Cathode Material For Li-O2 Batteries Research Articles

High-throughput computational screening of cathode materials for Li-O2 battery

Despite the tremendous specific energy of Li-O2 energy storage technology in theory, the practically achieved values are still inferior to compete with conventional Li-ion batteries. A key limitation is the inability to use thick gas diffusion electrodes made of carbonaceous materials because of the passivation by discharge products. In this study, a computational methodology for searching alternative positive electrode (cathode) materials for Li-O2 batteries based on density functional theory is proposed. The methodology includes a three-stage screening with a sequential selection of the best candidates according to their stability, band gap (electronic conductivity), normalized surface energies (stability in a highly porous form), and oxygen adsorption energies (resistance towards oxidation and passivation). The methodology is applied for the AxBy series of binary compounds with A = Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Ru, Ag, Mg, Al and B = B, C, N, O, Si, S and benchmarked with graphite. As a result, 33 potential compounds with low normalized surface energies are identified out of 2800 initial structures taken from the material database. It is shown that only several oxides can be stable towards surface oxidation among the selected compounds and thus considered as potential cathode materials.

Accelerating the Oxygen Reduction Reaction and Oxygen Evolution Reaction Activities of N and P Co-Doped Porous Activated Carbon for Li-O2 Batteries

Rechargeable lithium–oxygen (Li-O2) batteries represent state-of-the-art electrochemical energy storage devices that provide high energy densities. However, their commercialization is challenging owing to their low charging/discharging efficiencies, short battery lives, high overpotentials, and high cathode manufacturing costs. In this study, we prepared a metal-free, N,P co-doped, porous activated carbon (N,P-PAC) electrode via KOH activation and P doping for application as a Li-O2 battery cathode. When used in a rechargeable Li-O2 battery, the N,P-PAC cathode showed a high specific discharge capacity (3724 mA h g−1 at 100 mA g−1), an excellent cycling stability (25 cycles with a limit capacity of 1000 mA h g−1), and a low charge/discharge voltage gap (1.22 V at 1000 mA h g−1). The N,P-PAC electrode showed a low overpotential (EOER-ORR) of 1.54 V. The excellent electrochemical performance of the N,P-PAC electrode can mainly be attributed to its large active area and oxygen-containing functional groups generated via KOH activation and P-doping processes. Therefore, the N,P-PAC prepared in this study was found to be a promising eco-friendly and sustainable metal-free cathode material for Li-O2 batteries.

Ultra-small MnCo2O4 nanocrystals decorated on nitrogen-enriched carbon nanofibers as oxygen cathode for Li-O2 batteries

Ultra-small MnCo2O4 nanocrystals/nitrogen enriched carbon nanofiber composites, with particle size as small as 2–4[Formula: see text]nm and nitrogen content as high as [Formula: see text][Formula: see text]at.%, were designed and used as oxygen cathode materials for Li-O2 batteries, with an aprotic electrolyte. Via an in-situ nucleation and growth, the morphology, size and distribution of MnCo2O4 nanocrystals were well controlled on surface of nanofibers, with strong interfacial interactions between the two components. Benefitting from the unique microstructure and high-level nitrogen doping, the MnCo2O4/NCF composite can deliver discharge and charge capacities of 4147.8 and 3842.8[Formula: see text]mAh/g as oxygen cathode materials, and columbic efficiencies are about 92.6%. More importantly, the discharge products can completely decompose in charging process as evidenced by an ex-situ FESEM investigation, while for pure NCF cathode, particle and plate-like Li2O2 were still observed on its surface, which confirmed that the MnCo2O4/NCF composite has a superior electrocatalytic activity that that of NCF.

Preparation of low-load Au-Pd alloy decorated carbon fibers binder-free cathode for Li-O2 battery

Developing high-performance cathode is critical to facilitating the development of lithium-oxygen (Li-O2) batteries. In this work, a low-load (1.32%) Au-Pd alloy decorated carbon fibers binder-free cathode is prepared by facile magnetron sputtering (MS). It is the first use as a cathode material for Li-O2 batteries, and exhibits excellent electrochemical performance. During the second to the 30th cycle of the battery operation at a current density of 100 mA g−1 with a limited specific capacity density of 1000 mAh g−1, the charge and discharge polarization voltage is only about 0.6 V. In addition, the problem of abnormal charge polarization voltage in the first cycle is also investigated. The by-products Li2CO3 are formed during the first discharge, resulting in a significant increase in the charge polarization voltage. The facile preparation method we adopted and our findings may provide new ideas for the future development of Au-Pd alloy composites in Li-O2 or other metal-air batteries.

A free-standing CeO2/Co3O4 nanowires electrode featuring a controllable discharge/charge product evolution route with enhanced catalytic performance for Li-O2 batteries

Although transition metal oxides are important potential catalytic cathode materials for Li-O2 batteries (LOBs), their poor cycle durability at high current density, high overpotentials and side reaction are still the challenges to solve. Herein, CeO2/Co3O4 nanowire arrays grown on Ni foam were fabricated as a free standing cathode of LOBs, featuring a controllable discharge/charge products evolution route. CeO2 served as active sites for nucleation, initial growth and decomposition of Li2O2. The embedded CeO2 nanocrystalline on Co3O4 substrate dominated the initial discharge/charge product evolution with multi-formation kinetics of crystal Li2O2 and Li2-xO2 at high current densities which leading to low overpotentials and efficient decomposition of discharge products. Owing to the stable structure, the CeO2/Co3O4 nanowires were found to energetically favor the mass transport between the electrode/electrolyte interface during long cycle testing. As a consequence, excellent cyclability of 500 cycles at high current density (500 mA g−1) under a fixed capacity of 500 mA h g−1 with low overpotentials of 0.2 V and 1.0 V for discharge/charge process (after 500 cycles) were achieved. The present work provides a new strategy and intrinsic insight in designing high-performance metal oxides electrocatalysts with a fine-tuned structure for LOBs.

Se-doped carbon as highly stable cathode material for high energy nonaqueous Li-O2 batteries

Hetero-doped carbon materials have been generally applied as the cathode materials for Li-O2 batteries due to the improved catalytic activity. However, they commonly suffer from performance fading because of carbon corrosion under harsh oxidative environments in Li-O2 batteries. Herein, an elegant cathode material for Li-O2 batteries is developed through doping Se in carbon. The results show that the Se-doped carbon cathode reveals 53% higher discharge capacity, decreased overpotential and superior rate capability. Structure evolution analysis and density functional theory calculation reveal that the products mainly grow via solution driven mechanism, which can better sustain discharge. Se-doped carbon cathode also presents high stability due to its prominent antioxidant ability, efficiently suppressing the side reaction and cathode passivation.

Enhancing the electrocatalytic activity of 2D micro-assembly Co3O4 nanosheets for Li–O2 batteries by tuning oxygen vacancies and Co3+/Co2+ ratio

The Li–O2 batteries (LOBs) have arouse worldwide interests due to their superior energy density and environmental sustainability. The researches on Li–O2 battery electrocatalysts can contribute to the development of Li–O2 batteries. Among various catalysts, Co3O4 has arouse some interests owing to its high abundance, low cost and bifunctional activity. In this study, a vacancy engineering method was adopted to tune the contents of oxygen vacancy and metal ion vacancy in 2D micro-assembly Co3O4 nanosheets through changing annealing temperature. The synergistic effects of mesoporous morphology, oxygen vacancy and higher Co3+ content induce the best cycle performances of the Li–O2 cell, because structural defects can create a pathway for O2 to enter the interior of the 2D micro-assembly Co3O4 nanosheets, serve as active sites and thus accelerate ORR, and the diffusion of oxygen and electrolyte can be assisted by mesopores in Co3O4 nanosheets to prevent channel clogging by discharge products and facilitate rapid mass of oxygen and lithium ions, and the large surface area of nanosheets can accommodate substantial discharge products. This work offers new horizons in the consideration and design of transition-metal oxides as cathode materials for LOBs.

Synthesis of Cobalt Manganese Oxide-Carbon Nanotubes Composite Microspheres As a Bifunctional Cathode Catalyst for Li-O2 Batteries

Although lithium-ion batteries (LIBs) have been widely used in electronic devices, intercalation limits their energy densities, rendering them incapable of meeting the increasing demand for high energy density. Lithium-oxide (Li-O2) batteries are appealing energy storage and conversion devices owing to their remarkably high energy densities (~3500 Wh kg-1), ~8 times higher than those of current LIBs. However, despite great promise, Li-O2 batteries still suffer from low round-trip efficiency, poor cycle life, and voltage decay. It has been revealed that this limited electrochemical performance is mainly attributed to the sluggish kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) at the oxygen electrode. The electrochemical performance of Li-O2 batteries is highly dependent on both the materials and architecture of the oxygen electrode. Transition metal oxide nanomaterials have attracted enormous interest as low-cost alternative catalysts to noble-metal catalysts capable of catalyzing both the ORR and OER. In particular, materials with mixed metal compounds play an important role in electrocatalytic reactions, since mixed valence species enhance electrocatalytic activity. However, the poor electronic conductivity limits the catalytic activity of the mixed-valant transition metal oxide. Therefore, nanostructured materials comprising efficient catalyst nanocrystals and carbon materials with high electrical conductivity, high electrocatalytic activities for the ORR and OER, and a well-designed porous structure to accommodate the reaction products have to be developed. A commercial air electrode should be formed by a simple printing process from slurry containing the cathode powders, organic binder, and solvent. Macroporous cathode powders with uniform composition, several microns in size, spherical morphology, and narrow size distribution should be used to form the uniform air electrode in the commercial process. However, studies on cathode microspheres with optimum morphological characteristics and high electrocatalytic activities for ORRs and OERs for Li–O2 batteries are rare. In this study, Mn-Co-O catalyst was studied as electrocatalyst showing high efficiencies for the ORR and OER for Li-O2 batteries. In addition, the optimum structure of carbon nanotube (CNT) microspheres was designed as efficient support material for catalysts with high electrical conductivity and high accommodation ability for Li2O2 products. Consequently, novel and uniquely structured CNT microspheres decorated with Mn-Co-O nanocrystals with a high electrocatalytic activity were designed and synthesized as cathode material for high performance Li–O2 batteries. Macroporous Mn-Co-O-CNT composite microspheres designed as efficient cathode material for Li-O2 batteries were prepared by one-pot spray pyrolysis. The macroporous structure of CNTs facilitated the wetting of the electrolyte and diffusion of oxygen within the electrode and provided plenty of space to accommodate Li2O2 species formed during the discharge process. The Li-O2 batteries with macroporous Mn-CO-O-CNT composite microspheres showed superior properties as cathode material. The new concept studied in this project can be efficiently applied to the development of highly efficient cathode materials for advanced Li-O2 batteries. Figure 1

Highly Enhanced Cycleability from High Crystalline Biaxially Aligned CNT Web for Li-Air Cathode Applications

In recent years, Li-O2 batteries have attracted a great deal of attention due to the high theoretical capacity for applications as electronic vehicles. Carbon nanotubes are widely used as cathode materials for Li-O2 batteries because of their high conductivity, high surface area and low density. However, due to their hydrophobic surface properties and the high van der Waals interaction between their walls, they tend to aggregate on their own, failing full use of the CNT surface. Therefore, it is necessary to engineer the nanoscale space between the CNTs such that each CNT aligns with each other and the nanoscale spacing lies between them. In this way, it is possible for the ionic liquid electrolyte and oxygen to access individual CNTs and enable uniform Li2O2 formation. In this research, we developed a one-step process for biaxially oriented, highly crystalline CNT fabrics using a combination of floating catalyst chemical deposition (FCCVD) and one-step direct spinning technology. A highly crystalline DWCNT with a diameter of 6 nm was synthesized by FCCVD using ferrocene, thiophene, and methane, as precursors, and as-spun CNTs were woven into fabric by 2-directional spinning process. The intensity ratio between D and G bands (ID/IG) of 0.06 showed high crystallinity of CNT surface. The microscopic architectures of randomly oriented CNT fabric and biaxially oriented CNT fabric were investigated using a high-resolution scanning electron microscope (See Figure 1.(a) and (b)). The biaxial orientation was reconfirmed using a polarized Raman spectroscopy. In order to investigate the effect of the microscopic structure on the performance of the Li-O2 battery, we have assembled Li-O2 cells using a random CNT fabric and a biaxially aligned CNT fabric as a cathode. In the random CNT cathode, the charging overpotential gradually increases, leading to rapid decay of capacity upon cycling (Fig. 1(c)). The increase in overpotential is typically associated with the accumulation of insulating byproducts due to parasitic reactions involving the electrolyte or the carbon electrode. However, in the biaxially aligned CNT electrode, a much improved cycle stability and small charging overpotentials are demonstrated (Fig. 1.(d)). This dramatically improved cycling performance might result from the increase in the surface area corresponding to mesopores, providing more effective sites capable of carrying discharge products such as Li2O2. In addition, Li-O2 cathode using biaxially aligned CNT fabric can offer open framework in which O2 and electrolytes can access to the individual CNTs, facilitating reversible Li2O2 formation and removal, and consequently exhibiting enhanced cyclability. Figure 1

Highly efficient hierarchical multiroom-structured molybdenum carbide/carbon composite microspheres grafted with nickel-nanoparticle-embedded nitrogen-doped carbon nanotubes as air electrode for lithium-oxygen batteries

Designing an efficient and effective air electrode catalyst material is the most intrinsic requisite for rechargeable lithium-oxygen (Li-O2) batteries showing long cycling lives and high rate capacities. Here, we present for the first time, hierarchical multiroom-structured molybdenum carbide/carbon composite microspheres grafted with Ni nanoparticle-embedded N-doped carbon nanotubes (NCNTs) (i.e., mNi-NCNT-MoC-C), which are prepared by pilot-scale spray drying and subsequent surface growth of NCNT bundles. The designed mNi-NCNT-MoC-C microspheres show efficient bifunctional catalytic activities toward both oxygen reduction and evolution. In addition, the hierarchical multiroom structure of mNi-NCNT-MoC-C microspheres increases the discharge capacity by providing sufficient space to accommodate Li2O2, which forms during discharging. A Li-O2 battery prepared using mNi-NCNT-MoC-C microspheres as an air electrode exhibits excellent electrochemical performances including a long cycle life (199 cycles) and low overpotentials of 0.20 and 0.21 V for charging and discharging, respectively. The synergetic effects of efficient morphology (i.e., the multiroom structure), highly electrocatalytically active materials (i.e., NCNT, Ni, and MoC), and high electrical conductivity (imparted by the NCNTs) is responsible for the superior performance of mNi-NCNT-MoC-C microspheres as an air cathode material for Li-O2 batteries.

3D-ordered porous nitrogen and sulfur Co-Doped carbon supported PdCuW nanoparticles as efficient catalytic cathode materials for Li-O2 batteries

3D-ordered porous nitrogen and sulfur co-doped carbon (NS/C) is synthesized from the functionalized fluidic acrylonitrile telomer (ANT) as C, N and S source. The dispersed PdCuW nanoparticles with an average size of 5.25 nm are supported on the walls of the NS/C by a glycol reduction method. Compared with 20%Pd black (Pd/C), NS/C and 20%Pd-NS/C, it is found that PdCuW-NS/C composite with 11.4 wt% Pd exhibits superior catalytic activity for oxygen reduction reaction (ORR) in alkaline solution. Moreover, PdCuW-NS/C possesses higher capacities, lower overpotentials and better cyclic stability as cathode materials for Li-O2 batteries; the cyclic retention of more than 70 cycles are achieved at 300 mA g−1 with the restricted capacity at 500 mAh g−1.

MnCo2 O4 /MoO2 Nanosheets Grown on Ni foam as Carbon- and Binder-Free Cathode for Lithium-Oxygen Batteries.

Carbon is usually used as cathode material for Li-O2 batteries. However, the discharge product, such as Li2 O2 and LiO2 , could react with carbon to form an insulating lithium carbonate layer, resulting in cathode passivation and capacity fading. To solve this problem, the development of non-carbon cathodes is highly desirable. Herein, we successfully synthesized MnCo2 O4 (MCO) nanoparticles anchored on porous MoO2 nanosheets that are grown on Ni foam (current collector) (MCO/MoO2 @Ni), acting as a carbon- and binder-free cathode for Li-O2 batteries, in an attempt to improve the electrical conductivity, electrocatalytic activity, and durability. This MCO/MoO2 @Ni electrode delivers excellent cyclability (more than 400 cycles) and rate performance (voltage gap of 0.75 V at 5000 mA g-1 ). Notably, the battery with this electrode exhibits a high energy efficiency (higher than 85 %). The advanced electrochemical performance of MCO/MoO2 @Ni can be attributed to its high electrical conductivity, excellent stability, and outstanding electrocatalytic activity. This work offers a new strategy to fabricate high-performance Li-O2 batteries with non-carbon cathode materials.

Theoretical prediction of LiScO2 nanosheets as a cathode material for Li-O2 batteries.

Our previous study (J. Mater. Chem. A, 2018, 6, 3171-3180) theoretically predicted that a scandium oxide (ScO2) monolayer can deliver high specific capacity and energy density as the active material of a lithium-ion (Li-ion) battery, but the voltage will drop below 0.5 V when ScO2 is lithiated to LiScO2 during the discharge process. The current study predicts that the discharge product LiScO2 in the Li-ion battery mode can potentially work as the host material of Li-O2 batteries. It is found that the adsorption of O2 on the LiScO2 substrate is energetically favored. The LiScO2 substrate can also provide strong affinities to molecular LiO2 and Li2O2 species. It is interesting to find that the presence of an O2 molecule can oxidize the pre-adsorbed Li2O2 molecule and result in two LiO2 molecules. Hence, the final discharge product of the Li-O2 battery using the LiScO2 cathode is expected to be a crystalline-like LiO2 layer. The discharge voltage related to forming a LiO2 layer on the LiScO2 substrate is 3.50 V vs. Li+/Li according to the present theoretical calculation.

Pyridinic-N-dominated carbon frameworks with porous tungsten trioxide nano-lamellae as a promising bi-functional catalyst for Li-oxygen batteries.

The rational design and synthetic route to fabricate hybrid materials with desirable electrocatalytic functionalities remain critical but still challenging for sustainable energy devices. Here, we constructed a tungsten trioxide nano-lamellae chemically anchored with pyridinic-N-dominated doped CNT/graphene frameworks (W-NCG) via a general solution-based synthesis method. The detailed results indicated that this hybrid structure is composed of vacancy-defect abundant WO3 porous nanoflakes anchoring through or onto a 3D N-doped carbon matrix. After a facile post-annealing treatment, the W-NCG sample is utilized as a bi-functional catalyst for rechargeable lithium-oxygen batteries. The optimized sample with a large BET surface exhibits unprecedented ORR/OER activity in the cell, and satisfying specific capacity (∼7850 mA h g-1) and cycling stability. This excellent electrochemical performance can be ascribed to the pseudo 3D structure with sufficient microspace and good electrical conductivity, which facilitate the high dispersion of active components and effectivly relieve the formation of large/irreversible Li2O2. As such, this porous W-NCG framework is a prospective high-performance cathode material for Li-O2 batteries.

Nanoarchitectured CNTs-Grafted Graphene Foam with Hierarchical Pores as a Binder-Free Cathode for Lithium-Oxygen Batteries

In this study, a three-dimensional CNTs-grafted graphene foam has been fabricated by a chemical vapor deposition method. The nanoarchitectured composite is employed as a cathode for lithium-oxygen (Li-O2) batteries and exhibits a substantially enhanced capacity of 20300 mAh g−1 at 0.05 mA cm−2, the number of which is among the highest prototype Li-O2 batteries reported to date. And the cathode can still exhibit a capacity of 3000 mAh g−1 when the current density is increased to 0.15 mA cm−2. The improved performance, e.g., discharge capacity and rate capability, is mainly attributed to the synergistic effects of CNTs and graphene, which not only provide an effective channel for the transportation of oxygen and Li ion but also offer more and fast pathways for electron transport. In addition, the forest distribution of CNTs hinders the restack of graphene, contributing to the stability of Li-O2 batteries. This study demonstrated that the CNTs-grafted graphene foams would be likely to be a good alternative cathode material for Li-O2 batteries in the future.

Low Cost FeCo Bimetal Cathode for Li-Air Batteries

In spite of the extremely high theoretical specific capacity of lithium oxygen (Li-O2) electrochemistry,1 low energy efficiency makes this system impractical. Among the catalysts, noble metals and their derivatives have shown improved results with low overpotentials during an oxygen evolution reaction (OER).2 Moreover, bimetal noble metal alloys have also exhibited remarkable decreases of charge overpotentials.3 However, use of noble metal based catalysts makes overcoming their inherent high cost impossible. Therefore, low cost catalysts with acceptable performance are essential for practical Li-O2 battery use. In this report, an iron cobalt bimetal-carbon nanotube (FeCo-CNT) composite was synthesized as a catalytic air cathode material for Li-O2 batteries. An Li-O2 battery using FeCo-CNT air electrodes exhibited higher efficiency than that of pristine CNT as well as higher capacity. Spectroscopic and electron microscope analyses showed that the improved cell performances can be attributed to the catalytic effect of FeCo. As a cost-effective non-noble metal catalyst, FeCo-CNT demonstrated performance comparable to noble metal catalysts in Li-O2 systems.

MnCo2O4 Anchored on Magneli Phase Titanium Oxide As a Non-Carbon Cathode for Li-O2 Batteries

Development of highly effective and environmentally friendly battery technology is crucial to the energy storage system and electronic vehicle.[1,2] Recently, the aprotic lithium-oxygen (Li-O2) battery has attracted much attention due to its high specific energy density of 3500 Wh kg-1 (based on the mass of Li2O2) and a favorable potential of 3.0 V, which are defined by the chemistry of the reversible reaction Li + O2 ↔Li2O2.[3,4] The high specific energy density of Li-O2 battery renders it a promising candidate for the electrical vehicle with extended-range driving. One of the challenges in Li-O2battery is to find a catalyst that can improve the sluggish electrochemical kinetics of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) during the discharge and charge process, respectively.[5] Herein, we employ Magneli phase Ti4O7 as a support and construct a hybrid with MnCo2O4 (MCO) nanoparticles being anchored on the surface of Ti4O7 for the first time. This hybrid is employed as a non-carbon cathode material for Li-O2 batteries. The synergistic interaction between Ti4O7 support and MCO nanoparticles further promotes the catalytic activity and stability of the hybrid. Benefiting form these advantages, the as-synthesized Ti4O7-MCO hybrid exhibits excellent catalytic activity to decrease the voltage gap between discharge and charge. The Ti4O7-MCO presents excellent discharge performance when compared with other carbon-free cathode materials. Moreover, due to the excellent chemical durability of Ti4O7 in organic electrolyte and the interaction between Ti4O7and MCO, the hybird is very stable during the battery cycling without agglomeration and detachment. The Discharge/charge profile, rate capability and cycling performance of Ti4O7-MCO are shown in Figure 1. Figure 1. (a) Discharge/charge profile of Ti4O7-MCO cathode under different current densities; (b) First discharge/charge profile of Ti4O7-MCO, Ti4O7, C and C-MCO with a limited capacity of 500 mAh g-1; (c) Cycling performance of Ti4O7 cathode; (d) Cycling performance of Ti4O7-MCO cathode. Acknowledgements This work is supported by National Natural Science Foundation of China (51572181), the National Key Research and Development Program of China (2016YFB0100200)，Natural Science Foundation of Jiangsu Province, China (BK20151226).

Electrodeposition of MnO2-Based Nanomaterials As Carbon-Free and Binder-Free Cathodes for Li-O2 Battery

Carbon based nanomaterials have been widely used as cathode materials for Li-O2 batteries because of their high conductivity, large surface area, and good ORR catalytic activity. However, instability of carbon under Li-O2 operation poses problems. During recharge, carbon can decompose to form lithium carbonate at high charging potential1, but also react chemically with the discharge product Li2O2 2. Metal oxides, like MnO2 and RuO2, have been investigated as cathode catalysts in Li-O2 batteries3. In this study, MnO2 nanostructure is electrodeposited on stainless steel mesh and applied as a carbon-free and binder-free cathode for non-aqueous Li-O2 battery. To maintain good adhesion and electrical contact between deposited layer and the substrate, the mesh is pre-treated. Benefiting from enhanced electronic contact and large surface area of MnO2, the electrochemical performance is improved. Material characterization and electrochemical impedance spectroscopy (EIS) technique are also utilized to systematically study the electrochemical processes of these carbon-free MnO2-based electrodes.

Electrochemically synthesized nanoporous gold as a cathode material for Li-O2 batteries

Nanoporous gold (NPG) prepared via chemical de-alloying has been recently shown to dramatically improve the reversibility and kinetics of Li-O2 batteries, but high cost makes its use as practical electrode material difficult. Recently developed electrochemical routines for synthesis of very thin NPG layers (<100 nm) on various low-cost substrates could potentially provide a feasible economic alternative. In this work, NPG on both gold and glassy carbon (GC) substrates was successfully synthesized via electrochemical de-alloying method and tested as cathode material in Li-O2 batteries. The results show that electrochemically synthesized NPG cathode cycles repeatedly with LiFePO4 anode. The voltage hysteresis is also significantly reduced when NPG is used in comparison with plain GC. Along with these results, challenges that need to be addressed for future implementation of NPG cathode in practical Li-O2 batteries are also discussed.

The Carbon-Free RuO2-TiO2 Electrode for Li-Air Batteries

Li-O2 batteries offer the highest energy density among rechargeable batteries. This superb energy density makes the system one of the most attractive energy storage devices for electric vehicles (EV), energy storage systems (ESS), and other applications1 where high energy density is the most important figure of merit. There remain many challenges, however, for the practical application of Li-O2 batteries including low energy efficiency caused by the large energy loss during Li-O2 electrochemical cycles. The widely-accepted theoretical reaction of Li-O2 batteries is 2Li+ + O2 + 2e- ↔ Li2O2. The forward direction forming Li2O2 represents the discharge process and the reverse reaction of Li2O2 decomposition the charge process; however, many parasitic reactions have been reported, especially involving electrolyte and carbon materials.2 These undesired side reactions significantly reduce the round-trip efficiency of Li-O2 batteries. Carbon in particular, the most common material for air electrodes because of its high surface area and good electrical conductivity, may react to Li2CO3 in Li-O2 cells that impede the reversible Li-O2 reaction.2 Some of the alternative cathode materials suggested recently include TiC,3 indium tin oxide (ITO).4 Bruce and his co-workers3 ascribed the stability of TiC cathodes to the surface TiO2 layer on TiC. The TiO2 surface layer was stable from the attack of oxygen radicals during cycling. Recently, TiO2 nano tubes have been explored as a support for electrocatalysts.5 The resulting cells showed enhanced cycle life due to the higher stability of TiO2 when compared with carbon materials. The Pt or RuO2 catalyst-loaded TiO2 nanotube exhibited an enhanced cyclability with low charge potentials, especially under high current density. Despite the high specific capacity in their research, however, the actual capacity of Li-O2 batteries was not sufficient for practical application, since the specific capacity was calculated based on the catalyst loading mass, which is only 0.1 mg on a 15 mm support. Li-O2 batteries must be highly reversible with a high cell capacity for high energy applications. Here, we synthesized RuO2/mesoporous TiO2 composites, abbreviated as RuO2/mTiO2, and used as a cathode material for Li-O2 batteries. A carbon-free electrode of RuO2/mTiO2 showed 4 mAh of cell capacity with alow charge potential of under 4 V.