Effects of Fluoroethylene Carbonate (FEC) on Anode and Cathode Interfaces at Elevated Temperatures

Abstract

The use of electrolyte additives is one of the most effective and economical ways to improve battery performance by stabilizing the electrode/electrolyte interface. In this work, we identified that fluoroethylene carbonate (FEC), which is one of the important electrolyte additives, had different impacts on anode and cathode, by investigating a graphite anode and a LiMn 2 O 4 cathode through electrochemical analyses at room and elevated temperatures. In the anode side, the solid electrolyte interphase (SEI) layer derived from FEC exhibited a lower interfacial resistance and excellent thermal stability, showing excellent rate capability and improved cycle retention of cells. In contrast, poor cycling retention and a rapid increase in the interfacial resistance of the cathode were observed at elevated temperature. The poorer performance of the cathode in the FEC-containing cell at elevated temperature was attributed to the formation of a thicker surface layer and to increased Mn dissolution catalyzed by HF, which resulted from FEC dehydrofluorination initiated or accelerated by elevated temperature. Accordingly, it is suggested that the amount of FEC in a full cell must be optimized to minimize the adverse effects of FEC on cathode.It is well known that lithium-ion (Li-ion) batteries experience significant capacity fade during cycling or storage at elevated temperatures. However, the mechanisms responsible for the capacity fade at elevated temperatures are poorly understood because the capacity fade is caused by several interdependent factors. On anode side, most previous studies have identified the degradation of the solid electrolyte interphase (SEI) layer as the factor primarily responsible for the capacity loss seen at elevated temperatures. 1-4 Temperature-induced reactions, such as SEI decomposition, redox reaction, and electrolyte decomposition, cause changes in the morphology and composition of the SEI layer. 3 Deposition of transition metal ions, which are dissolved from the cathode active materials at elevated temperatures, onto the anode/electrolyte interface also contributes significantly to the degradation of the SEI layer. 5-7 Degradation of the SEI layer results in SEI reformation and growth, during which cyclable lithium ions are additionally consumed due to further electrolyte reduction reactions. Thus, the degraded SEI layer cannot sustain its original properties, which affects the degradation of anode performance. 1,3,7 For instance, an increase in the level of inorganic components present in the SEI layer can lower the ionic conductivity of the SEI layer, which hinders lithium ion transport into/from the anode. 4 While degradation of the SEI layer has been considered the main contributor to the fade in capacity at the anode side, several factors have been proposed as reasons for the capacity fade that comes from the cathode side. In particular, LiMn 2 O 4 shows the most severity in terms of (i) the dissolution of cathode materials due to the disproportionation reaction and hydrofluoric acid (HF) ...

Simple Green Synthesis and Electrochemical Performance of a New Fluorinated Carbonate as Additive for Lithium‐Ion Batteries

“…From the results, it is evident that FEC participates on the SEI formation as expected (the cell can be cycled), whereas TFEC and 1 do not show any stabilization. The SEI formation properties of FEC on graphite are well-known from literature [9,10,15,18,[21][22] and are confirmed by the present study. In contrast, according to the present tests TFEC as additive plays no role in the SEI formation.…”

“…[18] Further studies show a general shift of the first reduction peak in the presence of FEC, towards higher potential, being in the range 0.9-1.2 V vs. Li/Li + , even when altering the electrolyte composition and anode materials. [15,[19][20][21][22] Thus, our results are in line with the literature data. TFEC containing half-cells do not show any reactivity towards reduction except the pronounced peak at 0.55 V originating from the base electrolyte components.…”

“…Linear sweep voltammetric measurements with graphite as working electrode (Li as counter and reference electrode) and 1 M Li[PF 6 ] in EC/DEC/PC (30 : 65 : 5 by weight) showed a shift from 0.6 V (blank electrolyte) to 1.0 V (blank electrolyte+2 wt‐% FEC) . Further studies show a general shift of the first reduction peak in the presence of FEC, towards higher potential, being in the range 0.9–1.2 V vs. Li/Li + , even when altering the electrolyte composition and anode materials ,. Thus, our results are in line with the literature data.…”

Challenges and Recent Progress in the Development of Si Anodes for Lithium‐Ion Battery

“…[62] However, much attention should be paid to optimize amount of FEC because residual FEC results in gas evolution and HF formation, which causes significant dissolution of cathode materials in full cell, especially at elevated temperatures. [63,64] We summarize the initial Coulombic efficiency in Figure 6 by comparing the three main improve methods with original nanostructured silicon anodes. The enhancement of initial Coulombic efficiency is quite obvious.…”

Investigating the Solid Electrolyte Interphase Formed by Additive Reduction Using Physics-Based Modeling

“…16,17 However, the heterogeneity and difficult-to-measure physical properties of porous electrodes can lead to incorrect estimates of physical parameters. In addition to electrode complexity, experimental protocol can cause the onset of unintended capacity fade mechanisms during experimental testing.…”