Self-Healing Phenomenon Observed During Capacity-Control Cycling

Feb 8, 2018 - Aero-propulsion, Mechatronics and Energy Center (AME),. §. High-Performance Materials. Institute (HPMI), and. ∥. Center for Advanced ...
0 downloads 5 Views 4MB Size
Download PDF
Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 7155−7161

www.acsami.org

Self-Healing Phenomenon Observed During Capacity-Control Cycling of Freestanding Si-Based Composite Paper Anodes for Li-Ion Batteries Kang Yao,*,†,‡,§ Jim P. Zheng,†,‡,∥,⊥ and Zhiyong Liang†,§,# †

Materials Science & Engineering, ‡Aero-propulsion, Mechatronics and Energy Center (AME), §High-Performance Materials Institute (HPMI), and ∥Center for Advanced Power Systems (CAPS), Florida State University, Tallahassee, Florida 32310, United States ⊥ Department of Electrical & Computer Engineering and #Department of Industrial and Manufacturing Engineering, Florida A&M University-Florida State University College of Engineering, Tallahassee, Florida 32310, United States ABSTRACT: A different strategy of capacity-control cycling under fixed upper and lower capacity/voltage limits is used in an attempt to seek an extended cycle life for Si nanoparticle− multiwalled carbon nanotube (Si−MW) electrodes for Li-ion batteries. For cells using Si−MW 1:1 (w/w) electrodes in the electrolyte of ethylene carbonate−diethyl carbonate−fluoroethylene carbonate (EC−DEC−FEC, 45:45:10 w/w/w) tested at a current of 1 mA, stable 326 charge/discharge cycles at a designated capacity of 506 mA h g−1 are attained. The new cycling protocol allows for the observation of a self-healing phenomenon by studying the specific capacities and charge/discharge end voltages. Prolonged cycling under capacity control (500 mA h g−1) and the interesting pattern of variations in the discharge/ charge end voltage are successfully reproduced under different electrode/electrolyte and current conditions: Si−MW 3:2 in the electrolyte of DEC−FEC (1:1 w/w) at 1 mA (490 cycles), Si−MW 3:2 in DEC−FEC at 0.5 mA (483 cycles), and Si−MW 1:1 in DEC−FEC at 0.5 mA (576 cycles), which can be explained by applying the proposed self-healing mechanism as well. KEYWORDS: Li-ion battery, Si anode, carbon nanotube, capacity-control cycling, self-healing cycle life of 40 cycles was attained, whereas 430 mA h g−1 gave 1500 cycles. Liu et al.16,17 implemented charge/discharge cycling at designated discharge capacity on Si and carbon (C)coated Si electrodes. In some cases, reaching the full designated discharge capacity for both electrodes took a couple of induction cycles. Two fading modes were identified for Si anodes, including a local mode due to loss of electronic contact between individual particles and a global mode due to failure of the entire electrode structure. The C-coated Si anode showed only the global fading mode but not the local one. The latter maintained 1000 mA h g−1 designated capacity for 57 cycles. Obrovac and Krause9 first demonstrated a constant current− constant voltage charging method to reversibly cycle crystalline Si at a limited capacity, while maintaining a two-phase microstructure of amorphous and crystalline Si. By maintaining the two-phase microstructure during cycling, only the amorphous phase was active, and excellent cycle life and Coulombic efficiency were achieved. Mazouzi et al.18 reported that Si−carbon black−sodium carboxymethyl cellulose composite electrode achieved more than 700 cycles at a discharge capacity limited to 960 mA h g−1 when a pH 3 buffer solution

1. INTRODUCTION The strategy of limiting lithium (Li) insertion to a certain extent has been employed by researchers in the hope of reducing volume expansion and improving the cycle life of silicon (Si)-based electrodes.1−9 While it may seem counterproductive to exploit a high capacity Si anode and then limit its capacity, such an Si anode can still offer a capacity as high as or higher than a traditional carbon anode.10,11 In general, limitedcapacity cycling of Si is attempted by restricting each discharge (lithiation) half-cycle to a fixed capacity. If the electrode composite is poor, this results in the lithiation of a larger amount of Si every cycle. This occurs because some Si particles become disconnected from the composite during charge (delithiation). Then, during the discharge half-cycle, more Si must be lithiated to reach the fixed capacity limit. After all the Si is lithiated, the fixed capacity can no longer be reached, and the cell capacity begins to drop abruptly.9 Yoshio et al.12,13 found that if the discharge capacity is restricted less than the corresponding capacity of Li1.71Si (1640 mA h g−1-Si), the big expansion of Si lattice due to phase transition can be considerably suppressed, and thus, the cyclability of Si-based electrodes can be improved. Jung et al.14,15 reported the strong influence of the cycling voltage or capacity on the cycle life of 50 nm thick amorphous Si thin film anodes. At a designated discharge capacity of 4200 mA h g−1, a © 2018 American Chemical Society

Received: December 18, 2017 Accepted: February 8, 2018 Published: February 8, 2018 7155

DOI: 10.1021/acsami.7b19246 ACS Appl. Mater. Interfaces 2018, 10, 7155−7161

Research Article

ACS Applied Materials & Interfaces was used for the electrode preparation. Iwamura et al.19 discovered a wrinkled structure formed from Si nanoparticles (SiNPs) at relatively early numbers of cycling through transmission electron microscopy. The state of the wrinkled structure could be frozen by restricting the lithiation degree below Li1.9Si. The C-coated SiNPs showed a constant discharge capacity of 1500 mA h g−1 during 100 cycles and exhibited an excellent rate performance (1500 mA h g−1 even at a rate of 3.3 C). In the study by Chakrapani et al.,20 the charge capacity instead of discharge capacity was limited to evaluate Si nanowire (SiNW) anodes grown on a stainless steel current collector in an ionic liquid electrolyte. The shallow capacity cycling protocol reduced the fraction of Li intercalated into/ deintercalated from the lattice and thus limited the extent of volumetric expansion/contraction. A cycle life greater than 650 cycles and a Coulombic efficiency close to 100% were obtained at 321 mA h g−1 charge capacity (similar to a carbon anode). In all previous studies, either the discharge capacity was limited with an upper cutoff voltage (the majority of the cases), or the charge capacity was fixed while the cell was discharged to a lower cutoff voltage. Only the Si thin film, SiNW, or slurrycast Si electrodes have been investigated so far. Herein, we applied the concept of capacity-control cycling on binder-free freestanding SiNP-based electrodes to extend their cycle life while maintaining a desirable reversible capacity. The galvanostatic cycling procedure is different from those used in the literature in that both voltage and capacity limits are set for the discharge and the charge processes, and therefore, the discharge and charge stop when the voltage or capacity limit is reached, whichever comes first. By monitoring the discharge/ charge capacity and the voltage at the end of each discharge/ charge, a self-healing phenomenon is observed, where a schematic three-layer structure with coexistence of crystalline Si, lithiated amorphous Si, and amorphous Si is proposed to explain the prolonged cycling. The self-healing mechanism can be applied when the same capacity-control cycling protocol is repeated under different electrode/electrolyte composition and current conditions.

ethylene carbonate−diethyl carbonate−fluoroethylene carbonate (EC−DEC−FEC, 45:45:10 w/w/w) and 1 M LiPF6 in DEC−FEC (1:1 w/w). An Li metal foil was used as the reference and counter electrode, and a glass fiber separator (EL-CELL) was placed between the electrodes. 2.2. Characterization. The microstructural image analysis was conducted using JEOL JSM-7401F field emission scanning electron microscopy (SEM). The elemental composition of the prepared composite was examined quantitatively by energy-dispersive X-ray spectroscopy (EDX) integrated with SEM. The galvanostatic charge/discharge cycling performance was tested using MTI battery analyzer systems. Electrochemical impedance spectroscopy (EIS) was performed at an ac voltage amplitude of 10 mV in a frequency range of 0.1−106 Hz using Gamry Instruments on the cells before (2.8 V vs Li/Li+) and after cycling (0.5 V vs Li/Li+). All the specific capacity values are based on the total weight of SiNPs and MWCNTs in the electrode. The cells were opened in the glovebox for postcycling analyses. The electrodes were extracted from the cells and carefully rinsed with dimethyl carbonate and then dried overnight at 70 °C on a hotplate in the glovebox. The electrode samples were transferred from the glovebox to the analytical instrument using a tightly closed container to avoid exposure to the air.

3. RESULTS AND DISCUSSION 3.1. Prolonged Cycling: Self-Healing Mechanism. The Si−MW 1:1 electrode samples were assembled into CR2032type coin cells with the EC−DEC−FEC electrolyte. The galvanostatic cycling procedure is different from those used in the literature in that both voltage and capacity limits are set for the discharge and the charge processes. In other words, for the discharge half-cycle, the discharge capacity limit is set at 506 mA h g−1, and the lower cutoff voltage is 0.005 V versus Li/Li+; the discharge stops when the voltage or capacity limit is reached, whichever comes first. For the charge half-cycle, the charge capacity limit is set to be 506 mA h g−1 and the upper cutoff voltage is 1 V versus Li/Li+; the charge stops when the voltage or capacity limit is reached, whichever comes first. The discharge/charge capacity and the voltage at the end of each discharge/charge are measured. The constant current is 1 mA (280 mA g−1). Figure 1a,b shows the specific charge/discharge capacities and Coulombic efficiency versus cycle number of Si− MW 1:1 cells cycled between a fixed voltage range of 1−0.005 V versus Li/Li+ (no capacity control) and cycled with capacity control, respectively. Figure 1c shows a plot of the voltage at the end of each discharge/charge half-cycle during the capacitycontrol cycling. In the first cycle with capacity control (Figure 1b,c), the 506 mA h g−1 discharge capacity is easily reached at an end voltage of 0.45 V versus Li/Li+ (this is different from the case in the literature,16,17 where a couple of induction cycles were needed to reach the full designated discharge capacity because of incomplete wetting of the electrolyte solution within the electrode using the water-based binder), but the charge is stopped by the upper cutoff voltage without reaching the designated charge capacity. It takes a few cycles at the beginning to reach the charge capacity limit, and then, the reversible capacity remains stable at 506 mA h g−1, and the Coulombic efficiency is 100% until the 326th cycle (in a few cases, the discharge is stopped at 0.005 V versus Li/Li+ and the discharge capacity limit is not reached, resulting in a Coulombic efficiency greater than 100%). Comparatively, for cycling without capacity control (Figure 1a), the reversible capacity continuously fades from the original high values to below 500 mA h g−1 after 173 cycles, though the Coulombic efficiency remains ∼99−100% after 30 cycles. The 325th cycle capacity is

2. EXPERIMENTAL SECTION 2.1. Electrode and Coin Cell Fabrication. The bind-free freestanding composite electrodes were prepared from commercial SiNPs and multiwalled carbon nanotubes (MWCNTs) by ultrasonication and positive pressure filtration. The raw materials and detailed preparation of the SiNP−MWCNT (Si−MW) composite paper were reported elsewhere.21 Briefly, SiNPs and MWCNTs at a mass ratio of 1:1 or 3:2 were dispersed in 1 vol % deionized (DI) water solution of Triton X-100 surfactant through ultrasonic agitation using a probe sonicator. A filtration process under positive pressure on the resultant dispersion was conducted using a 0.4 μm pore size polycarbonate (PC) membrane. During filtration, the MWCNTs and SiNPs were deposited onto the membrane surface to generate a dark green composite thin film. Upon natural drying overnight, the thin film could be peeled off from the PC membrane, creating a freestanding Si−MW composite paper. To remove the residual surfactant, the composite paper was washed with DI water, isopropyl alcohol, and DI water sequentially, followed by heat treatment at 500 °C in a nitrogen gas atmosphere for 1 h. Prior to cell assembly, the freestanding composite paper was cut into circular electrode sheets using half-inch punch and dried at 120 °C in vacuum overnight to make a working electrode without any additives or a Cu current collector. CR2032-type coin cells were constructed in a glovebox filled with argon gas for battery performance test. Two electrolyte compositions were studied: 1 M LiPF6 in 7156

DOI: 10.1021/acsami.7b19246 ACS Appl. Mater. Interfaces 2018, 10, 7155−7161

Research Article

ACS Applied Materials & Interfaces

Figure 2. Schematic of microstructures of Si during capacity-control cycling. (a) More and more Si is lithiated upon discharge. After charge, the resulting particle is imagined to comprise a crystalline Si core− amorphous Si shell structure (reprinted with permission from ref 9. Copyright 2006, The Electrochemical Society). (b) Designated charge capacity is reached without all lithiated Si being delithiated, forming a self-healing three-layer structure where crystalline Si, lithiated amorphous Si, and amorphous Si exist simultaneously. The lithiated amorphous Si layer is gradually delithiated but is able to protect the remaining crystalline Si until complete lithiation, contributing to a prolonged cycle life.

(0.005 V vs Li/Li+) is about to be met, the electrode self-heals by delithiating more LixSi instead of lithiating more Si. This is reflected in the end voltage change. In stage-2, for example, as the electrode becomes deeply lithiated, the upper end voltage first starts to drop because not all lithiated Si needs to be delithiated to reach the set charge capacity. When the discharge end voltage is about to hit the lower limit, the charge end voltage begins to increase, such that the electrode self-heals to avoid deterioration. This can be seen from 150th to 192nd cycles, where the discharge end voltage remains ∼0.02−0.03 V versus Li/Li+ while the charge end voltage increases from 0.50− 0.65 V versus Li/Li+. The occurrence of self-healing in stage-3 is similar to stage-2, except that the discharge end voltage is maintained at ∼0.01 V versus Li/Li+, while the charge end voltage increases continuously until the upper cutoff voltage (1 V vs Li/Li+) is reached. Figure 2b illustrates the self-healing mechanism, where designated charge capacity is reached without all lithiated Si being delithiated, forming a three-layer structure composed of crystalline Si, lithiated amorphous Si, and amorphous Si. The lithiated amorphous Si layer is gradually delithiated but is able to protect the remaining crystalline Si, contributing to prolonged cycle life. In stage-4, the protective lithiated amorphous Si layer is depleted and the Si is completely lithiated. At this point, both the upper and lower voltage limits are met and the designated capacities can no longer be reached. The discharge and charge capacities then start to fade as in the case of normal cycling. The capacity decay is similar to the global fading mode proposed by Liu et al. In their study, the Si anode exhibited a local fading mode characterized by the attainment of the designated discharge capacity along with a Coulombic efficiency being significantly less than unity and a global fading mode characterized by accelerated decline in attainable discharge capacity.17 In contrast, their C−Si anode showed only the global fading mode, and the cycling stability was remarkably improved because of the presence of the surface C coating. It can thus be concluded that in our freestanding electrode without any binder or current collector, the absence of the local fading and the long cycle life can mainly be attributed to the CNT matrix. The continuous conductive network of CNTs not only serves as a buffer to accommodate

Figure 1. Charge/discharge capacities and Coulombic efficiency vs cycle number of Si−MW 1:1 cells cycled (a) without and (b) with capacity control in the EC−DEC−FEC electrolyte under 1 mA current, and (c) voltage at the end of charge/discharge vs cycle number during capacity-control cycling in (b).

352 mA h g−1, corresponding to a capacity retention of only 26%. Apparently, the capacity control method improves longterm cycle performance. Interestingly, monitoring of end voltages during the capacitycontrol cycling suggests the presence of four different stages. Stage-1 (1st to 140th cycles) is characterized by capacitydominated discharge and voltage-dominated charge, stage-2 (141st to 192nd cycles) and stage-3 (193rd to 325th cycles) are characterized by capacity-dominated discharge and charge, while stage-4 (326th−500th cycles) is characterized by voltagedominated discharge and charge. In stage-1, both discharge and charge capacity limits are met after a few cycles, but the gradual decrease in the lower end voltage reveals that not all Li ions inserted during discharge are released during charge and a higher degree of lithiation is needed for the remaining active Si to provide for the 506 mA h g−1 target discharge capacity. This indicates that some individual Si particles begin to lose electrical contact upon charge because these particles are expanding during discharge, while they begin to shrink upon charge, leading to limited degradation of the composite electrode upon cycling. Figure 2a illustrates the process, where more and more Si is lithiated upon discharge; after charge, the resulting particle is imagined to comprise a crystalline Si core−amorphous Si shell structure.9 A self-healing phenomenon is observed for the first time in stage-2 and 3that is, any time when the lower cut-off voltage 7157

DOI: 10.1021/acsami.7b19246 ACS Appl. Mater. Interfaces 2018, 10, 7155−7161

Research Article

ACS Applied Materials & Interfaces

Figure 4 compares the Nyquist plots of the cells cycled without and with capacity control: (a) Si−MW 1:1 with the

the volume change of Si during charge/discharge cycling so the structural stability of the electrode can be enhanced but also allows the Si particles to remain in good electrical contact until failure of the entire electrode structure commences. 3.2. Testing with Different Electrode/Electrolyte and Current Rate. The practice of capacity-control cycling was subjected to more tests using different electrode and electrolyte compositions. The Si−MW 3:2 samples were assembled into CR2032-type coin cells with the DEC−FEC electrolyte, and galvanostatic cycling under 1 mA was carried out. During cycling, both voltage (1−0.005 V vs Li/Li+) and capacity (500 mA h g−1) limits were set for the discharge/charge process, as described earlier. Figure 3 shows (a) charge/discharge capacities and Coulombic efficiency and (b) voltage at the end of charge/

Figure 4. Nyquist plots of the cells cycled without and with capacity control: (a) Si−MW 1:1 with the EC−DEC−FEC electrolyte and (b) Si−MW 3:2 with the DEC−FEC electrolyte.

EC−DEC−FEC electrolyte as the case in Figure 1 and (b) Si− MW 3:2 with the DEC−FEC electrolyte as the case in Figure 3. The Si−MW 3:2 cell in DEC−FEC with capacity control underwent 800 cycles to see what might happen after signs of capacity fading emerged. The other three were tested for 500 cycles. For the cases with capacity control, the total resistance developed after cycling is lower than those without capacity control. Especially for the Si−MW 3:2 cell in DEC−FEC, it exhibits lower resistance after capacity-control cycling, even though it is subjected to 300 more cycles than the normal cycling. This is probably because capacity-control cycling results in less solid-electrolyte interphase (SEI) formation owing to the restricted lithiation/delithiation. The lower resistance, in turn, contributes to the cycle life extension. Figure 5 shows SEM images of the Si−MW 1:1 electrodes cycled in the EC−DEC−FEC electrolyte without (a) and with

Figure 3. (a) Charge/discharge capacities and Coulombic efficiency and (b) voltage at the end of charge/discharge as a function of the cycle number of the Si−MW 3:2 cell cycled with capacity control in the DEC−FEC electrolyte under 1 mA current.

discharge as a function of the cycle number of the cycled cell. Except for a few cycles at the beginning, the discharge/charge capacity is maintained at the designated value of 500 mA h g−1 for 490 cycles before the fading begins. Monitoring of the discharge/charge end voltage (Figure 3b) reveals similar patterns to Figure 1c, showing the stage (1st to 60th cycles) with capacity-dominated discharge and voltage-dominated charge, the stages (60th to 490th cycles) with capacitydominated discharge and charge, and the stage (490th−800th cycles) with voltage-dominated discharge and charge. The last stage or the fading stage where the voltage limit dominates both discharge and charge is when the capacity limit cannot be reached for both charge and discharge, which is undesirable. On the contrary, the stages in the middle, or the self-healing stages as we call it, are the key to achieving a long cycle life without capacity decay. Therefore, for the cell with a different electrode composition and a different electrolyte system, prolonged cycling and the interesting pattern of variations in the discharge/charge end voltage are successfully reproduced and can be explained by the proposed self-healing mechanism.

Figure 5. SEM images of Si−MW 1:1 electrodes cycled in the EC− DEC−FEC electrolyte (a) without and (b) with capacity control and of Si−MW 3:2 electrodes cycled in the DEC−FEC electrolyte (c) without and (d) with capacity control.

capacity control (b) and of the Si−MW 3:2 electrodes cycled in the DEC−FEC electrolyte without (c) and with capacity control (d). Table 1 lists the corresponding chemical compositions at the surface of the electrodes after long-term cycling without and with capacity control measured by EDX. As 7158

DOI: 10.1021/acsami.7b19246 ACS Appl. Mater. Interfaces 2018, 10, 7155−7161

Research Article

ACS Applied Materials & Interfaces

Table 1. Chemical Compositions at the Surface of the Si−MW Electrodes After Long-Term Cycling without and with Capacity Control by EDX electrode

electrolyte

capacity control

C (wt %)

O (wt %)

F (wt %)

Si (wt %)

P (wt %)

Si−MW 1:1

EC−DEC−FEC

Si−MW 3:2

DEC−FEC

no yes no yes

36.32 36.37 30.15 36.70

28.52 25.46 25.79 24.01

17.97 18.30 22.86 17.04

14.13 17.25 14.62 18.78

3.06 2.63 6.58 3.47

expected, formed SEI layers can be seen from all the images because long-term cycling leads to the growth of a large amount of the SEI. In Figure 5c, the Si−MW 3:2 electrode cycled in DEC−FEC without capacity control exhibits a porous flowerlike structure characteristic of the SEI derived from FEC-based electrolytes, which is another study that will be published separately.22 A comparison of Figure 5a,b shows that Si−MW 1:1 cycled in EC−DEC−FEC without capacity control is heavily covered with blocks of the deposited substance, and the CNTs are buried and can hardly be seen, while the one cycled with capacity control displays a flower-like structure with more visible CNT structures, which is indicative of less SEI buildup. This supports our deduction from the EIS analysis. We may also infer that the special flower-like structure can be formed not only in FEC-based electrolytes but also under capacitycontrol cycling. Therefore, the Si−MW 3:2 electrode cycled in DEC−FEC with capacity control should exhibit a structure bearing a resemblance to Figure 5c without capacity control. Figure 5d supports this inference. However, compared to Figure 5b and c, Figure 5d seems to contain more SEI layers after the capacity-control cycling. This is reasonable because Figure 5d was taken after 800 cycles while others were after 500 cycles. For Si−MW 1:1 in EC−DEC−FEC and Si−MW 3:2 in DEC−FEC, the elemental compositions in Table 1 show a reduced concentration of O species and an increased concentration of Si species for cycling with capacity control. Because O components are big contributors to the SEI, the lower O concentrations can be indicative of less SEI formation. Meanwhile, the amount of Si at the electrode surface reflects the thickness of the SEI,23 and therefore, the higher Si concentrations of the electrodes after cycling with capacity control than those without provide further evidence supporting the fact that less severe SEI formation occurs under capacity control. Therefore, the EDX results are in accordance with the previous EIS analysis and SEM observation. It can be concluded that by setting the capacity/voltage limits for charge/discharge cycling, the growth of the SEI can be limited. Regarding the analysis of light elements, the EDX facility is equipped with a beryllium (Be) entrance window for its Si(Li) detector and elements heavier than Be can be detected in principle. Background subtraction and ZAF correction, which takes into account atomic number effects (Z), absorption (A), and fluorescence (F), are used for percentage calculation, and the accuracy of quantitative analysis is typically 1% for the system. In addition, EDX analysis is not used solely but as a complementary tool in our study. Using the DEC−FEC electrolyte, we carried out more tests of capacity-control cycling on the Si−MW 3:2 and Si−MW 1:1 electrodes with the same testing protocol but at a lower constant current of 0.5 mA (140 mA g−1). Figure 6 shows the specific capacity, Coulombic efficiency, and voltage at the end of charge/discharge as a function of the cycle number for the

Figure 6. (a) Charge/discharge capacities and Coulombic efficiency and (b) voltage at the end of charge/discharge as a function of the cycle number of the Si−MW 3:2 cell cycled with capacity control in the DEC−FEC electrolyte under 0.5 mA current.

Si−MW 3:2 cell. Figure 7 shows the same for the Si−MW 1:1 cell. Again, the self-healing phenomena are reproduced in both cases. The Si−MW 3:2 and Si−MW 1:1 cells last for 483 and 576 cycles, respectively, before reaching the last stage with voltage-dominated discharge and charge where their capacities start to decrease gradually. Figures 6b and 7b show similar patterns of discharge/charge end voltage change for the two cells, including the beginning stage with capacity-dominated discharge and voltage-dominated charge, the self-healing stage with capacity-dominated discharge and charge, and the fading stage, which are the same as shown in Figures 1c and 3b. However, there is another stage characterized by voltagedominated discharge and capacity-dominated charge for the two cells cycled at 0.5 mA, which is different from the previous two cases cycled at 1 mA. In this new stage, the discharge end voltage reaches the lower voltage limit of 0.005 V versus Li/Li+ without meeting the lower designated capacity of 500 mA h g−1, and the charge capacity reaches the upper capacity limit without hitting the upper voltage limit. Therefore, parts of the curves in Figures 6a and 7a show the discharge capacities below 500 mA h g−1 and the Coulombic efficiencies above 100%. The discharge end voltage then remains flat, and the charge end voltage increases gradually along cycling to the upper voltage limit of 1 V versus Li/Li+. The appearance of the new stage is believed to be due to the lower current that is used for testing. Lower cycling current means longer charge/discharge time and thus more thorough 7159

DOI: 10.1021/acsami.7b19246 ACS Appl. Mater. Interfaces 2018, 10, 7155−7161

Research Article

ACS Applied Materials & Interfaces ORCID

Kang Yao: 0000-0002-4540-7811 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work is supported by DOE Batteries for Advanced Transportation Technologies (BATT) Program through Pacific Northwest National Laboratory (PNNL) with contract no. 212964.

■

(1) Kasavajjula, U.; Wang, C.; Appleby, A. J. Nano- and bulk-siliconbased insertion anodes for lithium-ion secondary cells. J. Power Sources 2007, 163, 1003−1039. (2) Gauthier, M.; Mazouzi, D.; Reyter, D.; Lestriez, B.; Moreau, P.; Guyomard, D.; Roué, L. A low-cost and high performance ball-milled Si-based negative electrode for high-energy Li-ion batteries. Energy Environ. Sci. 2013, 6, 2145−2155. (3) Markevich, E.; Fridman, K.; Sharabi, R.; Elazari, R.; Salitra, G.; Gottlieb, H. E.; Gershinsky, G.; Garsuch, A.; Semrau, G.; Schmidt, M. A. Amorphous columnar silicon anodes for advanced high voltage lithium ion full cells: dominant factors governing cycling performance. J. Electrochem. Soc. 2013, 160, A1824−A1833. (4) Obrovac, M. N.; Chevrier, V. L. Alloy negative electrodes for Liion batteries. Chem. Rev. 2014, 114, 11444−11502. (5) Obrovac, M. N.; Christensen, L. Structural changes in silicon anodes during lithium insertion/extraction. Electrochem. Solid-State Lett. 2004, 7, A93−A96. (6) Li, J.; Dahn, J. R. An in situ X-ray diffraction study of the reaction of Li with crystalline Si. J. Electrochem. Soc. 2007, 154, A156−A161. (7) Iaboni, D. S. M.; Obrovac, M. N. Li15Si4 formation in silicon thin film negative electrodes. J. Electrochem. Soc. 2016, 163, A255−A261. (8) Klett, M.; Gilbert, J. A.; Pupek, K. Z.; Trask, S. E.; Abraham, D. P. Layered oxide, graphite and silicon-graphite electrodes for Lithium-ion cells: effect of electrolyte composition and cycling windows. J. Electrochem. Soc. 2017, 164, A6095−A6102. (9) Obrovac, M. N.; Krause, L. J. Reversible cycling of crystalline silicon powder. J. Electrochem. Soc. 2007, 154, A103−A108. (10) Li, J.-Y.; Xu, Q.; Li, G.; Yin, Y.-X.; Wan, L.-J.; Guo, Y.-G. Research progress regarding Si-based anode materials towards practical application in high energy density Li-ion batteries. Mater. Chem. Front. 2017, 1, 1691−1708. (11) Xu, Q.; Li, J.-Y.; Sun, J.-K.; Yin, Y.-X.; Wan, L.-J.; Guo, Y.-G. Watermelon-inspired Si/C microspheres with hierarchical buffer structures for densely compacted lithium-ion battery anodes. Adv. Energy Mater. 2017, 7, 1601481. (12) Umeno, T.; Fukuda, K.; Wang, H.; Dimov, N.; Iwao, T.; Yoshio, M. Novel anode material for lithium-ion batteries: carbon-coated silicon prepared by thermal vapor decomposition. Chem. Lett. 2001, 30, 1186−1187. (13) Yoshio, M.; Wang, H.; Fukuda, K.; Umeno, T.; Dimov, N.; Ogumi, Z. Carbon-coated Si as a lithium-ion battery anode material. J. Electrochem. Soc. 2002, 149, A1598−A1603. (14) Jung, H.; Park, M.; Yoon, Y.-G.; Kim, G.-B.; Joo, S.-K. Amorphous silicon anode for lithium-ion rechargeable batteries. J. Power Sources 2003, 115, 346−351. (15) Jung, H.; Park, M.; Han, S. H.; Lim, H.; Joo, S.-K. Amorphous silicon thin-film negative electrode prepared by low pressure chemical vapor deposition for lithium-ion batteries. Solid State Commun. 2003, 125, 387−390. (16) Liu, W.-R.; Yang, M.-H.; Wu, H.-C.; Chiao, S. M.; Wu, N.-L. Enhanced cycle life of Si anode for Li-ion batteries by using modified elastomeric binder. Electrochem. Solid-State Lett. 2005, 8, A100−A103. (17) Liu, W.-R.; Wang, J.-H.; Wu, H.-C.; Shieh, D.-T.; Yang, M.-H.; Wu, N.-L. Electrochemical characterizations on Si and C-coated Si

Figure 7. (a) Charge/discharge capacities and Coulombic efficiency and (b) voltage at the end of charge/discharge as a function of the cycle number of the Si−MW 1:1 cell cycled with capacity control in the DEC−FEC electrolyte under 0.5 mA current.

and complete reactions, which is why, when cycled at the lower current, Si can be lithiated to 0.005 V versus Li/Li+ at an earlier stage than when cycled at the higher current. However, the lithiated amorphous Si layer (see the self-healing mechanism in Figure 2) can be delithiated more and more along cycling, but slowly, as evidenced by the relatively long stage of gradual increase of the charge end voltage in Figures 6b and 7b, contributing to the prolonged cycling.

4. CONCLUSIONS In summary, a new strategy of capacity-control cycling under fixed upper and lower capacity/voltage limits is employed on the nonconventional binder-free freestanding Si−MW composite paper anodes to seek an extended cycle life. For the Si− MW 1:1 cell in EC−DEC−FEC cycled at 1 mA, stable 326 charge/discharge cycles at a designated capacity of 506 mA h g−1 are attained. A self-healing phenomenon is observed by monitoring the specific capacities and charge/discharge end voltage and proposed as the possible mechanism behind the improved cycling stability. Prolonged cycling under capacity control at 500 mA h g−1 and the interesting pattern of variations in the discharge/charge end voltage are successfully reproduced with different electrode/electrolyte and current rate conditions: Si−MW 3:2 in DEC−FEC at 1 mA (490 cycles), Si−MW 3:2 in DEC−FEC at 0.5 mA (483 cycles), and Si−MW 1:1 in DEC−FEC at 0.5 mA (576 cycles). The proposed selfhealing mechanism can be applied to explain the observed extension of the cycle life under varied conditions. In addition, EIS and SEM-EDX analyses suggest that by setting the capacity/voltage limits for charge/discharge cycling, the growth of the SEI can be limited.

■

REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 7160

DOI: 10.1021/acsami.7b19246 ACS Appl. Mater. Interfaces 2018, 10, 7155−7161

Research Article

ACS Applied Materials & Interfaces particle electrodes for lithium-ion batteries. J. Electrochem. Soc. 2005, 152, A1719−A1725. (18) Mazouzi, D.; Lestriez, B.; Roué, L.; Guyomard, D. Silicon composite electrode with high capacity and long cycle life. Electrochem. Solid-State Lett. 2009, 12, A215−A218. (19) Iwamura, S.; Nishihara, H.; Kyotani, T. Fast and reversible lithium storage in a wrinkled structure formed from Si nanoparticles during lithiation/delithiation cycling. J. Power Sources 2013, 222, 400− 409. (20) Chakrapani, V.; Rusli, F.; Filler, M. A.; Kohl, P. A. Silicon nanowire anode: improved battery life with capacity-limited cycling. J. Power Sources 2012, 205, 433−438. (21) Yao, K.; Liang, R.; Zheng, J. P. Freestanding flexible Si nanoparticles−multiwalled carbon nanotubes composite anodes for Liion batteries and their prelithiation by stabilized Li metal powder. J. Electrochem. Energy Convers. Storage 2016, 13, 011004. (22) Yao, K.; Zheng, J. P.; Liang, R. Ethylene carbonate-free fluoroethylene carbonate-based electrolyte works better for freestanding Si-based composite paper anodes for Li-ion batteries. J. Power Sources 2018, 381, 164−170. (23) Nakai, H.; Kubota, T.; Kita, A.; Kawashima, A. Investigation of the solid electrolyte interphase formed by fluoroethylene carbonate on Si electrodes. J. Electrochem. Soc. 2011, 158, A798−A801.

7161

DOI: 10.1021/acsami.7b19246 ACS Appl. Mater. Interfaces 2018, 10, 7155−7161