A solid electrolyte interphase (SEI) is generated on the anode of lithium-ion batteries during the first few charging cycles. The SEI provides a passivation layer on the anode surface, which inhibits further electrolyte decomposition and affords the long calendar life required for many applications. However, the SEI remains poorly understood. Recent investigations of the structure of the initial SEI, along with changes which occur to the SEI upon aging, have been conducted. The investigations provide significant new insight into the structure and evolution of the anode SEI. The initial reduction products of ethylene carbonate (EC) are lithium ethylene dicarbonate (LEDC) and ethylene. However, the instability of LEDC generates an intricate mixture of compounds, which greatly complicates the composition of the SEI. Mechanisms for the generation of the complicated mixture of products are presented along with the differences in the SEI structure in the presence of electrolyte additives.

The solid electrolyte interphase (SEI) acts as a protection layer on the surface the anodes of lithium ion batteries to prevent further electrolyte decomposition. Understanding the fundamental properties of the SEI is essential to the development of high capacity silicon anodes. However, the detailed mechanism of the generation of the evolution of the SEI on the silicon anodes is not fully understood. This manuscript reviews our recent investigations of the SEI on silicon anodes. We have studied the fundamental formation mechanism of the SEI on silicon anodes, along with the evolution which occurs to the SEI upon cycling.

A novel calculation model is devised to quantitatively assess two irreversible capacities evolved in Si negative electrodes: electrolyte decomposition and Li trapping. In this model, the capacity of the electrode reaction (Li-Si alloy formation, Q n alloy ), which is the only implicit value on the galvanostatic charge/discharge voltage profiles, is calculated with the data obtained from GITT (galvanostatic intermittent titration technique) experiment. When the calculation model is applied to two Si electrodes of different particle sizes, the particle size is found to significantly affect the nature of the irreversible reactions. In the bulk-sized Si electrode, Li trapping is dominant over electrolyte decomposition. This feature must be due to an electrical contact loss that is caused by crack formation, which is more vulnerable to bulk-sized Si particles. The hump behavior in the Coulombic efficiency profiles is also explained by the Li trapping. In the nano-sized Si electrode, electrolyte decomposition is the major irreversible reaction because of its larger surface area. Because of a stronger endurance against mechanical stress, crack formation and subsequently Li trapping are less severe than that of the bulk-sized one.Silicon has emerged as an alternative material to conventional carbonaceous materials for negative electrodes because of its high theoretical capacity (Li 15 Si 4 : 3579 mA h g −1 at room temperature) and working potential (<0.5 V vs. Li/Li + ) which is close to that of Li metal. 1-4 However, the use of Si in practical lithium-ion batteries (LIBs) is still not practical because of low Coulombic efficiency and poor cycle performance. The low Coulombic efficiency is because of irreversible consumption of Li + ions and the equivalent amount of electrons during charge/discharge. Here, the Li + ions/electrons are supplied from the positive electrode. Hence, if irreversible consumption continues to proceed, Li + ions/electrons are gradually exhausted in the positive electrode, which appears as a steady decrease in cell capacity (that is, poor cycle performance). 5 Two major routes for the irreversible consumption of Li + ions/electrons are assumed: reductive electrolyte decomposition and Li trapping, both of which are in turn deeply associated with severe volume change upon charge/discharge of Si electrodes. 6-9 Namely, the volume expansion of Si upon charging and contraction during discharge causes crack formation within the Si particles. The new Si surface is then exposed to the electrolyte solution, and subsequently, the electrolyte is reductively decomposed by taking Li + ions/electrons supplied from the positive electrode. [10][11][12] This electrolyte decomposition reaction is irreversible, such that the Li + ions/electrons cannot be recovered. Li trapping is also caused by a volume change in the Si particles. That is, crack formation as a result of volume change leads to electrically isolated Si particles, or poor contact between the Si particles themselves or with other conductive carbon parti...

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.

The beneficial role of lithium bis­(trimethylsilyl) phosphate (LiTMSP), which may act as a novel bifunctional additive for high-voltage LiNi1.5Mn0.5O4 (LNMO)/graphite cells, has been investigated. LiTMSP is synthesized by heating tris­(trimethylsilyl) phosphate with lithium tert-butoxide. The cycle performance of LNMO/graphite cells at 45 °C significantly improved upon incorporation of LiTMSP (0.5 wt %). Nuclear magnetic resonance analysis suggests that the trimethylsilyl (TMS) group in LiTMSP can react with hydrogen fluoride (HF), which is generated through the hydrolysis of lithium hexafluorophosphate (LiPF6) by residual water in an electrolyte solution or water generated via oxidative electrolyte decomposition reactions to form TMS fluoride. Inhibition of HF leads to a decrease in the concentration of transition-metal ion-dissolution (Ni and Mn) from the LNMO electrode, as determined by inductively coupled plasma mass spectrometry. In addition, the generation of the superior passivating surface film derived by LiTMSP on the graphite electrode, suppressing further electrolyte reductive decomposition as well as deterioration/reformation caused by migrated transition metal ions, is supported by a combination of chronoamperometry, X-ray photoelectron spectroscopy, and field-emission scanning electron microscopy. Furthermore, a LiTMSP-derived surface film has better lithium ion conductivity with a decrease in resistance of the graphite electrode, as confirmed by electrochemical impedance spectroscopy, leading to improvement in the rate performance of LNMO/graphite cells. The HF-scavenging and film-forming effects of LiTMPS are responsible for the less polarization of LNMO/graphite cells enabling improved cycle performance at 45 °C.