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Unusual Conformal Li Plating on Alloyable Nanofiber Frameworks to Enable Dendrite Suppression of Li Metal Anode Tiancun Liu, Jiulin Hu, Chilin Li, and Yong Wang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00573 • Publication Date (Web): 15 May 2019 Downloaded from http://pubs.acs.org on May 15, 2019
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ACS Applied Energy Materials
Unusual Conformal Li Plating on Alloyable Nanofiber Frameworks to Enable Dendrite Suppression of Li Metal Anode Tiancun Liu†, Jiulin Hu‡, Chilin Li‡* and Yong Wang†* †
Department of Chemical Engineering, School of Environmental and Chemical Engineering, Shanghai University, Shanghai, 200444, P. R. China ‡ State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, 200050, P. R. China Abstract: Li metal anode is deemed to the most promising candidate of anode for high-energy battery systems like Li-sulfur and Li-fluoride batteries. However, some severe challenges, e.g. facile formation and growth of Li dendrites, large volume evolution of hostless Li and low coulombic efficiency of Li plating/stripping, still hinder the commercialization of Li metal batteries (LMBs). Herein, a free-standing and highly flexible 3D current collector made of carbon nanofibers (CNFs) conformally coated by continuous Sn layer is synthesized by electrospinning method. Sn layer enables a lithiophilic and alloyable carbon skeleton surface and provides uniform and continuous Li nucleation sites, leading to unusual conformal Li plating behavior and effective inhibition of Li dendrites. The spatial confinement of Li plating mitigates the volume expansion and network distortion of CNFs. The electric contact reinforced by Sn interlayer achieves highly reversible Li stripping for more than 850 h for CNF-Sn@Li symmetric cell. The small nucleation overpotential (28 mV) and potential polarization (14 mV for symmetric cell) benefit from the low energy barrier of Li-Sn alloying and following Li nucleation on Li-Sn layer. For CNF-Sn@Li-LiFePO4 full cell, the capacity retention ratio is as high as 92.2% after 150 cycles at 0.5 C and the reversible capacities are maintained at 134.3 and 106.7 mAh g-1 at 2C and 5C respectively. The design of 3D lithiophilic current collector instead of planar Cu is a potential solution to highly safe LMBs.
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Keywords: Li metal anode; C-Sn nanofiber framework; conformal coating; dendrite suppression; Li metal batteries
Introduction With quick social development, energy demands are becoming more urgent. Some energy storage systems e.g. Li-ion batteries (LIBs) have been widely exploited in the past decades.1 However, the application of these battery systems in the fields of large-scale transportation and stationary energy storage is still retarded owing to the insufficient energy density.2,3 Li metal batteries (LMBs) with Li metal as anode are expected to have the potentiality of much higher energy density, in view of the high theoretical capacity (3860 mA h g-1) and low reduction potential (-3.04V vs. standard hydrogen electrode) of Li metal. They therefore are considered as “Holy Grail” for future energy systems.4,5 However the high chemical activity and hostless property of Li metal also introduce some drawbacks, e.g. uncontrollable Li dendrite growth, unavoidable side reactions with electrolyte and low Coulombic efficiency (CE).6 Li dendrite growth may further cause the formation of deactivated “dead Li” zone, as well as the danger of internal short circuit during repeated Li plating/stripping cycles.7 Tremendous efforts have been attempted to address the strategies on Li dendrite suppression. The modulation of electrolyte by functional additive enables the in-situ formation of desired solid electrolyte interphase (SEI) containing additive components (e.g. ionic conductive LiF and Li3N) on the surface of Li anode to homogenize the Li-ion fluxing across SEI.8-11 The painting of artificial SEI layer (e.g. two-dimensional robust BN and C3N4) can further harden the electrode-electrolyte
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ACS Applied Energy Materials
interface with high mechanical modulus to restrain Li dendrite growth.12-17 The separator decoration also takes effect on smoothening the Li plating by reducing the solvation effect of Li+ and enhancing the Li+ transference number in electrolyte.18-21 Constructing metallic composite anode containing deformable Li is also an effective strategy to realize steady Li plating/stripping.22-24 Rolling Li foil with lithiophilic grains or thermally (or electrochemically) injecting molten Li into lithiophilic skeleton has been investigated to inhibit the volume expansion in these composite anodes. Recently, the current collectors with three-dimensional (3D) porous framework
have
been
prepared
by
chemical
dealloying
or
textured
deposition-reduction methods to accommodate the extrusion of Li plating.25,26 Different from planar Cu sheet, 3D host can disperse the spatial distribution of electron charge and lower the current density so as to avoid the concentrated deposition and nucleation of Li-ions.27,28 Guo and Ji et al. proposed graphitized carbon fibers as 3D current collector to enhance the Li plating capacity and meantime avoid the dendrite formation.29,30 However, the aforementioned scaffold structures show poor affinity towards Li atoms, which likely causes large nucleation overpotential and voltage hysteresis. The 3D substrate endowed simultaneously with excellent lithiophilicity and sufficient voids is expected to achieve a better effect on Li plating and stripping behavior with the modification of polarization and cycling performance. Most recently, Hu and Zhang et al. have grown discrete and coarse Ag grains on carbon fibers to construct Li permeable host network, and the nucleation potential is remarkably mitigated based on the low formation energy barrier of Li-Ag
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alloy.31,32 However Ag grains are expensive and their high melting point (> 900oC) retards their conformal coating on carbon substrate. In this work, we propose a flexible and alloyable carbon nanofiber (CNF) network as a free-standing 3D current collector. These nanofibers are decorated by thin Sn layer in a form of conformal coating, which benefits from the low melting point ( 850 h and 3 mA cm-2 for symmetric cell) with lower nucleation overpotential (28 mV) and potential polarization (18 mV for asymmetric cell and 14 mV for symmetric cell) compared with naked 3D CNF and planar Cu films. For CNF-Sn@Li-LiFePO4 full cell, the capacity retention ratio is as high as 92.2% after 150 cycles at 0.5 C and the reversible capacities are as high as 134.3 and 106.7 mAh g-1 at 2C and 5C respectively. The rational design of 3D current collector capable of wetting and reshaping monolithic Li is a promising solution to the achievement of safe Li metal anode
and
practically
applicable
Li
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metal
batteries.
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Experimental Section Materials: Poly(acrylonitrile) with average molecule weight (Mw) of 150, 000 was purchased from J&K Scientific. N,N-dimethyl formamide (DMF) solvent was received from Sinopharm Chemical ReagentCo., Ltd. Tin (Sn) nanopowder (nanoparticle size < 100 nm) was obtained from Aladdin. CNF synthesis: 3D carbon nanofiber framework with Sn coating layer was prepared by the electrospinning method with following carbonization. Briefly, a certain amount of nano-sized Sn particles (
