# Development on Solid Polymer Electrolytes for Electrochemical Devices
## 3 Basic Solid Polymer Electrolytes
### Strategies to Improve Ionic Conductivity of SPEs
#### 3.2.2 Modification on Polymer Host
Besides polymer modification, the polymer can also undergo copolymerization, grafting, crosslinking and blending with a second polymer in order to lower the crystallization. [PERSON] and co-authors [65] have photo-copolymerized polyglycidyl methacrylate (P(GMA)) with PMMA as the polymer host. SPE with configuration P(GMA-co-MMA)-LiClO\({}_{4}\) displayed the ambient conductivity of 8.70 \(\times\) 10\({}^{-6}\) S cm\({}^{-1}\)[65]. The bonding of one polymer chain with that of another polymer is known as a crosslinking reaction. This process can take place physically and chemically (photo-induced polymerization, curing, etc.). In 2016, [PERSON] et al. [66] prepared SPE containing PEO (M\({}_{w}\) 2000 g mol\({}^{-1}\)) crosslinked with poly(ethyleneimine) (PEI) (M\({}_{w}\) 600 g mol\({}^{-1}\)) and LiTFSI salt and obtained the conductivity of 3.90 \(\times\) 10\({}^{-5}\) S cm\({}^{-1}\) at 40 \({}^{\circ}\)C. The values of \(T_{g}\), percentage of crystallinity, tensile strength and cationic transference number were \(-\)33 \({}^{\circ}\)C, 20%, 0.47 MPa and 0.76 (40 \({}^{\circ}\)C), respectively. Meanwhile, SPE, which comprises 600 M\({}_{w}\) PEI, with the same salt, attained the conductivity of 1.50 \(\times\) 10\({}^{-6}\) S cm\({}^{-1}\) at 40 \({}^{\circ}\)C and \(T_{g}\) of \(-\)65 \({}^{\circ}\)C [66]. In 2016, [PERSON] et al. [67] systematically studied the concentration effect of crosslinking agent (divinylbenzene (DVB)) on SPE based on PEO crosslinked poly(ethylene glycol) diacrylate (PEGDA) with LiTFSI salt that had been prepared via polymerization under ultraviolet (UV) irradiation, using a photo-initiator, -hydroxy-2-methylpropiophenone and acetonitrile solvent. At optimum concentration of 10%mol DVB, conductivity of 1.40 \(\times\) 10\({}^{-4}\) S cm\({}^{-1}\), lithium-ion transference number (\(t_{Li+}\)) of 0.21 and high electrochemical window of 4.30 V were obtained at 70 \({}^{\circ}\)C [67]. At the same temperature and C/10 rate, LiFePO\({}_{4}\)/Li battery employing this electrolyte delivered the first discharge capacity of 138 mAh g\({}^{-1}\) which was sustainable even after the 20 th cycle (138 mAh g\({}^{-1}\)) [67]. The SPE has also been tested in Li-sulfur battery under similar conditions and acquired the capacity of 175 mAh g\({}^{-1}\) after the 50 th cycle, with an initial capacity of 375 mAh g\({}^{-1}\)[67]. After cycling, there was no lithium dendrite formation, as evidenced through SEM micrograph. In addition to inhibit crystallinity percentage, the crosslinked electrolyte is said to have the ability to hamper lithium dendrite growth when being used in Li-ion cells and to improve dimensional stability at elevated temperatures [66; 67; 68; 69]. In a separate report, PEO underwent crosslinking reaction with tetraethylene glycol dimethyl ether (TEGDME) employing 4-methyl benzophenone (MBP) as crosslinker agent, LiTFSI salt under UV radiation without solvent usage [68]. It is said that MBP can remove H\({}^{+}\) from its methylene group via hydrogen abstraction to form free-radical chains [68]. Likewise, TEGDME that contained the methylene group produced free radicals. These free-radical chains have interlinked with -EO- unit of PEO chain under UV exposure to form SPE film that displayed features, such as being highly amorphous and having a uniform morphology, high mechanical strength, good flexibility, low \(T_{g}\) of \(-\)34 \({}^{\circ}\)C and compatibility with Li metal electrode. At 25 \({}^{\circ}\)C, the SPE showed a conductivity of 0.11 \(\times\) 10\({}^{-3}\) S cm\({}^{-1}\), \(t_{Li+}\) of 0.55 and diffusion coefficient of Li\({}^{+}\) ion of 5.6 \(\times\) 10\({}^{-6}\) cm\({}^{2}\) s\({}^{-1}\)[68]. The TiO\({}_{2}\)/Li cell having such an electrolyte had good cyclability over 100 cycles, with a capacity of \(\sim\)138 mAh g\({}^{-1}\) and almost 90% capacity retention [68]. [PERSON] et al.