1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 Poly(Ionic Liquid)-Based Composite Gel Electrolyte for Lithium Batteries Meer Safa, [a] Ebenezer Adelowo, [a] Amir Chamaani, [b] Neha Chawla, [c] Amin Rabiei Baboukani, [a] Marcus Herndon, [a] Chunlei Wang, [a] and Bilal El-Zahab* [a] Composite gel polymer electrolyte (cGPE) containing a poly (ionic liquid) (PIL) polymer, imidazolium cation-based ionic liquid as a solvent, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) as salt, and glass fillers with various concentrations have been developed and tested in lithium batteries. cGPEs with 1 wt% glass filler shows the highest ionic conductivity and lithium-ion transference number with 25% and 18.18% im- provement compared to gel polymer electrolyte (GPE), respec- tively. Raman results show that the improvement is due to the improved ion-pair dissociation of LiTFSI, which causes improve- ment of Li + mobility. Cyclic charge-discharge studies using binder-free LiFePO 4 /C cathode and lithium anode for 100 cycles at various C-rates and at a fixed rate of C/2 for 300 cycles show superior performances compared to other cGPEs and GPE. Electrochemical impedance spectroscopy and scanning electron microscopy confirm uniform deposition of reaction products on the cathode surface, which improves the charge-transfer reactions and hence improves cyclic performances for cGPE-1 cells with increasing cycles. 1. Introduction Rechargeable lithium metal batteries (LMBs) have been studied widely for the past decades due to its high demand in consumer electronics, stationary energy grids and electric vehicles. [1–3] The major advantages of using LIBs is its high energy-to-weight ratio (180 Whkg 1 ), power-to-weight ratios (1500Wkg 1 ) and low self-discharge rate. [4,5] However, safety remains one of the major concerns for LMBs due to its wide use of liquid organic electrolyte which has poor chemical stability, is highly flammable and has leakage concern. [6,7] A number of research has been going on to replace the electrolyte with a non-flammable, both chemically and thermally stable electrolyte. [8] Polymer electrolyte (PE) as electrolyte shows a safer route to LIB technology as there is no issue of leakage, flammability, chemical instability problems as associated with liquid electrolyte. [9–11] PEs for LIBs mainly consist of a polymer dissolved in a high concentration of lithium salts. Although PEs have advantages over liquid electrolytes, poor room temper- ature ionic conductivity (~10 8 10 5 Scm 1 ) due to ion diffusion restrictions causing a major problem for them to be applied in LIBs. [2] To overcome the issue, room temperature ionic liquid (RTIL) have emerged as a solvent for an electrolyte which is nonflammable and shows excellent chemical and thermal stabilities as well as good ionic conductivity at room temperature. [12,13] PE gelled with a solvent is known as a gel polymer electrolyte (GPE). Watanabe and Noda et al. for the first time reported RTIL based GPE using vinyl monomers as a polymer with imidazolium and pyridinium based IL solvent. [14] Since then, numerous researches have been reported using IL as a solvent for GPE and polymers based on PEO, [15] PAN/ PMMA, [16] PVDF-HFP, [17] PVA [18] for the use in LIB application. Another approach to making GPE is the application of polymeric ionic liquid (PIL) which is the polymers of IL monomers used as polymer matrices for GPE. [19] Major advan- tages of using PILs are their excellent chemical affinity with IL which results in improved compatibility, minimal phase separa- tion, and leakage. PILs also possess better electrochemical stability and room temperature ionic conductivity resulting in high cyclic stability when used as polymer matrices for GPE in energy storage devices. [12,20] Since the introduction of the concept of PIL by Ohno et al., [21] a number of researches have been reported using PIL as polymer matrices for the electrolyte in energy storage devices. [19,22] Appetecchi et al. reported pyrrolidinium cationic-based GPE using polydiallyldimethyl ammonium bis(trifluoro)methanesulfonimide (PDADMATFSI) PIL with pyrrolidinium bis(trifluoro)methanesulfonylimide (PYR 14 TFSI) IL with a reported capacity of 140 mAhg 1 at 40 °C for 70 cycles at C/10 rate. [23] In our earlier study, we investigated PIL (PDADMATFSI) combined with imidazolium-based IL electro- lyte using Li/LiFePO 4 cell and reported 166 mAhg 1 discharge capacity after 40 cycles at C/10 rate at room temperature. [12] Li et al. investigated GPE using guanidinium-based PILs and IL in LIBs with a reporting discharge capacities of 140 mAhg 1 at C/ 10 rate after 100 cycles at 80 °C. [24] Kun et al. reported imidazolium-based PIL using Li/LiFePO 4 cells with a discharge capacity of 157.5 mAhg 1 after 80 cycles at C/10 rate at 60 °C. [25] Ratios of the solvent and polymer play an important role to have a GPE which has higher ionic conductivity while maintain- ing dimensional stability. The ionic conductivity problem [a] Dr. M. Safa, E. Adelowo, A. R. Baboukani, M. Herndon, Prof. C. Wang, Prof. B. El-Zahab Mechanical & Materials Engineering Department Florida International University, Miami, FL 33174, USA E-mail: belzahab@fiu.edu [b] Dr. A. Chamaani University of Virginia, Charlottesville, VA 22904, USA [c] Dr. N. Chawla Carnegie Mellon University, Pittsburgh, PA 15213, USA Supporting information for this article is available on the WWW under https://doi.org/10.1002/celc.201900504 Articles DOI: 10.1002/celc.201900504 3319 ChemElectroChem 2019, 6,3319–3326 © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim