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 Na₁₅Sn₄ 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^-1. The symmetric cells of the Na-K-Sn@mesh electrodes cycled at 2.0 mA cm^-2 with 1.0 mAh cm^-2 showed little fluctuations with the stable overpotential of ∼200 mV for 550 h, and the full cell coupled with Na₃V₂(PO₄)₃ showed an initial discharge capacity of 103 mAh g^-1 at 2 C with a retention of 90% after 800 cycles. When the high-loading Na₃V₂(PO₄)₃ 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^-2) and 91% over 60 cycles (5.2 mAh cm^-2).