Ultrathin and capacity-tunable lithium metal wires for lithium-based fiber batteries

Ultrathin lithium (Li) metal wires with tunable capacities have great promise for precise prelithiation of fiber anodes and high-energy-density Li-based fiber batteries. However, the application of Li metal in fiber batteries faces great challenges due to its mechanical fragility and the resulting limited micro-dimension manufacturing capability. These challenges impede the production of ultrathin Li wires with adjustable Li contents to match the capacities of Li-based fiber batteries. Herein, silver-plated aramid yarns (Ag/AYs) are employed to load Li metal for producing ultrathin Li wires. The bundled structure of Ag/AYs leads to the adjustable volume of oriented voids within the fibers, thus resulting in accurately tunable capacities (0.0048-2.4 mAh cm^-1) and diameters (20-534 μm) of Li wires. Such thin Li wires are used to precisely compensate for Li loss during the formation cycle of the fiber graphite anodes, thereby improving the initial Coulombic efficiency from ∼88% to ∼100%. Additionally, when employed as anodes, these Li wires enabled the fiber batteries to exhibit exceptional cycling stability for 150 cycles under a relatively low negative/positive ratio of 2.06, while achieving a high energy density of 139.822 Wh kg^-1 based on the total mass of the battery.