Effect of TiO2 nano-filler on ionic conductivity of poly (ethylene oxide) based gel polymer electrolyte for magnesium ion batteries

Abstract

Gel polymer electrolytes (GPEs) have been identified as novel materials for magnesium ion rechargeable batteries. Among different polymers, poly(ethylene oxide) (PEO) based systems are the most studied candidates due to their solvation power, complexion ability, and proper (Dissanayake et al., 2012). However, ionic conductivity of PEO based polymer electrolytes at ambient temperature is not high enough for most practical applications. In order to improve the ionic conductivity, the addition of nano sized oxide fillers into the PEO-salt matrix have been regarded as the most promising method (Agrawal et al., 2013). We here present the synthesis and characterization of PEO based Mg

ion conducting GPE by incorporating titanium dioxide nano filler (TiO 2). Methodology

PEO (Mw~1×10 ), magnesium triflate (MgTf) (purity˃97%), ethylene carbonate (EC) (purity ˃99%), propylene carbonate (PC) (purity ˃99%) were purchased from sigma Aldrich and used as starting materials along with titanium dioxide (TiO2) nano-filler. Prior to use, polymer, salt and TiO2 were vacuum dried using appropriate temperatures. Nano composite polymer electrolytes were prepared by adding different amounts of TiO2 (2,5,7.5,10,15 wt.%) in the PEO:MgTf :EC/PC (1:1) mixture with weights of 0.20 g, 0.12 g, 0.30 g and 0.30 g respectivel y. The mixtures were heated to 80 C and magnetically stirred for 12 hours without heating until a homogenous gel was formed. The cathode film was prepared using vanadium pentoxide (V 2O5), carbon black and polyvinylidene fluoride (PVdF) with the weight percentages of 76%, 14% and 10% respectively with the solvent of 1-methyl -1, 2-dipyrrolidine using doctor blade method.

The prepared electrolytes were characterized in order to obtain their ionic conductivity using complex impedance spectroscopy whereas electrochemical tests were performed using the cells, Mg/electrolyte/V2O5:C. DC polarization tests were done using both blocking (stainless steel) and non-blocking (Mg) electrodes. Microscopic images of the electrolytes were taken using the polarization microscope to investigate the change of crystallinity with the addition of nano-filler.

Figure 01(a) shows the temperature dependence of ionic conductivity for the GPEs, PEO:MgTf:EC/PC (1:1), incorporating different wt. % of nano-sized TiO2 filler. Among studied

systems, highest ionic conductivity is observed for the GPE with 5 wt. % filler content. Figure 01(b) shows the variation of conductivity with different wt. % TiO2 at various temperatures

(conductivity isotherms) for the PEO:MgTf:EC /PC (1:1) GPEs. This plot also shows that the 5 wt.% filler containing electrolyte has the highest ionic conductivity in the studied temperature range. A possible explanation to this effect could be the availability of extra hopping sites for migrating ionic species due to formation of Lewis acid-base type interactions of ionic species with O/OH surface groups on TiO2 nano filler (Pitawala et al., 2008).