Fast Cu+ ion conducting phosphate iodide-glasses

Solid State lonics 13 (1984) 105-109 North-Holland, Amsterdam

FAST Cu + ION CONDUCTING PHOSPHATE IODIDE-GLASSES Changler LIU and C.A. ANGELL Department o f Chemistry, Purdue University, West Lafayette, Indiana 47907, USA Received 18 October 1983

The glass-forming region in the ternary system CuI-Cu20-P20 s , from which Cu2+ ion formation has been excluded, is described and compared with that in the corresponding Ag+-based system. The glasses are characterized by Tg values some 50°C higher than in the silver systems. Frequency independent (dc) conductivities along the join Cul + CuPO3 have been determined over a wide range, and found to be characterized by both higher activation energies and pre-exponential constants than the silver glasses. The combination leads to higher conductivities in the Cu+-based glasses in the composition range 20-30 mole% CuI at room temperature and above.

1. Introduction

In the search for new materials to serve as solid electrolytes in solid state battery and other electrochemical technology, much attention has recently been focused on glassy solids [ 1 ]. Compositions have been discovered in which the conductivities rival those of the best crystalline state conductors. Highest conductivities have been recorded in systems containing large mole fractions o f silver iodide combined with other silver compounds mostly o f the oxyanion type. While systems in which silver iodide is mixed with simple tetrahedral oxyanion salts, such as Ag3AsO4 or Ag2MoO4, yield the highest silver iodide content glasses (up to 80 mole%) (and concomitantly the highest conductivities [2,3]) somewhat more interesting systems are provided by ternary systems containing one o f the common Lewis acids B203, SIO2, or P205 . In the latter cases, wide glass-forming ranges have been described by several authors [4--8] (in greatest detail by Minami [7,8]). Within these, enormous and systematic variations of properties with composition may be observed. Clearly, in view of the high cost o f silver, it would be o f interest if Cu + ion equivalents o f these Ag+ ionbased conductors could be found. A problem is posed by the easy oxidation o f the Cu + ion to the divalent state, indeed in some cases a spontaneous disproportionation to Cu 0 + Cu 2+ can occur. However, there

is much evidence that high P205 content in a glass has a strong tendency to maintain reduced states of cations [9] suggesting phosphate glasses as a possible base for Cu + containing systems. Indeed, Bartholomew et al. [10] have briefly described in a patent disclosure a considerable number o f P205-based ternary glasses containing Cu 2 0 and various copper (I) halides :k. In the patent description, Bartholomew et al. gave approximate values for the conductivities o f their glasses at room temperature. These were generally some two orders of magnitude below the conductivities o f the known Ag+ analogs. However, no information was given for temperatures other than room temperatures, and the glass-forming regions themselves were only crudely described. In the interests of providing a more adequate scientific basis for the understanding o f Cu + ion-conducting glasses we have now carried out a more detailed study of both the glass-forming region and the electrical conductivities in the system C u 2 0 - P 2 05 CuI, and report herein the results. It will be seen that there are conditions under which Cu ÷ conductors can outperform their Ag+-based equivalents.

We are grateful to Dr. J.L. Souquet for drawing our attention to this work after we had independently prepared glasses in the present system.

106

C. Liu , C.A. Angell/Cu ÷ ion conducting phosphate iodide-glasses

2. Experimental Glasses were prepared by heating dry box-weighed mixtures of CuI (99.9%, Alfa Chemical Co.) Cu20 (99.9% from Cu metal oxidation) and P205 (MCB Co. 98%) in nitrogen-filled glass ampoules with capillary vent arms, at about 650°C for 30 min. The particular order of addition of components to the ampoule (P205 first, then ground mixture of Cu20 + CuI) and fusion (P205 layer first incorporating the oxideiodide layer slowly) was found important to avoid a pre-fusion disproportionation of the CuI. Cu + is also very sensitive to oxidation, and protection of the melt from air during melting and prior to the final quench was essential to avoid discoloration and devitrification. A small weight loss (<0.3 wt.%) occurred during melting, possibly moisture from the P205, as the Cu20 was dissolved. Successful preparations produced by quenching the melts between brass plates followed by annealing at 80°C for 30 min to relieve internal stresses, were colored brown to darkish red depending on CuI and Cu20 contents. Once formed, they showed no tendency to degrade or discolor in air. Glass transition and devitrification temperatures were determined using a Perkin-Elmer differential scanning calorimeter, model DSC-2. Electrical conductivities were determined by evaporating Pt blocking electrodes onto each side of the thin disc samples obtained from the quench and measuring the real and imaginary parts of the complex impedance as a function of frequency using a General Radio 1689 precision RLC Digibridge in frequency range 12-100000 Hz. In the electrode sputtering procedure it was important to configure the sample in such a way that the dc voltage was the same on both sides of the sample so that no Cu + ion current could be generated and/or disproportionation, 2Cu + = Cu 0 + Cu 2÷, induced.

3. Results The glass-forming region in this system is shown in fig. 1. We note that the shape of the glass-forming region has features in common with the corresponding Ag+ system studied by Minami [8], but the glass.forming range is less extensive in the MI coordinate. We

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Fig. 1. Glass-formingregion in the ternary system CuI'Cu20P2Os. Upper dashed region indicates the high-iodide glassforming region in the case of the corresponding Ag+-based system (from ref. [8]). Lower quasi-rectangular dashed region is the glass-formingdomain reported for the present system in ref. [101.

note that the region extending to the binary join AgI + P205 described in the patent claim of Bartholomew et al. [10] is not confirmed. This system is remarkable for the existence of glasses oxide contents beyond the stoichiometry of the PO43- ion. The chemical state and potential of the excess oxide must be a matter of interest, and will stimulate thermodynamic studies in the future. The glass transition temperatures which are collected in table 1, show a striking independence of composition, and there is a suggestion in some cases of a second glass transition at about 410-420 K, i.e. the constancy of the first Tg might be a consequence of microscopic liquid-liquid phase separation. Crystallization temperatures however, are very variable (table 1). Stability indices (T c - Tg)/Tg, show the stablest glasses are located along the orthophosphate edge. The Tg values are some 30 ° higher than those of their Ag÷ equivalents. This means that Cu ÷ based glasses will relax towards equilibrium states at room temperatures at a much slower rate than do their Ag+ equivalents. Complex impedance plots from which the electrical conductivities for the present system were determined, are shown in fig. 2. o 0 was shown to be independent of the ac measuring voltage in the range 0.05-1.25 V.

C. Liu, CA. Angell/Cu + ion conducting phosphate iodide.glasses

107

Table 1 Glass transition temperatures and devitrification temperatures in the system CuI + C u 2 0 - P 2 0 s . Composition

Tg

Tc

(Tc -

0.5 Cul-0.5(Cu20.P20 s)

385.5 384.5 384.0 384.0 382.0 386.0 381.5 381.0 386.0 386.0 382 388.5 383

444.5 437.0 452.5 455.0 470.0 479.0 472.5 476.0 537.0 501.0 470.0 510.0 -

0.15 0.18 0.18 0.23 0.24 0.25 0.39 0.30 0.23 0.31 -

0.45 0.40 0.40 0.35

Cul-0.55(CuzO.P2Os) CuI.0.6(Cu20.P2Os) CuI.0.4Cu20.0.2P20 s Cul.0.65(Cu20-P2Os)

0.3 0.3 0.3 0.25 0.2 0.2

CuI.O.7(Cu20.P20 s) CuI-O.5Cu20.O.2P20 s CuI.O.4Cu20.O.3P20 s CuI.O.8(Cu20-P2Os) CuI.O.8(Cu20-P2Os) CuI.O.6Cu20.O.2P20s

o 0 values from the c o m p l e x i m p e d a n c e diagrams o f fig. 2 are displayed in Arrhenius form in fig. 3. F o r selected c o m p o s i t i o n s these are contrasted w i t h earlier data for the corresponding Ag + glasses. It is m o s t no-

Tg)/Tg

ticeable that, while the silver glasses are better conductors at room temperature, this situation is reversed at higher temperatures. At 120°C, where silver-based

glasses of high AgI content would recrystallize, the 0.5 CuI 0.5 ( C u 2 0 + P 2 0 5 ) glass is the best vitreous phosphate fast ion c o n d u c t o r in existence.

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along the join CuI + C u P O 3 at mole fractions of CuI up to 0.5. Corresponding plots covering more restricted temperature regions for the Ag+-based glasses (from ref. [4]) are also shown. Note the superior conductivity of certain of the Cu + glasses (see also fig. 5).

108

C Liu, CA. Angell/Cu + ion conducting phosphate iodide.glasses

4. Discussion

The results presented in the preceding section show that although the highest room-temperature electrical conductivities clearly belong to the silver-based systems, the Cu+-based glasses constitute a most impressive class o f fast ion conductors with exceptional performance at higher temperatures. In the following we will compare their characteristics with those of some previously studied fast ion conducting iodide-phosphate systems. Of particular interest is the pattern of behavior revealed by the extended Arrhenius plot comparisons of the three MPO 3 + MI systems, where M = Li +, Ag+ and Cu ÷, seen in fig. 4. Each system may be well described by a fan of Arrhenius plots o 0 =A o e x p ( - E o / R T ) with a common (within 0.3 log units) origin at 1/T = 0, in which the slope o f the individual plots decreases with increasing MI content. The Cu + and I.i + systems have the same pre-exponent within uncertainty limits, while that of the Ag+ system is distinctly lower. Preliminary measurements [11] on 0.7AgPO 3 .0.3AgI glass over wider and lower temperature ranges than previously studied, confirm the Malugani et al. data [4] thereby eliminating the possibility that the difference is due to effects found near Tg in some higher AgI content cases (where a

decrease in Eo, hence also in A o , is encountered [2,12]). The pre-exponent for Ag+ glasses falls at a value of log A o = 2.0 which is typical of sodium silicate [13] and borate glasses [14]. The ordering of the pre-exponent values is roughly that of the cation radii though there is no agreement on precise radius values for Cu + and Ag+. As far as can be told the order is also that of the respective far IR spectrum peak frequencies [ 11 ], though the pre-exponents certainly vary by a wider margin. The physics underlying these differences is not understood in detail at this time. The variation with iodide content o f the Arrhenius activation energies is shown in fig. 5, along with the net dc conductivity at room temperature. All metaphosphate-iodide systems show the same qualitative behavior, i.e. a linear portion, and a final tendency to flatten out at different minimum values which seem to depend on the solubility of the halide in the metaphosphate matrix. From the insensitivity of Tg to composition and the existence of stable liquid-liquid immiscibility regions in some alkali chloride-borate mixtures beyond 20 mol% [15 ], it seems probable that the plateau in E a is a reflection o f sub-liquidus,

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Fig. 4. Extended Arrhenius plots of the data in the present system, and the corresponding Ag+- and Li+-systems showing common patterns with pre-exponents dependent on the mobile species.

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mole % (CuI, A g ] , L i I )

Fig. 5. Composition dependence of the activation energy in the Ml + MPO3 systems of fig. 4. Insert: composition dependence of the room temperature dc conductivity in the same three systems.

C Liu, C.A. Angell/Cu ÷ ion conducting phosphate iotfide-glasses

l i q u i d - l i q u i d phase separation in each system at variable MI concentrations near the c r y s t a l - l i q u i d solubility limit. The combined effect o f E o and A o on the ambient glass conductivity is shown in the insert to fig. 5 which shows another quirk o f the present system. Where most previous studies o f log o 0 versus composition have shown a simple linear relation [ 1 - 8 ] , the present system shows a strongly curved relationship with tendency to a conductance maximum. A similar effect has been seen in some G e S 2 A g 2 S - A g I glasses [16]. We have no simple explanation for this behavior. The effect o f mixing o f the monovalent cations in these glasses will be interesting to observe, and studies o f these effects with emphases on Li+ - C u + mixtures are in progress.

Acknowledgement This work was supported by the NSF-MRL program under Grant No. 8020249. We are indebted to Dr. E.I. Cooper for valuable discussions o f this work.

109

References [ 1 ] J.L. Tuller, D_p. Button and D.R. Uhlmann, J. NonCryst. Solids 40 (1980) 93; J.L. Souquet, Solid State Ionics 5 (1981) 77; C.A. Angell, Solid State lonics 9/10 (1983) 3. [2] R.J. Grant, M.D. Ingrain, L.D.S. Turner and C.A. Vincent, J. Phys. Chem. 82 (1978) 2838. [3] T. Takahashi, S. Ikeda and O. Yamamoto, J. Electrochem. Soc. 120 (1973) 647; A. Magistris and G. Chiodelli, Electrochem. Acta 26 (1981) 1241. [4] J.P. Malugani, A. Wasniewski, M. Doreau and G. Robert, Mat. Res. Bull. 13 (1978) 427. [5 ] G. Robert, J.P. Malugahi and A. Saida, Solid State Ionics 3/4 (1981) 311. [6] A. Magistris, G. Chiodelli and A. Sch£raldi, Electrochim. Acta 24 (1979) 203. [7] T. Minami, Y. Takauma and M. Tanaka, J. Electrochem. Soc. 124 (1977) 1659. [8] T. Minami, J. Non-Cryst. Solids 56 (1983) 15. [9] J. Wong and C.A. Angell, Glass: structure by spectroscopy (Dekker, New York, 1976) ch. 6. [10] R.F. Bartholomew, W.G. Dorfield, J.A. Murphy, J.E. Pierson, S.D. Stookey and P.A. Tick, U.S. Patent No. 4,226,628 (October 7, 1980). [ 11 ] Changler Liu and C.A. Angell, to be published. [ 12] M.D. Ingrain, C. Vincent and A.R. Wandless, J. NonCryst. Solids 53 (1982) 73. [ 13 ] K. Otto, Phys. Chem. Glasses 7 (196.6) 35. [14] S.W. Martin and C.A. Angell, J. Am. Ceram. Soc., to be published. [ 15 ] H. Tufter, private communication. [ 16] B. Carette, E. Robinel and M. Ribes, Glass Technol. 24 (1983) 157.