Understanding the Electrochemical Compatibility and Reaction

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Understanding the Electrochemical Compatibility and Reaction Mechanism on Na Metal and Hard Carbon Anodes of PC-Based Electrolytes for Sodium-Ion Batteries Kanghua Pan, Haiyan Lu, Faping Zhong, Xinping Ai, Hanxi Yang, and Yuliang Cao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b13236 • Publication Date (Web): 25 Oct 2018 Downloaded from http://pubs.acs.org on October 27, 2018

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ACS Applied Materials & Interfaces

Understanding the Electrochemical Compatibility and Reaction Mechanism on Na Metal and Hard Carbon Anodes of PC-Based Electrolytes for Sodium-Ion Batteries Kanghua Pan,† Haiyan Lu,† Faping Zhong,*‡ Xinping Ai,† Hanxi Yang,† Yuliang Cao*†

†

College of Chemistry and Molecular Sciences, Hubei International Scientiﬁc and Technological

Cooperation Base of Sustainable Resource and Energy, Wuhan University, Wuhan 430072, China. ‡

National Engineering Research Center of Advanced Energy Storage Materials, Changsha,

410205, China. *E-mail: [email protected] (Y.C.) *E-mail: [email protected] (F. Z.)

KEYWORDS: propylene carbonate, linear carbonates, stability, electrolyte, sodium-ion battery

ACS Paragon Plus Environment

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ABSTRACT: Electrolyte as an important part of sodium-ion battery has a pivotal role for capacity, rate and durability of electrode materials. On account of the high reduction activity of sodium metal with organic solvents, it is very important to optimize the electrolyte component to realize high stability on Na metal and hard carbon anodes. Herein, chemical and electrochemical stability of propylene carbonate (PC)-based electrolytes on sodium metal and hard carbon anodes are investigated systematically. The results demonstrate that whether using NaClO4 or NaPF6, the PC-based electrolytes are not stable on Na metal, but adding of FEC can immensely enhance the stability of the electrolyte due to compact SEI film formed. The electrolytes containing FEC also exhibit high electrochemical compatibility on hard carbon anode, showing high reversible capacity and excellent cycling performance. A reaction mechanism based on Na+ induction effect is proposed by spectrum and electrochemical measurements. This study can provide a new insight to optimize and develop stable PC-based electrolytes and be helpful for understanding the other electrolyte systems.

Introduction Sodium ion batteries (SIBs) are widely concerned for large-scale energy storage due to their resource abundance and environmentally friendly. In recent years, more attention has been paid to sodium ion batteries, so that electrode materials have been widely developed and explored. Some cathode and anode materials have been able to meet application requirement, such as transition metal oxides1-5 and phosphates6-11 for cathode materials, hard carbon12-19 and NaTi2(PO4)320-24 for anode materials. Electrolyte as an important part of the battery has a pivotal role in the electrochemical performance and practical application of SIBs. However, systematic


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studies on electrolyte are overlooked. Electrolytes of SIBs mainly includes: liquid electrolytes (non-aqueous liquid electrolytes25-32, aqueous liquid electrolytes24, 33-35, ionic liquids36-42, etc.), glass and ceramic electrolytes43-51, solid polymer electrolytes52-57, gel polymer electrolytes58-60, etc. In these electrolytes, non-aqueous liquid electrolytes have been widely investigated for SIB systems, due to their wide electrochemical window, high ionic conductivity, low viscosity, and chemical and electrochemical stability. The non-aqueous liquid electrolytes used for sodium ion battery

rely

on

sodium

salts

of

NaClO4,

NaPF6,

NaSO3CF3

or

sodium

bis(trifluoromethane)sulfonamide (NaTFSI), dissolved in organic carbonate solvents, consisting of cyclic and linear carbonates, or their mixtures. The cyclic carbonates, such as ethylene carbonate (EC) and propylene carbonate (PC), possess high dielectric constant and high boiling point, but low viscosity (for example, PC) or high melting (for example, EC). The linear carbonates, such as dimethyl carbonate (DMC), diethyl carbonate (DEC) and ethyl methyl carbonate (EMC), have low viscosity, but low boiling point and low dielectric constant. Therefore, general electrolyte should choose a mixture of the cyclic and linear carbonate to balance physical and chemical properties. Some of the studies have focused on the physical properties of different electrolytes, such as conductivity, viscosity, DSC, or electrochemical properties of different electrolytes on the electrode material. Ponrouch et al

25-26

investigated physical and chemical properties of the

electrolytes with three different kinds of sodium salt (NaClO4, NaPF6 and NaTFSI) dissolved into single or mixed organic solvents, such as PC, DMC, EC: PC, EC: DEC and so on. The results demonstrated that hard carbon anode showed the best electrochemical properties in the electrolyte with NaPF6 dissolved in EC: PC=1:1 solvent, which exhibited the best thermal stability in four kinds of mixed organic solvents (EC: DME, EC: DMC, EC: DEC and EC: PC).


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Bhide et al27 made a comparative study of different salts (NaClO4, NaPF6 and NaCF3SO3) in EC: DMC=3:7 mixed electrolyte respectively. The ionic conductivity of different concentrations at room temperature was recorded. The order of ionic conductivity is NaPF6 > NaClO4 > NaCF3SO3. In addition, some discussions about the function and mechanism of additives29, 61 and other characteristics were also reported.30-32 However, few studies have attempted to systematically analysis the physicochemical properties between the electrolyte and sodium metal. Because PC solvent has high dielectric constant (εr=65), low melting point (-49 oC) and high boiling point (242 oC), particularly excellent electrochemical compatibility with hard carbon anode, the PC-based electrolytes were widely used for SIBs.25-26, 61-62 To improve viscosity of PC solvent, some linear carbonates were added to ensure the good wettability of the electrode and improvement of ionic conductivity. However, systematical understanding of chemical and electrochemical stability of PC-based electrolytes on metal Na and hard carbon anodes is lake, which would lead to conflicting measurement data and confused results. In order to obtain universal and systematical understanding on PC-based electrolytes to avoid misunderstanding on the experiments, we studied chemical and electrochemical stability of the PC-based electrolytes with different solvents, salts and additives on metal Na and hard carbon anodes. The reaction mechanisms of PC-based electrolytes with sodium were investigated by means of

13

C nuclear

magnetic resonance (NMR)，infra-red spectrum (IR), gas chromatography mass spectrometry (GC-MS), ex situ X-ray photoelectron spectroscopy (XPS) measurements and so on.

Experimental section


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Preparation of the electrolytes PC and DMC, DEC, EMC (different proportions by volume) solvents with and without FEC (5% by volume) were prepared by mixing 0.8M NaClO4 (99%, Sigma-Aldrich) or NaPF6 (99%, AlfaAsear) salt. All the solvents come from Dongguan Shanshan Co., China, 99%. The composition of each electrolyte is listed in Table 1 and Table 2. Preparation of hard carbon electrodes To prepare the electrode, 80 wt % hard carbon (HC, LBV-1001, LIB Anode Material Business Development Dept), 10 wt % carbon black (SP), and 10 wt % Polyacrylic acid (PAA) in water solution was milled to form a slurry and then the slurry was pasted onto Cu foil to form HC electrode. This electrode was vacuum-dried at 100 oC for 7 h. The loading amount of active material was 1.8−2.0 mg cm−2. A CR2016 coin cell was assembled with a HC electrode as the working electrode, a Na foil as the counter electrode, a glass fiber (GF/F, Whatman) as the separator to test the electrochemical properties.

Characterizations The charge−discharge performance of the Na/HC and Na/Na half cells in various electrolytes was evaluated using a battery tester (Shenzhen Neware Electronics Co., China). The coin cells were tested in a constant temperature room (25 oC). The impedance properties of PC-based electrolytes were performed with Autolab PGSTAT128N (Eco Chemie, Netherlands). The ionic conductivity of electrolyte was examined by a conductivity meter (Lei-Ci DDS-307). Gas chromatograph-mass spectrometer (GC-MS) characterization was carried out on Varian 450GC-


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320MS test station. The infrared spectra analysis (IR) were recorded on a Thermo NICOLET FTIR5700 spectrometer at room temperature. Nuclear Magnetic Resonance Spectroscopy (NMR) measurements of 13C were carried out at room temperature in a Bruker AVANCE III 400 MHz spectrometer. The Morphological and structural features of the metal Na were characterized by Scanning Electron Microscope (SEM) (Zeiss Merlin Compact). The ex situ Xray photoelectron spectroscopy (XPS) was performed by Thermo Fisher ESCALAB 250Xi.

Results and discussion Chemical stability of PC-based electrolytes on metal Na Because the intercalation potential of hard carbon anode is very close to the potential of Na metal (0 V vs. Na/Na+), the chemical stability on Na metal should be investigated firstly. Table 1 and 3 summarized the reactivity between pure solvents (PC, DMC, DEC and EMC) or mixed solvents (PC: DMC, PC: DEC, PC: EMC in different proportions) with different salts (NaClO4 or NaPF6) and Na metal. It can be seen from Table 1 and Table 2, there is no discernible reactivity between pure or mixed solvents and Na metal. However, different reaction degrees in different solvents occurred when the salts were added as shown in Figure 1.


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Table 1. Experimental results of chemical stability of single solvent on Na metal. Solvents

PC

Salts

/

NaClO4

FEC

/

/

0

0

DMC

DEC

EMC

NaPF6

/

NaClO4

NaPF6

/

/

+

/

+

/

/

/

/

/

2

0

0

0

0

0

0

0

0

5

0

1

0

0

0

0

0

0

Reactivity after 7 days in 25 oC Reactivity after 3 days in 60 oC

Reaction degree

In the table: "0" indicates no reaction; “/” means not without; “+” means adding. The number indicates the reaction degree. From 1-5, the reaction degree increases gradually.


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Table 2. Experimental results of PC-based mixed solvents on Na metal Solvents

PC:DMC

PC:DEC

PC:EMC

Proportion

Reactivity

1:3

0

1:1

0

3:1

0

1:3

0

1:1

0

3:1

0

1:3

0

1:1

0

3:1

0

Salts NaClO4 NaClO4 NaClO4 NaPF6 NaClO4 NaPF6 NaClO4 NaClO4 NaClO4 NaClO4 NaClO4 NaPF6 NaClO4 NaPF6 NaClO4 NaClO4 NaClO4 NaClO4 NaClO4 NaPF6 NaClO4 NaPF6 NaClO4 NaClO4

FEC / + / / + + / + / + / / + + / + / + / / + + / +

25 oC 3 days 5 0 4 0 0 0 3 0 3 0 2 0 0 0 1 0 4 0 3 0 0 0 2 0

60 oC 9 days / 0 / 2 0 0 / 0 / 0 / 3 0 0 / 0 / 0 / 5 0 0 / 0

In the table: "0" indicates no reaction; “/” means without; “+” means adding. The number indicates the reaction degree. From 1-5, the reaction degree increases gradually. Figure 1 showed that the compatibility between the electrolytes with different ratios and Na metal is different when NaClO4 is added. At room temperature, the mixing electrolytes showed stronger reactivity than the pure PC electrolyte with 0.8 M NaClO4. Moreover, the larger ratios of the linear carbonate (e.g. linear carbonate: PC=1:3), the stronger reactivity. This indicates that adding the linear carbonate can accelerate the reaction between the mixing solvent and Na metal.


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For three linear carbonates, their stability ranges from high to low in order: DEC＞EMC＞DMC, which is in the same order with their producing free radicals63. The results demonstrate that whether pure PC solvent or mixing solvents, the electrolytes with NaClO4 all are unstable on Na metal, indicating that the PC-based electrolytes with NaClO4 cannot form stable SEI film to suppress the decomposition of the solvents.

Figure 1. Reaction state diagram between different NaClO4 electrolytes and Na metal. Figure 2 shows the reactivity of the NaPF6 electrolytes with Na metal at room temperature and 60 oC. No obvious reaction in PC-based electrolytes with NaPF6 was observed within two weeks under room temperature (25 oC). However, the different reactivity was detected under 60 oC. The pure PC electrolyte showed higher stability on Na metal than the mixing electrolytes at 60 oC, similar to the electrolytes with NaClO4 (Figure 1). The stability of the electrolytes with different linear carbonates range from high to low in order: DMC＞DEC＞EMC. This order is different from that of adding NaClO4 (Figure 1), which is possibly due to the effect of NaPF6 acting as a film-formed additive. Compared to the electrolytes with NaClO4 (Figure 1), the electrolytes with NaPF6 shows more stable on Na metal, which originates from the contribution of NaPF6 to form


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SIE film. However, the stability of the electrolytes with NaPF6 still cannot remain under 60 oC, suggesting that more stable SEI film on the electrode surface need to be constructed.

Figure 2. Reaction state diagram between different NaPF6 electrolytes and Na at 25 oC and 60 o

C. It is well known that the film-formed additives can construct stable SEI film to suppress the

decomposition of the solvent. In order to more clearly demonstrate the effects of the additives on the stability of the electrolytes, the electrolyte of PC: DMC (1:3) + NaClO4 with the strongest reactivity on Na metal was chosen to investigate the reactivity of the electrolyte after adding the additives (FEC, vinylene carbonate (VC), ethylene sulfite (ES) and lithium bis(oxalate)borate (LiBOB)). Table 3 summarized the reaction states of the electrolytes with different additives under 25 oC and 60 oC. It can be seen that adding additives can improve the compatibility of PCbased electrolytes and Na metal. But the effects of different additives are different. The


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electrolytes with VC and ES just react slowly under room temperature (25 oC), while the electrolyte with LiBOB keeps stable only at room temperature, but still reacts at high temperatures (60 oC). However, the addition of FEC can effectively inhibit the reaction of PCbased electrolytes and Na metal whether under 25 oC or 60 oC. In addition, the similar experiments in the NaPF6 system were also done, which is also in accordance with the result in the NaClO4 system. In a word, FEC is more favorable in terms of forming film ability and inhibiting the side reaction than VC, ES and LiBOB additives. Table 3. The reaction states of PC-based electrolytes with different additives on Na metal Additives FEC 5%

VC 5%

ES 5%

LiBOB 5%

No reaction

No reaction

No reaction

Electrolytes PC

No reaction

PC: DMC =1: 3 25 oC after a week

PC: DMC=1: 3 60 oC after a

——

——

week


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The ionic conductivity of electrolytes The ionic conductivity is an important factor for application of electrolyte. The electrolytes with good chemical compatibility with Na metal were chosen to measure their ionic conductivities shown in Figure 3. It is obviously found that the ionic conductivities of the electrolytes using NaPF6 is larger than those with NaClO4 in the same PC-based solvents. In addition, the effect of the addition of different linear carbonates on the ionic conductivity is different. The order of the conductivity of different solvents from large to small is PC: DMC > PC > PC: DEC. This result implies that the addition of DMC with high dielectric constant (εr=3.1) and low viscosity (ɳ (20 oC) =0.66 cP) is more effective in enhancing the ionic conductivity of the electrolyte than DEC (ε= 2.8 and ɳ (20 oC) =0.75 cP), which is governed by the concentration and mobility of charge carriers.

Figure 3. Ionic conductivities of the electrolytes based on 0.8 M NaClO4 and NaPF6 dissolved in various solvents (PC: linear carbonate=1:1).


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Electrochemical characteristics of PC-based electrolytes Electrochemical impedance spectroscopy (EIS) of the half cell was performed before and after cycling in order to grasp more information about the interface film between the electrode and electrolyte, and the electrochemical reaction characteristics.64 Figure 4a, 4b and Figure S1 showed the effects of different solvents (PC, PC: DMC=1:1 and PC: DEC=1:1), salts and additives on EIS in Na/Na symmetrical cells (NNSCs). It is discriminated that there is one depressed semicircle in all Nyquist plots, which represents mainly the migration resistance of Na+ through SEI film. For different salts, the resistances of the cells in NaClO4 electrolytes are much lower (100~200 Ω, Figure 4a, Figure S1a and c) than those (5000~10000 Ω, Figure 4b, Figure S1b and d) for the same electrolyte containing NaPF6, suggesting that the decomposition of NaPF6 forms a low-conductive SEI film. However, it is found that the impedances in the NaClO4 electrolyte significantly increase upon time, and even reach 800~1000 Ω after 20 cycles. The impedances in NaPF6 electrolyte decrease to less than 2000 Ω after 20 cycles. Those results demonstrate the chemical instability between the NaClO4 electrolyte and Na metal. When 5% FEC was added, the impedances in the cell with the NaClO4 electrolytes increase to 1000~1500 Ω, while in the cell with NaPF6 electrolytes decrease to ~2000 Ω. However, after 20 cycles, their values are almost the same (800~1000 Ω), implying that FEC can form preferentially a stable protective film on the surface of Na metal to suppress the decomposition of solvent and NaPF6. The time dependence of the potential experiments were carried out to observe the polarization properties of Na metal electrode by using the NNSCs in different PC-based electrolytes (Figure 4c, d and Figure S2). For NaClO4 electrolytes without FEC, the potential polarization of the cells is about 10~20 mV in initial cycles, and gradually increases with cycling (Figure 4c, Figure S2a and c), suggesting that the instable PC-based electrolytes continually react with Na metal with


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cycling. When adding FEC to NaClO4 electrolytes, it is surprising that the potential polarization of the cells becomes large (about 30~50 mV) in initial cycle, but keeps a stable value in subsequent cycles (Figure 4c). However, when using NaPF6 salt (Figure 4d), the cells without FEC show very large potential polarization of > 100 mV, which gradually decreases with cycling, implying the formation of a low-conductive SEI film and the reconstruction of SEI film. The presence of NaPF6–derived SEI film also further demonstrates that NaPF6 electrolytes are more stable on sodium metal than in NaClO4 electrolytes (Figure 1 and Figure 2). After adding FEC, the potential polarization of the cells decreases to about 50 mV and keeps stable with cycling, which are possibly because FEC forms stable SEI film prior to NaPF6. In general, unstable electrolyte would cause increase of the impedance and polarization of the cell during cycling, but film-forming agents (FEC and NaPF6) would remain stable impedance characteristics. The results of the current polarization experiments are consistent with those of EIS (Figure 4a and b).


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Figure 4. Impedance spectra of the NNSCs in PC-based electrolytes with 0.8 M (a) NaClO4 and (b) NaPF6. Time dependence of the potential of the NNSCs at 0.5 mA cm-2 in different PC-based electrolytes with 0.8 M (c) NaClO4 and (d) NaPF6. 5% vol. FEC is added for comparison.

Exploration of the reaction mechanism In order to study the reaction mechanism of the PC-based electrolytes with Na metal, pure PC solvent and PC solvent with different salts (0.8 M NaClO4 and NaPF6) including Na metal were analyzed by using GC–MS (Figure 5). The results show the presence of two other peaks around 4.8 and 6.4 minutes in the case of adding salts besides the characteristic peaks of PC (Figure 5b and c). And it is found that the extra peaks in different salt electrolytes are the same in the


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electrolytes with different salts, which is indicative of the similar reaction processes of Na and PC solvent. By comparing their mass peaks, it could be deduced that two produces are Diisopropyl carbonate (DIPC) and 1,1'-oxydi-Bis(2-hydroxypropyl) ether (OBHE), resulting from the decomposition of PC solvent. Also, it should be noted that no side reaction occurs in PC solvent without salts including Na metal (Figure 5a). Therefore, the occurrence of two produces (DIPC and OBHE) should originate from the induction effect of salt to accelerate the reaction of PC and Na metal.

Figure 5. GC–MS chromatograms of (a) PC+ Na, (b) PC+ NaPF6+Na, (c) PC+NaClO4+Na. To further verify the reaction of the PC-based electrolytes with Na metal, IR spectroscopy was conducted to investigate reaction produces in PC electrolyte with different salts (Figure 6a and Figure S3). A strong band at 1100 cm-1 differing from pure PC solvent was observed in PC+NaClO4, which corresponds to the characteristic peaks of ClO4- (Figure S3a). Similarly, two intense peaks absorption at 850 cm-1and 550 cm-1 for PC+NaPF6 is associated with PF6- (Figure


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S3b), which suggests that no reaction occurs when NaPF6 or NaClO4 is added to the PC solvent. Compared to the PC+NaClO4 electrolyte, the spectrum of the PC+NaClO4 electrolyte including Na metal showed different peaks mainly at about 1600 cm-1, which characterizes the COOasymmetric stretching vibration originating from the PC decomposition on the Na metal. Similarly, the characteristic absorption peak also was observed in PC+NaPF6 including Na metal (Figure S3c), which indicates a similar side reaction as PC+NaClO4 electrolyte (Figure 6a). To demonstrate the reaction mechanism and the reaction product of PC with Na metal, NMR measurements were carried out as shown in Figure 6b. For pure PC solvent, there is one-to-one match between four resonance peaks observed and four kinds of carbon in PC molecule. By contrast, there are other obvious resonance peaks observed except for the carbon resonance peaks of PC molecule in the spectra of the NaClO4 electrolyte including Na metal. Two resonance peaks at about 25.7 and 68.4 ppm correspond to the CH groups and CH3 groups of DIPC decomposed by the PC molecule, while other three resonance peaks around 23.2, 48.5 and 70.1 ppm can be assigned to the CH3, CH and CH2 position of OBHE. Likewise, for the NaPF6-added electrolyte including Na metal, these emerging resonance peaks were also observed to match the characteristics of DIPC and OBHE produces (Figure S3d).


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Figure 6. (a) IR spectra of PC+ NaClO4 and PC+NaClO4 including Na metal. (b)13C NMR spectra of PC including Na metal and PC+NaClO4 including Na metal. Based on the above results, it is reasonable to conclude that the NMR results are accordance with those of GC-MS and IR, indicating that in the presence of salt, PC electrolyte including Na metal is unstable and easy to decompose into DIPC and OBHE produces. Therefore, a cationic induction mechanism is proposed to support by the results of GC-MS, IR and NMR (Figure 7). The first assumption approach is that PC undergoes ring-opening reaction via a single-electron nucleophilic attack pathway under Na+ induction to produce linear alkyl carbonate (R1COOR2) (Figure 7a). PC molecule firstly gets electron and is reduced as propylene and oxygen. Then the propylene reacts with PC molecule to produce Diisopropyl carbonate (DIPC) through electrophilic addition assisted by the coordination of Na+ with the carbonyl group of PC molecule (Figure 7a). For the generation of 1,1'-oxydi-Bis(2-hydroxypropyl) ether (OBHE), a possible mechanism is that disodium 1, 2-propanediol first generates through nucleophilic attack of Na+ and initial electronation of the carbonate molecules (Figure 7b). Then two disodium 1, 2propanediol recombine to produce the OBHE and Na2O (Figure 7b). The above reaction pathways can explain the coexistence of DIPC and OBHE when the NaClO4 electrolyte


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including Na metal. Besides, it is noting that when adding the linear carbonate, the electrolytes become more unstable (Figure 1), indicating that the linear carbonate accelerates the decomposition of PC. It is possibly because the linear carbonates are more likely to produce free radicals under Na+ induction,29,65 such as CH3· from DMC and EMC, CH3CH2· from DEC and EMC, the free radicals can further trigger the decomposition of PC molecule through a succession of nucleophilic attack (Figure 7c). It can be obviously found that from the abovediscussed decomposing mechanisms, Na+ induction effect has played an important role on the decomposition reaction of PC, which is the reason why the electrolyte becomes more unstable after adding salt.


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Figure 7. The reaction mechanism of PC-based electrolytes system with Na metal (a) the generation mechanism of DIPC. (b) the generation mechanism of OBHE. (c) The reaction mechanism in the presence of linear carbonates. In order to further understand the influence of film-forming additive on the composition and morphology of SEI film, Scanning Electron Microscope (SEM) and X-ray photoelectron spectroscopy (XPS) measurements for sodium tablets in different electrolytes were performed (Figure 8 and Figure S4-6).

Figure 8. SEM images of different sodium tablets immersed (a) in the NaPF6 electrolyte and (b) in the NaClO4+FEC electrolyte. XPS spectra of the different sodium tablets immersed in the NaClO4+FEC electrolyte: (c) F 1s spectra and (d) C 1s spectra.


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In the NaClO4 electrolyte, the surface of the sodium tablet is porous and has no visible SEI formation (Figure S4). However, in the NaPF6 electrolyte (Figure 8a), the surface of sodium is covered with a relatively rough layer with a large number of pores, which is ascribed to the decomposition of NaPF6 to form porous SEI film. For NaClO4+FEC electrolyte (Figure 8b), some nanoparticals with spherical or flaky shape was observed, resulting from the decomposition of FEC to form the SEI film with NaF. The compact and dense SEI can inhibit decomposition of PC electrolyte. The decomposition products formed on the sodium tablets can be identified by XPS measurements (Figure 8c, d, S5 and S6). Compared to the results of sodium tablets in the NaClO4 electrolyte (Figure S5), main difference is that the SEI films contain F produce in the NaPF6 (Figure S6) and NaClO4+FEC electrolyte (Figure 8c). The peak at 683.5 eV assigned to NaF in F1s, which is an important component of SEI formed on the surface of sodium tablets, relating to the degradation of the NaPF6 or FEC. In addition, the C 1s spectra show that the peaks corresponding to the C=O, O-C=O and NaO-C=O become more intense in the NaClO4+FEC electrolyte, which is likely due to the decomposition of the FEC on surface of sodium tablets. These facts indicate that the addition of NaPF6 or FEC in the PC-based electrolytes can produce a protective film on the surface of sodium so as to hinder the occurrence of the side reaction. While there is no presence of the protective film in the NaClO4 electrolyte, the electrolyte would cause continuous side reaction. According to the above discussion, the reaction schematic of solvent and Na metal in the PCbased electrolytes is depicted in Figure 9. The instability of PC on Na metal mainly results from Na+ inductive effect (Figure 7a and Figure 7b). The addition of linear carbonate further promotes the decomposing reaction due to the presence of more free radicals (Figure 7c and Figure 9a). NaPF6 salt can form SEI film on the surface of Na metal to suppress the decomposition of PC to


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some extent, but the NaPF6-derived SEI film is not enough dense, leading to the continuous reaction of the decomposition of PC at 60 oC, as illustrated in Figure 9b. It is mainly because NaPF6-derived SEI film consists mainly of inorganic Na2O and NaF species and lacks some organic reduction products to fill the gaps among the inorganic species, resulting in the permeation of solvents to further trigger the decomposing reaction (Figure 9b). By contrast, FEC additive can form a compact SEI film on the surface of Na metal due to more organic reduction products combined with inorganic species, which strongly prevent the solvent from penetrating, so as to hinder more side reaction of PC with Na metal (Figure 9c). Additionally, the compact SEI film derived by FEC can efficiently decrease the interfacial resistance of Na metal to reduce the electrochemical polarization of Na metal and improve the electrochemical reversibility of Na metal.


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Figure 9. Schematic illustration of the impact on PC decomposition on the surface of the Na metal in the (a) NaClO4 electrolyte, (b) NaPF6 electrolyte, (c) electrolyte with FEC.

Electrochemical performance of hard carbon anode in different electrolytes The above studies focus on the electrochemical compatibility of the PC-based electrolyte on Na metal. However, hard carbon is the most promising anode material for the application of SIBs. Therefore, according to the above works on the electrochemical compatibility of the PC-based electrolytes on Na metal, the electrochemical performance of hard carbon in different electrolytes was also investigated, as shown in Figure 10. For PC electrolytes containing different film-formed additives (Figure 10a), the hard carbon electrodes cycled in the FEC-added electrolyte exhibited outstanding behavior compared to in other electrolytes with different additives (VC ， ES and LiBOB). Besides, it is found that the hard carbon electrodes can maintain stable cycleability in other PC-based electrolytes with FEC additive, while the cycling capacity dramatically decreases in the electrolytes without FEC additive (Figure b-d). In addition, the cycling performance of hard carbon anode in NaPF6-contained electrolyte is a little better than that in NaClO4-contained electrolyte. The results demonstrate that the electrolyte without FEC undergoes significant decomposition on account of the instable electrolyte, indicating that the chemical and electrochemical stability of the electrolyte directly affects cycling performance of hard carbon anode. Based on the results in Figure 10, for PC-based electrolytes, 0.8 M NaPF6 PC: DEC-5%FEC is an optimum choice in capacity and cycling stability of hard carbon anode.


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Figure 10. (a) Cycling performance of hard carbon anodes cycled at 50 mA g-1 in 0.8 M NaClO4 PC-based electrolytes with different additives. (b-d) Cycling performance of hard carbon anodes cycled at 50 mA g-1 in different electrolytes (PC, PC-DMC and PC-DEC). Salt concentration is 0.8 M. PC: DEC (DMC or EMC): FEC=50:50:5 (v/v).

Conclusions In this paper, the chemical and electrochemical compatibility of PC-based electrolytes were systematically investigated through color change of the electrolyte, EIS and current polarization experiments. The results found that some factors including salt, additives, linear carbonates and temperature all affect the stability of the electrolytes. The possible mechanism was proposed


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based on GC-MS, IR, NMR, XPS and SEM measurements. The following conclusions are drawn. 1. There is no obvious reaction observed between pure solvent and Na metal. However, after adding salt, the electrolytes show reactive activity to some extent based on different salts and cosolvents. The film-formed additives can efficiently suppress the decomposition of the solvents. Among them, FEC is optimum one to form the compact and dense SEI film. 2. A Na+ induction mechanism was proposed to demonstrate the reaction mechanism of PCbased electrolytes with Na metal. The experiments showed that Na+ induction effect prompts the formation of free radicals resulting from PC and linear carbonates to accelerate the side reaction. This mechanism can also explain well the inhibiting effect of the film-formed additive by forming compact SEI film to isolate solvent and Na metal. 3. Though the film-formed additives (such as NaPF6, FEC) can stabilize the electrolytes on Na metal, they increase the initial polarization and impedance of Na metal electrode. Therefore, it is very important to find a mild additive to decrease the impedance of the electrode. This work will be a focus of our future research. 4. It is particularly important that adding the FEC additive can improve the electrochemical performance of hard carbon anode, due to the instability of PC-based electrolytes with NaClO4 or NaPF6 salt on Na metal. An electrolyte including 0.8 M NaPF6 PC: DEC (1:1) +5% FEC was suggested as an optimum choice to realize excellent cycling performance for hard carbon anode.


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This study only explored the electrochemical properties of the PC-based electrolytes for Na metal and hard carbon anode. Though the optimum composition based on PC electrolyte is helpful for the future studies of materials in half cell, the understanding of extension electrolyte systems (EC, ether, et al.) is still lack. However, some proposed methods and mechanisms in this work should be referred to extend the studies on other electrolyte systems.

ASSOCIATED CONTENT The Supporting Information is available free of charge on the The basic physical properties of the Common organic solvents. Impedance spectra and time dependence of the potential of the NNSCs in different electrolytes. IR spectra and

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C NMR

spectra in different electrolytes. SEM image of different Sodium pretreated in the NaClO4 electrolyte. XPS spectra of the different sodium tablets in the NaClO4 and NaPF6 electrolytes.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Y.C.) *E-mail: [email protected] (F. Z.) Notes The authors declare no competing ﬁnancial interest. ACKNOWLEDGMENT


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This work was support by National Key Research Program of China (No. 2016YFB0100200) and the National Nature Science Foundation of China (Nos. 21673165 and 21333007). REFERENCES (1) Kim, H.; Kim, H.; Ding, Z.; Lee, M. H.; Lim, K.; Yoon, G.; Kang, K. Recent Progress in Electrode Materials for Sodium-Ion Batteries. Adv. Energy Mater. 2016, 6, 1600943.

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Understanding the Electrochemical Compatibility and Reaction

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