Lithium metal batteries due to their high energy densities assume significance in portable electronics and electric vehicles. Nevertheless, dendrite formation during deposition, instability of the Li metal interface, and huge volume change are the major limitations that need to be solved for their effective utilization. The uncontrolled dendrite-growth of Li is considered responsible for the low reversibility, short cycle life, internal short circuiting, and safety hazards. Researchers at Department of Materials Science and Engineering, University of Maryland at College Park, USA have demonstrated a rapid Joule heating method to anchor Ag nanoparticles (Ag NPs) on carbon nanofibers (CNFs) to guide seeded nucleation and growth of Li to obtain smooth Li metal anode without dendrite growth.
CNFs prepared by electrospinning served as the host material. They were soaked in silver acetate and rapidly heated by Joule heating setup (Fig. 1(a)). When heated above the melting point of Ag (962 K) for only 0.1 s, the molten Ag gets self-assembled as Ag NPs. The high temperature promotes a strong bonding between Ag NPs and CNFs. The defects in the CNFs constrain the migration of Ag NPs. Rapid quenching of the CNFs seeded with Ag NPs below the melting point of Ag prevents agglomeration of Ag NPs. Fortunately, the CNFs could withstand the thermal shock and preserved its graphitic structure.
Fig. 1 (a) Schematic of the Joule heating method for coating Ag NPs on CNFs (inset: morphology of CNFs prepared by electrospinning); (b) Digital image of the Joule heating set up. The sample was connected to Cu electrodes and heated by a current pulse in Ar-filled glove box.
The morphologies of Ag NPs on CNFs obtained by Joule heating for 0.05, 0.1, 0.5, and 4 s (Fig. 2) indicate that they are homogenous with an average size of 29–57 nm. The size of Ag NPs show a strong dependence on the thermal shock time; the shorter the time, the lesser the particle size (Fig. 2).
Fig. 2 SEM images of Ag NPs deposited on CNFs by Joule heating method for (a) 0.05 s; (b) 0.5 s; and (c) 4 s.
The nucleation and growth of Li seeded by Ag NPs on CNFs is schematically represented in Fig. 3(a). Due to the zero nucleation overpotential, selective nucleation of Li occurs on AgNP/CNFs (Fig. 3(c)). Plating of Li is proceeded by alloying of Li with Ag NPs. The strong anchoring of Ag NPs on CNFs guides the formation of a smooth Li coating. During the growth stage, Li from AgNP/CNFs gradually fills the voids between the CNFs, resulting in the formation of an even Li metal anode without dendrite growth (Fig. 3(d)). The ability of the Ag NPs strongly bound on to CNFs to retain itself on the surface of the anode even after stripping of Li (Fig. 3(e)), could repeatedly guide the seeded nucleation of Li. Figs. 3 (f) and 3(g) show the inability of bare CNFs to promote uniform deposition of Li metal, due to the poor wettability of CNFs with Li, thus justifying the beneficial role of Ag NPs.
Fig. 3 (a) Schematic of Li nucleation and growth seeded by Ag NPs on CNFs; (b-g) SEM images: (b) pristine AgNP/CNFs without Li deposition; (c) initial Li nucleation on AgNP/CNFs; (d) Li deposited on CNFs guided by Ag NPs at 1 mA h/cm2 of; (e) AgNP/CNFs after the first plating/stripping cycle: (f) bare CNFs without Ag nanoseeds; and (g) Li deposited on bare CNFs
The cycling performance of Li metal anodes using AgNP/CNFs as host (size of Ag NPs ≈40 nm) indicate an exceptional cycling stability at 0.5 mA/cm2 for 500 h without short-circuiting with a high Coulombic efficiency of ≈98%. In contrast, self-nucleation of Li resulting in the formation of pillars and dendrites of Li metal dramatically decrease cycling stability of bare CNFs to 100 h. The discharge/charge profiles of Li anode seeded by AgNP/CNFs show a low overpotential (≈25 mV) with a negligible nucleation overpotential at 0.5 mA/cm2, which is likely to promote a controlled growth of Li. In contrast, plating or stripping of Li on bare CNFs is accompanied with an initial nucleation overpotential.
The CNFs host modified by Ag NPs effectively regulates the deposition of Li, thus enabling the formation of a smooth Li anode without dendrites. The Li metal anodes developed using AgNP/CNFs exhibits a low voltage overpotential, an exceptional cycling stability and avoids problems due to short-circuiting.
T.S.N. Sankara Narayanan.
For more information, the reader may kindly refer: Chunpeng Yang et al., Ultrafine Silver Nanoparticles for Seeded Lithium Deposition toward Stable Lithium Metal Anode, Adv. Mater. 2017, 1702714, DOI: 10.1002/adma.201702714
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