Solid Polymer Electrolytes

Solid polymer electrolytes (SPEs) plays a potential applications in energy storage devices such as rechargeable batteries, supercapacitors, fuel cells, sensors etc., as they found to be an suitable alternate to liquid electrolytes.

1, 2  Poly(ethylene oxide) (PEO) based SPEs excel as an excellent candidate as polymer host because of their well solvating power, good complexation and ion transport mechanism.  However, the low ionic conductivity of these SPEs at ambient temperature limits their practical applications.

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3 This is due to the crystalline nature of PEO (i.e, monomers are closely arranged).  Hence, in order to improve the ionic conductivity, various strategies have been carried out.  The most common approach is the addition of low molecular weight plasticizers likes EC and PC.

3 Though, the addition of these plasticizers increases the ionic conductivity, it suffers from poor mechanical integrity and thus it could not be used for practical applications.  Another way to increase the ionic conductivity is by blending of polymers and it has received greater importance.  This is due to its easy preparation and easy control of their physical properties within the compositional regime.  So far, many polymer blend electrolytes have been prepared based on PEO/PVC2, PEO/PAN4, PEO/P(VdF-HFP)5 and so on.  Among them, PEO/P(VdF-HFP) is found to be more appealing, since this blend not only has good miscibility, but also act as a best separator in secondary battery applications and hence the same is chosen for the present investigation. 

In general, electrolytes with low lattice energy are preferable.  It means that the electrolytes to be used should have a polarizing cation and a large anion with a well delocalized charge distribution.

6  Up-to-date various lithium salts like lithium perchlorate (LiClO4), lithium triflate (LiCF3SO3), lithium bis(trifluoro methane sulfonyl) imide (Li(N(CF3SO2)2)), lithium tetrafluoro borate (LiBF4), lithium hexafluoro phosphate (LiPF6) etc, has been used for the preparation of SPEs.  Of these above mentioned salts, lithium bis(trifluoro methane sulfonyl) imide (Li(N(CF3SO2)2)) (LiTFSI) has attracted particular attention as a guest species in SPEs and is used in a variety of polymer hosts which showed good electrochemical performance.  While compared with other lithium salts listed above, LiTFSI contain large polarizable anions.  Further, it has low lattice energy and has a low tendency to form ion pairs, leading to an enhanced ionic mobility.  Due to these properties, LiTFSI is chosen here as the electrolyte.

Enhancement in the ionic conductivity can also be achieved by the addition of inorganic particles in nano dimension as nanofillers to the polymer electrolytes.  Following are the examples of nano fillers, SiO2, Al2O3, TiO2, CeO2, CdO2, SrTiO3, BaTiO3, etc.,  The addition of such nanofillers has not only improved the ionic conductivity, but also enhanced the optical, mechanical and thermal properties.

8  In general, these systems are referred to as “nanocomposite polymer electrolytes”.  These nanofillers act as a solid plasticizers inhibiting crystallization kinetics and promoting the retention of amorphous phase down to sub-ambient temperatures.

9  Due to these facts, we have added nano titania (TiO2) (size ~ 27.03 nm) as the filler for the present investigation.  The nano TiO2 is an attractive candidate for lithium ion batteries (LIBs) due to its low cost, ready availability, and eco-friendliness.  Further, nano TiO2 added SPEs found to show enhanced Li cyclability and Li storage for LIBs.

10, 11 It is well known that that the ion transport in SPEs is coupled with polymer segmental relaxation processes.  Yet, this mechanism is still to be understood.  In order to understand the above said mechanism, it is necessary to study the dielectric relaxation phenomena.  It is a useful tool to understand the ion transport behavior and for obtaining the information of ionic and molecular interactions.

12  The dielectric parameters associated with relaxation processes are of particular significance in ion-conducting polymers where the dielectric constant plays a role, which shows the ability of a polymer material to dissolve salts.13  Thus the effect of nano TiO2 on the dielectric and relaxation studies have been carried out in PEO/P(VdF-HFP)/Li(N(CF3SO2)2) based polymer blend electrolyte in addition to XRD, AFM, SEM and TG-DTA analyses.

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