Alumina + YSZ Composites, Ionic Conductivity and Stability

Article Sodium, Silver and Lithium-Ion Conducting β”-Alumina + YSZ Composites, Ionic Conductivity and Stability

Liangzhu Zhu 1,2,* and Anil V. Virkar 1

1 Materials Science & Engineering Department, University of Utah, Salt Lake City, UT 84112, USA; [email protected] 2 Ningbo Institute of Materials Technology & Engineering, Chinese Academy of Sciences, Ningbo 315201, China * Correspondence: [email protected]

Abstract: Na-β”-alumina (Na2O.~6Al2O3) is known to be an excellent sodium ion conductor in battery and sensor applications. In this study we report fabrication of Na- β”-alumina + YSZ dual

phase composite to mitigate moisture and CO2 corrosion that otherwise can lead to degradation in pure Na-β”-alumina conductor. Subsequently, we heat-treated the samples in molten AgNO3 and LiNO3 to respectively form Ag-β”-alumina + YSZ and Li-β”-alumina + YSZ to investigate their potential applications in silver- and lithium-ion solid state batteries. Ion exchange fronts were captured via SEM and EDS techniques. Their ionic conductivities were measured using electrochemical impedance spectroscopy. Both ion exchange rates and ionic conductivities of these composite ionic conductors were ﬁrstly reported here and measured as a function of ion exchange time and temperature. Keywords: β”-alumina; sodium battery; vapor phase conversion; ion exchange; ionic conductivity Citation: Zhu, L.; Virkar, A.V. Sodium, Silver and Lithium-Ion Conducting β”-Alumina + YSZ Composites, Ionic Conductivity and 1. Introduction Stability. Crystals 2021, 11, 293. Na-β”-alumina (Na O.~6Al O ) solid electrolyte (BASE) is known to be an excel- https://doi.org/10.3390/ 2 2 3 cryst11030293 lent sodium ion conductor with uses in sodium-sulfur batteries, sodium-nickel chloride (ZEBRA) batteries, alkali-metal thermoelectric convertors (AMTEC) and in sensors [1–8].

Academic Editor: Sebastian Among these applications, sodium sulfur batteries, originally developed by Weber and Wachowski Kummer at the Ford Motor Company in the 1960s [9] have demonstrated attractive applications in automobiles before lithium batteries became more popular. Due to the abundance Received: 10 February 2021 of sodium compared to lithium resulting in lower cost [10–12] and high ionic conductivity −1 −1 Accepted: 13 March 2021 of about 1 Scm in single crystal Na-β”-alumina and 0.2–0.4 Scm in polycrystalline β”- ◦ Published: 16 March 2021 alumina at 300 C, Na-β”-alumina still sees an encouraging future in certain applications. Conventional process for the fabrication of Na-β”-alumina involves ﬁrst calcining a ◦ Publisher’s Note: MDPI stays neutral mixture of alumina (Al2O3), Na2CO3 and LiNO3 (or MgO) at 1250 C for 2 h. This leads with regard to jurisdictional claims in to the formation of a mixture of Na-β”-alumina, Na-β-alumina (Na2O.~11Al2O3) and published maps and institutional afﬁl- NaAlO2. Discs, bars or tubes are formed by powder-pressing. The samples are then placed iations. in platinum containers or MgO containers and heated in air to ~1600 ◦C for a few minutes and rapidly cooled to ~1450 ◦C and heat-treated for an hour to convert Na-β-alumina into Na-β”-alumina by the reaction [13]:

Copyright: © 2021 by the authors. Na2O.~11Al2O3 + 2NaAlO2 → 2(Na2O.~6Al2O3) (1) Licensee MDPI, Basel, Switzerland. This article is an open access article Sintering occurs by a transient liquid phase mechanism. Some NaAlO2 remains at distributed under the terms and the grain boundaries, which makes the material susceptible to degradation by moisture conditions of the Creative Commons and CO2 present in the atmosphere. Thus, Na-β”-alumina made by conventional methods Attribution (CC BY) license (https:// typically needs to be stored in desiccators. creativecommons.org/licenses/by/ A vapor phase process was originally developed by one of the authors to fabricate 4.0/). Na-β”-alumina-containing materials in which two phase samples of α-alumina + YSZ

Crystals 2021, 11, 293. https://doi.org/10.3390/cryst11030293 https://www.mdpi.com/journal/crystals Crystals 2021, 11, 293 2 of 13

are fabricated by pressing and sintering [14–16]. The proportions of the two phases are such that both phases are contiguous. The sintered samples are then buried in Na-β”- alumina powder and heat-treated at 1250 ◦C to 1400 ◦C for several hours. This leads to the conversion of α-alumina into Na-β”-alumina thus forming a two phase composite of Na-β”-alumina + YSZ. Rapid conversion occurs since the incorporation of Na2O occurs by a coupled transport of 2Na+ through the formed Na-β”-alumina and O2− through YSZ, which is an oxygen ion conductor [14–16]. The overall reaction is:

6Al2O3 + Na2O → Na2O.~6Al2O3 (2)

In this process, NaAlO2 is not present and thus boundaries are free of any NaAlO2. Such samples are resistant to degradation by moisture and CO2 in the atmosphere. Samples of the composite are very ﬁne-grained and thus exhibit excellent strength. It has been known for a long time that β”-alumina is capable of transporting other alkali ions, silver ion, some divalent ions and even trivalent ions and can be made by ion exchange in which Na-β”-alumina is immersed in the respective molten salt for several hours [17–21]. For example, to form Ag-β”-alumina, Na-β”-alumina is immersed in molten AgNO3. The reaction:

+ + Na2O.~6Al2O3 + Ag → Ag2O.~6Al2O3 + Na (3)

occurs to some extent. Typically, partial replacement of Na+ occurs by Ag+, thus forming a silver ion-conducting β”-alumina. Many β”-alumina formed by ion exchange are kinetically stable at low temperatures because oxygen diffusion through β”-alumina is extremely sluggish. Thus, reactions such as:

1 Ag2O.~6Al2O3 → ~6Al2O3 + 2Ag + /2O2 (4)

do not occur at low temperatures (e.g., below about 600 ◦C). The objective of this work was to investigate ion-exchange of Na-β”-alumina + YSZ to form Ag-β”-alumina + YSZ and Li-β”-alumina + YSZ composites. At low temperatures, below about 500 ◦C, oxygen ion conductivity of YSZ is much lower than sodium ion conductivity of Na-β”-alumina. Thus, the composite is essentially a sodium ion conductor. However, at elevated temperatures, the oxygen ion conductivity is signiﬁcant. In samples made by ion exchange, the stability is kinetic. Thus, samples of Ag-β”-alumina + YSZ may become unstable. In order to explore this possibility, it was also the objective of this work to determine its stability at elevated temperatures.

2. Materials and Methods α-Alumina (Baikowski, France) powder and 3YSZ (Tosoh, Tokyo, Japan) powder in a volume proportion of ~70% and ~30% were mixed and ball-milled for 12 h. Discs were uniaxially pressed followed by isostatic pressing. The discs were sintered in air at 1600 ◦C for 5 h. Na-β”-alumina packing powder was made by mixing Na2CO3, LiNO3 and α-Al2O3 ◦ and calcined at 1250 C for 2 h. The composition of the powder was ~8.85 wt.% Na2O, 0.75 wt. Li2O% (as stabilizer) and balance Al2O3. Sintered α-alumina + YSZ samples were buried in Na-β”-alumina powder and heat-treated at 1250 ◦C and 1350 ◦C for several hours. Samples were periodically cooled to room temperature and weighed. After each treatment, sample weight was measured to monitor the conversion of α-alumina into Na-β”-alumina. The samples were heat treated until no further weight change occurred, indicating full conversion through the thickness of the sample. Flat surfaces of the discs were polished. The discs were then immersed in molten AgNO3 and LiNO3 for ion exchange to respectively form Ag-β”-alumina + YSZ and Li- ◦ ◦ ◦ ◦ β”-alumina + YSZ at 300 C and 365 C (in AgNO3) and 300 C and 370 C (in LiNO3). Samples were periodically removed, cooled to room temperature, washed in water to remove salt sticking to the samples, dried and weighed to determine the extent of ion Crystals 2021, 11, 293 3 of 13

exchange. Thin layers of gold were applied as a paste on the surfaces and fired at 500 ◦C for 1 h. Platinum mesh was placed on the two surfaces and then placed between ceramic honeycomb structures and spring-loaded. EIS was conducted over a range of temperatures. Upon removal from the fixture, the gold layers formed thin films bonded to the platinum mesh. The samples were further ion exchanged for a period of time, and weighed. In this manner, EIS measurements could be made after various ion exchange treatments, allowing a study of conductivity as a function of the degree of ion-exchange. After a certain period of time, samples were immersed in molten NaNO3 to reverse ion exchange back to predominantly Na-β”-alumina + YSZ. EIS measurements were also made after reverse ion exchange with sodium. At these relatively low temperatures, the oxygen ion conductivity of YSZ is much lower than that of β”-alumina. Thus, the only transport occurs through the β”-alumina phase. In order to determine the possible instability of Ag-β”-alumina at elevated temperatures, a disc of ion-exchanged sample was first weighed to the accuracy of 0.0001 g. One disk was then heated in air at 900 ◦C for 2 and 3 h. Another ion-exchanged disc was heated in air at 900 ◦C for 12 h. The samples were weighed after each treatment. After the final treatment, the sample heated for 3 h was leached in nitric acid. After drying, the sample once again was weighed.Samples were characterized using scanning electron microscopy (SEM, FEI Nova Nano, Hillsboro, USA), EDS (FEI Nova Nano, Hillsboro, USA) and X-ray diffraction (Philips PANalytical X’Pert, Amsterdam, Netherlands).

3. Results 3.1. Microstructure, Elemental Distribution, and Crystal Structure Figure1a shows an SEM micrograph of an as-converted Na- β”-alumina + YSZ sample. Figure1b shows an SEM image of the sample after ion exchange in LiNO 3 for 24 h. No microstructural changes are observed, consistent with expectations, since the only change that occurs is the partial replacement of Na+ by Li+ in Na-β”-alumina. Also shown in insets

Crystals 2021, 11, x FOR PEER REVIEWare EDS scans. Lithium is too light to be detected. However, it is seen4 of that15 in the partially ion-exchanged sample, the sodium count is lower, consistent with its partial replacement.

FigureFigure 1.1. (a)( aAn) An SEM SEM image image of a fully of aconverted fully converted sample before sample ion exchange. before ionThe exchange.light areas are The light areas are YSZ YSZ and the dark areas are Na-β″-alumina. The sample exhibits high density (negligible porosity) and fine microstructure. The EDS spectrum shows peaks corresponding to O, Na, Al, Zr and Y. (b) An SEM image of the sample after ion exchange in LiNO3 at 370 °C for 24 h. The microstructure is virtually unchanged. The EDS spectrum shows peaks corresponding to O, Na, Al, Zr and Y. The intensity of the Na peak is much lower in the ion-exchanged sample consistent with its partial replacement by Li.

Figure 2a shows an SEM image of a sample that had been ion-exchanged in LiNO3 at 300 °C for 2 h. Figure 2b–d show respectively, EDS maps of O, Al and Zr. All three are uniform through the width of the image. Figure 2e shows EDS map of Na. At about 80 microns from the left, a distinct change in color is observed. Figure 2f shows actual counts of sodium. Again at ~80 microns, a clear change in the sodium count is observed. This suggests that in 2 h, approximate depth of ion exchange is ~80 microns. In samples not exchanged throughout the thickness, the measured conductivity reflects total conductivity which includes contributions that are spatially dependent.

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and the dark areas are Na-β”-alumina. The sample exhibits high density (negligible porosity) and ﬁne microstructure. The EDS spectrum shows peaks corresponding to O, Na, Al, Zr and Y. (b) An SEM ◦ image of the sample after ion exchange in LiNO3 at 370 C for 24 h. The microstructure is virtually unchanged. The EDS spectrum shows peaks corresponding to O, Na, Al, Zr and Y. The intensity of the Na peak is much lower in the ion-exchanged sample consistent with its partial replacement by Li.

Figure2a shows an SEM image of a sample that had been ion-exchanged in LiNO 3 at 300 ◦C for 2 h. Figure2b–d show respectively, EDS maps of O, Al and Zr. All three are uniform through the width of the image. Figure2e shows EDS map of Na. At about 80 microns from the left, a distinct change in color is observed. Figure2f shows actual counts of sodium. Again at ~80 microns, a clear change in the sodium count is observed. This suggests that in 2 h, approximate depth of ion exchange is ~80 microns. In samples not Crystals 2021, 11, x FOR PEER REVIEWexchanged throughout the thickness, the measured conductivity reﬂects total5 conductivityof 15

which includes contributions that are spatially dependent.

Figure 2. (a) An SEM image of a sample that had been ion exchanged at 300 °C in LiNO3 for◦ 2 h. Figure 2. (a) An SEM image of a sample that had been ion exchanged at 300 C in LiNO3 for The micrograph shows ~250 microns of the sample. The microstructure is similar to that in Figure 2 h. The micrograph shows ~250 microns of the sample. The microstructure is similar to that in 1. (b–d) show respectively EDS maps of O, Al and Zr. All three are uniform through the width of Figurethe image.1.( (be)– And) showEDS map respectively of Na. As seen EDS in maps the map, of O,ther Ale is and a distinct Zr. All change three in are color uniform at approx- through the widthimately of 80 the microns. image. This (e) suggests An EDS that map in of 2 h, Na. lithium As seen ion inexchange the map, had there occurred is a distinctto a depth change of in color at approximatelyabout 80 microns. 80 The microns. demarcation This suggestsline is marked that by in 2a red h, lithium arrow. ( ionf) Actual exchange EDS counts had occurred of Na to a depth ofshowing about a 80 distinct microns. change The in demarcation counts at about line 80 ismicrons. marked by a red arrow. (f) Actual EDS counts of Na showing a distinct change in counts at about 80 microns. Figure 3 shows EDS line scans of a sample ion-exchanged in AgNO3 at 300 °C for 1 h. The measurements were performed at near the surface region since the sample is relatively thick. At both surfaces, high concentration of silver atoms and depletion of sodium atoms are observed, indicating sodium to silver ion exchange has occurred. Away from the surface, silver concentration gradually decreases to nearly zero while Na concentration gradually recovers to a steady value reaching bulk concentration. Comparing Figures 2 and 3 suggests that at the same temperature, ion exchange of Na+ by Ag+ can happen more rapidly, which can be understood as to be shown later as well as reported by Yao and Kummer that Na+ to Ag+ ion exchange has a much higher maximum ion exchange percentage (90%–100%) than that of maximum Na+ to Li+ ion exchange (40%–50%)[1]. And more importantly, even when ion-exchanged with only 10% AgNO3 in AgNO3/NaNO3

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◦ Figure3 shows EDS line scans of a sample ion-exchanged in AgNO 3 at 300 C for 1 h. The measurements were performed at near the surface region since the sample is relatively thick. At both surfaces, high concentration of silver atoms and depletion of sodium atoms are observed, indicating sodium to silver ion exchange has occurred. Away from the surface, silver concentration gradually decreases to nearly zero while Na concentration gradually recovers to a steady value reaching bulk concentration. Comparing Figures2 and3 suggests that at the same temperature, ion exchange of Na+ by Ag+ can Crystals 2021, 11, x FOR PEER REVIEW happen more rapidly, which can be understood as to be shown later as well as reported6 of 15 by Yao and Kummer that Na+ to Ag+ ion exchange has a much higher maximum ion exchange percentage (90–100%) than that of maximum Na+ to Li+ ion exchange (40–50%) [1]. And more importantly, even when ion-exchanged with only 10% AgNO in AgNO /NaNO + + 3 3 3 moltenmolten mixture, mixture, the the maximum maximum Na Na to+ toAg Ag ion+ ion exchange exchange percentage percentage can can exceed exceed 80% 80% while while Na+ to+ Li+ ion+ exchange percentage remains less than 10% in 80% LiNO3 in LiNO3/NaNO3 Na to Li ion exchange percentage remains less than 10% in 80% LiNO3 in LiNO3/NaNO3 moltenmolten mixture mixture [1]. [1 ].

◦ Figure 3. EDS line scans of a sample ion-exchanged in AgNO3 at 300 C for 1 h. The “left” surface Figurewas arbitrarily3. EDS line chosen scans of from a sample one of ion-exchanged the planar surfaces in AgNO of a disc3 at 300 sample. °C for 1 h. The “left” surface was arbitrarily chosen from one of the planar surfaces of a disc sample. Figure4 compares samples without (converted only) and with ion exchange for 24 h ◦ at 370FigureC in4 compares LiNO3 where samplesβ”-alumina, without (converted tetragonal andonly) monoclinic and with ion phases exchange of zirconia for 24 were h at observed.370 °C in LiNO Identiﬁcation3 where β″ of-alumina,β”-alumina tetragonal structure and usually monoclinic can bephases characterized of zirconia by were a few observed.single peaks Identification and a set of of β″ low-alumina scattered structure peaks with usually relatively can be lowcharacterized intensity atby 35 a◦ few–45 ◦sin-. The glesamples peaks and were a veryset of strong low scattered and no cracks peaks werewith observed.relatively Notelow inte in thensity samples at 35°–45°. before The and samplesafter ion were exchange, very strong both and tetragonal no cracks and were monoclinic observed. zirconia Note in were the samples observed. before This and is not aftersurprising ion exchange, since 3YSZ both is tetragonal not a fully and stabilized monoclinic zirconia. zirconia It is known were observed. that tetragonal This zirconiais not surprisingcan slowly since transforms 3YSZ is not to monoclinic a fully stabilized zirconia zirconia. (sometimes It is known referred that to as tetragonal t-m transformation) zirconia canover slowly a rather transforms narrow to but monoclinic important zirconia temperature (sometimes range, typicallyreferred roomto as temperaturet-m transfor- to ◦ mation)around over 400 a Crather over narrow extended but period, important which temperature is referred to range, as low-temperature typically room degradation temperature(LTD) to around [22]. This 400 phenomenon°C over extended had period, been an which issue is in referred biomedical to as applications, low-temperature such deg- as hip radationimplants (LTD) and [22]. dental This restorations, phenomenon before had the been LTD an mechanism issue in biomedical was discovered. applications, It is obvious such as t-mhip implants transformation and dental has occurredrestorations, after before ion exchange the LTD mechanism process based was on discovered. the suppressed It is obvious t-m transformation has occurred after ion exchange process based on the suppressed intensity of tetragonal phase and increasing intensity of monoclinic phase for a few major peaks (indicated by double arrows).

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intensity of tetragonal phase and increasing intensity of monoclinic phase for a few major peaks (indicated by double arrows).

◦ Figure 4. ComparisonComparison of samples without (converted(converted only)only) andand withwith ionion exchangeexchange for for 24 24 h h at at 370 370 C ◦ ◦ ◦ °Cin LiNOin LiNO3. The3. The characteristic characteristicβ”-alumina β″-alumina peaks peaks are are labeled labeled (mainly (mainly for for peaks peaks at at 16 16°,, 32 32°,, 46 46°.and and a agroup group of of low low scattered scattered between between 35 35°–45°).◦–45◦). Both Both tetragonal tetragonal and and monoclinic monoclinic phases phases of of zirconia zirconia can can be beseen seen in thein the ﬁgure. figure. Tetragonal Tetragonal to to monoclinic monoclinic phase phase transformation transformation was was observed observed based based on on relative rela- tiveintensity intensity change change of two of zirconiatwo zirconia phases. phases. The sample The sample was very was strong very strong and exhibited and exhibited no obvious no obvi- cracks. ous cracks. Double arrows indicate sign of tetragonal to monoclinic zirconia transformation after Double arrows indicate sign of tetragonal to monoclinic zirconia transformation after extended ion extended ion exchange period. exchange period.

3.2. Ion Ion Exchange Exchange Kinetics Figure 55 showsshows the extent of of ion ion exchange exchange as as a afunction function of of time. time. A Asample sample of ofNa- Na-β″- ◦ aluminaβ”-alumina + YSZ + YSZ was wasion ionexchanged exchanged in AgNO in AgNO3 at 3653 at 365°C forC up for to up 20 to h. 20 Subsequently, h. Subsequently, the samplethe sample was reverse was reverse ion exchanged ion exchanged in NaNO in NaNO3 for an3 additionalfor an additional 61 h. The 61 figure h. The shows ﬁgure an increaseshows an in increase Ag+/[Ag in+ + Ag Na+/[Ag+] ratio+ +eventually Na+] ratio reaching eventually saturation. reaching One saturation. sample Onewas samplefurther ion-exchangedwas further ion-exchanged for up to 60 h for with up no to 60change h with in nothe changeAg+/[Ag in+ + the Na Ag+] ratio+/[Ag indicating+ + Na+] satu- ratio + + + + + + rationindicating occurred saturation in 20 occurredh. After ion in 20 exchange h. After ionin NaNO exchange3, the in Ag NaNO/[Ag3, + the Na Ag] decreased/[Ag + Na and] eventuallydecreased andattained eventually a lower attained saturation a lower limit. saturationTable 1 gives limit. the Table weights1 gives of the the sample weights after of successivethe sample ion after exchange. successive The ion ‘before’ exchange. column The lists ‘before’ weight column of the sample lists weight before of subsequent the sample ionbefore exchange. subsequent The ‘after’ ion exchange. column lists The weight ‘after’ columnafter that lists ion weight exchange after step. that As ion seen exchange in the table,step. Asduring seen ion in the exchange table, during in AgNO ion3 exchange, the weight in AgNOincreases3, the since weight Ag increaseshas higher since atomic Ag masshas higher than Na. atomic For the mass same than reason, Na. For when the ion same exchanged reason, whenin NaNO ion3, exchanged the weight indecreases, NaNO3, asthe expected. weight decreases, The angled as expected.arrows show The angledweights arrows after removing show weights gold afterelectrode/platinum removing gold gridelectrode/platinum from EIS measurements. grid from EISThere measurements. is virtually no There change, is virtually indicating no change,that there indicating was no physicalthat there or was chemical no physical bonding or between chemical the bonding electrodes between and the the sample. electrodes Table and 2 theshows sample. sim- ilarTable data2 shows for ion similar exchange data for in ionLiNO exchange3 and reverse in LiNO ion3 and exchange reverse in ion NaNO exchange3. In inthis NaNO case,3. + + + weightIn this case,decreases weight with decreases increasing with Li increasing+/[Li+ + Na Li+] since/[Li lithium+ Na ] sincehas a lithiumsmaller hasatomic a smaller mass thanatomic sodium. massthan sodium.

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++ + + + + + ++ + + + FigureFigureFigure 5. 5.5. MeasuredMeasured Measured Ag Ag Ag/[Ag+/[Ag/[Ag ++ Na++ Na Na] +and] ]and and Li Li/[Li Li+/[Li/[Li ++ Na+ Na+] Na+ratios] ratios] ratios after after ion after ion exchange ionexchange exchange in inAgNO AgNO in AgNO3 and3 and3 and later after reverse ion exchange in NaNO3 as a function of time from five converted samples. Li ion laterlater afterafter reverse reverse ion ion exchange exchange in in NaNO NaNO3 3as as a a function function of of time time from from ﬁve five converted converted samples. samples. Li Li ion ion exchangeexchangeexchange was waswas conducted conducted conducted at at at two two two temperatures; temperatures; temperatures; 300 300 300 °C◦ C°C and and and 370 370 370 °C.◦ C.°C.

TableTable 1. 1. Summary Summary of of AgNO AgNO3 ion ion exchange exchange & &reverse reverse NaNO NaNO3 Ion Ion exchange exchange experiment experiment data. data. Table 1. Summary of AgNO3 3ion exchange & reverse NaNO3 3Ion exchange experiment data.

Note: Each change of arrow direction from an upper row to a lower row corresponds to an EIS measurement and Note:gentle Each removal change of Au of electrode. arrow direction from an upper row to a lower row corresponds to an EIS Note: Each change of arrow direction from an upper row to a lower row corresponds to an EIS measurement and gentle removal of Au electrode. measurement and gentle removal of Au electrode.

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Table 2. Summary of LiNO3 ion exchange & reverse NaNO3 Ion exchange experiment data. Table 2. Summary of LiNO3 ion exchange & reverse NaNO3 Ion exchange experiment data.

Note: Each change of arrow direction from an upper row to a lower row corresponds to an EIS measurement and gentleNote: removalEach change of Au electrode.of arrow direction from an upper row to a lower row corresponds to an EIS measurement and gentle removal of Au electrode. 3.3. Conductivity of as Converted and Ion-Exchanged Samples 3.3. ConductivityFigure6 shows of as the Converted ionic conductivity and Ion-Exchanged measured Samples by EIS on a fully converted sample ◦ and afterFigure ion 6 exchange shows the for ionic 24 h inconductivity LiNO3 at 300 measuredC. Representative by EIS on EISa fully spectra converted are shown sample in Figureand after6a,b. ion The exchange spectra showfor 24 noh in semicircles LiNO3 at which300 °C. are Representative usually seen inEIS electrochemical spectra are shown cells in suchFigure as 6a,b. in fuel The cells spectra with activeshow no electrodes. semicircles This wh isich partly are dueusually to poor seen electrode in electrochemical performance cells at low temperatures with pure Au electrodes. On the other hand, wire inductance can such as in fuel cells with active electrodes. This is partly due to poor electrode performance play a role too so that semicircles can be completely overlapped by inductance, the true at low temperatures with pure Au electrodes. On the other hand, wire inductance can play semicircle in this case may be recovered by ﬁrstly measuring wire/metal-contact EIS spectruma role too so and that followed semicircles by subtracting can be completely wire/metal-contact overlapped by spectrum inductance, point the by true point semicircle from measuredin this case wire/metal-contact may be recovered +by sample firstly EIS me spectrumasuring wire/metal-conta [23]. Removal ofct theEIS inductance spectrum and is particularlyfollowed by important subtracting if separationwire/metal-contact of grain andspectrum grain boundarypoint by resistancepoint from is required.measured Similarwire/metal-contact behavior has + sample been noted EIS spectrum for some [23] other. Removal solid state of the lithium inductance ion conductors[ is particularly23]. Sinceimportant in this if study,separation total of ionic grain conductivity and grain boun is ofdary most resistance interest, is we required. therefore Similar directly behavior take thehas intercept been noted in the for highsome frequency other solid range state with lithi realum axision forconductors[23]. the conductivity Since measurement. in this study, Figure6c shows the Arrhenius plots of the total ionic conductivity. Both samples exhibit an total ionic conductivity is of most interest, we therefore directly take the intercept in the high Arrhenius behavior. The activation energy for the converted sample is 0.13 eV while it is frequency range with real axis for the conductivity measurement. Figure 6c shows the 0.18 eV for the ion exchanged sample. These two values agrees relatively well with that of reportedArrhenius activation plots of energiesthe total ionic of single conductivity. crystal β”-alumina Both samples of 0.10–0.33 exhibit eVan andArrhenius polycrystalline behavior. βThe”-alumina activation of 0.16–0.22energy for eV the [1 ].Theconverted room sample temperature is 0.13 conductivities eV while it is were 0.18 estimatedeV for the byion extrapolation.exchanged sample. The estimated These two total values ionic agrees conductivities relatively at well room with temperature that of reported are 3.6 ×activation10−3 S cmenergies−1 and of 7.5 single× 10 crystal−4 S cm β″−1-aluminafor the as-converted of 0.10–0.33 eV and and ion-exchanged polycrystalline samples, β″-alumina respectively. of 0.16– + + Koh0.22 eteV al. [1].The reported room vapor temperature phase exchange conductiviti of Liesto were replace estimated Na [18 by]. Theextrapolation. extrapolated The × −5 −1 conductivityestimated total at roomionic conductivities temperature in at their room work temperature was ~7 are10 3.6 S× cm10−3 S .cm The−1 and conductivity 7.5 × 10−4 S of the Li-ion exchanged sample likely contains contributions from both Na+ and Li+ ions. cm−1 for the as-converted and ion-exchanged samples, respectively. Koh et al. reported However, it has been reported that the Li+ transport number is nearly one if the Li+/(Na+ + vapor phase exchange of Li+ to replace Na+ [18]. The extrapolated conductivity at room Li+) ratio is greater than about 0.5 in Li+/Na+ - β-alumina [17]. Since β and β” have similar −5 −1 structures,temperature we in expect their thework partially was ~7 ion-exchanged × 10 S cm . Li-Theβ”-alumina conductivity to also of the be aLi-ion predominantly exchanged Lisample+-ion conductorlikely contains if Li+ co/(Nantributions+ + Li+) ratiofrom isboth greater Na+ thanand Li 0.5+ ions. in Li +However,/Na+ - β ”-alumina.it has been Inreported Figure 4that, Li the+/(Na Li+ transport+ + Li+) rationumber is about is nearly 0.35, one the if compositethe Li+/(Na+ is + aLi mixed+) ratio Nais greater+ and Lithan+ conductorabout 0.5 in with Li+/Na Li+ +being - β-alumina the dominant [17]. Since conducting β and β″ species, have similar this may structures, partially we contribute expect the + + topartially the measured ion-exchanged higher conductivity Li-β″-alumina in theto ionalso exchanged be a predominantly Li /Na - βLi-alumina+-ion conductor than the if extrapolatedLi+/(Na+ + Li+) conductivity ratio is greater from than Koh 0.5et in al.’s Li+/Na work.+ - β″-alumina. In Figure 4, Li+/(Na+ + Li+) ratio is about 0.35, the composite is a mixed Na+ and Li+ conductor with Li+ being the dominant conducting species, this may partially contribute to the measured higher conductivity in the ion exchanged Li+/Na+ - β-alumina than the extrapolated conductivity from Koh et al.’s work.

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FigureFigure 6. 6. IonicIonic conductivity conductivity measured measured by by EIS EIS on ona fully a fully converted converted sample sample and and after after ion ionexchange exchange for 24 for ◦ ◦ h24 in hLiNO in LiNO3 at 3003 at °C. 300 (a) C.EIS (a spectra) EIS spectra for a fully for aconverted fully converted sample (EIS sample measured (EIS measured at 381 °C) at and 381 afterC) ion and ◦ ◦ exchangeafter ion for exchange 24 h in LiNO for 243 at h in300 LiNO °C (EIS3 at measured 300 C (EIS at 387 measured °C). (b) Enlarged at 387 C). EIS ( bspectra) Enlarged for a). EIS and spectra inter- ceptfor a).with and real intercept axis is taken with as real the axis total is ionic taken conductivity; as the total ( ionicc) Arrhenius conductivity; plots of ( cthe) Arrhenius measured plots conduc- of the tivity on a fully converted sample and after ion exchange in LiNO3 for 24 h at 300 °C. measured conductivity on a fully converted sample and after ion exchange in LiNO3 for 24 h at 300 ◦C. Figure 7 shows Arrhenius plots of the ionic conductivity of the as-converted Na-β″- aluminaFigure + YSZ7 shows sample, Arrhenius after ion plots exchange of the in ionic molten conductivity LiNO3, and of after the as-converted reverse exchange Na- β in”- NaNOalumina3 as + a YSZ function sample, of aftertemperature ion exchange and time in molten for two LiNO samples.3, and afterThe reverseconductivity exchange is the in highestNaNO 3foras the a function as converted of temperature Na-β″-alumina and time + YSZ, for and two the samples. lowest Theafter conductivity ion exchanged isthe in LiNOhighest3 and for reversely the as converted ion exchanged Na-β”-alumina in NaNO3 +. The YSZ, conductivity and the lowest is slightly after ion higher exchanged in the as-convertedin LiNO3 and Na- reverselyβ″-alumina ion exchanged+ YSZ sample in NaNOat 13503 .°C The indicating conductivity slightly is slightly higher higherNa+ con- in ◦ + centration,the as-converted but becomes Na-β”-alumina slightly lower + YSZ after sample LiNO at3 1350ion exchangeC indicating and NaNO slightly3 reverse higher Naion exchangeconcentration, in the but same becomes sample. slightly Since lowerwe have after seen LiNO a dramatic3 ion exchange reduction and of NaNO conductivity3 reverse afterion exchange LiNO3 ion in exchange, the same sample.a sample Since containing we have higher seen ainitial dramatic Na+ reductionconcentration of conductivity will likely + haveafter higher LiNO3 Li+ion concentration exchange, a sample after reaching containing Li+ higher saturation. initial Upon Na concentrationreverse ion exchange, will likely a + higherhave higher Na+ concentration Li+ concentration may be after replaced reaching by Lia highersaturation. Li+ concentration Upon reverse resulting ion exchange, a lower a + + overallhigher conductivity. Na concentration may be replaced by a higher Li concentration resulting a lower overall conductivity. Figure8 shows Arrhenius plots of the ionic conductivity after silver ion exchange for various periods of time. As seen in the ﬁgure, the conductivity decreases with increased + + + Ag /[Ag + Na ] ratio and saturated in about 3 h. Reverse ion exchange by NaNO3 did not recover the conductivity to the initial value in the as-converted state.

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FigureFigure 7. 7. ArrheniusArrhenius plots plots of ofthe the measured measured conductivity conductivity of the of as the fabricated as fabricated samples samples and after and suc- after cessivesuccessive lithium lithium ion exchange ion exchange and andafter after reverse reverse exchange exchange in NaNO in NaNO3. 3.

Figure 8 shows Arrhenius plots of the ionic conductivity after silver ion exchange for various periods of time. As seen in the figure, the conductivity decreases with increased Ag+/[Ag+ + Na+] ratio and saturated in about 3 h. Reverse ion exchange by NaNO3 did not recover the conductivity to the initial value in the as-converted state.

3.4. Stability of Ag-β″-Alumina + YSZ Composites Samples of Ag-β″-alumina + YSZ were heated in air at 900 °C for various periods of time (up to 20 h) and their weights were measured. The initial mass of one sample was 1.1797 g. After 3 h, the mass of the sample was 1.1784 g. The surface of the sample was metallic indicating some loss of oxygen and formation of Ag metal. Figure 9a shows a photograph of the sample before heat treatment. Figure 9b shows a photograph of the sample after the heat treatment. The loss of mass attributed to oxygen corresponds to 0.0013 g and the corresponding moles of oxygen lost was 8.13 × 10−5 mol. The sample was then etched in nitric acid to dissolve silver from the surface. The weight of the sample after leaching was 1.1665 g. This corresponds to a loss of silver of 0.0119 g. The corresponding moles of silver lost was 1.1 × 10−4 moles. Thus, the mole ratio of loss corresponds to Ag1.35O. Ideally, it should be Ag2O. Given the very small mass loss, especially of oxygen, the accuracy of the balance is not sufficient. So we will assume that the loss of mass of the two elements must be in 2(Ag) to 1(O) ratio.

FigureFigure 8. 8.ArrheniusArrhenius plots plots of the of measured the measured conductivity conductivity of the of as thefabricated as fabricated sample sample and after and suc- after cessivesuccessive silver silver ion exchange. ion exchange.

The vapor phase process for the conversion of -Al2O3 + YSZ is made possible because Na-β″-alumina is a sodium ion conductor and YSZ is an oxygen ion conductor. This allows the conversion by incorporation of Na2O as a coupled transport of Na+ through the formed Na-β″-alumina and of O2− through YSZ. Detailed mechanism of the process has been described by Parthasarathy and Virkar [16].

Figure 9. (a) A sample of Ag-β″-alumina + YSZ. (b) The sample after heat treating at 900 °C in air showing metallic color.

The process occurs because reaction (1) is thermodynamically favored. The formation of Ag-β″-alumina + YSZ by ion exchange in molten AgNO3 starting with Na-β″-alumina + YSZ is thermodynamically favored since it is only a reaction involving entropy change with essentially no enthalpy change. This makes it possible to make many β″-alumina capable of transporting other cations by ion exchange, even though such phases may not

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3.4. Stability of Ag-β”-Alumina + YSZ Composites Samples of Ag-β”-alumina + YSZ were heated in air at 900 ◦C for various periods of time (up to 20 h) and their weights were measured. The initial mass of one sample was 1.1797 g. After 3 h, the mass of the sample was 1.1784 g. The surface of the sample was metallic indicating some loss of oxygen and formation of Ag metal. Figure9a shows a photograph of the sample before heat treatment. Figure9b shows a photograph of the sampleFigure 8. after Arrhenius the heat plots treatment. of the measured The lossconductivity of mass of attributed the as fabricated to oxygen sample corresponds and after suc- to −5 0.0013cessive g silver and theion correspondingexchange. moles of oxygen lost was 8.13 × 10 mol. The sample was then etched in nitric acid to dissolve silver from the surface. The weight of the sample after leachingThe was vapor 1.1665 phase g. Thisprocess corresponds for the conversion to a loss of of silver -Al of2O 0.01193 + YSZ g. is The made corresponding possible be- − molescause Na- of silverβ″-alumina lost was is a 1.1 sodium× 10 ion4 moles. conductor Thus, and the YSZ mole is an ratio oxygen of loss ion correspondsconductor. This to Agallows1.35O. the Ideally, conversion it should by incorporation be Ag2O. Given of Na the2O very as a small coupled mass transport loss, especially of Na+ through of oxygen, the theformed accuracy Na-β″ of-alumina the balance and is of not O sufﬁcient.2− through So YSZ. we Detailed will assume mechanism that the lossof the of massprocess of thehas twobeen elements described must by Parthasarathy be in 2(Ag) to 1(O)and Virkar ratio. [16].

FigureFigure 9.9.( (aa)) AA samplesample ofof Ag-Ag-ββ″”-alumina-alumina + YSZ. YSZ. ( (bb)) The The sample sample after after heat heat treating treating at at 900 900 °C◦C in in air air showingshowing metallic metallic color. color.

TheThe vapor process phase occurs process because for thereaction conversion (1) is thermodynamically of α-Al2O3 + YSZ is madefavored. possible The formation because Na-of Ag-β”-aluminaβ″-alumina is a + sodium YSZ by ion ion conductor exchange and in molten YSZ is anAgNO oxygen3 starting ion conductor. with Na- Thisβ″-alumina allows + the+ YSZ conversion is thermodynamically by incorporation favored of Na2 Osince as a it coupled is only transporta reaction of involving Na through entropy the formed change Na-withβ ”-aluminaessentially and no enthalpy of O2− through change. YSZ. This Detailed makes it mechanism possible to ofmake the processmany β″ has-alumina been describedcapable of by transporting Parthasarathy other and cations Virkar by [16 ion]. exchange, even though such phases may not The process occurs because reaction (1) is thermodynamically favored. The formation of Ag-β”-alumina + YSZ by ion exchange in molten AgNO3 starting with Na-β”-alumina + YSZ is thermodynamically favored since it is only a reaction involving entropy change with essentially no enthalpy change. This makes it possible to make many β”-alumina capable of transporting other cations by ion exchange, even though such phases may not be thermodynamically stable. Since the alumina spinel blocks in β”-alumina and YSZ are very refractory materials, such phases are kinetically stable. When heated to a high temperature, e.g., 900 ◦C, there is sufﬁcient conductivity for oxygen ions in YSZ so that, what makes possible to make the original Na-β”-alumina + YSZ by coupled transport, also makes the thermodynamically unstable Ag-β”-alumina to decompose. In this process some of Ag+ is removed from Ag-β”-alumina and some of O2− is removed from YSZ. The β”-alumina structure as such does not decompose since oxygen ion diffusion in the spinel block is very sluggish. While the kinetics of the decomposition process is believed to be controlled by coupled diffusion of 2Ag+ through Ag-β”-alumina and O2− through YSZ. However, it is possible to remove oxygen from YSZ. So the process involves some removal of Ag+ from Ag-β”-alumina and some removal of O2− from YSZ. If this happens, it is conceivable the there may be some positive charge in Crystals 2021, 11, x FOR PEER REVIEW 13 of 15

be thermodynamically stable. Since the alumina spinel blocks in β″-alumina and YSZ are very refractory materials, such phases are kinetically stable. When heated to a high temperature, e.g., 900 °C, there is sufficient conductivity for oxygen ions in YSZ so that, what makes possible to make the original Na-β″-alumina + YSZ by coupled transport, also makes the thermodynamically unstable Ag-β″-alumina to decompose. In this process some of Ag+ is removed from Ag-β″-alumina and some of O2− is removed from YSZ. The β″-alumina structure as such does not decompose since oxygen ion diffusion in the spinel block is very sluggish. While the kinetics of the decomposition process is believed to be controlled by coupled diffusion of 2Ag+ through Ag-β″-alumina Crystals 2021, 11, 293 and O2− through YSZ. However, it is possible to remove oxygen from YSZ. So the process12 of 13 involves some removal of Ag+ from Ag-β″-alumina and some removal of O2− from YSZ. If this happens, it is conceivable the there may be some positive charge in YSZ grains and YSZbalancing grains negative and balancing charge negative in Ag-β″ charge-alumina, in Ag- whichβ”-alumina, may be similar which mayto that be of similar space tocharge that ofphenomenon space charge along phenomenon grain boundaries along grain of sinter boundariesed polycrystalline of sintered yttrium polycrystalline stabilized yttrium zirco- stabilizednia [24]. Alternatively, zirconia [24]. some Alternatively, electronic some transport electronic may transportoccur through may both occur Ag- throughβ″-alumina both Ag-andβ YSZ”-alumina to remove and YSZAg and to remove O by the Ag reactions: and O by the reactions: 2Ag+ + 2e  2Ag (5) 2Ag+ + 2e → 2Ag (5) and: and: 2− 1 O O→2- / 12/O2O2 2(g) (g) + + 2e2e (6) (6) + andand thethe 2e are transferred to Ag Ag+ atat the the surface. surface. Otherwise, Otherwise, O2 O and2 and Ag Ag may may form form inside inside the thesample. sample. Since Since Ag Ag was was observed observed only only at at the the surf surface,ace, it it means means the the charge transfer shouldshould mainlymainly havehave startedstarted fromfrom Ag-Ag-ββ″”-alumina/YSZ-alumina/YSZ grain grain boundary boundary at at the the surface. InIn suchsuch aa case,case, Ag-Ag-ββ″”-alumina-alumina becomesbecomes slightlyslightly silver-deﬁcientsilver-deficient andand YSZYSZ becomesbecomes slightlyslightly oxygenoxygen deﬁcient.deficient. FigureFigure 1010aa showsshows aa fracturedfractured samplesample whichwhich hashas turnedturned darkdark inin colorcolor inin thethe interiorinterior andand thethe surfacesurface isis metallic.metallic. FigureFigure 1010bb showsshows thethe samplesample afterafter etching.etching.

FigureFigure 10.10. ((aa)) AA fractured piece of the the sample sample showin showingg metallic metallic surface surface and and grey grey interior. interior. (b ()b The) The fractured sample after leaching in HNO3. The surface has turned dark in color. fractured sample after leaching in HNO3. The surface has turned dark in color. 4.4. ConclusionsConclusions Na-Na-ββ″”-alumina-alumina + + YSZ YSZ two two phase phase composite composite solid solid electrolytes electrolytes were were fabricated fabricated by va- by vaporpor phase phase conversion conversion of of sintered sintered α-alumina-alumina ++ YSZYSZ composites.composites. Na-Na-ββ″”-alumina-alumina ++ YSZYSZ compositescomposites were ion-exchanged in in AgNO AgNO33 andand LiNO LiNO3 3forfor various various periods periods of oftime time to re- to respectivelyspectively form form Ag- Ag-β″β-alumina”-alumina + YSZ + YSZ and and Li- Li-β″-aluminaβ”-alumina + YSZ + YSZ composites, composites, which which are arerespectively respectively silver silver and and lithium lithium ion ionconductors conductors.. Their Their conductivity conductivity was wasmeasured measured by elec- by electrochemicaltrochemical impedance impedance spectroscopy spectroscopy (EIS), (EIS), also also as a as function a function of the of thedegree degree of ion of ionex- exchange.change. EDS EDS analysis analysis was was performed performed on onthe the ion-exchanged ion-exchanged samples samples to further to further confirm conﬁrm the theoccurrence occurrence of ion of ionexchange. exchange. When When Ag-β″ Ag--aluminaβ”-alumina + YSZ + samples YSZ samples were heated were heated to 900 to°C 900 ◦C in air, a thin layer of metallic silver formed on the surface. This was attributed to the thermodynamic instability of Ag-β”-alumina/YSZ composite The kinetics of the decomposition process is believed to be controlled by coupled diffusion of 2Ag+ through Ag-β”-alumina and O2− through YSZ.

Author Contributions: L.Z. and A.V.V. designed the experiments, reviewed the data, and ﬁnalized the publication. L.Z. fabricated cells, executed tests, wrote the draft of the manuscript, and revised the manuscript. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported in part by the National Science Foundation under grant numbers NSF-DMR-1742696, and in part by the US Department of Energy under grant number DE-FG02- 03ER46086.The work also received ﬁnancial support from Zhejiang key research and development program under grant number 2021C01101 and National Natural Science Foundation of China under grant number U20A20251. Crystals 2021, 11, 293 13 of 13

Data Availability Statement: All data relevant to this work are reported in the corresponding figures and tables. Raw data for XRD or figures presented in this study are available from the corresponding author on reasonable request. Acknowledgments: The authors appreciate support from US National Science Foundation, US Department of Energy, Zhejiang Provincial Department of Science and Technology, and National Natural Science Foundation of China. Conflicts of Interest: The authors declare no conflict of interest.

References 1. Sudworth, J.L.; Tilley, A.R. The Sodium Sulfur Battery; Chapman and Hall: London, UK, 1985. 2. Sudworth, J.L. The sodium/nickel chloride (ZEBRA) battery. J. Power Sources 2001, 100, 149–163. [CrossRef] 3. Hunt, T.K.; Weber, N.; Cole, T. High efﬁciency thermoelectric conversion with beta”-alumina electrolytes, the sodium heat engine. Solid State Ion. 1981, 5, 263–265. [CrossRef] 4. Chang, H.-J.; Lu, X.; Bonnett, J.F.; Canﬁeld, N.L.; Han, K.; Engelhard, M.H.; Jung, K.; Sprenkle, V.L.; Li, G. Decorating β00-alumina solid-state electrolytes with micron Pb spherical particles for improving Na wettability at lower temperatures. J. Mater. Chem. A 2018, 6, 19703–19711. [CrossRef] 5. Sparks, T.D.; Ghadbeigi, L. Anisotropic properties of Na-β”-alumina + YSZ composite synthesized by vapor phase method. J. Mater. Res. 2018, 33, 81–89. [CrossRef] 6. Li, K.; Yang, Y.; Zhang, X.; Liang, S. Highly oriented β”-alumina ceramics with excellent ionic conductivity and mechanical performance obtained by spark plasma sintering technique. J. Mater. Sci. 2020, 55, 8435–8443. [CrossRef] 7. Ligon, S.C.; Bay, M.-C.; Heinz, M.V.F.; Battaglia, C.; Graule, T.; Blugan, G. Large planar Na-β”-Al(2)O(3) solid electrolytes for next generation na-batteries. Materials 2020, 13, 433. [CrossRef] 8. Bay, M.-C.; Wang, M.; Grissa, R.; Heinz, M.V.F.; Sakamoto, J.; Battaglia, C. Sodium plating from Na-β”-alumina ceramics at room temperature, paving the way for fast-charging all-solid-state batteries. Adv. Energy Mater. 2020, 10, 1902899. [CrossRef] 9. Weber, N.; Kummer, J.T. Sodium sulfur batteries. Annu. Power Sources Conf. 1967, 21, 37–39. 10. Long, W.; Yuhao, L.; Jue, L.; Maowen, X.; Jinguang, C.; Dawei, Z.; Goodenough, J.B. A superior low-cost cathode for a Na-Ion battery. Angew. Chem. Int. Ed. 2013, 52, 1964–1967. [CrossRef] 11. Jian, Z.; Han, W.; Lu, X.; Yang, H.; Hu, Y.-S.; Zhou, J.; Zhou, Z.; Li, J.; Chen, W.; Chen, D.; et al. Superior electrochemical performance and storage mechanism of Na3V2(PO4)3 cathode for room-temperature sodium-ion batteries. Adv. Energy Mater. 2013, 3, 156–160. [CrossRef] 12. Kubota, K.; Komaba, S. Review—Practical issues and future perspective for Na-Ion batteries. J. Electrochem. Soc. 2015, 162, A2538–A2550. [CrossRef] 13. Youngblood, G.E.; Virkar, A.V.; Cannon, W.R.; Gordon, R.S. Sintering processes and heat treatment schedules for conductive, lithia-stabilized β00-Al2O3. Am. Ceram. Soc. Bull. 1977, 56, 206–210. 14. Virkar, A.V.; Jue, J.-F.; Fung, K.-Z. Alkali Metal β and β”-Alumina and gallate polycrystalline ceramics by a vapor phase method. US Patent 6,117,807, 12 September 2000. 15. Parthasarthy, P.; Weber, N.; Virkar, A.V. High temperature sodium-zinc chloride batteries with sodium beta-alumina solid electrolyte. ECS Trans. 2007, 6, 67. [CrossRef] 16. Parthasarathy, P.; Virkar, A.V. Vapor phase conversion of α-alumina + zirconia composites into sodium ion conducting Na-β”- alumina + zirconia solid electrolytes. J. Electrochem. Soc. 2013, 160, A2268–A2280. [CrossRef] 17. Yao, Y.; Kummer, J.T. Ion exchange properties of and rates of ionic diffusion in beta-alumina. J. Inorg. Nucl. Chem. 1967, 29, 2453–2466. 18. Koh, J.-H.; Weber, N.; Virkar, A.V. Synthesis of lithium-beta-alumina by various ion-exchange and conversion processes. Solid State Ion. 2012, 220, 32–38. [CrossRef] 19. Roth, W.L.; Farrington, G.C. Lithium-Sodium Beta alumina: First of a Family of Co-ionic Conductors? Science 1977, 196, 1332. [CrossRef] 20. Dunn, B.; Farrington, G.C. Trivalent ion exchange in beta” alumina. Solid State Ion. 1983, 9–10, 223–225. [CrossRef] 21. Dunn, B.; Farrington, G.C. Fast divalent ion conduction in Ba++, Cd++ and Sr++ beta” aluminas. Mater. Res. Bull. 1980, 15, 1773–1777. [CrossRef] 22. Chevalier, J.r.m.; Gremillard, L.; Virkar, A.V.; Clarke, D.R. The tetragonal-monoclinic transformation in zirconia: Lessons learned and future trends. J. Am. Ceram. Soc. 2009, 92, 1901–1920. [CrossRef] 23. Allen, J.L.; Wolfenstine, J.; Ramaswamy, E.; Sakamoto, J. Effect of substitution (Ta, Al, Ga) on the conductivity of Li7La3Zr2O12. J. Power Sources 2012, 206, 315–319. [CrossRef] 24. Zhang, L.; Virkar, A.V. On space charge and spatial distribution of defects in yttria-stabilized zirconia. J. Electrochem. Soc. 2017, 164, F1506–F1523. [CrossRef]