LiAlO2-coated LiCoO2 as cathode material for lithium ion batteries

LiAlO2-coated LiCoO2 as cathode material for lithium ion batteries

Solid State Ionics 176 (2005) 911 – 914 www.elsevier.com/locate/ssi LiAlO2-coated LiCoO2 as cathode material for lithium ion batteries Hui Cao*, Baoj...

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Solid State Ionics 176 (2005) 911 – 914 www.elsevier.com/locate/ssi

LiAlO2-coated LiCoO2 as cathode material for lithium ion batteries Hui Cao*, Baojia Xia, Yao Zhang, Naixin Xu State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China Received 11 August 2004; received in revised form 1 December 2004; accepted 1 December 2004

Abstract 3% LiAlO2-coated LiCoO2 and 3% Al2O3-coated LiCoO2 were prepared from C9H21O3Al, LiOHd H2O and LiCoO2 by hydrolyzing and heating technique. XRD, SEM and other various electrochemical measurement methods were used to examine these structural and electrochemical characteristics. XRD results proved that no secondary phase appeared after coating. The LiAlO2-coated LiCoO2 showed a higher specific discharge capacity (170 mA h/g at the first cycle) and an excellent cycling performance (165 mA h/g after 20 cycles) when cycled between 2.8 V and 4.5 V. Cyclic voltammograms curves indicated that LiAlO2-coating didn’t suppress the phase transition from hexagonal to monoclinic. The conclusion was different from former literature’s reports. D 2004 Elsevier B.V. All rights reserved. Keywords: Lithium ion batteries; Cathode materials; Coating; LiCoO2

1. Introduction Although cathode materials of lithium ion batteries such as LiNix Me1x O2 (Me=Co, Al, etc.), LiMn2O4 and LiFePO4 have been investigated by a lot of candidates in recent years [1–4], hexagonal a-NaFeO2-type LiCoO2 is still most widely used due to its excellent electrochemical characteristics and stable structure. Unfortunately, its capacity fades sharply when cycled to an extended voltage (higher than 4.3 V). It was observed that lattice constant c of LiCoO2 decreased dramatically when charged to over 4.3 V, which led to a deterioration of its structure. In order to stabilize the structure of LiCoO2, the effects of coating with SnO2, Al2O3 and ZrO2 and so on have been investigated by Cho and Dahn et al. [5–10]. The coated materials showed a better structural stability than plain LiCoO2. However, the attached inactive oxides reduced its reversible cycling capacity.

* Corresponding author. Tel.: +86 2162511070 8807; fax: +86 2132200534. E-mail address: [email protected] (H. Cao). 0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2004.12.001

In this study, a novel LiAlO2-coated LiCoO2 cathode material was synthesized by means of hydrolyzing and heating technique. The structure of LiAlO2-coated LiCoO2 was proved to be as stable as Al2O3-coated LiCoO2, thus the former can extend its effective charge cut-off voltage to 4.5 V. In comparison with the reported Al2O3-coated LiCoO2 sample, this new material showed a higher reversible specific charge/discharge capacity because LiAlO2 can provide Li+ and accept Li+ in the cycling process while Al2O3 can’t.

2. Experimental 2.1. Preparation of cathode materials C9H21O3Al was dissolved in water first and stirred continuously for 1 h at room temperature. Then LiCoO2 was mixed with LiOHd H2O (1:1 to C9H21O3Al in molar ratio) and added into the C9H21O3Al solution to form sol. The sol was transformed into gel after drying at 120 8C. The dried gel was then heated at 600 8C for 3 h to produce LiAlO2coated LiCoO2. Al2O3-coated LiCoO2 was prepared [4] and used as control for comparison.

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2.2. Sample characterization

Intensity(CPS)

Powder X-ray diffraction (Japanese D/max-rA) with CuKa radiation was carried out to identify the crystalline phase of the as-prepared materials. Unit cell parameters were obtained from least-square fitting of all peak positions. The morphology and size of the powder particle were observed by TEM. The elemental composition was checked by ICP-AES.

003

104 101 006 105 102

3. Results and discussions 3.1. Physical characteristics of the coated and uncoated samples Fig. 1 shows the TEM image of the LiAlO2-coated LiCoO2 sample. It can be seen clearly that there exist a rough surface phase with the average thickness of about 5 nm on the surface of the particles, which is difficult to

110 107 108 113 b

2.3. Electrochemical evaluation A coin-type cell was assembled in a glove box. Lithium sheet with 0.02 mm thickness was cut as a disk in 12 mm diameter to use as negative electrode. A suspension was prepared by mixing the cathode active material, carbon black and binder (PVDF, polyvinylidene difluoride) in NMP (N-methyl-2-ketopyrrolidine) in mass ratio 85:10:5. The mixture was then coated onto an Al disk with 14 mm in diameter as current collector. The coated disk was dried in a vacuum dryer. The electrolyte was 1 M LiPF6 dissolved in volume ratio 1:1 of ethylene carbonate (EC) and dimethyl carbonate (DMC).

c

a 10

20

30

40

50

60

70

2-theta (degree) Fig. 2. XRD patterns of different cathode materials; (a) plain LiCoO2, (b) 3% LiAlO2-coated LiCoO2, (c) 3% Al2O3 coated LiCoO2.

recognize. The LiAlO2-coated LiCoO2, Al2O3-coated LiCoO2 and plain LiCoO2 display nearly the same XRD pattern, which implies that these three materials belong to the same hexagonal structure (Fig. 2). In the modified samples, no Al2O3 or LiAlO2 phase was detected. The lattice parameters a and c were calculated by the least-square method based on all characteristic XRD peaks, andp the unit cell volume was obtained according ffiffiffi hexagonal 2 = to 3 a c 2. Error estimates were calculated by s ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Pn 2 (n=36; X -Discrete lattice parame1 i i¼1 ð X i  X Þ n2 ters calculated by any two characteristic peaks; X-Lattice parameters calculated by the least-square method). The values in Table 1 are computed results based on Fig. 1. It has been reported that substitution of Al for Co in LiCoO2 by doping results in decrease in a and increase in c [11]. However, after coating, the results in Table 1 don’t show this tendency. The variations in lattice constant for Al2O3coated sample are smaller than Al-doping one. The a axis and c axis of 3% Al2O3-coated LiCoO2 decreased slightly compared to LiCoO2, while the lattice constant of 3% LiAlO2-coated sample is the same as that of pristine LiCoO2. This phenomenon indicates that the mechanism involved in coating reaction is different from doping mentioned above. In the Al2O3-coating process, C9H21O3Al is hydrolyzed into Al(OH)3 and C9H21(OH)3. The produced Al(OH)3 was deposited on the LiCoO2 surface and reacted with the Li and O atoms in the following heating process. Table 1 Crystallographic parameters from different cathode material samples

Fig. 1. Transmission electron micrographs of the LiAlO2-coated LiCoO2 powders calcined at 700 8C.

Sample

a

c

c/a

V

Error Error estimate estimate of a of c

LiCoO2 3% Al2O3-coated LiCoO2 3% LiAlO2-coated LiCoO2

2.817 14.075 4.996 96.725 0.00043 2.816 14.060 4.993 96.554 0.00051

0.00097 0.0011

2.817 14.074 4.996 96.718 0.00047

0.0089

H. Cao et al. / Solid State Ionics 176 (2005) 911–914

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Specific discharge capacity(mAh/g)

Specific discharge capacity(mAh/g)

150

140

130

Al2O3 coated LiCoO2 Plain LiCoO2

120

LiAlO2 coated LiCoO2

170 160 150 140

Plain LiCoO 2 3% Al2O 3-coated LiCoO 2 3% LiAlO 2-coated LiCoO 2

130 120 110 100 0

5

10

15

20

Cycle Number

110 0

5

10

15

20

Cycle number

Fig. 3. Cycling performances of LiCoO2, 3% Al2O3-coated LiCoO2 and 3% LiAlO2-coated LiCoO2 between 2.8 V and 4.3 V at 0.1 8C rate.

Thus the Al2O3-coated sample which can be described as LiCoAlx O2 was synthesized. In the LiAlO2 coating process, the final sample would be Li1+x CoAlx O2(1+x) because of the addition of LiOH. Although the molecular formula of the coated samples is the same as the doped samples, their inner structures are different. The Al atoms exist in the whole lattice structure of the doped LiCoO2 and just exist symmetrically on the surface of the coated LiCoO2 which causes different lattice constants for the coated LiCoO2 and the doped LiCoO2. 3.2. Electrochemical behavior of the coated and plain samples The cycling performances of LiCoO2, Al2O3-coated LiCoO2 and LiAlO2-coated LiCoO2 between 2.8 V and 4.3 V at 0.1 8C rate are shown in Fig. 3. Although the

Fig. 5. Cycling performances of LiCoO2, 3% Al2O3-coated LiCoO2 and 3% LiAlO2-coated LiCoO2 between 2.8 V and 4.5 V at 0.1 8C rate.

specific discharge capacity of the Al2O3-coated LiCoO2 is lower than that of pristine LiCoO2 by about 9 mA h/g, it shows a better cycling ability with the capacity retention ratio of 98.3% after 20 cycles than pristine LiCoO2 with 96.8% capacity retention ratio. The specific discharge capacity of the LiAlO2-coated LiCoO2 is lower than pristine LiCoO2 by only 4 mA h/g in the first cycle, then the gap becomes smaller in the following cycles. Among these three samples, the LiAlO2-coated LiCoO2 exhibits the best cyclability with 98.4% capacity retention ratio after 20 cycles. It is clear that both Al2O3-coated LiCoO2 and LiAlO2-coated LiCoO2 show a beneficial effect in stabilizing the lattice structure compared to pristine LiCoO2. The other noticeable phenomenon is that the specific discharge capacity of LiAlO2-coated LiCoO2 is larger by about 8 mA h/g than Al2O3-coated LiCoO2. In order to examine the cyclability of the LiAlO2-coated sample in a larger cycling range, charge cutoff voltage was

Fig. 4. Charge/discharge curves of 3% LiAlO2-coated LiCoO2 in the first 20 cycles between 2.8 V and 4.5 V at 0.1 rate.

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in Fig. 6. There are three redox peaks at 3.83 V, 4.15 V and 4.45 V in the first 3 cycles, which is the same as pristine LiCoO2. This implies that the phase transition from hexagonal to monoclinic still exist during cycling. As a result, LiAlO2-coating doesn’t suppress phase transition from hexagonal to monoclinic but improves the reversibility of phase transition in cycling.

4. Conclusion

Fig. 6. Cyclic voltammogram curves of LiAlO2-coated LiCoO2 in the first 3 cycles. Voltage scan rate=0.02 mV/s.

set at 4.5 V in the following experiments. The first 20 cycles of charge/discharge between 2.8 V and 4.5 V at 0.1 8C rate are shown in Fig. 4. The sample exhibits an excellent cycling stability. The specific discharge capacity was calculated to compare with 3% Al2O3-coated LiCoO2 and pristine LiCoO2. It reaches about 170 mA h/g and retains over 165 mA h/g after 20 cycles respectively (Fig. 5). On the other hand, pristine LiCoO2 and 3% Al2O3-coated LiCoO2 show that the first specific discharge capacity of 168 mA h/g and 165 mA h/g decline to135 mA h/g and about 160 mA h/g after 20 cycles. The Al2O3-coated LiCoO2 is a bit better than pristine LiCoO2. The results show clearly that both Al2O3 and LiAlO2 coating play an important role in stabilizing the lattice structure when charged to a higher cutoff voltage. Most of researchers attributed the performance improvement to the formation of a physical barrier that separates the electrolyte from the electrode and the reduction of electrolyte oxidation [5,6,12]. However, chemical role should be considered too. With Al atoms distributing symmetrically on the surface of LiCoO2, the Me–O bonding energy increased accordingly because of a higher Al–O bond energy. Thus the structural stability between layers is enhanced. The higher structural stability restrains the variations of lattice volume during cycling. It can be seen both in Fig. 3 and Fig. 5 that The LiAlO2-coated LiCoO2 showed a higher specific discharge capacity than Al2O3-coated LiCoO2. This should be attributed to the facts that Li atoms in LiAlO2 can take part in intercalation/deintercalation process when cycled. To investigate the phase transition of 3% LiAlO2-coated LiCoO2 during cycling, cyclic voltammogram tests between 2.8 V and 4.6 V were carried out. The results are illustrated

3% LiAlO2-coated LiCoO2 and 3% Al2O 3-coated LiCoO2 were synthesized by precipitation and heating technique. XRD studies showed that the variations of lattice constant caused by coating were different from which caused by doping. Electrochemical tests proved that both LiAlO2-coating and Al2O3-coating improved the cycling ability when charged to 4.5 V because of the presence of LiAlx Co1x O2 on the surface of LiCoO2. The LiAlO2coated LiCoO2 has a higher specific discharge capacity than Al2O3-coated LiCoO2 because LiAlO2 is active during cycling. Cyclic voltammogram experimental results showed that coating didn’t suppress phase transition in 4.4 V, but improved the reversibility of phase transition in cycling.

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