A Rechargeable Na/Cl Battery

doi:10.21203/rs.3.rs-82836/v1

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A Rechargeable Na/Cl Battery

https://doi.org/10.21203/rs.3.rs-82836/v1

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published 25 Aug, 2021

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Sodium is a promising anode material for batteries due to its low standard electrode potential, high abundance and low cost. In this work, we report a new rechargeable ~ 3.5 V sodium ion battery using Na anode, amorphous carbon-nanosphere cathode and a starting electrolyte comprised of AlCl₃ in SOCl₂ with fluoride-based additives. The battery, exhibiting ultrahigh ~ 2800 mAh/g first discharge capacity, could cycle with a high reversible capacity up to ~ 1000 mAh/g. Through battery cycling, the electrolyte evolved to contain NaCl, various sulfur and chlorine species that supported anode’s Na/Na⁺ redox and cathode’s chloride/chlorine redox. Fluoride-rich additives were important in forming a solid-electrolyte interface, affording reversibility of the Na anode for a new class of high capacity secondary Na battery.

Chemical Engineering

Energy Engineering

rechargeable battery

sodium ion battery

Na anode

high capacity

Devising new battery concepts is important to meeting society’s growing demand of energy storage. Different rechargeable batteries have been developed, including lithium ion batteries (LIBs), sodium ion batteries (SIBs) and aluminum ion batteries (AIBs)^1-9. Prior to the invention of secondary LIBs, a primary Li-metal battery was developed in the 1970’s using thionyl chloride (SOCl₂) as a catholyte, Li metal as anode and amorphous carbon as the positive electrode^10-16. The Li-SOCl₂ battery was attractive due to its high energy density, but did not receive sustained interest due to the lack of rechargeability^17,18. The battery discharges through Li anode oxidation and catholyte SOCl₂ reduction into sulfur (S), sulfur dioxide (SO₂), and chloride ion (Cl^-) on the carbon electrode^19,20. The Cl^- ions react with Li⁺ stripped from the Li anode to form LiCl deposited on the carbon surface until full coverage/passivation. This primary battery could deliver a high specific capacity of ~ 2300 mAh/g and high energy density of up to 710 mWh/g in a single discharge^19,20. In addition, the Li-SOCl₂ batteries exhibit a long shelf life (10 – 20 years) and wide operation temperature range, and have been used in applications for the military, utility metering, GPS tracking and professional electronics^21,22.

Sodium batteries have also been actively pursued as an alternative to lithium due to its low standard electrode potential (only ~ 0.34 V higher than lithium), much lower price and equal promise for high energy density batteries^23,24. Interestingly, there has been no report of sodium/SOCl₂ primary battery as in the lithium primary battery case, let alone rechargeable Na batteries in SOCl₂ based electrolyte.

Here, we report a sodium – amorphous carbon nanosphere (aCNS) battery with AlCl₃ and thionyl chloride as the main components in the starting electrolyte. The positive electrode contained a packed layer of ~ 60 nm high temperature CO₂ activated aCNS with an ultra-high surface area of 3000 – 3200 m²/g. The battery delivered a high first discharge capacity ~ 2800 mAh/g (based on the mass of aCNS) with an average discharge voltage of ~ 3.2 V. To our surprise the battery could be cycled reversibly at a specific capacity up to 1000 mAh/g with a discharge voltage of ~ 3.55 V and an average coulombic efficiency ~ 99.5%. It was found that the first discharge of battery involved thionyl chloride (SOCl₂) catholyte reduction to sulfur (S), sulfur dioxide (SO₂) and NaCl, with both NaCl and S deposited to passivate the positive aCNS electrode. When the battery was recharged, Cl^- in NaCl and SOCl₂ were oxidized to form Cl₂, SCl₂, S₂Cl₂, and SO₂Cl₂ in the system. Upon subsequent discharge, the dominant reaction was the reduction of Cl₂ to form NaCl, accompanied by SCl₂, S₂Cl₂ and SO₂Cl₂ reduction and SOCl₂ regeneration to complete the cycle. The Na/Cl secondary battery opens new possibilities for energy storage.

The amorphous carbon nanospheres were synthesized using a slightly modified Stöber method by first dissolving a triblock copolymer (F – 127), ammonia, resorcinol and formaldehyde in a mixture of deionized water and ethanol²⁵. Upon centrifugation, the collected solid was dried and carbonized by heating at 800 ^oC for 4 hours in a nitrogen (N₂) environment followed by 1000 ºC for 45 minutes in carbon dioxide (CO₂) (see Methods). Scanning electron microscope (SEM) imaging showed that aCNS was made of closely packed ~ 60 nm carbon nanospheres (Fig. 1a). Transmission electron microscopy (TEM) showed amorphous carbon without lattice fringes, consistent with X-ray diffraction (XRD) data lacking well-defined diffraction peaks (Fig. 1b, Extended Data Fig. 1a, b). The Brunauer – Emmett – Teller theory (BET) surface area measurement indicated that aCNS had an impressive surface area between 3000 – 3200 m²/g and high pore volume of 2.76 cm³/g (Extended Data Table 1).

A Na/Cl battery was made by using sodium metal as the negative electrode and packed carbon nanosphere (aCNS) with PTFE binder in a Ni foam as the positive electrode in coin cell configuration (see Methods, Extended Data Fig. 2). The starting electrolyte was 4 M aluminum chloride (AlCl₃) dissolved in SOCl₂ with a mixture of 2 wt% sodium trifluoromethanesulfonimide (NaTFSI) and 2 wt% sodium bis(fluorosulfonyl)imide (NaFSI) as additives (Fig. 1a). The as-made battery was first discharged to 2 V, exhibiting a high capacity of ~ 2810 mAh/g with two obvious plateaus at ~ 3.47 V and 3.27 V (Fig. 1c). In the beginning of the discharge, the electrolyte was highly acidic with 4 M AlCl₃ and the plateau at ~ 3.47 V was attributed to Na oxidation, with the NaCl formed reacting with AlCl₃ to neutralize the electrolyte,

4 Na + 4 AlCl₃ + 2 SOCl₂ 4 NaAlCl₄ + S + SO₂ (1)

As discharge progressed in a neutralized electrolyte, a lower discharge plateau of ~ 3.27 V appeared, corresponding to

4 Na + 2 SOCl₂ 4 NaCl + S + SO₂ (2)

with the NaCl product depositing onto the positive electrode and passivating the electrode. Similar two-plateau discharge was observed in Li/SOCl₂ cells in initially acidic electrolytes^11,17.

At the end of the first discharge sulfur dioxide (SO₂) formation was confirmed by mass-spectroscopy measurements of species inside the battery cell (Fig. 1d). Since SO₂ had a solubility of 2.47 M in SOCl₂ near room temperature at 22.3 ^oC and the molarity of SO₂ after the first discharge was ~ 1.34 M, most of the SO₂ was dissolved in the electrolyte without increasing the pressure in the battery²⁶. XRD and SEM imaging showed a thick layer of NaCl on the surface of the aCNS positive electrode (Fig. 1e), accompanied by a large increase in the electrochemical impedance of the battery (Extended Data Fig. 3a), consistent with passivation of the carbon electrode by deposited NaCl. The reduction chemistry of SOCl₂ to form S, SO₂ and Cl^- was similar to that in Li/SOCl₂ primary cells¹⁷. The proposed reactions (1) and (2) were supported by the fact that when an initially neutral electrolyte of 4 M NaCl + 4 M AlCl₃ in SOCl₂ was used, the higher plateau at ~ 3.47 V was not observed in the first discharge and only the plateau at ~ 3.25 V appeared throughout the whole discharge (Extended Data Fig. 3b).

When re-charging the battery after the first discharge, Na was deposited on the Na electrode and Cl^- from the deposited NaCl on the aCNS electrode was oxidized (at ~ 3.83 V, Fig. 2a) to form Cl₂ likely trapped in the abundant pores of aCNS (Extended Data Table 1). Oxidation of NaCl was probed by XPS showing that under increasing charging the percentage of NaCl on aCNS decreased, the NaCl crystallites on aCNS disappeared (revealed by SEM, Fig. 2b), and the XRD peaks of NaCl on aCNS decreased (Fig. 2c). In the beginning of charging, the voltage immediately spiked to ~ 4.16 V and then decreased to ~ 3.83 V. The voltage spike was observed in the beginning of charging in every cycle and was attributed to a high impedance caused by the NaCl coating layer on the aCNS electrode. Towards the end of charging (Fig. 2a), a slightly higher charging voltage plateau (~ 3.91 V) was observed and the reaction was attributed to the oxidation of SOCl₂ with formation of SCl₂, S₂Cl_2.and SO₂Cl₂^18,27,28. SO₂Cl₂ could also form by chemical reaction between Cl₂ generated through the main charging plateau and SO₂ generated in the first discharge step (Fig. 1d).

Using mass-spectroscopy (MS) we analyzed the speciation and composition of molecules in batteries by opening them in charged and discharged state (see Text S1 in SI: Mass Spectroscopy Analysis of Chemical Compositions in Na/Cl Battery Cells). The batteries were stopped at the 21^stcycle running at a stable coulombic efficiency (CE) ~ 100% and opened. The main discharge plateau of ~ 3.55 V was found to be due to Cl₂ reduction, based on that the detected Cl₂ species (excluding fragments from other molecules, see Extended Data Figure 4) decreased to ~ 0 in fully discharged state from ~ 100 % in fully charged state (see Text S1). Reduction of molecular Cl₂ species generated and trapped in the pores of aCNS contributed to the majority of the discharge capacity of the battery during stable cycling. The main battery reaction responsible for the main charge/discharge plateaus during cycling was Na + ½ Cl₂ NaCl.

The two small discharge plateaus at ~ 3.69 V and ~ 3.18 V were attributed to reduction of S₂Cl₂/SCl₂ (formed at the end of the charging plateau) and SO₂Cl₂ (formed at the end of the charging step) respectively (Fig. 2a), based on capacity retention and mass-spectrometry experiments. We observed that the main discharge plateau at ~ 3.55 V decreased in capacity while the lower discharge plateau at ~ 3.18 V extended as a battery was held at open-circuit in a charged state over longer times (up to 5 days) (Fig. 2d). Mass spectrometry showed that the detected Cl₂ in the battery decreased proportionally to the ~ 3.55 V plateau capacity, together with a decrease in SO₂ and increased SO₂Cl₂ and increased ~ 3.18 V SO₂Cl₂discharge plateau (Fig. 2e, Fig. 2f, Extended Data Fig. 5). During long retention times, Cl₂ and SO₂ in the system slowly recombined to form SO₂Cl₂, leading to the observed decrease in the 3.55 V Cl₂ reduction plateau and increase in the SO₂Cl₂ discharge plateau of ~ 3.18 V (Fig. 2d). Although open circuit holding of battery for days would sacrifice the higher discharge plateau at ~ 3.55 V, battery capacity retention was ~ 99.9%, and the 3.55 V plateau was immediately restored in subsequent cycles and the battery could continue to cycle with excellent reversibility (Fig. 2g, Fig. 2h).

Mass spectrometry also revealed that SCl₂ in the battery increased when fully charged and decreased to ~ 0 when the battery was discharged (Extended Data Fig. 4, Text S1). This and the fact that SCl₂ and S₂Cl₂ were known to be generated from oxidation of SOCl₂ at a higher potential suggested that the small plateau in the beginning of the high discharge voltage of ~ 3.69 V (~ 20 mAh/g or 4% of total capacity) corresponded to SCl₂/S₂Cl₂ reduction ^10,18,27,28.

The chemical compositions inside the Na/Cl battery evolved in charged and discharged state, but between cycles the composition was largely kept constant since the main redox reactions involving species of Cl₂, SCl₂, S₂Cl₂, SOCl₂ and SO₂Cl₂ were largely reversible. The reduction product of NaCl formed by discharge reacted with AlCl₄^-•SOCl⁺in the electrolyte to re-generate SOCl₂ oxidized in the charging step, which was important to rechargeability of the Na/Cl battery ¹⁸.

The Na/Cl battery cycled reversibly and stably for more than 100 cycles at a set specific capacity of 500 mAh/g at 150 mA/g current (Fig. 3a). The reversible battery capacity of the Na/Cl cell was found to be up to ~ 1000 mAh/g with the charge – discharge curve maintaining its overall shape as capacity varied from 375 mAh/g to 1000 mAh/g, with the main ~ 3.83 V charge and ~ 3.55 V discharge plateaus simply extended their length and capacity (Fig. 3b, Fig. 3d). The battery could cycle reversibly at various specific capacities and currents with no obvious decrease in coulombic efficiency (Fig. 3c). At lower currents the polarization voltage showed slight decreases (Fig. 3b, Fig. 3d). For a specific capacity of 1000 mAh/g, the energy density that the ~ 3.5 V Na/Cl battery could deliver was ~ 3.5 Wh/g (based on aCNS mass) with an impressive energy efficiency of 92.4%. Importantly, throughout cycling of all of our Na/Cl battery coin cells, we encountered no safety problems. We found no pressurizing problems due to strong solvation of SO₂, Cl₂ species by SOCl₂ SO₂Cl₂ and NaAlCl₄ in the electrolyte.

Since NaCl was the major species undergoing oxidation during charging, the upper limit of the charging capacity was set by the amount of NaCl deposited on the aCNS in the first discharge. During the first discharge, the high plateau at ~ 3.47 V produced NaCl reacting to neutralize the electrolyte (eq. 1). The lower plateau at ~ 3.27 V (~ 1400 mAh/g) led to the formation of NaCl deposits (eq. 2), which set an upper limit of the reversible Cl^-/Cl₂ capacity. We observed that the battery could cycle up to 1300 mAh/g close to the ~ 1400 mAh/g limit but with a poor cycle life likely caused by the reduced stability of the Na anode under higher charging conditions.

We found that the Na/Cl battery decay was due to the Na anode and not the cathode, based on that a battery using fresh Na, fresh electrolyte and a recycled aCNS electrode from a decayed battery could cycle stably again with normal charge – discharge behavior. The fluoride containing additives, known to facilitate the formation of robust solid electrolyte interphase (SEI) on alkaline metal battery anodes in general, were found important to prolong the cycle life of our Na/Cl battery^29-33. Batteries with no additive showed the worst cycle life (< 50 cycles). When only 2 wt% NaFSI was added, the cycle life of the battery improved to about 70 cycles, and when a mixture of 2 wt% NaFSI/NaTFSI was added, the battery showed the best cycling performance (> 100 cycles) (Fig. 4a). The composition on Na metal surface after immersion in electrolyte (4 M AlCl₃ in SOCl₂) with and without 2 wt% NaFSI/NaTFSI additives were analyzed by XPS. We found NaCl coating on Na metal due to reaction with SOCl₂ in both samples, analogous to LiCl formed on Li in Li – SOCl₂ primary batteries (Extended Data Fig. S6a, d)^34-36. Abundant elements of F, N and S, assigned to NaF, Na₂SO₄, Na₂S₂O₃, Na₂SO₃ were observed only on Na in the fluoride containing electrolyte, suggesting reactions between Na and anions FSI^- and TFSI^- forming a more protective SEI layer on Na (Extended Data Fig. S6a, b, c)^37-40.

The fluoride containing SEI was formed in the first few cycles over which the coulombic efficiency gradually increased to ~ 100% (Fig. 3a). A more robust SEI layer could prevent or slow down reactions with corrosive species in the electrolyte including Cl₂, SOCl₂, SCl₂and SO₂Cl₂. When the battery eventually decayed, the overpotential increased accompanied by a drop in coulombic efficiency (Extended Data Fig. 6e). The impedance of the battery increased significantly as well, caused by the build-up of thick insulating NaCl on the Na anode. Our observation was consistent with previous results that the SEI on alkaline metal anode was more robust when both FSI^- and TFSI^- anions were present, leading to better sodium battery cycle lives^1,41.

Lastly, our amorphous carbon nanosphere was a novel positive electrode material for the Na/Cl battery owing to its ultra-high surface area (3000 – 3200 m²/g) and high porosity (2.76 cm³/g). We compared several widely used commercially available amorphous carbon including acetylene black (AB) and ketjenblack carbon black (KJ) to aCNS as positive electrode (Fig. 4b). The battery using AB as positive electrode could only run for less than 20 cycles and battery using KJ as positive electrode showed a better cycle life than AB but the coulombic efficiency was always lower than 90%. We attributed aCNS’s high cycling performance to both high surface area and porosity that were superior to AB. On the other hand, KJ showed higher pore volume but exhibited much lower surface area than aCNS (Extended Data Table 1). High surface area and porosity helped the positive electrode to better accommodate insoluble discharge product, i.e., NaCl in our battery⁴². In addition, carbon with high surface area and porosity could trap the corrosive species such as Cl₂, SCl₂, SO₂Cl₂ in the electrolyte better, reducing corrosion of the anode and improving battery’s cycle life.

We employed the Na/Cl battery to light up of an LED light that required an operating voltage of 3.0 V – 3.2 V. The current measured through the LED was ~ 12.03 mA with a high current density of 4.57 mA/cm²Na, equivalent to a discharge rate of 1563.35 mA/g (based on aCNS mass) (Fig. 4c). The new rechargeable Na/Cl battery with ultra-high first discharge capacity and subsequent high reversible capacity gives a new life to decades old primary batteries. There is still room to further improve the battery performance. Novel electrolyte additives can be explored for more robust SEI to further prolong the battery cycle life. In addition, other positive electrode materials can also be investigated.

Data Availability the authors declare that the data supporting the findings of this study are available within the paper and its supplementary information files.

Synthesis of aCNS

50 mL of deionized water and 20 mL of ethanol (> 99.9 %, J. T. Baker) were mixed uniformly at room temperature. 0.25 g of triblock copolymer, F-127 (PEO106-PPO70-PEO106, MW: 14600, Aldrich), was then added in the mixture and stirred for about 10 minutes. After F-127 dissolved completely, 0.5 g of ammonia solution (25 %, Choneye, Taiwan) was then added in the solution and stirred for about 30 minutes followed by adding 0.5 g of resorcinol (99%, Alfa Aesar) into the solution. Finally, 0.763 g of formaldehyde solution (37 wt%, Aldrich) was added gradually into the solution and stirred for 24 hours at room temperature. The solution was centrifuged with 14,900 rpm to separate the solid and liquid. The solid was dried at 100 ^oC in oven and heated at 350 ^oC for 2 hours in N₂ to remove the template. The carbonization process was conducted at 800 ^oC for 4 hours in N₂followed by the activation process using CO₂ at 1000 °C for 45 minutes.

Fabrication of aCNS electrode

90% by weight of aCNS and 10% by weight of polytetrafluoroethylene (60% aqueous PTFE dispersion, FuelCellStore) were mixed in 100% ethanol (Fisher Scientific). The mixture was sonicated for 2 hours until the aCNS was uniformly dispersed in ethanol. Ni foam substrate was cut into circular shape with diameter of 1.5 cm using a compact precision disc cutter (MTI, MSK-T-07). The circular Ni foam substrate was sonicated in 100% ethanol for 15 minutes and dried in an 80 ^oC oven until all the ethanol evaporated. The weight of the Ni foam substrate was measured and then placed to hover over a hot plate. The aCNS, PTFE and ethanol mixture was then slowly dropped (180 μL each time) onto the Ni foam. Between each drop, we waited for approximately 4 minutes to allow all the ethanol from previous drop to fully evaporate. This process was repeated and stopped until the loading of the aCNS on Ni foam substrate reached 4.5 – 5 mg/cm². The electrodes were then dried in an 80 ^oC oven overnight. After drying, the electrode was pressed using a spaghetti roller and the final weight of the electrode was measured. After calculating the weight of aCNS, i.e. final weight of the electrode minus initial weight of the Ni foam times 90%, the electrode was ready to be used in a battery.

Electrolyte making

The electrolyte was made inside an argon-filled glovebox. NaFSI (TCI Chemical) and NaTFSI (Alfa Aesar) were dried at 100 ^oC vacuum oven overnight before use and stored in an argon – filled glovebox. Thionyl chloride (purified, Spectrum catalog # TH138) was used without any further purification. The appropriate amount of thionyl chloride liquid was added into a 20 mL scintillation vial (Fisher Scientific) and its weight was measured. 4 M aluminum chloride (Fluka, 99%, anhydrous, granular) were weighed and added to the thionyl chloride and stirred until all the aluminum chloride was fully dissolved. Then the appropriate amount of NaFSI and NaTFSI (2 wt% of the total weight of aluminum chloride and thionyl chloride) were added to the solution and stirred until both NaFSI and NaTFSI completely dissolved, after which the electrolyte was ready to be used.

Battery making

All batteries were made inside an argon-filled glovebox. Sodium metal block (Sigma Aldrich) was dried using kimwipes (Kimberly – Clark Professional^TM Kimtech Science^TM) to remove the mineral oil on the surface. Razor blade was then used to cut all sides of the Na block to expose the shiny Na metal. The sodium metal block was then placed inside a zip lock bag and pressed using a scintillation vial to make thin sodium foil. The sodium foil was then pasted onto the spacer in a coin cell. Any extra sodium was removed so that the sodium foil had the exact shape as the spacer and could be used as the negative electrode. aCNS loaded on Ni foam was used as the positive electrode. 2 layers of quartz fiber filters (Sterlitech, Advantec, QR – 100) were used as the separators and were dried in 120 ^oC vacuum oven overnight before each use. The aCNS positive electrode was put in the middle of the SS316 positive coin cell case. 2 layers of QR – 100 separators were then put on top of the aCNS positive electrode. 150 µL of the electrolyte (4 M AlCl₃ in SOCl₂ + 2 wt% NaFSI + 2 wt% NaTFSI) were then added to wet the QR – 100 separators. The Na negative electrode on spacer was then put on top of the separators, with Na foil directly facing the aCNS positive electrode. One piece of spring was put on top of the spacer. Lastly, one layer of PTFE foil was put on top of the spring and underneath the SS316 negative coin cell case to prevent corrosion from the electrolyte. After all the components of the coin cell were put together, the coin cell was pressed using a digital pressure controlled electric crimper (MTI, MSK-160E) with the pressure reading set to 9.23. Then the coin cell was taken out the glovebox and was tested using a battery tester from Neware, BTS80, Version 17.

Electrochemical Impedance Spectroscopy

The electrochemical impedance spectroscopy (EIS) of the battery was measured using a potentiostat/galvanostat (model CHI 760D, CH Instruments). The working electrode was connected to the aCNS positive electrode, and the counter and reference electrodes were connected to the sodium negative electrode. The initial voltage of the measurement was set to be the open circuit potential of the battery at the time of the measurement. The high frequency was 1 10⁵ Hz and the low frequency was 0.01 Hz. The amplitude of the measurement was 0.005 V.

Scanning Electron Microscope (SEM)

SEM imaging was measured using Hitachi/S-4800 SEM instrument. To conduct SEM imaging on aCNS, aCNS powder was first stuck on the sample stage of SEM using double-sided conductive carbon adhesive tapes and the stage was then loaded into the SEM chamber for measurement. To conduct SEM imaging on electrodes in actual battery, the battery was first opened inside an argon – filled glovebox. The electrodes were taken out from the opened battery and transferred into the argon – filled antechamber of the glovebox. The electrodes were vacuumed and dried inside the antechamber for approximately 3 hours to remove any electrolyte trapped in them. After drying, the electrodes were transferred back into the glovebox and ready to be characterized. The samples were stuck onto the SEM sample stage using double-sided conductive carbon adhesive tapes and introduced into the SEM chamber for measurement. The sample was observed by SEM with 15 kV acceleration voltage of an electron beam at a pressure of 10^-7 torr. A magnification of 200,000 could be achieved.

Transmission Electron Microscopy (TEM)

Transmission electron microscopy (TEM) imaging was conducted on a FEI EO Tecnai F20 G2 MAT S-TWIN field transmission electron microscopy. To prepare samples for TEM imaging, 0.02 g aCNS was dispersed in 10 mL deionized water in a 20 mL scintillation vial (Fisher Scientific). The mixture was sonicated for 30 minutes until a uniform dispersion of aCNS was achieved. After sonication, one drop of the mixture was dropped onto a Cu TEM grid using glass dropping pipette. The grid was then placed inside a 100 ^oC oven for 3 days. After drying, the Cu TEM grid with aCNS sample was introduced into the TEM instrument operating at 200 kV for measurement.

XPS Experiments

XPS measurement was conducted in SNSF facility, Stanford University and the XPS instrument used was PHI VersProbe 1. To conduct XPS on sodium immersed in different solutions (4 M AlCl₃ + 2 wt% NaFSI/NaTFSI in SOCl₂ or 4 M AlCl₃ in SOCl₂), the sample preparation was done inside an argon – filled glovebox. Na foil was prepared the same way as preparing Na electrode in battery (Battery Making). After immersion in the appropriate solution for 1 hour, the Na foil was taken out from the solution and any liquid remaining on the surface was dried using kimwipes (Kimberly – Clark Professional^TM Kimtech Science^TM). The antechamber of the glovebox was refilled with argon and the sample was transferred into the antechamber, in which the sample was vacuumed dried. After drying, the sample was transferred into the glovebox and was ready to be characterized by XPS. To conduct XPS on electrodes from battery, the sample preparation was the same as the sample preparation for SEM imaging. After sample preparation, the sample was clamped onto the XPS stage and was transferred into the main chamber of the XPS instrument for measurement. All the spectra reported were the spectra obtained after 20 nm argon ion sputtering to remove any possible surface contamination during sample handling.

X – ray Diffraction

X-ray diffraction (XRD) was conducted on an X-ray diffraction system (Rigaku Miniflex 600 Benchtop) with Cu Kα radiation. The aCNS powder was put on the XRD sample stage and a razor blade was used to press the powder until a flat surface was obtained and the powder was uniformly and firmly distributed over the sample stage. Any extra powder was carefully removed from the sample stage. The sample stage was then transferred into the center of the XRD instrument for measurement. The start angle and the stop angle were set to be 5^o and 90^o, respectively, with the scan speed of 3^o/min. To conduct XRD measurements of electrodes from battery, the sample preparation was the same as the sample preparation for SEM imaging, and XRD was performed after the samples were transferred out from glovebox into the XRD instrument.

pH Measurements of Carbon

pH was measured by dissolving 1 g of the carbon into 30 mL deionized water. The solution was then transferred into a round bottom flask and boiled under reflux for 5 minutes. After 5 minutes of boiling, the round bottom flask was removed from the heat source and allowed to cool down to room temperature. After all the carbon particles has sunk to the bottom of the round bottom flask, the pH of the clear liquid at top was measured.

Brunauer – Emmett – Teller (BET) Surface Area and Porosity

Brunauer–Emmett–Teller (BET) surface area and pore volume were measured by an 2020 Accelerated Surface Area and Porosimetry System from Micromeritics. Before each measurement, the appropriate amount of carbon (~ 0.14 g) was weighed and placed in the instrument for degas at (350 ^oC). After degassing, the weight of the carbon was measured again and this weight was input into the software for final surface area and porosity analysis. In the final analysis, the evacuation time was set to be (6 hours) and dose amount was set to be (10 cm³/g STP). After the measurement was done by the instrument, the surface area and porosity were reported.

Volatile Percent of Carbon

Volatile % was measured using a high gravimetric sensitivity thermogravimetric analysis (TGA) instrument. The initial weight of the carbon samples was measured before introducing the samples into the TGA instrument. Then the temperature of the instrument was increased to 80 ^oC in 5 minutes and held at 80 ^oC for 10 minutes. After the 10 minutes isothermal step, the temperature was increased to 160 ^oC in 8 minutes and then held at 160 ^oC for 10 minutes. The final weight of the carbon was measured and the volatile % of the carbon was equal to the percent difference between the initial weight and the final weight⁴³.

Acknowledgments part of this work was performed at the Stanford Nano Shared Facilities (SNSF), supported by the National Science Foundation under award ECCS-1542152.

Author contributions G. Zhu and H. Dai. conceived the main idea of the project. G. Zhu and X. Tian performed the experiments and contributed equally to this work. G.Z. performed the mass spectroscopy measurement of species in the battery. H. –C. Tai, Z. –L. Huang and Y. –Y. Li prepared the aCNS raw material. H. –C. Tai performed the characterizations of aCNS (SEM, TEM, XRD etc.). J. Li, C. –S. Ku, W. -H. Hung, J. Kai and B. –J. Hwang performed characterizations of electrodes in battery. G. Zhu and H. Sun performed the LED demo of the Na/Cl battery. G. Zhu, P. Liang and M. Angell performed X-ray photoelectron spectroscopy measurements. G. Zhu and H. Dai. prepared the manuscript. All authors participated in experimental data/results analysis and discussion;

Competing interests Authors declare no competing interests.

Supplementary Information is available for this paper.

Correspondence and requests for materials should be addressed to H. Dai. ([email protected]). Reprints and permissions information is available at www.nature.com/reprints.

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A Rechargeable Na/Cl Battery

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