Computational Insights into Oxygen Reduction Reaction and Initial Li2O2 Nucleation on Pristine and N-Doped Graphene in Li–O2 Batteries

Localized charge-induced ORR/OER activity in doped fullerenes for Li-air battery applications

Non-aqueous Li-air batteries have garnered significant interest in recent years. The key challenge lies in the development of efficient catalysts to overcome the sluggish kinetics associated with the oxygen reduction reaction (ORR) during discharge and the oxygen evolution reaction (OER) during charging at the cathode. In this work, we conducted a comprehensive study on B/N-doped and BN co-doped fullerenes using first-principles analysis. Our results show significant changes in the geometries, electronic properties, and catalytic behaviors of doped and co-doped fullerenes. The coexistence of boron and nitrogen boosts the formation energy, enhancing stability compared to pristine and single-doped structures. C179B exhibits minimal overpotentials (0.98 V), implying superior catalyst performance for ORR and OER in LABs and significantly better performance than Pt (111) (3.48 V) and standard graphene (3.51 V). The electron-deficient nature of the B atom makes it provide its vacant 2pz orbital for conjugation with the p-electrons of nearby carbon atoms. Consequently, boron serves as a highly active site due to the localization of positive charge, which improves the adsorption of intermediates through oxygen atoms. Moreover, the higher activity of B-doped systems than N-doped systems in lithium-rich environments is opposite to the observed trend in the reported PEM fuel cells. This work introduces doped and co-doped fullerenes as LAB catalysts, offering insights into their tunable ORR/OER activity via doping with various heteroatoms and fullerene size modulation.

Boosted Mg-CO2 Batteries by Amine-Mediated CO2 Capture Chemistry and Mg2+ -Conducting Solid-electrolyte Interphases

Mg-CO2 battery has been considered as an ideal system for energy conversion and CO2 fixation. However, its practical application is significantly limited by the poor reversibility and sluggish kinetics of CO2 cathode and Mg anode. Here, a new amine mediated chemistry strategy is proposed to realize a highly reversible and high-rate Mg-CO2 battery in conventional electrolyte. Judiciously combined experimental characterization and theoretical computation unveiled that the introduced amine could simultaneously modify the reactant state of CO2 and Mg2+ to accelerate CO2 cathodic reactions on the thermodynamic-kinetic levels and facilitate the formation of Mg2+ -conductive solid-electrolyte interphase (SEI) to enable highly reversible Mg anode. As a result, the Mg-CO2 battery exhibits boosted stable cyclability (70 cycles, more than 400 h at 200 mA g-1 ) and high-rate capability (from 100 to 2000 mA g-1 with 1.5 V overpotential) even at -15 °C. This work opens a newly promising avenue for advanced metal-CO2 batteries.

An experimental and computational study of graphene oxide functionalized with tris(hydroxymethyl)aminomethane as an electrode material for supercapacitors

In this research, graphene oxide (GO) functionalized with tris(hydroxymethyl)aminomethane (T) was synthesized with a simple one-pot method, and applied as an electrode material for supercapacitors. Electrochemical measurements on the synthesized tris(hydroxymethyl)aminomethane-functionalized graphene oxide (GO@T) indicated a specific capacitance of 549.8 F g- 1 at a specific current of 2.5 A g- 1 and a specific capacitance of 358 F g-1 at a specific current of 7 A g- 1 in the potential range of - 0.5-0.5 V versus Ag/AgCl. It also showed a high cyclic stability. According to the results, 80 and 68% of the initial capacitance was retained after 5500 and 9300 cycles, respectively. Density functional theory calculations were used to investigate the quantum capacitance, free energy change during functionalization reaction, and the layer distance of GO and GO@T.

Influence of Ion Diffusion on the Lithium-Oxygen Electrochemical Process and Battery Application Using Carbon Nanotubes-Graphene Substrate

Lithium-oxygen (Li-O2) batteries are nowadays among the most appealing next-generation energy storage systems in view of a high theoretical capacity and the use of transition-metal-free cathodes. Nevertheless, the practical application of these batteries is still hindered by limited understanding of the relationships between cell components and performances. In this work, we investigate a Li-O2 battery by originally screening different gas diffusion layers (GDLs) characterized by low specific surface area (<40 m2 g-1) with relatively large pores (absence of micropores), graphitic character, and the presence of a fraction of the hydrophobic PTFE polymer on their surface (<20 wt %). The electrochemical characterization of Li-O2 cells using bare GDLs as the support indicates that the oxygen reduction reaction (ORR) occurs at potentials below 2.8 V vs Li+/Li, while the oxygen evolution reaction (OER) takes place at potentials higher than 3.6 V vs Li+/Li. Furthermore, the relatively high impedance of the Li-O2 cells at the pristine state remarkably decreases upon electrochemical activation achieved by voltammetry. The Li-O2 cells deliver high reversible capacities, ranging from ∼6 to ∼8 mA h cm-2 (referred to the geometric area of the GDLs). The Li-O2 battery performances are rationalized by the investigation of a practical Li+ diffusion coefficient (D) within the cell configuration adopted herein. The study reveals that D is higher during ORR than during OER, with values depending on the characteristics of the GDL and on the cell state of charge. Overall, D values range from ∼10-10 to ∼10-8 cm2 s-1 during the ORR and ∼10-17 to ∼10-11 cm2 s-1 during the OER. The most performing GDL is used as the support for the deposition of a substrate formed by few-layer graphene and multiwalled carbon nanotubes to improve the reaction in a Li-O2 cell operating with a maximum specific capacity of 1250 mA h g-1 (1 mA h cm-2) at a current density of 0.33 mA cm-2. XPS on the electrode tested in our Li-O2 cell setup suggests the formation of a stable solid electrolyte interphase at the surface which extends the cycle life.

A review on development of metal-organic framework-derived bifunctional electrocatalysts for oxygen electrodes in metal-air batteries

Worldwide demand for oil, coal, and natural gas has increased recently because of odd weather patterns and economies recovering from the pandemic. By using these fuels at an astonishing rate, their reserves are running low with each passing decade. Increased reliance on these sources is contributing significantly to both global warming and power shortage problems. It is vital to highlight and focus on using renewable energy sources for power production and storage. This review aims to discuss one of the cutting-edge technologies, metal-air batteries, which are currently being researched for energy storage applications. A battery that employs an external cathode of ambient air and an anode constructed of pure metal in which an electrolyte can be aqueous or aprotic electrolyte is termed as a metal-air battery (MAB). Due to their reportedly higher energy density, MABs are frequently hailed as the electrochemical energy storage of the future for applications like grid storage or electric car energy storage. The demand of the upcoming energy storage technologies can be satisfied by these MABs. The usage of metal-organic frameworks (MOFs) in metal-air batteries as a bi-functional electrocatalyst has been widely studied in the last decade. Metal ions or arrays bound to organic ligands to create one, two, or three-dimensional structures make up the family of molecules known as MOFs. They are a subclass of coordination polymers; metal nodes and organic linkers form different classes of these porous materials. Because of their modular design, they offer excellent synthetic tunability, enabling precise chemical and structural control that is highly desirable in electrode materials of MABs.

Uncovering the Electrolyte-Dependent Transport Mechanism of LiO2 in Lithium-Oxygen Batteries

Lithium-oxygen batteries (LOBs) offer extremely high theoretical energy density and are therefore strong contenders for bringing conventional batteries into the next generation. To avoid deactivation and passivation of the electrode due to the gradual covering of the surface by discharge products, electrolytes with high donor number (DN) are becoming increasingly popular in LOBs. However, the mechanism of this electrolyte-assisted discharge process remains unclear in many aspects, including the lithium superoxide (LiO₂) intermediate transportation mechanism and stability at both electrode/electrolyte interfaces and in bulk electrolytes. Here, we performed a systematic Born-Oppenheimer molecular dynamics (BOMD)-level investigation of the LiO₂ solvation reactions at two interfaces with high- or low-DN electrolytes (dimethyl sulfoxide (DMSO) or acetonitrile (CH₃CN), respectively), followed by examinations of stability and condensation once the LiO₂ monomers are solvated. Release of partial discharge product LiO₂ is found to be energetically favorable into DMSO from the Co₃O₄ cathode with a small energy barrier. However, in the presence of CH₃CN electrolyte, the release of LiO₂ from the electrode surface is found to be energetically unfavorable. Dissolved LiO₂(sol) clusters in bulk DMSO solvents are found to be more favorable to dimerize and agglomerate into a toroidal shape rather than to decompose, which avoids the emergence of strong oxidant ions (O₂^-) and preserves the system stability. This study provides two complete molecular-level pathways (solution and surface) from first-principles understanding of LOBs, offering guidance for future selection and design of electrode catalysts and solvents.

Atomically Dispersed Uranium Enables an Unprecedentedly High NH3 Yield Rate

The low NH₃ yield rate is a grand challenge for electrocatalytic N₂ reduction to NH₃. Herein, we report the first uranium single-atom catalyst (SAC) capable of catalyzing the electrochemical N₂ reduction reaction (NRR). The uranium SAC features a low limiting potential (<0.5 V) and near-zero free energy changes for N₂ adsorption and NH₃ desorption. The integration of these merits enables the uranium SAC to afford an unprecedentedly high NH₃ yield rate, 3-4 orders of magnitude higher than that of the Ru(0001) surface, which is widely recognized as an excellent NRR electrocatalyst. Further theoretical analysis reveals that the N₂ reduction catalyzed by the uranium SAC is synergistically regulated by the d and f electrons of atomic uranium. This work proposes a promising solution (that is, atomically dispersed uranium) to the daunting challenge associated with the low NH₃ yield rate, thus enabling the scalable production of NH₃ via electrochemical N₂ reduction.

Ab Initio Exploration of Energetically and Kinetically Favorable ORR Activity on a 1T-ZrO2 Monolayer for a Nonaqueous Lithium-Oxygen Battery

Herein, we explore the potential applications of the experimentally synthesized ZrO₂ monolayer as the cathode catalyst for nonaqueous lithium-oxygen batteries. First, we show that a new peroxide-like adsorption geometry is the most stable configuration for LiO₂, which is distinct from the previously known O-Li-O triangular geometry. The proposed most stable adsorption configuration is because the Zr atoms in the substrate play a critical role in stabilizing the LiO₂ cluster. Second, our ab initio calculations indicate that both the ORR and OER catalytic activities are most likely to adopt the four-electron mechanism with a considerably low overpotential of only 0.44 and 0.76 V, respectively. Finally, we show that the adsorption energy of Li₂O₂ is a good descriptor for both ORR and OER catalytic activities, and weak Li₂O₂ adsorption behavior is positively related to low overpotentials and satisfactory catalytic performance.

Recent Advances in Metal-Gas Batteries with Carbon-Based Nonprecious Metal Catalysts

Metal-gas batteries draw a lot of attention due to their superiorities in high energy density and stable performance. However, the sluggish electrochemical reactions and associated side reactions in metal-gas batteries require suitable catalysts, which possess high catalytic activity and selectivity. Although precious metal catalysts show a higher catalytic activity, high cost of the precious metal catalysts hinders their commercial applications. In contrast, nonprecious metal catalysts complement the weakness of cost, and the gap in activity can be made up by increasing the amount of the nonprecious metal active centers. Herein, recent work on carbon-based nonprecious metal catalysts for metal-gas batteries is summarized. This review starts with introducing the advantages of carbon-based nonprecious metal catalysts, followed by a discussion of the synthetic strategy of carbon-based nonprecious metal catalysts and classification of active sites, and finally a summary of present metal-gas batteries with the carbon-based nonprecious metal catalysts is presented. The challenges and opportunities for carbon-based nonprecious metal catalysts in metal-gas batteries are also explored.

How Do Oxygen Vacancies Influence the Catalytic Performance of Two-Dimensional Nb2 O5 in Lithium- and Sodium-Oxygen Batteries?

Alkali metal-oxygen batteries possess a higher specific capacity than alkali-ion batteries and stand out as the most competitive next-generation energy source. The core reaction mechanism of the battery is mainly the formation of alkali metal oxide during the discharge process and the decomposition of these oxides during the charge process. A large number of researchers have devoted themselves to seeking promising catalysts for the reaction. Two-dimensional Nb₂ O₅ was discovered to be a highly potential catalyst that can promote the reaction of alkali-metal-oxygen batteries, but few studies focus on it. In this study, the catalytic performance of both pristine Nb₂ O₅ and oxygen-deficiency modified Nb₂ O₅ was investigated. Furthermore, the effect of oxygen defects on catalytic performance was analyzed from multiple angles, namely, the reaction mechanism, d-band center theory, and the diffusion behavior of alkali metals. The exploration revealed the microscopic mechanism of oxygen deficiency affecting the alkali-metal battery reaction and provided a theoretical basis for quantitatively changing the d-band center of the catalyst through oxygen deficiency to ultimately change the performance of the catalyst.

Enhanced catalytic performance of pillared δ-MnO2 with enlarged layer spaces for lithium- and sodium-oxygen batteries: a theoretical investigation

Facing the challenge of increasingly severe environmental issues, many researchers are committed to seeking "post lithium-ion batteries" that can replace fossil fuels, one of which is alkali-metal-oxygen batteries. Nevertheless, the main bottleneck restricting the development of these batteries is the need for suitable catalysts to facilitate the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). In this study, we attempt to modify the catalytic performance of δ-MnO₂ for lithium- and sodium-oxygen batteries (LOBs and SOBs) by constructing 1,4-benzenedisulfonic acid, 2-chloro-1,4-benzenedisulfonic acid, and 2-fluoro-1,4-benzenedisulfonic acid pillared structures (H-, Cl-, and F-MnO₂). Their dynamic stability and catalytic mechanism have been explored by employing density functional theory (DFT). H-MnO₂ possesses a theoretical discharge voltage of 2.645 V for LOBs, which is 0.293 V larger than that of Cl-MnO₂. The discharge voltage of Cl-MnO₂ for SOBs is 3.152 V; however, H-MnO₂ impedes the formation of sodium superoxide and can hardly promote the ORR in SOBs. Both H- and Cl-MnO₂ can prevent the parasitic disproportionation reaction in LOBs and SOBs that produce active singlet oxygen through different reaction mechanisms. We believe that the constructed pillared structures are efficient ORR/OER catalysts for alkali-metal-oxygen batteries. Our research provides a theoretical basis for the micro-level mechanism of LOBs and SOBs catalyzed by the pillared δ-MnO₂ and sheds light on ameliorating the properties of the catalyst by constructing pillared structures.

Single-side functionalized graphene as promising cathode catalysts in nonaqueous lithium-oxygen batteries

High-performance cathode catalysts are always desirable for nonaqueous lithium-oxygen (Li-O₂) batteries. Using density functional theory calculations, the structural, electronic, and magnetic properties of SSX-Gr with different C/X ratios (X = H or F) are systematically studied. Then, a detailed mechanism on the dissociation of O₂ and the migration of Li on the SSX-Gr is revealed, based on which C₆X and C₈X are confirmed as the potential candidates as cathode catalysts. The studies on reaction pathways suggest that the four-electron pathway is the more possible catalytic pathway for the selected SSX-Gr. The free energy diagrams for discharging and charging processes catalyzed by SSX-Gr reveal that C₆F exhibits the highest application potential due to its considerably low oxygen reduction overpotential (0.83 V) and oxygen evolution overpotential (1.47 V). The extra spins induced by single-side functionalization endow graphene with the electrocatalytic activity.

Enhancing the hydrogen evolution activity of MoS2 basal planes and edges using tunable carbon-based supports

The hydrogen adsorption free energy (ΔG_Hads) on the basal plane and edges of MoS₂ is studied using periodic density functional theory, with the catalyst supported by a range of two-dimensional carbon-based materials. Understanding how ΔG_Hads can be tuned with support gives insight into MoS₂ as a catalyst for the hydrogen evolution reaction. The supports studied here include graphene oxide materials, heteroatom doped (S, B, and N) graphene, and some insulator materials (hexagonal boron nitride and graphitic carbon nitride). For the basal plane of MoS₂, a wide range of values for ΔG_Hads are observed (between 1.4 and 2.2 eV) depending on the support material used. It is found that ΔG_Hads relates directly to the energy of occupied p-orbital states in the MoS₂ catalyst, which is modified by the support material. On the Mo-edge of MoS₂, different supports induce smaller variations in ΔG_Hads, with values ranging between -0.27 and 0.09 eV. However, a graphene support doped with graphitic N atoms produces a ΔG_Hads value of exactly 0 eV, which is thermodynamically ideal for hydrogen evolution. Furthermore, ΔG_Hads is found to relate closely and linearly to the amount of charge transfer between MoS₂ and support when they adhere together. The support-induced tuning of ΔG_Hads on MoS₂ observed here provides a useful tool for improving current MoS₂ catalysts, and the discovery of variables which mediate changes in ΔG_Hads contributes to the rational design of new hydrogen evolution catalysts.

Performance and Sustainability Tradeoffs of Oxidized Carbon Nanotubes as a Cathodic Material in Lithium-Oxygen Batteries

Climate change mitigation efforts will require a portfolio of solutions, including improvements to energy storage technologies in electric vehicles and renewable energy sources, such as the high-energy-density lithium-oxygen battery (LOB). However, if LOB technology will contribute to addressing climate change, improvements to LOB performance must not come at the cost of disproportionate increases in global warming potential (GWP) or cumulative energy demand (CED) over their lifecycle. Here, oxygen-functionalized multi-walled carbon nanotube (O-MWCNT) cathodes were produced and assessed for their initial discharge capacities and cyclability. Contrary to previous findings, the discharge capacity of O-MWCNT cathodes increased with the ratio of carbonyl/carboxyl moieties, outperforming pristine MWCNTs. However, increased oxygen concentrations decreased LOB cyclability, while high-temperature annealing increased both discharge capacity and cyclability. Improved performance resulting from MWCNT post-processing came at the cost of increased GWP and CED, which in some cases was disproportionately higher than the level of improved performance. Based on the findings presented here, there is a need to simultaneously advance research in improving LOB performance while minimizing or mitigating the environmental impacts of LOB production.

Kinetically Stable Oxide Overlayers on Mo3 P Nanoparticles Enabling Lithium-Air Batteries with Low Overpotentials and Long Cycle Life

The main drawbacks of today's state-of-the-art lithium-air (Li-air) batteries are their low energy efficiency and limited cycle life due to the lack of earth-abundant cathode catalysts that can drive both oxygen reduction and evolution reactions (ORR and OER) at high rates at thermodynamic potentials. Here, inexpensive trimolybdenum phosphide (Mo₃ P) nanoparticles with an exceptional activity-ORR and OER current densities of 7.21 and 6.85 mA cm^-2 at 2.0 and 4.2 V versus Li/Li⁺ , respectively-in an oxygen-saturated non-aqueous electrolyte are reported. The Tafel plots indicate remarkably low charge transfer resistance-Tafel slopes of 35 and 38 mV dec^-1 for ORR and OER, respectively-resulting in the lowest ORR overpotential of 4.0 mV and OER overpotential of 5.1 mV reported to date. Using this catalyst, a Li-air battery cell with low discharge and charge overpotentials of 80 and 270 mV, respectively, and high energy efficiency of 90.2% in the first cycle is demonstrated. A long cycle life of 1200 is also achieved for this cell. Density functional theory calculations of ORR and OER on Mo₃ P (110) reveal that an oxide overlayer formed on the surface gives rise to the observed high ORR and OER electrocatalytic activity and small discharge/charge overpotentials.

Porous Materials Applied in Nonaqueous Li-O2 Batteries: Status and Perspectives

Porous materials possessing high surface area, large pore volume, tunable pore structure, superior tailorability, and dimensional effect have been widely applied as components of lithium-oxygen (Li-O₂ ) batteries. Herein, the theoretical foundation of the porous materials applied in Li-O₂ batteries is provided, based on the present understanding of the battery mechanism and the challenges and advantageous qualities of porous materials. Furthermore, recent progress in porous materials applied as the cathode, anode, separator, and electrolyte in Li-O₂ batteries is summarized, together with corresponding approaches to address the critical issues that remain at present. Particular emphasis is placed on the importance of the correlation between the function-orientated design of porous materials and key challenges of Li-O₂ batteries in accelerating oxygen reduction reaction (ORR)/oxygen evolution reaction (OER) kinetics, improving the electrode stability, controlling lithium deposition, suppressing the shuttle effect of the dissolved redox mediators, and alleviating electrolyte decomposition. Finally, the rational design and innovative directions of porous materials are provided for their development and application in Li-O₂ battery systems.

Dissociation of (Li2O2)0,+ on graphene and boron-doped graphene: insights from first-principles calculations

Reducing charge overpotential is of great significance to enhance the efficiency and cyclability of Li-O₂ batteries. Here, a dramatically reduced charge overpotential via boron-doped graphene as a catalytic substrate is successfully predicted. By first-principles calculations, from the perspective of reaction thermodynamics and kinetics, the results show that the electrochemical oxidation of the Li₂O₂⁺ cation is easier than the chemical oxidation of the neutral Li₂O₂ molecule, and the oxidation of (Li₂O₂)^0,+ is facilitated by boron-doping in pristine graphene. More importantly, the results reveal the oxidation mechanism of (Li₂O₂)^0,+: two-step dissociation with the LiO₂ molecule as a reactive intermediate has advantages over one-step dissociation; the rate-determining step for the dissociation of (Li₂O₂⁺)_G is the oxygen evolution process, while the lithium removal process is the rate-determining step for the dissociation of (Li₂O₂⁰)_G, (Li₂O₂⁰)_BG, and (Li₂O₂⁺)_BG.

2 D Materials for Electrochemical Energy Storage: Design, Preparation, and Application

Electrochemical energy storage is a promising route to relieve the increasing energy and environment crises, owing to its high efficiency and environmentally friendly nature. However, it is still challenging to realize its widespread application because of unsatisfactory electrode materials, with either high cost or poor activity and new electrode materials are urgently needed. Two-dimensional (2 D) materials are possible candidates, owing to their unique geometry and physicochemical properties. This Review summarizes the latest advances in the development of 2 D materials for electrochemical energy storage. Computational investigation and design of 2 D materials are first introduced, and then preparation methods are presented in detail. Next, the application of such materials in supercapacitors, alkali metal-ion batteries, and metal-air batteries are summarized comprehensively. Finally, the challenges and perspectives are discussed to offer a guideline for future exploration of high-efficiency 2 D materials for electrochemical energy storage.

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 Li_xO_y (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.

Li2S growth on graphene: Impact on the electrochemical performance of Li-S batteries

Lithium-sulfur batteries show remarkable potential for energy storage applications due to their high-specific capacity and the low cost of active materials, especially sulfur. However, whereas there is a consensus about the use of lithium metal as the negative electrode, there is not a clear and widely accepted architectural design for the positive electrode of sulfur batteries. The difficulties arise when trying to find a balance between high-surface-area architectures and practical utilization of the sulfur content. Intensive understanding of the interfacial mechanisms becomes then crucial to design optimized carbon-hosted sulfur architectures with enhanced electrochemical performance. In this work, we use density functional theory (DFT)-based first principles calculations to describe and characterize the growing mechanisms of Li₂S active material on graphene, taken as an example of a nonencapsulated carbon host for the positive electrode of Li-S batteries. We first unravel the two growing mechanisms of Li₂S supported nanostructures, which explain recent experimental findings on real-time monitoring of interfacial deposition of lithium sulfides during discharge, obtained by means of in situ atomic force microscopy. Then, using a combination of mathematical tools and DFT calculations, we obtain the first cycle voltage plot, explaining the three different regions observed that ultimately lead to the formation of high-order polysulfides upon charge. Finally, we show how the different Li₂S supported nanostructures can be characterized in X-ray photoelectron spectroscopy measurements. Altogether, this work provides useful insights for the rational design of new carbon-hosted sulfur architectures with optimized characteristics for the positive electrode of lithium-sulfur batteries.

Theoretical investigation on graphene-supported single-atom catalysts for electrochemical CO2 reduction

TM atoms supported on the graphene sheet (TM@Gr_s) as promising CO₂ catalysts were investigated by first-principles calculations. Cr-, Co- and Rh@Gr_s show remarkable performance with the low limiting potentials for CO₂RR.

Boron and pyridinic nitrogen-doped graphene as potential catalysts for rechargeable non-aqueous sodium–air batteries

In this work, we performed density functional theory (DFT) analysis of nitrogen (N)- and boron (B)-doped graphene, and N,B-co-doped graphene as potential catalysts for rechargeable non-aqueous sodium–air batteries. Four steps of an NaO₂ growth and depletion mechanism model were implemented to study the effects of B- and N-doped and co-doped graphene on the reaction pathways, overpotentials, and equilibrium potentials. The DFT results revealed that two-boron- and three-nitrogen (pyridinic)-doped graphene exhibited plausible reaction pathways at the lowest overpotentials, especially during the charging process (approximately 200 mV), thus, significantly improving the oxygen reduction and oxidation reactions of pristine graphene. In addition, pyridinic nitrogen-doped graphene meaningfully increased the equilibrium potential by approximately 0.30 eV compared to the other graphene-based materials considered in this study. This detailed DFT study provides valuable data that can be used for the successful development of low-cost and efficient graphene-based catalysts for sodium–air battery systems operating with non-aqueous electrolyte.

We performed density functional theory analysis of heteroatom doped graphene as potential catalysts for rechargeable non-aqueous sodium–air batteries. Pyridinic nitrogen and boron doped graphene exhibited too low overpotential reaction pathways.

Hollow carbon nanofibers as high-performance anode materials for sodium-ion batteries

Hollow carbon nanofibers (HCNFs) are successfully fabricated by pyrolyzation of a polyaniline hollow nanofiber precursor. The as-prepared HCNFs as sodium storage anode materials exhibit a high reversible charge capacity of 326 mA h g-1 at 20 mA g-1, high rate capability (85 mA h g-1 at 1.6 A g-1) and superior cycling stability (a capacity retention of 70% even after 5000 cycles at 1.6 A g-1). Such a high performance of HCNFs can be ascribed to the special hollow structure characteristics; they possess a well fabricated electronic transport path and can shorten the ion diffusion distance. Therefore, the HCNFs can be regarded as promising anode materials for advanced sodium ion batteries (SIBs).

Potential Applications of Heterostructures of TMDs with MXenes in Sodium-Ion and Na-O2 Batteries

Na-ion batteries are viewed as the alternative to Li-ion batteries for similar electrochemical properties, while they always suffer from a low capacity. Na-O₂ batteries are important due to their high energy density; however, they are usually limited by high overpotential. In this manuscript, 16 different heterostructures of TMDs with MXenes (bare and O-terminated case) are constructed and their potential in the application of sodium-ion batteries (SIBs) and Na-O₂ batteries is explored. Among these structures, it is proved that only the heterostructures of VS₂ with O-terminated MXenes could load five layers of Na⁺ ions, while the others will have a distortion when Na⁺ ions intercalate or diffuse in the interlayer or the second adsorption layer. The ultrasmall diffusion barrier of Na⁺ ion denotes that these structures have a fast charge/discharge speed, and the ultrasmall open circuit voltages (OCVs) of 0.18 and 0.21 V prove that they are promising candidates for SIBs. The ultralow overpotential 0.55 V/0.20 V for the η_ORR/η_OER means that the O facet of the VS₂/Ti₂CO₂ heterostructure also has a great potential in the application of Na-O₂ batteries. These simulations prove that the heterostructures constructed by TMDs with MXenes have great potential in SIBs and Na-O₂ batteries and are important for future battery design.

Recent Progress on Catalysts for the Positive Electrode of Aprotic Lithium-Oxygen Batteries †

Rechargeable aprotic lithium-oxygen (Li-O2) batteries have attracted significant interest in recent years owing to their ultrahigh theoretical capacity, low cost, and environmental friendliness. However, the further development of Li-O2 batteries is hindered by some ineluctable issues, such as severe parasitic reactions, low energy efficiency, poor rate capability, short cycling life and potential safety hazards, which mainly stem from the high charging overpotential in the positive electrode side. Thus, it is of great significance to develop high-performance catalysts for the positive electrode in order to address these issues and to boost the commercialization of Li-O2 batteries. In this review, three main categories of catalyst for the positive electrode of Li-O2 batteries, including carbon materials, noble metals and their oxides, and transition metals and their oxides, are systematically summarized and discussed. We not only focus on the electrochemical performance of batteries, but also pay more attention to understanding the catalytic mechanism of these catalysts for the positive electrode. In closing, opportunities for the design of better catalysts for the positive electrode of high-performance Li-O2 batteries are discussed.

Nitrogen-doped graphene fiber webs for multi-battery energy storage

Freestanding carbon-based electrodes with large surface areas and pore volumes are essential to fast ion transport and long-term energy storage. Many of the current porous carbon substrates are composed of particulates, making it difficult to form a self-supported structure. Herein, novel highly porous nitrogen-doped graphene fiber webs (N-GFWs) are prepared using a facile wet-spinning method. The wet chemical process facilitates simultaneous N-doping and surface wrinkling of graphene fibers in a one-pot process. The atomic structure and electrical conductivity of N-GFWs are tailored by tuning the degree of N-doping and thermal reduction for multi-battery charge storage in both lithium-oxygen batteries (LOBs) and lithium-sulfur batteries (LSBs). The N-GFW900 electrode presents an excellent electrocatalytic activity and the cathode with a high areal loading of 7.5 mg cm-2 delivers a remarkable areal capacity of 2 mA h cm-2 at 0.2 mA cm-2 for LOBs. The N-GFW700 interlayer with abundant oxygenated and nitrogen functional groups demonstrates effective entrapment of polysulfides in LSBs, delivering a much improved specific capacity after 200 cycles at 0.5C with a remarkable decay rate of 0.04%. The current approach paves the way for rational design of porous graphene-based electrodes, satisfying multifunctional requirements for high-energy storage applications.

Phosphorene as a Catalyst for Highly Efficient Nonaqueous Li-Air Batteries

Li-air batteries can yield exceptionally high predicted energy densities. However, for this technology to become realizable, round-trip efficiency issues and slow kinetics at the cathode require implementation of a catalyst. With design parameters not well understood and limitations on material selection, choosing an ideal catalyst is complex. In Li-air batteries, energy storage is achieved by reactions between Li and O (oxygen reduction reaction for discharge and oxygen evolution reaction for charge). Here, phosphorene is proposed as a solution through simulations of its catalytic behavior toward discharge initiated via either O₂ dissociation or Li adsorption. After obtaining intermediate geometries for both nucleation paths leading to either Li₂O₂ or 2(Li₂O), free-energy diagrams are generated to predict the promoted discharge product of Li₂O₂. Furthermore, considering a final product of Li₂O₂, the overpotentials are predicted to be 1.44 V for discharge and 2.63 V for charge. Activation barriers for the catalytic decomposition of Li₂O₂ (during charge) are found to be 1.01 eV for phosphorene versus 2.06 eV for graphene. This leads to a major difference in reaction rate up to 10¹⁷ times in favor of phosphorene. These results, complemented by electronic analysis, establish phosphorene as a promising catalyst for Li-air batteries.

A Quasi-Solid-State Flexible Fiber-Shaped Li-CO2 Battery with Low Overpotential and High Energy Efficiency

The rapid development of wearable electronics requires a revolution of power accessories regarding flexibility and energy density. The Li-CO₂ battery was recently proposed as a novel and promising candidate for next-generation energy-storage systems. However, the current Li-CO₂ batteries usually suffer from the difficulties of poor stability, low energy efficiency, and leakage of liquid electrolyte, and few flexible Li-CO₂ batteries for wearable electronics have been reported so far. Herein, a quasi-solid-state flexible fiber-shaped Li-CO₂ battery with low overpotential and high energy efficiency, by employing ultrafine Mo₂ C nanoparticles anchored on a carbon nanotube (CNT) cloth freestanding hybrid film as the cathode, is demonstrated. Due to the synergistic effects of the CNT substrate and Mo₂ C catalyst, it achieves a low charge potential below 3.4 V, a high energy efficiency of ≈80%, and can be reversibly discharged and charged for 40 cycles. Experimental results and theoretical simulation show that the intermediate discharge product Li₂ C₂ O₄ stabilized by Mo₂ C via coordinative electrons transfer should be responsible for the reduction of overpotential. The as-fabricated quasi-solid-state flexible fiber-shaped Li-CO₂ battery can also keep working normally even under various deformation conditions, giving it great potential of becoming an advanced energy accessory for wearable electronics.

First-Principles Study of Magnesium Peroxide Nucleation for Mg-Air Battery

Recently, rechargeable non-aqueous Mg-air batteries have gained a lot of interest as the next-generation energy storage device due to the high theoretical volumetric density (3832 Ah L^-1 for Mg anode vs. 2062 Ah L^-1 for Li), low cost and safety. The field of Mg-air batteries is in the initial stage of development having a limited number of experimental and theoretical reports, in which mainly a carbon cathode is used; however, the information regarding the structural form of carbon is still missing. In this work, using first-principles density functional theory (DFT) calculations, we demonstrate the possibility of graphene and graphite as a cathode material towards Mg-air batteries by studying the initial MgO and MgO₂ nucleation processes on the surfaces of graphene and graphite. The calculated free energy diagrams for the redox reactions of oxygen are used to identify the rate-determining step controlling the overpotentials for initial nucleation of MgO and MgO₂ . We observe that graphene and graphite surfaces show similar reactivity towards the nucleation of MgO or MgO₂ , and the overpotential of the controlling steps for MgO₂ nucleation is comparatively less than that of MgO nucleation, which is supported by a recent experimental study, where a higher discharge voltage was observed in a cell having a mixed MgO/MgO₂ discharge product than MgO-based cells. Furthermore, the preferable formation of MgO₂ cluster compared to MgO on the graphene surface during the ab initio molecular dynamic (AIMD) simulations confirms the selectivity of MgO₂ formation over MgO as the final discharge product. We believe that our study will be helpful in understanding the initial nucleation processes during the oxygen reduction reaction (ORR) mechanism and development of suitable cathodes for the future Mg-air batteries.

Performance-improved Li-O2 batteries by tailoring the phases of MoxC porous nanorods as an efficient cathode

Novel nitrogen-doped porous molybdenum carbide (α-MoC1-x and β-Mo2C) architectures were prepared using Mo-based metal-organic frameworks (MOFs) as the precursor. The synthesized molybdenum carbides consist of numerous nanocrystals organized into micro-sized rods with interpenetrating mesoporous-channels and macroporous-tunnels along the axial direction. When employed as the cathode catalyst for Li-O2 batteries, this dual pore configuration offers abundant active sites for the electrochemical reaction and many nucleation sites for the discharge product of Li2O2; hence, decent performances were obtained. Among the two synthesized molybdenum carbides, the α-MoC1-x electrode stands out as being better due to its lower charge transfer resistance (395.8 Ω compared to 627.9 Ω) and better O2 adsorption (binding energy of -1.87 eV of α-(111)-Mo compared to -0.72 eV of β-(101)-Mo). It delivered a high full discharge of 20 212 mA h g-1 with a discharge voltage of 2.62 V at 200 mA g-1. A good cycling stability was also obtained: i.e. 100 stable cycles with a fixed capacity of 1000 mA h g-1 (at a current density of 200 mA g-1) with a charging voltage of 4.24 V and maintaining a respectable round-trip efficiency of ∼70%.

CxNy particles@N-doped porous graphene: a novel cathode catalyst with a remarkable cyclability for Li-O2 batteries

Despite the intrinsic advantages of ultra-high theoretical capacity and energy density of lithium-O2 batteries, there remain several critical issues to be resolved, especially the two concerning poor cyclability and rate capability. In this work, CxNy particles@N-doped porous graphene (CxNy@NPG) with a novel three-dimensional architecture is successfully synthesized via a simple template method and employed as the cathode catalyst of Li-O2 batteries. It is surprisingly found that the as-synthesized CxNy@NPG cathode not only demonstrates a remarkable cycling performance of 200 cycles at 1000 mA g-1 but also an intriguing high-rate capability with 8892 mA h g-1 at 1000 mA g-1, both of which can be attributed to a synergistic effect between the unique 3D porous structure and an effective N-doping. Specifically, it is believed that the unique porous 3D structure will, on one hand, build numerous microchannels, thus facilitating rapid O2 diffusion, and on the other hand, provide sufficient storage space to accommodate adequate discharge products. Indispensably, it is also believed that the N-doped porous graphene enables improved bifunctional catalytic activities towards both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), thus decreasing the discharge/charge overpotential, and reducing undesired side reactions. It is anticipated that the new 3D porous CxNy@NPG provides an inspiring route to design long cycling and high-rate performance cathodes for Li-O2 batteries.

Doped boron nitride surfaces: potential metal free bifunctional catalysts for non-aqueous Li-O2 batteries

The development of novel cathode catalysts is crucial for the practical application of lithium-oxygen (Li-O2) batteries. In this paper, we have evaluated the catalytic mechanism and activity of doped hexagonal boron nitride (h-BN) surfaces as cathode catalysts for nonaqueous Li-O2 batteries. From the free energy diagrams it is evident that the CN doped h-BN surface shows the best catalytic activity among the others and this arises due to its considerably lower oxygen reduction reaction (ORR) overpotential and lower oxygen evolution reaction (OER) overpotential. This is due to the weaker binding of the first product (LiO2) and stronger binding with the inserted Li atom. The computations predict that among the considered doped h-BN surfaces, the CN doped h-BN surface can be an efficient metal-free cathode material for nonaqueous Li-O2 batteries.

Near-Infrared-Driven Selective Photocatalytic Removal of Ammonia Based on Valence Band Recognition of an α-MnO2/N-Doped Graphene Hybrid Catalyst

Near-infrared (NIR)-response photocatalysts are desired to make use of 44% NIR solar irradiation. A flower-like α-MnO₂/N-doped graphene (NG) hybrid catalyst was synthesized and characterized by X-ray diffraction spectroscopy, transmission electron microscopy, Raman spectroscopy, UV-vis-NIR diffuse reflectance spectroscopy, and X-ray photoelectron spectroscopy. The flower-like material of α-MnO₂/NG was oval-shaped with the semi major axis of 140 nm and semi minor axis of 95 nm and the petal thickness of 3.5-8.0 nm. The indirect band gap was measured to be 1.16 eV, which is very close to 0.909 eV estimated by the first-principles calculation. The band gap can harvest NIR irradiation to 1069 nm. The coupling of α-MnO₂ with NG sheets to form α-MnO₂/NG can significantly extend the spectrum response up to 1722 nm, improving dramatically the photocatalytic activity. The experimental results displayed that the α-MnO₂/NG hybrid catalyst can recognize ammonia in methyl orange (MO)-ammonia, rhodamine B (RHB)-ammonia, and humic acid-ammonia mixed solutions and selectively degrade ammonia. The degradation ratio of ammonia reached over 93.0% upon NIR light irradiation in the mixed solutions, while those of MO, RHB, and humic acid were only 9.7, 9.4, and 15.7%, respectively. The products formed during the photocatalytic process were followed with ion chromatography, gas chromatography, and electrochemistry. The formed nitrogen gas has been identified during the photocatalytic process. A valence band recognition model was suggested based on the selective degradation of ammonia via α-MnO₂/NG.

Shuttle inhibition by chemical adsorption of lithium polysulfides in B and N co-doped graphene for Li-S batteries

The advance of lithium sulfur batteries is now greatly restricted by the fast capacity fading induced by shuttle effect. Using first-principles calculations, various vacancies, N doping, and B,N co-doping in graphene sheets have been systematically explored for lithium polysufides entrapped in Li-S batteries. The LiS, LiC, LiN and SB bonds and Hirshfeld charges in the Li₂S₆ adsorbed defective graphene systems have been analyzed to understand the intrinsic mechanism of retaining lithium polysulfides in these systems. Total and local densities of states analyses elucidate the strongest adsorption sites among the N and B-N co-doped graphene systems. The overall electrochemical performance of Li-S batteries varies with the types of defects in graphene. Among the defective graphene systems, only the reconstructed pyrrole-like vacancy is effective for retaining lithium polysulfides. N doping induces a strong LiN interaction in the defective graphene systems, in which the pyrrolic N rather than the pyridinic N plays a dominant role in trapping of lithium polysulfides. The shuttle effect can be further depressed via pyrrolic B,N co-doped defective graphene materials, especially the G-B-N-hex system with extremely strong adsorption of lithium polysulfides (4-5 eV), and simultaneous contribution from the strong LiN and SB interactions.

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

The electrochemical reaction producing crystalline LiO₂ on the LiScO₂ nanosheet can deliver a high discharge voltage of 3.50 V.

Functional and stability orientation synthesis of materials and structures in aprotic Li–O2batteries

This review presents the recent advances made in the functional and stability orientation synthesis of materials/structures for Li–O₂batteries.