High-Capacity Retention of Si Anodes Using a Mixed Lithium

High-Capacity Retention of Si Anodes Using a Mixed Lithium/Phosphonium Bis(fluorosulfonyl)imide Ionic Liquid Electrolyte Robert Kerr,*,† Driss Mazouzi,‡,§ Mojtaba Eftekharnia,† Bernard Lestriez,‡ Nicolas Dupré,‡ Maria Forsyth,† Dominique Guyomard,‡ and Patrick C. Howlett† †

Institute for Frontier Materials (IFM), Deakin University, 221 Burwood Highway, Burwood, Victoria 3125, Australia Institut des Matériaux Jean Rouxel (IMN), CNRS UMR 6502, Université de Nantes, 2 rue de la Houssinière, B.P. 32229, 44322 Nantes Cedex 3, France § Materials, Natural Substances, Environment and Modeling Laboratory, Multidisciplinary Faculty of Taza, University of Sidi Mohamed Ben Abdellah-Fes, Fes 30000, Morocco ‡

ABSTRACT: The commercialization of high-capacity Si electrodes for lithium batteries has stalled due to the inability to overcome the mechanical degradation and electrolyte consumption that occur as a result of the inherent volume expansion upon charging. Using an ionic liquid (IL) electrolyte, trimethylisobutylphosphonium bis(fluorosulfonyl)imide (P1,1,1,i4FSI) containing a high lithium bis(fluorosulfonyl)imide (LiFSI) salt content of 3.2 mol per kg of IL (50 mol %), inexpensive and high-capacity Si electrodes made from a facile and ball-milling process demonstrated outstanding capacity retention of around 3.5 mAh/cm2 after 300 cycles when cycled at current densities of ∼1500 mA/g (C/2.5) at room temperature. Moreover, highcapacity retention was maintained for 60 cycles at elevated temperatures up to 80 °C, where traditional electrolytes are unable to operate. SEM images suggest that the use of this highly concentrated IL electrolyte promotes the formation of a stable surface layer that accommodates the volume expansion of the Si electrode. This benchmark result suggests that tailoring of the electrolyte for advantageous solid−electrolyte interphase properties is a very promising route of premium interest.

S

bis(fluorosulfonyl)imide (FSI) anion having been recently shown to promote a more stable surface for Si electrode cycling compared to bis(trifluoromethansulfonyl)imide (TFSI) or tetrafluoroborate in propylene carbonate solvent.9 Ionic liquids (ILs) have been considered for battery applications as they possess many favorable properties such as low volatility and good electrochemical stability. In the field of lithium batteries, ILs composed of fluorinated sulfonylimide anions such as TFSI or FSI combined with alkylated pyrrolidinium (e.g., N-propyl-N-methylpyrrolidinium, C3mpyr) cations have recently received interest due to their improved conductivity and high cathodic stability.10−13 However, IL electrolytes such as these still suffer from low ionic conductivity owing to their relatively high viscosity at

ilicon is a promising candidate to replace graphite anodes in lithium battery technologies owing to its high theoretical gravimetric and volumetric capacities (3579 mAh/g and 8339 mAh/cm3, respectively) while bypassing the issue of lithium metal dendrite formation. However, accompanying the high storage capacity is a large volume expansion (280%),1 leading to mechanical pulverisation of the electrode and continual electrolyte consumption at the re-exposed silicon surface with each successive cycle.2 The formation of solid− electrolyte interface (SEI) layers in situ is an effective approach in mitigating these effects and has been demonstrated through the addition of vinylidene carbonate3 (VC) and fluoroethylene carbonate4,5 (FEC) to carbonate-based electrolytes such as 1 M LiPF6 in EC/(DMC or DEC) 1:1 v/v. It is proposed that these additives form a flexible and passivating SEI upon electropolymerization of VC at the silicon particle surface.6−8 The choice of ionic species in the electrolyte can also provide a direct means of controlling the surface properties, with the © 2017 American Chemical Society

Received: May 11, 2017 Accepted: July 12, 2017 Published: July 12, 2017 1804

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http://pubs.acs.org/journal/aelccp

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Figure 1. Specific capacity of lithiation (red) and delithiation (blue) during cycling (left) and rate capability (right) at 25 °C of a Si/graphene working electrode in a Li|Si half-cell using (a) 3.2 M LiFSI in P111i4FSI IL electrolyte, (b) LP30 electrolyte, and (c) LP30 + 10 wt % FEC electrolyte.

several promising results having been recently reported.21,22 Recent work by Piper demonstrated the cycling of optimized cyclized polyacrylonitrile-coated 1D silicon nanowires, achieving a half-cell capacity of around 2000 mAh/g after 250 cycles and 1500 mAh/g after 700 cycles for low mass loadings of 0.6 mg/cm2 and at slow cycling rates of C/5 (50 mol %) of lithium salt. This emerging class of mixed salt/IL electrolytes compensates for their lower bulk conductivity on two fronts; a change in the lithium transport mechanism at high salt content leads to a higher Li+ transport number, while the lithium metal SEI properties are vastly superior to that of the IL with low salt content.16,18−20 Li|Si half-cell cycling has been reported for a number of IL electrolytes using a variety of Si electrode morphologies, with 1805

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ACS Energy Letters 1.5 mg/cm2) are achieved here for the first time, representing a major step toward Si electrode development. Figure 1 displays the long-term cycling (C/2.5) and rate capability measurements conducted at room temperature for a Li|Si half-cell assembled using either the IL electrolyte (P1,1,1,i4FSI with 50 mol % LiFSI), a carbonate electrolyte (LP30), or an optimized carbonate electrolyte (LP30 + 10 wt % FEC) that has been previously shown to improve the cycle stability of the electrodes used here.26,27,26 The cycling performances in the IL electrolyte (Figure 1a) are remarkable for two reasons; first, the electrode capacity unexpectedly drops to 1600 mAh/g before reaching more than twice that value over the first 200 cycles. We tentatively assigned this behavior to poor electrode wetting during room-temperature cycling, supported by the typical cycling behavior observed at elevated temperatures in Figure 2 where the IL is more fluid. We do note that all cells were allowed to rest at 50 °C for 24 h prior to cycling and thus suspect that this delayed wetting is also related to the progressive SEI formation and volume expansion that occur during the initial cycles. Second, an outstanding storage capacity of around 3000 mAh/g is retained after 300 cycles. It

should also be noted that in each case the cells were stopped after 300 cycles for ex situ analysis, with these detailed compositional analyses to be published in a follow-up study. As far as we are aware, the capacity retention achieved here is the highest capacity after 300 full charge−discharge cycles for any Si-based electrode. Furthermore, the C rate of C/2.5 with a Si loading greater than 1 mg/cm2 corresponds to areal current densities of >1.5 mA/cm2, approaching the levels required for practical use.28 This anode + electrolyte system has thereby taken a leap toward satisfying a number of criteria for commercial use, namely, a cheap, high-capacity anode material produced via a facile and scalable method that demonstrates sufficient rate capability and promising long-term stability at high areal capacities (∼3.5 mAh/cm2). For the sake of comparison, the long-term cycling of Si/graphene in LP30 (1 M LiPF6) and LP30 + 10 wt % FEC (0.92 M LiPF6) in Figure 1b,c, respectively, displays the typical capacity fade that is associated with a combination of advanced mechanical degradation of the Si electrode and rapid passivation of the Li metal electrode in carbonate-based electrolytes.29 While the individual contributions of each electrode were not investigated further here, it is generally accepted that the consumption of Li+ ions during continual SEI formation invariably results in a progressive capacity fade of both Li metal and Si electrodes in such electrolytes. While this is the case for carbonate-based electrolytes, it is expected that Li metal passivation will not contribute to Si|Li capacity fade when cycled in the IL electrolyte, in line with previous results showing stable Li metal cycling.18 It is noted that the IL electrolyte does have a greater concentration of Li+ ions, a fact that one could suppose would alone result in a longer cycling lifetime. However, the initial postmortem electrode studies presented here indicate that superior SEI properties are instead the dominant contributing factor toward the drastically improved Si cycling performance,as was seen previously for Li metal electrodes using this IL electrolyte.18 Comparison of the rate capability tests in Figure 1 would indicate significantly inferior high C rate performance of the IL electrolyte with a capacity of around 1100 mAh/g at 1C versus nearly 2500 mAh/g for the LP30 + 10 wt % FEC electrolyte. Given that the capacity for the IL electrolyte nearly doubles after 300 cycles, the capacity at 1C would presumably also increase in a similar fashion, to give around 2000 mAh/g, a value only slightly inferior to that of the optimized carbonate electrolyte. Unfortunately, this has not yet been confirmed as the rate capability was not retested after full capacity was achieved. In addition to the aspect of improved room-temperature device safety, one of the most attractive features of using an IL electrolyte is the ability to operate at elevated temperatures due to increased thermal stability of the electrolyte. As an added advantage, the rate capability of IL electrolytes tends to increase with temperature due to the lower viscosity and increased conductivity. Figure 2a shows the rate capability test of the IL electrolyte at 50 °C, delivering a capacity of 3000 mAh/g at C/ 2.5. Such a capacity is much higher than that of the carbonate electrolytes at the same temperature (Figure 2b). Figure 2b also compares the curves of discharge capacities versus cycle number for the Si electrode cycled in IL, LP30, and LP30 + FEC at C/2.5 for various temperatures. As can be observed over 100 cycles, the cycling behavior of the silicon is clearly better at 50 °C for the phosphonium-based IL electrolyte, while there is drastic capacity loss during the first cycles for LP30

Figure 2. (a) Lithiation (blue) and delithiation (red) capacity during cycling of a Si/graphene working electrode in a Li|Si halfcell using 3.2 M LiFSI in P111i4FSI IL electrolyte at 50 °C and (b) long-term cycling at C/2.5 for the various electrolytes at elevated temperatures: ■ = IL at 50 °C, □ = IL at 80 °C, ● = LP30 at 50 °C), and ▲ = LP30 + FEC at 50 °C. 1806

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Figure 3. FTIR spectra of (a) 3.2 M LiFSI in P111i4FSI IL electrolyte, (b) LP30 electrolyte, and (c) LP30 + 10 wt % FEC electrolyte after cycling 100 cycles in a Li|Si half-cell at different temperatures (black = 25 °C, blue = 50 °C, red = 80 °C). A zoomed-in view of the critical domain of wave numbers is shown on the right.

Figure 4. Surface and cross-sectional SEM images of the Si/graphene electrodes (a) before cycling, (b) after cycling in IL electrolyte, (c) after cycling in LP30 + FEC, and (d) after cycling in LP30. Cells were cycled at room temperature for five cycles at C/20, and images were taken after the fifth delithiation.

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graphene sheets as a conductive additive (GM15, length ≈ 15 μm, width ≈ 5−10 nm, SBET = 74 m2/g, XGSciences) in an 80:8:12 weight ratio. pH 3 buffer solution was added to give a 30 wt % solids slurry, which was mixed further and used to cast onto electrode foils, as described in ref 26. Electrode loadings were between 1.0 and 1.5 mg/cm2, with all loadings expressed for the active Si only. Electrochemical Measurements. Swagelok and 2032 coin cell Li| Si half-cells were assembled using the dried Si/graphene composite electrodes (0.785 cm2 disc). A borosilicate glass-fiber separator (Whatman GF/D) was used and 0.6 mL of one of three electrolytes added; an IL electrolyte composed of trimethylisobutylphosphonium bis(fluorosulfonyl)imide (P111i4FSI, Cytec Industries) with 50 mol % LiFSI (3.2 mol of LiFSI added per kg of IL, Coorstek) or 1 M LiPF6 in ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1) either with or without 10 wt % fluoroethylene carbonate (note: addition of 10 wt % FEC dilutes LiPF6 concentration to 0.92 M) was used. Cells containing IL electrolyte were allowed to rest for 24 h at 50 °C after assembly and prior to cycling. Lithium foil (99.9%, Sigma) was used as the counter electrode. The cycling tests were performed in galvanostatic mode with voltage cutoffs of 1 and 0.005 V using a VMP3 multichannel electrochemical workstation (Bio-Logic). Rate capability testing was performed at various discharge rates while holding the charge rate constant at C/5. Long-term cycling was performed at a discharge−charge rate of C/2.5. Fourier-transform infrared (FTIR) spectroscopy spectra were recorded using a Vertex 70 Bruker Spectrometer operating in ATR mode inside of a glovebox with an inert argon atmosphere. For each sample, the spectra were recorded for 1000 scans with a spectral resolution of 4 cm−1. Scanning electron microscopy imaging was performed using a JEOL JSM 7600F microscope. Samples were preserved as much as possible from contacting the ambient atmosphere. We estimate that the total exposure time lasted less than 1 min.

electrolyte and a stronger capacity fade for LP30 + FEC. Moreover, Si electrodes cycled in an IL at 80 °C demonstrate both quite attractive and similar specific capacity and capacity retention compared to cycling at 50 °C, whereas LP30 and LP30 + FEC electrolytes decompose and thus cannot function at 80 °C. Kim et al. recently showed that FEC facilitates a more rapid HF-forming degradation of the PF6− anion at elevated temperatures, which could explain the deleterious effect of FEC addition to LP30 at 50 °C.29 The Si anode cycled in the IL electrolyte thus demonstrates clearly improved electrochemical performance at elevated temperatures, with higher capacity than those achieved using carbonate-based LP30 and LP30 + FEC electrolytes. FTIR spectra of electrolytes were recorded postcycling (100 cycles) in order to determine whether any breakdown of the IL electrolyte occurs as a result of cycling at elevated temperatures. Figure 3a shows spectra recorded for the IL electrolyte in cells cycled at 25, 50, and 80 °C, with no difference observed at the three temperatures. In contrast, the LP30 and the LP30 + FEC electrolytes both indicate significant changes (in the boxed regions) when cycled at elevated temperatures, consistent with the observed faster capacity fade of Li|Si half-cells. While the influence of the Si and Li surfaces on the Li|Si half-cell behavior at elevated temperatures is an area of interest (and should be considered), further studies into the precise failure mechanisms lie outside of the scope of this work. Comparison of the Si electrode surfaces after cycling in LP30 and LP30 + FEC (Figure 4c,d) reveals that a thick surface deposit is formed, as has been demonstrated in previous works.5 This is consistent with the corresponding high cumulative irreversible capacities of 2090 and 1850 mAh/g, respectively. These high values indicate that a significant amount of charge is being consumed in side-reactions such as electrolyte degradation at the electrode surface. In contrast, the surface of the electrode cycled in IL electrolyte appears to be much less altered, while the corresponding electrochemical behavior exhibits a much lower cumulative irreversible capacity of 790 mAh/g, thus indicating a thinner surface film. These observations are also consistent with the cross-sectional images, showing that the IL electrolyte minimizes the irreversible volume expansion of the electrode to approximately 25%. The results presented here mark a major step forward in the development of the silicon electrode for Li-ion batteries. Retention of 3000 mAh/g after 300 cycles at a current density of nearly 2 mA/cm2 (C/2.5) with capacities of around 3.5 mAh/cm2 at room temperature is now approaching practical values of performance. In addition, similar capacity retention is achieved up to 80 °C after 60 cycles. Preliminary electrode studies presented here strongly suggest that the increased cycling stability of the IL electrolyte is not simply due to there being a larger Li+ “reservoir” that is consumed at the Si electrode surface. Instead, the SEM results suggest that the use of this highly concentrated LiFSI IL promotes the formation of a stable surface layer that allows the Si electrode to remain intact upon cycling. Compositional analyses and physical characterization of this surface film are of particular interest and a topic for ongoing work.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Robert Kerr: 0000-0001-7499-3920 Patrick C. Howlett: 0000-0002-2151-2932 Notes

The authors declare no competing financial interest. Web site for the ST2E group at IMN: https://www.cnrs-imn. fr/index.php/en/scientific-groups/st2e-group-electrochemicalstorage-and-conversion-of-energy

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ACKNOWLEDGMENTS The authors thank Prof. Lionel Roue (INRS-EMT) and coworkers for kindly providing the ball-milled silicon samples used in this study. M.F. acknowledges the Australian Research Council for a Laureate fellowship through Grant FL110100013.

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EXPERIMENTAL METHODS Electrode Preparation. Slurries were prepared using optimized high-energy ball-milled silicon from a commercial source (99.999%, 20 mesh, Materion),27 carboxymethyl cellulose binder (CMC, DS = 0.9, MW = 700 000, Sigma-Aldrich), and

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