Sn-Based Nanocomposite for Li-Ion Battery Anode with High Energy

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A Sn-Based Nanocomposite for Li-Ion Battery Anode with High Energy Density, Rate Capability and Reversibility Min-Gu Park, Dong-Hun Lee, Heechul Jung, Jeong-Hee Choi, and Cheol-Min Park ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b00586 • Publication Date (Web): 05 Mar 2018 Downloaded from http://pubs.acs.org on March 7, 2018

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ACS Nano

A Sn-Based Nanocomposite for Li-Ion Battery Anode with High Energy Density, Rate Capability and Reversibility Min-Gu Parka,b, Dong-Hun Leea, Heechul Jungc, Jeong-Hee Choid∗ and Cheol-Min Parka∗ a

School of Materials Science and Engineering, Kumoh National Institute of Technology, 61

Daehak-ro, Gumi, Gyeongbuk 39177, Republic of Korea b

c

Battery Research Center, Bexel, 168 Sanho-daero, Gumi, Gyeongbuk 39376, Republic of Korea

Energy Material Laboratory and Analytical Engineering Group, Samsung Advanced Institute of

Technology, 130 Samsung-ro, Suwon, Gyeonggi 16678, Republic of Korea d

Battery Research Center, Korea Electrotechnology Research Institute, 12 Boolmosan-ro,

Changwon, Gyeongnam 51543, Republic of Korea

*

Corresponding authors. *

Jeong-Hee Choi. Tel.: +82-55-280-1367; Fax:+82-55-280-1590 E-mail: [email protected]

*

Cheol-Min Park. Tel.: +82-54-478-7746; Fax:+82-54-478-7769 E-mail: [email protected] 1

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ABSTRACT To design an easily manufactured, large energy density, highly reversible, and fast rate-capable Liion battery (LIB) anode, Co-Sn intermetallics (CoSn2, CoSn, and Co3Sn2) were synthesized and their potential as anode materials for LIBs was investigated. Based on their electrochemical performances, CoSn2 was selected and its C-modified nanocomposite (CoSn2/C) as well as Ti- and C-modified nanocomposite (CoSn2/a-TiC/C) was straightforwardly prepared. Interestingly, the CoSn2, CoSn2/C, and CoSn2/a-TiC/C showed conversion/non-recombination, conversion/partialrecombination, and conversion/full-recombination during Li-insertion/extraction, respectively, which were thoroughly investigated using ex situ X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) analyses. As a result of the interesting conversion/fullrecombination mechanism, the easily manufactured CoSn2/a-TiC/C nanocomposite for Sn-based Liion battery anode showed large energy density (1st reversible capacity of 1399 mAh cm-3), high reversibility (1st Coulombic efficiency of 83.2%), long cycling behavior (100% capacity retention after 180 cycling) and fast rate-capability (appoximately 1110 mAh cm-3 at 3 C-rate). In addition, degradation/enhancement mechanisms for high-capacity and high-performance Li-alloy-based anode materials for next-generation LIBs were also suggested.

KEYWORDS

lithium-ion batteries; anode materials; tin-based compound anodes; reaction mechanism; nanocomposite electrodes 2


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As the development of electric vehicles and portable electronic devices accelerates, the need for improved secondary batteries has risen considerably. Rechargeable Li-ion battery (LIB) is a representative energy storage system due to its high operating voltage and energy density. However, research and development has not achieved the desired enhancements required of the LIBs. Although commercial graphite anodes in LIBs show relatively good electrochemical performance thanks to Li intercalation reaction in its graphene layer gaps, it has a small theoretical capacity and slow rate capability. Therefore, ongoing research efforts have focused on finding alternatives that have a high initial Coulombic efficiency (ICE), large energy density, long cycling behavior, and fast rate capability.1–6 One of the most attractive anode materials for high-capacity LIBs is Sn-based materials because they can form higher Li-containing alloy (Li4.25Sn, 959 mAh g-1) at room temperature than do graphite (LiC6, 372 mAh g-1) for commercialized LIBs.7-25 However, Sn-based materials show undesirable capacity fading during cycling because of the pulverization resulting from large volume variation, which is caused by expansion and contraction during repeated Li insertion and extraction. Since the development of the Stalion battery by Fuji in 1997 and the Nexelion battery by Sony in 2005, various Sn-based materials have been extensively investigated.26,27 However, Sn-based materials that undergo an easy synthetic process and possess high reversibility, a large energy density, and a fast-rate capability are necessary for high-capacity LIB anodes. Nano-sized materials are being studied intensively as alternative anode materials for LIBs because they have a number of advantages: (1) high reversible capacity as a result of providing 3


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higher interfacial areas, (2) long cycling behaviors (attributable to their mechanical characteristics such as superplasticity and high ductility) that enable them to accommodate the strain generated during repeated cycling, and (3) fast rate capability as a result of shorter Li-ion diffusion paths.28-33 Nano-sized materials also have several disadvantages. For example, their high surface energy can lead to their agglomeration during repeated cycling. In addition, if chemical methods are employed, they exhibit a poor ICE as a result of the salts or oxides that remain after synthesis.34-37 Therefore, to overcome these problems, nanostructured composite materials modified with various C-based materials have been suggested, because C in the composites hinders the agglomeration of nanomaterials.11-20,38-40 To produce various nanostructured composite materials, several synthetic methods such as sol– gel, co–precipitation, chemical reduction, and ball milling (BM) have been reported.34-40 Among these, the BM method is a very simple solid-state synthetic process, which yields well-distributed, nano-sized metal or alloy crystallites in a carbon matrix through repeated fracturing, flattening, welding, and re-welding of particles.41 In addition, as a result of a solid-state synthetic feature, which results in enhanced ICE, it can produce nanostructured composite materials having fewer impurities than those produced by chemical routes.39,40,42,43 In this study, we hierarchically designed and synthesized Co-Sn intermetallics (CoSn2, CoSn, and Co3Sn2) based on Co-Sn binary phase diagram, as well as their C-modified (CoSn2/C and CoSn/C) and C- and Ti-modified (CoSn2/a-TiC/C) nanocomposites using a simple solid-state BM process and tested them as anode materials in LIBs to circumvent the problems associated with Sn 4


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when it is used solely as an electrode. In addition, their distinctive reaction mechanisms during Li insertion and extraction were evaluated based on ex situ XRD and EXAFS analyses and differential capacity (dQ/dV) plots. Furthermore, we suggest degradation and enhancement mechanisms for their use in next-generation high-capacity and high-performance LIBs.

RESULTS AND DISCUSSION

Based on the binary Co-Sn phase diagram (Fig. S1), three main intermetallic compounds, CoSn2, CoSn, and Co3Sn2, were synthesized using a simple solid-state BM process. The Sn and three synthesized intermetallic compounds were confirmed through XRD, the results of which are shown in Fig. 1a–d, respectively. All XRD peaks well corresponded to the standard crystalline tetragonal Sn (JCPDS No. 04-0673), tetragonal CoSn2 (JCPDS No. 25-0256), hexagonal CoSn (JCPDS No. 65-5600), and hexagonal Co3Sn2 (JCPDS No. 27-1124) phases without any impurities. The Sn and synthesized CoSn2, CoSn, and Co3Sn2 intermetallics were electrochemically tested and the results are shown in Fig. 2a–d, respectively, as voltage profiles and in Fig. S2 as cycling performances. Although the Sn electrode showed a high first Li-insertion/extraction (discharge/charge) capacity of 864/716 mAh g-1, the capacity dramatically decreased after a few cycles. The drastic decrease in capacity was caused by the extremely large volume change. This latter fact was itself due to the formation and release of Li4.25Sn during repeated Li insertion and extraction,7-9 which is associated with the pulverization of active materials and their electrical isolation from the current collector. 5


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The CoSn2 electrode showed a high reversible capacity of 727/555 mAh g-1 with a relatively enhanced capacity retention. By contrast, small discharge/charge capacities were observed in the CoSn and Co3Sn2 electrodes, corresponding to 457/344 and 124/68 mAh g-1, respectively, which derived from the high overpotential for their Co-Sn bonding to break during Li insertion. Overall, although the CoSn2 electrode showed the highest reversible capacity, the capacity decreased as cycling progressed. To understand the relatively poor cycling behavior of the CoSn2 electrode, ex situ Co K-edge EXAFS was performed based on the dQ/dV plot. The results are shown in Fig. S3a– S3b. 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), which means that CoSn2 was converted into Li4.25Sn and Co after full Li insertion (1). Then, the Li4.25Sn and Co transformed into Sn and Co after full Li extraction (2). 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 phasechange representations in Fig. S3c. CoSn2 electrode - Li-insertion: CoSn2 Li4.25Sn + Co; conversion reaction

(1)

- Li-extraction: Li4.25Sn + Co Sn + Co; non-recombination reaction

(2)

6


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As mentioned in the introduction, recent Li-alloy-based researches showed that C-modified nanocomposites experienced enhanced electrochemical performance because C acted as a buffering matrix for hindering the agglomeration of Li-alloy-able nanomaterials during repeated cycling and as a conducting additive to enhance the electric conductivity between the current collector and particles.11-20,38-40 Based on these electrochemical performance results, we selected CoSn2 and CoSn to produce their C-modified nanocomposites, CoSn2/C and CoSn/C, to enhance the electrochemical performance of CoSn2 and CoSn, respectively. The prepared CoSn2/C and CoSn/C nanocomposites were confirmed by XRD and transmission electron microscopy (TEM) analyses, which are shown in Fig. 3a–d. All XRD peaks of prepared CoSn2/C and CoSn/C nanocomposites well matched the standard crystalline CoSn2 (JCPDS No. 25-0256) and CoSn (JCPDS No. 65-5600) phases, respectively. Interestingly, significant peak broadening was observed for the nanocomposites. In addition, bright field (BF)- and high resolution (HR)-TEM images with selected-area diffraction patterns (DPs) confirmed that these two nanocomposites had significantly reduced CoSn2 (approximately 10–15 nm) and CoSn (approximately 5–10 nm) nanocrystallites within an amorphous C matrix, respectively.

The prepared CoSn2/C nanocomposite electrode showed a reversible initial discharge/charge capacity of 933/648 mAh g-1 with an ICE of 69.4% (Fig. 4a). Although the CoSn2/C electrode showed better electrochemical performance than that of the CoSn2 electrode, its capacity decreased to 520 mAh g-1, corresponding to 80.2% capacity retention, after 100 cycles. To understand the 7


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electrochemical performance of a CoSn2/C nanocomposite electrode, ex situ XRD and Co K-edge EXAFS analyses based on dQ/dV plots were performed and the results are shown in Fig. 4b–d. At the fully Li-inserted state (0.0 V), the ex situ XRD peaks were amorphized (Fig. 4c–ii) and the two main EXAFS peaks (approximately 2.0 and 2.5 Å, Fig. 4d–i) of CoSn2 were transformed to the main EXAFS peak (approximately 2.2 Å) of Co metal (Fig. 4d–ii), whose reaction mechanism coincided with that of the CoSn2 electrode. However, at the fully Li-extracted state (2.0 V), tiny XRD main peak of CoSn2 was observed (Fig. 4c–iii). In addition, the duplicated main Co (approximately 2.2 Å) and CoSn2 (approximately 2.0 and 2.5 Å) EXAFS peaks were observed, as shown in Fig. 4d–iii. The results demonstrate that CoSn2 in the nanocomposite was converted into Li4.25Sn and Co after full Li-insertion (3), and then Li4.25Sn and Co partially recombined into CoSn2 with partially non-recombined Sn and Co after full Li extraction (4). Based on the ex situ XRD and Co K-edge EXAFS results, the following electrochemical conversion/partial-recombination reaction of the CoSn2/C composite electrode during Li insertion/extraction was demonstrated and is shown schematically using the crystallographic phase-change representations in Fig. 4e. CoSn2/C nanocomposite electrode - Li-insertion: CoSn2 (in CoSn2/C) Li4.25Sn + Co; conversion reaction

(3)

- Li-extraction: Li4.25Sn + Co CoSn2 + Co + Sn; partial-recombination reaction

(4)

The prepared CoSn/C nanocomposite electrode was also electrochemically tested and the results showed a larger initial discharge/charge capacity of 759/546 mAh g-1 (with an ICE of 71.9%)

8


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than that of the CoSn electrode (Fig. 5a). This was achieved by the reduced overpotential for their Co-Sn bonding to break up during Li insertion as a result of nanocomposite preparation. Although the CoSn/C electrode showed smaller reversible capacity than that of the CoSn2/C electrode, it showed good cycling behavior, corresponding to 88.1% capacity retention, after 100 cycles. To understand the good cycling behavior of the CoSn/C electrode, ex situ XRD and Co K-edge EXAFS analyses based on dQ/dV plots were also performed and the results are shown in Fig. 5b–d. At the fully Li-inserted state (0.0 V, Fig. 5c–ii and 5d–ii), the ex situ XRD peaks were amorphized (Fig. 5c–ii) and the two main EXAFS peaks (approximately 1.9 and 2.4 Å) of CoSn were transformed to the main EXAFS peak (approximately 2.2 Å) of Co metal (Fig. 5d–ii), which means that CoSn in the nanocomposite was converted into Co and Li4.25Sn. This conversion reaction was similar to that of the CoSn2/C electrode. However, at the fully Li-extracted state (2.0 V, Fig. 5c–iii and 5d–iii), tiny XRD main peaks of CoSn were observed. Additionally, the two main EXAFS peaks (approximately 1.9 and 2.4 Å) of CoSn were also definitely observed, which demonstrates that CoSn in the nanocomposite was converted into Li4.25Sn and Co after full Li-insertion (5), and then they fully recombined into CoSn after full Li-extraction (6). Based on the ex situ XRD and Co Kedge EXAFS results, the following electrochemical conversion/full-recombination reaction of the CoSn/C electrode during Li-insertion/extraction was demonstrated and is shown schematically using the crystallographic phase-change representations in Fig. 5e. CoSn/C nanocomposite electrode - Li-insertion: CoSn (in CoSn/C) Li4.25Sn + Co; conversion reaction 9


(5)

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- Li-extraction: Li4.25Sn + Co CoSn; full-recombination reaction

Interestingly,

the

electrochemical

phase-change

mechanism

(6)

results

of

CoSn2/C

(conversion/partial-recombination reaction) and CoSn/C (conversion/full-recombination reaction) electrodes

demonstrate

that

the

conversion/full-recombination

reaction

during

Li

insertion/extraction contributes considerably to good cycling performance (Fig. S4). However, the first reversible charge capacities of CoSn2/C and CoSn/C electrodes were 648 and 546 mAh g-1, respectively. Therefore, if the CoSn2/C nanocomposite electrode can experience the conversion/fullrecombination reaction, its electrochemical performance will improve.

TiC is known as a very hard refractory ceramic (Mohs 9–9.5), which can have a grinding additive role during the BM process.44-46 Additionally, TiC was simply synthesized by the BM process and did not react with Li (Fig. S5), which serves as an inactive matrix and reduces irreversible capacity originated from BM-treated C.46 Therefore, on the basis of the reaction mechanism results, to enhance further the electrochemical performance of CoSn2/C, we straightforwardly prepared a Ti- and C-modified nanostructured composite, CoSn2/a-TiC/C, using a solid-state BM process. The prepared CoSn2/a-TiC/C nanocomposite was confirmed by XRD, which is shown in Figure 6a. All XRD peaks matched only the standard crystalline CoSn2 phase (JCPDS No. 25-0256) with no trace of TiC. Interestingly, the XRD peaks of CoSn2/a-TiC/C were more significantly broadened than those of CoSn2/C. To confirm the formation of TiC, X-ray photoelectron spectroscopy (XPS) analysis was performed, and the binding energy results of Ti 2p 10


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and C 1s are shown in Fig. 6b and c, respectively. The results definitely confirm that amorphous TiC (a-TiC) was well formed in the composite. To examine electronic conductivity, electrochemical impedance spectroscopy (EIS) measurement results of CoSn2, CoSn2/C, and CoSn2/a-TiC/C nanocomposite electrodes are compared in Fig. 6d. The semicircle reflects the charge transfer resistance of electrode-electrolyte interface reaction. The semicircles representing CoSn2/C and CoSn2/a-TiC/C nanocomposites are similar, which was attributed to the relatively high electronic conductivity of TiC.47 Additionally, they are much smaller than that of the CoSn2, which demonstrates that the solid-state BM process is very useful for reducing the charge transfer resistance after nanocomposite preparation. In addition, the microstructure of the CoSn2/a-TiC/C nanocomposite was observed through TEM analyses as shown in Fig. 6e–h. The BF- (Fig. 6e) and HR-TEM (Fig. 6f) images corresponding to selected-area DPs (Fig. 6g) for the CoSn2/a-TiC/C are shown. The tiny nano-sized (approximately 5 nm) CoSn2 crystallites and amorphous TiC were uniformly embedded in the amorphous C matrix. In addition, the images of EDS elemental (Sn, Co, Ti, and C) mapping demonstrates the even dispersion of tiny ~5 nm-sized CoSn2 nanocrystallites and a-TiC in the amorphous-C matrix (Fig. 6h), which implies that the grinding additive a-TiC had an effective role to obtain the tiny and well-dispersed CoSn2 nanocrystallites embedded in the carbon matrix. Therefore, based on various analysis results of XRD, XPS, EIS, and HR-TEM, the Ti- and C-modified CoSn2/a-TiC/C nanocomposite obtained through the simple and straightforward solid-state BM process confirms that it is a promising technology for preparing tiny and welldispersed Li-alloy-able nanocrystallites in the nanostructured composites. 11


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The CoSn2/a-TiC/C electrode showed a highly reversible discharge/charge capacity of 576/479 mAh g-1 or 1682/1399 mAh cm-3 with a very high ICE of 83.2% (Fig. 7a). The first irreversible capacity from the composition 15 wt% of BM-treated C within the CoSn2/a-TiC/C nanocomposite was approximately 60 mAh g-1 (Fig. S6), which demonstrates that the extremely high reversibility between Li and nanocrystalline CoSn2 in the composite was obtained. In addition, the capacity of CoSn2/a-TiC/C nanocomposite electrode was well retained without any decrease in capacity after 100 cycles. To understand the highly reversible and good electrochemical performance of CoSn2/aTiC/C electrode, ex situ XRD and Co K-edge EXAFS analyses based on dQ/dV plots were performed and the results are shown in Fig. 7b–d. At the fully Li–inserted (0.0 V, Fig. 7c–ii) and – extracted states (2.0 V, Fig. 7c–iii), the ex situ XRD peaks were amorphized. Therefore, Co K-edge EXAFS analyses were performed and the results are shown in Fig. 7d. The two main EXAFS peaks (approximately 2.0 and 2.5 Å) of CoSn2 (Fig. 7d–i) were transformed into the main EXAFS peak (approximately 2.2 Å) of Co metal (Fig. 7d–ii) at the fully Li–inserted state of 0.0 V. In addition, at the fully Li–extracted state of 2.0 V (Fig. 7d-iii), the two main EXAFS peaks (approximately 2.0 and 2.5 Å) of CoSn2 were definitely observed, which means that CoSn2 in the nanocomposite were converted into Li4.25Sn and Co after full Li insertion (7), and then they fully recombined into CoSn2 after full Li extraction (8). The results demonstrate that preparing extremely small crystallite-sized (approximately 5 nm) Li-alloy-able materials is crucial to induce conversion/full-recombination reaction during cycling. Based on the ex situ Co K-edge EXAFS results, the following electrochemical conversion/full-recombination reaction of CoSn2/a-TiC/C electrode during Li 12


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insertion/extraction was demonstrated and is shown schematically using the crystallographic phasechange representations in Fig. 7e. CoSn2/a-TiC/C nanocomposite electrode - Li insertion: CoSn2 (in CoSn2/a-TiC/C) Li4.25Sn + Co; conversion reaction

(7)

- Li extraction: Li4.25Sn + Co CoSn2; full-recombination reaction

(8)

To identify the advantage of conversion/full-recombination reaction during cycling, ex situ HR-TEM analyses were performed on the CoSn2/a-TiC/C nanocomposite electrode after cycling (Fig. 8a and b). It is noteworthy that the approximately 5 nm-sized CoSn2 crystallites before cycling (Fig. 6f) were reduced to 2–3 nm after 10 cycling (Fig. 8a) and maintained their size, even after 100 cycling (Fig. 8b). These results confirm that the repeated conversion/full-recombination reactions during repeated cycling contributed to excellent electrochemical performance of the stabilized, tiny (approximately 2–3 nm), and well-dispersed CoSn2 nanocrystallites in the CoSn2/a-TiC/C nanocomposite. The straightforward preparation methods as well as the features of CoSn2, CoSn2/C, and CoSn2/a-TiC/C nanocomposites are summarized schematically in Fig. 9a–c, respectively. Additionally, the crystallite size variation of CoSn2 in the CoSn2/a-TiC/C nanocomposite during cycling is also represented schematically in Fig. 9d.

The gravimetric and volumetric capacity as compared to the cycling number (current density: 100 mA g-1) of the CoSn2, CoSn2/C, CoSn2/a-TiC/C, and commercial MCMB (meso-carbon microbead) graphite electrodes for LIBs are compared in Fig. 10a and b. Although the CoSn2 electrode 13


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showed poor cycling behavior because of the large volume expansion/contraction resulting from the Li4.25Sn and Co conversion/non-recombination reaction during cycling, the CoSn2/C electrode showed enhanced cycling behavior as a result of the conversion/partial-recombination reaction during cycling. The CoSn2/a-TiC/C electrode showed a very high reversible charge capacity of 479 mAh g-1 or 1399 mAh cm-3. The high volumetric capacity was more than 3.5 times higher than that of the commercial MCMB graphite electrode (Fig. 10b). Furthermore, it had an extremely long cycling behavior and showed no diminished capacity. Notably, the CoSn2/a-TiC/C electrode had a high ICE of 83.2% and 100% capacity retention after 180 cycles (Fig. 10c). The rate-capabilities of CoSn2/a-TiC/C and MCMB graphite electrodes were examined at different current rates (C-rates) and are compared in Fig. 10d and S7. The CoSn2/a-TiC/C nanocomposite electrode had 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/fullrecombination during cycling, which results in a shorter Li-ion diffusion path. The high reversibility with a high ICE, large energy density, long capacity retention, and fast rate-capability for the CoSn2/a-TiC/C nanocomposite electrode is one of the best results among the Sn-based anodes for LIBs.36,37,42,43,48-57

As a result of the interesting conversion/full-recombination reaction during cycling, the CoSn2/a-TiC/C electrode showed the following excellent electrochemical performance: highly 14


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reversible capacity of ca. 1399 mAh cm-3 (479 mAh g-1) with a very high ICE of 83.2%, capacity retention of 100% after 180 cycling, and a fast rate capability of ca. 1110 mAh cm-3 at a 3 C-rate, which are the highest values among the reported Sn-based anode materials for LIBs. The excellent electrochemical behaviors were attributed to the recombinable CoSn2 after preparing tiny CoSn2 nanocrystallites (approximately 5 nm) and evenly dispersed a-TiC inactive matrix within the amorphous C.

CONCLUSIONS

To design an easily manufactured, highly reversible, large energy density, and fast rate capable LIB anode, we straightforwardly prepared Ti- and C-modified nanostructured composite, CoSn2/a-TiC/C, using a solid-state BM process. The CoSn2/a-TiC/C was comprised of evenly dispersed ~5 nm-sized CoSn2 nanocrystallites and a-TiC in the amorphous-C matrix, which was attained by employment of the grinding additive a-TiC. The CoSn2/a-TiC/C electrode showed an interesting conversion/fullrecombination during Li insertion/extraction. As a result of this interesting electrochemical reaction mechanism, an easily manufactured, highly reversible, large energy density, long cycling behavior, and fast rate-capable Sn-based Li-ion battery anode was successfully fabricated. Based on the electrochemical performance and phase-change mechanisms for the various CoSn2, CoSn2/C, and CoSn2/a-TiC/C electrodes, the enhancement mechanisms for high-performance Li-alloy-based anode materials for LIBs were suggested as follows: 1)

Conversion/full-recombination

reaction

during

cycling

15


induces

stabilized

and

tiny

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(approximately 2–3 nm) crystallites of Li-alloy-able compound materials. 2) Preparing less than 5 nm-sized crystallites of Li-alloy-able compound materials assists the conversion/full-recombination reaction during cycling. 3) The C matrix hinders the agglomeration of Li-alloy-able small nanocrystallites during cycling. We strongly believe that these findings represent a promising solution for an easily manufactured, highly reversible, large energy density, and fast rate-capable LIB anode.

METHODS

Material Synthesis. The Co-Sn intermetallic compounds CoSn2, CoSn, and Co3Sn2 were synthesized through a simple solid-state BM method (SPEX-8000M mixer/mill) as follows. Stoichiometric amounts of Co (Sigma-Aldrich, 99.9%, mean particle size = 150 µm) and Sn powders (Daejung Chemicals & Metals, 99.9%, mean particle size = 45 µm), and hardened steelballs (mass ratio of powder to ball = 1:20) were placed into an 80-mL Ar-filled hardened steel container and subjected to the BM process at room temperature for 6 h. Then, their corresponding C-modified nanocomposites, CoSn2/C and CoSn/C, were straightforwardly prepared by subjecting mixtures of the Co, Sn, and carbon black (Super-P, Timcal) powders (CoSn2/C; 48.1 wt% Sn, 11.9 wt% Co, 40 wt% C and CoSn/C; 40.1 wt% Sn, 19.9 wt% Co, 40 wt% C) to the same BM procedure. In addition, further C- and Ti-modified nanocomposite (CoSn2/a-TiC/C) was also straightforwardly prepared using the Co, Sn, Ti (High Purity Chemicals, 99.9%, mean particle size = 150 µm), and

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carbon black powders (48.1 wt% Sn, 11.9 wt% Co, 20 wt% Ti, 20 wt% C) to the same BM procedure. The average particles sizes of the CoSn2/C and CoSn/C, and CoSn2/a-TiC/C nanocomposites were 4.9, 5.0, and 5.7 µm, respectively (Figure S8), which were analyzed using particle size analyzer (Mastersizer-2000) and scanning electron microscopy (SEM, JSM-6701F, JEOL). Preliminary electrochemical characterizations demonstrated that the best performance results (ICE, reversible capacity, capacity retention per cycle, and cycling stability) of CoSn2/C and CoSn2/a-TiC/C were achieved at CoSn2:C and CoSn2:a-TiC:C weight ratios of 60:40 and 60:25:15, respectively.

Material Characterization. The phase and crystal structures of various Co-Sn intermetallic compounds, their corresponding C-modified nanocomposites (CoSn2/C and CoSn/C), and Ti- and C-modified nanocomposite (CoSn2/a-TiC/C) were characterized by XRD (X-Max/2000-PC, Cu Kαtarget), HR-TEM (FEI F20, accelerating voltage: 200 kV), and TEM-coupled EDS. Phase changes occurring in the electrodes during Li insertion and extraction were monitored by ex situ analyses such as XRD, EXAFS, and HR-TEM methods. The Co K-edge EXAFS spectra of the electrodes were investigated at the 7D-XAFS beamline (storage ring: 3.0 GeV) at the PLS in the Republic of Korea. For the ex situ HR-TEM analyses of the Li-extracted CoSn2/a-TiC/C electrodes after 10 and 100 cycling, the test samples scratched on the Cu substrate were introduced into a glass vial containing an anhydrous ethyl-alcohol solution. All sample preparation processes using ex situ XRD, HR-TEM, and EXAFS were performed in an Ar-filled glove box to avoid moisture in the air. The discharge/charge reaction was defined as Li-insertion/extraction reaction. 17


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Electrochemical Measurements. The electrochemical characteristics of various Sn-based electrodes were probed by galvanostatically driven charge-discharge tests that were conducted using coin-type electrochemical cells assembled in an Ar-filled glove box. In these cells, Li foil was used as counter and reference electrodes, with the electrolyte comprising 1 M LiPF6 in ethylene carbonate/diethyl carbonate (1:1, v/v) (Panax STARLYTE). A Celgard 2400 polypropylene membrane was used as a separator. The electrodes were prepared using various Sn-based powders as active materials, carbon black (Denka) as a conducting additive, and polyvinylidene fluoride as a binder (80:10:10, w/w/w, respectively). The aforementioned mixtures were dissolved in N-methyl2-pyrrolidone to afford slurries that were coated on Cu-foil using a doctor blade and that were vacuum-dried at 120 °C for 3 h. The average loadings of Sn-based electrodes were approximately 3.0 mg cm–2 (average active material weight: 2.4 mg, electrode area: 0.79 cm2, electrode thickness: 0.05 mm). EIS measurements were conducted using an impedance analyzer (ZIVE-MP2A, ZIVELAB), and potentiostatic impedance patterns were recorded in a frequency range of 105 to 10– 2

Hz at an amplitude of 5 mV. Electrochemical test cells were galvanostatically tested (constant

current density: 100 mA g–1, and potential window: 0–2 V vs. Li+/Li) using a battery cycling tester (Maccor-4000). The gravimetric capacity was calculated based on the weight of the main Li-alloyable materials, whereas the volumetric capacity was calculated by multiplying the gravimetric capacities by their tap densities (CoSn2/C: 2.71 g cm-3, CoSn2/a-TiC/C: 2.92 g cm-3, MCMB graphite: 1.27 g cm-3) measured using a tap density tester (BT-301, Bettersize). The rate capabilities were measured at different C-rates (1C-rate; CoSn2/a-TiC/C: 500 mA g-1, MCMB graphite: 320 mA 18


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g-1.

ASSOCIATED CONTENT

The authors declare no competing financial interest.

Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI:

Binary Co-Sn phase diagram, cycling behaviors of Sn and synthesized CoSn2, CoSn, and Co3Sn2 intermetallics, dQ/dV plots of 1st and 2nd cycle, Co K-edge EXAFS results, and crystallographic phase-change representations during cycling for CoSn2 electrode, cycling behaviors of Sn, CoSn, CoSn2, CoSn/C, and CoSn2/C electrodes, XRD and voltage profile of synthesized TiC, voltage profile and cycling behavior of BM-treated amorphous C, and voltage profiles at various C-rates for CoSn2/a-TiC/C nanocomposite electrode, SEM and particle size analyses of CoSn/C, CoSn2/C, and CoSn2/a-TiC/C nanocomposites.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.-H. Choi), [email protected] (C.-M. Park)

Author Contributions 19


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C.-M.P. initiated the study and outlined the experiments. M.-G.P. and D.-H.L. synthesized the samples and performed various analyses. H.J. and J.-H.C. provided assistance in the analyses of electrochemical data. C.-M.P. and J.-H.C. supervised the research work and wrote the manuscript. All authors contributed to the discussion of the results reported in the manuscript.

ACKNOWLEDGMENTS

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea Government (MSIP) (NRF-2018R1A2B6007112). This work was also supported by the MSIP (Ministry of Science, ICT and Future Planning), Korea, under the ITRC (Information Technology Research Center) support program (IITP-2017-2014-0-00639) supervised by the IITP (Institute for Information & communications Technology Promotion).

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FIGURE CAPTIONS

Figure 1. Synthesis of three main binary Co-Sn intermetallic compounds. (a) XRD data of Sn and its standard (JCPDS No. 04-0673). (b) XRD data of manufactured CoSn2 and its standard (JCPDS No. 25-0256). (c) XRD data of manufactured CoSn and its standard (JCPDS No. 65-5600). (d) XRD data of manufactured Co3Sn2 and its standard (JCPDS No. 27-1124).

Figure 2. Electrochemical performances of three main binary Co-Sn intermetallic compound electrodes. (a) Voltage profiles from the 1st to 10th cycles of the Sn electrode (constant current density: 100 mA g-1). (b) Voltage profiles from the 1st to 100th cycles of the CoSn2 electrode (constant current density: 100 mA g-1). (c) Voltage profiles from the 1st to 100th cycles of the CoSn electrode (constant current density: 100 mA g-1). (d) Voltage profiles from the 1st to 10th cycles of the Co3Sn2 electrode (constant current density: 100 mA g-1).

Figure 3. Preparation and morphological observation of CoSn2/C and CoSn/C nanocomposites. (a) XRD results of manufactured CoSn2/C nanocomposite with its standard (JCPDS No. 25-0256). (b) XRD result of manufactured CoSn/C nanocomposite with its standard (JCPDS No. 65-5600). (c) BF- and HR-TEM images and corresponding DP and FT patterns with CoSn2 crystals. (d) BF- and HR-TEM images and corresponding DP and FT patterns with CoSn crystals. 25


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Figure 4. Electrochemical performance and electrochemically driven phase-change mechanism of the CoSn2/C nanocomposite electrode during Li insertion/extraction. (a) Voltage profiles from the 1st to 100th cycles of the CoSn2/C electrode (constant current density: 100 mA g-1). (b) dQ/dV vs. potential (Li+/Li) plots of the CoSn2/C electrode during the 1st and 2nd cycles. (c) Ex situ XRD results of the CoSn2/C electrode during the 1st cycle. (d) Co K-edge EXAFS spectra of the CoSn2/C electrode during the 1st Li insertion/extraction. (e) Crystallographic conversion/partialrecombination mechanism of CoSn2 in the CoSn2/C nanocomposite during Li insertion/extraction.

Figure 5. Electrochemical performance and electrochemically driven phase-change mechanism of the CoSn/C nanocomposite electrode during Li insertion/extraction. (a) Voltage profiles from the 1st to 100th cycles of the CoSn/C electrode (constant current density: 100 mA g-1). (b) dQ/dV vs. potential (Li+/Li) plots of the CoSn/C electrode during the 1st and 2nd cycles. (c) Ex situ XRD results of the CoSn/C electrode during the 1st cycle. (d) Co K-edge EXAFS spectra of the CoSn/C electrode during the 1st Li insertion/extraction. (e) Crystallographic conversion/fullrecombination mechanism of CoSn in the CoSn/C nanocomposite during Li insertion/extraction.

Figure 6. Preparation, characterization, and morphological observation of the CoSn2/a-TiC/C nanocomposite. (a) XRD result of manufactured CoSn2/a-TiC/C with its standard (JCPDS No. 250256). (b) XPS Ti 2p binding energy result of manufactured CoSn2/a-TiC/C. (c) XPS C 1s binding energy result of manufactured CoSn2/a-TiC/C. (d) EIS comparison results for CoSn2, CoSn2/C, and CoSn2/a-TiC/C nanocomposite electrodes. (e) BF-TEM image of CoSn2/a-TiC/C. (f) HR-TEM images and (g) corresponding DPs with CoSn2 crystals. (h) Scanning-TEM image of CoSn2/aTiC/C and corresponding EDS mapping images of Sn, Co, Ti, and C.

Figure 7. Electrochemical performance and electrochemically driven phase-change mechanism of CoSn2/a-TiC/C nanocomposite electrode during Li insertion/extraction. (a) Voltage profiles from the 1st to 100th cycles of CoSn2/a-TiC/C electrode (constant current density: 100 mA g-1). (b) dQ/dV vs. potential (Li+/Li) plots of the CoSn2/a-TiC/C electrode during the 1st 26


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and 2nd cycles. (c) Ex situ XRD results of the CoSn2/a-TiC/C electrode during the 1st cycle. (d) Co K-edge EXAFS spectra of the CoSn2/a-TiC/C electrode during the 1st Li insertion/extraction. (e) Crystallographic conversion/full-recombination mechanism of CoSn2 in the CoSn2/a-TiC/C nanocomposite during Li insertion/extraction.

Figure 8. Observed Li-alloy-able CoSn2 nanocrystallites in CoSn2/a-TiC/C nanocomposite during cycling. (a) Ex situ HR-TEM image and corresponding DPs of the CoSn2/a-TiC/C nanocomposite electrode after the 10th cycling. (b) Ex situ HR-TEM image and corresponding DPs of the CoSn2/a-TiC/C nanocomposite electrode after the 100th cycling.

Figure 9. Schematic of the straightforward preparation method for and features of CoSn2based materials. (a) Straightforward preparation method of CoSn2. (b) Straightforward preparation method of CoSn2/C nanocomposite. (c) Straightforward preparation method of CoSn2/a-TiC/C nanocomposite. (d) Crystallite size variation of CoSn2 in the CoSn2/a-TiC/C nanocomposite during cycling.

Figure 10. Electrochemical performance of CoSn2-based electrodes. (a) Gravimetric capacity vs. cycle number (cycling rate: 100 mA g-1) of the MCMB graphite, CoSn2, CoSn2/C, and CoSn2/aTiC/C electrodes. (b) Volumetric capacity vs. cycle number (cycling rate: 100 mA g-1) of the MCMB graphite (tap density: 1.27 g cm-3), CoSn2/C (tap density: 2.71 g cm-3), CoSn2/a-TiC/C (tap density: 2.92 g cm-3) electrodes. (c) Comparison of ICEs and capacity retentions for Sn, CoSn2, CoSn2/C, and CoSn2/a-TiC/C electrodes. (d) C-rate capabilities of the MCMB graphite (1 C-rate: 300 mA g-1) and CoSn2/a-TiC/C nanocomposite electrode (1 C-rate: 500 mA g-1).

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Figure 1

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ACS Nano

Figure 2

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ACS Nano 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

Figure 3

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ACS Nano

Figure 4

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ACS Nano 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

Figure 5

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ACS Nano

Figure 6

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ACS Nano 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

Figure 7

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ACS Nano

Figure 8

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ACS Nano 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

Figure 9

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ACS Nano

Figure 10

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ACS Nano 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

SYNOPSIS TOC

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Sn-Based Nanocomposite for Li-Ion Battery Anode with High Energy

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