Silicon–Carbon Nanocomposite Semi-Solid Negolyte and Its

Aug 21, 2017 - Silicon–Carbon Nanocomposite Semi-Solid Negolyte and Its Application in Redox Flow Batteries. Hongning Chen†, Nien-Chu Lai†, and ...
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Silicon−Carbon Nanocomposite Semi-Solid Negolyte and Its Application in Redox Flow Batteries Hongning Chen,† Nien-Chu Lai,† and Yi-Chun Lu*,† †

Electrochemical Energy and Interfaces Laboratory, Department of Mechanical and Automation Engineering, The Chinese University of Hong Kong, Shatin, New Territories 999077, Hong Kong SAR, China S Supporting Information *

ABSTRACT: Nonaqueous redox flow battery (RFB) is a promising technology to improve the energy density of RFBs for both stationary grid energy storage and electric vehicle applications. Despite tremendous advancement made in highcapacity flow posolyte, lithium metal has been dominantly exploited as a nonflowable counter electrode in reported lithium-based RFBs to date due to the lack of efficient and high-capacity flow negolyte. Here, we report a silicon−carbon nanocomposite semi-solid negolyte, achieving a high reversible capacity (>1200 mAh g−1) and stable cycle life (>100 cycles). A facile process of incipient wetness impregnation followed by polymerization/carbonization is developed to synthesize monodispersed Si−C nanocomposite with ultrathin graphitic carbon coating. Exploiting Si−C nanocomposite as the negolyte effectively suppresses the volume change of Si particles and enhances the electrical conductivity of the negolyte. Coupling with highly concentrated LiI (5.0 M), we demonstrate the first lithium metal-free Si-based negolyte in a full all-flow cell configuration with a stable cycle life (>60 cycles), high Coulombic efficiency (>90%), and the highest full cell voltage (3.0 V) reported for Li metal-free RFBs. Our successful demonstration of lithium metal-free all-flow battery highlights the promising potential of Sibased negolyte to replace lithium metal for high-energy-density RFBs.

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Developing efficient and high-capacity flowable negolytes to replace the lithium metal is crucial in realizing practical and safer high-energy-density nonaqueous RFBs.13,18,27 For instance, Ding et al. recently reported a metallocene-based negolyte (CoCp2) with high reaction rate (10−3 cm s−1) and high Coulombic efficiency (>95%).13 Duduta et al. demonstrated the semi-solid negolyte utilizing Li4Ti5O12 in a carbon percolating conducting network to replace lithium metal and coupled with LiCoO2 posolyte as a full flow battery system.18 However, the low specific capacity (1.3 V vs Li/Li+) of these lithium metal-free negolyte candidates dramatically decrease the total energy density of the flow battery system compared with the use of lithium metal. Therefore, it is critical to develop low-voltage negolyte with high capacity for the high energy density lithium metal-free all flow battery system.13,28 Silicon has a relatively low reaction potential (0.3 V vs Li/Li+)29 with high specific capacity (theoretically 4200 mAh g−1),30 which makes it an attractive candidate for high-capacity, low-voltage negolyte. Silicon-based negolyte was first developed by mechanically mixing silicon nanoparticles (∼100 nm) with Ketjen black (KB).27 However, similar to other mechanically mixed semisolid flow posolytes, the poor contacts between the active

INTRODUCTION Redox flow batteries (RFBs) are promising energy storage technologies owing to their flexibility in decoupling power and energy.1−6 Applying RFBs as the energy storage systems for electric vehicles (EVs) provides an attractive advantage in short-time refueling and off-line charging integrating with renewable power sources.7 However, the low-energy-density nature of RFBs strongly limits their applications in both stationary grid storage and mobile EVs.8−12 Nonaqueous lithium-based flow battery (Li-RFB) is one of the most promising strategies in improving the energy density of RFBs,5,13 taking advantages of the large cell voltage against lithium and the high volumetric capacity of the posolyte. For instance, highly concentrated posolytes (>5 M) using semisolid posolytes (S/C,14 S/C−LiI,15 polysulfide,16,17 LiCoO2,18 LiFePO 4 19 suspensions, etc.), organic active materials (TEMPO,20 MeO−TEMPO,21 ferrocene-based liquid,22 etc.) and high-solubility aqueous posolytes (LiI,23 LiBr,24,25 etc.) were investigated as the posolyte. However, lithium metal was exploited in most of the Li-RFBs reported to date as the negative electrode, which faces issues related to dendrite formation and difficulty in decoupling power and energy of the RFBs.5,14 In addition, numerous safety concerns associated with the lithium metal anode including dendrite penetration (short circuit and thermal runaway) and severe chemical reaction with aqueous posolyte could be eliminated with alternative flowable negolyte.23,26 © 2017 American Chemical Society

Received: June 21, 2017 Revised: August 21, 2017 Published: August 21, 2017 7533

DOI: 10.1021/acs.chemmater.7b02561 Chem. Mater. 2017, 29, 7533−7542

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Chemistry of Materials

Figure 1. Schematic illustration of (a) silicon−carbon nanocomposite negolyte preparation process; (b) concept of Si-based flow battery; (c) cracking process of mechanically mixed Si−KB negolyte, and (d) cycling process of Si−C nanocomposite negolyte. composite was prepared via incipient wetness impregnation with liquid carbon precursor followed by heat treatment under inert atmosphere. FA was used as carbon precursor, and OA was added to the FA solution as a polymerization catalyst. The starting composition was prepared by mixing FA and OA at mass ratio of 19:1 followed by ultrasonication in a water bath for 30 min. FA−OA solution was infiltrated in Si nanopowders by incipient wetness impregnation at room temperature, followed by polymerization at 80 °C for 2 h under air. Afterward, the nanocomposite was treated at 350 °C for 4 h with a heating rate of 5 °C min−1. After cooling to room temperature, another FA−OA solution was added to the aforementioned powder again. The mixture was heated to 350 °C for 4 h with a heating rate of 5 °C min−1. During the synthesis of Si−C nanocomposites, the amount of FA/OA solution corresponding to the total pore volume was first added and infiltrated in Si nanopowders by incipient wetness impregnation, followed by adding additional FA/OA corresponding to 20% of pore volume. Finally the temperature was increased to 1000 °C with a heating rate of 1 °C min−1 and maintained at that temperature for 4 h. The carbonization procedure was performed under argon.31 Consequently, the silicon nanoparticles were covered with a uniform porous carbon layer on the surface to form the Si−C nanocomposite, which is designed to enhance the contacts between active materials and carbon conducting network in the negolyte. The chemical composition of the Si−C nanocomposite was determined to be 79 wt % silicon and 21 wt % carbon by thermogravimetric analysis (Figure S1a). All the negolytes reported herein were made with an electrolyte of 1 M LiPF6 in EC/DEC (volume ratio 1:1). The active materials (Si nanopowder or Si−C nanocomposite) and KB were mixed in a 10 mL glass container with the electrolyte, followed by sonication using the SLPt Cell Disruptor for 10 min before testing. The as-prepared carbon coating Si−C negolyte was designed to replace lithium metal for all flow nonaqueous RFBs (Figure 1b). Preparation of Si Solid Electrodes. Si solid electrodes were prepared by coating a slurry of Si nanopowder/Si−C nanocomposite, conducting carbon (Super P), and carboxymethyl cellulose (CMC) with a mass ratio of 1:1:1 onto copper foil. The slurry was prepared by 10 min sonication with a H2O/EtOH (volume ratio 1:1) as solvent and cast on a Cu foil surface.32,33 After vacuum drying at 110 °C for 12 h in the vacuum-dry oven, The Si solid electrodes with a diameter of 12 mm were punched out. The mass loading of Si was approximately 0.14 mg cm−2. Synthesis of NASICON-Type Structured Ceramic Membrane. A solid-state reaction method was employed to synthesize the NASICON-type structured Li1.5Al0.5Ge1.5(PO4)3 (LAGP) lithium ion conductor as reported in other literature.34−36 Stoichiometric amounts of LiOH·H2O (15 wt % excess), Al2O3, GeO2, and NH4H2PO4 were used as received. The precursors were ball-milled at 400 rpm for 12 h

materials and the carbon network remain a critical challenge. In addition, the significant volume change of silicon particles occurring during cycling presents a critical challenge in improving the stability of Si-based negolyte. To date, there is still a lack of full flow battery demonstration based on the silicon negolyte. In this work, we propose a carbon-coated silicon−carbon nanocomposite (Si−C) semi-solid negolyte to address the two critical issues of silicon-based negolyte: high interfacial resistance and large volume expansion. We develop a facile process of incipient wetness impregnation followed by polymerization/carbonization to synthesize monodispersed Si−C nanocomposite with ultrathin graphitic carbon coating. The carbon-coated Si−C nanocomposite enhances the contacts between the active materials and carbon conducting network and reduces the nonuniform silicon volume expansion. The Si− C nanocomposite negolyte demonstrates a reversible capacity of 1200 mAh g−1 with a stable cycle life (>100 cycles). Combining the designed Si−C nanocomposite negolyte with a high-capacity posolyte (5.0 M LiI), we demonstrate a lithium metal-free full all-flow nonaqueous RFB with a stable cycle life (>60 cycles) and high Coulombic efficiency (>90%). The Si−C nanocomposite negolyte shows promising potential in replacing lithium metal for high-energy nonaqueous Li-RFB applications.

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EXPERIMENTAL SECTION

Materials. Silicon nanopowder (99.9% metals basis, 100−120 nm), carboxymethyl cellulose (CMC, DS = 0.7, Mw = 90 000) were received from Aladdin. Super P carbon black was received from MTI. Ketjen Black EC-600JD (KB) was received from AzkoNobel. LiOH· H2O, GeO2, and NH4H2PO4 were received from Sinopharm Chemical Reagent. Furfuryl alcohol (FA, 98%) was received from Fluka. Oxalic acid (OA, 98%) was received from Acros. Aluminum oxide (Al2O3), lithium perchlorate (LiClO4, 99.99% trace metals basis), lithium iodide (LiI, anhydrous, 99.99% trace metals basis), lithium hexafluorophosphate (LiPF6, 99.99% trace metals basis), 1,3-dioxolane (DOL, anhydrous, 99.8%), 1,2-dimethoxyethane (DME, anhydrous, 99.5%), ethylene carbonate (EC, anhydrous, 99%), and diethyl carbonate (DEC, anhydrous, 99%) were received from Sigma-Aldrich. LiClO4 and LiPF6 were dried overnight under dynamic vacuum in a glass oven (Büchi, Switzerland) at 110 °C. Preparation of Si−C Nanocomposite and Si-Based Negolyte. Figure 1a shows the schematic illustration of the silicon−carbon nanocomposite negolyte preparation process. The Si−C nano7534

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Figure 2. TEM images of (a) as-received Si nanoparticle and (b) Si−C nanocomposite; (c) HRTEM image of Si−C nanocomposite; (d) 2nd galvanostatic discharge/charge profiles of 10Si-10KB-MM and 10Si-3C-7KB negolytes in Li half-cell; (e) 8th galvanostatic discharge/charge profiles of the cells shown in (d). The current density is 0.1 mA cm−2. in acetone and then heated at 600 °C for 2 h. The mixture was then cooled, ball milled, and heated at 800 °C for 6 h. After that, the milling process was repeated and the obtained powder was pressed into pellets and sintered at 850 °C for 6 h. Lithium ionic conductivity of the as prepared LAGP ceramic conductor was around 4 × 10−4 S cm−1 when measured at room temperature. The thickness and diameter of the asprepared LAGP are 0.3 mm and 19 mm, respectively. Preparation of Si-Based Negolyte Half Flow Cell. Figure S2b shows the structure of the half flow cell. One piece of lithium foil (2 × 10 mm) was attached to a bottom cell body, which acted as a current collector for the negative electrode. One Celgard 2325 separator (4 × 14 mm) was placed on the surface of the lithium foil followed by adding 20 μL of electrolyte. A polytetrafluoroethylene channel spacer (1 mm thickness) was placed between the bottom cell body and a piece of copper foil, which was attached to an outer top cell body as a current collector fixed by six bolts. A total of 20 μL of negolytes were injected into the channel (1 × 2 × 10 mm) from the inlet. The cell assembling process was conducted in an Ar-filled glovebox (Etelux, H2O < 1.0 ppm of O2 < 1.0 ppm). Preparation of Si-Based Negolyte and LiI Posolyte Full Cell. Figure S2c shows the structure of the full test cell. One piece of carbon paper (12 mm diameter) was placed onto the bottom stainless steel cell body as cathode current collector. A total of 20 μL of 5 M LiI in 0.2 M LiClO4 DOL/DME (volume ratio 1:1) posolytes was added to the zero gap channel. One Celgard 2325 separator (18 mm diameter) was placed on the gap. A total of 40 μL of 0.2 M LiClO4 DOL/DME electrolytes was added on the Celgard separator. One piece of LAGP ceramic membrane was placed on the Celgard 2325 separator. A total of 20 μL of Si−C nanocomposite semi-solid negolytes was added on the surface of LAGP with the zero gap channel. One piece of copper foil (12 mm diameter) was placed on the negolytes as a current collector followed by a stainless steel spring and a polytetrafluoroethylene O-ring. Two cell bodies (bottom and top) were separated by a polytetrafluoroethylene spacer to avoid a short circuit. The cell assembling process was conducted in an Ar-filled glovebox (Etelux, H2O < 1.0 ppm of O2 < 1.0 ppm).

Electrochemical Characterizations. All the electrochemical characterizations were performed using a VMP3 electrochemical testing unit (Bio-Logic). Galvanostatic discharge/charge tests of Sibased negolyte in the half flow cell were performed between 0.01 and 1.2 V versus Li/Li+, while they were between 2.6 and 3.4 V for the full cell. Specifically, the partial discharge/charge tests were shown in Figure S2a. When the discharge specific capacity (or charge specific capacity) reached the desired value (e.g., 3000 mAh g−1 or 1380 mAh g−1), the test returned to the charge (discharge) process until it fulfilled the cutoff voltage of 1.2 V for half flow cell (i.e., 2.6 V for full flow cell). The current density was calculated based on the current collector surface area. The specific capacity was calculated from the mass of active materials of silicon. Material Characterizations. SEM characterization was performed on Quanta 400 FEI. The SEM samples for different discharge/charge steps were collected from cells disassembled at a specified phase in the glovebox. The residual solvent was evaporated before SEM observing. TEM analysis was performed on FEI Tecnai F20. X-ray diffraction measurements were conducted on a Rigaku SmartLab diffractometer (Cu Kα radiation) at a scan rate of 1° min−1. Thermal gravity analysis of as prepared Si−C nanocomposite was conducted on Perkinelmer STA 6000. The temperature was controlled from room temperature to 650 °C at 10 °C min−1 with O2 purge. The electrical conductivity was measured by a standard four-point probe method37,38 with an RTS resistivity measurement system (RTS-8, China) on disc-shaped pellets with a diameter of 16 mm and a thickness of about 1.0 mm. Raman analysis was performed using a Micro Raman spectrometer (RM-1000, Renishaw Co. Ltd.). The specific surface area was determined using Brunauer−Emmett−Teller (BET) method with Micromeritics ASAP2020M+C.

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RESULTS AND DISCUSSION A novel carbon coating approach is proposed and applied to synthesize ultrathin carbon-coating on the Si nanoparticles. In previous work,39−41 carbon-coated Si nanoparticles were 7535

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Figure 3. SEM images of 10Si-10KB-MM negolytes at (a) pristine stage; (b) after 1st discharge (lithiated Si); and (c) after cycling (8 cycles at 0.1 mA cm−2). SEM images of 10Si-3C-7KB negolytes at (d) pristine stage; (e) after 1st discharge (lithiated SiC); and (c) after cycling (8 cycles at 0.1 mA cm−2).

nanoparticle core. The structure and morphology of the prepared Si−C nanocomposite were compared with the asreceived Si nanoparticles (Figure 2 (TEM) and Figure S3 (SEM). As shown in Figure 2b, the Si−C nanocomposite was uniformly covered by a porous carbon structure with 9 nm in thickness. The core−shell nanostructure was previously suggested as being beneficial for the cycling performance of Si anode in conventional Li-ion batteries.49 From the X-ray diffraction (XRD) measurement results (Figure S1b), we confirmed that Si−C nanocomposite has the same domain size (38 nm) with the same crystal structure (cubic) as the starting Si nanopowder. The XRD results show no change of silicon domain size between the silicon nanopowder and the Si−C composite. Considering factors that influence the silicon crystal domain (e.g., heating rate, target temperature, and heating duration50,51), we believe that it can be attributed to limited silicon surface reconstruction under the mild carbonization temperature (1000 °C) compared to the melting point of silicon 1680 K (∼1400 °C) as well as a slow heating rate of 1 °C min−1 and a short duration (4 h).52 To further confirm the structure of the as-synthesized Si−C nanocomposite, Raman spectroscopy was used to study the carbon structures of the Si−C nanocomposite. As shown in Figure S5, the peaks at 1340 cm−1 (D band)53 and 1580 cm−1 (G band)54 correspond to the amorphous carbon and crystalline graphite, respectively.55 The intensity ratio of the G/D bands of the Si−C nanocomposite is determined to be IG/ID = 1.2, which indicates a high degree of graphitization in the porous carbon material.40,56,57 We then characterized the electrical conductivities of the Si nanopowder and Si−C nanocomposite by fourpoint probe measurements with disc-shaped pellets (Experimental Section). The results showed that the electrical conductivity of Si−C nanocomposite is 2.1 S cm−1, which are above 5 orders of magnitude higher than that of the pristine Si nanopowder (20 nm reported in the literature39,40) and uniform carbon coating on the Si nanoparticles (Figure 2) with monodispersion (Figure S3). As shown in Figure S4, the specific surface area derived from the BET method increased from 23.4 m2 g−1 for the as-received Si nanoparticles to 214.4 m2 g−1 for the Si−C nanocomposites. The increase of surface area is attributed to the coating of the porous carbon. In addition, since the capillary force is not a chemical-specific interaction, the existence of oxide layers as well as etching of the layer are not necessary in our synthesis approach.48 While the hydroxyl group of furfuryl alcohol could react with the native oxide on the surface of the silicon nanoparticle, since no substantial oxide layer can be observed on the surface of the silicon nanoparticles (Figure 2), we believe that the nonchemical-specific capillary force is the main driving force to fill the large amount of liquid carbon precursor and is responsible for the uniform carbon shell coating on the individual Si 7536

DOI: 10.1021/acs.chemmater.7b02561 Chem. Mater. 2017, 29, 7533−7542

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After the 1st discharge, the charge transfer resistances of both negolytes were significantly decreased, which could be attributed to the improved reaction kinetics after lithiation of the silicon particles. However, the resistance of the 10Si-3C7KB negolyte is still smaller than that of the 10Si-10KB-MM negolyte. Interestingly, after the fifth discharge cycle, the resistance of the 10Si-10KB-MM negolyte increased compared to the 1st discharge cycle while the resistance of the 10Si-3C7KB negolyte decreased compared to the 1st discharge cycle. The decreased RCT value of Si−C after the fifth cycle compared to the 1st cycle was also found in conventional carbon-coated silicon anodes.62−64 We believe that the local contact between the Si particles and the KB conducting network could be enhanced (compared with the pristine state) after lithiation/ delithiation in the presence of carbon coating owing to stronger interaction between the surface carbon and the KB. On the contrary, for 10Si-10KB-MM negolyte, no carbon coating is available to serve as the linkage between the lithiated/ delithiated Si particles and the KB conducting network. In addition, the increase of the resistance observed in 10Si-10KBMM negolyte coincides with its capacity fade in the Li-cells (Figure 2e). These observations suggest that the charge transfer resistance of the Si-based negolyte was improved by our carbon coating method resulting in improved cycle performance. We further examine the electrochemical activities of the Si− C nanocomposite negolyte via galvanostatic discharge/charge cycling at various capacities. Figure 4a showed the full capacity

configuration (Figure S2b), in comparison with the mechanically mixed Si negolytes. Two types of Si-based negolytes were evaluated including 10 vol % Si−10 vol % KB prepared by mechanical mixing method (10Si-10KB-MM) and 10 vol % Si− 3 vol % C−7 vol % KB prepared by Si−C nanocomposite method (10Si-3C-7KB). The KB is used for forming a threedimensional continuous conducting network and to uniformly suspend the active materials.18,28,58 The carbon coating on the Si−C nanocomposite enhances the electrochemical activity (reduces the charge transfer resistance) of the individual silicon particles resulting in more uniform lithiation and volume expansion than the noncoated silicon particles. All the volumetric percentages were calculated using the tap density (Si: 0.11 g cm−3, KB: 0.12 g cm−3). Figure 2d,e shows the galvanostatic discharge/charge comparisons at the 2nd cycle and 8th cycle for 10Si-10KB-MM and 10Si-3C-7KB negolytes. The 10Si-3C-7KB negolyte showed lower overpotential (260 mV) and higher Coulombic efficiency (CE = 70%) compared with 10Si-10KB-MM negolyte (490 mV, CE = 50%), and the enhancement became more prominent at subsequent cycles. More importantly, the Si−C nanocomposite showed stable capacity retention (stable for 100 cycles) while the 10Si-10KBMM negolyte suffered from serious capacity fading from the 8th cycle. We believe that the increase of the electrical conductivity, the improved uniformity of the electrical conductive network, and the alleviation of cracks in the suspension network contributed to the enhancements, as illustrated in Figure 1c,d. More specifically, in the mechanically mixed Si negolyte, the Si nanoparticles located next to the KB particles could be lithiated first during discharge and thus undergo a higher degree of volume expansion compared to the rest of the Si nanoparticles. Consequently, the nonuniform volume expansion may lead to the formation of cracks in the electrode network (Figure 1c). On the other hand, for the Si− C nanocomposite negolyte, the Si nanoparticles can be lithiated homogeneously owing to higher electrical conductivity of the Si−C electrode, leading to homogeneous volume expansion (Figure 1d). This hypothesis is consistent with the reported conventional Si−C nanocomposite solid electrodes59−61 that the core−shell nanostructure can efficiently suppress the volume change of silicon during the cycling. To support our hypothesis, we investigate the morphological changes of the Si−C nanocomposite and mechanically mixed Si negolytes using scanning electron microscopy (SEM). Figure 3 shows the SEM images of 10Si-10KB-MM and 10Si-3C-7KB negolytes at pristine, after 1st discharge and after 8 cycles. Both 10Si-10KBMM and 10Si-3C-7KB negolytes show volume expansion and formation of surface film after the 1st discharge process. Interestingly, after cycling, the volume expansion of the 10Si10KB-MM negolyte is highly nonuniform as evidenced by the large difference in Si particle size after cycling (Figure 3c). In contrast, the volume expansion of the 10Si-3C-7KB negolyte after cycling is much more uniform compared to the 10Si10KB-MM negolyte with similar particle size in the suspension. These observations are consistent with the hypothesis illustrated in Figure 1c,d. We conducted electrochemical impedance spectroscopy (EIS) measurements for 10Si-3C-7KB and 10Si-10KB-MM negolytes at the pristine, 1st discharge, and fifth discharge stages (Figure S6). At the pristine stage, the charge transfer resistance (at lower frequency 100−1400 Hz) of the Si−C nanocomposite negolyte (10Si-3C-7KB) is much smaller than that of the mechanically mixed negolyte (10Si-10KB-MM).

Figure 4. Galvanostatic discharge/charge profiles of 10Si-3C-7KB negolytes in Li half-cells with (a) no capacity limit (0.01 V cut off); (b) a fixed capacity of 3000 mAh g−1; and (c) a fixed capacity of 1380 mAh g−1. The current density is 0.4 mA cm−2.

cycling of Si−C nanocomposite negolyte between 0.01 and 1.2 V. Large irreversible capacity (>4200 mAh g−1) at the 1st two cycles and serious capacity decay at the subsequent cycles were observed. We believe that the extra capacity, irreversible capacity, and capacity decay were caused by the large fraction of electrolyte (80 vol %) in the negolyte, acting as the source of electrolyte decomposition.27 In addition, the carbon particle could exhibit irreversible lithium intercalation reaction and/or electrolyte decomposition at such a low voltage.27,65,66 To support these hypotheses, we examined the deep cycling behavior of the Si−C nanocomposite in the conventional solid electrode where the volume ratio of the electrolyte to the solid material is much lower than that in the negolyte suspension. As 7537

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Figure 5. (a) Galvanostatic discharge/charge profiles of 40Si-10C-5KB negolyte in Li half-cells at 0.4 mA cm−2. (b) Comparison of the specific capacity and volumetric capacity of reported negolytes including the work of Madec et al.,28 Ding et al.,13 and Gong et al.74

Figure 6. (a) Galvanostatic discharge/charge profiles of 10Si-3C-7KB negolytes in Li half-cells at a fixed capacity of 1380 mAh g−1 at 0.1 mA cm−2. (b) Cycling retention in specific charge capacity and Coulombic efficiency of 10Si-3C-7KB negolytes. (c) Intermittent-flow mode test of 10Si-3C7KB negolytes in Li half-cells.

achieved a high reversible capacity (>1200 mAh g−1) and stable cycle life (>100 cycles) with high Coulombic efficiency (>90%) in the Li-half cells. Figure 6a shows the galvanostatic discharge/ charge profiles for 10Si-3C-7KB at 0.1 mA cm−2 with a fixed discharge capacity of 1380 mAh g−1. The reversible charge capacity and the Coulombic efficiency were improved gradually upon cycling (Figure 6b), which is also observed at other current densities (Figure S8). The improved reversible charge capacity and the Coulombic efficiency could be related to the reduced contribution of the irreversible capacity associated with the KB/electrolyte at the low voltage27,65,66 after the 1st few cycles. It was reported that a solid-electrolyte-interphase (SEI) layer can be formed on Si-based electrode at potentials lower than 0.4 V vs Li/Li+ in the first few cycles until a stable SEI layer is formed.67,68 Further electrolyte decompositions may occur when the potentials go below 0.1 V vs Li/Li+, leading to

shown in Figure S7b, no extra capacity and reasonable capacity retention was observed,30,32 which is consistent with our hypothesis that the large ratio of electrolyte to active materials in the negolyte suspension contributed to the extra capacity and the decay in the deep discharge/charge cycle of the Si−C nanocomposite negolyte (Figure 4a). Capacity-limited cycling was applied to avoid the electrolyte decomposition at low voltages. Figure 4b,c shows the cycling behavior of the Si−C nanocomposite negolyte at 3000 mAh g−1 and 1380 mAh g−1, respectively. The capacity retentions of both cells are improved compared to the full capacity cycling (Figure 4a) with an obvious trade-off between specific capacity and the stability. At a limited capacity of 1380 mAh g−1, the Si−C nanocomposite negolyte shows stable cycling, representing a promising negolyte compared with the reported candidates of negolyte (Figure 5b). The Si−C nanocomposite semi-solid negolyte 7538

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Figure 7. (a) Capacity limitation of galvanostatic charge/discharge profiles of full cells of 10Si-3C-7KB negolytes and 5 M LiI posolytes at 0.1−0.2 mA cm−2 at 2nd and 10th cycles; (b) cycling retention in specific capacity and Coulombic efficiency of the full cell of 10Si-3C-7KB negolytes and 5 M LiI posolytes at 0.2 mA cm−2.

large irreversible capacity.68 In our work, the low Coulombic efficiency in the first few cycles was attributed to the electrolyte decomposition for forming the SEI layer on the Si-based negolyte.68,69 No continuous electrolyte consumption occurred once the stable SEI layer was fully formed.68 In addition, we exploit the capacity-limited cycling method, which helps to prevent the Si−C negolyte from reaching potentials below 0.1 V vs Li/Li+ and avoids further electrolyte decomposition. The reversible charge capacity was stabilized at around 1200 mAh g−1 with a Coulombic efficiency (CE) more than 90% (Figure 6b). In contrast, the 10Si-10KB-MM negolyte suffered severe capacity decay and decreased to less than 400 mAh g−1 after 8 cycles (Figure 2e). We note that the current density is mainly limited by the ohmic resistance associated with the 1 mm gap of the flow channel, as illustrated in the EIS measurements (Figure S9). We believe that the current density can be further increased with improved cell channel design including dimensions, shapes, and surface properties.18,28 The flowability and reversibility of 10Si-3C-7KB negolyte were examined in an intermittent-flow mode as the intermittent-flow mode is suggested as a more effective operation mode for the semisolid flow battery/capacitor considering the pump power consumption.10,14,18,70 Slugs of negolytes were injected (flow rate 100 uL min−1) into the half flow cell for discharge and charge tests. Figure 6c shows iterations of discharge/charge profiles of the 10Si-3C-7KB negolyte in an intermittent-flow mode at 0.2 mA cm−2. We note that the open circuit voltage (OCV) of the cell jumped from 0.7 V to above 1.8 V immediately after each injection of fresh negolyte, confirming the sensitivity of the intermittent-flow mode. The demonstrated discharge specific capacity was stable above 1380 mAh g−1 for each slug. Reversible cycling can be achieved for more than 100 h. The volumetric capacity of the negolyte depends on the volumetric concentration of the silicon and the discharge capacity. We examine the negolyte at various depths-ofdischarge (DOD) using a high silicon concentration at 40% (40Si-10C-5KB). As shown in Figure 5a, the negolyte achieved superior volumetric capacities 30−72 Ah L−1, which represents one of the highest demonstrated negolyte volumetric capacities in the literature (Figure 5b). The rheology of the non-Newtonian fluid semi-solid based flow negolyte can greatly influence the flow field distribution, which results in different active material concentration distributions and gradients. In conventional RFBs (e.g.,

VRFBs), the active species, are soluble in the electrolyte and behave as a Newtonian fluid. On the other hand, the semi-solid flow suspensions are a highly non-Newtonian fluid. We study the flow rate distribution of the negolyte by the mathematical modeling method by applying the non-Newtonian power law with the incompressible Navier−Stokes equation71 as follows: Incompressible Navier−Stokes equation: ⎛ ∂u ⃗ ⎞ ρ⎜ + u ⃗ ·∇u ⃗⎟ = −∇p + ∇·(μ(∇u ⃗ + ∇u ⃗)T ) ⎝ ∂t ⎠

Non-Newtonian Power Law:

μ = N(γ )̇ n − 1 where μ is the non-Newtonian fluid viscosity, N and n are the rheological constants which can be obtained from the viscosity measurement experiment, and ρ is the negolyte density which can be calculated from the volume ratio and density of the negolyte components. The width of flow channel is set to be 0.5 mm, and the length of flow channel is set to be 3 mm. We study the flow rate distribution at the midplane of the channel for different flow properties at an initial flow rate of 0.02 m s−1. The velocity boundary layer was thinner and the main flow rate is uniform (0.1−0.4 mm channel width) for the non-Newtonian fluid compared with the Newtonian fluid (Figure S10). The flow rate distribution for the Newtonian fluid is completely nonuniform in the midplane. As suggested by some previous work,72,73 the uniformity of electrolyte through the flow channel is critical to decrease the overpotential and increase the energy efficiency in RFBs. To study the influence of the viscosity of non-Newtonian fluid, we increase the viscosity of the Si-based flow negolyte to four times of the initial value (N = 13.12 to 52.48). We evaluate the flow rate distribution at the midplane of the channel for the two non-Newtonian flow negolytes as shown in Figure S11. Interestingly, the flow rate of the high viscosity negolyte was less uniform compared with the low viscosity negolyte, which may lead to higher overpotential of the flow cell. This is consistent with our experimental results showing that the Si negolyte with a lower volume ratio of KB (lower viscosity) shows lower overpotential compared to that with higher volume ratio of KB, as shown in Figure S12. This result also implies that the optimized flow rate for uniform distribution will vary with the viscosity of the non-Newtonian fluid. These 7539

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Chemistry of Materials observations suggest that more delicate flow rate control mechanisms are required for semi-solid flow suspensions compared with conventional Newtonian fluid. We apply the Si−C semi-solid negolyte into a full flow battery system to demonstrate its applicability in Li-RFBs. Highly concentrated lithium iodide (5.0 M LiI) was selected as the posolyte to couple with the Si−C nanocomposite negolyte, forming a lithium metal-free nonaqueous RFB. LiI has a high solubility in the nonaqueous solvents (>5 M in DOL/DME) and a standard potential around 3.0 V vs Li/Li+ (I−/I3− ∼ 3.0 V vs Li/Li+). Highly concentrated 5.0 M LiI posolyte is used considering the balance between volumetric capacity and solubility.15,23 In addition, excess capacity from LiI will ensure the Si−C negolyte to be the limiting factor of the full cell capacity. When coupled with Si−C negolyte, LiI provides the source of Li ions and thus eliminates the need of Li metal, which makes it a promising posolyte candidate for high-energydensity Li metal-free RFBs.15,75 A piece of solid state Li+ conducting membrane Li1.5Al0.5Ge1.5(PO4)3 (LAGP, Experimental Section) was applied as the separator to avoid the crossover between posolyte (5.0 M Li in 0.2 M LiClO4 DOL/ DME (1:1)) and negolyte (10Si-3C-7KB in 1 M LiPF6 in EC/ DEC (1:1)). A full flow battery configuration was assembled as shown in Figure S2c. The 5 M LiI posolyte and 10Si-3C-7KB negolyte were tested at a limited capacity of 1380 mAh g−1 of silicon or 2.6 V vs silicon with the cutoff voltage in static mode. Figure 7a shows the charge/discharge voltage profile for 2nd cycle and 10th cycle of this full battery configuration at various current densities. The cycling tests were done with the fixed discharge capacity in the Li/Si−C half-cell and a fixed charge capacity in the LiI/Si−C full cell. The increase in reversible capacity in both half and full cells could be attributed to improved Coulombic efficiency resulting from reduced irreversible capacity of carbon.27,65,66 In other words, with a fixed capacity applied, the capacity dedicated to the lithiation of silicon will increase when the irreversible capacity associated with carbon decreases. The overpotential of the 2nd and 10th cycles at 0.1 mA cm−2 was less than 0.2 V and increased with increasing current density. A stable cycle performance was achieved for more than 60 cycles with the Coulombic efficiency higher than 90% in static mode (Figure 7b). The reversible capacity and the Coulombic efficiency of the Si−C negolyte in the lithium metal-free full cell are comparable with that in the Li half-cell (>1200 mAh g−1, >90%, Figure 6b). This is the first demonstration of Si-based negolyte in a full all-flow cell configuration with stable cycle life, high Coulombic efficiency, and the highest full cell voltage (3 V) reported for Li metal-free RFBs. It highlights that the Si-based negolyte is a promising candidate to replace lithium metal to achieve a full flow Li-RFB.

composite negolyte, a lithium metal-free 3 V full all-flow battery coupled with a highly concentrated posolyte (5.0 M LiI) was achieved with high Coulombic efficiency (>90%) and long cycle life (>60 cycles). Our successful demonstration of lithium metal-free all-flow battery highlights the promising potential of Si-based negolyte to replace lithium metal for high-energydensity RFBs.

CONCLUSION In summary, the Si−C nanocomposite semi-solid negolyte was proposed and demonstrated as a promising candidate to replace lithium metal in Li-RFBs. We developed a facile incipient wetness impregnation method to synthesize monodispersed Si−C materials with ultrathin (9 nm) graphitic carbon coating. The Si−C nanocomposite negolyte achieved high reversible capacity (>1200 mAh g−1) with high Coulombic efficiency (>90%) and long cycle life (>100 cycles) in the Li half-cell. The Si−C nanocomposite effectively enhances the electrical conductivity and suppresses the nonuniform volume expansion of the Si−C negolyte, which are critical requirements for semisolid type suspensions. Using the designed Si−C nano-

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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/acs.chemmater.7b02561. Thermal gravity analysis, XRD, BET, Raman results, schematic illustration of partial discharge/charge process and half flow cell configuration, SEM images of Si nanopowder and Si−C nanocomposite, galvanostatic discharge/charge profiles of Si−C nanocomposite based solid electrode, galvanostatic discharge/charge profiles of 10Si-3C-7KB negolytes in Li half-cells at various current densities, EIS measurements for Si−C nanocomposite solid electrode in zero-gap cell and negolyte in 1 mm-gap cell, flow field comparison between Newtonian fluid and non-Newtonian fluid, Flow field comparison between low viscous Si-based negolyte and high viscous Si-based negolyte, and EIS measurements of 10Si-3C-7KB and 10Si-10KB-MM in Li half-cells (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Yi-Chun Lu. E-mail: [email protected]. ORCID

Yi-Chun Lu: 0000-0003-1607-1615 Author Contributions

Y.-C.L. conceived the project, Y.-C.L. and H.C. designed the experiments, and H.C. performed experiments and analyzed the data. N.-C.L conducted the material synthesis and characterization. The manuscript was written through contributions of all authors. All authors agree with the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by a grant from the Research Grants Council (RGC) of the Hong Kong Administrative Region, China, with Project No. CUHK14200615 and Theme-based Research Scheme through Project No. T23-407/13-N.

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REFERENCES

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