 0.3 μm and the formation of multiple deposit integrities within a void at the sur-optimized 1.4 μm. Consequently, the critical void dimension at 0.8 μm was superior to other dimensions in terms of the void space utilization efficiency and the lithium peroxide decomposition efficiency, disallowing empty space and side reactions during discharging.

(Invited) The Role of Electrolyte Solution in Next-Generation Batteries

Lithium-ion electrolytes are predominantly base on carbonate solvents and have been instrumental in the success of the lithium-ion battery.1 Uniquely, their ability to form a stable solid-electrolyte interphase (SEI) at lithiated carbon electrodes underpins the battery.2 Unfortunately, the lithium-ion battery is approaching maturity and attention is turning to alterative next-generation batteries that are able to deliver enhancements in energy, power, sustainability and cost. Often, the challenges in next-generation batteries are very different from those found in the lithium-ion battery.3 New understanding and advancements in electrolyte chemistry will be needed to realize these technologies.Here, we look at how electrolyte chemistry and impurities can control charge/discharge reactions in next-generation batteries. For example, the lithium-air battery has one of the highest specific energies of any known secondary battery chemistry, but faces significant challenges.4 The battery operates via oxidation of lithium metal at the anode, coupled to reduction of oxygen and formation of lithium peroxide at the cathode and these reactions are heavily influenced by the electrolyte chemistry.5 Moreover, to operate in air they must tolerate H2O, and here we describe how solvent chemistry is able to control the discharge route from a 2-electron reaction to an irreversible 4-electron reaction. This theory explains why cells are able to tolerate 1 M H2O,6, 7 and we show that with judicious selection of the electrolyte, up to 4 M H2O can be tolerated at the positive electrode of the battery. The impact of electrolyte chemistry on anode reactions will also be discussed. Recently, there has been significant research into magnesium anodes due to their high volumetric capacity (twice that of metallic lithium) and limited dendrite formation.8 Here we consider the nature of SEI films at magnesium in simple salt glyme ether electrolytes and how this impacts the cyclability of magnesium metal. Our results demonstrate the formation of a dynamic SEI at magnesium. Reference s G. E. Blomgren, J. Electrochem. Soc., 164, A5019–A5025 (2017).E. Peled, J. Electrochem. Soc., 126 (1979).D. Larcher and J. M. Tarascon, Nat. Chem., 7, 19–29 (2015)W. J. Kwak et al., ACS Appl. Mater. Interfaces (2020).L. Johnson et al., Nat. Chem., 6, 1091–1099 (2014)K. U. Schwenke, M. Metzger, T. Restle, M. Piana, and H. A. Gasteiger, J. Electrochem. Soc., 162, A573–A584 (2015).N. B. Aetukuri et al., Nat. Chem., 7, 50–56 (2015)R. Attias, M. Salama, B. Hirsch, Y. Goffer, and D. Aurbach, Joule, 3, 27–52 (2019)

The mechanism of Li2O2-film formation and reoxidation – Influence of electrode roughness and single crystal surface structure

In this work, the role of the electrode surface structure on oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in lithium ion containing dimethyl sulfoxide (DMSO) was elucidated for Pt single crystal electrodes and deliberately roughened polycrystalline gold and platinum electrodes. Both rotating ring disk electrode (RRDE) measurements and results obtained by differential electrochemical mass spectrometry (DEMS) show that the amount of insoluble Li 2 O 2 formed is proportional to the roughness factor and corresponds to one or two monolayers if referenced to the true surface area. The amount of soluble LiO 2 is independent of roughness, although the formation of the passivating Li 2 O 2 layer takes longer. On gold electrodes less side products are formed as compared to platinum electrodes. It was furthermore found that ORR on single crystalline Pt(100) leads to two monolayers of Li 2 O 2 , while in case of Pt(111)/Pt(pc) only one monolayer was reoxidized. The shape of the reoxidation peaks in the anodic potential scan is independent of roughness, but it depends not only on the electrode material (Au or Pt) but also on the atomic surface structure. Since Tafelslopes cannot be determined directly by usual methods for such surface confined reactions, we used ac voltammetry to determine the apparent transfer coefficient ( α′ ) and gain some insight into the reoxidation mechanism; the obtained α ′ of 0.3 hints to a rate determining first charge transfer. This agrees well with the detection of superoxide during oxidation of Li 2 O 2 on the surface. • Strong effect of electrode surface structure on electrochemistry in Li + -containing DMSO. • Superoxide as intermediate during lithium peroxide oxidation in DMSO. • Apparent transfer coefficient of lithium peroxide oxidation in Li + -containing DMSO. • Direct correlation of lithium peroxide formation and surface roughness.

Tuning lithium-peroxide formation and decomposition routes with single-atom catalysts for lithium\u2013oxygen batteries

Lithium-oxygen batteries with ultrahigh energy density have received considerable attention as of the future energy storage technologies. The development of effective electrocatalysts and a corresponding working mechanism during cycling are critically important for lithium-oxygen batteries. Here, a single cobalt atom electrocatalyst is synthesized for lithium-oxygen batteries by a polymer encapsulation strategy. The isolated moieties of single atom catalysts can effectively regulate the distribution of active sites to form micrometre-sized flower-like lithium peroxide and promote the decomposition of lithium peroxide by a one-electron pathway. The battery with single cobalt atoms can operate with high round-trip efficiency (86.2%) and long-term stability (218 days), which is superior to a commercial 5 wt% platinum/carbon catalyst. We reveal that the synergy between a single atom and the support endows the catalyst with excellent stability and durability. The promising results provide insights into the design of highly efficient catalysts for lithium-oxygen batteries and greatly expand the scope of future investigation.

Oxygen Reaction at Carbonaceous Materials with Different Structure in Electrolytes Based on Lithium Perchlorate and Aprotic Solvents

The necessity of the studying of carbonaceous materials differing in their surface area and structure is called for by the fact that these materials are used until now in the designing of positive electrodes for lithium-oxygen current sources. Under the model conditions, the effect of some factors on the effectiveness of oxygen reduction reaction at the positive electrode is studied. Among them are: properties of the dimethylsulfoxide- and acetonitrile-based electrolytes, the carbonaceous material (ХС 72, Super P, and carbon nanotubes) structure and its relevant transport processes depending on the electrode active layer mass (thickness) and the polarization current density, which determines the oxygen reaction effectiveness at the carbonaceous material. The electrochemically active surface area is shown to increase with the specific surface area, which is determined by the carbonaceous material porous structure, its mass at the electrode, the solvent properties, and the reaction rate. The active layer thickness and current density must be chosen for each carbonaceous material individually, depending upon its structure. At that, the active layer entire surface must be electrochemically accessible; it must make possible the lithium peroxide formation and subsequent decomposition. In the dimethylsulfoxide-based electrolyte (high donor number), the oxygen reduction reaction is highly reversible; the lithium peroxide formation here occurs via disproportionation in the solution bulk and results in the formation of Li2O2 particles with disordered (in all probability, toroidal) structure. This facilitates the back reaction (Li2O2 anodic decomposition), in good agreement with literature data [1]. In acetonitrile-based electrolyte (low donor number), the oxygen reduction reaction occurs in adsorbed state, producing LiО2 that disproportionates at the electrode surface forming a lithium peroxide insulating film whose oxidation needs high overvoltage. On the strength of all the parameters, carbon nanotubes are most effective in the oxygen reduction reaction in the dimethylsulfoxide-based electrolyte, because the carbon nanotubes have large volume of mesopores for the reactant transport, high electrochemically active surface area for the Li2O2 accumulation, and thus provide high characteristics per electrode.

Biological Redox Mediation in Electron Transport Chain of Bacteria for Oxygen Reduction Reaction Catalysts in Lithium–Oxygen Batteries

AbstractGoverning the fundamental reaction in lithium–oxygen batteries is vital to realizing their potentially high energy density. Here, novel oxygen reduction reaction (ORR) catalysts capable of mediating the lithium and oxygen reaction within a solution‐driven discharge, which promotes the solution‐phase formation of lithium peroxide (Li2O2), are reported, thus enhancing the discharge capacity. The new catalysts are derived from mimicking the biological redox mediation in the electron transport chain in Escherichia coli, where vitamin K2 mediates the oxidation of flavin mononucleotide and the reduction of cytochrome b in the cell membrane. The redox potential of vitamin K2 is demonstrated to coincide with the suitable ORR potential range of lithium–oxygen batteries in aprotic solvent, thereby enabling its successful functioning as a redox mediator (RM) triggering the solution‐based discharge. The use of vitamin K2 prevents the growth of film‐like Li2O2 even in an ether‐based electrolyte, which has been reported to induce surface‐driven discharge and early passivation of the electrode, thus boosting the discharge capacity by ≈30 times. The similarity of the redox mediation in the biological cell and lithium–oxygen “cell” inspires the exploration of redox active bio‐organic compounds for potential high‐performance RMs toward achieving high specific energies for lithium–oxygen batteries.

The Effect of the Structure of a Positive Electrode on the Process of Discharge of a Lithium–Oxygen Power Source. The Monoporous Cathode Theory

The results of computerized simulation of the process of formation of lithium peroxide attending the discharge of lithium–oxygen power source, in individual pore of constant radius are presented. It is found that, in the model of porous cathode (pores are tortuous, noncrossing, and of the same radius), variation of specific surface of the pores (decrease of pore radius) does not enable a possibility to increase notably the value of specific capacity of the cathode. A necessity of presence of both macropores, and micro- and mesopores in the structure of the active material was discussed. The effect of porous structure of the cathode on the discharge characteristics of LOPS was experimentally demonstrated by the example of some cathode materials (carbon blacks and carbon nanotubes). The highest discharge capacity was achieved with use of the sample of CNT-TNaOH combining pores of various sizes, which corresponds to the formulated hypotheses about an optimal structure of the active cathode material.

Biological Redox Mediation for Oxygen Reduction Reaction Catalysts in Lithium–Oxygen Batteries

Governing the fundamental reaction of oxygen and lithium in lithium–oxygen batteries is vital to realizing their potentially high energy density. Here, we report novel oxygen reduction reaction (ORR) catalysts that are capable of mediating the lithium and oxygen reaction within a solution-driven discharge, which promotes the solution-phase formation of lithium peroxide (Li2O2), thus enhancing the discharge capacity. The new ORR catalysts are derived from mimicking the biological redox mediation occurring in the respiration system of a bacteria, where biomolecule mediates the oxidation of reduced flavin mononucleotide and the reduction of cytochrome b in the cell membrane. The redox potential of the molecule in the biological environment is demonstrated to coincide with the suitable ORR potential range of lithium–oxygen batteries in aprotic solvent, thereby enabling its successful functioning as a redox mediator (RM) that triggers the solution-based discharge. The use of the catalyst prevents the growth of film-like Li2O2 even in an ether-based electrolyte, which has been reported to induce surface-driven discharge and early passivation of the electrode, thus boosting the discharge capacity by ~30 times. The similarity of the redox mediation in the biological cell and lithium–oxygen ‘cell’ inspires the exploration of various redox active bio-organic compounds for potential high-performance RMs toward achieving high theoretical specific energies for lithium–oxygen batteries.

Highly Durable and Stable Sodium Superoxide in Concentrated Electrolytes for Sodium–Oxygen Batteries

A great interest has emerged on metal–oxygen batteries with a hope for ultra-high-energy-density batteries [1]; of these, the intrinsic high energy efficiency and reversibility make sodium–oxygen batteries as one of the most promising post-lithium-ion batteries. In contrast to the lithium peroxide formation in lithium–oxygen batteries, the formation of sodium superoxide discharge product brings out unique electrochemical properties of sodium–oxygen batteries. Intense studies have revealed that the high capacity and low polarization of sodium–oxygen batteries are attributable to the solution phase formation and decomposition of sodium superoxide, which has substantial stability and solubility in ethereal electrolytes.[2-4] However, recent studies have raised out the issue on chemical instability that sodium superoxide undergoes parasitic reactions with the ethereal electrolytes in sodium–oxygen batteries.[4-8] We have previously demonstrated the mechanism that the dissolution of sodium superoxide into the electrolytes liberates reactive superoxide anion and triggers the parasitic reactions.[4] Other products formed as a result of parasitic reactions require higher polarization to be recharged and do not involve oxygen evolution during the charge process, which seriously deteriorates cell efficiency and reversibility during storage period of batteries. These phenomena should be closely linked with the storage properties or shelf-life of batteries. The detrimental effects of storage after discharge in sodium–oxygen batteries may be amplified when it comes to be applied for practical batteries in electric vehicles (EVs) since EVs sometimes cannot be immediately recharged depending on the consumers’ utilization patterns. Therefore, proper strategies to accomplishment long living sodium superoxide have to be addressed for maintaining high efficiency and reversibility of sodium–oxygen batteries. A rational tuning of the electrolyte possesses the key to control the chemical stability of discharge products and improve storage properties of the sodium–oxygen batteries. In this study, we explore a concentrated electrolyte to prevent the dissolution and parasitic reactions of sodium superoxide in sodium–oxygen batteries. Through a couple of solution characterization techniques, unique solvation structures of concentrated electrolytes are revealed with the elimination of free solvents. Time-resolved ex situ characterizations confirm that sodium superoxide stored in the concentrated electrolyte exhibits prolonged lifetime compared to that in normal electrolytes. We finally demonstrate that highly durable sodium superoxide in the concentrated electrolyte succeeds to preserve both high energy efficiency and oxygen reversibility of sodium–oxygen batteries despite the insertion of storage period after discharge. This is the first study to successfully improve the chemical stability and lifetime of sodium superoxide by simple tuning of electrolyte concentrations in sodium–oxygen batteries. This finding is complementary to the previous understanding on the solution chemistry of sodium–oxygen batteries, highlighting the importance for understanding the role of electrolytes on the sodium–oxygen chemistry.

Gaining Control over the Mechanism of Oxygen Reduction in Organic Electrolytes: The Effect of Solvent Properties

In the present study, we employ differential electrochemical mass spectroscopy (DEMS) to investigate how additives to lithium containing organic electrolytes influence the mechanism of electrochemical oxygen reduction reaction. We find that some additives favor the formation of superoxide over the formation of lithium peroxide. We have tried to relate the influence of the additive on superoxide formation to properties of the pure solvent: Additives with a large donor number do not exert any influence. Although all additives with a large acceptor number facilitate superoxide formation, we have ascribed the effect to the polarity of the additive. We discuss in the present paper how the polarity of the additive increases solvent rearrangement energies. Following the Marcus–Hush theory this introduces an activation barrier for the second electron transfer. Therefore, electrochemical oxygen reduction stops at the superoxide stage, and larger overpotentials are required to facilitate the second electron transfe...

A New Architecture Design for Observing Lithium Oxide Growth-Evolution Employing Geometric Catalyst Positioning

A common route to overcoming the large overpotentials and poor reversibility in current Li-O2 batteries has been to develop various solid-state catalysts (i.e. noble metals, metal oxides) decorated on the oxygen electrodes.[1-2] Although these approaches have yielded incremental improvements, catalytic sites are easily deactivated by the continuous precipitation of solid products.[3] Understanding the mechanisms of discharge product formation and evolution and optimizing the catalyst systems are necessary to guide future Li-O2 cell design.[4] In our previous report,[5] we demonstrated that dispersing catalyst particles in an insulating membrane over the oxygen electrode helps to maintain their function by reducing passivation of the active surfaces during lithium peroxide formation. The major benefit of this approach is that the oxygen evolution reaction (lithium peroxide dissolution) is facilitated by these isolated catalytic sites. While the catalytic membrane architecture is quite effective in practical cells, it is important to investigate the exact chemistry and morphology of products far from the electrode surface and thereby elucidate the remarkable function of these electrically isolated sites. In this work we demonstrate a new architecture using anodic aluminum oxide (AAO) membranes as a porous scaffold with Pd nanoparticles dispersed throughout as the catalytic sites. This approach allows us to directly observe lithium-oxide product formation and evolution both in the presence of catalysts and without to better understand the mechanistic influence of catalyst positioning. We show that AAO membranes can be readily cross-sectioned (without destroying the well-defined pore structure), and can incorporate the formation of integrated lithium-oxide products. As a result, we clearly observe a planar-to-particulate morphological transition in Li-O2 products during cell discharge and the reverse during charge. The Pd-AAO interlayer facilitates the uniform growth of discharge products as well as the evolution reaction, minimizing the buildup of residues seen in the absence of Pd. In this talk, we will provide evidence for improved charge transfer and reduced overpotentials as a result of catalyst positioning inside the AAO membrane pores. The novelties of this work demonstrate how the catalyst positioning is important and how insolated catalyst sites can function separately from electrical contact to the electrode for improved columbic round trip efficiency for next generation Li-O2 cell designs. [1] Ryu, W. H.; Yoon, T. H.; Song, S. H.; Jeon, S.; Park, Y. J.; Kim, I. D. Nano Lett 2013,13, 4190-7. [2] Gittleson, F. S.; Ryu, W. H.; Schwab, M.; Xiao Tong, Andre D. Taylor, Chem Commun 2016, In press. DOI: 10.1039/C6CC01778A [3] Gittleson, F. S.; Ryu, W. H.; Taylor, A. D. ACS Appl Mater Interfaces 2014,6, 19017-19025. [4] Gittleson, F. S.; Yao, K. P. C.; Kwabi, D. G.; Sayed; Ryu, W.-H.; Shao-Horn, Y.; Taylor, A. D., ChemElectroChem 2015, 2, 1446-1457. [5] Ryu, W. H.; Gittleson, F. S.; Schwab, M.; Goh, T.; Taylor, A. D., Nano Lett 2015, 15, 434-441

Lattice Boltzmann Simulation of Lithium Peroxide Formation in Lithium–Oxygen Battery

Present research deals with multiphysics, pore-scale simulation of Li–O2 battery using multirelaxation time lattice Boltzmann method. A novel technique is utilized to generate an idealized electrode–electrolyte porous media from the known macroscopic variables. Present investigation focuses on the performance degradation of Li–O2 cell due to the blockage of the reaction sites via Li2O2 formation. Present simulations indicate that Li–air and Li–O2 batteries primarily suffer from mass transfer limitations. The study also emphasizes the importance of pore-scale simulations and shows that the morphology of the porous media has a significant impact on the cell performance. While lower porosity provides higher initial current, higher porosity maintains sustainable output.

Universality in Nonaqueous Alkali Oxygen Reduction on Metal Surfaces: Implications for Li–O2 and Na–O2 Batteries

Nonaqueous metal–oxygen batteries, particularly lithium–oxygen and sodium–oxygen, have emerged as possible high energy density alternatives to Li-ion batteries that could address the limited driving range issues faced by electric vehicles. Many fundamental questions remain unanswered, including the origin of the differences in the discharge product formed, i.e., Li2O2 versus Li2O in Li–O2 batteries and NaO2 versus Na2O2 in Na–O2 batteries. In this Letter, we analyze the role of the electrode (electrocatalyst) in determining the selectivity of the discharge product through a tuning of the nucleation overpotential for a given electrolyte. On the basis of a thermodynamic analysis using density functional theory calculations, we demonstrate that the free energy of adsorbed LiO2* is a descriptor determining the nucleation overpotential for the formation of lithium peroxide, Li2O2, the primary discharge product in Li–O2 batteries. Our analysis suggests that Au(100), Ag(111), and Au(111) are capable of nucleatin...

Influence of Electrolyte Solvent Properties on Li-O2 Battery Discharge Product Morphology

Li-air batteries (LAB) can theoretically enable energy storage with 5-fold increased specific energy in comparison to that of Li-ion systems. A number of fundamental problems prevent LAB from being reversibly rechargeable however. Precipitation of the discharge products, e.g. lithium peroxide (Li2O2), inside a positive electrode is one of the key processes that control battery performance [1], thus detailed understanding of product deposition mechanisms is required to achieve deeper discharge and complete reversibility. Product formation is a result of multistep chemical process ending up with crystallization. Depending on the discharge conditions and, especially, electrolyte solvent, different growth mechanisms occur giving rise to quite different morphologies. Recently, it has been reported that solvent’s donor number play a key role in lithium peroxide formation processes [2]. Here we test the proposed theory in a wider range of solvents, including DMA and sulfolane. Carbon paper was used as an electrode material with controlled porosity. Solvent list included DMSO, MeCN, DMA, DME and sulfolane and was demonstrated to be relatively stable to ORR products and intermediates. Composition, structure and morphology of the discharge products formed during potentiostatic discharge were analysed. It was shown that oxygen reduction in the presence of lithium ions in the electrolyte proceeds with the formation of plate-like particles Li2O2 that form a film or agglomerate complex morphology. The formation of Li2O2 crystalline plates proceeds via superoxide disproportionation in solution [3]. [1] Christensen J, Albertus P, Sanchez-Carrera RS et al. A Critical Review of Li-Air Batteries// J. Electrochem. Soc. 2012, 159:R1. [2] Johnson L, Li C, Liu Z et al. The role of LiO2 solubility in O2 reduction in aprotic solvents and its consequences for Li–O2batteries// Nature Chem. 2014, 6:1091–1099. [3] T.K. Zakharchenko, A. Y. Kozmenkova, D.M. Itkis et al. Lithium peroxide crystal clusters as a natural growth feature of discharge product in Li-O2 cell// Beilstein J. Nanotechnol., 2013:4.

Density Functional Theory (DFT) Study for Role of Ion-Conducting Lithium Salts Regarding the Oxygen Reduction Reaction (ORR) Kinetics in Li-air (O2) Batteries

Density functional theory calculations are performed to investigate the role of lithium salts regarding the oxygen reduction reaction (ORR) occurred in non-aqueous electrolyte in Li-air batteries. This study is useful to elucidate the influence of ion-conducting lithium salts on ORR kinetics. The Li+-O bonding, binding energy, Gibb's free energy, heat of formation energy, and IR spectra with increase in dimethyl sulfoxide (DMSO) solvent molecules in the primary solvation shell reveal that lithium trifluoromethanesulfonate (LiCF3SO3) is the most stable in dimethyl sulfoxide (DMSO) as compared to lithium hexafluorophosphate (LiPF6) and lithium bis(trifluoromethanesulfonimide (LiN(CF3SO2)2). Infrared spectroscopic (IR) data confirm the formation of lithium peroxide (Li2O2), lithium oxide (Li2O), and lithium carbonate (Li2CO3) as discharge products on the micro-electrode surface in non-aqueous electrolyte containing lithium salts. The infrared (IR) analysis reveals that the lithium trifluoromethanesulfonate/dimethyl sulfoxide (LiCF3SO3/DMSO) electrolyte is primarily assumed to be the most favorable for ORR kinetics. In order to study the kinetics of oxygen reduction reaction at the oxygen cathode of non-aqueous Li-air battery with three Li salts in dimethyl sulfoxide (DMSO), we calculate thermodynamic properties of the reactions held during the formation of lithium peroxide (Li2O2) and lithium oxide (Li2O). The Gibb's energy and heat of formation of these reactions indicate that lithium trifluoromethanesulfonate/dimethyl sulfoxide (LiCF3SO3/DMSO) electrolyte plays a key role in oxygen reduction reaction mechanism compared to other two.

Operando observation of the gold-electrolyte interface in Li-O2 batteries.

Observing the cathode interface in Li-O2 batteries during cycling is necessary to improve our understanding of discharge product formation and evolution in practical cells. In this work a gold electrode surface is monitored by operando surface-enhanced Raman spectroscopy during typical discharge and charge cycling. During discharge, we observe the precipitation of stable and reversible lithium superoxide (LiO2), in contrast to reports that suggest it is a mere intermediate in the formation of lithium peroxide (Li(2)O2). Some LiO2 is further reduced to Li(2)O2 producing a coating of insulating discharge products that renders the gold electrode inactive. Upon charging, a superficial layer of these species (∼ 1 nm) are preferentially oxidized at low overpotentials (<0.6 V), leaving residual products in poor contact with the electrode surface. In situ electrochemical impedance spectroscopy is also used to distinguish between LiO2 and Li(2)O2 products using frequency-dependent responses and to correlate their reduction and oxidation potentials to the accepted mechanism of Li(2)O2 formation. These operando and in situ studies of the oxygen electrode interface, coupled with ex situ characterization, illustrate that the composition of discharge products and their proximity to the catalytic surface are important factors in the reversibility of Li-O2 cells.

Mediation of the Oxygen Reduction in Non-Aqueous Lithium-Air Batteries

In recent years, non-aqueous lithium-air batteries have been intensively investigated due to their high theoretical gravimetric energy, which is up to 10 times higher than lithium-ion batteries [1-4]. The discharge reaction involves the oxygen reduction reaction (ORR), forming either superoxide or peroxide. Superoxide formation is highly detrimental, because not only would the charge delivered by the battery be halved, but most importantly, superoxide induces the degradation of most known types of organic electrolytes [5-8] and carbon-containing electrodes [9-13]. Therefore, new catalysts enhancing the 2-electron reduction of oxygen in non-aqueous electrolytes are essential for the development of lithium-air batteries. Another issue in lithium-air batteries is that the reaction product, lithium peroxide, is insoluble and insulating. As a result, lithium peroxide deposits on the electrode surface and forms a passivation layer [14], which leads to capacity fading.We have recently introduced the concept of redox shuttles as applied to solve the problem of electrode passivation in lithium-air batteries [15]. As illustrated in the figure on the example of ethyl viologen (EtV2+/EtV+), a redox shuttle will displace the oxygen reduction reaction a short distance from the electrode surface, thus avoiding passivation.This work continues by investigating another advantage, which is the homogeneous catalysis of the oxygen reduction reaction. By combining electrochemical and UV-visible measurements, we have shown that ethyl viologen can act as a mediator for the 2-electron reduction of oxygen. This has the remarkable advantage of decreasing the lifetime of the highly reactive superoxide, which would be beneficial for the stability of electrolyte and electrode materials against irreversible degradation. Under appropriate reaction conditions, it was shown that the extent of degradation reactions undergone by ethyl viologen is <2 %. In conclusion, ethyl viologen improves the selectivity of the oxygen reduction reaction towards the formation of lithium peroxide instead of the formation of degradation products or superoxide radical anions.

Surface Enhanced Raman Microscopy in the Study of Li-Ion and Li-Air Battery Interfaces

Surface layers on lithium battery electrodes play a vital role in affecting battery performance, ageing mechanics and safety. Particularly, the solid electrolyte interphase (SEI), which is a protecting layer formed on the negative electrode of lithium-ion batteries as a result of electrolyte decomposition, significantly impacts upon irreversible charge “loss”, rate capability, cyclability, exfoliation of graphite and safety.The investigation of surface layer formation is also important in the understanding of the operation of the lithium-oxygen battery. The non-aqueous lithium-oxygen cell is one of a host of emerging opportunities available for enhanced energy storage [1]. Unlike a conventional battery where the reagents are contained within the cell, the lithium-oxygen cell uses dioxygen from the atmosphere to electrochemically form the discharge product lithium peroxide. Degrees of reversible oxidation and formation of lithium peroxide has been demonstrated in a number of non-aqueous electrolyte classes, mostly notably in dimethysulfoxide based electrolytes [2], thus making the lithium-oxygen cell a potential energy storage device.The design and optimisation of techniques based on both surface-enhanced Raman spectroscopy (SERS) and shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) for ultra-sensitive analysis will be presented in the study the formation and composition of interfaces on lithium battery electrode materials. SHINERS, first reported by Prof. Tian and his co-workers [3] provides, as does SERS, an ultra-sensitive technique for understanding the formation and composition of interphases/ passivation layers on electrode materials and is considered to be a powerful technique for in situ surface analysis. The highest enhancement of Raman scattering using SHINERS appears at the junction between the particles and substrate as hot spots with magnitudes of up to 4x108 fold. Raman signals can be greatly enhanced by surface plasmon resonance generated on the surface of gold or silver nanoparticles. In SHINERS, the gold nanoparticles are coated with ultrathin silica or alumina layer. The inert shell is expected to be thin, uniform, fully enclosed and optically transparent so that it can retain the surface enhancement and ensure that there is no direct contact between the probed substance and the gold core. The shell-isolated nanoparticles are then spread onto the electrode surface and assembled into an in situ Raman electrochemical cell.The poster will present our groups recent results of the spectro-electrochemistry studies of dioxygen in non-aqueous electrolytes, as well the use of SHINERS to understand model electrode interfaces within Li-ion battery electrolytes.