All-Metal Phosphide Electrodes for High-Performance Quasi-Solid-State Fiber-Shaped Aqueous Rechargeable Ni–Fe Batteries

Aqueous secondary Ni–Fe batteries with superior energy density, cost-effectiveness, and outstanding safety contribute significantly toward the development of portable and wearable energy storage devices with high performance. However, the common electrode materials are nickel/iron or their oxides which have suffered from poor conductivity and cycle performance. As an ideal candidate to address these issues, metal phosphides may offer outstanding theoretical specific capacity, low conversion potential, and impressive redox. In this study, one novel type of high-performance flexible Ni–Fe battery with binder-free electrodes on conductive fiber substrates is successfully designed and fabricated. Carbon nanotube fibers with the direct grown hierarchical NiCoP nanosheet arrays and FeP nanowire arrays are fabricated first using hydrothermal synthesis and then the pursuant gas phosphating process. With the assistance of the PVA–KOH gel electrolyte, our fiber-shaped aqueous rechargeable battery (FARB) presents negligible capacity loss after bending 3000 times. Meanwhile, the assembled FARB has a significant capacity of 0.294 mA h/cm2 under the current density of 2 mA/cm2 and a high energy density of 235.6 μW h/cm2.