Surface Confined Water by Metal Oxide Cluster for Single Lithium-Ion Conduction

The innovation of water-in-salt electrolytes (WiSEs) broadens the electrochemical stability window (ESW) while preserving high ionic conductivity. However, the required high Li-salt concentrations incur high cost and device fabrication complexity, and the low Li + transference number limits long-term stability. Here, we report that sub-nm anionic metal oxide clusters (SiW 12 O 40 4– ) enable single Li + conduction in a low-salt WiSE (∼1 wt % Li loading) with robust antifreeze properties down to –80 °C. Surface nanoconfinement disrupts the hydrogen-bond network of water, accelerating local water relaxation dynamics to achieve high ionic conductivity (0.08 S cm –1 at 30 °C) while hindering water diffusivity to widen ESW to 2.2 V. The anionic clusters form a rigid framework that immobilizes anions, yielding a high Li + transference number of 0.92. This work establishes an economical and high-performance single-ion WiSE platform and provides a molecular understanding of the structure–property relationship via relaxation dynamics.