The Roles of Intermediate Phases of Li-Si Alloy as Anode Materials for Lithium-Ion Batteries

Rare Metal Materials and Engineering Volume 39, Issue 12, December 2010 Online English edition of the Chinese language journal

Cite this article as: Rare Metal Materials and Engineering, 2010, 39(12): 2079-2083.

ARTICLE

The Roles of Intermediate Phases of Li-Si Alloy as Anode Materials for Lithium-Ion Batteries Hou Xianhua,

Hu Shejun,

Ru Qiang,

Zhang Zhiwen

South China Normal University, Guangzhou 510006, China

Abstract: An ab initio method of the first-principles plane-wave pseudopotentials based on the density functional theory has been used to calculatethe physical character and electrochemical performance of various alloy phases in Li-Si alloy. The results show that besides the growth of solid electrolyte interphase (SEI), the formation of Li12Si7 alloy phase also partly leads to the initial irreversible capacity loss. In addition, the pure silicon thin film electrode was prepared by the radio frequency (RF) magnetic sputtering on copper foil collector as anode materials. The structural and electrochemical characteristics of Li-Si alloy were examined using X-ray diffraction (XRD), cyclic voltammogram (CV) and repeatedly constant current charge/discharge (CC). The results show that the first irreversible capacity loss is very large and amorphous structure can accommodate the large volume expansions and improve cyclic performance. Key words: lithium-ion batteries; lithiation formation energy; capacity loss; first-principle

Li-ion rechargeable batteries have become one of the most potential power sources in the 21st century[1]. The demand for lithium ion batteries as a power supply for portable electric devices increases continuously, but the lithium insertion capacity of graphite, the anode of commercial lithium ion battery, has already reached the theoretical limit (372 mAh/g as C6Li)[2]. In order to enhance the energy density of lithium ion batteries, new anode materials are required to be developed[3,4]. Metal-based anodes have received great interest as alternatives of carbon for lithium-ion rechargeable batteries due to their intrinsically high gravimetric and volumetric capacity[5]. Although the basic thermodynamics and electrochemistry of most of the simple alloys are known, themechanism of inse rtion/extraction lithium remains poorly understood[6]. Fracture, dislocation damage, and phase transformations are commonly observed in both cathodes and anodes subjected to electrochemical cycling in lithium ion battery. In particular, metal anodes have been observed to undergo increased disorder at the atomic level upon electrochemical cycling[7,8], the cause of which has not been clear yet.

Computer simulation provides the most efficient tool for reducing testing time and optimizing battery systems[9-12]. In this paper, we provide computationally electrochemical parameters in lithiated metals with a focus on silicon based on the first-principle plane-wave pseudopotential method, and a new mechanism is proposed to explain the initial irreversible capacity loss and phase transformation from stable alloy phases to metaphase amorphous structures in Li-Si alloy. Meanwhile, since the Li-Si alloy system has a high theoretical energy density and low electrochemical potential versus Li/Li+, the investigation on the lithium insertion mechanism of silicon-based anode materials is of far-reaching significance in preparing practical and effective commercial anode materials.

1 Computational and experimental details 1.1 Physics model According to the Li-Si phase diagram, there are eight intermetallic compounds of lithium and silicon, such as LiSi, Li12Si7, Li2Si, Li7Si3, Li13Si4, Li7Si2, Li21Si5 and Li22Si5. The experimentally determined structures of these alloy phases are listed in Table 1. There are several phase transitions between q u i t e

Received date˖December 27, 2009 Foundation item: National Natural Science Foundation (50771046) Corresponding author: Hou Xianhua, Ph.D., School of Physics and Telecommunication Engineering, South China Normal University, Guangzhou 510006, P. R. China, Tel: 0086-20-39310066, E-mail: [email protected] Copyright © 2010, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.

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Table1

Crystal structure information

Samples

Space group

Lattice constants/nm

Reference

LiSi

I41/aZ (No.88)

a=0.9354, b=0.9354, c=0.5746

[13]

Li12Si7

Pnma (No.62)

a=0.8566, b=1.9701, c=1.4299

[14]

Li2Si

C12/m1 (No.12)

a=0.770, b=0.441, c=0.656, β=113.4°

[15]

Li7Si3

R-3mH (No.166)

a=0.4435, b=0.4435, c=1.8134, γ=120°

[16]

Li13Si4

Pbam (No.55)

a=0.799, b=1.521, c=0.443

[17]

Li7Si2

Pbam (No.55)

a=0.799, b=1.521, c=0.443

[18]

Li21Si5

F-43m (No. 216)

a=b=c=1.871

[19]

Li22Si5

F23 (No.196)

a=b=c=1.875

[20]

different crystallographic structures, eight to be exact, including four metaphases and four stable phases, which make first principles calculation of the lithium-silicon system a reliable undertaking.

1.2 Computational methods In this work, the calculations were performed using plane-wave pseudopotentials method based on the density functional theory (DFT)[21]. The electron-ion coulomb interaction of the system is described by optimized ultrasoft pseudopotential (USPP) introduced by Vanderbilt[22]. Meanwhile, all calculations were performed within the generalized gradient approximation (GGA) proposed by Perdew et al. using Perdew-Wang (PW91) form for the exchange correlation energy[23]. The influence of different K-point samplings and plane-wave cutoff energies was explored in a series of test calculations. The Brillouin Zone integration is approximate using the special k-points sampling scheme of Monkhorst-Pack and 5×5×5 k-point grid is used. The cutoff energy of plane wave was set at 300 eV. The atom positions, lattice parameters and cell volumes were all relaxed with the conjugated gradient method using forces and stresses. The geometry optimization was stopped when forces all relaxed were less than 0.005 eV/nm. The maximum displacement error was within 0.005 nm and the maximum stress was less than 0.1 GPa. The maximum root-mean-square convergent tolerance was less than 2×10-6 eV/atom. All calculations were performed using Cambridge serial total energy package (CASTEP) program. In this work, the electrochemical parameters were calculated with an approximation method introduced by Courtney et al[24,25]. Assuming that the reaction process only undergoes initial state and final state and neglects the complicated medium processes. The insertion voltage is given by V ( x) =−ΔG / Δx , where ΔG =ΔE + pΔV −TΔS is the change of Gibbs free energy for the insertion reaction and Δx is the charge transported by lithium in the electrolyte. E, p, V, T, S denote energy, pressure, volume, temperature and entropy, respectively. It is assumed that the changes in volume and entropy associated with the insertion are negligibly small, ΔG can be approximated by the internal energy

term Δ E , V ( x ) ≈ −ΔE / Δx . Meanwhile, it is assumed that V0 and V are the equilibrium volumes before and after insertion reaction, so the coefficient of relative expansion is defined as: Δ V = (V − V0 ) V0 ×100% . The calculated specific capacity is defined as: Ccal = ΔxF ξ M Si , where F is the Faraday constant, ξ is the structural coefficient of materials, and M is the relative molecular weight of the anode materials.

1.3 Experimental methods The silicon thin film was deposited onto Cu-foil substrates in a deposition chamber to 5×10-3 Pa by muti-target magnetic sputtering apparatus (JGP560). The Cu-foil substrates were cleaned ultrasonically with acetone for 10 min. Radio frequency (RF) power of 100 W was used to bombard the substrates for 5 min to eliminate the impurity of the surface before the deposition of the thin film. The silicon thin film was obtained by sputtering silicon target with a RF power of 250 W for 15 min. Cyclic voltammetry (CV) and repeatedly constant current charge/discharge (CC) measurements were carried out by 8-channel testing system (Solartron1480, England) at room temperature with CR2016 button cells. The cells were assembled in an argon-filled glove box (Mikrouna, Sukei1220/750). Lithium foil was used as both counter and reference electrodes, and the silicon films were used as the working electrodes. The electrolyte was 1 mol/L LiPF6 dissolved in a 1:1:1 (vol.) mixture of ethylene carbonate (EC), diethyl carbonate (DEC) and ethyl methyl carbonate (EMC). Polypropylene membrane (celgard2400) was used as the separator. In the CC test, the current was 0.1 mA/cm2 and the potentials were between 0 and 1.5 V. In the CV test, the potentials were between 0 and 2.5 V and the sweep rate was 0.5 mV/s. In this paper, the charge and discharge processes were corresponding to lithium insertion and lithium extraction, respectively.

2

Results and Discussion

2.1 Formation energy of LixSi alloy phase X-ray diffraction pattern of the pure silicon displays crystalline structure in Fig.1. In order to study the effect of the lithiation formation energy on the mechanism of the phase transformation in LixSi alloy, thevarious physical and elec-

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Intensity/cps

Hou Xianhua et al. / Rare Metal Materials and Engineering, 2010, 39(12):2079-2083

5000 3000 1000 20

40

60

80

2θ/(º) Fig.1

XRD pattern of the pure silicon before charge

trochemical properties of LixSi (x=0.0, 1.0, 1.7143, 2.0, 2.333, 3.25, 3.5, 4.2, 4.4) are calculated using CASTEP software package, and the partial results are described in Table 2. From Table 2, it can be found that the poor-lithium phases (LiSi and Li12Si7) have the highest formation energy in all phases. According to the solid state theory, the formation energy is directly proportional to the stability of the matters. So the poor-lithium phases, e.g. LiSi and Li12Si7 will be formed easily during the initial charge process. Meanwhile, dealloying (extraction) is difficult once the phases are formed due to high formation energy of 54.3201 eV and 58.8344 eV, respectively[26]. Therefore, the large irreversible capacity loss may be caused by the formation of solid electrolyte interphase (SEI) and lithium-silicon difficultly dealloyed phases. Particularly, lots of lithium ions are consumed during the initial lithium insertion and extraction process. The theoretical results are consistent with the Ref.[27]. But Dimov et al[27] inferred that the reaction of Li-Si alloy system is shown as: charge xLi + + Si(crystal) + xe- ⎯⎯ ⎯ → Li xSi(amorphous) ( 1 ) + Li xSi(amorphous)  ( x - y)Li + Li ySi(amorphous) + ( x - y)e- (2) Eq. (1) is the reaction of first charge (lithiation) process, and then the amorphous LixSi alloy phases are formed gradually in the following charge process. The structure still remains the amorphous state when the lithium ions de-intercalate from the host materials. Moreover, some of lithium ions will be consumed due to formation of stable alloy phases and solid elec-

Table 2

trolyte interphase (SEI). Upon subsequent cycling, the electrochemical reaction will occur according to Eq.(2) during charge and discharge process. The Li-Si equilibrium phase diagram exhibits numerous intermetallic phases, mainly including four stable phases (Li12Si7, Li7Si3, Li13Si4 and Li22Si5). The other metastable phases (LiSi, Li2Si, Li7Si2 and Li21Si5) appear difficultly during the alloying process. Meanwhile, the lithiation formation energy for known stable crystalline phases is calculated in this work. Theresults indicate that intermetallic compound Li12Si7 has the highest formation energy in Li-Si alloy, which leads to the initial irreversible capacity loss in addition to the formation of solid electrolyte interphase (SEI). So the reversible capacity is mainly attributed to the low formation energy phases (Li7Si3, Li13Si4 and Li22Si5) as anode materials for lithium ion battery. Because the lower the formation energy is, the easier the decomposition of samples is during the discharge process. The order of their decomposition is Li22Si5, Li7Si3 and Li13Si4 in the lithium extraction process, but the order of their alloying is reverse in the lithium insertion process. On the other hand, the reversible capacity will decrease gradually due to the different formation energy for different alloy phases. In particular, the capacity fade increases dramatically with proceeding of rapid charge and discharge, the cause of which is that the alloy phases with higher formation energy decompose difficulty in a short space of time. Therefore, the sweep rate also plays an important role in the formation of the lithiation alloy phases. The intrinsic character of specific capacity fade can be explained commendably by the computational value.

2.2 Volume expansion and phase characteristic The volume expansion ratio was calculated before and after lithium insertion into the host materials. The results indicate that volume expansion ratio increases dramatically with increasing of the lithium concentration. So the pure silicon as anode material for lithium ion battery exhibits poor cycling performance, which leads to mechanical disintegration of the electrode during charge and discharge process, especially after

Total energy, formation energy, specific capacity, volume expansion ratio and average potentials for formations of LixSi phases Average

Volume/h10-3 nm3 Expansion ratio /%

Formation en-

Capacity /mA⋅h⋅g-1

Samples

Total energy/eV

Li

–190.252 0

ˉ

19.945 0

ˉ

ˉ

ˉ

Si

–107.346 0

ˉ

20.020 6

ˉ

ˉ

ˉ

potential/V

ergy/eV

LiSi

–241.048 7

54.320 1

31.422 5

56.950 8

54.320 1

954.13

Li12Si7

–434.187 8

82.366 5

43.090 7

115.231 8

58.834 4

1 635.66

Li2Si

–488.426 2

1.821 1

51.109 3

155.283 6

0.520 3

1 908.26

Li7Si3

–583.878 5

2.881 8

51.482 6

157.148 1

1.440 9

2 385.32

Li13Si4

–726.702 4

2.409 1

67.296 1

236.134 3

1.806 8

3 100.92

Li7Si2

–773.825 0

0.467 6

67.296 1

236.134 3

0.116 9

3 339.45

Li21Si5

–907.420 1

2.827 3

81.871 2

308.934 8

1.979 1

4 007.34

Li22Si5

–945.313 1

1.442 2

82.397 5

311.563 6

0.288 4

4 198.17

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2.3 Lithiation voltage and specific capacity The calculated charge potential profile as a function of theoretical capacity is plotted in Fig.2a. The stable voltage is around 2.5 V, and the practical and effective alloy phases should include Li7Si2, Li13Si4, Li22Si5 or the mixture of the above phases. Fig.2b shows that the experimental value is around 0.1 V for the lithiation of crystalline Si and 0.5 V for the subsequent delithiation. The curve of the potential vs the capacity has an essentially consistent tendency with the experimental value. However, the difference of the potential vs the capacity profile is attributed to the poor-lithium phases; namely LiSi and Li12Si7 have higher formation energy, which will be decomposed difficultly once these phases are formed in the range of low lithium concentration during the initial charge. So, the experimental values don’t display the characteristic of these phases. However, the calculated value is consistent with the long voltage plateau observed in the electrochemical test range of high lithium concentration. The electrode material should be the mixture of the Li7Si3, Li13Si4 and Li22Si5 combined with the phase diagram for crystalline structure. In fact, the non-crystalline amorphous structures affect largely the specific capacity and cyclic performance. In contrast, the specific capacity of LixSi alloy increases with decreasing of the cycling performance based on the above analysis. In this work, the typical cyclic voltammogram response of LixSi alloy anode material is presented in Fig.3. The scan potential is in the range from 0 to 2.5 V (vs. Li/Li+) at a sweep rate of 0.5 mV/s. According to the area difference of closed

90 a Potential/V

70 50 30 10 1000

2000

3000

4000

3.0

b

Potential/V

1st charge 2.0

1.0

1st discharge

0.0 –500

500

1500

2500

Capacity/mAh·g

3500

-1

Fig.2 Charge potential profile as a function of theoretical capacity (a) and the first chare-discharge profile as a function of experimental capacity (b) 4 I/h10-5 mA·cm-2

the depth of discharge. In general, the silicon-based anode material needs alloying with other components such as intermetallic compounds of Si-Ni[28,29], Si-Zr[30] etc. for accommodating a larger volume expansion ratio. On the other hand, another reason of the existence of large volume expansion would be crystalline phases occurring in the cycled products, because the relative volume expansion ratios of various phases were calculated based on the crystalline structures; generally, thermodynamically stable phases have much lower Gibbs energy than the amorphous alloy namely non-crystalline phases. The free energy construction is consistent with the experimental observations in every aspect. However, in our work, the calculated Gibbs energy is higher than the experimental value which was obtained from relation of voltage and formation energy based on the above equation ( V ( x ) ≈ − Δ E / Δ x ). Since many phases do not easily crystallize at room temperature, the silicon is lithiated. The metastable Li-Si phases form little crystalline LixSi and more amorphous alloy structures. These amorphous structures can improve effectively cyclic life and capacity. The calculated results of the physical and electrochemical characteristics of LixSi alloy play a guiding role in exploring of the Li-metal commercial electrode materials.

2 0 –2 –4

1st cycle 2nd cycle 5th cycle 10th cycle 20th cycle

–6 –8 –10

SEI

0.0

0.5

1.0

1.5

E/V vs. Li/Li

Fig.3

2.0

2.5

+

Cyclic voltammograms of Si film electrode

cyclic voltammogram curve, we can find that the capacity loss in the second cycle was mainly due to the formation of solid electrolyte interphase (SEI) on the surface of the electrodes and poor-lithium phase Li12Si7 alloy. The irreversible capacity decreases dramatically in the second cycle, which is consistent with the calculated results. On the other hand, no specific peaks occur from the second cycle, which indicated the characteristic of amorphous LixSi alloy. So non-crystalline amorphous structures exist intensively in Si thin film electrode, and the amorphous structure exhibit good cyclic performance since the second cycle.

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3 Conclusions 1) The first-principle investigation can give practical information and insights about the structural stability and electrochemical voltage profile. 2) The initial irreversible capacity loss is mainly attributed to the formation SEI film and poor-lithium phase Li12Si7 alloy phase. 3) The thin film electrode can be fabricated by the radio frequency (RF) magnetic sputtering on copper foil collector as anode materials. 4) The amorphous structure exhibits good cyclic performance; meanwhile, the experimental voltage profile is essentially consistent with that of the theoretical calculation.

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