Room-Temperature Fabrication of a Liquid NaK Alloy-Based Membrane Electrode for Sodium-Ion Batteries.

The room-temperature liquid anode is a feasible method for building dendrite-free alkali-metal-based batteries. The Na-K phase diagram shows a eutectic point as low as 260.53 K with a long liquid range below 298 K with the molar fraction of potassium ranging from 30.48 to 84.99%. However, the NaK alloy exhibits a very high surface tension preventing it from wetting the current collector surface. Herein, a novel homogeneous dual solid-liquid composite in which the liquid alloy is fixed by the solid NaSn phase and perfectly stuffed into the grid of the mesh has been designed and fabricated. Based on the liquid range of the NaK alloy, the Na-K-Sn mixture possesses a theoretical specific capacity of 768 mAh g. The symmetric cells of the Na-K-Sn@mesh electrodes cycled at 2.0 mA cm with 1.0 mAh cm showed little fluctuations with the stable overpotential of ∼200 mV for 550 h, and the full cell coupled with NaV(PO) showed an initial discharge capacity of 103 mAh g at 2 C with a retention of 90% after 800 cycles. When the high-loading NaV(PO) electrode is applied in the full cell, a stable cycling life is still maintained with a good capacity retention of 86% over 190 cycles (2.7 mAh cm) and 91% over 60 cycles (5.2 mAh cm).