Mitigating Succinonitrile-Li Molecular Crosstalk in In Situ Polymerization toward High-Voltage and Low-Temperature...

In situ polymerization technology presents a promising avenue for constructing solid-state Li metal batteries with tight electrode-electrolyte interfacial contact. However, its application is often compromised by slow Li⁺ kinetics at low temperatures and poor stability against high-voltage cathodes. While succinonitrile (SN) has been introduced as a plasticizer to enhance ionic conductivity and oxidation resistance, it exhibits high reactivity toward Li metal, causing continuous side reactions and an unstable interface. Herein, we propose a methoxy-functionalized strategy to address the SN-Li molecular crosstalk by methoxylating one terminal of SN, resulting in 3-methoxypropionitrile (MPN), which is incorporated into an in situ polymerized poly(1,3,5-trioxane) electrolyte. Among them, the methoxy group introduces a weakly positive dipole via its C-H bond, which not only significantly lowers the freezing point to -62.9 °C, much lower than that of SN (50 °C), resulting in enhanced low-temperature kinetics, but also limits anion mobility and further enhances Li⁺ kinetics through ion-dipole interactions with TFSI^- anions. Simultaneously, the electron-withdrawing cyano group is retained, endowing the electrolyte with high oxidation resistance and a stability window exceeding 5.0 V. Therefore, the obtained solid-state electrolyte exhibits a high ionic conductivity (0.9 × 10^-3 S cm^-1) and a high Li⁺ transfer number of 0.70 at -20 °C. As a result, the Li∥LiFePO₄ full cells can run stably and deliver high capacity retention (∼100%) after 1500 cycles at 10 C. Additionally, all the full cells (Li∥LFP, Li∥NCM811, Li∥NCM622 (high loading: 25.27 mg cm^-2), and Li∥LCO (cutoff voltage: 4.5 V)) can run stably at -20 °C and even -40 °C with high capacity retention. This work offers a feasible molecular functionalization strategy to overcome the interfacial incompatibility of traditional plasticizers in solid-state Li metal batteries, advancing their practical application.