Negligible âNegative Space-Charge Layer Effectsâ at Oxide-Electrolyte

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Negligible “negative space-charge layer effects” at oxideelectrolyte/electrode interfaces of thin-film batteries Masakazu Haruta, Susumu Shiraki, Thoru Suzuki, Akichika Kumatani, Takeo Ohsawa, Yoshitaka Takagi, Ryota Shimizu, and Taro Hitosugi Nano Lett., Just Accepted Manuscript • Publication Date (Web): 24 Feb 2015 Downloaded from http://pubs.acs.org on February 24, 2015

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Negligible “negative space-charge layer effects” at oxide-electrolyte/electrode interfaces of thin-film batteries Masakazu Haruta†,‡, Susumu Shiraki†, Tohru Suzuki†, Akichika Kumatani†, Takeo Ohsawa†, Yoshitaka Takagi†, Ryota Shimizu†, and Taro Hitosugi†,* †

Advanced Institute for Material Research (AIMR), Tohoku University, 2-1-1 Katahira, Aoba, Sendai 980-8577, Japan ‡

Organization for Research Initiatives and Development, Doshisha University, 1-3 TataraMiyakodani, Kyotanabe, Kyoto 610-0321, Japan

ABSTRACT: In this paper, we report the surprisingly low electrolyte/electrode interface resistance of 8.6 Ω cm2 observed in thin-film batteries. This value is an order of magnitude smaller than that presented in previous reports on all-solid-state lithium batteries. The value is also smaller than that found in a liquid electrolyte based batteries. The low interface resistance indicates that the negative space-charge layer effects at the Li3PO4-xNx/LiCoO2 interface are negligible, and demonstrates that it is possible to fabricate all-solid state batteries with faster charging/discharging properties.

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KEYWORDS: Li-ion battery, Thin-film battery, Charge-transfer resistance, Interface resistance, Space-charge layer effect, Solid electrolyte.


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All-solid-state lithium batteries are expected to become the next generation of batteries owing to their improved safety, larger capacity, and wider potential window relative to present Li-ion batteries based on organic liquid electrolytes.1,2 One of the major drawbacks for the practical use of all-solid-state batteries is their large interface resistance (Ri) at the solid electrolyte and electrode interface, which hinders fast charging/discharging. The negative effects of a space-charge layer was proposed as the origin of such large Ri, especially for interfaces of sulfide-electrolytes/oxide-electrodes.3-5 To reduce the Ri of electrolyte/electrode interfaces, electrodes coated with buffer layers have been tested in bulk-type all-solid-state batteries.3-6 However, details of the origin of the Ri and guidelines to reduce it have yet to be clarified. This is because it is extremely difficult to analyze quantitatively the ionic conduction across the interface in bulk-type batteries composed of powders since the interface area, crystal structures, and orientations cannot be defined. A well-defined thin-film interface has been the preferred platform for investigating Li-ion conductivity at electrolyte/electrode interfaces in battereies.1,2,7-12 Systematic and quantitative studies of interface resistance as a function of the growth process or film thickness would provide insight into the origins of interface resistance, and lead to its reduction. Moreover, it is not known whether all-solid-state batteries can achieve lower Ri values than liquid-electrolyte batteries. It would be encouraging for all-solid-state battery research if one can prove that Ri values can be made very small for fabricating higher power batteries that are superior to liquidelectrolyte batteries. In this study, we demonstrate a very low electrolyte/electrode Ri in thin-film batteries with Li/Li3PO4-xNx(LiPON)/LiCoO2 structures (Figure 1(a)). The thin-film batteries were never exposed to air throughout their preparation and evaluation, and thus, the electrolyte/electrode


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interfaces were free from contamination. We obtained a remarkably low Ri of 8.6 Ω cm2 when the LiPON films were deposited in an off-axis sputtering configuration. This value is an order of magnitude smaller than that presented in previous reports on all-solid-state batteries. The value is also smaller than that found in batteries using liquid electrolytes. We found that the sputtering deposition process of LiPON significantly affects the interfacial properties. The results indicate that damages at the interface are the main cause of the resistance, and the negative space-charge layer effect is negligible in this system. This study suggests that the internal resistance of allsolid-state batteries can be made very small, which is encouraging for fabricating all-solid-state batteries for high-power applications. To ensure a clean electrolyte/electrode interface in the thin-film batteries, the ultra-high vacuum (UHV) chambers used for film depositions and characterizations were all directly connected to each other. The fabricated thin-film batteries consisted of a LiCoO2 cathode, Li3PO4 or LiPON electrolyte, and Li anode, as shown in Figure 1(a) and (b). First, a polycrystalline Au film, to act as the current collector, was deposited on an unheated Al2O3 (0001) single-crystal substrate using DC magnetron sputtering. To improve the orientation of the crystallites, the deposited Au film was then annealed at 600°C for 10 min in a vacuum chamber. After being annealed, the Au film had a (111) orientation, and the full-width at half maximum (FWHM) of a rocking curve of the (111) peak was 1.7°. On this Au layer, the LiCoO2 (001) cathode film, in trigonal notation, was deposited using pulsed laser deposition (PLD).13,14 During the deposition of the LiCoO2, the substrate was kept at 400°C in an oxygen partial pressure of 0.13 Pa. A sintered ceramic target with a Li-rich composition of Li1.2CoO2 (Toshima Manufacturing Co. Ltd., Japan) was used to compensate for the loss of Li during deposition, and a KrF excimer laser (wavelength = 248 nm, repetition rate = 5 Hz, and energy density = 1.0 J cm-


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) irradiated the ceramic target. A typical surface roughness (Ra) of the LiCoO2 thin films was 2.5

nm, and in-plane X-ray diffraction (XRD) measurements showed the in-plane orientation of the LiCoO2 thin films was random. Subsequently, a solid electrolyte layer of either Li3PO4 or LiPON was deposited onto the LiCoO2 layer using RF magnetron sputtering at room temperature in a 0.5 Pa atmosphere of either Ar or N2, respectively. RF power was fixed at 100 W because when the RF power was above 200 W, the LiPON film was frequently peeled from the Al2O3 substrate with accompanying removal of the LiCoO2 films. The sputtering depositions were conducted in either on-axis or off-axis configurations (Figure 1(c)), using a 2 inch diameter Li3PO4 sputtering target (Toshima Manufacturing Co. Ltd., Japan). Finally, a metallic Li anode was deposited on the electrolyte using a conventional thermal evaporation method under 1 × 10-6 Pa. We measured the thickness of the Au, LiCoO2, and LiPON layers with a stylus profiler (VEECO DEKTAK150). The thickness of the Li layer was estimated with a quartz crystal microbalance (QCM) thickness monitor. The typical thicknesses of the Li, LiCoO2, and Au layers were 600, 100, and 100 nm, respectively. The active area was circular with a 0.25 mm radius (0.2 mm2). To evaluate the electrochemical properties, the fabricated samples were transferred to a characterization chamber without air-exposure, and the electrochemical properties were measured using a potentiostat/galvanostat with a frequency response analyzer (Biologic SP-150). The impedance spectra were obtained by applying an AC voltage with an amplitude of 50 mV at frequencies of 0.1 Hz to 1 MHz. All the thin-film batteries presented in this paper exhibited good battery performance. Figure 2(a) shows charge/discharge curves, in the voltage range of 3.5 to 4.3 V, of a thin-film battery with a Li3PO4 electrolyte deposited in the on-axis configuration (thickness = 500 nm). A current of 20 nA, corresponding to a 1 C rate, was applied for charging and discharging. The curves


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show a characteristic voltage plateau at 3.9 V and inflections at approximately 4.15 V. The former is attributed to the coexistence of two LiCoO2 hexagonal phases, and the latter originates from a phase transition between the hexagonal and monoclinic phases of LiCoO2.15,16 The capacity corresponded to 76% of the theoretical capacity (138 mAh g-1) for LiCoO2 utilizing 50% of the Li. As shown in Figure 2(b), the discharge capacity did not degrade, even up to the 100th cycle, and the charge/discharge Coulomb efficiency of the 100th cycle was 97%. These charge/discharge characteristics show the distinctive behavior of batteries composed of LiCoO2, and confirm the good performance of the fabricated thin-film batteries. We now discuss the charge transfer resistance and Ri evaluated with impedance spectroscopy. Figure 3(a) shows Nyquist plots of the impedance spectra of the thin-film battery with Li3PO4 electrolyte shown in Figure 2. The discharged state (open-circuit voltage, Voc = 3.8 V) shows one semicircle at a higher frequency region, originating from the electrolyte’s resistance. We confirmed this resistance corresponds to the electrolyte’s resistance by changing the electrolyte thickness. The ionic conductivity of the Li3PO4 electrolyte, estimated from the impedance spectra, was 2.4 × 10-8 S cm-1, which corresponded well with the value obtained from the Au/Li3PO4/Au blocking cell. In contrast, a smaller semicircle emerged at a lower frequency region in the case of the charged state (Voc = 4.2 V). This smaller semicircle is attributed to charge transfer resistance at the electrolyte/electrode interface because the electrochemical Li intercalation reaction only occurs above ∼3.9 V for LiCoO2.9-11,17 The value of Ri derived from the product of the charge transfer resistance and the active area was approximately 200 Ω cm2, which is a typical value for all-solid-state batteries. Next, a thin-film battery with a 1000-nm-thick LiPON electrolyte layer (on-axis configuration) was fabricated to investigate any correlation between the ionic conductivity of the


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electrolyte and Ri values. The performance of this battery was very similar to that shown in Figure 2(a), with further details shown in Figure S1. The semicircle observed at the higher frequency region in the Nyquist plot shown in Figure 3(b) was very small compared to that of the thin-film battery with the Li3PO4 electrolyte (Figure 3(a)). Using the impedance spectra to estimate the ionic conductivity, the conductivity of the LiPON electrolyte (1.8 × 10-6 S cm-1) was found to be almost two orders of magnitude greater than that of the Li3PO4 electrolyte. This difference in ionic conductivity is attributed to the incorporation of N in Li3PO4. In contrast to the high conductivity observed in the electrolyte, the derived Ri was unexpectedly large at 880 Ω cm2. These results indicate that an electrolyte with high ionic conductivity does not always lead to lower Ri values. Considering the large increase in Ri, despite the high conductivity of LiPON, we assumed that the large Ri values originated from sputtering damages in the LiCoO2 electrode surface due to N-ion bombardment. It has been reported that negatively charged ions induce damages in films deposited by sputtering.18,19 Thus, in our experiment, the Ri values of the thin-film batteries with Li3PO4 deposited in an Ar atmosphere are lower than that of the thinfilm batteries with LiPON deposited in a N2 atmosphere in the on-axis configuration. The negatively charged N ions induce larger damages in the samples compared to the positively charged Ar ions. As a result, we investigated interfaces prepared in the off-axis configuration to reduce the sputtering damages. A drastic decrease in Ri was confirmed in a LiPON thin-film battery (LiPON thickness = 1500 nm) that was deposited in the off-axis configuration (Figure 1(c)), indicating that the resistance of electrolyte/electrode interfaces strongly depends on the sputtering process. In contrast to the two semicircles measured in the charged state for the on-axis sample, the off-axis thin-film battery shows only one semicircle and a linear slope at low frequencies (Figure 3(c)).


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The absence of a semicircle corresponding to a charge transfer process at the electrolyte/electrode interface can be attributed to the very low Ri value. We speculate that the impedance component of Ri overlaps with the semicircle corresponding to the impedance of the LiPON electrolyte. To gain further insight into the LiPON/LiCoO2 interface prepared in the off-axis configuration, we investigated a thin-film battery with a thinner LiPON electrolyte layer. A 100nm-thick LiPON film was deposited to make the interface resistance component dominant by reducing the resistance of the LiPON film. As a result, two semicircles appeared in the impedance spectrum (Figure 3(d)), and the spectrum was fitted in order to determine the Ri values. Surprisingly, the thin-film battery showed a remarkably low interface resistance of 8.6 Ω cm2. We confirmed the reproducibility of this result, and at least ten thin-film batteries exhibited Ri values ranging from 8.6 to 10 Ω cm2. The ionic conductivity estimated from the impedance spectra of the LiPON electrolyte deposited in the off-axis configuration was 7.1 × 107

S cm-1, which was lower than that of the LiPON electrolyte deposited in the on-axis

configuration. The values of the N/P atomic ratio estimated from Rutherford backscattering spectrometry (RBS) were 0.05±0.03 and 0.09±0.04 for the on- and off-axis configurations, respectively, and details are described in the supporting information. Although these values are very close to the detection limit of RBS, the results indicate that the values of N content in the films prepared in two configurations are very small and similar. Consequently, it is evident that the N/P ratio is not the primary factor that determines the interface resistance. Moreover, we prepared a thin-film battery with the LiPON deposited under a higher N2 pressure, 2.0 Pa. The deposition of LiPON at a N2 pressure of 2.0 Pa (off-axis) resulted in a Ri value of 25 Ω cm2 (Figure S2). Although the sample had similar conductivity (6.0 × 10-7 S cm-1) with LiPON


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deposited at N2 pressure of 0.5 Pa (off-axis, 7.1 × 10-7 S cm-1), the samples exhibited different interface resistances. The interface resistance values ranging from 8.6 to 10 Ω cm2 are almost an order of magnitude smaller than that found in previous studies (Table 1). Kuwata et al. and Iriyama et al. reported that the Ri values for LiPON/LiCoO2 interfaces could be as low as 90 Ω cm2 and 125 Ω cm2, respectively.9,11 The extremely low interface resistance in our thin-film batteries indicate that it is possible to obtain a very low interface resistance in bulk all-solid-state batteries. To understand the nature of the interface, the activation energy (Ea) of Ri was evaluated (Figure 4). The Ea was derived from the temperature dependence of Ri using the Arrhenius equation below:

= exp −

(1)

where T, C, and k are the absolute temperature, the pre-exponential factor, and the Boltzmann constant, respectively. The Ea values were 0.46 and 0.38 eV for batteries with LiPON deposited in on- and off-axis configurations, respectively (Table 1). In the former case, the value is similar to those presented in earlier reports.10,11 The increased Ea of the on-axis sample can be attributed to damages introduced by the bombardment of ions during the sputtering process; it is well known that the on-axis configuration increases damage effects due to ion bombardment compared to the off-axis configuration.18,20 This speculation is further supported by the deposition rates of the LiPON films. For the on-axis configuration, the deposition rate was 40 nm/h, whereas the rate was 58 nm/h for the off-axis configuration. The lower deposition rate of on-axis sputtering implies that re-sputtering of LiPON is also occurring because of the highenergy O- and N-ion bombardment during the deposition process. Furthermore, we confirmed that the deposition rate varied little with target-substrate distances of 70 to 110 mm in the on-axis


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configuration. This result can again be interpreted as ion bombardment with negatively charged N ions; as the target-sample distance decreases, the effects of ion bombardment become more apparent and cancel out the increased deposition rate. Thus, it is most likely that the LiCoO2 surface is damaged in the initial deposition process of LiPON, leading to the formation of deep Li-trapping centers; possible origins are Co or O vacancies and/or the incorporation of N or P into the LiCoO2 surface. In order to achieve a lower Ea and Ri, optimization of the electrolyte deposition process is necessary so that sputtering damage and contamination of the electrolyte/electrode interface must be avoided. Moreover, our results show the Ea and Ri values are smaller than those of liquid-electrolyte batteries. The typical Ea and Ri values of a liquid-electrolyte/film-LiCoO2 interface are 0.64 eV and 25 Ω cm2, respectively, for the interface between 1 mol dm-3 LiClO4 dissolved in propylene carbonate (PC) and a LiCoO2 film.9,21 In general, batteries with a liquid electrolyte require a desolvation process for the charge transfer reaction at the liquid-electrolyte/electrode interface, and thus, contributions from the desolvation process dominate both Ea and Ri values. Consequently, lower Ea and Ri values are expected in all-solid-state batteries because of the absence of the desolvation reaction. However, there has been no experimental evidence of such low Ea and Ri values. In this study, for the first time, lower Ea and Ri values, relative to those for batteries with a liquid electrolyte, were successfully demonstrated. Although the origin of Ri is proposed to be the space-charge layer effect,3-5,22 the Ri values demonstrated here indicate that the main cause of resistance at the interface is dominated by the effect of defects. These results lead us to conclude that the space-charge layer effect, which has a negative impact on battery performance, is negligible in the LiPON/LiCoO2 system because the Ri value is very small. This conclusion is further supported by our Ri value being smaller than


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that of liquid-electrolyte batteries, which are free from space-charge layer effects. Our study implies that if bulk-type all-solid-state batteries are fabricated properly, a high conductance at the solid-electrolyte LiPON/LiCoO2 interface can be achieved. In summary, we have demonstrated that the interface resistance of all-solid-state lithium batteries can be as low as 8.6 Ω cm2. This value is an order of magnitude smaller than that presented in previous reports on all-solid-state batteries, and is smaller than that observed in liquid-electrolyte batteries. The key to obtaining a low interface resistance with low activation energy is to reduce sputtering damage by using an off-axis sputtering configuration. In addition, an all-in-vacuum fabrication process should be used to avoid contamination of the electrolyte/electrode interface. The low interface resistance obtained implies that the negative space-charge layer effect is negligible in the LiPON/LiCoO2 interface. These results are very encouraging for the application of all-solid-state batteries because our results indicate that it is possible to obtain a very low interface resistance in bulk all-solid-state batteries.


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Figure 1. (a) Schematic cross-sectional view and (b) microscope photograph of a fabricated thinfilm battery. (c) Configurations of a sample substrate and a Li3PO4 target in the sputtering chamber.


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Figure 2. (a) Charge and discharge curves, with a constant current of 20 nA corresponding to a 1 C rate, for the thin-film battery with a Li3PO4 electrode. (b) Cycle dependence of the discharge capacity with 1, 1.5, and 2 C rates (20, 30, and 40 nA, respectively) of the thin-film battery shown in (a).


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Figure 3. Impedance spectra of thin-film batteries deposited in the on-axis configuration in: (a) an Ar atmosphere (Li3PO4) and (b) a N2 atmosphere (LiPON). The inset of (b) shows the high frequency region magnified. Impedance spectra of batteries with LiPON deposited in the off-axis configuration with a thickness of (c) 1500 nm and (d) 100 nm. Square and circle symbols represent the data for the discharged and charged states, respectively. Note that the top x-axes are normalized with the activation area, A, of 0.0252π cm2, to determine the interface resistance.


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Figure 4. Temperature dependence of charge transfer resistance (interface resistance, Ri) for thin-film batteries deposited in the on- and off-axis sputtering configurations.


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Table 1. Summary of the conductivities (σEL) of electrolytes (EL), and the interface resistances (Ri) and activation energies (Ea) of EL/LiCoO2 interfaces. PC represents a liquid electrolyte of 1 mol dm-3 LiClO4 dissolved in propylene carbonate.

Sputtering

σEL [S cm-1]

EL

Position

2

Ri [Ω cm ]

Ea [eV]

-8

~200

0.38

-6

880

0.46

-7

8.6

0.38

4.6 × 10

-7

90

-

-

125

0.59

-

25

0.64

Li3PO4

2.3 × 10

LiPON

1.8 × 10

LiPON

7.1 × 10

On-axis

Off-axis

References

Li3PO4

a

LiPON

b

PC

a

c

Ref.11; b Ref. 9 and 10; c Ref. 9 and 20.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions M. H., S. S., A. K., R. S., and T. H. conceived the experimental strategies. M. H., S. S., T. S., T. O., R. S., and T. H. designed and constructed the fabrication/characterization system of thin-film batteries. M. H., T. S., A. K., T. O., and Y. T. fabricated the thin-film batteries and optimized the fabrication processes. M. H., S. S., T. S., and A. K. performed the electrochemical evaluations. M. H., S. S., and T. H. wrote the manuscript. All authors discussed the results.

ACKNOWLEDGMENT This research was supported by the World Premier International Research Center Initiative (WPI Initiative), the Japan Society for the Promotion of Science through its Funding Program for World-Leading Innovation R&D on Science and Technology (FIRST Program), and Toyota Corporation. We thank Dr. H. Oki and Dr. N. Kuwata for fruitful discussions. We also thank Toray Research Center, Inc. for RBS analysis. M. H. acknowledges a Grant-in-Aid for Scientific Research (No. 25820335) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan.


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111-116. (22)

Yada, C.; Ohmori, A.; Ide, K.; Yamasaki, H.; Kato, T.; Saito, T.; Sagane, F.; Iriyama, Y.

Adv. Energy Mater. 2014, 4, 1301416.


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