Potassium Doping Facilitated Formation of Tunable Superoxides in Li2O2 for Improved Electrochemical Kinetics

Co(acac)2-Mediated Regulation of Li2O2 Gradient Growth in Lithium-Oxygen Batteries

In Li-O2 batteries, Li2O2 serves as the primary cathodic material but its wide band gap imparts insulating properties. Regulating the composition and morphology of Li2O2 enables the construction of a novel cathodic structure with exceptional electrochemical performance. Herein, we introduce an organic salt containing cobalt ions, cobalt acetylacetonate (Co(acac)2), into the cathodic electrolyte. We exploit its cation migration characteristics under an electric field to facilitate the generation and decomposition of Co-doped Li2O2, achieving gradient control and optimized growth of Li2O2. Moreover, the Co(acac)2 molecule stabilizes the properties of LiOx species and mitigates side reaction. In situ UV-vis and XANES spectra reveal direct interactions between Co(acac)2 and O2-/LiO2, highlighting the superior reversibility of Co(acac)2 enhanced Li-O2 batteries. Both experimental and theoretical results indicate that this novel Li-O2 battery system exhibits rapid reaction kinetics, with a reduced overpotential of 520 mV and extended cyclability, surpassing 400 cycles with lower polarization.

A Self-Catalysis System Coupled with Redox Mediator Effect for Ultra-Long Cycle Life Li-O2 Batteries

The sluggish kinetics of Li-O2 batteries significantly limit their performance. To address this issue, the insulating characteristics of the discharge product Li2O2 and the reactivity of highly active superoxide species are examined. Herein, organic metal salts with weak electrolyte properties are utilized as bifunctional additives. The ionized metal ions can be reduced and doped Li2O2 through in situ electrochemical implantation, thereby altering its insulating properties. Additionally, organic metal salts function as redox mediators (RMs), stabilizing the intermediate LiO2 and facilitating its further disproportionation to Li2O2, as well as enhancing the decomposition reaction during charging, which are further proven by the in situ X-ray absorption spectroscopy and UV-vis spectroscopy. Notably, Li-O2 batteries incorporating Mn(acac)3 demonstrate an ultra-low overpotential of 0.43 V and sustain 250 long cycles at 1000 mA g-1. Furthermore, when combined with the optimized cathode, a remarkable cycle stability of 3850 cycles at 1000 mA g-1 is achieved. These findings offer novel insights into the design of advanced Li-O2 battery systems and the enhancement of their performance.

Long-Life Quasi-Solid-State Lithium-Oxygen Battery Enabled by the Gel Polymer Electrolyte and Redox Moieties Anchored in the Cathode

The practical development of Li-O2 batteries is often hindered by poor cycling stability, which arises from volatile liquid electrolytes, an unstable anode/electrolyte interface, and sluggish reaction kinetics related to Li2O2. In this study, we design a long-life quasi-solid-state Li-O2 battery by integrating a gel polymer electrolyte (GPE) with a tetramethylpiperidinyloxy (TEMPO) redox mediator anchored in a poly(2,2,6,6-tetramethylpiperidinyloxy-4-methacrylate) (PTMA) cathode. During cycling, the GPE stabilizes the lithium/electrolyte interface and retains the electrolyte, while the TEMPO moieties anchored in the PTMA cathode effectively enhance the catalytic selectivity for Li2O2 formation and decomposition. This innovative design significantly improves electrochemical performance, achieving an impressive lifespan of 800 h. The advancements in rechargeability and efficiency presented in this work are expected to pave the way for the development of long-lived solid-state Li-O2 batteries.

Fluorinated Amide-Based Electrolytes Induce a Sustained Low-Charging Voltage Plateau under Conditions Verifying the Feasibility of Achieving 500 Wh kg-1 Class Li-O2 Batteries

Although lithium-oxygen batteries (LOBs) hold the promise of high gravimetric energy density, this potential is hindered by high charging voltages. To ensure that the charging voltage remains low, it is crucial to generate discharge products that can be easily decomposed during the successive charging process. In this study, we discovered that the use of amide-based electrolyte solvents containing a fluorinated moiety can notably establish a sustained voltage plateau at low-charging voltages at around 3.5 V. This occurs under conditions that can verify the feasibility of achieving a benchmark energy density value of 500 Wh kg-1. Notably, the achievement of the low-voltage plateau was accomplished solely by relying on the intrinsic properties of the electrolyte solvent. Indeed, synchrotron X-ray diffraction measurements have shown that the use of fluorine-containing amide-based electrolyte solvents results in the formation of highly decomposable discharge products, such as amorphous and Li-deficient lithium peroxides.

The Role of Self-Catalysis Induced by Co Doping in Nonaqueous Li-O2 Batteries

This work systematically studies the product self-catalysis of in situ electrochemical cobalt doping of Li2O2 and reveals its potential mechanism for improving the performance of lithium-oxygen (Li-O2) batteries. Theoretical calculations demonstrate that the discharge products contain substituted and interstitial Co impurities, which serve as active sites to promote the formation of Li3O4 crystallization, thus switching the nucleation mechanism from the main discharge product Li2O2 to Li3O4. This Co-doping behavior leads to the thermodynamically favorable and dynamically stable formation of Li3O4 crystals during the discharge process. Through systematic investigation of the structural, energetic, electronic, diffusive, and catalytic properties of the Co-doped Li2O2 and Li3O4 compounds, we found that Li3O4 has better charge/mass transport and a lower overpotential for the Li3O4 formation/decomposition reaction. Consequently, this work elucidates that Co doping provides a simple and effective approach for increasing the proportion of Li3O4, which can significantly improve the Li-O2 battery performance.

Crystal Phase Conversion on Cobalt Oxide: Stable Adsorption toward LiO2 for Film-Like Discharge Products Generation in Li-O2 Battery

Regulating the structure and morphology of discharge product is one of the key points for developing high performance Li-O₂ batteries (LOBs). In this study, the reaction mechanism of LOB is successfully controlled by the regulated fine structure of cobalt oxide through tuning the crystallization process. It is demonstrated that the cobalt oxide with lower crystallinity shows stronger affinity toward LiO₂ , inducing the growth of film-like LiO₂ on the electrode surface and inhibiting the further conversion to Li₂ O₂ . The batteries catalyzed by the lower crystallinity cobalt oxide hollow spheres which pyrolyzed from ZIF-67 at 260 °C (ZIF-67-260), go through the generation and decomposition of amorphous film-like LiO₂ , which significantly reduces the charge overpotential and improves the cycle life. By contrast, the ZIF-67 hollow spheres pyrolyzed at 320 °C (ZIF-67-320) with better crystallinity are more likely to go through the solution-mediated mechanism and induce the aggregation of discharge product, resulting in the sluggish kinetics and limited performance. The combined density functional theory data also directly support the strong relationship between the adsorption toward LiO₂ by the electrocatalyst and the battery performance. This work provides an important way for tuning the intermediate and constructing the high-performance battery system.

Overlooked Factors Required for Electrolyte Solvents in Li-O2 Batteries: Capabilities of Quenching 1 O2 and Forming Highly-Decomposable Li2 O2

Although sufficient tolerance against attack by superoxide radicals (O₂ ^- ) has been mainly recognized as an important property for Li-O₂ battery (LOB) electrolytes, recent evidence has revealed that other critical factors also govern the cyclability, prompting a reconsideration of the basic design guidelines of LOB electrolytes. Here, we found that LOBs equipped with a N,N-dimethylacetamide (DMA)-based electrolyte exhibited better cyclability compared with other standard LOB electrolytes. This superior cyclability is attributable to the capabilities of quenching ¹ O₂ and forming highly decomposable Li₂ O₂ . The ¹ O₂ quenching capability is equivalent to that of a tetraglyme-based electrolyte containing a several millimolar concentration of a typical chemical quencher. Based on these overlooked factors, the DMA-based electrolyte led to superior cyclability despite its lower O₂ ^- tolerance. Thus, the present work provides a novel design guideline for the development of LOB electrolytes.

Monodispersed Ruthenium Nanoparticles on Nitrogen-Doped Reduced Graphene Oxide for an Efficient Lithium-Oxygen Battery

Lithium-oxygen batteries with ultrahigh energy densities have drawn considerable attention as next-generation energy storage devices. However, their practical applications are challenged by sluggish reaction kinetics aimed at the formation/decomposition of discharge products on battery cathodes. Developing effective catalysts and understanding the fundamental catalytic mechanism are vital to improve the electrochemical performance of lithium-oxygen batteries. Here, uniformly dispersed ruthenium nanoparticles anchored on nitrogen-doped reduced graphene oxide are prepared by using an in situ pyrolysis procedure as a bifunctional catalyst for lithium-oxygen batteries. The abundance of ruthenium active sites and strong ruthenium-support interaction enable a feasible discharge product formation/decomposition route by modulating the surface adsorption of lithium superoxide intermediates and the nucleation and growth of lithium peroxide species. Benefiting from these merits, the electrode provides a drastically increased discharge capacity (17,074 mA h g^-1), a decreased charge overpotential (0.51 V), and a long-term cyclability (100 cycles at 100 mA g^-1). Our observations reveal the significance of the dispersion and coordination of metal catalysts, shedding light on the rational design of efficient catalysts for future lithium-oxygen batteries.

Isotopic Depth Profiling of Discharge Products Identifies Reactive Interfaces in an Aprotic Li-O2 Battery with a Redox Mediator

Prior to the practical application of rechargeable aprotic Li-O₂ batteries, the high charging overpotentials of these devices (which inevitably cause irreversible parasitic reactions) must be addressed. The use of redox mediators (RMs) that oxidatively decompose the discharge product, Li₂O₂, is one promising solution to this problem. However, the mitigating effect of RMs is currently insufficient, and so it would be beneficial to clarify the Li₂O₂ reductive growth and oxidative decomposition mechanisms. In the present work, Nanoscale secondary ion mass spectrometry (Nano-SIMS) isotopic three-dimensional imaging and differential electrochemical mass spectrometry (DEMS) analyses of individual Li₂O₂ particles established that both growth and decomposition proceeded at the Li₂O₂/electrolyte interface in a system containing the Br^-/Br₃^- redox couple as the RM. The results of this study also indicated that the degree of oxidative decomposition of Li₂O₂ was highly dependent on the cell voltage. These data show that increasing the RM reaction rate at the Li₂O₂/electrolyte interface is critical to improve the cycle life of Li-O₂ batteries.