2017, Vol. 28 
b Shanghai Electrochemical Energy Devices Research Center, School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
Non-aqueous rechargeable Li-O2 batteries have received increasing attention due to their extremely high theoretical specific energy density (11, 000 Wh/kg), which is five times upon lithium-ion batteries [1, 2]. Li-O2 battery is one of the candidates that have the potential to lead the next generation of energy directions. Despite many researchers have achieved much progress on Li-O2 batteries, those significant obstacles are still restricting the application of Li-O2 batteries in practice [3]. One of the prominent problems is to find an electrolyte with high chemical and electrochemical stability for Li-O2 battery [4]. The reversible reaction (Li+O2—Li2O2) within battery causes reactive radical species as peroxide and superoxide leading the decomposition of electrolytes [5, 6].
Nowadays, dimethyl sulfoxide (DMSO) and tetraethylene glycol dimethyl ether (TEGDME) as most common solvents are used in liquid electrolytes for Li-O2 batteries [3]. Bruce et al. reported that DMSO based electrolyte could be used as cycling-stable electrolyte in Li-O2 batteries [7]. However, in the subsequent report, DMSO based electrolytes showed high reactivity with bare metallic lithium and DMSO solvent was also easily volatile [7, 8]. Besides, DMSO was not stable in cell cycling of Li-O2 battery due to the attack of superoxide radicals from the reduced oxygen within the electrochemical environment [9]. TEGDME-based electrolytes are the most widely used non-aqueous electrolytes for Li-O2 batteries due to their high oxygen solubility and relatively good chemical stability against superoxide radicals [10]. However, TEGDME is also not completely unreactive to superoxide radicals and will be decomposed during the cycling process of Li-O2 batteries [11, 12].
Organic carbonates like propylene carbonate (PC) or ethylene carbonate (EC) are usually used as solvents of liquid electrolytes in Li-ion batteries [13, 14]. But they are not proper to be used in Li-O2 batteries since the nucleophilic attack of superoxide radicals, which led to decomposition of them during the charge and discharge process of Li-O2 battery [15]. This decomposition was confirmed by both computational study and experiments in Li-O2 batteries [16, 17]. As for the mechanism of EC or PC decomposition, the superoxide radicals would attack the α-CH atoms in these carbonates, which was regarded as the start of decomposition [3, 18].
Herein, we reported a new stable electrolyte based chloromethyl pivalate (CP) as solvent for Li-O2 batteries. As shown in Scheme 1, the possibility of the nucleophilic attack of superoxide radicals will eliminate since there are no α-H atoms in the structure of CP. Then we think the CP liquid electrolyte will be stable in Li-O2 batteries. In our experiments, we prepared lithium bis(trifluoromethanesulfonyl)imide (LiTFSI, 1.0 mol/L) in CP as electrolyte, and evaluated its performance including chemical stability against the superoxide radicals, electrochemical stability, ionic conductivity, cycling stability and rate capability of Li-O2 batteries. To the best of our knowledge, it is the first time to report that chloromethyl pivalate as solvent in the liquid electrolytes for Li-O2 batteries.
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| Scheme 1. Chemical structure of chloromethyl pivalate. | |
The cathodes were composed with multi-wall carbon nanotubes and poly(vinylidene fluoride) (PVDF) (8:2, wt:wt). The loading of MWCNTs in a cathode was about 1.0mg/cm. A Li-O2 battery was comprised of a modified 2032 coin-cell put in a glove box under high-purity oxygen (99.9%) atmosphere, a lithium metal anode, a Whatman glass filter separator impregnated with electrolyte and a prepared cathode. After assembling in an argon-filled glove box, the batteries were placed in a gastight box continuously saturated with 1.0atm of high-purity oxygen and tested on Land CT2001A test system (Wuhan, China).
Lithium peroxide (Li2O2) screening method was employed to evaluate the chemical stability of CP based electrolyte [9]. NMR experiments were carried out on the AVANCE Ⅲ HD 400MHz Bruker (BioSpin Corp., Germany) instrument at room temperature. The 13C NMR spectra in CDCl3 was referenced to the residual CHCl3 at 77.2ppm.The 1H NMR spectra in CDCl3 was referenced to the residual CHCl3 at 7.26ppm. Mass spectrum was applied on Gas Chromatograph–Mass Spectrometer (GC–MS) (Agilent 7890A/5975C). Ionic conductivity was measured with a ZAHNER Zennium conductivity meter at room temperature. To evaluate the electrochemical window of the CP based electrolyte, linear sweep voltammetry (LSP) measurement was carried out in a threeelectrode glass cell on CHI 660D potentiostat at a scan rate of 1mV/s and at room temperature. A glass carbon electrode, platinum wire and Ag+/Ag non-aqueous reference electrode (10mmol/L AgNO3 in acetonitrile, from CHI, Inc.) were used as working, counter and reference electrodes, respectively. X-ray diffraction (XRD) measurements were applied on an X-ray diffractometer (D/max-2200/PC, Japan) with Cu Kα radiation from 10° to 80° at a scanning rate of 0.1°/s. After the cycled test in Li-O2 cells, the Whatman glass filter containing the CP based electrolyte was impregnated in CDCl3 for 24h, and then 0.6mL of the mixed solution was transferred to a NMR tube for 13C NMR and 1H NMR analysis.
In general, the α-H atoms in the carbonate solvents will be attacked by the superoxide radicals [19, 20], which finally results in the decomposition of the solvents. The CP has three methyl (-CH3) groups on the α-C as shown in Scheme 1. Due to no α-H, it will decrease the possibility of methylene hydrogen abstraction and highly improve the stability of CP against the superoxide radicals.In order to confirm the chemical stability of CP, we first studied by the lithium peroxide (Li2O2) screening method, which is a normal method to deduce the chemical stability of organic solvents before complete Li-O2 batteries are fabricated. To simulate the environment of Li-O2 battery, the discharge product (Li2O2) was added to the1mol/LLiTFSI of CP electrolyte.There is no change in the color of the liquid electrolyte after 72 h.Furthermore, 1HNMR measurement was used to determine if H atoms of CP were attacked by the superoxide or peroxide radicals, namely the stability of CP.
In the 1H NMR spectrum of the pristine CP as shown in Fig. 1a, there are two peaks at 5.67ppm and 1.18ppm (an integrated ratio of 2:9), respectively. The peak at 1.18ppm is the signature of the Hs on three methyl carbons, and the peak at 5.67ppm is the signature of 2Hs next to the ester functional group (CDCl3 is 7.26ppm). After 72h, it can be found that the there is no any change of the 1H NMR spectrum as shown in Fig. 1a, indicating that CP is unreactive against the peroxide or superoxide radicals. The 13C NMR also confirms that the stability of CP in the environment of Li2O2. The 13C NMR (Fig. 1b) spectrum of CP shows four peaks mainly. The peak at 38.62ppm is due to the one saturated carbon on the backbonenext tothe esterfunctional group.The peak at 68.80ppm is the signature of the one carbon of terminal methylene groups. The peak at 26.63ppm arises from the three "protecting" methyl carbons. The peak at 117.61ppm is the carbon atom of ester group (CDCl3 is 77.16ppm). After 72h, no new peak arises in 13C NMR spectrum, further demonstrated the chemical stability of CP in Li2O2 or superoxide radical environment.
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| Fig. 1. 1H NMR (a) and 13C NMR (b) spectrum of CP based electrolyte before and after reacting with Li2O2 for 72h. | |
The electrochemical stability of 1.0mol/L LiTFSI-CP electrolyte was evaluated by the LSP test, which was conducted in a threeelectrode cell under O2 atmosphere. As shown in Fig. 2, it can be found that there is no peak formed until 5 V, which indicated the CP based electrolyte is electrochemically stable enough in O2 atmosphere. Furthermore, ionic conductivity of the CP based electrolyte is 0.9 mS/cm at room temperature.
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| Fig. 2. LSP of 1.0 mol/L LiTFSI-CP electrolyte under O2 atmosphere at room temperature. | |
A Li-O2 battery with 1.0 mol/L LiTFSI in CP electrolyte was investigated at room temperature. Fig. 3a shows the initial discharge and charge curves of the Li-O2 batteries at different current densities from 2.0 V to 4.5 V vs. Li/Li+. It can be found that the discharge and charge plateaus are about 2.5 V. The discharge and charge capacities are about 5943.8 mAh/g and 3714.7 mAh/g at the current density of 0.2 A/g, 4543.9 mAh/g and 3468.3 mAh/g at the current density of 0.5 A/g, and 3805.2 mAh/g and 1560.7 mA h/g at the current density of 1.0 A/g, respectively. The discharge and charge products were further validated by X-ray diffraction (XRD) measurements. Fig. 3b shows the XRD patterns of the cathode at different discharge-charge stages in the Li–O2 batteries, which indicates that the main discharge product is Li2O2. After charging, all the peaks of Li2O2 disappeared, which demonstrated that the discharge and charge process are reversible electrochemical process and discharge product Li2O2 was completely reversed during the charging process. These results indicate that the Li-O2 battery using the CP based electrolyte has a good reversibility. Capacity-limited cycle method was used to evaluate the cycle performance of the Li-O2 batteries with the CP based electrolyte. Fig. 3c shows the cycling stability of the Li-O2 battery cycled at a fixed capacity of 1000 mA h/g and a current density of 1.0 A/g. It can be found that the cell can keep 20 cycles, and the discharge voltage remains up to 2.0 V and charge voltage remains below 4.5 V. All these results indicate that the Li-O2 batteries with CP based electrolyte show good cycling stability and rate capability.
In order to evaluate the chemical stability of the CP based electrolyte, we measured the electrolyte after 13 cycles in the Li-O2 battery by NMR and MS.As shown in Figs. 4a and b, there are no new peaks formed compared to the pristine CP as shown in Fig. 1. Mass spectrum is also applied to analyze the stability of the electrolyte, as shown in Fig. 4c. It is noticeable that CP does not decompose after 13 cycles of charge-discharge, which proves the satisfactory stability of this electrolyte. These results further demonstrated that the CP based electrolyte has a good chemical stability against the superoxide radicals during the charge and discharge process of Li-O2 battery. In addition, we also found the Li-O2 battery showed a performance decay since there is an increasing voltage gap between discharge and charge terminal voltages with increasing cycles. We think the main reasons of this decay is that both the MWCNTs and PVDF binder from the cathode will be attacked by the superoxide radicals and form some byproducts [3].
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| Fig. 4. 1H NMR (a), 13C NMR spectrum (b) and MS gas analysis (c) of the CP-based electrolyte after cycle test in Li-O2 battery. | |
In summary, we first demonstrated a novel stable liquid electrolyte with chloromethyl pivalate as solvent for non-aqueous Li-O2 batteries. The chemical stability of the CP based electrolyte was investigated in both superoxide radical solution and real Li-O2 battery environment. NMR and MS results confirmed that this new electrolyte showed good chemical and electrochemical stability. The Li-O2 battery with 1.0 mol/L LiTFSI in CP electrolyte showed good electrochemical performance. However, despite these benefits, the CP based electrolyte is characterized by a relatively low ionic conductivity and the batteries show poor cycle performance. Further work on the selection of stable polymer binders and catalysts for the cathode, the optimization of the structure of the cathode to improve cycle life and rate capability is under way. We would improve the ionic conductivity of CP by increasing the concentration or mixing it with different lithium salts in further experiment. And we are also trying to synthesize other esters with no α-H atoms which may have better performance.
AcknowledgmentsThis research was supported by the National Basic Research Program of China (No. 2014CB932303), National Natural Science Foundation of China (No. 21573145). The authors thank the Instrumental Analysis Center of Shanghai Jiao Tong University.
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