# Development on Solid Polymer Electrolytes for Electrochemical Devices
## 3 Basic Solid Polymer Electrolytes
### Strategies to Improve Ionic Conductivity of SPEs
#### 3.2.6 Composite Solid Polymer Electrolytes
Comparison was made with basic SPE configuration of PVDF-HFP-LiTFSI in electrochemical performance of Li-ion battery. The LLZO CSPE cell displayed good cyclability over 1000 cycles with specific capacity of 149 mAh g\({}^{-1}\), whereas the capacity decreased sharply after 50 th cycle for the Li-ion cell with basic SPE [153]. Synergistic effect of LLZO filler and poly(ethylene glycol) dimethyl ether (PEGDME) plasticizer was also studied in PEO-LiTFSI SPEs system [154]. Again, improvements in conductivity, Li-ion transference number and electrochemical performance were reported [154]. In the work of [PERSON] et al. [155], LLZO-added CSPE containing PEO and LiTFSI did not contribute to ionic conductivity but exhibited improved electrolyte-electrode interfacial contact and higher electrochemical performance in lithium battery. It was found that the addition of LLZO in SPEs suppressed lithium dendrites growth [152; 153; 154; 155; 156]. The lithium and zirconium sites of LLZO can be doped with other elements to boost its properties. [PERSON] and co-authors [157] employed tantalum (Ta) as dopant in LLZO to form Li\({}_{6.75}\)La\({}_{3}\)Zr\({}_{1.75}\)Ta\({}_{0.25}\)O\({}_{12}\) (LLZTO). The PAN-LiClO\({}_{4}\)-LLZTO CSPE showed conductivity increment by 12 times (2.20 \(\times\) 10\({}^{-4}\) S cm\({}^{-1}\) at 40 \({}^{\circ}\)C) and higher Li\({}^{+}\) transference number of 0.30 than filler-free SPE [157]. In addition, the CSPE was thermally and electrochemically stable with enhanced mechanical strength [157]. It is worthy to mention that Li\({}^{+}\) transference number as high as 0.75 was obtained in CSPE containing LLZTO, poly(propylene carbonate) and LiTFSI with conductivity of 5.20 \(\times\) 10\({}^{-4}\) S cm\({}^{-1}\) at 20 \({}^{\circ}\)C [158]. It is understood that the addition of nano-sized LLZTO promoted salt dissociation due to the interactions between polymer, cation and filler and subsequently impeded the diffusion of anions [158]. LLZO is preferred over NASICON-type Li\({}_{1.5}\)Al\({}_{0.5}\)Ge\({}_{1.5}\)(PO\({}_{4}\))\({}_{3}\) (LAGP) due to their inexpensive elements [155]. In contrast, germanium in the latter is costly [155]. Nevertheless, LLZO has shortcomings; for example, it has an unstable at room-temperature condition, and thus, it is reactive towards moisture and humidity [155]. Similar to LLZO, LAGP is also unreactive towards lithium metal electrodes and has been used as a filler in polymer electrolytes. PEO, LiTFSI and LAGP were mixed and ground without solvent before heating and hot-pressing to form CSPE as reported by [PERSON] et al. [159]. The CSPE showed improvements in term of conductivity, mechanical integrity and specific capacity as compared to electrolyte without filler [159]. On the other hand, the prepared CSPE in Reference [159] showed superior interfacial contact than pelletized LAGP solid electrolyte in lithium-ion cells. However, no significant change in lithium-ion transference number was observed with the introduction of LAGP in PEO-based electrolyte. On the contrary, LAGP added in poly(propylene carbonate)-based electrolyte increased lithium-ion transference number to 0.77 with conductivity of 0.56 \(\times\) 10\({}^{-3}\) S cm\({}^{-1}\) and improved cyclability and stability in Li-ion battery [160]. [PERSON] et al. [13] employed electrospinning technique to synthesize Li\({}_{0.33}\)La\({}_{0.55}\)TiO\({}_{3}\) nanodospbefore adding in PVDF-HFP electrolyte containing LiTFSI salt. The CSPE was more thermally stable and showed decreased crystallinity with higher ambient ionic conductivity (1.21 \(\times\) 10\({}^{-4}\) S cm\({}^{-1}\)) and tensile strength (39.77 MPa) than filler-free electrolyte (\(\sigma\) = 4.72 \(\times\) 10\({}^{-5}\) S cm\({}^{-1}\); tensile strength = 38.41 MPa) [13]. Ionic conductivity of PAN-based SPE has been shown to increase with the help of an active filler namely lithium titanate nanotubes (LiTNT) [161].