Polymer Electrolytes for Mg-ion Batteries
Safety is an important criterion for the fabrication of all types of batteries. Most of the conventional liquid electrolytes are incapable of solving this issue. In this scenario, researchers developed polymer electrolytes that are an excellent substitute for the organic electrolytes in batteries. The major highlights of polymer electrolytes are lightweight, flexible, noncombustible reaction products that avoid internal short circuiting and leakage [27, 28]. Polymer electrolytes, such as polyethylene oxide (PEO) [29], polyacrylonitrile (PAN) [30], polyvinylidene difluoride (PVdF) [31], polyvinyl acetate (PVAc) [32], etc., have been used in experiments in lithium-ion batteries. Solution casting [33], hot-press method [34], phase-inversion technique [35], and electrospinning [36] are some of the techniques used to prepare polymer electrolytes. The well-established polymer electrolytes in lithium-ion batteries led to the extension of the studies in Mg-ion batteries as well.
Solid Polymer Electrolytes for Mg-Ion Batteries
Oligo (ethylene oxide) grafted polymethyl acrylate-(PM A) based solid-polymer electrolyte was the first synthesized polymer electrolyte in Mg-ion batteries [37]. This polymer matrix provides ionic conductivity of 0.4 mS cnr1 even at 60° C, but the discharge capacity was very low. Additives such as plasticizers are helpful for the improvement of ionic conductivity. Later, Yoshimoto et al. [38] introduced a PEO- PMA blended matrix with the addition of plasticizers, providing an insignificant increase in the electrochemical performance. Sheha et al. [39] reported complexation polymeric materials that can also upgrade the electrical and electrochemical performance of the battery. (PVA)07(NaBr)0 3 solid polymer electrolyte doped with sulphuric acid (SA)enhances the chemical and mechanical stability of the polymer matrix owing to the strong intra molecular proton switch mechanism. (PVA)07(NaBr)03:xM (SA) delivers highest ionic conductivity of 6 104 Scnv1 at 2.6M SA addition and then decreases. However specific capacity was less (-90 mAh g-') compared to their theoretical capacity (-300 mAh g-') Biodegradable polymers are environmentally benign materials, and the introduction of PVA to PVP doped with Mg nitrates offers high thermal and mechanical stability even though it contributes less ionic mobility [40]. On increasing doped salts, ionic conductivity increases to an extent and then decreases because of the formation of ion pairs, triplets, etc. (Figure 8.4a). The ionic transference number is 0.98 measured from direct current (DC) polarization technique (Figure 8.4b). Similarly, studies also report blending PVA with PVdF-co-HFP (poly(vinylidene fluoride-co-hexafluoropropene) that has had polymer applied in dye sensitized solar cells for better electrical performance [41].
Gel Polymer Electrolytes for Mg-Ion Batteries
Gel-polymer electrolytes are promising candidates for the next generation of battery electrolytes. The major advantages of gel-polymer electrolytes over solid-polymer electrolytes include their easy ion mobility, flexibility, high ionic conductivity, and safety. Gel-polymer electrolytes, which are created by incorporating Mg salts mixed with a plasticizer, such as alkyl carbonates, were introduced by Yoshimoto et al. [42]. Flexible, free-standing gel polymer electrolytes were prepared by blending PEO with PMA (Figure 8.5b) and doping salt, as magnesium imide delivers ionic conductivity of 2.8 mS cm1. Impedance spectra of (PEO/PMA)/(ECY DMC)/ Mg[(CF3S02)2N]2 (25/5) at 60° C is given in Figure 8.5a. The prototype cell assembled with (PEO/PMA)-(EC-DMC))Mg[(CF,SO^)2N]2 polymer gel, Mg V205/ Mn02
FIGURE 8.4 Properties of polyvinyl alcohol/polyvinyl pyrrolidone (PVA/PVP) (50:50) polymer blend electrolyte: (a) effect of Mg nitrate content (wt%) on the ionic conductivity, and (b) DC polarization versus current plot. Adapted and reproduced with permission from reference [40]. Copyright 2015 Elsevier.
electrodes delivers speciifc capacity of 60 mAh g4 where as with MgV2Os/ V205 exhibits specific capacity of 140 mAh g4 (0.1 mA cm4); however, both the cells shows an open circuit voltage of 2.3 V. The rechargeablility of V205 cathode material is higher than MnO, and hence V205 was considered as suitable candidate for this polymer matrix.
FIGURE 8.5 (a) АС-impedance of PEO/grafted polymethyl acrylate (PEO/g-PMA) polymer blend electrolyte, and (b) optical image depicting the transparency and physical appearance of PEO/PMA blend electrolyte with Mg triflate (Mg[(CF,SO,)2N]2) in EC/DMC at room temperature. Adapted and reproduced with permission from reference [42]. Copyright 2003 Elsevier.
Polymer Composite Electrolytes for Mg-Ion Batteries
Composite gel polymer electrolytes are materials that contain polymers, doping salt, and inorganic fillers for better structural stability, ionic conductivity, mobility, etc. A Lewis acid-base interaction takes place between the Mg2+ ions and hydroxyl groups in metal oxides that increases the ionic mobility. Kim et al. [43] and Yu et al. [44] reported that adding filler to PVdF-co-HFP-based gel polymer electrolytes enhanced the ionic conductivity to 3.2 mS cm1. Gel polymer electrolytes of 15% PVdF-co-HFP,
73% Mg(C104)2-EC/PC and 15 % silica composites provide a high electrochemical stability window of 4.3 V. Silica filler reinforced the physical properties of the matrix. Pandey [45] investigated the same matrix with different silica content and found that ionic conductivity and cyclability was also improved by the addition of active filler MgO instead of passive filler silica. Ionic conductivity was increased to 8 mS cnv1 with good thermal and electrochemical stability. MgO facilitates the further dissociation of aggregates, undissociated particles, etc. [45]. Recently a PVA/ PAN-based matrix was introduced to Mg(C104)2 salt [46]. Strong interaction between the blend and the salt was clearly observed by the X-Ray Diffraction (XRD) studies. Still further studies on composite gel polymer electrolytes with the addition of plasticizers, fillers, etc. have been carried out. The addition of nano size filler increases the amorphous nature of the polymer matrix, thereby increasing the ionic mobility of the material. Biodegradable nano fillers are an important area in the research field that are environmentally friendly. Sreedevi et al. [47] reported the addition of biodegradable nanosize chitin and succinonitrile (SN) to the PEO-Mg perchlorate matrix. The maximum ionic conductivity was obtained for 10 wt% MgC104 chitin nanofiber, delivering in the order of 10 J S cnr1 at 60° C.
Room-Temperature Ionic Liquid-Based Electrolytes for Mg-ion Batteries
Safety is a major threat for the commercialization of Mg-ion batteries. Many measurements were taken in order to create a safer electrolyte. Room temperature ionic liquids (RTILs) are quaternary ammonium salts with an ambient temperature conduction range of 0.1 mS cnv'-18 mS cm1. In batteries, ionic liquid is always associated with lithium salts or some other salts, depending on the type of cations. Thus, there will be two cations and one anion that enhance the conduction [48] and possess electrochemical stability of 4-6 V. An earlier study dissolved (1-ethyl 3-methyl) imidaz- olium bis(trifluromethylsulphonyl)imide (EMITFSI) in polymeric matrix and ionic salts. The system consists of PEO-PMA matrix, ionic liquid, and Mg-conducting triflate. From the thermogravimetric analysis (TGA) analysis, a shift in value suggested ionic liquid acts as a plasticizing agent, and ionic conductivity of 3.5 mS cm-1 was observed. An increase in viscosity with a decrease in conductivity was observed by the addition of Mg salts [49]. The same group investigated the effect of the ionic liquid on polymer electrolytes [50] consisting of ethyl magnesium bromide in THF and ionic liquid. The study obtained a maximum conductivity of 7.4 mS cnv1 and optimized the ratio of Mg salt and ionic liquid of quaternary ammonium salts in the ratio of 3:1 volume. In 2007, a detailed study on the reduction of ionic liquids on Mg salt found that imidazolium salt was unsuitable for the reaction; however, 1-butyl 1-methyl pyrrolidinium systems (BMP) are a suitable candidate for reduction. Out of these systems, 1-butyl 1-methyl pyrrolidinium bis-trifluro methyl sulphonyl imide (BMPTf,N) has a better deposition when Grignard reagents are incorporated [38, 51]. Different ionic liquids were studied, and a single ionic liquid was used for the system. Wang et al. [52] attempted to mix two ionic liquids with conducting salt. Д'-methyl Д'-propyl piperidinium (bistrifluoromethylsulfonyl) imide (PP13-TFSI) and
FIGURE 8.6 FE-SEM images on surface morphology of Mg electrodeposits at different current densities (a) 0.5 raA cm'2 (b) 2.5 mA cm'2, in which no dendrite formation is noted. Adapted and reproduced with permission from reference [55]. Copyright 2015 Elsevier.
l-/bbutyl 3-methyl imidazolium tetrafluoroborate (BMIMBF4) were optimized by a 4:1 ratio. The study shows almost 200 cycles were processed with low overpotential value and concluded that these were compatible with Mg ions. BMIMBF4 was also chosen for mixing with Mg triflate, and the deposition and dissolution were studied on a silver substrate. It has been noted that deposition occurs in the pyramidal form and becomes thin and flat on dissolution, providing an electrochemical window of
4.2 V with Platinum electrode (Vs Pt) [53]. Different ionic liquids show different behavior in salts, and the behavior was later studied by choosing three ionic liquids and three Mg salts. The ionic liquids chosen were: N-methyl-/V-propyl piperidinium (PP13)Tf2N, N,(V-diethyl-(V-methyl(2-ethoxyethyl)-ammonium (DEME+) tetrafluoroborate (BF-1'), and l-n-butyl-3-methylimidazolium (BMIM)-Tf2N. The Mg salts chosen were bis(trifluoromethanesulfonyl)imide (Tf2N ), borohydride (BH4) and tri- fluoromethanesulfonate (TfO ). Mg plating is found to be quite difficult in Tf,N' and BH4- because of the dense charge density of Mg and greater coulombic attraction. Owing to these factors, these batteries always prefer the low charge density of ionic liquid cation [54]. Kitada et al. [55] synthesized halide-free ionic liquid-based electrolytes by choosing a magnesium imide. Out of the different concentrations chosen, an equimolar ratio of Mg/glymes shows good oxidative stability and was found to be safer. Field emission scanning electron microscope images (Figure 8.6) show no dendrite formation.
