Li2 O2 Formation Electrochemistry and Its Influence on Oxygen Reduction/Evolution Reaction Kinetics in Aprotic Li-O2 Batteries

Unlocking the critical roles of N, P Co-Doping in MXene for Lithium-Oxygen Batteries: Elevated d-Band center and expanded interlayer spacing

The design and fabrication of bifunctional catalysts with high electrocatalytic activity and stability are critical for developing highly reversible Li-O2 batteries (LOBs). Herein, the N, P co-doped MXene (NP-MXene) is prepared by one-step annealing method and evaluated as bifunctional catalyst for LOBs. The results suggest that the P doping plays a crucial role in increasing interlayer distance of MXene, thereby effectively providing more active sites, fast mass transfer, and ample space for the deposition/decomposition of Li2O2. Moreover, the N doping can significantly elevate the d-band center of Ti, thereby remarkably improving the adsorption of reaction intermediates and accelerating the deposition/decomposition of Li2O2 films. Consequently, the MXene-based LOBs deliver an ultrahigh specific capacity of 13,995 mAh/g at 500 mA g-1, a discharge/charge voltage gap of 0.89 V, and a cycle life up to 523 cycles with a limited capacity of 1000 mAh/g at 500 mA g-1. Impressively, the as-fabricated flexible LOBs with NP-MXene cathode display excellent cycling stability and ability to continuously power LEDs even after bending. Our findings pave the road of heteroatom doped MXenes as next-generation electrodes for high-performance energy storage and conversion systems.

Engineering Triple-Phase Interfaces around the Anode toward Practical Alkali Metal-Air Batteries

Alkali metal-air batteries (AMABs) promise ultrahigh gravimetric energy densities, while the inherent poor cycle stability hinders their practical application. To address this challenge, most previous efforts are devoted to advancing the air cathodes with high electrocatalytic activity. Recent studies have underlined the solid-liquid-gas triple-phase interface around the anode can play far more significant roles than previously acknowledged by the scientific community. Besides the bottlenecks of uncontrollable dendrite growth and gas evolution in conventional alkali metal batteries, the corrosive gases, intermediate oxygen species, and redox mediators in AMABs cause more severe anode corrosion and structural collapse, posing greater challenges to the stabilization of the anode triple-phase interface. This work aims to provide a timely perspective on the anode interface engineering for durable AMABs. Taking the Li-air battery as a typical example, this critical review shows the latest developed anode stabilization strategies, including formulating electrolytes to build protective interphases, fabricating advanced anodes to improve their anti-corrosion capability, and designing functional separator to shield the corrosive species. Finally, the remaining scientific and technical issues from the prospects of anode interface engineering are highlighted, particularly materials system engineering, for the practical use of AMABs.

Nature of Li2O2 and its relationship to the mechanisms of discharge/charge reactions of lithium-oxygen batteries

Lithium-air batteries (LABs) are considered one of the most promising energy storage devices because of their large theoretical energy density. However, low cyclability caused by battery degradation prevents its practical use. Thus, to realize practical LABs, it is essential to improve cyclability significantly by understanding how the degradation processes proceed. Here, we used online mass spectrometry for real-time monitoring of gaseous products generated during charging of lithium-oxygen batteries (LOBs), which was operated with pure oxygen not air, with 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) tetraethylene glycol dimethyl ether (TEGDME) electrolyte solution. Linear voltage sweep (LVS) and voltage step modes were employed for charge instead of constant current charge so that the energetics of the product formation during the charge process can be understood more quantitatively. The presence of two distinctly different types of Li2O2, one being decomposed in a wide range of relatively low cell voltages (2.8-4.16 V) (l-Li2O2) and the other being decomposed at higher cell voltages than ca. 4.16 V (h-Li2O2), was confirmed by both LVS and step experiments. H2O generation started when the O2 generation rate reached a first maximum and CO2 generation took place accompanied by the decomposition of h-Li2O2. Based on the above results and the effects of discharge time and the use of isotope oxygen during discharge on product distribution during charge, the generation mechanism of O2, H2O, and CO2 during charging is discussed in relation to the reactions during discharge.

Dynamic Modulation of Li2O2 Growth in Li-O2 Batteries through Regulating Oxygen Reduction Kinetics with Photo-Assisted Cathodes

Widely acknowledged that the capacity of Li-O2 batteries (LOBs) should be strongly determined by growth behaviors of the discharge product of lithium peroxide (Li2O2) that follows both coexisting surface and solution pathways. However until now, it remains still challenging to achieve dynamic modulation on Li2O2 morphologies. Herein, the photo-responsive Au nanoparticles (NPs) supported on reduced oxide graphene (Au/rGO) have been utilized as cathode to manipulate oxygen reduction reaction (ORR) kinetics by aid of surface plasmon resonance (SPR) effects. Thus, we can experimentally reveal the importance of matching ORR kinetics with Li+ migration towards battery performance. Moreover, it is found that Li+ concentration polarization caused "sudden death" of LOBs is supposed to be just a form of suspended animation that could timely recover under irradiation. This work provides us an in-depth explanation on the working mechanism of LOBs from a kinetic perspective, offering valuable insights for the future battery design.

Boosting the ORR/OER Activity of Cobalt-Based Nano-Catalysts by Co 3d Orbital Regulation

The transition metal oxides/sulfides are considered promising catalysts due to their abundant resources, facile synthesis, and reasonable electrocatalytic activity. Herein, a significantly improved intrinsic catalytic activity is achieved for constructing a Co-based nanocrystal (Co-S@NC) with the coordination of Co─S, Co─S─C, and Co─Nx─C. The calculational and experimental results demonstrate that the diversified chemical environment of Co-cations induces the transition of 3d orbitals to a high spin-state that exhibits the coexistence of Co2+ with fully occupied dπ orbitals and Co3+ with unpaired electrons in dπ orbitals. The diverse dπ orbitals occupation contributes to an elevated d-band center of Co ions, which accelerates oxygen reduction reaction and oxygen evolution reaction electrocatalytic kinetics of the Co-S@NC nanocrystal. Therefore, the Li-O2 batteries with Co-S@NC as cathode catalyst exhibit 300 cycles at the current density of 500 mA g-1 with a cut-off capacity of 1000 mAh g-1. Moreover, the ultrahigh discharge specific capacity of 34 587 mAh g-1 is obtained at a current density of 1000 mA g-1, corresponding to the energy density 949 Wh kg-1 of a prototype Li-O2 battery. The study on 3d orbital regulation of nanocrystals provides an innovative strategy for bifunctional electrocatalysts toward the practical application of metal-air batteries.

Hybrid Organic-Inorganic Additive for Robust Al Anode in Alkaline Aluminum-Air Battery

Aluminum-air batteries (AABs), known for their high energy density, environmental friendliness, and cost-effectiveness, show immense promise in the realm of energy conversion applications. Nonetheless, their commercialization has encountered inherent challenges of Al anode corrosion and material degradation. In this study, economical hybrid electrolyte additives to inhibit the Al corrosion are developed, safeguarding the integrity of the Al anode. Due to the synergistic interplay between the organic compound dithiothreitol, and inorganic compounds zinc chloride, a robust zinc film is formed on the Al surface This Zn film plays a pivotal role in quelling parasitic hydrogen evolution reactions that typically can plague the Al electrode. Consequently, the as-prepared hybrid additive culminates in a remarkable enhancement to AABs, delivering exceptional discharge capacity of 1793.37 mAh g-1 , high energy density of 2047 Wh kg-1 , and excellent battery longevity (over 20 h in on/off cycling tests). This study, therefore, introduces a novel approach in utilizing hybrid electrolyte additives to effectively counteract corrosion-related challenges and boost the stability and performance of AABs.

Enhanced TiO2/SiCx Active Layer Formed In Situ on Coal Gangue/Ti3C2 MXene Electrocatalyst as Catalytic Integrated Units for Efficient Li-O2 Batteries

The pursuit of efficient cathode catalysts to improve cycle stability at ultra-high rates plays an important role in boosting the practical utilization of Li-O2 batteries. Featured as industrial solid waste, coal gangue with rich electrochemical active components could be a promising candidate for electrocatalysts. Here, a coal gangue/Ti3C2 MXene hybrid with a TiO2/SiCx active layer is synthesized and applied as a cathode catalyst in Li-O2 batteries. The coal gangue/Ti3C2 MXene hybrid has a tailored amorphous/crystalline heterostructure, enhanced active TiO2 termination, and a stable SiCx protective layer; thereby, it achieved an excellent rate stability. The Li-O2 battery, assembled with a coal gangue/Ti3C2 MXene cathode catalyst, was found to obtain a competitive full discharge capacity of 3959 mAh g-1 and a considerable long-term endurance of 180 h (up to 175 cycles), with a stable voltage polarization of 1.72 V at 2500 mA g-1. Comprehensive characterization measurements (SEM, TEM, XPS, etc.) were applied; an in-depth analysis was conducted to reveal the critical role of TiO2/SiCX active units in regulating the micro-chemical constitution and the enhanced synergistic effect between coal gangue and Ti3C2 MXene. This work could provide considerable insights into the rational design of catalysts derived from solid waste gangue for high-rate Li-O2 batteries.

Exploring the Potential of Hierarchical Zeolite-Templated Carbon Materials for High-Performance Li-O2 Batteries: Insights from Molecular Simulations

The commercialization of ultrahigh capacity lithium-oxygen (Li-O2) batteries is highly dependent on the cathode architecture, and a better understanding of its role in species transport and solid discharge product (i.e., Li2O2) formation is critical to improving the discharge capacity. Tailoring the pore size distribution in the cathode structure can enhance the ion mobility and increase the number of reaction sites to improve the formation of solid Li2O2. In this work, the potential of hierarchical zeolite-templated carbon (ZTC) structures as novel electrodes for Li-O2 batteries was investigated by using reactive force field molecular dynamics simulation (reaxFF-MD). Initially, 47 microporous zeolite-templated carbon morphologies were screened based on microporosity and specific area. Among them, four structures (i.e., RHO-, BEA-, MFI-, and FAU-ZTCs) were selected for further investigation including hierarchical features in their structures. Discharge product cluster analysis, self-diffusivities, and density number profiles of Li+, O2, and dimethyl sulfoxide (DMSO) electrolyte were obtained to find that the RHO-type ZTC exhibited enhanced mass transfer compared to conventional microporous ZTC (approximately 31% for O2, 44% for Li+, and 91% for DMSO) electrodes. This is due to the promoted formation of small-sized product clusters, creating more accessible sites for oxygen reduction reaction and mass transport. These findings indicate how hierarchical ZTC electrodes with micro- and mesopores can enhance the discharge performance of aprotic Li-O2 batteries, providing molecular insights into the underlying phenomena.

Protecting Li-metal in O2 atmosphere by a sacrificial polymer additive in Li-O2 batteries

Li-O2 batteries (LOBs) with Li-metal as the anode are characterized by their high theoretical energy density of 3500 W h kg-1 and are thus considered next-generation batteries with an unlimited potential. However, upon cycling in a harsh O2 atmosphere, the poor-quality solid electrolyte interphase (SEI) film formed on the surface of the Li-metal anode cannot effectively suppress the shuttle effect from O2, superoxide species, protons, and soluble side products. These issues lead to aggravated Li-metal corrosion and hinder the practical development of LOBs. In this work, a polyacrylamide-co-polymethyl acrylate (PAMMA) copolymer was innovatively introduced in an ether-based electrolyte as a sacrificial additive. PAMMA was found to preferentially decompose and promote the formation of a dense and Li3N-rich SEI film on the Li-metal surface, which could effectively prohibit the shuttle effect from a series of detrimental species in the Li-O2 cell during the discharge/charge process. Using PAMMA, well-protected Li-metal in a harsh O2 atmosphere and significantly enhanced cycling performance of the Li-O2 cell could be achieved. Thus, the use of a sacrificial polymer additive provides a promising strategy for the effective protection of Li-metal in Li-O2 cells in a severe O2 atmosphere during practical applications.

Water-Trapping Single-Atom Co-N4 /Graphene Triggering Direct 4e- LiOH Chemistry for Rechargeable Aprotic Li-O2 Batteries

Lithium-oxygen (Li-O2 ) batteries have received extensive attention owing to ultrahigh theoretical energy density. Compared to typical discharge product Li2 O2 , LiOH has attracted much attention for its better chemical and electrochemical stability. Large-scale applications of Li-O2 batteries with LiOH chemistry are hampered by the serious internal shuttling of the water additives with the desired 4e- electrochemical reactions. Here, a metal organic framework-derived "water-trapping" single-atom-Co-N4 /graphene catalyst (Co-SA-rGO) is provided that successfully mitigates the water shuttling and enables the direct 4e- catalytic reaction of LiOH in the aprotic Li-O2 battery. The Co-N4 center is more active toward proton-coupled electron transfer, benefiting - direction 4e- formation of LiOH. 3D interlinked networks also provide large surface area and mesoporous structures to trap ≈12 wt% H2 O molecules and offer rapid tunnels for O2 diffusion and Li+ transportation. With these unique features, the Co-SA-rGO based Li-O2 battery delivers a high discharge platform of 2.83 V and a large discharge capacity of 12 760.8 mAh g-1 . Also, the battery can withstand corrosion in the air and maintain a stable discharge platform for 220 cycles. This work points out the direction of enhanced electron/proton transfer for the single-atom catalyst design in Li-O2 batteries.

Boosting Li-O2 Battery Performance via Coupling of P-N Site-Rich N, P Co-Doped Graphene-Like Carbon Nanosheets with Nano-CePO4

Development of efficient and robust cathode catalysts is critical for the commercialization of Li-O₂ batteries (LOBs). Herein, a well-designed CePO₄ @N-P-CNSs cathode catalyst for LOBs via coupling P-N site-rich N, P co-doped graphene-like carbon nanosheets (N-P-CNSs) with nano-CePO₄ via a novel "in situ derivation" coupling strategy by in situ transforming the P atoms of P-C sites in N-P-CNSs to CePO₄ is reported. The CePO₄ @N-P-CNSs exhibit superior bifunctional ORR/OER activity relative to commercial Pt/C-RuO₂ with an overall overpotential of 0.64 V (vs RHE). Moreover, the LOB with CePO₄ @N-P-CNSs as the cathode catalyst delivers a low charge overpotential of 0.67 V (vs Li/Li⁺ ), high discharge capacity of 29774 mAh g^-1 at 100 mA g^-1 and long cycling stability of 415 cycles, respectively. The remarkably enhanced LOB performance is attributable to the in situ derived CePO₄ nanoparticles and the P-N sites in N-P-CNSs, which facilitate increased bifunctional ORR/OER activity, promote the rapid and effective decomposition of Li₂ O₂ and inhibit the formation of Li₂ CO₃ . This work may provide new inspiration for designing efficient, durable, and cost-effective cathode catalysts for LOBs.

Reversible Discharge Products in Li-Air Batteries

Lithium-air (Li-air) batteries stand out among the post-Li-ion batteries due to their high energy density, which has rapidly progressed in the past years. Regarding the fundamental mechanism of Li-air batteries that discharge products produced and decomposed during charging and recharging progress, the reversibility of products closely affects the battery performance. Along with the upsurge of the mainstream discharge products lithium peroxide, with devoted efforts to screening electrolytes, constructing high-efficiency cathodes, and optimizing anodes, much progress is made in the fundamental understanding and performance. However, the limited advancement is insufficient. In this case, the investigations of other discharge products, including lithium hydroxide, lithium superoxide, lithium oxide, and lithium carbonate, emerge and bring breakthroughs for the Li-air battery technologies. To deepen the understanding of the electrochemical reactions and conversions of discharge products in the battery, recent advances in the various discharge products, mainly focusing on the growth and decomposition mechanisms and the determining factors are systematically reviewed. The perspectives for Li-air batteries on the fundamental development of discharge products and future applications are also provided.

An Organic Redox Mediator with a Defense-Donor for Lithium Anode in Lithium-Oxygen Batteries

Lithium-oxygen batteries (LOBs) suffer from large charge overpotential and unstable Li metal interface, which can be attributed to the inefficient charge transport at the insulating Li₂ O₂ /cathode interface and the severe oxygen corrosion issue on the Li anode surface. The use of soluble redox mediators (RMs) can effectively enhance the charge transport between Li₂ O₂ and cathode, thus greatly reducing the charge overpotential. However, oxidized RMs will also shuttle to the anode side and react with the Li metal, which not only results in the loss of both the RMs and the electrical energy efficiency but also exacerbates the Li anode corrosion. Herein, an organic compound-acetylthiocholine iodide (ATCI), in which a big cation group is contained, is proposed as a defense-donor RM for lithium anode in LOBs to simultaneously address the above issues. During charge, it can accelerate the oxidation kinetics of Li₂ O₂ via its iodide anion redox couple (I^- /I₃ ^- ). Meanwhile, its cation segment (ATC⁺ ) can move to the anode surface via electric attraction and in situ forms a protective interfacial layer, which prevents the Li anode from the attack of oxidized RM and oxygen species. Consequently, the ATCI-containing LOBs can achieve both a low charge potential (≈3.49 V) and a long cycle life (≈190 cycles).

Organic-inorganic Hybrid Perovskite-Based Light-Assisted Li-oxygen Battery with Low Overpotential

Organic-inorganic hybrid perovskites have emerged in the last decade as promising semiconductors due to the excellent optoelectronic properties. This kind of perovskites exhibited respectable photocatalytic activities toward potential application in battery; however, the instability issue still hindered their practical use. Herein, a hybrid perovskite material, 4,4'-ethylenedipyridinium lead bromide [(4,4'-EDP)Pb₂ Br₆ ], was assembled onto the carbon materials to function as photoelectrode of the Li-oxygen battery. The strong cation-π interactions between the A-site cations enabled this hybrid perovskite to endure the cycling process as well as the exposure to battery electrolyte and oxygen. Benefitting from the photo-generated carriers of the photoelectrode under illumination, the formation/decomposition of the discharge product was accelerated, thus leading to a reduced overpotential from 1.3 V to an optimized 0.5 V compared to the Li-oxygen battery without illumination. The overpotential could be maintained lower than 0.9 V after cycling for 170 h. Furthermore, when exposed to the sunlight, the charging voltage was reduced by over 0.2 V. The intrinsic stability and strong light absorption of perovskites together with the optimized perovskite/carbon cathode interfaces contributed to the improved performance under different light sources without complex material design, which shed light on the exploration of organic-inorganic hybrid perovskites in Li-oxygen battery applications.

Sacrificial Co-solvent Electrolyte to Construct a Stable Solid Electrolyte Interphase in Lithium-Oxygen Batteries

Lithium-oxygen batteries are vital devices for electrochemical energy storage. The electrolyte is a crucial factor for improving battery performance. The high reactivity of lithium metal induces side reactions with organic electrolytes, thus leading to an unstable interface between the anode and electrolyte and poor performance of batteries. In this work, to compensate for the above shortcomings, 1-methylimidazole (MeIm) is introduced to the tetraethylene glycol dimethyl ether (TEGDME) electrolyte to form the TEGDME/MeIm co-solvent electrolyte. Because of the high donor number value of MeIm, the solution-based pathway of discharge products can be triggered. Compared with the single TEGDME electrolyte, the discharge capacity with the TEGDME/MeIm co-solvent electrolyte is increased by more than 2 times. Moreover, the TEGDME/MeIm co-solvent electrolyte can promote the dissociation of Li salt due to the high dielectric constant of MeIm and thus make up for the shortcomings of TEGDME. In addition, due to the lower energy than the lowest unoccupied molecular orbital (LUMO) level of TEGDME, MeIm is decomposed preferentially, and a dense solid electrolyte interphase (SEI) layer is constructed. Then, the decomposition of TEGDME is suppressed. Therefore, the cycle performance of the battery with the TEGDME/MeIm co-solvent electrolyte is 18 times compared to that with the single TEGDME electrolyte.

Strategies toward anode stabilization in nonaqueous alkali metal-oxygen batteries

Alkali metal-O2 batteries exhibit ultra-high theoretical energy density which is even on a par with to fossil energy and expected to become the next generation of energy storage devices. However,...