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Fabrication and ionic conductivity of amorphous Li–Al–Ti–P–O thin film
Journal of Non-Crystalline Solids 357 (2011) 3267–3271
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l
Fabrication and ionic conductivity of amorphous Li–Al–Ti–P–O thin film Hongping Chen, Haizheng Tao, Xiujian Zhao ⁎, Qide Wu State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan, Hubei 430070, People's Republic of China
a r t i c l e
i n f o
Article history: Received 22 February 2011 Received in revised form 21 May 2011 Available online 14 June 2011 Keywords: RF magnetron sputtering; Ionic conductivity; Inorganic electrolyte; Li–Al–Ti–P–O film; ITO glass
a b s t r a c t Li + ion conducting Li–Al–Ti–P–O thin films were fabricated on ITO-glass substrates at various temperatures from 25 to 400 °C by RF magnetron sputtering method. When the substrate temperature is higher than 300 °C, severe destruction of ITO films were confirmed by XRD (X-ray diffraction) and the abrupt transformation of one semi-circle into two semi-circles on the impedance spectra. These as-deposited Li–Al–Ti–P–O solid state electrolyte thin films have an amorphous structure confirmed by XRD and a single semicircle on the impedance spectra. Good transmission higher than 80% in the visible light range of these electrolyte thin films can fulfill the demand of electro-chromic devices. Field emission scanning electron microscopy and atomic force microscopy showed the denser, smoother and more uniform film structure with the enhanced substrate temperature. Measurements of impedance spectra indicate that the gradual increased conductivity of these Li–Al–Ti–P–O thin films with the elevation of substrate temperature from room temperature to 300 °C is originated from the increase of the pre-exponential factor (σ0). The largest Li-ion conductivity can come to 2.46 × 10− 5 S cm − 1. This inorganic solid lithium ion conductor film will have a potential application as an electrolyte layer in the field such as lithium batteries or all-solid-state EC devices. © 2011 Elsevier B.V. All rights reserved.
1. Introduction During the past two decades, with the developments of solid state ionic power devices, such as CMOS, FE-RAM, liquid crystal displays and micro-electric devices, it looks more and more emphasis on micro-power sources with high energy density devices [1–4]. For improving safety and reliability of the devices, all-solid-state devices are much desired. As a key material of all-solid-state devices, ion conductive layer has been extensively studied in the field of materials science, polymer science, electrochemistry, and solid state chemistry [5]. The inorganic solid electrolytes possess many potential advantages, such as electrochemical stability window, thermal stability, the possibility of easy miniaturization, especially for thin-film technologies [4]. In addition, single ion conduction is also an important feature of inorganic materials. Thin film fabrication can reduce the thickness of the electrolyte layer, resulting in a reduction in internal resistance of the devices. So Li-ion inorganic solid electrolyte films have attracted more and more attentions. Compared with other lithium oxide or sulfide based glasses, LiPON thin films prepared by RF sputtering using Li3PO4 target in nitrogen gas [6] is rather stable in spite of moderate room temperature ionic conductivity of 3.3 × 10 − 6 S cm − 1 and activation energy of 0.54 eV [7]. Ion conductivity of these LiPON thin films can increase to 1.24 × 10 − 5 S cm − 1 with the incorporation of Si [8]. The NASICON⁎ Corresponding author. E-mail addresses: [email protected] (H.P. Chen), [email protected] (H.Z. Tao), [email protected] (X.J. Zhao). 0022-3093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2011.05.023
structured film material LiTi2(PO4)3 exhibits high ionic conductivity of 9.2 × 10 − 6 S cm − 1 at 25 °C, which can be further improved by Si 4+ ion substitution for P 5+ corresponding with increasing Li + ion concentration (Li–Ti–Si–P–O–N) and N incorporated [9,10]. However, the ion conductivity is still small. The NASICON crystallographic structure Na(AIV)2(PO4)3 (AIV = Ge, Ti and Zr) was identified in 1968 [1,11]. Among the Al-doped ceramics, the nominal composition Li1.3Al0.3Ti1.7(PO4)3 (LATP) was reported to show very high conductivity of over 10 − 4 S cm − 1[12–14], but they are not easily fabricated into a desired thickness or shape to accommodate micro-electric devices. In this study, utilizing radio frequency magnetron sputtering method, we focused on the preparation and characterization of Alsubstituted LiTi2(PO4)3 thin films, which can be considered as a potential thin film solid electrolyte for all solid state micro-batteries. 2. Experimental 2.1. Preparation of nominal composition LATP thin film electrolyte The ceramic target was prepared with a conventional cold press and sintering method. Powder of LATP was synthesized by the solidstate reaction of stoichiometric amount of lithium carbonate Li2CO3, aluminum hydroxide Al(OH)3, titanium oxide TiO2 and ammonium dihydrogen phosphate NH4H2PO4 (0.85:0.3:1.7:3 in mol% and an 10% excess of Li2CO3 to compensate for the Li loss in heat treatment and sputtering) as starting materials. The powders were calcined in an alumina crucible for 10 h at 1100 °C. Then it was thoroughly reground, subsequently the powders were pressed into pellets under 200 MPa
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pressure and heated at 950 °C for 5 h, cooled to room temperature naturally. The ITO-coated glass slides were used as substrates. The LATP thin films layers were deposited by RF magnetron sputtering using the above target in an Ar atmosphere at a pressure as high as 1.0 Pa (65% Ar + O2 35%, base pressure 2.5 × 10 − 3 Pa) with a substrate rotation speed of 5 rpm at a power of 150 W. The distance between substrate and target was 6 cm and the deposition time was 1 h. The substrate temperatures were raised from room temperature to 400 °C (25, 100, 200, 300 and 400 °C). For the measurement of electrochemical properties, a sandwich structure was formed on ITO glass which a layer of ITO as the bottom electrode, a layer of the LATP thin film electrolyte, and a layer of aluminum thin film as the tip electrode. The aluminum thin film was deposited by direct current (dc) sputtering. 2.2. Characterization of the materials The structure of the target and thin films was characterized by Xray diffraction (XRD) using Cu Ka radiation and X-ray diffractometer equipped with a thin film diffraction attachment (Rigaku), 2θ scanning from 10 to 70°. The morphology of the film was observed by scanning electron microscopy, field-emission scanning electron microscopy (FESEM, S-4800, HITACHI, Japan) and atomic force microscopy (AFM, NanoScope (R) IV model, Veeco Metrology Group, USA). Conductivity of the thin film was measured by electrochemical a.c. impedance analyzer (CHI 760d) with a small perturbation voltage of 5 mV in a frequency range from 10 5 to 1 Hz. The UV–vis spectrophotometer (UV1601, SHIMADZU, Japan) were carried out to measure the optical transmittance for the LATP films in the 250–1000 nm region.
reaches 400 °C, other unexpected crystal phases such as ITO come out [20], which indicate the destruction of the ITO films. SEM and FESEM images of surfaces and cross-sections of the Li–Al– Ti–P–O thin films deposited at different temperatures are shown in Fig. 2 and Fig. 3 respectively. Thin films deposited at room temperature show a uniform texture of column-shaped rods with a diameter of about 20 nm and a length of about 100 nm. With the enhancement of the substrate temperature, the diameter of columnshaped rods gradually increases and the films become denser and denser. To further characterize the surface morphology, AFM measurement was employed. With the gradual elevation of substrate temperature, Fig. 4 further shows the slow transformation from a quite rough structure with very regular column of ~20 nm diameter that are homogeneously distributed at room temperature to the denser and smoother state at 300 °C together with the decrease of surface RMS (roughness mean square) from 11 nm at 25 °C to 3 nm at 300 °C. 3.2. Electrochemical properties of Li–Al–Ti–P–O thin films Utilizing the structure of ITO/Li–Al–Ti–P–O/Al sandwich, the electrochemical impedance spectroscopy of the Li–Al–Ti–P–O thin films at 25 °C is shown in Fig. 5. High-frequency part is the contribution of electrolyte film, while the low-frequency part corresponds to the impedance of the electrode/electrolyte interface. According to the previous report [21], the impedance spectroscopy of Li–Al–Ti–P–O ceramic can be divided into two semi-circles which attributed to the contributions of the bulk part and the grain boundary respectively. Only one semi-circle comes out on the impedance spectroscopy of Li–Al–Ti–P–O films fabricated below 300 °C, which is
3. Results 3.1. Structure and morphology of the thin films As seen from the X-ray diffraction patterns (Fig. 1), the target with the nominal composition Li1.3Al0.3Ti1.7(PO4)3 elaborated clearly the main diffraction peaks due to the lithium-analog of NASICON LiTi2 (PO4)3 structure [13,15–19] (JCPDS card 35–754) and no any peak of other crystalline phases can be found. However, the XRD patterns for the thin film electrolytes fabricated at the substrate temperatures below 300 °C only have broad haloes, showing that the thin films should be of amorphous phase. When the substrate temperature
Fig. 1. XRD patterns of the Li1.3Al0.3Ti1.7(PO4)3 target and the LATP thin-film deposited at 25 °C, 100 °C, 200 °C, 300 °C and 400 °C, respectively.
Fig. 2. SEM images of surface (a) and cross-section (b) of the Li–Al–Ti–P–O thin film prepared at 25 °C.
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Fig. 3. FESEM images of the LATP films deposited at (a) 25 °C, (b) 100 °C, (c) 200 °C, (d) 300 °C.
due to their homogeneous amorphous structure confirmed by XRD as shown in Fig. 1. The ionic conductivity was calculated according to the resistance value (R) determined from the impedance diagrams (Fig. 5) by means of the following equation: h i −1 σ = d = ðR × AÞ S⋅cm
ð1Þ
where d is the electrolyte film thickness, R the electrolyte resistance and A is the valid film area. To illustrate the effect of the temperature on the charge transfer at the ITO/LATP/Al interface, the temperature dependence of the alternating current impedance was measured. The results abided by the rule of Arrhenius equation. The Arrhenius plots of conductivity multiplied by absolute temperature, against reciprocal temperature, are shown in Fig. 6. These data were fitted to a straight line, indicating that the charge transfer resistance multiplied by absolute temperature can be expressed as follows [22], σ=
σ0 −Ea exp T kT
ð2Þ
where Ea is the activation energy, T is the absolute temperature, k is the gas constant and σ0 is the pre-exponential factor. According to Eq. (1) and Eq. (2), the ionic conductivity at room temperature, activation energy and pre-exponential factor were calculated and summarized in Table 1. It is found that the activation energy was close to 31 kJ mol − 1 and almost not changed with the substrate temperature, however the ionic conductivity at room temperature and the pre-exponential factor increased with increasing substrate temperature.
3.3. Optical transmission As seen in Fig. 7, except for the films obtained at 400 °C which have lower transmittance because of the crystallization of ITO films, optical transmittance of other samples varied little and was above 80% in the range of visible light from 400 nm to 800 nm. For the electrochromic applications, this feature is very important [23]. 4. Discussion The gradual transformation of the decrease of surface RMS can be easily explained by the basic micro-processes of film growth, which can be generalized into three basic events: 1) The incident atom deposits on the growing surface and is attached, namely the absorption events; 2) All kinds of atoms diffuse on the growing surface, namely the diffusion events; 3) The adatoms detach from the growing surface, namely the evaporation events. In fact, this process could be incorporated into a special kind of diffusion event. At lower temperature just as in this work (lower than 400 °C), considering the facts that atoms hardly conquer the characteristic activation barrier and thus are considered to be essentially immobile, film morphology is mainly controlled by molecular mobility. To fill in a dip, a molecular would have to pass several steps, during which it is more probable to dissociate into essentially immobile atoms. Therefore, once formed on the surface, these small dips will tend to sharpen, separated from its neighbors by gaps and further lead to the morphology of small “island-like” growth just as shown in Fig. 3 and Fig. 4, especially at the lowest temperature at 25 °C. With the enhancement of temperature, atomic mobility will gradually increase, which will generate more and more activated atoms. These activated atoms will more easily fill in the inner voids of the film and lead to a denser and smoother film [24–27].
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Fig. 4. 3D topographical AFM images of the sample obtained at (a) 25 °C, (b) 100 °C, (c) 200 °C and (d) 300 °C, respectively.
In addition, from the surface morphology shown in Figs. 2–4, it can be seen that there was no fracture phenomenon on the surface of thin films and the obtained films are uniform and dense, which is very important to avoid safety problems of short circuit or uneven current density as electrolyte thin films. Based on the cross-section diagram shown in Fig. 2, the prepared film has a thickness of approximately 275 nm and is uniform comparatively. According to the Nernst–Einstein relationship, the ionic conductivity can be given by, σ=
NðZe Þ2 ξ2 −Ea υ0 exp f kT kT
where N is the number density of mobile ions, Ze the charge of a mobile ion (in the case of the Li–Al–Ti–P–O films, as the mobile ion is Li +, then Ze = 1), ξ the hopping distance of the ion, ν0 the attempt frequency and f the correlation factor which value would be about 1. Comparing Eqs.(2) and (3), we can see that the pre-exponential factor is connected with the number density, hopping distance and attempt frequency of mobile ions [28,29]. Combining the pre-exponential factor with the number density by N ~ ρ and the hopping distance ξ, it can be concluded that the higher pre-exponential factor should be due
ð3Þ
Fig. 5. AC impedance spectroscopy of ITO/LATP/Al sandwich measured at different temperatures 25, 100, 200, 300 and 400 °C.
Fig. 6. Arrhenius plot of ionic conductivities of the Li–Al–Ti–P–O thin-film electrolytes deposited in 25, 100, 200 and 300 °C, respectively.
H.P. Chen et al. / Journal of Non-Crystalline Solids 357 (2011) 3267–3271
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Table 1 Ionic conductivity at 25 °C, pre-exponential factor and activation energy of thin films deposited at various temperatures. LATP film sample
σ (S cm− 1)
σ0 ± 0.01a × 103 (S cm− 1 K− 1)
Ea ± 0.1a(KJ/mol)
Deposited at 25 °C Deposited at 100 °C Deposited at 200 °C Deposited at 300 °C
0.34 × 10− 5 0.76 × 10− 5 1.32 × 10− 5 2.46 × 10− 5
0.67 × 103 1.93 × 103 2.33 × 103 4.65 × 103
31.1 31.7 30.9 31.1
a
Linear regression error for both Ea and σ0 is also indicated.
to the denser film structure and/or longer hopping distance. As confirmed by the above-mentioned characterization of film structure, with the increased substrate temperature, the thin-films become smoother and denser. Meanwhile, the film grows from column to round and surface roughness decreases, which lead to the decrease of ion migration barrier and the increase of hopping distance. However, when the substrate temperature increased to 400 °C, it can be clearly seen that the impedance spectra possess two semicircles in high- and medium-frequency region, which is due to the destruction of ITO films confirmed by the previous XRD pattern shown in Fig. 1. It is believed that SnO2 reacts with lithium ions according to the following two step process [30,31]: þ
−
SnO2 + 4Li + 4e →Sn + 2Li2 O þ
−
Sn + xLi + ne ↔Lix Sn; 0 ≦ x ≦ 4:4
ð4Þ ð5Þ
The irreversible process in Eq. (4) results in metallic tin dispersed in Li2O matrix, and then the alloying and de-alloying process of Li and Sn in Eq. (5) provides the reversible capacity of the material [32]. Accordingly, the electronic conductivity of ITO films declined which will affect the measured value of the ionic conductivity of the LATP film. Since the faradic reaction is determined by ion transfer and electron conduction, the increase in the resistance can be attributed to the reduced electronic conductivity of the ITO electrodes which crystallized and reacted with Li–Al–Ti–P–O films. Nevertheless, maybe there are other reasons about this phenomenon which need further investigation. 5. Conclusions The lithium aluminum titanium phosphate film electrolytes were prepared by radio-frequency (RF) magnetron sputtering using Li1.3Al0.3Ti1.7(PO4)3 target on ITO conductive glass substrate. Firstly, due to the crystallization of ITO films and its interaction with the electrolyte film simultaneity, the conductivity descended and the grain boundary conductivity came out when the sample was prepared at higher temperature of 400 °C. Results of electrochemical analysis are consistent with the morphological and structural properties for these samples. The films obtained have an excellent transparence in the visible range and have an amorphous structure. With the enhancement of substrate temperature, these films become denser, smoother and more uniform, which naturally lead to the increased ionic conductivity. The maximum ionic conductivity at room temperature obtained at the substrate temperature of 300 °C can come up to 2.46 × 10− 5 S cm− 1. All of the characteristics of these Li–Al–Ti–P–O thin films show that it is a potential candidate for the electrolyte used in all solid-state film lithium ion batteries, sensors and electrochemical devices. Acknowledgements The authors wish to acknowledge financial support by National Basic Research Program of China (2009CB939704), Natural Science Founda-
Fig. 7. Optical transmittance for LATP thin films deposited on slide glass substrates for 60 min, and at different temperatures 25, 100, 200, 300 and 400 °C.
tion of China (nos. 51032005 and 60808024) and the Fundamental Research Funds for the Central Universities (Wuhan University of Technology).
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Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l
Fabrication and ionic conductivity of amorphous Li–Al–Ti–P–O thin film Hongping Chen, Haizheng Tao, Xiujian Zhao ⁎, Qide Wu State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan, Hubei 430070, People's Republic of China
a r t i c l e
i n f o
Article history: Received 22 February 2011 Received in revised form 21 May 2011 Available online 14 June 2011 Keywords: RF magnetron sputtering; Ionic conductivity; Inorganic electrolyte; Li–Al–Ti–P–O film; ITO glass
a b s t r a c t Li + ion conducting Li–Al–Ti–P–O thin films were fabricated on ITO-glass substrates at various temperatures from 25 to 400 °C by RF magnetron sputtering method. When the substrate temperature is higher than 300 °C, severe destruction of ITO films were confirmed by XRD (X-ray diffraction) and the abrupt transformation of one semi-circle into two semi-circles on the impedance spectra. These as-deposited Li–Al–Ti–P–O solid state electrolyte thin films have an amorphous structure confirmed by XRD and a single semicircle on the impedance spectra. Good transmission higher than 80% in the visible light range of these electrolyte thin films can fulfill the demand of electro-chromic devices. Field emission scanning electron microscopy and atomic force microscopy showed the denser, smoother and more uniform film structure with the enhanced substrate temperature. Measurements of impedance spectra indicate that the gradual increased conductivity of these Li–Al–Ti–P–O thin films with the elevation of substrate temperature from room temperature to 300 °C is originated from the increase of the pre-exponential factor (σ0). The largest Li-ion conductivity can come to 2.46 × 10− 5 S cm − 1. This inorganic solid lithium ion conductor film will have a potential application as an electrolyte layer in the field such as lithium batteries or all-solid-state EC devices. © 2011 Elsevier B.V. All rights reserved.
1. Introduction During the past two decades, with the developments of solid state ionic power devices, such as CMOS, FE-RAM, liquid crystal displays and micro-electric devices, it looks more and more emphasis on micro-power sources with high energy density devices [1–4]. For improving safety and reliability of the devices, all-solid-state devices are much desired. As a key material of all-solid-state devices, ion conductive layer has been extensively studied in the field of materials science, polymer science, electrochemistry, and solid state chemistry [5]. The inorganic solid electrolytes possess many potential advantages, such as electrochemical stability window, thermal stability, the possibility of easy miniaturization, especially for thin-film technologies [4]. In addition, single ion conduction is also an important feature of inorganic materials. Thin film fabrication can reduce the thickness of the electrolyte layer, resulting in a reduction in internal resistance of the devices. So Li-ion inorganic solid electrolyte films have attracted more and more attentions. Compared with other lithium oxide or sulfide based glasses, LiPON thin films prepared by RF sputtering using Li3PO4 target in nitrogen gas [6] is rather stable in spite of moderate room temperature ionic conductivity of 3.3 × 10 − 6 S cm − 1 and activation energy of 0.54 eV [7]. Ion conductivity of these LiPON thin films can increase to 1.24 × 10 − 5 S cm − 1 with the incorporation of Si [8]. The NASICON⁎ Corresponding author. E-mail addresses: [email protected] (H.P. Chen), [email protected] (H.Z. Tao), [email protected] (X.J. Zhao). 0022-3093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2011.05.023
structured film material LiTi2(PO4)3 exhibits high ionic conductivity of 9.2 × 10 − 6 S cm − 1 at 25 °C, which can be further improved by Si 4+ ion substitution for P 5+ corresponding with increasing Li + ion concentration (Li–Ti–Si–P–O–N) and N incorporated [9,10]. However, the ion conductivity is still small. The NASICON crystallographic structure Na(AIV)2(PO4)3 (AIV = Ge, Ti and Zr) was identified in 1968 [1,11]. Among the Al-doped ceramics, the nominal composition Li1.3Al0.3Ti1.7(PO4)3 (LATP) was reported to show very high conductivity of over 10 − 4 S cm − 1[12–14], but they are not easily fabricated into a desired thickness or shape to accommodate micro-electric devices. In this study, utilizing radio frequency magnetron sputtering method, we focused on the preparation and characterization of Alsubstituted LiTi2(PO4)3 thin films, which can be considered as a potential thin film solid electrolyte for all solid state micro-batteries. 2. Experimental 2.1. Preparation of nominal composition LATP thin film electrolyte The ceramic target was prepared with a conventional cold press and sintering method. Powder of LATP was synthesized by the solidstate reaction of stoichiometric amount of lithium carbonate Li2CO3, aluminum hydroxide Al(OH)3, titanium oxide TiO2 and ammonium dihydrogen phosphate NH4H2PO4 (0.85:0.3:1.7:3 in mol% and an 10% excess of Li2CO3 to compensate for the Li loss in heat treatment and sputtering) as starting materials. The powders were calcined in an alumina crucible for 10 h at 1100 °C. Then it was thoroughly reground, subsequently the powders were pressed into pellets under 200 MPa
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pressure and heated at 950 °C for 5 h, cooled to room temperature naturally. The ITO-coated glass slides were used as substrates. The LATP thin films layers were deposited by RF magnetron sputtering using the above target in an Ar atmosphere at a pressure as high as 1.0 Pa (65% Ar + O2 35%, base pressure 2.5 × 10 − 3 Pa) with a substrate rotation speed of 5 rpm at a power of 150 W. The distance between substrate and target was 6 cm and the deposition time was 1 h. The substrate temperatures were raised from room temperature to 400 °C (25, 100, 200, 300 and 400 °C). For the measurement of electrochemical properties, a sandwich structure was formed on ITO glass which a layer of ITO as the bottom electrode, a layer of the LATP thin film electrolyte, and a layer of aluminum thin film as the tip electrode. The aluminum thin film was deposited by direct current (dc) sputtering. 2.2. Characterization of the materials The structure of the target and thin films was characterized by Xray diffraction (XRD) using Cu Ka radiation and X-ray diffractometer equipped with a thin film diffraction attachment (Rigaku), 2θ scanning from 10 to 70°. The morphology of the film was observed by scanning electron microscopy, field-emission scanning electron microscopy (FESEM, S-4800, HITACHI, Japan) and atomic force microscopy (AFM, NanoScope (R) IV model, Veeco Metrology Group, USA). Conductivity of the thin film was measured by electrochemical a.c. impedance analyzer (CHI 760d) with a small perturbation voltage of 5 mV in a frequency range from 10 5 to 1 Hz. The UV–vis spectrophotometer (UV1601, SHIMADZU, Japan) were carried out to measure the optical transmittance for the LATP films in the 250–1000 nm region.
reaches 400 °C, other unexpected crystal phases such as ITO come out [20], which indicate the destruction of the ITO films. SEM and FESEM images of surfaces and cross-sections of the Li–Al– Ti–P–O thin films deposited at different temperatures are shown in Fig. 2 and Fig. 3 respectively. Thin films deposited at room temperature show a uniform texture of column-shaped rods with a diameter of about 20 nm and a length of about 100 nm. With the enhancement of the substrate temperature, the diameter of columnshaped rods gradually increases and the films become denser and denser. To further characterize the surface morphology, AFM measurement was employed. With the gradual elevation of substrate temperature, Fig. 4 further shows the slow transformation from a quite rough structure with very regular column of ~20 nm diameter that are homogeneously distributed at room temperature to the denser and smoother state at 300 °C together with the decrease of surface RMS (roughness mean square) from 11 nm at 25 °C to 3 nm at 300 °C. 3.2. Electrochemical properties of Li–Al–Ti–P–O thin films Utilizing the structure of ITO/Li–Al–Ti–P–O/Al sandwich, the electrochemical impedance spectroscopy of the Li–Al–Ti–P–O thin films at 25 °C is shown in Fig. 5. High-frequency part is the contribution of electrolyte film, while the low-frequency part corresponds to the impedance of the electrode/electrolyte interface. According to the previous report [21], the impedance spectroscopy of Li–Al–Ti–P–O ceramic can be divided into two semi-circles which attributed to the contributions of the bulk part and the grain boundary respectively. Only one semi-circle comes out on the impedance spectroscopy of Li–Al–Ti–P–O films fabricated below 300 °C, which is
3. Results 3.1. Structure and morphology of the thin films As seen from the X-ray diffraction patterns (Fig. 1), the target with the nominal composition Li1.3Al0.3Ti1.7(PO4)3 elaborated clearly the main diffraction peaks due to the lithium-analog of NASICON LiTi2 (PO4)3 structure [13,15–19] (JCPDS card 35–754) and no any peak of other crystalline phases can be found. However, the XRD patterns for the thin film electrolytes fabricated at the substrate temperatures below 300 °C only have broad haloes, showing that the thin films should be of amorphous phase. When the substrate temperature
Fig. 1. XRD patterns of the Li1.3Al0.3Ti1.7(PO4)3 target and the LATP thin-film deposited at 25 °C, 100 °C, 200 °C, 300 °C and 400 °C, respectively.
Fig. 2. SEM images of surface (a) and cross-section (b) of the Li–Al–Ti–P–O thin film prepared at 25 °C.
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Fig. 3. FESEM images of the LATP films deposited at (a) 25 °C, (b) 100 °C, (c) 200 °C, (d) 300 °C.
due to their homogeneous amorphous structure confirmed by XRD as shown in Fig. 1. The ionic conductivity was calculated according to the resistance value (R) determined from the impedance diagrams (Fig. 5) by means of the following equation: h i −1 σ = d = ðR × AÞ S⋅cm
ð1Þ
where d is the electrolyte film thickness, R the electrolyte resistance and A is the valid film area. To illustrate the effect of the temperature on the charge transfer at the ITO/LATP/Al interface, the temperature dependence of the alternating current impedance was measured. The results abided by the rule of Arrhenius equation. The Arrhenius plots of conductivity multiplied by absolute temperature, against reciprocal temperature, are shown in Fig. 6. These data were fitted to a straight line, indicating that the charge transfer resistance multiplied by absolute temperature can be expressed as follows [22], σ=
σ0 −Ea exp T kT
ð2Þ
where Ea is the activation energy, T is the absolute temperature, k is the gas constant and σ0 is the pre-exponential factor. According to Eq. (1) and Eq. (2), the ionic conductivity at room temperature, activation energy and pre-exponential factor were calculated and summarized in Table 1. It is found that the activation energy was close to 31 kJ mol − 1 and almost not changed with the substrate temperature, however the ionic conductivity at room temperature and the pre-exponential factor increased with increasing substrate temperature.
3.3. Optical transmission As seen in Fig. 7, except for the films obtained at 400 °C which have lower transmittance because of the crystallization of ITO films, optical transmittance of other samples varied little and was above 80% in the range of visible light from 400 nm to 800 nm. For the electrochromic applications, this feature is very important [23]. 4. Discussion The gradual transformation of the decrease of surface RMS can be easily explained by the basic micro-processes of film growth, which can be generalized into three basic events: 1) The incident atom deposits on the growing surface and is attached, namely the absorption events; 2) All kinds of atoms diffuse on the growing surface, namely the diffusion events; 3) The adatoms detach from the growing surface, namely the evaporation events. In fact, this process could be incorporated into a special kind of diffusion event. At lower temperature just as in this work (lower than 400 °C), considering the facts that atoms hardly conquer the characteristic activation barrier and thus are considered to be essentially immobile, film morphology is mainly controlled by molecular mobility. To fill in a dip, a molecular would have to pass several steps, during which it is more probable to dissociate into essentially immobile atoms. Therefore, once formed on the surface, these small dips will tend to sharpen, separated from its neighbors by gaps and further lead to the morphology of small “island-like” growth just as shown in Fig. 3 and Fig. 4, especially at the lowest temperature at 25 °C. With the enhancement of temperature, atomic mobility will gradually increase, which will generate more and more activated atoms. These activated atoms will more easily fill in the inner voids of the film and lead to a denser and smoother film [24–27].
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Fig. 4. 3D topographical AFM images of the sample obtained at (a) 25 °C, (b) 100 °C, (c) 200 °C and (d) 300 °C, respectively.
In addition, from the surface morphology shown in Figs. 2–4, it can be seen that there was no fracture phenomenon on the surface of thin films and the obtained films are uniform and dense, which is very important to avoid safety problems of short circuit or uneven current density as electrolyte thin films. Based on the cross-section diagram shown in Fig. 2, the prepared film has a thickness of approximately 275 nm and is uniform comparatively. According to the Nernst–Einstein relationship, the ionic conductivity can be given by, σ=
NðZe Þ2 ξ2 −Ea υ0 exp f kT kT
where N is the number density of mobile ions, Ze the charge of a mobile ion (in the case of the Li–Al–Ti–P–O films, as the mobile ion is Li +, then Ze = 1), ξ the hopping distance of the ion, ν0 the attempt frequency and f the correlation factor which value would be about 1. Comparing Eqs.(2) and (3), we can see that the pre-exponential factor is connected with the number density, hopping distance and attempt frequency of mobile ions [28,29]. Combining the pre-exponential factor with the number density by N ~ ρ and the hopping distance ξ, it can be concluded that the higher pre-exponential factor should be due
ð3Þ
Fig. 5. AC impedance spectroscopy of ITO/LATP/Al sandwich measured at different temperatures 25, 100, 200, 300 and 400 °C.
Fig. 6. Arrhenius plot of ionic conductivities of the Li–Al–Ti–P–O thin-film electrolytes deposited in 25, 100, 200 and 300 °C, respectively.
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Table 1 Ionic conductivity at 25 °C, pre-exponential factor and activation energy of thin films deposited at various temperatures. LATP film sample
σ (S cm− 1)
σ0 ± 0.01a × 103 (S cm− 1 K− 1)
Ea ± 0.1a(KJ/mol)
Deposited at 25 °C Deposited at 100 °C Deposited at 200 °C Deposited at 300 °C
0.34 × 10− 5 0.76 × 10− 5 1.32 × 10− 5 2.46 × 10− 5
0.67 × 103 1.93 × 103 2.33 × 103 4.65 × 103
31.1 31.7 30.9 31.1
a
Linear regression error for both Ea and σ0 is also indicated.
to the denser film structure and/or longer hopping distance. As confirmed by the above-mentioned characterization of film structure, with the increased substrate temperature, the thin-films become smoother and denser. Meanwhile, the film grows from column to round and surface roughness decreases, which lead to the decrease of ion migration barrier and the increase of hopping distance. However, when the substrate temperature increased to 400 °C, it can be clearly seen that the impedance spectra possess two semicircles in high- and medium-frequency region, which is due to the destruction of ITO films confirmed by the previous XRD pattern shown in Fig. 1. It is believed that SnO2 reacts with lithium ions according to the following two step process [30,31]: þ
−
SnO2 + 4Li + 4e →Sn + 2Li2 O þ
−
Sn + xLi + ne ↔Lix Sn; 0 ≦ x ≦ 4:4
ð4Þ ð5Þ
The irreversible process in Eq. (4) results in metallic tin dispersed in Li2O matrix, and then the alloying and de-alloying process of Li and Sn in Eq. (5) provides the reversible capacity of the material [32]. Accordingly, the electronic conductivity of ITO films declined which will affect the measured value of the ionic conductivity of the LATP film. Since the faradic reaction is determined by ion transfer and electron conduction, the increase in the resistance can be attributed to the reduced electronic conductivity of the ITO electrodes which crystallized and reacted with Li–Al–Ti–P–O films. Nevertheless, maybe there are other reasons about this phenomenon which need further investigation. 5. Conclusions The lithium aluminum titanium phosphate film electrolytes were prepared by radio-frequency (RF) magnetron sputtering using Li1.3Al0.3Ti1.7(PO4)3 target on ITO conductive glass substrate. Firstly, due to the crystallization of ITO films and its interaction with the electrolyte film simultaneity, the conductivity descended and the grain boundary conductivity came out when the sample was prepared at higher temperature of 400 °C. Results of electrochemical analysis are consistent with the morphological and structural properties for these samples. The films obtained have an excellent transparence in the visible range and have an amorphous structure. With the enhancement of substrate temperature, these films become denser, smoother and more uniform, which naturally lead to the increased ionic conductivity. The maximum ionic conductivity at room temperature obtained at the substrate temperature of 300 °C can come up to 2.46 × 10− 5 S cm− 1. All of the characteristics of these Li–Al–Ti–P–O thin films show that it is a potential candidate for the electrolyte used in all solid-state film lithium ion batteries, sensors and electrochemical devices. Acknowledgements The authors wish to acknowledge financial support by National Basic Research Program of China (2009CB939704), Natural Science Founda-
Fig. 7. Optical transmittance for LATP thin films deposited on slide glass substrates for 60 min, and at different temperatures 25, 100, 200, 300 and 400 °C.
tion of China (nos. 51032005 and 60808024) and the Fundamental Research Funds for the Central Universities (Wuhan University of Technology).
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