An investigation of silicon-doped LiCoO2 as cathode in lithium-ion secondary batteries

Solid State Ionics 177 (2006) 317 – 322 www.elsevier.com/locate/ssi

An investigation of silicon-doped LiCoO2 as cathode in lithium-ion secondary batteries Y. Jin, P. Lin, C.H. Chen * Department of Materials Science and Engineering, University of Science and Technology of China, Anhui Hefei 230026, P.R. China Received 21 July 2005; received in revised form 22 September 2005; accepted 8 November 2005

Abstract Powders of layered – structured LiCo(1 x)Six O2 (x = 0, 0.01, 0.05, 0.10, 0.35) were synthesized by a co-precipitation method followed by a 950 -C sintering. Their structures were analyzed by the X-ray diffraction (XRD) and scanning electron microscopy (SEM). Iodine titration method was also employed to measure the average valence of cobalt ions. With these powders as the active materials of positive electrodes versus lithium, the electrochemical behaviors of the cells were investigated using charge – discharge cycling, AC impedance spectroscopy and DC resistance measurement. It is found that the Si-doping results in the decrease of cobalt valence and the crystallite size. When the Si-content is less than 10%, pure LiCo1 x Six O2 phases are obtained. A second phase Li2CoSiO4 is also obtained when the Si-content is 35%. Among the five compositions, the non-doped LiCoO2 exhibits a high initial specific capacity (about 150 and 185 mA h/g at a current density of 0.4 mA/cm2 from 2.8 to 4.2 and from 2.8 to 4.5 V, respectively), but degrades in the following cycles; while the Si-doped LiCo1 x Six O2 electrodes especially LiCo0.99Si0.01O2 show the best performance of long-life cycling. And all of the doped powders have better stability of the 3.6 V-plateau efficiency due to the improved stability effect on the cell impedance. D 2005 Elsevier B.V. All rights reserved. PACS: 81.20.Fw Keywords: Lithium cobalt oxide; Silicon; Doping; Rechargeable battery; Impedance

1. Introduction Among the four primary lithiated transition metal oxides, i.e. LiCoO2, LiNiO2, LiMn2O4 and LiFePO4 as the positive electrodes for rechargeable lithium batteries, the layered compound LiCoO2 is the preferred one used in commercial batteries industry because of its relatively high and particularly reversible specific capacity, reasonably good safety characteristics, and simple synthesis methods. Because the structure stability of Lix CoO2 during charge–discharge cycles is limited to x = 0.5, the practical specific capacity of LiCoO2 is about 137 mA h/g, which is only half of its theoretical capacity 273 mA h/g [1,2]. During cycling, the highly oxidizing Co4+ in the delithiated Li1 x CoO2 may oxidize the organic solvent of the electrolyte on the cathode surface, causing an obvious capacity fading [3]. Ways to ameliorate this issue include surface modification [4] and bulk doping of the LiCoO2.

* Corresponding author. Tel.: +86 551 3602938; fax: +86 5513602940. E-mail address: [email protected] (C.H. Chen). 0167-2738/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2005.11.009

Till now, some 3-d transition metal elements such as Ni [5], Cr [6], Mn [7], Fe [8], and Ti [9] have been successfully used to partially substitute the cobalt in LiCoO2. Besides, doping with other elements like Al [10], B [11], Zr [12], Mg [13] and mixed Zr + Mg [14] has also been investigated. Out of these studies, it is believed that the bond energy between the doping element and O plays an important role in stabilizing the rock-salt structure. For example, Myung et al. reported that the Al – O bond is much stronger than Co – O so that the distance of the interlayers in the doped LiCoO2 could be restricted and the structure stability would retain [15]. The bond energy values of nine M– Os (M = Li, Fe, Ti, Co, Ni, Mn, Al, Si, B) are listed in Table 1. Among these M – Os, Si– O bond has the second highest energy. In addition, Si is the most abundant element on earth and silicon dioxide is a very stable compound. Therefore, we are much interested in the Si substitution of Co in the LiCoO2 and the consequent changes in its electrochemical performance. It also adds to the few non-metal dopants investigated so far. In our investigation, Si-doped LiCoO2 was synthesized by a co-precipitation method. We have found that a small amount of

Y. Jin et al. / Solid State Ionics 177 (2006) 317 – 322 003

Li CoSiO 009

35% 10% 5% 1% 0% 10

20

30

40

50

60

70

1-min of zero current pulse through the process of charge and recording the voltage change before and after interruption. Thus, the DC resistance of a cell (R dc) at a certain state-of-charge (SOC) can be calculated as R dc = DU / DI (DU = U after U before). 3. Results and discussion Fig. 1 shows the X-ray diffraction (XRD) patterns of the five powders synthesized by a co-precipitation method and

(a) 2.820

Lattice parameter c (Å)

14.090

c -- Ο − a

14.085

2.819

14.080 14.075

2.818

14.070 2.817 14.065 14.060

0.0

0.1

0.2

0.3

511 T 3 808.8 T 20.9 384.5 T 13.4

Fe – O Li – O Mn – O

390.4 T 17.2 333.5 T 8.4 402.9 T 41.8

Ni – O Si – O Ti – O

382.0 T 16.7 799.6 T 13.4 776.1 T13.4

* From: ‘‘Handbook of Chemistry and Physics’’ 1913 – 1995, David R. Lide, 75th Edition, CRC press.

2.816

0.4

x in LiCo1-xSixO2

(b) 5.000

c/a value of Co ion

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-0.1

0.0

0.1

0.2

0.3

2.5 0.4

Averange Valence of Co ion

Al – O B–O Co – O

90

Fig. 1. XRD patterns of the LiCo(1 x)Six O2 (x = 0, 0.01, 0.05, 0.10, 0.35) powders sintered at 950 -C in air for 25 h.

4.992 Table 1 Bond energy of M – O (M = Li, Fe, Ti, Co, Ni, Mn, Al, Si, B) (kJ/mol)*

80

2 Theta (Degree)

Lattice parameter a (Å)

A co-precipitation reaction process was employed to synthesize the LiCo(1 x)Six O2 powders. Lithium hydroxide (LiOH) and ammonia (NH3IH2O) with molar ratios of Li : NH3 equal to 1 : 1 were mixed and dissolved in de-ionized water to obtain a 0.1 M aqueous solution. In the same time, cobalt nitrate (Co(NO3)2I6H2O) and tetraethyl silicate (Si(OC2H5)4, TEOS) were dissolved in ethanol with molar ratios of Co : Si equal to 0.99 : 0.01, 0.95 : 0.05, 0.90 : 0.10 and 0.65 : 0.35, respectively. Each ethanol solution was stirred up till the solution became completely transparent. Then, according to the Li : (Co + Si) molar ratio equal to 1.075 : 1, each ethanol solution was added dropwise into the aqueous LiOH solution in order to co-precipitate the cobalt and silicon. It was observed that the color of the precipitates changed quickly from rose-red to black. The mixtures of the precipitates and the mother solution were heated together at about 70 -C to obtain gel-like products by evaporating the solvent and excess NH3. Then they were dried at 200 -C for 10 h and annealed at 400 -C for 5 h. Then the products were ground into powders that were subsequently calcined at 950 -C in air for 25 h. The structure of the powders was analyzed by X-ray diffraction (XRD) method (Philips X’per Pro Super, Cu Ka radiation, 15 –75-) and scanning electron microscopy (SEM, Hitachi X-650). The valence of the Co in each sample was analyzed through an iodometry method [4]. The powders (84 wt.%) were mixed with 8 wt.% acetylene black and 8 wt.% PVDF binder in NMP to obtain slurries. Each of these slurries was cast onto an aluminum foil to form an electrode laminate. After vacuum drying the laminates, CR2032 coin-cells with the configuration LiCo1 x Six O2/LiPF6 (1 : 1 EC : DEC)/Li were assembled in an argon-filled glove-box (MBraun Lab Master130). The mass of LiCo1 x Six O2 in every cell was about 7 mg. These cells were cycled on a multi-channel battery cycler (Neware BTS-610) at a current density of 0.4 mA/cm2 in the voltage ranges of 2.8– 4.2 and 2.8 – 4.5 V. In order to compare the internal resistance of cells with different Sicontent and after different cycles, the cells were charged to 4.2 V at the 5th and 25th cycles and rested for a 2-h relaxation. They were measured by AC impedance spectroscopy. The instrument used as an electrochemical workstation (CHI 604B). The AC signal applied was with the voltage amplitude of 5 mV and a frequency range from 10 mHz to 100 kHz. The internal resistance of the cells was also measured by a current interruption technique. This was done by applying intermittently

4

108 110 113

2. Experimental

2

104

101 006 102

Si-doping leads to a significant improvement in the cycling stability of LiCoO2.

105

318

x in LiCo(1-x)SixO2 Fig. 2. The lattice parameters of the Si-doped LiCoO2. (a) The variation in the measured lattice parameter of LiCo(1 x)Six O2 (x = 0, 0.01, 0.05, 0.10, 0.35) powders calcined at 950 -C in air for 25 h: (g) c-axis; (>) a-axis. (b) The lattice parameter c / a ratio and average value of Co ionic as a function of x in LiCo(1 x)Six O2 powder samples.

Y. Jin et al. / Solid State Ionics 177 (2006) 317 – 322

(a)

319

(b)

8 µm

30 µm

(c)

8 µm Fig. 3. SEM micrographs of (a) LiCoO2; (b) LiCo0.99Si0.01O2; (c) LiCo0.65Si0.35O2.

calcined at 950 -C. A pure a-NaFeO2 phase belonging to R3m space group is obtained for the LiCo1 x Six O2 powders with x up to 0.10. However, at the doping level of 35% Si, a second phase, i.e. Li2CoSiO4, can be detected in addition to the aNaFeO2 main phase. Thus, the upper limit of Si-doping in the LiCoO2 lattice is at least 10%. As shown in Fig. 2, the lattice parameters of the Si-doped LiCoO2 can be calculated from these XRD patterns. It can be seen that the parameters a and c of the hexagonal lattice decrease with increasing the Si-doping

Single crystal size (nm)

90

80

70

60

0.0

0.1

0.2

0.3

0.4

0.5

x in LiCo(1-x)SixO2 Fig. 4. The single crystal sizes of five LiCo(1 0.05, 0.10, 0.35).

x)Six O2

samples (x = 0, 0.01,

level till 10% (Fig. 2a). And with the second phase Li2CoSiO4 appears in the 35% Si-doped sample, the parameters a and particularly c increase again. The general trend of lattice parameter change with the Si-content is just on the contrary to the Si-doped layered electrode Li(Co1 / 3Mn1 / 3Ni1 / 3)1 x Six O2 reported by Na and Moon [16]. In their case, the lattice parameters increase with the Si-content. Nevertheless, since the ˚, Si4+ is markedly smaller than Co3+ (ion radius: Si4+ 0.42 A 3+ ˚ Co 0.63 A), and Si– O bond is much stronger than Co – O bond (Table 1), it is more reasonable that the cell parameters decrease with the Si-content. At the same time, the volume distortion of the unit cell, represented by c / a, displays a slightly different change compared with the individual lattice parameters (Fig. 2b). Among the five compositions, LiCo0.95Si0.05O2 reaches a minimum. The results of the iodometry titration are also shown in Fig. 2b. With increasing the Sidoping level, the average valence of cobalt ions decreases from 2.94 for the non-doped LiCoO2 to 2.54 for the nominal composition LiCo0.65Si0.35O2. Such a valence change is easy to understand because the introduction of Si4+ on the Co3+ sites I ) will bring about more Co2+ ions (i.e. CoCo (i.e. SiCo V ) to keep the valence balance. Fig. 3 displays the SEM photos of the powders of LiCoO2 (Fig. 3a), LiCo0.99Si0.01O2 (Fig. 3b) and LiCo0.65Si0.35O2 (Fig. 3c). It can be seen that the LiCoO2 powder is composed of irregular-shaped particles of an average size between 20 and 30

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Y. Jin et al. / Solid State Ionics 177 (2006) 317 – 322

(b) 0 1% 5% 10% 35%

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Specific capacity (mAh/g)

Specific Capacity (mAh/g)

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0.4 mA/cm 2.8 V- 4.5 V

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Cycle number

Cycle Number

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3.6V

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26th cycle

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Depth of Discharge (%)

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Depth of Discharge (%)

Fig. 5. The capacity retentions and discharge profiles of the samples (a) capacity retention of five samples at current density of 0.4 mA/cm2 in the voltage range of 2.8 – 4.2 V; (b) capacity retention of five samples at current density of 0.4 mA/cm2 in the voltage range of 2.8 – 4.5 V; (c) the 3rd cycle; (d) the 25th cycle.

reveals higher capacity than LiCoO2 after 30 cycles. This result is likely related to the stabilization effect of the electrode surface caused by the stronger Si– O bonding and less amount of oxidizing Co4+ in the Si-doped electrodes. For the 35% Sidoped sample, its discharge capacity keeps virtually unchanged over 70 cycles measured (Fig. 5a). Apparently, the existence of the second phase Li2CoSiO4 also helps to stabilize the capacity. Since Li2CoSiO4 is a new lithium-containing material that has been rarely reported in literature, it is worthwhile to investigate the detailed stabilization mechanism of Li2CoSiO4 as an independent topic. On the other hand, according to the quantum chemistry calculation [10], the doping of Al or Si in LiCoO2 would increase the discharge potential. Hence, it is understood that in the same relatively 100 80

-Z'' (ohm)

Am, while the LiCo0.99Si0.01O2 powder is composed of particles of about 5 – 6 Am. The Si-doping apparently leads to the reduction of the particle size. This trend is also true for the crystallite size of the Si-doped compositions, as shown in Fig. 4 calculated based on the XRD diffraction peak width (Fig. 1) by the Scherrer’s equation. In addition, the surface of the LiCoO2 particles looks smoother and cleaner than that of the LiCo0.99Si0.01O2 powder. This is also due to the presence of many tiny particles on the surface of big particles in the LiCo0.99Si0.01O2 powder. As is well known, the small particle size is usually favorable for the electrochemical performances of the electrode materials used in lithium-ion batteries due to the short diffusion distance of Li-ions [17,18]. The charge– discharge cycling characteristics of LiCo(1 x) Six O2 are shown in Fig. 5. Obviously, among all of the compositions with Si-content from 0% to 35%, the non-doped LiCoO2 possesses the highest initial capacity, which is 148 mA h/g at the current density of 0.4 mA/cm2 (ca. C / 2 –C / 1.5 rate) (Fig. 5a). The initial specific capacity for LiCo0.99Si0.01O2, LiCo0.95Si0.05O2, LiCo0.90Si0.10O2 and LiCo0.65Si0.35O2 is 137, 130, 115 and 55 mA h/g, respectively. Unlike other compositions, the LiCo0.99Si0.01O2 displays a very good capability of capacity retention in the first fifty cycles or so and then experiences a slow capacity drop in the subsequent cycles. Compared to the non-doped LiCoO2, all of the Si-doped samples exhibit a better capacity stability. The capacity fading rate in the first 50 cycles for LiCoO2, LiCo0.99Si0.01O2, LiCo0.95Si0.05O2, LiCo0.90Si0.10O2 and LiCo0.65Si0.35O2 is 0.24%, 0%, 0.1%, 0.2% and 0.02% per cycle, respectively. LiCo0.99Si0.01O2

5th cycle

LiCo0.99Si0.01O2

25th cycle

60 40

25th cycle

LiCoO2

20 5th cycle

0 0

20

40

60

80

100

120

Z' (ohm) Fig. 6. AC impedance spectra of two LiCo(1 5th and 25th cycles.

x)Six O2/Li

cells (x = 0, 0.01) at the

Y. Jin et al. / Solid State Ionics 177 (2006) 317 – 322

(a)

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(b)

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23rd cycle 10%

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35% 50 0

0

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100

Depth of Discharge (%) Fig. 7. DC resistance measurement of five LiCo(1 x)Six O2/Li cells (x = 0, 0.01, 0.05, 0.10, 0.35) at the 23rd cycle by the current interruption measurement: (a) the discharge profile of 1%-doped LiCoO2 at a DC measurement process; (b) the DC resistance of five LiCo(1 x)Six O2/Li cells.

low potential range (i.e. 2.8 to 4.2 V), the capacity of Si-doped LiCoO2 is reduced. To investigate the effect of Si-doping on the higher voltage cycling performance, the cells were also cycled in a range from 2.8 to 4.5 V (Fig. 5b). In the high voltage range, the 1% Sidoped LiCoO2 exhibits again the best capability of capacity retention among the five electrode compositions. Nevertheless, with the higher cut-off voltage (4.5 V), the capacity of all five cells fades faster than with the lower cut-off voltage (4.2 V). To check if there is really a potential rise caused by the Sidoping, the normalized discharge voltage profiles of LiCo(1 x)Six O2/Li cells (Fig. 5c and d) are examined. It is clearly seen from the ‘‘tails’’ of the 4 V-plateau, especially for the 4th cycle (Fig. 5c) that the potentials of all of the four Sidoped compositions are higher than that of LiCoO2. Nevertheless, the potential appears not monotonously proportional to the Si-content. On the other hand, the discharge profiles can also present the so-called 3.6 V-plateau efficiency, a useful parameter which is the concern of the cellular phone industry [4]. It can be seen that all of the samples have a 3.6 V-plateau efficiency of over 90% during the beginning cycles (Fig. 5c). Specifically, the 3.6 V-plateau efficiency of a cell is 91%

321

(LiCoO2), 98% (LiCo0.99Si0.01O2), 98% (LiCo0.95Si0.05O2), 97% (LiCo0.9Si0.1O2) and 92% (LiCo0.65Si0.35O2), respectively. After 25 cycles, it changes to 78% (LiCoO2), 97% (LiCo0.99Si0.01O2), 91% (LiCo0.95Si0.05O2), 95% (LiCo0.9Si0.1O2) and 90% (LiCo0.65Si0.35O2), respectively (Fig. 5d). Obviously, the biggest efficiency drop, i.e. from 91% to 78%, is found for the non-doped LiCoO2. Hence, the Si-doped LiCoO2 has a positive effect on stabilizing the 3.6 V-plateau efficiency, and is thus favorable to maintain the power density as well as the effective energy density of a cell. The changes of the 3.6 V-plateau efficiency are consistent with the relative changes of the cell impedance (Fig. 6). In general, the higher percentage of the impedance change, the larger change of the 3.6 V-plateau efficiency is found. From the comparison of the impedance spectra of the LiCoO2/Li and the LiCo0.99Si0.01O2/Li cells at the 5th and 25th cycles, the Sidoping has obviously resulted in much better impedance stability. Once again, the stabilization mechanism of Si-doping is probably the origin of the more stable impedance. In addition, the DC resistance of the cells measured at the 23rd cycles is shown in Fig. 7. A typical discharge voltage profile during the current interruption measurement is given by Fig. 7a. The spikes on the curve are due to the rapid voltage rises when the current is cut into zero. The shape of the voltage response is also inserted in Fig. 7a. The DC resistance of the cells (Fig. 7b) has clearly indicated that the non-doped LiCoO2 electrode leads to substantially higher resistance than that of Sidoped LiCoO2 electrodes at their 23rd cycle. This result is in agreement with the above change of the 3.6 V-plateau efficiency (Fig. 5c and d) as well as the capacity fading rates (Fig. 5a and b). 4. Conclusions The Si-doped LiCoO2 powders were synthesized through a co-precipitation method. The particle size decreases with increasing the Si-content. The Si-doping leads to the lower impedance and better electrochemical performances including the capacity retention and stability of the 3.6 V-plateau efficiency. The optimal composition is LiCo0.99Si0.01O2. Acknowledgements This study was supported by 100 Talents Program of Academia Sinica and National Science Foundation of China (grant Nos. 50372064 and 20471057). We are also grateful to the China Education Ministry (SRFDP No. 20030358057). References [1] J.N. Reimers, J.R. Dahn, J. Electrochem. Soc. 139 (1992) 2091. [2] T. Ohzuku, A. Ueda, N. Nagayama, Y. Iwakoshi, H. Komori, Electrochim. Acta 38 (1993) 1159. [3] H.F. Wang, Y.I. Jang, B.Y. Huang, D.R. Sadoway, Y.M. Chiang, J. Electrochem. Soc. 146 (1999) 473. [4] J. Zhang, Y.J. Xiang, Y. Yu, S. Xie, G.S. Jiang, C.H. Chen, J. Power Sources 132 (2004) 187. [5] W.W. Huang, R. Frech, Solid State Ionics 86 – 88 (1996) 395.

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