Prelithiation of Silicon–Carbon Nanotube Anodes for Lithium Ion

Letter pubs.acs.org/NanoLett

Prelithiation of Silicon−Carbon Nanotube Anodes for Lithium Ion Batteries by Stabilized Lithium Metal Powder (SLMP) Michael W. Forney,† Matthew J. Ganter,† Jason W. Staub,† Richard D. Ridgley,§ and Brian J. Landi*,†,‡ †

NanoPower Research Laboratories and ‡Chemical and Biomedical Engineering, Rochester Institute of Technology, Rochester, New York 14623, United States § United States Government, Washington, DC, United States ABSTRACT: Stabilized lithium metal powder (SLMP) has been applied during battery assembly to effectively prelithiate high capacity (1500−2500 mAh/g) silicon−carbon nanotube (Si-CNT) anodes, eliminating the 20−40% first cycle irreversible capacity loss. Pressure-activation of SLMP is shown to enhance prelithiation and enable capacity matching between Si-CNT anodes and lithium nickel cobalt aluminum oxide (NCA) cathodes in full batteries with minimal added mass. The prelithiation approach enables high energy density NCA/Si-CNT batteries achieving >1000 cycles at 20% depthof-discharge. KEYWORDS: Solid electrolyte interphase, SEI formation, irreversible capacity, prelithiation

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from being consumed during SEI formation by providing an alternative source of lithium. There are several reports23−25 of SLMP prelithiation of conventional electrode composites, but this Letter is the first report on SLMP prelithiation of nanostructured Si- or CNT-based electrodes. The Si-CNT anodes described in this letter were fabricated by plasma-enhanced chemical vapor deposition (PECVD) of silicon onto commercially available CNT sheets (Nanocomp Technologies, Inc.), as described elsewhere.9 The Si-CNT anodes have areal capacities of 1.9−7.2 mAh/cm2, based on the Si loading.9 Lithium nickel cobalt aluminum oxide (NCA) cathodes were fabricated with a standard slurry procedure (92% NCA, 4% PVDF binder, 4% SuperC) and coated onto aluminum foil. The NCA and Si-CNT electrodes were fabricated to have 5−10% excess anode capacity, when capacity matching, to reduce the possibility of lithium plating. Electrodes were cut into 6 mm × 6 mm squares and used in coin cells (2032 size) that were assembled with a Celgard 2325 separator and 1.2 M LiPF6 in 3:7 EC/EMC electrolyte. All coin cells were fabricated in an argon-filled glovebox. First cycle voltage profiles for Si-CNT anodes are shown in Figure 1. In Figure 1a, the Si loading is 36% Si w/w (1.9 mAh/cm2), and the CE is only 58% due to the high surface area of the Si and CNT current collector. The large voltage plateau that results from SEI formation during the first cycle lithium insertion is observed at ∼0.8−0.9 V versus Li/Li+. As a result, the lithium insertion capacity is 2498 mAh/g, whereas the extraction capacity is only 1459 mAh/g. It is important to note that >90−95% of this capacity comes from

igh capacity alloying anode materials for lithium ion batteries, such as silicon and germanium, have generated significant research interest in the last 10 years. Silicon, which has a lithium storage specific capacity of 4200 mAh/g,1 has been the topic of over 500 research papers in the last 3 years.2 Although silicon is a promising anode active material due to its extremely high specific capacity, one of its primary disadvantages is that large volume changes (>400%)3 can lead to rapid pulverization of Si particles and loss of capacity during cycling. A common approach to improving the cycling performance of silicon-based anodes is to use nanostructured silicon,1,4−7 but a drawback to this approach is that the high surface area of nanostructured materials significantly increases solid electrolyte interphase (SEI) formation on the first cycle.8 SEI formation on silicon anodes during the first cycle causes a high irreversible capacity loss and can result in low Coulombic efficiency (CE) values such as 25−75%, depending on the structure of the silicon and the composition of the anode composite.1 In addition to using high capacity active materials like silicon, free-standing electrodes have generated significant interest due to their potential to eliminate electrochemically inactive mass from metal foil current collectors.8−22 Similar to composite electrodes based on nanostructured silicon, free-standing electrodes often incorporate high surface area nanomaterials like CNTs or graphene, which causes low CE values (90%. There are several approaches that can be used to mitigate first cycle SEI loss, including (1) a sacrificial cathode that is replaced after one cycle, (2) prelithiation in a temporary battery using lithium foil, (3) excess cathode to offset the SEI loss, and (4) prelithiation with SLMP. Approaches (1) and (2) are undesirable, as they require a temporary battery to be fabricated, cycled, disassembled, and the anode inserted into a new battery. Approaches (3) and (4) only require a single battery to be built, but the use of excess cathode results in additional inactive mass after the first cycle, which lowers the energy density. Figure 2a demonstrates several cases of Si-CNT anodes compared to a state-of-the-art meso-carbon microbead (MCMB) anode, when paired with an NCA cathode, to illustrate the impact of the methods for managing SEI on the specific capacity of the entire electrode pack. In these

calculations, the mass of the NCA cathode, Celgard separator, and the anode are considered, along with the designed areal capacity of the electrodes, to determine the electrode pack specific capacity in units of mAh/mg. On the right axis, the improvement for each anode case is indicated as a percent improvement over the case of MCMB coated onto a copper foil current collector at 7.2 mAh/cm2 (black data), which is the baseline case. The red data shows the case where MCMB on copper is replaced with Si-CNT without any form of prelithiation. Below 4 mAh/cm2, the SEI loss on the first cycle is large enough to reduce the electrode pack specific capacity below that of conventional MCMB on copper, but at higher areal capacities the high specific capacity of the asprepared Si-CNT anode does lead to some improvement over the baseline. Using excess cathode (purple data) to compensate for the SEI loss enables some improvement at all areal B

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capacities, with 36% improvement at 7.2 mAh/cm2. Finally, prelithiation of the anode by SLMP (green data) yields a 67% increase in electrode pack specific capacity at 7.2 mAh/cm2. This increase nearly reaches the expected improvement for the ideal case, where a Si-CNT anode does not actually have any SEI loss (blue data), which would yield a 69% improvement relative to MCMB on copper at 7.2 mAh/cm2. In Figure 2b, the mass distribution of electrode pack components are shown for two cases at 7.2 mAh/cm2 areal capacity: state-of-the-art MCMB anode and an SLMP prelithiated Si-CNT. At this areal capacity, the mass of the electrode pack with an MCMB on copper anode is 93.9 mg/cm2, compared to only 56.2 mg/cm2 for the SLMP prelithiated Si-CNT anode. It is important to note that the SLMP mass only contributes 1% of the total electrode pack mass, despite the fact that it is compensating for ∼20% first cycle loss in the anode, since pure lithium has a specific capacity of 3860 mAh/g. In contrast, when excess NCA cathode is used, it only contributes 180 mAh/g, which underscores the advantage of using SLMP over excess cathode. The prelithiation process is described schematically in Figure 3 for an example 6 mm × 6 mm Si-CNT anode at ∼3−4 mAh/cm2

toluene, necessitating a small stir bar to keep the SLMP mixed in the toluene during SLMP deposition. For SLMP deposition, the Si-CNT anode is placed inside the coin cell can and a micropipet is used to drop-cast a 10 μL drop of SLMP/toluene onto the SiCNT anode (Figure 3b). At this volume, the droplet is small enough to stay on top of the Si-CNT while the toluene evaporates, depositing ∼250−300 μg of SLMP on the surface of the Si-CNT anode, as measured by an ultramicrobalance, which is the estimated amount of SLMP required to fully prelithiate the SiCNT anode based on the predicted irreversible capacity loss on the first cycle. After depositing the SLMP onto the Si-CNT anode, it is essential to pressure-activate the SLMP to properly utilize the lithium.23 This can be achieved through a mechanical press, as shown in Figure 3c. The pressure-activation step is done in a dry state, before adding electrolyte to the coin cell, where 100− 300 PSI is applied for 30−60 s. The effect of pressure-activation of SLMP is shown as a cartoon in Figure 3d, where the contact area between the SLMP and the Si surface is maximized by compressing the SLMP spheres onto the surface. This step ensures proper lithium diffusion into the anode and increases the consistency among batteries for the present case. After a coin cell has been fabricated with pressure-activated SLMP (p-SLMP), the anode is allowed to equilibrate for optimal utilization of the SLMP. Battery voltage and electrochemical impedance spectroscopy (EIS) were used to monitor the coin cell during this equilibration period. The change in the coin cell voltage as a function of equilibration time is plotted in Figure 4a, and the data suggests that the coin cell has equilibrated after 15−20 h. In addition, the EIS spectra were analyzed to determine when the prelithiation process had reached equilibrium. The spectra were fitted using a modified Randles circuit model that can be used to study interfacial impedance in battery electrodes and cells.26−30 The modified version of the model replaces the capacitor element with a constant phase element (CPE) and the Warburg element with a “modified restricted diffusion” element that can be physically related to electron diffusion and recombination at the interface31 (see Figure 4b for circuit diagram and EIS spectra). The interfacial diffusion and recombination rates are expected to change during the prelithiation process when lithium inserts into Si, until a stable state has been reached (i.e., maximum prelithiation prior to the application of charging current). Spectra were fitted in EC-Lab (Bio-Logic) with |Z| weighting; the fit quality was χ2 < 0.02 for all spectra, indicating a high quality fit to the data. Figure 4c shows the change in three fitting parameters as a function of equilibration time for a representative NCA/Si-CNT coin cell that has been p-SLMP prelithiated: RS is the solution resistance, RCT is the charge transfer resistance, and RMRD is one of the parameters of the “modified restricted diffusion” circuit element. The RS is measured where −Zim(Ω) = 0 and it is stable from 0 to 100 h at ∼5.5 Ω. The RCT is defined by the semiellipse in the spectrum, and it decreases during the first 40 h and restabilizes around 50 h near the original value of 115 Ω. The tails of the EIS spectra are defined by the “modified restricted diffusion” element, which is represented by the change in the RMRD fitting parameter. RMRD drops by a factor of 4 during the first 30 h of equilibration time and has essentially stabilized by 40−50 h. The derivative of the EIS fitting parameter data is plotted in Figure 4d, which confirms that the fitting parameters have stabilized by 40−50 h. These results suggest that it takes approximately 40−50 h of equilibration time for the p-SLMP to

Figure 3. (a) The 3% SLMP w/w is suspended in toluene and Si-CNT anode is cut to 6 mm × 6 mm, then (b) 10 μL of SLMP suspension is drop-cast onto the anode (sitting inside coin cell can) and the toluene is allowed to evaporate. (c) Manual press is used to pressure-activate the SLMP. (d) Cartoon of the effect of pressure-activation of the SLMP.

areal capacity. Note: the values listed below are only valid for this example and need to be customized based on the expected SEI loss for a given anode. First, SLMP powder is added to toluene to form an SLMP/toluene suspension at a 3% SLMP w/w loading (Figure 3a). Because of the low density of pure lithium (0.53 g/mL), the SLMP particles (97% Li) rise to the top of the C

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Figure 4. (a) Coin cell voltage as a function of equilibration time for a coin cell that has an NCA cathode paired with a Si-CNT anode that has been prelithiated by SLMP, (b) the impedance spectra of the same coin cell, (c) the stabilization of fitting parameters when the EIS spectra are fitted with a modified Randles circuit, and (d) the derivative of the fitting parameters plotted in (c).

fully prelithiate this type of Si-CNT anode through lithium diffusion and any spontaneous SEI formation that may occur, and that battery voltage may not be sufficient to determine when the prelithiation is complete for these Si-CNT anodes. Consequently, all coin cells were allowed to equilibrate for at least that long such that all components within the battery (i.e., SLMP prelithiation of the anode active material, cathode, etc.) have stabilized before electrochemical cycling. Representative first cycle voltage profiles of coin cells that have NCA cathodes paired with Si-CNT anodes are shown in Figure 5. Figure 5a shows the charge profiles of coin cells under three fabrication conditions: no SLMP prelithiation (red), SLMP dropcast onto the Si-CNT anode (black), and pressure-activated SLMP (p-SLMP, blue). The charge profiles for drop-cast SLMP and anodes without SLMP show a pronounced sloping voltage plateau during the first 0.5 mAh of charge, which is attributed to SEI formation on the silicon and CNT current collector. As mentioned previously, the SEI formation will cause irreversible capacity loss, and the presence of such a voltage plateau indicates that an Si-CNT anode has not been properly prelithiated. When an Si-CNT has been properly prelithiated, the voltage profile from SEI formation is not present, as shown for the p-SLMP (blue) in Figure 5a. The resulting impact of prelithiation on the coin cell discharge capacity has been compared in Figure 5b−d. Figure 5b

shows three representative discharge voltage profiles for NCA/SiCNT coin cells where the anode was not prelithiated. The discharge capacities were 0.58 ± 0.06 mAh, which averages only 52% of the target discharge capacity of 1.12 mAh due to the irreversible SEI loss on the first cycle. Some improvement is observed when the SLMP is simply drop-cast onto the anode (no pressure-activation), and Figure 5c has three discharge voltage profiles with discharge capacities of 0.81 ± 0.11 mAh, which averages to 72% of the target discharge capacity. Under this fabrication protocol, the SLMP appears to have varied effectiveness, which results in a wider distribution of discharge capacities. In contrast, when the SLMP is pressure-activated, the discharge voltage profiles in Figure 5d are tightly grouped and have discharge capacities of 1.14 ± 0.02 mAh, which matches the target discharge capacity, within error. Thus, these results demonstrate that SLMP is a viable approach for prelithiating high surface area silicon-based free-standing anodes. The data shows that it is critical to properly apply and pressure-activate the SLMP during fabrication, and that the coin cells must also be allowed to equilibrate for 40−50 h for the SLMP to properly diffuse into the anode under the current conditions. Post-mortem cross-sectional SEM images were analyzed to determine how SLMP prelithiation affects the Si-CNT anode thickness and morphology during the 40−50 h equilibration, D

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(∼40 to ∼100 μm), whereas prelithiated Si-CNT anodes only expand approximately 50% (∼80 to ∼120 μm), as shown in Figure 6b and 6e, respectively. When discharged to 2.5 V, the Si-CNT anodes with (Figure 6c) and without (Figure 6f) prelithiation both decrease in thickness by approximately 20% (∼120 to ∼100 μm and ∼100 to ∼90 μm, respectively), suggesting that the variation in Si-CNT anode thickness during cycling is approximately 10−20%, independent of whether the anode was prelithiated. After prelithiation, test coin cells were subjected to prolonged cycling under two cycling regimes: (1) 100% depth-ofdischarge (DOD), after 10 conditioning cycles at a C/10 rate, and (2) 20% DOD, after 5 conditioning cycles at a C/10 rate. For the 100% DOD testing, the coin cells were cycled at a C/5 rate (charge and discharge), relative to the discharge capacity at the end of the conditioning cycles. Cycling results at 100% DOD are

Figure 5. (a) First cycle charge voltage profiles, and the first cycle discharge voltage profiles for three replicates of coin cells (b) without prelithiation, (c) with SLMP prelithiation, and (d) with pressureactivated SLMP prelithiation.

as well as after charging a coin cell to 4.3 V and discharging to 2.5 V. All Si-CNT anodes were paired with NCA cathodes in full coin cells, then equilibrated, equilibrated/charged, or equilibrated/charged/discharged. The coin cells were then disassembled, and the electrodes were rinsed with EMC and vacuum-dried overnight (100 °C). Samples were prepared for cross-sectional SEM by razor cutting and were mounted orthogonally to the SEM sample holder. During the 40−50 h equilibration, the lithium from the SLMP inserts into the Si-CNT anode and results in a thickness that is 2× greater than without prelithiation (∼80 μm versus ∼40 μm, respectively), which can be seen by comparing Figure 6a to Figure 6d. Upon charging to 4.3 V, Si-CNT anodes without prelithiation must expand approximately 250% in thickness

Figure 7. (a) One hundred percent depth-of-discharge cycling of a coin cell with NCA versus Si-CNT (p-SLMP prelithiation) electrodes at C/5, after 10 cycles at C/10. (b) Twenty percent depth-of-discharge cycling at more demanding rates (charge: C/4; discharge: C/3).

Figure 6. (a−c) are post-mortem cross-sectional SEM images of Si-CNT anodes which were not prelithiated (500× magnification). The anodes have been (a) equilibrated for 50 h, then subsequently (b) charged to 4.3 V and (c) discharged to 2.5 V. (d−f) Corresponding images for Si-CNT anodes that have been prelithiated with SLMP. E

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(5) Liu, N.; Hu, L.; McDowell, M. T.; Jackson, A.; Cui, Y. ACS Nano 2011, 5 (8), 6487−6493. (6) Liu, X. H.; Zhong, L.; Huang, S.; Mao, S. X.; Zhu, T.; Huang, J. Y. ACS Nano 2012, 6 (2), 1522−1531. (7) Song, T.; Lee, D. H.; Kwon, M. S.; Choi, J. M.; Han, H.; Doo, S. G.; Chang, H.; Park, W. I.; Sigmund, W.; Kim, H.; Paik, U. J. Mater. Chem. 2011, 21 (34), 12619−12621. (8) DiLeo, R. A.; Ganter, M. J.; Thone, M. N.; Forney, M. W.; Staub, J. W.; Rogers, R. E.; Landi, B. J. Nano Energy 2012, 1−8. (9) Forney, M. W.; DiLeo, R. A.; Raisanen, A.; Ganter, M. J.; Staub, J. W.; Rogers, R. E.; Ridgley, R. D.; Landi, B. J. J. Power Sources 2013, 228 (0), 270−280. (10) DiLeo, R. A.; Frisco, S.; Ganter, M. J.; Rogers, R. E.; Raffaelle, R. P.; Landi, B. J. J. Phys. Chem. C 2011, 115 (45), 22609−22614. (11) Wang, J. Z.; Zhong, C.; Chou, S. L.; Liu, H. K. Electrochem. Commun. 2010, 12 (11), 1467−1470. (12) Landi, B. J.; Cress, C. D.; Raffaelle, R. P. J. Mater. Res. 2010, 25 (8), 1636−1644. (13) Hu, L. B.; Wu, H.; La Mantia, F.; Yang, Y. A.; Cui, Y. ACS Nano 2010, 4 (10), 5843−5848. (14) DiLeo, R. A.; Ganter, M. J.; Raffaelle, R. P.; Landi, B. J. J. Mater. Res. 2010, 25 (8), 1441−1446. (15) Dileo, R. A.; Castiglia, A.; Ganter, M. J.; Rogers, R. E.; Cress, C. D.; Raffaelle, R. P.; Landi, B. J. ACS Nano 2010, 4 (10), 6121−6131. (16) Cui, L. F.; Hu, L. B.; Choi, J. W.; Cui, Y. ACS Nano 2010, 4 (7), 3671−3678. (17) Choi, J. W.; Hu, L. B.; Cui, L. F.; McDonough, J. R.; Cui, Y. J. Power Sources 2010, 195 (24), 8311−8316. (18) Landi, B. J.; Ganter, M. J.; Cress, C. D.; DiLeo, R. A.; Raffaelle, R. P. Energy Environ. Sci. 2009, 2 (6), 638−654. (19) Chew, S. Y.; Ng, S. H.; Wang, J. Z.; Novak, P.; Krumeich, F.; Chou, S. L.; Chen, J.; Liu, H. K. Carbon 2009, 47 (13), 2976−2983. (20) Landi, B. J.; Ganter, M. J.; Schauerman, C. M.; Cress, C. D.; Raffaelle, R. P. J. Phys. Chem. C 2008, 112 (19), 7509−7515. (21) Ng, S. H.; Wang, J.; Guo, Z. P.; Wang, G. X.; Liu, H. K. Electrochim. Acta 2005, 51 (1), 23−28. (22) Kim, D. W. J. Power Sources 2000, 87 (1−2), 78−83. (23) Li, Y. X.; Fitch, B. Electrochem. Commun. 2011, 13 (7), 664− 667. (24) Jarvis, C. R.; Lain, M. J.; Yakovleva, M. V.; Gao, Y. J. Power Sources 2006, 162 (2), 800−802. (25) Jarvis, C. R.; Lain, M. J.; Gao, Y.; Yakovleva, M. J. Power Sources 2005, 146 (1−2), 331−334. (26) Dokko, K.; Mohamedi, M.; Umeda, M.; Uchida, I. J. Electrochem. Soc. 2003, 150 (4), A425−A429. (27) Roto, R.; Villemure, G. J. Electroanal. Chem. 2002, 527 (1−2), 123−130. (28) Dokko, K.; Mohamedi, M.; Fujita, Y.; Itoh, T.; Nishizawa, M.; Umeda, M.; Uchida, I. J. Electrochem. Soc. 2001, 148 (5), A422−A426. (29) Funabiki, A.; Inaba, M.; Ogumi, Z.; Yuasa, S.; Otsuji, J.; Tasaka, A. J. Electrochem. Soc. 1998, 145 (1), 172−178. (30) Mauracher, P.; Karden, E. J. Power Sources 1997, 67 (1−2), 69− 84. (31) Bisquert, J. J. Phys. Chem. B 2002, 106 (2), 325−333.

shown in Figure 7a. At 50 cycles, the coin cell retained 93% of the original capacity, which is promising for extended cycling. Separate coin cells were tested at 20% DOD with charge and discharge rates of C/4 and C/3, respectively, meaning that the coin cell was discharged to 20% of the discharge capacity at a C/10 rate . The cycling results are shown in Figure 7b, where the cycling performance is assessed by the end-of-discharge voltage (EODV) after discharging 20% of the coin cell capacity. Under these cycling conditions, NCA/Si-CNT coin cells show stable cycling with EODV values at ∼3.1 V after >1000 cycles. These results demonstrate that the prelithiated Si-CNT anodes can be cycled for hundreds of cycles at reduced DOD. In summary, irreversible capacity loss due to SEI formation during the first cycle is an obstacle that must be overcome for commercial application of any electrodes that incorporate nanostructured materials, including the Si-CNT anodes presented here. It has been shown that Si-CNT anodes can be effectively prelithiated with SLMP to counteract first cycle capacity loss, and this method should be applicable to any other advanced electrode systems that have high surface area nanomaterial components. The prelithiated Si-CNT anodes presented in this Letter required 40−50 h of equilibration time to complete prelithiation after fabrication, achieved the target coin cell capacities with prelithiated anodes, and demonstrated stable cycling at 20 and 100% DOD. Thus, prelithiation of nanostructured electrodes using SLMP is a promising approach that should enable commercial implementation of nanostructured electrode materials that have a large irreversible capacity loss on the first cycle, yet can achieve dramatically higher battery energy densities than are possible with conventional electrodes.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +1 585-475-4726. Fax: +1 585-475-7890. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the U.S. Government, including a grant from the Intelligence Community Postdoctoral Research Fellowship Program through funding from the Office of the Director of National Intelligence. This material is based upon work funded in whole or in part by the U.S. Government and any opinions, findings, conclusions, or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the U.S. Government. The authors thank Reginald Rogers and Roberta DiLeo for helpful discussions regarding this work. The authors also wish to acknowledge Al Raisanen from the Manufacturing and Mechanical Engineering Technology/Packaging Science Department at RIT for his assistance with the PECVD system.

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REFERENCES

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