Systematic Investigation of the Alucone-Coating Enhancement on

Sep 26, 2017 - Polyvinylidene fluoride (PVDF) is the most popular binder in commercial lithium-ion batteries, but is incompatible with a silicon (Sdi)...
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Research Article Cite This: ACS Appl. Mater. Interfaces 2017, 9, 40143-40150

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Systematic Investigation of the Alucone-Coating Enhancement on Silicon Anodes Seoung-Bum Son,† Yikai Wang,‡ Jiagang Xu,‡ Xuemin Li,†,§ Markus Groner,∥ Adam Stokes,⊥ Yongan Yang,§ Yang-Tse Cheng,‡ and Chunmei Ban*,† †

National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado 80401, Unites States Department of Chemical and Materials Engineering, University of Kentucky, Lexington, Kentucky 40506, United States § Department of Chemistry, Colorado School of Mines, 1012 14th Street, Golden, Colorado 80401, United States ∥ ALD NanoSolution, 580 Burbank Street, Unit 100, Broomfield, Colorado 80020, United States ⊥ Department of Materials Science, Colorado School of Mines, 1600 Illinois Street, Golden, Colorado 80401, United States ‡

S Supporting Information *

ABSTRACT: Polyvinylidene fluoride (PVDF) is the most popular binder in commercial lithium-ion batteries but is incompatible with a silicon (Si) anode because it fails to maintain the mechanical integrity of the Si electrode upon cycling. Herein, an alucone coating synthesized by molecular layer deposition has been applied on the laminated electrode fabricated with PVDF to systematically study the sole impact of the surface modification on the electrochemical and mechanical properties of the Si electrode, without the interference of other functional polymer binders. The enhanced mechanical properties of the coated electrodes, confirmed by mechanical characterization, can help accommodate the repeated volume fluctuations, preserve the electrode structure during electrochemical reactions, and thereby, leading to a remarkable improvement of the electrochemical performance. Owing to the alucone coating, the Si electrodes achieve highly reversible cycling performance with a specific capacity of 1490 mA h g−1 (0.90 mA h cm−2) as compared to 550 mA h g−1 (0.19 mA h cm−2) observed in the uncoated Si electrode. This research elucidates the important role of surface modification in stabilizing the cycling performance and enabling a high level of material utilization at high mass loading. It also provides insights for the future development of Si anodes. KEYWORDS: energy storage, lithium-ion batteries, silicon-based anodes, polyvinylidene fluoride binder, molecular layer deposition

1. INTRODUCTION Sustainable high-energy-density rechargeable batteries are critical in promoting electrification of transportation.1,2 Lithium-ion batteries (LIBs) have been considered as the most attractive energy storage system; however, there are increasing global demands for higher gravimetric and volumetric energy-dense systems with reduced cost and environmentally benign materials.3,4 These demands have led to the exploration of new materials, and silicon (Si) has been considered a promising candidate that can replace the conventional graphite anode. Si has a high theoretical capacity (3579 mA h g−1) compared to graphite (372 mA h g−1).5,6 However, the utilization of Si in LIBs has been hindered by several issues. First, Si particles experience huge volume expansionup to 300%which causes mechanical disintegration of the electrodes.7−9 Second, the formation of solid electrolyte interphases (SEIs) on the Si surface results in the loss of lithium inventory and low coulombic efficiency (CE).10,11 Furthermore, the unstable SEI and the newly formed interface induced by repeated volumetric expansion/ © 2017 American Chemical Society

contraction lead to the continuous formation of SEIs, which makes the electrode more insulated and leads to rapid degradation of cycling performance. To circumvent these issues, scientists have exploited various nano-Si structures to accommodate the volume changes of Si,12−16 explored functional polymer binders for the enhanced mechanical integrity,17−23 and developed electrolyte additives to help stabilize the SEIs of Si electrodes.24−27 On the other hand, our previous work demonstrated the success of using molecular layer deposition (MLD) to grow a flexible and mechanically robust surface coating for Si anodes to improve electrochemical cycling performance.28−32 Inspired by the success of the MLD technique, we here (1) pursue an indepth investigation of the role of surface modification in electrochemical behavior of the Si anode prepared with a conventional binderpolyvinylidene fluoride (PVDF)in Received: June 21, 2017 Accepted: September 26, 2017 Published: September 26, 2017 40143

DOI: 10.1021/acsami.7b08960 ACS Appl. Mater. Interfaces 2017, 9, 40143−40150

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ACS Applied Materials & Interfaces

reaction sequence is as follows: (i) dose TMA, (ii) hold TMA pressure static for 60 s, (iii) pump down, (iv) repeat i−iii until the mass spectrometer indicates a saturated substrate surface, (v) pump down and nitrogen purge, and (vi−x) repeat the above procedure with glycerol. This sequence constitutes one cycle of the alucone-MLD process. The electrodes were coated with 10 cycles of alucone. The alucone reaction sequence is as follows: (i) dose TMA until the mass spectrometer indicates saturation of the TMA surface reaction, (ii) purge with nitrogen, (iii) dose glycerol until the mass spectrometer indicates saturation of the TMA surface reaction, and (iv) purge with nitrogen. This sequence constitutes one cycle of the alucone-MLD process. Ten MLD cycles of TMA and glycerol reactions were applied on the laminated Si−CB−PVDF electrode. The growth rate for the alucone coating was determined to be 2.5 Å per cycle by using quartz crystal microbalance and X-ray reflectometry on the flat substrate.31 The high-resolution transmission electron microscopy (HRTEM) image shows the thickness of the amorphous layer on Si to be around 5 nm, including the native oxide layer, as indicated in Figure S1. The mass of the 10-MLD-cycle alucone coating is about 0.039 mg cm−2. 2.4. Nanoindentation Tests. Electrodes were mounted to thermosetting holders for nanoindentation tests with a diamond Berkovich indenter (tip radius around 200 nm) using Nano Indenter G200 (Keysight, U.S.). A load-controlled test mode was used. Before nanoindentation tests, the thermal drift was calibrated to be less than 0.5 nm s−1. The loading rate and the maximum load were 0.0735 mN s−1 and 1.47 mN, respectively. The indenter was held for 10 s at the maximum load and then was unloaded to 10% of the maximum load for thermal drift calibration (100 s). The maximum depth is much larger than the diameter of the Si particles. Therefore, the overall mechanical response of the composite electrode was probed. The substrate effect can be neglected because the maximum depth of the coated electrodes is less than 1/10 of the thickness of the electrode. The elastic modulus and hardness were determined based on the Oliver−Pharr’s method.33 For each sample, nanoindentation tests were conducted 20 times to guarantee the repeatability. 2.5. Scratch Tests. Nanoscratch testing was carried out using the NanoTest Vantage system (Micro Materials, U.K.) with a conical indenter (cone angle 60° and tip radius 5 μm). A multipass mode was used for the scratch tests. The surface roughness was detected during the prescan with a constant load of 0.1 mN. The scratch distance was set to be 2000 μm. The normal load was applied after 50 μm with the loading rate of 0.4 mN s−1, the maximum load of 50 mN, and the scanning rate of 10 μm s−1. After the maximum normal load was reached at the scratch distance of 1300 μm, the scratch test then continued with the maximum normal load. The scratch topography was obtained from the postscan with a constant load of 0.1 mN and was calibrated with the prescan data. For each sample, scratch tests were conducted five times to guarantee reproducibility.

terms of the mass loading and material utilization and (2) shed light on the future development of Si anodes. Gravimetric capacity is a widely used figure-of-merit to quantify the electrochemical behavior of battery materials, such as the Si material herein. However, at the cell level, the areal capacity should be considered to demonstrate the energy stored by the electrode. In addition to comparing the gravimetric capacity, we studied the areal capacity to investigate the coating effect on the correlation between the mass loadings and the level of Si utilization. PVDF failed to maintain the mechanical integrity of Si-based electrodes, but it offered a great opportunity to understand the impact of the alucone coating on the electrochemical property at the electrode level, without being influenced by other factors such as the interaction between binder and Si. Not surprisingly, a low areal capacity of 0.19 mA h cm−2 was observed in the pristine uncoated electrodes as a result of the low level of material utilization. Fast degradation in electrochemical cycling performance has been observed for the uncoated electrodes with higher mass loading. In addition, the areal capacity of the thick electrodes is even lower than that of the thin electrode. By contrast, remarkable success in improving the electrochemical property is achieved with the alucone-MLD coating. Specifically, the areal capacity of Si anodes is increased from 0.19 to 0.64 mA h cm−2 with the alucone coating. Moreover, alucone even enables the use of Si materials in high mass-loaded electrodes, and it results in an areal capacity of 0.90 mA h cm−2. Systematic investigation using nanoindentation and scratch tests further confirms that the alucone coating on the electrodes enhances the mechanical properties of Si anodeswith higher elastic modulus and hardness and improved cohesion strength between electrode components. The enhanced mechanical properties ensure the mechanical integrity of Si anodes during cycling and correlatively result in remarkably improved electrochemical performance of Si anodes.

2. EXPERIMENTAL SECTION 2.1. Microstructure Analysis. A focused ion beam (FIB; FEI, NOVA 200 dual-beam system) was used for the cross-sectional scanning electron microscopy (SEM) observation and energydispersive spectroscopy (EDS) line scanning. A Ga ion source was used for FIB sectioning. 2.2. Preparation of Electrodes and Coin Cells. Standard types of 2032 half-coin cells with Li metal foils as counter electrodes were prepared for these experiments. The anode mixture was composed of Si (Alfa Aesar, 50 nm), carbon black, and PVDF binder with a wt % ratio of 60−20−20% and mixed with a 1-methyl-2-pyrrolidinone (NMP) solution. The mixture was coated on the Cu foil and then dried under air. Before assembling the cells, punched electrodes (with a diameter of 1.4 cm) were dried overnight (120 °C) in a vacuum oven. LiPF6 (1.5 M) in ethylene carbonate, diethyl carbonate, and fluoroethylene carbonate (5:70:25) was used as the electrolyte. Cells were assembled in an Ar-filled glovebox and tested at room temperature. Constant current was applied during discharge and charge between the voltage range of 0.01 and 1.0 V. Electrochemical measurements were carried out using an Arbin 2000 battery test station, and the cycling stability data were plotted with the standard deviations method using three cells to confirm the reproducibility of the results. 2.3. Alucone Coating on the Electrodes. Alucone films were grown directly on nano-Si electrodes using the trimethylaluminum (TMA)/glycerol MLD process. Static precursor exposures were used to give the precursors sufficient time to diffuse into the high-aspectratio electrode structures. The electrodes were heated to 150 °C and dried under a low-pressure nitrogen flow in a hot-wall MLD reactor chamber. The glycerol was heated to 100−120 °C. The alucone

3. RESULTS AND DISCUSSIONS Figure 1a schematically shows the application of an MLD coating on a laminated electrode. TMA (Al(CH3)3) and

Figure 1. Alucone coating on Si−CB−PVDF electrode. (a) Schematic of alucone-MLD coating on Si−CB−PVDF-laminated anode. (b) HRTEM observation of the individual Si particles randomly selected from alucone-coated electrodes. 40144

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anode with a thickness of 12 μm (thin electrode) and a thickness of 17 μm (thick electrode) on the Cu current collector. Platinum (Pt) was deposited on top of the electrodes during FIB operation to protect the electrode surface from Ga+ ion milling. In both thin and thick electrodes, the aluminum (Al) signal from the alucone coating has been detected along the thickness, showing uniform distribution throughout the electrode. This result demonstrates that the alucone coating has been applied consistently to the whole region of the electrodes. This is one of the strongest features of the MLD technique that the coating can disperse through the whole electrodes despite the tortuous structure. Further investigation was performed on the Si anodes with a thickness of 25 μm to demonstrate the ability of the MLD technique to coat thicker electrodes, as shown in Figure S2. Even for the thick electrode with a tortuous, porous architecture, the uniform distribution of the alucone coating was observed in the MLD-coated electrode, verifying the uniqueness of the MLD technique in coating ultrathin hybrid organic−inorganic materials on a complex structure. Electrochemical performance was compared using galvanostatic discharge and charge tests in a half-cell configuration with a Li metal electrode as the counter electrode. The electrochemical cycling data are compared and plotted in Figures 3a and S3. To investigate the impact of coating on electrochemical properties, we compared the electrochemical cycling performance of the Si anodes with/without the alucone coating; we also studied the electrochemical cycling performance of the Si anodes as a function of electrode thickness/mass loading. Cells were cycled at a rate of 0.04 C (143 mA g−1) for the first five cycles, then 0.1 C (358 mA g−1) for the following five cycles, and finally 0.2 C (716 mA g−1) for the subsequent cycles between 10 mV and 1 V. PVDF is notorious for failing to maintain the mechanical integrity of Si electrodes upon cycling, and rapid capacity decay has been reported repeatedly in Si electrodes made with the PVDF binder.39,40 However, it was surprising to find that the uncoated thin Si anode can actually deliver the reversible capacity of ∼550 mA h g−1 for over hundreds of cycles. Therefore, if using thin electrodes (herein, less than 0.35 mg cm−2), the PVDF binder can maintain a somewhat ionic/electrical conductive matrix for some Si particles participating in the electrochemical reaction. As expected, the uncoated thick Si anode shows rapid capacity degradation, losing all of the capacity within 50 discharge/ charge cycles. By contrast, the alucone-coated Si anodes exhibit significant improvements in the specific capacity and CE for both thin and thick electrodes (as shown in Figure S3), achieving reversible capacity of 1490 and 1410 mA h g−1 at the 500th cycles, respectively. The difference in cycling performance is negligible for the coated thin and thick electrodes as compared to the vast difference observed in the uncoated thin and thick electrodes. Note that the gravimetric capacity for the coated electrodes was calculated based on the overall mass of the coated Si particles, without considering the mass contribution from the surface coating. The real gravimetric capacity of the Si particles should be ∼6.5% higher than the values in the figures because the mass of the electrodes increases by about 0.039 mg cm−2 after the MLD coating. Details of electrochemical reactions in each cycle can be found in the voltage profile and differential capacity (dQ/dV) plots, as shown in Figure 3b−i. A single voltage plateau at around 0.1 V, observed in the first lithiation for all of the electrodes, indicates the Li−Si alloy reaction. After the first

glycerol (C3H5(OH)3) were used as precursors to form an aluminum alkoxide coating,34 by repeatedly performing the following pair of self-limiting surface reactions, which define one MLD cycle: Step A: R−OH* + Al(CH3)3 → R−O−Al(CH3)*2 + CH4 Step B: R−O−Al(CH3)* + C3H5(OH)3 → R−O−AlC3H5(OH)*2 + CH4

where asterisks indicate surface species and R represents the substrate. Such aluminum alkoxides, also referred to as alucones, are part of a range of new type of metalcone materials, developed by MLD.34,35 Similar to atomic layer deposition, the MLD technique is based on repeating a set of sequential, self-limiting surface reactions to grow a film. Our research group has reported remarkable electrochemical performance improvements of Si anodes via alucone coatings, which has led to a significant interest in MLD for battery electrode applications.29,31,36−38 The alucone coating was applied on the laminated electrodes comprising Si, PVDF, and carbon black. Figure 1b shows the amorphous alucone coating on a crystalline Si particle, from an image obtained with HRTEM. The as-received Si particles were covered by the native oxide layer with a thickness of ∼2 nm, as shown in Figure S1a. The surface morphology of the pristine particles is quite rough as compared to the coated particles (Figure S1b). The surface morphology has been largely changed after the MLD coating, and the particles were conformally covered by the alucone coating. Intimate contact at the interface of the alucone coating and bulk Si was clearly observed with HRTEM, confirming that the coating is well-adhered on the Si particle with the MLD technique. Figure 2a,b presents the cross-sectional SEM images and EDS line scans of the MLD-coated Si anodes. We fabricated Si

Figure 2. Cross-sectional SEM observation on the alucone-coated electrodes with EDS line scanning. (a) Thin electrode with 12 μm thickness. (b) Thick electrode with 17 μm thickness.

anodes with different thickness and mass loading as shown in Table 1, and then applied the MLD alucone coating on the laminated Si anode. A tortuous structure is observed in this Si Table 1. Specific Details of the Laminated Electrodes (Thin and Thick Electrodes) electrodes

compositions

mass of Si (mg cm−2)

thickness (μm)

thin thick

Si 60 wt % CB 20 wt % PVDF 20 wt %

0.35 0.60

12 17

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Figure 3. Comparisons of the electrochemical cycling performance of the laminated electrodes. The alucone coating has been applied on both thin and thick electrodes. (a) Specific capacity as a function of cycling number. Specific charge (delithiation) capacity was considered for comparisons. Cycling stability data were plotted using three cells with the standard deviations method, and the error limits are ±7.2%. (b) Voltage profiles of the thin-uncoated electrode. (c) Voltage profiles of the thin-coated electrode. (d) Voltage profiles of the thick-uncoated electrode. (e) Voltage profiles of the thick-coated electrode. (f) dQ/dV plots of the thin-uncoated electrode. (g) dQ/dV plots of the thin-coated electrode. (h) dQ/dV plots of the thick-uncoated electrode. (i) dQ/dV plots of the thick-coated electrode.

(Figure 3g,i), which is consistent to the voltage profile (Figure 3c,e). The further lithiation reaction results in the much higher capacity observed in the coated electrodes as compared with the capacity obtained in the uncoated electrodes. The differential capacity (dQ/dV) plots in Figure 3f−i mirror the voltage profiles, but they more clearly show how the potentialswhere lithiation/delithiation occursevolve with cycling. After the first couple of cycles, the lithiation occurs at a much lower voltage in both thin- and thick-uncoated electrodes as compared to those in both thin- and thick-coated electrodes. Much higher overpotential associated with the alloy/dealloy reaction is established in the uncoated electrodes compared to that of the coated electrodes. No electrochemical activities were observed in the thick-uncoated electrode after 100 cycles. By contrast, the alucone coating preserved the electrochemical behavior for both thin and thick electrodes and sustained the cycling performance.

cycle, the lithiation curves for the subsequent lithiation exhibit sloping voltage curves. However, the voltage profile of the coated electrodes shows much higher capacity and lower voltage hysteresis than those of the uncoated electrodes. With cycling, the voltage profiles of uncoated Si gradually degrade and develop overpotentials, showing only about 550 mA h g−1 for thin electrodes and negligible specific capacity for thick electrodes after 50 cycles, whereas voltage profiles for uncoated Si anodes vary rapidly during cycles, and in strong contrast, stable electrochemical reactions have been achieved from the alucone-coated electrodes. Both thin- and thick-coated Si anodes show very similar voltage profiles from 50 to 500 cycles, indicating greatly improved cycling reversibility. The sudden voltage drop at around 0.01 V for the coated electrodes are related to further lithiation in Si electrodesfrom Li3.5Si to Li3.75Si.41 A peak at 0.01 V was clearly observed in the differential capacity (dQ/dV) plots for the coated electrodes 40146

DOI: 10.1021/acsami.7b08960 ACS Appl. Mater. Interfaces 2017, 9, 40143−40150

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are considered as an elastic−plastic solid, the elastic reversibility, that is, the ratio of reversible work to total work, is proportional to the ratio of hardness to the reduced modulus (H/E*)42

The capacity degradation of Si anodes (with PVDF as the binder) is commonly attributed to the destruction of the electrode microstructure because of the expansion/shrinkage of the Si particles during electrochemical cycling. Our previous work has proven that MLD coating could help maintain the microstructure integrity by improving the mechanical properties of Si electrodes. Here, we used nanoindentation to quantitatively investigate the effect of alucone coating on the mechanical properties of Si electrodes. Electrodes with 2.5 nm alucone coating (10 MLD cycles) and 5 nm alucone coating (20 MLD, this sample was only used for mechanical study) were compared with the uncoated electrodes. Typical nanoindentation load−displacement (L−D) curves are shown in Figure 4a. We found that the indentation depth in the electrode

W H =k u E* Wtot

(1)

where k is a constant related to the geometry of the indenter. Wtot is the total work (equal to the total area under the loading curve), and Wu is the reversible work (equal to the total area under the unloading curve). The ratio of Wu/Wtot indicates the fraction of reversible work over the total work during the indentation measurements. The higher the Wu/Wtot ratio, the larger the extent that indentation deformation can elastically recover. As shown in Figure 4c, the H/E* values of the 20 MLD-coated electrodes increased by 150% compared with the uncoated electrode. Therefore, the alucone-coated electrode with higher H/E* values is more resistant to the permanent plastic deformation. In other words, the microstructure of MLD-coated electrodes would be more recoverable and will be well-preserved during electrochemical cycling. Scratch tests have been performed on the as-prepared electrodes to evaluate the cohesion between electrode components as well as the adhesion strength of electrode materials to the copper (Cu) current collector. Figure 5g shows the scratch depth versus the scratch distance profiles (S−D curves). At the same normal load (including the maximum normal load), the scratch depth of the uncoated electrode is larger than that of the 10 and 20 MLD-coated electrodes, which indicates that the alucone coating improves the scratch resistance of the Si anodes. Delamination of Si anodes from the Cu current collector is clearly observed in the uncoated electrode at a scratch distance ranging from 900 to 1050 μm, as shown in Figure 5d. The fluctuations in the S−D curves are usually considered an indication of the fracture events in the film.43,44 Although there are fluctuations in the S−D curves of the MLD-coated electrodes, no obvious fracture was observed on their surfaces. In addition, there is no delamination of the coated electrodes from the Cu current collector in the whole range of scratch distances, which could be attributed to the improved mechanical properties by the alucone coating. Even though the thickness of the alucone coating is only a few nanometers, it is uniformly distributed throughout the depth of the electrode, and therefore, it can effectively improve the cohesive strength between electrode components and the scratch resistance of the electrode. Figure 6 shows the cycling performance of the uncoated and the alucone-coated electrode using the areal capacity (mA h cm−2) unit. Considering the amount of areal capacity achieved from the cell is important because it indicates the achievable energy at the electrode level. Theoretically, high areal capacity can be achieved by the combination of high specific capacity and high mass loading. However, in reality, a low areal capacity has been usually observed in the electrodes prepared using the PVDF binder because of the low material utilization. Not surprisingly, both uncoated electrodes deliver a negligible amount of areal capacity, whereas the uncoated thin electrode shows a slightly higher areal capacity (0.19 mA h cm−2 at 500 cycles) than the uncoated thick electrode, owing to the relatively higher material utilization at a very low mass loading. Surface modification with the alucone coating significantly increases the areal capacity of Si anodes up to 0.64 mA h cm−2

Figure 4. Nanoindentation results of the uncoated and coated electrodes. The alucone coating with a thickness of 2.5 nm (as noted with 10 MLD) and 5 nm (as noted with 20 MLD) has been applied on the Si-laminated electrodes. (a) Load−displacement (L−D) curves, (b) elastic modulus and hardness of the uncoated and coated electrodes, and (c) ratio of H/E* of the uncoated and coated electrodes.

corresponding to the same maximum load decreased by 41% after applying 10 MLD alucone coating. A 20 MLD coating further improved the deformation resistance of the electrode, with its maximum indentation depth being less than half of the 10 MLD-coated electrode. The elastic modulus and hardness were determined from the L−D curves based on the Oliver− Pharr method.33 As shown in Figure 4b, both elastic modulus and hardness increased after the MLD coating was applied. Remarkably, the elastic modulus of the electrode increased by more than 125% with only a 2.5 nm alucone coating (10 MLD). The improved mechanical properties are expected to help sustain the microstructure of the electrodes to volumetric changes during cycling. The elastic reversibility of the electrode is another important parameter to understand the relationship between mechanical properties and electrochemical performances. If Si electrodes 40147

DOI: 10.1021/acsami.7b08960 ACS Appl. Mater. Interfaces 2017, 9, 40143−40150

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Figure 5. Scratch test results of the uncoated and coated electrodes. Optical images of the uncoated and coated electrodes corresponding to a scratch distance of 250−300 μm: (a) uncoated electrode, (b) electrode coated with 2.5 nm of alucone (10 MLD), and (c) electrode coated with 5 nm of alucone (20 MLD). Optical images of the uncoated and coated electrodes corresponding to scratch distance of 900−1050 μm: (d) bare electrode, (e) electrode coated with 2.5 nm of alucone (10 MLD), (f) electrode coated with 5 nm of alucone (20 MLD), and (g) depth profiles of the uncoated and coated electrodes as a function of scratch distance.

Figure 6. Comparisons of the electrochemical cycling performance in units of mA h cm−2. The alucone coating greatly improves the material utilization, leading to increased areal capacity for the thick electrodes. Cycling stability data were plotted using three cells with the standard deviations method, and the error limits are ±7.2%.

for the thin-coated electrodes and 0.90 mA h cm−2 for the thick-coated electrodes. Both the coated thin and thick electrodes exhibit similar gravimetric capacities of 1490 and 1410 mA h g−1 at 500 cycles, respectively. This indicates a similar level of material utilization and a linear increase with the mass loading. In addition, the electrodes with a mass loading even higher than 0.60 mg cm−2 have also been fabricated to investigate the limit of using PVDF as the binder for Si electrodes. Interestingly, the areal capacity is independent of the mass loading after the mass loading reaches the limit (1 mA h cm−2), as indicated in Figure S4. The electrodes with a mass loading capacity above 1 mA h cm−2 have a decreased gravimetric capacity. Considering the high electrical/ionic resistance in high-mass-loading Si electrodes, further improvement in materials utilization requires a conductive network for high mass loading.

alucone-MLD coating, outstanding improvements are observed from both thin and thick electrodes; the areal capacity has been improved from 0.19 mA h cm−2 for the uncoated electrode to 0.90 mA h cm−2 for the coated electrode. The results demonstrate the importance of using surface modification for building a mechanically robust and resilient electrode and for high material utilization at high mass loading.

4. CONCLUSIONS The surface modification with the alucone-MLD coating offers significant improvements in mechanical properties and electrochemical performances of Si anodes fabricated with the PVDF binder. An in-depth investigation of the alucone-coating effect confirms that the enhancements in mechanical properties mechanical robustness and elastic reversibilitycontribute to the improved cycling performance. As a result, the highly reversible cycling performance has been achieved for the PVDF-enabled electrode with a capacity over 1490 mA h g−1 at the 500th cycles. Moreover, the alucone coating greatly increases the material utilization, so as to enable cycling thick Si electrodes with high mass loading. Correspondingly, with the

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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.7b08960. HRTEM images of the Si particles with and without alucone-MLD coating; cross-sectional SEM images and EDS line-scanning of the alucone-MLD-coated electrode; and the detailed electrochemical performances for both thin and thick electrodes with the areal capacity (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 303-384-6504. Fax: 303-384-6490. ORCID

Xuemin Li: 0000-0002-5214-8117 Yongan Yang: 0000-0003-1451-2923 Chunmei Ban: 0000-0002-1472-1496 Notes

The authors declare no competing financial interest. 40148

DOI: 10.1021/acsami.7b08960 ACS Appl. Mater. Interfaces 2017, 9, 40143−40150

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ACKNOWLEDGMENTS The support from the Vehicle Technologies Office, Hybrid Electric Systems Program, David Howell (Manager), Battery R&D, Brian Cunningham and Peter Faguy (Technology Managers), at the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy are gratefully acknowledged.

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DOI: 10.1021/acsami.7b08960 ACS Appl. Mater. Interfaces 2017, 9, 40143−40150