A fundamental study of interfacial phenomena on a Si(100) single crystal electrode in organic carbonate-based electrolytes was carried out. The SEI formation on the Si(100) single crystal electrode was investigated as a function of the electrolyte composition, electrode potential and Li x Si lithiation degree. Fourier transform infrared spectroscopy (FTIR) and X-ray photon spectroscopy (XPS) studies of the SEI layer during early stages of SEI formation indicate a strong dependence of the SEI composition on the electrolyte composition. However, the influence of the electrolyte composition becomes negligible at low potentials, when lithium alloys with Si and forms amorphous Li Currently, graphite is the most common negative electrode material in commercial lithium ion batteries for portable electronics. [1][2][3] However, lithium ion batteries for large-scale transportation applications require new inexpensive electrode materials with higher specific capacity. For decades, silicon has been considered as a promising negative electrode material, mainly because of its high specific capacity of ca. 3500 mAh/g 4 and its abundance in the earth crust. [4][5][6] The key problems that prevent its widespread use are large, up to ∼300 % volumetric changes during lithiation/delithiation processes and interfacial instability of lithium silicides in organic solvent based electrolytes. 6-12The resultant poor electrochemical cycling performance and large irreversible capacities during formation and operation of the Si negative electrode contribute to rapid degradation and failure of the battery. 6,7,13 The solid electrolyte interphase (SEI) layer, which forms at the electrode/electrolyte interface during the initial charge/discharge cycles, is the key component that determines the long-term stability and cycling behavior of carbonaceous and intermetallic negative Li-ion battery electrodes.14-16 Electrolyte reduction and SEI layer formation on a Si electrode usually take place at potentials below 1.8 V vs. Li/Li + and accompany the formation of Li-Si phases, the so-called "Si-Li alloying" process at E <0.4 V vs. Li/Li + . 17 The exact mechanism of the SEI formation processes on Si, the SEI composition and the effect on the Si electrode electrochemical cycling performance is not well understood. 18 On the other hand, SEI-forming electrolyte additives such as vinylene carbonate (VC) and fluoroethylene carbonate (FEC) are known to alter the composition and properties of the SEI on Si and improve the electrode electrochemical performance. [19][20][21] Interestingly, recent model studies on Sn single crystal electrodes in organic carbonate electrolytes revealed a strong correlation between the crystal surface orientation and the SEI composition.22,23 A similar study of the composition of the SEI on a silicon monocrystal electrode showed strong effects of different SEI formation protocols, presence/absence of intrinsic SiO 2 , electrolyte composition and impurities e.g., HF. [24][25][26] Also graphite exhibits different SEI layer compos...

Subsequent to our previous studies on the SEI formation mechanism on the single crystal silicon (100) surface, here we report on complementary studies of the SEI formation on Si surfaces with the crystal orientations ( 111) and ( 110). The differences in electrochemical behavior of the different crystal orientations are discussed -especially with regard to the effect of the SEI forming electrolyte additives fluoroethylene carbonate (FEC) and vinylene carbonate (VC) added to ethylene carbonate (EC)/diethyl carbonate (DEC) based electrolytes. Fourier transform infrared spectroscopy (FTIR) of the SEI during early stages of SEI formation and physico-chemical investigations (wetting behavior) indicate a strong dependence of the chemical composition of the SEI on the surface orientation and the electrolyte composition during the early stages of lithiation of Si. However, at a higher lithiation degree less difference in the chemical composition of the SEI can be observed. These findings are in agreement with those made for the SEI formation on the Si(100) surface.

We report on overcharge protection characteristics of N-ethyl-2-pyrrolidone (NEP) as electrolyte additive. The electrochemical stability of this organic compound was investigated on a platinum working electrode using linear sweep voltammetry. We investigated the overcharge protection effect on a lithium iron phosphate (LFP), a lithium nickel manganese cobalt oxide (NMC) and on a LiMn 2 O 4 (LMO) cathode. Via electrochemical quartz crystal microbalance (EQCMB) measurements the effects of NEP during overcharge were studied and revealed that NEP acts not like a redox shuttle, but is forming a protecting polymeric film. The oxidation products on the cathode were studied by analyzing the surface of the lithium iron phosphate (LFP) and LiMn 2 O 4 (LMO) cathode using XPS spectroscopy.

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