Analysis of the Reduction and Oxidation of Vinylene Carbonate (VC) and Fluoroethylene Carbonate (FEC) in Graphite/LiNi0.5Mn1.5O4 Cells By Electrochemical Impedance Spectroscopy

Abstract

Vinylene carbonate (VC) is one of the most effective additives to produce an effective solid electrolyte interphase (SEI) on graphite anodes, increasing battery life [1, 2]. VC is reduced on the anode, forming poly(VC), which leads to a stable electrolyte interface [3]. Furthermore, there is evidence that VC polymer films are also formed on the cathode side [1, 3 ,4]. Burns et al. [4] have shown that VC reduces the charge endpoint slippage and capacity fade in Graphite/LiCoO2 cells at elevated temperatures and correlate this fact to reduced electrolyte oxidation and transition metal dissolution at the positive electrode. When VC is used in Li-Ion cells with high-voltage cathode materials like LiNi0.5Mn1.5O4 (LNMO), the anodic decomposition of VC is reported to drastically lower performance, as was observed when 1 M LiPF6 in EC/EMC (3/7 by volume) with 1% VC is used in Graphite/LNMO cells [5]. Solchenbach et al. [6] have shown that VC is decomposed at 4.6 V vs. Li/Li+ using carbon black model electrodes and an electrolyte containing VC with 1M LiPF6. These results suggest that VC is unsuitable for high-voltage Li-Ion batteries. One strategy to overcome this issue is to determine the exact amount of VC which can be reduced at the graphite anode, so that there is no excess amount of VC in the electrolyte which would be oxidized at the cathode above 4.6 V vs. Li/Li+. In this work, we use electrochemical impedance spectroscopy (EIS) with a novel reference electrode to independently determine the impedance of anode and cathode in a full-cell. Electrolytes (1M LiPF6 in EC/EMC, 3/7 by weight) with different amounts of VC are tested in Graphite/LiNi0.5Mn1.5O4 cells. Firstly, the impedance spectra are collected at 50% state of charge (SOC) after one formation cycle at C/10. Figure 1 shows the result for electrolytes containing 0% VC and 0.17% VC. The charge transfer resistance RCT both of anode and cathode is increased when VC is added to the electrolyte. The increase of the RCT of the negative electrode is caused by the reduction of VC and formation of polymeric compounds at the surface [2]. The increase of the positive electrode’s RCT indicates that VC is oxidized at the surface of the LMNO particles at high voltages, leading to oxidation products which form a passivating layer at the cathode side. Next, cells with different amounts of VC are cycled at 40°C, and both capacity retention and impedance spectra are analyzed up to 100 cycles. Lastly, the fluorinated cyclic carbonate FEC is investigated as an additive in Graphite/LNMO cells. Even though FEC also decomposes at high potentials [6], in contrast to VC it does not lead to strongly passivating films on the cathode. This will be shown by examining anode and cathode RCT as well as cell performance of Graphite/LMNO cells with FEC, VC, and FEC+VC containing electrolytes. Figure 1: Nyquist plots of Graphite/LNMO cells after one formation cycle at 25°C with 1M LiPF6 EC/EMC (3/7 by weight) containing 0% and 0.17% VC. 7 mg Graphite/cm2 (0.95 cm2 area) and 13 mgLNMO/cm2 (0.95 cm2 area) in a Swagelok T-cell with 60 ml electrolyte and two glass fiber separators. Impedance spectra are collected at 50% SOC at 10°C from 100 kHz to 100 mHz with a perturbation of 0.5 mA.