Surface fluorination of reactive battery anode materials for enhanced stability
Jie Zhao1,†, Lei Liao1,†, Feifei Shi1, Ting Lei2, Guangxu Chen1, Allen Pei1, Jie Sun1, Kai Yan1, Jin Xie1, Chong Liu1, Yuzhang Li1, Zheng Liang1, Zhenan Bao2, and Yi Cui1,3*
1 Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, USA.
2 Department of Chemical Engineering, Stanford University, California 94305, USA.
3 Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, CA 94025, USA.
† These authors contributed equally.
Significant increase in energy density of batteries must be achieved by exploring new materials and cell configurations. Lithium metal and lithiated silicon are two promising high-capacity anode materials. Unfortunately, both these anodes require reliable passivating layer to survive the serious environmental corrosion during handling and cycling. Here we developed a surface fluorination process to form a homogenous and dense LiF coating on reactive anode materials, with in situ generated fluorine gas by using a fluoropolymer, CYTOP, as the precursor. The process is effectively a “reaction in the beaker”, avoiding developing specialized equipment and handling highly-toxic fluorine gas. For lithium metal, this LiF coating serves as a chemically stable and mechanically strong interphase, which minimizes the corrosion reaction with carbonate electrolytes and suppresses dendrite formation, enabling dendrite-free and stable cycling over 300 cycles with current densities up to 5 mA/cm2. Lithiated silicon can serve as either an anode prelithiation additive to compensate the initial lithium loss in existing lithium-ion batteries or a replacement for lithium metal in Li–O2 and Li–S batteries. However, lithiated silicon reacts vigorously with the standard slurry solvent N-methyl-2-pyrrolidinone (NMP), indicating it is not compatible with the real battery fabrication process. With the protection of crystalline and dense LiF coating, LixSi can be processed in NMP with a high extraction capacity of 2504 mAh/g. With low solubility of LiF in water, this protection layer also enables the stability of LixSi in humid air (~40% relative humidity). Therefore, this environmental-friendly surface fluorination process brings huge benefit to both the existing lithium-ion batteries and next-generation lithium metal batteries.
 J. Zhao†, L. Liao†, F. Shi, T. Lei, Z. Bao, Y. Cui. et al, “Surface fluorination of reactive battery anode materials for enhanced stability”, JACS 139, 11550 (2017).