Ionic Liquid–Organic Carbonate Electrolyte Blends To Stabilize Silicon

May 30, 2016 - To address these challenges, we evaluated novel high energy density three-dimensional nano-silicon electrodes paired with 1-methyl-1-pr...
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Ionic Liquid−Organic Carbonate Electrolyte Blends To Stabilize Silicon Electrodes for Extending Lithium Ion Battery Operability to 100 °C Khalid Ababtain,† Ganguli Babu,†,∥ Xinrong Lin,‡,∥ Marco-Tulio F. Rodrigues,§ Hemtej Gullapalli,§ Pulickel M. Ajayan,§ Mark W. Grinstaff,‡ and Leela Mohana Reddy Arava*,† †

Department of Mechanical Engineering, Wayne State University, Detroit, Michigan 48202, United States Department of Biomedical Engineering and Chemistry, Boston University, Boston, Massachusetts 02115, United States § Department of Materials Science and NanoEngineering, Rice University, Houston, Texas 77005, United States ‡

S Supporting Information *

ABSTRACT: Fabrication of lithium-ion batteries that operate from room temperature to elevated temperatures entails development and subsequent identification of electrolytes and electrodes. Room temperature ionic liquids (RTILs) can address the thermal stability issues, but their poor ionic conductivity at room temperature and compatibility with traditional graphite anodes limit their practical application. To address these challenges, we evaluated novel high energy density three-dimensional nano-silicon electrodes paired with 1-methyl-1-propylpiperidinium bis(trifluoromethanesulfonyl)imide (Pip) ionic liquid/propylene carbonate (PC)/LiTFSI electrolytes. We observed that addition of PC had no detrimental effects on the thermal stability and flammability of the reported electrolytes, while largely improving the transport properties at lower temperatures. Detailed investigation of the electrochemical properties of silicon half-cells as a function of PC content, temperature, and current rates reveal that capacity increases with PC content and temperature and decreases with increased current rates. For example, addition of 20% PC led to a drastic improvement in capacity as observed for the Si electrodes at 25 °C, with stability over 100 charge/discharge cycles. At 100 °C, the capacity further increases by 3−4 times to 0.52 mA h cm−2 (2230 mA h g−1) with minimal loss during cycling. KEYWORDS: silicon electrodes, ionic liquid electrolytes, solid−electrolyte interface, additive, high temperature Li-ion batteries

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INTRODUCTION

temperature range without compromising the electrochemical performance.6−8 Room temperature ionic liquids (ILs) offer a potential solution to this challenge due to their negligible vapor pressure, wide electrochemical potential window, and structural stability across a large temperature range.9−16 However, many ILs exhibit high viscosity and poor Li-ion conductivity at room temperature, along with reduced cathodic stability.17,18 Among the studied ILs, piperidinium-based ionic liquids are promising for LIB due to their wide electrochemical potential stability up to 5.0 V, high thermal stability up to 385 °C, and moderate Liion conductivity at room temperature (RT) (1.4 mS cm−1).19−21 However, co-intercalation of the piperidinium cation, at lower potentials, along with the Li-ion during the charge−discharge process significantly limits its use with intercalation graphite-based electrodes.22 Hence, in order to effectively use piperidinium ILs in Li-ion batteries, a non-

With continued success in the portable electronic device market, Li-ion batteries (LIBs) are of increasing interest for applications in electric and hybrid vehicles, surgical tools, and oil and gas drilling, etc., due to their superior energy density and long cycle life. However, current LIBs employ conventional liquid electrolytes based on organic solvents, which poses a safety concern, especially at elevated temperatures. Specifically, the use of carbonate solvents such as ethylene carbonate (EC), dimethyl carbonate (DMC), or diethyl carbonate (DEC) restricts battery operation to less than 60 °C due to their volatile and highly flammable nature.1,2 Moreover, when these solvents are used with Li salts, such as lithium hexafluorophosphate (LiPF6), a resistive film forms on the electrode surface affording poor cycle life.3,4 These side reactions become more dominating at higher temperatures as the rate of chemical reaction between the dissolved lithium salt and electrolyte solvent increases.3−5 Thus, there is an unmet need for alternative electrolytes with superior thermal and chemical stability to expand the use of LIBs to a wider working © XXXX American Chemical Society

Received: March 2, 2016 Accepted: May 30, 2016

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DOI: 10.1021/acsami.6b02620 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. Characterization of ionic liquid based electrolytes and SEM images of 3D Ni current collectors at different conditions: (a) viscosity of the Pip electrolytes and its variation upon an addition of PC at different temperatures, (b) linear sweep voltammetry traces for pure Pip and PC-Pip electrolytes at 100 °C from −0.1 to 5.0 V, (c) ionic conductivity of the Pip and PC added Pip electrolytes, and (d−g) morphology of 3D Ni at different pH and deposited currents.

electrochemical process. Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt was added (0.8 M) to the above mixture to afford the final electrolyte. The viscosity, electrochemical stability, and ionic conductivity were subsequently measured as a function of PC addition (10, 20, and 30% (v/v)) to the Pip/LiTFSI electrolyte (Figure 1). The Pip electrolyte exhibited thermal stability up to 385 °C as evidenced from negligible weight loss during the TGA experiment (Supporting Information Figure S1). However, with addition of PC, these electrolytes (PC/Pip mixtures) were thermally stable up to 120 °C, and the stability decreased with increasing PC concentration from 10 to 30% (v/v) to Pip electrolyte, due to solvent vaporization. The PC/Pip mixtures were not flammable when exposed to an open flame, demonstrating the safety of the electrolytes even with doping of PC as high as 30% (Figure S2). The viscosity of the three electrolyte formulations as a function of PC content and temperature was determined. The viscosity decreased with increasing PC addition from 0.50 to 0.13 Pa·s with the addition of 30% PC at 25 °C (Figure 1a). The most significant temperature dependence was observed for the neat Pip ionic liquid, decreasing from 0.50 (25 °C) to 0.11 Pa·s (95 °C). As a comparison, the viscosity of 20PC-Pip decreased from 0.14 Pa·s at 25 °C to 0.084 Pa·s at 95 °C. The values for the viscosity of 20PC-Pip and 30PC-Pip converged at about 0.084 Pa·s at 95 °C. Next, the electrochemical stability of the Pip, 10PC-Pip, 20PC-Pip, and 30PC-Pip electrolytes was determined. A linear sweep voltammetry (LSV) experiment was performed using a three-electrode system consisting of a stainless steel working electrode, and lithium as a counter and a reference electrode over the temperature range of 25−100 °C. At 25 °C, comparable results were observed for all of the electrolytes with anodic stability up to 4.35 V vs Li/Li+ (Figure S3a). The observed increase in the current for the 30PC-Pip electrolyte, at relatively low voltage, may be due to carbonate solvent decomposition on the stainless steel working electrode. Upon increasing the electrolyte temperature to 100 °C, the overall electrochemical stability remained with a slight decrease in anodic stability at about 0.2 V (Figure 1b). Similarly, LSV test has been conducted to understand the effect of TFSI− anion on Al current collector at 100 °C (Figure S3b). The system is stable up to 4.2 V, indicating the feasibility of currently

intercalating anode is required such as one based on Si, Sn, or Ge. Silicon is an attractive material for the anode because it has high thermal stability and low lithiation potential (∼0.3 V vs Li/Li+), and its theoretical capacity is 10 times that of carbon anodes (4200 mA h g−1). The Si particles undergo large volume expansion during lithiation and the electrodes need to be designed in a way to preserve the mechanical integrity of the silicon structures, extending the cycle life of the cell. Nanostructured silicon has attracted attention in the past few years as a solution for this problem, as these structures can allow a better accommodation of the strains generated by cycling and offer the space necessary to allow volumetric expansion of the silicon structures without structural deterioration.23−29 Various engineered structures have been reported for this purpose, ranging from nanoparticles, 2D nanorods, and 3D architectures.30−36 We recently reported porous three-dimensional (3D) nano-Si electrodes, which exhibit minimal volume expansion, and studied their electrochemical performance in the presence of conventional organic electrolytes at room temperature.37 Yet their performance in the presence of ionic liquids at elevated temperatures is unknown. Herein, we combine a piperidinium IL/propylene carbonate electrolyte with a 3D nano-Si anode and construct a cell that operates between 25 and 100 °C. Specifically, we report the (1) ionic conductivity, viscosity, and lithium-ion transference number of 1-methyl-1-propyl piperidinium bis(trifluoromethanesulfonyl)imide doped with propylene carbonate; (2) thermal and electrochemical stability of the electrolyte; (3) battery operation over a wide temperature range from room temperature up to 100 °C; (4) rate capability as a function of temperature; and (5) morphological investigation and compositional study of the SEI on the Si anode.

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RESULTS AND DISCUSSION 1-Methyl-1-propylpiperidinum bis(trifluoromethanesulfonyl)imide (Pip) was chosen as the base solvent for the electrolyte formulation due to its wide electrochemical potential window and thermal stability. Propylene carbonate (PC) was chosen as an additive to enhance room temperature ionic conductivity of highly viscous ILs38,39 and also to contribute to the formation a stable solid−electrolyte interphase (SEI) film19 during the B

DOI: 10.1021/acsami.6b02620 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 2. Room temperature electrochemical studies of 3D Si: (a) comparison of cyclic voltammograms (all four electrolytes), (b) charge−discharge profiles at different C-rates, (c) C-rate plot at RT, and (d) comparison of cycle life vs capacity at C/5 of all electrolytes.

optimizing bath solution pH and deposited currents (Figure 1d−g) and subsequently coated with Si using a PECVD technique (Figure S5). Microscopy studies revealed that 3D Ni current collector which is electrodeposited at pH 1.5 and current −10 mA cm−2 exhibited the desired porosity of 1−2 μm. The details pertinent to the fabrication and fine-tuning of porosity and thickness are provided as Supporting Information. First, a CR2032 coin cell was fabricated using the 3D Si as the working electrode, metallic lithium as counter/reference electrode, celgard membrane as separator, and a conventional organic electrolyte consisting of 1 M LiPF6 in 1:1 (v/v) mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC). The electrochemical performance of 3D Si anode at 25 °C was verified, and the results agreed with literature values (Figures S6 and S7).37 Also, it has been confirmed that unsuitability of organic electrolyte for high temperature applications as cell failed after few cycles at 100 °C (Figure S8). Having confirmed structural and electrochemical properties of the 3D nano-Si electrodes with conventional organic electrolytes, the bestidentified electrodes were subsequently studied with the novel PC/Pip electrolytes. For the high temperature studies, the separator was replaced by a quartz membrane, and coin cells were fabricated with PC/ Pip electrolyte following a procedure similar to that outlined in the previous section. Cyclic voltammetry (CV) experiments were performed on the PC/Pip electrolytes at room temperature with a scan rate of 0.2 mV s−1 as shown in Figure 2a. The absence of prominent reduction and oxidation peaks in the Pip electrolyte was a consequence of its poor ionic conductivity at room temperature. Upon the addition of PC to Pip (e.g., 10PCPip electrolyte), a broad cathodic peak was observed around 0.4 V, likely due to the formation of a solid electrolyte interphase (SEI), as commonly seen in organic electrolytes based systems.31,43−45 However, the broad reduction and oxidation peaks attributed to the lithiation/delithiation of silicon suggested that the electrolyte kinetics were still slow. Upon

investigated electrolyte (20PC-Pip) for silicon-based full cell applications. The ionic conductivity of the electrolytes depended heavily on temperature. As shown in Figure 1c, the Arrhenius plots for the ionic conductivities of Pip, 10PC-Pip, 20PC-Pip, and 30PCPip revealed increased conductivity at higher temperatures. The ionic conductivity of the Pip electrolyte was 0.23 mS cm−1 at 25 °C and increased by 1 order of magnitude with the addition of PC. Although the slopes were slightly different between the electrolyte PC/Pip compositions, the ionic conductivities converged at about 10 mS cm−1 at 100 °C (373 K). The pseudo-activation-energy (Ea) value initially dropped with addition of PC, later increasing at larger volume fraction of PC (Table S1). Although a preferential solvation of Li+ by PC explained the initial behavior, the interplay between free volume and increased PC content is still under investigation. For all of the PC/Pip compositions, the conductivity dependence with temperature was described by the Vogel− Tammann−Fulcher (VTF) model (Figure S4). Addition of PC to the Pip enhanced the ionic mobility at lower temperatures without critically compromising the overall thermal stability up to 100 °C.40,38 Next, the lithium-ion transference number (TLi+) was measured, using the polarization technique described by Evans et al.,41 and the values were determined to be 0.05, 0.20, and 0.33 for Pip, 10PC-Pip, and 20PC-Pip, respectively. An increase in the TLi+ was found with addition of PC to the electrolyte, consistent with the reduction in viscosity and solvation of the lithium ions resulting in an increase in the relative diffusion coefficient of Li+. The observed increase in lithium-ion transport with addition of organic solvents to ILbased electrolytes agrees with previous reports in the literature.42 As discussed earlier, our motivation is to evaluate the feasibility of using Si as an anode for an IL−electrolyte based high temperature Li-ion battery. Following our previously reported method,37 3D Ni current collectors were prepared by C

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Figure 3. Temperature-dependent performance of 3D Si electrodes with Pip and 20PC-Pip electrolytes: (a and b) cyclic voltammograms and (c and d) capacity vs voltage profiles at different temperatures.

Figure 4. High temperature electrochemical properties: (a and b) capacity vs cycle number studies of electrolytes at different temperatures, (c) comparative cycling behavior of Pip and 20PC-Pip electrolytes, and (d) rate capability of 20PC-Pip electrolyte at 100 °C.

To understand the rate capability and cycle life of the 3D Si anode, galvanostatic charge/discharge measurements were conducted by operating cells between C-rates of C/40 and C/5 at 25 °C (Figure 2b,c). The Pip electrolyte in concert with the Si anode exhibited excellent charge−discharge properties with capacity of 0.3 mA h cm−2 and voltage plateaus around 0.4 V/0.2 V, when the cell operated at C/40 (Figure 2b). The trend in voltage plateaus continued with slightly higher C-rates up to C/10 with a gradual decrease in specific capacity values.

increasing the concentration of PC to 20%, a significant enhancement in redox peaks current was observed, indicating improved electrochemical properties. Further increases in PC concentration to 30% afforded only a marginal enhancement in electrochemical performances over the 20PC-Pip electrolyte. Hence, in order to retain the best performance of the electrolyte while minimizing the PC content, we selected the 20PC-Pip electrolyte for the subsequent experiments. D

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Figure 5. XPS spectra and SEM image of 3D silicon electrode with Pip and 20PC-Pip electrolytes interface: (a) deconvoluted high resolution C 1s spectra, (b) O 1s spectra, and (c and d) morphology of 3D Si electrode with pip and 20PC-Pip electrolytes respectively on discharged state.

shown in Figure 3c,d. During the first discharge, there was a small plateau region around the potential of 0.6 V and then a stable large plateau occurred below 0.4 V corresponding to SEI formation and lithium−silicon alloy formation, respectively.46,47 In view of volume expansion driven electrode destabilization, the discharge cutoff voltage was restricted to 50 mV at the expense of deep discharge capacity. Upon cycling at different temperatures the charge−discharge plateau, corresponding to alloy/dealloy of Li−Si, remained unaltered due to improved ion transport properties of the PC-Pip electrolyte. In comparison, the large polarization observed at 25 °C, as a consequence of the high viscosity impeding Li-ion conductivity, afforded a lower specific capacity of 0.18 mA h cm−2. The capacity versus cycle number as a function of temperature from 100 to 25 °C was investigated at a constant current rate of C/5 (Figure 4a,b). As the ionic conductivities of both electrolytes were comparable at higher temperatures, their capacities were also expected to be of similar magnitudes. At 100 °C, silicon exhibited high specific capacity of 0.41 (1912 mA h g−1) and 0.52 mA h cm−2 (2230 mAh g−1) when using the Pip and 20PC-Pip electrolytes, respectively (Figure S10). A similar result is observed at 80 °C, with the cell containing the 20PC-Pip electrolyte showing slightly higher capacities. At high temperatures, the ionic conductivity did not limit the cell operation. The higher TLi+ values measured in the presence of PC-Pip were in agreement with the improved performance of the mixed electrolytes. Differences in the specific capacity values were more apparent at working temperatures below 60 °C. For instance, at 25 °C, the Pip electrolyte containing cell afforded a capacity of 0.01 mA h cm−2 whereas the 20PC-Pip electrolyte cell presented a capacity of 0.19 mA h cm−2. The poor electrochemical performance of the Pip at 25 °C was consistent with its low conductivity and transference number. Importantly, both cells regained their high electro-

However, at higher C-rates such as C/5, a significant drop in the specific capacity was observed (95%) and attributed to the poor ionic conductivity of the Pip electrolyte at 25 °C. On the other hand, the 20PC-Pip electrolyte exhibited excellent characteristic features of lithiation and delithiation of silicon at 25 °C with desired potentials of 0.2 and 0.45 V vs Li/Li+ respectively (Figure S9). Furthermore, comparative cycle life tests at a C/5 rate for the Pip, 10PC-Pip, and 20PC-Pip highlight this point (Figure 2d). At the end of the 100th cycle, the specific capacities of 3D Si electrode are 0.01, 0.04, and 0.21 mA h cm−2 for Pip, 10PC-Pip, and 20PC-Pip electrolytes, respectively. Addition of 20% PC to Pip resulted in improved electrochemical performance of the Si anode at a C/5 rate with a drastic enhancement in capacity and stability over 100 cycles. Based on the encouraging results above, we next evaluated the performance of the 3D Si/20PC-Pip LiTFSI/Li coin cell configuration at higher temperatures, as the compatibility of Si anodes with ILs in general and Pip IL in particular are unknown at higher temperatures. First, we performed CV studies with the Pip and 20PC-Pip electrolytes at 25, 60, 80, and 100 °C. The electrochemical activity of the 3D Si anode in the presence of both electrolytes improved at higher temperatures, due to the increased ionic conductivity and reaction kinetics (Figure 3a,b). The enhanced reduction and oxidation peak currents observed in the case of 20PC-Pip, without changing peak position at 100 °C, confirmed not only the improved electrochemical properties but also the unaltered thermal stability upon PC addition to the Pip electrolyte (Figure 3b). The lithiation/delithiation of silicon at elevated temperatures occurred in two stages. Interestingly, these two stage lithiation/delithiation peaks overlapped with each other in the CV cycles without altering their position, which further confirmed the thermal stability of the system. The charge−discharge profiles of Si electrodes with Pip and 20PC-Pip electrolytes at different temperatures are E

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PC. The structure, porosity, and morphology of the 3D silicon anode were maintained even upon lithiation, confirming the robustness of structure (Figure 5c,d and Figure S16).

chemical performance and high energy density when returned to operation at 100 °C. In order to evaluate the long-term performance of the electrolytes at high temperature, cycling stability tests were conducted at 100 °C (Figure 4c and Figure S10c). Excellent capacity retention over 30 cycles at 0.2 C rate with specific capacity values of 0.42 and 0.46 mA h cm−2 were observed for the Pip and 20PC-Pip electrolytes, respectively. Further, at 2C, specific capacity of 0.21 mA h cm−2 was obtained for 60 charge−discharge cycles with Coulombic efficiency of 96%. Additional rate capability tests were performed to understand the suitability of the PC-containing cell for high power applications at 100 °C (Figure 4d). Moreover, electrochemical impedance spectroscopy (EIS) studies were performed at RT and at 100 °C to probe the charge transfer resistance for cells using the investigated electrolytes (Figure S11). The drastic difference on the diameter of the semicircles after addition of 20% of PC (195 Ω) compared to that with pure Pip electrolyte (2750 Ω) highlight the improved reaction kinetics in the presence of organic solvents. Interestingly, the impedance spectra for both systems presented a perfect overlap at 100 °C, showing that the role of propylene carbonate is to improve the cell performance at low temperatures without affecting it at a large extent once the temperature goes up. The suitability of 20PC-Pip electrolyte for silicon-based full cell configurations was also verified by fabricating half-cells with conventional cathode material (LiCoO2), and the obtained results have been displayed in Figure S12. Finally, X-ray photoelectron spectroscopy (XPS) experiments were performed on the silicon anodes used with the Li metal cells containing the Pip, 20PC-Pip, or organic (1 M LiPF6 in EC: DEC = 1:1 (v/v)) electrolytes to probe the chemical nature of solid-electrolyte interphase (SEI) (Figure 5 and Figures S13−S15). The disassembled silicon electrodes (on lithiated state) were carefully transferred from the glovebox to XPS fast entry lock using a transferable vacuum chamber without exposing them to atmospheric air. The chemical composition of the SEI formed was strongly dependent on the type of electrolyte employed for cell preparation,48 as listed in Table S2. The C 1s spectra data collected for both electrolytes indicated that C−C and C−F were mostly from the reduction of the alkyl carbonates and LiTFSI salt (Figure 5a). The oxygen content, determined from the O 1s was found to be higher when the carbonate-based organic electrolyte was used, resulting in formation of Li2CO3 and oxygenated organic compounds (Figure 5b). Surprisingly the intensities of the S 2p and F 1s peaks were only appreciable for the 20PC-Pip electrolyte, but still with a very reduced atomic fraction, possibly in the form of LiF and other products from the decomposition of TFSI−49 (Figure S15). One of the most interesting features in the spectrum is that the Si 2p peak is nearly invisible for the electrode cycled with the Pip, indicating the formation of a thicker SEI (Figure S14). Although contributing for cyclic stability, the growth of a thick surface layer further increased the cell resistance, in support of the observed lower capacities obtained for cells using a low conductive electrolyte. The absence of detectable amounts of S and F may suggest that the SEI surface was formed primarily by decomposition products of the cation or carbonate solvents. Addition of 20% PC, however, led to an increased contribution of TFSI− decomposition products to the SEI and formation of a thinner passivation layer. Such changes are likely a consequence of changes in the molecular environment of Li+ after addition of

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EXPERIMENTAL DETAILS

Preparation of Electrolyte Mixtures. Thermally stable room temperature ionic liquid electrolyte is prepared using 0.8 M of lithium bis(trifuoromethanesulfonyl)imide (99.8%, Sigma-Aldrich) salt dissolved in 1-methyl-1-propylpiperidinum bis(trifluoromethylsulfonyl)imide (99%, io-li-tec) solvent. The electrolyte mixtures are prepared by mixing propylene carbonate (99.7%, Sigma-Aldrich) and RTIL (v/v) with constant lithium salt concentration (0.8M). All electrolytes used in the study were prepared in an argon-filled glovebox with oxygen and water contents lower than 0.1 ppm. Preparation and Evaluation of 3D Si Electrode. First, 3D porous Ni current collectors are prepared by galvanostatic electrodeposition method as reported previously (also see Supporting Information). Then, Si deposition process was carried out on the 3D porous Ni current collectors (deposited at pH 1.5 and current −10 mA cm−2) using plasma enhanced chemical vapor deposition (PECVD) process, which has a capability of remotely cracking Si precursor (Figure S5). The silane precursor was used to deposit Si thin films. To achieve conformal Si coating on 3D porous Ni, experimental parameters such as carrier/reacting gas mixture flow, substrate temperature, chamber pressure, and deposition time were tuned. Prior to testing ionic liquid based electrolytes, electrochemical performance of 3D Si has been verified by fabricating cells with 1 M of LiPF6 in 1:1 (v/v) mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) as an electrolyte with celgard separator and the results are displayed in Figures S6 and S7. The electrochemical measurements of the ionic liquid based electrolytes with 3D Si were performed on CR2032 coin cells in the potential range between 1.5 and 0.05 V vs Li/Li+ at different current rates from room temperature (RT) to 100 °C. Cell Fabrication and Characterizations. Coin cells of standard 2032 were fabricated using prepared pure RTIL and RTIL-based electrolyte mixtures with 3D porous silicon as working electrode, metallic lithium as counter/reference electrode, and quartz membrane separator. Cyclic voltammograms are recorded in the potential range from 1.5 to 0.05 V with different electrolytes using Biologic (VM3) electrochemical workstation. Charge− discharge studies at different current rates (from C/10 to 1 C-rate) were carried out in the potential range of 1.5−0.05 V using ARBIN charge−discharge cycle life tester. The morphology of the samples were characterized by a JSM 401F (JEOL Ltd., Tokyo, Japan) scanning electron microscope operated at 3.0 kV and a JEM 2010 (JEOL Ltd., Tokyo, Japan).

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CONCLUSIONS Enhancing room temperature Li-ion transport properties of ILbased electrolytes and maintaining high energy density, without compromising thermal stability, is key for developing safe Liion batteries and extending their use to additional applications that require operation at >60 °C. To address these challenges, we are evaluating novel high energy density 3D nano-Si electrodes paired with 1-methyl-1-propyl piperidinium bis(trifluoromethanesulfonyl)imide (Pip) ionic liquid/propylene carbonate (PC)/LiTFSI electrolytes. A systematic study on the physical and electrochemical properties of PC-containing Pip electrolytes shows that the 20% PC formulation provides Li-ion battery operation from 25 to 100 °C. Measurement of transference numbers and ionic conductivity reveals that the addition of PC overcomes the slow kinetics of Pip electrolyte, due to its positive contribution in reducing the viscosity of the electrolyte. For the 20PC-Pip electrolyte, a drastic improvement in capacity is observed for the Si electrodes at 25 °C with stability over a 100 charge/discharge cycles. At 100 °C, the capacity further increases by 3−4 times to 0.52 mA h cm−2 F

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ACS Applied Materials & Interfaces (2230 mA h g−1) with minimal loss during cycling. The use of high capacity Si anodes in combination with IL-based electrolytes demonstrates a validated approach to address both the safety and energy density limitations of current Li-ion batteries. Continued research on thermally stable and high energy density energy storage devices, especially those operating over a wide temperature range, will provide advances in basic science and novel devices for use in routine day-to-day operation as well as those for broader industrial applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b02620. Additional electrolyte characterizations, electrode behavior with organic electrolyte, XPS, and microscopy studies of cycled electrodes (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions ∥

G.B. and X.L. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge funding support from the Advanced Energy Consortium (AEC) under Project No. BEG10-02. REFERENCES

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DOI: 10.1021/acsami.6b02620 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX