A pinboard by
Jie Zhao

Phd student, Stanford University


Air-stable and freestanding lithium alloy/graphene foil as an alternative to lithium metal anodes

Substantial improvements in energy density for lithium-ion batteries require the development of high-capacity electrodes. Alloy anodes with much higher capacity have been recognized as promising alternatives to commercial graphite. Without pre-stored lithium in alloy anodes, the energy density is limited by the low capacity of lithium metal oxide cathodes. Recently, Li metal has been revived as a high-capacity anode, but faces many challenges resulting from its high reactivity and uncontrolled dendrite growth. Here we develop a series of Li-containing anodes as alternatives to Li metal, inheriting the desirable properties of alloy anodes and pure metal anodes. This large-scale freestanding LixM/graphene foil (M = Si, Sn, Al etc.) consists of fine nanostructures of densely-packed LixM nanoparticles encapsulated by large graphene sheets. With fully-expanded LixM confined in the highly-conductive and chemically-stable graphene matrix, this foil maintains a stable structure and cyclability in half cells. The LixSi/graphene foil is successfully paired with high-capacity Li-free cathodes, such as V2O5 and Sulphur, to achieve stable full-cell cycling. LixM/graphene foils are stable in air, owing to their unique structure as well as the hydrophobicity and gas impermeability of graphene sheets. By addressing electrochemical and environmental stability simultaneously, these Li metal alternatives represent a significant breakthrough in battery research.

Reference: [1] J. Zhao, G. Zhou, K. Yan, J. Xie, H.-M. Cheng, and Y. Cui, “Air-stable and freestanding lithium alloy/graphene foil as an alternative to lithium metal anodes”, Nat. Nanotech, doi:10.1038/nnano.2017.129.


Surface fluorination of reactive battery anode materials for enhanced stability.

Abstract: 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 homogeneous 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 handling highly-toxic fluorine gas directly. 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 a prelithiation additive for 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 anhydrous NMP with a high 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 facile surface fluorination process brings huge benefit to both the existing lithium-ion batteries and next-generation lithium metal batteries.

Pub.: 27 Jul '17, Pinned: 29 Sep '17