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

Abstract

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,...

Interfacially Induced Cascading Failure in Graphite‐Silicon Composite Anodes

“…The G–Si electrodes with 100 cycles reveal notable electrochemical performance differences depending on whether G–Si electrodes are alucone‐MLD‐coated or not. Infrared (IR) and XPS analyses on SEI components on the graphite and Si surfaces have been studied extensively, and the chemical components of lithium ethylene dicarbonate (LEDC), LiCO 3 , and LiF are known as main components of the SEI . These SEI components are consistently observed in our XPS analysis in Figure .…”

The Electrochemical Performance of Silicon Nanoparticles in Concentrated Electrolyte

“…[24] From the resultso f the electrode surfacea fter one cycle and 50 cycles (Figure 10), the SEI layer formed in LiFSI-(PC) 8 is mainly organic compounds derived from the reduction of PC, such as RCOLi, ROCOLi, and ROCO 2 Li/Li 2 CO 3 .W ith the increasing LiFSI concentration, the relative contento fF SI anion reduction species( LiF and sulfurbased compounds,e tc.) Based on the report by Yoon et al, [26] CIP and AGG would be preferentially reduced compared with free solventa nd SSIP,w hich might be attributed to the higher reduction potential of the CIP or AGG solvate, or lower kinetic barrier for the electron transfer to the CIP or AGG. According to previous research, [25] these compounds play an influential role in forming robust SEI layers.…”

“…In combination with the Ramanr esults (Figure 1), the ratio control of organic to inorganic components is acquired though modifying the solution structure. Based on the report by Yoon et al, [26] CIP and AGG would be preferentially reduced compared with free solventa nd SSIP,w hich might be attributed to the higher reduction potential of the CIP or AGG solvate, or lower kinetic barrier for the electron transfer to the CIP or AGG. We adopt the method of increasing lithium salt concentrationt oi ncrease the concentration of CIP or AGG, and further construct complex SEI layers.…”

Understanding the nature of the passivation layer enabling reversible calcium plating

“…The cation solvation shell in the electrolyte impacts, also, the chemical composition and properties of the SEI layer. Such a relation has been broadly studied in Li systems 35,36 and recently demonstrated by real-time mass spectroscopy. 37 The lithium ions accumulate at the surface of the negatively polarized electrode and carry their solvation shells with them.…”