Improved Battery Performance of Nano-crystalline Si Anodes Utilized

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Improved Battery Performance of Nano-crystalline Si Anodes Utilized by Radio Frequency (RF) Sputtered Multifunctional Amorphous Si Coating Layers In-Kyoung Ahn, Young-Joo Lee, Sekwon Na, So-Yeon Lee, Dae-Hyun Nam, Ji-Hoon Lee, and Young-Chang Joo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17890 • Publication Date (Web): 08 Jan 2018 Downloaded from http://pubs.acs.org on January 8, 2018

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ACS Applied Materials & Interfaces

Improved Battery Performance of Nano-crystalline Si Anodes Utilized by Radio Frequency (RF) Sputtered Multifunctional Amorphous Si Coating Layers

In-Kyoung Ahn,‚ Young-Joo Lee,‚ $ Sekwon Na,‚ So-Yeon Lee,‚ † Dae-Hyun Nam,‚ Ji-Hoon Lee,* ” and Young-Chang Joo* ‚ †

‚Department

of Materials Science & Engineering, Seoul National University, Seoul 151-742, Republic of Korea

$Institute

of Advanced Machines and Design, Seoul National University, Seoul 151-742, Republic of Korea

ACS Paragon Plus Environment

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ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

†Research

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Institute of Advanced Materials (RIAM), Seoul National University, Seoul 151-742, Republic of Korea

”Advanced

Functional Thin Films Department, Surface Technology Division, Korea Institute of

Materials Science (KIMS), Changwon, Gyeongnam 51508, Republic of Korea

KEYWORDS. Si anode, RF sputter, Coating, Amorphous Si, Li-ion battery

ABSTRACT. Despite the high theoretical specific capacity of Si, commercial Li-ion batteries (LIBs) based on Si are still not feasible due to unsatisfactory cycling stability. Herein, amorphous Si (a-Si)-coated nanocrystalline Si (nc-Si) formed by versatile radio frequency (RF) sputtering systems is proposed as a promising anode material for LIBs. Compared to uncoated nc-Si (retention of 0.6% and coulombic efficiency (CE) of 79.7%), the a-Si-coated nc-Si (ncSi@a-Si) anodes show greatly improved cycling retention (C50th/C1st) of ~ 50% and a 1st CE of 86.6%. From the ex situ investigation with electrochemical impedance spectroscopy (EIS) and cracked morphology during cycling, the a-Si layer was found to be highly effective at protecting the surface of the nc-Si from the formation of solid-state electrolyte interphases (SEI) and to dissipate the mechanical stress upon de-/lithiation due to the high fracture toughness.


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Introduction

With the growing demand of high energy and power density batteries for electric vehicles, developing high-performance electrode materials has been strongly urged. Although Si-based anode materials exhibit an order of magnitude higher specific capacity (3579 mAh g-1) compared to graphite (372 mAh g-1), constructing commercial Li-ion batteries based on Si anodes is still not feasible. This is because of the poor cycling stability and the microstructural failure, e.g., cracking or delamination, of Si anodes caused by catastrophic volume changes of ~ 400% during the repeated insertion and extraction of Li ions.1 An effective strategy for enhancing the cycling performance of Si anodes is material design, because the volume expansion and contraction is an intrinsic property of Si after it electrochemically reacts with Li ions. To address this issue, a number of trials for nanostructuring2-4 or surface modification with oxides (SiO2,5,6 Al2O37-9 etc.) or carbon-based materials (carbon10,11 or graphene12,13) have been conducted. Nanostructuring of Si is beneficial for improving the cycling stability, due to the stresses released by the additional free volume located in the interparticular space and the enhanced rupture resistance by critical size effects.14,15 Unfortunately, this approach requires highly complicated processes and is difficult to apply to mass production. In contrast, surface modification can be a more effective way to enhance the cycling performance as well as the productivity because of the inhibition of volume expansion and the possibility of slurry casting cells. However, to date, most coating layers of electrode materials for Li-ion batteries have been fabricated through a solution process or atomic layer deposition (ALD), which includes complicated processing steps and limits the candidates


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for coating materials.16 In this case, the surface coating layer reduces the total capacity, while improving the capacity retention. On the other hand, the a-Si is a highly effective multifunctional coating material from the viewpoints of cycling stability and energy storage performance. First, a-Si experiences less volume changes compared to nc-Si due to eliminating the existence of the two phase regions during the de-/lithiation17 and isotropic lithiation processes,18 indicating a higher resistance to cracking followed by pulverization and delamination. In addition, due to the restrained volume expansion of a-Si, the a-Si coated nc-Si particles could be directly applied as anode layer without making artificial free volume space to release the tensile stress during delithiation, which have been provided by the complicated nanostructuring process. Second, a-Si also delivers a high theoretical capacity, which is comparable with that of nc-Si. Furthermore, there are no issues caused by unstable or poorly bonded interfaces between the surface coating layer and core regions because the chemical compositions for both regions are identical.19 Herein, a novel surface modification method to fabricate a hierarchically designed core-shell type anode of nano-crystalline Si (nc-Si)@amorphous Si (a-Si) is suggested using the modified radio frequency (RF) sputtering system developed by our group.16 By systematically coating the a-Si layer on nc-Si, we found that a-Si layer can be delivered a positive effect on cycling performance such as superior initial specific capacity, coulombic efficiency, and cycling stability. Furthermore, from the investigation based on ex situ electrochemical and microstructural observations of the cycled anodes, it is revealed that a-Si coating layers were highly effective at restraining the volume change upon de-/lithiation, formation of solid electrolyte interphase (SEI), and the accumulation of microstructural damages. As a result, the hierarchically designed nc-Si@a-Si anodes are the most promising materials in that the


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composite anodes exhibit exceptional cycling stability and electrochemical reversibility, while the total specific capacity is not sacrificed by introducing the surface coating layer. This study provided new directions into nanometer-scale powder Si anodes with a-Si coating layers based on a simple physical deposition method with systematically controllable coating thickness. Also, we believed that a-Si coating layer and the coating method applied to this study will aid in the establishment of versatile large-scale energy devices.

Results and Discussion

Figure 1. (a) Schematics of the modified RF sputtering system (left) and the nc-Si@a-Si particle after sputtering process (right). (b – d) Transmission electron microscopy (TEM) images of the uncoated nc-Si powder (b) and nc-Si@a-Si powders sputtered for 5 (c) and 10 (d) min. The


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bottom of (d) shows the fast Fourier transform (FFT) pattern images of the 10-min-coated ncSi@a-Si powder (the 1 and 2 regions indicate the core and the surface coating layer, respectively). (e) Calculated thickness of the a-Si coating layer as a function of sputtering time.

Figure 1a displays a schematic illustration of the modified RF sputtering system to coat the surface of arbitrarily shaped powders. The system was designed to prevent the loss of loaded powders during the deposition process and give a continuous motion of the nc-Si powders to facilitate a uniform surface coating. Briefly, a planar-type holder, which has been generally used for conventional sputtering, was replaced by a dome-shaped one where four vibrational motors were attached to the edge of the holder. The inset (red box) shows a schematic representation of the core/shell-type nc-Si@a-Si particles after the sputtering process. Figures 1b – d shows the TEM images of the uncoated nc-Si and the nc-Si@a-Si coated for 5 and 10 min, respectively. To systematically control the thickness of the a-Si coating layer, only the sputtering time was varied while the other processing conditions, including sputtering power, working pressure, and Ar gas flow rate, were fixed. As shown in Figure 1b, the average diameter of the uncoated nc-Si powder is ~ 120 nm. From Figures 1c and d, it was confirmed that the sputtered coating layer (brighter region compared to the core) fully covered the surface of the ncSi powder with a high uniformity. Also, sputtered a-Si coating layers were completely deposited on the all loaded particles (Figure S1). From investigating the fast Fourier transform (FFT) pattern obtained from the core (red box ‘1’) and shell (blue box ‘2’) regions (bottom of Figure 1d), it was revealed that the surface covered Si layer was in the amorphous phase and the microstructure of the core region was maintained as crystalline after the sputtering process. In Figure 1e, it was confirmed that the thickness of the coating layer monotonically increased with


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sputtering time (TEM images for 2-, 15-, and 30-min-coated nc-Si@a-Si anodes are depicted in Figure S2). From the above investigation, the modified sputtering system proved to be a promising methodology for inducing high coating uniformity with a systematically controllable coating layer thickness, even for nanometer-scale powders.

Figure 2. The electrochemical performances of the uncoated nc-Si and nc-Si@a-Si anodes for Li-ion batteries. (a) Delithiation capacity and coulombic efficiency of the anodes as functions of the cycle number and a-Si sputtering time. (b) Capacity retention (C50th/C1st, %) and 1st coulombic efficiency (%) according to the a-Si sputtering time. Cyclic voltammetry curves of the uncoated nc-Si (c) and nc-Si@a-Si (5 min) (d) for the first five cycles.


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To examine the effects of the a-Si coating layers on the cycling stability of the nc-Si anodes, galvanostatic cycling testing was conducted for the uncoated nc-Si and nc-Si@a-Si (sputtering time of 2, 5, 10, and 30 min) anodes, as shown in Figure 2. Notably, the first 5 cycles were cycled with a C-rate of C/20 as a formation step, and the cells were cycled with a C/10 rate during the remaining cycles. Comparing the 1st delithiation capacity for the uncoated nc-Si and the 2-, 5-, 10-, and 30-mincoated nc-Si@a-Si anodes, the capacities were calculated as 2238, 2239, 2413, 2138, and 2360 mAh g-1, respectively (Figure 2a). Notably, the gravimetric specific capacity normalized by the total mass of the active materials (nc-Si and a-Si) was not sacrificed after the introduction of the surface a-Si coating layers, while previously reported surface-decorated Si anodes exhibited ~ 30% (carbon-coated Si)20 and ~ 71% (Si/Al2O3 foam)8 lower specific capacity compared to the uncoated anode. During the first 5 cycles, the specific capacity of all the anodes was abruptly reduced, and it was assumed that the formation of the solid electrolyte interphase (SEI) layers was responsible for the capacity fading at the initial stages of the cycling. However, compared to the uncoated anode, the nc-Si@a-Si anode exhibited a retarded capacity degradation because the formation of the SEI layers was less favorable on the surface of the a-Si.21 Quantitatively, the improved coulombic efficiency (CE) of the 1st cycle, which is closely related to the degree of formation of the SEI layers, was observed at the nc-Si@a-Si anodes, indicating the inhibited irreversible side reactions by the a-Si surface layers. The 1st cycle CE for the uncoated nc-Si, and the 2-, 5-, 10-, 15-, and 30-min-coated nc-Si@a-Si anodes were 79.7, 76.3, 86.6, 81.1, 81.0, and 82.0%, respectively. As a result, it was confirmed that the a-Si coating layer simultaneously acts as an effective physical/chemical protection barrier and electrochemically active material.


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The effect of the a-Si surface coating on the cycling stability becomes more prominent after the first 5 cycles. Figure 2b shows the capacity retention, defined as C50th/C1st (%), for the uncoated nc-Si and the nc-Si@a-Si anodes. The uncoated nc-Si electrode exhibited the poorest capacity retention of ~ 0.6%. In contrast, the capacity retention of the nc-Si@a-Si electrodes was greatly improved, except for the 2-min-coated sample, in which the thickness of the surface coating layer was insufficient to protect the nc-Si surface from irreversible electrochemical reactions. Among the nc-Si@a-Si anodes, the 5-min-coated electrode showed the best capacity retention of 50% under a C-rate of C/10. Upon further increases in the thickness of the a-Si coating layers, the capacity retention was somewhat reduced. This trend was closely related to the critical size to initiate crack formation. Cracking was continuously formed until the size of each particle or grain reached the critical size where the mechanical stress upon volume change was totally dissipated through the interfacial shear stress.22,23 The optimal capacity retention of the nc-Si@a-Si was relatively higher or comparable with previous reports for surface-decorated crystalline Si-based anodes, with retentions of 47% (Si nanoparticles@polyaniline)24 and 51.8% (Si/TiSi2 nanonet).25 Notably, the synthesis procedures of previous studies were complex and time-consuming processes, while the nc-Si@a-Si anodes were simply synthesized using modified RF sputtering, which could be readily applicable to the conventional fabrication process of the battery industry. Figures 2c and d compare the cyclic voltammograms (CVs) of the uncoated nc-Si and ncSi@a-Si (5 min) for the initial 5 cycles. In both cases, unpaired, broad, cathodic current peaks were detected within the potential range from 1.6 V to 0.4 V during the first cathodic sweep (lithiation), as shown in the figure insets. These peaks originated from the formation of the SEI layer that accompanied the electrolyte decomposition.1,6,21 After the first cycle, the two cathodic


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peaks were observed centered at ~ 0.01 and ~ 0.16 V and corresponded to the formation of LixSi alloys upon insertion of Li+ ions, which were paired with the two anodic peaks at ~ 0.36 and ~ 0.53 V, respectively. Since the nc-Si@a-Si surface coating layer was also Si, the characteristics of its current peaks were almost identical with those of the uncoated nc-Si. However, there were two major differences in the CV curves when comparing the two electrodes. First, the peak intensity corresponding to the formation of the SEI layers for the uncoated nc-Si was more prominent compared to that of the nc-Si@a-Si (5 min). Second, the pair of anodic and cathodic peaks for the nc-Si@a-Si (5 min) was relatively sharper than that of the uncoated nc-Si. These results indicated that the a-Si surface coating layers were beneficial for improving the electrochemical reversibility and kinetics of the de-/lithiation process.


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Figure 3. Nyquist plots during cycling for the uncoated nc-Si (a) and 5-min coated nc-Si@a-Si (b) measured at the fully delithiated state at 1.0 V. (c) The equivalent circuit model to simulate the measured impedance curves. (d) Changes in the resistance (Re (electrolyte), Rsurface (film), and Rct (charge transfer)) with the cycle number calculated from the simulation of the Nyquist plots.

To further clarify the effect of the a-Si coating on the formation of then SEI layer and the charge transfer behavior upon cycling, an analysis using electrochemical impedance


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spectroscopy (EIS) was performed in the frequency range from 40,000 Hz to 0.01 Hz. Figure 3 shows the Nyquist plots of the uncoated nc-Si (a) and 5-min coated nc-Si@a-Si (b) anodes obtained at the 5th, 6th, 10th, 30th cycles in the fully delithiated state. To separate the resistance components contributing to the entire electrochemical reaction, the observed Nyquist plots were simulated using the equivalent circuit model shown in Figure 3c. The circuit consists of three resistances and two capacitances. Note that a Warburg impedance, which appears at the low frequency region, was not observed within the investigated frequency range. The three resistance components include the electrolyte resistance (Re), surface resistance (Rsurface), and charge transfer resistance (Rct). Each resistance component as a function of cycle number for the uncoated nc-Si and 5-min-coated nc-Si@a-Si anodes are depicted in Figure 3d. In both electrodes, Re was unchanged, indicating electrolytes remained stable within the cycling potential window. In contrast, the Rsurface, which is displayed as the semicircle at the high frequency region (inset of Figures 3a and b) of the uncoated nc-Si and nc-Si@a-Si anodes, exhibited different changing behaviors. Initially, the Rsurface of the uncoated nc-Si shows an ~ 3 times higher resistance compared to that of the nc-Si@a-Si. This result proves that the nc-Si@aSi can effectively protect the surface from the formation of an SEI layer, even at the initial cycles. At the following cycles, the Rsurface of the uncoated nc-Si quickly increased, while the Rsurface of the nc-Si@a-Si was readily saturated after a slight increase in the resistance during the initial stages of the cycles, and the nc-Si@a-Si anodes exhibited a much lower Rsurface compared to the uncoated anode at each cycle. Changes in Rct along with the cycle number were affected by the degree of SEI layer formation and microstructural fractures in the charge transfer process upon the insertion/extraction of Li ions. In case of uncoated nc-Si anodes, the SEI layers were thicker and cracking followed by delamination evolved by volume change is much severe


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compared to nc-Si@a-Si. Therefore, transfer of electron from the current collectors and supply of Li+ ions become poor. Accordingly, the Rct of the nc-Si@a-Si was smaller than that of the uncoated ns-Si at each cycle, as shown in Figure 3d. The improved electrochemical reaction kinetics were due to the stabilized surface and reduced microstructural damage with the aid of the surface a-Si coating layers. In the case of the uncoated nc-Si, cracking easily evolved from volume changes during the repeated de-/lithiation due to the absence of a physical barrier. This cracking led to delamination of the Si active materials from the current collector. In addition, a fresh surface of the uncoated nc-Si was continuously formed due to the cracking and pulverization, and the newly exposed surface behaved as reaction sites for the formation of additional SEI layers. Therefore, upon repeated cycling, the uncoated portion of the electrochemically inactive sites, i.e., dead volume, in the uncoated nc-Si anode became more prominent, leading to a poor cycling stability.28-30 In contrast, the a-Si coating layers restrained the formation of cracks because the nc-Si@a-Si electrode experienced a reduced volume change compared with the uncoated nc-Si due to the isotropic lithiation behavior of a-Si.23 Furthermore, a-Si is more resistive to the formation of SEI layers, and the SEI layers that do form on the a-Si are more dense and stable compared to those on crystalline Si.19,26,27 For this reason, the Rfilm and Rct of the nc-Si@a-Si remained lower than those of the uncoated nc-Si. From these results, it was confirmed that the capacity degradation associated with the formation of the SEI layers as well as the increase in charge transfer resistance was highly restricted with a-Si coating layers of only a few nanometers thick.


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Figure 4. FE-SEM (top of each row) and energy dispersive X-ray spectrometry (EDS) mapping (bottom of each row) images of the uncoated nc-Si (a) and 5-min-coated nc-Si@a-Si (b) anodes observed before the cycling, after the 1st de-/lithiation, 10th delithiation, and 50th delithiation.

The effect of the a-Si coating on the mechanical stability and integrity of the electrodes, i.e., macroscopic cracking morphology and delamination behavior, was investigated ex situ by SEM observation and energy dispersive X-ray spectrometry (EDS) analysis on the surface microstructure of the uncoated and a-Si-coated electrodes, which were disassembled at certain cycles of galvanostatic cycling (Figure 4). Before cycling, both electrodes showed a smooth surface, and Si was uniformly distributed over the entire surface of the electrodes. The smooth surface was preserved after the 1st lithiation process for both cases. The volume shrinkage evolved during the following delithiation process caused tensile stress on the electrode materials, because the electrode was constrained by the


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current collectors. In this condition, cracks were generated to dissipate the accumulated mechanical energy. However, after the 1st delithiation, the surface morphology and cracking behavior of the nc-Si@a-Si anodes were clearly distinguishable from those of the uncoated ncSi, as shown in the third column images of Figures 4a and b. The nc-Si@a-Si anodes exhibited a higher crack density, smaller size of crack islands, and narrower spacing between adjacent cracks compared to uncoated nc-Si. The reason for the distinction of the cracking morphology was attributed to the differences in the fracture toughness between the crystalline and amorphous Si. Owing to the higher fracture toughness of the a-Si compared to that of the c-Si,17,19 the ncSi@a-Si had a higher resistance to crack formation and propagation than uncoated nc-Si. For this reason, the formation of cracks is highly hindered and the propagation of generated cracks encounters a number of obstacles inducing a detour of the path for crack propagation for the case of the nc-Si@a-Si. In contrast, wide channeling cracks were formed in the uncoated nc-Si anode. Therefore, the nc-Si@a-Si (5 min) showed a smaller crack island and narrower crack spacing when a tensile stress was applied to the anodes. During the subsequent cycles, the evolution behavior of cracking more obviously differed, depending on the a-Si coating. In case of the uncoated nc-Si, almost all the Si islands were delaminated from the Cu current collectors, even at the 10th cycle, because of the continuously evolved cracks (top fourth column of Figure 4). The loss of active materials coincided with the specific capacity observed in Figure 2a where the capacity reached almost 0 after the 10th cycle. However, the cracking morphology formed after the 1st delithiation remained stable and each islands of active materials were perfectly adhered on the current collectors for the case of ncSi@a-Si anodes up to the end of the cycling (fourth and fifth column images of Figure 4b). Correspondingly, the nc-Si@a-Si anodes showed a superior cycling stability and higher specific


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capacity compared to the uncoated nc-Si. The delamination phenomenon was closely related to the critical crack size (Lcr), which is the maximum crack size where additional crack formation does not occur between the existing cracks.28 When the observed crack size was larger than Lcr, plastic strain localization that occurred in the films (Si) induced high dislocation activities at the interfaces. For this reason, the films were delaminated from the substrate (Cu current collectors) upon the mechanical deformation (repetitive de-/lithiation in this case). In contrast, if the observed crack size was below Lcr, the strain localization within the film did not exist because the stress level applied on the film was smaller than the plastic flow stress. Based on the above consideration, it was assumed that the average crack size of the nc-Si@a-Si anodes was smaller than Lcr, while that of uncoated nc-Si was larger than Lcr. Therefore, the nc-Si@a-Si anodes wellretained the initially generated crack morphology during the cycling without any sign of delamination, leading to an excellent cycling stability.

In conclusion, we demonstrated that a-Si surface coating layers were highly effective at enhancing the cycling stability and electrochemical reversibility of the nc-Si anodes. Few nanometer-thick a-Si layers were successfully coated on the surface of the nc-Si powders using a modified RF sputtering process, which exhibited high coating uniformity and a highly controllable thickness of surface coating layers. From the investigation on the cycling performance of the nc-Si@a-Si anodes, the capacity retention was greatly improved by introducing the a-Si surface coating layers. In addition, there was an optimal thickness to protect the surface of the nc-Si and reduce the side reactions, e.g., formation of SEI layers, which was demonstrated by the enhanced CE at the 1st cycle and the reduced peak intensity related to the irreversible reactions observed in the CV curves during the first cathodic potential sweep. The ex


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situ EIS investigations of the uncoated nc-Si and nc-Si@a-Si anodes confirmed that the capacity degradation due to the SEI layers and the accumulation of charge transfer resistance was effectively increased by nanometer scale a-Si coating layers. During repeated cycling, the cracking morphology and delamination behavior was clearly distinguishable, depending on the aSi surface layer. Due to the difference in the fracture toughness between the a-Si and c-Si, a stable cracked morphology (average size of cracked islands < Lcr) was formed after the 1st delithiation process for the nc-Si@a-Si, and all the Si-based materials were perfectly adhered to the current collectors, leading to a superior cycling stability. However, the uncoated nc-Si islands were mostly delaminated after 10 cycles.


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FIGURES.

Figure 1. (a) Schematics of the modified RF sputtering system (left) and the nc-Si@a-Si particle after sputtering process (right). (b – d) Transmission electron microscopy (TEM) images of the uncoated nc-Si powder (b) and nc-Si@a-Si powders sputtered for 5 (c) and 10 (d) min. The bottom of (d) shows the fast Fourier transform (FFT) pattern images of the 10-min-coated ncSi@a-Si powder (the 1 and 2 regions indicate the core and the surface coating layer, respectively). (e) Calculated thickness of the a-Si coating layer as a function of sputtering time.


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Figure 2. The electrochemical performances of the uncoated nc-Si and nc-Si@a-Si anodes for Li-ion batteries. (a) Delithiation capacity and coulombic efficiency of the anodes as functions of the cycle number and a-Si sputtering time. (b) Capacity retention (C50th/C1st, %) and 1st coulombic efficiency (%) according to the a-Si sputtering time. Cyclic voltammetry curves of the uncoated nc-Si (c) and nc-Si@a-Si (5 min) (d) for the first five cycles.


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Figure 3. Nyquist plots during cycling for the uncoated nc-Si (a) and 5-min coated nc-Si@a-Si (b) measured at the fully delithiated state at 1.0 V. (c) The equivalent circuit model to simulate the measured impedance curves. (d) Changes in the resistance (Re (electrolyte), Rsurface (film), and Rct (charge transfer)) with the cycle number calculated from the simulation of the Nyquist plots.


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Figure 4. FE-SEM (top of each row) and energy dispersive X-ray spectrometry (EDS) mapping (bottom of each row) images of the uncoated nc-Si (a) and 5-min-coated nc-Si@a-Si (b) anodes observed before the cycling, after the 1st de-/lithiation, 10th delithiation, and 50th delithiation.

ASSOCIATED CONTENT Supporting

Information.

The

following

files

are

available

free

of

charge.

Preparation of nc-Si@a-Si anode powders, Microstructural investigation, Electrochemical investigation, TEM images of the nc-Si@a-Si anode powders coated for 5 min at low magnification, TEM images of the nc-Si@a-Si anode powders coated for 2, 15, and 30 min, Potential profile of the uncoated nc-Si and nc-Si@a-Si anode coated for 2, 5, 10, 15, and 30 min recorded at the 1st, 6th, 30th, and 50th cycle.


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brief description (file type, i.e., PDF) brief description (file type, i.e., PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This research was supported by Creative Materials Discovery Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2017M3D1A1040688). SEM and TEM analysis was supported by the Research Institute of Advanced Materials (RIAM) in Seoul National University. This work was supported by internal grant from Korea Institute of Materials Science (PNK5460).


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ABBREVIATIONS

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(7) Kim, S.-O.; Manthiram, A. Low-cost carbon-coated Si-Cu3Si-Al2O3 nanocomposite anodes for high-performance lithium-ion batteries. Journal of Power Sources 2016, 332, 222-229. (8) Hwang, G.; Park, H.; Bok, T.; Choi, S.; Lee, S.; Hwang, I.; Choi, N. S.; Seo, K.; Park, S. A high-performance nanoporous Si/Al2O3 foam lithium-ion battery anode fabricated by selective chemical etching of the Al-Si alloy and subsequent thermal oxidation. Chem Commun 2015, 51, 4429-4432. (9) Li, B.; Qi, R.; Zai, J.; Du, F.; Xue, C.; Jin, Y.; Jin, C.; Ma, Z.; Qian, X. Silica Wastes to High-Performance Lithium Storage Materials: A Rational Designed Al2O3 Coating Assisted Magnesiothermic Process. Small 2016, 12, 5281-5287. (10) Li, X.; Meduri, P.; Chen, X.; Qi, W.; Engelhard, M. H.; Xu, W.; Ding, F.; Xiao, J.; Wang, W.; Wang, C.; Zhang, J.-G.; Liu, J. Hollow core–shell structured porous Si–C nanocomposites for Li-ion battery anodes. J. Mater. Chem. 2012, 22, 11014-11017. (11) Magasinski, A.; Dixon, P.; Hertzberg, B.; Kvit, A.; Ayala, J.; Yushin, G. Highperformance lithium-ion anodes using a hierarchical bottom-up approach. Nat Mater 2010, 9, 353-358. (12) Son, I. H.; Hwan Park, J.; Kwon, S.; Park, S.; Rummeli, M. H.; Bachmatiuk, A.; Song, H. J.; Ku, J.; Choi, J. W.; Choi, J. M.; Doo, S. G.; Chang, H. Silicon carbide-free graphene growth on silicon for lithium-ion battery with high volumetric energy density. Nat Commun 2015, 6, 7393-7400.


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(13) Son, I. H.; Park, J. H.; Kwon, S.; Choi, J. W.; Rummeli, M. H. Graphene Coating of Silicon Nanoparticles with CO2 -Enhanced Chemical Vapor Deposition. Small 2016, 12, 658667. (14) Jung, M.-S.; Seo, J.-H.; Moon, M.-W.; Choi, J. W.; Joo, Y.-C.; Choi, I.-S. A Bendable LiIon Battery with a Nano-Hairy Electrode: Direct Integration Scheme on the Polymer Substrate. Advanced Energy Materials 2015, 5, 1400611-1400618. (15) Takamura, T.; Ohara, S.; Uehara, M.; Suzuki, J.; Sekine, K. A vacuum deposited Si film having a Li extraction capacity over 2000 mAh/g with a long cycle life. Journal of Power Sources 2004, 129, 96-100. (16) Lee, J.-H.; Kim, J. W.; Kang, H.-Y.; Kim, S. C.; Han, S. S.; Oh, K. H.; Lee, S.-H.; Joo, Y.-C. The effect of energetically coated ZrOx on enhanced electrochemical performances of Li(Ni1/3Co1/3Mn1/3)O2 cathodes using modified radio frequency (RF) sputtering. J. Mater. Chem. A 2015, 3, 12982-12991. (17) Maranchi, J. P.; Hepp, A. F.; Kumtaa, P. N. High Capacity, Reversible Silicon Thin-Film Anodes for Lithium-Ion Batteries. Electrochemical and Solid-State Letters 2003, 6, A198-A201. (18) Berla, L. A.; Lee, S. W.; Ryu, I.; Cui, Y.; Nix, W. D. Robustness of amorphous silicon during the initial lithiation/delithiation cycle. Journal of Power Sources 2014, 258, 253-259. (19) Cui, L.-F.; Ruffo, R.; Chan, C. K.; Peng, H.; Cui, Y. Crystalline-Amorphous Core-Shell Silicon Nanowires for High Capacity and High Current Battery Electrodes. Nano Lett 2009, 9, 491-495.


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(20) Xue, L.; Xu, G.; Li, Y.; Li, S.; Fu, K.; Shi, Q.; Zhang, X. Carbon-Coated Si Nanoparticles Dispersed in Carbon Nanotube Networks As Anode Material for Lithium-Ion Batteries. ACS Appl Mater Interfaces 2013, 5, 21-25. (21) Schroder, K. W.; Celio, H.; Webb, L. J.; Stevenson, K. J. Examining Solid Electrolyte Interphase Formation on Crystalline Silicon Electrodes: Influence of Electrochemical Preparation and Ambient Exposure Conditions. The Journal of Physical Chemistry C 2012, 116, 19737-19747. (22) Graetz, J.; Ahn, C. C.; Yazami, R.; Fultz, B. Highly Reversible Lithium Storage in Nanostructured Silicon. Electrochemical and Solid-State Letters 2003, 6, A194-A197. (23) McDowell, M. T.; Lee, S. W.; Harris, J. T.; Korgel, B. A.; Wang, C.; Nix, W. D.; Cui, Y. In Situ TEM of Two-Phase Lithiation of Amorphous Silicon Nanospheres. Nano Lett 2013, 13, 758-764. (24) Lin, H. Y.; Li, C. H.; Wang, D. Y.; Chen, C. C. Chemical doping of a core-shell silicon nanoparticles@polyaniline nanocomposite for the performance enhancement of a lithium ion battery anode. Nanoscale 2016, 8, 1280-1287. (25) Zhou, S.; Liu, X.; Wang, D. Si/TiSi2 Heteronanostructures as High-Capacity Anode Material for Li Ion Batteries. Nano Lett 2010, 10, 860-863. (26) Martinez de la Hoz, J. M.; Leung, K.; Balbuena, P. B. Reduction Mechanisms of Ethylene Carbonate on Si Anodes of Lithium-Ion Batteries: Effects of Degree of Lithiation and Nature of Exposed Surface. ACS Appl Mater Interfaces 2013, 5, 13457-13465.


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A hierarchically-designed nc-Si@a-Si anodes was fabricated by sputtering and the anodes showed improved cycling stability without scarification of specific capacity.


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Figure 1. (a) Schematics of the modified RF sputtering system (left) and the nc-Si@a-Si particle after sputtering process (right). (b – d) Transmission electron microscopy (TEM) images of the uncoated nc-Si powder (b) and nc-Si@a-Si powders sputtered for 5 (c) and 10 (d) min. The bottom of (d) shows the fast Fourier transform (FFT) pattern images of the 10-min-coated nc-Si@a-Si powder (the 1 and 2 regions indicate the core and the surface coating layer, respectively). (e) Calculated thickness of the a-Si coating layer as a function of sputtering time. 190x125mm (300 x 300 DPI)



Figure 2. The electrochemical performances of the uncoated nc-Si and nc-Si@a-Si anodes for Li-ion batteries. (a) Delithiation capacity and coulombic efficiency of the anodes as functions of the cycle number and a-Si sputtering time. (b) Capacity retention (C50th/C1st, %) and 1st coulombic efficiency (%) according to the a-Si sputtering time. Cyclic voltammetry curves of the uncoated nc-Si (c) and nc-Si@a-Si (5 min) (d) for the first five cycles. 211x156mm (300 x 300 DPI)


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Figure 3. Nyquist plots during cycling for the uncoated nc-Si (a) and 5-min coated nc-Si@a-Si (b) measured at the fully delithiated state at 1.0 V. (c) The equivalent circuit model to simulate the measured impedance curves. (d) Changes in the resistance (Re (electrolyte), Rsurface (film), and Rct (charge transfer)) with the cycle number calculated from the simulation of the Nyquist plots. 190x169mm (300 x 300 DPI)



Figure 4. FE-SEM (top of each row) and energy dispersive X-ray spectrometry (EDS) mapping (bottom of each row) images of the uncoated nc-Si (a) and 5-min-coated nc-Si@a-Si (b) anodes observed before the cycling, after the 1st de-/lithiation, 10th delithiation, and 50th delithiation. 187x105mm (300 x 300 DPI)


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Improved Battery Performance of Nano-crystalline Si Anodes Utilized

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