Nonaqueous Li-O2 Batteries Research Articles

In-situ formed free-standing Ir nanocatalysts as carbon- and binder-free cathode for rechargeable nonaqueous Li–O2 batteries

Abstract The design of carbon- and binder-free cathodes to replace the vulnerable carbon cathodes has become very imperative to further enhance the cycling performance of Li–O2 battery. Herein, free-standing Ir nanocatalyst as a novel carbon- and binder-free cathode is first proposed for rechargeable nonaqueous Li–O2 batteries. Impressively, the Li–O2 battery with freestanding iridium cathodes can maintain more than 67 stable cycles with a limited capacity of 800 mAh g−1 at a current density of 400 mA g−1. Greatly improved energy efficiency and rate capability are also successfully achieved. The iridium nanocatalyst is revealed to promote the discharge product formation via a newly desirable surface growth coupled with solution driven mechanism. This study sheds a new light on the role of iridium nanocatalyst, and provides a rational approach for the design of carbon- and binder-free cathodes of Li–O2 batteries.

Three-dimensional self-standing Co@NC octahedron/biochar cathode for non-aqueous Li-O2 batteries: efficient catalysis for reversible formation and decomposition of LiOH

Lithium–oxygen (Li-O2) batteries have received wide attention due to their high specific capacity. In non-aqueous Li-O2 batteries, water is generally considered as a harmful pollutant, which will cause parasitic reaction to generate LiOH. However, it is difficult to decompose in the charging process, resulting in premature failure of battery and serious impact on the cycle stability. Therefore, it is of great significance to explore efficient catalysts to decompose LiOH for the future commercialization of Li-air batteries. In this work, a novel three-dimensional (3D) self-standing Co@NC octahedron/biochar is used as the cathode of Li-O2 battery. Ex situ XRD and XPS identify that the discharge products were LiOH rather than Li2O2. However, the specific capacity of Li-O2 battery with Co@NC still reaches up to 14.1 mAh cm-2 at a current density of 0.1 mA cm-2 and sustains a stable cycling of 136 times with a limited capacity of 0.5 mAh cm−2 at 0.1 mA cm−2. It is found that the charge specific capacity of the battery with preloading LiOH in the cathode is close to the actual theoretical value, proving that Co@NC could effectively decompose LiOH.

Parameter sensitivity analysis and cathode structure optimization of a non-aqueous Li–O2 battery model

Non-aqueous Li–O2 batteries have gained more attention and are promising power sources for next-generation electric vehicles due to their high energy density. In the present study, a comprehensive one-dimensional model is developed to investigate the performance of non-aqueous Li–O2 batteries. Two different models for insoluble discharge product growth are compared and the tunneling-effect model is in better agreement with the published experimental data. In order to evaluate the effects of various parameters on battery specific capacity in the model, the parameter sensitivity analysis is conducted under different current densities at a cutoff voltage of 2.4 V. The calculated average sensitivities are classified as three levels, including insensitive, sensitive, and very sensitive. The porosity of the cathode is found to be the most sensitive parameter to specific capacity. Based on this, a hierarchical porous cathode structure with gradient initial porosity distribution is proposed according to the distribution of cathode product and effective porosity. Our model shows that the hierarchical cathode structure can improve specific capacity by up to 10.38%.

Computational Insights into LixOy Formation, Nucleation, and Adsorption on Carbon Nanotube Electrodes in Nonaqueous Li-O2 Batteries.

Recent theoretical and experimental studies have shown that the formation of Li2O2, the main discharge product of nonaqueous Li-O2 batteries, is a complex multistep reaction process. The formation, nucleation, and adsorption of LixOy (x and y = 0, 1, and 2) and (Li2O2)n clusters with n = 1-4 on the surface of carbon nanotubes (CNTs) were investigated by periodic density functional theory calculation. The results showed that both Li2O2 and Li2O on CNT electrodes are preferentially generated by lithiation reaction rather than disproportionation reaction. The free energy profiles demonstrate that the discharge potentials of 2.54 and 1.29 V are the threshold values of spontaneous nucleation of (Li2O2)2 and (Li2O)2 on a CNT surface, respectively. The electronic structure indicates that Li2O2 is a p-type semiconductor, while Li2O exhibits the properties of an insulator. Interestingly, once Li2O2 molecules condense into large clusters, they will be repelled away from the CNT surface and continue to grow into large-sized Li2O2. Our results provide insights into the full understanding of the electrochemical reaction mechanism and product formation processes of lithium oxides in the cathodes of Li-O2 batteries.

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.

Remarkable improvement of cyclic stability in Li–O2 batteries using ruthenocene as a redox mediator

Nonaqueous Li–O2 batteries attract attention for their theoretical specific energy density. However, due to the difficulty of decomposition of Li2O2, Li–O2 batteries have high charge overpotential and poor cycling life. So all kinds of catalysts have been studied on the cathode. Compared to heterogeneous solid catalysts, soluble catalysts achieve faster and more effective transport of electrons by reversible redox pairs. Here, we first report ruthenocene (Ruc) as a mobile redox mediator in a Li–O2 battery. 0.01 mol/L Ruc in the electrolyte effectively reduces the charging voltage by 610 mV. Additionally, Ruc greatly increases the cycling life by four-fold (up to 83 cycles) with a simple ketjen black (KB) cathode. The results of SEM, XPS and XRD confirm that less discharge product residue accumulated after recharge. To verify the reaction mechanisms of the mediator, free energy profiles of the possible reaction pathways based on DFT are provided.

Pt/NiO Microsphere Composite as Efficient Multifunctional Catalysts for Nonaqueous Lithium-Oxygen Batteries and Alkaline Fuel Cells: The Synergistic Effect of Pt and Ni.

Developing efficient and low-cost multifunctional electrocatalysts is important for electrochemical devices. In this work, a cost-effective Pt/NiO composite with very limited Pt loading (from 0.5 to 3%) was controllably synthesized through facile hydrothermal procedures. The composite demonstrated the improved catalytic performance as applied to the nonaqueous Li-O2 batteries and the alkaline fuel cells. Regarding the alkaline fuel cells, 1% Pt/NiO composite gave rise to the best Pt distribution and thus exhibited the optimized electrochemical conductivity and properties as suggested by the significantly improved electrochemical reversibility. Meanwhile, the demonstrated 1% Pt/NiO composite presented high catalytic capability as electrode for Li-O2 batteries, which allowed for much improved capacity utilization, high cycling stability, high initial capacity (2329 mAh/g), and no obvious voltage drop during cycling. Such multiple advantages of prepared composite electrode material offer new prospects and application as multifunctional electrocatalysts for both Li-O2 batteries and alkaline fuel cells.

An efficient electrocatalyst of NiO supported on carbon paper for nonaqueous Li–O2 batteries

A Li–O2 battery has been considered as one of the most promising energy storage systems owing to their ultrahigh theoretical energy densities. However, low energy efficiency (high polarization) during discharge/charge and resulting cycle stabilities have severely limited the development of this type of battery. Here, we demonstrate a simple preparation of NiO supported on carbon paper by dipping carbon paper in Ni acetate solution and heating it to apply NiO directly to the carbon as a cathode material for nonaqueous Li–O2 batteries. The prepared sample was confirmed as the structure of NiO-incorporated carbon using X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and high-resolution transmission electron microscopy (HR-TEM) analysis despite amorphous patterns seen in XRD (~ 10% NiO in NiO supported on carbon paper). A Li–O2 cell in which the NiO supported on carbon paper was applied as an electrocatalyst showing an initial ~ 6% increase in energy efficiency and a subsequent 50 cycle retention.

Nitrogen/sulfur dual-doped porous carbon nanofibers with Co9S8 nanoparticles encapsulated by graphitic shells: A highly active stable free-standing air electrode for rechargeable non-aqueous Li-O2 batteries and primary alkaline Al-air batteries

The rational design of economical air cathodes or electrocatalysts with high electrocatalytic activity and durability for both non-aqueous lithium-oxygen (Li-O2) and alkaline electrolyte aluminum-air (Al-air) batteries is a critical factor for promoting the practical applications. Herein, we report the fabrication of graphitic layers covered with Co9S8 nanoparticles immobilized inside N,S dual-doped hierarchical porous carbon nanofibers by a facile and one-step electrospinning method combined with a pyrolytic process. When employed as free-standing electrodes for a Li-O2 battery and an Al-air battery, the as-prepared Co9S8@G/NS-PCNFs deliver excellent battery performances and afford comparable catalytic performance as that of a 20 wt% Pt/C electrode. The results show that the superior battery performance of the Co9S8@G/NS-PCNF cathode can be attributed to the synergistic effect between the catalytic behavior of the uniformly distributed Co9S8 nanoparticles and the textural properties of the N,S dual-doped hierarchical porous carbon nanofibers. Our findings open a new direction for designing nonprecious hybrid metal sulfide carbon architectures to replace costly Pt/C electrodes and indicate their promising application in next-generation metal-air batteries.

Decoupling the Cumulative Contributions of Capacity Fade in Ethereal-Based Li-O2 Batteries.

In the loop of numerous challenges and ambiguities, Li-O2 batteries are crawling to reach their commercialization phase. To achieve the progressive milestones, along with the developments in the architecture of cathodes, anodes, and electrolytes, understanding its failure mode is equally important. Under an unrestricted charge-discharge protocol, cyclability of nonaqueous Li-O2 batteries are limited to only a few cycles. This report examines an additive-free ether-based Li-O2 battery in the perspective of identifying the origin of possible side reactions and their affiliations to integral components of the battery. Structural and compositional changes during every charge-discharge sequence are studied using bottom-up sequential tear-down analysis. The substantial increase in impedance and corresponding decrease in capacities after every cycle are interrelated to the amount of electrode passivation resulting from the discharge products and electrolyte decomposition. From the tear-down analysis, it is approximated that, among the total capacity loss, ≈55% is attributed to the cathode, ≈28% of the loss corresponds to the anode, and ≈17% is attributed to the electrolyte, given that battery failure instigates from the "reactive oxygen species". Electrochemically formed Li2O2 via the superoxide pathway induces large decomposition overpotentials up to 4.6 V versus Li/Li+ because of its overrated reactivity with electrolytes and carbon supports. On the contrary, efficient decomposition of chemically formed Li2O2 below 3.9 V proves that the extra charge potential observed for electrochemically formed Li2O2 is in fact consumed for the decomposition of irreversibly formed side products via the superoxide pathway. Spontaneous reactivity of Li2O2 and trivial reactivity of Li2O highlight the need of advanced strategies to maneuver oxygen red-ox in selective pathways that unalter the electrolyte and electrodes, and the necessity of their synchronized performance for the evolution of practical Li-O2 batteries.

High rate and long cycle life in Li-O2 batteries with highly efficient catalytic cathode configured with Co3O4 nanoflower

The reaction mechanism of non-aqueous Li-O2 batteries is based on the deposition and decomposition of Li2O2. The polarization of Li-O2 batteries can be rapidly increased by operation under a high rate condition, resulting in the early capacity fade of the cells. Therefore, a well-designed catalyst with a unique structure and excellent catalytic ability is an important way to boost the round-trip performance of Li-O2 batteries, especially under high current density. In this work, a unique nanoflower structure assembled with Co3O4 nanosheets is synthesized by using 2-methylimidazole (2-MIM) as a structural directing agent. X-ray photoelectron spectroscopy (XPS) and Raman spectra reveal abundant oxygen vacancies on the surface of the Co3O4 nanoflower, which are beneficial for oxygen reduction and evolution reactions and long round-trip lifetime. Density functional theory results demonstrate that Co3O4 catalyst with oxygen vacancies could promote the wetting of Li2O2 on substrate and formation of a Li2O2 nanofilm, thereby boosting the discharge capacity of Li-O2 batteries. On account of the synergistic effect of abundant oxygen vacancies, the unique structure, and excellent oxygen evolution reaction, Co3O4 nanoflower-based cells could deliver ultralong lifetime of 276 and 248 cycles with a discharge capacity of 1000 mAh g−1 under charge/discharge current densities of 0.5 A g−1 and 1 A g−1, respectively. This study has shed light on a new strategy for catalyst preparation for long lifetime Li-O2 batteries.

Computational study on catalytic performance of BC3 and NC3 nanosheets as cathode electrocatalysts for nonaqueous Li–O2 batteries

The development of high-performance cathode electrocatalysts in nonaqueous lithium-oxygen (Li–O2) battery is mainly restricted by high charge/discharge overpotentials. Based on first-principles calculations, we theoretically explore the application potential of BC3 and NC3 nanosheets as the suitable cathode electrocatalysts. By analyzing the stable adsorption configurations of LixO2y (x = 0–4, y = 0–2) intermediates on BC3/NC3, it is confirmed that oxygen reduction reaction (ORR) on BC3 follows four-electron pathway with (Li2O)2 as the discharge product, while for ORR on NC3, (Li2O2)2 is the discharge product following two-electron pathway. We then plot the corresponding free energy diagrams of charge/discharge processes and calculate the theoretical discharge and charge overpotentials (ηORR and ηOER) along the optimal reaction pathways. Our results show that both the ηORR and ηOER on BC3 are considerably low (0.21 V and 0.36 V), indicating that BC3 can serve as a high-performance cathode electrocatalyst. Meanwhile, the ηORR and ηOER on NC3 (0.37 V and 0.95 V) are comparable to previously reported materials following two-electron pathway (such as N-doped graphene), exhibiting practical values. Both the BC3 and NC3 nanosheets are stable enough for application in Li–O2 battery, and dimethyl sulfoxide (DMSO) isn't recommended as an organic electrolyte when BC3 works as the cathode catalyst.

Expansion of the Potential Region for Sustained Discharge of Non-aqueous Li-O2 Batteries Using an Oxygen-enriched Carbon Cathode

This study demonstrates that surface modification of a carbon-paper cathode by oxygen functional groups expands the potential region for the sustained discharge of non-aqueous Li-O2 batteries. Expa...

Potential Dependences of Discharge Characteristics of Li-O2 Batteries with Various Electrolytes

Practical application of non-aqueous Li-O2 batteries is strongly expected, as its theoretical energy density is much larger than Li ion batteries. However, due to non-conductive Li2O2 formed on the cathode during the discharge, the discharge capacity is limited to be much smaller than the theoretical value. Two different pathways, the surface and solution pathway, have been proposed for the discharge process forming Li2O2, the latter being considered preferred for the sustained discharge. Recent works revealed that the formation pathway of Li2O2 is determined by the adsorption/desorption equilibrium of LiO2, a reaction intermediate of the discharge reaction. Considering that the operating potential of the cathode is one of the factors which determines the adsorption/desorption equilibrium of LiO2, it is effective to conduct the discharge tests under potential-controlled modes for deeper understanding of the electrochemistry of the discharge reaction. In the present work, we examined the discharge profiles of non-aqueous Li-O2 batteries under potential-controlled conditions. We put a particular interest on revealing the generality and specificity of the potential dependences of the discharge characteristics among Li-O2 batteries carrying different solvents, including dimethyl sulfoxide (DMSO), acetonitrile (MeCN), and tetraglyme (G4). For all the systems investigated qualitatively similar potential dependences were observed. Namely, the higher (or the lower) the potential was, the smaller (or larger) the current value was and the larger (or smaller) the discharge capacity was, suggesting that the solution and surface pathway preferentially proceeds in higher and lower potentials, respectively, irrespective of the solvents. On the other hand, it was also found that the sharpness of the potential dependence strongly depends on the kind of solvents used. For example, the potential dependence of the discharge capacity in MeCN was much sharper than that in DMSO. These results indicate that it is important to tune the working potential of the cathode so that the discharge capacity can be larger, especially in a solvent which has the strong potential dependency. Figure 1

Self-Supported ZIF-Derived Co3 O4 Nanoparticles-Decorated Porous N-Doped Carbon Fibers as Oxygen Reduction Catalyst.

Oxygen reduction is a significant cathodic reaction in the state-of-art clean energy devices such as fuel cell and metal-oxygen battery. Here, ZIF-incorporated hybrid polymeric fibres have been fabricated by using a dual-phase electrospinning method. These are then transformed into Co3 O4 -nanoparticle-decorated porous N-doped carbon fibres (ZIF-Co3 O4 /NCF) through multi-step annealing treatment. The resultant ZIF-Co3 O4 /NCF is interweaved into a self-supported film and can be used as a bi-functional catalyst for catalysing oxygen reduction in both aqueous and non-aqueous electrolytes. Electrochemical tests demonstrate that ZIF-Co3 O4 /NCF displays a high catalytic activity for oxygen reduction with a half-wave potential (E1/2 ) of 0.813 V (vs. RHE) and an almost favourable four-electron reduction pathway in alkaline medium. ZIF-Co3 O4 /NCF catalyst only shows 4 mV negative shift of E1/2 after 5000 continuous CV cycles. In addition, the ZIF-Co3 O4 /NCF can be applied as the cathode catalyst of non-aqueous Li-O2 battery, exhibiting a remarkable ORR activity in LiPF6 contained 1,2-dimethoxyethane electrolyte. The excellent electrocatalytic performance of ZIF-Co3 O4 /NCF is probably due to the abundance of active sites of graphitic carbon-wrapped Co3 O4 nanoparticles, as well as the cross-linked fibrous nitrogen-doped carbon texture.

Co3O4 nanocage derived from metal-organic frameworks: An excellent cathode catalyst for rechargeable Li-O2 battery

Rechargeable non-aqueous Li-O2 battery is regarded as one of the most promising energy-storage technologies on account of its high energy density. It is believed that the rational design of three-dimensional (3D) architecture for catalyst is a key factor for the remarkable performance. Metal-organic frameworks (MOFs) derived materials possess excellent architecture, which is beneficial for Li-O2 batteries. In this work, ZIF-67 is used as precursor template and calcinated under different temperature to produce Co3O4 crystals. When the anneal treatment is under 350 °C, the derived Co3O4 nanocage holds the most complete skeleton, which provides better charge transfer ability as well as O2 and Li+ diffusion. Meanwhile, the Co3O4 nanocage owns more oxygen vacancies, offering more active sites. With the synergistic effect of nanocage structure and active sites, the Co3O4 nanocage stably delivers a large specific capacity of 15,500 mAh·g-1 as well as a long cycle-life of 132 cycles at limited discharge capacity of 1,000 mAh·g-1 under discharge/charge current density of 0.5 A·g-1.

Oxygen-free cell formation process obtaining LiF protected electrodes for improved stability in lithium-oxygen batteries

The unsatisfactory stabilities of carbon-based cathodes and lithium anodes are the major hurdles limiting the development of Li-air (O2) batteries. Herein, we propose an extremely simple cell formation process for non-aqueous Li-O2 batteries, by a discharge-charge process in argon prior to O2 atmosphere, to produce protective films on both CNTs-cathode and lithium-anode surfaces. The films are mainly composed of lithium fluoride derived from LiTFSI decomposition and endows lithium oxygen batteries with enhanced cycling stability (>200 cycles) under a consistent capacity condition (1000 mAh g−1). This study reveals that LiF-rich films could effectually suppress the parasitic reactions of electrodes against the reactive intermediates and electrolyte attacks. This simple approach provides a new strategy to protect both electrodes for lithium oxygen batteries.

Enhanced Li2O2 Decomposition in Rechargeable Li–O2 Battery by Incorporating WO3 Nanowire Array Photocatalyst

Reducing the high charging overpotential of nonaqueous Li–O2 batteries is very important for their energy storage ability. Herein, we propose a newly photoassisted Li–O2 battery system, in which a ...

An all-nanosheet OER/ORR bifunctional electrocatalyst for both aprotic and aqueous Li–O2 batteries

Rechargeable lithium-oxygen (Li-O2) batteries are receiving intense interest because of their high energy density. A highly efficient catalyst for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is a key factor influencing the performance of Li-O2 batteries. In this work, a facile synthesis of an all-nanosheet architecture electrocatalyst constructed from a monolayer ruthenium dioxide (RuO2) nanosheet with a nitrogen doped sulfonated graphene nanosheet (RuO2-NS-GNS) has been developed for Li-O2 batteries. This complex catalyst displays excellent activity towards the ORR and OER in both aprotic and aqueous Li-O2 batteries. A low overpotential around 1.0 V during the discharge/recharge process is obtained for the aprotic Li-O2 battery with RuO2-NS-GNS. Meanwhile, linear sweep voltammetry curves show that the OER and ORR potentials are 1.45 V and 0.81 V in an alkaline solution (1 M LiOH-5 M LiNO3) for RuO2-NS-GNS, respectively. Both aprotic and aqueous Li-O2 batteries with RuO2-NS-GNS exhibit stable cyclability and low round-trip overpotential without obvious degradation at a limited specific capacity of 1000 mA g-1. The advanced electrochemical performance of RuO2-NS-GNS in both aprotic and aqueous Li-O2 batteries can be attributed to the increased catalytic sites and synergistic effect arising from RuO2 and NS-GNS nanosheets.

A bi-functional redox mediator promoting the ORR and OER in non-aqueous Li-O2 batteries.

Herein, a bi-functional homogeneous redox mediator is employed to promote the rate capability of Li-O2 batteries. The addition of this redox mediator not only facilitates the formation of toroid-shaped Li2O2 at high current densities, but also significantly reduces the charging over-potential caused by the bulky Li2O2.