Space-Charge-Mediated Superionic Transport in

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Journal of The Electrochemical Society, 156 共7兲 A506-A513 共2009兲 0013-4651/2009/156共7兲/A506/8/$25.00 © The Electrochemical Society

Space-Charge-Mediated Superionic Transport in Lithium Ion Conducting Glass–Ceramics B. Kumar,*,z D. Thomas,** and J. Kumarz Electrochemical Power Group, University of Dayton Research Institute, Dayton, Ohio 45469-0170, USA This paper describes an investigation of the properties of superionic glass–ceramic specimens synthesized from the lithium– aluminum–germanium–phosphate system. The specimens were characterized using differential scanning calorimetry, X-ray diffraction, scanning electron microscopy, and impedance spectroscopy. The concentration of lithium oxide in the glass–ceramic formulations was the primary variable investigated. It is shown that the ionic conductivity of the specimens was dependent on the lithium oxide concentration. An optimized composition exhibited a conductivity approaching 10−2 S cm−1 at around room temperature. The superionic conductivity in these specimens is attributed to the space-charge-mediated effect resulting from the presence of the dielectric Li2O phase. The specimens also displayed different conductivities during heating and cooling scans, referred to in this paper as hysteresis effect. © 2009 The Electrochemical Society. 关DOI: 10.1149/1.3122903兴 All rights reserved. Manuscript submitted January 12, 2009; revised manuscript received March 23, 2009. Published May 4, 2009.

The space charge alludes to an accumulation or depletion of local, uncompensated charges in bulk, heterogeneous solids. The charges may result from ionization, migration, and adsorption of electroactive species onto a dielectric surface, or interfaces such as grain boundaries. The glass–ceramics are polycrystalline solids that are formed by the conventional glass-making process and subsequently nucleated and crystallized at appropriate temperatures without altering the shape of the formed object. In effect, the glass– ceramics are materials with an abundance of crystallites 共of rather uniform grain size and distribution兲 and grain boundaries. The microstructure of the glass–ceramics allows it to be fine tuned so as to control and investigate the space-charge formation. The lithium ion conducting superionic glass–ceramics are of recent origin. The synthesis and ionic conductivity of lithium– aluminum–titanium–phosphate 共LATP兲 and lithium–aluminum– germanium–phosphate 共LAGP兲 glass–ceramics were originally reported by Fu1,2 and subsequently by other investigators.3,4 The LATP and LAGP glass–ceramics primarily consist of highly conductive Li1+xTi2−xAlx共PO4兲3 and Li1+xGe2−xAlx共PO4兲3 crystalline phases, respectively. The phases are derivatives of the 123 structures 关LiTi2共PO4兲3 and LiGe2共PO4兲3兴, which possess a rhombohedral lattice 共space group R3C兲 with an open three-dimensional framework of TiO6 or GeO6 octahedra sharing all corners with PO4 tetrahedra. The lithium ion occupies interstitial sites, and its transport takes place along the c axis. These glass–ceramics are single lithium ion conductors. Therefore, they are of special interest for investigating space-charge-related effects. The glass–ceramics are heterogeneous solids in which constituent phases may have different electrical properties. For example, in the LATP glass–ceramic, the lithium ion conducting phase is Li1+xTi2−xAlx共PO4兲3 共x = 0.275兲, and it is mixed with a minor dielectric phase, AlPO4.5 The AlPO4 phase becomes a substrate for the adsorption of the free and mobile lithium ions. The accumulation of the space charge results in the creation of the localized electric fields that in turn influences the transport of the remaining conduction ions. The authors characterize this as space-charge-mediated ionic conduction. The adsorbed lithium ions on the AlPO4 surface can also be desorbed by heating the specimens to a higher temperature. The adsorption and desorption processes have been experimentally investigated in the LATP glass–ceramics that allowed a quantitative measurement of the space-charge-mediated ionic conduction.6

* Electrochemical Society Active Member. ** Electrochemical Society Student Member. z

E-mail: [email protected]; [email protected]

A nanoscopic view of a glass–ceramic ionic conductor is schematically shown in Fig. 1. The conducting ions of the glass–ceramic are shown by arrows identifying them as vectors. They can be represented by a vector as they have a magnitude of velocity and direction. Before an electric field is applied, these arrows have localized motion and random direction. The diffusion coefficient of these materials is the fundamental parameter that characterizes them. Subsequent to the application of an electric field, these ions participate in the long-range conduction process. Also shown in Fig. 1 are solid circles depicting a dielectric phase that is distributed uniformly in the ionically conducting matrix. The ionic-conductor–dielectric interfaces are electroactive regions, which immobilize ions due to an electrical interaction. The immobilization reduces the number of available conducting ions. The consequence of the immobilization is also the creation of a metastable, localized internal electric field that will influence the transport of the remaining conducting ions. The metastable electric field is shown by arcs in the vicinity of the immobilized ions in Fig. 1. The metastable electric fields of Fig. 1 may disappear if the immobilized ions become free due either to the increased thermal energy or to the magnitude of the external electric field. The metastable electric field has an effect on the measured conductivity values. The interaction of the conductive ions and the dielectric phase is thus an important variable, which may enhance the ionic conductivity.

Figure 1. Schematic presentation of the interaction between conducting ion and dielectric phase in a glass–ceramic matrix.

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Journal of The Electrochemical Society, 156 共7兲 A506-A513 共2009兲 The first demonstration of the space-charge-mediated ionic transport may be traced to the work of Liang.7 In this pioneering paper, Liang7 reported that lithium iodide doped with 35–45 mol % aluminum oxide enhanced conductivity by almost a factor of 50 at 25°C. However, no significant amount of aluminum oxide was determined to be soluble in LiI. Subsequently, many investigations have reported an enhanced conductivity of silver in the AgI–Al2O3 system,8 copper in the CuCl–Al2O3 system,9 fluorine in the PbF2–SiO2 and PbF2–Al2O3 systems,10 and lithium in polymer–ceramic composite electrolytes.11 Three review papers12-14 also documented the developmental history and general properties of these heterogeneous ionic conductors. The theory of ionically conducting composites, which was developed in the 1980s,15-17 highlights the importance of the space-charge region or phase boundaries in ionic conductivity. The boundaries provide a three-dimensional percolation pathway for the transport of charge carriers. The boundaries are created through the interaction of two distinct crystalline phases; for example, LiI–Al2O3 and Li1+xTi2−xAlx共PO4兲3–AlPO4. The bulk structure of these systems remains crystalline, yet the conductivity increases because of the creation of phase boundaries containing electroactive defects. In spite of the considerable history of the space-charge-mediated ionic transport, its application for developing commercially useful materials has been nonexistent. The lack of commercial application may have resulted from unresolved fundamental issues, one of which is the coexistence of the blocking and space-charge effects. The intuitive wisdom suggests that when a dielectric phase is introduced in an ionic conducting matrix, the conductivity should decrease because the motion of the conducting ions is impeded by the existence of the dielectric phase. This physical reality is the basis of the blocking effect. The coexistence of the two effects has led to an erroneous interpretation of experimental data and substantial confusion in the literature. The author’s group recently developed an experimental technique to delineate the blocking and space-charge effects,6 which has facilitated the formulation and development of heterogeneous ionic conductors with diminished blocking effect and enhanced spacecharge-mediated ionic transport. This paper describes the formulation, processing, and superionic conductivity 共10−2 S cm−1 at 25°C兲 in LAGP glass–ceramics. It also reveals a technique to enhance the space-charge-mediated ionic transport in the glass– ceramics. LAGP glass–ceramic membranes were also used to fabricate working lithium–oxygen/air cells operating in the 25–100°C temperature range.18,19 However, due to space limitations, the performance of these cells is not covered in this paper. Experimental Melting and crystallization.— A 40 g batch of a 19.75 Li2O·6.17Al2O3·37.04GeO2·37.04P2O5 共mol %兲 composition was prepared by using reagent-grade chemicals such as Li2CO3 共Alfa Aesar兲, Al2O3 共Aldrich, particle size ⬍10 mm兲, GeO2 共Alfa Aesar兲, and NH4H2PO4 共Acros Organics兲. The chemicals were weighed, mixed, and ground for 10 min with an agate mortar and pestle. For further homogenization, the batch was milled in a glass jar for 1 h using a roller mill. The milled batch was contained in a platinum crucible and transferred to an electric furnace. Initially, the furnace was heated to 350°C at the rate of 1°C/min and held at that tem-

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perature for 1 h to release the volatile components of the batch before raising the furnace temperature to 1350°C at the rate of 1°C/min after which the glass was melted for 2 h. A clear, homogeneous, viscous melt was poured onto a stainless steel 共SS兲 plate at room temperature and pressed by another SS plate to yield ⬍1 mm thick transparent glass sheets. Subsequently, the cast and pressed glass sheets were annealed at 500°C for 2 h to release thermal stresses and were then allowed to cool to room temperature. These annealed specimens remained in the glassy state as noted by visual observation. The annealed glass specimens were subsequently crystallized in the 750–850°C temperature range for selected times. The crystallization transformed the glass to a glass–ceramic that led to a change in the appearance of the glass from colorless transparent to bluish opaque. DSC.— Thermal characterization was conducted on 10 mg of LAGP glass specimens using differential scanning calorimetry 共DSC兲 共MDSC 2920, TA Instruments, USA兲. The specimens were placed on a gold pan, while an empty gold pan was used as a reference. The samples were heated to 725°C from room temperature at the rate of 10°C/min. The cell was purged with dry nitrogen at the rate of 50 cm3 /min during the measurement. XRD.— X-ray diffraction 共XRD兲 data for the specimens were obtained using a Rigaku Miniflex XRD 共The Woodlands, TX兲, operated at 40 kV, 150 mA current with a copper target in the range of 10–60° in increments of 0.05°. Scanning electron microscopy.— Fractured and thermally etched specimens were coated with gold–palladium and subsequently characterized using a high resolution scanning electron microscope 共Hitachi S-4800; Hitachi High Technologies America, Inc., Schaumburg, IL兲. The thermal etching of the polished specimens was carried out by heating the LAGP-1 and -2 at 750° for 3 h and LAGP-3 and -4 at 650° for 3 h and subsequently quenching it down to room temperature. AC impedance measurements.— The ac impedance measurements of specimens were carried out using a Solartron instrument 共model 1260 with an electrochemical interface; Solartron US, Houston, TX兲 in the 0.1–106 Hz frequency range. A gold coating of ⬃0.5 ␮m thickness was sputtered on both sides of these specimens. The gold-coated specimens were assembled into a cell using SS blocking electrodes in a cell fixture. The fixture containing the SS/ electrolyte/SS cell was subsequently placed in a stable fixture holder with attached electrical wires leading to the impedance spectrometer. The ac impedance of the electrolyte was measured in the ⫺40 to 500°C temperature range. At each temperature, the specimen was equilibrated for 1 h before the impedance measurement. The conductivity of the specimens was computed from the ac impedance spectra. Z Plot and Z View software were used for the impedance data acquisition and analysis. The impedance spectra normally showed one semicircle and in some cases two. The diameter of these semicircles was further normalized with respect to thickness and cross-sectional area of the specimen to obtain the total 共grain and grain boundary兲 conductivity, ␴t 共s/cm兲 of the specimen. Results and Discussion Table I presents the molar composition of the LAGP glasses in which lithium oxide concentration is a variable. The LAGP-1 com-

Table I. Molar compositions of LAGP glass–ceramic specimens. Composition

LAGP-1

LAGP-2

LAGP-3

LAGP-4

Li2O Al2O3 GeO2 P 2O 5 Stoichiometry Li1+xAlxGe2−x共PO4兲3

18.75 6.25 37.50 37.50 x = 0.5

19.75 6.17 37.04 37.04 x = 0.5 +Li2O = 1.00%

21.12 6.07 36.41 36.41 x = 0.5 +Li2O = 2.37%

21.56 6.03 36.20 36.20 x = 0.5 +Li2O = 2.81%

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Journal of The Electrochemical Society, 156 共7兲 A506-A513 共2009兲

Figure 2. DSC scans of LAGP-1, -2, -3, and -4 glass specimens.

position represents a baseline formulation corresponding to the Li1.5Al0.5Ge1.5共PO4兲3 stoichiometry. The composition has been investigated by Fu2 and has shown to exhibit high conductivity around the ambient temperature, 10−4–10−3 S cm−1. In the remaining compositions, LAGP-2, -3, and -4, the Li2O concentration was gradually increased with an intent to precipitate Li2O as a dielectric phase in the ionically conducting glass–ceramic matrix. Li2O is an oxide similar to H2O and is therefore expected to be highly polar. The polar Li2O should show a high propensity to interact with the free lithium ions, as shown by Eq. 1. The formation of the Li2O:Li+ complex should lead to the space-charge-mediated ionic transport and therefore should affect ionic conductivity Li2O + Li+ ⇔ Li2O:Li+

关1兴

The crystallization temperature and time are additional parameters 共in addition to the composition兲 that can control the size and distribution of Li2O in the glass–ceramic matrix. These parameters have considerable influence on the microstructure and subsequently should affect the ionic conductivity of specimens. Figure 2 shows the DSC scans of LAGP-1, -2, -3, and -4 specimens from room temperature to 700°C. All the specimens show an inflection at around 500°C, which is related to the glass transition

temperature 共Tg兲. The estimated Tg of the LAGP-1 specimen is 522°C. An exothermic peak 共Tc兲 at 640°C is attributed to the crystallization of the glass during its conversion to glass–ceramic. As the Li2O concentration was increased progressively from LAGP-1 to LAGP-4, Tg decreased slightly from 522 to 511°C in all specimens, with the exception of LAGP-2. Tc also decreased to 626, 635, and 630°C for LAGP-2, -3, and -4 specimens, respectively. The LAGP-4 specimen showed two crystallization peaks; the first one is located at 610°C, whereas the second one is situated at 630°C. The two peaks suggest precipitation of two different crystalline phases. The DSC data of Fig. 2 suggest that the Li2O addition slightly reduced the viscosity of the glass that resulted in the lowering of Tg and Tc. Figure 3a-d shows the XRD patterns of the crystallized glass LAGP-1, -2, -3, and -4 specimens. The crystallization temperatures and time varied as indicated in the caption of Fig. 3a-d for each specimen. The Li1+xAlxGe2−x共PO4兲3 共x = 0.5兲 phase is identified with all the major reflections reported for the LiGe共PO4兲3 lattice.4 Even after an extensive substitution of the Al at the Ge site, the diffraction patterns of the Li1+xAlxGe2−x共PO4兲3 共x = 0.5兲 and LiGe2共PO4兲3 phases match closely, which is attributed to the similar ionic radii of Ge4+ and Al3+ 共Ge4+ = 0.39IV, 0.53VI, Al3+ − 0.39IV, 0.54VI Å兲. Although the LAGP-1, -3, and -4 specimens show the presence of Li2O and AlPO4, the predominance of the major crystalline phase, 关Li1+xAlxGe2−x共PO4兲3兴, is very much evident in these specimens. In terms of the conductive phase 关Li1+xAlxGe2−x共PO4兲3兴 purity, the LAGP-2 specimen may be considered to be superior as compared to other specimens. A slight shift in the peak locations toward a lower angle was noted as the Li2O concentration was increased from LAGP-1 to LAGP-4. The scanning electron microscopy 共SEM兲 images of fractured and thermally etched surfaces of the specimens LAGP-1, -2, -3, and -4 are shown in Fig. 4a-d. Figure 4a shows a dense packing of a large number of cube-shaped crystals. The lateral dimension of these cube-shaped crystals varies from about 0.25 to 3.00 ␮m. The existence of porosity is also apparent in the microstructure of Fig. 4a. The microstructure of the LAGP-2 specimen is shown in Fig. 4b. The microstructure reveals the existence of a few larger crystals with sizes ranging from 3 to 5 ␮m. The individual crystallites are less defined in terms of a cube-shaped morphology as compared to the LAGP-1 specimen. The microstructure of LAGP-3 共Fig. 4c兲 depicts a bimodal distribution of crystallites. The smaller crystals are about 1 ␮m, whereas the larger crystals are about 20 ␮m. The higher concentration of Li2O in the LAGP-3 batch appears to promote crystal growth. Figure 4d shows the microstructure of the LAGP-4 specimen. This specimen shows the existence of only large crystals with an average size of about 10–20 ␮m. The existence of monodistribution of larger-size crystals is associated with the highest concentration of Li2O among the four specimens. This micrograph 共Fig. 4d兲 also points out the possibility of liquid phase formation and fluxing effect resulting from a high concentration of Li2O. The morphology of the LAGP-1 and -2 specimens at a higher magnification 共50,000⫻兲 is shown in Fig. 5a and b. These micrographs show cracks along the grain boundaries, indicating a mechanical weakness that developed during the thermal etching process. The existence of a stretched material across the grain boundaries 共shown by an arrow兲 in Fig. 5b shows the presence of a glassy phase along the grain boundaries in LAGP-2 specimens. The LAGP-1 specimen exhibited little evidence of the glassy phase. The SEM investigation revealed that the progressive increase in Li2O from LAGP-1 to -4 led to a residual glassy phase after the transformation of the glass to glass–ceramic, even though the crystallization temperature was reduced from 850 to 750°C. Also, the size of the crystalline phase progressively increased from LAGP-1 to -4. The DSC data revealed the existence of two crystalline precipitates in the LAGP-4 specimen; however, the XRD and SEM data showed only one type of the crystalline phase. The two peaks in the DSC curve may have resulted from the crystalline precipitates of the

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Journal of The Electrochemical Society, 156 共7兲 A506-A513 共2009兲

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as circuit resistance external to the specimen. Originating from the intersection of the ⫺3 and 23°C spectra is a semicircle 共Fig. 6a and b兲. The diameter of the semicircle is the resistance of the specimen, and this resistance includes contribution of the grain, Rg, and grain boundary, Rgb. In some cases and specifically at lower temperatures 共⬍−20°C兲, the existence of two semicircles corresponding to Rg and Rgb has been noted and varies with the composition and thermal treatments applied to the specimens.4 At higher temperatures 共⬎100°C兲, only a straight line spike at an angle is observed, such as that shown in Fig. 6c. In this case, the intersection on the z⬘ axis includes contributions of the circuit and specimen resistances. To obtain the specimen resistance, the circuit resistance is extrapolated from the low temperature measurements 共assuming it to be linearly dependent on temperature兲 and subtracted from the intersection on the z⬘ axis. The technique of obtaining specimen resistance from an impedance measurement such as that shown in Fig. 6c can be found elsewhere.20 Figure 7 shows Arrhenius plots of bulk 共grain and grain boundary兲 conductivity in the ⫺40 to 150°C temperature range for LAGP-1, -2, -3, and -4 glass–ceramic specimens. The crystallization temperature and time were selected to achieve the highest conductivities from these specimens. The LAGP-1 and -2 specimens were crystallized at 850°C for 12 h. The observations of DSC measurements and prior work4 led us to lower the crystallization temperature for the LAGP-3 and -4 glass–ceramic specimens to 750°C; however, the hold time was increased to 36 and 48 h for these two specimens. The LAGP-2 specimen provided the highest conductivity, with the room-temperature conductivity approaching 10−2 S cm−1. All of the specimens exhibited an inflection at around 70°C. This inflection point is attributed to the presence of AlPO4. The XRD data for LAGP-1, -3, and -4 show the existence of AlPO4. Although the XRD data did not reveal the presence of AlPO4 in LAGP-2, the Arrhenius plot clearly shows its existence. In all probability, the concentration and size of AlPO4 in LAGP-2 were below the XRD detection limit. The presence of AlPO4 in these specimens led to the reaction as expressed by Eq. 2 AlPO4 + Li+ ⇔ AlPO4:Li+

Figure 3. XRD patterns of 共a兲 LAGP-1 specimen crystallized at 850°C for 12 h, 共b兲 LAGP-2 specimen crystallized at 850°C for 12 h, 共c兲 LAGP-3 specimen crystallized at 750°C for 36 h, and 共d兲 LAGP-4 specimen crystallized at 750°C for 48 h.

Li1+xAlxGe2−x共PO4兲3 structure with two different values of x. The phase with a lower value of x allowed the available lithium to form a glassy phase with other components. Typical ac impedance spectra of the LAGP-2 specimen at ⫺3, 23, and 150°C are shown in Fig. 6a-c. These spectra intersect at the high frequency side of the z⬘ axis. The intersections are interpreted

关2兴

The AlPO4:Li+ complex is a source of space charge, and its existence below 70°C increases the activation energy for lithium ion transport. The dissociation of the AlPO4:Li+ complex above 70°C eliminates space-charge-mediated ionic transport, and the transport takes place primarily through the LAGP crystallites and grain boundaries. The activation energies for the ionic transport of the specimens below and above 70°C are presented in Table II. The activation energies for LAGP-1 and -2 below 70°C remain similar but decrease for LAGP-3 and -4 to 0.50 and 0.45 eV, respectively. Similarly, above 70°C the activation energies for LAGP-1 and -2 remain identical, whereas it decreases to 0.16 for both LAGP-3 and -4 specimens. Although the concentration of Li2O in these LAGP-1, -2, -3, and -4 specimens was progressively increased with an intent to create a stronger space-charge effect and therefore to achieve substantially enhanced conductivities, the Li2O concentration can only be increased up to a certain level—up to the level of the LAGP-2 specimen to create the space-charge effect. Subsequent increases lead to the excessive presence of a glassy phase along the grain boundaries and lower conductivities in LAGP-3 and -4 specimens. It should also be noted that the Arrhenius plots of LAGP-1 and -2 appear to be close in Fig. 7; nonetheless their absolute measured conductivity values differ by a factor of 3. The increase in conductivity from LAGP-1 to LAGP-2 is attributed to the excess of Li2O, as shown in Table I. The Li2O precipitates and creates the spacecharge effect similar to AlPO4, which enhances the conductivity of LAGP-2. Additional Li2O in LAGP-3 and -4 is detrimental to the

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Journal of The Electrochemical Society, 156 共7兲 A506-A513 共2009兲

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(a)

(c)

conductivity because it forms excess of the glassy phase, which is an impediment to the transport of lithium ions through the grain boundaries. The measured conductivity values for the LAGP-2 specimen crystallized for 12 h at 850°C are presented in Table III, which primarily reflects the data presented in Fig. 7; nonetheless, it further illustrates the superionic characteristics of the LAGP-2 specimen. An experimentally measured conductivity value of 4 S cm−1 of a solid at any temperature has not been reported. The conductivity value of 4 S cm−1 of the specimen at 310°C is attributed to the space-charge mediation induced by the Li2O:Li+ complex. It is possible to further enhance the conductivity by forming space charges having higher field strengths 共ⰇLi2O:Li+兲. Such an ionic conductor would be similar to many metals in terms of electrical conductivity, with the possibility that someday one may discover superconductivity in a solid ionic conductor. The drop in conductivity of the LAGP-2 specimen above 310°C will be explained in the next paragraph. An evidence of the existence of the Li2O:Li+ complex is presented in Fig. 8, showing Arrhenius plots of LAGP-1 and 2 in the temperature range of 150–500°C. The LAGP-1 sample shows a conductivity plateau above 350°C. The LAGP-2 specimen shows a peak at around 400°C and a subsequent drop in conductivity above 400°C. At around 400°C, the Li2O:Li+ complex 共formed as per Eq. 1兲 dissociates, and the space-charge effect mediated by the Li2O:Li+ complex is destroyed. The result is a decrease in conductivity, as shown in Fig. 8 and Table III. The LAGP specimens exhibit two space-charge-mediated effects: one at around 70°C due to the presence of AlPO4 and a second at around 400°C resulting from the existence of Li2O. The AlPO4 effect is small; it just shows an inflection in the Arrhenius plot, whereas the large contribution of the Li2O is reflected by a major drop in conductivity. It is also interesting to note that these space-

(b)

Figure 4. SEM micrographs of polished and thermally etched specimens from 共a兲 LAGP-1, 共b兲 LAGP-2, 共c兲 LAGP-3, and 共d兲 LAGP-4.

(d)

-charge-mediated effects influence conductivities in different ways in low and high temperature regions because of their influence on activation energies. The formation and destruction of the space-charge-mediated effects also lead to a hysteresis effect on conductivity; i.e., the measured conductivity data differ during heating and cooling scans if the temperature range of the scans contains a space-charge signature. For example, there is only one space-charge signature attributed to the Li2O:Li+ complex in the 150–450°C range of Fig. 8. Therefore, a thermal cycling in the 150–450°C temperature range leads to a hysteresis effect totally attributed to the Li2O:Li+ complex. In Fig. 9, the range of temperature cycling is 150–450°C. In this case the magnitude of the hysteresis effect is equal to the combined contributions of the Li2O:Li+ and AlPO4:Li+ complexes. The thermal cycling in a narrow temperature range, for example, 0–60°C, does not exhibit any hysteresis from the LAGP specimens, which have been thermally annealed 共stabilized兲. The conductivity data of the LAGP-3 specimen shown in Fig. 9 show that during heating and cooling scans conductivity could differ by a factor of 5. Figure 9 also shows characteristic space-charge-mediated transitions at 70 and 400°C in the Arrhenius plot. The hysteresis effect in LAGP specimens is similar to that in the polymer–ceramic composite electrolytes reported in an earlier publication.21

Conclusions Ionically conducting glass–ceramic specimens from the LAGP system were prepared and characterized using DSC, XRD, SEM, and ac impedance spectroscopy. The concentration of lithium oxide is varied so as to precipitate Li2O as a dielectric phase in the ionically conducting glass–ceramic matrix to create the space-chargemediated effect on the ionic transport. Specific conclusions of the investigation are presented in the following paragraphs.

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Journal of The Electrochemical Society, 156 共7兲 A506-A513 共2009兲 (a)

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0

LAGP-2 at -3 C

-1800

-1500

Z''

-1200

-900

-600

Rb= R2-R = 317 ohm 1

-300

R1

R2

0 0

300

600

900

1200

1500

1800

Z' -500 0

LAGP-2 at 23 C

(b) -400

Z''

-300

-200

Rb= R2-R1=75 ohm

-100

R2

R1 0 0

Figure 5. SEM micrographs of polished and thermally etched specimens from 共a兲 LAGP-1 specimen crystallized at 850°C for 12 h and 共b兲 LAGP-2 specimen crystallized at 850°C for 12 h.

100

200

Z'

300

400

500

40

50

-50 0

LAGP-2 at 150 C

(c) -40

-30

Z''

XRD revealed that in addition to the primary conductive crystalline phase, Li1+xAlxGe2−x共PO4兲3 共x = 0.5兲, AlPO4 and Li2O also exist as minor phases. The concentrations of these two minor phases varied from one specimen to another; however, the XRD data did not delineate the variation as their concentrations were near the XRD detection limit. SEM data demonstrated that the size of the primary crystalline phase, Li1+xAlxGe2−x共PO4兲3 共x = 0.5兲, increased with increasing concentration of Li2O. The effect was attributed to the fluxing effect of Li2O in these glass–ceramic specimens. A glassy phase is also present at the grain boundaries in the specimens other than LAGP-1. The ionic conductivity as measured by the ac impedance technique was highest for the LAGP-2 specimen. The conductivity of this specimen approached 10−2 S cm−1 at around the ambient temperature. The minor phases, AlPO4 and Li2O, gave rise to specific signatures in the Arrhenius plots at around 70 and 400°C.

-20

-10

0 0

10

20

30

Z'

Figure 6. Impedance spectra of LAGP-2 glass–ceramic at 共a兲 −3°C, 共b兲 23°C, and 共c兲 150°C.

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Journal of The Electrochemical Society, 156 共7兲 A506-A513 共2009兲

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0

0

T( C)

Temperature ( C) 227

1

127

60

-23

12

-51

0.2 560

441

352

227

283

LAGP-1(850 C-12 hrs) 0 LAGP-2(850 C-12 hrs) 0 LAGP-3(750 C-36 hrs) 0 LAGP-4(750 C-48 hrs)

0

144

182 0

LAGP-1 (850 C-12 hrs) 0 LAGP-2 (850 C-12 hrs)

0

0.1 0.0

-1

log σ (S/cm)

log σ (S/cm)

-0.1

-2

-3

-4

-0.2 -0.3 -0.4

-5

-0.5

-6 2.0

2.5

3.0

3.5

4.0

-0.6

4.5

1.2

1.4

1.6

1.8

0

2.0

2.2

2.4

0

1000/T ( K)

1000/T ( K)

Figure 7. 共Color online兲 Arrhenius plots of total conductivity of LAGP-1, -2, -3, and -4 glass–ceramic specimens crystallized at different temperatures and times.

Figure 8. 共Color online兲 Arrhenius plots of total conductivity of LAGP-1 and 2 glass–ceramic specimens crystallized at different temperatures and times in the high temperature region.

0

Temperature C

Table II. Activation energies for ionic transport in LAGP specimens.

727

394

227

127

-1.0

Specimens

Below 70°C 共eV兲

Above 70°C 共eV兲

0.62 0.62 0.50 0.45

0.30 0.30 0.16 0.16

Conductivity 共S cm−1兲

Temperature 共°C兲

Conductivity 共S cm−1兲

23 30 50 70 110 150

4.48 ⫻ 10−3 6.18 ⫻ 10−3 3.81 ⫻ 10−2 1.69 ⫻ 10−1 5.19 ⫻ 10−1 1.07

210 250 310 360 390 450

2.06 2.63 4 1.58 ⫻ 10−1 2 ⫻ 10−1 2.85 ⫻ 10−1

Li2O and AlPO4 contributed to the space-charge-mediated effect in conductivity. The enhancement resulted from the formation of Li2O:Li+ complex. The space-charge effect is also manifested by a hysteresis effect as observed in the conductivity measurement during heating and cooling scans. Acknowledgments D.T. gratefully acknowledges the financial support by Dayton Area Graduate Studies Institute 共DAGSI兲. The authors also express

Li2O mediated

Log σ (S/cm)

Heating

Cooling

-2.0

-2.5

-3.0 AlPO4 mediated

Table III. Temperature dependent conductivity of LAGP-2 specimen crystallized at 850°C for 12 h. Temperature 共°C兲

-23

LAGP-3(850C-12 hrs)

-1.5

LAGP-1 LAGP-2 LAGP-3 LAGP-4

13

60

-3.5

-4.0 1.0

1.5

2.0

2.5

3.0

3.5

4.0

0

1000/T ( K)

Figure 9. Hysteresis effect in the Arrhenius plot of conductivity of the LAGP-3 specimen in the ⫺13 to 450°C temperature range.

their appreciation to Dr. N. Hecht for reviewing the manuscript and Dale Grant for conducting the SEM work. University of Dayton Research Institute assisted in meeting the publication costs of this article.

References 1. 2. 3. 4. 5. 6.

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