Characterization of ion transport property and study of

Eur. Phys. J. Appl. Phys. 43, 209–216 (2008)

DOI: 10.1051/epjap:2008135

Characterization of ion transport property and study of solid state battery discharge performance on newly synthesized Ag+ ion conducting quaternary solid electrolyte systems: x[0.75AgI: 0.25AgCl]: (1–x)KI R.C. Agrawal, A. Chandra, A. Bhatt and Y.K. Mahipal

Eur. Phys. J. Appl. Phys. 43, 209–216 (2008) DOI: 10.1051/epjap:2008135

THE EUROPEAN PHYSICAL JOURNAL APPLIED PHYSICS

Characterization of ion transport property and study of solid state battery discharge performance on newly synthesized Ag+ ion conducting quaternary solid electrolyte systems: x[0.75AgI: 0.25AgCl]: (1–x)KI R.C. Agrawal1,a , A. Chandra, A. Bhatt2 , and Y.K. Mahipal 1 2

Solid State Ionics Research Laboratory, School of Studies in Physics Pt. RSS University, Raipur 492010, C.G., India M.P. Christian College of Engg. & Tech., Bhilai 490026, India Received: 19 December 2007 / Accepted: 14 May 2008 c EDP Sciences Published online: 19 July 2008 – Abstract. Investigations on ion transport mechanism in the new Ag+ ion conducting quaternary solid electrolyte systems: (1 − x)[0.75AgI: 0.25AgCl]: xKI, where 0 < x < 1 in mol. wt.%, are reported. The quaternary systems were prepared by solid solution reaction of an alternate host: ‘a quenched [0.75AgI: 0.25AgCl] mixed system/solid solution’, instead of the traditional host salt AgI, and KI. The compositional dependent conductivity studies on the quaternary systems identified the composition: 0.7[0.75AgI: 0.25AgCl]: 0.3KI, as the Optimum Conducting Composition (OCC) having conductivity σ ∼ 5.9 × 10−3 S cm−1 at 27 ◦ C. For direct comparison of the room temperature conductivity behaviour of the quaternary OCC, AgI-based ternary solid solution composition: 0.8AgI: 0.2KI, resulting into the well-known superionic system: KAg4 I5 , has also been synthesized in the identical manner which exhibited σ ∼ 1.12 × 10−2 S cm−1 at 27 ◦ C. The conductivity of newly synthesized quaternary OCC was slightly lower than that of as prepared ternary KAg4 I5 . However, in the open ambient conditions σ of OCC sample remained practically stable for over hundred hours while that of KAg4 I5 decreased by more than two orders of magnitude in the same duration. The phase identification and material characterization studies on the quaternary OCC have been carried out using XRD and DTA techniques. The ion transport mechanism has been characterized on the basis of experimental studies on some basic ionic parameters viz. conductivity (σ), ionic mobility (μ), mobile ion concentration (n), ionic transference number (tion ) and ionic drift velocity (vd ). Solid state batteries have been fabricated using the newly synthesized quaternary OCC as well as KAg4 I5 as electrolytes, sandwiched between Ag/I2 electrode couple and the cell potential discharge performances have been studied under varying load conditions. PACS. 60. Condensed matter: structural, mechanical, and thermal properties

1 Introduction Solid State Ionics, relatively a new branch of material science, emerged around 1967 after the discovery of two new groups of solid systems viz. MAg4 I5 (M = Rb, K, NH4 ) and Na-β-alumina exhibiting exceptionally high Ag+ - Na+ -ion conduction (∼10−1 S cm−1 ) at room/moderately high temperatures respectively [1–3]. The field evolved with rapid pace since then and a large number of high ion conducting solids involving different kinds of mobile ion species viz. H+ , Li+ , Ag+ , Cu+ , Na+ , F− , O2− , etc. has been discovered which has been grouped presently into a variety of phases such as crystalline/polycrystalline, glassy/amorphous, composite, polymeric electrolytes, etc. [4–10]. Solid state ionic materia

e-mail: rakesh c [email protected]

als, widely referred to as ‘Superionic Solids’ or ‘Solid Electrolytes’ or ‘Fast Ion Conductors’, show room temperature conductivity very close to that of liquid/aqueous electrolytes. Consequently, these materials have great technological potentials to develop solid state electrochemical devices viz. batteries, fuel cells, supercapacitors, etc. Amongst various solid state ionic materials, fast Ag+ ion conductors always attracted special attention due to the fact that they exhibit relatively higher room temperature conductivity (∼10−1 − 10−2 S cm−1 ) and involve easy synthesis/material handling. Traditionally, AgI is used as a common host salt for the preparation of fast Ag+ ion conductors in polycrystalline/glassy/composite phases [11]. In 1994, Agrawal and coworkers [12] discovered an alternate host salt: ‘a quenched/annealed [0.75AgI: 0.25AgCl] mixed system/solid solution’ which

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not only exhibited transport characteristics akin to those of the traditional host AgI but yielded number of glassy/composite/polycrystalline solid electrolyte systems with superior electrolyte properties as compared to those analogous systems based on AgI [13]. Further, amongst the known AgI -based fast Ag+ ion conductors, the ternary superionic solids: MAg4 I5 (where M = Rb, K, NH4 ) retain the status of highest conducting systems at room temperature [1,2]. However, MAg4 I5 solid systems have been observed to be thermodynamically unstable around room temperature, especially in the humid ambience. The present paper reports the solid solution synthesis of relatively more stable Ag+ ion conducting quaternary systems: x[0.75AgI: 0.25AgCl]: (1 − x)KI in which the traditional host AgI has been replaced by the alternate host, as mentioned. The highest conducting composition, referred to as the Optimum Conducting Composition (OCC), has been identified from the compositional-dependent conductivity studies. To enable a direct comparison of the room temperature conductivity behaviour as well as device performance, the traditional host salt AgI-based well-known ternary superionic material: KAg4 I5 has also been synthesized following identical routes as for the quaternary OCC. The material characterization was done on the quaternary electrolyte OCC sample only and the formation of a new compound having polycrystalline phase has been confirmed. The mechanism of ion transport in OCC, has been explained on the basis of some basic ionic parameters viz. conductivity (σ), ionic mobility (μ), mobile ion contraction (n), ionic transference number (tion ) and ionic drift velocity (vd ) measured with the help of different experimental techniques, as mentioned below in Section 2. Solid state batteries have been fabricated by sandwiching the quaternary OCC as well as ternary KAg4 I5 solid electrolytes between Ag/I2 electrode couple and the cell potential discharge performances have been studied at room temperature under varying load conditions.

2 Experimental Pre-dried starting chemicals: AgI, AgCl [Purity > 98%, Redial (India) Chemicals] and KI [purity > 99%, Merck] were used for the preparation of quaternary systems: x[0.75AgI: 0.25AgCl]: (1 − x)KI as well as the ternary superionic solid: KAg4 I5 . These systems were synthesized adapting following four routes of solid solution reaction: Route #1: homogeneously-mixed compositions: x[0.75AgI: 0.25AgCl]: (1 − x)KI, where 0.1 < x < 1 in mol wt.%, kept in separate silica tubes, were heated ∼700 ◦C (well above the melting points of the constituent chemicals), shook the melt well for ∼15 min., then allowed to cool slowly in air to room temperature(∼27 ◦ C). Route #2: the above homogeneously mixed compositions, in different silica tubes in the molten state, were kept in the furnace ∼700 ◦C for 48 h then cooled to room temperature as in #1. Route #3: different compositional ratios, as in #1, were prepared by simple homogeneous mixing by mortar and pestle for 30 min. at room temperature.

Route #4: only the Optimum Conducting Composition (OCC) of the quaternary system, identified from ‘logσ − x’ studies (to be discussed below), was heated ∼700 ◦C for one week, then left in the oven to cool to room temperature. In addition to the synthesis of the above quaternary solid electrolyte systems based on alternate host, the solid solution reactions of the ternary composition: 0.8AgI: 0.2KI, resulting into the well-known superionic solid KAg4 I5 , have also been carried out using traditional host AgI following three routes viz. #1, #2 and #4. However, as reported in the literature [1,2], one can obtain the superionic KAg4 I5 only through synthesis route #4. The finished products were thoroughly ground, then pressed ∼4 t cm−2 to form pellets of diameter ∼1.28 cm and thickness ∼1–3 mm. Colloidal silver paint was applied on both the surfaces of pellets as reversible electrodes for conductivity (σ) measurements and conducting graphite was used as blocking electrodes for mobility (μ) and transference number (tion ) measurements. As mentioned, to identify quaternary electrolyte OCC, the compositional dependent conductivity measurements were carried out at room temperature using a LCR bridge (Model: HIOKI 3520, Japan) at a fixed frequency (∼5 KHz). Likewise, the conductivity of ternary superionic solid KAg4 I5 was also measured at room temperature and compared with that of quaternary OCC sample. The material characterization/phase identification studies on the newly synthesized quaternary OCC sample was done by XRD and DTA. The ionic mobility (μ) measurements for different composition (x) were also carried out at room temperature on the quaternary solid electrolyte systems using the Transient Ionic Current (TIC) d.c. polarization technique [14,15]. Subsequently, the mobile ion concentration (n) values were evaluated from σ and μ data. The ionic transference number (tion ) was determined for the quaternary OCC sample only by Wagner’s dc polarization method [16]. An x.y.t recorder (Model: Graphtec WX 2300-1L, Japan) was employed in both the measurements. The ionic drift velocity (vd ) values for different compositions were estimated using the peak ionic current (IT ) values, obtained from the ‘current–time’ plots of d.c. polarization studies and n data. The details related to these experimental procedures have been discussed at length in our earlier communications [12,13]. The temperature dependent conductivity studies on the newly synthesized quaternary systems: x[0.75AgI: 0.25AgCl]: (1 − x)KI were done and the activation energy values were computed from ‘log σ − 1/T ’ Arrhenius plots. The temperature variation of μ, n and vd were also studied on the quaternary OCC sample only and the energies involved in the different thermally activated processes were computed from the respective Arrhenius type plots. Using the quaternary OCC as well as the ternary superionic solid KAg4 I5 as electrolytes, solid state batteries were fabricated in the cell configuration: Ag/(quaternary OCC or KAg4 I5 )/(C+I2 ), where the elemental iodine (I2 ) mixed with the conducting graphite (C) in 1: 1 weight ratio and silver metal were used as cathode and anode respectively. A high input impedance digital

R.C. Agrawal et al.: Ion transport property and solid state battery study

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-1

multimeter was employed for the cell potential measurements under varying load conditions.

RT = 27 oC -2

3 Results and discussion

Figure 1 shows ‘log σ − x’ variations at room temperature for the quaternary systems: x[0.75AgI: 0.25AgCl]: (1 − x)KI, prepared by routes ##1, 2, 3. All the three plots look almost alike and followed the usual variation reported for a number of double salt solution compounds. A conductivity maxima was found at x = 0.7 in all the three plots which was followed by a minima at x = 0.8. For direct visualization, σ-values of the quaternary composition: 0.7[0.75AgI: 0.25AgCl]: 0.3KI, prepared by route #4 as well as ternary composition: 0.8AgI: 0.2KI,prepared by routes ##1, 2, 4 (as mentioned in Sect. 2) are also plotted in Figure 1. Some significant features of these studies are outlined as below: • For the quaternary system i.e. the solid solution of the alternate host: [0.75AgI: 0.25AgCl] with KI, the molar composition at x = 0.7 prepared by route #1, exhibited highest conductivity σ ∼ 5.9 × 10−3 S cm−1 at 27 ◦ C. This has been referred to as the Optimum Conducting Composition (OCC). The solid solution reactions of this composition involving prolonged heating of the melt (routes ##2 and 4) as well as the sample prepared by simple physical mixing (route #3) resulted into the quaternary systems of relatively lower conductivity values. • As reported in the literature, for the ternary systems: xAgI: (1 − x)KI, the composition at x = 0.8 results into the well-known superionic solid KAg4 I5 with highest room temperature conductivity [1,2]. We carried out the solid solution reactions for this ternary composition: 0.8AgI: 0.2KI only following routes ##1, 2, 4, as mentioned. It was observed that the conductivity results of these ternary solid solutions are just other way round from those for the quaternary OCC obtained at x = 0.7. The optimum conducting superionic phase: KAg4 I5 was obtained only when the solid solution reaction was done for prolonged (∼ 1 week) heating (route #4), akin to what has been reported [1,2]. The conductivity σ ∼ 1.1 × 10−2 S cm−1 at 27 ◦ C, obtained for the ternary superionic solid: KAg4 I5 prepared by us, is lower than that reported in the literature. This may be due to the reason that the precursor chemicals of relatively lower purities were used by us during the solid solution synthesis. These chemicals are much cheaper and hence, cost effective. The ternary system: 0.8AgI: 0.2KI, prepared by other two routes (##1, 2) exhibited substantially low conductivity values. This is probably due to the insufficient heating and hence, incomplete solid solution reaction.

-1

3.1 Compositional dependent conductivity studies on quaternary systems: x[0.75AgI: 0.25AgCl]: (1 − x)KI

Log σ (S cm )

-3

-4

-5

-6

-7 0.1

0.3

0.5

0.7

0.9

x mol (%)

Fig. 1. (Color online) ‘ log σ − x’ plots for quaternary systems: x[0.75AgI:0.25AgCl]: (1 − x)KI prepared by routes #1 (), #2 (), #3 (). The conductivity values for quaternary OCC: 0.7[0.75AgI: 0.25AgCl]: 0.3KI prepared by route #4 (×) and ternary system: 0.8AgI: 0.2KI prepared by routes: #1 (•), #2 (◦), #4 (+) are also plotted. (Refer to the text for details.)

The conductivity of the newly synthesized quaternary OCC: 0.7[0.75AgI: 0.25AgCl]: 0.3KI is approximately onehalf times lower than that of the ternary KAg4 I5 . However, in the prevailing ambient conditions, the conductivity of the quaternary system remained almost stable for more than 100 h. While the conductivity of KAg4 I5 decreased from 10−2 →10−5 in the order of magnitude in this duration. This can be clearly visualized in the conductivitystability plot, shown in Figure 2. As already pointed out in Section 1, the ternary superionic solid: KAg4 I5 is unstable thermodynamically around room temperature particularly in the humid ambience and gets dissociated into low conducting compounds through the chemical reaction as below [1,2]: 2KAg4 I5 → K2 AgI3 + 7AgI. In order to identify the reason for σ-maxima in ‘ log σ − x’ plot of Figure 1 giving rise to the quaternary electrolyte OCC, we carried out μ and n measurements at room temperature as a function of x. Figure 3 shows ‘ log μ − x’ and ‘ log n − x’ variations for the quaternary systems: x[0.75AgI: 0.25AgCl]: (1 − x)KI prepared by route #1. One can obviously note that μ remained almost unaltered for different compositions, while ‘ log n − x’ followed the variation almost analogous to ‘ log σ − x’, with n-maxima appearing at x = 0.7. Table 1 lists the room temperature values of these ionic parameters for the quaternary OCC as well as those of the pure host salt [12], along with the values of ionic transference number (tion ) and activation energy (Ea ) (to be discussed below). The sets of data, listed in the table for the pure host and superionic OCC, are self explanatory. However, on the basis of these

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Table 1. Room temperature (27 ◦ C) values of some basic ion transport parameters of the quaternary electrolyte OCC: 0.7[0.75AgI: 0.25AgCl]: 0.3KI and the quenched host salt: [0.75AgI: 0.25AgCl]. σ (S cm−1 )

System Host salt: [0.75AgI: 0.25AgCl] Newly synthesized quaternary electrolyte OCC: 0.7[0.75AgI: 0.25AgCl]: 0.3KI

μ(cm2 V−1 s−1 )

tion

Ea (eV)

Ref.

(2.41 ± 1) × 10

8.0 × 1016

∼1

0.23

[12]

5.9 × 10−3

(7.6 ± 1) × 10−2

4.87 × 1017

∼1

0.11

Present work

−2

0

3.2 Phase identification and material characterization studies on the quaternary electrolyte OCC: 0.7[0.75AgI: 0.25AgCl]: 0.3KI

RT = 27 oC

-1

Log σ (S.cm )

-2

-4

-6 0

20

40

60

80

100

120

Time (Hours)

Fig. 2. (Color online) Conductivity- stability plots for quaternary electrolyte OCC: 0.7[0.75AgI: 0.25AgCl] : 0.3KI (•) and ternary superionic solid: KAg4 I5 (). 0.0

20

ionic mobility

o

RT = 27 C

mobile ion concentration

18

2

-1.0

-3

16

Log n (cm )

-1

-1

Log μ (cm .V .s )

-0.5

-1.5

-2.0

14 0.0

0.2

0.4

0.6

n(cm−3 )

3.1 × 10

−4

0.8

x (mol. wt. %)

Fig. 3. Compositional dependence of room temperature mobility (μ) () and mobile ion concentration (n) (•) for the newly synthesized quaternary systems: x[0.75AgI: 0.25AgC]: (1 − x)KI.

studies, one can clearly identify that the overall increase in σ for the quaternary electrolyte OCC is predominantly due to the increase in n. This, in turn, indicated the fact that a relatively larger number of mobile ions got available for conduction in the OCC composition.

Figure 4 shows the X-ray diffraction patterns for the quaternary electrolyte OCC: 0.7[0.75AgI: 0.25AgCl]: 0.3KI along with the constituent compounds i.e. pure host: ‘a quenched/annealed [0.75AgI: 0.25AgCl] mixed system/solid solution’; pure KI. Table 2 lists d-values and the relative intensities for some prominent reflection peaks of the three XRD patterns. Comparing these results, one can clearly observe in XRD diffractogram for quaternary electrolyte OCC the appearance of some new peaks/disappearance of some peaks/shifting in some peak positions/variations in the peak intensities, etc. It is wellknown that such features occurring in XRD pattern, give clear indication of the formation of a new compound. Moreover, the appearance of well-defined sharp reflection peaks in the XRD diffractogram of OCC further confirmed that the new compound exists in the polycrystalline phase. The Differential thermal analysis (DTA) was done on the newly synthesized quaternary electrolyte OCC: 0.7[0.75AgI: 0.25AgCl]: 0.3KI. DTA thermogram for OCC is shown in Figure 5. A small shallow endothermic peak at ∼140 ◦ C is indicative of presence of minute fraction of unreacted host salt (see the discussion in the section below) [12], while the second endothermic peak at ∼250 ◦ C may be attributed to the melting point temperature of the newly formed compound. 3.3 Temperature dependence of σ, μ, n, vd for quaternary electrolyte OCC: 0.7[0.75AgI: 0.25AgCl]: 0.3KI Figure 6 shows ‘ log σ − 1/T ’ plots for different compositions of the quaternary systems: x[0.75AgI: 0.25AgCl]: (1 − x)KI synthesized through route #1. Similar plot for the pure quenched host: [0.75AgI: 0.25AgCl] has been reproduced in Figure 6 where an abrupt jump in the conductivity ∼135−140 ◦ C corresponds to β → α like phase transition, akin to AgI [12]. One can note that the conductivity of different compositions of the quaternary system increased gradually as the temperature increased and no abrupt changes from the straight line behavior were observed in any of ‘ log σ − 1/T ’ plots. This is a clear indication of stabilization of the host salt during solid solution reaction as well as formation of new compounds. This has been confirmed by XRD analysis on the quaternary electrolyte OCC: 0.7[0.75AgI: 0.25AgCl]: 0.3KI, as discussed


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Table 2. XRD data for pure host: [0.75AgI: 0.25AgCl], pure KI and the quaternary electrolyte OCC: 0.7[0.75AgI: 0.25AgCl]: 0.3KI. Host (a)

d – 3.93 3.74 – 2.80 – 2.40 – 2.29 2.10 2.00 1.95 –

(b)

(c)

KI I/Io – 32 78 – 38 – 18 – 100 20 19 65 –

d 4.06 – – 3.52 – 2.49 – – – – 2.03 – 1.57

OCC I/Io 12 – 100 – 28 – – – – 10 – 20

d – 3.95 3.71 3.49 – – – 2.31 2.29 2.11 – 1.95 –

I/Io – 55 22 61 – – – 40 100 90 – 57 –

Fig. 4. XRD patterns: (a) quaternary electrolyte OCC: 0.7[0.75AgI: 0.25AgCl]: 0.3KI; (b) pure host: [0.75AgI: 0.25AgCl; (c) pure KI.

Ea (eV)

1 0

0.6 0.4 0.2 0 0

0.2 0.4 0.6 0.8

Log σ (S.cm-1)

1

X (m ol. %)

-1 -2 -3 -4 -5 -6 -7 2

2.2

2.4

2.6

2.8

3

3.2

3.4

(103/T) K-1

Fig. 5. DTA thermogram for the quaternary OCC: 0.7[0.75AgI: 0.25AgCl]: 0.3KI.

above. It can also be noted that the ‘ log σ − 1/T ’ plot of OCC is almost temperature independent and the conductivity values at different temperatures are much higher than those of the other compositions. However, a slight upward deviation from the straight behaviour can be noticed in ‘ log σ − 1/T ’ plot for OCC around 130–140 ◦ C. This may be due to the presence of a fractional amount of the host salt which was also reflected in DTA thermogram of Figure 5. The Arrhenius equation governing the thermally activated conductivity process in the quaternary electrolyte OCC: 0.7[0.75AgI: 0.25AgCl]: 0.3KI can be expressed as: σ(T ) = 5.7 × 10−1 exp[−0.11/kT ]

(S cm−1 )

(1)

where Ea = 0.11 eV, the activation energy (also listed in Tab. 1), computed by least square linear fitting of the

Fig. 6. (Color online) ‘Log σ − 1/T ’ Arrhenius plots for different molar compositions of quaternary solid electrolyte systems: x[0.75AgI: 0.25AgCl] : (1−x)KI, prepared by route #1; x = 0.1 (), 0.2 (), 0.3 (×), 0.4 (Δ), 0.5 (•), 0.6 (+), 0.7 (♦), 0.8 (–), 0.9 () and pure quenched host (o). Top inset: Variation of activation energy (Ea ) as a function of x.

slope. Similar equations were set-up for the Arrhenius plots for the rest of the compositions. The activation energy (Ea ) values, computed likewise, are plotted as a function composition (x) in the top-inset of Figure 6. It can be clearly noticed that the activation energy for the quaternary electrolyte OCC is minimum. This is indicative of a relatively easier ion transport in this composition. μ and n values for the quaternary electrolyte OCC: 0.7[0.75AgI: 0.25AgCl]: 0.3KI were also measured as a function of temperature, as mentioned in Section 2 and plotted in Figure 7. It can be noticed that as temperature increased, μ increased substantially while n decreased slightly. The increase in ionic mobility can be attributed as a consequence of opening up of the lattice structure as well as increase in

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The European Physical Journal Applied Physics 18.0

ionic mobility mobile ion concentration

1.0

180

) -3

-0.5

120

Current (µ A)

17.0

0.0

Log n (cm

-1 2

o

IT (90 C)

140

-1

Log μ (cm .V .s )

160 17.5

0.5

IT = Peak Current

O

IT (110 c)

16.5

-1.0 16.0 2.0

2.2

2.4

2.6

2.8 3

3.0

3.2

o

100

IT (60 C)

80 o

60

IT (40 C) o

IT (30 C)

40

3.4

-1

(10 / T) K

20

Fig. 7. ‘Log μ − 1/T ’ () and ‘log n − 1/T ’ (•) plot for the quaternary electrolyte OCC: 0.7[0.75AgI: 0.25AgCl]: 0.3KI.

0 0

2

4

6

8

10

12

Time (h)

μ(T ) = 5.75 × 101 exp(−0.173/kT ) (cm2 V−1 s−1 ) (2) n(T ) = 1.33 × 1017 exp(+0.043/kT )

(cm−3 )

(3)

where 0.173 and 0.043 in eV are the energies, designated as energies of migration (Em ) and formation (Ef ) respectively, involved in the two separate thermally activated processes. The (−) ve or (+) ve signs appearing in the argument of the exponential terms indicate the increase or decrease respectively of the factor on the left hand side of the equation with increasing temperature. The ionic transference number (tion ), an another basic parameter, gives a quantitative extent of ionic contribution to the total conductivity. For a pure ionic/superionic system with ions as the sole current carriers: tion = σion /σT = Iion /IT = 1. tion measurements were carried out at various temperatures on the quaternary electrolyte OCC sample using d.c. polarization method, as mentioned in Section 2. Figure 8 shows the Wagner’s ‘current vs time’ plots for the quaternary electrolyte OCC: 0.7[0.75AgI: 0.25AgCl]: 0.3KI recorded at various temperatures. It can be clearly noticed that the initial peak current (IT ) in all the plots finally approached zero with the lapse of time. This is indicative of the fact that the quaternary electrolyte OCC is a pure ion conducting system in whole range of temperature (i.e. 27–110 ◦ C), with Ag+ ions as the sole carriers and tion 1. It was observed that the time duration for IT → 0 gets longer and longer as the sample gets hotter and hotter. This may due to the reason that at high temperatures, the mobile ions would be more thermally agitated

Fig. 8. ‘Current vs. time’ plot for the quaternary electrolyte OCC: 0.7[0.75 AgI: 0.25 AgCl]: 0.3KI at different temperatures. 4

3.5 -1

Log vd (cm.s )

the kinetic energy of the mobile ions, while the decrease in the number of mobile ion concentration with increasing temperature may be due to the reason that some of the mobile ions get them associated with the lattice and become immobile. Similar kind of temperature dependent variations in μ and n has recently been observed by us for the quaternary solid electrolyte: 0.7[0.75AgI: 0.25AgCl]: 0.3RbI (Agrawal et al. 2007, Ref. [13]). Arrhenius equations governing ‘ log μ − 1/T ’ and ‘ log n − 1/T ’ plots of Figure 7 can be expressed as:

3

2.5

2 2.4

2.6

2.8

3 3

(10 /T) K

3.2

3.4

-1

Fig. 9. ‘Log vd −1/T ’ plot for the quaternary electrolyte OCC: 0.7[0.75AgI: 0.25AgCl]: 0.3KI.

and hence, would be requiring longer time to get polarized with a fixed value of applied d.c. potential. Using the peak current (IT ) and mobile ion concentration (n) data, obtained from Figures 8 and 7 respectively, we also evaluated the ionic drift velocity (vd ) at different temperatures with the help of the well-known equation: vd = IT /A n q, where ‘A’ is the cross-sectional area of the sample pellet, ‘q’ is the charge on the mobile ion. Figure 9 shows ‘log vd − 1/T ’ plot for the quaternary electrolyte OCC: 0.7[0.75AgI: 0.25AgCl]: 0.3KI, which can be represented by following Arrhenius equation: vd = 6.81 × 105 exp(−0.19/kT )

(cm s−1 )

(4)

where 0.19 eV is the energy (Ed ) involved in this thermally activated process. At a fixed value of external d.c. electric field across the sample, vd is directly proportional to μ. Hence, both the parameters would vary analogously with the temperature and the energies involved in the two thermally activated processes would be identical. On comparing the Arrhenius plots: ‘ log μ−1/T ’ and ‘ log vd −1/T ’


215

Table 3. Some important cell parameters, calculated in the plateau region of the discharge profiles of the solid state battery: Ag -anode/0.7[0.75AgI: 0.25AgCl]: 0.3KI –solid electrolyte/(C+I2 ) –cathode. Load (KΩ) 100 50

Working Voltage (V) 0.636 0.556

Current Density (μA cm−2 ) 0.594 0.519

Discharge Capacity (μA h) 66.78 41.7

Energy Density (m Wh kg−1 )

0.108 0.090

11.34 6.81

0.8

(Figs. 7 and 9), it can be clearly noted that two variations are almost identical and the two energy values are fairly close to each other.

RT = 27 oC

0.6

Solid state batteries were fabricated using the newly synthesized quaternary OCC: 0.7[0.75 AgI: 0.25 AgCl]: 0.3KI as electrolyte, sandwiched between Ag-metal as anode and (C + I2 ) as cathode. An open circuit voltage (OCV) ∼ 0.685 V was obtained which is very close to the theoretical OCV ∼ 0.687 V [17]. The cell potential discharge performances were studied at room temperature under different load conditions viz. 100, 50 and 20 kΩ. Figure 10 shows the discharge profiles for these batteries. Similar plot for the battery fabricated using ternary superionic solid KAg4 I5 synthesized by us and discharge through 100 kΩ load is also included in Figure 10 for direct visualization of the discharge performances. On comparing the discharge profile under 100 kΩ for the two batteries based on the quaternary OCC and ternary KAg4 I5 , one can clearly note that the latter battery discharged relatively quicker than the former battery. The cell potentials of the batteries, based on the newly synthesized OCC, remained practically constant at 0.636 V for more than 100 h with 100 kΩ and at 0.560 V for more than 70 h with 50 kΩ, except for the small initial potential drop. However, the cell potential decreased relatively rapidly with 20 kΩ load. Some relevant cell parameters, calculated in the plateau regions of 50 and 100 kΩ discharge profiles of the batteries based on the newly synthesized OCC, are listed in Table 3. The discharge performance studies on these batteries clearly indicated that they can be used fairly satisfactorily under the low current drain states. Employing this electrochemical cell potential measurement, one can also determine the ionic transference number (tion ) alternatively using the following equation: (5)

where E and E are the measured and theoretical values of OCV respectively. On substituting E’ and E in equation (5), we obtained tion ∼ 0.997 V, which is extremely close to the unity and in excellent agreement with the value obtained earlier through Wagner’s method.

4 Conclusion A new fast Ag+ ion conducting quaternary solid electrolyte system: 0.7[0.75 AgI: 0.25 AgCl]: 0.3KI, has been

Cell Potential (V)

3.4 Solid state battery applications

tion = E /E

Power Density (m W kg−1 )

0.4

50 kΩ 100 kΩ 20 kΩ

0.2

100 kΩ

0 0

20

40

60

80

100

120

140

Time (h)

Fig. 10. Cell potential discharge profiles of the solid state battery: Ag (anode)/0.7[0.75AgI: 0.25AgCl]: 0.3KI (solid electrolyte)/(C+I2 ) (cathode) under different load conditions viz. 100 (o), 50 (), 20 (•) kΩ. Discharge profile under 100 kΩ () for the battery using KAg4 I5 as electrolyte is also included for direct comparison.

investigated which exhibited relatively superior electrical and electrolytic characteristics than those of the wellknown ternary superionic solid: KAg4 I5 . The room temperature conductivity of the newly synthesized quaternary electrolyte system is slightly lower as compared to that of KAg4 I5 . However, the conductivity of the quaternary solid electrolyte remained practically stable for very long time in the prevailing ambient conditions, while that of the ternary superionic solid KAg4 I5 decreased by more than two orders of magnitude in 24 h. Moreover, the quaternary OCC electrolyte system can be prepared much quicker than KAg4 I5 . Hence, on the basis of these investigations it can be concluded that the quaternary OCC: 0.7[0.75 AgI: 0.25 AgCl]: 0.3KI, synthesized using an alternate host salt: ‘a quenched/annealed [0.75AgI: 0.25AgCl] mixed system/solid solution’, is a superior electrolyte system. Material characterization on quaternary OCC using XRD analysis confirmed the formation of a new polycrystalline compound by solid solution reaction. The ion transport behavior in OCC has been explained by characterizing various ionic parameters viz. σ, μ, n, tion , vd , etc.

216


The discharge characteristic studies on the solid state battery based on the newly synthesized OCC exhibited relatively superior performance especially under low current drain states.

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