Sodium Oxygen Batteries Research Articles

Phosphorous and Nitrogen Dual-Doped Carbon as a Highly Efficient Electrocatalyst for Sodium-Oxygen Batteries.

Sodium-oxygen batteries have been regarded as promising energy storage devices due to their low overpotential and high energy density. Its applications, however, still face formidable challenges due to the lack of understanding about the influence of electrocatalysts on the discharge products. Here, a phosphorous and nitrogen dual-doped carbon (PNDC) based cathode is synthesized to increase the electrocatalytic activity and to stabilize the NaO2 superoxide nanoparticle discharge products, leading to enhanced cycling stability when compared to the nitrogen-doped carbon (NDC). The PNDC air cathode exhibits a low overpotential (0.36 V) and long cycling stability (120 cycles). The reversible formation/decomposition and stabilization of the NaO2 discharge products are clearly proven by in-situ synchrotron X-ray diffraction and ex-situ X-ray diffraction. Based on the density functional theory calculation, the PNDC has much stronger adsorption (-2.85 eV) for NaO2 than that of NDC (-1.80 eV), which could efficiently stabilize the NaO2 discharge products.

A Rechargeable Solid-State Sodium-Oxygen Battery with Enhanced Energy Efficiency and Cycle Life

Sodium-oxygen (Na-O2) batteries offer a promising path for the advancement of high-energy-density inexpensive storage systems to replace current state of the art Li-based batteries owing to the natural abundance and low-cost Na metal. Despite numerous studies to improve the performance of liquid-based electrolyte Na-O2 batteries, this technology suffers from poor cycle life, electrolyte stability issues, and limited choice of cell design as well as growing safety concerns associated with liquid electrolytes. Recently, solid electrolytes received a great attention to substitute traditional liquid electrolytes used in Na-O2 batteries, due to their ability of improving safety and energy density. However, low ionic conductivity and high impedance in contact with the anode and cathode remain as major challenges for the development suitable solid-state electrolyte for Na-O2 batteries.In addressing these challenges, we recently have established a solid-state Na-O2 battery cell that comprises a highly conductive composite polymer-ceramic solid electrolyte with a conductivity of 0.7 mS/cm. This solid electrolyte works in synergy with vanadium phosphide (VP) nanoparticles, serving as oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) catalysts at the cathode and Na metal anode for more than 500 reversible charge-discharge cycles at a capacity of 1000 mAh/g, achieving a low polarization gap of approximately 20 mV in the first cycle. Various electrochemical and physicochemical characterization techniques, such as Raman spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), differential electrochemical mass spectrometry (DEMS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), were employed to gain insight into the cell chemistry and solid-state electrolyte role in the formation and decomposition of sodium oxides components such as superoxide (NaO2), peroxide (Na2O2). The findings of this study underscore the significance of proper cell component design and possible mechanisms in solid-state Na-O2 battery technologies. This research represents a promising avenue in advancing energy conversion and storage systems, showcasing the potential of solid-state batteries with enhanced safety and performance characteristics.

Advances In Borophene: Synthesis, Tunable Properties, and Energy Storage Applications.

Monolayer boron nanosheet, commonly known as borophene, has garnered significant attention in recent years due to its unique structural, electronic, mechanical, and thermal properties. This review paper provides a comprehensive overview of the advancements in the synthetic strategies, tunable properties, and prospective applications of borophene, specifically focusing on its potential in energy storage devices. The review begins by discussing the various synthesis techniques for borophene, including molecular beam epitaxy (MBE), chemical vapor deposition (CVD), and chemical methods, such as ultrasonic exfoliation and thermal decomposition of boron-containing precursors. The tunable properties of borophene, including its electronic, mechanical, and thermal characteristics, are extensively reviewed, with discussions on its bandgap engineering, plasmonic behavior, and thermal conductivity. Moreover, the potential applications of borophene in energy storage devices, particularly as anode materials in metal-ion batteries and supercapacitors, along with its prospects in other energy storage systems, such as sodium-oxygen batteries, are succinctly, discussed. Hence, this review provides valuable insights into the synthesis, properties, and applications of borophene, offering much-desired guidance for further research and development in this promising area of nanomaterials science.

PVDF–HFP@Nafion-based quasisolid polymer electrolyte for high migration number in working rechargeable Na–O2 batteries

Rechargeable sodium-oxygen (Na-O2) battery is deemed as a promising high-energy storage device due to the abundant sodium resources and high theoretical energy density (1,108 Wh kg-1). A series of quasisolid electrolytes are constantly being designed to restrain the dendrites growth, the volatile and leaking risks of liquid electrolytes due to the open system of Na-O2 batteries. However, the ticklish problem about low operating current density for quasisolid electrolytes still hasn't been conquered. Herein, we report a rechargeable Na-O2 battery with polyvinylidene fluoride-hexafluoropropylene recombination Nafion (PVDF-HFP@Nafion) based quasisolid polymer electrolyte (QPE) and MXene-based Na anode with gradient sodiophilic structure (M-GSS/Na). QPE displays good flame resistance, locking liquid and hydrophobic properties. The introduction of Nafion can lead to a high Na+ migration number (tNa+ = 0.68) by blocking the motion of anion and promote the formation of NaF-rich solid electrolyte interphase, resulting in excellent cycling stability at relatively high current density under quasisolid environment. In the meantime, the M-GSS/Na anode exhibits excellent dendrite inhibition ability and cycling stability. Therefore, with the synergistic effect of QPE and M-GSS/Na, constructed Na-O2 batteries run more stably and exhibit a low potential gap (0.166 V) after an initial 80 cycles at 1,000 mA g-1 and 1,000 mAh g-1. This work provides the reference basis for building quasisolid state Na-O2 batteries with long-term cycling stability.

Unveiling the Electro‐Chemo‐Mechanical Failure Mechanism of Sodium Metal Anodes in Sodium–Oxygen Batteries by Synchrotron X‐Ray Computed Tomography

AbstractRechargeable sodium–oxygen batteries (NaOBs) are receiving extensive research interests because of their advantages such as ultrahigh energy density and cost efficiency. However, the severe failure of Na metal anodes has impeded the commercial development of NaOBs. Herein, combining in situ synchrotron X‐ray computed tomography (SXCT) and other complementary characterizations, a novel electro‐chemo‐mechanical failure mechanism of sodium metal anode in NaOBs is elucidated. It is visually showcased that the Na metal anodes involve a three‐stage decay evolution of a porous Na reactive interphase layer (NRIL): from the initially dot‐shaped voids evolved into the spindle‐shaped voids and the eventually‐developed ruptured cracks. The initiation of this three‐stage evolution begins with chemical‐resting and is exacerbated by further electrochemical cycling. From corrosion science and fracture mechanics, theoretical simulations suggest that the evolution of porous NRIL is driven by the concentrated stress at crack tips. The findings illustrate the importance of preventing electro‐chemo‐mechanical degradation of Na anodes in practically rechargeable NaOBs.

Investigating the effectiveness of borophene on anchoring and influence on kinetics of sodium superoxide in sodium–oxygen batteries

Owing to the increasing cost and limited resources of lithium metal, there is a growing interest in developing energy storage systems that are more affordable, environmentally friendly, and have high energy densities. This is also driven by the need to significantly reduce carbon dioxide emissions resulting from the use of fossil fuels. The sodium–oxygen battery has emerged as a potential alternative, as the materials used for both of its electrodes are among the most abundant and inexpensive elements on Earth. However, despite these advantages, there are technical challenges in implementing it, such as the insulation of the cathode electrode and failure to prevent discharge products from desorbing into electrolytes during discharging. In this study, using density functional theory based on first principles, we investigated the electronic properties of free-standing β12 and χ3-borophenes after adsorption of sodium superoxide (NaO2). Our findings showed that the adsorption energy of sodium superoxide on β12 and χ3 borophene are −3.85 eV and −3.24 eV, respectively. The large negative values of adsorption energy suggest that sodium superoxide is anchored spontaneously, which is significant as it can be prevented from migrating to the negative electrode via the electrolyte during the discharging process. Furthermore, the β12 and χ3 structures showed moderate diffusion energy barriers of 0.89 eV and 1.37 eV and decomposition energies of 0.73 eV and 0.52 eV, respectively. The latter demonstrates the catalytic effects of nanosheets on the decomposition of the sodium superoxide into separated sodium (Na+) and oxygen (O2). Moreover, the decomposition energies being lower than sodium superoxide formation energy (3.90 eV) in a vacuum, suggests nanosheets effects during the charging process. Most importantly, the metallic characteristics of both crystal structures were preserved after the adsorption of sodium superoxide, and the electronic conductivities were enhanced. This is significant to improve the cycle life of the battery, as the materials can charge back the decomposed species during the charging process, thereby preventing the formation of dendrites at the surface of the cathode. Ultimately, the predicted electronic properties for both structures demonstrate their potential as cathode electrode materials for enhancing the electrochemical processes of sodium–oxygen batteries.

Electrochemical Analysis of Charge Overpotentials in Non-Aqueous Lithium and Sodium Oxygen Batteries

Aprotic metal-oxygen batteries, especially Li-O2 and Na-O2 batteries, could afford theoretical specific energies of more than 500 Wh/kg. However, while not compromising on rechargeability, the practical realization of the theoretically possible high specific energies has been elusive. A better understanding of the differences and similarities between Li–O2 and Na–O2 battery systems in terms of charge-discharge mechanisms and parasitic chemistry will be meaningful in solving these challenges. Here, we explore the differences between the two systems using a combination of galvanostatic charge-discharge measurements, Differential Electrochemical Mass spectrometry (DEMS), and Potentiostatic Electrochemical Impedance Spectroscopy (PEIS) analysis. The discharge profiles of these battery chemistries are similar in terms of overpotential and have been widely studied. But the origins of the differences in charge overpotentials are not clear. Using in-situ pressure decay measurements during discharge, we calculated the number of electrons/oxygen (e-/O2) to be 2.02 and 1.01 for Li-O2 and Na-O2 cells, respectively. Further, DEMS analysis during charge has shown that the ideal 2 e-/O2 is attained only during the initial phases of charging for Li-O2 cells. However, for Na-O2, the ideal 1 e-/O2 is achieved during a signification portion of the galvanostatic charge step. We corroborate these findings with PEIS and distribution of relaxation times (DRT) analysis to develop a mechanistic view of the processes that lead to poor coulombic and oxygen evolution efficiencies in metal-oxygen cells. Based on DRT analysis, we identify a lower time constant process that is common to both Li-O2 and Na-O2 cells. Interestingly, we observed that voltage polarization is preceded by the observation of this lower time constant peak. In the case of Li-O2 cells, this lower time constant process was observed at the end of discharge and throughout the recharge process, while for the Na-O2 cells, this was only observed after about 70% of recharge. We discuss the possible origins of this low time-constant process and its implications for recharging metal-oxygen batteries. We will also discuss the role of singlet oxygen, if any, in the recharge efficiencies of these two aprotic metal-oxygen batteries. Our results imply that the identification of the origins of this lower time constant process and possible suppression of this process will be key for rechargeable metal-oxygen batteries.

(Invited) High-Rate and High-Efficiency Molten Salt Li-O2 and Na-O2 Batteries Enabled by Nitrate Redox

The decarbonization of the electrical grid requires advancements in affordable, long-term energy storage to address the intermittency of energy sources like solar and wind, while the continued decarbonization of the transportation sector necessitates advancements in energy dense storage devices such as batteries, with low cost and made from abundant materials. Alkali metal-oxygen batteries can provide greater specific energy than Li-ion batteries and can potentially lower cost by reducing reliance on late transition metals like nickel and cobalt. In this talk, we share our recent work on molten-salt Li-O2 1 and Na-O2 2 batteries operating at 150-170 °C which can deliver high areal energy (33 mWh/cm2 geo) and power densities (19 mW/cm2 geo), with high energy efficiency (~90% at 5 mA/cm2 geo). We show that nitrate redox plays a critical role in these batteries, where the apparent four-electron oxygen reduction to form Li2O in Li-O2 batteries is facilitated by the electrochemical reduction of nitrate to nitrite, and subsequent chemical oxidation of nitrite to nitrate by molecular oxygen. In Na-O2 batteries, on the other hand, Na2O is further oxidized by O2 to Na2O2 to give an apparent two-electron oxygen reduction rection. We demonstrate the role of transition metal oxide catalysts for reducing the overpotential of nitrate redox in the positive electrode, as well as strategies for enhancing the power density and specific energy of these molten-salt metal-oxygen batteries. References (1) Zhu, Y. G.; Leverick, G.; Giordano, L.; Feng, S.; Zhang, Y.; Yu, Y.; Tatara, R.; Lunger, J. R.; Shao-Horn, Y. Nitrate-Mediated Four-Electron Oxygen Reduction on Metal Oxides for Lithium-Oxygen Batteries. Joule 2022, 6 (8), 1887–1903.(2) Zhu, Y. G.; Leverick, G.; Accogli, A.; Gordiz, K.; Zhang, Y.; Shao-Horn, Y. A High-Rate and High-Efficiency Molten-Salt Sodium–Oxygen Battery. Energy Env. Sci 2022, 15, 4636–4646.

Controlling the triple phase boundary on Na-O2 battery cathodes with perfluorinated polymers

Sodium-oxygen batteries hold great promise for the transition to a non-fossil fuel economy due to their high theoretical energy density. One of the most important components of these devices is the air-cathode, where the electrons available at the solid electrode, the Na+ ions present in the liquid electrolyte and oxygen gas react to form sodium oxides as discharge products. The kinetics of the discharge/charge reactions depend significantly on the boundary points between the solid-liquid-gas reaction phases, known as triple phase boundary (TPB). The density of TPB points (and therefore the battery efficiency) can be maximized by incorporating perfluorinated polymers on the cathode formulation. Thus, this type of polymers enhance oxygen transport properties which favour the diffusion of gaseous components in detriment to liquid electrolytes on solid electrodes. In this work, polytetrafluoroethylene (PTFE) and fluorinated ethylene propylene (FEP) polymers were added in different weight ratio to commercial graphene nanoplatelets (GNPs) cathodes. The critical physical properties affecting the formation of the TPB have been identified and correlated to sodium-oxygen battery performance. These key properties, which are crucial to modulate the oxygen diffusion within the cathode structure, have been identified for the first time in this work for aprotic metal air devices. This approach is of outmost importance for the development or efficient electrochemical storage devices where oxygen gas is involved.

Sodium‐Containing Surface Film Formation on Planar Metal–Oxide Electrodes with Potential Application for Sodium‐Ion and Sodium–Oxygen Batteries

Excellent, self‐improving sodiation rate capabilities in combination with high capacity retention upon galvanostatic charge/discharge cycling are found for oxygen‐deficient, carburized, and self‐organized titanium dioxide (TiO2−x) nanotubes (NTs). The sodiation mechanism is attributed to the formation of an acicular surface film as the active storage material with sodium (Na) peroxide (Na2O2) being the main component. Whether the proposed surface chemistry is unique for TiO2 NTs or serves as a common scheme for Na‐ion storage at metal oxide surfaces, in general, is not clear by now. Herein, three different materials, titanium(IV) oxide in the anatase and rutile phase and molybdenum(IV) oxide, are investigated in a planar electrode geometry toward their capability for Na‐ion storage. It is shown that all three materials under investigation demonstrate a significant progression of capacity increase upon cycling in combination with the formation of a Na‐oxide containing surface film. These “self‐improving” characteristics are found to significantly enhance the Na‐ion storage performance of the electrodes during long‐term galvanostatic cycling in a Na‐containing electrolyte.

A Reachable Sodium-Oxygen Battery Based on Sodium Superoxide Chemistry

Sodium-oxygen (Na-O2) batteries offer a great potential to provide high energy density storage systems needed for small-sized and inexpensive electric vehicles owing to the abundance of sodium compared with lithium. Yet, their development is hindered by the low cycle life and poor energy efficiencies due to (i) the formation of singlet oxygen, resulting in parasitic reactions with the air cathode and the organic electrolyte, (ii) the formation of unstable SEI layers and dendrites associated with the metallic sodium anode, and (iii) lack of an active, stable cathode catalyst to reduce the overpotentials and improve the cycle stability.Here, we have developed a Na-O2 battery cell composed of a highly active cathode catalyst that works well in synergy with an ether-based ionic-liquid electrolyte with specific redox mediators to act as co-catalysts to reversibly form and decompose sodium superoxide (NaO2) via surface-mediated pathway at a low polarization gap of about 40 mV at a capacity of 1000 mAh/g. Different electrochemical and physicochemical characterization techniques, i.e., Raman spectroscopy, XRD, XPS, DEMS, SEM, and TEM were used to understand the cell chemistry. Moreover, a chemically synthesized Na anode protection layer implemented in this battery cell enabled a long cycle life of 900 with all-time energy efficiencies more than 80%, exceeding state-of-art Na-O2 and Na-air batteries. The outcome of our study reveals the significance of the proper cell components design in Na-O2 battery technologies as a promising venue in energy conversion and storage systems.

A high-rate and high-efficiency molten-salt sodium–oxygen battery

A molten-salt Na–O2 battery featuring a liquid Na negative electrode and Ni positive electrode was found to form Na2O2 on discharge, enabled by nitrate redox where NO3− is reduced to Na2O and NO2−, then reactions with O2 produce Na2O2 and reform NO3−.

Tunable multi-doped carbon nanofiber air cathodes based on a poly(ionic liquid) for sodium oxygen batteries with diglyme/ionic liquid-based hybrid electrolytes

Multi-doped carbon nanofibers are used as self-standing air cathodes in Na–O2 batteries. The synergetic effect of multiple heteroatoms greatly enhances oxygen reduction, and diglyme-based hybrid electrolytes remarkably improve cycling performance.

Driving the sodium-oxygen battery chemistry towards the efficient formation of discharge products: The importance of sodium superoxide quantification

Sodium-oxygen batteries (SOBs) have the potential to provide energy densities higher than the state-of-the-art Li-ion batteries. However, controlling the formation of sodium superoxide (NaO2) as the sole discharge product on the cathode side is crucial to achieve durable and efficient SOBs. In this work, the discharge efficiency of two graphene-based cathodes was evaluated and compared with that of a commercial gas diffusion layer. The discharge products formed at the surface of these cathodes in a glyme-based electrolyte were carefully studied using a range of characterization techniques. NaO2 was detected as the main discharge product regardless of the specific cathode material while small amounts of Na2O2⋅2H2O and carbonate-like side-products were detected by X-ray diffraction as well as by Raman and infrared spectroscopies. This work leverages the use of X-ray diffraction to determine the actual yield of NaO2 which is usually overlooked in this type of batteries. Thus, the proper quantification of the superoxide formed on the cathode surface is widely underestimated; even though is crucial for determining the efficiency of the battery while eliminating the parasitic chemistry in SOBs. Here, we develop an ex-situ analysis method to determine the amount of NaO2 generated upon discharge in SOBs by transmission X-ray diffraction and quantitative Rietveld analysis. This work unveils that the yield of NaO2 depends on the depth of discharge where high capacities lead to very low discharge efficiency, regardless of the used cathode. We anticipate that the methodology developed herein will provide a convenient diagnosis tool in future efforts to optimize the performance of the different cell components in SOBs.

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.

Functional Binders Based on Polymeric Ionic Liquids for Sodium Oxygen Batteries Using Ionic Liquid Electrolytes

The polymeric ionic liquid poly(diallyldimethylammonium) bis(trifluoromethanesulfonyl)imide (PDADMA TFSI) is herein reported as a functional binder for sodium oxygen (Na–O2) batteries using ionic l...

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.

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.

Atomic/nano-scale in-situ probing the shuttling effect of redox mediator in Na–O2 batteries

Abstract Sodium-oxygen batteries (Na-O2) have attracted extensive attention as promising energy storage systems due to their high energy density and low cost. Redox mediators are often employed to improve Na-O2 battery performance, however, their effect on the formation mechanism of the oxygen reduction product (NaO2) is still unclear. Here, we have investigated the formation mechanism of NaO2 during the discharge process in the presence of a redox mediator with the help of atomic/nano-scale in-situ characterization tools used in concert (e.g. atomic force microscope, electrochemical quartz crystal microbalance (EQCM) and laser nano-particle analyzer). As a result, real-time observations on different time scales show that by shuttling electrons to the electrolyte, the redox mediator enables formation of NaO2 in the solution-phase instead of within a finite region near the electrode surface. These findings provide new fundamental insights on the understanding of Na-O2 batteries and new consequently perspectives on designing high performance metal-O2 batteries and other related functions.

In-situ imaging Co3O4 catalyzed oxygen reduction and evolution reactions in a solid state Na-O2 battery

Co3O4 nano materials have attracted tremendous attention as effective catalysts for sodium oxygen batteries (SOBs). However, their electrochemical processes and fundamental catalytic mechanism remain unclear till now. Herein, in-situ environmental transmission electron microscopy technique was used to study the catalysis mechanism of the Co3O4 nanocubes in SOBs during discharge and charge processes. It is found that during the 1st discharge and charge processes, Na2O2 formed and decomposed, respectively, around the Co3O4 nanocubes, but the following discharge and charge processes were very difficult. In order to promote the charge kinetics, we increased the charging temperature up to 500 °C, when the decomposition of Na2O2 became facile. Aberration corrected high-angle annular dark field imaging indicated that a thin layer of CoO grew epitaxially on the surface of Co3O4 nanocubes after the first discharge. Density functional theory calculations indicate that the CoO surface is energetically more favorable than Co3O4 for the nucleation of Na2O2. This study provides not only new fundamental understandings to the electrochemical reaction mechanisms of SOBs, but also strategies to improve the cycling performance of solid state SOBs.