Silicon-Encapsulating Spherical Carbon Microbeads for Lithium ion Batteries

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Silicon-Encapsulating Spherical Carbon Microbeads for Lithium ion Batteries

Duck Gyun Mok

Department of Energy Engineering Battery Science and Technology

Graduate School of UNIST

2014

Silicon-Encapsulating Spherical Carbon Microbeads for Lithium ion Batteries

Duck Gyun Mok

Department of Energy Engineering Battery Science and Technology

Graduate School of UNIST

2014

Silicon-Encapsulating Spherical Carbon Microbeads for Lithium ion Batteries

A thesis

submitted to the Graduate School of UNIST in partial fulfillment of the

requirements for the degree of Master of Science

Duck Gyun Mok

6. 20. 2014.

Approved by

___________________________

Major Advisor

Kyu Tae Lee

Silicon-Encapsulating Spherical Carbon Microbeads for Lithium ion Batteries

Duck Gyun Mok

This certifies that the thesis of Duck Gyun Mok is approved.

6. 20. 2014.

Signature

__________________________

Thesis supervisor: Kyu Tae Lee

Signature

__________________________

Nam-Soon Choi

Signature

__________________________

Yoon Seok Jung

Abstract

Recently, remarkable improvements in the electrochemical performance of Si materials have been achieved through several strategies including the use of a buffer matrix such as Si/carbon composites and control of the morphology. However, the inherent volume change of Si still induces electrode expansion and external cell deformation, although the electrical contact loss is strongly inhibited. The cell deformation is the critical factor limiting the commercialization of Si-based anode materials, and is as important as electrochemical performance from a practical point of view. An acceptable degree of volume change for the electrodes is about 10 %, similar to that of commercialized graphite electrodes. A few approaches have been taken to alleviate cell deformation, including control of electrode porosity and the use of functional binders.

In this paper, Silicon-Encapsulating Spherical Carbon Microbeads are synthesized not only to inhibit the electrode degradation caused by electrode thickness change during cycling, but also to increase tap density of electrodes

Contents

1. Introduction --- 1

1. 1. Lithium-ion batteries --- 1

1. 2. Anode materials --- 4

1. 3 Alloy-based anode materials for lithium-ion batteries --- 5

1.4 Silicon anode materials --- 7

1.4.1 Silicon carbon composites --- 9

1.4.2 Silicon Encapsulating Hollow Carbon --- 11

2. Experimental ------ 13

2.1. Synthesis --- 13

2.2. Characterization analysis--- --- 14

2.3. Electrochemical characterization--- 15

3. Results & Discussion --- 16

4. Conclusion ---- 34

Reference --- 35

List of figures

Figure 1 Global LIB Market status and forecast (2011 ~ 2018) – Sale Amount

Figure 2 Diagram comparing the rechargeable battery technologies as a function of volumetric and specific energy densities

Figure 3 Schematic diagram of the lithium ion batteries

Figure 4 Diagram compared to alloy-based materials and (*) lihthiation (**) delithiation Figure 5 Crystal structure of alloy-base materials^13b

Figure 6 Voltage profiles of pure silicon anode ²⁵

Figure 7 (a) TEM image of Si powders (b) T EM image of Si-C composites.¹¹

Figure 8 (a) Voltage profile of the Si-C composites (b) cycle performance of the Si-C composites ¹¹

Figure 9 TEM image of (a) phthalocyanine coated silicon and silicon nanopowder (b) chemical etching and (c, d) electroless etching.

Figure 10 (a) cycl eability of Si-encapsulating hollow carbon via chemical et ching (circle) and electrol ess (square). (b) Rate perform ance of the Si-encapsulating hollow carbon via chemical etching (black) and electroless etching (white).

Figure 11 XRD pattern of synthesized Si-Encapsulating spherical carbon microbeads

Figure 12 TG curve of Si-Encapsulating spherical carbon microbeads. Silicon nanopowder and phenolic resin ratio (a) 100:20, wt. % (b) 100:40, wt. %

Figure 13 SEM image of the Si-Encapsulating spherical carbon microbeads before HF treatment

Figure 14 SEM image of FIB cross section (a~c) before HF treatment (d~f) after HF treatment

Figure 15 pore size distribution analysis of Si-Encapsulating spherical carbon microbeads (a) BET curve (b) BJH pore size distribution

Figure 16 Voltage profile (a) Si-Encapsulating spherical carbon microbeads (b) Fe-phthalocyanine coated Si-Encapsulating spherical carbon microbeads (c) cycleability

Figure 17 XRD pattern of synthesized carbon coated silicon.

Figure 18 TG curve of Fe-phthalocyanine coated silicon (a) 1:1, wt. % (b) 2:1, wt. % (c) 4:1 wt. % under an air atmosphere.

Figure 19 SEM image of Fe-Phthalocyanine coated silicon (a) bare Si nanopowder (b) Silicon nanopowder and Fe-phthalocyanine mixed (1:1, wt.%) (c) 2:1, wt. % (d) 4:1, wt. %

Figure 20 Voltage curve of Fe-phthalocyanine coated silicon (a) 1:1, wt.% (b) 2:1, wt.% (c) 4:1, wt.%

Figure 21 Electrochemical performances of Fe-Phthalocyanine coated silicon cycleability

List of tables

Table 1 Global LIB Market Status and Forecast (2011 – 2018) – Sale Amount

Table 2 Comparison of property various anode materials.⁹

Table 3 Crystal structure, unit cell volume and volume per silicon atom for the Li-Si system ²⁴

List of scheme

Scheme 1 Schematic illustration of R-F gel reaction.¹¹

Scheme 2 Schematic illustration for the synthesis methods (a) chemical etching and (b) electroless etching.

Scheme 3 Schematic figure for the synthesis of Si-Encapsulating Spherical carbon microbeads.

List of equation

Equation 1 Electro etching mechanism of Si

1

1. Introduction

1.1 Lithium-ion batteries

In 2013, total sales of lithium-ion batteries have grown up to 140 million dollar from 133 million dollar, compared to 2012. Demand of portable devices such as smart phones and laptops computer and tablet PC have increased. Besides, EV and ESS market rapidly expanding.¹ It is expected to increase sale amount about 70 percent in 2018. A history of the Lithium-ion batteries, First of all, American scientist John B. Goodenough invented lithium cobalt oxide at Oxford University. The graphite anode materials were discovered by Rachid Yazami who is research scientist. In 1985, Akira Yoshino could lead a research team, who invented prototype of the first batteries using by lithium-ion.

The inventions were used in the commercial lithium-ion battery. After Sony released a lithium-ion battery, The company has become the first to commercialize lithium-ion batteries in 1991.^{2 3} The motivation is based on Li-metal as anode for using a battery technology on the fact that Li is not only the lightest (molecular weight 6.94 g mol^–1 and density 0.53 g cm^–3) but also the most electropositive metal (standard redox potential –3.04 V), therefore lithium metal can reveal the high capacity and used in the storage system. In 1970, these advantages were first applied at assembly of primary batteries using Li metal. Because of their variable discharge rate and high energy density, they could quickly applied these to power sources such as calculators, watches with medical devices. In the same age, inorganic materials and transition metals were discovered and was examined at lihium-ion battery systems.^44b

The primary lithium-ion batteries are examined by layered structure Li⁺ (Li_xC₆) anode and (Li_1−xCoO₂) cathode. The theoretical capacity is 274 mA h g⁻¹ at a response voltage of 3.7 V, which is only a better factor of better than that stored by the much older batteries such as lead-acid. The lithium cobalt oxide have lattice parameter a = 2.8141 Å and c = 14.0436 Å. During the lithiation/delithiation process, Li-ion move to between cathode and anode through electrolyte.⁵ During the charge, lithium ion are eliminated at the cathode, of which working voltage is higher than 2 V vs. Li/Li⁺ and are moved into the anode, of which working voltage is lower than 3 V vs. Li/Li⁺. During the electrochemical process, the electrons moved from cathode to anode used by and external circuit. In the theoretical lithium-ion batteries, all of the Li-ions have to move between the cathode and anode however it doesn’t do. Actually, columbic efficiency is lower than 100%, because side effect was occurred by such as resistance at between electrode and electrolyte or polarization phenomena.^{66d 6b} The metal oxide and phospate used as a cathode materials and graphite employed as a anode material.

They are used in commercialized lithium-ion batteries and are indicated intercalation reaction.

2

Representation of the cathode materials are lithium cobalt oxide, whose layered structure show disposition of intercalation reaction. Lithium-ions were stored between layer as an electrochemical reaction in the host lattice. Intercalation mechanism has a property of not only small stains but also low irreversible structure. As a result, lithium cobalt oxide has good capacity and long cycleability.^7,8,

9,10 Carbonaceous materials are used as commercialized in lithium-ion batteries. Graphite anode materials indicate a theoretical capacity of 372 mA h g^-1 at formation of LiC₆ at a response voltage of 0.0 ~0.35 V¹¹.

Figure 1 Global LIB Market status and forecast (2011 ~ 2018) – Sale Amount

Table 1 Global LIB Market Status and Forecast (2011 – 2018) – Sale Amount (Unit = USD)

2011 2012 2013 2014 2015 2016 2017 2018

IT 10,479 11,562 11,609 13,127 14,170 14,862 14,973 14,963

xEV 882 1,522 2,058 2,726 3,786 5,559 7,630 10,469

ESS 125 269 341 507 868 1,249 2,082 2,666

Total 11,486 13,353 14,008 16,360 18,824 21,670 24,685 28,098

2011 2012 2013 2014 2015 2016 2017 2018

11,486 13,353 14,008 16,360 18,824 21,670

24,685 28,098 IT xEV ESS

3

Figure 2 Diagram comparing the rechargeable battery technologies as a function of volumetric and specific energy densities

Figure 3 Schematic diagram of the lithium ion batteries

4

1.2 Anode materials

Initially, lithium metal was widely used when using anode material. But lithium metal has many problems such as formation dendrite and possibility of explosion. After a round of cycle, dendrite has grown by lithium metal ionization. As a result, eternal short circuit has become because of this safety problems. Also, lithium metal was exposed to moisture. The reaction is highly exothermic, and can lead to an explosion. To overcome the disadvantage of the lithium metal, research to replace lithium metal was currently underway worldwide. ¹²

In particular, carbon-based anode material, it is possible that the lithium ion are inserted therein, it can also be present in a stable state. Providing a way to be able to solve the problem of safety by lithium metal. Carbon-based materials are very close to the lithium metal potential of the electrochemical reaction. Crystal structure were rarely changed during the lithiation/delithiation, it is possible that oxidation and reduction reaction were continued from the electrode. As a result, carbon-based materials indicate good capacity and good cycleability. But carbonaceous anodes had been lauded for their advantage, but lamented for their low power output. Since the demand for the market of the high capacity has increased, there is a need to invented new materials. Accordingly, plans are underway to develop such as alloy materials.¹³

Table 2 Comparison of property various anode materials.⁹

Materials

Theoretical specific capacity (mA h g

^-1

)

Theoretical charge density (mA h cm

^-3

)

Potential vs. Li

(V)

Density (g cm

^-3

)

Volume change (%)

Li 3862 2047 0 0.53 100

C 372 837 0.05 2.25 12

La4Ti5O12 175 613 1.6 3.5 1

Si 4200 9786 0.4 2.33 320

Sn 994 7246 0.6 7.29 260

Sb 660 4422 0.9 6.7 200

Al 993 2681 0.3 2.7 96

M g 3350 4355 0.1 1.3 100

Bi 385 3765 0.8 9.78 215

5

1.3. Alloy-based anode materials for lithium-ion batteries

Alloy-based materials are announced that for the characteristics of the safety and the high specific capacity. Table 2 shows compare to property of the anode materials. It is better for alloy-based materials than such as carbon-based material and metal oxide on theoretical capacity. Moreover, it had a similar operation potential vs Li/Li

⁺

. However, alloy materials are expanded about 300% on full lithiation state

¹⁴

.

Large volume expansion was announced for the main issue of the alloy-based materials during lithiation state. Crack of the electrode surface was occurred by the volume change. As a result, alloy materials were indicated poor cycleability and the 1st cycle irreversible capacity fading.

^{14d, 15}

. These research has been examined to solve problems and significant process has been made

^{10, 12, 16 17}

.

T his report focuses on the electrochemical performance of alloy anodes including such as capacity fading, cycleability. So, problems were prevented through control of the morphology and carbon coated. In this work the electrochemical performance of silicon because which has abundant resources with high capacity.

Figure 4 Diagram compared to alloy-based materials and (*) lihthiation (**) delithiation

6

18

Figure 5 Crystal structure of alloy-base materials^13b

7

1.4 Silicon anode materials

Next generation anode materials promise silicon for lithium-ion batteries. This has high energy density and abundant resources. However, silicon has short cycleability due to extreme volume change (300%) that occurs during process of the lithiation. These problems cause crack on the surface of the silicon¹⁹²⁰²¹. Silicon particles were exposed to new surface and then occurring continuous SEI formation²². As a result, particles are occurred to loss electric contact between the active materials and Cu foil or decomposed. This appears capacity fading and low cycle life. To solve these problems, many studies have been conducted such as control the morphology and silicon/carbon composite with silicon nanowire and silicon nanotube so on. In this figure 6 shows voltage profile of pure silicon. It indicates extremely irreversible capacity between charge and discharge at 1^st cycle. It shows that columbic efficiency is 25%. Table 3 indicates the crystal structure for Li-Si system. It exhibits that silicon atom for full of the charge state is much higher than initial silicon atom. ^{8, 23} Try to reduce capacity fading and the 1^st cycle irreversible capacity of silicon anode materials, various strategies have been conducted to decrease effect of the volume expansion.

Table 3 Crystal structure, unit cell volume and volume per silicon atom for the Li-Si system ²⁴

Compound and crystal structure Unit cell

Volume (ų)

Volume per silicon atom (ų)

Silicon cubic 160.2 20.0

Li₁₂Si₇, orthorhombic 243.6 58.0

Li₁₄Si₆, rhombohedral 308.9 51.5

Li₁₃Si₄, orthorhombic 538.4 67.3

Li₂₂Si₅, cubic 659.2 82.4

8 Figure 6 Voltage profiles of pure silicon anode ²⁵

9

1.4.1 Silicon carbon composite

Many studies focus on increase the electrochemical performance of alloy anodes using by carbon- based composites. ^26,27 Silicon and carbon composite were prepared using by RF-gel and synthesis, in this scheme 1 shows that reaction of the resorcinol and formaldehyde. Through the reaction, carbon aerogel embedded silicon was obtained. ²⁵ Synthesized powers were confirmed by T EM image as shown in Figure 7(b).

Scheme 1 Schematic illustration of R-F gel reaction.¹¹

Figure 7 (a) TEM image of Si powders (b) T EM image of Si-C composites.¹¹

10

Si-C composites are obtained for electrochemical performance such as voltage profile and cycleability as shown in figure 8. It is better for Si-C composites than pure silicon powder. In this figure 8(a) shows irreversible capacity of between 1^st cycle and 2^nd cycle. The discharge capacity showed approximately 50 percent rises, compare with figure 6. Cycleability also shows good cycle life. Si powders are inserted into a three-dimensional carbon network matrix. Volume expansion of silicon nanopowder was preserved by carbon matrix for repeated charge and discharge processes. T his report was solved using by Si-C composites, and good cycle life should be attributed to carbon matrix coated silicon.¹¹

Figure 8 (a) Voltage profile of the Si-C composites (b) cycle performance of the Si-C composites ¹¹

11

1.4.2 Silicon Encapsulating Hollow carbon

Si-Encapsulating hollow carbon was synthesized, in order to prevent volume expansion for silicon anode materials. ²⁸ Through the chemical etching and the electroless etching prepare, both of them are indicated as shown in scheme 2. ²⁸ First of all, silica components are removed by HF solution as shown formation of the void. However, sample has critical problem such as high electric resistance and lack of between hollow carbon and silicon nanoparticles. Secondary, sample was obtained by electroless method. Fe-phthalocyanine coated silicon and then addition of the HF solution, iron carbide components react HF solution. As a result, iron carbide was removed as shown in equation 1.^29,30 It is better for electroless etching than chemical etching such as electric resistance and contact of between hollow carbon and silicon nanoparticles. Electrochemical performance was improved by two methods as shown in figure 10. Through the electroless method, silicon encapsulating hollow carbon can indicate good cycleability and rate property.

Scheme 2 Schematic illustration for the synthesis methods (a) chemical etching and (b) electroless etching.

Equation 1 Electro etching mechanism of Si

Si + Fe³⁺ + 3HF + 3HF^-₂→ H₂SiF₆ + 2H⁺ + H₂ + 3F^- + e^- + Fe²⁺ H⁺ e^- → 1/2H₂

12

Figure 9 TEM image of (a) phthalocyanine coated silicon and silicon nanopowder (b) chemical etching and (c, d) electroless etching.

Figure 10 (a) cycleability of Si-encapsulating hollow carbon via chemical etching (circle) and electroless (square). (b) Rate performance of the Si-encapsulating hollow carbon via chemical etching (black) and electroless etching (white).

13

2. Experimental

2.1 Synthesis

2.1.1 Si-Encapsulating spherical carbon microbeads

Silicon nanopowders were heated at 700℃ for 2 h under an air atmosphere to obtain Si-silica core- shell nanoparticles. Silicon nanopowder and phenolic resin and fumed silica were mixed using by a mortar. The Fumed silica (CAB-O-SIL) coated phenolic resin powder and Si-silica core-shell nanoparticles was then heat-threated under an argon atmosphere from 30 to 1000℃ for 1h. After carbonization, synthesized sample was washed using by aqueous hydrofloric acid (10 wt. %) for remove the silica component.

2.1.2 Supporting experimental

Fe-phthalocyanine coated silicon anode active material was prepared as follows. Silicon nanopowders ( < 100 nm, Sigma Aldrich) and Fe-phthalocyanine (Sigma Aldrich) were blend using by a mortar. They were mixed (1:1, 2:1, 4:1, wt.%) and then the mixture was heated 900℃ for 2h under Ar atmosphere

14

2.2 Characterization analysis

Power X-ray diffraction (XRD) measurement was executed by D/MAZX 2500V/PC (Rigaku) using Cu-Kα radiation (λ = 1.5405Å) and 2θ range from 10^o to 80^o in steps of 0.02^o. Thermogravimetric analysis (T GA) and differential scanning calorimetry (DSC) was observed using SDT Q600 Simultaneous DSC/T GA (TA Instruments) at the heating rate of 10℃ min^-1 until 900℃ in air. The morphology of sample was observed by Quanta 200 field emission scanning electron microscope (FE- SEM). Cross section was executed by FIB. Pore size distribution measurement for calculated BET surface area and BJH desorption distribution was examined by BET method. Electrochemical impedance spectroscopy (EIS) of symmetric cells was performed by using SP-150 (Biologic) instrument at frequencies range from 300 kHz to 1 mHz with an amplitude of 5 mV.

15

2.3 Electrochemical characterization

The galvanostatic charge-discharge cycling (WonATech WBCS 3000 battery measurement system) was measured in the potential from 0 to 2 V vs. Li/Li⁺ with fabricate half-cell using by coin cell type (CR2016), the counter electrode employ Li metal. T he synthesized active materials were mixed with Super P and the polyacrylic acid(Aldrich) in an 8:0.5:1.5 weight ratio to make the working electrode on Cu foil. The first lithiation and delithiation capacities were determined at a current density of 100 mA g^-1 and the electrochemical performance of the half cells was monitored in convention oven at 30℃. The electrolyte examines 1.3M LiPF₆ in a mixture of ethylene carbonate and diethyl carbonate (30:70, v/v) with 5 wt. % fluoroethylene carbonate (FEC, Soulbrain Co. Ltd.). Microporous polyethylene film was examined as a separator. Cells were fabricated in an Ar atmosphere glove box and oxygen and moisture are less than 1ppm.

16

3. Result & discussion

3.1 Si-encapsulating spherical carbon microbeads

Scheme 3 indicates how to synthesize Si-encapsulating spherical carbon microbeads. Phenolic resin and fumed silicon with Si-silica core-shell nanoparticles was used to obtain samples. All of them are mixed. Phenolic resin can inflict heat for 80 °C or more. Particles transformed to a sphere form and fumed silica cover with surface of the sample. The reason is that both of them are having similar property of hydrophobic. Also, particles become spherical shape in order to reduce surface energy. In order to carbonize the powder, samples were heating at 1000 °C. XRD measurements were conducted to examine the synthesized spherical carbon. Samples showed typical diffraction peaks at 2θ of about 28°, 48°, and 56°. Figure 11 shows XRD peak position, it indicates silica component at about 20°.

There is figure explain reduced fumed silica. In Figure 12. T GA graph was heat treatment from 30 to 900℃ under an air atmosphere. Each T GA curves indicated removed carbon content. T he silicon content can be calculated to be 12.8%, 19%.

17

Scheme 3 Schematic figure for the synthesis of Si-Encapsulating Spherical carbon microbeads.

18

Figure 11 XRD pattern of synthesized Si-Encapsulating spherical carbon microbeads

19

Figure 12 TG curve of Si-Encapsulating spherical carbon microbeads. Silicon nanopowder and phenolic resin ratio (a) 100:20, wt. % (b) 100:40, wt. %

20

Figure 13 indicates SEM image of the synthesized sample. Most of all shows spherical shape. And diameter of the synthesized sample is approximately 10um. Particle shape is not spherical when synthesis increase silicon ratio. Figure 14 shows SEM image of the cross section using by FIB. In this image of the cross section indicates spherical carbon inside silicon particles. It seems like pomegranate shape. Cross section of the silicon shows core shell structure. Silicon particle has thin silica layer, as shown in the SEM image of figure 14 (c) Figure 14 (f) indicates voids when after HF treatment. T he powder obtained using by HF treatment in order to make void from chemical etching.

21

Figure 13 SEM image of the Si-Encapsulating spherical carbon microbeads before HF treatment

22

Figure 14 SEM image of FIB cross section (a~c) before HF treatment (d~f) after HF treatment

23

Figure 15 shows pore size distribution analysis using by Brunauer - Emmett - Teller method. T hrough the BJH pore size distribution, samples confirm that synthesized sample has pore size of 10nm. BET surface area increase 60%. Value of the calculated BET surface area are each 355 m² g^-1, 568 m² g^-1 when before and after HF treatment. As shown in the figure created pore that size approximately 4nm.

24

Figure 15 pore size distribution analysis of Si-Encapsulating spherical carbon microbeads (a) BET curve (b) BJH pore size distribution

25

Figure 16 shows the voltage profile of Si-Encapsulating spherical carbon microbeads. T he cell was discharged to 0 V. T he cell discharged to 0.1C-rate for the cycles. The voltage profile indicated plateau during charge at 0 V. It can decrease effect of volume expansion compare to pure silicon. The void can decrease effect of the volume expansion. However, chemical etching process occur lack of the electric contact between spherical carbon and silicon nanoparticles as shown in figure 16 (a). Lack of the electric contact indicates capacity fade. Silicon nanoparticles non participate reaction of the electrochemical. In order to increase electric contact of between spherical carbon and silicon, we are using Fe-phthalocyanine coated silicon. It shows increase capacity more than non-carbon coated Si- encapsulating spherical carbon. However, this method was indicated low cycleability as shown in figure 16 (c)

26

Figure 16 Voltage profile (a) Si-Encapsulating spherical carbon microbeads (b) Fe-phthalocyanine coated Si-Encapsulating spherical carbon microbeads (c) cycleability

27

3.2 Supporting experimental of Fe-phthalocyanine coated silicon

In order to decrease silicon volume expansion, Fe-phthalocyanine coated silicon nanopowder.

Silicon and Fe-phthalocyanine are mixed by using mortar. And carbonization at 900℃. Figure 17 shows XRD measurements were conducted to examine the synthesized carbon coated silicon.

Samples showed typical diffraction peaks at 2θ of about 28°, 48°, and 56°. And whole region showed amorphous peak from 10° to 80°. T GA graph was heat treatment from 30 to 900℃ under an air atmosphere as shown in figure 18. Each T GA curves indicated carbon content. The silicon content can be calculated to be 71.8%, 85.4%, 92.5%. SEM image of bare silicon nanopowder and Fe- phthalocyanine coated silicon. Figure 18 shows that the Silicon nanopowder/Fe-phthalocyanine weight ratio for the three cases prepared are 1:1 ratio, 2:1 ratio, and 4:1 ratio. Figure 19 (a) indicates bare silicon nanopowder that submicron particle size. Figure 19 (b) shows covered silicon surface.

28

Figure 17 XRD pattern of synthesized carbon coated silicon.

29

Figure 18 TG curve of Fe-phthalocyanine coated silicon (a) 1:1, wt. % (b) 2:1, wt. % (c) 4:1 wt. % under an air atmosphere.

30

Figure 19 SEM image of Fe-Phthalocyanine coated silicon (a) bare Si nanopowder (b) Silicon nanopowder and Fe-phthalocyanine mixed (1:1, wt.%) (c) 2:1, wt. % (d) 4:1, wt. %

31

Figure 19 shows the voltage profile of carbon coated silicon. The cell was discharged to 0 V. The cell discharged to 0.1C-rate for the cycles. The voltage profile indicated plateau during charge at 0 V. It seems to decrease silicon effect of volume expansion. Homogeneous carbon prevents volume expansion on the silicon surface. Synthesized silicon for 1:1 wt. % more decreased polarization than other sample. Figure 20 indicates cycle performance for synthesized carbon coated silicon. In case bare silicon powder, this cell shows high capacity fading and short cycle life. Although prepared Silicon and carbon 1:1 ratio shows low specific capacity. The cell shows low capacity fading during charge and discharge

32

Figure 20 Voltage curve of Fe-phthalocyanine coated silicon (a) 1:1, wt.% (b) 2:1, wt.% (c) 4:1, wt.%

33

Figure 21 Electrochemical performances of Fe-Phthalocyanine coated silicon cycleability

34

4. Conclusion

We have demonstrated that Silicon-Encapsulating Spherical Carbon Microbeads improved the cycling performance due to their void space to buffer volume change of Si during repeated cycle. Void structure of Spherical Carbon Microbeads obtained via chemical etching was investigated by XRD patterns, cross-section SEM images, and pore size distribution analysis from N₂ sorption. It is considered that Silicon-Encapsulating Spherical Carbon Microbeads are a promising design to minimize the volume expansion of Si-based anodes upon Li de/insertion and to increase tap density of electrodes

Also, the lithiation and delithiation electrochemical performance of Fe-phthalocyanine coated silicon for lithium-ion batteries was examined. Si/Carbon 50 wt. % ratio shows improve than the other, through the carbon coated silicon.

35

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