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:  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.
Abstract: A common issue plaguing battery anodes is the large consumption of lithium in the initial cycle as a result of the formation of a solid electrolyte interphase followed by gradual loss in subsequent cycles. It presents a need for prelithiation to compensate for the loss. However, anode prelithiation faces the challenge of high chemical reactivity because of the low anode potential. Previous efforts have produced prelithiated Si nanoparticles with dry air stability, which cannot be stabilized under ambient air. Here, we developed a one-pot metallurgical process to synthesize LixSi/Li2O composites by using low-cost SiO or SiO2 as the starting material. The resulting composites consist of homogeneously dispersed LixSi nanodomains embedded in a highly crystalline Li2O matrix, providing the composite excellent stability even in ambient air with 40% relative humidity. The composites are readily mixed with various anode materials to achieve high first cycle Coulombic efficiency (CE) of >100% or serve as an excellent anode material by itself with stable cyclability and consistently high CEs (99.81% at the seventh cycle and ∼99.87% for subsequent cycles). Therefore, LixSi/Li2O composites achieved balanced reactivity and stability, promising a significant boost to lithium ion batteries.
Pub.: 16 Jun '16, Pinned: 29 Sep '17
Abstract: Rapid progress has been made in realizing battery electrode materials with high capacity and long-term cyclability in the past decade. However, low first-cycle Coulombic efficiency as a result of the formation of a solid electrolyte interphase and Li trapping at the anodes, remains unresolved. Here we report LixSi-Li2O core-shell nanoparticles as an excellent prelithiation reagent with high specific capacity to compensate the first-cycle capacity loss. These nanoparticles are produced via a one-step thermal alloying process. LixSi-Li2O core-shell nanoparticles are processible in a slurry and exhibit high capacity under dry-air conditions with the protection of a Li2O passivation shell, indicating that these nanoparticles are potentially compatible with industrial battery fabrication processes. Both Si and graphite anodes are successfully prelithiated with these nanoparticles to achieve high first-cycle Coulombic efficiencies of 94% to >100%. The LixSi-Li2O core-shell nanoparticles enable the practical implementation of high-performance electrode materials in lithium-ion batteries.
Pub.: 04 Oct '14, Pinned: 29 Sep '17
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
Abstract: Developing high-capacity anodes is a must to improve the energy density of lithium batteries for electric vehicle applications. Alloy anodes are one promising option, but without pre-stored lithium, the overall energy density is limited by the low-capacity lithium metal oxide cathodes. Recently, lithium metal has been revived as a high-capacity anode, but faces several challenges owing to its high reactivity and uncontrolled dendrite growth. Here, we show a series of Li-containing foils inheriting the desirable properties of alloy anodes and pure metal anodes. They consist of densely packed LixM (M = Si, Sn, or Al) nanoparticles encapsulated by large graphene sheets. With the protection of graphene sheets, the large and freestanding LixM/graphene foils are stable in different air conditions. With fully expanded LixSi confined in the highly conductive and chemically stable graphene matrix, this LixSi/graphene foil maintains a stable structure and cyclability in half cells (400 cycles with 98% capacity retention). This foil is also paired with high-capacity Li-free V2O5 and sulfur cathodes to achieve stable full-cell cycling.
Pub.: 12 Jul '17, Pinned: 29 Sep '17
Join Sparrho today to stay on top of science
Discover, organise and share research that matters to you