Perfluoroalkyl-substituted ethylene carbonates: Novel electrolyte additives for high-voltage lithium-ion batteries
Introduction
High-capacity lithium- and manganese-rich metal oxides are gaining increased attention because of their ability to deliver high rechargeable capacities; when cycled between 2.0 and 4.7 V vs. Li, a rechargeable capacity of 270 mAh-g−1 can be routinely obtained [1]. Hence lithium-ion cells containing the Li2MnO3-stabilized LiMO2 (M = Mn, Ni, Co) positive electrodes, graphite negative electrodes and EC:EMC (3:7 by wt.) + 1.2 M LiPF6-based electrolyte (henceforth referred as Gen2 electrolyte) can be designed to meet the target cell specific capacity of 300 Wh kg−1 for transportation applications [2], [3]. However, the target battery cycle life of up to 1000 charge–discharge cycles at 80% depth of discharge (DOD) can only be achieved through new electrolyte formulations because these cells show significant performance degradation on extended cycling [4]. Extensive diagnostic studies indicate that cell impedance rise mainly arises at the positive electrode, and cell capacity fade mainly results from lithium trapping in the solid electrolyte interphase (SEI) at the negative electrode [4], [5], [6]. During electrochemical aging, both electrodes undergo a cycle of surface film formation, decomposition, dissolution, and redeposition; this process results in the continuous consumption of lithium ions, thereby reducing cell capacity and often increasing cell impedance [7], [8], [9].
Electrolyte additives are known to be an effective and economic approach to improving the stability of electrode surface films [10]. In the past two decades, many organic and inorganic compounds have been identified as effective electrolyte additives: examples include vinylene carbonate (VC) [11], [12], ethylene sulfite (ES) [13], vinyl ethylene carbonate (VEC) [14], [15], and fluoroethylene carbonate (FEC) [16]. In recent years, with the emergence of many high-voltage cathode materials, the anodic stability of common electrolytes is recognized as the main bottleneck limiting the calendar- and cycle-life of high-energy lithium-ion cells [17]. Therefore, more attention has been devoted to improving stability of the cathode–electrolyte interface [6], [18], [19], [20], [21], [22], [23], [24], [25]. As part of DOE's Advanced Battery Research (ABR) program, we have been examining ways to mitigate performance degradation of cells containing Li1.2Ni0.15Mn0.55Co0.1O2 (0.5Li2MnO3·0.5LiMn0.375Ni0.375Co0.25O2)-based positive electrodes (LMR-NMC) that are cycled at voltages beyond 4.5 V versus Li. Initial studies indicate that common electrolyte additives such as VC, VEC, and FEC are not effective at enhancing long-term cycling performance of these cells, i.e. stable electrode passivation could not be achieved with traditional SEI-forming additives. This observation underscores the need for new electrolyte additives that effectively form stable electrode passivation films in high-energy and high-voltage lithium-ion cells.
Polyfluoroalkyl (PFA) compounds are well known for their high chemical stabilities, and exhibit both hydrophobic and lipophobic behaviors. Upon dispersing in organic solvents, solvophobic PFAs tend to aggregate and form micelles in solution [26]. These types of compounds have been extensively used as fluorosurfactants, and are especially valuable as additives in stain repellents [27]. In light of these facts, we envision that compounds containing PFAs could serve as a new type of electrolyte additive, forming double-layered passivating layers that reduce both electrode surface degradation and electrolyte decomposition. In our design, the PFA additive has two components: (i) a reactive headgroup for attachment onto electrode surfaces via either reductive or oxidative decomposition, so that it becomes an integral part of the surface layer (inner layer); (ii) a polyfluoroalkyl chain that self-assembles on this inner layer as a solvophobic layer (outer layer) that is highly stable and impermeable to the electrolyte solvent. A schematic representation of this idea is shown in Scheme 1.
To explore this novel idea, we synthesized a series of PFA-substituted ethylene carbonates (PFA-EC) and studied them as electrolyte additives in our lithium-ion cells. Cell performance was characterized using a combination of galvanostatic cycling and electrochemical impedance spectroscopy (EIS) techniques, and supplemented by linear sweep voltammetry (LSV), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy data. Of the various PFA-EC compounds studied, we determined that perfluorooctyl-substituted ethylene carbonate (PFO-EC) most significantly improves the long-term cycling performance of our cells.
Section snippets
Materials and synthesis
All chemicals used in the synthesis of polyfluoroalkyl compounds were purchased from commercial suppliers and used without further purification. 4-(trifluoromethyl)-1,3-dioxolan-2-one (TFM-EC) was purchased from Synquest Laboratories, Inc. (United States); all other polyfluoroalkyl-ECs were synthesized by a two-step reaction sequence (Scheme 2). Newly synthesized compounds were characterized by 1H, 13C, and 19F NMR spectroscopy, using a 300- or 400-MHz spectrometer. All chemical shift values (δ
Cycling performance of Li1.2Ni0.15Mn0.55Co0.1O2//graphite full cells
During our cycling tests, cell to cell variations are always observed. These variations arise from various factors that include (i) small differences in active material weight between similar electrodes, (ii) small differences in the glove box environment when assembling the cells, (iii) differing amount of impurities in the electrolytes, additives, etc. However, the overall data trends observed across multiple cells are consistent. Therefore, only representative data and trends are reported
Conclusions
A new family of polyfluoroalkyl-substituted ECs is examined as electrolyte additives in full cells using Li1.2Ni0.15Mn0.55Co0.1O2-based cathodes and graphite anodes. Addition of 0.5 wt% 4-(perfluorooctyl)-1,3-dioxolan-2-one (PFO-EC) to a standardized electrolyte solution, Gen 2 (1.2 M LiPF6 in a 3:7 mixture of EC:EMC), is found to improve capacity retention and reduce impedance rise in full cells cycled between 2.2 and 4.6 V. The LSV and XPS data suggest that PFO-EC is sacrificially oxidized on
Acknowledgments
Support from the U.S. Department of Energy's Vehicle Technologies Program, specifically from Dave Howell and Peter Faguy, is gratefully acknowledged. The submitted manuscript has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. We are grateful to B. Polzin, A. Jansen, and S. Trask from the U.S. Department of Energy's (DOE) Cell
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