Study of Polymer Electrolyte for Zn Rechargeable Nanostructured Galvanic Cells via Combined in Situ SAXS/DSC/WAXD Measurements

A. Turković1, P. Dubček1, B. Etlinger1 and S. Bernstorff2
1Institute "Ruđer Bošković", P.O.Box 180, HR-10002 Zagreb, Croatia
2Sincrotrone ELETTRA, 5 Basovizza, 34012 Trieste, Italy

SAXS experiments are suitable to determine the microstructure of nanocomposite polymer electrolyte. Solid electrolyte poly(ethyleneoxide) (PEO) is one of the most extensively studied systems due to its relatively low melting point and glass transition temperature, Tg, its ability to play host to varied metal salt systems in a range of concentrations and to act as a binder for other phases. For these reasons PEO has been the basis of many investigations in the area based on composites of a polymer and an insulating ceramic. Polymeric complexes of (PEO)n with ZnCl2 have been used, due to their stability and a very high conductivity as compared to other complexes [1]. The mechanical properties of amorphous of PEO-based electrolytes are poor and attempts to improve these have included addition of inert filler. Many kinds of ceramic fillers such as micron and nanosized TiO2 or Al2O3 particles, as for instance these nano-particles with particle sizes of 5.8–13 nm in (PEO)8LiClO4 [2], were used. An increase in conductivity of three orders of magnitude was seen at temperatures up to the melting point of the PEO complex. Above the melting point of the complex, the effect of the filler on conductivity was only moderate. We intend to improve the electrical conductivity of polymer electrolyte (PEO)8ZnCl2 by introducing nano-particles of TiO2 and as a new project nano-particles of ZnO, Al2O3, MgO2 and V2O5. An alternative approach is to bring about interchain linking by exposure of the polymer to high-energy radiation [3]. As polymer is a composite of an amorphous and a crystalline part and conductivity is occurring in the amorphous part, both treatments were directed towards inhibition of the crystalline phase in the polymer matrix [4-6].
The aims of the present investigation were to study SAXS/DSC/WAXD temperature behaviour of (PEO)8ZnCl2 electrolyte, the same prepared as nanocomposite using 10 % of nano-sized Degussa P-25 TiO2 and the other different nano-fillers as ZnO, Al2O3, MgO2 and V2O5 and that of cross linked (PEO)8ZnCl2 obtained by γ-radiation.

Figure 1.  SAXS results for nanocomposite polyelectrolyte (PEO)8ZnCl2  in the temperature range from 20°C to 100°C at rate of ½°C/min, for a) PEO)8ZnCl2 /Al2O3; b) PEO)8ZnCl2 /MgO and c) PEO)8ZnCl2 /V2O5. 
Figure 1 shows the SAXS  measurements on polymer electrolyte (PEO)8ZnCl2 nanocomposite with three nanofillers which were performed simultaneously at the SAXS-beamline of  ELETTRA. The  evolution  of  the  average  radii  of  grain  sizes  obtained  by applying Guinier law is compared to the corresponding DSC and WAXD spectra behaviour. These results are presented in Table 1.

The combined SAXS/WAXD/DSC measurements have shown that the nanostructure of the nanopolymer electrolyte (PEO)8ZnCl2 is changing during the crystalline-amorphous phase transition to a highly conductive superionic phase. The conductivity is higher and the phase transition temperature lower than for the (PEO)8ZnCl2, which are desirable properties for application in batteries. The significant role that the nanodimensions of the electrolyte material play in the Zn2+-ion mobility was discussed. The combined SAXS/WAXD information about the evolution of the average grain sizes during the phase transition gave insight into the nanomorphology, which influences the ionic transport in a nanocomposite polymer electrolyte. Further optimizations of the electrolyte properties are in progress since these nanostructured materials are very attractive for batteries or other types of electronic devices.

TABLE 1. Changes of average grain radius <R>/nm by SAXS and phase transition temperatures T (in °C) in (PEO)8ZnCl2 polyelectrolyte with different nanofillers (x) during heating and cooling as determined by SAXS/WAXS/DSC measurements.    	

(PEO)8ZnCl2 + x 	heating 	
	SAXS 	WAXS 	DSC 	
              x/nm	t (°C) 	<R> (nm) 	t (°C) 	t (°C) 	
Al2O3 /5.1	66.9 	8.6-6.1		 	70.1 	66.8 	
MgO /13.3	66.4 	11.7-9.8 	71.2 	65.3 	
V2O5 /9.1	45.4 	10.1-9.9 	48.0 	64.1 	

(PEO)8ZnCl2 +x 	cooling 	
	SAXS 	WAXS 	DSC 	
                x /nm	t (°C) 	<R> (nm) 	t (°C) 	t (°C) 	
Al2O3/5.1	42.5 	5.7 	39.6 	52.5 	
MgO/13.3 	39.3 	6.3 	39.8 	54.4; 48.9 	
V2O5 /9.1	40.6 	6.2 	40.4 	49.9; 45.3; 44.1 	

The combined WAXD, SAXS and DCS results are summarized in the Table 1.

References:
[1] T.M.A. Abrantes, LJ.Alcacer and C.A.C. Sequeira, Solid State Ionics 18/19, 315 (1986).
[2] F. Croce, G.B. Appetecchi, L. Persi and B. Scrosati, Nature 394, 456-458 (1998). 
[3] I. Pucić, A. Turković, Solid State Ionics 176, 1797-1800 (2005).
[4] A. Turković,  P. Dubček, K. Juraić, A. Drašner and S. Bernstorff,  Materials,  3/11, 4979 (2010).
[5] A. Turković,  P. Dubček, K. Juraić, M. Rakić and S. Bernstorff, Journal of The Organic and Inorganic Polymers, 21, 1-5 (2011) DOI: 10.1007/s10904-011-9469-7, Online First.
[6] A. Turković, P. Dubček, M. Rakić, M. Lončarić, B.  Etlinger and  S.  Bernstorff, Vacuum, in press 2011.

Fig. 1