Solid State Ionics 18 & 19 (1986) 1063-1067 North-Holland, Amsterdam 1063

NOVEL SOLID STATE POLYMERIC BATTERIES

Andrew PATRICK, Malcolm GLASSE, Roger LATHAM and Roger LINFORD

School of Chemistry, Leicester Polytechnic, P.O. Box 143, Leicester LE1 9BH, UK.

AC conductivity measurements have been performed on a number of polymeric electrolytes containing Mg, Ca, Sr and Zn perchlorates and Mg and Ca thiocyanates. The electrolytes were characterised using DSC. Results are reported of preliminary tests of cells incorporating anodes of the above metals.

1. INTRODUCTION Over the last ten years many polymeric

electrolytes, based on metal salts dissolved in

polyethers particularly Polyethylene Oxide (PEO)

have been widely investigated because of their

potential v iab i l i t y in high performance

batteries. Such electrolytes have mainly been

based on the alkal i metal salt systems, with

particular attention being focused on lithium.

Comparatively l i t t l e work has been reported on

other metals, and the alkaline earth metals have

been particularly neglected. Some early

studies 1'2 suggested that a number of inorganic

salts of non alkal i metals may be suitable for

use in polymeric electrolytes, and more recently

complexes of calcium and barium thiocyanates

have been r e p o r t e d 3.

Of the metals not widely studied, magnesium

is one of particular interest. Its diagonal

relationship in the periodic table with lithium,

is shown for example by the simi lar i ty in ionic

radii (table 1).

Table 1.

Ion Ionic radii/pm

Li + 68

Na + 98

K + 133

Rb + 14~

Cs + 167 +

NH 4 148

Ionic radii

Ion Ionic radii/pm

Mg 2+ 65

Ca 2+ 94

Sr 2+ 110

Ba 2+ 134

Zn 2+ 74

0 167-2738/86/$ 03.50 © Elsevier Science Publishers B.V. (North-HoUand Physics Publishing Division)

The analogous PEO/magnesium salt complexes are of

considerable interest because of the divalent

charge and the consequent increase in anion to

cation ratio of 2:1.

Results previously reported from Leicester

Polytechnic 4-7 have shown the potential v iab i l i t y

of magnesium based cells using compacted powder

electrolytes and iodine based cathodes. The

investigations reported here extend the use of

magnesium anodes to cells incorporating polymeric

electrolytes and various cathodes R'g Inter-

calation compounds such as TiS 2 and v6013 are of particular interest, and have already been shown I0 to intercalate ions such as Ca 2+.

2. EXPERIMENTAL

PEO, relative molar mass 4 x 106 , supplied by

BDH (Polyox wsr 30) was used throughout the

experiments. The inorganic salts were supplied

by BDH [Mg(SCN)2.4H20; Mg(ClO4)2.6H20]; Alfa

[Ca,Sr,Zn (CI04)2.6H20] ; Fluka [Ca(SCN)2.3H20]. Both the salt and polymer were dissolved in a

suitable solvent (usually methanol) at about 45-

50°C in various mass ratio combinations, to give

a 2-3% solution with a PEO :Salt ratio usually in n

the range n = 4 to 20. The solution was cast on

polyethylene, teflon or silicone paper. The

materials were then allowed to dry in a vacuum

desiccator at room temperature, rather than at

elevated temperatures since these could induce

1064 A. Patrick et al. / Novel solid state polymeric batteries

crystal l ine growth. 3

In common with Fontanella et al. , no special

precautions were taken to dry the polymer, salts

or solvents before use.

2.1 DSC

Samples were investigated using a Perkin Elmer

DSC4 microcomputer controlled instrument over the

temperature range -100 to +200°C, at 20°C/min.

2.2 AC Conductivity Measurements

AC measurements were made over the range 65.5

kHz to 1Hz using a combination of a Solartron

1250 Frequency Response Analyser and a 1186

Electrochemical Interface controlled by a BBC

model B microcomputer. Stainless steel blocking

electrodes, 13 mm in diameter, were used, the

same size as the anode pellets employed in the

cell tests.

From the complex impedance plots and related

admittance data, the bulk resistance and thus the

conductance was obtained for samples of di f ferent

stoichiometry over the temperature range 20 to

140°C under vacuum.

2.3 Cell Tests

The cells were tested under constant load in

an a i r thermostat at 30°C. Voltages were

measured using a Keithley Electrometer with an

input impedance of 1014 ohms.

3. RESULTS AND DISCUSSION

3.1 DSC

I t was anticipated that there might be high

melting endotherms associated with the alkaline-

earth complexes since they have been reported 11,

with ethylene oxide (repeat units 4 to 8).

Although Fontanella et al.3 reported no high

melting points in an examination of calcium and

barium complexes, they did find signi f icant ly

raised glass transit ion temperatures.

The results of the DSC studies of the PEO/

alkaline earth metal complexes are summarised in

Table 2.

Table 2. DSC data

n Tg/°C Tm Tm Tm onset max. offset

PEOn:Mg(CI04) 2 20.0 -2 36 73 81 15.1 -4 33 68 74 12.0 0 27 66 77

9.0 0 31 58 66 6.0 i 49 55 59

PEOn:Mg(SCN) 2 18.0 I I 31 64 70 15.0 13 46 65 69 12.0 4 36 65 76 9.0 12 - 6.0 14 -

PEO :Ca(Cl04) 2 n 18.0 7 40 67 72 15.0 0 35 56 63 12.0 3 44 48 54

9.0 23 6.0 i i

PEO :Ca(SCN) 2 n 18.1 6 28 68 77 15.0 13 19 69 73 12.0 17 36 69 78

9.2 6 43 59 67 6.0 26

PEO :Zn(Cl04) 2 n 18.0 -6 33 63 68 15.0 -3 32 63 85 12.0 -3 3F~ 65 73 9.0 2 - - 6.0 4 - -

PEOn:Sr(CI04) 2 18.0 - 47 65 74 15.0 - 49 65 73 12.0 - 50 67 75

The glass transit ion temperature is taken as the

central point on the step, and the melting

temperature (Tm) is given in the form of onset,

maximum and offset of the peak.

In every case the Tg is elevated compared with

the value of about -60°C for amorphous PEO. The

melting peaks are in a temperature range

corresponding approximately to that of pure PEO.

There are no higher melting endotherms that could

be associated with a crystal l ine complex of PEO

and the alkaline earth metals. This evidence,

combined with the results of i n i t i a l variable

A. Patrick et al. / Novel solid state polymeric batteries 1065

temperature polarising microscopy studies

indicate that these electrolytes are very

amorphous. I t is now accepted that the

amorphous regions are responsible for the ionic

conductivity of polymer/salt complex films.

3.2 AC Conductivity Studies

A plot of log (conductivity) against

reciprocal temperature for a Sr complex is shown

in figure la and for the highest conductivity Mg

complex in figure lb. From similar plots for

the other salts, data were interpolated at 10°C

intervals and are presented as isotherms of log

(conductivity) v. composition (figure 2a-e).

The data were always taken from the heating

cycle since this reflects the conductivities

that have been attained after standing for

prolonged periods.

3.3 Cell Tests

The results reported here are of i n i t i a l cell

studies. Open c i rcu i t voltages (ocv) are

given in tables 3 and 4.

Table 3. Recorded ocv with Mg anode and Mg polymeric electrolyte

Cathode ocv/volts

TiS 2 1.7

V6013 2.0 MnO 2 2.0

NiO 2 1.5

CoO 2 1.65

MoO 2 1.75

MoO 3 1.75

V205 1.45 WO 3 1.8

Table 4. Recorded ocv with other anodes

ocv/volts

Cathode Ca Zn Al

TiS 2 2.28 0.87 0.87

V6013 2.75 1.35 1.25 MnO 2 2.5 1.23 1.25

-6

--L

T/°C 140 120 100 80 60 I I I I 1

40 20 I J

o = hes. t i n 9 + = c o o l i n9

o% q-o

4-0 + o

'.¢: e-

C3 ,4- _ J -p

+ %

.4-

i

2 ' ' 2 ' ' '. 3 ' L • 4 , 5 2 8 . 0

1 0 0 0 / T (K "1)

++ 4- ° %

\ 4- -h-

o ++

3 t 2 ' 314

T / ° C i 4 0 128 180 80 6 0

I I I I I

b C

- 6

- 7

° ° ' - ~ o -4- ° ~

+ O

O +

• 4- 0 0 0 0

q- O 4-

E +

\ + CO + ~ +

b =

0 J

i

214 2 :6 ' 2 . 8 . . . . 3 . 0 1 0 0 0 / T [ K "1]

FIGURE 1 Log (conductivity) against reciprocal temperature for (a) strontium polymer electrolyte, PEOIp:Sr(CIOa) p (b) magnesium polymer electrolyte, PEOI2:Mg(C104) 2

40 20

o = h e ~ . t i n 9 + = C o o l in9

o o

o

o

4- +

+

+ o

+

+ o

\ .4-

+

4.

3 1 I , 2 3" ,4

1066 A. Patrick et al. / Novel solid state polymeric batteries

"4

a) P E O n : M g ( C I 0 4 ) 2

1 2 0 ~

I 1 I I ! 6 9 12 15 18

"4

E'5

~-7 3

"9

d) P E O n : M g ( S C N ) 2

12o

J J

J I

201 I ¢ ~ ~ 1 I 6 9 12 15 18

"4

E ' 5 K=

~,'e

n " 7 3

w - 8

"9

b) P E O n : C a ( C l 0 4 ) 2

,2o

1 I I I I 6 9 12 15 18

"4

E ' 5

e - q " 6 9

"9

e) PEO n:Ca(SCN) 2

120

I I I I 1 6 9 12 15 18

"4

0

em-7

"8

"9

C) PEOn: Zn(Cl04) 2 1 2 0 ~ FIGURE 2

Isotherms at lOOC intervals of log (conductivity)as a function of composition for the temperature range 20-120°C. a) Mg(CI04)2; b) Ca(CI04)2; c) Zn(Cl04)2; d) Mg(SCN)2; e) Ca(SCN)2.

I I I I I n 6 9 12 15 18

A. Patrick et al. / Novel solid state polymeric batteries 1067

Discharge tests were then carried out on a

number of these cells. For example a PE015:

Mg(SCN) 2 electrolyte in contact with a 13 mm

diameter polished Mg pellet and a TiS 2 composite

cathode, passed a current of about 1 ~A for over

3000 hours with about 45% cathode ut i l izat ion.

Higher current densities could be sustained for

shorter periods of time. Various unencapsulated

cell systems were used to power a LCD digital

clock for several hundred hours, at ambient

temperatures as low as 15°C.

The best results obtained to date have been

from cells u t i l i s ing Mg anodes2~nd TiS 2 or V6013 cathodes. I t is found that Mg ions wi l l

conduct through other alkaline earth PEO complex

electrolytes and also through the alkal i metal

complexes. Indeed metals such as zinc and

aluminium wi l l conduct through the PEO/magnesium

electrolyte, although voltages and cell

efficiencies are lower.

4. CONCLUSIONS

The PEO/alkaline earth complexes are more

amorphous than their equivalent PEO/alkali metal

complexes. There is no evidence for the

presence of high melting complexes. The glass

transition temperatures of the divalent

electrolytes are much higher than the value for

pure PEO.

At and above room temperature these polymeric

electrolytes have greater conductivities than

pure PEO. The complex PEO :Mg(ClO.)^ has a conductivity of 10 -5- 10 -6 1~ cm_ 1 a~ ~O°C,

which is comparable with that for Li polymeric

electrolytes at similar temperatures. I t

appears that electrolytes of perchlorates are

generally better conductors than thiocyanates,

as with the alkal i metals. In al l cases for

divalent perchlorate electrolytes, the 12:1

ratio is the highest conductor at room

temperture, but this does change as the

temperature is raised.

At 20°C with the 12:1 ratios, the order of

decreasing conductivity is Mg 2+ Ca 2+ 2+ > Zn 2+ > > Sr ,

which corresponds to increase in ionic radius.

Magnesium polymeric electrolytes offer an

interesting alternative to lithium systems for

room temperature solid state battery systems.

ACKNOWLEDGEMENT

Duracell Batteries (UK) are thanked for a

Research Studentship to AJP.

REFERENCES

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2. R.E. Wetton, J.Polym.Sci. PL 14 (1976) 577.

3. J.J. Fontanella, M.C. Wintersgill and J.P. Calarne. J.Polym.Sci.PP 23 (1985) 113.

4. S. Hackwood and R.G. Linford. Chem.lnd. (1980) 323.

5. C. Johnson, R.J. Latham and R.G. Linford. Solid State Ionics 7 (1982) 331.

6. C. Johnson, Ph.D. Thesis, (Leicester Polytechnic 1984).

7. UK Patent Application no's 8134427 and 8134428.

8. UK Patent Application no. 8508841.

9. US Patent Application no. 718,141.

10. A. LeBlanc-Soreau and J. Rouxel, C.R.Acad.Sci. Paris (Series C) 279(8) (1974) 303.

11. S. Yanagida, K. Takahashi and M. Okahama, Bull. Chem.Soc. Japan, 51(11) (1978) 3111.