Li2O2 Particles Research Articles

Cathode Chemistries of Lithium-Oxygen Batteries in Nanoconfined Space.

In lithium-oxygen batteries, although the porous carbon cathodes are widely utilized to tailor the properties of discharged Li2O2, the impact of nanopore size on the Li2O2 formation and decomposition reactions remain incompletely understood. Here, we provide the straightforward elucidation on the effect of pore size in a range of 25-200 nm, using a highly ordered porous cathode matrix based on the carbon-coated anodic aluminum oxide membrane formed on an Al substrate (C/AAO_Al). When the nanopore size is 25 nm, film-like Li2O2 with a thickness of 2-5 nm is formed, possibly via a surface-driven mechanism. When the nanochannel becomes larger, the Li2O2 film thickness saturates at ca. 10 nm, along with crystalline Li2O2 particles possibly formed by a solution-mediated mechanism.

RuO2-Incorporated Co3O4 Nanoneedles Grown on Carbon Cloth as Binder-Free Integrated Cathodes for Tuning Favorable Li2O2 Formation.

Developing ideal Li-O2 batteries (LOBs) requires the discharge product to have a large quantity, have large contact area with the cathode, and not passivate the porous surface after discharge, which put forward high requirement for the design of cathodes. Herein, combining the rational structural design and high activity catalyst selection, minor amounts of RuO2-incorporated Co3O4 nanoneedles grown on carbon cloth are successfully synthesized as binder-free integrated cathodes for LOBs. With this unique design, plenty of electron-ion-oxygen tri-phase reaction interface is created, the side reaction from carbon is isolated, and oxygen reduction reaction/oxygen evolution reaction (OER) kinetics are significantly facilitated. Upon discharge, film-like Li2O2 is observed growing on the needle surface first and eventually ball-like Li2O2 particles form at each tip of the needle. The cathode surface remains porous after discharge, which is beneficial to the OER and is rare in the previous reports. The battery exhibits a high specific discharge capacity (7.64 mAh cm-2) and a long lifespan (500 h at 0.1 mA cm-2). Even with a high current of 0.3 mA cm-2, the battery achieves a cycling life of 200 h. In addition, punch-type LOBs are fabricated and successfully operated, suggesting that the cathode material can be utilized in ultralight, flexible electronic devices.

Disentangling plasmonic and catalytic effects in a practical plasmon-enhanced Lithium–Oxygen battery

Despite possessing high theoretical energy density, rechargeable Li–O2 batteries face critical drawbacks towards commercialization. In line with recent attempts to integrate solar energy exploitation in high-energy storage, here we investigate the promise of plasmonic materials with unique light-interacting properties (localized surface plasmon resonance, LSPR) and emerging application in catalysis. Au nanoparticles (NPs) at increasing contents/sizes are incorporated on conventional Ketjen Black cathodes, with preliminary half-cell measurements underlining the promise of LSPR-generated hot-carriers on the O2 electrochemistry. The illuminated battery with facile Li2O2 formation/decomposition, small Li2O2 particles, and suppressed carboxylate side-products unlocks a round-trip efficiency boost from 75.2 to 80.2% (first cycle) and a ∼1.2-fold full capacity enhancement. Even more remarkably, with continuous cycling (30 cycles), a 680 mV-overpotential suppression is here reported. Comparatively, dark conditions reveal negligible Au-driven catalytic effects, whereas LSPR-induced local heat effects are ruled out upon meticulous assessment of the product selectivity in cells at increasing temperatures. These outstanding efficiencies are ensured even with larger particles (5–100 nm), as corroborated by corresponding galvanostatic profiles and finite-difference time-domain simulations, pinpointing the practicality of our cathodes towards scale-up. This contribution is the first to disentangle catalytic effects and plasmon relaxation pathways over practical carbon-based cathodes for high-energy storage.

Direct Visualization of Dynamic Mobility of Li2O2 in Li-O2 Batteries: A Differential Interference Microscopy Study.

The reversibility and the discharge/charge performance in nonaqueous lithium-oxygen (Li-O2) batteries are critically dependent on the kinetics of interfacial reactions. However, the interfacial reaction dynamic behaviors, especially the quantitative analysis, are still far from deep understanding. Using the method of laser confocal microscopy combined with differential interference contrast microscopy (LCM-DIM), we monitored the Li-O2 interfacial reaction and in situ traced the Li2O2 migration processes promoted by the solution catalyst. Different dynamic behaviors exist when regulating the concentration of the redox mediator. Quantitative analysis of the discharged Li2O2 particles shows high mobility at the early discharge stage and decayed motion in the subsequent process, indicating the solution-mediated pathway participating Li2O2 formation in the low-concentration redox mediator addition, while particles/aggregates confined into the amorphous film demonstrate simultaneous solution and surface route-mediated pathway participation in the high-concentration case. These distinctive observations of Li2O2 formation and decomposition processes present the advantage of LCM-DIM to fundamentally understand the dynamic evolution in Li-O2 batteries.

MIL-53 Metal-Organic Framework as a Flexible Cathode for Lithium-Oxygen Batteries.

Li-air batteries possess higher specific energies than the current Li-ion batteries. Major drawbacks of the air cathode include the sluggish kinetics of the oxygen reduction (OER), high overpotentials and pore clogging during discharge processes. Metal–Organic Frameworks (MOFs) appear as promising materials because of their high surface areas, tailorable pore sizes and catalytic centers. In this work, we propose to use, for the first time, aluminum terephthalate (well known as MIL-53) as a flexible air cathode for Li-O2 batteries. This compound was synthetized through hydrothermal and microwave-assisted routes, leading to different particle sizes with different aspect ratios. The electrochemical properties of both materials seem to be equivalent. Several behaviors are observed depending on the initial value of the first discharge capacity. When the first discharge capacity is higher, no OER occurs, leading to a fast decrease in the capacity during cycling. The nature and the morphology of the discharge products are investigated using ex situ analysis (XRD, SEM and XPS). For both MIL-53 materials, lithium peroxide Li2O2 is found as the main discharge product. A morphological evolution of the Li2O2 particles occurs upon cycling (stacked thin plates, toroids or pseudo-spheres).

Effects of Oxygen Cathode Substrates with Various Carbon Catalysts on the Discharge Capacity of Lithium-Oxygen Battery

Lithium-oxygen batteries have been considered strong candidates to supersede current rechargeable batteries especially lithium-ion batteries. Despite having tremendously high theoretical energy density, the practically achievable energy density is far from commercial application till date. The oxygen electrode is one of the major bottlenecks in maximizing the battery capacity. It is comprised of a porous structure (mainly carbon) with high surface area and sufficient reaction sites. High porosity facilitates lithium ion and oxygen mass transfer and results in high storage capacity for the insoluble discharge product during discharging. The cathode electrode is fabricated by coating a catalyst layer on a porous carbon substrate. The substrate, which contacts with oxygen channels, plays a vital role in mass transfer of oxygen from outside to the reaction sites. Therefore, it is important to select a substrate that does not clog with deposition of Li2O2 particles immediately after discharging begins and allows oxygen to reach the catalyst side for longer period of time.This study investigates the effects of oxygen electrode substrates with various coating materials on the discharge capacity of lithium-oxygen battery. For this purpose, we have compared the discharge capacity of plain carbon cloth, plain carbon paper and carbon cloth or carbon paper coated with carbon catalysts including Vulcan XC, Super P and Graphene Oxide. 1.5mg/cm2 (+/-0.3mg/ cm2) is used as catalyst loading. The electrolyte used in this study is 1M LiTFSI in TEGDME. PTFE (30wt%) is used as a binder. All experiments are conducted at room temperature with pure oxygen supply.With carbon cloth as a substrate, the discharge capacities obtained were 3 to 6 times higher (depending on the carbon catalysts coated) than the capacities obtained with carbon paper. The bare carbon cloth, Vulcan XC, Super P and Graphene oxide with carbon cloth as a substrate have discharge capacities of 0.11 mAh/mg, 13 mAh/mg, 6.8 mAh/mg, 1.5 mAh/mg respectively. On the other hand, the discharge capacities achieved with bare carbon paper, Vulcan XC, Super P and graphene oxide with carbon paper as substrate were 0.02 mAh/mg, 2.4 mAh/mg, 2.7 mAh/mg, 0.28 mAh/mg respectively. The results clearly indicate that the cathode with carbon cloth as a substrate yields significantly higher discharge capacity with all the coating materials as compared with the carbon paper. The different performances of two substrates can be attributed to their structure and pore size distribution. Carbon cloth is composed of flexible woven threads with longer lengths ( ̴ 30µm) and broader pore size distribution (5-200 µm) with peaks at 10, 70 and 100 µm. This type of structure is favorable in reducing tortuosity, enhancing mass transport of oxygen and lithium ion and improving ionic conductivity. On the other hand, the structure of carbon paper comprises of non-woven short carbon fibers ( ̴ 15µm) and smaller pore size distribution range (10-70 µm) with a peak at 50 µm. This renders poor transport of oxygen from outside to the reaction sites leading to lower discharge capacity.The primary function of coating materials is to increase surface area of cathode thereby providing more reaction sites and creating tri-phase boundaries for reaction to take place. These catalysts also have different pore size distributions and resistance to mass transfer. Therefore, various carbon catalysts used yield different discharge capacity with same substrate.

First-Principles Study of the Surfaces and Equilibrium Shape of Discharge Products in Li-Air Batteries.

Li-air batteries are a promising alternative to Li-ion batteries as they theoretically provide the highest possible specific energy density. Mainly, Li2O2 (lithium peroxide) and to a lesser extent, Li2O (lithium oxide) are assumed to be the discharge products of these batteries formed with the soluble LiO2 (lithium superoxide) considered to be an intermediate product. Bulk Li2O2 is an electronic insulator, and the precipitation of this compound on the cathode is thought to be the main limiting factor in achieving high capacities in lithium-oxygen cells. For the most promising electrolytes including solvents with high donor numbers, microscopy observations frequently reveal crystallite morphologies of Li2O2 compounds, rather than uniform layers covering the electrode surface. The precise morphologies of Li2O and Li2O2 particles, and their effect and their extent of contact with the electrode, which may all affect the capacity and rechargeability, however, remain largely undetermined. Here, we address the stability of various Li2O and Li2O2 surfaces and consequently, their crystallite morphologies using density functional theory calculations and ab initio thermodynamics. In contrast to previous studies, we also consider high-index surface terminations, which exhibit surprisingly low surface energies. We carefully analyze the reasons for the stability of these high-index surfaces, which also prominently influence the equilibrium shape of the particles, at least for Li2O2, and discuss the consequences for the observed morphology of the reaction products.

Isotopic Depth Profiling of Discharge Products Identifies Reactive Interfaces in an Aprotic Li-O2 Battery with a Redox Mediator.

Prior to the practical application of rechargeable aprotic Li-O2 batteries, the high charging overpotentials of these devices (which inevitably cause irreversible parasitic reactions) must be addressed. The use of redox mediators (RMs) that oxidatively decompose the discharge product, Li2O2, is one promising solution to this problem. However, the mitigating effect of RMs is currently insufficient, and so it would be beneficial to clarify the Li2O2 reductive growth and oxidative decomposition mechanisms. In the present work, Nanoscale secondary ion mass spectrometry (Nano-SIMS) isotopic three-dimensional imaging and differential electrochemical mass spectrometry (DEMS) analyses of individual Li2O2 particles established that both growth and decomposition proceeded at the Li2O2/electrolyte interface in a system containing the Br-/Br3- redox couple as the RM. The results of this study also indicated that the degree of oxidative decomposition of Li2O2 was highly dependent on the cell voltage. These data show that increasing the RM reaction rate at the Li2O2/electrolyte interface is critical to improve the cycle life of Li-O2 batteries.

In situ small-angle X-ray scattering reveals solution phase discharge of Li–O2 batteries with weakly solvating electrolytes

Electrodepositing insulating lithium peroxide (Li2O2) is the key process during discharge of aprotic Li-O2 batteries and determines rate, capacity, and reversibility. Current understanding states that the partition between surface adsorbed and dissolved lithium superoxide governs whether Li2O2 grows as a conformal surface film or larger particles, leading to low or high capacities, respectively. However, better understanding governing factors for Li2O2 packing density and capacity requires structural sensitive in situ metrologies. Here, we establish in situ small- and wide-angle X-ray scattering (SAXS/WAXS) as a suitable method to record the Li2O2 phase evolution with atomic to submicrometer resolution during cycling a custom-built in situ Li-O2 cell. Combined with sophisticated data analysis, SAXS allows retrieving rich quantitative structural information from complex multiphase systems. Surprisingly, we find that features are absent that would point at a Li2O2 surface film formed via two consecutive electron transfers, even in poorly solvating electrolytes thought to be prototypical for surface growth. All scattering data can be modeled by stacks of thin Li2O2 platelets potentially forming large toroidal particles. Li2O2 solution growth is further justified by rotating ring-disk electrode measurements and electron microscopy. Higher discharge overpotentials lead to smaller Li2O2 particles, but there is no transition to an electronically passivating, conformal Li2O2 coating. Hence, mass transport of reactive species rather than electronic transport through a Li2O2 film limits the discharge capacity. Provided that species mobilities and carbon surface areas are high, this allows for high discharge capacities even in weakly solvating electrolytes. The currently accepted Li-O2 reaction mechanism ought to be reconsidered.

High‐Capacity and Stable Li‐O2 Batteries Enabled by a Trifunctional Soluble Redox Mediator

AbstractLi‐O2 batteries with ultrahigh theoretical energy densities usually suffer from low practical discharge capacities and inferior cycling stability owing to the cathode passivation caused by insulating discharge products and by‐products. Here, a trifunctional ether‐based redox mediator, 2,5‐di‐tert‐butyl‐1,4‐dimethoxybenzene (DBDMB), is introduced into the electrolyte to capture reactive O2− and alleviate the rigorous oxidative environment of Li‐O2 batteries. Thanks to the strong solvation effect of DBDMB towards Li+ and O2−, it not only reduces the formation of by‐products (a high Li2O2 yield of 96.6 %), but also promotes the solution growth of large‐sized Li2O2 particles, avoiding the passivation of cathode as well as enabling a large discharge capacity. Moreover, DBDMB makes the oxidization of Li2O2 and the decomposition of main by‐products (Li2CO3 and LiOH) proceed in a highly effective manner, prolonging the stability of Li‐O2 batteries (243 cycles at 1000 mAh g−1 and 1000 mA g−1).

High-Capacity and Stable Li-O2 Batteries Enabled by a Trifunctional Soluble Redox Mediator.

Li-O2 batteries with ultrahigh theoretical energy densities usually suffer from low practical discharge capacities and inferior cycling stability owing to the cathode passivation caused by insulating discharge products and by-products. Here, a trifunctional ether-based redox mediator, 2,5-di-tert-butyl-1,4-dimethoxybenzene (DBDMB), is introduced into the electrolyte to capture reactive O2 - and alleviate the rigorous oxidative environment of Li-O2 batteries. Thanks to the strong solvation effect of DBDMB towards Li+ and O2 - , it not only reduces the formation of by-products (a high Li2 O2 yield of 96.6 %), but also promotes the solution growth of large-sized Li2 O2 particles, avoiding the passivation of cathode as well as enabling a large discharge capacity. Moreover, DBDMB makes the oxidization of Li2 O2 and the decomposition of main by-products (Li2 CO3 and LiOH) proceed in a highly effective manner, prolonging the stability of Li-O2 batteries (243 cycles at 1000 mAh g-1 and 1000 mA g-1 ).

Boosting the electrochemical performance of Li–O2 batteries with DPPH redox mediator and graphene-luteolin-protected lithium anode

Aprotic Li-O2 batteries with high theoretical energy density are regarded as promising next-generation energy storage devices. However, the accumulation of discharge products (Li2O2) during the discharge/charge processes would lead to high overpotential, low round-trip efficiency and poor cycling stability of the batteries. Soluble redox mediators (RMs) have been proved to be efficient in promoting the oxidation of Li2O2 particles. However, the self-discharge of the electrochemically oxidized RMs (RMsox) on the surface of Li metal would accelerate the formation of lithium dendrites. In this work, nitrogenous radicals, 1,1-Diphenyl-2-picrylhydrazyl radical 2,2-Diphenyl-1-(2,4,6-trinitrophenyl) hydrazyl (DPPH), is designed and employed as a RM to lower the overpotential of Li–O2 batteries. Moreover, the self-discharge of the oxidized DPPH on the surface of Li metal and the formation of lithium dendrites are successfully suppressed by the formation of graphene/luteolin protective layer on Li metal. Consequently, an ultrastable cycle stability (>150 cycles) is achieved with DPPH and the stable graphene/luteolin protective layer for the Li metal anodes. These synergistic effect of RMs and stable Li protective layer may inspire the development of sustainable and durable Li–O2 batteries.

Coupling solid and soluble catalysts toward stable Li anode for high-performance Li–O2 batteries

Promoting the Li2O2 particles formation in the electrolyte by soluble catalysts could achieve high capacities for Li–O2 cell. However, side reactions between the soluble catalysts and lithium metal anode and more importantly, the deposition of the solid Li2O2 particle on which side (lithium anode or oxygen cathode) is inevitable for the solution phase discharge process in Li–O2 batteries, leading to the obvious capacity decay. Here we design and create a partial soluble metal nitroxides frameworks (MNOFs) to promote the “marriage” between the soluble and solid catalysts to overcome the most stubborn of these challenges. Taking advantage of the partial soluble property in the electrolyte, the prepared MNOFs can be simultaneously applied as the soluble and solid catalysts. MNOFs dissolving balance between electrolyte and cathode could construct a bridge between the electrolyte and cathode, which can not only suppress the corrosion reactions on Li anode, but also target discharge products deposition on the cathode. A 1Ah Li–O2 cell with a high areal specific capacity of 40 ​mA ​h ​cm-2 was demonstrated under a relatively low overpotential (~ 0.6 ​V). By ideally bridging the soluble and solid catalyst, the reversibility is significantly improved.

Revealing Origins for Sluggish Reaction Kinetics in Li-O2 Battery Via Operando Liquid Electron Microscopy

Rechargeable lithium-oxygen (Li-O2) battery has the potential to replace conventional lithium ion battery for high-energy applications. However, this system is plagued by its sluggish kinetics during the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) with the mechanisms not fully understood. Here, we explored the discharge/charge (i.e. ORR/OER) fundamentals in a working Li-O2 battery using in situ liquid transmission electron microscopy. During ORR, Li2O2 is observed to nucleate at the carbon electrode/electrolyte interface, and its growth kinetics is controlled by Li+ diffusion in the electrolyte. Li2O2 nucleation and growth are also observed within the electrolyte where there is no direct contact with the carbon electrode, and its growth exhibits O2 - diffusion-limited kinetics, indicating the existence of non-Faradaic disproportionation reaction of intermediate LiO2 into Li2O2. Li2O2 at the carbon electrode surface and within the electrolyte are both actively decomposed during OER. For Li2O2 particles attached to the carbon electrode surface, their decomposition initiates at the electrode/ Li2O2 interface, indicating electron-conduction limited charge kinetics. Interestingly, for Li2O2 isolated within the electrolyte, we observed a side-to-side decomposition mode, which can be associated with the non-Faradaic formation of dissolvable O2 -, whose diffusion in the electrolyte controls the overall charge kinetics. The mechanisms are summarized in Figure 1. This work reveals the ORR/OER fundamentals in Li-O2 battery system in terms of the various limiting factors controlling the discharge and charge process. We expect findings here to guide both the electrode design and the electrolyte screening aiming for enhancement of the ORR and OER kinetics. Figure 1

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.

Isotopic Labeling Reveals Active Reaction Interfaces for Electrochemical Oxidation of Lithium Peroxide.

The unresolved debate on the active reaction interface of electrochemical oxidation of lithium peroxide (Li2 O2 ) prevents rational electrode and catalyst design for lithium-oxygen (Li-O2 ) batteries. The reaction interface is studied by using isotope-labeling techniques combined with time-of-flight secondary ion mass spectrometry (ToF-SIMS) and on-line electrochemical mass spectroscopy (OEMS) under practical cell operation conditions. Isotopically labelled microsized Li2 O2 particles with an Li2 16 O2 /electrode interface and an Li2 18 O2 /electrolyte interface were fabricated. Upon oxidation, 18 O2 was evolved for the first quarter of the charge capacity followed by 16 O2 . These observations unambiguously demonstrate that oxygen loss starts from the Li2 O2 /electrolyte interface instead of the Li2 O2 /electrode interface. The Li2 O2 particles are in continuous contact with the catalyst/electrode, explaining why the solid catalyst is effective in oxidizing solid Li2 O2 without losing contact.

Isotopic Labeling Reveals Active Reaction Interfaces for Electrochemical Oxidation of Lithium Peroxide

AbstractThe unresolved debate on the active reaction interface of electrochemical oxidation of lithium peroxide (Li2O2) prevents rational electrode and catalyst design for lithium‐oxygen (Li‐O2) batteries. The reaction interface is studied by using isotope‐labeling techniques combined with time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) and on‐line electrochemical mass spectroscopy (OEMS) under practical cell operation conditions. Isotopically labelled microsized Li2O2 particles with an Li216O2/electrode interface and an Li218O2/electrolyte interface were fabricated. Upon oxidation, 18O2 was evolved for the first quarter of the charge capacity followed by 16O2. These observations unambiguously demonstrate that oxygen loss starts from the Li2O2/electrolyte interface instead of the Li2O2/electrode interface. The Li2O2 particles are in continuous contact with the catalyst/electrode, explaining why the solid catalyst is effective in oxidizing solid Li2O2 without losing contact.

Aprotic Li/O2 Batteries: Reactions and Products in Electrolytes with and without a Redox Mediator

The interest in secondary Li/O2 batteries has grown rapidly over the past two decades, as they exhibit a practically achieveable specific energy of about 1,700 Wh/kg, which equals that of gasoline and is well beyond those of conventional lithium ion, Ni metal hydride and Zn/air batteries.1,2 The large energy density is due to the very light-weight cell components: a Li metal anode, a porous C cathode (gas diffusion electrode, GDE) and gaseous O2 as active cathode material. During discharge LiO2 and Li2O2 (as well as Li2O) are formed via the oxygen reduction reaction (ORR) in the porous carbon matrix. These products are oxidized again upon recharge. This seemingly simple system, however, has some major drawbacks due to the decomposition of the organic electrolytes during battery cycling – which is caused by the reaction of the respective electrolyte with either the Li anode, different components of the GDE and/or even the desired ORR products. The decomposition products are deposited in the GDE pores, leading to a minimized active electrode surface, decreasing capacities and ultimately cell failure upon continued cycling.3 There are two major reaction mechanisms leading to the formation if Li2O2, and the prevailing mechanism depends on the electrolyte. Li2O2 toroids are a common discharge product of the so-called solution growth pathway proceeding via an EC mechanism4: O2 is reduced to O2 -, which is dissolved and forms LiO2. The solvated LiO2 intermediates subsequently disproportionate to solid Li2O2 particles with different morphologies (which depend on the nature of the electrolyte)5. The ORR proceeds primarily via this mechanism when high DN electrolyte solvents are used and/or discharge parameters, such as low discharge current densities and low overpotentials, are applied6. With low DN solvents and/or high current densities and overpotentials, the discharge reaction proceeds via the surface growth pathway. In this work, galvanostatic cycling and CV measurements were conducted with 1 M mixtures of LiTFSI/DMSO and LiTFSI/TEGDME, with and without additions of the redox mediator dimethylphenazine, DMPZ7, to analyze the electrolyte influence on the cycling behavior. At different states of charge, the GDE surface and pores were investigated by SEM and XRD to obtain information on the nature, the morphology and the distribution of the product deposits on and in the carbon structure. Galvanostatic cycling measurements show a larger discharge capacity and lower discharge overpotential with LiTFSI/DMSO as well as a better recharge behavior in the first cycle, although both electrolytes exhibit very high recharge potentials. The TEGDME-based electrolyte on the other hand exhibits a higher stability over the first five cycles, whereas the discharge capacity of LiTFSI/DMSO decreases drastically already upon second discharge. CV measurements confirm these absolute and relative trends of the discharge capacities. A SEM investigation of the GDE surfaces after first discharge reveals a considerable difference in discharge product morphology depending on the electrolyte. This is in agreement to previously published results: with DMSO as solvent the product consists of toroidal particles with diameters of approximately 200 nm, whereas discharge with the TEGDME-based electrolyte yields a product layer on the porous cathode surface.5,8 Differential capacity analysis confirmed the difference in discharge product formation: with LiTFSI/TEGDME two reduction processes were detected to contribute to the discharge capacity, whereas discharge with LiTFSI/DMSO contains only one reduction process. X-ray diffraction after the first discharge with LiTFSI/DMSO shows the presence of crystalline Li2O2 in the GDE and the absence of crystalline LiOH. The discharge with the TEGDME-based electrolyte, on the other hand, did not yield any crystalline Li2O2 (or LiOH). FIB-SEM measurements after the first discharge with each electrolyte furthermore showed that the GDE pores were almost completely blocked by bulk product. The addition of the DMPZ significantly reduced the charging potential, both for the DMSO and the TEGDME electrolyte. The cycling performance was, however, only improved for DMSO, and not for TEGDME. Bruce et al., Nat. Mater. (2012) 11, 19-29.Yao et al., Angew. Chem. Int. Ed. (2016) 55, 11344-11353.Sharon et al., Isr. J. Chem. (2015) 55, 508-520.Johnson, L. et al., Nat. Chem. 6, 1091–1099 (2014).Mitchell, R. R., et al., J. Phys. Chem. Lett. 4, 1060–1064 (2013).Aurbach, D., et al., Nat. Energy 1, 16128 (2016).Lim et al., Nature Energy (2016) 1, 16066.Aetukuri et al., Nat. Chem. (2014) 7, 50-56.

3D‐Printed MOF‐Derived Hierarchically Porous Frameworks for Practical High‐Energy Density Li–O2 Batteries

AbstractAprotic Li–O2 batteries are promising candidates for next‐generation energy storage technologies owing to their high theoretical energy densities. However, their practically achievable specific energy is largely limited by the need for porous conducting matrices as cathode support and the passivation of cathode surface by the insulating Li2O2 product. Herein, a self‐standing and hierarchically porous carbon framework is reported with Co nanoparticles embedded within developed by 3D‐printing of cobalt‐based metal–organic framework (Co‐MOF) using an extrusion‐based printer, followed by appropriate annealing. The novel self‐standing framework possesses good conductivity and necessary mechanical stability, so that it can act as a porous conducting matrix. Moreover, the porous framework consists of abundant micrometer‐sized pores formed between Co‐MOF‐derived carbon flakes and meso‐ and micropores formed within the flakes, which together significantly benefit the efficient deposition of Li2O2 particles and facilitate their decomposition due to the confinement of insulating Li2O2 within the pores and the presence of Co electrocatalysts. Therefore, the self‐standing porous architecture significantly enhances the cell's practical specific energy, achieving a high value of 798 Wh kg−1cell. This study provides an effective approach to increase the practical specific energy for Li–O2 batteries by constructing 3D‐printed framework cathodes.

Facile preparation of unique three-dimensional (3D) α-MnO2/MWCNTs macroporous hybrid as the high-performance cathode of rechargeable Li-O2 batteries

Undoubtedly, there remains an urgent prerequisite to achieve significant advances in both the specific capacity and cyclability of Li-O2 batteries for their practical application. In this work, a series of unique three-dimensional (3D) α-MnO2/MWCNTs hybrids are successfully prepared using a facile lyophilization method and investigated as the cathode of Li-O2 batteries. Thereinto, cross-linked α-MnO2/MWCNTs nanocomposites are first synthesized via a modified chemical route. Results demonstrate that MnO2 nanorods in the nanocomposites have a length of 100–400 nm and a diameter ranging from 5 to 10 nm, and more attractively, the as-lyophilized 3D MnO2/MWCNTs hybrids is uniquely constructed with large amounts of interconnected macroporous channels. The Li-O2 battery with the 3D macroporous hybrid cathode that has a mass percentage of 50% of α-MnO2 delivers a high discharge specific capacity of 8,643 mAh·g−1 at 100 mA·g−1, and maintains over 90 cycles before the discharge voltage drops to 2.0 V under a controlled specific capacity of 1,000 mAh·g−1. It is observed that when being recharged, the product of toroidal Li2O2 particles disappears and electrode surfaces are well recovered, thus confirming a good reversibility. The excellent performance of Li-O2 battery with the 3D α-MnO2/MWCNTs macroporous hybrid cathode is ascribed to a synergistic combination between the unique macroporous architecture and highly efficient bi-functional α-MnO2/MWCNTs electrocatalyst.