High Capacity and Cycle-Stable Hard Carbon Anode for

Subscriber access provided by Kaohsiung Medical University

Energy, Environmental, and Catalysis Applications

A High Capacity and Cycle-Stable Hard Carbon Anode for Non-Flammable Sodium-Ion Batteries Xingwei Liu, Xiaoyu Jiang, Ziqi Zeng, Xinping Ai, Hanxi Yang, Faping Zhong, Yong-Yao Xia, and Yuliang Cao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b16129 • Publication Date (Web): 17 Oct 2018 Downloaded from http://pubs.acs.org on October 17, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

A High Capacity and Cycle-Stable Hard Carbon Anode for Non-Flammable Sodium-Ion Batteries Xingwei Liua, Xiaoyu Jianga, Ziqi Zenga, Xinping Aia, Hanxi Yanga, Faping Zhongb,*, Yongyao Xiac, Yuliang Caoa,*

a

College of Chemistry and Molecular Sciences, Hubei Key Laboratory of Electrochemical

Power Sources, Wuhan University, Wuhan 430072, China. b

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

410205, China. c

Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative

Materials, Institute of New Energy, Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), Fudan University, Shanghai 200433, China.

KEYWORDS: phosphate electrolytes, high molar ratio, safety, hard carbon anode, sodium-ion batteries

1

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: Non-flammable phosphate electrolytes are in principle able to build intrinsically safe Na-ion batteries, but their electrochemical incompatibility with anodic materials, especially hard carbon anode, restricts their battery applications. Here, we propose a new strategy to enable high capacity utilization and cycle-stability of hard carbon anode in the non-flammable phosphate electrolyte by use of low-cost Na+ salt with a high molar ratio of salt/solvent combine with SEI film-forming additive. As a result, the carbon anode in the trimethyl phosphate (TMP) electrolyte with a high molar ratio of [NaClO4]: [TMP] and 5% FEC additive demonstrates a high reversible capacity of 238 mAh g−1, considerable rate capability and long-term cycling life with 84% capacity retention over 1500 cycles. More significantly, this work provides a promising route to build intrinsically safe and low-cost sodium-ion batteries for large-scale energy storage applications.

2


Page 2 of 31

Page 3 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. INTRODUCTION Sodium-ion batteries (SIBs) have recently attracted renewed interest as low-cost alternatives or complements to Li-ion batteries (LIBs) for large-scale electric energy storage applications, because of the widespread availability of sodium resources.1-3 In the past few years, considerable efforts have been devoted to exploring Na-host compounds with high Na storage capacity and long-term cyclability.4-6 A large number of Na storage cathodes and anodes such as Prussian blue analogs,7-9 metal oxide cathodes,10-12 transition-metal phosphate cathodes13-18 and hard carbon19-24 have been identified to be capable to deliver considerably high redox capacity with acceptable cycle life, possibly being implemented for battery application. In contrast, less attention has been paid to the interfacial chemistry and electrochemical compatibility of these Na storage materials with electrolyte.25-26 Particularly, the SIBs developed so far are mostly based on volatile and flammable organic carbonate electrolytes, which still impose a safety hazard for practical applications.27 In principle, the use of non-flammable electrolyte can completely eliminate the firing and explosion of the batteries. In the light of previous studies of safer electrolytes for LIBs,28-29 several types of non-flammable organic solvents have been used to partially or fully replace the organic

carbonate

electrolyte

for

SIBs.30-33

Butylmethylpyrrolidinium

bis(trifluoromethanesulfonyl)imide based ionic liquid electrolyte with 1 mol L−1 NaClO4 have demonstrated an electrochemical compatibility with Na0.44MnO2 cathode, which could deliver its full reversible capacity of 115 mAh g−1 at 0.05 C.31 A non-flammable electrolyte composed of 3



Page 4 of 31

5% ethoxy(pentafluoro)-cyclotriphosphazene and 95% conventional ethylene carbonate (EC)/diethyl carbonate (DEC) electrolyte was shown to be applicable for SIBs as well.32 Our recent works revealed that the non-flammable trimethyl phosphate-based electrolytes were electrochemically

compatible

with

non-carbonaceous

anode

materials

in

SIBs.33-34

Unfortunately, these non-flammable electrolytes are all failed to work with hard carbon, a most attractive high capacity and low-cost anode for SIBs, due to their inability to form a compact and stable solid electrolyte interphase (SEI) on the hard carbon surface. Therefore, it remains a challenge to enhance the electrochemical stability of non-flammable electrolytes toward the Na+ insertion reaction of hard carbon anode as for establishing intrinsically safe SIBs. Highly concentrated electrolytes have demonstrated a strong ability to promote the formation of stable anion-derived SEI films on a number of electrode materials for LIBs and SIBs.35-38 Wang et al reported that a high-concentrated LiFSI flame-retardant electrolyte can form a robust inorganic passivation film by LiFSI to realize sodium ion reversible insertion/extraction into hard carbon.39 However, the high salt-concentrated electrolyte (> 3 mol L−1) with film-formed salts (LiFSI or LiTFSI) is too expensive to be used in practical application. Thus, it is important to use low-cost salts (e.g., NaClO4) to realize reversible electrochemistry of Na+ ion intercalation into hard carbon in phosphate electrolyte, which will further expand on low-cost and safe SIB application for large-scale energy storage. Recently, Zeng et al. reported that increasing the salt-to-solvent molar ratio (MR) can suppress efficiently the decomposition of the solvent to improve greatly the electrochemical compatibility between 4


Page 5 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


phosphate electrolytes and carbonaceous electrodes, due to the significantly decreased population of free solvent molecules.40 A LiFSI:TEP (triethyl phosphate) electrolyte with high MR (1:2) but not high molar concentration (~2.2 mol L−1) exhibits high electrochemical compatibility on Li+ ion insertion into graphite.40 Thus, it is possible to enhance the electrochemical stability of carbon anodes in the non-flammable electrolytes with non-film-formed salts by increasing the molar ratio of salt to solvent molecules. In this paper, we report a non-flammable phosphate electrolyte with a high MR of salt/solvent by using low-cost NaClO4, in which hard carbon (HC) displays a reversible and stable charge/discharge performance as a Na+-insertion anode. As a result, the HC anode in the non-flammable electrolyte with [NaClO4]: [TMP] = 1:3 and 5 vol% FEC additive demonstrated a high Na+ insertion capacity of 238 mAh g−1, a superior cycling performance with 84% capacity retention over 1500 cycles at 200 mA g−1 and a strong rate capability at 1000 mA g−1, suggesting a great prospect for battery application. More significantly, this work validates the feasibility to effectively improve the electrochemical stability of organic phosphate electrolytes by increasing the MR for building intrinsically safe SIBs.

2. EXPERIMENTAL METHODS 2.1 Materials. Trimethyl phosphate (TMP) was obtained from Aladdin and purified by vacuum distillation prior to use. The tested non-flammable electrolytes were prepared by dissolving NaClO4 (Aldrich, 98%) in TMP with the MR of 1:9, 1:4.5 and 1:3, corresponding to 0.91 mol L−1, 1.74 mol L−1 and 2.50 mol L−1, respectively. To help the SEI formation, 5 vol% FEC was 5



additived into the electrolytes. For comparison, 1mol L−1 NaClO4 in ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 v/v) with and without 5 vol% FEC were supplied by Guotai Huarong Co. Ltd., China, and used as a control electrolyte. The hard carbon (LBV-10010) was purchased from Sumitomo Bakelite Co., Ltd.) and used as received. 2.2 Materials characterization. Raman spectroscopy of the samples was performed with a laser micro-Raman spectrometer with 532 nm excitation wavelength (Renishaw in Via, Renishaw). Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were recorded in a field-emission scanning electron microscope (FE-SEM, ZEISS Merlin Compact VP, Germany; EDS, Oxford Instruments Link ISIS). X-ray photoelectron spectroscopy (XPS) was recorded with a Thermo Fisher ESCALAB 250Xi using monochromic Al Kα X-ray source. All samples were taken out from the half cells after electrochemical cycles. The electrodes were rinsed by dimethyl carbonate (DMC) for three times and dried under vacuum for 1 hour before SEM/EDS and XPS measurement. 2.3 Electrochemical measurement. The conductivities of the electrolytes were measured by using a conductivity measuring meter (DDS-307, Leici, China) at the temperature range of 5 – 65 °C. The HC electrode was prepared by mixing the HC, Super P and polyacrylic acid (PAA) in deionized water with a weight ratio of 8:1:1 to form electrode slurry. The slurry was pasted on a Cu foil followed by drying in a vacuum oven at 100 °C overnight. The loading of the active material in the electrode is about 1.5 mg cm−2. The half cells were assembled in 2016-type coin cell in an argon-ﬁlled glove box with a hard carbon working electrode, a sodium metal counter 6


Page 6 of 31

Page 7 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


electrode and a glass fiber filter separator, filled with the TMP electrolytes. The cells were galvanostatically cycled on a LAND cycler (Wuhan LAND Electronics Co., China) between 0 V and 2 V. Cyclic voltammetry (CV) measurements and electrochemical impendence spectroscopy (EIS) tests were carried out with Autolab PGSTAT128N (Eco Chemie, Netherlands).

3. RESULTS AND DISCUSSION 3.1 Physical properties of the electrolytes with different NaClO4/TMP MRs The chemical stability, electrochemical window and Na+ ion conductivity are the basic criteria for judging whether the non-flammable TMP electrolyte is appropriate for SIB applications. The chemical reactivity of the TMP electrolyte towards Na metal can be seen by color changes of the Na metal stored in the TMP electrolyte. Figure 1a shows three samples of Na metals in the TMP electrolytes with different MRs of NaClO4:TMP. After stored at room temperature (25 °C) for 90 days, the surface of sodium metal in the electrolyte with NaClO4:TMP = 1:9 appeared tarnished and the electrolyte became slightly turbid, implying that a slow chemical corrosion took place on the Na surface by the electrolyte. Meanwhile, in the electrolytes with NaClO4:TMP = 1:4.5 and 1:3, the surface of Na metal remained bright, reflecting a depressed reactivity of Na metal with the TMP electrolytes at a higher molar ratio of NaClO4:TMP. This observation suggests a simple way to improve the chemical stability of the TMP electrolyte by increasing the MR of NaClO4:TMP. Electrochemical windows of the TMP electrolytes are the largest available potential range 7



that is usable for the operation of SIBs. Figure 1b displays the CV curves of a Pt microelectrode in the TMP electrolytes with different MRs of NaClO4:TMP. When scanning over a wide potential range from +5.0 V to -0.25V (vs. Na/Na+), only an irreversible reduction current emerged below the potential of 0 V in the TMP electrolyte with MR = 1:9, corresponding to the electrochemical decomposition of the electrolyte, whereas this reduction current became reversible with increasing the MRs of NaClO4:TMP in the electrolyte, resembling very much the electrochemical plating/ stripping peaks of Na. Except for these CV peaks, there were no any redox peaks observed in the potential region of 0 to 5.0 V, indicating a wide electrochemical stability window of 5.0 V of the TMP electrolytes. It is also interesting to note that when the MR was increased to 1:3 in the electrolyte, the CV peaks at low potentials became greatly enlarged and fully reversible, implying a greatly suppressed reduction of the electrolyte along with the emergence of fully reversible electrochemical deposition/dissolution of Na. This phenomenon further confirms that increasing the MR of NaClO4:TMP can increase not only the chemical stability but also the electrochemical stability of the TMP electrolytes. Room temperature ionic conductivities of the TMP electrolytes with various NaClO4:TMP are shown in Figure 1c. In general, the ionic conductivities of the electrolytes increase with increasing temperature and decrease with increasing MRs of NaClO4:TMP, as expected from the Vogel–Tammann–Fulcher (VTF) equation. This is simply because the higher temperature can accelerate the dissociation of ion pairs and produce a larger amount of free ions, while higher MRs favors the formation of ion clusters that have no contribution to the conductivity.30, 41 The 8


Page 8 of 31

Page 9 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


room temperature conductivities of the TMP electrolytes are found to be 5.75, 4.22 and 2.35 mS cm−1 at the MRs of NaClO4:TMP = 1:9, 1:4.5 and 1:3, respectively, all of which are sufficiently high for SIB applications. The results presented above demonstrate a considerable enhancement in the chemical and electrochemical stability of the TMP electrolytes with increased MRs of NaClO4:TMP. These stability enhancements could be explained by the change in the solvation structure of the electrolytes.40,

42

Figure 1d shows the Raman spectra of the TMP electrolytes with different

NaClO4:TMP ratios. In a relatively diluted TMP electrolyte (MR of NaClO4:TMP = 1:9), the P-O-C stretching band did not show much change as compared to the 737 cm−1 band of pure TMP, suggesting the existence of a large amount of free TMP molecules in the electrolyte. When the MR increased to 1:3, this P-O-C band was positively shifted to 743 cm−1, reflecting a strong bonding between Na+ ions and TMP molecules.43 This Raman shift suggests that with increasing the MR of NaClO4:TMP, the structure of the solvation spheres of Na+ and ClO4− ions changes considerably due to the gradual depletion of free TMP molecules. In the MR of NaClO4:TMP =1:3, almost all the TMP molecules are coordinated with Na+ ions and even ClO4− participate in the coordination since the Na+ ions are at lease 4-coordinated (inset in Figure 1d), thus greatly decreasing the chemical reactivity and electrochemical activity of the TMP solvent.

9



Figure 1. Physicochemical properties of the TMP electrolytes with different NaClO4/TMP ratios. (a) Room temperature storage behaviors. (b) CV curves of a Pt microelectrode in the TMP electrolytes at a scan ratio of 10 mV s−1. (c) Arrhenius plots and (d) Raman spectra of the TMP electrolytes. The insets show Na+ solvation structure of the TMP electrolytes with minimum energy. 3.2 Electrochemical behaviors of hard carbon in the TMP electrolytes A major obstacle for battery applications of non-flammable TMP electrolytes is their failure to form a desirable SEI film on the carbon anodes, which leads to ceaseless electrochemical decomposition (reduction) of TMP molecules at low potentials and then depresses the ionic insertion reaction. Therefore, development of effective strategies to suppress the cathodic reduction of the TMP electrolytes is vital for successful use of carbon-based anodes in LIBs and SIBs. Figure 2a shows the CV curves of HC electrodes in the TMP electrolytes with different 10


Page 10 of 31

Page 11 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


MRs. In the low MR TMP electrolyte (NaClO4:TMP = 1: 9), a huge and broad reduction band appears at the cathodic scan from +0.5 V to 0 V, while no oxidation peak is detected on the reversed anodic scan, reflecting an irreversible electrochemical reduction of the TMP molecules. When the MR of NaClO4:TMP is increased to the higher ratios of 1:4.5 and 1:3, the cathodic peaks are greatly reduced due to the less decomposition of TMP molecules and accordingly a distinct oxidation peak come out at around 0.2 V, corresponding to a reversible Na-insertion/extraction process on the HC anode. Nevertheless, the cathodic peaks appearing in the higher MR TMP electrolytes have much larger areas than their corresponding anodic peaks, suggesting that there still exists a considerable electrochemical reduction of TMP molecules accompanying with the reversible Na+-insertion reaction. The charge/discharge curves of the TMP electrolytes show similar electrochemical features to the CV curves. As displayed in Figure 2b, the HC anode in the TMP electrolyte with NaClO4:TMP = 1:9 is hard to be charged and therefore give only a negligible charge capacity, which is ascribed to the irreversible reduction of TMP molecules on the HC anode. With increasing the MR to 1:4.5 and 1:3 in the TMP electrolytes, the reversible charge capacities of the HC anode increase remarkably to 232.3 mAh g−1 and 256.1 mAh g−1, and the initial Coulombic efficiencies (ICE) of the HC anode rise to 45.6 and 67.1%, respectively. Though these ICE are still lower as compared to that (72%) in carbonate electrolyte (Figure S1), the observation of reversible Na+-insertion reaction on the HC anode suggests a possible way to enhance the electrochemical utilization by increasing the MRs of the non-flammable TMP 11



electrolytes even using non-film-forming salt (NaClO4). In such a high MR electrolyte, the TMP solvent exists less tendency of decomposition while the NaClO4 shows high reductive stability. Thus, the 1:3 NaClO4:TMP electrolyte displays good electrochemical compatibility. It should be noticed that after the initial cycle, the HC electrode in the high MR electrolyte (1:3) exhibits a stable reversible capacity of ~ 240 mAh g−1 (Figure 2c) and a quite high Coulombic efficiency of ~ 98.5% during subsequent cycles (Figure 2d), reflecting considerably electrochemical reversibility and cyclability for Na+ insertion reaction. Even so, the HC electrode cannot give a satisfactory cycling performance and show a serious capacity fading after 50 cycles. The reason is that the NaClO4 and TMP cannot construct a compact and stable SEI film which completely prevent the further reduction of the electrolyte at subsequent cycles, despite its irreversible decomposition is greatly suppressed.

12


Page 12 of 31

Page 13 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. Electrochemical performances of the HC anode in the TMP electrolytes with different MRs of NaClO4:TMP. (a) Initial CV curves at a scan rate of 0.1 mV s−1. (b) Initial charge/discharge curves at 20 mA g−1. (c) Charge/discharge curves of HC at 50 mA g−1. (d) Cycling performance of HC at 200 mA g−1. 3.3 Improved cyclability of HC anode in the FEC-added TMP electrolytes Since both NaClO4 and TMP cannot form a stable SEI film on the surface of HC anodes, we tried to use a highly efficient SEI-forming additive, fluoroethylene carbonate (FEC), to promote the SEI formation and thereby enhance the cycling performance of the HC anode in the TMP electrolytes. The basic chemical and electrochemical properties of the TMP electrolytes with the addition of 5 vol. % FEC are given in Figure S2. Compared with blank TMP electrolytes, the TMP electrolytes with 5 vol. % FEC show no discernible change in the chemical stability, ionic conductivity and solvation structure but much improved electrochemical reversibility. Figure 3a shows the initial CV curve of HC anode in the electrolyte of 1:3 NaClO4:TMP-5 vol.% FEC. It exhibits a two stepped CV response with broad redox currents in the potential region of 1.0 ~ 0.25 V, followed with a pair of sharp redox peaks at ~0.1 V. The cathodic and anodic branches are symmetric with very similar shape and area, reflecting a greatly improved electrochemical reversibility of the redox reactions if compared with the CV curve obtained in the absence of FEC additive (Figure 2a). In accord with the CV feature, the discharge curve of the HC anode in the electrolyte of 1:3 13



NaClO4:TMP-5 vol.% FEC gives a sloping profile from 1.0 V to 0.25 V, followed with a low potential plateau at ~0.1V, which is proved to be a characteristic of Na ion insertion reaction (Figure 3b).44-45 On the other hand, the charge-discharge profiles of HC anodes cycled in low-MR FEC-added electrolyte display a shorter plateau (for NaClO4:TMP=1:4.5) or even no plateau (NaClO4:TMP=1:9) (Figure S3), demonstrating higher polarization in these cells. The HC anode in the electrolyte of 1:3 NaClO4:TMP-5 vol.% FEC exhibits a reversible capacity of 243.8 mAh g−1 and a Coulombic efficiency of 68.4%, higher than those in 1:9 NaClO4:TMP-5 vol.% FEC (114.4 mAh g−1 and 47.6%) and 1:4.5 NaClO4:TMP-5 vol.% FEC (210.9 mAh g−1 and 62.5%) electrolytes, indicating high MR electrolyte benefit for good electrochemical performance is also valid in the FEC-added electrolyte. Note that all the HC anodes using FEC-added electrolytes show higher ICE than those cells using corresponding FEC-free electrolytes, illustrating that the FEC additive could reduce the undesired decomposition reaction. Figure 3c displays the rate capability of the HC anode in the 1:3 NaClO4:TMP-5 vol.% FEC electrolyte at various current densities. When the current rate changes from 20 mA g−1 to 50, 100, 200, 1000 and 2000 mA g−1, the reversible capacity gradually decreases from 238.3 mAh g−1 to 225.9, 163.1, 88.2, 52.3 and 33 mAh g−1, respectively, and this reversible capacity can fully recover its original value when the current rate turns back to 20 mA g−1. This rate capability is comparable to those of the HC anodes in 1 mol L−1 NaClO4 EC+DEC electrolyte,46-47 despite the high-MR TMP electrolyte has a lower ionic conductivity (2.60 mS cm−1) than that of EC/DEC-based electrolyte (~6.0 mS cm−1). This phenomenon seems to 14


Page 14 of 31

Page 15 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


indicate that the SEI film formed by FEC additive in the TMP electrolyte is sufficiently thin, compact and ion-conductive, enabling the HC anodes to achieve a high rate performance. The stable SEI formation leads to a long-term cycle stability of the HC anode in the TMP electrolyte. As shown in Figure 3d, the reversible capacity of the HC anode can retain 84% of its initial capacity over 1500 cycles with the average Coulombic efficiency of ~99.8%. If compared with poor cycling performance (50% capacity retention after 100 cycles, Figure 2d) obtained from the blank TMP electrolyte, the cycling capacity and cycle life of the HC anode are surprisingly enhanced by adding 5% FEC in TMP electrolyte. Obviously, such an excellent cycling performance is attributed to the synergistic effect between the high-MR electrolyte and the stable FEC-derived SEI film that suppressed solvent decomposition

Figure 3. Electrochemical performances of HC anode in the TMP electrolyte with MR of 1:3 15



and 5% FEC. (a) initial CV curve at 0.1 mV s−1; (b) Charge-discharge profiles at 50 mA g−1; (c) Rate capability and (d) cycling performance at a current density of 200 mA g−1. 3.4 Investigating SEI film in the TMP electrolytes with FEC additive To get an insight into the mechanism for the enhanced cyclability, the changes in the surface structures of the HC electrode cycled in the FEC-added and FEC-free TMP electrolytes with a high-MR of NaClO4:TMP were characterized by SEM, EDS, XPS and EIS measurements. Figure 4 shows the morphological changes of the cycled HC anodes with various MRs of NaClO4:TMP in the TMP electrolytes. It is clearly seen in the SEM images that the surface deposition becomes smaller and thinner with the increased MRs. As shown in Figure 4a, the surface of HC anode is covered with large granules of deposition when cycled in the TMP electrolyte with a lower ratio of NaClO4:TMP = 1:9, exclusively due to the electrochemical decomposition of TMP solvent, as confirmed by EDS mapping (Figure S4a). Once cycled in the high MR TMP electrolyte (1:4.5), the surface deposition on the HC anodes becomes a thin layer of small particles (Figure 4b). Furthermore, when cycled in the higher MR TMP electrolyte (1:3), the surface morphology of the HC anodes (Figure 4c) almost remain the same as the uncycled HC anode (Figure S4b), which confirms the aforementioned suggestion that the electrolyte with high MR of NaClO4 to TMP can effectively suppress the electrochemical decomposition of TMP solvent. When the FEC additive was introduced into the TMP electrolyte, the surface compositions and morphologies of the cycled HC anodes change considerably with the formation of an intact 16


Page 16 of 31

Page 17 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


SEI layer. As shown in Figure 4d, the surface of HC anode even cycled in low MR TMP electrolyte is spread of microspherical particles in the presence of FEC molecules, totally different from the large granules observed in the absence of FEC (Figure 4a), which arises most likely from the prior formation of the FEC-derived SEI film that suppresses the decomposition of TMP solvent. In the TMP electrolyte with higher MRs of NaClO4:TMP =1:3, the cycled HC anode shows a very similar morphology as the uncycled HC anode but the surface appears to be covered by a very thin layer of SEI film (Figure 4f). Since the electrochemical reduction of FEC molecules for SEI formation starts at much higher potentials of ~0.7 V (Figure 3a) than the decomposition potential (

High Capacity and Cycle-Stable Hard Carbon Anode for

Recommend Documents