Superoxide stability for reversible Na-O2 electrochemistry

A Critical Review on Room-Temperature Sodium-Sulfur Batteries: From Research Advances to Practical Perspectives

Room-temperature sodium-sulfur (RT-Na/S) batteries are promising alternatives for next-generation energy storage systems with high energy density and high power density. However, some notorious issues are hampering the practical application of RT-Na/S batteries. Besides, the working mechanism of RT-Na/S batteries under practical conditions such as high sulfur loading, lean electrolyte, and low capacity ratio between the negative and positive electrode (N/P ratio), is of essential importance for practical applications, yet the significance of these parameters has long been disregarded. Herein, it is comprehensively reviewed recent advances on Na metal anode, S cathode, electrolyte, and separator engineering for RT-Na/S batteries. The discrepancies between laboratory research and practical conditions are elaborately discussed, endeavors toward practical applications are highlighted, and suggestions for the practical values of the crucial parameters are rationally proposed. Furthermore, an empirical equation to estimate the actual energy density of RT-Na/S pouch cells under practical conditions is rationally proposed for the first time, making it possible to evaluate the gravimetric energy density of the cells under practical conditions. This review aims to reemphasize the vital importance of the crucial parameters for RT-Na/S batteries to bridge the gaps between laboratory research and practical applications.

Real-Time Monitoring of Surface Effects on the Oxygen Reduction Reaction Mechanism for Aprotic Na-O2 Batteries

Discharging of aprotic sodium-oxygen (Na-O₂) batteries is driven by the cathodic oxygen reduction reaction in the presence of sodium cations (Na⁺-ORR). However, the mechanism of aprotic Na⁺-ORR remains ambiguous and is system dependent. In-situ electrochemical Raman spectroscopy has been employed to study the aprotic Na⁺-ORR processes at three atomically ordered Au(hkl) single-crystal surfaces for the first time, and the structure-intermediates/mechanism relationship has been identified at a molecular level. Direct spectroscopic evidence of superoxide on Au(110) and peroxide on Au(100) and Au(111) as intermediates/products has been obtained. Combining these experimental results with theoretical simulation has revealed that the surface effect of Au(hkl) electrodes on aprotic Na⁺-ORR activity is mainly caused by the different adsorption of Na⁺ and O₂. This work enhances our understanding of aprotic Na⁺-ORR on Au(hkl) surfaces and provides further guidance for the design of improved Na-O₂ batteries.

Mixed Lithium and Sodium Ion Aprotic DMSO Electrolytes for Oxygen Reduction on Au and Pt Studied by DEMS and RRDE

In order to advance the development of metal-air batteries and solve possible problems, it is necessary to gain a fundamental understanding of the underlying reaction mechanisms. In this study we investigate the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER, from species formed during ORR) in Na+ containing dimethyl sulfoxide (DMSO) on poly and single crystalline Pt and Au electrodes. Using a rotating ring disk electrode (RRDE) generator collector setup and additional differential electrochemical mass spectrometry (DEMS), we investigate the ORR mechanism and product distribution. We found that the formation of adsorbed Na2O2, which inhibits further oxygen reduction, is kinetically favored on Pt overadsorption on Au. Peroxide formation occurs to a smaller extent on the single crystal electrodes of Pt than on the polycrystalline surface. Utilizing two different approaches, we were able to calculate the heterogeneous rate constants of the O2/O2− redox couple on Pt and Au and found a higher rate for Pt electrodes compared to Au. We will show that on both electrodes the first electron transfer (formation of superoxide) is the rate-determining step in the reaction mechanism. Small amounts of added Li+ in the electrolyte reduce the reversibility of the O2/O2− redox couples due to faster and more efficient blocking of the electrode by peroxide. Another effect is the positive potential shift of the peroxide formation on both electrodes. The reaction rate of the peroxide formation on the Au electrode increases when increasing the Li+ content in the electrolyte, whereas it remains unaffected on the Pt electrode. However, we can show that the mixed electrolytes promote the activity of peroxide oxidation on the Pt electrode compared to a pure Li+ electrolyte. Overall, we found that the addition of Li+ leads to a Li+-dominated mechanism (ORR onset and product distribution) as soon as the Li+ concentration exceeds the oxygen concentration.

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Bi Nanoparticles Embedded in 2D Carbon Nanosheets as an Interfacial Layer for Advanced Sodium Metal Anodes

Sodium metal is regarded as one of the most prospective next-generation anodes material owing to its high theoretical capacity, low redox potential, low cost, and natural abundance. Its most notable problem is the dendrite growth during Na plating/striping, which causes not only the safety concern but also the generation of inactive Na. Here, it is demonstrated that 2D carbon nanosheets embedded by bismuth nanoparticles (NPs) (denoted as Bi⊂CNs) serve as a robust nucleation buffer layer to endow the sodium metal anodes (SMAs) with high Coulombic efficiencies (CEs) and dendrite-free deposition during long-term cycling. The embedded Bi nanoparticles significantly reduce the nucleation barrier through the "sodiophilic" Na-Bi alloy. Meanwhile, the carbon frameworks effectively circumvent the gradual failure of those Na-Bi nucleation sites. As a result, the metallic Na on the Bi⊂CNs nucleation layer is repeatedly plated/stripped for nearly 7700 h (1287 cycles) at 3 mA h cm^-2 with an average CE of 99.92%. Moreover, the Na||Na symmetric cells with the Bi⊂CNs buffer layer are stably plated/stripped for 4000 h at 1 mA cm^-2 and 1 mA h cm^-2 . It is found that the cycling stability is closely related to the Na utilization of SMAs and current rate.

Pulse Discharging of Sodium-Oxygen Batteries to Enhance Cathode Utilization

Using sodium metal in sodium-oxygen batteries with aprotic electrolyte enables achieving a very high theoretical energy density. However, the promised values for energy density and capacity are not met in practical studies yet due to poor utilization of the void space in the cathode during battery discharge. In this work, we achieve better cathode utilization and higher discharge capacities by using pulse discharging. We optimize the chosen resting-to-pulse times, the applied current density, and elucidate that three-dimensional cathode materials yield higher capacities compared to two-dimensional ones. By implication, the pulse discharging mode ensures better supply with dissolved oxygen within the cathode. The higher amount of dissolved oxygen accumulated during the resting period after a current pulse is essential to form more of the discharge product, i.e., the metal oxide sodium superoxide. Interestingly, we show for the first time that the superoxide is deposited in a very unusual form of stacked and highly oriented crystal layers. Our findings on the pulse discharging can be transferred to other metal-oxygen battery systems and might assist in achieving their full potential regarding practical energy density.

Impact of anion shape on Li+ solvation and on transport properties for lithium–air batteries: a molecular dynamics study

Here we report the influence of the anion shape over the solvation structure and transport properties over commonly employed Li–O₂ electrolytes and discuss their implications for the device.

Biomimetic Superoxide Disproportionation Catalyst for Anti-Aging Lithium-Oxygen Batteries

Reactive oxygen species or superoxide (O₂^-), which damages or ages biological cells, is generated during metabolic pathways using oxygen as an electron acceptor in biological systems. Superoxide dismutase (SOD) protects cells from superoxide-triggered apoptosis by converting superoxide to oxygen and peroxide. Lithium-oxygen battery (LOB) cells have the same aging problems caused by superoxide-triggered side reactions. We transplanted the function of SOD of biological systems into LOB cells. Malonic acid-decorated fullerene (MA-C₆₀) was used as a superoxide disproportionation chemocatalyst mimicking the function of SOD. As expected, MA-C₆₀ as the superoxide scavenger improved capacity retention along charge/discharge cycles successfully. A LOB cell that failed to provide a meaningful capacity just after several cycles at high current (0.5 mA cm^-2) with 0.5 mAh cm^-2 cutoff survived up to 50 cycles after MA-C₆₀ was introduced to the electrolyte. Moreover, the SOD-mimetic catalyst increased capacity, e.g., more than a 6-fold increase at 0.2 mA cm^-2. The experimentally observed toroidal morphology of the final discharge product of oxygen reduction (Li₂O₂) and density functional theory calculation confirmed that the solution mechanism of Li₂O₂ formation, more beneficial than the surface mechanism from the capacity-gain standpoint, was preferred in the presence of MA-C₆₀.

Tuning anion solvation energetics enhances potassium-oxygen battery performance

The oxygen reduction reaction (ORR) is a critical reaction in secondary batteries based on alkali metal chemistries. The nonaqueous electrolyte mediates ion and oxygen transport and determines the heterogeneous charge transfer rates by controlling the nature and degree of solvation. This study shows that the solvent reorganization energy (λ) correlates well with the oxygen diffusion coefficient [Formula: see text] and with the ORR rate constant [Formula: see text] in nonaqueous Li-, Na-, and K-O₂ cells, thereby elucidating the impact of variations in the solvation shell on the ORR. Increasing cation size (from Li⁺ to K⁺) doubled [Formula: see text], indicating an increased sensitivity of k to the choice of anion, while variations in [Formula: see text]were minimal over this cation size range. At the level of a symmetric K-O₂ cell, both the formation of solvent-separated ion pairs [K⁺-(DMSO)_n-ClO₄ ^- + (DMSO)_m-ClO₄ ^-] and the anions being unsolvated (in case of PF₆ ^-) lowered ORR activation barriers with a 200-mV lower overpotential for the PF₆ ^- and ClO₄ ^- electrolytes compared with OTf^- and TFSI^- electrolytes with partial anion solvation [predominantly K⁺-(DMSO)_n-OTf^-]. Balancing transport and kinetic requirements, KPF₆ in DMSO is identified as a promising electrolyte for K-O₂ batteries.

A high-rate and long-life organic-oxygen battery

Alkali metal-oxygen batteries promise high gravimetric energy densities but suffer from low rate capability, poor cycle life and safety hazards associated with metal anodes. Here we describe a safe, high-rate and long-life oxygen battery that exploits a potassium biphenyl complex anode and a dimethylsulfoxide-mediated potassium superoxide cathode. The proposed potassium biphenyl complex-oxygen battery exhibits an unprecedented cycle life (3,000 cycles) with a superior average coulombic efficiency of more than 99.84% at a high current density of 4.0 mA cm^-2. We further reduce the redox potential of biphenyl by adding the electron-donating methyl group to the benzene ring, which successfully achieved a redox potential of 0.14 V versus K/K⁺. This demonstrates the direction and opportunities to further improve the cell voltage and energy density of the alkali-metal organic-oxygen batteries.