Electrochemical performance of nano-SiO2 modified LiCoO2 thin films fabricated by electrostatic spray deposition (ESD)

Electrochimica Acta 51 (2006) 3292–3296

Electrochemical performance of nano-SiO2 modified LiCoO2 thin films fabricated by electrostatic spray deposition (ESD) Y. Yu, J.L. Shui, Y. Jin, C.H. Chen ∗ Department of Materials Science and Engineering, University of Science and Technology of China, Anhui Hefei 230026, PR China Received 11 July 2005; received in revised form 15 September 2005; accepted 16 September 2005 Available online 27 October 2005

Abstract With a mixture of a SiO2 sol and a solution of lithium and cobalt acetates as the precursor, nano-SiO2 modified LiCoO2 films were fabricated by the electrostatic spray deposition (ESD) technique. The SiO2 content of these films was 0, 5, 10, 15 and 20 wt%, respectively. Their structure and electrochemical properties were characterized by means of X-ray diffraction, scanning electron microscopy, galvanostatic cell cycling, AC impedance spectroscopy and cyclic voltammetry. Li2 CoSiO4 was found formed in the SiO2 -containing films. The film with 15 wt% SiO2 shows the best cycling stability with the capacity of 130 mAh/g in the voltage range between 2.7 and 4.3 V at the current density of 0.1 mA/cm2 . Due to its resulted small cell impedance, it has excellent rate capability. A LiCoO2 (shell)/SiO2 (core) structure model is proposed to explain the improved properties of these films. © 2005 Elsevier Ltd. All rights reserved. Keywords: Lithium cobalt oxide; Silicon oxide; Thin film; Electrostatic spray deposition; Lithium battery

1. Introduction The development of thin film lithium-ion batteries has been very important for upgrading smart cards, on-chip power sources and implanted medical elements. Various types of LiMO2 (M = Co, Ni and Mn) with a layered structure, such as LiCoO2 , LiMnNiO2 and LiNi1−x Cox O2 , have attracted great attention as cathode film materials for secondary lithium microbatteries [1–5]. Among them, LiCoO2 is the most widely used positive electrode material in lithium-ion batteries. The deintercalation of lithium from LiCoO2 is accompanied by an expansion of the hexagonal lattice in the c-direction, as a result of the increased electrostatic repulsion between adjacent oxygen layers [6,7], and a contraction in the Co–Co distance [8,9]. This anisotropic volume change during repeated cycling leads to a structure degradation of the host material [10], which consequently results in large capacity fading [11,12]. Many studies have been carried out to improve the structure stability of LiCoO2 [13–16]. Modifica∗

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0013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2005.09.021

tion of the surface properties of the cathode materials by coating with some metal oxides such as ZnO, MgO and Al2 O3 have been recognized as one of the most effective techniques [17–19]. It is suggested that the coating materials form substitutional oxides on the cathode surface, which bestow improved structural stability to the core material and thus enhance its cyclability [20]. Cho et al. suggested that coatings with high fracture-tough materials suppress the cycle-limiting phase transitions associated with the intercalation–deintercalation processes [21]. On the other hand, the surface modification may be performed in an opposite fashion, i.e. the modifier is placed in the core while the active material is designed to locate as the shell. Such a core/shell structure may lead to the increase of the specific area and possible decrease of the cell impedance. This structure change may be termed as an inner surface modification. In this study, the active material LiCoO2 is attempted to coat on the surface of SiO2 nano-beads by electrostatic spray deposition (ESD) technique [22–26]. Different from usual ESD process where the precursor is either a solution [22–26] or a sol [27,28], a mixture of a SiO2 sol and a solution of lithium and cobalt salts was used for the deposition. With an optimal nano-SiO2 content,

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the electrochemical performance of LiCoO2 can be significantly improved. 2. Experimental Tetraethyl orthosilicate (TEOS) [Si(OC2 H5 )4 ] was converted into a 0.2 M silica sol by the hydrolysis and polycondensation of ethanol (EtOH) diluted TEOS in the presence of ammonia (NH3 ·H2 O). A mixed solution of 0.021 M Li(CH3 COO)·2H2 O and 0.02 M Co(CH3 COO)2 ·4H2 O with a mixture of ethanol (C2 H5 OH, 80 vol.%) and glycol (C2 H4 (OH)2 , 20 vol.%) as the solvent was also prepared separately. The excess of lithium in solution was designed to compensate the loss of lithium during the post-deposition high temperature annealing of the films and to avoid the formation of impurity phases. Then the silica sol was mixed with above solution of lithium and cobalt acetates to obtain precursor solutions with different silica content. The content of SiO2 in the targeting SiO2 /LiCoO2 films was controlled as 0, 5, 10, 15 and 20 wt%, respectively. The films of LiCoO2 were prepared by the ESD technique. Details about the experimental procedure were described in a previous paper [22]. A distance of 2–3 cm was kept between the needle and the substrate. The applied voltage was 9–11 kV. Platinum was used as the substrate. The precursor solution was pumped through the metal capillary nozzle at a rate of about 4 ml/h. The Pt substrate (30 ␮m in thickness and 14 mm in diameter) was kept at 350 ◦ C. The film deposition took place under ambient atmosphere. The deposition time was about 50 min. Then the films were annealed at 700 ◦ C in air for 2 h. The mass of Pt disc was recorded before the deposition and after the annealing to calculate the mass of the LiCoO2 layer. The adhesion of the films to the Pt substrate was good. The surface morphology of the deposited films was studied using a scanning electron microscope (Hitachi X-650), while the crystal structure was analyzed with X-ray diffraction (XRD) (Cu K␣ radiation, Philips X’Pert Pro Super). Cycling test was carried out on coin-type cells (2032) of LiCoO2 /1 M LiPF6 in ethylene carbonate and diethyl carbonate (EC–DEC, 1:1, v/v)/Li, which were assembled in an argon-filled glove box (MBRAUN LAB MASTER 130) where both moisture and oxygen levels were less than 1 ppm. The cells were cycled in the voltage range between 2.7 and 4.3 V with a battery test system (NEWARE BTS-610). Note that the specific capacity of the cell in this paper is calculated based on the mass of LiCoO2 . The cyclic voltammetry analysis of the LiCoO2 film cells was performed on a CHI 604A Electrochemical Workstation using a voltage scan rate of 0.1 mV/s. AC impedance measurement was also performed on this CHI 604A Electrochemical Workstation using a voltage amplitude of 5 mV in the frequency range between 100 kHz and 0.001 Hz. 3. Results and discussion Fig. 1 shows the XRD patterns of the ESD-derived LiCoO2 thin films with different SiO2 contents that have been annealed at 700 ◦ C. Except for the peaks from the Pt substrate, all films are composed of LiCoO2 as the main phase. When the SiO2

Fig. 1. X-ray diffraction patterns of the LiCoO2 thin films with 0 wt% (a), 5 wt% (b), 10 wt% (c), 15 wt% (d) and 20 wt% (e) of SiO2 that were annealed at 700 ◦ C for 2 h. Platinum is the substrate material. The diffraction peaks from LiCoO2 were indexed in the patterns.

content is equal to or less than 15 wt% (Fig. 1, patterns a–d), no diffraction peaks from SiO2 is discernable. Thus, a LiCoO2 (shell)/SiO2 (core) microstructure is probably formed in these films as designed so that the X-ray does not reach the SiO2 cores. When the SiO2 content is 20 wt% (Fig. 1, pattern e), a diffraction peak from SiO2 is detected, which suggests that, when the number of SiO2 sol particles in the precursor solution increases to a certain extent, the SiO2 cores may not be completely covered by LiCoO2 shells. In addition, in each SiO2 -containing film a small diffraction peak from Li2 CoSiO4 is detected (JCPDS file number 70-2351). The origin of Li2 CoSiO4 may come from the reaction between some un-hydrolyzed tetraethyl orthosilicate and lithium and cobalt acetates during the 700 ◦ C annealing. The SEM images of two LiCoO2 thin films with 0 and 20 wt% of SiO2 -content are shown in Fig. 2. The morphologies of both films are very similar and rather porous. The texture of the thin film does not appear distinctly changed upon the addition of SiO2 . Usually, the electron beam impinges on non-conducting material will increase the brightness of the SEM images. However, under the same measurement conditions the brightness of the SiO2 (20 wt%)–LiCoO2 thin film (Fig. 2b and d) is not greater than that of the LiCoO2 thin film (Fig. 2a and c). Therefore, it is also believed that the silica particles are coated by some LiCoO2 precursors such as CoO and Li2 O during the spraying and annealing process. After the 700 ◦ C annealing, a LiCoO2 shell is formed. This LiCoO2 (shell)/SiO2 (core) structure model is also in agreement with the above XRD results. Fig. 3 shows the electrochemical characteristics of the LiCoO2 thin films. Obviously, their voltage profiles (Fig. 3a) are typical for the LiCoO2 electrodes. The specific discharge capacity increases with increasing the SiO2 content within the composition range between 0 and 15% SiO2 . The discharge capacity of the LiCoO2 –SiO2 (15%) film reaches about 130 mAh/g at the current density of 0.1 mA/cm2 . Nevertheless, at the composition

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Fig. 2. Scanning electron microcopy pictures of a LiCoO2 thin film (a and c) and a LiCoO2 –15 wt% SiO2 thin film (b and d).

of 20 wt% SiO2 , the capacity falls back to about 120 mAh/g. Apparently, the presence of the non-covered insulating SiO2 between the LiCoO2 particles results in the decrease of the capacity. The cycling behavior of these films (Fig. 3b) also indicates that the introduction of SiO2 cores helps to obtain excellent cycleability. In particular, the capacity of the LiCoO2 –SiO2 (15%) film hardly fades over 60 cycles. In general, the capacity retention of the films get better with increasing the SiO2 content except for the film containing 20% SiO2 . This trend may be related to the possible stabilization effect of Li2 CoSiO4 . Nevertheless, a detailed investigation on the electrochemical

characteristics is needed to confirm this point and is underway in our group. In addition, to the absolute discharge capacity in the voltage range of 2.7–4.3 V, the capability of retaining the 3.6 V-plateauefficiency differs for the films with different SiO2 contents. The 3.6 V-plateau-efficiency is a control parameter required by the battery industry in order to reflect the real effectiveness of a cell [29]. The discharge curves versus the depth-of-discharge (DOD) of the 1st cycle and the 50th cycles are shown in Fig. 4. It can be seen that at the first cycle, the 3.6 V-plateau-efficiency for all of the films is over 95% that is for the LiCoO2 –SiO2 (20%) film

Fig. 3. The galvanostatic cycling property of LiCoO2 –SiO2 films: the first cycle discharge curves (a) and their capacity vs. cycle number (b). The voltage range was 2.7–4.3 V at 0.1 mA/cm2 current density. The SiO2 content is indicated in the graphs.

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Fig. 6. AC impedance spectra of LiCoO2 –SiO2 /Li cells after 10 cycles. The open-circuit-voltage was approximately 4.2 V. The SiO2 content is indicated in the graphs.

Fig. 4. The 3.6 V-plateau-efficiency of LiCoO2 –SiO2 /Li cells at the 1st (a) and 50th (b) cycles. The SiO2 content is indicated in the graphs.

(Fig. 4a). At the 50th cycle, the 3.6 V-plateau-efficiency declines to different extents; it is 90 and 94.5% for the films containing 0 and 15% SiO2 , respectively (Fig. 4b). Hence, the LiCoO2 –SiO2 (15%) is the optimal composition for a long cycling life. Fig. 5 shows the rate capability of these electrode films. Obviously, the LiCoO2 –SiO2 (15%) film has the best rate capa-

Fig. 5. Rate capability of LiCoO2 –SiO2 /Li cells. The voltage range was 2.7–4.3 V. The SiO2 content is indicated in the graphs.

bility while the pure LiCoO2 film has the worst rate capability. Both the relationship of the rate capability and the 3.6 V-plateau-efficiency versus the SiO2 content must correspond to the effect of SiO2 on the cell impedance. Fig. 6 presents the impedance spectra of five cells constituted with the LiCoO2 –SiO2 films against lithium. It can be seen that the LiCoO2 –SiO2 (15%)/Li cell gives the lowest impedance while the pure LiCoO2 /Li cell gives the highest value. This impedance sequence matches exactly with above results of rate capability (Fig. 5) and 3.6 V-plateau-efficiency (Fig. 4). Also, in a general trend the impedance decreases with increasing the SiO2 content with an exception that the impedance of the LiCoO2 –SiO2 (20%)/Li cell is slightly higher than that of the LiCoO2 –SiO2 (15%)/Li cell. To understand the relationship between the cell impedance and the SiO2 content, we should consider the thickness of the LiCoO2 shells in the SiO2 (core)/LiCoO2 (shell) structure. The more the SiO2 content, the thinner the LiCoO2 shell is. Consequently, the lithium-ion diffusion path from the outer surface to the inner surface becomes shorter with increasing the SiO2 content. Thus, the impedance decreases accordingly. The slightly higher impedance of the LiCoO2 –SiO2 (20%)/Li cell lies in the presence of SiO2 on the LiCoO2 surface or between the LiCoO2 particles. Fig. 7 shows the cyclic voltammograms of a LiCoO2 /Li cell and a LiCoO2 –SiO2 (15%) cell. Both are typical for LiCoO2 electrode with three sets of cathodic and anodic peaks in the potential range between 2.7 and 4.3 V [30]. The major deintercalation and intercalation take place at the main peaks around 3.94 V. Two sets of minor peaks at around 4.07 and 4.2 V correspond to order–disorder phase transitions [8]. For the LiCoO2 /Li cell (Fig. 7a), these three sets of peaks are well separated initially. The major cathodic peak at 4.0 V shifts toward higher potentials upon cycling and its corresponding anodic peak shifts toward to the lower potential. Their peak current also decreases gradually so that the first two peaks are largely overlapped. This result suggests that the impedance of the cell increases with cycling. On the other hand, for the LiCoO2 –SiO2 (15%) cell (Fig. 7b),

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Fig. 7. Cyclic voltammogram of the LiCoO2 /Li cell (a) and LiCoO2 –15% SiO2 /Li cell (b) for their first three cycles. The scan rate was 0.1 mV/s.

the positions and current of the three peak sets are rather stable during cycling, meaning that the introduction of SiO2 in the LiCoO2 film is helpful for stabilizing the cell impedance. The stabilization mechanism is likely the same for stabilizing the cell capacity (Fig. 3b). 4. Conclusions We have successfully utilized the mixture of nano-SiO2 sol and lithium acetate and cobalt acetate solution as precursor to synthesize active LiCoO2 thin film electrodes by the ESD technique. The optimal SiO2 content is found to be 15 wt% to obtain the highest specific capacity and excellent cyclability and rate capability. A LiCoO2 (shell)/SiO2 (core) structure model is proposed to explain the effect of the SiO2 . 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. 2003035057). References [1] N.J. Dudney, J.B. Bates, R.A. Zuhr, S. Young, J.D. Robertson, H.P. Jun, S.A. Hackney, J. Electrochem. Soc. 146 (1999) 2455. [2] B. Wang, J.B. Bates, F.X. Hart, B.C. Sales, R.A. Zuhr, J.D. Robertson, J. Electrochem. Soc. 143 (1996) 3203. [3] B.J. Neudecker, R.A. Zuhr, B.S. Kwak, J.B. Bates, J.D. Robertson, J. Electrochem. Soc. 145 (1998) 4148. [4] B.J. Neudecker, R.A. Zuhr, J.D. Robertson, J.B. Bates, J. Electrochem. Soc. 145 (1998) 4160. [5] S.J. Lee, J.K. Lee, D.W. Kim, H.-K. Baik, S.M. Lee, J. Electrochem. Soc. 143 (1996) L268. [6] K. Mizushima, P.C. Jones, P.J. Wiseman, J.B. Goodenough, Mater. Res. Bull. 15 (1980) 783.

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