Dissolution and Ionization of Sodium Superoxide in Sodium&Ndash

ARTICLE Received 21 Aug 2015 | Accepted 8 Jan 2016 | Published 19 Feb 2016 DOI: 10.1038/ncomms10670 OPEN Dissolution and ionization of sodium superoxide in sodium–oxygen batteries Jinsoo Kim1,*, Hyeokjun Park1,*, Byungju Lee1,2, Won Mo Seong1, Hee-Dae Lim1, Youngjoon Bae1, Haegyeom Kim1,2, Won Keun Kim3, Kyoung Han Ryu3 & Kisuk Kang1,2 With the demand for high-energy-storage devices, the rechargeable metal–oxygen battery has attracted attention recently. Sodium–oxygen batteries have been regarded as the most promising candidates because of their lower-charge overpotential compared with that of lithium–oxygen system. However, conflicting observations with different discharge products have inhibited the understanding of precise reactions in the battery. Here we demonstrate that the competition between the electrochemical and chemical reactions in sodium–oxygen batteries leads to the dissolution and ionization of sodium superoxide, liberating superoxide anion and triggering the formation of sodium peroxide dihydrate (Na2O2 Á 2H2O). On the formation of Na2O2 Á 2H2O, the charge overpotential of sodium–oxygen cells significantly increases. This verification addresses the origin of conflicting discharge products and overpotentials observed in sodium–oxygen systems. Our proposed model provides guidelines to help direct the reactions in sodium–oxygen batteries to achieve high efficiency and rechargeability. 1 Department of Materials Science and Engineering, Research Institute of Advanced Materials, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 151-742, Republic of Korea. 2 Center for Nanoparticle Research at Institute for Basic Science, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 151-742, Republic of Korea. 3 Environment and Energy Research Team, Division of Automotive Research and Development, Hyundai Motor Company, 37 Cheoldobangmulgwan-ro, Uiwang, Gyeonggi-do 437-815, Republic of Korea. * These authors contributed equally to this work. Correspondence and requests for materials should be addressed to K.K. (email: [email protected]). NATURE COMMUNICATIONS | 7:10670 | DOI: 10.1038/ncomms10670 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms10670 o address the increasing use of renewable energy and Results launch of electric vehicles, the need for rechargeable Electrochemical profile depending on the operating condition. Tbatteries with high-energy densities has been growing more To address the previous conflicting results on the discharge 1,2 rapidly than ever before . Among the available battery products and overpotentials of Na–O2 cells, we carefully assessed chemistries, metal–oxygen systems offer the highest energy the effects of operating parameters on the resulting electro- density with the largest theoretical capacities. Unlike chemical profiles. Among the various parameters examined conventional lithium-ion batteries, the direct reaction between (Supplementary Figs 1–3 and Supplementary Notes 1–3), we oxygen and light metals such as lithium and sodium in metal– observed that the charge/discharge profiles were most sensitively oxygen systems circumvents the need for a heavy transition metal affected by the applied current and rest time between the dis- redox couple in their operation, thereby making a high charge and charge, which was analogous to the report by Yadegari gravimetric energy density achievable3–7. The most intensively et al. as a function of discharge current or limited capacities19. studied metal–oxygen system to date is the lithium–oxygen Figure 1 presents and compares the electrochemical profiles (Li–O2) battery, which shares a similar lithium chemistry with obtained under various conditions. Although the discharge lithium-ion batteries. However, this system suffers from poor profiles are similar, with a single plateau at B2.1 V, there are cycle stability and efficiency, which has retarded the feasibility of roughly three different charging plateaus observed at (i) B2.5 V, its use in practical systems8,9. In particular, the large charge (ii) B3.0 V and (iii) 3.8 V, which agree with recent reports under overpotential over 1 V, the main reason for the low efficiency, also certain settings19,20. However, the relative lengths of each plateau accelerates the degradation of the electrode and electrolyte9. markedly vary under differing operating conditions. For the cases As an alternative, Na has been introduced to replace Li in of controlled discharge currents followed by a constant current 10 Li–O2 batteries with a few important merits . Despite the charging in Fig. 1a, it was observed that the length of the lower reduction in the energy density resulting from the lower redox plateau (B2.5 V) in the charge profiles was reduced as the applied potential of Na/Na þ , Na resources are readily available, are less discharge current decreased from 0.5 to 0.02 mA. However, the expensive than Li and can easily replace Li in the battery lengths of the plateaus at higher voltages, that is, B3.0 and 3.8 V, chemistry. It has been reported that the redox reactions in the were substantially increased, resulting in an overall larger sodium–oxygen (Na–O2) battery result in an extremely overpotential. Similar behaviours were observed in Fig. 1b when low charge overpotential (B0.2 V) despite involving the varying the charge currents after a constant current discharge. formation of micrometre-sized sodium superoxide (NaO2) With the lower applied charge currents, the cell exhibited a higher cubic crystallites11–14. This unique phenomenon supports the charging overpotential with shortened plateau length at 2.5 V. idea that this system is a promising alternative not only in This result contrasts with the general observation that slow terms of the cost of materials but also regarding the potential charging/discharging of electrochemical cells results in a voltage practical performance advantages. close to the equilibrium potential, thereby resulting in smaller Notably, however, the reactions of Na–O2 cells appear to be overpotentials. In addition, this result strongly indicates that the more diverse than those of Li–O2 cells. Contrary to the initial different discharge products might undergo the charging process report of NaO2 discharge products, some recent lines of work at each case. Notably, the shapes of the electrochemical charge could not reproduce either the formation of the discharge product profiles provide important clues to determine the discharge 15–17 19,22 NaO2 or the low charge overpotential . It was reported that products of Na–O2 reactions . Even though the discharge 15–17 sodium peroxide (Na2O2) or sodium peroxide dihydrate products should be identical for the cases of the same protocol of 18,19 (Na2O2 Á 2H2O) was formed instead. In addition, these cells discharge, each charge profile was distinct with different charge exhibited high overpotential during charge, similar to that currents. This finding implies that the initial discharge products observed in the Li–O2 system. Many groups have attempted are gradually transformed into other phases during the charge to determine the reasons for these discrepancies; however, process via time-dependent reactions. To verify whether this to date, the main cause of the divergence of reactions has not transformation occurs via an electrochemical or chemical been identified. Janek et al. investigated the effect of different reaction, we also controlled the rest time between the discharge carbon electrodes; Guo et al. and Shao-Horn et al. addressed and charge processes. As observed in Fig. 1c, the lowest voltage this issue but observed no critical differences among the region in the charge profiles systematically decreases on cases12,20,21. increasing the rest time from 0 to 12 h. The change in the In this work, we demonstrate the interplay of the diverse electrochemical profile in the absence of the applied current reactions in Na–O2 batteries involving a series of electrochemical clearly indicates that the time-dependent chemical reactions and chemical reactions as a function of time. Under systematic occurred during the rest period, affecting the subsequent control of the operating conditions, we observe that the charging. This behaviour was also confirmed in similar tests for galvanostatic charge/discharge profiles are sensitively affected by the higher charging currents with the resting time after the the conditions and durations of the electrochemical operations. discharge, which revealed the growth of the charge polarizations It is also revealed that the electrochemically formed NaO2 is on increasing the rest time (Supplementary Fig. 1). unstable and degrades into Na2O2 Á 2H2Ointheabsence The time-dependent chemical reactions can be more clearly of an applied current. The spontaneous dissolution and visualized by plotting all the voltage profiles as a function of the À ionization of NaO2 liberates the free O2 in the electrolyte time. Figure 1d–j illustrates the voltage evolution of each cell after and promotes side reactions involving the formation of the completion of the discharge at different operating conditions. Na2O2 Á 2H2O. On the basis of these observations, we propose The first inflection points of the voltage profiles at charge B reaction mechanisms of Na–O2 batteries under various operat- (indicated with arrows) occur at 10 h regardless of the rest or ing conditions. This report is the first to reveal the relationships charge protocols. This result indicates that a specific time of B among the different discharge products

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