Formation of Stable SolidâElectrolyte Interphase Layer on Few-Layer

Subscriber access provided by GRIFFITH UNIVERSITY

Article

Formation of Stable Solid-Electrolyte Interphase Layer on Few- Layer Graphene-Coated Silicon Nanoparticles for High-Capacity Li-Ion Battery Anodes Jong Hwan Park, Junhyuk Moon, Sangil Han, Seongyong Park, Ju Wan Lim, Dong-Jin Yun, Dong Young Kim, Kwangjin Park, and In Hyuk Son J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b05876 • Publication Date (Web): 26 Oct 2017 Downloaded from http://pubs.acs.org on October 26, 2017

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 free 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 accessible to all readers and 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.

The Journal of Physical Chemistry C 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 29

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

The Journal of Physical Chemistry

Formation of Stable Solid-Electrolyte Interphase Layer on FewLayer Graphene-Coated Silicon Nanoparticles for High-Capacity Li-Ion Battery Anodes

Jong Hwan Park†, ‡,#, Junhyuk Moon†,#, Sangil Han§, Seongyong Park∥, Ju Wan Lim†, DongJin Yun§, Dong Young Kim†, Kwangjin Park*,†, and In Hyuk Son*,†

†

Energy Material Lab, Material Research Center, Samsung Advanced Institute of Technology, Samsung Electronics Co., LTD, 130 Samsung-ro, Yeongtong-gu, Suwon-si, Gyeonggi-do 443-803, Republic of Korea. ‡

Nano Hybrid Technology Research Center, Creative and Fundamental Research Division,

Korea Electrotechnology Research Institute (KERI), 12, Bulmosan-ro 10 beon-gil, Seongsangu, Changwon-si, Gyeongsangnam-do, 51543, Republic of Korea. §

Materials R&D Center, Samsung SDI Co., LTD, 130 Samsung-ro, Yeongtong-gu, Suwon-si,

Gyeonggi-do 16678, Republic of Korea. ∥

Analytical Science Laboratory, Samsung Advanced Institute of Technology (SAIT),

Samsung Electronics Co., LTD, 130 Samsung-ro, Yeongtong-gu, Suwon-si, Gyeonggi-do 16678, Republic of Korea.

1 ACS Paragon Plus Environment


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 Silicon-based anode materials exhibit higher specific and volumetric capacities than other materials and have therefore received much attention for potential use in lithium-ion batteries. However, the continuous growth of a solid-electrolyte interphase at the surface of silicon is a primary cause of chronic capacity fading of silicon electrodes. In this paper, we report the formation of an electrochemically stable solid-electrolyte interphase layer on the surfaces of the few-layer graphene-coated silicon nanoparticles. During the first lithiation, electrolyte molecules were electrochemically decomposed and deposited on the surface of few-layer graphene, thus forming a stable protective layer. When combined with an ionic liquid electrolyte based on pyrrolidinium and bis(fluorosulfonyl)imide, an anode containing 75% few-layer graphene-coated silicon delivered a reversible capacity of 1770 mAh g-1 (1430 mAh/ccelectrode) at a current density of 400 mAh g-1 (2 mAh cm-2) after 200 cycles. Averaged over the first 200 cycles, the half-cell exhibits a capacity loss of only 7.2% with a columbic efficiency of 99.4%. The results of our study demonstrate that the few-layer graphene coating may lead to an ideal candidate for the generation of a stable protecting layer for a silicon anode that is otherwise harmed by side reactions with electrolytes during cycling.


Page 2 of 29

Page 3 of 29

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


INTRODUCTION Of the various possible anode materials for lithium-ion batteries (LIBs) currently being explored, Silicon (Si) exhibits the highest gravimetric (4,200 mAh g-1) and volumetric (9,800 mAh cm-3) capacities.1-10 However, achieving stable cycling remains a challenge due to pulverization of Si particles and the continuous growth of a solid-electrolyte interphase (SEI). Pulverization is caused by volume changes of up to 300% during the transition between fully charged and discharged states.11 For a decade, various nanostructured Si materials have been developed to accommodate large volume changes, resulting in significantly improved cycling performance.12-22 While the formation of SEI layers on Si and their effects on electrode performance have been studied by several groups,23-33 development of stable SEI layers remains a challenge. The SEI layer formed during the first lithiation can be damaged during cycling due to the large Si volume change. Electrolytes can then contact the bare Si surface, which leads to continual electrolyte breakdown and eventual cell failure.34,35 Therefore, a robust, elastic SEI layer is needed. The pure Si/graphite composite structure currently used in most practical approaches is unable to solve the aforementioned problems. Conformal coatings and composites with conductive materials such as amorphous carbon14,36-38 are among the most promising approaches for commercialization of Si anodes. Thus, stable SEI layer formation on these conductive coatings is important to the goal of improving Si anode performance. Most conductive coatings fail to deliver stable long-term cycling performance because of the pulverization or sintering of Si materials. We recently reported a few-layer graphene-coated Si nanoparticles (FLG-Si NPs) that exhibit very high volumetric energy densities.39 The silicon carbide-free graphene coating layer can accommodate the volume expansion of Si NPs during the first lithiation via sliding 3 ACS Paragon Plus Environment


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

between adjacent graphene layers. The graphene layers are anchored to the Si surface. The overlapping graphene layers can slide apart during lithiation as the core particles swell to overcome friction between the layers. This process helps to maintain the structural integrity of the Si nanoparticles and thus pulverization is avoided. Moreover, the intercalation-based diffusion of Li ion species through the graphene coating to the Si core is exceedingly efficient and thus the exciting properties of Si nanoparticles for Li ion batteries are not compromised. Indeed, addition of 1 wt% graphene increased the electrode film conductivity to 12.8 cm s-1 (~108 times higher than a bare Si film) via an efficient percolation network. We found individual FLG-Si NPs with clear particle boundaries and graphene coatings remaining on the surfaces of Si NPs even after 200 cycles. Accordingly, the conformal graphene coating allowed us to increase the volumetric energy density of the Si anode significantly even at the commercial loading level (>2 mAh/cm2 of an areal capacity).40 Our initial approach to optimizing SEI layers on FLG-Si NPs was based on knowledge of the structure of graphene, which is simply one atomic layer of graphite. Electrolytes can decompose on the basal plane and edge of graphene, producing a SEI layer with the same composition and structure as graphite. The mechanism of SEI layer formation on graphite and its effect on cell performance have been studied extensively in order to accelerate commercialization of LIBs.41-42 Co-solvent electrolyte systems, which include cyclic and linear carbonates mixed with lithium hexafluorophosphate (LiPF6) salts, are commonly used in commercial LIBs that employ graphite anodes. In particular, ethylene carbonate (EC) and diethyl carbonate (DEC) decompose on graphite edges and can transport only Li ions. Fluoroethylene carbonate (FEC), which is well-known as an additive, was also used to address F- and lithium fluoride (LiF) formation and thus stabilize the surfaces of negative Si electrodes.24-26 However, the columbic efficiencies of half-cells made using carbonate


Page 4 of 29

Page 5 of 29

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


electrolytes were still lower than 99.0%. In addition, the half-cells exhibited continuous capacity loss during cycling. The interface between the carbonate electrolyte and FLG-Si NPs remains unstable. Therefore, further research on the formation and electrochemical stability of SEI layers is needed to improve cell performance. Ionic liquid electrolytes have attracted significant attention as nonvolatile, nonflammable, high voltage electrolytes for use in LIBs, and are expected to serve as possible candidates for use with graphite and Si anodes.43-46 Recently, D. M. Piper et al. reported the formation of a stable SEI layer between ionic liquid electrolytes and cyclized polyacrylronitile-coated Si NPs.45,46 However, little attention has been paid thus far to SEI layers formed between ionic liquid electrolytes and Si NPs coated with conductive materials. In this work, the formation and stability of the ionic liquid electrolyte-based SEI layers that cover FLG-Si NPs are studied. Stable SEI layers can be formed on the surfaces of FLGSi NPs by ionic liquid electrolytes, and these layers can significantly suppress capacity loss that originates from continuous electrolyte decomposition. Irreversible SEI formation during the first lithiation was confirmed via electrochemical analyses, and the compositions and structures of ionic liquid-mediated SEI layers were investigated. With the aid of an ionic liquid electrolyte consisting of the N-methyl-N-propyl pyrrolidinium cation (PYR13+), bis(fluorosulfonyl)imide anion (FSI-), and lithium bis(fluorosulfonyl)imide salt (LiFSI), FLG-Si NPs exhibit reversible cycling with a capacity of 1770 mAh g-1 (1430 mAh ccelectrode1

) after 200 cycles (92.8% retention) and average Coulombic efficiency (CE) of 99.42%. Due

to decomposition of the ionic liquid electrolytes, relatively thick (>20 nm) SEI layers composed of sulfur mediated molecules are formed on the surfaces of FLG-Si NPs. This study contributes to knowledge of stabilizing interfaces between electrolytes and highcapacity, carbon-coated Si anodes.



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

Page 6 of 29

EXPERIMENTAL SECTION Electrode and electrolyte preparation. Pristine Si NPs with 50–100 nm diameters (Nanostructure and Amorphous Materials, Inc.) were used in this study (p-Si NPs). FLG-Si NPs were fabricated via chemical vapor deposition (CVD) of carbon dioxide and methane.39 The lithium-substituted polyacrylic acid (Li-PAA) binder was prepared by adding lithium hydroxide to a solution of polyacrylic acid (PAA, Sigma Aldrich) in water. Slurries consisting of FLG-Si NPs and LiPAA at a 75:25 weight ratio were coated on Cu foil. The LiPF6-EC/DEC electrolyte was prepared by dissolving 1.0 M LiPF6 (Sigma Aldrich) in a mixture of 20 vol% EC (Sigma Aldrich) and 80 vol% DEC (Sigma Aldrich). The LiPF6EC/DMC/FEC and LiFSI-EC/DMC/FEC electrolytes contained FEC (Sigma Aldrich) in an EC/DEC/FEC volume ratio of 2:6:2. A mixture of 75 vol% DEC and 25 vol% FEC was used to prepare the LiFSI-DEC/FEC (1.0 M LiFSI) electrolyte. The ionic liquid electrolytes were 1.0 M LiFSI in PYR13FSI (LiFSI-PYR13FSI), 1.0 M LiPF6 in PYR13FSI (LiPF6-PYR13FSI), and 1.0M LiTFSI in PYR13FSI (LiTFSI-PYR13FSI). Li metal foil was used as reference electrode, and the areal capacities of all samples were set to ~2 mAh cm-2 (~10 μm).

Characterization of the SEI layers. The morphologies of the p-Si and FLG-Si NPs were examined before and after cycling via ultrahigh-resolution field emission scanning electron microscopy (UHR-FE-SEM, Hitachi S-5500) and high-resolution transmission electron microscopy (HR-TEM, FEI Titan Cubed 60-300). XPS spectra were obtained using a Physical

Electronics

Quantum

2000

scanning

ESCA

microprobe

spectrometer.

Photoelectrons were excited with an Al Kα (1486.6 eV) anode operating at a constant power of 100 W (15 kV and 10 mA), with an X-ray spot diameter of 400 µm.


Page 7 of 29

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


Electrochemical Analysis. A TOSCAT 3100U battery test station was used to charge and discharge half-cells using a current of 100 mA g-1 (0.05 C, 1 C = 2000 mA h g-1) between 0.01 and 1.5 V (versus Li/Li+) during precycling, and then at 400 mA g-1 (0.2 C) between 0.05 and 1.5 V (versus Li/Li+) during subsequent cycling. Cyclic voltammetry (CV) was tested using an electrochemical analyzer model Biologic VMP3, Nano Québec using a CR2032 coin cell. The voltage range was 0.01–3.0 V and the scan rate was 0.2 mV s-1.

RESULTS AND DISCUSSION The effect of electrolyte composition on the electrochemical performance of prepared FLG-Si NPs was examined by preparing CR2032 coin half cells. The cycling performances and columbic efficiencies of FLG-Si NPs with various electrolytes are shown in Figure 1 and Table 1. For comparison, in addition to conventional 1.0M LiPF6, EC/DEC (LiPF6-EC/DEC) electrolyte, 1.0M LiPF6, EC/DEC/FEC (LiPF6-EC/DEC/FEC) and 1.0M LiFSI, PYR13FSI (LiFSI-PYR13FSI) were also investigated. In precycling (0.05 C, 1 C = 2000 mA g-1, 0.05~1.5 V vs. Li/Li+), the FLG-Si NPs+LiPF6-EC/DEC, FLG-Si NPs+LiPF6-EC/DEC/FEC, and FLG-Si NPs+LiFSI-PYR13FSI exhibit lithiation/delithiation capacities of 1652/1407, 2670/2216, and 2602/2251 mAh g-1, leading to initial columbic efficiencies (ICE) of 85.1%, 83.0%, and 86.5%, respectively. FLG-Si NPs+LiPF6-EC/DEC exhibits the lowest capacity, as well as significant capacity loss after 20 cycles. This may be caused by a continuous side reaction between the FLG-Si NPs and electrolyte. Both the initial capacitance and the cycling performance of FLG-Si NPs+LiPF6-EC/DEC/FEC are superior to those of the sample without FEC because formation of a LiF layer via side reactions between Li salts and FEC enhance the stability of the Si anode.24-26 This is confirmed by the higher CE exhibited after additional cycles (Figure 1 (b)). FLG-Si NPs+LiFSI-PYR13FSI exhibits more stable cycling 7 ACS Paragon Plus Environment


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

performance than FLG-Si NPs+LiPF6-EC/DEC/FEC. Capacity retention of 92.8% was exhibited after 200 cycles. The improved cycling performance of FLG-Si NPs+LiFSIPYR13FSI can be ascribed to its average stable CE of 99.5%, which is better than that of FLG-Si NPs+LiPF6-EC/DEC/FEC, which did not exceed 99.2% until 100 cycles. The effect of electrolyte composition on the electrochemical stability of the SEI layers was determined via SEM (Figure 2) and TEM (Figure 3). Here, we focus on the first 50 cycles because they represent the period when the difference in CE is most significant. We found that all three electrodes with FLG-Si NPs can maintain their mechanical integrity even after 50 cycles. Therefore, capacity fading in the FLG-Si electrodes originates primarily from degradation of the SEI layer. SEM images of both pristine (Figure 2a) and cycled FLG-Si NPs in LiFSI-PYR13FSI (Figure 2b) show their clear particle boundaries. However, FLG-Si NPs+EC/DEC (Figure 2c) and FLG-Si NPs+LiPF6-EC/DEC/FEC (Figure 2d) include irregularly shaped FLG-Si NPs with overgrown SEI layers. There is more SEI growth on the FLG-Si NPs in the topmost region of the electrode film, which implies a vertical potential gradient. Unlike in other coating approaches, we take advantage of the high mechanical strength of graphene and the ability of the graphene layers to slide apart upon volume expansion (lithiation).39 Moreover, the layered nature of graphene easily allows Li ions to intercalate for the inward (and outward) diffusion of Li to the Si particles during cycling. Thus, this clamping process and easy intercalation in few-layer graphene combine to create an ideal coating for Si in LIB applications. Moreover, the conductive nature of the graphene that coats all of the Si particles allows it to provide a homogenous conduction pathway in the anode material. Before cycling, 2–10 layers of graphene were directly grown via carbon dioxideenhanced CVD at 1000 °C (Figures 3a, b). After precycling, FLG-Si NPs exhibit enhanced 8 ACS Paragon Plus Environment

Page 8 of 29

Page 9 of 29

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


contrast in their centers. Volume shrinkage during the first delithiation is known to be far smaller than volume expansion during the first lithiation, resulting in porous Si with decreased density (Figure 3c, e). In addition, FLG-Si NPs became distorted spheres coated by wrinkled graphene layers (Figure 3d, f). Therefore, The SEI layers formed on the FLG-Si NPs surfaces may be damaged by movement of the graphene layers during Si volume changes. These results also indicate that graphene sliding is not fully reversible, and that the innermost graphene layer is physically adsorbed to the Si surface. Another notable result verified via TEM with energy-dispersive X-ray spectroscopy analysis is the formation of relatively thick (>20 nm) carbonaceous SEI layers on the surface of the FLG-Si NPs+LiFSIPYR13FSI sample (Figure 3g-j). To investigate SEI formation during cycling, CV curves were measured as shown in Figure 4. For all samples, a large reduction peak corresponds to the transformation of crystalline Si (c-Si) to crystalline amorphous lithium silicide (a-LixSi), as shown at 0.01 V during precycling.47 With FLG-Si NPs+LiPF6-EC/DEC, the sharp reduction peak at 0.6 V is related to the decomposition of EC.48-50 The small, high-voltage peaks at 1.4 V can be attributed to DEC or water.50,51 After the first cycle, the reduction peak at approximately 1.4 V (blue arrow) remains distinct and may be evidence of a continuous side reaction at the interface between FLG-Si NPs and the electrolyte. When FEC is added to LiPF6/EC/DEC, the reduction peak from EC (~0.6 V) becomes much smaller, and a strong reduction peak at 1.1 V is measured. This result indicates that FEC molecules decompose primarily on the graphene surface, generating a SEI layer.52,53 However, the reduction peak at approximately 1.5 V remains during subsequent cycles. Neither EC/DEC nor EC/DEC/FEC produces SEI layers that protect the FLG-Si NP surface efficiently. The CV curve of FLG-Si NPs+LiFSIPYR13FSI exhibits distinct reduction peaks at 0.7 V that can be attributed to FSI-



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

decomposition.54 In contrast to the other electrolytes, LiFSI-PYR13FSI produces no distinct reduction peak after the first cycle, indicating formation of a stable SEI layer via a prior reaction involving FSI- on the graphene surface. The compositions of the SEI layers on the FLG-Si NPs surfaces were confirmed via XPS. The XPS data reported in Figure 5 shows the compositions of the SEI layers after precycling. The SEI layers from all three electrodes contain C-O (C1s ~ 286 eV) and C=O (C1s ~ 287.5 eV).41 However, there are differences in the XPS peaks at C1s ~289.0 eV and at F1s ~ 686 eV, which indicate the presence of carbonates (LiCO3R or Li2CO3) and LiF respectively. SEI layers from FLG-Si cycled in EC/DEC contain fewer carbonates but more LiFs (F1s ~ 686 eV) than the other electrodes. SEI layers based on LiF can be produced via the decomposition of LiPF6 salts or FEC and improve battery cycle life.24-26 Therefore, the poor cyclablility of FLG-Si NPs+LiPF6-EC/DEC may be attributed to the absence of carbonates in the SEI layer. For the FLG-Si NPs cycled in LiFSI-PYR13FSI, the presence of sulfur-mediated species such as Li2S, Li2SO4 and SOx45 also suggests decomposition of the FSI- . By contrast, the LiF content of the SEI layers declines sharply. The electrochemical performances of electrodes based on p-Si NPs were tested to investigate interactions between ionic liquid electrolytes and the bare Si surface. To facilitate this comparison, 5 wt% of a conductive auxiliary (Super P) was added to the slurry during preparation of the p-Si NPs anode.39 While the p-Si NPs+LiPF6-EC/DEC/FEC and p-Si NPs+LiFSI-PYR13FSI samples exhibited high initial capacities of 2811 and 2623 mAh g-1, they showed much lower ICEs of 81.14 and 76.66 %. The capacity retention of each sample decreased significantly, even though FEC or an ionic liquid electrolyte was used (Figures 6a, b). In addition, during cycling, the CE values of both samples were below 98% and unstable. Super P nanoparticles could not protect the Si NP surfaces effectively in order to 10 ACS Paragon Plus Environment

Page 10 of 29

Page 11 of 29

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


accommodate the volume expansion and consequent fracture of Si over repeated cycles. Figures 6c and d show both samples of fractured Si NPs with overgrown SEI layers, even after precycling. We further investigated the role of Li salts in SEI formation. When LiPF6 (LiPF6PYR13FSI, 1.0M) was used as a salt in the ionic liquid electrolyte, overall capacity retention was ~60% after 100 cycles, although the CE values were higher than those of LiPF6EC/DEC/FEC (Figure 7a, b). This result indicates that the presence of LiPF6 hinders the formation of a stable SEI from ionic liquid electrolytes. Previous studies revealed that reduction of LiPF6 also contributed to the formation of SEI layers on graphite electrodes.55-57 T. Kawaguchi et al. reported that the reduction potential of LiPF6 is 2.6 V vs Li/Li+.51 PF6can be reduced to PF5, and LiF can form from free F- ions on the electrode surface. We also used LiFSI (1.0 M) as a salt in the carbonate electrolytes (LiFSI-EC/DEC/FEC and LiFSIDEC/FEC), and confirmed that there was no significant improvement in electrochemical performance. In the differential capacity analysis (dQ/dV) data from the LiFSI-EC/DEC/FEC and LiFSI-DEC/FEC sample (Figure 7c), we find that the reduction peak at 0.7 V that corresponds to the decomposition of the FSI- is negligible. DEC or FEC molecules decompose to form unstable SEI layers on the surfaces of FLG-Si NPs at 1.4~1.5 V, and prevent

decomposition

of

FSI-.

FLG-Si

NP

electrodes

with

lithium

bis(trifluoromethanesulfonyl)imide (LiTFSI) salts exhibit very low capacities (~10 mAh g-1, Figure 7d). Previous reports have indicated that TFSI- cannot prevent exfoliation of graphite caused by cation intercalation.58 During the first lithiation, we observed a voltage plateau at around 1.0 V, which can be ascribed to irreversible PYR13+ intercalation between FLG layers. CONCLUSIONS



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

Page 12 of 29

In summary, we successfully carried out a systematic study of SEI layer formation on FLG-Si NPs. We confirmed that more stable SEI layers can form from ionic liquid electrolytes on the surfaces of the FLG-Si NPs, and that these layers can significantly suppress

capacity

loss

originating

from

continuous

electrolyte

decomposition.

Electrochemical analyses indicated that FSI- based ionic liquid molecules can generate thicker, more efficient protecting layers then those that originate from conventional carbonates. After combining FLG-Si NP powders with the LiFSI-PYR13FSI electrolyte, reversible cycling was achieved with a capacity of 1770 mAh g-1 after 200 cycles (92.8% retention) and average columbic efficiency of 99.42%. Due to decomposition of the ionic liquid electrolytes, relatively thick (>20 nm) SEI layers composed of sulfur-mediated molecules were formed on the FLG-Si NP surfaces. We also found that Li salts play a key role in the formation of stable SEI layers.

ASSOCIATED CONTENT

AUTHOR INFORMATION

Corresponding Authors *E-mail: [email protected] *E-mail: [email protected]

Author Contributions #

These authors contributed equally. 12 ACS Paragon Plus Environment

Page 13 of 29

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


Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by funds from Samsung Electronics Co. Ltd. A relevant patent application is in progress by Samsung Electronics Co. Ltd.



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

REFERENCES (1) Etacheri, V.; Marom, R.; Elazari, R.; Salitra, G.; Aurbach, D. Challenges in the development of advanced Li-ion batteries: A review. Energy Environ. Sci. 2011, 4, 32433262. (2) Yoo, H. D.; Markevich, E.; Salitra, G.; Sharon, D.; Aurbach, D. On the challenge of developing advanced technologies for electrochemical energy storage and conversion. Mater. Today 2014, 17, 110−121. (3) Sun, Y.; Liu, N.; Cui, Y. Promises and challenges of nanomaterials for lithium-based rechargeable batteries. Nat. Energy 2016, 1, 16071. (4) Choi, J. W.; Aurbach, D. Promise and reality of post-lithium-ion batteries with high energy densities. Nat. Rev. Mater. 2016, 1, 16013. (5) Thackeray, M. M.; Wolverton, C.; Isaacs, E. D. Electrical energy storage for transportation—approaching the limits of, and going beyond, lithium-ion batteries. Energy Environ. Sci. 2012, 5, 7854–7863. (6) Goodenough, J. B.; Kim, Y. Challenges for rechargeable Li batteries. Chem. Mater. 2010, 22, 587–603. (7) Armand, M.; Tarascon, J.-M. Building better batteries. Nature 2008, 451, 652−657. (8) Choi, N.-S.; Chen, Z.; Freunberger, S. A.; Ji, X.; Sun, Y.-K.; Amine, K.; Yushin, G.; Nazar, L. F.; Cho, J.; Bruce, P. G. Challenges facing lithium batteries and electrical doublelayer capacitors. Angew. Chem., Int. Ed. 2012, 51, 9994−10024. (9) Kang, B.; Ceder, G. Battery materials for ultrafast charging and discharging. Nature 2009, 458, 190–193. (10) M. S. Whittingham, Lithium batteries and cathode Materials. Chem. Rev. 2004, 104, 4271–4301.


Page 14 of 29

Page 15 of 29

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


(11) Wu, H.; Cui, Y. Designing nanostructured Si anodes for high energy lithium ion batteries. Nano Today 2012, 7, 414–429. (12) Wang, C.-M.; Li, X.; Wang, Z.; Xu, W.; Liu, J.; Gao, F.; Kovarik, L.; Zhang, J.-G.; Howe, J.; Burton, D. J.; et al. In situ TEM investigation of congruent phase transition and structural evolution of nanostructured silicon/carbon anode for lithium ion batteries. Nano Lett. 2012, 12, 1624–1632. (13) Yao, Y.; McDowell, M. T.; Ryu, I.; Wu, H.; Liu, N.; Hu, L.; Nix, W. D.; Cui, Y. Interconnected silicon hollow nanospheres for lithium-ion battery anodes with long cycle life. Nano Lett. 2011, 11, 2949–2954. (14) Liu, N.; Lu, Z.; Zhao, J.; McDowell, M. T.; Lee, H. W.; Zhao, W.; Cui, Y. A pomegranate-inspired nanoscale design for large-volume-change lithium battery anodes. Nat. Nanotech. 2014, 9, 187–192. (15) Park, M. H.; Kim, M. G.; Joo, J.; Kim, K.; Kim, J.; Ahn, S.; Cui, Y.; Cho, J. Silicon nanotube battery anodes. Nano Lett. 2009, 9, 3844–3847. (16) Zhou, X.; Yin, Y.-X.; Wan, L.-J.; Guo, Y.-G. Facile synthesis of silicon nanoparticles inserted into graphene sheets as improved anode materials for lithium-ion batteries. Chem. Comm. 2012, 48, 2198–2200. (17) Wu, H.; Chan, G.; Choi, J. W.; Ryu, I.; Yao, Y.; McDowell, M. T.; Lee, S. W.; Jackson, A.; Yang, Y.; Hu, L.; et al. Stable cycling of double-walled silicon nanotube battery anodes through solid–electrolyte interphase control. Nat. Nanotech. 2012, 7, 310–315. (18) Magasinski, A.; Dixon, P.; Hertzberg, B.; Kvit, A.; Ayala, J.; Yushin, G. Highperformance lithium-ion anodes using a hierarchical bottom-up approach. Nat. Mater. 2010, 9, 353–358.



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

(19) Westover, A. S.; Freudiger, D.; Gani, Z. S.; Share, K.; Oakes, L.; Cartera, R. E.; Pint, C. L. On-chip high power porous silicon lithium ion batteries with stable capacity over 10,000 cycles. Nanoscale 2015, 7, 98–103. (20) Zhang, R.; Du, Y.; Li, D.; Shen, D.; Yang, J.; Guo, Z.; Liu, H. K.; Elzatahry, A. A.; Zhao, D. Highly reversible and large lithium storage in mesoporous Si/C nanocomposite anodes with silicon nanoparticles embedded in a carbon framework. Adv. Mater. 2014, 26, 6749–6755. (21) Chen, S.; Bao, P.; Huang, X.; Sun, B.; Wang, G. Hierarchical 3D mesoporous silicon@graphene nanoarchitectures for lithium ion batteries with superior performance. Nano Res. 2014, 7, 85–94. (22) Xie, J.; Wang, G.; Huo, Y.; Zhang, S.; Cao, G.; Zhao, X. Nanostructured silicon spheres prepared by a controllable magnesiothermic reduction as anode for lithium ion batteries. Electrochim. Acta. 2014, 135, 94–100. (23) Cho, J. H.; Picraux, S. T. Silicon nanowire degradation and stabilization during lithium cycling by SEI layer formation. Nano Lett. 2014, 14, 3088-3095. (24) Xu, C.; Lindgren, F.; Philippe, B.; Gorgoi, M.; Björefors, F.; Edström, K.; Gustafsson, T. Improved performance of the silicon anode for Li-ion batteries: Understanding the surface modification mechanism of fluoroethylene carbonate as an effective electrolyte additive. Chem. Mater. 2015, 27, 2591−2599. (25) Etacheri, V.; Haik, O.; Goffer, Y.; Roberts, G. A.; Stefan, I. C.; Fasching, R.; Aurbach, D. Effect of fluoroethylene carbonate (FEC) on the performance and surface chemistry of Sinanowire Li-ion battery anodes. Langmuir 2012, 28, 965–976. (26) Bordes, A.; Eom, K.-S.; Fuller, T. F. The effect of fluoroethylene carbonate additive content on the formation of the solid-electrolyte interphase and capacity fade of Li-ion fullcell employing nano Si–graphene composite anodes. J. Power Sources 2014, 257, 163-169. 16 ACS Paragon Plus Environment

Page 16 of 29

Page 17 of 29

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


(27) Arreaga-Salas, D. E.; Sra, A. K.; Roodenko, K.; Chabal, Y. J.; Hinkle, C. L. Progression of solid electrolyte interphase formation on hydrogenated amorphous silicon anodes for lithium-ion batteries. J. Phys. Chem. C 2012, 116, 9072−9077. (28) Ruffo, R.; Hong, S. S.; Chan, C. K.; Huggins, R. A.; Cui, Y. Impedance analysis of silicon nanowire lithium ion battery anodes. J. Phys. Chem. C 2009, 113, 11390–11398. (29) Yen, Y.-C.; Chao, S.-C.; Wu, H.-C.; Wu, N.-L. Study on solid-electrolyte-interphase of Si and C-coated Si electrodes in lithium cells. J. Electrochem. Soc. 2009, 156, A95-A102. (30) Liu, Y.; Guo, X.; Li, J.; Lv, Q.; Ma, T.; Zhu, W.; Qiu, X. Improving coulombic efficiency by confinement of solid electrolyte interphase film in pores of silicon/carbon composite. J. Mater. Chem. A, 2013, 1, 14075–14079. (31) Van Havenbergh, K.; Turner, S.; Driesen, K.; Bridel, J.-S.; Van Tendeloo, G. Solid– electrolyte interphase evolution of carbon-coated silicon nanoparticles for lithium-ion batteries monitored by transmission electron microscopy and impedance spectroscopy. Energy Technol. 2015, 3, 699–708. (32) Li, B.; Yao, F.; Bae, J. J.; Chang, J.; Zamfir, M. R.; Le, D. T.; Pham, D. T.; Yue, H.; Lee, Y. H. Hollow carbon nanospheres/silicon/alumina core-shell film as an anode for lithium-ion batteries. Sci. Rep. 2015, 5, 7659. (33) Zheng, J.; Zheng, H.; Wang, R.; Ben, L.; Lu, W.; Chen, L.; Chena, L.; Li, H. 3D visualization of inhomogeneous multi-layered structure and Young's modulus of the solid electrolyte interphase (SEI) on silicon anodes for lithium ion batteries. Phys. Chem. Chem. Phys. 2014, 16, 13229–13238. (34) Ryu, J. H.; Kim, J. W.; Sung, Y.-E.; Oh, S. M. Failure modes of silicon powder negative electrode in lithium secondary batteries. Electrochem. Solid-State Lett. 2004, 7, A306-A309.



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

(35) Radvanyi, E.; Porcher, W.; Vito, E. D.; Montani, A.; Franger, S.; Larbi, S. J. S. Failure mechanisms of nano-silicon anodes upon cycling: an electrode porosity evolution model. Phys. Chem. Chem. Phys. 2014, 16, 17142-17153. (36) Hu, Y. S.; Demir-Cakan, R.; Titirici, M. M.; Müller, J. O.; Schlögl, R.; Antonietti, M.; Maier J., Superior storage performance of a Si@SiOx nanocomposite as anode material for lithium-ion batteries. Angew. Chem. Int. Ed. 2008, 47, 1645–1649. (37) Gu, M.; Li, Y.; Li, X.; Hu, S.; Zhang, X.; Xu, W.; Thevuthasan, S.; Baer, D. R.; Zhang, J. G.; Liu, J.; et al. In situ TEM study of lithiation behavior of silicon nanoparticles attached to and embedded in a carbon matrix. ACS Nano 2012, 6, 8439–8447. (38) Ng, S. H.; Wang, J.; Wexler, D.; Konstantinov, K.; Guo, Z. P.; Liu, H. K. Highly reversible lithium storage in spheroidal carbon-coated silicon nanocomposites as anodes for lithium-ion batteries. Angew. Chem. Int. Ed. 2006, 45, 6896–6899. (39) Son, I. H.; Park, J. H.; Kwon, S.; Park, S.; Rümmeli, M. H.; Bachmatiuk, A.; Song, H. J.; Ku, J.; Choi, J. W.; Choi, J.-M.; et al. Silicon carbide-free graphene growth on silicon for lithium-ion battery with high volumetric energy density. Nat. Commun. 2015, 6, 7393. (40) Leveau, L.; Laïk, B.; Pereira-Ramos, J.-P.; Gohier, A.; Tran-Van, P.; Cojocaru, C.-S. Silicon nano-trees as high areal capacity anodes for lithium-ion batteries. J. Power Sources 2016, 316, 1-7. (41) K. Xu, Nonaqueous liquid electrolytes for lithium-based rechargeable batteries. Chem. Rev. 2004, 104, 4303–4417. (42) Verma, P.; Maire, P.; Novák, P. A review of the features and analyses of the solid electrolyte interphase in Li-ion batteries. Electrochim. Acta 2010, 55, 6332–6341. (43) Baranchugov, V.; Markevich, E.; Pollak, E.; Salitra, G.; Aurbach. D. Amorphous silicon thin films as a high capacity anodes for Li-ion batteries in ionic liquid electrolytes. Electrochem. Commun. 2007, 9, 796–800. 18 ACS Paragon Plus Environment

Page 18 of 29

Page 19 of 29

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


(44) Nguyen, C. C.; Song, S.-W. Characterization of SEI layer formed on high performance Si–Cu anode in ionic liquid battery electrolyte. Electrochem. Commun. 2010, 12, 1593–1595. (45) Piper, D. M.; Evans, T.; Leung, K.; Watkins, T.; Olson, J.; Kim, S. C.; Han, S. S.; Bhat, V.; Oh, K. H.; Buttry, D. A.; et al. Stable silicon-ionic liquid interface for next-generation lithium-ion batteries. Nat. Commun. 2015, 6, 6230. (46) Piper, D. M.; Evans, T.; Xu, S.; Kim, S. C.; Han, S. S.; Liu, K. L.; Oh, K. H.; Yang, R.; Lee, S.-H. Optimized silicon electrode architecture, interface, and microgeometry for nextgeneration lithium-ion batteries. Adv. Mater. 2016, 28, 188–193. (47) Schroder, K. W.; Celio, H.; Webb, L. J.; Stevenson, K. J. Examining solid electrolyte interphase formation on crystalline silicon electrodes: influence of electrochemical preparation and ambient exposure conditions. J. Phys. Chem. C 2012, 116, 19737−19747. (48) Gnanaraj, J. S.; Thompson, R. W.; Di Carlo, J. F.; Abraham, K. M. The role of carbonate solvents on lithium intercalation into graphite. J. Electrochem. Soc. 2007, 154, A185–A191. (49) Jeong, S.-K.; Inaba, M.; Abe, T.; Ogumi, Z. Surface film formation on graphite negative electrode in lithium-ion batteries: AFM study in an ethylene carbonate-based solution. J. Electrochem. Soc. 2001, 148, A989–A993. (50) Striebel, K. A.; Sierra, A.; Shima, J.; Wang, C.-W.; Sastry, A. M. The effect of compression on natural graphite anode performance and matrix conductivity. J. Power Sources 2004, 134, 241–251. (51) Kawaguchi, T.; Shimada, K.; Ichitsubo, T.; Yagi, S.; Matsubara, E. Surface-layer formation by reductive decomposition of LiPF6 at relatively high potentials on negative electrodes in lithium ion batteries and its suppression. J. Power Sources 2014, 271, 431–436. (52) Jeong, S.-K.; Inaba, M.; Mogi, R.; Iriyama, Y.; Abe, T.; Ogumi, Z. Surface film formation on a graphite negative electrode in lithium-ion batteries: Atomic force microscopy



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

study on the effects of film-forming additives in propylene carbonate solutions. Langmuir 2001, 17, 8281–8286. (53) Zhao, X.; Zhuang, Q.-C.; Xu, S.-D.; Xu, Y.-X.; Shi, Y.-L.; Zhang, X.-X. A new insight into the content effect of fluoroethylene carbonate as a film forming additive for lithium-ion batteries. Int. J. Electrochem. Sci. 2015, 10, 2515–2534. (54) Hu, X.; Chen, C.; Yan, J.; Mao, B. Electrochemical and in-situ scanning tunneling microscopy studies of bis(fluorosulfonyl)imide and bis(trifluoromethanesulfonyl)imide based ionic liquids on graphite and gold electrodes and lithium salt influence. J. Power Sources 2015, 293, 187–195. (55) Aurbach, D.; Markovsky, B.; Shechter, A.; Ein-E1i, Y.; Cohen, H. A comparative study of synthetic graphite and Li electrodes in electrolyte solutions based on ethylene carbonate‐dimethyl carbonate mixtures. J. Electrochem. Soc. 1996, 143, 3809–3820. (56) Tasaki, K.; Goldberg, A.; Lian, J.-J.; Walker, M.; Timmons, A.; Harris, S. J. Solubility of lithium salts formed on the lithium-ion battery negative electrode surface in organic solvents. J. Electrochem. Soc. 2009, 156, A1019–A1027. (57) Ein-Eli, Y.; Markovsky, B.; Aurbach, D.; Carmeli, Y.; Yamin, H.; Luski, S. The dependence of the performance of Li-C intercalation anodes for Li-ion secondary batteries on the electrolyte solution composition. Electrochimica Acta 1994, 39, 2559–2569. (58) Ishikawa, M.; Sugimoto, T.; Kikuta, M.; Ishiko, E.; Kono, M. Pure ionic liquid electrolytes compatible with a graphitized carbon negative electrode in rechargeable lithiumion batteries. J. Power Sources 2016, 162, 658–662.


Page 20 of 29

Page 21 of 29

Figures and Tables

(b) 100 Coulombic Efficiency (%)

(a) 2,500 Specific Capacity (mAh/g)

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


2,000 1,500 LiPF6-EC/DEC

1,000

LiPF6-EC/DEC/FEC LiFSI-PYR13FSI

500 0

0

50

100

150

200

99 98 97 96 LiPF6-EC/DEC

95

LiPF6-EC/DEC/FEC

94 93

LiFSI-PYR13FSI

0

50

Cycle Number

100 150 Cycle Number

200

Figure 1. (a) The delithiation capacities and (b) coulombic efficiencies of FLG-Si NPs+LiPF6-EC/DEC, FLG-Si NPs+LiPF6-EC/DEC/FEC, and FLG-Si NPs+LiFSI-PYR13FSI.



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

(a)

(b)

(c)

(d)

Figure 2. Top-view SEM images: (a) as-prepared FLG-Si NPs, (b) FLG-Si NPs+LiFSIPYR13FSI, (c) FLG-Si NPs+LiPF6-EC/DEC, and (d) FLG-Si NPs+LiPF6-EC/DEC/FEC electrodes after 50 cycles. Scale bars: 1 µm for (a-d).


Page 22 of 29

Page 23 of 29

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


(a)

(b)

(c)

(d)

(g)

Si (24-28 eV)

(h)

(e)

(f)

(i)

C (279-315 eV)

( j)

O (528-557 eV)

Si O C

Figure 3. TEM images of (a-b) pristine FLG-Si NPs. (c, e) FLG-Si NPs+LiPF6EC/DEC/FEC and (d, f) FLG-Si NPs+LiFSI-PYR13FSI after precycling. (g) Si, (h) O, (i) C, and (j) combined TEM-EDX elemental mapping of precycled FLG-Si NPs in LiFSIPYR13FSI (red box in (d)). Scale bars: 50 nm for (a), 10 nm for (b), 500 nm for (c-d), and 100 nm for (e-j).



(a) 0.00

Current (mA)

-0.01

c-Si a-Si

DEC water

-0.02

LiPF6-EC/DEC st

-0.03 -0.04 -0.05

1 nd 2 rd 3 th 4

EC

0

1 2 3 + Potential (V vs. Li/Li )

(b) 0.00

Current (mA)

-0.01

DEC water

-0.02

LiPF6-EC/DEC/FEC st

-0.03 EC

-0.04 FEC

-0.05

0

1 nd 2 rd 3 th 4


(c) 0.00 -0.01 Current (mA)

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

-0.02

LiFSI-PYR13FSI st

-0.03

1 nd 2 rd 3 th 4

FSI

-0.04 -0.05

0


Figure 4. Cyclic voltammograms of FLG-Si NPs electrodes in (a) LiPF6-EC/DEC, (b) LiPF6EC/DEC/FEC, and (c) LiFSI-PYR13FSI. 24 ACS Paragon Plus Environment

Page 24 of 29

Page 25 of 29

(a)

LiPF6-EC/DEC

C-C C=C

(b)

C1s

FLG-Si NPs

C-O C=O

LiFSI-PYR13FSI FLG-Si NPs

Si3N

NSO2

(d)

690

685

LiPF6-EC/DEC

680

S2p

LiPF6-EC/DEC/FEC

Intenstiy (a.u.)

N1s

LiPF6-EC/DEC/FEC

FLG-Si NPs

Binding Energy (eV)

Binding Energy (eV) LiPF6-EC/DEC

LiF

LiFSI-PYR13FSI

695

292 290 288 286 284 282 280

(c)

F1s

LiPF6-EC/DEC/FEC

LiFSI-PYR13FSI

O-C=O

LiPF6-EC/DEC

Intenstiy (a.u.)

Intenstiy (a.u.)

LiPF6-EC/DEC/FEC

Intenstiy (a.u.)

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


404 402 400 398 396 394 392

LiFSI-PYR13FSI FLG-Si NPs

Li2SO4 SOx

170

Binding Energy (eV)

165

Li2S

160

155

Binding Energy (eV)

Figure 5. XPS spectra of (a) C1s, (b) F1s, (c) N1s, and (d) S2p bands from FLG-Si NPs, FLG-Si NPs+LiPF6-EC/DEC, FLG-Si NPs+LiPF6-EC/DEC/FEC, and FLG-Si NPs+LiFSI-PYR13FSI samples.



(b)100.0

3,000

p-Si NPs+LiPF6-EC/DEC/FEC p-Si NPs+LiFSI-PYR13FSI

2,500

Coulombic Efficiency (%)

(a) Specific Capacity (mAh/g)

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

2,000 1,500 1,000 500 0

Page 26 of 29

0

(c)

50 100 150 Cycle Number

99.5 99.0 98.5 98.0

97.0

200

p-Si NPs+LiPF6-EC/DEC/FEC p-Si NPs+LiFSI-PYR13FSI

97.5 0

50 100 150 Cycle Number

200

(d)

Figure 6. (a) Delithiation capacity retentions and (b) columbic efficiencies of p-Si electrodes cycled in LiPF6-EC/DEC/FEC and LiFSI-PYR13FSI. Top-view SEM images: p-Si NP electrodes precycled in (c) LiPF6-EC/DEC/FEC and (d) LiFSI-PYR13FSI. Scale bars: 1 µm for (c) and (d).


Page 27 of 29

(b) 100.0

(a)

99.5 Coulombic Efficiency (%)

2

Specific Capacity (mAh/cm )

2,200 2,000 1,800 1,600 LiFSI-PYR13FSI LiPF6-PYR13FSI

1,400 1,200

LiFSI-EC/DEC/FEC LiFSI-DEC/FEC

0

(c) 0.00

20

40 60 Cycle Number

80

100

99.0 98.5 98.0 97.5 LiFSI-PYR13FSI

97.0

LiPF6-PYR13FSI

96.5 96.0

LiFSI-EC/DEC/FEC LiFSI-DEC/FEC

0

(d)

20

40 60 Cycle Number

80

100

+

Potential (V vs Li/Li )

3.0 -0.05

2

dQ/dV (mAh/cm V)

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


-0.10

-0.15

LiPF6-PYR13FSI LiFSI-DEC/FEC

-0.20 0.0

0.5 1.0 1.5 2.0 2.5 + Potential (V, vs Li/Li )

2.5 2.0 1.5 1.0 0.5 0.0

3.0

LiTFSI-PYR13FSI

0

10 20 30 40 Specific Capacity (mAh/g)

Figure 7. (a) Delithiation capacity retentions and (b) columbic efficiencies of FLG-Si electrodes cycled in LiFSI-EC/DEC/FEC, LiPF6-PYR13FSI, and LiTFSI-PYR13TFSI. (c) dQ/dV plots of lithium cells with FLG-Si NPs electrodes precycled in LiPF6-PYR13FSI and LiFSI-DEC/FEC. (d) Voltage profiles from FLG-Si electrodes precycled in LiTFSIPYR13TFSI.



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

Page 28 of 29

Table 1. Summary of the electrochemical performances of FLG-Si NPs with various electrolytes. Initial Lithiation

Average

Initial Cycle Retention

/Delithiation

Coulombic

Coulombic

Initial Capacity after 200th Cycle

Samples Capacitya)

Efficiency

(mAh g-1)b)

Efficiencyc) (%)

-1

(mAh g )

(%)

(%)

1652/1407

85.17

816

18.6

97.66

2670/2216

83.00

2043

72.1

99.08

2602/2251

86.51

1909

92.8

99.42

LiPF6EC/DEC LiPF6EC/DEC /FEC LiFSIPYR13F SI a) 0.05C = 100 mA g-1 b) 0.2C = 400 mA g-1 c) 1~200 cycles


Page 29 of 29

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


TOC GRAPHIC


Formation of Stable SolidâElectrolyte Interphase Layer on Few-Layer

Recommend Documents

Formation of Stable SolidâElectrolyte Interphase Layer on Few-Layer