Polymer Nanofiber-Guided Uniform Lithium Deposition for Battery Electrodes

Transcription

1 Polymer Nanofiber-Guided Uniform Lithium Deposition for Battery Electrodes Zheng Liang, Guangyuan Zheng, Chong Liu, Nian Liu, Weiyang Li, Kai Yan, Hongbin Yao, Po-Chun Hsu, Steven Chu, and Yi Cui *,, Department of Materials Science and Engineering, Department of Chemical Engineering, and Department of Physics, Stanford University, Stanford, California 94305, United States Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, United States Corresponding Author *

2 SUPPORTING INFORMATION Figure S1. Schematics of the fabrication processes of the polymer nanofiber mat. (a) Precursor solution synthesis; (b) PAN/PVP hybrid nanofiber by co-electrospinning and its

3 corresponding SEM image. The resulting nanofiber forms a core-shell structure with PAN as the core and PVP as the shell; (c) Proposed structural formula for oxidized PAN and its corresponding SEM image. PVP is removed from the hybrid fiber through thermal decomposition. Scale bars are 2 µm. Figure S2. SEM characterization and cycling performance of Li deposition on Cu-OxPAN* electrode. The Cu-OxPAN* electrode utilizes oxidized PAN nanofiber from precursors without the addition of PVP. (a) SEM image of oxidized PAN* nanofiber before electrochemical cycling. The non-uniform fiber morphology is due to the absence of PVP; (b) Top-view SEM image of Li deposition on Cu-OxPAN* electrode; (c) Top-view SEM image of internal structure of Li deposition on Cu-OxPAN* electrode after removing the top layer; (d) Cycling performance of Li deposition on Cu-OxPAN* electrode at 3 ma/cm 2 for a total of 1 mah/cm 2 of Li. The Cu-OxPAN* delivers an average Coulombic efficiency of 97% over 120 cycles. It exhibits similar Li deposition morphology and cycling performance compared to the Cu-OxPAN electrode with the addition of PVP, suggesting that PVP is added only to assist the electrospinning process, with little influence on the Li deposition. Scale bars are 2 µm in (a) and (b), 5 µm in (c).

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5 Figure S3. Statistics of the PAN/PVP fiber diameter distribution. (a) Pristine PAN/PVP before cycling; (b) Pristine PAN/PVP after 5 cycles of Li-plating-stripping at 3 ma/cm 2 ; (c) Oxidized PAN/PVP before cycling; (d) Oxidized PAN/PVP after 5 cycles of Li-platingstripping at 3 ma/cm 2. The oxidized fiber shows a volume expansion of ~15% after electrochemical cycling, compared with ~45% for pristine fiber, suggesting an enhanced chemical stability after oxidation. The narrow diameter distribution of PAN/PVP fiber is due to the existence of PVP. Scale bars are 2 µm.

6 Figure S4. Statistics of the PAN fiber diameter distribution. (a) Pristine PAN before cycling; (b) Pristine PAN after 5 cycles of Li-plating-stripping at 3 ma/cm 2 ; (c) Oxidized PAN before cycling; (d) Oxidized PAN after 5 cycles of Li-plating-stripping at 3 ma/cm 2. The

7 oxidized fiber shows a volume expansion of ~13% after electrochemical cycling, compared with ~40% for pristine fiber, suggesting an enhanced chemical stability after oxidation. The non-uniform fiber morphology is due to the absence of PVP. Scale bars are 2 µm. Figure S5. Statistics of the oxidized PAN fiber porosity distribution. The average porosity of the nanofiber is 85%, with the following electrospinning parameters: 15 kv of conducted voltage, 12 cm of nozzle-to-collector distance, and 0.3 ml/h of liquid pump rate. The calculated volumetric specific capacity of the polymer fiber Li composite anode is 1740 mah/cm 3, which is much larger than that of commercial graphite anode S1 (833 mah/cm 3 ).

8 Figure S6. SEM characterization and cycling performances of Li deposition on Cu-OxPAN electrode and Cu-PP electrode. PP fiber was purchased from 3M, and PAN fiber was made by electrospinning with similar fiber dimensions. (a) and (b) SEM images of Li deposition on Cu-OxPAN electrode at different magnifications; (c) and (d) SEM images of Li deposition on Cu-PP electrode at different magnifications. The images show the morphology difference of Li deposition. Because of the strong adhesion between Li ions and the oxidized PAN fiber, Li could wet the PAN fiber surface completely and form a uniform layer of flat Li embedded in the fiber mat. On the contrary, weak interaction between Li ions and the PP fiber leads to a poor wettability of Li on the PP surface. Hence the deposited Li on Cu-PP electrode exhibits a dendritic morphology; (e) and (f) Comparison of Coulombic efficiency of Cu-OxPAN (solid symbols) and Cu-PP (hollow symbols) electrode at 3 ma/cm 2 for a total of 1 mah/cm 2 of Li. The Cu-OxPAN electrode

9 shows a superior cycling stability (96.3% over 100 cycles) compared to the Cu-PP electrode. Scale bars are 50 µm in (a) and (c), and 20 µm in (b) and (d). Figure S7. FTIR of oxidized PAN fiber and oxidized PAN/PVP fiber. Oxidized PAN and oxidized PAN/PVP show the same spectrum, indicating that PVP is decomposed during the thermal stabilization process, with only or prominently oxidized PAN remained.

10 Figure S8. High areal capacity electrochemical cycling. (a) Comparison of Coulombic efficiency of control electrode and modified electrode cycled at 3 ma/cm 2 for a total of 3 mah/cm 2 of Li. The Coulombic efficiency of control electrode rapidly drops to below 90% after 20 cycles; (b) Comparison of voltage hysteresis of control electrode and modified electrode at 3 ma/cm 2 for a total of 3 mah/cm 2 of Li. Figure S9. Symmetrical cell testing of bare Li electrode and Li-OxPAN electrode in DOL/DME electrolyte without the addition of LiNO3. The Li-OxPAN displays a more stable voltage profile and smaller hysteresis compared to the control Li electrode. After the removal of LiNO3, both modified electrode and control electrode exhibit inferior cycling stability due to the fact that LiNO3 has a passivating effect on Li metal. The current density applied is 3 ma/cm 2 and the amount of Li plated in each cycle is 1 mah/cm 2.

11 Figure S10. XPS study of the oxidized PAN before and after electrochemical cycling. The intensity was normalized and binding energies were calibrated according to the C 1s peak at ev. (a) Normalized C 1s XPS spectra of oxidized PAN before cycling. The C 1s signal is curve-resolved to three dominant peaks, which are assigned to neutral carbon at ev, S3 C-O and C=N groups at ev, S3 and carbonyl group at ev. S3 (b) Normalized C 1s XPS spectra of oxidized PAN after cycling. The C 1s signal consists of three components, showing the same surface functional groups. Surface chemistry of the fiber remained unchanged before and after the Li-plating-stripping process at a current density of 3 ma/cm 2, confirming the stability of the PAN fiber towards electrochemical cycling. Methods Polymer Fiber Fabrication PAN (Mw = 150,000), PVP (Mw = 1,300,000), and DMF were purchased from Sigma- Aldrich Chemical Corporation. 0.5 g PAN and 0.5 g PVP were added into 10 ml of DMF. The as-prepared precursor solution was subject to vigorous stirring at 80 o C for 6 hours (400 rpm). Afterwards the solution obtained was electrospun into non-woven nanofiber by a conventional electrospinning machine. A piece of graphite paper (10 cm x 8 cm) was used as the collector. The electrospinning parameters are as follows: 15 kv of conducted voltage, 12 cm of nozzle-to-collector distance, and 0.3 ml/h of liquid pump rate. After electrospun for 6 hours, the freshly prepared PAN/PVP nanofiber mat was then heated in a box furnace (Thermo Electron Corporation, Lindberg/Blue M) under air atmosphere. The temperature was increased from 20 o C at a rate of 10 o C/min and held at 300 o C for 2 hours. PAN*

12 precursor solution was prepared by adding 0.5 g PAN into 10 ml DMF, without the addition of PVP. The factors for the following electrospinning process are: 10 kv of conducted voltage, 15 cm of nozzle-to-collector distance, and 0.2 ml/h of liquid pump rate. PP fiber could not be synthesized from electrospinning in our lab. We adopted a commercial PP fiber mat (fiber diameter of ~ 2 µm) from 3M. The oxidized PAN fiber with similar dimension was made from the precursor solution of 1.5 g PAN in 10 ml DMF. The following electrospinning factors are: 10 kv of conducted voltage, 20 cm of nozzle-tocollector distance, and 1 ml/h of liquid pimp rate. The experimental details of thermal stabilization process for all polymer fibers are the same. Electrochemical Measurement A metallic Cu foil (1 cm 2 ) was directly utilized as the control working electrode. A metallic Cu covered with polymer fiber mat (1 cm 2 ) was utilized as the modified working electrode type coin cells (MTI) were constructed in argon-filled glove box (MB-200B, Mbraun) with lithium foil (Alfa Aesar) as counter/reference electrode. No extra binder or additives were employed. Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI, 1M) in co-solvent of 1,3-dioxolane and 1,2-dimethoxyethane (volume ratio 1:1) with lithium nitrite (2 wt%) was used as electrolyte. Galvanostatic cycling was conducted using a 96-channel battery tester (Arbin Instruments). A fixed amount of Li (1 mah/cm 2 or 3 mah/cm 2 ) was deposited onto the working electrode and then stripped away up to 1 V. For the symmetrical cell test, symmetrical coin cells (2032-Type) were assembled in argon-filled glove box (MB-200B, Mbraun) with two identical electrodes (1 cm 2 ). To standardize the measurement, a fixed amount (60 µl) of electrolyte was used in each coin cell. Characterization A FEI XL30 Sirion scanning electron microscope (SEM) with a field emission gun (FEG) source was used to conduct the SEM characterization. The FTIR spectrum was measured by Bruker Vertex 70 FTIR spectrometer. The oxidized PAN nanofiber after cycling was collected by dissembling the symmetrical cells. The obtained nanofiber was washed with DOL and hydrochloric acid (1 M in water), followed by distilled water rinsing. XPS characterization was performed using Phi5000 VersaProbe (Ulvac-Phi) with AlKα radiation. Volumetric Specific Capacity Calculation In the calculation of volumetric specific capacity of our polymer fiber Li composite anode, P = 1 Td/Tf, where P is porosity of the oxidized PAN fiber network, Td is the polymer fiber layer thickness and Tf is the thickness of equivalent weight polymer dense film. Thickness of the polymer fiber layer was measured using a micrometer (General Tools & Instruments, No. 102) while thickness of the equivalent weight dense film was calculated based on the density of oxidized PAN (1.45 g/cm 3 ). S2 The calculated porosity has an average of ~85% and the corresponding volumetric specific capacity of the polymer fiber Li composite anode could reach 85% 2046 mah/cm 3 = 1740 mah/cm 3.

13 Reference (S1) Chen, J. Materials 2013, 1, 156. (S2) Mittal, J.; Mathur, R. B.; Bahl, O. P. Carbon 1997, 8, (S3) Takahagi, T.; Shimada, I.; Fukuhara, M.; Morita, K.; Ishitani, A. Journal of Polymer Science Part A: Polymer Chemistry 1986, 11, 3101.