Influence of fluoroethylene carbonate as co-solvent on the high-voltage performance of LiNi1/3Co1/3Mn1/3O2 cathode for lithium-ion batteries
Introduction
Lithium-ion batteries (LIBs) have been applied widely as the power source for portable electronic devices because of their high specific capacity and excellent cyclic performance [1], [2]. In recent years, LIBs are developing towards high energy and high power density to meet the increasing demands of their applications in hybrid electric vehicles (HEVs) and electric vehicles (EVs) [3], [4], [5]. It is well known that elevating the working voltage of the cathode is one of the most effective approaches to increase the energy density for LIBs [6]. Currently, ternary composition layered LiNi1/3Co1/3Mn1/3O2 is regarded as a promising cathode material for the commercial power LIBs by virtue of its appropriate operate voltage and other merits, such as low cost, high capacity and safety features [7], [8]. It was reported that LiNi1/3Co1/3Mn1/3O2 cathode delivered a higher discharge capacity (over 160 mAh g−1) after improving the cut-off voltage to 4.5 V (vs. Li/Li+) [9], [10], [11]. Unfortunately, the LiNi1/3Co1/3Mn1/3O2 cathode suffered serious capacity decay under high voltage, which is mainly attributed to the violent decomposition of electrolyte and severe structural destruction of cathode in the conventional carbonate-based electrolyte during long cycling process [12], [13].
In order to resolve these problems, various methods have been proposed to enhance the electrochemical performance of high voltage cathode materials, including surface coating of cathode, adding functional additives into electrolytes and novel solvent substitution in the electrolytes [14], [15], [16], [17]. Despite the fact that the former strategies of surface coating [18], [19], [20] and functional additives incorporation [21], [22], [23], [24] have been widely reported to improve the electrochemical properties of LiNi1/3Co1/3Mn1/3O2 cathode under high voltage, the electrolyte decomposition issue could not be settled thoroughly based on conventional carbonate-based electrolyte system. On the other hand, solvent substitution, such as sulfones, nitriles, ionic liquids, phosphazenes and fluorinated carbonates, have been identified as the optimal substitutes to carbonate-based electrolytes due to their intrinsic high oxidation potential (>5 V vs. Li/Li+) and good safety [25], [26], [27], [28], [29]. Among these solvents, fluorinated carbonates have gained more attention for their superior physical properties and higher oxidation potential than carbonates. At present, fluoroethylene carbonate (FEC) has been served as co-solvent in the electrolyte solution for two types of cathodes (LiCoPO4, LiNi0.5Mn1.5O4), which demonstrated a variety of benefits in terms of improving the cycling performance, enhancing the voltage stability at elevated temperature and suppressing the structure degradation of the high-voltage cathode materials [30], [31], [32]. Best to our knowledge, the impacts of FEC-based electrolyte on high-voltage electrochemical behavior of commercial LiNi1/3Co1/3Mn1/3O2 has seldomly been reported.
In this study, the effects of FEC as co-solvent on the high-voltage electrochemical performance of LiNi1/3Co1/3Mn1/3O2 cathode have been investigated under the cut-off charge voltage of 4.6 V. The role of FEC in preventing the electrolyte decomposition and improving the high-voltage cycling stability of LiNi1/3Co1/3Mn1/3O2 cathodes was systematically investigated by battery performance testing, electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD).
Section snippets
Preparation of electrolytes and electrode
The prepared electrolytes were 1 M LiPF6 dissolved in ethylene carbonate (EC)/dimethyl carbonate (DMC) (1/1, vol.%), FEC/DMC (1/1, vol.%) and FEC/DMC (1/4, vol.%) solution, respectively. The electrolyte solvents were Li-battery grade purchased from Guangzhou Tinci Materials Technology Co., Ltd. and purified by drying with 4A molecular sieves before using. The electrolyte solutions were prepared in an argon-filled glove box with the content of water and oxygen less than 5 ppm.
The LiNi1/3Co1/3Mn1/3O
Electrochemical stability of electrolyte
The oxidative decomposition behaviors of the electrolyte with and without FEC were investigated by linear sweep voltammetry (LSV) with a Pt electrode over open circuit voltage (OCV) to 6.0 V. As shown in Fig. 1, for the electrolyte containing EC, the oxidation current appears at 4.2 V (vs. Li/Li+) and then a distinct increase of the oxidation current was observed around 4.7 V (vs. Li/Li+), which corresponds to electrolyte decomposition and demonstrates that the oxidation stability of the
Conclusion
LiNi1/3Co1/3Mn1/3O2 electrode exhibits a better electrochemical performance in terms of rate capacity and cycling stability by employing FEC as co-solvent compared with the EC-containing electrolyte solution. After 100 cycles at 0.2C rate, the capacity retention ratio of the LiNi1/3Co1/3Mn1/3O2 with FEC/DMC (1/4, vol.%) solution reaches 82.7%, while it is only 41.3% for the EC-containing electrolyte. The prominent enhancement in the electrochemical performance of LiNi1/3Co1/3Mn1/3O2 is mainly
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China (Grant No. 21373072 and No. 51202047).
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Cited by (51)
Fluoroethylene carbonate as co-solvent for Li(Ni<inf>0.8</inf>Mn<inf>0.1</inf>Co<inf>0.1</inf>)O<inf>2</inf> lithium-ion cells with enhanced high-voltage and safety performance
2022, Journal of Power SourcesCitation Excerpt :Therefore, it has become a challenging task to find a solvent to replace traditional carbonates to propose lithium-ion cells with advanced high-voltage and high-safety characteristics. To date, fluorinated solvents have been paid widespread focus owing to their superior properties such as high flash point, high boiling point, high oxidation potential, and so on [8–13]. Im et al. [8] proposed a fluorinated electrolyte (1 M LiPF6 in fluoroethylene carbonate (FEC) and methyl (2,2,2-trifluoroethyl) carbonate (FEMC), 1:9 v/v) for high-voltage LiNi0.5Mn0.3Co0.2O2/graphite cells.
Machine learning technique-based data-driven model of exploring effects of electrolyte additives on LiNi<inf>0.6</inf>Mn<inf>0.2</inf>Co<inf>0.2</inf>O<inf>2</inf>/graphite cell
2021, Journal of Energy StorageCitation Excerpt :Moreover, FEC can be utilized as the high voltage solvent to improve the battery's performance. For instance, Wang et al. reported that the capacities retention of LiNi1/3Co1/3Mn1/3O2 in FEC: DMC (1: 4, v/v) was 82.7% after 100 cycles in a potential window of 3.0 − 4.6 V at 0.2 C, while the non-additive electrolyte EC: DMC (1/4, v/v) exhibits capacity retention of 41.3% [30]. Besides, most reported studies have been investigated the effect of a single electrolyte additive for either anode or cathode.