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Sen Xin, Ya-Xia Yin, Yu-Guo Guo,* and Li-Jun Wan Efficient energy storage represents one of the most attractive topics in this power-demanding world.[1] Among various advanced energy storage devices, metal-sulfur batteries, including Li-S, Na-S and Mg-S batteries, are especially attractive due to their high specific energies.[1c,n,2] The traditional Na-S battery holds notable advantages, including high energy density (theoretical value: 760 W h kg−1) and efficiency (approaching 100%), low material cost (rich abundances of Na and S in nature), and long life.[1c,n,o,2g,3] All these benefits make them especially promising for stationary storage applications, for example, utility-based load-leveling and peak-shaving in smart grid, and emergency/uninterruptible power supply.[1c,o] Traditional Na-S batteries employ sodium β-alumina (NaAl11O17) as the electrolyte, which is a ceramic electrolyte able to yield sufficient Na+ conductivity only when the temperature goes beyond 300 °C.[1c,3a] As a result, they are usually operated between 300 and 350 °C,[1c,n,2g,3] at which the conversion between the chemical potential and the electrical energy is achieved via the following reactions:[1c,n,2g,3a,4] 2Na + xS ↔ Na2 Sn (n ≥ 3) (Voltage output : 1.78 − 2.08 V)
Since the temperatures far exceed the melting points of Na (98 °C) and S (115 °C), both electrodes are in their molten states, which makes them more reactive and corrosive than their solid counterparts, and may bring serious safety problems.[2g,4,5] Moreover, a considerable amount of power is consumed to maintain the high working temperature of Na-S battery.[1c,4,5] Therefore, people turn to Na-S batteries working at lower temperatures (especially the ambient temperature), with hope that such batteries with solid electrodes can offer better durability, improved safety and higher energy output.[4–6] Since solid electrolyte with satisfactory Na+ conductivity at room temperature is not available, such batteries have to use liquid electrolytes. However, solid sulfur usually exhibits low reactivity with sodium in liquid electrolyte, leading to limited material utilization and incomplete reduction to form polysulfides (Na2Sx, x ≥ 2) rather than Na2S. Another problem lies in the dissolution of polysulfides into the liquid electrolyte, which results in low efficiency and rapid capacity fade upon cycling. Efforts have been made to address these issues, including the combination of sulfur with polymer matrix, and the use of polymer electrolyte, carbonate electrolyte and glyme electrolyte.[4–6] Though certain Prof. Y.-G. Guo, Dr. S. Xin, Dr. Y.-X. Yin, Prof. L.-J. Wan Key Laboratory of Molecular Nanostructure and Nanotechnology, and Beijing National Laboratory for Molecular Sciences Institute of Chemistry Chinese Academy of Sciences (CAS) Beijing, 100190, P. R. China E-mail: [email protected]
DOI: 10.1002/adma.201304126
Adv. Mater. 2014, 26, 1261–1265
improvements have been achieved, the performances of RT Na-S batteries are still limited by the poor electrochemical activity and cyclability. In our previous work, we reported a sulfur-carbon composite cathode with metastable small sulfur molecules (S2–4) confined in microporous carbon.[7] Due to the high electrochemical activity of S2–4 and the fine confinement of microporous carbon, the cathode exhibited a high lithium electroactivity and a stable cycling ability in a Li-S battery.[7] Given that these sulfur molecules may also bring opportunities to Na-S battery, we designed and realized a RT Na-S battery with them. It is found that the S2–4 cathode exhibits a high electrochemical reactivity with Na, enabling a complete reduction to Na2S at room temperature. The as-assembled RT Na-S battery shows a long lifespan of 200 cycles and a high specific energy of 955 W h kg−1 (based on the mass of S and Na, same below) far exceeding that of the traditional HT Na-S battery. A sulfur-microporous carbon composite was synthesized as previously reported and directly used as the cathode material in the Na-S battery (Figure S1a, Supporting Information).[7] The composite has a coaxial cable-like structure, with a carbon nanotube inside and a sulfur-containing microporous carbon sheath outside (S/(CNT@MPC)) (Figure S1b-d, Supporting Information). The mean diameter of carbon micropores in the carbon substrate is 0.5 nm. The S content in the composite is 40 wt-%. S cathodes were prepared using the S/(CNT@ MPC) composite (Figure S2a,c, Supporting Information), and then paired with Na anodes to assemble the Na-S coin cells and placed in ambient environment for electrochemical tests (Figure 1a). Figure 1b shows the cyclic voltammograms (CVs) of the sulfur cathode. There are two reduction peaks in the first cycle at 1.45 V and 1.1 V vs. Na+/Na, respectively, suggesting a stepped reduction reaction between S and Na. However, only one sharp anodic peak appears at 1.75 V in the reverse oxidation process. From the second scan, the oxidation peaks shift to higher potentials (1.55 V and 1.1 V), suggesting a much reduced polarization. The CV curves gradually stabilize in the subsequent scans, indicating a stable electrochemistry of the battery. To further test the performances of the battery, galvanostatic discharge-charge (GDC) tests were performed on the assembled RT Na-S battery. Figure 1c shows the GDC profiles of a Na-S battery cycled at 0.1 C (167 mA g−1 based on S mass, same below). The battery exhibits two slopped discharge plateaus with almost equal capacity contributions in the first cycle (above and below 1.4 V, as divided by the dashed line), and one plateau upon charging, which are consistent with the CVs. The electrochemical behavior of the RT Na-S battery is different from those reported in previous literatures with its much lowered discharge/charge voltage. The two plateaus together contribute to a specific capacity of 1610 mA h g−1 (based on S mass, same below), close to the theoretical capacity of S (1675 mA h g−1 to yield Na2S). Given that carbon makes little capacity contribution
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A High-Energy Room-Temperature Sodium-Sulfur Battery
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Figure 1. Configuration of the RT Na-S battery and electrochemical behaviors of the sulfur cathode. (a), Schematic illustration of the Na-S battery, (b), The first five CVs of the S/(CNT@MPC) cathode at 0.05 mV s−1, (c), GDC voltage profiles of the S/(CNT@MPC) cathode at 0.1 C.
above 0.8 V,[8] the result indicates that sulfur in the S/(CNT@ MPC) composite is completely reduced to Na2S. Since the capacity contributions on the two plateaus are equal, the discharge products at the ends of the two plateaus could be Na2S2 and Na2S, respectively. To confirm the feasibility of sulfur reduction in micropores of ~0.5 nm, theoretical calculations have been performed to investigate the molecular structures of Na2S2 and Na2S. Figure 2a simulates the molecular structures of Na2S2 and Na2S from their crystals.[9] Though Na2S2 has two crystal forms (α-Na2S2 and β-Na2S2, which respectively belong to P62m and P63mmc space groups), only α-Na2S2 is adopted due to its higher stability at ambient temperature.[9b] Simulation results show that the dimensions of Na2S2 and Na2S molecules are close to the size of the micropore, suggesting that these
molecules can be accommodated as single molecules in the carbon channels. Since Na2S2 and Na2S crystals can not form in the micropore, the interaction between the molecule and adjacent ions is weak, and the molecular structures of Na2S2 and Na2S may vary from those in their crystals. To clarify this point, geometric optimizations have been performed on these molecules free of the interactions with adjacent ions. As shown in Figure 2a, the optimized Na2S2 and Na2S molecules show denser and flattened structures, and each of them has at least two perpendicular dimensions smaller than or very close to the diameter of carbon micropore (0.5 nm), suggesting that they can be accommodated. Therefore, the reduction from S to Na2S is theoretically feasible in the carbon micropore. To clarify the final product at the end of the reduction process, ex-situ transmission electronic microscopy (TEM) and
Figure 2. Molecular structure simulation and electrochemistry of the RT Na-S battery. (a), Structures and dimensions of Na2S2 and Na2S molecules in their crystals and after geometry optimizations (unit of length: angstrom), (b), Electrochemical reactions between S and Na+ during the discharge process, (c) XPS S2p spectrum of the sulfur cathode before use, (d), XPS S2p spectrum of the same cathode after being discharged to 0.8 V.
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COMMUNICATION Figure 3. Performances of the S/(CNT@MPC) cathode and corresponding specific energies of the RT Na-S battery at different GDC rates. (a), Cycling performances of the S/(CNT@MPC) cathode at 0.1 C, 1 C and 2 C. (b), GDC voltage profiles of the S/(CNT@MPC) cathode in the 4th cycle at these rates. Specific energy profiles vs. voltage of the RT Na-S battery (c), in different cycles at 0.1 C and (d), in the 4th cycle at 0.1 C, 1 C and 2 C.
X-ray photoelectron spectroscopy (XPS) characterizations were performed on a sulfur electrode after its discharge to 0.8 V. The energy dispersive X-ray (EDX) spectrum reveals a rich Na abundance on the S/(CNT@MPC) cathode at the discharged state (Figure S2b,d, Supporting Information). The elemental mappings of C, S and Na overlap each other, indicating a uniform Na intercalation in the composite (Figure S3, Supporting Information). On the EDX pattern collected from the discharged S/ (CNT@MPC) nanocables (Figure S4, Supporting Information), the atom ratio between Na and S is calculated to be ~2:1, which suggests a chemical formula of Na2S of the product. The XPS spectra further demonstrate that the originally zero-valent sulfur has been fully reduced to Na2S (Figure 2c,d).[10] These results prove that the final discharge product of S is Na2S in the RT Na-S battery. It is reported that sulfur in HT Na-S batteries usually exhibits one discharge plateau with an incomplete reduction product of Na2Sn (n ≥ 3), which reduces the specific capacity of sulfur (≤ 558 mA h g−1) and the specific energy of the battery.[3a,4] In the present RT Na-S battery, it is the high electrochemical activity of confined S2–4 molecules that leads to a complete reduction of sulfur even at room temperature. In this way, we can take full advantage of the electrochemistry of Na-S battery.
Adv. Mater. 2014, 26, 1261–1265
Given the discharge products of the two plateaus are Na2S2 and Na2S, respectively, the stepwise reductions of S can be described by the following reactions (Figure 1b): 2S + 2Na → Na2 S2 (>1.4 V)
(1)
Na2 S2 + 2Na → 2Na2 S (
Sen Xin, Ya-Xia Yin, Yu-Guo Guo,* and Li-Jun Wan Efficient energy storage represents one of the most attractive topics in this power-demanding world.[1] Among various advanced energy storage devices, metal-sulfur batteries, including Li-S, Na-S and Mg-S batteries, are especially attractive due to their high specific energies.[1c,n,2] The traditional Na-S battery holds notable advantages, including high energy density (theoretical value: 760 W h kg−1) and efficiency (approaching 100%), low material cost (rich abundances of Na and S in nature), and long life.[1c,n,o,2g,3] All these benefits make them especially promising for stationary storage applications, for example, utility-based load-leveling and peak-shaving in smart grid, and emergency/uninterruptible power supply.[1c,o] Traditional Na-S batteries employ sodium β-alumina (NaAl11O17) as the electrolyte, which is a ceramic electrolyte able to yield sufficient Na+ conductivity only when the temperature goes beyond 300 °C.[1c,3a] As a result, they are usually operated between 300 and 350 °C,[1c,n,2g,3] at which the conversion between the chemical potential and the electrical energy is achieved via the following reactions:[1c,n,2g,3a,4] 2Na + xS ↔ Na2 Sn (n ≥ 3) (Voltage output : 1.78 − 2.08 V)
Since the temperatures far exceed the melting points of Na (98 °C) and S (115 °C), both electrodes are in their molten states, which makes them more reactive and corrosive than their solid counterparts, and may bring serious safety problems.[2g,4,5] Moreover, a considerable amount of power is consumed to maintain the high working temperature of Na-S battery.[1c,4,5] Therefore, people turn to Na-S batteries working at lower temperatures (especially the ambient temperature), with hope that such batteries with solid electrodes can offer better durability, improved safety and higher energy output.[4–6] Since solid electrolyte with satisfactory Na+ conductivity at room temperature is not available, such batteries have to use liquid electrolytes. However, solid sulfur usually exhibits low reactivity with sodium in liquid electrolyte, leading to limited material utilization and incomplete reduction to form polysulfides (Na2Sx, x ≥ 2) rather than Na2S. Another problem lies in the dissolution of polysulfides into the liquid electrolyte, which results in low efficiency and rapid capacity fade upon cycling. Efforts have been made to address these issues, including the combination of sulfur with polymer matrix, and the use of polymer electrolyte, carbonate electrolyte and glyme electrolyte.[4–6] Though certain Prof. Y.-G. Guo, Dr. S. Xin, Dr. Y.-X. Yin, Prof. L.-J. Wan Key Laboratory of Molecular Nanostructure and Nanotechnology, and Beijing National Laboratory for Molecular Sciences Institute of Chemistry Chinese Academy of Sciences (CAS) Beijing, 100190, P. R. China E-mail: [email protected]
DOI: 10.1002/adma.201304126
Adv. Mater. 2014, 26, 1261–1265
improvements have been achieved, the performances of RT Na-S batteries are still limited by the poor electrochemical activity and cyclability. In our previous work, we reported a sulfur-carbon composite cathode with metastable small sulfur molecules (S2–4) confined in microporous carbon.[7] Due to the high electrochemical activity of S2–4 and the fine confinement of microporous carbon, the cathode exhibited a high lithium electroactivity and a stable cycling ability in a Li-S battery.[7] Given that these sulfur molecules may also bring opportunities to Na-S battery, we designed and realized a RT Na-S battery with them. It is found that the S2–4 cathode exhibits a high electrochemical reactivity with Na, enabling a complete reduction to Na2S at room temperature. The as-assembled RT Na-S battery shows a long lifespan of 200 cycles and a high specific energy of 955 W h kg−1 (based on the mass of S and Na, same below) far exceeding that of the traditional HT Na-S battery. A sulfur-microporous carbon composite was synthesized as previously reported and directly used as the cathode material in the Na-S battery (Figure S1a, Supporting Information).[7] The composite has a coaxial cable-like structure, with a carbon nanotube inside and a sulfur-containing microporous carbon sheath outside (S/(CNT@MPC)) (Figure S1b-d, Supporting Information). The mean diameter of carbon micropores in the carbon substrate is 0.5 nm. The S content in the composite is 40 wt-%. S cathodes were prepared using the S/(CNT@ MPC) composite (Figure S2a,c, Supporting Information), and then paired with Na anodes to assemble the Na-S coin cells and placed in ambient environment for electrochemical tests (Figure 1a). Figure 1b shows the cyclic voltammograms (CVs) of the sulfur cathode. There are two reduction peaks in the first cycle at 1.45 V and 1.1 V vs. Na+/Na, respectively, suggesting a stepped reduction reaction between S and Na. However, only one sharp anodic peak appears at 1.75 V in the reverse oxidation process. From the second scan, the oxidation peaks shift to higher potentials (1.55 V and 1.1 V), suggesting a much reduced polarization. The CV curves gradually stabilize in the subsequent scans, indicating a stable electrochemistry of the battery. To further test the performances of the battery, galvanostatic discharge-charge (GDC) tests were performed on the assembled RT Na-S battery. Figure 1c shows the GDC profiles of a Na-S battery cycled at 0.1 C (167 mA g−1 based on S mass, same below). The battery exhibits two slopped discharge plateaus with almost equal capacity contributions in the first cycle (above and below 1.4 V, as divided by the dashed line), and one plateau upon charging, which are consistent with the CVs. The electrochemical behavior of the RT Na-S battery is different from those reported in previous literatures with its much lowered discharge/charge voltage. The two plateaus together contribute to a specific capacity of 1610 mA h g−1 (based on S mass, same below), close to the theoretical capacity of S (1675 mA h g−1 to yield Na2S). Given that carbon makes little capacity contribution
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A High-Energy Room-Temperature Sodium-Sulfur Battery
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Figure 1. Configuration of the RT Na-S battery and electrochemical behaviors of the sulfur cathode. (a), Schematic illustration of the Na-S battery, (b), The first five CVs of the S/(CNT@MPC) cathode at 0.05 mV s−1, (c), GDC voltage profiles of the S/(CNT@MPC) cathode at 0.1 C.
above 0.8 V,[8] the result indicates that sulfur in the S/(CNT@ MPC) composite is completely reduced to Na2S. Since the capacity contributions on the two plateaus are equal, the discharge products at the ends of the two plateaus could be Na2S2 and Na2S, respectively. To confirm the feasibility of sulfur reduction in micropores of ~0.5 nm, theoretical calculations have been performed to investigate the molecular structures of Na2S2 and Na2S. Figure 2a simulates the molecular structures of Na2S2 and Na2S from their crystals.[9] Though Na2S2 has two crystal forms (α-Na2S2 and β-Na2S2, which respectively belong to P62m and P63mmc space groups), only α-Na2S2 is adopted due to its higher stability at ambient temperature.[9b] Simulation results show that the dimensions of Na2S2 and Na2S molecules are close to the size of the micropore, suggesting that these
molecules can be accommodated as single molecules in the carbon channels. Since Na2S2 and Na2S crystals can not form in the micropore, the interaction between the molecule and adjacent ions is weak, and the molecular structures of Na2S2 and Na2S may vary from those in their crystals. To clarify this point, geometric optimizations have been performed on these molecules free of the interactions with adjacent ions. As shown in Figure 2a, the optimized Na2S2 and Na2S molecules show denser and flattened structures, and each of them has at least two perpendicular dimensions smaller than or very close to the diameter of carbon micropore (0.5 nm), suggesting that they can be accommodated. Therefore, the reduction from S to Na2S is theoretically feasible in the carbon micropore. To clarify the final product at the end of the reduction process, ex-situ transmission electronic microscopy (TEM) and
Figure 2. Molecular structure simulation and electrochemistry of the RT Na-S battery. (a), Structures and dimensions of Na2S2 and Na2S molecules in their crystals and after geometry optimizations (unit of length: angstrom), (b), Electrochemical reactions between S and Na+ during the discharge process, (c) XPS S2p spectrum of the sulfur cathode before use, (d), XPS S2p spectrum of the same cathode after being discharged to 0.8 V.
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COMMUNICATION Figure 3. Performances of the S/(CNT@MPC) cathode and corresponding specific energies of the RT Na-S battery at different GDC rates. (a), Cycling performances of the S/(CNT@MPC) cathode at 0.1 C, 1 C and 2 C. (b), GDC voltage profiles of the S/(CNT@MPC) cathode in the 4th cycle at these rates. Specific energy profiles vs. voltage of the RT Na-S battery (c), in different cycles at 0.1 C and (d), in the 4th cycle at 0.1 C, 1 C and 2 C.
X-ray photoelectron spectroscopy (XPS) characterizations were performed on a sulfur electrode after its discharge to 0.8 V. The energy dispersive X-ray (EDX) spectrum reveals a rich Na abundance on the S/(CNT@MPC) cathode at the discharged state (Figure S2b,d, Supporting Information). The elemental mappings of C, S and Na overlap each other, indicating a uniform Na intercalation in the composite (Figure S3, Supporting Information). On the EDX pattern collected from the discharged S/ (CNT@MPC) nanocables (Figure S4, Supporting Information), the atom ratio between Na and S is calculated to be ~2:1, which suggests a chemical formula of Na2S of the product. The XPS spectra further demonstrate that the originally zero-valent sulfur has been fully reduced to Na2S (Figure 2c,d).[10] These results prove that the final discharge product of S is Na2S in the RT Na-S battery. It is reported that sulfur in HT Na-S batteries usually exhibits one discharge plateau with an incomplete reduction product of Na2Sn (n ≥ 3), which reduces the specific capacity of sulfur (≤ 558 mA h g−1) and the specific energy of the battery.[3a,4] In the present RT Na-S battery, it is the high electrochemical activity of confined S2–4 molecules that leads to a complete reduction of sulfur even at room temperature. In this way, we can take full advantage of the electrochemistry of Na-S battery.
Adv. Mater. 2014, 26, 1261–1265
Given the discharge products of the two plateaus are Na2S2 and Na2S, respectively, the stepwise reductions of S can be described by the following reactions (Figure 1b): 2S + 2Na → Na2 S2 (>1.4 V)
(1)
Na2 S2 + 2Na → 2Na2 S (
