A superhigh discharge capacity induced by a synergetic effect between high-surface-area carbons and a carbon paper current collector in a lithium–oxygen battery

This paper invesitages the synergetic effect between high-surface-area carbons, such as Ketjan Black(KB) or Super P(SP) carbon materials, and low-surface-area carbon paper(CP) current collectors and it also examines their influence on the discharge performance of nonaqueous Li–O2cells. Ultra-large specific discharge capacities are found in the KB/CP cathodes, which are much greater than those observed in the individual KB or CP cathodes. Detailed analysis indicates that such unexpectedly large capacities result from the synergetic effect between the two components. During the initial discharges of KB or SP materials, a large number of superoxide radical(O·-2) species in the electrolytes and Li2O2 nuclei at the CP surfaces are formed, which activate the CP current collectors to contribute considerable capacities. These results imply that CP could be a superior material for current collectors in terms of its contribution to the overall discharge capacity.On the other hand, we should be careful to calculate the specific capacities of the oxygen cathodes when using CP as a current collector; i.e., ignoring the contribution from the CP may cause overstated discharge capacities.

Atomically Dispersed Ruthenium Catalysts with Open Hollow Structure for Lithium-Oxygen Batteries

Lithium–oxygen battery with ultrahigh theoretical energy density is considered a highly competitive next-generation energy storage device,but its practical application is severely hindered by issues such as difficult decomposition of discharge products at present.Here,we have developed N-doped carbon anchored atomically dispersed Ru sites cathode catalyst with open hollow structure(h-RuNC)for Lithium–oxygen battery.On one hand,the abundance of atomically dispersed Ru sites can effectively catalyze the formation and decomposition of discharge products,thereby greatly enhancing the redox kinetics.On the other hand,the open hollow structure not only enhances the mass activity of atomically dispersed Ru sites but also improves the diffusion efficiency of catalytic molecules.Therefore,the excellent activity from atomically dispersed Ru sites and the enhanced diffusion from open hollow structure respectively improve the redox kinetics and cycling stability,ultimately achieving a high-performance lithium–oxygen battery.

Preparation and performance of hierarchically porous carbons as oxygen electrodes for lithium oxygen batteries

The hierarchically porous carbons （HPCs） were prepared by sol-gel selassembly technology in different surfactant concentrations and were used as the potential electrode for lithium oxygen batteries. The physical and electrochemical properties of the as-prepared HPCs were investigated by filed emission scanning electron microscopy （FE-SEM）, transmission electron microscopy （TEM）, nitrogen adsorption-desorption isotherm and galvanostatic charge/discharge. The results indicate that all of the HPCs mainly possess mesoporous structure with nearly similar pore size distribution. Using the HPCs as the electrode, a high discharge capacity for lithium oxygen battery can be achieved, and the discharge capacity increases with the specific surface area. Especially, the HPCs-3 oxygen electrode with CTAB concentration of 0.27 mol/L exhibits good capacity retention through controlling discharge depth to 800 mA·h/g and the highest discharge capacity of 2050 mA·h/g at a rate of 0.1 mA/cm2.

Recent Progress of Catalytic Cathodes for Lithium-oxygen Batteries

Lithium-oxygen batteries are among the most promising electrochemical energy storage systems,which have attracted significant attention in the past few years duo to its far more energy density than lithium-ion batteries.Lithium oxygen battery energy storage is a reactive storage mechanism,and the discharge and charge processes are usually called oxygen reduction reaction(ORR)and oxygen evolution reaction(OER).Consequently,complex systems usually create complex problems,lithium oxygen batteries also face many problems,such as excessive accumulation of discharge products(Li_(2)O_(2))in the cathode pores,resulting in reduced capacity,unstable cycling performance and so on.Cathode catalyst,which could influence the kinetics of OER and ORR in lithium oxygen(Li-O_(2))battery,is one of the decisive factors to determine the electrochemical performance of the battery,so the design of cathode catalyst is vitally important.This review discusses the catalytic cathode materials,which are divided into four parts,carbon based materials,metals and metal oxides,composite materials and other materials.

CuCr_2O_4@rGO Nanocomposites as High-Performance Cathode Catalyst for Rechargeable Lithium–Oxygen Batteries

Rechargeable lithium–oxygen batteries have been considered as a promising energy storage technology because of their ultra-high theoretical energy densities which are comparable to gasoline. In order to improve the electrochemical properties of lithium–oxygen batteries(LOBs), especially the cycling performance, a high-efficiency cathode catalyst is the most important component.Hence, we aim to demonstrate that CuCr_2O_4@rGO(CCO@rGO) nanocomposites, which are synthesized using a facile hydrothermal method and followed by a series of calcination processes, are an effective cathode catalyst. The obtained CCO@rGO nanocomposites which served as the cathode catalyst of the LOBs exhibited an outstanding cycling performance for over 100 cycles with a fixed capacity of 1000 mAh g^(-1) at a current density of 200 mA g^(-1). The enhanced properties were attributed to the synergistic effect between the high catalytic efficiency of the spinel-structured CCO nanoparticles, the high specific surface area, and high conductivity of the rGO.

A novel bifunctional oxygen electrode architecture enabled by heterostructures self-scaffolding for lithium–oxygen batteries

In recent years, as one of the most promising chemical power sources for future society, lithium–oxygen (Li–O2) battery receives great attention due to its extremely high theoretical energy density of 3505 Wh kg^(–1)[1–4]. In practice, large polarization and consequent low energy efficiency currently still hinder the application of Li–O2batteries, which mainly results from the sluggish electrochemical reaction kinetics of oxygen diffusion electrodes in aprotic electrolytes [5]. On one hand, oxygen reduction reaction (ORR)in aprotic electrolytes is intrinsically sluggish due to the difficult charge transfer, the low solubility of oxygen.

Effective exposure of nitrogen heteroatoms in 3D porous graphene framework for oxygen reduction reaction and lithium–sulfur batteries

The introduction of nitrogen heteroatoms into carbon materials is a facile and efficient strategy to regulate their reactivities and facilitate their potential applications in energy conversion and storage. However,most of nitrogen heteroatoms are doped into the bulk phase of carbon without site selectivity, which significantly reduces the contacts of feedstocks with the active dopants in a conductive scaffold. Herein we proposed the chemical vapor deposition of a nitrogen-doped graphene skin on the 3D porous graphene framework and donated the carbon/carbon composite as surface N-doped grapheme（SNG）. In contrast with routine N-doped graphene framework（NGF） with bulk distribution of N heteroatoms, the SNG renders a high surface N content of 1.81 at%, enhanced electrical conductivity of 31 S cm^（-1）, a large surface area of 1531 m^2 g^（-1）, a low defect density with a low I_D/I_G ratio of 1.55 calculated from Raman spectrum, and a high oxidation peak of 532.7 ℃ in oxygen atmosphere. The selective distribution of N heteroatoms on the surface of SNG affords the effective exposure of active sites at the interfaces of the electrode/electrolyte, so that more N heteroatoms are able to contact with oxygen feedstocks in oxygen reduction reaction or serve as polysulfide anchoring sites to retard the shuttle of polysulfides in a lithium–sulfur battery. This work opens a fresh viewpoint on the manipulation of active site distribution in a conductive scaffolds for multi-electron redox reaction based energy conversion and storage.

Revealing alkali metal ions transport mechanism in the atomic channels of Au@a-MnO_(2)

Understanding alkali metal ions’(e.g.,Li^(+)/Na^(+)/K^(+))transport mechanism is challenging but critical to improving the performance of alkali metal batteries.Herein using a-MnO_(2)nanowires as cathodes,the transport kinetics of Li^(+)/Na^(+)/K^(+)in the 2×2 channels of a-MnO_(2)with a growth direction of[001]is revealed.We show that ion radius plays a decisive role in determining the ion transport and electrochemistry.Regardless of the ion radii,Li^(+)/Na^(+)/K^(+)can all go through the 2×2 channels of a-MnO_(2),generating large stress and causing channel merging or opening.However,smaller ions such as Li^(+)and Na^(+)cannot only transport along the[001]direction but also migrate along the<110>direction to the nanowire surface;for large ion such as K^(+),diffusion along the<110>direction is prohibited.The different ion transport behavior has grand consequences in the electrochemistry of metal oxygen batteries(MOBs).For Li-O_(2)battery,Li^(+)transports uniformly to the nanowire surface,forming a uniform layer of oxide;Na^(+)also transports to the nanowire surface but may be clogged locally due to its larger radius,therefore sporadic pearl-like oxides form on the nanowire surface;K^(+)cannot transport to the nanowire surface due to its large radius,instead,it breaks the nanowire locally,causing local deposition of potassium oxides.The study provides atomic scale understanding of the alkali metal ion transport mechanism which may be harnessed to improve the performance of MOBs.

In-situ imaging electrocatalysis in a solid-state Li-O_(2) battery with CuSe nanosheets as air cathode

The development of highly efficient catalysts in the cathodes of rechargeable Li-O_(2) batteries is a considerable challenge.To enhance the electrochemical performance of the Li-O_(2) battery,it is essential to choose a suitable catalyst material.Copper selenide(CuSe)is considered as a more promising cathode catalyst material for Li-O_(2) battery due to its better conductivity and rich electrochemical active sites.However,its electrochemical reaction and fundamental catalytic mechanism remain unclear till now.Herein,in-situ environmental transmission electron microscopy technique was used to study the catalysis mechanism of the CuSe nanosheets in Li-O_(2) batteries during discharge and charge processes.It is found that Li_(2)O was formed and decomposed around the ultrafine-grained Cu during the discharge and charge processes,respectively,demonstrating excellent cycling.This indicate that the freshly formed ultrafine-grained Cu in the conversion reaction catalyzed the latter four-electron-transfer oxygen reduction reaction,leading to the formation of Li_(2)O.Our study provides important understanding of the electrochemistry of the LiO_(2) nanobatteries,which will aid the development of high-performance Li-O_(2) batteries for energy storage applications.

Carbon nanotube-supported mixed-valence Mn_(3)O_(4) electrodes for high-performance lithium-oxygen batteries

Lithium-oxygen batteries(LOBs)have extensive applications because of their ultra-high energy densities.However,the practical application of LOBs is limited by several factors,such as a high overpotential,poor cycle stability,and limited rate capacity.In this paper,we describe the successful uniform loading of Mn_(3)O_(4) nanoparticles onto multi-walled carbon nanotubes(Mn_(3)O_(4)@CNT).CNTs form a conductive network and expose numerous catalytically active sites,and the one-dimensional porous structure provides a convenient channel for the transmission of Li+and O2 in LOBs.The electronic conductivity and electrocatalytic activity of Mn_(3)O_(4)@CNT are significantly better than those of MnO@CNT because of the inherent driving force facilitating charge transfer between different valence metal ions.Therefore,the Mn_(3)O_(4)@CNT cathode obtains a low overpotential(0.76 V at a limited capacity of 1000 mAh g^(-1)),high initial discharge capacity(16895 mAh g^(-1) at 200 mA g^(-1)),and long cycle life(97 cycles at 200 mA g^(-1)).This study provides evidence that transition metal oxides with mixed-valence states are suitable for application as efficient cathodes for LOBs.

Optimal geometrical configuration and oxidation state of cobalt cations in spinel oxides to promote the performance of Li-O_(2) battery

Co_(3)O_(4) is considered as one of promising cathode catalysts for lithium oxygen(Li-O_(2))batteries,which contains both tetrahedral Co^(2+)sites(Co^(2+)Td)and octahedral Co^(3+)sites(Co^(3+)Oh).It is important to reveal the effect of optimal geometric configuration and oxidation state of cobalt ion in Co_(3)O_(4) to improve the performance of Li-O_(2) batteries.Herein,through regulating the synthesis process,Co^(2+)and Co^(3+)sites in Co_(3)O_(4) were replaced with Zn and Al atoms to form materials with a unique Co site.The Li-O_(2) batteries based on ZnCo_(2)O_(4) showed longer cycle life than that of CoAl_(2)O_(4),suggesting that in Co_(3)O_(4),the Co^(3+)Oh site is a relatively better geometric configuration than Co^(2+)Td site for Li-O_(2) batteries.Theoretical calculations revealed that Co^(3+)Oh sites provide higher catalysis activity,regulating the adsorption energy of the intermediate LiO_(2) and accelerating the kinetics of the reaction in batteries,which further leads to the change of the morphology of the discharge product and ultimately improves the electrochemical performance of the batteries.

Binder-free Li-O_(2)battery cathodes using Ni-and PtRu-coated vertically aligned carbon nanofibers as electrocatalysts for enhanced stability

The development of an ideal cathode for Li-O_(2)battery(LOB)has been a great challenge in achieving high discharge capacity,enhanced stability,and longevity.The formation of undesired and irreversible discharge products on the surface of current cathode materials limit the life span of the LOB.In this study,we report the systematic electrochemical study to compare the performance of LOB employing a unique graphitic nanostructured carbon architecture,i.e.,vertically aligned carbon nanofiber(VACNF)arrays,as the cathode materials.Transition metal(Ni)and noble metal alloy(PtRu)are further deposited on the VACNF array as electrocatalysts to improve the discharge/charge processes at the cathode.The structure of as-prepared electrodes was examined with the field emission scanning electron microscopy,high-resolution transmission electron microscopy,and X-ray photoelectron spectroscopy(XPS).The LOB with VACNF-Ni electrode delivered the highest specific and areal discharge capacities(14.92 Ah·g^(−1),4.32 mAh·cm^(−2))at 0.1 mA·cm^(−2)current density as compared with VACNF-PtRu(9.07 Ah·g^(−1),2.62 mAh·cm^(−2)),bare VACNF(5.55 Ah·g^(−1),1.60 mAh·cm^(−2))and commercial Vulcan XC(3.83 Ah·g^(−1),1.91 mAh·cm^(−2)).Cycling stability tests revealed the superior performance of VACNF-PtRu with 27 cycles as compared with VACNF-Ni(13 cycles),VACNF(8 cycles),and Vulcan XC(3 cycles).The superior cycling stability of VACNF-PtRu can be attributed to its ability to suppress the formation of Li2CO3 during the discharge cycle,as elucidated by XPS analysis of discharged samples.We also investigated the impact of carbon cloth and carbon fiber as cathode electrode substrate on the performance of LOB.

Nanostructured energy materials for electrochemical energy conversion and storage: A review

Nanostructured materials have received tremendous interest due to their unique mechanical/electrical properties and overall behavior contributed by the complex synergy of bulk and interfacial properties for efficient and effective energy conversion and storage. The booming development of nanotechnology affords emerging but effective tools in designing advanced energy material. We reviewed the significant progress and dominated nanostructured energy materials in electrochemical energy conversion and storage devices, including lithium ion batteries, lithium-sulfur batteries, lithium-oxygen batteries, lithium metal batteries, and supercapacitors. The use of nanostructured electrocatalyst for effective electrocatalysis in oxygen reduction and oxygen evolution reactions for fuel cells and metal-air batteries was also included. The challenges in the undesirable side reactions between electrolytes and electrode due to high electrode/electrolyte contact area, low volumetric energy density of electrode owing to low tap density, and uniform production of complex energy materials in working devices should be overcome to fully demonstrate the advanced energy nanostructures for electrochemical energy conversion and storage. The energy chemistry at the interfaces of nanostructured electrode/electrolyte is highly expected to guide the rational design and full demonstration of energy materials in a working device. (C) 2016 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.

Boron-doped Ketjenblack based high performances cathode for rechargeable Li–O2 batteries

Boron-doped Ketjenblack is attempted as cathode catalyst for non-aqueous rechargeable Li–O2 batteries. The boron-doped Ketjenblack delivers an extremely high discharge capacity of 7193 m Ah/g at a current density of 0.1 m A/cm2, and the capacity is about 2.3 times as that of the pristine KB. When the batteries are cycled with different restricted capacity, the boron-doped Ketjenblack based cathodes exhibits higher discharge platform and longer cycle life than Ketjenblack based cathodes. Additionally, the boron-doped Ketjenblack also shows a superior electrocatalytic activity for oxygen reduction in 0.1 mol/L KOH aqueous solution. The improvement in catalytic activity results from the defects and activation sites introduced by boron doping.

Recent advances in cathode catalyst architecture for lithium–oxygen batteries

Lithium–oxygen(Li–O_(2))batteries have great potential for applications in electric devices and vehicles due to their high theoretical energy density of 3500 Wh kg^(-1).Unfortunately,their practical use is seriously limited by the sluggish decomposition of insulating Li_(2)O_(2),leading to high OER overpotentials and the decomposition of cathodes and electrolytes.Cathode electrocatalysts with high oxygen reduction reaction(ORR)and oxygen evolution re-action(OER)activities are critical to alleviate high charge overpotentials and promote cycling stability in Li–O_(2)batteries.However,constructing catalysts for high OER performance and energy efficiency is always challenging.In this mini-review,we first outline the employment of advanced electrocatalysts such as carbon materials,noble and non-noble metals,and metal–organic frameworks to improve battery performance.We then detail the ORR and OER mechanisms of photo-assisted electrocatalysts and single-atom catalysts for superior Li–O_(2)battery performance.Finally,we offer perspectives on future development directions for cathode electrocatalysts that will boost the OER kinetics.

Microbe-derived carbon materials for electrical energy storage and conversion

Microbes are microscopic living organisms that surround us which include bacteria, archaea, most protozoa, and some fungi and algae. In recent years, microbes have been explored as novel precursors to synthesize carbon-based（nano）materials and as substrates or templates to produce carbon-containing（nano）composites. Being greener and more affordable, microbe-derived carbons（MDCs） offer good potential for energy applications. In this review, we describe the unique advantages of MDCs and outline the common procedures to prepare them. We also extensively discuss the energy applications of MDCs including their use as electrodes in supercapacitors and lithium-ion batteries, and as electrocatalysts for processes such as oxygen reduction, oxygen evolution, and hydrogen evolution reactions which are essential for fuel cell and water electrochemical splitting cells. Based on the literature trend and our group＇s expertise, we propose potential research directions for developing new types of MDCs. This review, therefore, provides the state-of-the-art of a new energy chemistry concept. We expect to stimulate future research on the applications of MDCs that may address energy and environmental challenges that our societies are facing.

Highly dispersed silver nanoparticles for performance-enhanced lithium oxygen batteries

Aprotic lithium-oxygen batteries possess ultrahigh energy density but suffer from the sluggish decomposition of discharge product,quick depletion of Li anode and cleavage of electrolyte,in close association with oxygen reduction reaction at the cathode.Herein,highly dispersed silver nanoparticles are used to enhance the lithium-oxygen battery with 1.0 M lithium perchlorate in dimethyl sulfoxide.It is observed that film-like amorphous lithium peroxide is formed through surface pathway instead of bulk crystals,due to the incorporation of silver nanoparticles dispersed in the electrolyte,which subsequently accelerates the decomposition of the discharge product by offering more active sites and improved conductivity.The released silver nanoparticles after battery charging can be re-used in the following cycles.Experiments and theoretical calculation further indicate that the suspended silver nanoparticles can adsorb the soluble oxygen reduction intermediates,which are responsible for the alleviation of oxidative cleavage of electrolyte and corrosion of lithium anode.The lifespan of lithium oxygen batteries is therefore significantly extended from 55 to 390 cycles,and the rate performance and full-discharge capacity are also largely enhanced.The battery failure is attributed to the coalescence and growth of silver nanoparticles in the electrolyte,and further improvement on colloid stability is underway.

Greatly promoted oxygen reduction reaction activity of solid catalysts by regulating the stability of superoxide in metal-O_(2)batteries

Oxygen reduction reactions(ORRs)with one-or two-electron-transfer pathways are the essential process for aprotic metal-oxygen batteries,in which the stability of superoxide intermediates/products(O_(2)^(-),LiO_(2),NaO_(2),etc.)mainly dominates the ORR activity/stability and battery performance.However,little success in regulating the stability of the superoxides has been achieved due to their highly reactive characteristics.Herein,we identified and modulated the stability of superoxides by introducing anthraquinone derivatives as cocatalysts which functioned as superoxide trapper adsorbing the superoxides generated via surface-mediated ORR and then transferring them from the solid catalyst surface into electrolyte.Among the studied trappers,1,4-difluoroanthraquinone(DFAQ)with electron-withdrawing groups showed the highest adsorption towards superoxides and could efficiently stabilize LiO_(2)in electrolyte,which greatly promoted the surface-mediated ORR rate and stability.This highlighted the magnitude of adsorption between the trapper and LiO_(2)on the ORR activity/stability.Using an aprotic Li-O_(2)battery as a model metal-O_(2)battery,the overall performance of the cell with DFAQ was substantially improved in terms of cell capacity,rate capability and cyclic stability.These results represent a significant advance in the understanding of ORR mechanisms and promoting the performance of metal-O_(2)batteries.

Free-standing porous carbon electrodes derived from wood for high-performance Li-O2 battery applications

Porous carbon materials are widely used in particulate forms for energy applications such as fuel cells, batteries, and （super） capacitors. To better hold the particles together, polymeric additives are utilized as binders, which not only increase the weight and volume of the devices, but also cause adverse side effects. We developed a wood-derived, free-standing porous carbon electrode and successfully applied it as a cathode in Li-O2 batteries. The spontaneously formed hierarchical porous structure exhibits good performance in facilitating the mass transport and hosting the discharge products of Li202. Heteroatom （N） doping further improves the catalytic activity of the carbon cathode with lower overpotential and higher capacity. Overall, the Li-O2 battery based on the new carbon cathode affords a stable energy efficiency of 65% and can be operated for 20 cycles at a discharge depth of 70%. The wood-derived free-standing carbon represents a new, unique structure for energy applications.

Three-dimensional nitrogen and phosphorous Co-doped graphene aerogel electrocatalysts for efficient oxygen reduction reaction

The development of efficient electrocatalysts for oxygen reduction reaction(ORR) is of importance for fuel cells and metal-air batteries. Herein, three-dimensional nitrogen and phosphorous co-doped graphene aerogel(NPGA) was prepared via the pyrolysis of polyaniline(PANi) coated graphene oxide aerogel synthesized by oxidative polymerization of aniline on graphene oxide(GO) sheets in the presence of phytic acid. The uniform coating of PANi thin layer on the surface of GO sheets enables the formation of highly porous composite aerogel of PANi and GO. The subsequent thermal treatment is able to prepare the porous NPGA due to the carbonization of PANi and phytic acid as nitrogen and phosphorous resources. When used as electrocatalysts,the as-prepared NPGA electrocatalysts exhibited good catalytic activity to ORR via an efficient four-electron pathway with good stability, benefiting from the highly porous structure and the heteroatom co-doping. More importantly, Zn-air batteries operated in ambient air have been fabricated by coupling a Zn plate with the NPGA electrocatalyst in an air electrode, demonstrating the maximal power density as high as ~260 W/g and a good long-term stability with slightly potential decay for over 450 h. The facile method for preparing efficient carbon based ORR electrocatalysts would generate other potential applications including fuel cells and others.