Synthesis of nano-sized lithium cobalt oxide via a sol–gel method

Applied Surface Science 258 (2012) 7612–7616

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Synthesis of nano-sized lithium cobalt oxide via a sol–gel method Guangfen Li a,b,∗ , Jing Zhang a,b a b

State Key Laboratory of Hollow Fiber Membrane Materials and Processes, Tianjin Polytechnic University, 300387 Tianjin, PR China College of Material Science Technology, Tianjin Polytechnic University, 300387 Tianjin, PR China

a r t i c l e

i n f o

Article history: Received 21 April 2011 Received in revised form 16 April 2012 Accepted 16 April 2012 Available online 22 April 2012 Keywords: Spin-coating LiCoO2 Polyacrylic acid Thin film

a b s t r a c t In this study, nano-structured LiCoO2 thin film were synthesized by coupling a sol–gel process with a spin-coating method using polyacrylic acid (PAA) as chelating agent. The optimized conditions for obtaining a better gel formulation and subsequent homogenous dense film were investigated by varying the calcination temperature, the molar mass of PAA, and the precursor’s molar ratios of PAA, lithium, and cobalt ions. The gel films on the silicon substrate surfaces were deposited by multi-step spin-coating process for either increasing the density of the gel film or adjusting the quantity of PAA in the film. The gel film was calcined by an optimized two-step heating procedure in order to obtain regular nano-structured LiCoO2 materials. Both atomic force microscopy (AFM) and scanning electron microscopy (SEM) were utilized to analyze the crystalline and the morphology of the films, respectively. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Due to the rapid development of micro-device and microtechnology for the past two decades, micro-scale lithium-ion batteries have attracted many attentions [1–3]. Lithium-based layered metal oxides, such as LiMnO2 , LiNiO2 , and LiCoO2 have been utilized in the micro-device as power sources [4–6]. The disadvantage of layered LiMnO2 is the crystallographic transformation to spinel structure during electrochemical cycling [7,8]. Whereas the layered LiNiO2 has difficulties in preparation a stoichiometric LiNiO2 powders without metal ions mixing and has the structure degradation caused by irreversible phase transition during electrochemical cycling [8]. LiCoO2 has been considered as a prime material for positive electrode in the batteries owing to its predominant characteristics, i.e. high capacity, high specific energy, and material structural stability [9]. Several approaches i.e. plasma vacuum deposition method, radio frequency (RF) sputtering [10,11], pulsed laser deposition (PLD) [12,13], electron cyclotron resonance (ECR) sputtering [14] and the sol–gel method [15–17] have been developed for preparing LiCoO2 thin films. The sol–gel method is a widely applied technique for films preparation because of its well-known advantages, i.e. better stoichiometric control, lower calcination temperature, shorter sintering time, finer particle size with homogeneous distribution and high surface area. These parameters are believed to be

∗ Corresponding author at: Tianjin Polytechnic University, College of Material Science and Technology, State Key Laboratory of Hollow Fiber Membrane Materials and Processes, Xiqing District, Binshui East Road 399, Tianjin 300387, PR China. E-mail address: [email protected] (G. Li). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.04.102

crucial for achieving a higher electrode activity and better cell performance. The sol–gel method can also be integrated with either spin-coating or dip-coating process to obtain thin films. In this study, LiCoO2 thin films were prepared by a spin-coating method. Polycrylic acid was used as chelating agent to prevent the problems of pH controlling in the gel formation and fluffiness during the calcination process. For obtaining dense and homogeneous films with regular nano-sized particles, the experimental conditions as the quantity and the molecular weight of polyacrylic acid (PAA), the calcination temperature, and the multiple-layers prepared by spin-coating process were optimized. The morphology and crystalline of the thin films were characterized by either atomic force microscopy (AFM) or scanning electron microscopy (SEM), respectively.

2. Experiment LiCoO2 thin films were prepared by a sol–gel process according to the procedure as follows:LiNO3 (A.R., 99%) and Co(NO3 )2 ·6H2 O (A.R., 99%) were dissolved in de-ionized water and then mixed with PAA (A.R., Mw. 800–1000). PAA acts as chelating agent for a better gel formation. The molar ratio of Li:Co was varied from 1:2 to 2:1, whereas the molar ratio of the total metallic ions charges (M+ ) to polyacrylic acid (M+ :PAA) was set as 1:2. The mixture was continuously stirred at 50 ◦ C for a few hours until a homogeneous solution was obtained. Before spin-coating process, the silicon substrates were cleaned in solvent of ethanol (99.7%) and then acetone (99.5%) in the ultrasonic bath for 2 min to remove dust and organic residue on the silicon surface. Afterwards, the cleaned silicon substrate was placed

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Fig. 1. AFM images of LiCoO2 thin films. The dosage of PAA per 100 ml solution is: (a) 40 g; (b) 60 g; (c) 80 g.

on the holder of the spin-coating equipment, and then a drop of solution was deposited on the substrate surface by a pipette. The formed thin films during spin-coating process were subsequently dried in the oven at 110 ◦ C for 1 h for removing the solvent. Finally, these samples were sintered in the furnace programmed by a twostep calcination process, which will be latter described in details in the study. The crystalline and morphology of the thin films were mainly analyzed by either scanning electron microscope (SEM, Quanta 200, FEI company, U.S.A.) or atomic force microscopy (AFM, AFMSTM5500, Agilent, U.S.A.). 3. Result and discussion

thin films appear more homogeneous. The particles z-height ranges from 105 nm to 185 nm, and the particles sizes are uniform with a narrow size distribution. When further increasing the PAA content to 80 g, the average z-height of the particles decreases extremely, and particles morphology can hardly be identified by AFM analysis. The reason why the crystalline and the morphology of thin film changes with the quantity of PAA can attribute to the combustion heat created by PAA decomposition, which is necessary for the synthesize of LiCoO2 . However, it has disadvantage for the formation of LiCoO2 , especially when excessive PAA is used in the process. The CO and CO2 generated by thermal decomposition of PAA lead to void formed in the LiCoO2 phase, which is unfavorable for the crystal formation.

3.1. The crystalline and morphology of the thin films in dependent of the quantities of polyacrylic acid

3.2. The influence of calcination process on the morphology of the thin films

PAA is primarily used as chelating agent for a better dispersion of metal ions in the gel film. The quantities of PAA have impacts on the rheological properties of the solution, the gel film thickness, and then LiCoO2 crystallization process. An appropriate addition of PAA can improve the film quality and favor LiCoO2 crystallization. In this study, the solution consists of lithium and cobalt in proportion of 2:1, whereas the total metallic ions concentration (M+ ) is 0.3 mol/l. The quantities of PAA varying from 40 g/100 ml, 60 g/100 ml, to 80 g/100 ml were chosen for the investigation. The calcination procedure is performed as (i) increasing the temperature from room temperature to 300 ◦ C with a speed of 1 ◦ C/min, and keeping for 2 h, (ii) increasing temperature from 300 ◦ C to 600 ◦ C with a speed of 5 ◦ C/min, and keeping for another 2 h. The purpose of setting the initial temperature as 300 ◦ C is to expel H2 O and CO2 from gel film produced during the organic materials decomposition. The AFM analyses of the thin films were revealed in Fig. 1. For obtaining the thickness of the various films, the film surface was scratched by a sharp needle and then the depth of the scratch was measured by AFM. The film thickness was in the range from 10 nm to 100 nm. As the quantities of PAA in 100 ml solution are 40 g, the thin films with thickness of 20 nm are resulted. The fine particles size is about 200 nm, and the particle z-height varies from 75 nm to 155 nm. As the PAA content increases to 60 g, the

The investigation of the influence of two-step calcination process on the morphology of thin film was carried out by extending the calcination time from 2 h to 4 h for each step. At second step, we reduced the heating speed to 2 ◦ C/min. The SEM micrographs of LiCoO2 thin films calcined at temperature of 700 ◦ C, 800 ◦ C, and 900 ◦ C in air, are given in Fig. 2. When LiCoO2 thin films were calcined at 700 ◦ C, a smooth and less dense thin film was formed on the substrate surface, where the particles grew in parallel with the substrate surfaces. The rodlike particulates scattered with a broad size distribution, shown in Fig. 2a. However, when the calcination temperature increased to 800 ◦ C, LiCoO2 thin film became homogenous, which consists of extremely dense layers and monodispersed rod-like particulates with an average particle size of about 5 ␮m. The particulates size distribution is rather narrow, shown in Fig. 2b. As the calcination temperature increased to 900 ◦ C, less dispersed irregular particulates are observed, the particles tends to aggregate and the surface congeries appears porous. The combination effects of the fast growth of LiCoO2 particles at high calcined temperature, and the rapid releases of CO2 gas from the decomposition of PAA under higher temperature cause the substantial particulates agglomeration, which hinders the formation of LiCoO2 crystals, in Fig. 2c. These observations suggest that LiCoO2 thin films calcined at an

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Fig. 2. SEM micrographs of the samples calcined at different temperatures: (a) 700 ◦ C; (b) 800 ◦ C; (c) 900 ◦ C. The scale bars in the images and the insets of a and b are 20 ␮m, 10 ␮m and 5 ␮m, respectively.

appropriate temperature as 800 ◦ C have better crystalline than the films calcined at other temperature intervals. 3.3. The influence of the total metallic ion density on the thin film morphology The deposition of thin films was actualized by spin-coating pure PAA solution and then the solution containing components of PAA, lithium, and cobalt metallic ions on the silicon substrate surface. Owing to the excess addition of PAA, thicker gel films were expected. In this study, the second layer was deposited several minutes after the first PAA layer was prepared, a thin film with better quality can be resulted. The calcination procedure was performed as described before. Fig. 3 reveals SEM micrographs of LiCoO2 prepared at various proportions of Li:Co from 1:2 1.1:1, 1.5:1, to 2:1.

The average particle size decreases from 5 ␮m to 1 ␮m with increasing proportions of Li:Co. When the proportion Li:Co is set as 1:2, larger LiCoO2 particles were resulted. The particle size ranges from nanometer to micrometer due to the insufficient of lithium ions in the gel film, in Fig. 3a. A cluster of rod-like particles is observed as Li:Co is set as 1.1:1 and 1.5:1, in Fig. 3b and c. However, a dense film with multiple-layer can be observed in Fig. 3d. The decreases in particle size and the size distribution indicate that the suitable molar ratio between lithium and cobalt ions is essential for producing homogenous LiCoO2 particles. In contrast to the thin film without pre-coated by PAA solution, the morphology of the films pre-covered with PAA are more sensitive to the changes of Li:Co ratio. This suggests that a potential experimental parameter for better controlling of LiCoO2 morphology should be taken into consideration.

Fig. 3. The influence of the total metallic ion density on the thin film appearance: the proportion of Li:Co is set as: (a) 1:2; (b) 1.1:1; (c) 1.5:1; (d) 2:1, respectively.

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Fig. 4. The influences of multiple spin-coating on the morphology of thin films: (a) the solution is coated once; (b) PAA and the solution are alternately coated on silicon substrate once; (c) PAA and the solution are alternately coated on silicon substrate for three times; (d) the solution is coated for three times.

3.4. The influence of the multiple spin-coating on the morphology of thin film For finely controlling the crystalline and the morphology of LiCoO2 film, the deposition of thin films was realized by selectively spin-coating PAA solution and the solution. The solution consists of lithium and cobalt ions in proportion of 2:1, and PAA as 60 g/100 ml. The calcination procedure was used as described before. The preparation of the multiple-layer thin film on the silicon substrate was basically performed through one of the three procedures: (i) spincoating PAA solution and then the solution; (ii) spin-coating PAA solution and the solution alternately for three times; (iii) spincoating the solution for three times. Fig. 4 clearly shows that the multiple spin-coating can significantly affect the morphology of LiCoO2 thin film. Without pre-coating PAA on the substrate surface, the porous thin films contain several layers on the surface. The particles

aggregates and the particle sizes are nonuniform, and non-crystals can be observed in Fig. 4a and d. However, for the substrate precoated by PAA, the thin film surface has a few dense layers. Despite of the distribution of the particles is uneven, the particle sizes are uniform and particles are in regular rectangle shape in Fig. 4b and c. This is due to the enhancement of the adhesion force between the substrate surface and the solution after the substrate pre-covered with PAA solution. The multiple-layer solution deposits facilitate gradual decomposition of the organic components, which prevents the porous structure of thin film and large particles formation. Fig. 4c shows that the particulates size is smaller than 5 ␮m. For the substrate covered with PAA, the increase of the multiplelayers favors heating transferring homogenously inside of thin film, therefore lowers the crystal growth rate. This is again, proved in Fig. 4d. Comparing with the previous results, the thin film surface has a few dense layers and the particle numbers increase significantly, and the particle distributes evenly. The particle size

Fig. 5. LiCoO2 thin film formed on the PAA (Mw. 2000) pre-coated silicon surface at different scales.

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is down to 1 ␮m. Since the increases of density of ions enhances the crystal nucleation rate and growth rate, which prevents large crystals formation. Therefore, thin films prepared by multiple spincoating process are inclined to the formation of small particle of LiCoO2 . 3.5. The morphology of thin films in dependence of the molecular weight of PAA As studied before, the quantity of PAA has primarily influence on the formation of LiCoO2 . Here, the dependence of LiCoO2 morphology on PAA is further studied by varying the molecular weight of PAA from 1000 to 2000, while the rest experimental parameters remain unchanged. A circle-shape area containing with numerous particulates is found in the middle of dense film, in Fig. 5a and b. By zooming in this area, we could see that LiCoO2 particulates grew perpendicularly to the substrate surface. The average size of LiCoO2 particulates is about 150 nm. In the more dense area, the smallest particles size is about 50 nm. By using Mw. 2000 of PAA, nano-structured LiCoO2 is obtained. 4. Conclusions LiCoO2 thin films were prepared by a combination of a sol–gel process and spin-coating deposition. The analysis of both atomic force microscopy and scanning electron microscopy show how the morphology and crystalline of calcined LiCoO2 thin film change with the experimental parameters, i.e. the molar ratio of metal ions, the amount of poly-acrylic acid, the calcination process, and the numbers of the multiple-layer. It demonstrates that the morphology of the crystals formed on the substrate surface dramatically depends on the molar ratio between lithium and cobalt ions as the ratio of total metallic ion density and the PAA density is kept at 1:2. When the molar ratio of Li:Co increases from 1:2 to 2:1, the average particle size decreases from 5 ␮m to 1 ␮m, a homogeneous thin film with narrow size distribution of LiCoO2 crystals is resulted. That is because that excess amounts of Li are inclined to reduce the aggregation of particles, smooth particle surface, and produce dense materials. The multiple-layer is found to be in favor of the formation of nano-sized LiCoO2 .

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