Abstract:

In past decades, lithium-ion batteries (LIBs) were the dominant energy storage systems for powering portable electronic devices because of their reliable cyclability. However, further increase in the energy density of LIBs was met by a bottleneck when low-specific- capacity graphite was used at the anode. Li metal has long been regarded as the ideal anode material for building the next high-energy-density batteries due to its ultrahigh capacity of 3860 mAh·g^-1, which is ten times higher than that of graphite. However, using Li metal as an anode in rechargeable batteries is challenging due to its high uncontrolled volume expansion and aggressive side reactions with liquid electrolytes. In this study, we demonstrate the effect of a three-dimensional (3D) framework with enriched fluorinated sites for Li metal protection. This framework is obtained via a facile integration of down-sized fluorinated graphite (CF_x) particles into Li⁺ conducting channels. Thermogravimetry, energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy results show that Li⁺ conducting channels rich in lithium fluoride (LiF) are formed in situ across the embedded CF_x particles during the initial lithiation process, leading to fast Li⁺ transfer. Scanning electron microscopy results show that residual CF_x particles could act as high-quality nucleation sites for uniform Li deposition inside the framework. These features could not be achieved with a 2D structure consisting of large CF_x flakes, due to the limited Li⁺ transfer paths and low utilization ratio of CF_x for conversion into LiF-based solid electrolyte interphase (SEI) layers. Consequently, better performance of Li metal anodes in a 3D framework with enriched fluorinated sites is demonstrated. Stable Li plating/stripping over 240 cycles is obtained at a current density of 0.5 mA·cm^-2 for a fixed capacity of 1 mAh·cm^-2 by maintaining a voltage hysteresis below 80 mV. Improved Li-LiFePO₄ full cell performance with a practical negative/positive capacity ratio of 3 is also demonstrated. These results show the rational combination of well-developed 3D Li⁺ transfer channels and enriched fluorinated sites as an optimized interfacial design beyond the single use of a 2D fluorinated interface, giving new insight into the protection of Li metal anodes in high-energy-density batteries.

Key words: Li metal battery, Li metal anode, Fluorinated graphite, Solid electrolyte interphase, Coulombic efficiency