Activating reversible carbonate reactions in Nasicon solid electrolyte-based Na-air battery in ambient air via in-situ formed catholyte

doi:10.21203/rs.3.rs-1635525/v1

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Activating reversible carbonate reactions in Nasicon solid electrolyte-based Na-air battery in ambient air via in-situ formed catholyte

https://doi.org/10.21203/rs.3.rs-1635525/v1

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published 05 Apr, 2024

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Out of practicality, ambient air rather than oxygen is preferred as a fuel in electrochemical systems, but CO₂ and H₂O present in air cause severe irreversible reactions, such as the formation of carbonates and hydroxides, which typically degrades performance. Herein, we report on a Na-air battery enabled by a reversible carbonate Na₂CO₃·xH₂O (x = 0 or 1) reaction in Nasicon solid electrolyte (Na₃Zr₂Si₂PO₁₂) that delivers a much higher discharge potential of 3.4 V than other metal-air batteries leading to high energy density and achieves > 86% energy efficiency at 0.1 mA cm^− 2 over 100 cycles. This cell design takes advantage of moisture in ambient air which the deliquescence of NaOH forms an in-situ catholyte that catalyzes the activation of reversible electrochemical reaction of Na₂CO₃·xH₂O. Our result demonstrates that free ambient air can be used reversibly as a fuel when enabled by reversible electrochemical reaction of carbonates with a solid electrolyte.

Extending from portable devices to electric vehicles and large-scale energy storage systems, the demand for Li-ion batteries with high energy density is continuously increasing and will overwhelm the pace of their development as the thermodynamic limits of Li-ion are reached. Metal-air batteries (with Li or Na) have been considered as a promising next generation energy storage system due to remarkably high theoretical gravimetric energy density.^1–4 However, the metal (Li/Na)–air batteries typically exploit the formation/decomposition of metal oxides by using pure oxygen rather than the ambient air^5–7 or by suppressing chemical reactions between discharge products and CO₂/H₂O in air (Supplementary Table S1).⁸ In the latter case, operating under ambient air as an atmosphere or reacting gas inevitably forms carbonates or hydroxides due to chemical reactions with CO₂ or H₂O present in air,^9–11 and these are not electrochemically decomposed with ease. As a result, additional devices such as gas selective devices for removing CO₂ and moisture from air and gas storage tanks are required but sacrifice the gravimetric and volumetric energy densities. In metal-air batteries that are based on liquid or solid electrolytes, the formation of carbonates is not desirable for reversible electrochemical performance and thus should be avoided if possible.^{12, 13} Since metal carbonates and hydroxides have high thermodynamic stability, they need much higher voltage for the decomposition than upper limit of electrochemical window in aprotic liquid electrolytes.¹³ As a result, it causes poor round-trip energy efficiency (= E_discharge/E_charge), the ratio of input energy in the charge to output energy in the discharge.^{10, 13} In addition, the use of ambient air in metal–air batteries can cause severe side reactions with the cathodes that are composed of aprotic electrolytes and carbon, and can contaminate the anode¹⁴ leading to poor electrochemical performance.

Several studies have sought to address the challenges of operating in ambient air by using an all-solid-state Li-air battery design with oxide-based solid electrolytes that are chemically stable against air and have a wide electrochemical potential window.^{13, 15–18} Even though lithium oxides and hydroxides are electrochemically formed as discharge products in all-solid-state Li-air cells, these discharge products can still react chemically with CO₂/H₂O in air to form lithium hydroxides and carbonates. Since the oxide-based solid electrolyte has a wide electrochemical window, the hydroxides/carbonates can be electrochemically decomposed in the charge process by applying a high over-potential¹³ leading to the increase in coulombic efficiency. However, it can cause poor energy efficiency because different electrochemical reactions occur during charge (hydroxide and/or carbonate reactions) and discharge (oxides and/or hydroxide reactions) (Supplementary Table S2), and thereby large potential gap between charge and discharge cannot be eliminated. Even though a quasi-solid-state Na-air cell, which has a Nasicon SE with anolyte and an ionic liquid gel electrolyte, was recently reported,¹⁹ there have not been any reports yet on the all-solid-state Na-O₂/air batteries in part because of difficulties in their fabrication, and high interfacial resistance in using oxide-based solid electrolytes caused by the lack of a triple phase boundary (TPB), which is an active reaction area between a gas (air), an electronic conductor, and an ionic conductor.

Here, we report on the use of ambient air as fuel in a Nasicon (Na₃Zr₂Si₂PO₁₂) solid electrolyte-based Na-air battery requiring no additional devices by exploiting reversible Na₂CO₃∙ xH₂O (x = 0 or 1) reactions during cycling, leading to the highest operating voltage (~ 3.4 V) among metal-air batteries based on metal oxides or carbonate/hydroxide reactions. In contrast to previous reports, moisture in air reacts with the discharge products such as NaOH leading to an in-situ formed catholyte, acting as both an electrolyte and an active material, and the in-situ formed catholyte unexpectedly activates reversible carbonate reactions in a large active reaction area, leading to increased achievable capacity and enhanced electrochemical kinetics. The formation of the catholyte as the product of reaction with ambient air allows the cell to undergo the same electrochemical reaction pathway with sodium carbonates in both the charge and discharge without forming any intermediate phases. As a result, the potential gap between charge and discharge reaction shrinks, leading to high round-trip energy efficiency. The resulting Na-air battery delivers high reversible energy density for 100 cycles with high coulombic and energy efficiencies.

To use ambient air as a fuel, we constructed Nasicon solid electrolyte (SE)-based Na-air cells with an air-electrode, which consist of Nasicon (Na₃Zr₂Si₂PO₁₂) as an ionic conductor and nickel (Ni) metal instead of carbon as an electronic conductor (Fig. 1a-d) because Nasicon SE is chemically stable in ambient air.²⁰ Previous reports studying Na-air batteries with hybrid electrolytes (aqueous and solid) clearly show that the side reactions between Nasicon and the other components (H₂O, O₂, and CO₂) are not observed, and reliable electrochemical performance was possible even though dense Nasicon SE was in direct contact with H₂O as well as flowing gases such as O₂ and CO₂.^{21, 22} Chemical reactions between air and Na metal anode are also prevented by solid electrolytes. The carbon-free air electrode can help to preclude unpredictable side reactions induced by carbon (Supplementary Table S1).¹⁴ Dense Nasicon electrolyte was synthesized by a solid-state reaction and had ionic conductivity ~ 0.2 mS cm^− 1 at room temperature (RT) (Supplementary Fig. 1). To reduce the interfacial resistance between dense solid electrolyte and the air-electrode, and increase the activity in the air-electrode, a duplex solid electrolyte that is composed of the dense solid electrolyte and a porous solid electrolyte was prepared. The dense solid electrolyte was firstly prepared and then the porous Nasicon layer was fabricated on the dense Nasicon by a screen-printing process using a Nasicon slurry with a pore former (Di-ethylene glycol butyl ether). After this, the duplex solid electrolyte (Fig. 1A) was heated at 1100°C to sinter the Nasicon slurry and enhance contact between porous and dense Nasicon SE. For electron conduction, Ni nanoparticles were formed inside the porous Nasicon of the duplex structure by an infiltration process using Ni(NO₃)₂∙6H₂O aqueous solution (Fig. 1B). Finally, a porous Ni current collector was laminated on the porous Nasicon with infiltrated Ni nanoparticles by the screen-printing process with Ni slurry containing the pore former (Fig. 1A). The Ni current collector was chosen because it has relatively high electrochemical stability with water compared to other metals; no reactions or changes in chemical nature of the air-cathode (Nasicon + Ni metal) were observed even after linear sweep test (Supplementary Fig. 2). The resulting assemblage had ~ 50 µm thickness of the air-electrode, comprised of porous Nasicon, Ni nanoparticle, and porous Ni layer as a current collector.

To reduce interfacial resistance on the anode side, a thin gold (Au) layer (thickness ~ 10 nm) was deposited between Na metal and the dense Nasicon (Fig. 1a, e) using an ion coater. The interfacial resistance significantly decreased (Supplementary Fig. 3) because the formation of Na-Au alloy can make a homogeneous contact at the interface.^23–26 The cell was assembled (Supplementary Fig. 4), and then its air-electrode side was purged with O₂ gas (99.995% purity) for 3h before electrochemical test to increase the cell’s integrity (Supplementary Fig. 5). The purged O₂ can induce the formation of a protection layer that can shield the Na metal anode. We speculate that ambient air, especially moisture, can cross over to the Na anode side through pores in dense solid electrolyte or through non-perfect sealing of the cell, overall causing severe contamination of Na metal.

The Nasicon SE-based Na-air cell was electrochemically cycled for 10 cycles in open air with 70% relative humidity (RH) at 25°C (Fig. 2a). The current densities were 0.02 mA cm^− 2 at the 1st cycle, and increased to 0.1 mA cm^− 2 in subsequent cycles. This indicates that ambient air can indeed be used as a fuel for reversibly operating the Na-air cell. It should be noted that the cell shows higher electrochemical activity in ambient air than in other gases without moisture (O₂, CO₂, N₂, and their mixtures) (Supplementary Fig. 6) implying the importance of moisture in activating reversible electrochemical reaction with the ambient air. In the 1st cycle, the discharge proceeds through electrochemical reaction with a voltage plateau at ~ 2.5 V, whereas the charge process shows a reaction with a voltage plateau at ~ 3.6 V. As cycle proceeds, additional voltage plateaus at ~ 3.2 and ~ 3.4 V in the discharge appear (dQ/dV curve in Fig. 2a) and then begin to increase in capacity. After 10 cycles, the plateaus at 3.2 V and 3.4 V are almost saturated without further increase to capacity. This indicates that the electrochemical reactions in subsequent cycles have changed from the 1st cycle in the Na-air cell with the ambient air.

To characterize the reaction products from the 1st cycle, ex-situ X-ray diffraction (XRD) measurements were performed on the two air-electrodes (Fig. 2b). One electrode was discharged to 0.8 mAh cm^− 2 and the other was charged up to 3.85 V just after the discharge (Supplementary Fig. 7). The XRD clearly shows that Na₂CO₃·H₂O and NaOH in the discharged electrode are formed and then their content diminishes due to electrochemical decomposition during charge. Synchrotron high resolution X-ray powder diffraction (Fig. 2c) further confirms the co-existence of NaOH and Na₂CO₃·H₂O in the discharged electrode by observing the separation of the peak at ~ 37° in XRD. Ex-situ Raman spectra of the air-electrodes after discharge and charge in 1st cycle further confirms that the formation and decomposition of Na₂CO₃·H₂O (x = 0 or 1) in the SE-based Na-air cell (Supplementary Fig. 8). In-situ Differential electrochemical mass spectrometry (DEMS) analysis was also performed to confirm decomposition of Na₂CO₃·H₂O (Supplementary Fig. 9). In-situ DEMS clearly shows that a CO₂ gas is evolved during charge. Considering gas evolution rate based on the reaction, some of the evolved CO₂ is stored in the air-electrode (especially in the absorbed H₂O).

To take a closer look at the electrochemical reactions upon cycling in the Nasicon SE-based Na-air cell, galvanostatic intermittent titration technique (GITT) measurements were performed at 25°C under open air of 70% RH for three cycles (Fig. 2d-f). The applied current density was 0.02 mA cm^− 2 during the 1st cycle and increased to 0.1 mA cm^− 2 during the 2nd and 3rd cycles. All currents were applied for 10 pulses with durations of 1 h during the 1st cycle and with durations of 12 min during the 2nd and 3rd cycles, followed by a rest for 1 h after each pulse. Open circuit voltages (OCVs) of the Na-air cell in the 1st cycle (Fig. 2d) were quite different from those in the subsequent cycles. This clearly indicates that different electrochemical redox reactions occur in the 1st cycle compared to the subsequent cycles, and the discharge products can vary during cycling. In the 1st discharge, a single discharge voltage plateau appears at ~ 2.7 V, whereas there are two main voltage plateaus during the 1st charge: one at ~ 2.7 V, which can be the corresponding reaction of the observed discharge redox reaction, and another at ~ 3.4 V, which may be an additional redox reaction. These two distinct voltages in the 1st charge suggest that two Na compounds can form as reaction products during or after the 1st discharge. Given that both Na₂CO₃ˑH₂O and NaOH in the discharged electrode are observed in XRD (Fig. 2b, c) and Raman spectra (Supplementary Fig. 8), and only a single electrochemical redox reaction at ~ 2.7 V in the discharge is observed in the 1st discharge, only one of the two Na compounds is formed electrochemically during the 1st discharge while the other is not. Considering the thermodynamic redox potential of NaOH (Supplementary Table 2) and the relatively high concentration of O₂ and H₂O in ambient air, NaOH could be electrochemically formed at ~ 2.7 V during the 1st discharge (Eq. 1). The following reaction describes the origin of the single voltage plateau observed.

Na + 0.5H₂O (g) + 0.25O₂ (g) \(\text{↔}\) NaOH (s), E = 2.75 V vs. Na/Na⁺ (1)

Noting that two Na compounds were observed in the 1st discharge and sodium carbonates have higher thermodynamic stability than NaOH (Supplementary Table 2), the formation of Na₂CO₃·H₂O may be caused by chemical reactions of NaOH with CO₂ and H₂O from the ambient air during or after the 1st discharge by following the below reactions (equations. 2–3):

2NaOH + CO₂ ◊ Na₂CO₃ + H₂O (2)

2NaOH + CO₂ ◊ Na₂CO₃·H₂O (3)

Therefore, the two plateaus observed in the 1st charge (Fig. 2d) can be ascribed to the electrochemical decomposition of NaOH at ~ 2.7 V (Eq. 1) and Na₂CO₃·xH₂O (x = 0 or 1) at ~ 3.4 V (equations 4–5), respectively.

2Na + CO₂ (g) + 0.5O₂ (g) \(\text{↔}\) Na₂CO₃ (s), E = 3.37 V vs. Na/Na⁺ (4)

2Na + CO₂ (g) + 0.5O₂ (g) + H₂O (g) \(\text{↔}\) Na₂CO₃ˑH₂O (s), E = 3.43 V vs. Na/Na⁺ (5)

The OCVs of the first charge plateau slightly increases from ~ 2.7 V to ~ 2.9 V (Fig. 2D). Considering that NaOH·H₂O tends to be formed when NaOH is exposed in humid air,²⁷ we speculate that NaOH·H₂O is also chemically formed at the end of discharge and it can lead to the increase in the OCVs of the first charge plateau (see NaOH·H₂O reactions in Supplementary Table S2). After the 1st cycle, the additional redox reaction at ~ 3.4 V in subsequent discharge cycles starts to grow and becomes dominant, whereas the redox reaction at ~ 2.7 V in subsequent charge cycles disappeares after the 1st cycle (Fig. 2e, f). Given the thermodynamic potentials and the differential capacity (dQ/dV) plot (Fig. 2a), the voltage plateau at ~ 3.4 V can be a result of the formation of sodium carbonates such as Na₂CO₃·xH₂O (x = 0 or 1) (equations 4–5). Electrochemical formation of Na₂CO₃·xH₂O at ~ 3.4 V was confirmed using a hybrid (aqueous + solid electrolytes) Na-air battery with Na₂CO₃ aqueous solution (Supplementary Fig. 10) with Raman spectroscopy. Details of the hybrid battery will be elucidated in the following session. Raman spectroscopy measurements of the electrodes show that carbonates and hydroxides were formed at ~ 3.4 V and ~ 2.0 V, respectively during discharging the cell. Given that the charged state the hybrid Na-air cell with Na₂CO₃ aqueous solution is quite similar with that of the SE-based Na-air cell, the discharge process of the hybrid cell would be identical to that of the SE-based Na-air cell. Only Na₂CO₃ is observed by discharging the hybrid cell to ~ 3.4 V (Supplementary Fig. 10A), and it supports that Na₂CO₃ is electrochemically formed at ~ 3.4 V in the SE-based Na-air cell. The electrochemical reaction of Na₂CO₃·xH₂O in the Na-air cell is activated and becomes the predominant redox reaction as cycling proceeds. This is the first report of a reversible reaction via the electrochemical formation of carbonate compounds in the Li/Na-ambient air cells. It should be noted that the electrochemical decomposition/formation of Na₂CO₃·xH₂O (equations 4–5) in the Na-air cell occurs at much higher redox potential than reported carbonate reactions^{11, 28–30} partly because the sodium carbonate reactions in the Na-air cell can occur directly without any intermediate phases or reactions.

Surprisingly, the electrochemical reactions of the Na₂CO₃·xH₂O exhibit much smaller polarization than that of the NaOH. In the 1st cycle, tested at 0.02 mA cm^− 2, the polarizations of the NaOH redox reaction were ~ 0.3 V for the discharge and ~ 0.6 V for the charge, whereas the polarization of the Na₂CO₃·xH₂O reactions was ~ 0.25 V for the 1st charge (Fig. 2d). Also, even at increased the current density of 0.1 mA cm^− 2 after 1st cycle (five times higher than the 1st cycle), the polarization of the Na₂CO₃·xH₂O reactions was almost similar to that of the 1st cycle. This indicates that the electrochemical reactions of Na₂CO₃·xH₂O can be kinetically facile, unlike previous results.^{9, 13} In contrast, the polarization of the NaOH reaction significantly increased from ~ 0.3 V at 0.02 mA cm^− 2 to ~ 0.9 V at 0.1 mA cm^− 2 (Fig. 2e, f). The use of ambient air in the Nasicon SE-based Na-air cells can activate the electrochemical reaction of sodium carbonates via a thermodynamic reaction pathway during charge/discharge, and can enable facile kinetic of electrochemical formation of the sodium carbonates. As a result, the Na-air battery with the ambient air in subsequent cycles will deliver higher redox potential (~ 3.4 V) than Na-O₂ cell,^{5, 6} and achieve low polarization even with main electrochemical reactions of sodium carbonates.

To further characterize electrochemical/chemical reactions during cycling, we carried out scanning electron microscopy (SEM) measurements for the air-electrodes of two cells. One was discharged to 0.8 mAh cm^− 2 and the other was charged to 3.85 V after the discharge (Supplementary Fig. 7). The air-electrodes of these two cells were compared with the pristine cell. Before the cell test, the air-electrode was observed as a particulate form (Fig. 3a and Supplementary Fig. 8a). However, the discharge products in the air-electrode showed a film-like morphology that covers the entire electrode after 1st discharge (Fig. 3b and Supplementary Fig. 8b) indicating that the electrochemical/chemical reaction is widespread in the electrode. Most of the film-like discharge products disappeared after 1st charge. It can demonstrate that the discharge products can be electrochemically decomposed (Fig. 3c and Supplementary Fig. 8c). However, the morphology of particles in the charged cell is still slightly blurred compared to that of the pristine cell. It might be originated from a residual water because the charge reaction is not complete during 1st charge (Supplementary Fig. 7). We also note that the discharge products electrochemically formed and decomposed even in 10th cycle (Supplementary Fig. 9) revealing the reversibility of the electrochemical reactions (equations 1, 4–5). Furthermore, the film-like morphology clearly shows that the reacted products in the 1st discharge can be formed through the absorption of liquid reactants such as H₂O from the ambient air. Given that NaOH, electrochemically-formed during 1st discharge, has a strong deliquescent property, it easily absorbs H₂O from the ambient air until the NaOH dissolves into the absorbed H₂O⁹ leading to the formation of catholyte. The film-like morphology of the reacted products can be the result of the in-situ formed catholyte. The existence of H₂O in the air-electrode was further confirmed by electrochemical test (Supplementary Fig. 10a-b). When the cell was discharged to 0.2 mAh cm^− 2 and charged to > 3.9 V, additional large capacity was obtained at ~ 3.9 V, which is quite close to the decomposition thermodynamic potential of water.^{21, 22} This indicates that H₂O even after 1st charge can exist inside the air-electrode unless high voltage (over ~ 3.9 V) is applied. This residual water can lead to slightly blurred morphology of particles even after the charge. Removal of the film-like morphology by performing vacuum drying the discharged cell further supports that the blurred particle boundaries are formed by residual water (Supplementary Fig. 10c). Therefore, moisture present in ambient air can help form a catholyte through chemical reaction with the discharged products of the SE-based Na-air battery unlike previous Na-air reports, which show a deteriorated electrochemical reaction under the existence of moisture.⁹

The in-situ formed catholyte helps not only the chemical formation of Na₂CO₃·xH₂O, but also the electrochemical activation of the Na₂CO₃·xH₂O reactions. Considering that a CO₂ concentration in the ambient air is negligible to activate the electrochemical discharge reactions of Na₂CO₃·xH₂O, it would make sense that CO₂ gas evolved by electrochemical decomposition of the Na₂CO₃·xH₂O reactions (equations 4 and 5) during charge would not release to the ambient air but be captured inside the air-electrode (especially nearby the reaction sites) leading to the increase in the local concentration of CO₂ gas inside the air-electrode. This can be enabled by the absorbed H₂O because the CO₂ gas is about two orders of magnitude more soluble in H₂O than the O₂ gas is.³¹ The in-situ DEMS measurement also shows that the actual amount of evolved CO₂ during charge is lower than the theoretical value calculated by the Faraday’s law of electrolysis (Supplementary Fig. 9), and it indirectly confirm that the CO₂ dissolution by the absorbed H₂O in the air-electrode. As a result, the local concentration of CO₂ gas in the air-electrode can be increased, enabling the electrochemical formation of Na₂CO₃·xH₂O (equations 4 and 5) in the following discharge process. Considering that the electrochemical formation and decomposition reactions of Na₂CO₃·xH₂O continuously increase for 10 cycles and then is saturated, the electrochemical decomposition of Na₂CO₃·xH₂O can keep increasing the amount of CO₂ gas inside the air-electrode but finally is saturated.

To understand the effect of the absorbed H₂O in the air-electrode on the activation of the Na₂CO₃·xH₂O electrochemical reactions, the Na-air cell was dried up by a vacuum drying at 80°C for 6 h after the charge (Fig. 3d) to remove H₂O inside the air-electrode. Before performing the vacuum drying process, the cell was cycled for five times to sufficiently activate the electrochemical reactions of Na₂CO₃·xH₂O. The current densities were 0.02 mA cm^− 2 at the 1st cycle and 0.1 mA cm^− 2 at the other cycles. After the activation process, the cell at the end of 5th charge was dried up at 80°C for 6 h under vacuum condition. This vacuum drying process severely reduced the discharge capacity of the Na₂CO₃\(\bullet\)xH₂O from 0.13 mAh cm^− 2 at 5th cycle to 0.06 mAh cm^− 2 at 6th cycle. The result indicates that the loss of the absorbed H₂O in the air-electrode strongly reduces the electrochemical formation of Na₂CO₃\(\bullet\)xH₂O partly due to a decrease in the amount of the in-situ formed catholyte, which can lower of the CO₂ concentration nearby the reaction sites inside the air-electrode. This demonstrates that the existence of the absorbed H₂O significantly affects for the activation of the electrochemical reaction of Na₂CO₃\(\bullet\)xH₂O.

To further understand the origin of the carbonate reactions, the cells were constructed with the ‘hybrid (aqueous electrolyte with NaOH or Na₂CO₃ and Nasicon solid electrolyte)’ electrolyte system; it is similar to the discharged state of the Nasicon SE based Na-air battery with the in-situ formed catholyte. The cell with the hybrid electrolyte was prepared by filling the aqueous solution (1 mL) to the air-cathode side. In addition, the air-electrode side of the cell was closed to suppress water evaporation during cell tests (Supplementary Fig. 11a) unlike the Na-air cell, which is open to the ambient air. Electrochemical tests were carried out at RT. The ~ 3.4 V discharge plateau barely increased by cycling the cell with the NaOH (aq) hybrid electrolyte (Supplementary Fig. 11b). On the contray, the cell with the Na₂CO₃ (aq) hybrid electrolyte showed the significant increase of the ~ 3.4 V plateau and exhibited a similar voltage curve to the Nasicon SE based Na-air cell operating in the ambient air (Supplementary Fig. 11c). To further understand the role of CO₂ for the electrochemical formation/decomposition of Na₂CO₃·xH₂O, the SE-based Na-O₂ bubbled H₂O cell, which is CO₂-free cell, was prepared and then tested (Supplementary Fig. 14D). It did not show the ~ 3.4 V voltage reaction, and did not have the continuous activation of ~ 3.4 V reaction during cycles compared to the SE-based Na-air cell. This result clearly demonstrates that the ~ 3.4 V reaction is related to the formation of Na₂CO₃·xH₂O and the CO₂ from the air is an essential component for activating this carbonate reaction. Considering that electrochemical decomposition of Na₂CO₃ in the presence of water during charge can activate the electrochemical reaction at ~ 3.4 V in the subsequent discharge, it can be demonstrated that the carbonate reactions in the Nasicon SE-based Na-air cell can be activated by the dissolution of CO₂ in H₂O. In consequence, the in-situ formed catholyte in the Nasicon SE-based Na-air cell during cycles critically affects the activation of the Na₂CO₃·xH₂O reactions and its reversible electrochemical reaction.

Figure 3e shows a schematic diagram describing the activation of electrochemical reactions with Na₂CO₃ˑxH₂O and their reversibility in the SE based Na-air cell with the ambient air. At the beginning of the 1st discharge, NaOH is formed electrochemically through reacting with O₂/H₂O from the ambient air at the triple phase boundary of Nasicon, Ni metal, and air (Eq. 1). The discharge product, NaOH, spontaneously absorbs H₂O from the ambient air until it is dissolved in the absorbed H₂O leading to the formation of the catholyte. Then, the in-situ formed catholyte (NaOH + H₂O) can chemically react with CO₂ in the air to yield Na₂CO₃\(\bullet\)xH₂O during and after 1st discharge (equations 2–3). These discharged products can easily cover the entire electrode leading to the formation of the film-like morphology that can significantly increase the active reaction area due to in-situ formed catholyte. During 1st charge, the Na₂CO₃\(\bullet\)xH₂O and NaOH are electrochemically decomposed and then release CO₂ and O₂ or H₂O (equations 1, 4–5). If the CO₂ and O₂ gases are evolved during 1st charge, the absorbed H₂O might capture these gases (especially CO₂) nearby the reaction sites due to high CO₂ gas solubility in H₂O.³¹ As a result, at the end of charge, CO₂ concentration at the inside of the air-electrode can increase drastically, and then in the following (2nd ) discharge the electrochemical formation of Na₂CO₃\(\bullet\)xH₂O (equations 4–5) can be activated. The appearance of multiple peaks in the dQ/dV plot at ~ 3.4 V (Fig. 2a) is likely related not only to the electrochemical formation of Na₂CO₃ but also the change in OCV from the Na₂CO₃\(\bullet\)xH₂O reactions (equations 1, 4–5) caused by various states (i.e. solid, gas, or aqueous) of reactants/products (Supplementary Table 2). During the 2nd discharge, the redox reactions consume most of the dissolved CO₂ gas in the catholyte and then subsequently, the electrochemical reaction of NaOH (Eq. 1) occurs at the end of discharge. The formation of NaOH at the end of each discharge, which corresponds to the voltage plateau below 2 V after 2nd cycle, can lead to additional absorption of H₂O and CO₂ from the air at each cycle (Supplementary Fig. 15), which can enable to form the catholyte and help electrochemical decomposition of Na₂CO₃\(\bullet\)xH₂O in subsequent charge processes (Supplementary Table 2). Also, kinetics of the chemical reactions between NaOH and CO₂ (equations 2–3) are also improved by the presence of the absorbed H₂O.⁹ This allows most of the NaOH to form Na₂CO₃\(\bullet\)xH₂O at the end of the 2nd and 3rd discharge as observed in the GITT test (Fig. 2e, f). As a result, the absolute amount of CO₂ gas inside the air-electrode can be increased, resulting in the gradual increase of the Na₂CO₃\(\text{∙}\)xH₂O reactions in subsequent discharge cycles (Fig. 2a).

Furthermore, the in-situ formed catholyte can substantially improve other critical electrochemical properties of the Nasicon SE-based Na-air battery. The catholyte, which increases the active area, enables an increase of the achievable discharge capacity (Supplementary Fig. 13) by around 1.5 times (~ 6.3 mAh cm^− 2) when compared to the maximum capacity calculated based on the pore volume in the air-electrode (Supplementary Table 3). In addition, an excess amount of the discharge product was found even on the Pt mesh (4 in Supplementary Fig. 4) that was on the porous Ni current collector (3 in Fig. 1a) and was not in contact with any ionic conductor (Supplementary Fig. 14). This result indicates that the in-situ formed catholyte can act as a new ionic conductor for increasing the active reaction area.

The SE-based Na-air cell shows excellent electrochemical performances including superior capacity retention and high rate capability under open air conditions with 70% RH (Fig. 4a-c) and 40% RH (Fig. 4d-f) due to the in-situ formed catholyte despite operating through the electrochemical reaction of carbonates and hydroxides.

In the cycle tests, the cells were discharged to 0.2 mAh cm^− 2 and charged to 3.85 V. The current densities were 0.02 mA cm^− 2 at 1st cycle and 0.1 mA cm^− 2 in subsequent cycles. When the cell was tested in ambient air with 70% RH, the voltage plateau at ~ 3.4 V (corresponding to the Na₂CO₃·xH₂O reactions) extended with increasing cycle number. This extension was saturated at the 10th cycle with the capacity of the plateau reaching ~ 0.16 mAh cm^− 2 and the energy efficiency (= E_discharge/E_charge) converging to ~ 80% (Fig. 4a, b). It should be noted that the NaOH reaction at the end of discharge cycles still appears indicating the formation of the catholyte via the delinquency of NaOH can be sustained (Supplementary Fig. 12). After the activation of the carbonate reactions in the first 10 cycles, the cell tested in 70% RH showed excellent cycle stability. The cell showed 99% average coulombic efficiency and 82% average energy efficiency for 50 cycles. The coulombic efficiency between 10th and 25th cycle is higher than 100% (Fig. 4b), which may be attributed to residual discharge products forming between the 1st and 10th cycles where the efficiency is lower than 100%. After the activation process for 10 cycles, the kinetics of the Na-air cell are improved, allowing the residual discharge products to decompose during 10th ~ 25th cycles and thus the coulombic efficiency can be increased. The energy efficiency is much higher than the other metal-air cells in previous studies^{3, 6, 8, 11, 13} because the electrochemical reaction pathways during charge and discharge are the same (Na₂CO₃\(\bullet\)xH₂O reactions) in the Na-air cell. High energy efficiency can be achieved since the potential gap of the Na₂CO₃\(\bullet\)xH₂O reactions between charge and discharge, ~ 0.4 V, is small (Fig. 4b). This illustrates that the Na-air cell operated in ambient air can be reversibly cycled via reversible Na₂CO₃\(\bullet\)xH₂O reactions and can increase energy efficiency.

The rate capability test was performed by discharging the cells with the current densities from 0.1 mA cm^− 2 to 2.0 mA cm^− 2 and charging the cells using a constant current - constant voltage (CCCV) protocol: charging to 3.85 V at constant current, and then applying a voltage hold at 3.85 V until the current reaches < 30% of the applied current (Fig. 4c). The CCCV method was conducted to fully charge the cell without the decomposition of water above ~ 3.9 V (Supplementary Fig. 10).

Before the rate capability test, the cell was pre-activated for 10 cycles in ambient air with 70% RH (Supplementary Fig. 15a) to ensure that the Na₂CO₃\(\bullet\)xH₂O reaction was fully saturated. During the rate capability test, the cell was cycled five times at each current density (Supplementary Fig. 15b); only the voltage profile of the last cycle at each current density is shown (Fig. 4c). The cell could be operated at a high current density of 2.0 mA cm^− 2 even though an increased polarization was observed. At 2.0 mA cm^− 2 the coulombic efficiency was ~ 88, and the energy efficiency was ~ 66. This result clearly demonstrates that the electrochemical reactions involving carbonates and hydroxides in the Nasicon SE based Na-air cell are kinetically facile, partly due to the in-situ formed catholyte through chemical reaction of the discharged products with the air.

The cycle and rate capability tests were also performed in air with reduced RH, from 70–40% (Fig. 4d-f), in order to understand the effect of RH on the electrochemical performance. When the cell was cycled in air with only 40% RH, the Na₂CO₃\(\bullet\)xH₂O reactions were barely activated and increased even with repeated cycles (Supplementary Fig. 16a), and thereby the polarization was much higher than the cell in air with 70% RH. This indicates that the amount of H₂O strongly affects the electrochemical activation of the carbonate reactions at the beginning cycles. To activate the electrochemical reactions of Na₂CO₃\(\bullet\)xH₂O in ambient air with 40% RH, the SE-based Na-air cell was pre-cycled in air with 70% RH for 10 cycles (Fig. 2a and Supplementary Fig. 15a). After pre-cycling the cell in air with 70% RH, the cycle and rate capability tests in air with 40% RH were carried out. The electrochemical properties of the cells in air with 40% RH (Fig. 4d) were comparable to those in air with 70% RH (Fig. 4a). This result implies that once the sufficient amount of the catholyte is formed and the sodium carbonate reaction is activated in the air-electrode under the air with high RH, the reversible Na₂CO₃\(\bullet\)xH₂O reaction is well maintained even in air with low RH in subsequent cycles. Surprisingly, the cell with 40% RH showed significantly improved cycle stability compared to the cell with 70% RH (Fig. 4e); the cell with 40% RH air showed stable capacity retention to 100 cycles with 97.2 % verage coulombic efficiency and 86.5 % verage energy efficiency. Also, the potential gap between the charge and discharge during the cycle test (~ 0.4 V) was similar to that of the cell in air with 70% RH (Fig. 4e). Long extended cycle stability indicates that the reversible electrochemical reactions of Na₂CO₃\(\bullet\)xH₂O can be sustained even with less amount of H₂O in air if the cell is fully activated by pre-cycling in air with high RH.

Furthermore, the cell in air with 40% RH also showed reasonable rate capability (Fig. 4f and Supplementary Fig. 16b). The cell was cycled up to a high current density of 2.0 mA cm^− 2 even though it caused higher polarization than the cell with 70% RH. Also, the portion of the Na₂CO₃\(\bullet\)xH₂O reactions during the discharge decreased more rapidly as the current density increased, compared to the cell in air with 70% RH.

The cells with 70% RH showed the degradation in the capacity retention after ~ 50th cycles (Fig. 4b). When the Na metal was replaced by new metal, the cell was recovered and showed a typical voltage curve of the Na₂CO₃\(\bullet\)xH₂O reaction (Supplementary Fig. 17). This result strongly suggests that the degradation can be mainly originated from the corrosion of the Na metal rather than the air-electrode. Since the cycle life of the cells was significantly extended by operating them in air with low RH = 40% 8(Fig. 4e), so we speculate that influx of air (especially H₂O in it) in cycles into the Na metal anode might severely degrade the Na metal.

The electrochemical performances of the Nasicon SE-based Na-air batteries in ambient air as a fuel are superior than those of all-solid-state Li-air (O₂) batteries even without any liquid additives in air-electrodes,^{7, 16, 18} because of the higher redox potential of the Na₂CO₃\(\bullet\)xH₂O reactions (~ 3.4 V) and much smaller potential gap between the charge and discharge, which is originated from the same electrochemical reaction pathway in the charge/discharge. The substantial improvement in electrochemical performance in the the Nasicon SE-based Na-air battery originates from the in-situ formed catholyte that can lead to the chemical reactions of the discharge products formed in 1st discharge with the ambient air, and then can activate the reversible electrochemical reactions of Na₂CO₃\(\bullet\)xH₂O (Fig. 4) in subsequent cycles. In consequence, these superior electrochemical performances demonstrates that the SE-based Na-air battery can be operated in ambient air with a wide range of humidity by exploiting reversible electrochemical reactions of sodium carbonates/hydroxides which can deliver high energy density with facile kinetic.

Furthermore, the carbonate reactions with high operating voltage and a low polarization can deliver higher theoretical energy density than that of MO₂ (M = Li or Na) (Fig. 4g and Supplementary Table 4) leading to small potential gap between charge and discharge than M₂O₂ or M₂O (Supplementary Table 5). Compared to reported hybrid (aqueous + solid) electrolyte systems that have a large amount of H₂O, the Nasicon SE based Na-air battery is quite different because it exploits only the absorbed H₂O obtained from the humidity of the ambient air, which can provide a very limited amount of H₂O, and thereby can have much higher volumetric and gravimetric energy density than the hybrid electrolyte systems. Furthermore, the SE-based Na-air cell uses the reversible electrochemical Na₂CO₃\(\bullet\)xH₂O reactions during the charge/discharge that have never been observed in the reported hybrid electrolyte systems with flowing the air (not pure O₂) into the aqueous electrolyte.^{20, 21, 32} Since a small amount of water is absorbed, high CO₂ concentration can be maintained, and it would be the reason why the Na₂CO₃∙xH₂O reaction is activated only in the SE-based Na-air battery.

Utilizing ambient air without any additional devices has several advantages for practical use of metal-air batteries. Firstly, the cell design can be simplified because additional devices such as gas selective devices and gas tanks for storing purified gas are not necessary.^{6, 33} As a result, it will be helpful increase the gravimetric/volumetric energy densities of the metal-air batteries and step toward into the practical applications. Secondly, the Na-air cell in ambient air can achieve the lowest energy cost among various energy storage systems³⁴ because sodium is an earth-abundant material, the ambient air is free, and additional cost for preparing purified gas is not necessary. By using oxide-based solid electrolytes, the Na-air battery allows to use ambient air reversibly as a fuel, and enables to have chemical reactions between discharge products and the air that can lead to the formation of catholyte and can activate reversible electrochemical carbonate reactions, in contrast to previous approaches that try to suppress these chemical reactions.

By applying a chemically stable solid electrolyte, we were able to employ ambient air as a fuel in a Nasicon SE-based Na-air cell capable of delivering reversible capacity by activating electrochemical reactions with carbonates and hydroxides, which are commonly believed to degrade the reversibility and induce high polarization in reported metal-air batteries. This counter-intuitive result can be explained by considering the role of an in-situ formed catholyte caused by the chemical reaction of the discharge products with H₂O, which activates the reversible electrochemical reaction of carbonates and facilitate its kinetics. Consequently, the Nasicon SE-based Na-air cell delivers high energy density due to high redox potential of the carbonate reaction as well as high energy efficiency due to low polarization by operating on the same electrochemical pathway during charge and discharge. This first demonstration of ambient air as a fuel in a SE-based Na-air battery provides steps toward developing beyond conventional metal-O₂ batteries and demonstrates the basis of employing reversible carbonate reactions to enable novel electrochemical energy platforms with solid electrolytes that can achieve much higher energy density than conventional batteries.

Preparation of the Nasicon SE based Na-air batteries. Na₃Zr₂Si₂PO₁₂ (Nasicon) solid electrolyte was synthesized by a solid-state reaction. A stoichiometric mixture of Na₂CO₃, ZrO₂, SiO₂, and NH₄H₂PO₄ (Na/Zr/Si/P = 3/2/2/1) was ball-milled for 24 h with 3, 5, and 10 mm ZrO₂ balls with ethanol solvent at 300 rpm. Then, the starting mixture was calcined at 1150°C for 5 h under air to synthesize Nasicon phase, and the calcined sample was pulverized for 3 h by a planetary ball-mill at 500 rpm with 0.5 mm ZrO₂ balls with ethanol solvent in a ZrO₂ container. After that, the pulverized Nasicon was pressed in a mold with ~ 300 MPa. Lastly, the pressed sample was sintered at 1100°C for 10 h under air atmosphere. The sintered Nasicon samples had a disk-shape with the density of ~ 3.0 g cm^− 3, the diameter of ~ 12.20 mm, and the thickness of 1.00 mm.

To prepare the duplex structure of the Nasicon solid electrolyte, a porous Nasicon layer with 8 mm diameter was laminated on top of the dense Nasicon surface by screen printing process. The screen-printing process proceeded by printing a Nasicon paste with pore former on the sintered Nasicon, and then heating the screen-printed sample. First, a screen-printing solution was prepared by mixing alpha-terpineol as a solvent, ethylene cellulose as a binder, and di-ethylene glycol butyl ether as a pore former at a weight ratio of 60 : 9 : 31. Then, the Nasicon paste was prepared by mixing the pulverized Nasicon powder (40 wt%), obtained by the planetary ball-mill process, and the screen-printing solution (60 wt%) using an agate mortar. The screen-printed paste on the sintered Nasicon was fired at 1100°C for 5 h,

The air-electrodes were fabricated by forming with Ni nanoparticles inside the porous Nasicon of the duplex structure. The Ni nanoparticles were formed by using a solution-based infiltration process. A droplet (12 µl) of Ni solution (0.6M Ni(NO₃)₂\(\bullet\)6H₂O in distilled water) was injected into the porous Nasicon layer, and then the combination was heated at 700°C for 10 min in ambient air to form NiO nanoparticles. After that, a porous Ni layer was stacked on the air-electrode as a current collector. To make the porous Ni layer, an Ni paste composed of Ni particles (40 wt%, < 1 µm particle size) and the screen-printing solution (60 wt%) was mixed by using agate mortar, and then printed on the top of porous Nasicon layer of the duplex structure. The samples were heated to 300°C for 10 min in ambient air, and then reheated to 700°C for 10 min in a reducing atmosphere (95% Ar + 5% H₂) to transform residual NiO to Ni in the porous Nasicon layer and the current collector. A thin layer (~ 10 nm) of Au was deposited on the anode side of the dense Nasicon electrolyte in the duplex by using ion-coater (PS-1200, ParaOne) to make a homogeneous interface between Na metal anode and the dense Nasicon electrolyte.

Cell assembly. The cell was composed of various hardware (Supplementary Fig. 4). The resulting SE-based Na-air cells were assembled in an Ar-filled glove box at O₂ and H₂O contents < 0.1 ppm by attaching a Na metal foil to the anode side of the cell (on the top of the Au layer). After assembling the cell, pure O₂ gas (99.995% purity) was blown toward the cathode side for 3h to improve the integrity of the components of the cell. The flow rate of the gas was 0.3 mL min^− 1.

For the hybrid electrolyte systems, the same cell structure as the Nasicon SE based Na-air cell was used but the cathode side of the cell is closed to the air unlike the Na-air battery, which has the air-electrode open to the air, to suppress the evaporation of the aqueous electrolytes during the test. The aqueous solutions of 0.6M NaOH and 0.3M Na₂CO₃ in distilled water were prepared at first. After that, the hybrid electrolyte cell was assembled in the Ar-filled glove box. After that, the aqueous solutions (1 mL) were injected to the cathode side of the battery, and then the cells were tested at room temperature.

Electrochemical measurements. The assembled Na-air batteries were tested using a battery test system (Maccor 2200, Maccor, Inc.). The cell tests were performed at 25°C in a constant-temperature-and-humidity chamber (TH-KE-025, JeioTech). The cells were discharged to 0.1, 0.2 or 0.8 mAh cm^− 2 and charged up to 3.85 V at current densities of 0.02 for 1st cycle or 0.1 mA cm^− 2 for subsequent cycles. For the hybrid electrolyte systems, the cells were charged to 0.2 mAh cm^− 2, and then discharged to 0.2 mAh cm^− 2 at a current density of 0.2 mA cm^− 2. In rate capability tests, the cells were charged with the CCCV mode; after charging the cell up to 3.85 V at constant current and then the voltage was held at 3.85 V until the current reached < 30% of the applied current. Electrochemical procedures used for each cell are explained in the captions of the corresponding figures.

Characterization techniques. Scanning electron microscopy (SEM) was performed to investigate the microstructure of the air-cathodes in the Na-air batteries with a field emission scanning electron microscopy (XL30s FEG, Philips Electron Optics B.V.) The reaction products of the air-electrodes were characterized using X-ray diffraction (XRD) measurements. The XRD spectra of the air-electrodes were recorded using diffractometer (D/MAX-2500/PC, Rigaku), using a step size of 0.02° at 10 ≤ 2\(\text{θ}\) ≤ 70°. The synchrotron XRD pattern of the discharged air-electrode was also collected using beamline 9B at the Pohang Accelerator Laboratory (PAL) in Pohang, South Korea. Readings were recorded every 0.01° for 4 s. Confocal Raman spectroscopy (laser excitation wavelength λ = 532 nm; power = 1 mW; resolution = 1 µm; integration time = 10 s; confocal mode, Alpha 300R, Witec, Ulm, Germany) was used to detect the discharged products in the air-electrode (Supplementary Fig. 9). The instrument is equipped with a microscope with a focal spot size of 500 nm. The real-time gas evolution was monitored by performing in-situ DEMS analysis under galvanostatic charge of the home-built gas-tight cell (Supplementary Fig. 9). The details of the DEMS system are described elsewhere.³⁵

Author contributions

B. Kang and H. Park conceived the idea and design of the experiments, and in drafting the article for important intellectual content. H. Park carried out the experiments and data analysis and B. Kang supervised the experiments and analysis. H. Park and M. Kang performed ex-situ analyses. H. Park and B. Kang cowrote the paper.

Acknowledgement

This study was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2017M3A7B8065394 and NRF-2019R1A2C2007933), and Brain Korea 21 PLUS project for Center for Creative Industrial Materials (no. F19SN25D1706). This work was supported by Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government (MOTIE) (20215610100040, Development of 20Wh seawater secondary battery unit cell). We are grateful to Dr. Kunjoong Kim in POSTECH for the insightful discussions on screen printing process.

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Activating reversible carbonate reactions in Nasicon solid electrolyte-based Na-air battery in ambient air via in-situ formed catholyte

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