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Electrochemical reaction of lithium with ruthenium nitride thin films prepared by pulsed-DC magnetron sputtering
Electrochimica Acta 164 (2015) 12–20
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Electrochemical reaction of lithium with ruthenium nitride thin films prepared by pulsed-DC magnetron sputtering Barbara Laïk a, * , Samantha Bourg a , Jean-Pierre Pereira-Ramos a , Stéphanie Bruyère b , Jean-François Pierson b a b
Institut de Chimie et des Matériaux Paris-Est GESMAT, UMR 7182CNRS-Université Paris Est-Créteil, 2 rue Henri Dunant, 94320 Thiais, France Institut Jean Lamour, Département CP2S, UMR 7198CNRS-Université de Lorraine, Parc de Saurupt, CS 50840, 54011 Nancy cedex, France
A R T I C L E I N F O
A B S T R A C T
Article history: Received 11 December 2014 Received in revised form 18 February 2015 Accepted 19 February 2015 Available online 21 February 2015
Ruthenium nitride thin films with ZnS type cubic structure are successfully and reproducibly deposited by pulsed-DC magnetron sputtering. Their electrochemical behavior as negative electrode for Li-ion microbatteries is investigated by cyclic voltammetry and galvanostatic experiments. The influence of the film thickness and the rate capability are reported and discussed. A specific capacity of about 400 mA h g1 is proved to be stable at C/2 over 20 cycles at a mean voltage of 0.25 V vs. Li+/Li and attractive specific capacities varying from 100 to 300 mA h cm2 can be reached depending on the thickness in the range 250 nm–850 nm. XRD, SEM, HRTEM, electron diffraction and EELS are used to understand the electrochemical mechanism of RuN with lithium. Electrochemical properties and structural characterization support the occurrence of a reversible conversion reaction of RuN into Ru metal and Li3N. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: Ruthenium nitride Lithium-ion microbatteries Anode material Conversion reaction Pulsed-DC magnetron sputtering
1. Introduction Today, a constant increase in global energy demand is observed. In this context, rechargeable batteries, and more especially Lithium-ion batteries (LIBs), are increasingly attractive and many researches are devoted to the improvement of such lithium-ion based electrochemical systems. In the same time, a variety of applications requiring power microsources has been developed including medical implants, remote sensors, smart cards or other portable electronic devices. To satisfy these requirements, lithiumion thin film micro batteries (TFBs) are emerging. The quest of higher energy densities concomitantly with the necessity of stable electrode materials is become a challenge. Therefore an intensive research devoted to high performance thin film materials, mainly transition metal oxides, as positive electrode is made [1–3]. Nowadays, lithium metal is widely used as anode material due to its high specific capacity. However because of its reactivity, safety and packaging issues exist and lead to the study and development of alternative anode materials that can react reversibly with lithium [1,4–8] operating either by alloying or conversion. Among these materials, the binary metal nitrides MxNy are promising candidates due to their high specific capacity in the range of 300–1000 mA h g1, a high melting point and a high chemical stability. They have been demonstrated to
* Corresponding author. http://dx.doi.org/10.1016/j.electacta.2015.02.171 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
electrochemically react through conversion reactions [1,9]. Sn3N4 and Zn3N2 thin films have been proposed as negative electrode in TFBs in the early 2000s [1]. After these pioneer works, a large variety of binary metallic nitrides has been screened, including Zn3N2 [9], Cu3N [10], Ge3N4 [11], Co3N [12], Fe3N [12], Ni3N [13], CrN [14], VN [15], CoN [16], Mn3N2 [17], MoN [18], TiN [19] . . . Most of them have been prepared as thin films, 100–500 nm thick, mainly by RF sputtering [14–17,19] but also by pulsed laser deposition [12,13] or atomic layer deposition [18]. In these studies, the conversion reaction with lithium has been examined. It has been reported to be a two-step process. The reduction process yields irreversibly to the formation of metallic nanoparticles embedded in a Li3N matrix. The subsequent charge process depicted in Eq. (1), where M = transition metal in the MxNy nitride, provides the reversible capacity. x M + y Li3N ! MxNy + 3y e + 3y Li+ (1) The mechanism is similar to that observed with metal oxides but these nitride materials present a main advantage over them: Li3N produced by the conversion reaction is an excellent ionic conductor, better than Li2O, thereby satisfies one of the essential criteria for a desirable anode. According to this mechanism, as the first irreversible reaction involves the full reduction of the transition metal into its metallic state, remarkable high capacity values are delivered, particularly for nitrides with high chemical valence such CrN, VN and CoN that exhibit capacity above 1000 mA h g1 when cycled between 0.01 and 3.5 V vs Li+/Li.
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Ruthenium nitride RuN is one of these compounds with valence (+III) but has never been studied in detail as negative electrode for Li-ion batteries. Its theoretical specific capacity reaches 700 mA h g1 considering 3 lithium mol involved per mol of ruthenium. Previous works have shown that NaCl-like RuN can be synthesized by pulsed laser ablation [20] while RuN with ZnS-like structure can be deposited by RF-magnetron sputtering [21,22]. Only one recent study [23] reports the deposition of stoichiometric ZnS-like RuN by pulsed-DC magnetron sputtering and that these ruthenium nitride thin films can be regarded as possible anode material in lithiumion microbatteries since high capacity value close to the theoretical one was achieved at C/2 rate [23]. The present paper aims at investigating the electrochemical properties of RuN thin films and at providing more information on the discharge/charge processes involved during reaction of RuN with lithium. Electrochemical properties are investigated by cyclic voltammetry (CV) and galvanostatic cycling using a half-cell configuration versus lithium and discussed in relation with structural evolutions evidenced by X-ray diffraction (XRD), scanning electronic microscopy (SEM), transmission electronic microscopy (TEM), electron diffraction and electron energy loss spectroscopy (EELS). The cyclability, the rate capability and the influence of the thin film thickness on electrochemical performances are also evaluated. 2. Experimental section Ruthenium nitride thin films were deposited on steel substrate by pulsed-DC magnetron sputtering of a metallic ruthenium target (99.99% purity, 50 mm diameter and 3 mm thick). The 40-L sputtering chamber was pumped down via a mechanical pump and a turbomolecular one allowing a base vacuum of 104 Pa. Prior to their introduction in the deposition chamber, the substrates were ultrasonically cleaned in ethanol. The distance between the substrates and the target was 50 mm. The target was powered by an Advanced Energy Pinnacle+ 5 kW pulsed-DC supply. The discharge frequency and the off-time were fixed at 50 kHz and at 4 ms, respectively. The current applied to the target was 0.1 A. Due to the low reactivity of sputtered ruthenium atoms with nitrogen ones, RuN films was successfully synthesized in pure nitrogen atmosphere [23]. The total pressure during the film deposition was fixed to 1 Pa and no intentional heating was applied during the film growth (deposition temperature about 50 C). The thickness of the RuN films measured by tactile profilometry was controlled by adjusting the deposition duration. Within our deposition conditions, the film growth rate was about 250 nm h1 and the relative uncertainty on films thickness about 3%. Electrochemical measurements were carried out in twoelectrode coin cells (CR2032 type). The deposited films of RuN on stainless steel spacers (1 mm thick, 16 mm diameter) were used as working electrode. The weight of active material was determined by difference weighing before and after deposition. The absolute error on the RuN film weight was 0.01 mg. Lithium sheets (0.75 mm thick, 16 mm diameter, 99.9%, Alfa Aesar) were used as both reference and counter electrodes. Whatman (GF/C) glass fiber separators were saturated with 1 mol L1 LiPF6 in a nonaqueous solution of ethylene carbonate (EC) and dimethyl carbonate (DMC) with a volume ratio 1:1 (Merck). The cells were sealed in an Arfilled glovebox (O2 and H2O contents less than 0.1 ppm). Voltamperommetry and galvanostatic charge/discharge cycles were performed in an air-conditionned room (T = 20.0 0.1 C) with either a Solartron (AMETEK) or VMP3 (Bio-logic, France) galvanostat/potentiostat. For galvanostatic measurements, the charge/discharge rate is calculated based on the transfer of one mol of electron per mol of RuN per hour, consequently 1 C rate matches a constant current of 0.23 A g1.
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For ex-situ measurements, after galvanostatic cycles stopped at different potential values, coin cells were dismantled in the glove box to avoid exposure to oxygen and water. The more or less lithiated RuN electrodes are rinsed in anhydrous DMC to remove residual traces of electrolyte. The surface morphology of the RuN electrode was studied by scanning electron microscopy (SEM) using a Zeiss Merlin. The film structure was studied by X-ray diffraction (XRD) in u/2u mode using a Brucker D8 Advance diffractometer (Cu Ka1 radiation, l = 0.154056 nm) and in grazing incidence (1 ) using a Brucker D8 Discover diffractometer (Co Ka radiation, l = 0.179026 nm). For transmission electron microscopy (TEM) characterization, samples were prepared by scratching off the surface with a holey carbon copper grid. The experiments were performed using Philips CM200 and JEOL ARM 200 F microscopes with 200 kV acceleration voltage. The crystallographic structure was studied using the microdiffraction mode that allowed focusing the electron beam in a 50 nm diameter zone. Electron energy loss spectroscopy (EELS) experiments were carried out using the JEOL ARM 200 F in scanning transmission electron microscopy (STEM) mode with a GIF Quantum post-column energy filter from GATAN and the zero loss peak resolution was 0.8 eV. 3. Results and discussion 3.1. Structural characterization of deposited ruthenium nitride thin films The diffractogram of a RuN film deposited on a steel substrate is presented in Fig. 1. In addition to the a/g biphased steel substrate peaks at 43.6, 44.5 and 50.6 , one diffraction peak close to 34.6 is evidenced. It has been assigned to the (111) plan of the RuN phase that crystallizes in a ZnS-like structure with lattice constant of approximately 0.451 nm [23]. Depending on the deposition conditions, Cattaruzza et al. have estimated the lattice constant of RF-sputtered RuN films at 0.450 or 0.453 nm [21]. Since only one diffraction peak of RuN is evidenced in Fig. 1, this film grows with a strong preferred orientation in the [111] direction as confirmed by pole figure measurements (not shown here). However, there is no in plane orientation indicating that the films grow with a fiber texture along the [111] direction. As commonly observed for sputtered nitride coatings [24], RuN films grow with a columnar microstructure (Fig. 2a). At the top surface of the 450 nm-thick film, the column width is about 40–60 nm. The electronic diffraction pattern of one column obtained in microdiffraction mode is presented in Fig. 2b. It has been indexed considering the cubic RuN phase with a [–11 2] zone axis. Since diffraction spots are evidenced in Fig. 2b, one column can be considered as a single crystal.
Fig. 1. X-ray diffractogram (Cu Ka) of a 450 nm-thick RuN film deposited on steel substrate.
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Fig. 2. (a) TEM bright field image of a RuN thin film, (b) electronic diffraction pattern of the area marked by the white circle in Fig. 2a.
3.2. Electrochemical properties 3.2.1. Cyclic voltammetry and galvanostatic cycling experiments The lithium storage capacities and cycle performances of RuN 250 nm-thick thin films were investigated by discharge/charge measurements in Li/RuN cells. Fig. 3 shows the first two cyclic voltammograms recorded between 0.02 and 3 V measured at 50 mV s1. There are three main cathodic peaks at 0.64, 0.46 and 0.22 V in the initial reduction process and three main steps at 0.57, 0.81 and 1.40 V in the consecutive oxidation process. These multipeaks imply that the electrochemical behavior of RuN versus lithium is a complicated process involving a series of insertion/conversion and decomposition reactions. The coulombic charge recovered for the oxidation process is much lower than that involved in reduction because the reduction reaction includes the SEI (Solid Electrolyte Interphase) formation as expected in this voltage range. This explanation is confirmed by the better rechargeability found during further cycles. Indeed, the two main cathodic peaks of cycle 1 at 0.64 and 0.46 V have disappeared in cycle 2 to the benefit of less intense and broad peaks at 1.50 and 1.03 V followed by a sharp peak around 0.25 V. This finding is consistent with the occurrence of an irreversible process taking place during the first cycle and described as a conversion reaction as reported in the literature for other nitride compounds [12,17].
Fig. 3. Cyclic voltammograms of 250 nm-thick thin film measured at 50 mV s1 between 0.02 and 3 V (first cycle: full red line; second cycle: dashed blue line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Conversely the shape of the anodic voltammogram is maintained with peaks located at 0.81, 1.24 and 1.59 V. The charge/discharge profiles recorded at a moderate rate C/2 (117 mA g1) in the 0.02–3 V voltage window are presented in Fig. 4. It represents the potential evolution as a function of both the weight capacity on the bottom scale (Q) and the number of mol of lithium transferred per mol of ruthenium on the upper scale (x). The maximum theoretical specific capacity, 700 mA h g1, corresponds to x = 3. After a rapid potential drop entailing the consumption of 0.3 mol of lithium per mol of RuN, the first cycle presents a well-defined discharge plateau at 0.65 V that disappears in the second and following cycles. It corresponds to the main peak observed on the first cyclic voltammogram and discussed above. At the end of the voltage plateau, the capacity is about 300 m Ah g1 which corresponds to the consumption of 1.3 mol of lithium per mol of RuN. After this, a gradual potential decrease appears up to the deep discharge limit of 0.02 V. The total first discharge specific capacity is 940 mA h g1, about 240 mA h g1 higher than the theoretical specific capacity of RuN. This is probably due to the SEI formation all along the process [17]. During the charge process, the curve presents several inflexions that may rely on the different peaks observed on the
Fig. 4. Galvanostatic discharge/charge curves of 250 nm-thick thin film measured at C/2 (117 mA g1) between 0.02 and 3 V (first cycle: full red line; second cycle: dashed blue line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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cyclic voltammogram (Fig. 3, full red curve). The overall first charge capacity is about 570 mA h g1, 130 mA h g1 lower than the theoretical specific capacity. This result supports the faradaic yield in the reduction reaction includes the SEI formation. However the fact that a small part of RuN particles do not react cannot be discarded. The second discharge profile is different from the first one, confirming the starting material undergoes an irreversible and deep change after one cycle. The main plateau at 0.65 V and the less pronounced one at around 0.50 V have disappeared to give way to a signal at 0.25 V. It is consistent with the peak on the dashed blue line of voltammetric curves (Fig. 3). Conversely, the second charge profile completely superimposes with the first one. That means that, the same processes and chemical species are involved during the oxidation whatever the cycle number.
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and the charge curve shape are kept. After 10 cycles, the charge/ discharge profile does not evolve anymore (cycles 20–50 and 70 in Fig. 5b). This behavior has been called “formatting” of the electrode and corresponds to the structuring of the electrode before stability is attained as reported in the case of CoN nanoflakes [16].
3.2.2. Cyclability Cyclability of a 250 nm-thick sample is presented at C/2 rate in Fig. 5a. During the first ten cycles, the capacity rapidly decreases from 940 to 400 mA h g1. Thereafter the capacity value practically stabilizes with cycles. The capacity is about 330 mA h g1 after more than 70 cycles. Corresponding cycles are presented in Fig. 5b. Though decreasing capacities, the shape of the first ten cycles is the same. The step at 0.25 V during the reduction is conserved but shortened, whereas the shoulder noticeable at 0.8 V in reduction
3.2.3. Influence of the thickness As it appears possible to consider ruthenium nitride as negative electrode material for TFBs, the increase of the available capacity is a challenge. Indeed, for a 250 nm thick thin film, when stability is achieved during cycling, a capacity of around 330 mA h g1 44 mA h cm2 can be stored (Fig. 5). Therefore several samples were electrochemically tested with increasing thicknesses and compared. The first and second discharge–charge profiles are presented in Fig. 6a for 250, 450, 650 and 850 nm thick thin films cycled in the 0.02–3 V potential range at C/2. Regardless the sample’s thickness, the electrochemical signal is the same: the long flat plateau observed during the first discharging step is absent during the second one and the charge profile does not change at all. The effective capacities expressed both in mAh and mAh cm2 are reported too (Fig. 6b). A linear relationship is observed between capacities and thicknesses. For instance, the discharge capacity increases from 90 mA h cm2 with 250 nm-thick film to 300 mA h cm2 for 850 nm one, these values comparing very well with the best values reported for transition metal oxide thin films [3].
Fig 5. (a) Capacity versus cycle number for a 250 nm-thick thin film measured at C/2 (117 mA g1) between 0.02 and 3 V. (b) Evolution of discharge/charge profiles with cycles.
Fig 6. (a) First (full lines) and second (dashed lines) discharge/charge cycles at C/2 (117 mA g1) of 250, 450, 650 and 850 nm-thick thin films in the 0.02–3 V potential range. (b) Corresponding discharge/charge capacities versus film thickness.
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Besides, this result is very interesting as it demonstrates that the deposited material homogeneously participates to the electrochemical processes, even for the thickest samples, without limitation by lithium diffusion or electronic transfer. The higher slope obtained during the first discharge is related to the SEI formation reaction mentioned before. The influence of the film thickness on the cycle life of the electrode is presented in Fig. 7. The evolution of the capacity up to cycle 20 is presented for the four samples. Whatever the thickness, two domains can be distinguished: in the first one, the capacity rapidly decreases up to cycle 10 (corresponding to the formatting step mentioned above) whereas in the second one, some stability is achieved. Besides, the 850 nm thick sample exhibits a more rapid decrease in capacity than the other ones probably due to the fact that all particles do not work homogeneously upon further cycling, leading to lack of adhesion from the steel substrate. The influence of the film thickness has never been addressed for other binary nitrides, the thickness being usually limited to the 200–400 nm range [14–16,18].
3.2.4. Rate capability The influence of the current density has been also investigated. Results obtained for different C rates are presented for a 450 nm thick sample (Fig. 8a). In order to take into account the previous statement that about ten cycles are required before capacity stabilization, the first ten cycles wereperformed at C/2 to ensure the stabilization of the capacity. After its stabilization about 390 mA h g1, different current densities were successively applied. It can be seen that a lower rate (C/5) entails an increase of capacity up to 500 mA h g1 (Fig. 8) whereas higher cycling rates lead to lower but satisfactory values of 320 and 260 mA h g1 for 2 C and 5 C, respectively. These results highlight a good rate capability as high capacities are still available at high rates. The subsequent return to cycling at C/2 allows to recover the stable capacity (400 mA h g1) previously achieved at the same rate (Fig. 8b) suggesting the electrode material is not damaged at high rates. 3.3. SEM observations The SEM images of the as-deposited, the first discharged to 0.02 V and the first full charged to 3 V samples at C/2 are presented in Fig. 9. The as-deposited (a) thin film presents an uniform smooth surface with particles of about a few tens nanometers. The surface of the discharged film (b) is homogeneous too, but the morphology is slightly modified. Particles appear more divided than in the
Fig. 7. (a) Capacity versus cycle number for 250 nm (red), 450 nm (blue), 650 nm (green) and 850 nm (black)-thick thin films measured at C/2 (117 mA g1) between 0.02 and 3 V (b) corresponding 10th (full lines) and 20th (dashed lines) galvanostatic discharge/charge cycles. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 8. (a) Discharge capacity versus cycle number for a 450 nm-thick thin film measured successively at C/2 (117 mA g1), C/5, 2C, 5 C and C/2 between 0.02 and 3 V. (b) Corresponding discharge/charge profiles.
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Fig. 9. Surface morphology of RuN thin film electrodes as-deposited (a), discharged to 20 mV (b) and charged to 3 V at the end of the first cycle (c) and at the end of the second cycle (d).
starting film. At the end of the first cycle (c), when the sample has been oxidized up to 3 V, the surface morphology is strongly modified. Some plates are uniformly dispersed at the surface. This supports the change in the discharge profiles of the first two voltamperommetric and galvanostatic cycles reported in Figs. 3 and 4. Fig. 9d confirms that the new morphology is maintained after an additional cycle. 3.4. Ex-situ XRD, TEM and SAED observations XRD spectra of partially and fully discharged RuN thin-film electrodes are gathered in Fig. 10. Dark dashed lines are related to the diffraction peaks of the a/g biphased steel substrate at approximately 43.6, 44.5 and 50.6 (Fig. 10a). In addition to the substrate peaks, only one diffraction peak ascribed to ruthenium nitride RuN (111) is observed at 34.5 . Its intensity progressively decreases during the first discharge. This indicates that RuN is progressively transformed into other species, probably amorphous or nano-sized. For the fully discharged sample, more relevant information about the film structure is obtained by XRD in grazing incidence (1 ) (Fig. 10b). Broad peaks detected at 45.0 and 51.5 fit well with the (1 0 0) and (1 0 1) diffraction peaks of metallic ruthenium (JCPDS # 00-006-0663). This result indicates that metallic ruthenium is present in the fully lithiated sample and confirms the nanocrystalline size of the electrochemically formed Ru particles. A further examination of this sample was carried out using TEM and SAED. The lithiated RuN sample still exhibits a columnar microstructure (Fig. 11a). However, the single crystal character of the column is not observed after the first discharge anymore (Fig. 11b–c). The dark field image (Fig. 11b) clearly evidences that the columns are composed of nanograins with a mean grain size of approx. 2–3 nm.
The selected area electron diffraction pattern presented in Fig. 11c has been assigned to diffraction rings of metallic ruthenium. Li3N was not observed in SAED pattern probably due to poorly crystallized particles and to the lower atomic scattering factors of lithium and nitrogen compared to that of ruthenium metal. The TEM results well agree with those previously obtained by XRD. The strong decrease of the intensity of the (111) diffraction peak of RuN when x = 1 can be interpreted by the formation of nanograins of Ru into the whole thickness of RuN films. Complementary measurements were carried out on a delithiated sample. The TEM and SAED image of the thin film electrode after charging to 3 V are shown in Fig 12. The nanocrystalline character of the sample is clearly evidenced on the bright and dark field images presented on Fig. 12a and 12b. From the corresponding SAED pattern (Fig. 12c), it is not easy to clearly determine the crystalline nature of the delithiated sample. In order to obtain more detailed information about the structure of delithiated samples, EELS analyses were carried out. Two 50 nm-thick films of metallic Ru and RuN were used as references and Ru M edges and N K edge were recorded. TEM used in scanning mode (STEM) confirmed the nanocrystalline microstructure of the delithiated sample (Fig. 13a). Grains of about 2–4 nm diameter are clearly evidenced on this high resolution image. Two grains marked A and B have been characterized by EELS and the related spectra have been compared to those of the Ru and RuN reference samples (Fig. 13b). The EELS spectrum of the grain marked A does not show the N K edge while the Ru M ones are clearly evidenced. On the other hand, both Ru and N elements are detected by EELS on the grain marked B. The characterization of a larger zone evidenced that the delithiated sample is mostly composed of RuN nanograins with some Ru nanograins. Thus, the oxidation of the Ru electrode into RuN is not quantitative and electroformed nanosized Ru grains
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disappearance of the diffraction peak of RuN (111) (Fig 10). In the light of the significant intensity decrease of the RuN (111) diffraction peak at x = 1, a similar step to that proposed by Q. Sun and by B. Das respectively for CrN [14], VN [15] and CoN [16] can be suggested. It consists in the destruction/amorphisation of the crystal structure and in the formation of an intermediate state as presented in Eq. (2). This process involves one mol of lithium per mol of RuN and is characterized by the flat plateau observed at 0.65 V in the first galvanostatic cycle (Fig 4): RuN + e + Li+ ! LiRuN (2) Characterization of a RuN thin film electrode lithiated to 20 mV by grazing angle XRD and SAED evidences only metallic ruthenium. The concomitant formation of Li3N can be identified neither by X-ray diffraction nor by SAED analysis probably because of poor crystallization due to very small particle size. The further reduction step, leading to metallic particles dispersion with Li3N formation can also be written by Eq. (3): LiRuN + 2e + 2 Li+ ! Ru + Li3N (3)
Fig. 10. (a) Ex-situ XRD patterns (Cu Ka) of RuN thin film electrodes as deposited (top grey), partly discharged (x = 1 blue, x = 2 red) and fully discharged to 0.02 V (down black). (b) Grazing angle X-ray diffractogramm (Co Ka) of fully discharged electrode. Red vertical lines correspond to the theoretical position of ruthenium diffraction peaks (JCPDS # 00-006-0663). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
are available for further cycling. The occurrence of metallic ruthenium in the delithiated sample explains the origin of the irreversible capacity in addition to SEI formation.
This electrochemical process does not affect drastically the morphology of the thin film (Fig 9). The fractal aspect is conserved, with only a decrease of the particle size. During the consecutive delithiation to 3 V, the electrochemical processes involve a noticeable lower coulombic charge than that observed during the first discharge. It implies that the coulombic efficiency in the first cycle is low, in the order of 60% for the 250 nm thick thin film at C/2. The large irreversible capacity loss, that has been already observed in studies devoted to electrochemical behaviors of other nitrides or oxides, may be mainly assigned to the SEI formation. Indeed, the reaction of the electrolyte components, namely EC and DMC, with lithium can be promoted by the high reactive surface area generated by the ruthenium nitride structure destruction. Besides, the reverse reaction leading back to RuN may be regarded as not quantitative. One part of transition metal formed during the first reduction step is supposed to act as a catalyst that may play a major role in driving the further Li3N decomposition/formation [12,13,25]. This view is supported by EELS analyses of delithiated RuN electrode (Fig 13) that gives evidence of nanograins of metallic ruthenium still mixed with nanograins of ruthenium nitride, what is illustrated by Eq. (4):
3.5. Discussion
Ru + Li3N = RuN + 3e + 3 Li+ (4)
Both the electrochemical and structural sets of data examined clearly show that ruthenium nitride undergoes a conversion reaction during the discharge process. During the first lithiation, RuN of the deposited thin film is progressively consumed from the early stages according to the
This reaction is characterized by multi peaks in the voltammetric cycle (Fig. 3). By analogy with the theoretical investigations on the reaction of transition metal with Li3N based on energies of weak metallic bonds, interactions between lithium and metal, between nitrogen and metal, the peaks could be assigned to the
Fig. 11. Ex-situ TEM bright (a)/dark (b) field images and corresponding SAED pattern (c) of the lithiated RuN thin film electrode at the first discharge to 20 mV leading to metallic Ru nanoparticles.
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Fig. 12. Ex-situ TEM bright (a)/dark (b) field images and corresponding SAED pattern (c) of the delithiated electrode. The SAED pattern has been indexed considering only RuN nanograins.
the nanograins of metallic particles are the smallest. They permit a reduced diffusion length of Li+ and facilitate the ionic and electronic conduction. Afterward, the SEI and the restored capacity get stabilized, with values from 400 mA h g1 for cycle 10 to 330 mA h g1 after 70 cycles at C/2 (Fig. 5). 4. Conclusion RuN thin films with the ZnS type cubic structure have been successfully deposited on steel substrate by using pulsed-DC magnetron sputtering. This reproducible procedure has been applied to prepare RuN films electrodes with thickness in the range 250–850 nm. Their electrochemical behavior has been investigated and discussed in relation with structural characterization by XRD, SEM, HRTEM, SAED. The results demonstrate they undergo a conversion reaction mechanism upon lithiation. Nano-sized metallic Ru particles and Li3N are formed during the discharge while RuN is formed in the charge process. For thick films between 250 and 650 nm, a stable specific capacity of about 400 mA h g1 at C/2 rate is achieved over 20 cycles combined with a good rate capability with still 300 and 250 mA h g1 at 2 C and 5 C respectively without structural or electrochemical damage. The specific capacity delivered by RuN thin films is in the range 100 to 300 mA h cm2 depending on the thickness which makes these films very attractive as negative electrode for Li-ion thin films microbatteries. Further investigation is under progress in our lab to optimize the cycle life of thick films and the rechargeability of the electrochemical process by studying the influence of the upper voltage suggested being interesting as in the case of cobalt and vanadium nitrides [15,16]. Fig. 13. (a) Ex-situ STEM image of delithiated sample with localizations of EELS spectra (b) EELS spectra of Ru M edges and N edge recorded on delithiated sample and Ru and RuN References.
electrochemical reactivity of stable complexes such as LiRuNLi2, Li2RuNLi, LiNRuLi, RuNLi2 . . . [25]. SEM images of a full delithiated sample reveal a huge modification of the surface morphology (Fig. 9). This renewed surface may be related to the strongly modified discharge profile and cathodic voltammogram of cycle 2 compared with that of cycle 1 (Figs. 3 and 4). In both cases, the peak at 0.64 V and the plateau at 0.65 V related to Eq. (1) have disappeared. In subsequent cycles, a capacity fading is observed from cycle 2 to the tenth cycle (Fig. 5). This feature has already been reported in literature in the case of VN [15] and CoN [16]. It is justified by what is called electrode “formatting”. During those few cycles, the charge/ discharge process still entails morphology transformation. It may result in the crushing and powdering of the thin film electrode material till a stable electrode structuring is attained [15]. Then after,
Acknowledgements This work has been financially supported by the Agence Nationale de la Recherche under the project Advanced NIBACA (ANR-09-Stock-E-01). The authors thank Dr. S. Bouhtiyya and S. Migot for their helpful discussions and technical assistance in thin film preparation and TEM characterization. References [1] J.B. Bates, N.J. Dudney, B. Neudecker, A. Ueda, C.D. Evans, Thin-film lithium and lithium-ion batteries, Solid State Ionics 135 (2000) 33–45. [2] B.J. Neudecker, N.J. Dudney, J.B. Bates, “Lithium-free” thin-film battery with in situ plated Li anode, Journal of the Electrochemical Society 147 (2000) 517–523. [3] J.-P. Pereira-Ramos, R. Baddour-Hadjean, Thin-film metal-oxide electrodes for lithium microbatteries, in: K. Ozawa (Ed.), Lithium ion rechargeable batteries, Wiley-VCH, Weinheim, 2009, pp. 257–311. [4] J. Graetz, C.C. Ahn, R. Yazami, B. Fultz, Nanocrystalline and thin film germanium electrodes with high lithium capacity and high rate capabilities, Journal of the Electrochemical Society 151 (2004) A698–A702.
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[5] J. Cabana, L. Monconduit, D. Larcher, M.R. Palacin, Beyond Intercalation-Based Li-Ion Batteries: The State of the Art and Challenges of Electrode Materials Reacting Through Conversion Reactions, Advanced Materials 22 (2010) E170–E192. [6] N.G. Rudawski, B.L. Darby, B.R. Yates, K.S. Jones, R.G. Elliman, A.A. Volinsky, Nanostructured ion beam-modified Ge films for high capacity Li ion battery anodes, Applied Physics Letters 100 (2012) 4. [7] M.T. McDowell, S.W. Lee, W.D. Nix, Y. Cui, 25th Anniversary Article: Understanding the Lithiation of Silicon and Other Alloying Anodes for Lithium-Ion Batteries, Advanced Materials 25 (2013) 4966–4984. [8] B. Jerliu, E. Huger, L. Dorrer, B.K. Seidlhofer, R. Steitz, V. Oberst, U. Geckle, M. Bruns, H. Schmidt, Volume Expansion during Lithiation of Amorphous Silicon Thin Film Electrodes Studied by In-Operando Neutron Reflectometry, Journal of Physical Chemistry C 118 (2014) 9395–9399. [9] N. Pereira, L.C. Klein, G.G. Amatucci, The electrochemistry of Zn3N2 and LiZnN – A lithium reaction mechanism for metal nitride electrodes, Journal of the Electrochemical Society 149 (2002) A262–A271. [10] N. Pereira, L. Dupont, J.M. Tarascon, L.C. Klein, G.G. Amatucci, Electrochemistry of Cu3N with lithium – A complex system with parallel processes, Journal of the Electrochemical Society 150 (2003) A1273–A1280. [11] N. Pereira, M. Balasubramanian, L. Dupont, J. McBreen, L.C. Klein, G.G. Amatucci, The Electrochemistry of germanium nitride with lithium, Journal of the Electrochemical Society 150 (2003) A1118–A1128. [12] Z.W. Fu, Y. Wang, X.L. Yue, S.L. Zhao, Q.Z. Qin, Electrochemical reactions of lithium with transition metal nitride electrodes, Journal of Physical Chemistry B 108 (2004) 2236–2244. [13] Y. Wang, Z.W. Fu, X.L. Yue, Q.Z. Qin, Electrochemical reactivity mechanism of Ni3N with lithium, Journal of the Electrochemical Society 151 (2004) E162–E167. [14] Q. Sun, Z.W. Fu, An anode material of CrN for lithium-ion batteries, Electrochemical and Solid State Letters 10 (2007) A189–A193.
[15] Q. Sun, Z.W. Fu, Vanadium nitride as a novel thin film anode material for rechargeable lithium batteries, Electrochimica Acta 54 (2008) 403–409. [16] B. Das, M.V. Reddy, P. Malar, T. Osipowicz, G.V.S. Rao, B.V.R. Chowdari, Nanoflake CoN as a high capacity anode for Li-ion batteries, Solid State Ionics 180 (2009) 1061–1068. [17] Q. Sun, Z.W. Fu, Mn3N2 as a novel negative electrode material for rechargeable lithium batteries, Applied Surface Science 258 (2012) 3197–3201. [18] D.K. Nandi, U.K. Sen, D. Choudhury, S. Mitra, S.K. Sarkar, Atomic Layer Deposited Molybdenum Nitride Thin Film: A Promising Anode Material for Li Ion Batteries, ACS Applied Materials & Interfaces 6 (2014) 6606–6615. [19] K.H.T. Raman, T.R. Penki, N. Munichandraiah, G.M. Rao, Titanium nitride thin film anode: chemical and microstructural evaluation during electrochemical studies, Electrochimica Acta 125 (2014) 282–287. [20] M.G. Moreno-Armenta, J. Diaz, A. Martinez-Ruiz, G. Soto, Synthesis of cubic ruthenium nitride by reactive pulsed laser ablation, Journal of Physics and Chemistry of Solids 68 (2007) 1989–1994. [21] E. Cattaruzza, G. Battaglin, P. Riello, D. Cristofori, M. Tamisari, On the synthesis of a compound with positive enthalpy of formation: Zinc-blende-like RuN thin films obtained by rf-magnetron sputtering, Applied Surface Science 320 (2014) 863–870. [22] D. Rosestolato, G. Battaglin, S. Ferro, Electrochemical properties of stoichiometric RuN film prepared by rf-magnetron sputtering: A preliminary study, Electrochemistry Communications 49 (2014) 9–13. [23] S. Bouhtiyya, R. Lucio Porto, B. Laik, P. Boulet, F. Capon, J.P. Pereira-Ramos, T. Brousse, J.F. Pierson, Application of sputtered ruthenium nitride thin films as electrode material for energy-storage devices, Scripta Materialia 68 (2013) 659–662. [24] J.E. Sundgren, Structure and properties of tin coatings, Thin Solid Films 128 (1985) 21–44. [25] J. Ma, L. Yu, Z.W. Fu, Electrochemical and theoretical investigation on the reaction of transition metals with Li3N, Electrochimica Acta 51 (2006) 4802–4814.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electrochemical reaction of lithium with ruthenium nitride thin films prepared by pulsed-DC magnetron sputtering Barbara Laïk a, * , Samantha Bourg a , Jean-Pierre Pereira-Ramos a , Stéphanie Bruyère b , Jean-François Pierson b a b
Institut de Chimie et des Matériaux Paris-Est GESMAT, UMR 7182CNRS-Université Paris Est-Créteil, 2 rue Henri Dunant, 94320 Thiais, France Institut Jean Lamour, Département CP2S, UMR 7198CNRS-Université de Lorraine, Parc de Saurupt, CS 50840, 54011 Nancy cedex, France
A R T I C L E I N F O
A B S T R A C T
Article history: Received 11 December 2014 Received in revised form 18 February 2015 Accepted 19 February 2015 Available online 21 February 2015
Ruthenium nitride thin films with ZnS type cubic structure are successfully and reproducibly deposited by pulsed-DC magnetron sputtering. Their electrochemical behavior as negative electrode for Li-ion microbatteries is investigated by cyclic voltammetry and galvanostatic experiments. The influence of the film thickness and the rate capability are reported and discussed. A specific capacity of about 400 mA h g1 is proved to be stable at C/2 over 20 cycles at a mean voltage of 0.25 V vs. Li+/Li and attractive specific capacities varying from 100 to 300 mA h cm2 can be reached depending on the thickness in the range 250 nm–850 nm. XRD, SEM, HRTEM, electron diffraction and EELS are used to understand the electrochemical mechanism of RuN with lithium. Electrochemical properties and structural characterization support the occurrence of a reversible conversion reaction of RuN into Ru metal and Li3N. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: Ruthenium nitride Lithium-ion microbatteries Anode material Conversion reaction Pulsed-DC magnetron sputtering
1. Introduction Today, a constant increase in global energy demand is observed. In this context, rechargeable batteries, and more especially Lithium-ion batteries (LIBs), are increasingly attractive and many researches are devoted to the improvement of such lithium-ion based electrochemical systems. In the same time, a variety of applications requiring power microsources has been developed including medical implants, remote sensors, smart cards or other portable electronic devices. To satisfy these requirements, lithiumion thin film micro batteries (TFBs) are emerging. The quest of higher energy densities concomitantly with the necessity of stable electrode materials is become a challenge. Therefore an intensive research devoted to high performance thin film materials, mainly transition metal oxides, as positive electrode is made [1–3]. Nowadays, lithium metal is widely used as anode material due to its high specific capacity. However because of its reactivity, safety and packaging issues exist and lead to the study and development of alternative anode materials that can react reversibly with lithium [1,4–8] operating either by alloying or conversion. Among these materials, the binary metal nitrides MxNy are promising candidates due to their high specific capacity in the range of 300–1000 mA h g1, a high melting point and a high chemical stability. They have been demonstrated to
* Corresponding author. http://dx.doi.org/10.1016/j.electacta.2015.02.171 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
electrochemically react through conversion reactions [1,9]. Sn3N4 and Zn3N2 thin films have been proposed as negative electrode in TFBs in the early 2000s [1]. After these pioneer works, a large variety of binary metallic nitrides has been screened, including Zn3N2 [9], Cu3N [10], Ge3N4 [11], Co3N [12], Fe3N [12], Ni3N [13], CrN [14], VN [15], CoN [16], Mn3N2 [17], MoN [18], TiN [19] . . . Most of them have been prepared as thin films, 100–500 nm thick, mainly by RF sputtering [14–17,19] but also by pulsed laser deposition [12,13] or atomic layer deposition [18]. In these studies, the conversion reaction with lithium has been examined. It has been reported to be a two-step process. The reduction process yields irreversibly to the formation of metallic nanoparticles embedded in a Li3N matrix. The subsequent charge process depicted in Eq. (1), where M = transition metal in the MxNy nitride, provides the reversible capacity. x M + y Li3N ! MxNy + 3y e + 3y Li+ (1) The mechanism is similar to that observed with metal oxides but these nitride materials present a main advantage over them: Li3N produced by the conversion reaction is an excellent ionic conductor, better than Li2O, thereby satisfies one of the essential criteria for a desirable anode. According to this mechanism, as the first irreversible reaction involves the full reduction of the transition metal into its metallic state, remarkable high capacity values are delivered, particularly for nitrides with high chemical valence such CrN, VN and CoN that exhibit capacity above 1000 mA h g1 when cycled between 0.01 and 3.5 V vs Li+/Li.
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Ruthenium nitride RuN is one of these compounds with valence (+III) but has never been studied in detail as negative electrode for Li-ion batteries. Its theoretical specific capacity reaches 700 mA h g1 considering 3 lithium mol involved per mol of ruthenium. Previous works have shown that NaCl-like RuN can be synthesized by pulsed laser ablation [20] while RuN with ZnS-like structure can be deposited by RF-magnetron sputtering [21,22]. Only one recent study [23] reports the deposition of stoichiometric ZnS-like RuN by pulsed-DC magnetron sputtering and that these ruthenium nitride thin films can be regarded as possible anode material in lithiumion microbatteries since high capacity value close to the theoretical one was achieved at C/2 rate [23]. The present paper aims at investigating the electrochemical properties of RuN thin films and at providing more information on the discharge/charge processes involved during reaction of RuN with lithium. Electrochemical properties are investigated by cyclic voltammetry (CV) and galvanostatic cycling using a half-cell configuration versus lithium and discussed in relation with structural evolutions evidenced by X-ray diffraction (XRD), scanning electronic microscopy (SEM), transmission electronic microscopy (TEM), electron diffraction and electron energy loss spectroscopy (EELS). The cyclability, the rate capability and the influence of the thin film thickness on electrochemical performances are also evaluated. 2. Experimental section Ruthenium nitride thin films were deposited on steel substrate by pulsed-DC magnetron sputtering of a metallic ruthenium target (99.99% purity, 50 mm diameter and 3 mm thick). The 40-L sputtering chamber was pumped down via a mechanical pump and a turbomolecular one allowing a base vacuum of 104 Pa. Prior to their introduction in the deposition chamber, the substrates were ultrasonically cleaned in ethanol. The distance between the substrates and the target was 50 mm. The target was powered by an Advanced Energy Pinnacle+ 5 kW pulsed-DC supply. The discharge frequency and the off-time were fixed at 50 kHz and at 4 ms, respectively. The current applied to the target was 0.1 A. Due to the low reactivity of sputtered ruthenium atoms with nitrogen ones, RuN films was successfully synthesized in pure nitrogen atmosphere [23]. The total pressure during the film deposition was fixed to 1 Pa and no intentional heating was applied during the film growth (deposition temperature about 50 C). The thickness of the RuN films measured by tactile profilometry was controlled by adjusting the deposition duration. Within our deposition conditions, the film growth rate was about 250 nm h1 and the relative uncertainty on films thickness about 3%. Electrochemical measurements were carried out in twoelectrode coin cells (CR2032 type). The deposited films of RuN on stainless steel spacers (1 mm thick, 16 mm diameter) were used as working electrode. The weight of active material was determined by difference weighing before and after deposition. The absolute error on the RuN film weight was 0.01 mg. Lithium sheets (0.75 mm thick, 16 mm diameter, 99.9%, Alfa Aesar) were used as both reference and counter electrodes. Whatman (GF/C) glass fiber separators were saturated with 1 mol L1 LiPF6 in a nonaqueous solution of ethylene carbonate (EC) and dimethyl carbonate (DMC) with a volume ratio 1:1 (Merck). The cells were sealed in an Arfilled glovebox (O2 and H2O contents less than 0.1 ppm). Voltamperommetry and galvanostatic charge/discharge cycles were performed in an air-conditionned room (T = 20.0 0.1 C) with either a Solartron (AMETEK) or VMP3 (Bio-logic, France) galvanostat/potentiostat. For galvanostatic measurements, the charge/discharge rate is calculated based on the transfer of one mol of electron per mol of RuN per hour, consequently 1 C rate matches a constant current of 0.23 A g1.
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For ex-situ measurements, after galvanostatic cycles stopped at different potential values, coin cells were dismantled in the glove box to avoid exposure to oxygen and water. The more or less lithiated RuN electrodes are rinsed in anhydrous DMC to remove residual traces of electrolyte. The surface morphology of the RuN electrode was studied by scanning electron microscopy (SEM) using a Zeiss Merlin. The film structure was studied by X-ray diffraction (XRD) in u/2u mode using a Brucker D8 Advance diffractometer (Cu Ka1 radiation, l = 0.154056 nm) and in grazing incidence (1 ) using a Brucker D8 Discover diffractometer (Co Ka radiation, l = 0.179026 nm). For transmission electron microscopy (TEM) characterization, samples were prepared by scratching off the surface with a holey carbon copper grid. The experiments were performed using Philips CM200 and JEOL ARM 200 F microscopes with 200 kV acceleration voltage. The crystallographic structure was studied using the microdiffraction mode that allowed focusing the electron beam in a 50 nm diameter zone. Electron energy loss spectroscopy (EELS) experiments were carried out using the JEOL ARM 200 F in scanning transmission electron microscopy (STEM) mode with a GIF Quantum post-column energy filter from GATAN and the zero loss peak resolution was 0.8 eV. 3. Results and discussion 3.1. Structural characterization of deposited ruthenium nitride thin films The diffractogram of a RuN film deposited on a steel substrate is presented in Fig. 1. In addition to the a/g biphased steel substrate peaks at 43.6, 44.5 and 50.6 , one diffraction peak close to 34.6 is evidenced. It has been assigned to the (111) plan of the RuN phase that crystallizes in a ZnS-like structure with lattice constant of approximately 0.451 nm [23]. Depending on the deposition conditions, Cattaruzza et al. have estimated the lattice constant of RF-sputtered RuN films at 0.450 or 0.453 nm [21]. Since only one diffraction peak of RuN is evidenced in Fig. 1, this film grows with a strong preferred orientation in the [111] direction as confirmed by pole figure measurements (not shown here). However, there is no in plane orientation indicating that the films grow with a fiber texture along the [111] direction. As commonly observed for sputtered nitride coatings [24], RuN films grow with a columnar microstructure (Fig. 2a). At the top surface of the 450 nm-thick film, the column width is about 40–60 nm. The electronic diffraction pattern of one column obtained in microdiffraction mode is presented in Fig. 2b. It has been indexed considering the cubic RuN phase with a [–11 2] zone axis. Since diffraction spots are evidenced in Fig. 2b, one column can be considered as a single crystal.
Fig. 1. X-ray diffractogram (Cu Ka) of a 450 nm-thick RuN film deposited on steel substrate.
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Fig. 2. (a) TEM bright field image of a RuN thin film, (b) electronic diffraction pattern of the area marked by the white circle in Fig. 2a.
3.2. Electrochemical properties 3.2.1. Cyclic voltammetry and galvanostatic cycling experiments The lithium storage capacities and cycle performances of RuN 250 nm-thick thin films were investigated by discharge/charge measurements in Li/RuN cells. Fig. 3 shows the first two cyclic voltammograms recorded between 0.02 and 3 V measured at 50 mV s1. There are three main cathodic peaks at 0.64, 0.46 and 0.22 V in the initial reduction process and three main steps at 0.57, 0.81 and 1.40 V in the consecutive oxidation process. These multipeaks imply that the electrochemical behavior of RuN versus lithium is a complicated process involving a series of insertion/conversion and decomposition reactions. The coulombic charge recovered for the oxidation process is much lower than that involved in reduction because the reduction reaction includes the SEI (Solid Electrolyte Interphase) formation as expected in this voltage range. This explanation is confirmed by the better rechargeability found during further cycles. Indeed, the two main cathodic peaks of cycle 1 at 0.64 and 0.46 V have disappeared in cycle 2 to the benefit of less intense and broad peaks at 1.50 and 1.03 V followed by a sharp peak around 0.25 V. This finding is consistent with the occurrence of an irreversible process taking place during the first cycle and described as a conversion reaction as reported in the literature for other nitride compounds [12,17].
Fig. 3. Cyclic voltammograms of 250 nm-thick thin film measured at 50 mV s1 between 0.02 and 3 V (first cycle: full red line; second cycle: dashed blue line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Conversely the shape of the anodic voltammogram is maintained with peaks located at 0.81, 1.24 and 1.59 V. The charge/discharge profiles recorded at a moderate rate C/2 (117 mA g1) in the 0.02–3 V voltage window are presented in Fig. 4. It represents the potential evolution as a function of both the weight capacity on the bottom scale (Q) and the number of mol of lithium transferred per mol of ruthenium on the upper scale (x). The maximum theoretical specific capacity, 700 mA h g1, corresponds to x = 3. After a rapid potential drop entailing the consumption of 0.3 mol of lithium per mol of RuN, the first cycle presents a well-defined discharge plateau at 0.65 V that disappears in the second and following cycles. It corresponds to the main peak observed on the first cyclic voltammogram and discussed above. At the end of the voltage plateau, the capacity is about 300 m Ah g1 which corresponds to the consumption of 1.3 mol of lithium per mol of RuN. After this, a gradual potential decrease appears up to the deep discharge limit of 0.02 V. The total first discharge specific capacity is 940 mA h g1, about 240 mA h g1 higher than the theoretical specific capacity of RuN. This is probably due to the SEI formation all along the process [17]. During the charge process, the curve presents several inflexions that may rely on the different peaks observed on the
Fig. 4. Galvanostatic discharge/charge curves of 250 nm-thick thin film measured at C/2 (117 mA g1) between 0.02 and 3 V (first cycle: full red line; second cycle: dashed blue line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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cyclic voltammogram (Fig. 3, full red curve). The overall first charge capacity is about 570 mA h g1, 130 mA h g1 lower than the theoretical specific capacity. This result supports the faradaic yield in the reduction reaction includes the SEI formation. However the fact that a small part of RuN particles do not react cannot be discarded. The second discharge profile is different from the first one, confirming the starting material undergoes an irreversible and deep change after one cycle. The main plateau at 0.65 V and the less pronounced one at around 0.50 V have disappeared to give way to a signal at 0.25 V. It is consistent with the peak on the dashed blue line of voltammetric curves (Fig. 3). Conversely, the second charge profile completely superimposes with the first one. That means that, the same processes and chemical species are involved during the oxidation whatever the cycle number.
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and the charge curve shape are kept. After 10 cycles, the charge/ discharge profile does not evolve anymore (cycles 20–50 and 70 in Fig. 5b). This behavior has been called “formatting” of the electrode and corresponds to the structuring of the electrode before stability is attained as reported in the case of CoN nanoflakes [16].
3.2.2. Cyclability Cyclability of a 250 nm-thick sample is presented at C/2 rate in Fig. 5a. During the first ten cycles, the capacity rapidly decreases from 940 to 400 mA h g1. Thereafter the capacity value practically stabilizes with cycles. The capacity is about 330 mA h g1 after more than 70 cycles. Corresponding cycles are presented in Fig. 5b. Though decreasing capacities, the shape of the first ten cycles is the same. The step at 0.25 V during the reduction is conserved but shortened, whereas the shoulder noticeable at 0.8 V in reduction
3.2.3. Influence of the thickness As it appears possible to consider ruthenium nitride as negative electrode material for TFBs, the increase of the available capacity is a challenge. Indeed, for a 250 nm thick thin film, when stability is achieved during cycling, a capacity of around 330 mA h g1 44 mA h cm2 can be stored (Fig. 5). Therefore several samples were electrochemically tested with increasing thicknesses and compared. The first and second discharge–charge profiles are presented in Fig. 6a for 250, 450, 650 and 850 nm thick thin films cycled in the 0.02–3 V potential range at C/2. Regardless the sample’s thickness, the electrochemical signal is the same: the long flat plateau observed during the first discharging step is absent during the second one and the charge profile does not change at all. The effective capacities expressed both in mAh and mAh cm2 are reported too (Fig. 6b). A linear relationship is observed between capacities and thicknesses. For instance, the discharge capacity increases from 90 mA h cm2 with 250 nm-thick film to 300 mA h cm2 for 850 nm one, these values comparing very well with the best values reported for transition metal oxide thin films [3].
Fig 5. (a) Capacity versus cycle number for a 250 nm-thick thin film measured at C/2 (117 mA g1) between 0.02 and 3 V. (b) Evolution of discharge/charge profiles with cycles.
Fig 6. (a) First (full lines) and second (dashed lines) discharge/charge cycles at C/2 (117 mA g1) of 250, 450, 650 and 850 nm-thick thin films in the 0.02–3 V potential range. (b) Corresponding discharge/charge capacities versus film thickness.
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Besides, this result is very interesting as it demonstrates that the deposited material homogeneously participates to the electrochemical processes, even for the thickest samples, without limitation by lithium diffusion or electronic transfer. The higher slope obtained during the first discharge is related to the SEI formation reaction mentioned before. The influence of the film thickness on the cycle life of the electrode is presented in Fig. 7. The evolution of the capacity up to cycle 20 is presented for the four samples. Whatever the thickness, two domains can be distinguished: in the first one, the capacity rapidly decreases up to cycle 10 (corresponding to the formatting step mentioned above) whereas in the second one, some stability is achieved. Besides, the 850 nm thick sample exhibits a more rapid decrease in capacity than the other ones probably due to the fact that all particles do not work homogeneously upon further cycling, leading to lack of adhesion from the steel substrate. The influence of the film thickness has never been addressed for other binary nitrides, the thickness being usually limited to the 200–400 nm range [14–16,18].
3.2.4. Rate capability The influence of the current density has been also investigated. Results obtained for different C rates are presented for a 450 nm thick sample (Fig. 8a). In order to take into account the previous statement that about ten cycles are required before capacity stabilization, the first ten cycles wereperformed at C/2 to ensure the stabilization of the capacity. After its stabilization about 390 mA h g1, different current densities were successively applied. It can be seen that a lower rate (C/5) entails an increase of capacity up to 500 mA h g1 (Fig. 8) whereas higher cycling rates lead to lower but satisfactory values of 320 and 260 mA h g1 for 2 C and 5 C, respectively. These results highlight a good rate capability as high capacities are still available at high rates. The subsequent return to cycling at C/2 allows to recover the stable capacity (400 mA h g1) previously achieved at the same rate (Fig. 8b) suggesting the electrode material is not damaged at high rates. 3.3. SEM observations The SEM images of the as-deposited, the first discharged to 0.02 V and the first full charged to 3 V samples at C/2 are presented in Fig. 9. The as-deposited (a) thin film presents an uniform smooth surface with particles of about a few tens nanometers. The surface of the discharged film (b) is homogeneous too, but the morphology is slightly modified. Particles appear more divided than in the
Fig. 7. (a) Capacity versus cycle number for 250 nm (red), 450 nm (blue), 650 nm (green) and 850 nm (black)-thick thin films measured at C/2 (117 mA g1) between 0.02 and 3 V (b) corresponding 10th (full lines) and 20th (dashed lines) galvanostatic discharge/charge cycles. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 8. (a) Discharge capacity versus cycle number for a 450 nm-thick thin film measured successively at C/2 (117 mA g1), C/5, 2C, 5 C and C/2 between 0.02 and 3 V. (b) Corresponding discharge/charge profiles.
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Fig. 9. Surface morphology of RuN thin film electrodes as-deposited (a), discharged to 20 mV (b) and charged to 3 V at the end of the first cycle (c) and at the end of the second cycle (d).
starting film. At the end of the first cycle (c), when the sample has been oxidized up to 3 V, the surface morphology is strongly modified. Some plates are uniformly dispersed at the surface. This supports the change in the discharge profiles of the first two voltamperommetric and galvanostatic cycles reported in Figs. 3 and 4. Fig. 9d confirms that the new morphology is maintained after an additional cycle. 3.4. Ex-situ XRD, TEM and SAED observations XRD spectra of partially and fully discharged RuN thin-film electrodes are gathered in Fig. 10. Dark dashed lines are related to the diffraction peaks of the a/g biphased steel substrate at approximately 43.6, 44.5 and 50.6 (Fig. 10a). In addition to the substrate peaks, only one diffraction peak ascribed to ruthenium nitride RuN (111) is observed at 34.5 . Its intensity progressively decreases during the first discharge. This indicates that RuN is progressively transformed into other species, probably amorphous or nano-sized. For the fully discharged sample, more relevant information about the film structure is obtained by XRD in grazing incidence (1 ) (Fig. 10b). Broad peaks detected at 45.0 and 51.5 fit well with the (1 0 0) and (1 0 1) diffraction peaks of metallic ruthenium (JCPDS # 00-006-0663). This result indicates that metallic ruthenium is present in the fully lithiated sample and confirms the nanocrystalline size of the electrochemically formed Ru particles. A further examination of this sample was carried out using TEM and SAED. The lithiated RuN sample still exhibits a columnar microstructure (Fig. 11a). However, the single crystal character of the column is not observed after the first discharge anymore (Fig. 11b–c). The dark field image (Fig. 11b) clearly evidences that the columns are composed of nanograins with a mean grain size of approx. 2–3 nm.
The selected area electron diffraction pattern presented in Fig. 11c has been assigned to diffraction rings of metallic ruthenium. Li3N was not observed in SAED pattern probably due to poorly crystallized particles and to the lower atomic scattering factors of lithium and nitrogen compared to that of ruthenium metal. The TEM results well agree with those previously obtained by XRD. The strong decrease of the intensity of the (111) diffraction peak of RuN when x = 1 can be interpreted by the formation of nanograins of Ru into the whole thickness of RuN films. Complementary measurements were carried out on a delithiated sample. The TEM and SAED image of the thin film electrode after charging to 3 V are shown in Fig 12. The nanocrystalline character of the sample is clearly evidenced on the bright and dark field images presented on Fig. 12a and 12b. From the corresponding SAED pattern (Fig. 12c), it is not easy to clearly determine the crystalline nature of the delithiated sample. In order to obtain more detailed information about the structure of delithiated samples, EELS analyses were carried out. Two 50 nm-thick films of metallic Ru and RuN were used as references and Ru M edges and N K edge were recorded. TEM used in scanning mode (STEM) confirmed the nanocrystalline microstructure of the delithiated sample (Fig. 13a). Grains of about 2–4 nm diameter are clearly evidenced on this high resolution image. Two grains marked A and B have been characterized by EELS and the related spectra have been compared to those of the Ru and RuN reference samples (Fig. 13b). The EELS spectrum of the grain marked A does not show the N K edge while the Ru M ones are clearly evidenced. On the other hand, both Ru and N elements are detected by EELS on the grain marked B. The characterization of a larger zone evidenced that the delithiated sample is mostly composed of RuN nanograins with some Ru nanograins. Thus, the oxidation of the Ru electrode into RuN is not quantitative and electroformed nanosized Ru grains
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disappearance of the diffraction peak of RuN (111) (Fig 10). In the light of the significant intensity decrease of the RuN (111) diffraction peak at x = 1, a similar step to that proposed by Q. Sun and by B. Das respectively for CrN [14], VN [15] and CoN [16] can be suggested. It consists in the destruction/amorphisation of the crystal structure and in the formation of an intermediate state as presented in Eq. (2). This process involves one mol of lithium per mol of RuN and is characterized by the flat plateau observed at 0.65 V in the first galvanostatic cycle (Fig 4): RuN + e + Li+ ! LiRuN (2) Characterization of a RuN thin film electrode lithiated to 20 mV by grazing angle XRD and SAED evidences only metallic ruthenium. The concomitant formation of Li3N can be identified neither by X-ray diffraction nor by SAED analysis probably because of poor crystallization due to very small particle size. The further reduction step, leading to metallic particles dispersion with Li3N formation can also be written by Eq. (3): LiRuN + 2e + 2 Li+ ! Ru + Li3N (3)
Fig. 10. (a) Ex-situ XRD patterns (Cu Ka) of RuN thin film electrodes as deposited (top grey), partly discharged (x = 1 blue, x = 2 red) and fully discharged to 0.02 V (down black). (b) Grazing angle X-ray diffractogramm (Co Ka) of fully discharged electrode. Red vertical lines correspond to the theoretical position of ruthenium diffraction peaks (JCPDS # 00-006-0663). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
are available for further cycling. The occurrence of metallic ruthenium in the delithiated sample explains the origin of the irreversible capacity in addition to SEI formation.
This electrochemical process does not affect drastically the morphology of the thin film (Fig 9). The fractal aspect is conserved, with only a decrease of the particle size. During the consecutive delithiation to 3 V, the electrochemical processes involve a noticeable lower coulombic charge than that observed during the first discharge. It implies that the coulombic efficiency in the first cycle is low, in the order of 60% for the 250 nm thick thin film at C/2. The large irreversible capacity loss, that has been already observed in studies devoted to electrochemical behaviors of other nitrides or oxides, may be mainly assigned to the SEI formation. Indeed, the reaction of the electrolyte components, namely EC and DMC, with lithium can be promoted by the high reactive surface area generated by the ruthenium nitride structure destruction. Besides, the reverse reaction leading back to RuN may be regarded as not quantitative. One part of transition metal formed during the first reduction step is supposed to act as a catalyst that may play a major role in driving the further Li3N decomposition/formation [12,13,25]. This view is supported by EELS analyses of delithiated RuN electrode (Fig 13) that gives evidence of nanograins of metallic ruthenium still mixed with nanograins of ruthenium nitride, what is illustrated by Eq. (4):
3.5. Discussion
Ru + Li3N = RuN + 3e + 3 Li+ (4)
Both the electrochemical and structural sets of data examined clearly show that ruthenium nitride undergoes a conversion reaction during the discharge process. During the first lithiation, RuN of the deposited thin film is progressively consumed from the early stages according to the
This reaction is characterized by multi peaks in the voltammetric cycle (Fig. 3). By analogy with the theoretical investigations on the reaction of transition metal with Li3N based on energies of weak metallic bonds, interactions between lithium and metal, between nitrogen and metal, the peaks could be assigned to the
Fig. 11. Ex-situ TEM bright (a)/dark (b) field images and corresponding SAED pattern (c) of the lithiated RuN thin film electrode at the first discharge to 20 mV leading to metallic Ru nanoparticles.
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Fig. 12. Ex-situ TEM bright (a)/dark (b) field images and corresponding SAED pattern (c) of the delithiated electrode. The SAED pattern has been indexed considering only RuN nanograins.
the nanograins of metallic particles are the smallest. They permit a reduced diffusion length of Li+ and facilitate the ionic and electronic conduction. Afterward, the SEI and the restored capacity get stabilized, with values from 400 mA h g1 for cycle 10 to 330 mA h g1 after 70 cycles at C/2 (Fig. 5). 4. Conclusion RuN thin films with the ZnS type cubic structure have been successfully deposited on steel substrate by using pulsed-DC magnetron sputtering. This reproducible procedure has been applied to prepare RuN films electrodes with thickness in the range 250–850 nm. Their electrochemical behavior has been investigated and discussed in relation with structural characterization by XRD, SEM, HRTEM, SAED. The results demonstrate they undergo a conversion reaction mechanism upon lithiation. Nano-sized metallic Ru particles and Li3N are formed during the discharge while RuN is formed in the charge process. For thick films between 250 and 650 nm, a stable specific capacity of about 400 mA h g1 at C/2 rate is achieved over 20 cycles combined with a good rate capability with still 300 and 250 mA h g1 at 2 C and 5 C respectively without structural or electrochemical damage. The specific capacity delivered by RuN thin films is in the range 100 to 300 mA h cm2 depending on the thickness which makes these films very attractive as negative electrode for Li-ion thin films microbatteries. Further investigation is under progress in our lab to optimize the cycle life of thick films and the rechargeability of the electrochemical process by studying the influence of the upper voltage suggested being interesting as in the case of cobalt and vanadium nitrides [15,16]. Fig. 13. (a) Ex-situ STEM image of delithiated sample with localizations of EELS spectra (b) EELS spectra of Ru M edges and N edge recorded on delithiated sample and Ru and RuN References.
electrochemical reactivity of stable complexes such as LiRuNLi2, Li2RuNLi, LiNRuLi, RuNLi2 . . . [25]. SEM images of a full delithiated sample reveal a huge modification of the surface morphology (Fig. 9). This renewed surface may be related to the strongly modified discharge profile and cathodic voltammogram of cycle 2 compared with that of cycle 1 (Figs. 3 and 4). In both cases, the peak at 0.64 V and the plateau at 0.65 V related to Eq. (1) have disappeared. In subsequent cycles, a capacity fading is observed from cycle 2 to the tenth cycle (Fig. 5). This feature has already been reported in literature in the case of VN [15] and CoN [16]. It is justified by what is called electrode “formatting”. During those few cycles, the charge/ discharge process still entails morphology transformation. It may result in the crushing and powdering of the thin film electrode material till a stable electrode structuring is attained [15]. Then after,
Acknowledgements This work has been financially supported by the Agence Nationale de la Recherche under the project Advanced NIBACA (ANR-09-Stock-E-01). The authors thank Dr. S. Bouhtiyya and S. Migot for their helpful discussions and technical assistance in thin film preparation and TEM characterization. References [1] J.B. Bates, N.J. Dudney, B. Neudecker, A. Ueda, C.D. Evans, Thin-film lithium and lithium-ion batteries, Solid State Ionics 135 (2000) 33–45. [2] B.J. Neudecker, N.J. Dudney, J.B. Bates, “Lithium-free” thin-film battery with in situ plated Li anode, Journal of the Electrochemical Society 147 (2000) 517–523. [3] J.-P. Pereira-Ramos, R. Baddour-Hadjean, Thin-film metal-oxide electrodes for lithium microbatteries, in: K. Ozawa (Ed.), Lithium ion rechargeable batteries, Wiley-VCH, Weinheim, 2009, pp. 257–311. [4] J. Graetz, C.C. Ahn, R. Yazami, B. Fultz, Nanocrystalline and thin film germanium electrodes with high lithium capacity and high rate capabilities, Journal of the Electrochemical Society 151 (2004) A698–A702.
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