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Artificial Solid Electrolyte Interphase Protected LixSi Nanoparticles: An Efficient and Stable Prelithiation Reagent for Lithium-ion Batteries Jie Zhao, Zhenda Lu, Haotian Wang, Wei Liu, Hyun-Wook Lee, Kai Yan, Denys Zhuo, Dingchang Lin, Nian Liu, and Yi Cui J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b04526 • Publication Date (Web): 19 Jun 2015 Downloaded from http://pubs.acs.org on June 24, 2015
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Journal of the American Chemical Society
Artificial Solid Electrolyte Interphase Protected LixSi Nanoparticles: An Efficient and Stable Prelithiation Reagent for Lithium-ion Batteries Jie Zhao,† Zhenda Lu,† Haotian Wang,‡ Wei Liu,† Hyun-Wook Lee,† Kai Yan,† Denys Zhuo,† Dingchang Lin,† Nian Liu,# and Yi Cui†,§,* †
Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, United States
‡
Department of Applied Physics, Stanford University, Stanford, California 94305, United States
#
Department of Chemistry, Stanford University, Stanford, California 94305, United States
§
Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, United States
Supporting Information Placeholder
pensate for lithium loss in lithium-ion batteries, particularly during the formation of solid electrolyte interphase (SEI) from reduced electrolytes in the first charging cycle. We recently demonstrated that thermal-alloying synthesized LixSi nanoparticles (NPs) can serve as a high-capacity prelithiation reagent although its chemical stability in the battery processing environment remained to be improved. Here we successfully developed a surface modification method to enhance the stability of LixSi NPs by exploiting the reduction of 1-fluorodecane on the LixSi surface to form a continuous and dense coating, through a similar reaction process to SEI formation. The coating, consisting of LiF and Li alkyl carbonate with long hydrophobic carbon chains, serves as an effective passivation layer under ambient environment. Remarkably, artificial-SEI protected LixSi NPs show a high prelithiation capacity of 2100 mAh/g with negligible capacity decay in dry air after 5 days and maintain a high capacity of 1600 mAh/g in humid air (~10% relative humidity (RH)). Silicon, tin and graphite were successfully prelithiated with these NPs to eliminate the irreversible first cycle capacity loss. The use of prelithiation reagents offers a new approach to realize next generation high-energy-density lithium-ion batteries.
to address the issues associated with large volume change is 5,6 to use nanostructured materials, such as porous materials, 7,8 9 nanowires and nanotubes, and Si/C composites. The drawback of these nanostructures is their high surface area significantly increases SEI formation in the first and later 8,10 cycles. At the working potential of anodes, electrolytes are not stable and consequently reduced on the anode surface to form SEI, which consists of a complex composition of inor11,12 ganic and organic lithium compounds. The formation of SEI results in large irreversible capacity loss. The amorphous 9,13 carbon used in the nanostructures further consumes Li. st Accordingly, the first-cycle Coulombic efficiency (1 cycle CE) of alloying anode materials is low, typically in the range 14,15 st of 50%-80%. Although the 1 cycle CE of commercial graphite is consistently >90%, the capacity of the anode is usually 10% greater than that of the cathode to reduce the probability of Li deposition, which further exacerbates the 16,17 irreversible capacity loss of the full cell. As common Li metal oxide cathodes have relatively low specific capacities 18 ( 300%) during cycling usually results in rapid capacity decay as a result of high stress and mechanical damage. A common approach
Currently, the only commercial prelithiation reagent in powder form is micro scale stabilized lithium metal powder (SLMP, FMC Lithium Corp.), which is effective for compenst sating the 1 cycle irreversible capacity loss of different anode 20,21 materials, such as SiO and Si-CNT composite. However, it is difficult to synthesize SLMP in research laboratories and 17 other practical challenges still remain to be addressed. To minimize the disturbance of prelithiation reagents to the whole structure of the electrodes, we recently explored LixSi 22 NPs. Due to the chemical reactivity of LixSi, an appreciable amount of capacity is sacrificed to form a Li2O passivation layer to stabilize LixSi as LixSi-Li2O core-shell NPs. However,
ABSTRACT: Prelithiation is an important strategy to com-
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LixSi-Li2O NPs only maintain their capacity in air with low humidity for relatively short durations, which limits its potential use in large scale applications. Therefore, nanoscale prelithiation reagents with higher capacity and improved stability should be explored. Here we report a facile reaction process utilizing the highly reactive nature of LixSi NPs to reduce 1-fluorodecane, thereby producing a continuous and dense coating over the NPs (Figure 1a). The synthesis is inspired by the SEI formation process in battery anodes. The conformal coating, consisting of LiF and Li alkyl carbonate with long hydrophobic carbon chains, effectively suppresses the reactivity of LixSi NPs under ambient conditions, which allows for safe storage and handling. The passivated NPs can be reactivated by contact with the electrolyte during battery fabrication. These artificial-SEI protected LixSi NPs exhibit a high capacity of ~2100 mAh/g and can be mixed with various st anode materials during slurry processing to eliminate 1 cycle irreversible capacity loss. These particles show negligible capacity decay in dry air after 5 days and still exhibit a capacity of 1600 mAh/g in humid air (~10% RH) after 6 h.
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under non-polar solvent, which eliminates the possible ca22 pacity loss of LixSi in polar solvents. In step 2, 1fluorodecane was dissolved in anhydrous cyclohexane, followed by the addition of LixSi NPs which is reacted for two hours at room temperature. Dissolved 1-fluorodecane was directly reduced on the surface of these NPs, forming a conformal coating as shown in the TEM and SEM images (Figure 1c and Figure S2f). The selective and self-limiting reaction ensures a uniform and continuous coating on the surface. TEM image indicates each individual particle is wrapped in a uniform ~13 nm thickness coating. The dispersion of NPs is also improved after coating. By simply doubling the concentration of 1-fluorodecane in cyclohexane, the thickness becomes ~ 30 nm (Figure S3b), indicating the tunability of the coating layer thickness.
Figure 2. (a) XRD pattern of artificial-SEI coated LixSi NPs sealed in Kapton tape. (b) XPS of artificial-SEI coated LixSi NPs. Corresponding high-resolution XPS spectrum around F 1s peak region is shown in the inset. (c) High-resolution XPS spectra of C 1s. (d) Raman spectrum reveals the peak near -1 1762 cm as the stretching vibration mode of C=O. Figure 1. (a) Schematic diagram of the artificial-SEI coating formed by reducing 1-fluorodecane on the surface of LixSi NPs in cyclohexane. TEM images of LixSi NPs (b) before and (c) after coating. Artificial SEI-protected LixSi NPs were prepared via two 22 synthetic steps. Similar to our previous study, crystalline LixSi NPs were synthesized by heating a stoichiometric mixture (1:4.4) of Si NPs and Li metal foil at 200 ºC under mechanical stirring inside a tantalum crucible at 200 rpm for 3 days in a glove box (Ar-atmosphere, O2 level
