Sodium–Oxygen Battery: Steps Toward Reality - ACS Publications

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Sodium−Oxygen Battery: Steps Toward Reality Imanol Landa-Medrano,† Chunmei Li,‡ Nagore Ortiz-Vitoriano,‡ Idoia Ruiz de Larramendi,† Javier Carrasco,‡ and Teófilo Rojo*,†,‡ †

Departamento de Química Inorgánica, Facultad de Ciencia y Tecnología, Universidad del País Vasco UPV/EHU, 48080 Bilbao, Spain CIC Energigune, Albert Einstein 48, 01510 Miñano, Á lava, Spain

‡

ABSTRACT: Rechargeable metal−oxygen batteries are receiving significant interest as a possible alternative to current state of the art lithium ion batteries due to their potential to provide higher gravimetric energies, giving significantly lighter or longer-lasting batteries. Recent advances suggest that the Na−O2 battery, in many ways analogous to Li−O2 yet based on the reversible formation of sodium superoxide (NaO2), has many advantages such as a low charge overpotential (∼100 mV) resulting in improved efficiency. In this Perspective, we discuss the current state of knowledge in Na−O2 battery technology, with an emphasis on the latest experimental studies, as well as theoretical models. We offer special focus on the principle outstanding challenges and issues and address the advantages/disadvantages of the technology when compared with Li−O2 batteries as well as other state-of-the-art battery technologies. We finish by detailing the direction required to make Na−O2 batteries both commercially and technologically viable.

A

to have a suitable chemical structure to host the Li ions. The chemical structure is not directly involved in the electrochemical reaction, only adding “dead weight” to the battery. In contrast to conventional battery systems based on intercalation reactions,6,7 M−O2 batteries rely on the electrochemical reduction of molecular oxygen at the cathode surface. The advantages of M−O2 batteries are that negligible amount of “dead weight” is needed and the theoretical energy density is the highest among other battery systems (Figure 1). Since the beginning of the 21st century, worldwide researchers have mainly focused their efforts on the study of the Li−O2 battery, the M−O2 battery with the highest theoretical energy density. However, several underlying physical and chemical mechanisms during battery operation have prevented the system from reaching high efficiency and good stability.7 Li−O2 batteries therefore are not yet competitive enough to make this technology feasible. These limitations are partially due to (i) short circuit of the cell caused by Li metal anode upon cycling (precipitate growth of Li dendrites); (ii) reaction of Li metal with contaminants from air (e.g., H2O and CO2) and some electrolyte molecules; (iii) positive oxygen electrode reactions with the discharge product lithium peroxide (Li2O2), which can be further oxidized on charging (>3.5 V vs Li+/Li); and (iv) electrolyte stability with oxygen reduction products or intermediates. In addition, the Li−O2 battery presents low discharge/charge Coulombic efficiency and poor cycle life. In order to overcome these challenges, fundamental understanding of the system is critical. Recently, several researchers have focused their efforts on understanding the mechanisms governing the

ccording to the current trend in society’s energy consumption habits, the use of energy storage systems in the automotive industry and renewable energy infrastructures, far from being just an alternative, is arising as a necessity. Electrochemical energy storage, especially batteries, could bring significant improvements in pollution reduction and efficient use of finite resources associated with this issue. Rechargeable batteries have actually played a key role in the technological progress of recent years. A clear example of this is the rechargeable Li-ion battery, which has facilitated the revolution of portable electronics. More than previous achievements, rechargeable batteries pose significant potential to bring a number of new economic and environmental benefits, such as replacing systems powered by fossil fuels and balancing the fluctuating generation of renewable power sources for their integration into the electrical grid.1,2 The use of efficient rechargeable batteries would lead to important technological turning points in these industries. However, ubiquitous Li-ion batteries have almost reached their limits in terms of energy density (75−200 Wh kg−1), cycle life (1000 cycles at >80% of capacity), and charge/discharge rate capabilities (1C).3−5 These values need to be increased in order to develop new emerging applications such as electric vehicle (EV) or stationary grid storage. For example, current Li-ion batteries for EVs offer a driving range of around 200 km. Hence, higher energy density batteries with superior performance are necessary in order to deploy EVs with the sufficient driving range to compete with alternative technologies (fuel cells, non-renewable energy sources, etc.). Metal−oxygen (M−O2) batteries are considered the most attractive alternative to Li-ion batteries when high energy density is a critical requirement. In Li-ion batteries, the energy is stored and released by the intercalation reaction of Li ion in both anode and cathode materials; hence, such electrodes need © 2016 American Chemical Society

Received: December 22, 2015 Accepted: March 10, 2016 Published: March 10, 2016 1161

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energy density of a Na−O2 battery is ∼1108 Wh kg−1, which is ∼30% lower than that of a Li−O2 cell.15 One of the main advantages of a Na−O2 battery is its much lower charge overpotential (95% upon cycling)14,17 compared to Li−O2 batteries. In terms of cost, substituting lithium by sodium offers additional benefits: (i) sodium salts used in the electrolyte are more abundant than equivalent lithium salts, making them both cheap and easily obtainable18 and (ii) in contrast to lithium, sodium does not dissolve in aluminum, which enables the use of thin aluminum foil as a light and low-cost anode current collector (Li−O2 batteries require the use of copper or nickel). In addition, the electrochemistry of a Na−O2 cell might share similarities with that of Li−O2 cells and, therefore, the extensive knowledge gathered during the last two decades on Li−O2 batteries19 can be applied, with a degree of caution, to Na−O2 batteries. For example, stability of electrolytes, effective architectures of cathode supports, and even some degradation aspects from Li−O2 cells pave the way for the initial design of Na−O2 systems. All these advantages make the development of a potentially sustainable and affordable Na−O2 battery of high technological interest. In this Perspective, we briefly overview recent progress in the field and explore how recent experimental and theoretical insight is shedding light onto the development of the nextgeneration Na−O2 battery. Challenges and issues that remain to be addressed before a fully competitive Na−O2 battery can be achieved are our main scope. We will do so by addressing a

Figure 1. Practical energy density vs power density for M−O2 batteries and some other rechargeable batteries. The specifications of key applications is also highlighted: Uninterruptible Power Systems (UPS) is an electricity storage system that is used to reduce or avoid negative effects and costs associated with electrical service outgases and poor power quality. A hybrid electric vehicle (HEV) is a type of vehicle that combines a conventional internal combustion engine propulsion system with an electric propulsion system.

growth (decomposition) of Li2O2, upon discharge (charge). These studies have revealed a very complex system, which presents serious concerns that hinder the development of a practical, safe, and cheap Li−O2 battery.8−13 The substitution of lithium by sodium has recently emerged as a promising approach to surpass Li−O2 battery limitations.14 In its common configuration, the Na−O2 battery comprises a sodium-containing anode (currently Na-metal), a sodiumconducting organic electrolyte and an air cathode. Cell discharge occurs when the sodium metal anode electrochemically oxidizes and the Na+ ions migrate across the electrolyte. It is believed that the reaction mechanism involves the oxygen gas (O2) dissolving in the regions of the electrolyte left exposed by the porous cathode (generally a carbonaceous material), forming superoxide species (O2−) (in the case of a suitable nonaqueous electrolyte). Sodium superoxide (NaO2) is then formed as the final discharge product in a reaction between the Na+ and O2−. This is subsequently decomposed upon charging, in the reverse reaction (Na+ + O2 + e− ↔ NaO2). Other discharge products such as sodium peroxide (Na2O2) and peroxide dehydrate (Na2O2·2H2O) have also been reported in literature which will be discussed below. In Table 1, we compare the main characteristics of Na−O2 and Li−O2 systems. In terms of cell chemistry, the theoretical

Challenges and issues that remain to be addressed before a fully competitive Na−O2 battery can be achieved are our main scope. series of key open questions. What advantages and drawbacks Na−O2 batteries offer in comparison with Li−O2 batteries? Which new problems arise in Na−O2 batteries that need to be solved? Could Na−O2 batteries significantly outperform other state-of-the-art battery systems? The interest in Na−O2 batteries has unequivocally increased since Peled et al.20 reported the first cell in 2011 using molten sodium as anode. High cell impedance and dendrite formation, previously reported for Li−O2 batteries, were overcome at 100 °C

Table 1. Comparison of Theoretical Data between Na−O2 and Li−O2 Batteries for Different Discharge Products Na−O2 cell chemistry cell voltage overpotential (discharge/charge) theoretical capacity

energy density

Li−O2

Na+ + O2 + e− → NaO2 2Na+ + O2 + 2e− → Na2O2 E° (2NaO2) = 2.27 V (ΔG° = −437.5 kJ mol−1) E° (Na2O2) = 2.33 V (ΔG° = −449.7 kJ mol−1) ηdis < 100 mV ηch ≈ 30−100 mV 1165 mAh g−1 (Na) 488 mAh g−1 (NaO2) 689 mAh g−1 (Na2O2) 1108 Wh kg−1 (NaO2) 1605 Wh kg−1 (Na2O2) 1162

2Li+ + O2 + 2e− → Li2O2 E° (Li2O2) = 2.96 V (ΔG° = −570.8 kJ mol−1) ηdis ≈ 300 mV ηch ≈ 1300 mV 3861 mAh g−1 (Li) 1168 mAh g−1 (Li2O2) 3458 Wh kg−1 (Li2O2)

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leading to Coulombic efficiencies of ∼85%. In 2012, Prof. Janek’s group successfully constructed a Na−O2 battery at room temperature14 where cubic NaO2 was found as the sole discharge product. This is in contrast to Li−O2 batteries where Li2O2 is the main discharge product. This was the first indication of the important differences between these two, otherwise similar, alkali M−O2 systems, which prompted a flurry of interest. NaO2 is not, however, the sole discharge product described in literature since then. Several authors have reported the formation of Na2O2 on the cathode after discharge, usually crystallized with two water molecules (Na2O2·2H2O).21−24 This has, therefore, motivated several works in order to clarify such controversy. Density functional theory (DFT) calculations performed by Kang et al.25 argued that due to the very similar equilibrium potentials for the formation of both NaO2 (2.27 V vs Na+/Na) and Na2O2 (2.33 V vs Na+/Na), the competition between surface and bulk energy should ultimately drive the preference to form one of them. These authors concluded that Na2O2 is the most stable bulk phase, whereas NaO2 is preferred at the nanoscale. These results, however, are in contrast with the DFT results of Lee et al.,26 where NaO2 was predicted to be the most stable phase at standard conditions (300 K and 1 atm). From an experimental viewpoint, Yadegari et al.27 found that both superoxide and peroxide were formed during discharge. The charge profile revealed three plateaus, where the superoxide is first removed in the lower plateau, whereas peroxide oxidation takes place at higher potentials. This study is in stark contrast with those reported by Janek’s,28 Luntz’s16 and Shao-Horn’s17 groups where NaO2 was the only discharge product. In addition, Janek et al.28 compared different carbon electrodes (H2315, SFG-44, Super PLi, Kentjenblack EC 600JD, HSAG 500 and SCR-1) without any evidence of peroxide formation. Furthermore, McCloskey et al.16 confirmed that, in Na−O2 cell, both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) were 1 e−/O2 processes by means of differential electrochemical mass spectrometry (DEMS), unequivocally demonstrating the exclusive formation and decomposition of NaO2. In contrast, in the Li−O2 cell, e−/O2 ratio during OER increased evidencing the presence of side reactions. The lower efficiency in the Li−O2 cell could be attributed to the higher reactivity of the discharge product Li2O2 compared to NaO2. Ortiz-Vitoriano et al.17 also showed NaO2 as the only discharge product from Na−O2 cells with carbon nanotubes from X-ray diffraction and Raman spectroscopy. Na2O2·2H2O was detected, however, when operating at ambient air exposure, even though it was not found with up to 6000 ppm of H2O added to the electrolyte. Despite the evidence provided by Ortiz-Vitoriano et al.,17 it cannot be ensured that the formation of Na2O2·2H2O is due, in all cases, to water vapor contamination. What is clear is that both the overpotential and instability are lower when the discharge product is NaO214,16,17 than Na2O2·2H2O.21−24 This highlights the need to develop in situ methods to monitor and analyze the discharge product chemistry and to give insight into the poor cyclability of Na2O2·2H2O. Janek and co-workers have proposed two possible mechanisms for NaO2 growth:14,29 (i) a solution mediated path where superoxide ions formed on the electrode surface are dissolved in the electrolyte until they precipitate and grow on previously formed nuclei and (ii) a surface mediated path where the ORR directly occurs on the surface of these nuclei. An indispensable condition for the surface mechanism is that the discharge product

has to conduct electrons from the electrode surface to the NaO2/ electrolyte interface. However, DFT studies on the intrinsic conductivity of NaO2 bulk, surface, and nanoparticles30,31 have found that NaO2 is an electronic insulator. The solution mediated mechanism, therefore, arises as the most realistic approach for NaO2 growth, which has been recently confirmed by charge/discharge studies using a dual-working cell.29 Interestingly, Nazar’s group32 investigated the role of residual water in the electrolyte and proposed a mechanism where oxygen is reduced on the electrode surface and “accommodated” in the solution forming HO2 with protons of the residual water, driving to a solution mediated growth of NaO2 crystals. Essentially, water acts as a phase-transfer catalyst on both ORR and OER. In absence of water, NaO2 grows in thin-film shape, resulting in a negligible discharge capacity due to the insulating nature of NaO2. Despite the low charge overpotential and the minimal side reactions of Na−O2 systems, cell chemistry can significantly affect battery performance. In this regard, several researchers

Despite the low charge overpotential and the minimal side reactions of Na−O2 systems, cell chemistry can significantly affect battery performance. have reported the use of catalysts mixed with carbon33−35 or carbon-free electrodes to improve cell capacity.36 NaI redox mediator has also been investigated by Yin et al.,37 which could replace water and act as a phase-transfer catalyst avoiding humidity problems and improving the cycle life.38 In addition, Bi et al. have suggested a suitable strategy to avoid the premature death of the battery by limiting the growth of dendrites at the sodium anode.39 Another key aspect influencing battery performance is the design of the cathode. Janek and co-workers identified the limiting effect of oxygen transport in discharge capacity using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS).14 They concluded that electrode areas in direct contact with oxygen gas during discharge showed high concentrations of discharge product, whereas less oxygenexposed areas showed negligible concentrations. This highlights the need for achieving an extensive utilization of the cathode material in order to maximize discharge capacity. Consequently, carbon electrode design has been widely optimized in this field to enhance the capacity and cycle life.40−43 From the anode side, Janek and co-workers have recently investigated the replacement of sodium metal with sodiated carbon, which gives a capacity of 120 mAh g−1.44 Additionally, short-circuits attributed to metal plating and dendrite formation were overcome. On the basis of the aforementioned information, Na−O2 batteries can be considered as a promising alternative to traditional energy storage systems. The past four years have seen tremendous progress in fundamental knowledge of these

Na−O2 batteries can be considered as a promising alternative to traditional energy storage systems. 1163

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Figure 2. Schematic summary of the reactions in a Na−O2 cell and the challenges that still need to be overcome in order to take a step forward on their development.

systems, but much still remains to be understood to facilitate commercialization (Figure 2). Below, we briefly discuss the key challenges. (a) Understand Coulombic efficiency. It is essential to determine the origin of the lower Coulombic efficiency during the first galvanostatic cycle compared to later cycles.36,45 In this regard, it is necessary to develop an in situ and operando system capable of analyzing the possible side/degradation reactions at the electrode/ electrolyte interface. (b) Identify the discharge products. The factors leading to the formation of superoxide and peroxide and to their associated charge overpotentials need to be fully identified. (c) Stabilization of Na2O2 vs NaO2. Both discharge products have been reported in literature. Low overpotential on OER, however, has only been proven for NaO2. If it were possible to recharge Na2O2 at low overpotential, the 2-electron reaction would give greater energy density. (d) NaO2 growth mechanism. Nucleation, growth mechanisms and the origin of the low overpotential on charging of NaO2, opposite to both Li2O2 and Na2O2, needs to be understood in order to gain insights into ORR and OER. Electronic-structure computer simulations have great potential to speed up the characterization of surface reactions in general and NaO2 growth mechanism in particular. Gaining such molecular-level insight could enable new strategies for maximizing energy density of practical Na−O2 electrodes. (e) Cathode design. The design needs to be optimized as porosity and pore distribution have been demonstrated to be key factors in the discharge capacity and cycle life. It is, therefore, important to develop oxygen cathodes to minimize the effect of the pore clogging during discharge. Oxygen supply channels through the air cathode also need to be optimized in order to enhance oxygen availability.

(f) Anode materials. The current commonly used anode material is metallic sodium, which is reactive toward oxygen and moisture as well as being prone to form severe dendrite structures46 with the resulting degradation upon cycling. In order to overcome the stability issues with many solvents (DMSO, MeCN, etc.), alternative anode materials must be developed. (g) Solid−electrolyte interphase (SEI). Little is known about the SEI formation and its influence on the battery performance. Deeper studies into this matter will provide considerable insight into the function of this SEI. How does this barrier interact? How do the electrons interact with the sodium? (h) Investigation of new electrolytes. The problems related to liquid electrolytes such as flammability, volatility, and decomposition on cycling might be solved by the design of solid electrolytes, leading to safer devices. Up till now, there are few researchers trying to construct a solid-state battery that, we believe, could provide great advancement of this technology. (i) Improve the cyclability. As previously mentioned, Li-ion batteries are capable of performing 1000 cycles at >80% of capacity. In terms of Li−O2 batteries, Liu et al. have recently cycled a Li−O2 battery for over 2000 cycles at a limited discharge capacity of 1000 mAh g−1 using a redox mediator.47 Such an outstanding cyclability has not still been achieved in Na−O2 technology. The poor cyclability of Na−O2 batteries could arise from reactivity of metallic sodium anode, or decomposition reactions of the liquid electrolytes. In summary, Na−O2 systems are gaining interest in the R&D community due principally to their high energy density, relatively low polarization, and low cost. Conceivably, the low polarization compared to Li−O2 system makes Na−O2 battery a possible candidate for their potential application in EVs and HEVs. We are inspired by recent electrochemical results and claims of improved efficiency, but more effort is needed to develop robust tests and verifications. Toward this end, further insights into the chemistry governing the reaction in Na−O2 1164

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group’s research combines first-principles methods and statistical mechanics to explore molecular-level processes in energy materials. http://www.cicenergigune.com/es/sobre-energigune/persona/javiercarrasco/

batteries will be rewarding. A promising but challenging frontier is understanding and controlling the growth mechanisms, discharge product morphology, and the influence of carbon, electrolyte, and so forth on the performance of the battery. Theoretical energy density enhancement has yet to be proved at lab-scale when comparing with state-of-the-art lithium-ion batteries. Packaged Na−O2 batteries have not been developed and the true gravimetric energy advantage of practical devices is not known. This technology is in its early steps and accurate predictions about its future and commercialization are difficult or impossible to make. In addition, Na−O2 batteries need oxygen which could be obtained from the air but the tolerance to the constituents in the air is still unknown. Some prior key questions that need to be addressed at lab-scale are Why both NaO2 and Na2O2 discharge products have been reported for Na−O2 cells? What about the anode/electrolyte interface? How does it form a solid-electrolyte interphase (SEI)? Is the formation of this SEI positive? On the practical side, performance and reversibility unequivocally depends on the discharge product. Which would therefore be the most appropriate cell design and conditions in order to optimize the efficiency of the battery? On the horizon, a cooperative and coordinated effort across a broad spectrum of disciplines can help to provide answer to all these questions, facilitating Na−O2 batteries to take a step forward from the laboratory research to commercial devices.

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Prof. Teófilo Rojo has been Full Professor of Inorganic Chemistry at the Universidad del Paiś Vasco since 1992 (UPV-EHU) and Scientific Director of the CIC Energigune since 2010. His research focuses on solid-state chemistry, materials science, and the study of materials for both lithium- and non-lithium-based batteries. http://www. cicenergigune.com/es/sobre-energigune/persona/teofilo-rojo/

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ACKNOWLEDGMENTS

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REFERENCES

The authors thank the “Ministerio de Educación y Ciencia” of Spain (under project MAT2013-41128-R), the “Fondo Europeo de Desarrollo Regional” (FEDER) and the Eusko Jaurlaritza/Gobierno Vasco (under project IT-570-13) for their support on this work. I.L.M. thanks the Universidad del Paiś Vasco (UPV/EHU) for his predoctoral fellowship. N.O.V. acknowledges a Marie Curie International Outgoing Fellowship within the EU Seventh Framework Programme for Research and Technological Development (2007−2013). J.C. is supported by the MINECO through a Ramón y Cajal Fellowship and acknowledges support by the Marie Curie Career Integration Grant FP7-PEOPLE-2011-CIG: Project NanoWGS and The Royal Society through the Newton Alumnus scheme.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

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The authors declare no competing financial interest. Biographies Imanol Landa obtained his M.S. in Novel Materials at the University of the Basque Country (UPV/EHU). Currently, he is carrying out Ph.D. studies at the Inorganic Chemistry Department of UPV/EHU. His primary research interests are cathode reactions at oxygen electrode in metal−oxygen systems, particularly in nonaqueous lithium− and sodium−oxygen batteries. http://www.ehu.eus/en/ web/dqi/home Dr. Chunmei Li joined CIC Energigune in 2014. She currently works as a postdoc researcher on Na−O2 and Li−S batteries. She obtained her Ph.D. degree in 2014, between University of St. Andrews, U.K., and Laboratoire De Réactivité Et Chimie Des Solides (LRCS), Universite de Picardie Jules Verne, France. http://www.cicenergigune. com/es/sobre-energigune/persona/chunmei-li/ Dr. Nagore Ortiz-Vitoriano is a Marie Curie fellow at CIC Energigune who joined in September 2015 after a two-year stay at MIT. Her primary research interests are materials for metal−air batteries, catalysis of water splitting, and a range of other applications in electrochemical energy storage and conversion. http://www. cicenergigune.com/es/sobre-energigune/persona/nagore-ortizvitoriano/ Dr. Idoia Ruiz de Larramendi is a lecturer in Inorganic Chemistry at the University of the Basque Country (UPV/EHU). Her research focuses on the control of materials microstructure to improve their efficiency in energy-related devices. She also studies the design of nanohybrid materials for metal recovery from aqueous media. http:// www.ehu.eus/en/web/dqi/home Dr. Javier Carrasco is a Ramón y Cajal fellow at CIC Energigune and has led its Computational Studies group since 2013. He obtained his Ph.D. degree at the University of Barcelona, Spain, in 2006. His 1165

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The Journal of Physical Chemistry Letters

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DOI: 10.1021/acs.jpclett.5b02845 J. Phys. Chem. Lett. 2016, 7, 1161−1166