Failure mechanisms of nano-silicon anodes upon cycling: an electrode porosity evolution model

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: Resultsmentioning

confidence: 99%

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

Yohannes

Lin

2017

“…5,7 The contractions of the volume and surface area of Si results in damage to the SEI, which further increases the contact resis- tance and incomplete delithiation. 5,7,29 The continuous and significant decrease of the peak at 0.23 V in Fig. 3a indicates that the incomplete delithiation is the major fading mechanism of the Si electrode.…”

Section: Methodsmentioning

confidence: 99%

Lithium Salt Effects on Silicon Electrode Performance and Solid Electrolyte Interphase (SEI) Structure, Role of Solution Structure on SEI Formation

Yoon

Chapman

Seo

et al. 2017

Silicon electrodes were cycled with electrolytes containing different salts to investigate the effect of salt on the electrochemical performance and SEI structure. Comparable capacity retention were observed for the 1.2 M LiPF 6 , LiTFSI and LiClO 4 electrolytes in ethylene carbonate (EC):dimethyl carbonate (DEC), 1:1, but severe fading was observed for the 1.2 M LiBF 4 electrolyte. The differential capacity plots and EIS analysis reveals that failure of the 1.2 M LiBF 4 electrolyte is attributed to large surface resistance and increasing polarization upon cycling. However, when LiBF 4 was added as an electrolyte additive (10% LiBF 4 and 90% LiPF 6 ), the capacity retention and Coulombic efficiency were improved. The SEI was analyzed by FTIR and XPS for each electrolyte. Both spectroscopic methods suggest that the main components of the SEI are lithium ethylene dicarbonate (LEDC) and Li 2 CO 3 in the 1.2 M LiPF 6 , LiTFSI and LiClO 4 electrolytes, while an inorganic-rich SEI, composed of LiF and borates, was generated for both the Silicon negative electrodes for lithium ion batteries have attracted academic and industrial interest, since they provide ∼10 times more specific capacity (3579 mA g −1 ) than graphite (372 mA g −1 ). However, the large volumetric changes during lithiation and delithiation limits commercial application.1 The volume changes result in mechanical stress to individual Si particles and the binder which maintains physical contact between electrode components, thus degenerating the electrode laminate upon repeated lithiation/delithiation. 2,3In particular, it has been demonstrated that the electric contact loss becomes severe during delithiation when the Si particles are contracted. Thus, incomplete delithiation due to contact resistance has been reported as one of dominant failure mechanisms. [4][5][6][7] In addition to the volume contraction, the solid electrolyte interphase (SEI) has been reported to be another factor that impedes the reversibility of lithiation. 5,7 When the SEI on silicon is modified by fluoroethylene carbonate (FEC), the capacity retention, reversibility of lithiation, and suppression of electrolyte decomposition are observed. 5,7,8 Since the improvement of the SEI is critical for improving the electrochemical performance of Si electrodes, great efforts have been devoted to modify the SEI by using electrolyte additives, surface coatings, or concentrated electrolytes. [9][10][11][12][13][14][15][16] Recently, it has been reported that the SEI can be significantly modified by changing the electrolyte concentration. [16][17][18] For instance, propylene carbonate (PC)-based electrolytes do not generate a stable passivation layer on graphite at low salt concentration. However, upon dissolving high concentrations of either LiPF 6 or LiTFSI into PC a LiF rich passivation layer is generated on graphite affording electrochemical reversibility of the graphite. 16,18 The change in salt concentration has been reported to result in a change solution structure of the electrolyte. 16,...

“…Among others, the most prominent feature in Si electrodes is that the Coulombic efficiency is poor not only during the initial few cycles, but also in later cycles even under normal operating conditions. [10][11][12] The poor Coulombic efficiency over the entire cycling period is attributed to two irreversible reactions. 5,6 Li trapping occurs as a result of incomplete Li removal from lithiated Si particles that reside outside the electrically conductive network formed among Si particles, conductive carbon, and the current collector during composite electrode fabrication.…”

mentioning

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.