Insight into the Solid Electrolyte Interphase on Si Nanowires in Lithium-Ion Battery: Chemical and Morphological Modifications upon Cycling

The effect of fluoroethylene carbonate (FEC) additive has been studied in the formation of solid electrolyte interphase (SEI) over Si-based anode using in-situ DRIFTS (diffuse reflectance infrared Fourier transform spectroscopy). SEI species were observed in the first lithiation cycle at an onset potential of 1.4 V in electrolyte containing 2 wt% vinylene carbonate (VC) + 10 wt% FEC and at 1.1 V in electrolyte without FEC additive. With blended VC and FEC, high carbon containing species including poly (FEC), poly (VC), and polycarbonates were identified, while poly (VC) and polycarbonates formed in the absence of FEC. The FEC additive also led to a higher content of organic phosphorous fluorides as compared to the electrolyte containing no FEC. Electrochemical analyses indicated that the combination of 2 wt% VC and 10 wt% FEC resulted in lower impedances and improved the stability of the Si-electrode through cycling as compared to that without FEC. DRIFTS provided evidence that similar SEI species formed after the initiation in the first cycle, and this formation was recorded for five cycles. With the increasing demand for battery systems with high energy density and high power for applications such as hybrid and electric vehicles, the development of Li-ion batteries has attracted much attention in recent years.1,2 This self-supporting energy storage system 3 works based on the principle of redox reactions that can reversibly convert chemical energy into electrical energy. In Li-ion battery operation, the negative electrodes function at low potentials close to the potential of metallic lithium, where electrolytes are not stable thermodynamically. As a result, the reduction of solvents and salts of the electrolyte will result in the formation of a surface film on the electrode. Typically, this surface film is composed of organic reduction products (closer to the electrolyte) and inorganic species (closer to the electrode). 4 This film, called the solid electrolyte interphase (SEI), is known to be an important feature of graphite anodes that can allow for reversible cycling and long-term stability due to surface passivation. 5,6 Numerous replacements for the graphite anode have been investigated, with silicon 7,8 being one of the most attractive for its high theoretical specific capacity of 4200 mAhg −1 , which is more than ten times larger than that of graphite (372 mAhg −1 ). 9 However, Si undergoes large volume changes during lithiation and delithiation cycling, which cause capacity fading.10 Many studies have tried to improve the capacity retention of Si anodes by using nano-structured silicon, [11][12][13] silicon composite electrodes, 14,15 carbon coated silicon, 16 thin film silicon, [17][18][19] and new binders. [20][21][22] The formation of a stable surface layer for Si-based anodes would have an impact on achieving a long cycle life. One way to address this issue is to use a new electrolyte and/or to add a small amount (< 10 wt%) of electrolyte additives. Different types of electrolyte additives, such as...

Section: Resultssupporting

confidence: 91%

In Situ DRIFTS Analysis of Solid Electrolyte Interphase of Si-Based Anode with and without Fluoroethylene Carbonate Additive

Yohannes

Lin

2017

“…During the volume expansion non-passivated Si surfaces are expected to be exposed which are immediately covered by a SEI layer because of ongoing electrolyte decomposition. 5 Thus, similar to metallic Li electrodes the SEI layer is continuously re-formed and remains instable during each lithiation process. 6 The properties of the SEI are important for the performance of Si negative electrodes.…”

mentioning

confidence: 99%

Investigation of the Electron Transfer at Si Electrodes: Impact and Removal of the Native SiO₂Layer

Bülter

Sternad

Sardinha

et al. 2015

Silicon is considered as one of the promising alternatives to graphite as negative electrode material in lithium-ion batteries. The electron transfer at uncharged microstructured and planar Si was characterized using the feedback mode of scanning electrochemical microscopy (SECM) and 2,5-di-tert-butyl-1,4-dimethoxybenzene as redox mediator. Approach curves and images demonstrate that the electron transfer rate constants at pristine Si are relatively small due to the native SiO 2 surface layer. In addition, the electron transfer rate constants show local variations because of the heterogeneous coverage of SiO 2 . The SiO 2 layer is at least partially removed by mechanical contact and abrasion with the microelectrode probe. After SiO 2 removal by the microelectrode or by a hydrofluoric acid dip, the electron transfer rate constants increase strongly and remain heterogeneous. Moreover, the surface of the Si electrodes is at least stable over hours after SiO 2 removal. The consequences for investigating the formation of the solid electrolyte interphase (SEI) on Si are discussed. Silicon is considered as one of the promising alternatives to graphite as negative electrode material in lithium-ion batteries (LIBs). This is due to its wide abundance, low voltage and high theoretical gravimetric capacity of 3579 mAh g −1 .1 During the lithiation, the electrochemical potential of the Si electrode exceeds the stability window of the electrolyte and, consequently, the electrolyte is reductively decomposed.2 The decomposition products form a solid electrolyte interphase (SEI) covering the Si material lying beneath.3 A major challenge of Si as negative battery electrodes is the large volume change of ca. 270% 4 upon lithiation. Recurring lithiation/delithiation causes the electrode material to crack leading, in the worst case, to irreversible loss of active material. During the volume expansion non-passivated Si surfaces are expected to be exposed which are immediately covered by a SEI layer because of ongoing electrolyte decomposition. 5 Thus, similar to metallic Li electrodes the SEI layer is continuously re-formed and remains instable during each lithiation process. 6 The properties of the SEI are important for the performance of Si negative electrodes. 5,7 In order to evaluate the Si electrode performance, reliable in situ characterization of the SEI is definitely a key issue for the progress in this field. In general, scanning probe techniques are frequently applied for various aspects of battery research. 8,9 Especially atomic force microscopy (AFM) has been used to characterize in situ and ex situ the SEI at amorphous Si (a-Si) thin films, [10][11][12][13][14] Si-based thin films, 11,15 a-Si nanopillars 16 and Si nanowires. 17,18 AFM provides information about the morphology evolution and physical properties of the SEI. The present study aims at characterizing the functional properties of Si electrodes in situ by the feedback mode of scanning electrochemical microscopy (SECM), which probes the local electron transfer ra...

“…25,26 Several reports have proposed the possibility of mechanical degradation of the SEI layers without solid evidence. 14,26,27 Pereira-Nabais et al proposed crack formation on SEI films. 27 Wu and his coworkers reported the breakdown of the SEI layers into separate pieces.…”

Section: Resultsmentioning

confidence: 99%

“…14,26,27 Pereira-Nabais et al proposed crack formation on SEI films. 27 Wu and his coworkers reported the breakdown of the SEI layers into separate pieces.…”

Section: Resultsmentioning

confidence: 99%

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

Lee

Kim

Lee

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.