Abstract
The electrolyte additives vinylene carbonate (VC) and fluoroethylene carbonate (FEC) are well known for increasing the lifetime of a Li-ion battery cell by supporting the formation of an effective solid electrolyte interphase (SEI) at the anode. In this study combined simultaneous electrochemical impedance spectroscopy (EIS) and operando electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D) are employed together with in situ gas analysis (OEMS) to study the influence of VC and FEC on the passivation process and the interphase properties at carbon-based anodes. In small quantities both additives reduce the initial interphase mass loading by 30–50%, but only VC also effectively prevents continuous side reactions and improves anode passivation significantly. VC and FEC are both reduced at potentials above 1 V vs. Li+/Li in the first cycle and change the SEI composition which causes an increase of the SEI shear storage modulus by over one order of magnitude in both cases. As a consequence, the ion diffusion coefficient and conductivity in the interphase is also significantly affected. While small quantities of VC in the initial electrolyte increase the SEI conductivity, FEC decomposition products hinder charge transport through the SEI and thus increase overall anode impedance significantly.
Highlights
The electrolyte additives vinylene carbonate (VC) and fluoroethylene carbonate (FEC) are well known for increasing the lifetime of a Li-ion battery cell by supporting the formation of an effective solid electrolyte interphase (SEI) at the anode
The decomposition mechanism of EC is still under debate: While many studies show that the reduction of EC results in the formation of lithium ethylene dicarbonate (LEDC), recent results by Wang et al suggest that lithium ethylene mono-carbonate (LEMC) is instead the main SEI forming decomposition product of this process [2
Small amounts of VC or FEC added to the electrolyte significantly alter the interphase formation process at the C anode in a Li-ion cell
Summary
Carbon (C) based QCM-sensors are prepared by coating commercial 5 MHz Au type sensors (Q-Sense, Biolin Scientific AB, Sweden) with first a 50 nm thick Cu adhesion film and a 50 nm thick C active material layer by sputter deposition at room temperature. LFP counter electrodes (CEs) are prepared by coating a slurry containing 80 wt% LiFePO4 (LFP, Clariant AG, Switzerland), 10 wt% Super C65 carbon black (SC65, Imerys, Switzerland), and 10 wt% polyvinylidene fluoride (PVdF, Kynar HSV 900, France) dispersed in NMP (N-Meth yl-2-pyrrolidone, Sigma-Aldrich, Switzerland) on an Al current collector foil with 500 μm wet thickness. Porous C based working electrodes (WE) are prepared by coating a Cu current collector mesh (Dexmet Corporation, USA) with a mixture of 80 wt% SC65 and 20 wt% PVdF dispersed in NMP. In order to facilitate comparable results in research cells it is advantageous to use not the same electrolyte concentrations but rather similar ratios of overall additive amount to electrode surface area (flooding factor) as discussed in previous publications [12,16,17, 21]. The electrolyte additive concentrations in this work are adjusted (0.1 wt% for EIS/EQCM-D and 0.4 wt% for OEMS) and corre spond to ~3.5 wt% additive used in a typical commercial electrolyte according to Pritzl et al [12] and Burns et al [11]
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