Niobium garnet/polyethylene oxide composite as a solid electrolyte for all-solid-state batteries (ASSB) with high-nickel cathodes

All-solid-state lithium batteries (ASSB) are emerging as an effective and promising alternative to current technologies that use organic liquid electrolytes. Its main proposition is to mitigate the safety and environmental issues caused by the leakages and explosions of conventional cells through the development and use of solid electrolytes, in the form of polymer membranes, ceramic pellets, or even composites, which are a combination of both. In the present work, composite electrolytes of polyethylene oxide (PEO), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and Zr-doped niobium garnet oxides (Li_5+xLa₃Nb_2-xZr_xO₁₂ - LLNZ) were prepared. The addition of ceramic reduced the melting point and inhibited the formation of spherulite-type crystallization of the polymer. The ionic conductivities of the composites were slightly lower than the polymer but still high for composite electrolytes of this composition, around 10⁻⁴ S.cm⁻¹. The obtained results were analyzed considering the findings reported by other researchers, and some factors for a high-performance composite electrolyte were detailed. Additionally, all the fabricated composites showed a broad electrochemical window, some even above 5.0 V. Thus, electrochemical measurements were conducted with NMC811 as the cathode. The half-cell exhibited a specific capacity of 185 mAh.g⁻¹ at C/20 at 60 °C, and a capacity retention of 68% after 50 cycles at C/5. The results are promising and indicate the possibility of the use of high‑nickel cathodes in all-solid-state batteries to increase their energy density.