A Rechargeable Na/Cl Battery

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 AlCl 3 in SOCl 2 with uoride-based additives. The battery, exhibiting ultrahigh ~ 2800 mAh/g rst 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.

reduction to sulfur (S), sulfur dioxide (SO 2 ) and NaCl, with both NaCl and S deposited to passivate the positive aCNS electrode. When the battery was recharged, Clin NaCl and SOCl 2 were oxidized to form Cl 2 , SCl 2 , S 2 Cl 2 , and SO 2 Cl 2 in the system. Upon subsequent discharge, the dominant reaction was the reduction of Cl 2 to form NaCl, accompanied by SCl 2 , S 2 Cl 2 and SO 2 Cl 2 reduction and SOCl 2 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 modi ed Stöber method by rst dissolving a triblock copolymer (F -127), ammonia, resorcinol and formaldehyde in a mixture of deionized water and ethanol 25 . Upon centrifugation, the collected solid was dried and carbonized by heating at 800 o C for 4 hours in a nitrogen (N 2 ) environment followed by 1000 ºC for 45 minutes in carbon dioxide (CO 2 ) (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-de ned 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 m 2 /g and high pore volume of 2.76 cm 3 /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 con guration (see Methods, Extended Data Fig. 2). The starting electrolyte was 4 M aluminum chloride (AlCl 3 ) dissolved in SOCl 2 with a mixture of 2 wt% sodium tri uoromethanesulfonimide (NaTFSI) and 2 wt% sodium bis( uorosulfonyl)imide (NaFSI) as additives (Fig. 1a). The as-made battery was rst 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 AlCl 3 and the plateau at 3 .47 V was attributed to Na oxidation, with the NaCl formed reacting with AlCl 3 to neutralize the electrolyte, 4 Na + 4 AlCl 3 + 2 SOCl 2 4 NaAlCl 4 + S + SO 2 (1) As discharge progressed in a neutralized electrolyte, a lower discharge plateau of ~ 3.27 V appeared, corresponding to 4 Na + 2 SOCl 2 4 NaCl + S + SO 2 (2) with the NaCl product depositing onto the positive electrode and passivating the electrode. Similar twoplateau discharge was observed in Li/SOCl 2 cells in initially acidic electrolytes 11,17 .
At the end of the rst discharge sulfur dioxide (SO 2 ) formation was con rmed by mass-spectroscopy measurements of species inside the battery cell (Fig. 1d). Since SO 2 had a solubility of 2.47 M in SOCl 2 near room temperature at 22.3 o C and the molarity of SO 2 after the rst discharge was ~ 1.34 M, most of the SO 2 was dissolved in the electrolyte without increasing the pressure in the battery 26 . 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 SOCl 2 to form S, SO 2 and Clwas similar to that in Li/SOCl 2 primary cells 17 . The proposed reactions (1) and (2) were supported by the fact that when an initially neutral electrolyte of 4 M NaCl + 4 M AlCl 3 in SOCl 2 was used, the higher plateau at ~ 3.47 V was not observed in the rst 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 rst discharge, Na was deposited on the Na electrode and Clfrom the deposited NaCl on the aCNS electrode was oxidized (at ~ 3.83 V, Fig. 2a) to form Cl 2 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 SOCl 2 with formation of SCl 2 , S 2 Cl 2. and SO 2 Cl 2 18,27,28 . SO 2 Cl 2 could also form by chemical reaction between Cl 2 generated through the main charging plateau and SO 2 generated in the rst 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 st cycle running at a stable coulombic e ciency (CE) ~ 100% and opened. The main discharge plateau of ~ 3.55 V was found to be due to Cl 2 reduction, based on that the detected Cl 2 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 Cl 2 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 Cl 2 NaCl.
The two small discharge plateaus at ~ 3.69 V and ~ 3.18 V were attributed to reduction of S 2 Cl 2 /SCl 2 (formed at the end of the charging plateau) and SO 2 Cl 2 (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 Cl 2 in the battery decreased proportionally to the ~ 3.55 V plateau capacity, together with a decrease in SO 2 and increased SO 2 Cl 2 and increased ~ 3.18 V SO 2 Cl 2 discharge plateau (Fig. 2e, Fig. 2f, Extended Data Fig. 5). During long retention times, Cl 2 and SO 2 in the system slowly recombined to form SO 2 Cl 2 , leading to the observed decrease in the 3.55 V Cl 2 reduction plateau and increase in the SO 2 Cl 2 discharge plateau of ~ 3.18 V (Fig. 2d). Although open circuit holding of battery for days would sacri ce 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 SCl 2 in the battery increased when fully charged and decreased tõ 0 when the battery was discharged (Extended Data Fig. 4, Text S1). This and the fact that SCl 2 and S 2 Cl 2 were known to be generated from oxidation of SOCl 2 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 SCl 2 /S 2 Cl 2 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 Cl 2 , SCl 2 , S 2 Cl 2 , SOCl 2 and SO 2 Cl 2 were largely reversible. The reduction product of NaCl formed by discharge reacted with AlCl 4 -•SOCl + in the electrolyte to re-generate SOCl 2 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 speci c 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 speci c capacities and currents with no obvious decrease in coulombic e ciency ( Fig. 3c). At lower currents the polarization voltage showed slight decreases (Fig. 3b, Fig. 3d). For a speci c 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 e ciency 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 SO 2 , Cl 2 species by SOCl 2 SO 2 Cl 2 and NaAlCl 4 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 rst discharge. During the rst 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 -/Cl 2 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 uoride 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][30][31][32][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 AlCl 3 in SOCl 2 ) with and without 2 wt% NaFSI/NaTFSI additives were analyzed by XPS. We found NaCl coating on Na metal due to reaction with SOCl 2 in both samples, analogous to LiCl formed on Li in Li -SOCl 2 primary batteries (Extended Data Fig. S6a, d) [34][35][36] . Abundant elements of F, N and S, assigned to NaF, Na 2 SO 4 , Na 2 S 2 O 3 , Na 2 SO 3 were observed only on Na in the uoride containing electrolyte, suggesting reactions between Na and anions FSIand TFSIforming a more protective SEI layer on Na (Extended Data Fig. S6a, b, c) [37][38][39][40] .
The uoride containing SEI was formed in the rst few cycles over which the coulombic e ciency gradually increased to ~ 100% (Fig. 3a). A more robust SEI layer could prevent or slow down reactions with corrosive species in the electrolyte including Cl 2 , SOCl 2 , SCl 2 and SO 2 Cl 2 . When the battery eventually decayed, the overpotential increased accompanied by a drop in coulombic e ciency (Extended Data Fig.  6e). The impedance of the battery increased signi cantly 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 FSIand TFSIanions 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 m 2 /g) and high porosity (2.76 cm 3 /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 e ciency 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 battery 42 . In addition, carbon with high surface area and porosity could trap the corrosive species such as Cl 2 , SCl 2 , SO 2 Cl 2 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/cm 2 Na, 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 rst 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 ndings of this study are available within the paper and its supplementary information les.

Methods
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 o C in oven and heated at 350 o C for 2 hours in N 2 to remove the template. The carbonization process was conducted at 800 o C for 4 hours in N 2 followed by the activation process using CO 2 at 1000 °C for 45 minutes.
Fabrication of aCNS electrode 90% by weight of aCNS and 10% by weight of polytetra uoroethylene (60% aqueous PTFE dispersion, FuelCellStore) were mixed in 100% ethanol (Fisher Scienti c). 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 o C 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/cm 2 . The electrodes were then dried in an 80 o C oven overnight. After drying, the electrode was pressed using a spaghetti roller and the nal weight of the electrode was measured. After calculating the weight of aCNS, i.e. nal 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-lled glovebox. NaFSI (TCI Chemical) and NaTFSI (Alfa Aesar) were dried at 100 o C vacuum oven overnight before use and stored in an argon -lled glovebox. Thionyl chloride (puri ed, Spectrum catalog # TH138) was used without any further puri cation. The appropriate amount of thionyl chloride liquid was added into a 20 mL scintillation vial (Fisher Scienti c) 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-lled 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 ber lters (Sterlitech, Advantec, QR -100) were used as the separators and were dried in 120 o C 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 AlCl 3 in SOCl 2 + 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 10 5 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 rst 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 rst opened inside an argon -lled glovebox.
The electrodes were taken out from the opened battery and transferred into the argon -lled 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 magni cation 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 eld 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 Scienti c). 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 o C 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 AlCl 3 + 2 wt% NaFSI/NaTFSI in SOCl 2 or 4 M AlCl 3 in SOCl 2 ), the sample preparation was done inside an argon -lled 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 re lled 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 Mini ex 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 at surface was obtained and the powder was uniformly and rmly 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 ask and boiled under re ux for 5 minutes. After 5 minutes of boiling, the round bottom ask 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 ask, 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 o C). After degassing, the weight of the carbon was measured again and this weight was input into the software for nal surface area and porosity analysis. In the nal analysis, the evacuation time was set to be (6 hours) and dose amount was set to be (10 cm 3 /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 o C in 5 minutes and held at 80 o C for 10 minutes. After the 10 minutes isothermal step, the temperature was increased to 160 o C in 8 minutes and then held at 160 o C for 10 minutes. The nal weight of the carbon was measured and the volatile % of the carbon was equal to the percent difference between the initial weight and the nal weight 43 . Figure 1 A high capacity Na/Cl battery through the rst discharge. a, schematic drawing of the Na/Cl battery with initial electrolyte composition and SEM imaging of amorphous carbon nanosphere (aCNS) in the cathode. b, TEM imaging of aCNS. c, rst discharge curve of the Na/Cl battery. d, Ar normalized mass spectroscopy data of as made electrolyte vs. species in an opened battery after rst discharge. e, XRD spectrum of aCNS after rst discharge, unlabeled peaks were Ni current collector (inset: SEM imaging of aCNS after rst discharge).

Figure 2
Rechargeable Na/Cl battery at different battery states through cycling. a, Charge -discharge curve of the battery at 500 mAh/g (150 mA/g). b, Atomic percentages of Na and Cl from XPS Survey spectra recorded on the aCNS cathode after the battery was charged to different capacities (inset: SEM imaging of the cathode charged to 600 mAh/g with most of the NaCl removed). c, XRD spectra (normalized to Ni current collector) of aCNS cathodes showing evolution of the (220) diffraction peak of NaCl coating on the cathode when batteries in discharged state were charged to various capacities. NaCl was increasingly oxidized/removed from the cathode. d, Charge -discharge curve of a Na/Cl battery, with the discharge curves recorded after the battery was held at open circuit for different retention times in fully charged state. (e, f), Percentage changes for the parameters indicated versus battery retention time in open-circuit charged state for 0 hour, 24 hours, 72 hours and 120 hours before discharging. g, Charge -discharge curves (red) of a Na/Cl battery recorded after discharging the battery (black) post 120 hours retention in charged state. h, cycling performance of the Na/Cl battery with different retention cycles at 500 mAh/g (150 mA/g).

Figure 3
Cycling performance of Na/Cl battery at different capacities a, Cycling performance of a Na/Cl battery as the charging capacity was set to 500 mAh/g (150 mA/g). b, Charge -discharge curves of a Na/Cl battery