采用丁二腈拓宽Li/CFx一次电池的使用温度范围

1 Introduction

Lithium primary batteries (LPBs) are widely used in portable devices, medical electronics and military installations due to the advantages of high energy density, environmental adaptability and long storage life [1-2]. Compared with other LPBs, such as Li/SO2 and Li/MnO2 batteries, carbon materials make Li/CFx primary batteries possessing the advantage of high theoretical specific capacity     (865 mA·h/g), high energy density (2180 W·h/kg), low self-discharge rate, and stable voltage platform   [3-5]. However, lithium ions (Li+) migrate slowly in traditional electrolyte and CFx cathode, which results in the poor electrochemical performance at low temperature [6]. Therefore, exploring appropriate widen-temperature electrolyte for Li/CFx batteries become one of the research hotspots [7-8].

Electrolyte is the key factor for broadening the operating temperature of lithium-ion batteries [9-11]. At subzero temperature, the discharge capacity and voltage platform of batteries decrease sharply because of the slower transfer kinetics of Li+ and lower ionic conductivity [12-14]. However, the reactions between electrodes and electrolyte under high temperature will result in local overheating or short circuits,which lead to a fire or explosion [15]. The effective efforts in expanding the operating temperature range mainly depend on minimizing the Li+ transport resistance, improving the conductivity or forming the stable solid-electrolyte interface (SEI) [16-17]. Functional additives can cost-effectively improve the electrochemical performance [18-21]. The SEI layer not only improves the electrochemical stability of the electrode, but also restrains the growth of lithium dendrite [22-23]. IGNATOVA et al [24] introduced 15-crown-5 additive into LiBF4 GBL (γ-butyl lactone) and LiPF6 EC/DMC/EMC for improving the discharge capacity at -45 ℃. The quantum chemical calculations show that the oriented layers on the electrode surface with lithium-ion conductive performance, which is formed at low temperature. LI et al [25] added BF3 additive to 1 mol/L LiBF4 EC/DMC for dissolving the discharge product of LiF, which improves the rate performance of Li/CFx batteries at low temperature. Although several electrolyte additives were used to improve the electrochemical performance of Li/CFx batteries, there are still some problems, such as voltage lag effect, capacity attenuation and poor rate performance. Hence, it is necessary to explore the additives that can improve the electrochemical performance at both low and high temperatures.

Herein, we report a novel electrolyte additive succinonitrile (SN) to improve the capacity of the Li/CFx batteries at both low and high temperatures. The electrolyte with SN additive possesses low viscosity and good wettability, which reveals high discharge capacity throughout the temperature range between -20 ℃ and 60 ℃. The results of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy level indicates that SN can increase the oxidation potential of electrolyte. Specifically, the CFx cathode in 10% SN-modified electrolyte reveals a high specific capacity of 571.5 mA·h/g at 0 ℃, which is superior to the electrolyte without additive. The addition of SN can reach the demand of excellent wide-temperature electrochemical properties for Li/CFx batteries, showing a good application prospect.

2 Experimental

2.1 Materials and reagents

Lithium tetrafluoroborate (LiBF4), propylene carbonate (PC), 1,2-dimethoxyethane (DME) and the electrolyte additive SN were purchased from Guangdong CJ New Energy Technology Co., Ltd. The electrolyte was prepared in a glove box filled with Ar φ(O2)<0.1×10-6, φ(H2O)<0.1×10-6). The electrolyte of 1 mol/L LiBF4 in PC and DME  (1:1, in volume) was used as the control group. The SN with various concentrations was then added into the above electrolyte and stored in glove box for   24 h, which was used as the research group. In this work, all of the chemical reagents were not purified further.

2.2 Electrochemical tests

CR2016 type coin cells were assembled in Ar filled glovebox with 2500 Celgard as the separator and 100 μL electrolyte. To obtain the cathode, CFx, Super-P and PVDF were mixed with a mass ratio of 8:1:1. The obtained slurries were coated onto a carbon-coated aluminum foil and dried at 60 ℃. Then the dried pole plates were cut into circle pieces of 12 mm size and the areal active material loading was 1.7 mg. CFx (Shandong Chongshan Optoelectronics) electrodes were used as the working electrode and lithium foils with a diameter of 15.6 mm were used as the counter electrode. The galvanostatic charge/discharge performances were measured by the Neware test system. The discharge performance at low temperature was tested using the temperature controlling box (HXL-30L, China). Cyclic voltammetry (CV) was performed in the voltage range of 1.0-4.0 V with a scanning rate of 0.1 mV/s. Electrochemical impedance spectroscopy (EIS) was carried out by the electrochemical workstation (CHI660 Chenhua) in the frequency range of 0.01 Hz to 105 Hz with an amplitude of      5 mV.

2.3 Characterization

To further investigate the morphology difference of electrode after discharge, Li/CFx batteries were disassembled in an argon-filled glove box. The CFx cathodes after discharge were washed with DME and dried in glove box overnight. The surface morphologies of the CFx were observed by scanning electron microscopy (SEM). The valence states of the elemental and composition of the CFx cathodes were analyzed by X-ray photoelectron spectroscopy (XPS) and XRD (Bruker D8 Advance) with Cu Kα radiation. The wettability of the electrolyte for the separator was recorded by a contact angle tester (250-F1) at room temperature.

3 Results and discussion

The highest occupied molecular orbital (HOMO) energy level and the lowest unoccupied molecular orbital (LUMO) energy level are calculated by molecular orbital theory, which reflect the ability of gain and lose electrons. As shown in Figures 1(a) and (b), SN exhibits lower HOMO and LUMO energy values compared with PC and DME solvents, which indicates that the SN molecules have high oxidation potential, and broaden the oxidation potential of the electrolyte effectively.

Figure 1  (a) HOMO and (b) LUMO orbitals of PC, DME and SN molecules

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To investigate the effect of the additive on electrochemical properties, the discharge performance of Li/CFx batteries was tested by adding different amounts of SN to 1 mol/L         LiBF4 PC/DME. It is observed that the electrolyte with 10% SN in mass additive exhibits the highest discharge capacity of 527.3 mA·h/g (Figure 2(a)). As shown in Figures 2(b)-(d), the Li/CFx batteries with 10% SN deliver a higher capacity of          869.9 mA·h/g at 60 ℃, 801.9 mA·h/g at 40 ℃ and 761.8 mA·h/g at 25 ℃, and the corresponding capacities without SN are 792 mA·h/g at 60 ℃, 766.6 mA·h/g at 40 ℃, 726.3 mA·h/g at 25 ℃, respectively. Obviously, the discharge capacity and voltage of batteries decrease sharply at low temperature. At high current rate of 0.5C, Li/CFx batteries with 10% SN perform discharge capacity of 527.3 mA·h /g at 0 ℃, 299.4 mA·h/g at -10 ℃ and 141 mA·h/g at -20 ℃ as shown in Figures 2(e)-(g), which is higher than the battery sample without SN. The electrochemical performance also shows similar results at -10 ℃ (Figure S1). SN not only reduces the viscosity and permeability of electrolyte, but also enhances the stability of CFx cathode interphase at low temperature.

Figure 2  (a) The effect of additive content on batteries performance at 0 ℃; (b)-(f) Discharge curves with and without SN additive at the temperature of 60 ℃ to -20 ℃; (g) The capacity of batteries at different temperatures; The CV curves of CFx in the electrolyte with and without SN at 25 ℃ (h) and at -20 ℃ (i)

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Functional additives were used to construct a low impedance SEI on the electrode surface, which was beneficial to improve the performance at low temperature. Probing electrochemical kinetics of the CFx cathodes at different temperatures with the CV test. As shown in Figures 2(h) and (i), SN is an essential additive for the SEI formation and the intense peak at 2.3 V vs Li+/Li is associated with the reduction of this species. A significant reduction peak and a higher reaction current with SN additive, suggesting more rapid charge transfer kinetics and Li+ transport.

It is widely accepted that the viscosity of electrolytes is influenced by temperature. As shown in Figure 3(a), the corresponding viscosities of the electrolyte without additive are 5.309, 9.530, 13.04, 14.55 mPa·s at 25 ℃, 0 ℃, -10 ℃, -20 ℃, respectively. After adding SN to electrolyte, the corresponding viscosities are 3.832, 6.116, 7.866, 9.416 mPa·s. The viscosity of the electrolyte with SN is about 0.6 times of that without SN at -20 ℃, due to solubility and dissociation of LiBF4 lithium salts in the electrolyte solvents with high permittivity of SN. Moreover, the wettability between the separator and electrolyte was performed by the contact angle experiment, as shown in Figures 3(b) and (c). The contact angle of electrolyte without SN is 51.9° at room temperature, the addition of SN decreases the corresponding contact angle to 47.9°, which demonstrates better migration of Li+ and faster discharge reaction.

Figure 3  (a) Viscosity of different electrolytes from -20 ℃ to 25 ℃; Contact angles at room temperature without SN (b) and with SN (c)

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Electrochemical impedance spectroscopy (EIS) was used to analyze the interfacial reaction kinetics in the electrolytes without and with SN additive as shown in Figures 4(a)-(f). The impedance spectrum consists of a semicircle in the high-medium frequency regions and a straight line in the low-frequency regions, which correspond to the charge transfer resistance (Rct) and the Li+ diffusion coefficient, respectively. It is observed in         Figures 4(a)-(f) that the impedance increases with decreasing temperature, which impedes lithium ions migration. As shown in Figure 4(f), the Rct of Li/CFx batteries containing SN is 321 Ω, lower than 410 Ω of without SN additive during the discharge process at -20 ℃. Meanwhile, the slope of the straight line with SN additive is higher, indicating that the SN additive accelerates the interfacial kinetics and sustains the Li+ diffusion within CFx electrode. The electrolyte additives for low-temperature batteries are concentrated in lithium-ion batteries with organic electrolyte system [26]. The performance of low-temperature Li/CFx primary batteries mainly depends on the ion transport in the electrolyte and ion diffusion in the electrode. In general, the freezing point, viscosity, ionic conductivity and other related physicochemical properties of the electrolyte determined whether Li/CFx primary batteries could remain in normal operation. From the perspective of reaction kinetics, these properties are mainly related to the ion solvating structure. The addition of SN additives decrease the salvation energy of ions, avoiding the high freezing point and the significant decreasing of ionic conductivity. The interaction of coordination and hydrogen bonds inhibits the direct contact between lithium metal and SN, weakens the interaction between Li+ and solvent, facilitates the debolytication of lithium ions, and forms a low impedance conducive to rapid Li+ flow, then improves the kinetics of wide temperature regions.

Figure 4  EIS spectra of CFx batteries at different temperatures before discharge: (a) 25 ℃, (b) 0 ℃ and (c) -20 ℃; after discharge: (d) 25 ℃, (e) 0 ℃ and (f) -20 ℃

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The morphology and structure of the discharged CFx cathode with different electrolytes at low-temperature were investigated by SEM and XRD. The structure destroy during discharge is due to the LiF generation, which deposits into the interlayers of CFx cathode, leading to the cracks of CFx particles. Usually, the structural failure is not conducive to the electrochemical performance of secondary batteries. However, the stripping of the CFx can make more active substances exposed [27], which is beneficial to improving Li+ intercalation kinetics. As shown in Figure S2, the CFx material has a typical lamellar stacked structure and irregular particle size distribution, which is related to the selection of precursor materials and fiuoridation conditions. The irregular particle size and low surface energy of CFx material make it difficult to be infiltrated by the electrolyte, which hinders the migration of Li+. Additives are added to improve the compatibility between the electrolyte and the cathode. The CFx electrodes present similar structure and particle size after discharge in electrolyte without and with SN additive at 60 ℃ (Figure S3). Figures 5(a)-(c) present the SEM images of discharged CFx cathode without SN. It is observed that the discharged cathode showed different degrees of broken. There exist a large number of cracks, which decrease the utilization of the active materials. Conversely, the discharged cathode with SN additive show less cracks and smaller-size CFx particles (Figures 5(d)-(f)), which facilitates the rapid diffusion of Li+ at low temperatures. From Figure S4, a layer of LiF discharge product is pasted on the surface of the CFx cathode, which impedes further discharge reaction. As illustrated, the LiF on the cathode surface without SN electrolyte is loosely arranged with uneven distribution, while the LiF generated on the cathode surface with SN electrolyte is denser and homogeneous, which promotes the charge transfer on the interface [28]. The use of electrolyte additives did not affect the pole plates adversely, and the normal operation of the pole plates was ensured when the electrolyte performance was improved [29]. The XRD pattern of samples in the two electrolytes (Figure S5) show the peak of LiF at 30°, 45°, 65° and 79°, which demonstrate the amorphous crystalline nature. The results indicate that the SN additive does not affect the discharge mechanism of Li/CFx batteries.

Figure 5  The surface SEM images of discharged CFx cathodes at different temperatures without SN additive: (a) 25 ℃,     (b) 0 ℃ and (c) -20 ℃; with SN additive: (d) 25 ℃, (e) 0 ℃ and (f) -20 ℃

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To further understand the effect of SN additive on the CFx cathode surface, XPS was used to investigate the chemical states of carbon (C), fluorine (F) and nitrogen (N), as shown in       Figures 6(a)-(f), as derived from C 1s spectra, the C—F bond at 290.8 eV and the C—C bond at 284.53 eV corresponded to PVDF binder and super-P [30-31]. The peaks at 286.53 eV and 289.1 eV are the results of the C—O and C=O bonds [32], which are the product of the decomposition of solvents. It can be seen that the C—O bond intensity with SN additive is much lower than that without additive, which means that SN inhibits the oxidative decomposition of the electrolyte effectively. Moreover, the C—N bond (287 eV) is only observed in the electrolyte with SN additive. Previous results have shown that a suitable increase of LiF content is beneficial to reducing the interface impedance, then improves the low-temperature performance of batteries [33]. In the F 1s spectrum, the intensity of LiF (685 eV) with SN is much stronger than that without additive, suggesting that more CFx participate in the discharge reaction. Finally, the C—N bond (399 eV) is detected only on the CFx cathode with SN additive [34]. This indicated that the SN selectively decomposes to form a low impedance SEI, that is consistent with previous results of EIS and CV.

Figure 6  XPS spectra of CFx cathodes after being discharged at -20 ℃ without SN additive: (a) C 1s, (b) F 1s and         (c) N 1s; with SN additive: (d) C 1s, (e) F 1s and (f) N 1s

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SN additive effectively accelerated the reaction kinetics of Li/CFx batteries at low temperature. As present in Table 1, even at a discharge rate of 0.5C, the Li/CFx batteries with SN additive exhibits excellent low-temperature performance than other additives.

Table 1  Recent reported works with similar electrolyte additive in Li/CFx primary batteries at low temperature

Source F/C ratio Electrolyte Temperature/℃ Discharge current Cut off voltage/V Capacity/(mA·h·g-1)

This work 1 1 mol/L LiBF4 PC/DME+SN 0 1C/2C 1.5 571.5

-10 486

-20 141

Ref. [24] — 1 mol/L LiBF4 EC/DMC/EMC+15-crown-5 -45 1C/2C 0.5 150

1 mol/L LiPF6 EC/DMC/EMC +15-crown-5 -50 0.7 120

Ref. [27] 0.99-1.08 1 mol/L LiClO4 PC/DME+TTE -50 20C/173C 1.5 299

Ref. [35] 0.99-1.08 0.5 mol/L LiBF4 AN/GBL -50 1C/50C 1.5 325

Ref. [36] 0.467 0.5 mol/L LiBF4 PC/DME+TTFEB -40 1C/5C 0.5 690

-50 500

-60 275

Ref. [37] — 0.5 mol/L LiClO4 PC/DME/THF+FEC -40 1C/120C 2 210

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4 Conclusions

SN is used as a functional additive for Li/CFx batteries. SN effectively inhibits the oxidative decomposition of electrolyte at low temperature, and promotes the formation of low impedance SEI film on the CFx cathode surface, which accelerates the electrochemical reaction kinetics, thus improving the discharge capacity and platform of the batteries. The electrolyte with 10% SN improves the electrochemical performance of batteies at wide temperature effectively, and the Li/CFx batteries with 10% SN reveal a discharge capacity of       527.3 mA·h/g at 0 ℃ under 0.5C current rate.