The Effect of Fluoroethylene Carbonate as an Additive on the Solid Electrolyte Interphase on Silicon Lithium-Ion Electrodes

Kim

et al. 2016

This work demonstrates that the mechanical damage of surface passivation films plays an underlying role in the failure of nano-sized Si electrodes in lithium-ion batteries. The surface film derived from the standard electrolyte (1.3 M LiPF 6 dissolved in ethylene carbonate/diethyl carbonate) during the first lithiation step is damaged by the mechanical stress caused by the volume contraction of Si particles during the subsequent de-lithiation period. The electrolyte decomposes on the newly exposed Si surface and film deposition occurs, which is then mechanically damaged again owing to volume change of the Si particles. Such film deposition/damage cycles are repeated until the mechanical stress becomes insignificant as a result of capacity decay. Continued electrolyte decomposition, which prevails in the early cycling period, produces electronically insulating films located between Si particles, which cause Li trapping within the Si matrix. Li trapping is found to be responsible for the rapid decrease in capacity and Coulombic efficiency in the intermediate period of cycling. When fluoroethylene carbonate (FEC) is added to the electrolyte, a surface film that is robust against mechanical stress is produced. As a result, the FEC-derived surface film maintains its passivating ability and suppresses the irreversible reactions, resulting in a better cycling performance.

“…A relatively stable passivating ability of the surface film has been assumed. 21,22,28,29 This assumption can be validated from the results shown in Figs. 4d and 4e.…”

Section: Resultsmentioning

confidence: 55%

Mechanical Damage of Surface Films and Failure of Nano-Sized Silicon Electrodes in Lithium Ion Batteries

Kim

et al. 2016

“…22,[26][27][28]30,31,36,[46][47][48][49] A mechanism which would be consistent with this β-value will be presented below. Another important aspect of the FEC consumption analysis in Si-Li vs. Li-Li cells is that the FEC consumption per electrode during charge/discharge cycling only depends on the overall exchanged coulombs.…”

Section: Cellmentioning

confidence: 67%

“…In contrast, cells containing ≥5% wt FEC show substantially improved coulombic efficiency and a fast decrease in specific capacity until cycle 15, which is likely due to the formation of an initially more resistive SEI with FEC, as was suggested before. 36 After this initially fast capacity loss, the capacity fading in the presence of FEC is much less than in electrolytes with <5% wt FEC. However, this trend is interrupted when a rapid capacity drop is observed at roughly cycle 70 for 5% wt FEC and at roughly cycle 225 for 10% wt FEC.…”

Section: Resultsmentioning

confidence: 99%

“…In particular, LiF was observed or proposed independently by several authors, 22,[26][27][28]30,36,[46][47][48][49] whereby the study by Schroder et al indicates that the LiF content of the SEI is higher in the presence of FEC. 36 Formation of LiF also is consistent with our observations since we did not see any soluble fluorine containing decomposition products in the electrolyte by 19 F-NMR, even though we cannot exclude the formation of other fluoride containing solids. In recent publications by Balbuena and co-workers, applying ab initio modeling, the 1-electron reduction of FEC was suggested to yield an FEC-radical anion, decomposing into CO 2 , F − , and a vinoxylradical.…”

Section: Cellmentioning

confidence: 94%

See 1 more Smart Citation

Consumption of Fluoroethylene Carbonate (FEC) on Si-C Composite Electrodes for Li-Ion Batteries

Jung

Metzger

Haering

et al. 2016

262

311

The electrolyte additive fluoroethylene carbonate (FEC) is known to significantly improve the lifetime of Li-ion batteries with silicon anodes. In this work, we show that FEC can indeed improve the lifetime of silicon-carbon composite anodes but is continuously consumed during electrochemical cycling. By the use of 19 F-NMR spectroscopy and charge/discharge cycling we demonstrate that FEC is only capable to stabilize the cell performance as long as FEC is still remaining in the cell. Its total consumption causes a significant increase of the cell polarization leading to a rapid capacity drop. We show with On-line Electrochemical Mass Spectrometry (OEMS) that the presence of FEC in the electrolyte prohibits the reduction of other electrolyte components almost entirely. Consequently, the cumulative irreversible capacity until the rapid capacity drop correlates linearly with the specific amount of FEC (in units of μmol FEC /mg electrode ) in the cell. The latter quantity therefore determines the lifetime of silicon anodes rather than the concentration of FEC in the electrolyte. By correlating the cumulative irreversible capacity and the specific amount of FEC in the cell, we present an easy tool to predict how much cumulative irreversible capacity can be tolerated until all FEC will be consumed in either half-cells or full-cells. We further demonstrate that four electrons are consumed for the reduction of one FEC molecule and that one carbon dioxide molecule is released for every FEC molecule that is reduced. Using all information from this study and combining it with previous reports in literature, a new reductive decomposition mechanism for FEC is proposed yielding CO 2 , LiF, In the emerging market of electric vehicles (EVs), the development of batteries with higher energy density and improved cycle-life is essential.1 However, their penetration of the mass market significantly depends on cost and the available driving range.2 The US Advanced Battery Consortium (USABC) defined the target value of 235 Wh/kg (at a C/3 rate) on a battery level until 2020.3 As outlined in the recent review by Andre et al., 4 reaching this goal requires an increase of the energy density of today's batteries by a factor of roughly 2 to 2.5 and can only be achieved by the development and integration of novel anode and cathode active materials. A critical element to reach this goal is the implementation of anode active materials with much higher specific capacity than currently used graphite anodes (372 mAh/g 1,5,6 ), with silicon being considered as the most likely next generation anode material due to its high natural abundance and very high theoretical specific capacity of roughly 3600 mAh/g (corresponding to the Li 15 Si 4 phase 7 ). The alloying of silicon with lithium is accompanied by large structural changes, resulting in a volume increase by 310% upon full lithiation.5,7-11 These huge volumetric changes upon lithium insertion and extraction are responsible for the generally shorter cycle-life of silicon electrode materials comp...

“…[3][4][5][6][7][8] Advanced imaging and characterizations such as transmission electron microscopy (TEM), nuclear magnetic resonance (NMR), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), timeof-flight secondary ion mass spectrometry (TOF-SIMS) and in-situ dilatometry have helped scientists understand the underlying physical and mechanical interactions, as well as the chemical reactions in silicon electrodes during charge and discharge. [9][10][11][12][13] Though, eventually, it is the capacities that can be obtained from charge and discharge in each cycle, and how they can be sustained for a large number of cycles, that matters the most for battery applications. In this respect, there are hardly any studies that are able to tell how much of the measured capacity comes from different processes that are occurring in an electrode.…”

mentioning

confidence: 99%

Insights from Studying the Origins of Reversible and Irreversible Capacities on Silicon Electrodes

2016

Silicon electrodes can give high capacity as anodes for lithium-ion batteries. However, there has not been much work quantifying the different contributions to the reversible and irreversible capacities. Here, we report the use of an electrochemical approachdepth of discharge test -to separate the charge-discharge capacities of crystalline silicon electrodes into four contributions: (1) SEI formation, (2) lithium accommodation in carbon and binder, (3) lithiation and delithiation of the Si active material, and (4) capacity loss associated with particle cracking and isolation. We find that the intrinsic coulombic efficiency for the crystalline-to-amorphous transition of Si during initial cycle is about 90%, which is independent of particle size. SEI formation is estimated to be about 10 mAh per square meters of active material surface and scales with BET surface area. Mechanical issues and particle isolation are observed in fully discharged electrode when the amount of binder is less than 20%. Capacity limitation prolongs lifetime of Si electrode, but the overall performance is governed by the coulombic efficiency (CE) during cycle. Low CE is due to continual SEI formation with cycling, increase utilization of Si upon cycling, trapping of Li in the material and mechanical failure of the electrode. Silicon has received much attention as a next-generation anode material for lithium-ion battery (LIB) because it can alloy with Li electrochemically with a theoretical capacity of ∼3572 mAh g −1 , corresponding to the formation of Li 15 Si 4 .1,2 Typically, silicon electrodes show poor cycle performance, which is typically attributed to the large volume expansion during lithiation, pulverization of the active material, continuous solid electrolyte interphase (SEI) layer growth, etc. 3-8Advanced imaging and characterizations such as transmission electron microscopy (TEM), nuclear magnetic resonance (NMR), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), timeof-flight secondary ion mass spectrometry (TOF-SIMS) and in-situ dilatometry have helped scientists understand the underlying physical and mechanical interactions, as well as the chemical reactions in silicon electrodes during charge and discharge.9-13 Though, eventually, it is the capacities that can be obtained from charge and discharge in each cycle, and how they can be sustained for a large number of cycles, that matters the most for battery applications. In this respect, there are hardly any studies that are able to tell how much of the measured capacity comes from different processes that are occurring in an electrode. The aim of this study is therefore to establish a method to understand and quantify the contributions to the reversible and irreversible capacities of silicon anodes during the initial cycle and subsequent cycles in order to develop strategies to improve the stability of the electrodes.In this paper, we report the use of an electrochemical approachdepth of discharge test -to separate the charge-discharge capacities of crystal...