Electrochemical Stabilization of Self-Extinguishing Electrolyte

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Electrochemical Stabilization of Self-Extinguishing Electrolyte Solutions with Trimethyl Phosphate by Adding Potassium Salts Shigetaka Tsubouchi, Shohei Suzuki, Katsunori Nishimura, Takefumi Okumura, and Takeshi Abe J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10708 • Publication Date (Web): 01 Jun 2018 Downloaded from http://pubs.acs.org on June 1, 2018

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The Journal of Physical Chemistry

Electrochemical Stabilization of Self-Extinguishing Electrolyte Solutions with Trimethyl Phosphate by Adding Potassium Salts

Shigetaka Tsubouchi,*,†,‡ Shohei Suzuki,† Katsunori Nishimura,† Takefumi Okumura,† Takeshi Abe,‡

†Hitachi,

Ltd., Research & Development Group, 7-1-1, Omika, Hitachi, Ibaraki, 319-1292, Japan

‡Graduate

School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan

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ACS Paragon Plus Environment

The Journal of Physical Chemistry 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 A potassium salt, potassium bis(trifluoromethanesulfonyl)amide (KTFSA), with a weak Lewis acid in a self-extinguishing electrolyte solution containing trimethylphosphate (TMP) can improve the coulombic efficiency of charge-discharge in lithium-ion batteries with graphite negative electrodes.

17

O nuclear magnetic resonance (NMR) analysis of electrolyte solutions with KTFSA

indicated that weak Lewis acid K+ decreased the strength of the EC-Li+ interaction and improved the coulombic efficiency. Furthermore, three X-ray analyses, X-ray diffraction (XRD), X-ray fluorescence (XRF), and X-ray photoelectron spectroscopy (XPS), clarified that the addition of KTFSA derived from the stabilization of the electrolyte solution by forming an effective protective film on the graphite surface had the greatest effect on the coulombic efficiency.

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1. INTRODUCTION Lithium-ion batteries are now widely used in high-power and large-scale applications such as electric-load leveling and electric vehicle systems. The lithium-ion battery, which has higher energy than the lead-acid battery, nickel-metal-hydride battery, and so on, needs to have a high level of safety. However, conventional alkyl carbonate-based electrolyte solutions in lithium-ion batteries can easily catch fire in air. Recently, a flame retardant electrolyte solution has been achieved by including

organophosphorus

compounds,

such

as

phosphates1-5,

phosphonates6,7,

and

phosphazenes8,9. In particular, trimethylphosphate (TMP) suppresses the flammability of electrolyte solutions when the TMP content is 30% or more in an ethylene carbonate (EC) and ethylmethyl carbonate (EMC) mixed solution.

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However, TMP tends to co-intercalate with lithium ions into

the graphite anode because its donor number is higher than that of EC11, resulting in continuous decomposition and a large irreversible capacity loss during the initial cycling12-16. In our previous study10, the co-intercalation was suppressed by adding calcium bis(trifluoromethanesulfonyl)amide

(Ca(TFSA)2)15, magnesium bis(trifluoromethanesulfonyl)-

amide (Mg(TFSA)2), or sodium bis(trifluoromethanesulfonyl)amide (NaTFSA) to these solutions. Moreover, the coulombic efficiencies of the electrolyte solutions with different Lewis acids were different in the first charge-discharging process. We clarified that the strength of the EC-Li+ interaction correlated with the coulombic efficiency through

17

O nuclear magnetic resonance

(NMR) analysis of these electrolyte solutions. The strength of the EC-Li+ interaction was affected

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by the acidity of the Lewis acid, electron-donating ability of the organophosphorus compound, and solvated structure of the organophosphorus compound with the Lewis acid. In this study, we found potassium bis(trifluoromethanesulfonyl)amide (KTFSA) with fewer Lewis acid K+ than Na+ demonstrated a superior coulombic efficiency in the first charge-discharging process. However, the improvement in coulombic efficiency by adding KTFSA has not been known yet, and we could not simply interpret improvement based on our previous study on salts with strong Lewis acids10. Therefore, we investigated the dominant factor to improve the coulombic efficiency by adding KTFSA.

2. EXPERIMENTAL SECTION A charge-discharge (TOSCAT3000, Toyo System) was measured using a three-electrode cell. The working electrode with a diameter of 16 mmφ was prepared by applying a mixture of natural graphite powder (Hitachi Chemical) as the active material, carboxymethylcellulose (Daicel) as dispersant, and styrene-butadiene rubber (JSR) as binder, onto copper foil. The counter and reference electrodes were made of Li foil. The electrolyte solution was 1 mol dm-3 LiPF6 dissolved in a mixture of EC, EMC, and TMP (Kishida Chemical) and did or did not contain 0.5 mol dm-3 KTFSA, NaTFSA, Ca(TFSA)2, and Mg(TFSA)2. The mole fractions of EC, EMC, and TMP are 4.0, 3.9, and 3.3 in the solutions contained 1 mol LiPF6 and 0.5 mol TFSA salts. The water content in the solution was less than 30 ppm. The separator was polyolefin film consisting of polypropylene and

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polyethylene (Ube Industries). The graphite electrode was charged at a constant current of 1 mA cm-2 (1/3 C-rate) or 0.1 mA cm-2 (1/30 C-rate) to 0.01 V vs. Li/Li+, kept at the charged potential for 7 hours, and discharged at a constant current of 1 mA cm-2 to 1.5 V vs. Li/Li+. The coulombic efficiency was calculated from each capacity in the first charge-discharge. All potentials in this paper reflect V versus Li/Li+.

In an EC-based electrolyte solution with LiPF6, Li+ coordinates with the O of the carbonyl group (C=O) in EC and forms a stable solvated structure.17 It is possible to figure out the change in solvated structure by investigating the change in magnetic screening in the O nucleus in an electrolyte solution with a flame retardant and an additive. All

17

O NMR spectra were obtained

using an NMR spectrometer (ECA-500 FT-NMR, JEOL) operating at 67.8 MHz for the 17O nucleus with a recycle delay of 0.2 s and an accumulation of 65536 scans. All spectra were recorded at 25°C. An NMR sample was sealed in a sample tube in a glove box and measured without being exposed to air. A liquid sample of H2O was used for radio frequency (RF) power calibration as well as for 17

O chemical shift referencing. X-ray diffraction (XRD) was applied to investigate the transition of a crystal structure in

graphite after charging and discharging. All XRD spectra were obtained by wide-angle X-ray diffraction device (RINT2500HL, Rigaku) with an output of 50 kV, 250 mA with a Cu X-ray source. A concentrated beam produced by monochromator was used for the optical system. A divergence slit of 0.5 degrees, receiving slit of 0.5 degrees, and scattering slit of 0.15 mm were selected. The

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scan axis of the X-ray diffraction was 2θ / θ. The sample was scanned at a rate of 0.5 degrees per minute, and with an interval of 0.01 degrees per step in the range of 5 < 2θ < 100 degrees. The obtained spectrum was identified based on data from the International Centre for Diffraction Data (ICDD). X-ray fluorescence (XRF) analysis was applied to investigate the abundance of potassium in a charged graphite negative electrode. XRF spectra were obtained by X-ray fluorescence spectrometer (ZSX Primus II, Rigaku). The charged graphite sample was irradiated with an output of 3 kW using a rhodium target X-ray tube. To prepare the sample for analysis, the electrode composite was stripped from the copper foil, powdered in a mortar, mixed with boric acid powder, and finally press-molded. The detectable elements in this analysis are those from boron to uranium in atomic number order. Since boric acid powder was applied as the molding material in the sample preparation, boron and oxygen were not analyzed. The chemical compositions of the surface films on the graphite electrode were investigated by X-ray photoelectron spectroscopy (XPS) using a PHI5000 VersaProbeII (ULVAC-PHI) with an Al Kα excitation source. The electrode was sealed in a vessel in the glove box to be transferred into the sample chamber without being exposed to air. F 1s, O 1s, N 1s, K 2p, C 1s, S 2p, P 2p, and Li 1s spectra were obtained with a resolution of 0.1 eV. The X-ray output, time per step, and pass energy were set at 25 W, 200 ms, and 47 eV, respectively. The peak positions were calibrated by reference to the C 1s level of graphite at 284.5 eV.

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3. RESULTS AND DISCUSSION In our previous study10, we found that any salts with bis(trifluoromethanesulfonyl)amide (TFSA) anion, can improve the coulombic efficiency in self-extinguishing electrolyte solutions with TMP. Figure 1 shows the first charge-discharging curves in graphite/Li metal half cells. The electrolyte solutions were composed of 1.0 mol cm-3 LiPF6 dissolved in a mixture of EC+EMC+TMP (1:2:3 v/v/v%) with no additives, 0.5 mol cm-3 NaTFSA, and KTFSA. The graphite electrodes were charged at a constant current of 0.1 mA cm-2 (1/30 C-rate) to 0.01 V and discharged at the same current to 1.5 V. Figures 1a and 1b indicate potential changes vs. capacity and the number of Li ions intercalated into graphite, respectively. In Fig. 1a, it can be seen that the charging capacity was over 600 mAh g-1 in the electrolyte solution without additives. However, the discharging capacity was nearly 100 mAh g-1. Therefore, this charging capacity, which exceeds the theoretical capacity of graphite of 372 mAh g-1, is caused by reductive decomposition in the graphite electrode. This decomposition can be attributed to co-intercalation of Li+ with TMP into the graphite interlayer 13. On the other hand, adding NaTFSA to the electrolyte solutions improved the coulombic efficiency to approximately 40% by partially suppressing the co-intercalation of Li+ with TMP. In particular, the solution to which KTFSA was added showed a coulombic efficiency of over 90%. According to a study on potassium-ion batteries by Jiang18, potassium ions can

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electrochemically intercalate into synthetic graphite. However, in Fig. 1a, it can be seen that the capacity is close to the theoretical value of the lithium intercalated compound LiC6. In Fig. 1b, as is known from Dahn’s report

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, the charging curve shows the change in the stage structure of the

graphite intercalated compound. In Fig. 1, the charge-discharging curves for the self-extinguishing electrolyte to which KTFSA was added imply that Li+ can only intercalate into graphite by adding KTFSA. In other words, it seems that K ions do not intercalate into graphite. It has not been reported yet how adding KTFSA suppresses co-intercalation. Furthermore, in our previous study on the effect of TFSA salts, we had not been able to understand the suppression mechanism. Therefore, we attempted to ascertain the suppression mechanism by adding KTFSA. Figure 2 shows the effect of the concentration of KTFSA on the charge-discharging in graphite/Li metal half cells. The electrolyte solutions were composed of 1.0 mol cm-3 LiPF6 dissolved in a mixture of EC+EMC+TMP (1:2:3 v/v/v%) with 0-0.5 mol cm-3 KTFSA. The graphite electrodes were charged-discharged at a constant current of 1 mA cm-2 (1/3 C-rate). With an increase in the concentration of KTFSA, the discharge capacity was increased. As a result, the coulombic efficiency was also improved with an increase in the concentration of KTFSA. With the addition of 0.5 mol cm-3 KTFSA especially, the coulombic efficiency and discharge capacity were 90% and 287 mAh g-1, respectively, even when the graphite electrode was charged-discharged at a practical current rate of 1 mA cm-2 (1/3 C-rate). The coulombic efficiency exceeded 64%, and the

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discharge capacity exceeded the 254 mAh g-1 of Ca(TFSA)2 with strong Lewis acid Ca2+ in our previous study10. To clarify the factors causing an improvement in coulombic efficiency, the relative strength of the EC-Li+ interaction in the electrolyte solution was examined from the chemical shift changes in the oxygen of the C=O bond in EC in the 17O NMR spectra. Figure 3 shows the 17O NMR spectra in 1.0 mol cm-3 LiPF6 dissolved in a mixture of EC+EMC+TMP (1:2:3 v/v/v%) with 0-0.5 mol cm-3 KTFSA. The observed spectral peaks show the chemical shifts of the oxygen of the C=O bond in EC under different KTFSA concentrations in the electrolyte solution. The spectrum in the 1 mol dm-3 LiPF6/ EC+EMC (1:2, v/v%) mixture (a) is a reference to identify spectral peaks. Curves (b)-(e) show the change in the 17O NMR chemical shifts of the oxygen of the C=O bond in EC in the 1 mol dm-3 LiPF6/ EC+EMC+TMP (1:2:3, v/v/v%) mixture containing no additives (b), 0.1 (c), 0.3 (d), and 0.5 (e) mol dm-3 KTFSA. The decrease in the chemical shift in Fig. 3 means an increase in the strength of the EC-Li+ interaction20. When TMP was added (b) to EC+EMC (a), the strength of the EC-Li+ interaction decreased due to the selective solvation of TMP with Li+. As a result, the chemical shift in EC+EMC+TMP (b) was larger than that in EC+EMC (a). Therefore, the EC-Li+ interaction in the electrolyte solution was slightly strengthened by adding KTFSA. We have already obtained the results that the EC-Li+ interaction in the electrolyte solution was strengthened by adding Mg(TFSA)2, Ca(TFSA)2, or NaTFSA.10 Figure 4 shows the correlation between the chemical shift, which means the strength of the

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EC-Li+ interaction, and the coulombic efficiency in charge-discharging at the 1/30C-rate (open and filled squares) and 1/3C-rate (open and filled circles). Firstly, we focus on the plots on the solid line. They show the results obtained in our previous study and in this study by adding Mg(TFSA)2, Ca(TFSA)2, or NaTFSA to the same electrolyte solution including 50 vol% TMP. The TFSA salts of the other Lewis acids, with the exception of KTFSA, showed a correlation between the EC-Li+ interaction and coulombic efficiency. The correlation shown by the solid line in Fig. 4 will be explained by the diagram in Fig. 5. When TMP was added to the EC-based solution, the strength of EC-Li+ interaction decreased due to selective solvation of TMP with Li ions. As a result, the TMP with Li ions co-intercalate into the graphite negative electrode. By adding a Lewis acid to the electrolyte solution including TMP, the strength of EC-Li+ interaction increased due to selective solvation of TMP with a Lewis acid. Strong Lewis acids like magnesium ion can selectively solvate and re-enhance the EC-Li+ interaction. As a result, these Lewis acids can suppress the co-intercalation of TMP with Li ions and improve the coulombic efficiency. Secondly, we focus on the plots on the dotted line in Fig. 4. These plots are the results in the 1 mol dm-3 LiPF6/ EC+EMC+TMP (1:2:3, v/v/v%) mixture with 0-0.5 mol cm-3 KTFSA. The figures in the square brackets indicate the concentration of KTFSA. The plots on the dotted line show the correlation between chemical shift, which means the strength of the EC-Li+ interaction in the electrolyte solution with added KTFSA, and the coulombic efficiency obtained by

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charge-discharging regardless of the current rate. The correlation indicated that weak Lewis acid K+ can also decrease the strength of the EC-Li+ interaction and increase the coulombic efficiency. However, the correlation was not consistent with the solid line that represents the electrolyte solution including Mg(TFSA)2, Ca(TFSA)2 or NaTFSA with a stronger Lewis acid than K+. Although the strength of the EC-Li+ interaction caused by adding KTFSA was weaker than “NaTFSA” in the chemical shift, the coulombic efficiency was improved to the same extent as when using the EC+EMC electrolyte solution, “No add.” filled square in Fig. 4. This result indicated that the effect of KTFSA is not only weakening the interaction. A main factor for improving coulombic efficiency are considered to be cations, because EC-Li+ interaction change by those cations with the same TFSA anion. The two pen circles, which represent adding Mg(TFSA)2 and Ca(TFSA)2, deviated from the solid line in Fig. 4. This decrease in coulombic efficiency at the 1/3C-rate was derived from the reaction overvoltage assuming that the two filled squares and “No add.” representing the EC+EMC electrolyte solution without TMP perfectly overlapped. Furthermore, the rising of the reaction overvoltage caused by adding Mg(TFSA)2 and Ca(TFSA)2 can also be understood from the charge-discharge curves in our previous research10. On the other hand, the two open circles and squares representing the adding of 0.5 mol cm-3 KTFSA are close to each other. This result indicates that the reaction overvoltage in the cell hardly rose by adding KTFSA. From the above results, it is considered that KTFSA will have a different reaction with other additives. Therefore, the role of

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KTFSA was investigated to find out why the KTFSA can suppress the co-intercalation of Li ions with TMP more strongly than with other salts with a strong Lewis acid. Figure 6 shows the X-ray diffraction peaks of the graphite negative electrode in the charging state in the electrolyte solution including TMP with 0.5 mol cm-3 KTFSA. The strong peak of Cu (200) derived from the current collector of the electrode. The lower graph is an extension of the upper one. In the upper graph, several peaks derived from the Li intercalated compound, LiC6 and LiC12, were observed. The lattice spacing of LiC6 (001) was 0.3699 nm. This value is consistent with the crystal lattice size of the graphite Li intercalated compound. In the lower graph, the peaks representing graphite were observed. In these results, peaks representing K intercalated structure, which is known from past studies on graphite intercalated compounds of alkali metals21, 22, were not observed. Instead, a small amount of KPF6 was observed in the charging state. This result indicated that most of the K ions do not exist in the layers of graphite in the charged state. Figure 7 shows the X-ray diffraction peaks of the graphite negative electrode in the discharged state in the same electrolyte solution. The lower graph is also an extension of the upper one. In these graphs, several peaks mainly derived from graphite were observed. The existence of these peaks means this charge-discharging reaction in this electrolyte solution is reversible. However, KPF6 remained after discharge. We considered that KPF6 can suppress co-intercalation of TMP with Li ion. The abundance of K in the graphite negative electrode was investigated by XRF. Figure 8 shows

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the existence ratio of the elements in the graphite negative electrode detected by the XRF. The graphite negative electrode was charged in a 1 mol dm-3 LiPF6/EC+EMC+TMP (1:2:3, v/v/v%) mixture with 0.5 mol dm-3 KTFSA. The O is excluded from the observation element due to the use of boric acid powder to prepare a sample. F, P, K, and S were detected in the charged graphite negative electrode, but C derived from graphite was not. This shows that the K content was less than the P content originating from LiPF6. This indicates that hardly any K exists in the electrode. Therefore, KPF6 should not contribute to suppressing the co-intercalation. The surface film on the graphite negative electrode was also analyzed by XPS. We considered that if KTFSA forms an effective protective film on the graphite surface, the film can suppress the Li ion co-intercalation with TMP. Figure 9 shows the detected components on the graphite surface in the electrolyte solution including TMP but without KTFSA. The pie chart in Fig. 9 indicates the constituents on the surface of the discharged graphite negative electrode. The components of the surface film in the electrolyte were inorganic-rich with Li, F, and P accounting for 53% of all the detected elements. The XPS spectra of C1s, O1s, Li1s, F1s, and P2p are also shown in Fig. 9. In the C1s spectrum, the peak intensity of binding species, C-O, -CFHCFH-, and CO3, derived from the decomposed product of carbonate solvents and LiPF6 is stronger than the peak intensity of the C-C from graphite. Therefore, the surface film was thick enough to cover the surface of the graphite. In the O1s and Li1s spectra, the surface film components were mainly Li2CO3, Li2O, and LiF with low ion conductivity. From the results, we consider that the components of the surface film were

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decomposition products originating from EC, EMC, and LiPF6. But in the pie chart in Figure 10, which shows the surface film composition in the electrolyte solution including TMP with KTFSA, the components of the surface film were organic-rich. The C and O account for 65% of all the detected elements. The XPS spectra of C1s, O1s, Li1s, F1s, and P2p are also shown in Fig. 10. In the C1s spectrum, the peak intensity of binding species, C-O, O-C=O, and CF, derived from the decomposed product of carbonate solvents and LiPF6 is weaker than the peak intensity of the C-C from graphite. Therefore, the surface film was thin enough to detect the surface of the graphite. In the C1s and O1s spectra, the main surface film component was alkyl ester (RCO2R’) with high ion conductivity. In the Li1s spectrum, only LiF was detected, but the amount was smaller than that of the alkyl ester as shown in the comparison between the Li and the O in the pie chart in Fig. 10. From the results, we consider that KTFSA can help to form an effective surface film to suppress co-intercalation of TMP with Li+. Figure 11 shows the reaction mechanism that occurs with the addition of KTFSA. These images show the charging process in the graphite electrode. The upper flow shows the interfacial reaction in an electrolyte solution including TMP but without KTFSA. The lower flow shows the reaction with KTFSA. In the charging process with no KTFSA added, Li ions co-intercalate with TMP and decompose. As a result, the decomposed products are deposited on the graphite surface. But with KTFSA, an effective protective film is formed in the first charging process. As a result, the surface film can suppress the co-intercalation and let Li ions through smoothly.

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In this study, the coulombic efficiencies in self-extinguishing electrolyte solutions with TMP by forming effective protective films by adding KTFSA almost reach the coulombic efficiencies of EC+EMC at a practical current, 1/3C-rate, as shown in Fig. 4. The addition of KTFSA can enhance the electrochemical stability of self-extinguishing electrolyte solutions. The KTFSA in this study can enhance the electrochemical stability by suppressing co-intercalation of Li+ with a high donor solvent like TMP. The KTFSA works as a substitute for conventional additives such as vinylene carbonate23-25 and vinyl ethylene carbonate23-26, which can form a protective film on the surface of a graphite negative electrode. To commercialize KTFSA as an electrolyte in lithium-ion batteries, several challenges still remain. For example, the discharge capacity in the electrolyte solution with 0.5 mol dm-3 KTFSA added was lower than that of the conventional flammable electrolyte solution27 as shown in Fig. 2.

4. CONCLUSION The electrolyte solutions containing 50 vol% of TMP that have self-extinguishing properties decomposed in the first charging process due to the co-intercalation of Li+ with TMP into the graphite interlayer. In this study, the co-intercalation was suppressed by adding KTFSA to the solution. With an increase in the concentration of KTFSA in the electrolyte solution, the discharge capacity and coulombic efficiency was increased. In the

17

O NMR analysis of the electrolyte

solution with different concentrations of KTFSA, we found there is a correlation between the

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strength of the EC-Li+ interaction and the coulombic efficiency. However, the correlation is not consistent with our previous study on electrolyte solutions including Mg(TFSA)2, Ca(TFSA)2, or NaTFSA with a stronger Lewis acid than K+. Although the effect of adding KTFSA on EC-Li+ interaction was smallest in one of the other TFSA salts, the coulombic efficiency was improved a great deal as well as when using the conventional electrolyte solution without TMP. The XRD, XRF, and XPS analysis indicate that the effect of adding KTFSA on the electrochemical stabilization of the self-extinguishing electrolyte solution not only weakens the EC-Li+ interaction but also forms an effective film on the graphite surface to suppress the co-intercalation. Therefore, we will continue to study to further enhance the electrochemical stability of the self-extinguishing electrolyte solution for practical use. We believe that this study will contribute to the development of electrolytes for lithium-ion batteries.

AUTHOR INFORMATION Corresponding author: Shigetaka Tsubouchi *Phone: +81-50-3176-3380; fax: +81-0294-52-7636; e-mail: [email protected]

ACKNOWLEDGMENTS This research was conducted with financial support from Hitachi, Ltd.

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Intercalation/De-Intercalation of Lithium Ions at Graphite Negative Electrode in TMP-Based Electrolyte Solution. J. Electrochem. Soc. 2012, 159, A2089-A2091. (16) Xu, K.; Ding, M. S.; Zhang, S.; Allen, J. L.; Jow, T. R. Evaluation of Fluorinated Alkyl Phosphates as Flame Retardants in Electrolytes for Li-Ion Batteries: I. Physical and Electrochemical Properties. J. Electrochem. Soc. 2003, 150, A161-A169. (17) Tasaki, K. Computational Study of Salt Association in Li-Ion Battery Electrolyte. J. Electrochem. Soc. 2002, 149, A418-A425. (18) Jian, Z.; Luo, W.; Ji, X. Carbon Electrodes for K-Ion Batteries. J. Am. Chem. Soc. 2015, 137, 11566-11569. (19) Dahn, J. R. Phase diagram of LixC6. Phys. Rev. B 1991, 44, 9170-9177. (20) Bogle, X.; Vazquez, R.; Greenbaum, S.; von Cresce, A.; Xu, K. Understanding Li+– Solvent Interaction in Nonaqueous Carbonate Electrolytes with 17O NMR. J. Phys. Chem. Lett. 2013, 2013 4, 1664-1668.

(21) Mizutani, Y.; Abe, T.; Ikeda, K.; Ihara, E.; Asano, M.; Harada, T.; Inaba, M.; Ogumi, Z.; Graphite intercalation compounds prepared in solutions of alkali metals in 2-methyltetrahydrofuran and 2,5-dimethyltetrahydrofuran. Carbon 1997, 35, 61-65. (22) Mizutani, Y.; Abe, T.; Inaba, M.; Ogumi, Z. Creation of nanospaces by intercalation of alkali metals into graphite in organic solutions. Synth. Met. 2001, 125, 153-159. (23) Ota, H.; Sakata, Y.; Inoue, Y.; Yamaguchi, S. Analysis of Vinylene Carbonate Derived SEI

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Layers on Graphite Anode. J. Electrochem. Soc. 2004, 151, A1659-A1669. (24) Bhatt, M. D.; O’Dwyer, C. The Role of Carbonate and Sulfite Additives in Propylene Carbonate-Based Electrolytes on the Formation of SEI Layers at Graphitic Li-Ion Battery Anodes. J. Electrochem. Soc. 2014, 161, A1415-A1421. (25) Tsubouchi, S.; Domi, Y.; Doi, T.; Ochida, M.; Nakagawa, H.; Yamanaka, T.; Abe, T.; Ogumi, Z. Spectroscopic Characterization of Surface Films Formed on Edge Plane Graphite in Ethylene Carbonate-Based Electrolytes Containing Film-Forming Additives. J. Electrochem. Soc. 2012, 159,

A1786-A1790. (26) Khasanov, M.; Pazhetnov, E.; Shin, W. C. Dicarboxylate-Substituted Ethylene Carbonate as an SEI-Forming Additive for Lithium-Ion Batteries. J. Electrochem. Soc. 2015, 162, A1892-A1898. (27) Xu, K. Electrolytes and Interphases in Li-Ion Batteries and Beyond. Chem. Rev. 2014, 114, 11503-11618.

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Page 21 of 34 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


Figure captions Figure 1. Potential changes vs. capacity (a) and Li-intercalated ratio in graphite (b) in first charge-discharge curves in 1 mol dm-3 LiPF6/ EC+EMC+TMP (1:2:3, v/v/v%) with no additives, 0.5 mol dm-3 NaTFSA, and KTFSA.

Figure 2. Correlation between coulombic efficiency or discharge capacity and concentration of KTFSA in first cycle in 1 mol dm-3 LiPF6/ EC+EMC+TMP (1:2:3, v/v/v%) with 0.1-0.5 mol dm-3 KTFSA.

Figure 3.

17

O NMR spectra of C=O in EC and EMC in 1 mol dm-3 LiPF6/EC+EMC+TMP(1:2:3,

v/v/v%) with 0.1-0.5 mol dm-3 KTFSA.

Figure 4. Correlation between

17

O NMR chemical shifts and coulombic efficiency in 1 mol dm-3

LiPF6/EC+EMC (1:2, v/v %) and LiPF6/EC+EMC+TMP(1:2:3, v/v/v%) with no additives, 0.5 mol dm-3 Mg(TFSA)2, Ca(TFSA)2, NaTFSA, and KTFSA. Figures in square brackets indicate molarity of KTFSA in electrolyte solutions.

Figure 5. Diagram of interaction between solvents and Lewis acids.

21



Page 22 of 34

Figure 6. XRD patterns for charged graphite in 1 mol dm-3 LiPF6/EC+EMC+TMP(1:2:3, v/v/v%) with 0.5 mol dm-3 KTFSA.

Figure 7. XRD patterns for discharged graphite in 1 mol dm-3 LiPF6/EC+EMC+TMP(1:2:3, v/v/v%) with 0.5 mol dm-3 KTFSA.

Figure 8. XRF analysis of charged graphite in 1 mol dm-3 LiPF6/EC+EMC+TMP(1:2:3, v/v/v%) with 0.5 mol dm-3 KTFSA.

Figure

9.

Components

of

surface

film

on

discharged

graphite

in

1

mol

dm-3

LiPF6/EC+EMC+TMP(1:2:3, v/v/v%).

Figure 10. Components of surface film on discharged graphite in 1 mol dm-3 LiPF6/EC+EMC+ TMP(1:2:3, v/v/v%) with 0.5 mol dm-3 KTFSA.

Figure 11. Reaction mechanism on graphite negative electrodes in self-extinguishing electrolyte solution with or without KTFSA.

22


Page 23 of 34

(a) Capacity

(b) Li-intercalated ratio

1.5

0.3

No additives NaTFSA KTFSA 1

0.5

With KTFSA

1' + 4

Potential / V vs. Li/Li +

Potential / V vs. Li/Li +

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


0

4+3

0.2

3+2 2+1

0.1

Stage 4

Stage 3

Stage 2

Stage 1

0

0

100

200

300

400

Capacity / mAh

500

600

0

0.2

g-1

0.4

0.6

0.8

1

X in LixC6

Figure 1. Potential changes vs. capacity (a) and Li-intercalated ratio in graphite (b) in first charge-discharge curves in 1 mol dm-3 LiPF6/ EC+EMC+TMP(1:2:3, v/v/v%) with no additives, 0.5 mol dm-3 NaTFSA, and KTFSA.

23



400

80

300

60 200 40 100 20

0

Discharge capacity / mAh g-1

100

Coulombic efficiency / %

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

Page 24 of 34

0 0

0.1

0.2

0.3

0.4

0.5

0.6

Concentration of KTFSA / mol dm-3

Figure 2. Correlation between coulombic efficiency or discharge capacity and concentration of KTFSA in first cycle in 1 mol dm-3 LiPF6/ EC+EMC+TMP(1:2:3, v/v/v%) with 0.1-0.5 mol dm-3 KTFSA.

24


Page 25 of 34

C=O (EC)

C=O (EMC)

(e) 0.5

Intensity / A.U.

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


(d) 0.3

(c) 0.1 (b) 0 (a) EC+EMC 260

250

240

230

220

210

200

190

Chemical shift / ppm

Figure 3.

17

O NMR spectra of C=O in EC and EMC in 1 mol dm-3 LiPF6/EC+EMC+TMP(1:2:3,

v/v/v%) with 0.1-0.5 mol dm-3 KTFSA.

25



C=O in EC

100

KTFSA [0.5]

No add.

Coulombic efficiency / %

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

80

Mg(TFSA)2

Ca(TFSA)2

KTFSA [0.3]

60

NaTFSA

40

KTFSA [0.1]

1/30C 1/3C

20

EC+EMC EC+EMC+TMP

No add.

0 -5

Change in

0

5

17O

10

15

NMR chemical shifts / ppm

EC-Li+ interaction

Figure 4. Correlation between

17

O NMR chemical shifts and coulombic efficiency in 1 mol dm-3

LiPF6/EC+EMC (1:2, v/v %) and LiPF6/EC+EMC+TMP(1:2:3, v/v/v%) with no additives, 0.5 mol dm-3 Mg(TFSA)2, Ca(TFSA)2, NaTFSA, and KTFSA. Figures in square brackets indicate molarity of KTFSA in electrolyte solutions.

26


Page 26 of 34

Page 27 of 34 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


EC EC+TMP with Mg2+ EC+TMP with Na+ EC+TMP with K+

Li+

TMP Mg2+ (Strong) Na+ (Weak) K+ (Weaker)

EC+TMP (No add.)

EC (Flammable)

Figure 5. Diagram of interaction between solvents and Lewis acids.

27


30

40

50

Cu (220)

LiC12 (003) 20

LiC6 (002) Cu (200) LiC12 (004)

LiC12 (002)

LiC12 (001)

Intensity / A.U. 10

60

70

80

10

20

30

40

50

60

70

Graphite-2H (110)

LiC12 (005)

LiC12 (004)

Cu (220)

Cu (200) 6

LiC12 (003) Graphite-2H (100) Cu (111) LiC (002)

LiC12 (002)

KPF6 (111) KPF6 (200)

LiC12 (001)

LiC6 (001)

2θ(CuKα) / deg.

Intensity / A.U.

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

LiC6 (001)


80

2θ(CuKα) / deg.

Figure 6. XRD patterns for charged graphite in 1 mol dm-3 LiPF6/EC+EMC+TMP(1:2:3, v/v/v%) with 0.5 mol dm-3 KTFSA.

28


Page 28 of 34

20

30

40

50

Cu (220)

Cu (200) Graphite-2H (004)

KPF6 (200)

Intensity / A.U. 10

60

70

80

10

20

30

40

Graphite-2H (110)

Cu (220)

Graphite-2H (004) Graphite-2H (103)

Graphite-2H (100) Cu (111) Graphite-2H (101)

Cu (200)

Graphite-2H (002)

KPF6 (111)

KPF6 (220)

KPF6 (200)

2θ(CuKα) / deg.

Intensity / A.U.

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


Graphite-2H (002)

Page 29 of 34

50

60

70

80

2θ(CuKα) / deg.

Figure 7. XRD patterns for discharged graphite in 1 mol dm-3 LiPF6/EC+EMC+TMP(1:2:3, v/v/v%) with 0.5 mol dm-3 KTFSA.

29



K S P 0.8% 0.5% 1.9% F 2.7%

C 94.1%

Figure 8. XRF analysis of the charged graphite in 1 mol dm-3 LiPF6/EC+EMC+TMP(1:2:3, v/v/v%) with 0.5 mol dm-3 KTFSA.

30


Page 30 of 34

Page 31 of 34

O1s

C-O

Intensity / A.U.

Intensity / A.U.

C1s

-CFHCFHCO3

295

C-C

290

285

Oxide

540

280

C-O, CO3

CO-F

Binding energy / eV

535

530

525

Binding energy / eV

Li1s

F1s

LiF

Intensity / A.U.

Intensity / A.U.

LiF, Li 2CO3

Li 2O

65

60

55

50

F-C

695

Binding energy / eV

690

685

680

Binding energy / eV P 4.1%

P2p 3/2 P-O, P-F

Intensity / A.U.

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


C 17.4%

1/2 P-O, P-F

Li 30.7%

F 18.1% O 29.8% 145

140

135

130

125

Binding energy / eV

Figure

9.

Components

of

surface

film

on

discharged

LiPF6/EC+EMC+TMP(1:2:3, v/v/v%). 31


graphite

in

1

mol

dm-3


O1s

C-C

Intensity / A.U.

Intensity / A.U.

C1s

C-O O-C=O C-F

295

C-O, C=O

CO-F Oxide

290

285

280

540

535

Binding energy / eV

530

525

Binding energy / eV

Li1s

F1s

LiF

Intensity / A.U.

Intensity / A.U.

LiF

65

60

55

50

F-C

695

Binding energy / eV

690

685

680

Binding energy / eV N S 0.5% 0.9%

P2p

P 3.1%

K 0.2%

3/2 P-O, P-F

Intensity / A.U.

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

Page 32 of 34

F 12.7%

1/2 P-O, P-F

C 42.4%

Li 17.6% O 22.6% 145

140

135

130

125

Binding energy / eV

Figure 10. Components of surface film on discharged graphite in 1 mol dm-3 LiPF6/EC+EMC+ TMP(1:2:3, v/v/v%) with 0.5 mol dm-3 KTFSA.

32


Page 33 of 34 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


No KTFSA added

Co-intercalation Decomposition EC

LiF

Li+ Li CO 2 3

TMP

Li2O

Graphite K+ LiF

RCO2R′ With KTFSA

Film formation

Li+ intercalation

Figure 11. Reaction mechanism on graphite negative electrodes in self-extinguishing electrolyte solution with or without KTFSA.

33



TOC Graphic

No KTFSA added

Co-intercalation Decomposition EC

LiF

Li+ Li CO 2 3

TMP

Li2O

Graphite K+ LiF

RCO2R′ With KTFSA

Film formation

Li+ intercalation

34


Page 34 of 34

Electrochemical Stabilization of Self-Extinguishing Electrolyte

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