In Situ Synthesis of Multilayer Carbon Matrix Decorated with Copper

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In-Situ Synthesis of Multilayer Carbon Matrix Decorated with Copper Particles: Enhancing the Performance of Si as Anode for Li-Ion Batteries Hui Zhang, Ping Zong, Mi Chen, Hong Jin, Yu Bai, Shiwei Li, Fei Ma, Hui Xu, and Kun Lian ACS Nano, Just Accepted Manuscript • Publication Date (Web): 05 Mar 2019 Downloaded from http://pubs.acs.org on March 5, 2019

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ACS Nano

In-Situ Synthesis of Multilayer Carbon Matrix Decorated with Copper Particles: Enhancing the Performance of Si as Anode for Li-Ion Batteries

Hui Zhang, †, #, ‡, ǁ Ping Zong, #, ‡, ǁ Mi Chen, †, #, ‡ Hong Jin, †, #, ‡ Yu Bai, †, #, ‡ Shiwei Li, #, ‡ Fei Ma, † * Hui Xu †, #, ‡, * and Kun Lian §, * †

State Key Laboratory for Mechanical Behaviour of Materials, Xi'an Jiaotong

University, Xi'an 710049, People's Republic of China #

Xi'an Jiaotong University Suzhou Academy, Suzhou 215123, People's Republic of

China ‡

School of Nano-Science and Nano-Engineering (Suzhou), Xi'an Jiaotong University,

Suzhou 215123, People's Republic of China §

Suzhou GuanJie Nano Antibacterial Coating Technology CO., LTD., Suzhou

215123, People's Republic of China

*E-mail: [email protected]; [email protected]; [email protected] ǁ These authors contributed equally to the work.

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Abstract A 3D structured composite was designed to improve the conductivity and to ease the volume problems of Si anode during cycling for lithium-ions batteries. An in-situ method via a controllable gelation process was explored to fabricate the 3D composite of multilayer carbon matrix toughened by crosslinked CNTs and decorated with conductive Cu agents. Structurally, bi-functional carbon shell was formed on the surface of Si to improve the conductivity but alleviate side reactions. Cu particles as conducting agents decorated in the carbon matrix is also used to further improve the conductivity. The volume issue of Si particles can be effectively released via toughening carbon matrix through the multilayered structure and crosslinked CNTs. Moreover, the carbon matrix might prevent silicon particles from agglomeration. Consequently, Si@C@Cu composite is expected to exhibit benign electrochemical performances with a commendable capacity of 1500 mAhg-1 (900 cycles, 1 Ag-1) and a high rate performance (1035 mAhg-1, 4 Ag-1). The DLi+ ranging from 10-11 to 10-9 cm-2s-1 of Si@C@Cu anode is obtained via GITT test, which is higher than the most reported data.

Keywords: in-situ synthesis; Cu conducting agents; multilayer carbon; silicon anode; lithium-ion batteries

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Si anode attracts great interests owing to its highest theoretical capacity of 4200 mAhg-1 among all the known anode materials.1-4 However, the application is still a challenge owing to several factors: (a) poor electrochemical performances are caused by large expansion, which might lead to an unstable solid electrolyte interphase (SEI) layer;5 (b) Si anode has a poor electrical conductivity and low lithium diffusivity.6 A great number of approaches have been proposed to effectively solve these problems: (a) embedding Si particles in the carbon matrix such as CNTs, GO, etc. to stabilize the structural stability of Si and to improve the electrical conductivity of the whole electrodes;7,

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(b) designing Si electrode with different morphologies (including

nano-sphere, nano-wire, nano-film, porous structure, etc.) to release the stress induced by Si volume change;9-15 (c) adding some binders to fix the active material.16-18

Among these strategies, three-dimensional (3D) porous structure is one of the research focuses. Compared with 1D and 2D structures, 3D structure has lots of merits: (a) more efficient ionic and electronic transport pathway can be provided by designing 3D porous structure; (b) it is easier for electrolyte to infiltrate; (c) 3D anode materials exhibit good cycling performance because they can accommodate volume changes of Si and allow stress relaxation with mechanical stability upon lithium intercalation.12 The 3D composites of Si and carbon are appealing, because suitable carbon-based composites are of great use for Si anode to release stress. In addition, the electrical conductivity and Coulombic Efficiency (CE) of Si electrode can also be continuously improved. For example, a 3D Si/C composite was fabricated by Du et al. 3


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through combining calcination and acid soaking method, and it was demonstrated that this 3D composite delivered a benign capacity of 1700 mAhg-1 (70 cycles, 0.8 Ag-1).19 Wang et al. obtained a commendable electrochemical performance of 1336 mAhg-1 after 200 circles at 1 Ag-1 via magnesiothermic reduction SBA-15, which is low cost and controllable.7 Cao et al. proposed a self-assemble method to fabricate 3D porous Si/C composite, which exhibited a comparably stable reversible capacity after 200 cycles at 0.1 Ag-1.20 However, better electrical conductivity of 3D structure is still needed to improve the performance of Si anode and some efforts, including the utilization of nanoparticles and layer decorated in the materials, such as SiOx, P, Ge, Ti, Cu, etc. were devoted.11, 21-26 Cu particles have been widely adopted to stabilize the anode structure, to improve the rate capability and the electrical conductivity of the whole composite.21 Sputtering, electrodepositing, electroless plating, CVD etc. are commonly adopted to prepare Cu particles decorated materials.27-29 However, some disadvantages are faced: (a) it is difficult to obtain a uniformly distributed particle size by ex-situ method; (b) Cu particles are inclined to be oxidized during post-treatment; (c) the fabrication processes are complex. Thus in-situ methods of producing Cu particles are appealing.

Herein, an in-situ method using a controllable gelation process with sodium alginate gel as the template is proposed to fabricate Cu particles embedded 3D multilayer Si/C composite. The Si particles as the active material can ensure the high specific capacity. The carbon shells prevent the contacting of Si and electrode with each other, 4


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which is capable to alleviate some side reactions to form SEI layer. Moreover, carbon shells are also helpful to improve the electrical conductivity of electrode. Although Cu particles contribute little to the storage of Li+, Cu decorated in the carbon matrix of Si/C composite can work as conductive agents and improve the transportation of Li+. The Si@C@Cu anode exhibits a commendable capacity of 1500 mAhg-1 at large current (1 Ag-1) and keeps stable during 900 cycles. High rate performance of 1035 mAhg-1 (4 Ag-1) has also showed its superiorities. Results & Discussion

Figure 1(a) Schematic gelation process and the picture as well as the model of the Si@C@Cu composite; (b)-(d) SEM images of Si@C@Cu; (e)-(g) TEM images of Si@C@Cu; (h) STEM image of Si@C@Cu; (i) STEM mapping images of Si@C@Cu. 5


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Sodium alginate (SA) is a crude exopolysaccharide substance which is commonly from brown seaweeds.30 Generally, SA is a kind of linear copolymer with different G/M ratio (see Figure 1(a)).30 When some divalent ions such as Cu2+, Ca2+, Ni2+ or trivalent ions such as Fe3+, Al3+ are introduced into alginate aqueous solution, a strong coordination bonds will be formed through G-G blocks and the adjacent alginate chains will be connected.31 Because of the sufficient sources, nontoxic and excellent conductivity, Cu2+ is selected as the crosslink ion in this work. Although Cu2+ has been employed as the crosslink ion, for example, in the work by Guo et.al.,32 there are still some superiorities in our work: (a) the reaction rate of crosslink is adjusted via a Cu2+ releasing-controllable process, which results in comparable uniform hydrogel; (b) glucolactone reagent is used to chemically react with CuO to release Cu2+ slowly, which is crucial to the formation of a uniform gel and consequently evenly distributed Cu particles. A homogenous slurry will be formed when the SA-Cu gelatin is blended with Si particles and CNTs. Structurally, a polymeric network consisting of well dispersive Si and CNTs is synthesized.30 As reported previously, a hydrogen bond formed between the hydroxylated-Si (Si-OH) and carboxylic groups (-COOH) is helpful to the stability of Si.31,

33, 34

Since the gelation process is comparably slow

through controlling the chemical reaction between CuO and glucolactone, a uniform gel of Si and CNTs is formed after crosslinking. The final Si@C@Cu material is obtained after freeze drying and treated by carbonization. The overall synthesis details of Si@C@Cu can be found in the experimental section. Different contents of CuO are also prepared for comparison and the nomination of each sample can be found in 6


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Table S1. Figure 1(b)-(d) show the SEM images of Si@C@Cu composite. The carbon matrix tends to form a 3D multilayer porous structure which might provide the transport channels for electrons and ions. As shown in Figure 1(e), well dispersive Si particles are tightly chelated on the carbon matrix and CNTs, and as a result, the stability of the whole anode material is greatly improved. Carbon encapsulated on Si is bi-functional (see Figure 1(f)). Importantly, expansion stress can be alleviated via carbon shells and the huge expansion issue of Si electrode will be eased for better properties.23 Meanwhile, as displayed by the high-resolution TEM images in Figure 1(g), Cu particles are embedded in the carbon matrix. In such a case, the electrical conductivity of the whole Si@C@Cu composite is significantly enhanced and thus improving the accessibility and diffusivity of Li+ ions.35 Cu particles come from two aspects: one is from the crosslinking Cu2+ ions and the other is the extra CuO reagents. Cu2+ in the composite will be reduced into Cu nanoparticles evenly distributed in the matrix. The excess micro-sized CuO in the pre-Si@C@Cu composite will be reduced into coarse Cu particles which play roles as the additives to improve the electrical conductivity of the composite. Nevertheless, it is difficult to characterize the lattice structure of Cu particles because they are deeply embedded in the carbon matrix. An elemental mapping is done to demonstrate the distribution of Cu and Si to well understand the whole structure of Si@C@Cu. It can be observed from the STEM and EDX mapping images that Cu particles are located near the surface of Si, which will be favorable to the electrical conductivity of Si and carbon matrix.

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Figure 2 XRD patterns of Si@C@Cu with increasing temperature

The XRD results (Figure 2) show the transformation of Cu in the order of CuO → CuO+Cu2O→Cu2O→Cu with increasing heating temperature. Cu particles in the final composite come from the crosslinking Cu2+ ions and CuO. Since Cu particles as the conductive additives could improve the electrical conductivity of carbon matrix, it would be good to augment the ratio of Cu in the whole composite. From the XRD pattern in Figure 2, it can be found that the final products consist of Si and Cu, which meets all the requirements we designed. Figure S5 shows the XRD pattern of dry gel after being carbonized at 600 °C and all the characteristic peaks match well with Si (JCPDS Card no. 65-1060) and Cu (JCPDS Card no. 04-0836). The relative ratio of Si and Cu is estimated to be 3.38:1 in weight by using TOPAS software (Brucker), and the precise content of Si in the Si@C@Cu composite is measured to be 69.24 % via ICP-AES. Although Cu particles are inactive towards lithium, it not only makes the volume change of the composite controllable through reducing the amount of 8


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lithium-active phase but also sets up a large number of electrical conducting channels between the Si and carbon matrix.

Figure 3 (a) FTIR spectra of Si@C@Cu composite with or without carbonized at N2 atmosphere; (b) N2 adsorption/desorption isotherm curves and (c) Raman spectra of Si@C@Cu composite.

Fourier transform infrared (FTIR) spectra are measured after crosslinking by Cu2+ ions. As compared with pure SA in Figure S8, the FTIR peaks are significantly broadened but the relative intensity decreases, indicating the strong coordination bonds between Cu2+ ions and alginate chains.30 After being carbonized in N2 atmosphere, all the organic groups disappeared, resulting in improved conductivity. The specific surface area of the Si@C@Cu composite calculated via BET equation (p/p0=0.05-0.3) is ~129.22 m2g-1 and the isotherm is type II, as shown in Figure 3(b). The pore size in sample Si@C@Cu is in the range of 1-15 nm, characteristic of porous structure. The contact between electrode and electrolyte are enhanced due to the large surface area which is helpful to the migration of Li+. The porous structure makes the infiltration of the electrolyte into Si@C@Cu easier and the diffusion pathway of Li+ to Si shorter.36 Figure 3(c) illustrates the Raman spectra of Si@C@Cu. The weak peak at 290 cm-1 can be ascribed to the second acoustic phonon mode of Si,7, 37 while the strong peak at 516 cm-1 is characteristic of crystalline Si, 9


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which is blue shifted by 21 cm-1 due to the carbon shell.38 And two broad peaks at 1364 and 1592 cm-1 are from the D and G peaks of carbon. Overall, a 3D multilayer CNTs/C matrix decorated with conducting agents is generated by an in-situ method via a controllable gelation process. The Si@C@Cu composite is used for LIBs to evaluate the potential to be used as the anode material.

Figure 4 (a) Rate performance from 0.1 Ag-1 to 4 Ag-1; (b) Voltage profile at different current density; (c) Long cycling property of Si@C@Cu anode at 1 Ag-1; (d) Cycling performance of Si@C@Cu (0.5 Ag-1); (e) CV curves of the Si@C@Cu electrode (0.02-1 V, 5*10-5 Vs-1); (f) EIS results of of Si@C@Cu electrode (5th, 0.2 V, charge state); (g) Relationship between Z′ and ω-1/2. 10


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The rate capability of Si@C@Cu electrode is measured from 0.1 Ag-1 to 4 Ag-1 whilst maintaining the current density at 0.1 Ag-1 (see Figure 4(a)). Discharge capacities of the Si@C@Cu electrode is measured at ~2341, 2168, 1837, 1577, 1236, 942 mAhg-1 (0.1, 0.2, 0.5, 1, 2, 4 Ag-1), respectively. The specific capacity can be retrieved to the initial value as the current density is reduced down to 0.1 Ag-1, proving the efficient kinetics for lithium storage. The voltage plateaus and profiles of Si@C@Cu are in accordance with other research results (Figure 4(b)).39 Surprisingly, the specific capacity is up to 1500 mAhg-1 at 1 Ag-1 even after 900 cycles with a CE of nearly 99 % (Figure 4(c)). Figure 4(d) displays the cycling performance of the Si@C@Cu electrode measured at 0.5 Ag-1 for 200 cycles, and the capacity of the electrode is maintained at 1773 mAhg-1. To testify the significant effect of the carbon shell and CNTs, pure SiNPs, Si@C@Cu-No (without CNTs), further investigations are done under the same condition (see Figure S10, Figure S11, Figure S13). The Si electrode shows a sharply fade after a few cycles, which is attributed to the huge volume change during the cycles. It is proved that carbon shell on Si is useful to enhance the cycling performance of Si anode.5 The role of CNTs is not only to alleviate the expansion of Si but also to guarantee a robust electrical contact for fast ion transportation as well as toughening the carbon matrix. The composite without Cu decoration has also been examined and the capacity of Si anode is greatly increased because of the high conductivity and ion transportation rate. However, too much Cu particles in the active material will deteriorate the specific capacity (Figure S14) because the specific 11


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capacity is counted by added active material and overmuch inactive component would affect the capacity value. So an appropriate Cu content is required to form a stable and continuous gel due to the content of active group -COOH in SA.

Figure 4(e) shows the cyclic voltammetry (CV) curves of Si@C@Cu electrode. A broad and weak peak is evidenced at about 0.9 V in the first discharge scan but it fades away in the next 8 cycles, indicating the formation of SEI.40 The gradual intensity increment of the reductive peak (0.22 V) and oxidative peaks (0.36 V and 0.51 V) suggests the kinetic enhancement of Si@C@Cu electrode before stabilization.41 EIS tests are adopted to confirm the conductivity and calculate the diffusion coefficient of Li+ ions (DLi+) in Si@C@Cu electrode. As shown in Figure 4(f), there are two parts of the Nyquist: a half circle (high frequency) and an approximate straight-line (low frequency).20 The high frequency part is characteristic of the SEI in which the resistance of the electrodes and electrolyte can be ignored.42 The increment of Rct values from 175 Ω to 196 Ω suggests the thickened SEI layer during the cycling.43 The specific value of DLi+ is evaluated as:

R 2T 2 , 2 A2 n 4 F 4C 2 2

(1)

Z '  Re  Rct   1/2 ,

(2)

D Li 

in which the gas constant R is 8.314 J·(mol·K)-1, T is 298.15 K, A is the surface area of Si@C@Cu electrode, n stands for the number of electrons transferred (per molecule) during the electrochemical reaction, F denotes the Faraday constant of 96485 C·mol-1, C means the concentration of Li+ in the electrode, ω is of the angular 12


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frequency, and σ is the Warburg factor (the slope of Z ′ vs ω-1/2).41 The DLi+ of the Si@C@Cu is calculated to be about 4.543*10-12 cm2s-1 (3rd) and 2.262*10-12 cm2s-11 (200th). In addition, the Rct of Si@C@Cu-removing Cu is nearly 380 Ω which is larger than that of Si@C@Cu composite (see Figure S15 & Figure S16). The DLi+ of Si@C@Cu-removing Cu electrode is 4.476*10-14 which is lower than Si@C@Cu electrode. This demonstrates that the Si@C@Cu electrode material has a superior structural stability and a comparable benign kinetic behavior. The results are in accord with the cycling performances depicted in Figure 4. It further proves that the properties of Si can be greatly improved via building up the structure of Si@C@Cu.

The galvanostatic intermittent titration technique (GITT) method is used to measure DLi+ of Si@C@Cu and the results are shown in Figure 5 and Table S3 (tested at 0.1 Ag-1, pulse time 20 min, relaxation time 30 min). The calculation is done as:

D

4 L2 Es 2 ( ) ,  Et

(3)

in which L is Li+ diffusion length (equals to the electrode thickness, cm), τ stands for the relaxation time (s), ΔEs represents the steady-state potential change via the current pulse, ΔEt is the potential change in a current pulse after subtracting iR drop.23 From Eq. (3) and the obtained GITT curves of Si@C@Cu (Figure 5(a)), the calculated DLi+ corresponding to different delithiation and lithiation states are shown in Figure 5(c), 5(d) and Table S3. The DLi+ of Si@C@Cu at delithiation states ranges from 9.20150*10-11 to 3.21343*10-9 cm-2s-1, and varies from 3.2475*10-10 to 2.03493*10-9 cm-2s-1 at the lithiation state, characteristic of a “W” type. Pure Si anodes of different structures, such as, nano-Si, Si film, Si nanorods, have different DLi+, varying from 10-13 to 10-9 cm-2s-1.44-48 The enhanced DLi+ means that Si@C@Cu is more suitable to 13


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be used as an anode material because of improved transport of ions as compared with pure Si.

Figure 5 (a) GITT curves of Si@C@Cu electrode (discharge/charge state); (b) E vs. t profile for one GITT test; (c) and (d) DLi+ of Si@C@Cu during the charge and discharge processes.

The Si@C@Cu electrode and pure Si anode materials are collected after several cycles and examined by SEM, the results are shown in Figure 6(b). The thickness of Si@C@Cu electrode changes only by 22 % after 200 cycles, but that of pure Si electrode is increased by 55 %. It indicates that the multilayer carbon matrix decorated with copper can ease the stress of Si active material effectively.

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Figure 6 SEM images of Si@C@Cu (a) Before cycling; (b) After 200 cycles (0.5 Ag-1); (c) Bare Si after 50 cycles (0.1 Ag-1)

Table S4 summarizes the latest researches on the performance of Si materials. Apparently, the Si@C@Cu electrode exhibits a comparable benign electrochemical performance. Figure 7(a) and 7(b) schematically show the mechanisms. The commercial Si anode commonly exhibits low intrinsic electron conductivity with unstable SEI layer formed and huge volume change during intercalation and deintercalation processes. All the problems can be solved once and for all via our design. Initially, the bi-functional carbon shells on Si enhance the connection between the conductive carbon matrix and Si particles but alleviate the side reaction on the liquid/solid-electrolyte/electrode interface. A stable SEI layer is formed to stabilize the electrode performance, resulting in long-term cycling properties, and the intrinsic conductivity of Si is substantially improved by the carbon matrix with embedded CNTs. The crosslinked CNTs serve as the conductive interlaced highway for electrons, and the Cu particles work as conducting agents. This gives rise to excellent performances with a specific capacity up to 1500 mAhg-1 at 1 Ag-1 and a CE nearly 99 % after 900 cycles. In addition, the volume change induced stress during cycling can be effectively released by the well dispersion of Si nanoparticles. The mechanical properties of the electrode can also be strengthened by the porous multilayered 15


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structure of carbon matrix and further toughened by the crosslinked CNTs. In essential, the porous multilayer structure of the carbon matrix will be filled with electrolyte, so that the diffusion path of Li+ ions is shortened and the electrolyte penetration is accelerated to some degree. The DLi+ value calculated by GITT method is competitive as compared to the reported results. Moreover, a stable SEI layer is formed around Si on account of carbon shells, resulting in further improved DLi+. Accordingly, the synergistic effects of porous carbon matrix, CNTs and Cu particles boost the performance of Si anode, of which the whole conductivity network is maintained and, the Li+ diffusion efficiency and the structural stability can be improved to the optimal extent.49 Furthermore, the controllable-releasing process of Cu2+ during fabrication is crucial to the formation of dispersive Cu particles. This tunable gelatin fabrication method can not only prevent the oxidation of metallic copper, but also maintain the dispersion of Cu particles. So it is a promising approach to deal with Si and other anode materials with volume change issues.

Figure 7 (a) Electrons transport on toughen carbon matrix decorated by Cu conducting agents; (b) Multilayer carbon matrix helps to release volume stress of Si. 16


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Conclusion A 3D structured Si@C@Cu composite is fabricated by in-situ method via a controllable gelation process. A layer of bi-functional carbon shell was formed on Si nanoparticles, which improves the intrinsic conductivity of Si, but alleviates side reactions on electrode/electrolyte interphase. The electrical conductivity of the Si electrode is improved by Cu particles as conducting agents decorated in the carbon matrix, and the stress induced by Si particles can be released via toughening carbon matrix through the multilayered structure and crosslinked CNTs. Moreover, the carbon matrix prevents Si nanoparticles from agglomeration. Consequently, the composite displays a commendable capacity of 1500 mAhg-1 over 900 cycles (1 Ag-1) and a rate performance of 1035 mAhg-1 (4 Ag-1). This low-cost and tunable gelatinization fabrication method is an optional method to deal with anode materials with volume change issue.

Experimental Material preparation Firstly, Si (Diameter

In Situ Synthesis of Multilayer Carbon Matrix Decorated with Copper

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