Rational Design of Hybrid Electrolyte for All-Solid-State Lithium Battery Based on Investigation of Lithium-Ion Transport Mechanism

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Lithium-ion batteries (LIBs) are widely used in various applications, including personal electronic devices, electric vehicles, and energy storage systems. Given the increasing demand for LIBs, there is need to develop rechargeable batteries with high energy densities, long cycle life, and enhanced safety. Lithium all-solid-state batteries (ASSBs) could improve operational safety by eliminating flammable liquid electrolytes and using non-flammable solid electrolytes. Among the various types of solid electrolytes explored, solid polymer electrolytes (SPEs) have garnered significant attention owing to their desirable properties, such as good processability, light weight, flexibility, and favorable interfacial contact with the electrodes. To develop practical battery systems using SPEs, improving the ionic conductivity mechanism of lithium-ion (Li+) in SPEs is crucial. Li+ transport mechanism of SPE is typically known as segmentation motion and ion hopping. In SPE, crystalline and amorphous phases coexist, and movement of Li+ occurs mainly in the amorphous phase. Therefore, the use of SPE at high temperatures improves ionic conductivity, but still that have limitation by low ionic conductivity (10-7~ 10-8 S cm-1) at ambient temperatures, low Li+ transference numbers, a restricted electrochemical window, and low mechanical strength. Hybrid polymer electrolytes (HPEs) combine an inorganic ceramic electrolyte (ICE) and SPE to create a ceramic-in-polymer or a polymer-in-ceramic hybrid and have improved ionic conductivity and mechanical stability compared to SPE. Ceramic materials of HPE can be classified into oxide-based materials and NASICON-type materials. Oxide-based materials such as SiO2, Al2O3, etc. induce an increase in the amorphous area of PEO and improve segmental mobility by preventing recrystallization of the polymer, thereby increasing ionic conductivity. NASICON-type materials, such as Li7La3Zr2O12 (LLZO), Li1+xAlxTi2-x(PO4)3 (LATP), etc. are promising candidates for improving ionic conductivity by increasing the mobility of Li+ because they form ionic conductive channels within the ceramic bulk. However, despite the observed improvements in performance, the Li+ ion transport mechanism in HPEs and its correlation with the electrochemical cell results remain unclear.In this study, we present a new approach to improve the performance of ASSB by establishing the Li+ transport mechanism through a designed HPE system proven through experimental results and density functional theory (DFT) calculations. To achieve high energy density ASSB, LiNi0.8Co0.1Mn0.1O2 (NCM811) Li ASSB cell was applied, and through a combination of experiment and DFT calculation, HPE was manufactured by PEO-based semi-interpenetrating polymer network (semi-IPN) SPE and Li1+xAlxTi2-x(PO4)3 (LATP) of NASICON-type. As a result, we confirmed that Li+ migrates through the interface of SPE and LATP, which has a significant effect on the improved ionic conductivity (7.23 × 10-4 S cm-1 at 45 ℃). In addition, it was confirmed that Li dendrites were suppressed and cycle performance were improved due to the improved physical strength of the electrolyte with the addition of ceramics. This suggests a promising design strategy for high-performance Li ASSB.