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Li–O2 batteries can provide greater gravimetric energy than Li-ion batteries but suffer from poor efficiency and cycle life due to the instability of aprotic electrolytes. In this study, we show that the apparent four-electron oxygen reduction to form Li2O in Li–O2 batteries with molten nitrate is facilitated by the electrochemical reduction of nitrate to nitrite, and subsequent chemical oxidation of nitrite to nitrate by molecular oxygen, instead of a four-electron oxygen reduction aided by disproportionation of Li2O2 generated from two-electron reduction of molecular oxygen. By examining a series of transition metal catalysts using experiments and computation, optimizing the surface binding of nitrate to enhance the kinetics of the electrochemical reduction of nitrate to nitrite, as well as increasing the kinetics of nitrite oxidation by O2 was shown to increase the discharge voltage and render the observed high-rate capability for NiO-based surfaces in Li–O2 batteries.

The oxygen evolution reaction is central to making chemicals and energy carriers using electrons. Combining the great tunability of enzymatic systems with known oxide-based catalysts can create breakthrough opportunities to achieve both high activity and stability. Here we report a series of metal hydroxide–organic frameworks (MHOFs) synthesized by transforming layered hydroxides into two-dimensional sheets crosslinked using aromatic carboxylate linkers. MHOFs act as a tunable catalytic platform for the oxygen evolution reaction, where the π–π interactions between adjacent stacked linkers dictate stability, while the nature of transition metals in the hydroxides modulates catalytic activity. Substituting Ni-based MHOFs with acidic cations or electron-withdrawing linkers enhances oxygen evolution reaction activity by over three orders of magnitude per metal site, with Fe substitution achieving a mass activity of 80 A gcatalyst-1 at 0.3 V overpotential for 20 h. Density functional theory calculationscorrelate the enhanced oxygen evolution reaction activity with the MHOF-based modulation of Ni redox and the optimized binding of oxygenated intermediates.

A molten-salt Na–O2 battery featuring a liquid Na negative electrode and Ni positive electrode was found to form Na2O2 on discharge, enabled by nitrate redox where NO3− is reduced to Na2O and NO2−, then reactions with O2 produce Na2O2 and reform NO3−.

Summary Manganese dioxide is abundant, low-cost, and has the potential to be utilized as a semi-solid electrode for long-duration energy storage technologies such as flow batteries. However, the more stringent pumping requirements of semi-solid electrodes compared to the electrolytes of all-liquid flow battery might limit their techno-economic feasibility. Here, we developed a rechargeable MnO2 semi-solid electrode, performed electrochemical and rheological characterizations, and bottom-up techno-economic analysis of the Zn-MnO2 semi-solid flow battery (SSFB) system. The high power needed for pumping (ranging from 8% to 50% of the power output) leads to a system with high cost of power. Using our experimental results, we suggest strategies to minimize the pumping power requirement for Zn-MnO2 SSFB. As a result of the low cost of its chemical constituents, we show that a Zn-MnO2 SSFB can be cheaper than Li-ion and vanadium redox flow battery solutions for long discharge durations (e.g., >24 h per cycle).

By increasing the charging voltage, a cell specific energy of >400 W h kg−1 is achievable with LiNi0.8Mn0.1Co0.1O2 in Li metal batteries. However, stable cycling of high-nickel cathodes at ultra-high voltages is extremely challenging. Here we report that a rationally designed sulfonamide-based electrolyte enables stable cycling of commercial LiNi0.8Co0.1Mn0.1O2 with a cut-off voltage up to 4.7 V in Li metal batteries. In contrast to commercial carbonate electrolytes, the electrolyte not only suppresses side reactions, stress-corrosion cracking, transition-metal dissolution and impedance growth on the cathode side, but also enables highly reversible Li metal stripping and plating leading to a compact morphology and low pulverization. Our lithium-metal battery delivers a specific capacity >230 mA h g−1 and an average Coulombic efficiency >99.65% over 100 cycles. Even under harsh testing conditions, the 4.7 V lithium-metal battery can retain >88% capacity for 90 cycles, advancing practical lithium-metal batteries. Charging at high voltages in principle makes batteries energy dense, but this is often achieved at the cost of the cycling stability. Here the authors design a sulfonamide-based electrolyte to enable a Li metal battery with a state-of-the-art cathode at an ultra-high voltage of 4.7 V while maintaining cyclability.

Critical factors that govern the composition and morphology of discharge products are largely unknown for Na-O2 battery. Here we report a reversible ORR and OER process in a sodium-oxygen battery o...

We present the discovery of a novel radical cation formed through one-electron oxidation of an N-heterocyclic carbene-carbodiimide (NHC-CDI) zwitterionic adduct. This compound possesses a distonic electronic structure (spatially separate spin and charge regions) and displays persistence under ambient conditions. We demonstrate its application in a redox-flow battery exhibiting minimal voltage hysteresis, a flat voltage plateau, high Coulombic efficiency, and no performance decay for at least 100 cycles. The chemical tunability of NHCs and CDIs suggests that this approach could provide a general entry to redox-active NHC-CDI adducts and their persistent radical ions for various applications.

Flow battery technology offers a promising low-cost option for stationary energy storage applications. Aqueous zinc–nickel battery chemistry is intrinsically safer than non-aqueous battery chemistry (e.g. lithium-based batteries) and offers comparable energy density. In this work, we show how combining high power density and low-yield stress electrodes can minimize energy loss due to pumping, and have demonstrate methods to achieve high energy and power density for ZnO/Ni(OH)2 electrodes by changing composition and optimizing testing protocols. Firstly, mechanically stable and homogeneous Ni(OH)2/carbon and ZnO/Zn flowable electrodes in 7 M KOH electrolyte were designed using a microgel dispersion as the suspending matrix. By determining the critical volume fractions for conductivity percolation, colloidal suspensions with 6.2 vol% of carbon and 23.1 vol% of Zn were selected for preparing catholytes and anolytes to ensure that these semi-solid electrodes possess high voltage and high coulombic efficiencies. The resulting flowable electrodes exhibited non-Newtonian rheology with a yield stress of approximately ∼200 Pa, which assists in maintaining mechanical stability of the suspensions. An energy density of up to 134 W h Lcatholyte−1 and power density up to ∼159 mW cmgeo.−2 was demonstrated for semi-solid ZnO/Ni(OH)2 electrodes, and coulombic efficiency of 94% was achieved during cycling by optimizing the charging protocol to 60% SOC of Ni(OH)2. Lastly, semi-solid ZnO and Ni(OH)2 flow cells were built and tested using an intermittent mode of operation. The high energy and power densities, high coulombic efficiency, and negligible pumping loss of the Zn–Ni semi-solid electrodes developed in the present work present a promising system for further development.

Large-scale electrical energy storage with high energy density and round-trip efficiency is important to the resilience of power grids and the effective use of intermittent renewable energy such as solar and wind. Lithium-oxygen battery, due to its high energy density, is believed to be one of the most promising energy storage systems for the future. However, large overpotentials, poor cycling stability, and degradation of electrolytes and cathodes have been hindering the development of lithium-oxygen batteries. Numerous heterogeneous oxygen electrocatalysts have been investigated to lower the overpotentials and enhance the cycling stability of lithium-oxygen batteries. Unfortunately, the prevailing issues of electrode passivation and clogging remain. Over the past few years, redox mediators were explored as homogenous catalysts to address the issues, while only limited success has been achieved for these soluble catalysts. In conjunction with a flowing electrolyte system, a new redox flow lithium-oxygen battery (RFLOB) has been devised to tackle the aforementioned issues. The working mechanism and schematic processes will be elaborated in this review. In addition, the performance gap of RFLOB with respect to practical requirements will be analysed. With the above, we anticipate RFLOB would be a credible solution for the implementation of lithium-oxygen battery chemistry for the next generation energy storage. Keywords: Lithium-air battery, Redox catalysis, Oxygen reduction reaction, Oxygen evolution reaction, Redox flow cell

Though low-cost and environmentally friendly, Li-Mn-O cathodes suffer from low energy density. Although synthesized Li4Mn5O12-like overlithiated spinel cathode with reversible hybrid anion- and cation-redox (HACR) activities has a high initial capacity, it degrades rapidly due to oxygen loss and side-reaction-induced electrolyte decomposition. Herein, we develop a two-step heat treatment to promote local decomposition as Li4Mn5O12 → 2LiMn2O4 + Li2MnO3 + 1/2 O2↑, which releases near-surface reactive oxygen that is harmful to cycling stability. The produced nanocomposite delivers a high discharge capacity of 225 mAh/g and energy density of over 700 Wh/kg at active-material level at a current density of 100 mA/g between 1.8 to 4.7 V. Benefiting from suppressed oxygen loss and side reactions, 80% capacity retention is achieved after 214 cycles in half cells. With industrially acceptable electrolyte amount (6 g/Ah), full cells paired with Li4Ti5O12 anode have a good retention over 100 cycles.

Ni-rich positive electrodes for Li-ion batteries can provide enhanced initial discharge capacity yet suffer from significant capacity degradation upon cycling. Fluorination of Ni-rich NMC811 positive electrodes results in a capacity retention of more than 90% after 100 cycles upon cycling to 4.4 VLi. The increased cycling stability of F-modified NMC811 can be attributed to the modification of the oxide electronic structures, where density functional theory calculations shows that incorporating fluorine into the oxide lattice decrease the driving force of carbonate dissociation on the oxide surface. In situ infrared (IR) spectroscopy and ex situ X-ray photoelectron spectroscopy (XPS) further supports this argument by showing less carbonate oxidation for F-modified LiNi0.8Mn0.1Co0.1O2 (NMC811) than as-received NMC811 upon charging. The reduced carbonate oxidation is coupled with minimal salt decomposition on the electrode surface, as revealed by XPS. Further comparing in situ IR and XPS with that of the heat-treated NMC811 and Li2CO3 allows for decoupling the solvent decomposition products, where oxides are responsible for the vinylene carbonate formation with two hydrogens removed whereas surface metal carbonates promote dehydrogenated ethylene carbonate with just one hydrogen removed. This work points towards the importance of the anion engineering to increase the cycling stability of Ni-rich NMC positive electrodes.

Electrolyte instability is one of the most challenging impediments to enabling Lithium-Oxygen (Li-O(2)) batteries for practical use. The use of physical organic chemistry principles to rationally design new molecular components may enable the discovery of electrolytes with stability profiles that cannot be achieved with existing formulations. Here, we report on the development of sulfamide- and sulfonamide-based small molecules that are liquids at room temperature, capable of dissolving reasonably high concentration of Li salts (e.g., LiTFSI), and are exceptionally stable under the harsh chemical and electrochemical conditions of aprotic Li-O(2) batteries. In particular, N,N-dimethyl-trifluoromethanesulfonamide was found to be highly resistant to chemical degradation by peroxide and superoxide, stable against electrochemical oxidation up to 4.5 V(Li), and stable for > 90 cycles in a Li-O(2) cell when cycled at < 4.2 V(Li). This study provides guiding principles for the development of next-generation electrolyte components based on sulfamides and sulfonamides.

Critical factors that govern the composition and morphology of discharge products are largely unknown for Na-O

A stable Zn metal electrode can develop rechargeable zinc metal batteries (RZMBs) which have the high theoretical capacity (820 mAh g−1), low redox potential, and intrinsic safety. However, the corrosion of Zn metal in aqueous electrolytes and Zn dendrite formation during the plating process lead to poor cycling and thus hinder the development of RZMBs. Here, we employed ionic liquid-based gel polymer (poly(vinylidene fluoride)-co-hexafluoropropylene, PVDF-HFP) and acetylene black (AB) to achieve a stable and flexible three-dimensional (3D) Zn/AB/PVDF-HFP film electrode with ionic and electronic conductive networks and high surface area, showing 26 times higher plating/stripping current than planar Zn plate. By developing a continuous structure between the ionic liquid-based gel polymer membrane and the flexible 3D Zn/AB/PVDF-HFP electrode, the low resistance, high rate capability and long cycle life (> 800 h) was obtained. Our work shows a flexible Zn film electrode and ionic liquid-based gel polymer electrolyte, could pave the path for rechargeable and high-cycle life thin-film RZMBs.

Summary Redox flow batteries have great system scalability and operational flexibility but relatively low energy density because of the limited solubility of redox molecules in the electrolytes. By storing energy in solid materials while producing power from redox fluids, the redox-targeting concept provides an effective way to significantly increase the energy density of flow batteries. Redox targeting generally involves multiple redox reactions between the molecules and materials, which inevitably brings about additional complexity in electrolyte composition and low voltage efficiency. The Nernstian-potential-driven redox-targeting reaction reported here considerably eliminates the voltage loss of a LiFePO 4 -based redox flow lithium battery. Driven by the Nernstian potential difference, the redox molecule, a ferrocene-grafted ionic liquid with standard potential identical to that of LiFePO 4 , reacts with the solid material both anodically and cathodically and exhibits near-unity material utilization, a voltage efficiency of 95%, and an energy density of 330 Wh/L (up to 942 Wh/L).

Redox flow batteries, despite great operation flexibility and scalability for large-scale energy storage, suffer from low energy density and relatively high cost as compared to the state-of-the-art Li-ion batteries. Here we report a redox flow lithium battery, which operates via the redox targeting reactions of LiFePO4 with a bifunctional redox mediator, 2,3,5,6-tetramethyl-p-phenylenediamine, and presents superb energy density as the Li-ion battery and system flexibility as the redox flow battery. The battery has achieved a tank energy density as high as 1023 Wh/L, power density of 61 mW/cm2, and voltage efficiency of 91%. Operando X-ray absorption near-edge structure measurements were conducted to monitor the evolution of LiFePO4, which provides insightful information on the redox targeting process, critical to the device operation and optimization.

The ever-increasing penetration of solar energy demands high energy density, safe and affordable energy storage systems. We demonstrated a redox flow battery on the basis of the redox targeting reactions of a bifunctional redox mediator – ethyl viologen diiodide (EVI2) with a low-cost pigment material – Prussian blue (PB). Sharing the same I–/I3– based electrolyte, this new battery system can be feasibly integrated with a dye-sensitized TiO2 photoelectrode for photoassisted charging process. The introduction of PB not only increases the energy density up to 117 Wh/L with good cycling performance and capacity retention, also instantaneously regenerates I– ensuring a stable I–/I3– concentration which is crucial to the operation of the photoelectrode. We anticipate that the work presented here may open an intriguing and credible way for the development of integrated all-in-one solar energy conversion and storage devices for large-scale applications.

Water contamination is generally considered to be detrimental to the performance of aprotic lithium–air batteries, whereas this view is challenged by recent contrasting observations. This has provoked a range of discussions on the role of water and its impact on batteries. In this work, a distinct battery chemistry that prevails in water-contaminated aprotic lithium–oxygen batteries is revealed. Both lithium ions and protons are found to be involved in the oxygen reduction and evolution reactions, and lithium hydroperoxide and lithium hydroxide are identified as predominant discharge products. The crystallographic and spectroscopic characteristics of lithium hydroperoxide monohydrate are scrutinized both experimentally and theoretically. Intriguingly, the reaction of lithium hydroperoxide with triiodide exhibits a faster kinetics, which enables a considerably lower overpotential during the charging process. The battery chemistry unveiled in this mechanistic study could provide important insights into the understanding of nominally aprotic lithium–oxygen batteries and help to tackle the critical issues confronted., Water is believed to undermine the performance of aprotic lithium–air batteries. However, the authors here disclose different battery chemistry, showing that both lithium ions and protons are involved in the battery reactions in the presence of water, leading to an unprecedented dynamic product.

We demonstrate a redox flow lithium oxygen battery (RFLOB) using a pair of soluble redox catalysts, 2,5-di-tert-butyl-p-benzoquinone (DTBBQ) and tris{4-[2-(2-methoxyethoxy)ethoxy]phenyl}amine (TMPPA). The catalytic effects of DTBBQ on the oxygen reduction reaction (ORR) and TMPPA on the oxygen evolution reaction (OER) were investigated and unambiguously substantiated with various electrochemical, morphological, and spectroscopic characterization methods. It is observed that, upon discharging, oxygen is rapidly reduced by DTBBQ•– and Li2O2 is formed in the presence of Li+; upon charging, Li2O2 is oxidized by TMPPA•+, releasing oxygen. Such redox-mediated ORR and OER reactions enable the formation and oxidation of Li2O2 in a separate gas diffusion tank (GDT) other than on the cathode of the cell, thus obviating surface passivation and pore clogging of the electrode. The cell presents a high energy density with DTBBQ as the ORR redox catalyst and good rechargeability when paired with TMPPA as the OER redox c...

A TCO (transparent conductive oxide)-free redox-flow electrochromic window (RFEW) with considerably improved performance is demonstrated. The new RFEW employs Prussian blue (PB)/Prussian white (PW) as the electrochromic (EC) material, in which the decoloration/coloration is achieved by the redox targeting reactions of anthraquinone-2,6-disulfonate (AQDS) and FeCl3 with PB/PW, respectively. These redox molecules are electrochemically oxidized/reduced in an external stack cell and circulated through the window cavity. Hence, the costly TCO glass, indispensable in conventional EC windows, becomes redundant in RFEWs. Owing to the fast charge transfer kinetics of AQDS and Fe3+, the RFEW design investigated in this study presents substantially reduced response time. For a window with a size exceeding 100 cm2, coloration/decoloration switching with a change in absorbance by more than 80% is achieved within approximately 2 min. The static and dynamic optical characteristics of the window pane and the electrochemical behavior of the stack cell were examined in detail to understand the operation of the RFEW.

The development of rechargeable Li–air batteries has been confronted by the critical challenges of large overpotential loss, low achievable capacity, and prohibitively poor cycling and power performance. Surface passivation and pore clogging of the cathode due to the formation of Li2O2 during discharge result in sluggish interfacial charge transfer and have an impact on the mass transport of Li+ ions and O2 in the electrode, consequently giving rise to large voltage hysteresis and premature termination of discharge with low power performance. Here we report a redox flow lithium–oxygen cell with a modified redox electrolyte to tackle these issues. With the assistance of redox mediators, the cell presents substantially enhanced power performance in O2 and dry air during discharge. Through in situ spectroelectrochemical measurements and theoretical calculations, an oxygen reduction intermediate was unequivocally identified. By judiciously optimizing the redox electrolyte, the cell operates at near complete utilization of Li metal upon multiple refueling. The redox flow lithium–oxygen cell demonstrated here is envisaged to provide a pragmatic approach for the implementation of lithium–oxygen battery chemistry and to pave the way for advanced large-scale energy storage.

In this work, SnO2 quantum dots and graphene (G) nanocomposites (SnO2/G) were synthesized via a facile one-step hydrothermal method for the application of anode material in lithium ion batteries (LIBs). SnO2 quantum dots with a diameter ranging from 5 to 10 nm were uniformly anchored on graphene nanosheets. SnO2/G nanocomposites can deliver high specific capacities of 860 mAh g−1 after 50 cycles at a current density of 200 mA g−1, and 683 mAh g−1 at a current density of 1600 mA g−1. Compared to the pure SnO2, the excellent improved electrochemical performance of SnO2/G nanocomposites for lithium ion storage can be attributed to the stable structure and the improved electron conduction channel provided by graphene with functional groups (FGs), such as C-OH, C=O and C-OOH. In order to further improve the SnO2/G nancomposites electrochemical performance, carbon thin layers were coated by the reduction of C2H2 gas via CVD technology (SnO2/G-C). However, SnO2/G-C showed an inferior performance compared to that of the SnO2/G without carbon coating. Through X-ray photoelectron spectroscopy (XPS) analysis, it was found that function groups play a critical role on the stability of SnO2/G nanocomposites. Such observation may thus instruct the design and implementation of graphene/metal oxides nanocomposites for high-performance LIBs anode materials.

Redox flow batteries, despite great operation flexibility and scalability for large-scale energy storage, suffer from low energy density and relatively high cost as compared to the state-of-the-art Li-ion batteries. Here we report a redox flow lithium battery, which operates via the redox targeting reactions of LiFePO

Miniaturized energy storage devices have attracted considerable research attention due to their promising applications such as power-on-chip units in various smart electronic devices. In this work, a printable micro-supercapacitor (MSC) device was designed and fabricated wherein a novel three dimensional (3D) nanocomposite consisting of cobalt oxide (CoO) nanoflowers woven with carbon nanotubes (CNTs) networks were used as the active material. The CoO/CNT nanocomposites were synthesized via a high-throughput hydrothermal method. High capacitance of 17.4 F/cm 3 and energy density of ~3.48 mWh/cm 3 were achieved for the CoO/CNT MSC at a current density of 0.25 A/cm 3 . The high volumetric energy density is attributed to the widened operation voltage window ranging from 0 to 1.2 V. Moreover, the printed CoO/CNT MSCs also showed remarkable cycling stability with ~85% energy density retention after 1700 cycles and high mechanical flexibility which can function well even after bending up to 180°. As a result, the printed CoO/CNT MSC is a possible contender in future energy storage devices for low-cost on-chip power applications.

In this work, we succeeded in synthesis of spinel LiMn2O4 via a facile self-template method. The product displays a micro-/nanohybrid structure. Nanoparticles/plates act as the primary nanoblocks to build the secondary microarchitecture. There is the open space between the nanoblocks and the void space between the secondary structures. Electrochemical tests demonstrate that the as-synthesized sample exhibits superior rate capability and high-rate cycleability when contrasted with its solid counterpart. The initial discharge capacity is 126 mAh/g at 0.1 C, 110 mAh/g at 10 C, and 84 mAh/g at 20 C. The discharge capacity retention of about 80% is obtained after 800 cycles at 10 C. The high capacity and excellent cycling life of the material shows its potential for application as high-power batteries. The improved rate capability and cycleability can be attributed to its secondary structure that can facilitate fast Li-insertion/extraction and buffer the volume expansion/contraction upon cycling.

A Co(OH)2/graphene sheet-on-sheet hybrid has been fabricated by in situ one-step hydrothermal growth for electrochemical pseudocapacitors application. The hybrid delivers a specific capacitance of 436 F g−1 at a current density of 50 A g−1. Besides, it can keep a specific capacitance of 651 F g−1 after 10,000 cycles at 10 A g−1. The excellent performance can be ascribed to the high-quality graphene matrix, regular morphology and high crystallinity of Co(OH)2, and unique sheet-on-sheet structure of the hybrid, endowing enhanced transportation of electrons and Faradic redox reactions. The results demonstrate that the Co(OH)2/graphene hybrid with a sheet-on-sheet structure is promising for high-performance energy storage applications.

An all redox flow lithium battery with strikingly high energy density is successfully demonstrated., Redox flow batteries (RFBs) are considered one of the most promising large-scale energy storage technologies. However, conventional RFBs suffer from low energy density due to the low solubility of the active materials in electrolyte. On the basis of the redox targeting reactions of battery materials, the redox flow lithium battery (RFLB) demonstrated in this report presents a disruptive approach to drastically enhancing the energy density of flow batteries. With LiFePO4 and TiO2 as the cathodic and anodic Li storage materials, respectively, the tank energy density of RFLB could reach ~500 watt-hours per liter (50% porosity), which is 10 times higher than that of a vanadium redox flow battery. The cell exhibits good electrochemical performance under a prolonged cycling test. Our prototype RFLB full cell paves the way toward the development of a new generation of flow batteries for large-scale energy storage.

A redox flow lithium-oxygen battery (RFLOB) by using soluble redox catalysts with good performance was demonstrated for large-scale energy storage. The new device enables the reversible formation and decomposition of Li2O2 via redox targeting reactions in a gas diffusion tank, spatially separated from the electrode, which obviates the passivation and pore clogging of the cathode.

The catalytic role of germanium (Ge) was investigated to improve the electrochemical performance of tin dioxide grown on graphene (SnO(2)/G) nanocomposites as an anode material of lithium ion batteries (LIBs). Germanium dioxide (GeO(20) and SnO(2) nanoparticles (

In this work, we report the synthesis of MoO2 nanoparticles grown on three dimensional graphene (3DG) via the reduction of α-MoO3 nanobelts through a facile chemical vapor deposition (CVD) approach under argon protection gas environment. In this synthesis approach, the presence of hydrophobic 3DG promoted the Volmer-Weber growth of MoO2 nanoparticles (30-60 nm). The as-prepared MoO2-3DG nanocomposite was directly used as a binder-free anode electrode for lithium ion batteries (LIBs) without additives and exhibited excellent electrochemical performance. It delivered high reversible capacities of 975.4 mA h g(-1) and 537.3 mA h g(-1) at the current densities of 50 and 1000 mA g(-1), respectively. Moreover, the electrode also showed an increased capacity from 763.7 mA h g(-1) to 986.9 mA h g(-1) after 150 discharge and charge cycles at a current density of 200 mA g(-1). The enhanced electrochemical performance of MoO2-3DG nanocomposite electrode may be attributed to the synergistic effects of MoO2 nanoparticles and 3DG layers. This facile CVD synthesis process presents a feasible route for large-scale production of high performance, environmentally friendly electrode. In addition, this process also has the potential of being utilized in other materials for energy storage devices application.

Lithium oxygen battery (LOB), as a promising next generation energy storage system, has attracted tremendous attention. However, for aprotic systems due to the deposition of insoluble and insulating discharging product Li2O2, surface passivation and pore clogging of the cathode occur. As a result, the charging overpotential of LOB is generally intolerably high. While people have tried to use homogeneous catalysts — redox mediators dissolved in the electrolyte to address the above issues, the passivation and clogging problems remain especially at deep discharge. Inspired by the redox targeting concept in redox flow lithium battery, we have recently proposed a new battery concept — redox flow lithium oxygen battery (RFLOB), which elegantly resolves the surface passivation and pore clogging issues.1-3 As shown in Figure 1, with a pair of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) redox catalysts, the formation of Li2O2 during discharge and oxidation during charge could be performed in a separate gas diffusion tank rather than on the cathode in the cell. Therefore, the passivation and clogging problems of the electrode are feasibly obviated.4 Here, we will report a series of robust redox mediators as efficient ORR and OER catalysts in the RFLOB system. By optimizing the electrolyte compositions, extremely low charging overpotential (~0.20 V) has been achieved, which results of unprecedently high voltage efficiency. The underlying mechanism of the redox catalytic processes toward ORR and OER will be discussed in detail. We anticipate RFLOB provide a new vista to the development of Li-air battery with high round-trip efficiency for practical applications. Figure 1. The structure of a redox flow lithium oxygen battery. Reference: (1) Wang, Q.; Zakeeruddin, S. M.; Wang, D.; Exnar, I.; Grätzel, M. Angewandte Chemie International Edition 2006, 45, 8197. (2) Huang, Q.; Li, H.; Gratzel, M.; Wang, Q. Physical Chemistry Chemical Physics 2013, 15, 1793. (3) Pan, F.; Yang, J.; Huang, Q.; Wang, X.; Huang, H.; Wang, Q. Advanced Energy Materials 2014, 4, n/a. (4) Zhu, Y. G.; Jia, C.; Yang, J.; Pan, F.; Huang, Q.; Wang, Q. Chemical Communications 2015. Figure 1

A challenge still remains to develop high-performance and cost-effective air electrode for Li-O2 batteries with high capacity, enhanced rate capability and long cycle life (100 times or above) despite recent advances in this field. In this work, a new design of binder-free air electrode composed of three-dimensional (3D) graphene (G) and flower-like δ-MnO2 (3D-G-MnO2) has been proposed. In this design, graphene and δ-MnO2 grow directly on the skeleton of Ni foam that inherits the interconnected 3D scaffold of Ni foam. Li-O2 batteries with 3D-G-MnO2 electrode can yield a high discharge capacity of 3660 mAh g−1 at 0.083 mA cm−2. The battery can sustain 132 cycles at a capacity of 492 mAh g−1 (1000 mAh gcarbon −1) with low overpotentials under a high current density of 0.333 mA cm−2. A high average energy density of 1350 Wh Kg−1 is maintained over 110 cycles at this high current density. The excellent catalytic activity of 3D-G-MnO2 makes it an attractive air electrode for high-performance Li-O2 batteries.

Development of a pseudocapacitor over the integration of metal oxide on carbonaceous materials is a promising step towards energy storage devices with high energy and power densities. Here, a self-assembled cobalt oxide (CoO) nanorod cluster on three-dimensional graphene (CoO-3DG) is synthesized through a facile hydrothermal method followed by heat treatment. As an additive-free electrode, CoO-3DG exhibits good electrochemical performance. Compared with CoO nanorod clusters grown on Ni foam (i.e., CoO-Ni, ≈680 F g−1 at 1 A g−1 and ≈400 F g−1 at 20 A g−1), CoO-3DG achieves much higher capacitance (i.e., ≈980 F g−1 at 1 A g−1 and ≈600 F g−1 at 20 A g−1) with excellent cycling stability of 103% retention of specific capacitance after 10 000 cycles. Furthermore, it shows an interesting activation process and instability with a redox reaction for CoO. In addition, the phase transformation from CoO nanorods to Co3O4 nanostructures was observed and investigated after charge and discharge process, which suggests the activation kinetics and the phase transformable nature of CoO based nanostructure. These observations demonstrate phase transformation with morphological change induced capacitance increasement in the emergent class of metal oxide materials for electrochemical energy storage device.

In this contribution, nickel oxide (NiO) nanoflakes/graphene (NiO/G) nanocomposites have been prepared by a simple hydrothermal method followed by a thermal treatment with N2 gas. NiO nanoflakes (∼30–80 nm in diameter) are uniformly anchored on graphene sheets in a layer-by-layer form, which effectively prevents the aggregation of NiO nanoflakes and offers two-dimensional (2D) diffusion channels for the transportation of electrons and ions. Compared to bare NiO nanoflakes, the NiO/G composite electrode exhibits improved electrochemical properties. The specific capacitances of the NiO/G electrode are 240 F g−1 at 5 A g−1 and 220 F g−1 at 10 A g−1, which are much higher than those of the NiO electrode (i.e., 100 F g−1 at 5 A g−1 and 90 F g−1 at 10 A g−1). In addition, the synergistic effect from this hybrid structure has led to the significantly improved cycling stability of the NiO/G supercapacitor, which exhibits a superior cycling stability of 100–120% retention of specific capacitance after 1500 cycles. This approach may advance the design and implementation of hybrid nanostructures in high-performance reversible supercapacitors.

In this work we present a controllable synthesis of hollow nanostructures of α-Fe2O3 by a facile hydrothermal route using FeCl3·6H2O as the iron source and NH4H2PO4 as the shape-directing agent. We found that the morphology of α-Fe2O3 experienced a continuous change from thin nanospindles to flat nanodisks via finely tuning the phosphate ion concentration due to the selective absorption of phosphate ions onto the Fe2O3 nanostructures. At a given phosphate ion concentration, prolonging the reaction time led to the formation of hollow nanostructures (nanotubes, nanobeads, and nanorings) due to the dissolution and re-crystallization of Fe2O3. We also found that morphology-preserved conversion from α-Fe2O3 to magnetic Fe3O4/C can be achieved during the CVD reactions in C2H2. The Fe3O4/C nanocomposites with uniformly coated carbon layers exhibited morphology-dependent magnetic properties.