Comparative study of fluoroethylene carbonate and succinic anhydride as electrolyte additive for hard carbon anodes of Na-ion batteries

Journal of Power Sources 423 (2019) 137–143

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Comparative study of fluoroethylene carbonate and succinic anhydride as electrolyte additive for hard carbon anodes of Na-ion batteries

T

Dong-Hui Kim, Byungsun Kang, Hochun Lee∗ Department of Energy Science & Engineering, DGIST, Daegu, 42988, Republic of Korea

H I GH L IG H T S

G R A P H I C A L A B S T R A C T

and SA were compared as elec• FEC trolyte additives for hard carbon (HC) anodes.

hardly improves the cycle per• FEC formance of HC anodes. markedly improves the cycle and • SA storage performance of HC anodes at 60 °C.

A R T I C LE I N FO

A B S T R A C T

Keywords: Sodium battery Hard carbon Solid electrolyte interphase Succinic anhydride Fluoroethylene carbonate Symmetric cell

Hard carbon is the most popular anode material for Na-ion batteries, but its long-term reliability has still to be further improved. We herein examine fluoroethylene carbonate and succinic anhydride as electrolyte additives for improving the cycle performance of hard carbon anodes. For a proper evaluation, hard carbon symmetric cells as well as hard carbon/Na half-cells are employed to discriminate the influence of the additives on the hard carbon and Na electrodes. We confirm that fluoroethylene carbonate hardly improves the cyclability of hard carbon symmetric cells, but it does enable far better cycle behavior of Na metal, and thus improves the cycle performance of hard carbon/Na half-cells by mitigating the degradation of the hard carbon anodes caused by harmful products derived from Na metal. In contrast, succinic anhydride markedly improves the performance of hard carbon symmetric cells in the cycle and storage tests at 60 °C. The superior thermal stability of succinic anhydride is attributed to a Na2CO3/sodium alkyl carbonates-rich solid-electrolyte interphase layer formed on the hard carbon surface, as evidenced by X-ray photoelectron spectroscopy measurements.

1. Introduction The demand for next-generation batteries, featuring superior energy/power density, long-term stability, and cost competitiveness, is increasing along with the rapid growth of large-scale applications including electric vehicles and grid storage. The Na-ion battery (NIB) is one of the most promising candidates among the post-Li ion chemistries, because of abundant Na resources [1–6]. Furthermore, recent

∗

rapid progresses in NIB cathode materials, including layered O3 and P2 type structures, have enabled NIBs to be competitive with Li-ion batteries (LIBs) in terms of energy/power density [5,6]. Graphite, the most common LIB anode material, cannot be adopted in NIBs because of the thermodynamically unstable nature of Na-intercalated graphite [7–10]. Hard carbon (HC) has been suggested as an alternative anode material to supersede graphite's low cost and high energy density, based on its Na storage ability via intercalation between

Corresponding author. E-mail address: [email protected] (H. Lee).

https://doi.org/10.1016/j.jpowsour.2019.03.047 Received 3 January 2019; Received in revised form 22 February 2019; Accepted 13 March 2019 0378-7753/ © 2019 Elsevier B.V. All rights reserved.

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integrating an Al foil (20 μm, 1.54 cm2) and a Na electrode (2 cm2) for Al/Na half-cells, a HC (1.54 cm2) and a Na electrode for HC/Na halfcells, or two HC electrodes for symmetric cells, with a glass microfiber separator (GF/A, Whatman), and 1 M NaPF6 in EC/DEC (1/1, v/v) with/without 0.5 wt % FEC or SA. Na metal deposition experiments were carried out using Al/Na coin cells. Cyclic voltammetry (CV) was performed over −0.2 to −0.6 V (vs Na/Na+) with scan rate of 0.5 mV s−1. The SEI formation behavior was examined by differential capacity curves (dQ/dV vs V) during the first sodiation process of HC/ Na half-cells at 0.2C current. The cyclability tests were performed at 0.5C constant current (CC) charge (sodiation) followed by constant voltage (CV) charge with 5% cut-off of the charging current, and 0.5C CC discharge (de-sodiation) over 0–2.0 V for HC/Na half-cells at 25 °C, and over ± 1.0 V for HC symmetric cells at 25 or 60 °C. The HC symmetric cells were assembled using two HC anodes with a state-of-charge (SOC) of 50, collected from two HC/Na half-cells pre-sodiated at 0.2C current. To confirm the origin of the degradation of HC/Na half-cells, cycled-Na/fresh-HC and fresh-Na/cycled-HC cells were reassembled with an additional electrolyte and a new separator. The cycled electrodes were collected from a fully discharge HC/Na cell that had been cycled ten times. The high-temperature storage tests were performed at 60 °C for 550 h using nearly fully charged HC symmetric cells. After storage, the recovery capacity of the cells was measured at 25 °C. The cyclabilities were examined using a temperature-chamber-equipped battery cycler (Toscat-3000, TOYO SYSTEM). In-situ Na dendritic growth was examined using a custom-built optical cell (Fig. S1) employing an ECLIPSE LV150NL optical microscope (Nikon) equipped with an IMTcam3-LP camera with TU Plan Fluor EPI 10× objective lens, NA 0.3, WD 17.5 mm. The optical cell assembly was air-sealed with a glass window and O-ring and promptly removed from the glove box after assembly, for optical observation. The videos were recorded during deposition/dissolution of Na metal at a current density of ± 2 mA cm−2. Electrochemical impedance spectra (EIS) were obtained for symmetric cells with SOC 50 before and after 100 cycles at 60 °C. The frequency range was 300 kHz–10 mHz, the ac amplitude was 5 mV, and the dc bias voltage was 0.0 V.

graphene layers and nano-void filling [11–13]. Even though HC presents adequate reversible capacity (ca. 290 mAh g−1), it suffers from large irreversible capacity at the initial cycle and poor cycle performance, due to considerable side reactions with Na electrolytes [14,15]. To address this issue, extensive efforts have been made to optimize the electrolyte composition including the use of cyclic carbonates/co-solvent mixtures, and Na salts such as NaClO4, NaPF6, and NaTFSI [14,16–19]. In addition, electrolyte additives such as fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), ethylene sulfite, and vinylene carbonate, have been investigated with the aim of improving the cyclability of the HC anode [15]. In particular, it has been claimed that FEC can markedly improve cyclability [15,19]. Currently, FEC is widely employed as an indispensable electrolyte component in various NIB anode materials including P, Sn, Sb, and Sb/C electrodes [20–24]. However, this claimed positive influence of FEC on the cycle performance of NIB anodes remains controversial. Notably, the previous reports claiming the beneficial effect of FEC employed flooded cells (beaker or Swagelok cell) containing much larger amounts of electrolyte than the practical batteries, or half-cells where cross-talk between the Na metal electrode and the anode materials was highly possible [15,19]. In this regard, a crucial lesson can be taken from previous studies where the claimed beneficial influence of the electrolyte component on the cyclability of Li or Na half-cells turned out to be a consequence of the improvement of the Li or Na metal rather than of the active electrode material itself [25,26]. Indeed, some recent studies revealed that FEC markedly suppresses the Na reactivity toward the electrolyte by forming NaF-rich passivation layers [27–29]. It was also reported that half-cells are not useful for evaluating the cyclability of Na-cathodes because of the severe electrolyte decomposition at the Na electrode, and that symmetric cells should be used for accurate evaluation of NIB materials [30]. In this context, through a systematic electrochemical analysis, we have investigated the roles of FEC and succinic anhydride (SA) additives in the cycle performance of HC and Na metal electrodes. To distinguish the influence of the electrolyte additives on the HC and Na electrodes, we employed HC symmetric cells in addition to HC/Na halfcells. Importantly, it was elucidated that FEC markedly improves the reversibility of the Na deposition/dissolution reaction and suppresses the formation of the by-products derived from Na metal that intensifies the degradation of the HC anode. The impact of FEC and SA on the cyclability of HC anodes was properly evaluated using HC symmetric cells, revealing that FEC hardly affects the cyclability of the HC anode, but that SA markedly improves its cyclability and thermal stability at elevated temperatures. Furthermore, the origin of the excellent hightemperature performance obtained with the SA additive was correlated, through X-ray photoelectron spectroscopy (XPS) analysis, with the chemical composition of the solid electrolyte interphase (SEI) layer derived from SA.

2.3. X-ray photoelectron spectroscopy (XPS) The chemical compositions of the HC surfaces were examined using X-ray photoelectron spectroscopy (ESCALAB 250Xi, Thermo Scientific). For the sample preparation, fully de-sodiated HC anodes were collected from HC symmetric cells before and after 100 cycles at 60 °C, rinsed in DEC, and dried for 20 h inside a glove box. 3. Results and discussions 3.1. Cycling performance of HC/Na half-cells First of all, the effects of the additives on SEI formation during the first cycle were examined employing HC/Na half-cells using 1 M NaPF6 EC/DEC (1/1, v/v) without and with 0.5 wt % FEC or SA (hereafter denoted: base, FEC, and SA electrolytes, respectively). In Fig. S2a, the voltage profiles of the HC/Na half-cells at the 1st sodiation/de-sodiation cycle are presented. The addition of the additives increased the overpotentials and the de-sodiation capacity and coulombic efficiency (CE) decreased in the order of base, FEC, and SA (255 mAh g−1/88%, 251 mAh g−1/84%, and 168 mAh g−1/80%, respectively). As seen in Fig. S2b, the cells displayed different differential capacity (dQ/dV) curves at the first charge (the sodiation process of the HC anodes). The cell with base electrolyte showed a small reduction peak at 0.53 V due to EC reduction [31–33]. In contrast, the cell with FEC exhibited a reduction peak at 0.63 V and the one with SA at 0.70 V, attributed to the reduction of the corresponding additive [33,34]. These results signify different characters of the SEI layers derived in the three electrolytes, which details will be discussed in section 3.3.

2. Experimental 2.1. Chemicals and electrode preparation Battery grade 1 M NaPF6 in ethylene carbonate (EC)/diethyl carbonate (DEC) (1/1, v/v), FEC, and SA were obtained from Panaxetec (Korea). The water content of all chemicals was controlled below 20 ppm. Na metal was purchased from Alfa. The HC anode was fabricated with HC (Kuraray Co., Japan, 92 wt %), polyvinylidene fluoride binder (PVDF, 6.5 wt %), and conductive carbon (super P, 1.5 wt %). The anode loading was 5.2 ± 0.2 mg cm−2. The fabricated HC anodes were dried in a vacuum oven (110 °C) for 24 h. 2.2. Electrochemical measurements Type 2032 coin cells were fabricated for the electrochemical tests by 138

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relatively low initial capacity. The superior cyclability of the HC/Na half-cell with FEC additive is consistent with the previous studies [15,19]. However, it should be noted that a good cycle result of HC/Na half-cells may not ensure that FEC is a beneficial additive toward the HC anode itself [26–28,30]. To elucidate this point, the effects of the additives on the cycle behavior of the Na metal electrode were examined. To this end, cyclic voltammetry was performed for Al/Na cells (Fig. S3). As seen in Fig. 1b, the Coulombic efficiency (CE) obtained from the CV measurements are in the following order: FEC > SA > base (at 30th cycle, 84.4%, 24.1%, and 8.7%, respectively). The beneficial effect of FEC on the Na deposition/dissolution is consistent with previous reports [27,28]. In the cyclic voltammograms (Fig. S3), it is noted that the current density decreases in the following order: Base > FEC > SA. This indicates that the FEC and SA electrolytes hamper facile charge transport at the Na/electrolyte interface, either by developing more resistive SEI layers on Na electrodes than the base electrolyte or by preventing the increase of the surface area of the Na electrodes through suppressed dendritic growth of Na deposit. The determination of the individual contributions of the SEI resistance and surface area deserves further study. As the next step, the dendrite growth behavior of the Na electrode was examined using an in-situ optical cell (Fig. 1c, see also Videos S1eS3). In the base electrolyte, severe dendritic growth (> 400 μm thick) was observed from the first deposition/dissolution cycle along with massive gas evolution possibly caused by electrolyte decomposition on the Na dendrite. Importantly, it was noted that a significant amount of Na fragments was dislodged from the Na surface (Video S1), which can be referred to as “dead Na’, following LIB terminology [35,36]. In contrast, in the FEC and SA electrolytes, the dendritic growth was markedly suppressed: the dendritic layer was much thinner than that with the base electrolyte (after the 3rd cycle, ca. 80 and 150 μm for FEC and SA, respectively). In addition, FEC and SA electrolytes led to a dense Na layer, preventing the formation of dead Na (Videos S2 and S3, respectively). Supplementary video related to this article can be found at https:// doi.org/10.1016/j.jpowsour.2019.03.047. The extensive gas evolution and dead Na formation in the base electrolyte may have a detrimental influence on the HC anode in the half-cells. To confirm this, Na metal and HC electrodes were retrieved from a HC/Na half-cell cycled ten times and reassembled using fresh Na metal or HC electrodes with an additional electrolyte and a new separator (Fig. 2a, hereafter denoted: rebuilt cell). As presented in Fig. 2b, the rebuilt cell with fresh HC and cycled Na recovers the initial capacity of the pristine HC/Na cell (255 mAh g−1), while its cycle performance is rather inferior, probably due to the inferior performance of the cycled Na electrode. In sharp contrast, however, the rebuilt cell with cycled HC and fresh Na failed to recover the initial capacity of a pristine HC/Na cell, showing a capacity similar to that of the ten-times cycled HC/Na cell (146 mAh g−1), and suffered from rapid capacity fading. These results suggest that the poor cycle performance of HC/Na half-cells is mainly due to the degradation of the HC anode, which is triggered by the side reaction between Na metal and the electrolyte. The minor damage of Na metal relative to HC anode is likely due to the intact inner part of Na metal beneath the dendritic surface layer: The overall capacity of the Na metal (∼500 μm thick) used in the half-cells is ca. two order of magnitude higher than that of HC anode. This also reveals the presence of cross-talk between the Na metal electrode and the HC anode in the half-cell configuration, a detail which deserves further study. Importantly, these results clearly support the conclusion that the influence of the electrolyte additive on the HC anode cannot be evaluated properly by employing HC/Na half-cells because of the interference from the Na metal. Therefore, as a next step, the cycle test was carried out by employing HC/HC symmetric cells (hereafter denoted: HC-sym cell).

Fig. 1. (a) Cycle performances of HC/Na metal half-cells with base, FEC, and SA electrolytes. (b) Coulombic efficiency of the Na deposition/dissolution reaction during cyclic voltammograms of Al/Na metal cells with scan rate of 0.5 mV s−1. (c) In-situ cell images before cycling, and after the 1st and 3rd deposition/ dissolution cycle at a current density of 2 mA cm−2. Scale bar: 200 μm.

The cycle performance of HC/Na half-cells with base, FEC, and SA electrolytes are compared in Fig. 1a. The cell with base electrolyte shows a rapid capacity fading with only 22.6% of its initial discharge capacity after 30 cycles. The cell with FEC electrolyte exhibits an initial discharge capacity similar to the cell with base electrolyte, but it shows much improved capacity retention (92.5%) over 3210 cycles. The cell with SA electrolyte presents reasonable capacity retention (47.2%) but 139

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Fig. 2. (a) Schematic of rebuilt cell. (b) Cycle performance of a pristine HC/Na metal cell (black), a rebuilt cell with fresh HC/cycled Na metal (red), and a rebuilt cell with cycled HC/fresh Na metal (blue). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

3.2. Cycling and storage performance of HC-sym cells Fig. 3a compares the cycle behavior at 25 °C of HC-sym cells with base, FEC, and SA electrolytes. In sharp contrast to the HC/Na half-cells (Fig. 1a), the HC-sym cells display quite similar cycle behaviors with reasonable capacity retention (over ca. 85% after 100 cycles), irrespective of the presence of FEC or SA additives. These results indicate that the cyclability of the HC anode at 25 °C is hardly affected by the additives. In contrast, the performance in the cycle and storage tests at 60 °C is quite dependent on the additives. As presented in Fig. 3b, the 60 °C cyclability of the HC-sym cell is in the following order: SA > FEC ≥ base. The voltage profiles of HC-sym cells at 60 °C with different electrolytes are compared at the 1st, 3rd, and 100th cycles in Fig. S4. The HC-sym cells show similar voltage profiles at the 1st cycle, but the cell with SA exhibits much smaller overpotential than the other cells. We also examined the effect of the additive content on the cyclability of the HC anode at 25 °C. As seen in Fig. S5, the HC-sym cells with 2 wt % FEC and SA displayed lower initial capacity and poor capacity retention than the cell without additive. This suggests that 0.5 wt % is closer to the optimum additive content, while a systematic investigation is needed to determine the exact optimum dose of the additives. In addition, we compared the self-discharge behaviors of fully charged (ca. 1.0 V) HC-sym cells during 60 °C storage (Fig. 3c). The

Fig. 3. Cycle performance of HC-sym cells with base, FEC, and SA electrolytes (a) at 25 °C, and (b) at 60 °C. (c) Open-circuit voltage changes during 60 °C storage of HC symmetric cells.

thermal degradation will lead to an increase in the potential of the HC anode, due to the loss of stored Na ions (de-sodiation). As depicted in Fig. S6a, due to the voltage-capacity profile of the HC anodes, the potential of de-sodiated HC (VDH) increases much more steeply than the potential of sodiated HC (VSH), for a given capacity loss. As a consequence, the open circuit voltage (OCV) of the HC-sym cell (VDH − VSH) increases as the self-discharge of the HC anode proceeds. As shown in Fig. 3c, during 60 °C storage, a HC-sym cell without additive, or with FEC, displayed rapid increase of OCV from 0.9 to above 140

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Fig. 6. Schematic of the formation of an SEI layer on HC anodes by FEC and SA additives.

exhibited higher RSEI and Rct values than the one with base electrolyte, which is likely due to the additives forming a resistive SEI layer. After 60 °C cycling, the overall resistance of the cell with FEC was much larger than that of the cells without this additive, mainly due to a drastic increase of Rct. In contrast, the increase of the overall cell resistance was markedly suppressed in the cell with SA. These results suggest that SA, unlike FEC, mitigates the thermal degradation of the HC anodes, possibly by forming a thermally stable SEI layer.

Fig. 4. Fitted resistance values of the HC-sym cells before and after 100 cycles at 60 °C.

2.0 V within 200 h. In contrast, the OCV increase of a HC-sym cell with SA was markedly mitigated (from 0.9 to 1.7 V over 550 h). Moreover, a cell with SA electrolyte recovered much higher capacity after storage than the other cells (Fig. S6b; 68, 90, and 115 mAh g−1 for base, FEC, and SA, respectively). These results unequivocally suggest that SA effectively suppresses the self-discharge and the accompanying thermal degradation of the HC anodes. To confirm the positive role of SA on the enhanced thermal stability of the HC anode, EIS measurements were performed for the HC-sym cells before and after 100 cycles at 60 °C. Fig. 6 and Table S1 summarize the EIS spectra and the resistance values that were obtained by fitting them to an equivalent circuit (Fig. S6d), where Rohm is the ohmic resistance, RSEI is the resistance related to the migration of Na-ions through the surface film, and Rct is the interfacial charge transfer resistance [37,38]. All of the fitted resistance values are presented in Fig. 4. Before 60 °C cycling, the cells with FEC and SA electrolytes

3.3. XPS study of SEI layers derived by the SA additive To find the origin of the thermally robust SEI derived by SA, the chemical composition of the surface layer of the cycled HC anodes with base, FEC, and SA electrolytes were compared using XPS measurements. The XPS spectra of C 1s, F 1s, and P 2p for the HC anodes before and after 100 cycles at 60 °C are presented in Fig. 5a and b, respectively. The atomic composition of the HC anodes is summarized in Table 1. The C 1s spectra shows the peaks corresponding to the sp2 CeC bonds of HC (285 eV)/hydrocarbons (286.5 eV); CeO bond in sodium alkyl carbonate (NaeO–(C]O)eOeCH2–R, 288.5 eV); and C]O bond in sodium alkyl carbonate (NaeO–(C]O)eOeCH2–R), sodium carbonate

Fig. 5. XPS results in the C 1s, F 1s, and P 2p regions for the fully de-sodiated HC anode collected from cycled HC-sym cells with base (black), FEC (red), and SA (blue) electrolytes (a) before and (b) after 100 cycles at 60 °C. The experimental spectra are denoted with dotted-lines and the fitted results are indicated by solid lines. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 141

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Table 1 Atomic composition, determined from XPS measurements, of the HC anodes collected from HC-sym cells before and after 100 cycles at 60 °C. C 1s Chemical species Peak position Before 60 °C cycling

After 60 °C cycling

Base FEC SA Base FEC SA

F 1s

P 2p

sp CeC 285 eV

eCH2e 286.5 eV

NaeOe(CO)eOeCH2–R 288.5 eV

NaeOe(CO)eOeCH2–R, Na2CO3 290.5 eV

eCF2e 292 eV

NaF 686 eV

NaPF6, NaxPFy 689.5 eV

NaxPOyFz 135 eV

NaPF6 139.5 eV

3.8 0 0 0 0 0

21.7 13.9 27.5 13.9 22.3 27.3

4.9 4.6 7.1 5.5 8.0 10.8

4.9 3.8 10.9 4.5 8.9 13.7

5.2 3.8 4.4 4.1 6.7 10.8

40.3 52.5 36.0 53.5 39.6 27.7

16.8 18.3 12.6 14.3 11.4 7.6

1.9 2.1 1.8 2.8 1.7 1.0

0.5 1.0 0.8 1.4 1.4 1.1

2

inhibited the degradation of the HC anodes. In contrast, SA markedly improved the cycle and storage performances of the HC anode at 60 °C, which was attributed to the ability of SA to form a thermally robust SEI rich in Na2CO3/sodium alkyl carbonates. While this study confirmed these beneficial effects of the SA additive on HC anodes, its impact on full cells employing Na-cathode materials merits future study.

(Na2CO3, 290.5 eV), and PVDF binder (292 eV). The F 1s peaks at 686 and 689.5 eV are attributed to NaF and fluorinated phosphate, respectively, derived from NaPF6 decomposition. The P 2p peak at 135 eV was assigned to NaxPOyFz and the peak at 139.5 eV to NaPF6 [17,19,39]. For a HC with base electrolyte, before 60 °C cycling, the CeC peak of bare HC (285 eV) is clearly observed in the C 1s spectra (Fig. 5a and Table 1), whereas that peak is absent in a cycled HC with FEC or SA electrolyte. This suggests that a thin or imperfect SEI layer is formed with base electrolyte, but that the SEI derived by FEC or SA fully covers the HC surface. In addition, it is noted that the FEC-derived SEI is rich in NaF, which is consistent with previous studies [15,40,41]. In contrast, the chemical species corresponding to NaeOeC]O)eOeCH2–R at 288.5 eV and NaeO–(C]O)eOeCH2–R and Na2CO3 at 290.5 eV appeared different in the SEI present with SA from those present with the other electrolytes. These results suggest that SA leads to the formation of a SEI rich in Na2CO3 and sodium alkyl carbonates. It was reported that high content of alkali carbonate (Li2CO3 and Na2CO3) is beneficial for improving the thermal stability of SEI of LIB anodes [42,43]. Similarly, the superior thermal stability enabled by SA seems to be associated with the high amounts of Na2CO3 and sodium alkyl carbonates in its SEI. For a cycled HC with base electrolyte, after 60 °C cycling, the NaF and NaxPOyFz peaks in the F 1s and P 2p spectra markedly increased (Fig. 5b and Table 1). However, for a cycled HC with SA electrolyte, there was only a marginal increase in the peaks in the F 1s and P 2p spectra. These results indicate that the SA-derived SEI is efficient in preventing salt decomposition during cycling at elevated temperatures. The growth of the NaF and NaxPOyFz peaks was, to some extent, suppressed by FEC, but not as much as by SA. The relatively poor thermal properties of the SEI present with FEC is also manifested in the marked increase in SEI thickness during 60 °C cycling. After 30 s etching, the CeC peak of the bare HC was observed in a cycled HC with base or SA electrolyte, but it was still absent in a HC with FEC, indicating a thicker SEI in the FEC electrolyte (Fig. S7). The thick SEI formation with FEC is also consistent with the large increase in Rct value after 60 °C cycling (Fig. 4). Combining all the surface-analysis results, the SEI formation on HC anodes by FEC and SA can be schematized as in Fig. 6. The base electrolyte generates a thin or imperfect SEI, which is vulnerable to thermal damage. FEC derives a NaF-rich SEI, which is beneficial in the cycling of Na metal anodes but not in that of HC anodes, especially at elevated temperatures. In contrast, SA forms an SEI rich in Na2CO3 and sodium alkyl carbonates, which enables superior high-temperature performance of the HC anodes.

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4. Conclusions This study systematically investigated the effects of FEC and SA additives on the cyclability of HC and Na-metal electrodes using HC/ Na-metal half-cells and HC-sym cells. Notably, FEC hardly affected the cyclability of HC symmetric cells, but it suppressed the formation of byproducts from the Na metal decomposition in HC/Na half-cells and thus 142

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