액적화학증착법을 이용한 LiMn2O4 박막 제조 및 특성 분석

Theories and Application of Chem. Eng., 2002, Vol. 8 No.1

화학공학의 이론과 응용 제8권 제1호 2002년

김기웅, 한규승*, 우성일

한국과학기술원 생명화학공학과/ 초미세화학공정시스템연구센터,

*충남대학교 정밀공업화학과

Characterization of LiMn2O4 thin films prepared by

Liquid Source Misted Chemical Deposition(LSMCD)

Ki Woong Kim , Kyoo-Seung Han* and Seong Ihl Woo

Department of Biomolecular and Chemical Engineering /

Center for Ultramicrochemical Process Systems, KAIST

* Department of Fine Chemicals Engineering and Chemistry, Chungnam National Univ.

Introduction

As the concerns and demands for portable electronics and microelectronic systems are increased rapidly, the fabrication of thin film Lithium rechargeable microbatteries have been studied in the worldwide [1-6]. Lithium microbatteries can be used as power sources of microelectronics such as implantable medical device, MEMS, and chip card. To fabricate all-solid-state Li rechargeable microbatteries, the effective deposition methods of cathode films such as LiCoO2, LiNiO2, and LiMn2O4 must be investigated. Recently, many research groups including ORNL prepared LiCoO2 and LiMn2O4 films by RF sputtering and pulsed laser ablation[2-4]. However, it is difficult to control the chemical composition in preparation to lithiated ternary and quaternary metal oxides due to the difference in sputtering yields of compositional chemical species. And it was reported that these techniques had many problems to commercialize because of narrow deposition conditions and high cost of vacuum apparatuses. To overcome these problems, this research developed one of wet deposition techniques, Liquid Source Misted Chemical Deposition (LSMCD) to prepare cathode films. LSMCD is the process that uses the ultrafine mists (diameter: 0.1-3□) generated by ultrasonic nebulizer as the deposition source. Therefore, it is easy to control chemical composition and microstructure of films and to synthesize films that have various properties in various deposition conditions. Therefore, it can be more effective to deposit lithiated multi-component metal oxide thin films than any other conventional deposition techniques. In this research, the effect of crystallization temperature on the structure and electrochemical properties of LiMn2O4 films was investigated

Experimental part

(1) Thin film Deposition (LSMCD)

Pt(1500□)/SiO2(3000□)/Si was used as a substrate. The precursor solution was 0.3M solution of LiNO3 and Mn(NO3)2□nH2O(4<n<6) in 2-methoxyethanol and ethylene glycol and the cationic ratio Li/Mn was 1.05/2. The mists generated by using ultrasonic nebulizer (frequency: 1.63MHz) were deposited on the substrate at 700torr in Ar atmosphere for 15min. After deposition, the solution films were baked at 200oC for 2min and were dried at 400oC in air for 5min sequentially. And then, heat treatment for crystallization of films was carried out at 600-800oC in RTA for 5min in the oxygen atmosphere. Therefore, LiMn2O4 films with 1670 □ thickness were obtained.

(2) Characterization

Scanning electron microscope (SEM) images were obtained using a Philips 533M. The X-ray diffraction (XRD) was performed using a Rigaku D-MAX-RC diffractometer and Cu K□ radiation (□= 1.5405□) operated at 60 mA and 30 kV. The diffractograms were recorded in the range of 2 between 10 and 80 with 0.01 resolutions. Raman spectrums were obtained using a Jobin Yvon U-1000 spectrometer. Electrochemical test cells were assembled in a beaker under an Ar atmosphere. The electrolyte was 1.0M LiClO4/PC solution. Lithium foil was used as an anode. The cyclic voltammogram(CV) of the prepared film was recorded to test its electrochemical activity in the voltage range between 3.5 and 4.3 V using a Biologic Macpile II. In addition, the cells were cycled at ±10□/cm2 using a Biologic Macpile II

Results and Discussion

As shown in Fig.1, the scanning electron microscope (SEM) images of the surface morphology and the cross section of the film directly demonstrate the formation of the homogeneous film with 0.167□ thickness without discontinuity and peeling of the film when LiMn2O4 was prepared at 800oC. However, LiMn2O4 prepared at 600oC and 700oC can be observed some cracks in the film and it has lower degree of crystallinity than LiMn2O4 film prepared at 800oC. Fig.2. shows the XRD pattern of LiMn2O4 film prepared by LSMCD in the function of crystallization temperature. When the heat treatments of LiMn2O4 for crystallization were carried out in RTA in O2 atmosphere for 5 min, impurity phases such as Mn2O3 did not exist in the prepared film regardless of crystallization temperature on the basis of the location of principle peak (111) that shows the prepared film spinel-LiMn2O4. However, These XRD patterns showed that short heat treatment suppressed the formation of impurity phases by reducing the loss of lithium generated by lithium overevaporation and oxidation to Li2O. However, when the heat treatment of LiMn2O4 film was carried out at 600oC for crystallization, the degree of crystallinity of prepared film was decreased on the basis of the lower intensity of diffraction line (111) at 2□=18.68o as expected from SEM image.

According to the cyclic voltammogram(Fig.3), the prepared films can be electrochemically oxidized and reduced in 1.0M LiClO4/PC. Open-circuit voltages (OCV) of freshly prepared cells are 2.95 to 3.0 V, which are similar to those of spinel LiMn2O4 phase. Here, we tested LiMn2O4 films that crystallized at 700oC and 800oC because LiMn2O4 film crystallized at 600oC have low crystallinity and did not have spinel phase. Two characteristic peaks were observed in both anodic and cathodic scans, respectively. Regardless of crystallization temperature of LiMn2O4 thin film, the first anodic and cathodic peaks at 4.03V and 4.0 V correspond to Li ion extraction/insertion between LiMn2O4 and Li0.5Mn2O4. And the second anodic and cathodic peaks at 4.15 and 4.12V correspond to Li ion extraction/reinsertion between Li0.5Mn2O4 and □-MnO2. These two-phase reactions can be recognized by charge/discharge curve that has two plateaus at 4V and 4.1V regions, respectively. As shown in fig.3 (a), cyclic voltammogram showed the maintenance of cathodic and anodic peak current on cycling without a shift in the potentials in the case of LiMn2O4 prepared at 800oC. It means Li+ extraction and insertion was performed efficiently without reducing Li+ diffusivity on cycling assuming that peak current was in proportion to Li+ diffusivity.  In addition, it can be recognized that the cathodic peaks due to the insertion of Li+ is equal to the anodic peaks due to the extraction of Li+. It means that this redox process is completely reversible and lithium ions occupy half of the total available crystallographic sites in each stage. However, in the case of LiMn2O4 prepared at 700oC(Fig 3.(b)), the fluctuation of peak current in low voltage region between 3.5 and 3.8V as well as the reduction of peak currents were observed. It means the porous morphology and low degree of crystallinity of prepared film have a bad effect on the cycling behavior. As shown in fig. 4., LiMn2O4 film prepared at 800oC had much better initial discharge capacity and capacity retention than LiMn2O4 films prepared by other deposition techniques such as RF sputtering, laser ablation and sol-gel coating etc. The prepared film exhibits the initial discharge capacity of 62.4 □h/cm2□□ which is 98.3% of theoretical discharge capacity (63.5□h/cm2□□) and the electrochemical cell which LiClO4(1M)/PC was used as a electrolyte and LiMn2O4 film was used as a cathode exhibits a capacity fade over 100cycles of only 8%(0.08%/cycle) because of structural stability during electrochemical cycling. Therefore, LiMn2O4 film did not show peeling phenomena like LiCoO2 film that show large capacity fade(0.5%/cycle).[7] LiMn2O4 film prepared at 700oC exhibits the initial discharge capacity of 60.7 □h/cm2□□ but rapid capacity fade over 100cycles (0.74%/cycle). It means that small amount of mechanical stress generated by a little lattice parameter change from LiMn2O4 (a = 8.24 □) to □-MnO2 (a = 8.03 □) was concentrated on the tip of cracks in the prepared film so that the cathode film was peeled from the substrate partially. Therefore, we concluded that morphology and film-density as well as low degree of crystallinity had a bad effect on the cycling behavior in the case of thin film cathode.

Reference

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(a)                      (b)                          (c)

Fig.1 Scanning electron microscope images of the surface of the LiMn2O4 film synthesized as a function of crystallization temperature (a) 600oC, (b) 700oC (c) 800oC.

(Heat treatment in RTA for 5min in oxygen atmosphere)

Fig.2 X-ray diffraction patterns of LiMn2O4 films annealed at (a) 600oC (b) 700oC (c) 800oC

Fig.3. Cyclic Voltammogram taken 1.0M LiClO4/PC electrolyte (potential is referred to Li/Li+ electrode) with the scan rate of 7.0mV/min of LiMn2O4 films synthesized as a function of crystallization temperature. (a) 800oC (b) 700oC

Fig.4. Charge/Discharge behavior of LiMn2O4 films annealed at (a) 800oC (b) 700oC