Ultrathin Nanofibrillar Cu Sheet Covered with Directly Grown Lithiophobic CNT for Anode-Free Li-Metal Battery Anode Current Collectors

Li-metal batteries (LMBs) are in the spotlight because of their low electrochemical potential and high energy density (3860 mAh/g) of Li-metal. However, low electrochemical reversibility and side reaction including SEI formation still remain as challenges. LMBs with excessive Li in the anode, thus having high N/P ratios, has been proposed to compensate the low reversibility and fast capacity loss, yet their energy density becomes even lower than that of Li-ion batteries when the N/P ratio is higher than 3. Therefore, to maximize the advantages of Li-metal while minimizing safety problems caused by Li, anode-free Li-metal batteries (AFLMBs) with N/P ratio = 0 is ideal.<br/>In AFLMBs, unlike the conventional LMBs, the anode current collectors (CCs) are not completely covered by Li. In addition, only limited amount of Li is cycled. As a result, fast capacity loss and side reactions including galvanic corrosion at the anode arise as new challenges. In this research, we study the effect of three-dimensional surface structures in copper anode CCs in AFLMB, and suggest novel fabrication strategy to enhance the electrochemical reversibility of Li and minimizing side reactions. The nanofibrillar Cu network in a form of thin sheet is fabricated through a rearrangement process of Cu on the woven Cu wiremesh via electrochemical etching and electrodeposition. The electrodeposited nanofibers exhibit approximately 400 nm in diameter. We can control the nanofibrillar sheet thickness to be 15 μm in minimum, which equivalent to 8 μm thick Cu foils in mass per unit area, yet the surface area is yet significantly greater. In addition, an optimized thermal reduction treatment method is suggested to interconnect the Cu nanofibers, effectively increasing the electrical conductivity in nano-scale. We quantitatively evaluate the capacity loss, the charge transfer resistance, and the side reaction including galvanic corrosion according to the sheet thickness and surface treatment. By introduction of a rationally-designed ultrathin nanofibrillar sheet as the Cu CC, we experimentally confirm reductions in overpotential and in side reaction in AFLMB by 0.3 V, and 90%, respectively. Accordingly, the discharge capacity at the first cycle can be enhanced by 36%. Finally, we decorate the top of the three-dimensional anode CC with carbon nanotubes (CNTs), as a lithiophobic nanoporous layer. Multi-walled CNTs with diameter of ~10-20 nm are directly grown on the surface of Cu nanofibrillar sheet via thermal chemical vapor deposition. The top CNT layer can further enforce the growth of Li inside the nanofibriallar Cu CCs, and thereby maximizes the reversible Li cycling in AFLMBs.