Binder free (BF) graphite electrodes were utilized to investigate the effect of electrolyte additives fluoroethylene carbonate (FEC) and vinylene carbonate (VC) on the structure of the solid electrolyte interface (SEI). The structure of the SEI has been investigated via ex-situ surface analysis including X-ray Photoelectron spectroscopy (XPS), Hard XPS (HAXPES), Infrared spectroscopy (IR) and transmission electron microscopy (TEM). The components of the SEI have been further investigated via nuclear magnetic resonance (NMR) spectroscopy of D 2 O extractions. The SEI generated on the BF-graphite anode with a standard electrolyte (1.2 M LiPF 6 in ethylene carbonate (EC) / ethyl methyl carbonate (EMC), 3/7 (v/v)) is composed primarily of lithium alkyl carbonates (LAC) and LiF. Incorporation of VC (3% wt) results in the generation of a thinner SEI composed of Li 2 CO 3 , poly(VC), LAC, and LiF. Incorporation of VC inhibits the generation of LAC and LiF. Incorporation of FEC (3% wt) also results in the generation of a thinner SEI composed of Li 2 CO 3 , poly(FEC), LAC, and LiF. The concentration of poly(FEC) is lower than the concentration of poly(VC) and the generation of LAC is inhibited in the presence of FEC. The SEI appears to be a homogeneous film for all electrolytes investigated. Lithium ion batteries have been used to power portable electronic devices for decades. Interest in lithium ion batteries has expanded to include electric vehicles (EV) due to their high energy density. [1][2][3] However, the calendar life of many lithium ion batteries is insufficient for the >10 year life expectancy of an EV.1 Thus there have been many recent investigations on methods to improve the calendar life of lithium ion batteries. Graphite is the most widely used anode material in lithium ion batteries. 4,5 During the initial charging cycles of the lithium ion battery a Solid Electrolyte Interphase (SEI) is generated on the graphite surface.6,7 The SEI acts as a passivation layer to inhibit further electrolyte reduction. 8 The SEI generated from standard ethylene carbonate based electrolytes has moderate thermal stability which leads to moderate calendar life. 9 In an effort to improve the stability of the SEI many film forming additives have been investigated. 10,11 Vinylene Carbonate (VC) and Fluoroethylene Carbonate (FEC) are among the most widely investigated electrolyte additives.12 VC has been used in many lithium ion batteries to increase first cycle efficiency, improve the high temperature stability, and improve the calendar life.13-16 FEC has largely been used in silicon-based anode materials to improve capacity retention, but has also been investigated with graphite anodes.15,17 However, there have been limited direct comparisons of the effects from FEC and VC on graphite electrodes especially related to differences in the structure of the anode SEI.The investigations of the components and morphology changes on graphite anode surfaces upon incorporation of small quantities of additives in a standard electrolyte 1....

Binder-free silicon (BF-Si) nanoparticle anodes were cycled with 1.2 M LiPF6 in ethylene carbonate (EC), fluoroethylene carbonate (FEC), or EC with 15% FEC (EC:FEC), extracted from cells and analyzed by Hard X-ray Photoelectron Spectroscopy (HAXPES). All of the electrolytes generate an SEI which is integrated with Si containing species. The EC and EC:FEC electrolytes result in the generation of LixSiOy after the first cycle while LixSiOy is only observed after five cycles for the FEC electrolyte. The SEI initially generated from the EC electrolyte is primarily composed of lithium ethylene dicarbonate (LEDC) and LiF. However, after five cycles, the composition changes, especially near the surface of silicon because of decomposition of the LEDC. The SEI generated from the EC:FEC electrolytes contains LEDC, LiF, and poly(FEC) and small changes are observed upon additional cycling. The SEI generated with the FEC electrolyte contains LiF and poly(FEC) and small changes are observed upon additional cycling. The stability of the SEI correlates with the observed capacity retention of the cells.

Tin (Sn) nanoparticle electrodes have been prepared and battery cycling performance has been investigated with 1.2 M LiPF 6 in ethylene carbonate (EC) / diethyl carbonate (DEC) electrolyte (1:1, w/w) with and without added vinylene carbonate (VC) or fluoroethylene carbonate (FEC). Incorporation of either VC or FEC improves the capacity retention of Sn nanoparticle electrodes although incorporation of VC also results in a significant increase in cell impedance. The best electrochemical performance was observed with electrolyte containing 10% of added FEC. In order to develop a better understanding of the role of the electrolyte in capacity retention and solid electrolyte interface (SEI) structure, ex-situ surface analysis has been performed on cycled electrodes with infrared (IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and Hard XPS (HAXPES). The ex-situ analysis reveals a correlation between electrochemical performance, electrolyte composition, and SEI structure. Graphite has been widely used as an anode material in lithium ion batteries. However, there is significant interest in increasing the energy density of lithium ion batteries for electric vehicle applications.1 One method of current interest for increasing the energy density of lithium ion batteries includes the use of high capacity metal alloy anode materials, such as silicon (Si) and tin (Sn).2-6 Sn has almost three times more capacity (944 mAh/g) than graphite (372 mAh/g). However, a major challenge for the use Sn as an anode material is the large volume expansion/contraction during lithium insertion and extraction. The large surface area changes result in damage to the anode solid electrolyte interface (SEI) and continuous decomposition of the electrolyte. There have been many investigations of novel fabrication methods for Sn-based anodes to mitigate the problems with the SEI due to the volume changes.2,7-9 However, few investigations have focused on developing a better understanding of SEI formation on Sn anodes. [10][11][12] In order to develop Sn anodes for lithium ion batteries, a better understanding of the structure and function of the SEI on Sn is required.In this investigation, Sn nanoparticle electrodes were prepared and tested with different electrolytes. A standard electrolyte composed of 1.2 M LiPF 6 in EC/DEC with and without 5 or 10% of the SEI film forming additives FEC or VC has been investigated for optimization of an electrolyte formulation for Sn electrodes. 13 The cells have been analyzed via electrochemical cycling and electrochemical impedance spectroscopy. In order to develop a better understanding of the role of the electrolyte in SEI formation and stability, the Sn nanoparticle electrodes were extracted from cells and ex-situ surface analysis with infrared with attenuated total reflectance (IR-ATR), X-ray photoelectron spectroscopy (XPS) and Hard XPS (HAXPES) was conducted. While IR-ATR and XPS are frequently utilized analytical techniques for the investigation of the SEI, 14 HAXPES has been less utilized due to...

A direct comparison of the cathode-electrolyte interface (CEI) generated on high-voltage LiNiMnO cathodes with three different lithium borate electrolyte additives, lithium bis(oxalato)borate (LiBOB), lithium 4-pyridyl trimethyl borate (LPTB), and lithium catechol dimethyl borate (LiCDMB), has been conducted. The lithium borate electrolyte additives have been previously reported to improve the capacity retention and efficiency of graphite/LiNiMnO cells due to the formation of passivating CEI. Linear sweep voltammetry (LSV) suggests that incorporation of the lithium borates into 1.2 M LiPF in EC/EMC (3/7) electrolyte results in borate oxidation on the cathode surface at high potential. The reaction of the borates on the cathode surface leads to an increase in impedance as determined by electrochemical impedance spectroscopy (EIS), consistent with the formation of a cathode surface film. Ex-situ surface analysis of the electrode via a combination of SEM, TEM, IR-ATR, XPS, and high energy XPS (HAXPES) suggests that oxidation of all borate additives results in deposition of a passivation layer on the surface of LiNiMnO which inhibits transition metal ion dissolution from the cathode. The passivation layer thickness increases as a function of additive structure LiCDMB > LPTB > LiBOB. The results suggest that the CEI thickness can be controlled by the structure and reactivity of the electrolyte additive.