Sn-Based Nanocomposite for Li-Ion Battery Anode with High Energy Density, Rate Capability, and Reversibility

Transcription

1 SUPPORTING INFORMATION Sn-Based Nanocomposite for Li-Ion Battery Anode with High Energy Density, Rate Capability, and Reversibility Min-Gu Park a,b, Dong-Hun Lee a, Heechul Jung c, Jeong-Hee Choi d and Cheol-Min Park a a School of Materials Science and Engineering, Kumoh National Institute of Technology, 61 Daehak-ro, Gumi, Gyeongbuk 39177, Republic of Korea b Battery Research Center, Bexel, 168 Sanho-daero, Gumi, Gyeongbuk 39376, Republic of Korea c Energy Laboratory, Samsung Advanced Institute of Technology, Suwon, Gyeonggi 16678, Republic of Korea d Battery Research Center, Korea Electrotechnology Research Institute, Changwon, Gyeongnam 51543, Republic of Korea * Corresponding authors. * Jeong-Hee Choi. Tel.: ; Fax: dodgers@keri.re.kr * Cheol-Min Park. Tel.: ; Fax: cmpark@kumoh.ac.kr 1

2 1. Intermetallic compounds of Co-Sn Three main intermetallic compounds, Co3Sn2, CoSn, and CoSn2, are shown in the binary Co-Sn phase diagram (Fig. S1). Based on the binary Co-Sn phase diagram, Co3Sn2, CoSn, and CoSn2 were synthesized using a simple solid-state BM process. Figure S1. The Co-Sn binary phase diagram with the three main intermetallic Co3Sn2, CoSn, and CoSn2 compounds. 2

3 2. Cycling behaviors of Sn and Co-Sn intermetallic compound electrodes The Sn and synthesized Co3Sn2, CoSn, and CoSn2 intermetallic compounds were electrochemically tested and the cycling behaviors are compared in Fig. S2, respectively. Although the Sn electrode showed a high first discharge/charge capacity of 864/716 mah g -1, the capacity dramatically decreased after a few cycles. Although the CoSn2 electrode showed the highest reversible capacity, its cycling behavior is poor. Whereas, the CoSn electrode showed better cycling behavior than that of CoSn2 with relatively small reversible capacity. Figure S2. Gravimetric capacity vs. cycle number (cycling rate: 100 ma g -1 ) of the Sn, CoSn2, CoSn, and Co3Sn2 compound electrodes. 3

4 3. Electrochemically driven phase-change mechanism of the CoSn2 electrode To understand the electrochemically driven phase-change mechanism of the CoSn2 electrode, ex situ Co K-edge EXAFS was performed based on the dq/dv plot and the results are shown in Fig. S3a and S3b, respectively. At the fully Li-inserted state (0.0 V), the two main EXAFS peaks (approximately 2.0 and 2.5 Å, Fig. S3b i) of CoSn2 were transformed to the main EXAFS peak (approximately 2.2 Å, Fig. S3b ii) of Co metal. In addition, at the fully Li-extracted state (2.0 V), the main Co EXAFS peak (approximately 2.2 Å) still remained (Fig. S3b iii). The EXAFS results demonstrate that the CoSn2 was converted into Li4.25Sn and Co after full Li insertion. Then, the Li4.25Sn and Co transformed into Sn and Co after full Li extraction. Based on the ex situ Co K-edge EXAFS results, the following electrochemical conversion/non-recombination reaction of the CoSn2 electrode during Li insertion/extraction was demonstrated and shown schematically using the crystallographic phase-change representations in Fig. S3c. Figure S3. Electrochemically driven phase-change mechanism of the CoSn2 electrode during Li insertion/extraction. (a) dq/dv vs. potential (Li + /Li) plots of the CoSn2 electrode during the 1st and 2nd cycles. (b) Co K-edge EXAFS spectra of the CoSn2 electrode during the 1st Li insertion/extraction. (c) Crystallographic conversion/non-recombination mechanism of CoSn2 electrode during Li insertion/extraction. 4

5 4. Cycling behaviors of Co-Sn intermetallics and their nanocomposite electrodes The Sn, CoSn, CoSn2, CoSn/C nanocomposite, and CoSn2/C nanocomposite electrodes were electrochemically tested and the cycling behaviors are compared in Fig. S4, respectively. The cycling behavior of CoSn/C nanocomposite electrode was better than that of CoSn2/C nanocomposite electrode, which was caused by the different electrochemical phase-change mechanism results of CoSn2/C (conversion/partial-recombination reaction) and CoSn/C (conversion/full-recombination reaction) electrodes. The conversion/full-recombination reaction during Li insertion/extraction contributes considerably to good cycling performance (Fig. S4). Figure S4. Gravimetric capacity vs. cycle number (cycling rate: 100 ma g -1 ) of the Sn, CoSn, CoSn2, CoSn/C nanocomposite, and CoSn2/C nanocomposite electrodes. 5

6 5. Cycling behaviors of Co-Sn intermetallics and their nanocomposite electrodes TiC was simply synthesized by the BM process (6 h) using Ti and C powders, which was confirmed by XRD result (Fig. S5a). Additionally, the voltage profile of TiC demonstrates that it did not react with Li (Fig. S5b), which demonstrates that it serves as an inactive matrix within the composite and reduces irreversible capacity originated from BM-treated C. Figure S5. Synthesis and electrochemical performance of TiC. (a) XRD data of the TiC and its standard (JCPDS No ). (b) Voltage profile of the TiC electrode (current rate: 100 ma g -1 ). 6

7 6. Electrochemical performance of the ball-milled carbon (Super P) electrode Fig. S6 shows the electrochemical performance of the ball-milled amorphous-c (Super P) electrode. Fig. S6a shows the voltage profile of the ball-milled amorphous-c electrode at a current density of 100 ma g -1. The ball-milled amorphous-c electrode showed high initial discharge and charge capacities of 916 and 560 mah g -1, respectively, with a Coulombic efficiency of 61.1%. The ball-milled amorphous-c electrode also showed relatively stable capacity retention, corresponding to approximately 81.1% of the initial charge capacity after the 100th cycle (Fig. S6b). Figure S6. Electrochemical performances of BM-treated C (Super P) electrode. (a) Voltage profile at current rate of 100 ma g -1. (b) Cycling behavior at cycling rate of 100 ma g -1. 7

8 7. Voltage profiles at different C rates of the CoSn2/a-TiC/C nanocomposite electrode Fig. S7 shows the voltage profiles at different C-rates of the CoSn2/a-TiC/C nanocomposite electrode. It showed large volumetric capacities at fast C-rates: 1290, 1190, and 1110 mah cm -3 at 1, 2, and 3 C-rates, respectively. The fast C-rate capability was attained by the provision of tiny Li-alloy-able CoSn2 nanocrystallites through the straightforward BM process and repeated conversion/full-recombination reactions during cycling, which results in a shorter Li-ion diffusion path. Figure S7. Voltage profiles at different C-rates of the CoSn2/a-TiC/C nanocomposite electrode (1C-rate: 500 ma g -1 ). 8

9 8. Morphologies for the CoSn/C, CoSn2/C, and CoSn2/a-TiC/C nanocomposites To analyze the morphologies and particle size for the CoSn/C, CoSn2/C, and CoSn2/a-TiC/C nanocomposites, SEM and particle size distribution analyses were performed and the results are shown in Fig. S8a-S8c, respectively. The average particle sizes of the CoSn/C, CoSn2/C, and CoSn2/a-TiC/C nanocomposites were 4.9, 5.0, and 5.7 µm, respectively. The secondary nanocomposite particles were comprised of the various small-sized primary particles agglomerated. Figure S8. SEM and particle size analyses for the various nanocomposites. (a) SEM and particle size distribution result for CoSn/C nanocomposite. (b) SEM and particle size distribution result for CoSn2/C nanocomposite. (c) SEM and particle size distribution result for CoSn2/a- TiC/C nanocomposite. 9