Carbon Nanofiber Host with Directed Gradient Pore Size for Dendrite-Free Lithium Metal Anodes

Three-dimensional carbon nanofibers with high conductivity, high specific surface area, and high porosity (>= 70%) can be used as hosts for lithium metal storage. However, lithium ions easily concentrate at the top surface of the conductive carbon hosts during electrodeposition, making the internal space of the hosts largely inaccessible and ultimately leading to the growth of lithium dendrites on the top surface. Therefore, a conductive carbon nanofiber host with a directed gradient pore structure is reported in this work (signed as GPCF), based on two carbon nanofiber networks with both a small pore size (similar to 2.6 mu m) as the current collector side and a large pore size (similar to 3.9 mu m) facing the separator side. As a result, the uniform lithium-ion flow with a small ion concentration gradient from the separator to the current collector side is formed within GPCF, to enable the dendrite-free lithium deposition morphology. The GPCF anode can run for more than 160 cycles at 0.5 mA cm-2 and 1 mAh cm-2, and the Coulombic efficiency is up to 99%. In contrast, the carbon nanofiber hosts with only small, large, or inverse gradient pore size structures easily form significant "dead Li" after 80, 120, and 100 cycles, respectively, and the Coulombic efficiency fluctuates severely. The above results reveal that the elaborately designed carbon nanofiber hosts with a directed gradient pore size can significantly homogenize the lithium-ion flow at the anode side to improve the electrochemical performance of lithium metal batteries.