Effect of Vinylene Carbonate and Fluoroethylene Carbonate on SEI Formation on Graphitic Anodes in Li-Ion Batteries

Abstract

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....

Analysis of Vinylene Carbonate (VC) as Additive in Graphite/LiNi0.5Mn1.5O4Cells

“…Figure 1 shows the current profile (a) and the gas evolution (b) of 12 + . While the current signal includes processes like capacitive currents related to the electrode surface or PF 6 − intercalation into the graphitic domains of the conductive carbon, which is reported to start around 4.6 V vs. Li/Li + , 25,26 we believe that the evolution of gaseous electrolyte oxidation products is a more meaningful indicator for the onset of electrolyte oxidation. As expected, Figure 1b shows that the oxidative CO 2 -release of VC starts at significantly lower potentials (∼4.3 V vs. Li/Li + ) compared to EC (∼4.8 V vs. Li/Li + ).…”

“…[14][15][16][17] In order to investigate the onset potential for VC oxidation and its products, we performed on-line electrochemical mass spectrometry on electrolytes based on only VC or EC mixed with 1 M LiPF 6 . For these experiments, we used carbon black electrodes made from isotopically labelled 13 Ccarbon, so that we can track the gas evolution from the unlabeled 12 C-electrolyte by monitoring the corresponding 12 C-related signals of CO 2 and CO. 22 As Jung et al 24 recently showed that the onset and extent of electrolyte oxidation on LNMO and carbon black is identical, it is safe to transfer the results obtained from the 13 C-carbon model electrodes to real LNMO cathodes later on.…”

“…The overall SEI will be thicker than ∼0.4 nm as also EC and the PF 6 − anion can be reduced after the initial cycles, however a sufficient fraction of the SEI consists of poly(VC) leading to enhanced stability.…”

“…As shown by Reaction 2a in Scheme 1, the oxidation of VC will release cations into the solution (presumably the cation radicals proposed in Reaction 2a) which will result in one or several of the following processes: i) electroneutrality in the electrolyte requires that lithium ions from the solution must intercalate into the graphite anode (under the reasonable assumption that no intercalation of the radical cations into graphite and/or the PF 6 − anions into LNMO can occur; this is a reasonable assumption, since the amount of C65 is small and as it is not fully graphitized), which would lead to a depletion of lithium ions in the electrolyte; ii) if the released radical cations stabilize by the release of a proton (as evidenced in our previous study 21 ), the proton concentration in the electrolyte would increase (simultaneously decreasing the lithium ion concentration), unless proton intercalation into graphite (during charge) and/or LNMO (during discharge) can occur to a significant degree; iii) VC oxidation and the formation of protons could lead to enhanced dissolution of transition metal ions (as this process is known to correlate with the electrolyte oxidation potential), 37 which -together with oxidation products like HF themselves -could damage the SEI and lead to additional irreversible lithium loss at the anode; and, iv) VC oxidation could lead to impedance growth on anode and/or cathode due to reactions involving VC oxidation products. Regarding the latter, we will see in the following that while the observed impedance growth is substantial, it seems too low to explain the dramatic observed capacity fading.…”

“…The first part of our investigation aims to understand the drastic decrease in cell performance when large amounts of VC are added to graphite/LNMO cells (i.e., at very high ratios of VC mass to graphite surface area). To this end, we will examine the anodic stability of VC via on-line electrochemical mass spectrometry (OEMS) using carbon black model electrodes and a VC-only electrolyte with 1 M LiPF 6 . As a next step, we conduct impedance measurements in graphite/LNMO cells using a micro-reference electrode, 13 quantifying anode and cathode impedance after formation in electrolytes with different amounts of VC (0.09, 0.17, 0.52, and 2 wt%).…”