Ternary Poly(ethylene oxide)/Poly(l,l‑lactide) PEO/PLA Blends as High-Temperature Solid Polymer Electrolytes for Lithium Batteries

Lithium batteries are in high demand in different technological fields. However, the operating temperature is required to be below 70 °C, and this limits their use in applications demanding high-energy rechargeable batteries that are able to operate at temperatures above 100 °C. Poly­(ethylene oxide) (PEO) is, currently, the reference solid polymer electrolyte (SPE) employed in solid-state lithium batteries. However, the application of PEO at higher temperatures is restricted due to the loss of mechanical properties. In this article, we show that the polymer blending strategy of blending PEO with poly­(l,l-lactide) (PLA) allows extending its use in batteries at high temperatures (100 °C). This improvement is due to the mechanical reinforcement of PEO solid electrolytes associated with the presence of PLA crystals. Thus, two solid electrolyte systems based on PEO/PLA blends with either a LiTFSI salt or a lithium single-ion polymer (poly­(lithium-1-[3-(methacryloyloxy)­propylsulfonyl]-1-(trifluoromethanesulfonyl)­imide), PLiMTFSI) were investigated and compared. Differential scanning calorimetry (DSC) results indicate that regardless of the concentration of LiTFSI or PLiMTFSI in the blend, crystals of PLA are present with melting peaks at 160–170 °C and the lithium salt distributes preferentially in the PEO-rich amorphous phases. The ionic conductivity is negatively affected by the incorporation of PLA in the blends. However, at high temperatures (>70 °C), ionic conductivities of ∼10^–4 S cm^–1 were obtained for both systems. DMTA results showed that PLA addition increases the mechanical properties of the electrolytes, yielding storage modulus values of ∼10⁶ Pa for the PEO/PLA/LiTFSI blend and ∼10⁷ Pa or higher for the PEO/PLA/PLiMTFSI blend at high temperatures (100 °C). Finally, both ternary blends were compared in a symmetrical lithium battery at 100 °C, and the single-ion conducting PEO/PLA/PLiMTFSI system presented lower overpotentials, which is reflected in a lower polarization inside the lithium battery.