Development of Solid Nanocomposite Polymer Electrolytes Using Modified Graphene Oxide and Polyhedral Oligomeric Silsesquioxane Polymers for Lithium-Ion Battery Application

Abstract

Lithium-ion batteries (LIBs) are widely used as a reliable and efficient power source in many portable electronic devices, such as mobile phones, laptops, and hybrid and electric vehicles. Traditionally, these LIBs employ liquid organic carbonates as electrolytes, however, these liquid electrolytes are facing some very serious safety and stability challenges, such as occurrence of fires and explosions during operation. Therefore, there is a need for alternative solid electrolytes, which are as efficient and reliable as liquid electrolytes but offer more safety, stability, and that could expand the application horizon of the LIBs. The current research project deals with the development of solid nanocomposite polymer electrolytes (NSPEs) with polymer (PEO) as matrix, LiClO4 as the source of lithium ions, and the preparation and application of various nanofillers: (i) graphene oxide (GO) or (ii) PEG grafted onto GO surface (GO-graft-PEG6k) or (iii) PEG and MA-POSS diblock copolymer grafted onto GO surface {GO-graft-PEG6k-block-P(MA-POSS)} or (iv) MA-POSS molecule grafted onto GO surface {GO-graft-P(MA-POSS)} and (v) diblock (hybrid) copolymer PEG5k-b-P(MA-POSS). Based on the applied nanofillers, the developed NSPEs can be grouped into three series. Here, PEG6k and MA-POSS represent, respectively, poly(ethylene glycol) having molar mass of 6000 g/mol and methacrylisobutyl polyhedral oligomeric silsesquioxane. In the first series, NSPEs of various compositions with PEO as polymer matrix and (i) GO or (ii) GO-graft-PEG6k or (iii) GO-graft-PEG6k-block-P(MA-POSS) as nanofillers have been fabricated, using solvent casting method. GO-graft-PEG6k is achieved via esterification by grafting PEG6k onto GO. GO-graft-PEG6k-block-P(MA-POSS) has been prepared via surface initiated-atom transfer radical polymerization (SI-ATRP). The morphology of the prepared fillers is confirmed by 13C MAS-NMR and FTIR spectroscopy. For NSPEs, FTIR spectroscopy reveals improved salt dissociation and PEO host and filler complexation that may be due to the Lewis acid-base interactions. Electrochemical impedance spectroscopy (EIS) confirms that the fabricated NSPEs have enhanced ion conductivity as compared to the neat PEO-LiClO4. As an example, at 50 oC, the ionic conductivity increased from 2.36 x 10-5 S cm-1 of PEO-LiClO4 to 4.01 x 10-5 S cm-1 with 0.3% GO and 6.31 x 10-5 S cm-1 with 0.3% GO-graft-PEG6k, xviii Abstract respectively, suggesting that brush-like architecture of filler (GO-graft-PEG6k) is more effective in improving the ion conductivity. However, the decrease in conductivity observed when the filler content was further increased could be attributed to filler aggregation. When the PEG and MA-POSS grafted brush like architecture nanofiller {GO-graft-PEG6k-block-P(MA-POSS)} was introduced into the PEO matrix, the conductivity was dramatically increased to 3.0 x 10-4 S cm-1 at 50 oC with 1.0 wt.% filler content. The nanocages of POSS molecule might probably enhance the free volume at the matrix interface that is associated with higher ionic and chain mobility, as a result having higher ionic conductivity as compared to GO and GO-graft-PEG6k. The dielectric studies of the fabricated films further verified the faster ion dynamics in 1.0 wt.% GO-graft-PEG6k-block-P(MA-POSS) NSPE. Thus, POSS and PEG integration onto GO-grafted brush-like architecture provides a new way for tuning the lithium-ion conductivity. In the second series, NSPEs of various compositions with PEO as polymer matrix and P(MA-POSS) grafted onto GO {GO-graft-P(MA-POSS)} as nanofiller have been fabricated. GO is modified by converting carboxylic acid groups to hydroxyls by treating with thionyl chloride and ethylene glycol. The modified GO is then transformed to multifunctional atom transfer radical polymerization (ATRP) macroinitiator that is used to achieve GO-graft-P(MA-POSS) by SI-ATRP of MA POSS. Solid-state NMR and FTIR spectroscopy are used to confirm the structure of the nanofiller. In nanocomposites solid polymer electrolytes, FTIR spectroscopy shows the complexation of lithium ion with the PEO matrix and reveals that the filler assists in LiClO4 dissociation in PEO matrix. The maximum lithium salt dissociation and hence the highest charge carrier concentration is calculated for 0.3 wt.% GO graft-P(MA-POSS) NSPE as revealed by the FTIR data analysis. Further increase in filler content does not increase salt dissociation rather it decreases slightly that is attributed to the nanofiller aggregation and less interaction with the salt. TGA analysis shows that the fabricated PEO-LiClO4/xGO-graft-P(MA-POSS) has higher thermal stability as compared to neat PEO-LiClO4. PEO-LiClO4/xGO-graft-P(MA-POSS) NSPEs exhibit higher ionic conductivity as compared to neat PEO-LiCLO4. The maximum ionic conductivity of 5.11 x 10-5 S cm-1 is recorded for NSPE with 0.3 wt.% GO-graft-P(MA-POSS) filler content. This is attributed to the higher charge carrier concentration and brush-like filler architecture which may form lower energy xix Abstract ion conducting channels at the filler host interface. The dielectric properties have been investigated and the data confirms the faster dynamics of ions in 0.3 wt.% GO-graft P(MA-POSS) NSPE. The electric modulus data displays a single conduction relaxation peak for the fabricated NSPEs that shifts to higher frequency (fast relaxation) as the temperature increases. In the third series, blend solid polymer electrolytes with PEO/PEG5k-b-P(MA-POSS) as blend matrix have been fabricated, using solvent casting method. PEG5k-b-P(MA POSS) is achieved by ATRP. 1H NMR and SEC confirm the formation of well-defined PEG5k-b-P(MA-POSS)22 (the subscript 22 is the degree of polymerization). FTIR spectroscopy reveals complexation between the PEO host and Li+ ions due to Lewis’s acid-base interactions. The XRD data shows a lowering of crystallinity of the host upon blending with salt and PEG5k-b-P(MA-POSS). The 10% blend [10 wt.% PEG5k b-P(MA-POSS) + 90 wt.% PEO] SPE displays superior ion conductivity (4.12 x 10-5 Scm-1) in comparison to PEO-LiClO4 (1.90 x 10-5 Scm-1) at 45 oC. This enhancement is ascribed to the additional free volume induced by the POSS cages of the diblock copolymer. However, further increase in the block copolymer content decreases the conductivity. This is assigned to the agglomeration of the POSS cages that block the ion conducting paths. Dielectric properties of the blend SPEs have been investigated and the data are well correlated with ion conducting behavior - showing the fastest ionic dynamics for 10% blend SPE. To get insight into the ion conducting mechanism, the electric modulus formalism is employed. The comparison of the spectrum of the dielectric loss, ɛ'' (with no dielectric relaxation peak) with the corresponding spectrum of the electric modulus, M'' (with single conductivity relaxation peak) suggest a strong coupling between the ion conductivity and the host polymer chain segmental motion., i.e., the ion conductivity in the fabricated blend SPEs takes place via the transfer of ions between the coordinated sites of the polymer chain coupled with the segmental relaxation.