2,3,4,5,6-Pentafluorophenyl Methanesulfonate as a Versatile

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Surfaces, Interfaces, and Applications

2,3,4,5,6-Pentafluorophenyl Methanesulfonate as a Versatile Electrolyte Additive Matches LiNi0.5Co0.2Mn0.3O2/ Graphite Batteries Working in a Wide-Temperature Range Tianxiang Yang, Weizhen Fan, Chengyun Wang, Qiufen Lei, Zhen Ma, Le Yu, Xiaoxi Zuo, and Junmin Nan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04743 • Publication Date (Web): 21 Aug 2018 Downloaded from http://pubs.acs.org on August 22, 2018

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ACS Applied Materials & Interfaces

2,3,4,5,6-Pentafluorophenyl Methanesulfonate as a Versatile Electrolyte Additive Matches LiNi0.5Co0.2Mn0.3O2/Graphite Batteries Working in a Wide-Temperature Range

Tianxiang Yang1, Weizhen Fan2, Chengyun Wang1, Qiufen Lei2, Zhen Ma1, Le Yu2, Xiaoxi Zuo1, Junmin Nan1* 1. School of Chemistry and Environment, South China Normal University, Guangzhou 510006, PR China 2. Guangzhou Tinci Materials Technology Co., Ltd., Guangzhou 510760, PR China

*[email protected] (J. Nan)

Keywords: electrolyte

2,3,4,5,6-pentafluorophenyl composition;

methanesulfonate;

wide-temperature

range;

LiNi0.5Co0.2Mn0.3O2/graphite battery. 1

ACS Paragon Plus Environment

versatile

comparative

additive;

performances;

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract An electrolyte using 2,3,4,5,6-pentafluorophenyl methanesulfonate (PFPMS) as a versatile additive is investigated through calculating the molecular orbital energies of additives and solvents and designing the electrolyte composition, and the comparative performances of LiNi0.5Co0.2Mn0.3O2/graphite cells operating in a wide-temperature range are improved. It is revealed that PFPMS can form interfacial films on both the cathode and anode surfaces, resulting in a decrease of the cell impedance and the side reactions between the active materials and electrolyte. Compared to the cells without additive of 74.9% and those with vinylene carbonate (VC) of 76.7%, the cycling retention of the cell with 1.0 wt.% PFPMS reaches 91.7% after 400 cycles at room temperature. In particular, for the high-temperature stored at 60 °C for 7 d, the cell containing 1.0 wt.% PFPMS exhibits optimal capacity retention of 86.3% and capacity recovery of 90.6%; for the low-temperature discharge capacity retention at -20 °C, the cell with 1.0 wt.% PFPMS maintains at 66.3% at 0.5 C, while for the cells without additive and containing 1.0 wt.% VC, their retention values are 55.0% and 62.1%, respectively. The excellent cycling, wide-temperature practicability, and rate capability of the cells with PFPMS demonstrate that the electrolyte with PFPMS additive is promising for applications in LiNi0.5Co0.2Mn0.3O2/graphite batteries.

2


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1. Introduction Lithium-ion batteries (LIBs) with lithium nickel cobalt manganese oxide (NCM) cathode have been increasingly used due to the higher energy and power density compared to the batteries with LiCoO2 cathode,1-3 but the absence of a wide-temperature electrolyte is still a barrier for the application of LIBs with high-nickel-content NCM cathodes.4-6 At low temperatures, there are well-known technical problems for LIBs, namely, the migration rate of lithium ions is significantly reduced and the interfacial impedance is obviously increased. They result in an obvious polarization of the electrode reactions, a reduction of discharge voltage plateau, and an energy fading of LIBs.7-8 Furthermore, at high temperatures, the thermal instability and decomposition of LiPF6 in the electrolyte and the reactions between the cathode materials and electrolyte will also give rise to material degradation, a shorter cycle life, and safety issues.9,10 Especially compared to the LIBs with LiCoO2 cathode, the HF generated by thermal decomposition and the hydrolysis reaction of LiPF6 in the electrolyte will more easily attack the Mn-contained cathode materials, leading to the destruction of material structure.11,12 In addition, the dissolved Mn ions in the electrolyte can migrate through the separator and subsequently deposit on the anode surface, bringing about the instability of solid electrolyte interphase (SEI) and an increased impedance of LIBs. This generates the further decomposition of electrolyte to rehabilitate the damaged SEI film, increasing the thickness of SEI film and further enhancing the impedance of LIBs.13,14 In practice, the LIBs with high-nickel-content NCM cathode should be adapted to the wide-temperature ranges according to their applications in portable intelligent electrical devices and electric vehicles. For these reasons, the electrochemical performances of LIBs with high-nickel-content NCM cathode in high- and low-temperature environments must be improved. 3



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To enhance the wide-temperature performance of LIBs, an alternative strategy has focused on the optimization of the cathode electrolyte interphase (CEI) and SEI films formed by the added functional additives in the electrolyte.15-18 It was demonstrated that some CEI and SEI layers not only improve the lithium ions diffusion and reduce the polarization and interface impedance of LIBs at low temperatures but also prevent the electrolyte decomposition and the reactions between the cathode materials and electrolyte at high temperatures. These typical additives include organic and inorganic functional molecules such as vinylene carbonate (VC),19,20 propane sultone (PS),21,22 butane sultone (BS),23 vinylethylene carbonate (VEC),24,25 lithium bis(oxalate)borate (LiBOB),26 lithium oxalatodigluoro borate (LiODFB),27 and lithium difluorophosphate (Li2PO2F2).28 Among them, PS, BS, and VEC generally act as high-temperature-type additives, while

LiBOB,

LiODFB, and

Li2PO2F2

are commonly used

as

low-temperature-type additives. In particular, as a versatile electrolyte additive used for the film formation on graphite anode, VC can enhance the electrochemical performances of LIBs with LiCoO2 cathode to a certain extent, but it does not appear to be optimal for the LIBs with NCM cathode.29-31 For long-term cycling at room temperature, the interface film formed by VC is imperfect, and its higher impedance leads to the capacity fading and shorter cycle life. For the full cells with NCM cathode materials, VC in the electrolyte shows poor compatibility. Similar to the graphite anode, VC suffers from oxidative decomposition to generate a CEI layer on the cathode surface.32,33 Nevertheless, the VC-derived CEI layer is unstable and cannot prevent the continuous decomposition of the electrolyte at high operating voltages and the reactions between the active materials and electrolyte at high temperatures. Thus, the long-term cycling and elevated-temperature storage performances of the full cells using VC additive are considered to be an open and debatable topic. In addition, only a few 4


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molecules such as 1,1-diphenylethylene and 1,1-diphenylethane have been investigated as cathode additives that can suppress the degradation of cathode electrodes at high temperatures.34 These studies on the electrolyte additives provide a suggestive solution for obtaining the desired performances of LIBs with high-nickel-content NCM cathode. It is noteworthy that most electrolyte additives cannot simultaneously enhance the performances of LIBs under high, low, and room temperatures.35-37 This means that novel electrolyte additives that can form stable interfacial films on both cathode and anode surfaces should be developed to meet the requirement of LIBs operating in a wide-temperature range. Considering the high electronegativity, low polarity, and intrinsic element properties, some fluorinated functional molecules are expected to be suitable electrolyte additives for enhancing the performance of LIBs with high-nickel-content

NCM

cathode.

In

this

work,

a

fluorinated

2,3,4,5,6-pentafluorophenyl methanesulfonate (PFPMS) functional molecule was evaluated as a versatile electrolyte additive to enhance the performances of LIBs at high, low, and room temperatures. It was demonstrated that PFPMS additive can form stable interfacial films on both high-nickel-content NCM cathode and graphite surfaces, and all electrochemical performances of LIBs stored at 60 °C, discharged at -20 °C, and cycled at room temperature can be significantly improved. In addition, the composition and morphology on the electrode surface were investigated to reveal its reaction mechanism.

2. Experimental section 2.1 Materials and preparation of the electrolyte and cells Ethylene

carbonate

(EC),

ethyl

methyl

carbonate

(EMC),

lithium

hexafluorophosphate (LiPF6), and vinylene carbonate (VC) were provided by the 5



Guangzhou Tinci High-Tech Materials Co. Ltd. and were used to prepare the blank and reference electrolytes. PFPMS was obtained from the Zhangjiagang Transchem Technology Co. Ltd. The electrolyte was prepared by dissolving 1 M LiPF6 in a mixture of EC and EMC (1:2 by w/w), and this electrolyte was designated as a blank electrolyte. The blank electrolyte was added to 1.0 wt.% VC and 1.0 wt.% PFPMS additives to obtain the reference and PFPMS-containing electrolytes, respectively. All electrolytes were prepared in a dry argon-filled glove box with both moisture and oxygen concentrations below 0.1 ppm. LiNi0.5Mn0.2Co0.3O2 (NCM523) cathode and artificial graphite (AG) anode materials were purchased from the Shenzhen Tianjiao Technology Co. Ltd. and the Shenzhen BTR Battery Materials Co. Ltd., respectively. The graphite anodes were prepared by 95.0 wt.% AG, 1.5 wt.% carboxymethyl cellulose (CMC), 2.0 wt.% styrene butadiene rubber (SBR), and 1.5 wt.% Super P. The cathode electrodes consisted of 96.8 wt.% NCM523, 1.2 wt.% PVdF, and 2.0 wt.% super P. The cathode paste and the anode paste were coated on an aluminum foil and a copper foil, respectively, then dried at 120 °C for 12 h in vacuum drying oven. The areal densities of obtained cathode and anode electrodes were 350 and 175 g m-2, respectively. In addition to the soft-pack LiNi0.5Co0.2Mn0.3O2/graphite full cells were assembled using the above-prepared electrodes, LiNi0.5Co0.2Mn0.3O2/Li and Li/graphite 2032 coin half cells were prepared with the NCM523 and graphite as the working electrode, respectively, and lithium as the counter electrode. The LiNi0.5Co0.2Mn0.3O2/graphite cells were designed to a nominal capacity of approximately 1900 mAh according to the cathode and anode capacity ratio of 1 to 1.1.

2.2 Electrochemical measurements 6


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The cyclic voltammetry (CV) tests of the Li/graphite coin half cells were conducted by an Instrumental Electrochemical Workstation (CHI660, Chenhua) at a sweep rate of 0.05 mV s-1 over the potential range of 0.01-3 V. Electrochemical impedance spectroscopy (EIS) of the LiNi0.5Co0.2Mn0.3O2/graphite full cells and half cells was carried out using a frequency response analyzer (FRA, Solartron 1455A) in the frequency range of 0.01-105 Hz. The charge-discharge measurements of the LiNi0.5Co0.2Mn0.3O2/graphite cells were conducted using a battery testing system (CT-3008W, Neware) between 2.75 and 4.2 V. And all full cells were pre-cycled at 0.1 C for first 3 cycles. The rate capability of full cells was performed at charge rate of 1C and discharge using various C rates: 0.1, 0.2, 0.5, 1, 2, 3, 4, and 5C. The discharge performance of the LiNi0.5Co0.2Mn0.3O2/graphite cells at -20 °C was measured in a temperature-controlled test chamber. Prior to the discharging at low temperature and storage at high temperature, the cells were cycled for 5 cycles to obtain the discharge capacities at room temperature. Subsequently, the fully charged cells were placed in the test chamber at -20 °C for 4 h and discharged at the discharge rates of 0.2 and 0.5 C, respectively. The high-temperature storage performance of the LiNi0.5Co0.2Mn0.3O2/graphite cells was tested at 60 °C in a high-temperature oven. The fully charged cells were stored at 60 °C for 7 d and then recovered by 3 cycles at room temperature. The capacity retention of the cells was obtained from the ratio of the initial discharge capacity recovered at room temperature to the discharge capacity at room temperature prior to the high-temperature storage. The capacity recovery is the ratio of the third discharge capacity recovered at room temperature to the discharge capacity prior to the high temperature storage.

2.3 Characterization of the functional molecules and the cycled electrode surfaces 7



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To evaluate the oxidative and reductive properties of the functional groups, the highest

occupied

molecular

orbital/lowest

unoccupied

molecular

orbital

(HOMO/LUMO) energies of the solvent molecules and additives were calculated using density functional theory (DFT) with the B3LYP/6-31G (d, p) functional/basis set. The cycled LiNi0.5Co0.2Mn0.3O2/graphite cells were disassembled in the argon-filled glove box, and then, the NCM523 and graphite electrodes were rinsed with dimethyl carbonate and dried in the glove box. The interface films of cycled cathode and anode surfaces were characterized using transmission electron microscopy (TEM, JEM-2100, JOEL). The pre-cycled cathode of full cells was soaked in the blank electrolyte and stored at 60 °C for 5d before ICP-MS analysis. The surface elemental compositions of the cycled cathode and anode were analyzed in detail by X-ray photoelectron spectroscopy (XPS, Axis Ultra DLD, Kratos).

3. Results and discussion 3.1 Film formation evaluation of the electrolyte additives For the selection of an electrolyte with excellent properties to match the LIBs with specified electrode materials, the additive and the solvent composition are all important. After an overall consideration of solvent candidates and different additives, EC and EMC were chosen as the solvent components, and PFPMS was optimized as the additive. Their frontier molecular orbital, relating with the oxidative and reductive properties of functional groups, is considered so that the interface films could be formed on both electrode surfaces. The results of the HOMO/LUMO energies of solvent molecules and additives are shown in Figure 1. For comparison with a typical electrolyte, the data of VC are also listed. Based on frontier molecular orbital theory, the greater the HOMO energy of the 8


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molecule, the more easily the molecule is oxidized. Similarly, the smaller the LUMO energy of the molecule, the more easily is reduced.38,39 It can be seen from the results of DFT calculations that the HOMO energy of VC is higher than those of solvent molecules, and the LUMO energy is lower than those of solvent molecules. This indicates that VC can take precedence over the oxidation and reduction reactions of the solvent molecules to form interface films, which is consistent with the observed electrochemical behavior of VC in the electrolyte.40,41 Correspondingly, in contrast to the LUMO energy of EC (0.9440 eV), EMC (1.2123 eV), and VC (-0.0191 eV) molecules, PFPMS exhibits a quite low LUMO energy (-1.3113 eV), suggesting that these molecules will be preferentially reduced in the same electrolyte. In addition, the HOMO energy (-7.3182 eV) of PFPMS additive is higher than those of EC (-8.0026 eV) and EMC (-7.6304 eV), also showing that PFPMS is more susceptible to oxidation on the cathode. This theoretical calculation proves that PFPMS additive could be preferentially oxidized and reduced to form an interfacial film on the surfaces of cathode and anode electrodes. To investigate the film forming behavior of the additives on the graphite surface, CV tests were performed at a sweep rate of 0.05 mV s-1 using Li/graphite cells, as displayed in Figure 2. The redox peaks below 0.3 V could be described to the lithiation/delithiation process of lithium ions on the graphite electrode.42,43 The reduction peak at approximately 0.75 V signifies the reductive decomposition of EC. Compared to the CV curve of the Li/graphite cell without the additive, there is an obvious irreversible reduction peak occurring from about 1.2 V with 1.0 w.t.% VC additive, which could be due to the priority reduction of VC additive. Similarly, the CV curve of the half cell with 1.0 wt.% PFPMS has an apparent irreversible reduction peak occurring from approximately 2.0-1.0 V at the first cycle, which could be attributed to 9



the preferential reduction of PFPMS to form the SEI film, which is consistent with the theoretical calculations. Additionally, a new plateau appears at approximately 1.3 V in the initial discharge curve of the Li/graphite half cell with PFPMS additive is revealed in Figure S1. These results demonstrate that PFPMS additive could be reduced in advance to form the SEI films on the anode surface. The pre-charge curves and dQ/dE versus voltage curves during pre-charge process of LiNi0.5Co0.2Mn0.3O2/graphite full cells with different electrolytes are presented in Figure 3. Comparing to the pre-charge curves of the full cell without the additive, it can be clearly found that the new plateau appears on the pre-charge curves of the full cell with PFPMS additive. Moreover, previous studies have shown that the plateaus below 3.5 V in the full cells are due to the reduction of electrolyte rather than its oxidation.44,45 Therefore, the plateau at 2.5 V displays the process of reducing PFPMS molecule and forming the SEI film on the anode, consistent with the reduction peak observed in the CV curve at approximately 2.0-1.0 V. Furthermore, the peak at 2.5 V in the dQ/dE versus voltage curve of the full cell with PFPMS additive also confirms the preferential reduction of PFPMS additive, and a peak appeared at approximately 3 V without and with VC additive, which can be assigned to the EC reductive process, indicating that the preferential reduction of PFPMS can inhibit the reduction of EC. As a comparison, VC additive could be preferentially reduced to form SEI films, but it does not prevent the further reduction of EC, and the reduction plateau and peak of VC does not appear in the pre-charge and dQ/dE versus voltage curves. All the results support that PFPMS as a better additive can preferentially form an SEI film with excellent performance on the graphite anode, therefore, it can effectively passivate the graphite surface and suppress the further reductive decomposition of the electrolyte, possibly leading to significant enhancement of the long-term cycling stability of full cells. 10


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3.2 Electrochemical performances of the cells at the room temperature Figure 4a shows the cycling performance of LiNi0.5Co0.2Mn0.3O2/graphite full cells with different electrolytes at 1 C between 2.75 V and 4.2 V. It is obvious that the long-term stable cycling performance of the cell containing PFPMS additive is superior to the cells without and with reference VC additive. The initial discharge capacity values of the full cells using blank, reference and PFPMS-containing electrolytes are 1765.6, 1792 and 1835 mAh, respectively. The full cell with PFPMS exhibits the highest discharge capacity, indicating that the addition of the additive contributes to the capacity performance. After 400 cycles, the cycling retention of the full cell containing 1.0 wt.% PFPMS electrolyte reaches 91.7% with the capacity of 1681.8 mAh. In contrast, the full cells with the blank and reference electrolytes show capacity retentions of 74.3% with 1312.5 mAh and 76.7% with 1373.9 mAh. And the parallel test for cycling performance of the full cells with different electrolytes is displayed in Figure S2, demonstrating the reliability of the result. It can be found that the cycling retention of full cell with 1.0 wt.% VC additive is only slightly increased. As a commonly used electrolyte additive, it is generally known that VC can be reduced and form the SEI interphase on the anode surface before EC. Whereas, the existing results show that VC haven’t exhibited significant enhancement in the cycling performance of full cells. This may be attributed to the SEI interphase formed by VC on graphite surface being non-dense. In addition, the CEI layer formed by the oxidative decomposition of VC on cathode surface is unstable and cannot effectively prevent the continuous decomposition of the electrolyte and protect the cathode materials from HF erosion. Furthermore, the interface impedance of the full cell with VC is higher. However, the addition of 1.0 wt.% PFPMS additive can greatly improve the long-term cycling performance at room 11



temperature. Above results indicate that the interphase layers derived from the PFPMS are more stable and can effectively protect the electrode/electrolyte interphase to prevent continuous electrolyte decomposition, alleviate the side reactions between the active materials and electrolyte and allow fast lithium ions transport during long-term cycling. The rate capability of the LiNi0.5Co0.2Mn0.3O2/graphite full cells with different electrolytes are shown in Figure 4b. The rate capability of the cell with VC is slightly worse than that of the cell without additive, which can be ascribed to the higher impedance of the cell with VC additive. Moreover, in all cells with different electrolytes, the cell with PFPMS additive delivers a much superior discharge capacity than the blank and reference electrolytes at various current densities, especially at high current densities, which shows that the PFPMS-added electrolyte has the best rate capability. It is because that the PFPMS-derived interface layer increases lithium ions conductivity, reduces the charge-transfer resistance and promotes the rapid transfer of lithium ions at various high current densities. This could be confirmed by the EIS of the full cell that shows that the lowest interfacial impedance is obtained with PFPMS additive after pre-cycling, therefore facilitating the rapid transport and diffusion of lithium ions at the electrode-electrolyte interface at high current density. The EIS was used to analyze changes in cell impedance during the cycling process at room temperature, as shown in Figures 4c and 4d. The EIS of the cell is usually composed of three parts, the high and intermediate frequency range each exist as a semicircle, and the low frequency range is a straight line. It is usually considered that the high frequency range semicircle in the EIS is assigned to the impedance of the SEI (Rf), the intermediate frequency range semicircle is related to the charge transfer resistance (Rct) and double layer capacitance (Cdl), and the low frequency range straight 12


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line reflects the Warburg impedance (Zw).46 Figure 4c shows the impedance of full cells using different electrolytes after pre-cycling. It is clearly indicated that the cells with PFPMS exhibit the lowest impedance after pre-cycling, while the impedance of cells with VC additive is increased. This shows that PFPMS additive forms low-resistance interface films on the electrode surface after pre-cycling. In addition, the impedance of LiNi0.5Co0.2Mn0.3O2/Li and Li/graphite half cells was tested after pre-cycling, the results are displayed in Figure S3, indicating that the impedance of all half cells with PFPMS additive is obviously decreased after pre-cycling. It can be demonstrated that PFPMS can optimize both cathode and anode interfaces, which contributes to reduce the impedance of full cell. Figure 4d shows the impedance of full cells after 400 cycles at room temperature. There isn’t obvious difference for the Rf values of cells with different electrolytes, while a large difference for the Rct values can be obtained. The Rct of the full cell with PFPMS is much lower than that with VC and that without the additive. Combined with the cycling performance of the cells, CV curves and pre-charge curves, it further indicates that the addition of PFPMS could form stable and ionically conductive interface layers on the electrode surfaces to minimize electrolyte decomposition and formation of by-products, and thereby decreasing the impedance of full cells.

3.3 Electrochemical performances of the cells at the elevated temperature To evaluate the storage performance at elevated temperature, the full cells at a fully charged state were stored at 60 °C for 7 d, and then, the change in the impedance and capacity retention and recovery were obtained by cyclic recovery at room temperature, the results are displayed in Figure 5. Figures 5a and 5b show the first discharge curves and the third charge-discharge curves at room temperature after the storage at 60 °C for 13



7 days. For the first discharge curves, the discharge capacity with PFPMS additive was significantly improved, despite the lack of significant changes in the discharge plateau. For the 3rd charge-discharge curves, the cell containing 1.0 wt.% PFPMS not only has the highest capacity recovery but also shows the smallest voltage polarization. Figure 5c displays the obtained capacity retention and recovery by cyclic recovery at room temperature after the storage at 60 °C for 7 d. The capacity retention of the full cells without additive and with VC and PFPMS is 79.7, 82.9, and 86.3%, and the corresponding capacity recovery is 86.5, 88.2, and 90.6%, respectively. The cycling performance of full cells with different electrolytes at 45 oC also was tested to evaluate the cycling stability at elevated temperature, as displayed in Figure S4. The cell containing 1.0 w.t.% PFPMS exhibits the best cycling stability at elevated temperature. And the cycling retention of full cell containing 1.0 wt.% PFPMS maintains at 96.2% after 100 cycles at 45 oC, higher than that of the cells without additive (92.8%) and with VC additive (94.3%). For NCM cathode materials, there is a problem in that metal ions are dissolved in the electrolyte and deposited on the graphite surface, resulting in rapid capacity fading and even cell failure. While the HF generated by the thermal decomposition and the hydrolysis reaction of LiPF6 generally attacks the NCM cathode material, resulting in the dissolution of metal ions, this phenomenon is more pronounced at elevated temperature.47,48 A comparison of transition metal dissolution from NCM523 cathode stored at 60 °C for 5d is shown in the Figure S5. The ICP-MS result shows that the cathode with PFPMS additive has the lowest metal ions (Ni, Co, Mn) concentration in the electrolyte, which reveals that the PFPMS-derived CEI layer imparts resistance to transition metal dissolution at elevated temperature. Therefore, the high-capacity retention and recovery of full cell containing PFPMS could be believed that the 14


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passivation film derived from the PFPMS additive on the cathode surface, which prevents the HF from attacking the cathode and thus reducing the dissolution of transition metal cations. The impedance of the cells with different electrolytes after 60 °C storage for 7 days, as illustrated in Figure 5d. After the storage at 60 °C for 7d, the impedance of the cells with different electrolytes shows significant differences. The impedance of the cell with PFPMS is much lower than that of the cells without and with VC additive. It has been reported that the thermal decomposition and the hydrolysis reaction of LiPF6 is more severe and contributes to the generation of more HF at elevated temperature, which could attack the cathode material and lead to the dissolution of more metal ions and deposition on the graphite surface, thus further increasing the impedance of LIBs. Therefore, the significantly lower impedance of the cell containing PFPMS is attributed to the fact that the additive forms a protective interface layer on the electrode surface, alleviating the formation and accumulation of by-products from the high temperature decomposition of the electrolyte and reducing the dissolution of the metal ions and the deposition on the graphite anode that result from the undesirable reactions between the electrode and the electrolyte.

3.4 Electrochemical performances of the cells at the low temperature To evaluate the discharge performance of the cells at a low temperature, the discharge tests were performed at -20 °C. The discharge curves, capacity retention and EIS spectra of full cells with different electrolytes at -20 °C are shown in Figure 6. According to the discharge curves, the initial discharge voltage of the cell with 1.0 wt.% PFPMS is significantly higher than that without the additive and with VC at discharge rates of 0.2 and 0.5 C, indicating that PFPMS could reduce the polarization of the cell at 15



low temperature. At the discharge rate of 0.2 C, the discharge capacity retention of the cell with 1.0 wt.% PFPMS reaches 72.3%, and the retention values of the cells without the additive and with 1.0 wt.% VC are 67.8 and 70.5%, respectively. Along with the improvement in the current density, the difference in the discharge capacity is more obvious. At the discharge rate of 0.5 C, the discharge capacities of all full cells decreased. Nevertheless, the discharge capacity retention of the cell containing 1.0 wt.% PFPMS is still the highest, reaching 66.3%. Meanwhile the cells without the additive and with 1.0 wt.% VC are only 55.0 and 62.1%, respectively. Therefore, the addition of PFPMS could reduce the cell polarization, improves the discharge voltage platform and discharge capacity retention at low temperature. From the EIS result of full cells at low temperature, it can be found that the full cell with PFPMS additive has the lowest interfacial impedance. It is generally considered that the factors that limit the electrochemical performances of LIBs at low temperature include the Li+ conductivity in the electrolyte, the interface impedance of the cells and the process of Li+-desolvation.49-51 However, most reported results suggest that the electrochemical performances of LIBs are mainly limited due to the high interface impedance of the cells at low temperature.51 These results demonstrate that the interfacial films with low-resistance formed by the participation of PFPMS additive can reduce the interface impedance and increase the conduction of lithium ions, which greatly enhances the diffusion and migration kinetics of lithium ions and the electrochemical performances of LIBs at low temperature.

3.5 Surface analysis of the cycled electrodes TEM analysis is the most direct method for observing the electrode surface features that can further support the effectiveness of PFPMS additive for enhancing the 16


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electrochemical performances of full cells operating in a wide-temperature range. Figure 7 displays the TEM images of the cathode and anode electrodes with different electrolytes after 400 cycles at room temperature. For TEM analysis of the cycled cathode, the surface morphology of the cathode with different electrolytes shows significant differences. The surface morphology of the cycled cathode without additive has a clear contour edge, indicating that the CEI layer was not formed on the cathode. However, the cycled cathode surface with VC and PFPMS was covered with the interface film. In addition, the interface film formed by VC is incomplete and less compact, and the PFPMS-derived interface film is more uniform and dense, further supporting that the CEI layer with PFPMS is more stable and can effectively protect the cathode material from the corrosion of electrolyte. For TEM analysis of cycled anode, all the cycled anode surfaces were covered with SEI film. Similarly, the morphologies of three SEI films on the graphite surface were also different. It is generally considered that the SEI film on the graphite surface without additive is derived from the reduction of EC, which is un-uniform and incomplete. And the SEI film on the graphite surface with VC and PFPMS additives originates from the preferential reduction of the additive, which is complete and uniform. Differently, the SEI film with PFPMS additive is more compact and thinner, which is conducive to reducing the impact of the metal ions on the graphite anode and decreasing the impedance of the cell. By contrast, the SEI film with VC additive is thicker, corresponding to its higher interface impedance, which may hinder the transport of lithium ions. To investigate the differences between the cathode interface without and with the additives, the XPS analysis of the cathode after 400 cycles at room temperature was performed, the results are presented in Figure 8. As seen from these results, the composition of the cathode interface is significantly different without and with the 17



additives. First, the cathode surface with PFPMS additive exhibits the unique element S of the additive molecule, while the S element was not detected for the other cathode surfaces, proving that PFPMS additive was involved in the formation of the CEI layer, and the TEM results also clearly show the presence of the CEI layer on the cathode surface with PFPMS additive. Second, compared to the cathode surface without additive (37.21%) and that with VC additive (37.71%), the C element content in the cathode surface increases to 41.19% when PFPMS additive is used, indicating that more organic components are included in the CEI layer of cathode. In addition, the concentration of Li and F elements is slightly decreased, further showing that the additive forms more organic components at the expense of inorganic components.52 It is generally believed that the inorganic components contribute to the increase in the cell impedance. Therefore, the smaller amount of inorganic components and the greater amount of organic components of the CEI layer on cathode surface contribute to the reduction in the interface impedance, which is consistent with EIS results. Finally, a lower amount of transition metal elements was detected on the cathode surface with PFPMS additive than for the cathodes without additive and with VC additive. This indicates that the additive forms an effective passivation film, which could effectively suppress the dissolution of transition metal ions and the migration to anode surface during long-term cycling. The detailed analysis of individual XPS spectra further explains the mechanism of the effect of the additive. Examination of the C 1s spectra for the cathode cycled in PFPMS-containing electrolyte shows that the intensities of C=O (288.5 eV) and C-O (286.2 eV) signals are decreased when compared with the cycled cathode without and with VC additive. Corresponding to the O 1s spectra, the intensities of C=O (531.5 eV) and C-O (533.3 eV) signals also decrease when adding PFPMS additive, and the results 18


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of the O1s spectra are consistent with those of the C 1s spectra. These results indicate that the electrolyte with PFPMS additive exhibits less oxidative decomposition of the organic solvent and lower levels of polymeric C-O compounds during long-term cycling.53,54 In addition, the intensity of the transition metal oxide peak (Me-O) at approximately 529.6 eV is weaker for the cathode with PFPMS additive than that without and with VC additive in the O 1s spectra. Furthermore, the cathode surface with PFPMS additive exhibits a lower content of Ni 2p and Mn 2p than the cathodes without additive and those with VC additive. These results support that PFPMS forms a protective CEI layer on the cathode surface to prevent the dissolution of transition metal ions. For F 1s spectra, the peaks appearing at approximately 684.8, 686.2, 687.3, and 688.3 eV correspond to LiF, LixPOyF, LiPxFy and C-F, respectively. Compared to the cathode without and with VC additive, the intensities of the LiF and LiPxFy peaks at 684.8 and 687.3 eV are decreased with the addition of PFPMS, indicating that the cathode surface contains less LiF content and decomposition products of LiPF6. The result indicates that the CEI layer formed by PFPMS additive can better prevent the decomposition of LiPF6 to reduce the LiF content and facilitates the decrease in the interface impedance. The P 2p spectra also illustrate this point, the amount of LixPFy (approximately 136.5 eV) from the decomposition products of LiPF6 is less than that of the cathode without additive. From S 2p spectra, the peaks at 169.2 eV and 168 eV respectively correspond to the presence of Li2SO4, ROSO2Li, which is related to the oxidation of PFPMS additive on the cathode.55,56 The XPS analysis of the graphite anode after 400 cycles under room temperature condition is presented in Figure S6. In comparison with the anode without and with VC additive, the unique S element of PFPMS additive was detected on the graphite surface, further supporting the additive to form SEI film on the anode surface. Furthermore, 19



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referring the results of previous works with a similar S containing additives as the investigated PFPMS molecule,56,57 the observed peaks at 169.6 eV (Li2SO4), 168.5 eV (ROSO2Li) and 163.2 eV (Li2S) can be assigned to products related to the reduction of PFPMS additive. For C 1s spectra, the intensity of Li2CO3 at 290 eV is weaker or even disappears for the cycled anode surface with PFPMS,43,58 which is consistent with the results of the O 1s spectra. What’s more, the LiF content of cycled graphite surface with PFPMS is lower than that without and with VC additive in the F 1s spectra. Above results suggest that the SEI film with PFPMS additive has a lower inorganic component, which corresponds to a low interface impedance of the full cell. Based on the surface analysis of the cycled electrodes, all these results provide evidence that the passivation film derived from the PFPMS additive on the cathode and anode surfaces could effectively protect the electrode interfaces to mitigate the side reactions and promote the superior electrochemical performances of full cells operating in a wide temperature range.

4. Conclusions PFPMS was optimized and evaluated as a novel versatile electrolyte additive in the EC and EMC solvents to broaden the operational temperature range of the LIBs with a high-nickel-content

NCM

cathode.

As

an

example

application,

the

LiNi0.5Co0.2Mn0.3O2/graphite full cells with the PFPMS-containing electrolyte exhibited excellent electrochemical performances at high, low, and room temperatures. Based on the DFT calculations, electrochemical and TEM analysis, it was demonstrated that PFPMS additive could form a stable interfacial film on both the cathode and anode surfaces. In addition, the EIS, ICP-MS and XPS results indicated that the interfacial film formed by PFPMS additive can reduce the cell impedance and prevent the 20


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undesired reactions between the cathode materials and electrolyte. Based on its excellent cycling, wide temperature range and rate capability of the cell with PFPMS additive, the as-prepared electrolyte with PFPMS additive is promising for use in applications of the LIBs high-nickel-content NCM cathode.

Supporting Information Available Initial discharge curves and EIS spectra of half cells, parallel test of cycling performance, high temperature performance, results of ICP-MS analysis, and XPS spectra of cycled anode.

Acknowledgements This work was financially supported by the science and technology projects of Guangdong Province (2015B010135008), and the science and technology projects of Guangzhou (2014J4500025, 201604016131, 201704030020).

21



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27



Figure 1. Chemical structures and calculated HOMO/LUMO energies of EC, EMC, VC, and PFPMS molecules.

28


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Figure 2. CV curves of Li/graphite half cells at the scan rate of 0.05 mV s-1 over the potential range of 0.01-3 V, (a) without additive, (b) with 1.0 wt.% VC additive, (c) with 1.0 wt.% PFPMS additive.

29



Figure

3.

(a)

Pre-charge

curves

and

(b)

dQ/dE

Page 30 of 37

vs.

LiNi0.5Co0.2Mn0.3O2/graphite full cells with different electrolytes.

30


voltage

curves

of

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Figure 4. (a) The cycling performance of LiNi0.5Co0.2Mn0.3O2/graphite full cells with different electrolytes at rate of 1C over the potential range of 2.75-4.2 V at room temperature, (b) rate performance of the full cells with different electrolytes. The EIS spectra of the full cells with different electrolytes: (c) after the pre-cycling, (d) after 400 cycles.

31



Figure 5. Electrochemical performances of LiNi0.5Co0.2Mn0.3O2/graphite full cells with different electrolytes after the storage at 60 °C for 7 d: (a) 1st discharge curves at room temperature, (b) 3rd charge-discharge curves at room temperature, (c) capacity retention and recovery, (d) EIS spectra.

32


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Figure 6. Electrochemical performances of LiNi0.5Co0.2Mn0.3O2/graphite full cells with different electrolytes at -20 °C: (a) discharge curves at 0.2 C, (b) discharge curves at 0.5 C, (c) capacity retention at 0.2 C and 0.5 C. (d) EIS spectra.

33



Figure 7. TEM images of cycled cathode (a) without additive, (b) with 1.0 wt.% VC, (c) with 1.0 wt.% PFPMS. TEM images of cycled anode (d) without additive, (e) with 1.0 wt.% VC, (f) with 1.0 wt.% PFPMS.

34


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Figure 8. XPS spectra of (a) C 1s, (b) O 1s, (c) F 1s, (d) P 2p, (e) Ni 2p, (f) Mn 2p measured from the cycled cathode (A) without additive, (B) with VC, and (C) with PFPMS, (g) S 2p XPS spectra of cycled cathode with PFPMS additive, (h) element contents of the cycled cathode with different electrolytes by XPS analysis.

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Table of Contents

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228x152mm (300 x 300 DPI)


2,3,4,5,6-Pentafluorophenyl Methanesulfonate as a Versatile

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