237

한국표면공학회지J. Korean Inst. Surf. Eng.

Vol. 50, No. 4, 2017.

https://doi.org/10.5695/JKISE.2017.50.4.237<연구논문>

ISSN 1225-8024(Print)

ISSN 2288-8403(Online)

Characterization of RF Sputter-deposited Sodium Phosphorous

Oxynitride Thin Films as a Solid-state Sodium-ion Conductor

Sang-Eun Chun*

School of Materials Sciences and Engineering, Kyungpook National University, Daegu 41566, South Korea

(Received August 21, 2017 ; revised August 27, 2017 ; accepted August 28, 2017)

Abstract

We demonstrated the thin film deposition of sodium phosphorous oxynitride (NaPON) via RF magnetronsputtering of Na3PO4, as a solid-state Na-ion conductor similar to lithium phosphorous oxynitride (LiPON),which is a commonly used solid electrolyte. The deposited NaPON thin film was characterized by scanningelectron microscopy, X-ray diffractometry, and electrochemical impedance spectroscopy, to investigate thefeasibility of the solid-state electrolyte in several different cell configurations. The key properties of a solid-state electrolyte, i.e., ionic conductivity and activation energy, were estimated from the complex non-linearleast square fitting of the measured impedance spectra at various temperatures in the range of 27-90 oC. Theionic conductivity of the NaPON film was measured to be 8.73 × 10-6 S cm-1 at 27 oC, which was comparableto that of the LiPON film. The activation energy was estimated to be 0.164 eV, which was lower than thatof the LiPON film (0.672 eV). The obtained values encourage the use of a NaPON thin film in the futureas a reasonable solid-state electrolyte.

Keywords : Sodium phosphorous oxynitride (NaPON), RF magnetron sputtering, Ion conductor, Ionic conductivity,

Activation energy

1. Introduction

All-solid-state batteries have acquired significantattention as a safer energy storage device in light ofseveral accidental explosions involving theconventional “wet” Li-ion batteries [1,2]. Safety canbe achieved by replacing flammable organicelectrolytes with nonflammable solid-state electrolytesin the current Li-ion based energy devices. To satisfythe ever-growing demand for miniaturized electroniccomponents, the use of solid-state electrolytes, inwhich the volumetric energy density of the solidphase is higher than that of the liquid phase, ispreferred over liquid electrolytes. In this perspective,microbatteries have been developed by employing a

thin solid-state electrolyte and solid electrodes [3-5].In addition to nonflammability and a higher energydensity, solid electrolytes offer several advantagessuch as chemical stability and no leakage, makingthem a promising candidate for use in future energydevices.

Various solid-state lithium electrolytes have beenresearched with the goal of developing a Li-ionbased all-solid-state battery. Lithium phosphorousoxynitride (LiPON) is one of primarily used solidelectrolytes, because it can provide a high energyoutput due to the wide stable potential window of aLiPON film (0-5 V vs Li+/Li) [6,7]. Thin films ofLiPON have been successfully incorporated into all-solid-state rechargeable lithium batteries [6,8-11]. Inspite of the multiple benefits of the LiPONelectrolyte as a solid phase, there are issues that needto be addressed when assembling a practical solid-state battery: the low ionic mobility through the solidphase, the reduced interfacial contact area with anelectrode material, and the greater complexity in the

*Corresponding Author: Sang-Eun Chun

School of Materials Science and Engineering, KyungpookNational University.Tel: +82-053-950-5566 ; Fax: +82-053-950-6559E-mail: sangeun@knu.ac.kr

238 Sang-Eun Chun/한국표면공학회 50 (2017) 237-243

manufacturing process of solid electrolytes ascompared to liquid electrolytes. Ongoing studies areattempting to overcome these issues by reducing thefilm thickness and incorporating more nitrogen intothe film [12-14].

Most of the currently used solid electrolytes,including LiPON, were designed to transport Li ionsthrough solid electrolytes, because the most widelyused electrodes donate/accept Li ions to store/releaseenergy, respectively. However, as lithium is a limitedresource and is expensive, there is a growing demandin developing energy devices based on ions otherthan lithium for future energy system [15-18].Sodium is a potential alternative to lithium, due to itslower cost, non-toxicity, and greater abundance thanlithium. Various sodium-based electrodes andelectrolytes have been developed to fabricate sodium-based energy systems [19-21]. However, theavailability of solid-state ionic conductors for Naions is very limited. To achieve Na-ion based highenergy density systems, a solid-state Na-ionconductor is required. Thin film Na-ion conductorscan also be used in a number of other applicationslike commercial molten Na-based batteries andsensors [19,22-24].

Similar to Li-ion solid electrolytes, solid-state Na-ion conductors also require a high ionic conductivity,electrochemical stability, and mechanical stability. Naions, being larger in size than Li ions, have a higherresistance to migration than Li ions, resulting in aslower charging/discharge rate (and correspondingly,lower power density). Therefore, a reasonableconductivity should be ensured to achieve a goodpower performance. Electrochemical compatibility ofthe solid electrolyte with Na ions is necessary forstable long-term cyclability. The dendritic structureof sodium may grow during cycling; this isdetrimental to long-term cycling. Therefore, themechanical stability of a solid-electrolyte needs to bechecked. Typically, solid electrolytes with a low ionicconductivity have an increased conductivity at hightemperatures due to a lower activation energy.Therefore, the ionic conductivity at high temperaturesneeds to be estimated.

Herein, we report the application of a solid-statesodium phosphorous oxynitride (NaPON) thin filmanalog to LiPON prepared by radio frequency (RF)magnetron sputtering, as a solid-state Na-ionconductor. The feasibility of this deposited RF filmas a Na-ion conductor was explored based on theconductivity and activation energy, since good ionic

conductivity and low activation energy are essentialfor a high energy output and good powerperformance. Electrochemical analysis using ac-impedance spectroscopy was carried out on theNaPON film to assess the abovementionedproperties. The results were discussed by comparingthe impedance spectra data for LiPON and NaPON.

2. Experimental

2.1. Preparation of NaPON and LiPON films

The sodium phosphorous oxynitride (NaPON) thinfilm was sputtered on a glass substrate by RFmagnetron sputtering of a sodium phosphate(Na3PO4) target using a similar manufacturingprocess as described elsewhere [6]. As a controlsolid-state electrolyte thin film, a lithiumphosphorous oxynitride (LiPON) film was alsoprepared on the same substrate via RF magnetronsputtering of a lithium phosphate (Li3PO4) target.

The targets for depositing the LiPON and NaPONfilms were manufactured by two-step calcination ofthe Li3PO4 and Na3PO4 powders, respectively. First,the as-received Li3PO4 (Sigma-Aldrich) and Na3PO4

(96 %, Sigma-Aldrich) powders were heated to900 oC for 1 h in a horizontal tube furnace (LindbergBlue M, Thermo Scientific) under air (first calcinationstep). The calcined powder was then grinded with aceramic mortar and pestle. The grounded powder wasmixed with 5 mL of N-Methyl-2-pyrrolidone (NMP,anhydrous 99.5%, Sigma-Aldrich) and 2 mL ofmethanol (99 %), and then poured into a stainlesssteel mold to obtain a pellet-type target. The mixedpowder was then pressed with a force of 100 kg cm-2.Both pressed targets were heat-treated at 200 oC for1 h, and then heated at 900 oC for 1 h under air(second calcination step). The volume of the Na3PO4

target slightly contracted during calcination. Incontrast, no noticeable volume contraction wasobserved for the Li3PO4 target.

The sputtering power for deposition is an importantfactor that determines the surface topography andgrowth rate of films. The optimum power forsuccessful deposition was estimated to be 50 W and100 W for NaPON and LiPON, respectively. ForNaPON, application of a sputtering power higherthan 80 W during the deposition resulted in crackingand burning of the target surface. During filmsputtering, the sputtering chamber was maintainedwith nitrogen to the grow the oxynitride layer that isreported to show a good ionic conductivity [6].

Sang-Eun Chun/한국표면공학회 50 (2017) 237-243 239

2.2. Characterization

The surface morphology and cross-sections of thedeposited films were analyzed by scanning electronmicroscopy (SEM-EDS, Philips XL30) at a workingdistance of 10 mm and acceleration voltage of 5 kV.The crystal structure of the specimen was determinedby X-ray diffractometry (XRD) (Rigaku, powder X-ray diffractometer, Cu Kα radiation source(λ = 1.5418 Å)) within the 2θ range of 8o-90o.

For electrochemical measurements, the thin filmwas sandwiched between two molybdenum (Mo)films on a glass substrate. Thin Mo films weredeposited as current collector/blocking electrodes onthe top and bottom of the NaPON film. The Mo filmwas sequentially deposited after sputtering a titanium(Ti) bonding layer on the glass substrate. Aschematic of the specimen structure for theelectrochemical experiments is illustrated in Fig. 1.Since both the NaPON and LiPON thin films canundergo degradation in a humid and oxygen-richenvironment, the prepared specimen was quicklytransferred to a transparent jar filled with dry Arimmediately upon completion of the film depositionin the sputtering chamber. The overlapped geometricarea between the bottom and top layers of Mo layerswas designed to be 16 mm2 (4 mm × 4 mm).

The conductivity and activation energy of thedeposited specimens were analyzed byelectrochemical impedance spectroscopy to studytheir feasibility as a solid-state electrolyte. Theimpedance spectra were taken at a specific potentialwith a sine wave of amplitude 5 mV over the

frequency range of 0.05 Hz-5 × 105 Hz. Themeasured impedance spectra were fitted to theequivalent circuit model to extract the physicalparameters of the electrolyte. The impedancebehavior of the specimens was measured at varioustemperatures between room temperature (27 oC) and120 oC. Li-ion transport in the solid electrolyte filmwas tested with cyclic voltammetry (CV) at a scanrate of 5 mV/s. Impedance spectra and voltammetrymeasurements were performed with a potentiostat/galvanostat (SP-300, Bio-logic).

3. Results and Discussion

The surface structure of the deposited filmcharacterized by SEM is illustrated in Fig. 2(a) and(b). By sputtering the lab-made Na3PO4 and Li3PO4

targets for 10 h at 50 W and 100 W, 1.24 µm and1.22 µm thick films, respectively, were grown on thesubstrate, based on the SEM images. The growthrates of the RF sputtered films were calculated to be124 nm h-1 and 122 nm h-1 for the NaPON andLiPON films, respectively. The translucent thin filmswere then deposited. The cross-sectional imagesshow that continuous homogeneous films weresputtered by RF-sputtering the Na3PO4 and Li3PO4

targets for both thin films. A rapid growth rate ispreferred in the preparation of solid films formanufacturing efficiency. Hence, RF sputtering ofNa3PO4 is a suitable technique to deposit solid-stateNaPON thin films.

To investigate whether the NaPON film reacts withmoisture, it was exposed to air for 1 h. The SEMimage in Fig. 3 shows that the surface morphologyof the NaPON film underwent a transformation afterexposure to air for 1 h. After exposure to air, thesurface of the NaPON thin film became swollen dueto the reactive property of oxynitride to moisture,which is similar to that in the LiPON thin film [25].

Fig. 1. Schematic diagram of the plain and cross-

sectional views of the Mo/NaPON/Mo sandwich cell

configuration.

Fig. 2. SEM micrographs of the cross-section of (a) Mo/

NaPON and (b) Mo/LiPON/Mo sandwich structure

deposited on the glass substrate by RF magnetron

sputtering method.

240 Sang-Eun Chun/한국표면공학회 50 (2017) 237-243

Electrical measurements on the swollen NaPON filmgave inconsistent results; hence, prolonged exposureto air should be avoided while handling as-sputteredNaPON samples.

The crystallinity of the prepared NaPON film wasanalyzed by overlaying the XRD patterns of theNaPON and LiPON thin films, for comparison (Fig.4). No characteristic diffraction peak was observedfor the NaPON film deposited on glass; only a broaddiffraction peak was observed at around 25o, which isindicative of the formation of an amorphous NaPONfilm. Analog to the amorphous LiPON film preparedby sputtering Li3PO4 in N2, an amorphous NaPONfilm can be deposited by Na3PO4 in a N2

environment at ambient temperature [6,26]. Non-crystallinity is crucial for a solid film to be used as

an electrolyte, since ions can move more rapidlythrough a solid film of an amorphous structure thanthat of a crystalline phase [7]. Incorporation ofnitrogen into the RF-sputtered NaPON film cansignificantly increase the ionic conductivity of thefilm [6]. Both the amorphous structure and nitridedglass network contribute toward enhancement of theionic conductivity of the solid-state NaPONelectrolyte.

As an electrolyte material in an energy system, akey property is sufficient ionic conductivity at anambient temperature, because poor ionic conductivityleads to low power energy and low energy efficiency[8]. The ionic conductivity of a thin film can bequantitatively determined by the followingconductivity equation.

(1)

where σ is the conductivity, d is the film thickness, Ais the geometric area of the Mo contact, and Rel isthe thin film electrolyte resistance. The resistance ofthe solid electrolyte can be obtained by fitting theequivalent circuit model to the experimentallymeasured impedance spectra at room temperature.

Figure 5(a) illustrates the Nyquist plots of theimpedance spectra measured for the NaPON andLiPON thin films. Both impedance spectra have theshape of two successive semi-circles in the mediumfrequency range with an intercept on the real axis atlow frequencies without a blocking region. Bothsemi-circles can be modeled as the resistor and theconstant phase element (CPE) in parallel, where CPEwas used to describe the non-ideal behavior of thecapacitor component [27]. The first arc correspondsto the electrochemical behavior in the electrolyte,while the second one involves the grain boundary[6]. The modified CPA model was used to model theimpedance behavior of both RF-sputtered thin filmsbetween the top and bottom layers of Mo, as shownin Fig. 5(b) [28]. In the equivalent circuit model, Zinter

is the interfacial impedance between the currentcollectors and the electrolyte, Cg is the geometriccapacitance, Zel is the electrolyte capacitance, Rel isthe electrolyte resistance, ZGB is the grain boundarycapacitance, and RGB is the grain boundary resistance.

The impedance spectra were fitted to theequivalent circuit model introduced above based onthe complex non-linear least square (CNLS) fitting toestimate the electrolyte resistance (Rel) [27]. Theanalyzed conductivities are listed in Table 1. The RF-

σd

A---

1Rel

------⋅=

Fig. 3. Surface morphology transformation of the RF-

sputtered NaPON thin film after exposure to ambient

environment for 1 h.

Fig. 4. X-ray diffraction (XRD) patterns of NaPON and

LiPON solid films RF sputtered on the glass substrate.

Sang-Eun Chun/한국표면공학회 50 (2017) 237-243 241

sputtered NaPON sample showed reasonableconductivity (8.73 × 10-6 S cm-1) as a solid electrolyte,which is comparable to the previously reportedconductivities of a number of solid electrolytes [3-5,7-9,28,29]. Furthermore, the LiPON thin filmprepared for comparison also showed a similarconductivity (8.11 × 10-6 S cm-1).

Another critical parameter as a solid electrolyte isthe activation energy, because this value is related tothe kinetics of ionic migration. In a solid-stateelectrolyte, ionic species can move through empty

sites such as vacancies or accessible interstitial sites.For a large number of ionic species to move, theempty sites should have a low activation energybarrier to movement. Thus, a lower activation energyindicates a more facile movement of the mobile ionsthrough the ionic conductor. The activation energycan be calculated from the least square fit of theArrhenius equation (2) based on the plot ofconductivity measured at the various temperatures forboth the NaPON and LiPON films, as:

(2)

where σ is the ionic conductivity measured from theimpedance spectra at a specific temperature, k is theBoltzmann constant (1.38 × 10-23 J/K), and Ea is theactivation energy. When ln(σT) is plotted as afunction of 1/T, the y-intercept is ln(σ0) and the valueof the slope corresponds to the activation energy.Before measuring the conductivity at differenttemperatures, the thin films samples were maintainedat a measurement temperature for 1 h.

Figure 6 shows the Nyquist plots of the impedancespectra obtained from both the NaPON and LiPONfilms at various temperatures between 27 oC and90 oC. All Nyquist plots for both samples show twosuccessive semi-circles with the x-axis intercept atlow frequencies. The arc of the first semicircle wasfound to decrease in size with an increase intemperature for both thin films. The decreasing arcsize indicates that the electrolyte would have had alower resistance at higher temperatures, because thefirst semi-circle in the impedance spectra representsthe ionic conductivity of the electrolyte film. Thisresult is good agreement with our expectation thatthe empty sites would move fast in the solid film dueto the applied thermal energy [30]. Conductivity wasdetermined by fitting the impedance spectra obtainedat various temperatures to the circuit model of Fig.5(b) based on the CNLS fitting model [27].

Figure 7 shows a graph of the natural logarithm ofσT as a function of 1000/T. A linear relationship wasobserved between the two parameters in the low-temperature range (room temperature-70 oC); thislinear trend deviated at higher temperatures. Hence,the CNLS fitting was carried out for the measuredvalues obtained between room temperature and70 oC. The activation energies obtained for both filmsare summarized in Table 1. The NaPON thin filmhas a lower activation energy than the LiPON thinfilm (0.164 eV vs. 0.672 eV). The lower activation

ln σT( ) ln σ0

( )Ea

kT------–=

Fig. 5. (a) Nyquist plots of the impedance spectra

experimentally measured on Mo/NaPON/Mo and Mo/

LiPON/Mo film structures at an open circuit potential at

an ambient temperature (inset: enlarged impedance

data of NaPON and LiPON films) and (b) equivalent

circuit model used to estimate the electrolyte resistance

for both oxynitride films (LiPON and NaPON). In the

Nyquist plot of (a), the empty dots indicate that the

experimentally measured impedance data and the solid

lines are the fitting values based on the equivalent circuit

of (b).

Table 1. Parameters of the RF-sputtered NaPON andLiPON thin films estimated from the impedancespectra

Sputtered filmConductivity,σ (27 oC)

Activation energy, Ea

NaPON 8.73×10-6 S cm-1 0.164 eV

LiPON 8.11×10-6 S cm-1 0.672 eV

242 Sang-Eun Chun/한국표면공학회 50 (2017) 237-243

energy of the NaPON film demonstrates its potentialfor use as a solid-state electrolyte film, since a lowactivation energy implies facile kinetics of the emptysites through the solid film. In addition, ionicconductivity can rapidly increase with increasingtemperature due to the lower activation energy.Therefore, a much higher conductivity can beobtained at a higher temperature in the NaPON thinfilm as compared to the LiPON thin film.

The reasonably high ionic conductivity and lowactivation energy of the RF-sputtered NaPON filmsare indicative of a good solid electrolyte. Similar tothe solid-state LiPON thin films, it is believed thatthe amorphous structure of NaPON films couldcontribute to the improvement in ionic conductivity.

In addition, the nitrogen incorporated into the filmcould also contribute to an enhancement in theconductivity.

4. Conclusions

We reported the preparation of a solid-state sodiumphosphorous oxynitride (NaPON) film deposited atroom temperature via RF magnetron sputtering of aNa3PO4 target at 50 W in a pure N2 environment.The deposited NaPON film was continuous andhomogenous, and the growth rate was measured tobe 124 nm h-1. The RF-sputtered NaPON film wasfound to be a potential solid-state electrolyte totransport Na ions based on the electrochemicalimpedance spectroscopy analysis. For comparison, alithium phosphorous oxynitride (LiPON) film,commonly used as a Li-ion conductor, was depositedwith the same RF magnetron sputtering techniquefrom a Li3PO4 target. The NaPON film was found tohave a reasonable ionic conductivity of 8.73 × 10-6 Scm-1 similar to that of the LiPON film (8.11 × 10-6 Scm-1). This relatively high conductivity is a keycharacteristic for a good solid-state electrolyte.Moreover, the NaPON film showed a loweractivation energy of 0.164 eV than the 0.672 eV forthe LiPON film. The lower activation energyindicates a facile movement of the conducting ionsthrough the film. Both, the reasonable ionicconductivity and the activation energy for theNaPON film, provides a lower series resistance forthe electrolyte in the electrochemical cell. The lowerresistance of the electrolyte allows for a higher

Fig. 6. Nyquist plots of the impedance spectra experimentally measured on (a) Mo/NaPON/Mo and (b) Mo/LiPON/Mo

sandwich configuration at an open circuit potential between room temperature and 90 oC (temperature step: 10 oC).

The empty dot indicates the experimentally measured impedance value. The solid lines in (a) and (b) were determined

by CNLS fitting of the experimental impedance spectra to the equivalent circuit model of (c).

Fig. 7. Arrhenius plots of the conductivity in NaPON and

LiPON thin films as a function of temperature between

27 oC and 90 oC.

Sang-Eun Chun/한국표면공학회 50 (2017) 237-243 243

power density and energy efficiency. A full-cellcharacterization needs to be performed todemonstrate the role of the NaPON film as anelectrolyte in a Na-ion full-cell.

Acknowledgement

This work was supported by the National ResearchFoundation of Korea (NRF) grant funded by theKorea government (Ministry of Science, ICT &Future Planning) (NRF-2017R1C1B2005470).

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