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A chemically stable PVD multilayer encapsulation for lithium microbatteries
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 J. Phys. D: Appl. Phys. 48 395306 (http://iopscience.iop.org/0022-3727/48/39/395306) View the table of contents for this issue, or go to the journal homepage for more
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Journal of Physics D: Applied Physics J. Phys. D: Appl. Phys. 48 (2015) 395306 (7pp)
doi:10.1088/0022-3727/48/39/395306
A chemically stable PVD multilayer encapsulation for lithium microbatteries J F Ribeiro1, R Sousa2, D J Cunha2, E M F Vieira3, M M Silva4, L Dupont5 and L M Goncalves3 1
University of Minho, Algoritmi Center, Guimaraes, Portugal University of Minho, DEI, Guimaraes, Portugal 3 University of Minho, MEMS UMINHO, Guimaraes, Portugal 4 University of Minho, Chemistry Center, Braga, Portugal 5 Université de Picardie Jules Verne, LRCS, UMR CNRS 7314, Amiens, France 2
E-mail:
[email protected] Received 5 June 2015, revised 3 August 2015 Accepted for publication 11 August 2015 Published 4 September 2015 Abstract
A multilayer physical vapour deposition (PVD) thin-film encapsulation method for lithium microbatteries is presented. Lithium microbatteries with a lithium cobalt oxide (LiCoO2) cathode, a lithium phosphorous oxynitride (LiPON) electrolyte and a metallic lithium anode are under development, using PVD deposition techniques. Metallic lithium film is still the most common anode on this battery technology; however, it presents a huge challenge in terms of material encapsulation (lithium reacts with almost any materials deposited on top and almost instantly begins oxidizing in contact with atmosphere). To prove the encapsulation concept and perform all the experiments, lithium films were deposited by thermal evaporation technique on top of a glass substrate, with previously patterned Al/Ti contacts. Three distinct materials, in a multilayer combination, were tested to prevent lithium from reacting with protection materials and atmosphere. These multilayer films were deposited by RF sputtering and were composed of lithium phosphorous oxide (LiPO), LiPON and silicon nitride (Si3N4). To complete the long-term encapsulation after breaking the vacuum, an epoxy was applied on top of the PVD multilayer. In order to evaluate oxidation state of lithium films, the lithium resistance was measured in a four probe setup (cancelling wires/contact resistances) and resistivity calculated, considering physical dimensions. A lithium resistivity of 0.16 Ω μm was maintained for more than a week. This PVD multilayer exonerates the use of chemical vapour deposition (CVD), glove-box chambers and sample manipulation between them, significantly reducing the fabrication cost, since battery and its encapsulation are fabricated in the same PVD chamber. Keywords: microbatteries, lithium, encapsulation, PVD, multilayer (Some figures may appear in colour only in the online journal)
1. Introduction
around the world and is still the best way to store electrical energy from intermittent power sources. However, improvements in terms of energy density, higher number of cycle life, flexibility and safety are still needed, in comparison with other battery types [3, 5, 8]. Improved and innovative materials chemistry and downhill to nanoscale is crucial to deliver more energy [1, 3, 9] and bring many improvements [3–5, 10–12]. Lithium–sulphur and lithium–air battery technologies are in the front line technology for lithium-ion battery replacement.
One of the most popular discussion topics in modern society is efficient energy storage [1, 2]. Isolated and intermittent renewable energy sources (e.g. solar, wind, etc…) are common, but improved energy-storage systems are necessary for their best use [1–5]. Rechargeable energy storage relies on lithium-ion battery technology, the same that supports all the mobile world [3, 6, 7]. This technology is under research by many groups 0022-3727/15/395306+7$33.00
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© 2015 IOP Publishing Ltd Printed in the UK
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J. Phys. D: Appl. Phys. 48 (2015) 395306
Figure 1. Battery art design (not on scale for better visualization). Zoom on battery encapsulation with materials and thickness indication.
the replacement of these metal layers by insulation layers [7]. This replacement facilitates the design and fabrication of lithium microbatteries, since the use of insulator layers makes short circuits unfeasible [7]. SiOX, SiXNY and others, deposited by plasma-enhanced chemical vapour deposition (PECVD), were investigated as replacement for Ti metal layer [7]. Plasma treated SiOX multilayers presented similar barrier results as metal layers [7] in multilayer protective coatings. Ubrig et al also investigate the use of SiOX layers with surface plasma treatment as protective layers for lithium microbatteries [16]. They deposited SiOX layers by PECVD on top of a thick (5 μm) parylene-C layer [16]. This type of organic/ inorganic multilayer seems to be effective for a reasonable time, but imposes a minimum of two high vacuum chambers (PVD and CVD) interconnected by a glove-box, where the sample never contacts with the atmosphere during the process [15, 17]. Song et al fabricated lithium microbatteries on flexible plastic substrate and proved that multilayer encapsulation is also essential to achieve high-rate performance [12]. Chemical vapour deposition (CVD) and sputtering techniques were used to fabricate an intercalated multilayer of polymeric and oxide films [12]. The flexible plastic substrate exhibits poor barrier properties and permeation of oxygen and water vapour (through the substrate) could lead to deterioration of active materials [18]. Si3N4 barrier layer is normally deposited on top of plastic substrates, before the battery fabrication, because of its excellent barrier properties [18, 19]. We propose an encapsulation using only physical vapour deposition techniques (PVD) that allows the fabrication of all battery materials in the same PVD vacuum chamber (including encapsulation) and excludes the need for sample transport/ manipulation between PVD, CVD and glove-box chambers.
Lithium-sulphur technology can theoretically storage five times more energy by weight than lithium-ion technology and is presented by many groups as the future battery technology [2, 6]. The common factor between these three technologies is the negative high-energy metallic lithium electrode [6, 8, 10]. The broad use of lithium intensifies the problem of encapsulation investigated in this work. Metallic lithium has electrochemical potential of 3.05 V versus H2, is the lightest metal (7 g mol−1) but is highly sensitive to atmosphere elements [8]. The Li2O stoichiometric surface is the most stable lithium oxide at ambient conditions and the main product of lithium oxidation in atmosphere [13, 14]. Lithium microbatteries are normally deposited by successive layers with different shadow masks onto insulating substrates [10, 12, 15]. The basic components of a battery are two metallic collectors, the positive electrode (cathode), the negative electrode (anode) and the electrolyte, which is responsible for the lithium ions transport inside the battery. Current collectors (normally platinum or titanium) and encapsulation layers are also essential for battery operation in real applications, providing connection but also protection of the battery. The most common materials used in cathode, electrolyte and anode are lithium cobalt oxide (LiCoO2), lithium phosphorous oxynitride (LiPON) and metallic lithium, respectively. Figure 1 presents an art design of the encapsulated lithium battery, with materials indication. After the deposition of lithium battery layers, the multilayer encapsulation is deposited on top [12, 15]. A polymer/metal multilayer is typically used as protective encapsulation for lithium microbatteries, as found in literature [5, 7, 11]. Dudney proposed a multilayer protective coating of intercalated Ti and parylene-C which allows battery exposing to the air for up to 3 months [10, 11]. Salot et al investigated 2
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J. Phys. D: Appl. Phys. 48 (2015) 395306
Table 1. Deposition parameters used in samples preparation.
Gas (sccm) Film
Dep. technique
Target
Ar
N
Power
Dep. pressure Thickness (mbar) (nm)
Dep. rate Temp. (°C) (Å s−1)
Al Ti Li LiPO LiPON Si3N4
E-beam E-beam Thermal evaporation RF sputtering RF sputtering RF sputtering
— — — Li3PO4 Li3PO4 Si
— — — 40
— — — — 20 13
(7 kV, 110 mA) (7 kV, 80 mA) 190 A 100 W 100 W 100 W
— — — 3 × 10−3 6 × 10−3 6 × 10−3
23.8 9.8 47.8 0.2 0.2 0.2
7
1000 1000 6000 20 20 20
60 127 85 79 73 114
Figure 2. Photographs of oxidation measurements setup: on top the Al/Ti contacts on a glass substrate, with connections to recording equipment; on the right, same sample after lithium deposition and oxidation; on the left, after PVD encapsulation and epoxy application.
chamber by RF sputtering (13.56 MHz), without breaking the vacuum. The three 20 nm layers (LiPO, LiPON and Si3N4, deposited in this order) were deposited on top of lithium film. All the deposition parameters are presented on table 1. After the Si3N4 deposition, an epoxy (LOCTITE® 3430 A&B Hysol® [23]) was applied on PVD multilayer, finalizing the encapsulation (figure 1). Epoxy was applied in atmospheric conditions, after removing sample from vacuum chamber, to demonstrate that a long-term encapsulation can be applied on top of presented PVD multilayer. Figure 2 presents three stages of sample fabrication process, Al/Ti contacts deposited onto glass substrates (top image), lithium deposition (on the right) and the finished PVD multilayer plus epoxy encapsulation (on the left). Ubrig et al evaluated the oxidation state of lithium by measuring the variation of weight versus time [16]. In this work, the film resistance value was measured in order to evaluate the oxidation state, under the premise that metallic lithium is an electrical conductor and lithium oxide (Li2O) is an electrical insulator [14, 24]. Four point resistance measurement setup was used in order to increase measurement accuracy. Current is injected in outer contacts and voltage measured in inner contacts (figure 2), thus avoiding wire and contact resistances. Measurements were performed with an Agilent 34401A multimeter. Considering the physical dimensions (visible in figure 2—top), the resistivity of lithium ‘ρ’ was calculated using equation (1), where ‘R’ is the resistance
This prevents the facilities costs inherent to the use of extra vacuum chambers and glove-box and the inherent manipulation problems. The encapsulation proposed contains three sputtered layers: lithium phosphorous oxide (LiPO), lithium phosphorous oxynitride (LiPON) and silicon nitride (Si3N4), each 20 nm thick. After these depositions and at atmospheric conditions, an epoxy was applied on the PVD multilayer to complete the encapsulation for long term protection. An art design of completed battery, including the encapsulation is presented on figure 1.
2. Samples preparation and measurement procedure Previous lithium oxidation measurements techniques were based on variation of lithium weight versus time [7, 16]. As far as authors know, the first time that the evolution of lithium film oxidation was measured by its electrical resistivity was presented in [20–22]. In this work, the protection of a 6 μm lithium film, on top of glass substrate is addressed. Al/Ti contacts with 2 μm thickness (see table 1) were deposited on top of a glass substrate by e-beam technique. Ti is used to avoid chemical reaction of lithium with the Al contact and Al is used for its high electrical conductivity. Metallic lithium was deposited by thermal evaporation on top of these contacts. The protective layers were then deposited in the same 3
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J. Phys. D: Appl. Phys. 48 (2015) 395306
Figure 3. Theoretical values of lithium resistivity at several temperatures [25] and linear approximation used in this work.
measured, ‘W’ is the width, ‘L’ is the length and ‘HiLi’ is the initial (before any oxidation) lithium thickness measured. The ‘HiLi’ of lithium films was measured in situ by a quartz crystal microbalance (Sycon STM-100, 6 MHz). R * W * HiLi ρ= (1) L
Lithium electric resistivity was documented before by Chi [25] and used in this work to compare data. Since samples were heated during deposition process and cooling occurs during measurement time, temperature dependence of resistivity was considered. Figure 3 presents theoretical values of lithium resistivity in function of temperature obtained in [25]. We used a linear approximation to calculate resistivity as function of temperature, as presented also in figure 3. Lithium films were exposed to air and resistance recorded in small time intervals until resistance value exceeded 1 MΩ. Using resistance value as input, the remaining non-oxidized lithium thickness film was calculated. Lithium oxide (Li2O) was assumed as perfect insulator [14, 24] when compared to metallic lithium (ρ ≈ 0.1 Ω μm), as presented elsewhere [25]. It was also considered that oxidation occurs at same rate all over the film, thus a layer of lithium oxide is created on top, consuming available metallic lithium. This calculation considered that lithium only oxidizes on top and forms a two layer structure of Li/Li2O. Equation (2) is used to calculate the remaining non-oxidized lithium film thickness ‘HLi’, considering the sample temperature ‘T’, dimensions of sample ‘W’ and ‘L’ and measured resistance ‘R’. −4
Figure 4. Oxidation evolution of lithium films exposed to air, with initial thickness of 3 μm, 4.5 μm and 6 μm: (a) film resistivity (with temperature correction from equation (1)) and (b) thickness of nonoxidized lithium film (calculated by equation (2)). Vacuum broken and lithium was exposed to atmosphere at 0 min.
lithium thickness) registered on figure 5. Samples with epoxy (‘Li/LiPO/LiPON/Si3N4/epoxy’) and without epoxy (‘Li/ LiPO/LiPON/Si3N4’) were also prepared and results compared on figure 6. 3. Results and discussion The first step was to evaluate the oxidation of lithium film at air (without multilayer protection). Figure 4 presents a comparison of three lithium films with different thickness. In this test, lithium films with thickness of 3 μm, 4.5 μm and 6 μm were deposited and exposed to air right after the deposition. Figure 4(a) presents the resistivity of the whole film during oxidation, calculated with equation (1) (air was injected to the vacuum chamber during 1 min and the vacuum chamber and instant 0 refers to the moment the chamber is firstly completely open). Resistivity graph (figure 4(a)) shows a start resistivity of lithium of 0.16 Ω μm, almost the same as the theoretical
−2
L * (4.07 × 10 * T + 8.49 × 10 ) HLi = (2) R*W
Several samples were prepared, all in glass substrate with Al/ Ti contacts (see figure 2—top). Samples with 3 μm, 4.5 μm and 6 μm of lithium film thickness were deposited and the oxidation evolution registered on figure 4. LiPO plus Si3N4 (‘Li/LiPO/Si3N4’) and LiPO plus LiPON (‘Li/LiPO/LiPON’) were deposited on top of a 6 μm lithium film and the oxidation evolution (lithium resistivity and non-oxidized calculated 4
J F Ribeiro et al
J. Phys. D: Appl. Phys. 48 (2015) 395306
Figure 6. Oxidation evolution of 6 μm lithium films encapsulated with LiPO/LiPON/Si3N4 and LiPO/LiPON/Si3N4/epoxy: (a) film resistivity and (b) calculated thickness of non-oxidized lithium film. Vacuum broken and lithium was exposed to atmosphere at 0 min.
Figure 5. Oxidation evolution of 6 μm lithium films encapsulated with LiPO/Si3N4 and LiPO/LiPON: (a) film resistivity and (b) calculated thickness of the non-oxidized lithium film. Vacuum broken and lithium was exposed to atmosphere at 0 min.
and dielectric protection to the battery. Si3N4 was deposited in a N2 rich atmosphere, as presented in table 1. Figures 5(a) and (b) presents oxidation results of this encapsulation (resistivity and lithium thickness, respectively). Despite the good protection grade obtained with a LiPO/Si3N4 film (resistivity is constant for more than 4 h), LiPO thin-film wasn’t enough to protect metallic lithium from the reaction that occurs during Si3N4 deposition. In figure 5(a) the increase of lithium resistivity (from 0.16 to 0.5 Ω μm) with the deposition of Si3N4 is visible, corresponding to a thickness of non-oxidized lithium of 1 μm, as stated in figure 5(b). Due to this reaction, LiPON was used as intermediate thin-film between LiPO and Si3N4 because it is a good protective layer, even few nanometers thick [26] and because it is chemically stable with LiPO and Si3N4 thin-films. Oxidation results are also presented in figures 5(a) and (b) demonstrating that lithium resistivity wasn’t affected by LiPON deposition. The large variations visible in the first minutes of test were attributed to the rough opening of vacuum chamber, in order to be fast, and consequently rapid variation on lithium temperature, not accurately measured due to different thermal inertia of films and the temperature sensor.
value of 0.1 Ω μm [25]. The difference is attributed to the time that takes to completely open the chamber (about 1 min), since lithium is already in contact with some air, before instant 0s in the graph. Figure 4(b) presents the non-oxidized lithium thickness, calculated with equation (2). When lithium oxide grows on top of the lithium film, it will automatically serve as protection for the lithium remaining bellow. This explains why the thinner films’ (3 μm) oxidation rate is higher than the other thicker films’ (6 μm). All the other experiences presented here were performed with 6 μm thick lithium film and all the resistivity and thickness graphs begin when samples were in full contact with atmosphere. The first thin-film deposited on top of metallic lithium was LiPO (20 nm) because it is chemically stable in contact with lithium and only uses argon during deposition. The use of nitrogen or other non-inert gas in contact with lithium, could lead to unwanted reactions with the lithium film during deposition. After LiPO deposition, a thin-film of Si3N4 (20 nm) was deposited. Si3N4 is a common material used in microelectronics to encapsulate devices and also allows mechanical 5
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J. Phys. D: Appl. Phys. 48 (2015) 395306
Figure 7. Long term (9 d) oxidation evolution of 6 μm lithium films after complete encapsulation (PVD films and epoxy). Film resistivity (left scale) and calculated thickness of non-oxidized lithium film (right scale).
Considering results of LiPO/LiPON films, Si3N4 thin-film (20 nm) was then deposited on top of LiPO/LiPON thin-film and proved to be an efficient protection for at least two hours (see figures 6(a) and (b)). After this time (140 min), an abrupt increase in lithium resistivity (figure 6(a)) and decrease in lithium remaining thickness (figure 6(b)) were measured, meaning that Si3N4 barrier lost the ability to protect lithium. Two hours is enough time to open the vacuum chamber and apply epoxy in order to complete the encapsulation without the need for any extra chamber (like CVD or glove-box chambers). After epoxy application the lithium maintains nonoxidized for more than a week, as demonstrated in figure 7. Further electrochemical investigations should be performed to prove the encapsulation performance during battery operation.
This multilayer film protected the lithium for more than 2 h in contact with atmosphere. After, an epoxy was applied at atmospheric conditions, finalizing the encapsulation. To evaluate oxidation state of lithium films, the lithium resistance was measured in a four probe setup. Resistivity was calculated, considering physical dimensions and temperature dependence. To better interpret the results, a calculation on the remaining non-oxidized lithium film was also performed. The presented PVD multilayer encapsulation exonerates the use of chemical vapour deposition (CVD) techniques and glove-box chambers, significantly reducing the fabrication cost.
4. Conclusions
This work was financially supported by FEDER/COMPETE and FCT funds with the projects PTDC/EEA-ELC/114713/2009, PEST-C/QUI/UI0686/2013 and UID/EEA/04436/2013, first author scholarship SFRH/BD/78217/2011, fourth author scholarship SFRH/BPD/95905/2013, and CRUP AI TC-09_14.
Acknowledgments
A chemically stable encapsulation method for lithium microbatteries was developed using only PVD techniques, in the same vacuum chamber that lithium microbatteries are fabricated. LiCoO2 cathode, LiPON electrolyte and metallic lithium anode are the most common materials used in this battery technology and are deposited by RF sputtering and thermal evaporation. Lithium anode is a challenging material in terms of encapsulation because it reacts with many materials deposited on top and almost instantaneously oxidizes in contact with atmosphere (oxygen, water). Metallic lithium anode was deposited and encapsulated, in a single vacuum chamber, and a standard lithium resistivity of 0.16 Ω μm was maintained for more than a week, demonstrating that oxidation was reduced to low levels. Samples with Al/Ti contacts on glass substrate were used to deposit metallic lithium and the protection layers in order to evaluate encapsulation efficiency. Encapsulation multilayer film was composed of LiPO, LIPON and Si3N4 layers, each 20 nm thick, deposited by RF sputtering.
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