Surface passivation of lithium metal via In situ polymerization

Abstract

Lithium (Li) metal is considered a promising anode material for high energy density secondary Li metal batteries because it has the highest specific energy (3860 mAh·g−1) and lowest redox potential (−3.04 V compared to standard hydrogen electrodes. However, the development of high-performance Li metal batteries is challenging. Firstly, Li dendrites tend to grow on the surface of Li metal foil, leading to a limited anodic coulombic efficiency (CE), poor cyclability, and even explosion hazards when an internal cell short circuit occurs. Moreover, Li metal suffers from serious surface stability problems and is easily corroded by electrolytes during cycling, further resulting in low CE, thus shortening the life cycle. We have developed a Li-carbon nanotube (Li-CNT) composite microsphere via a facile molten impregnation method. The Li-CNT composite’s CNT framework can suppress volume changes during the charge/discharge process and help stabilize the solid electrolyte interphase (SEI), which is typically mechanically fragile. As a result, Li-CNT shows a high specific capacity (2000 mAh·g−1) and can significantly suppress dendrite formation by reducing the current density, resulting in enhanced safety and cycling stability. However, the large specific surface area of the Li-CNT microspheres also enables increased reaction with the air and the electrolyte. A passivation layer is critical for the practical application of Li-CNT during the electrochemical cycling and manufacturing process. LiF is an important component of SEI in the liquid electrolyte system, and a uniform and dense LiF-rich SEI film can enable stable cycling. Moreover, LiF has been widely used as the preferred coating material to protect Li metal anodes through different methods. In this study, we improved the Li-CNT composite stability by constructing a uniform LiF-rich protecting layer on the surface through in situ polymerization of 4-fluorostyrene. The F functional group of 4-fluorostyrene, which is a lithiophilic group, reacts with the Li-CNT to produce a uniform LiF-rich layer on the surface of the Li-CNT via a facile and scalable liquid-phase reaction. The resulting passivation layer effectively suppresses the Li-CNT corrosion by the electrolyte and air, leading to better environmental and electrochemical stability. Consequently, after exposure to dryair with a dew point −40 ℃ for 24 h, the specific capacity of the surface passivated Li-CNT is still as high as 1129 mAh·g−1, corresponding to a capacity retention of 52.85%. When the surface passivated Li-CNT is paired with a LiFePO4 cathode (the capacity ratio of cathode and anode is 1: 6), a prolonged lifespan of over 280 cycles at 0.5C was reached, corresponding to a CE of 97.7%. The in situ polymerization passivation is simple and easy to be scale up; thus, it is a promising method for developing Li metal anodes towards the practical Li metal batteries.