(Phenylsulfonyl)acetonitrile as a High-Voltage Electrolyte Additive to

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C: Energy Conversion and Storage; Energy and Charge Transport

(Phenylsulfonyl)acetonitrile as a High-Voltage Electrolyte Additive to Form a Sulfide Solid Electrolyte Interface Film to Improve the Performance of Lithium-Ion Batteries Xiao Deng, Xiaoxi Zuo, Huiying Liang, Lengdan Zhang, Jiansheng Liu, and Junmin Nan J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 23 Apr 2019 Downloaded from http://pubs.acs.org on April 23, 2019

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The Journal of Physical Chemistry

(Phenylsulfonyl)acetonitrile as a High-Voltage Electrolyte Additive to Form a Sulfide Solid Electrolyte Interface Film to Improve the Performance of Lithium-Ion Batteries Xiao Denga, Xiaoxi Zuoa,, Huiyin Liang a, Lengdan Zhanga, Jiansheng Liu b, Junmin Nana,* a

School of Chemistry and Environment, MOE Key Laboratory of Theoretical Chemistry of

Environment, Guangzhou Key Laboratory of Materials for Energy Conversion and Storage, South China Normal University, Guangzhou 510006, PR China; b Guangzhou

Great Power Energy Technology Co., Ltd., Guangzhou 511483, PR China

Abstract: The effect of (phenylsulfonyl)acetonitrile (PSPAN) is investigated as a high voltage additive to form a solid electrolyte interface (SEI) on a LiCoO2 cathode. The results show that after 100 cycles, the capacity retention of Li/LiCoO2 cells containing 1.0 wt.% PSPAN increase from 66.34% to 91.84%, and the rate performance of the cells also significantly increases. Linear sweep voltammetry (LSV) results show that PSPAN can be preferentially oxidized over other solvents. X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) analysis confirm that PSPAN can form the SEI film containing sulfide on the electrode. The rapidly produced Li2S in combination with other components such as Li2O and LiF form smaller size particles on the electrode surface and provide more channels for the diffusion of lithium ions, resulting a more excellent cell performance.



E-mail address: [email protected] (X. Zuo), [email protected] (J. Nan) 1

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1. Introduction In recent years, lithium-ion batteries (LIBs) have been widely used in electronic devices and vehicles, however, they cannot be applied to electric vehicles due to their low energy density limitations. The use of high-voltage cathode materials can increase the energy density of LIBs. The most commonly used cathode materials for commercial LIBs are LiCoO2, LiNi0.5Co0.2Mn0.3O2 and LiNi1/3Co1/3Mn1/3O2. The working voltage of these active materials can usually reach 4.4 V, showing an acceptable high capacity of 140-180 mAh g-1.1-3 Unfortunately, at such high voltages, the electrolyte is susceptible to oxidative decomposition and produces a series of harmful byproducts that affect the performance of the battery. Therefore, many researchers are striving to develop electrolyte with good electrochemical stability during high-voltage operation.46

To improve the quality of high-voltage electrolyte, researchers have focused substantial effort to identify new stable solvents.7 However, alternative solvents often have some nonnegligible disadvantages such as a high viscosity, an instability at low voltages, and a high cost. In addition, they also lack the formation of a solid electrolyte interface (SEI) to protect the graphite anode.8-15 Currently, electrolyte solvents are still primarily selected from carbonates in consideration of the cost and overall performance. For example, ethylene carbonate (EC) is an essential component due to its excellent electrochemical properties. It is known that EC undergoes a decomposition reaction and forms an interfacial film on the electrode surface during the first few charge cycles, protecting the electrolyte from further decomposition.16 Unfortunately, the SEI film is unstable when in the presence of only EC, which is easily corroded by HF derived from LiPF6. When the protective film is damaged, PF6- and other solvents can come into contact with the cathode, producing polymer deposits, gaseous products and other harmful fluorine-containing compounds on the electrodes.17 As the charge-discharge progresses, these harmful reactions become increasingly serious and eventually lead to an increment in the interfacial impedance of the battery and a poor cycle performance.1, 12

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Previously, additives were successfully used to avoid further reduction of the electrolyte by forming the SEI film on the graphite surface. Similarly, recent work has demonstrated that electrolyte additives also show some particular advantages in improving battery performance by forming the SEI layer on the cathode.18-20 It is well known that the interface of the electrode/electrolyte is unstable under high-voltages. On the one hand, the electrolyte is inevitably continuously oxidative decomposed, and the byproducts produced further exacerbate the oxidation of the electrolyte. On the other hand, the HF generated by the complicated reaction on the surface of the cathode promotes the dissolution of transition metal ions and results in the poor cycle performance of the batteries. As a result, stable interfacial films formed by additives and other components that can provide better cycle performance for lithium-ion batteries than those formed without additives in the electrolyte are becoming more and more crucial.6, 21 According to some reported works, aromatic compounds have been widely studied because the aromatic conjugated structure allows the resulting passivation films to conduct electrons. In addition, it has been reported that aromatic compounds are very stable, which is conducive for the stability of the cathode interfacial film. Simultaneously, several substances that include sulfur oxides (S=O) have been considered as cathode film-forming electrolyte additives to improve the performance of the battery because the S=O group can act as a weakly basic site and inhibit the reactivity of PF5, thereby effectively inhibiting the formation of LiF and the generation of HF.22 A recent study proposed by Dong et al demonstrated that ptoluenesulfonyl isocyanate (PTSI) could be used as a film-forming additive to improve the cycling performance of the Li/LiNi0.5Co0.2Mn0.3O2 cell. Electrochemical test results showed that the capacity retention of the cell with 0.5 wt. % PTSI was improved from 71.4% to 86.2% at the range of 3.0-4.5 V after cycling 100 times. The improved performance was due to not only the thin protected layer derived from decomposition of PTSI but also the S=O group in PTSI that can reduce the reactivity of PF5, thereby suppressing the formation of LiF and HF.23 Zheng et al studied the use of phenyl vinyl sulfone (PVS) as a new electrolyte additive to build a passivation film to protect the layered lithium-rich cathode. Charge-discharge tests demonstrated that the capacity 3



retention of the battery with 1.0 wt. % PVS reaches approximately 80% after 240 cycles at 0.5 C between 2.0 and 4.8 V (vs Li/Li+). The results also indicated that the two doubles bond, the sulfur and the aromatic ring in the PVS molecule work together to endow its excellent chemical properties.22 Therefore, it is important to develop a compound with special chemical properties as a film-forming additive to improve the performance of the cell in the future.24 In this work, we proposed (phenylsulfonyl)acetonitrile (PSPAN) as a novel highvoltage film-forming additive to improve the performance of the battery. According to this work, the introduction of the benzene ring improves the chemical stability of the interfacial film, and the S=O group in the molecule helps to suppress LiF and HF and thus enhance the ionic conductivity of the battery. A commercial cathode (LiCoO2) was selected as the cathode material because of its special superiority, electrochemical tests were used to verify the contribution of PSPAN, and physical characterization was used to explore the possible role of PSPAN. 2. Experimental procedure 2.1 Preparation of the electrolyte, electrode and cells The base electrolyte in this study was 1.0 M LiPF6-EC/EMC (3: 7, wt.%), which was provided by Guangzhou Shantou Jinguang Technology Co., Ltd. The PSPAN (purity > 99%) used in this study was purchased from Alfa Aesar (China) Chemical Co., Ltd. The PSPAN-containing electrolyte was 1.0 wt.% PSPAN in a mixture of 1.0 M LiPF6-EC/EMC (3: 7, wt.%) electrolyte. A slurry containing LiCoO2 (obtained from Hunan Shanshan Toda Advanced Materials Co., Ltd.), acetylene black and polyvinylidene fluoride (PVDF) at a weight ratio of 8:1:1 was added in N-methyl pyrrolidinone (NMP) solvent and stirred 6 h, then it was coated on aluminum foil and subsequently dried at 80 °C in air for 2 h. Next, it was placed under vacuum conditions at 120 °C for 10 h to obtain a LiCoO2 cathode used in half-cell. The 2032-type batteries were assembled using the LiCoO2 cathode electrode with an active mass loading of approximately 2.48~2.78 mg cm-2 and a Celgard 2500 as a separator in an argon-filled glove box. As for full cell, the cathode was obtained by coating a slurry containing 97.5 wt.% LiCoO2, 0.5 wt.% carbon nanotubes(CNT, provided from Cnano Technology, 4


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China), 1.0 wt.% Super P and 1.0 wt.% PVDF onto an aluminum foil, and the graphite was composed of 95.4 wt.% graphite, 1.5 wt.% carboxy methyl cellulose (CMC), 1.0 wt.% Super P and 2.1 wt.% styrenebutadiene rubber (SBR), coating onto a piece of Cu foil. A porous polyethylene film with a thickness of 16 μm was used as the separator. All processes for the full cells, including the preparation of electrode and electrolyte, cells assembly and performance testing, were accomplished at the battery factory (Guangzhou Great Power Energy Technology Co., Ltd.). 2.2 Electrochemical Measurements of the cells The oxidation stability of the electrolyte was evaluated by the linear sweep voltammetry (LSV) using a three-electrode system including Pt working electrode and lithium metal as both the counter and reference electrode. The scan rate was 1.0 mV s1

from open voltages (OCV) to 6.0 V (vs. Li/Li+). The electrochemical impedance

spectroscopy (EIS) of the Li/LiCoO2 batteries with and without PSPAN was measured in the frequency range from 100 kHz to 0.01 Hz with a rate of 10 mV s-1. The LSV and EIS were performed on an instrumental electrochemical workstation (CHI660E, Chenhua, Shanghai). The cyclic stability of Li/LiCoO2 batteries were tested on NEWARE battery measurement device (NEWAREBTS2300, Shenzhen) at 25 °C for 100 cycles with 0.5 C (1C= 147.2 mA·g-1) between 3.0-4.4 V (vs. Li/Li+). The LiCoO2/graphite batteries were charged to 4.4V on NEWARE battery measurement device (BTS-PWM5V10A, Shenzhen, China) at 25 °C with 1 C (1C= 273.8 mA·g-1) , followed by a constant voltage at 4.4V until the current decreased to C/20, then charged and discharged at a constant current of 1 C for 160 cycles between 3.0-4.4 V (vs. Li/Li+).The rate performance of the Li/LiCoO2 battery was tested by cycling the cell for four cycles at different rates of 0.2 C, 0.5 C, 1 C, 2 C, 3 C, 0.5 C and 0.2 C, respectively. 2.3 Characterization of electrodes The cycled LiCoO2 electrodes were carefully separated in a glove box and washed three times by anhydrous DMC to remove the residual electrolyte and by-products on the surface, and then the samples were dried in the glove box all night at room temperature. The structure of LiCoO2 was investigated by XRD (Bruker D8 Advance, 5



Germany) using Cu Kα radiation in the angle range of 15-90°. The surface morphology of the LiCoO2 was characterized by SEM (ZEISS Ultra 55, Germany). The elements composition on the Surface were determined by X-ray photoelectron spectroscopy (XPS, ESCALAB 250) with an Al Kα line as the X-ray source, the obtained spectra were separated into several peaks by XPS peak software (version 4.1). 3. Results and discussion 3.1 Effects of PSPAN on the electrochemical performance of cells To investigate the impact of PSPAN on the half-cell performance, the Li/LiCoO2 cells with different contents of PSPAN were cycled under room temperature at 0.5 C for 100 cycles, as shown in Fig. 1(a). It can be seen that the capacities of the cells without PSPAN rapidly reduced after 100 cycles from 158.75 mAh g-1 to 106.37 mAh g-1. Surprisingly, when PSPAN was used, especially for batteries containing 1.0 wt.% PSPAN, a significant improvement in the cycling performance of the battery was observed. The capacities of the cells with 0.5 wt.%, 1.0 wt.%, 2.0 wt.% PSPAN were 139.77, 145.12 and 137.54 mAh g-1 after 100 cycles, respectively. This result shows that a suitable concentration of PSPAN has a positive effect on the cycling performance of the battery. Based on this knowledge, an electrolyte with 1.0 wt.% PSPAN was selected for further investigation. Fig. 1(b) presents the coulombic efficiencies (CEs) of the Li/LiCoO2 cells with and without PSPAN after 100 cycles. Although the CE of the base cell was higher at the beginning, after the 20th cycle, the CE of the cell with PSPAN was higher than that of the base battery because the oxidative decomposition of PSPAN required consumption of some Li+ to form the SEI in the first few cycles, which leads to a lower coulombic efficiency. However, as the battery was charged and discharged, a stable SEI film was formed on the cathode, and the cycle performance of the battery was improved. The more convincing evidence of PSPAN can improve performance of the battery were obtained from Fig. 1(c), the initial discharge capacity of the LiCoO2/graphite batteries with and without PSPAN were 165.86 mAh g-1 and 166.91 mAh g-1. As the charge and discharge process progresses, the capacity retention of the LiCoO2/graphite battery with PSPAN was 92.82%. While the battery in the base electrolyte rapidly decayed from 166.91 mAh g-1 to 59.61 mAh g-1, and the capacity 6


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retention rate was only 35.71% after 160 cycles. Obviously, this result confirms that PSPAN shows positive effect for the battery. To investigate the possibility of PSPAN as a cathode additive, LSV was used to test the oxidation potential of the electrolyte, as shown in Fig. 2, the result showed that an additional oxidation current peak was observed at 3.2 V (vs. Li/Li+) with 1.0 wt.% PSPAN in electrolyte. No corresponding current peak was observed for the electrolyte without PSPAN. This result indicates that PSPAN is more easily decomposed than the carbonate solvents and may be employed as a cathode additive. Fig. 3 shows the effect of PSPAN at different discharge rates on the Li/LiCoO2 cells. At the initial low discharge rates (C/5), the discharge capacity of the battery containing PSPAN was lower than that of the base battery. This result may be because the impedance of the SEI film is too large, resulting in an additional irreversible capacity loss.25 With the charge/discharge rate reaching 3 C, the capacity of the battery with PSPAN was 96.8 mAh g-1, which was higher than that of the base cell. In addition, when the charge/discharge rates returned to 0.5 C and 0.2 C, the capacity of the battery with PSPAN reached a normal level. The base battery, however, could not be restored to its original state, indicating that the base battery undergoes an irreversible capacity loss after high rates of charge and discharge. To confirm the interfacial resistance between the LiCoO2 and electrolyte, the impedance spectra of the battery cycled in electrolyte with and without PSPAN were obtained from EIS, as shown in Fig. 4(a). It can be seen that the spectra consist of a semicircle at high frequencies, which represents the interfacial impedance including the SEI and charge-transfer resistances, and a sloped line at low frequencies belonging to the Warburg impedance.26 As shown in Fig. 4(b), compared to the base cell, the cell with PSPAN after 5 cycles shows a slightly smaller semicircle at high frequencies, corresponding to the resistance of the SEI film. After 100 cycles, the impedance of the base cell was greatly increased, while that of the battery cycled in the electrolyte with PSPAN only slightly increased. It is apparent that the presence of PSPAN in the electrolyte reduces the impedance of the cathode, leading to a better cycle performance of the LIB.27 7



3.2 Effects of PSPAN on the morphology and composition of LiCoO2 SEM imagery of the fresh LiCoO2 and cathode cycled in the electrolyte with and without PSPAN for 200 times are shown in Fig. 5. It can be clearly observed from Fig. 5(a1, b1) that the surface of the fresh LiCoO2 is clean and smooth. However, after cycling in the base electrolyte, the surface of the cathode is covered with a small amount of particulate deposits (Fig. 5(a2)). It can be found from Fig. 5(b2) that these deposits, which are loose in shape and uneven in thickness, are the products of the oxidative decomposition of the electrolyte during charging and discharging.28 However, this film is too thin to protect the electrode material effectively. Based on Fig. 5(a3, b3), the LiCoO2 cycled in the electrolyte with PSPAN is obviously covered with a more uniform and dense protective film, which can protect the cathode electrode material from being destroyed and suppress the continuous decomposition of the electrolyte, thereby effectively improving the cycle performance of the battery.29 In addition, the protective layer derived from PSPAN has a lower impedance and higher Li+ ion conductivity, as demonstrated by the impedance spectra discussed above. Taken together, these results show that PSPAN plays a positive role in the performance of the battery. Fig. 6 presents the XRD patterns of the fresh LiCoO2 and cathode cycled in the electrolyte with and without PSPAN for 200 times. It can be seen that the groups of diffraction peaks (006)/(012) and (018)/(110) clearly split from the XRD patterns of the fresh LiCoO2 that are related to the typical layer structure.30 Compared with the fresh LiCoO2, all the characteristic peaks of the LiCoO2 layer are weakened or even disappeared in the electrolyte with PSPAN, suggesting that the LiCoO2 crystal structure has changed. In addition, due to the volumetric change in the LiCoO2, the stronger peak intensity of aluminum is the exposed surface of the aluminum foil. In contrast, the XRD pattern of the cathode with PSPAN was almost unchanged from the fresh cathode, suggesting that the structure of LiCoO2 could be maintained by adding PSPAN. To fully understand the effect of the PSPAN additive on the formation mechanism of the SEI on the electrode surface, the fresh LiCoO2 electrode and the LiCoO2 electrodes cycled in electrolyte with and without PSPAN were examined by XPS. Fig. 7 shows the C 1s, O1s, F 1s, P 2p and S 2p spectra of the LiCoO2 electrode. The C 1s 8


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spectrum associated with the PVDF binder (291.2 eV) and acetylene (284.6 eV). The peaks at 286.5 eV and 288.7 eV are characteristic of the C-O and C=O bonds, respectively, which are due to the lithium alkyl (ROLi), lithium carbonate alkyl ester (ROCO2Li) and Li2CO3 species derived from oxidative decomposition of the electrolyte.31-32 The peak at 285 eV from C-C in the electrolyte with PSPAN was significantly weaker than that of the base electrolyte under the same condition, indicating that the surface of the electrode containing the additive had a covering surface to weaken the signal of the C-C peak. Similarly, from the F 1s and C 1s spectra, the electrode surface of the PSPAN electrolyte was observed. It is almost impossible to distinguish the C-F peak belonging to PVDF, and it is more powerful to prove that a protective film is formed on the cathode interface. For the O 1s spectrum, the 532.0 eV and 533.1 eV peaks are mainly attributed to Li2CO3/carboxylate/carbonates and ethers.33 Furthermore, the peak located at 529.4 eV is assigned to the M-O bond on the surface of the cathode, which almost disappears in the electrolyte with PSPAN, suggesting that the film produced by PSPAN can inhibit the oxidative decomposition of the carbonate solvent and inhibit the deposition of the transition metal on the cathode surface. Additionally, the intensity of the C=O peak formed by the decomposition of the electrolyte at the presence of PSPAN is much weaker than the base electrolyte, confirming that the PSPAN can effectively inhibit the oxidative decomposition of the electrolyte. In the F 1s spectrum, the peak of LiF (685.5 eV) from base electrolyte was much stronger than that in the electrolyte with PSPAN. This result indicates that in the electrolyte without PSPAN, a higher concentration of LiF is deposited on the surface of the LiCoO2 electrode. It is well known that high concentrations of LiF cause a high interfacial impedance of the battery, which adversely affects the cells. This result is also consistent with the results of EIS analysis.34 Furthermore, LixPFy (137.4 eV) and LixPOyFz (133.6 eV), which are produced by the decomposition of LiPF6, are stronger in the base electrolyte than in the electrolyte with PSPAN as shown in the P 2p spectrum, suggesting that the decomposition of LiPF6 could be alleviated due to the influence of PSPAN.35, 36 In the S 2p spectrum, additional S elements were detected in the electrolyte 9



in the presence of the PSPAN additive. The peaks at 170.3 eV, 169.1 eV and 164.9 eV are assigned to Li2SO3, ROSO2Li and Li2S, respectively, confirming that the PSPAN participates in the formation of a solid electrolyte interface film, which could inhibit the decomposition of the solvents and the collapse of the structure of the LiCoO2 electrode, thereby improving the life of the battery.29, 37 3.3 Possible Mechanism of PSPAN on the Li/LiCoO2 cells For the base electrolyte, the decomposition products of the alkyl carbonate include Li2CO3, RCO2Li and ROCO2, which constitute the SEI film, as shown in Fig. 8(a). Unfortunately, the SEI film is unstable and susceptible to corrosion by PF5 and HF derived from the decomposition of LiPF6.38 Additionally, the resulting LiF product could increase the interfacial resistance on the cathode.39 Once the SEI film is destroyed, the LiCoO2 cathode is directly exposed to the electrolyte, which is further corroded by HF, causing the collapse of the LiCoO2 layer structure, as shown in Fig. 8(c). In addition, PF5 is an initiator of electrolyte oxidation, which could initiate ring-opening polymerization of EC to form poly(ethylene carbonate) (PEC) or poly(ethylene oxide) (PEO). It has been reported that both PEC and PEO polymers could continue to react with PF5, eventually leading to the decomposition of the electrolyte, as shown in Fig. 8(b).40 From the above analysis, it can be concluded that PF5 and HF are the main culprits for the decomposition of the electrolyte.41 When PSPAN is added, PSPAN containing S=O groups can inhibit the reactivity of PF5, which can reduce not only the formation of HF and LiF, but also the decomposition rate of EC, as shown in Fig. 8(d). As the first few charge-discharge processes continue, PSPAN formed a radical anion by one electron transfer and then combined with lithium ions and an alkyl radical or RO• radical generated by the decomposition of the electrolyte to produce ROSO2Li, Li2SO3 and Li2S, as is detected by XPS (Fig. 7). Compared with the products from the alkyl carbonate, those sulfurcontaining products constitute a more stable SEI film, effectively reducing the direct contact between the electrode/electrolyte and alleviating the decomposition of the electrolyte, as shown in Fig. 8(e).22, 29, 42 In addition, PSPAN could form a sulfide SEI film containing Li2S, which reduces the impedance of the cell and significantly 10


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improves the capacity retention of the cell. Many reports have been published stating that the rapid formation of Li2S and the large ionic radius of S2- can interrupt the growth of other inorganic crystals.43-44 Therefore, the rate of crystal growth could be controlled during SEI formation, resulting in the SEI component with a smaller crystal size and poorer crystallinity. Commonly, a poor crystallinity will lead to lithium ions with a higher conductivity

42.

Moreover, SEI components with a smaller crystal size can

provide more channels for lithium ion diffusion 44. Both factors could affect the SEI film, resulting in the excellent cycling performance of the lithium-ion battery.44, 46-47 4. Conclusion PSPAN was first proposed as a novel film-forming additive. By preferential oxidation reactions, a multichannel sulfurized SEI film is formed on the surface of the LiCoO2 cathode, which not only improved the Li+ diffusion efficiency, but also prevented the corrosion of the electrode. In addition, S=O in PSPAN can reduce the activity of PF5, thereby inhibiting the decomposition of the electrolyte and improving the cycle performance of the battery under high voltages. All electrochemical tests show that PSPAN is a promising electrolyte additive. Compared to published articles, PSPAN is a more excellent film-forming additive.48, 49

Acknowledgements This work was financially supported by the Natural Science Foundation of Guangdong Province (No. 2018A030313886), the National Natural Science Foundation (No. 21875077), the Science and Technology Projects of Guangdong Province (No. 2015A040404043) and the Science and Technology Projects of Guangzhou (No. 201604016131).

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Fig. 1. Cycle performances of Li/LiCoO2 cells (a), coulombic efficiencies of Li/LiCoO2 cells (b) and cycle performances of LiCoO2/graphite cells (c) in the electrolyte with and without PSPAN.

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Fig. 2. Linear sweep voltammetry (LSV) of Pt electrodes in the electrolyte with and without PSPAN.

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Fig. 3. Rate performances of the Li/LiCoO2 cells in the electrolyte with and without PSPAN.

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Fig. 4. Electrochemical impedance spectra of the Li/LiCoO2 batteries after 5 cycles (a) and 100 cycles (b) in the electrolyte with and without PSPAN.

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Fig. 5. SEM and TEM morphologies of the electrode cathodes: (a1, b1) fresh LiCoO2 cathode, (a2, b2) LiCoO2 cathode without PSPAN after 100 cycles, and (a3, b3) LiCoO2 cathode with PSPAN after 100 cycles

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Fig. 6. XRD patterns of LiCoO2 cathodes in the electrolyte with and without PSPAN after 100 cycles.

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Fig. 7. XPS spectra of the fresh LiCoO2 and the LiCoO2 cathodes cycled in the electrolyte with and without PSPAN after 100 cycles.

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Fig. 8. Possible mechanism for SEI formation by redox reactions of PSPAN and for enhancing the performance of Li/LiCoO2 cells.

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(Phenylsulfonyl)acetonitrile as a High-Voltage Electrolyte Additive to

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