Titanium nitride thin film anode: chemical and

Electrochimica Acta 125 (2014) 282–287

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Titanium nitride thin film anode: chemical and microstructural evaluation during electrochemical studies K.H. Thulasi Raman a , Tirupathi Rao Penki b , N. Munichandraiah b , G. Mohan Rao a,∗ a b

Department of Instrumentation and Applied Physics, Indian Institute of Science, Bangalore- 560012, Karnataka, India Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore- 560012, Karnataka, India

a r t i c l e

i n f o

Article history: Received 5 November 2013 Received in revised form 1 January 2014 Accepted 17 January 2014 Available online 31 January 2014 Keywords: Li- ion battery thin film battery TiN anode electrochemical performance

a b s t r a c t TiN thin films with (200) fibre texture are deposited on Cu substrate at room temperature using reactive magnetron sputtering. They exhibit a discharge capacity of 172 ␮Ah cm−2 ␮m−1 (300 mAh g−1 ) in a non-aqueous electrolyte containing a Li salt. There is a graded decrease in discharge capacity when cycled between 0.01 and 3.0 V. Electron microscopy investigations indicate significant changes in surface morphology of the cycled TiN electrodes in comparison with the as deposited TiN films. From XPS depth profile analysis, it is inferred that Li intercalated TiN films consist of lithium compounds, hydroxyl groups, titanium sub oxides and TiN. Lithium diffusivity and reactivity decrease with increase in depth and the major reaction with lithium takes place at film surface and grain boundaries. © 2014 Elsevier Ltd. All rights reserved.

1. Introduction Dramatic evolution in micron and submicron scale fabrication technology has enabled increasingly miniaturized autonomous micro systems for applications ranging from distributed sensing and communication networks to implantable micro devices [1,2]. However, power sources such as thin film micro batteries (TFB) have not advanced paralellaly to enable their widespread adoption. One limitation to TFB technology is the requirement of stable electrode materials which need to be explored further. Li is widely used anode material in TFB technology because of its high specific capacity. However, high reactive nature of Li, safety and packaging issues prevent this material usage [3,4]. For application in TFB, an anode material should be deposited on a solid state electrolyte. Carbon allotropes (graphene, carbon nanotubes, and carbon nanostructures), transition metal oxides and Si are new emerging anode materials for such applications [5,6]. But the deposition of above materials on solid state electrolyte is restricted by the following reasons: high processing temperature, poor adhesion to substrates, poor electronic conductivity and volume expansion [7–9]. Due to the low intercalation and deintercalation potentials of metal nitrides with respect to Li metal, and good reversible capacities, there has been a surge of interest in developing metal nitrides such as Sn3 N4 , Zn3 N4 ,Zn3 N2 , Cu3 N, Ge3 N4 , Fe3 N, Co3 N, Ni3 N, CrN and VN as a class of attractive lithium-ion storage

∗ Corresponding author. Tel.: +91 80 22932349; fax: +91 80 23600135. E-mail addresses: [email protected], [email protected] (G.M. Rao). 0013-4686/$ – see front matter © 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2014.01.104

materials for rechargeable lithium batteries [10–18]. Advantages with TiN among all transition metal nitrides are good electrical conductivity, very good adhesion with most of the materials, good diffusion barrier and matured low cost processing technology. TiN has been used in microelectronics, electrodes for semiconducting devices, catalytic materials, fuel cells, hard and wear resistance coatings [19–25]. In recent reports TiN has been used as diffusion barrier in TFB fabrication and it was shown that reactivity of Li with TiN is quite low [26]. In recent years, bulk TiN material is being used as a conducting media for anode materials. However, bulk TiN has poor electrochemical performance [27,28]. Oxidation and reduction process during intercalation and de-intercalation are not well understood in TiN. In this paper, we report TiN deposition using reactive magnetron sputtering and its physical and electrochemical properties while using as an anode material for thin film Li-ion battery. 2. Experimental TiN films were deposited by reactive magnetron sputtering on copper and fused quartz substrates. The target to substrate distance was kept constant at 100 mm. Metallic Ti was sputtered from pure Ti (99.99%) target of 75 mm diameter in the presence of Nitrogen (N2 ) of purity 99.999% and Argon (Ar) of purity 99.999%. The base pressure of the sputtering chamber was maintained at 4.0x10−6 mbar using turbo molecular pump and rotary pump. The target was pre-sputtered for 10 min to remove the surface contamination. The coatings were deposited at N2 partial pressure of 2x10−4 mbar. During deposition, the total pressure (Ar+N2 ) was maintained at

K.H.T. Raman et al. / Electrochimica Acta 125 (2014) 282–287

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Electrochemical cells were assembled in home-made Swagelok type cell containers made of Teflon. The cells were assembled in an Ar atmosphere glove box (MBraun model UNILAB). Lithium metal was used as the counter cum reference electrode, Cu side of TiN coated Cu disk contacted to stainless steel rod as the current collector. An adsorbed glass mat (AGM) was used as the separator between the TiN and Li foil. The separator was soaked in electrolyte of 1 M LiPF6 in ethylene carbonate, diethyl carbonate and dimethyl carbonate (2:1:2 v/v %). Cyclic voltammetry, galvanostatic charge discharge and rate capability studies were performed in the potential between 0.01 and 3.0 V. All electrochemical experiments were carried out using a Biologic potentiostat/galvanostat model VMP3. 3. Results and discussion

Fig. 1. X-ray diffraction spectra of TiN as deposited films on Cu substrate.

3.0x10−3 mbar. The substrates were maintained at room temperature and the target power density was optimized at 3.62 Wcm−2 . The deposited film thickness was kept constant at 500 nm, measured using Veeco Dektak 150 stylus profilometer, so as to avoid the effect of thickness on the physical properties of the films. Crystalline nature of the films was studied by X-ray diffraction (XRD) using an advanced SmartLab (Rigaku) diffractometer with Cu-K˛1 radiation. The microstructure of the films was analyzed with the aid of a field emission scanning electron microscope (Ultra 55, Carl Zeiss, GmbH). Surface chemistry of the as deposited TiN film and electrochemically experimented samples was analyzed using Xray photoelectron spectroscopy (XPS) (SPECS GmbH spectrometer with Phobios 100MCD Energy analyzer) depth profiling facility.

The as deposited films appeared with lustrous golden yellow colour. XRD results show the crystalline nature of TiN with (200) texture and it is shown in Fig. 1. The (200) planes have low surface energy and the energetics during the growth are completely dominated by surface and interface minimization energy. Fig. 2 (a) and (b) are the surface morphology and cross-sectional image of TiN film respectively and they show square edge shaped dense grains with non-equiaxed columnar structure. Since films are of nonequiaxed structure, the grain size increases with film thickness and grain boundary columnar gap is of the order of 0.2-0.3 nm. The average grain size is 10-20 nm. Surface chemistry of TiN films i.e., XPS results reveal that the films contain 4 at % of oxygen bonded as Titanium oxynitride in the films (discussed later and shown in Fig. 11). For the XRD studies, the peak positions are compared with standard data ICSD CC No. 26947, and there are no additional peaks corresponding to oxynitride and it can be concluded that oxygen is substituted in some of the nitrogen sites. Substitutional shifts cannot be measured in XRD because; measurements may

Fig. 2. SEM images shows, a) as deposited TiN film surface morphology, b) as deposited TiN film cross section and c) surface morphology after 190 cycles.

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Fig. 3. Cyclic voltammogram of TiN thin films.

be perturbed by intrinsic stresses generated during the film deposition are common in sputter deposited films. In the case of as deposited films there is no carbon, except for atmospheric carbon which was etched out before acquiring XPS data. Cyclic voltammograms of TiN thin films are shown in Fig. 3 for repeated cycles. In the first negative sweep form 3.00 to 0.01 V the current peaks are present at 1.40 and 0.87 V. These cathodic peaks could be related to different reduction reactions between TiN and Li. In the positive sweep, oxidation peeks are observed at 1.35 and 1.90 V. In subsequent dominant cycles the cathodic peak shifted to 0.65 V and the dominant anodic peak to 1.20 V. There is a gradual decrease in peak current of both anodic and cathodic peaks on repeated cycling. Results of the galvanostatic charge/discharge experiments studied between 0.01 and 3.00 V at 25 ␮A current are shown in Fig. 4(a). The plateaus are clearly observed corresponding to the peak potential regions of cyclic voltammograms. The discharge specific capacity of TiN was calculated by using the following Eq. Specific capacity =

It Ahf

(1)

where I is charge-discharge current, t is the discharge time, A is area and hf is the thickness of the film. The first discharge capacity was 172 ␮Ah cm−2 ␮m−1 and in the 2nd cycle it decreased to 148 ␮Ah cm−2 ␮m−1 . There is a graded decrease in capacity thereafter and reaches to 118 ␮Ah cm−2 ␮m−1 at 50th cycle. The capacity reduced to 85 ␮Ah cm−2 ␮m−1 by 180th cycle (not shown). Fig. 2(c) shows surface morphology of the TiN film after completing 180 cycles. The reduction in capacity could be due to changes in electrode surface morphology. Reproducibility of TiN deposition process and electrochemical performance was tested by depositing several TiN films and assembling electrochemical cells. It was found that there was no significant change in cyclic voltammetry and charge/discharge cycling performance from cell to cell. In order to check the rate capability, a TiN thin film electrode was subjected to charge-discharge cycling at several current densities in the range of 25-1000 ␮Ah cm−2 and the results are shown in Fig. 5(a) and (b). On increasing the current density from 25 to 1000 ␮Ah cm−2 , the discharge specific capacity decreases from 172 to 90 ␮Ah cm−2 ␮m−1 . Although the decrease in discharge capacity with an increasing current is as expected, the capacity of 90 ␮Ah cm−2 ␮m−1 at 1 mA is considered as high rate capability for a thin film electrode. Significant changes in morphology and colour were observed after 200 charge-discharge cycles. The colour changed from golden yellow colour to maroon. To understand the surface chemistry during reduction reaction, samples with two different states of charge (charging voltages were 0.05 and 0.1 V and charged cells are held

Fig. 4. a) Representative galvanostatic charge/discharge curves between 0.01 and 3.00 V at 25 ␮A current for different cycle numbers and b) Variation of specific capacity with cycle number.

for 24 hours) were subjected to ex-situ XPS depth profile analysis and the corresponding charged state SEM surface morphology is show in Fig. 6(a). TiN anode was removed from Swagelok assembly, rinsed in ethanol solution and vacuum dried before transferring to XPS chamber. Depth profiling analysis was done for 280 minutes using ion-beam sputtering technique (with the operating parameters of 45 ␮A ion current and 2 kV accelerating voltage) and the etched surface is shown in Fig. 6(b). In all XPS measurements the Li1s core level peak at 56.1 eV merges with lower binding energy shoulder of Ti3s peak, which prevented quantification and core level analysis of Li1s in intercalated TiN films and hence we have shown variation of Li peak intensity with depth. Variation of C, F, N, O, Ti and N atomic percentage with depth is tabulated in Table 1 and shown in Fig. 7. C1s, F1s, N1s, Ti2p and O1s core level peaks have been taken for core level spectra de-convolution. Fig. 8 shows XPS survey spectra of as intercalated surface and it shows the presence of O, Ti, C, F and N. De-convolution of O1s, Ti2p and N1s core level peaks reveals the existence of TiN, Ti-O-N, TiO2 , LiF, hydroxyl groups, and other sub oxides of Ti (Ti-O and Ti2 O3 ) (Figs. 9-11) Fig. 8 shows XPS survey spectra after 10 min etch, indicating the reduction in carbon peak intensity and de-convoluted O1s peak contains two peaks, one peak at 532.8 eV that belongs to hydroxyl bond and second peak at 530 eV corresponding to TiO2 . Fig. 10 shows O1s core level spectra after 30 min etch, de-convoluted O1s peak intensity at 530 eV increased and 532.8 eV peak intensity remains


285

Table 1 Variation of Ti, N, O, C and F at different stages of etching. Etch time(mints)

As deposited 0 10 20 30 40 52 66 78 90 110 200 230 280

Elemental composition (at %) Ti

N

0

C

F

44 3 13 18 20 20 24 27 31 31 33 44 43 46

52 5 15 16 20 24 26 28 30 35 37 41 47 44

4 44 59 51 47 46 37 36 33 30 25 14 10 10

0 42 1 3 4 1 4 3 0 0 0 0 0 0

0 6 11 10 10 8 7 6 6 4 5 0 0 0

Fig. 5. a) Representative galvanostatic charge/discharge curves between 0.01 and 3.00 V at different current densities and b) Variation of specific capacity with current density and cycle number.

unchanged. As mentioned earlier, the merging of Li 1s peak with Ti 3s prevents quantification and identification of lithium chemistry is difficult. Fig. 12 shows presence of Li and variation in Li with depth. After 280 minutes of etching the peaks related to F1s, C1s and Li1s almost disappeared and O1s atomic percentage was found to be 10%. The same oxygen content was maintained with further increase in depth and the de-convoluted O1s core level peaks show presence of Ti sub oxides and hydroxyl groups as shown in Fig. 10. Thus Li diffusion and reactivity with TiN decreases with depth and as a result entire TiN is not utilized in this reaction. Sun et al. first reported reaction mechanisms in CrN and VN and electrochemical performance in CrN and VN. It was reported that during lithiation total film converts into Cr and Li3 N and during delithiation again it

Fig. 7. Variation of Ti, N, O, C and F composition with depth.

forms CrN [17,18]. In our case, the Ti metal as such was not observed and it suggests that there could be possible conversion of Ti metal into sub oxides, because it is exposed to ambient atmosphere while taking from Swagelok to XPS chamber and during the depth profiling Ti may be oxidized because of its high reactive nature. Based on the above XPS analysis and cyclic voltammetry study it is suggested that, the reaction mechanism during lithiation/de-lithiation process could be similar to that of CrN, VN [17,18]. Lithium peak intensity in XPS spectra vanishes with decrease in etch depth, but

Fig. 6. TiN thin film morphology a) after 0.01 V charged state and b) surface morphology after depth profiling (after 280 minutes etching).

286


Fig. 10. XPS O1s core level spectra of as deposited and galvanostatic charged TiN film at different stages of etching.

Fig. 8. XPS survey spectra of as deposited and galvanostatic charged TiN film at different stages of etching.

there is no decrease in Ti sub oxide bond density. At higher depths, the surface analysis predominantly shows the presence of TiN and Ti sub oxides and at higher depths lithium presence is uncertain because of its lower scatter cross section of core level (Li 1s). Above results suggest that at higher depth lithium reactivity is not completely vanished but it is decreased. Because of diffusion barrier nature of TiN, access to lithium reactivity is limited to surface, grain boundaries and it can be enhanced by making higher surface area microstructure. Based on above depth profile XPS analysis, it is proposed that Li reacting surface in TiN films microstructure is surface and grain boundary volume (grain boundary gaps and it would be in the order of 0.2 nm) during Li intercalation and de-intercalation in TiN films and same mechanism was observed in TiN high temperature O2 oxidation[29,30]. The schematic of above mechanism is shown in Fig. 13.

Fig. 9. XPS Ti3s core level spectra of as deposited and galvanostatic charged TiN film at different stages of etching.

Fig. 11. XPS N1s core level spectra of as deposited and galvanostatic charged TiN film at different stages of etching.

Based on this analysis it could be concluded that TiN film microstructure plays an important role in its electrochemical performance. Dense nature of the film prevents Li diffusivity and reactivity with TiN and this problem would be addressed by making film more porous, increasing the shadowing effects during the growth and it can be achieved by implementing Glancing Angle deposition (GLAD) technique. During half cell characterization, front surface of as deposited thin film is subjected to electrochemical performance and it gives fundamental idea about the reaction

Fig. 12. XPS Ti3s+Li1s core level spectra of as deposited and galvanostatic charged TiN film at different stages of etching.


287

volume. Depth profile XPS measurement infers that Lithium diffusion and reactivity decreases gradually with thickness. Acknowledgements This work is supported by the Ministry of Communication and Information Technology, Govt. of India under a grant for the Centre of Excellence in Nano electronics, Phase II. References

Fig. 13. Schematic representation of as deposited, lithiated microstructre and during different stages of etching: a) As deposited TiN thin film on Cu substrate with columnar structure, b) After lithiation and lithiated sites in TiN microstructure, c) & d) Different stages of etching, e) After 280 mints etching shows absence of lithiation in structure.

mechanism of materials and other parameters. But in real application, the anode films are deposited on solid state electrolytes and hence instead of the front surface the rear surface of the film will be subjected to electrochemical performance. One could see that the rear surface morphology and columnar structure is entirely different from front surface as shown in Fig. 2(b). According to structure zone model, films deposited at low homologous temperature will have rear surface always more porous and the front surface will be a fine columnar structure [31]. Since TiN films are deposited at low homologous temperature it would be more advantageous micro structure for enhanced diffusivity and reactivity. 4. Conclusions In summary, TiN films are deposited using reactive magnetron sputtering at room temperature. The as deposited films are of single phase TiN with non-equiaxed columnar microstructure. Galvanostatic charge/discharge, cyclic voltammogram and cyclability performance at different currents show that TiN would be a promising material for long stable life of lithium ion batteries. Exsitu XPS depth profile measurements suggest the formation of Ti during intercalation and it forms sub oxides of Ti. Lithium reactive sites in TiN microstructure are film surface and grain boundary

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