Understanding the Improved High-Temperature Cycling Stability of a

Article pubs.acs.org/JPCC

Cite This: J. Phys. Chem. C 2018, 122, 5864−5870

Understanding the Improved High-Temperature Cycling Stability of a LiNi0.5Mn0.3Co0.2O2/Graphite Cell with Vinylene Carbonate: A Comprehensive Analysis Approach Utilizing LC-MS and DART-MS Yi-Hung Liu,*,†,§ Sahori Takeda,† Ikue Kaneko,‡ Hideya Yoshitake,‡ Takashi Mukai,† Masahiro Yanagida,† Yuria Saito,† and Tetsuo Sakai†,‡ †

National Institute of Advanced Industrial Science and Technology (AIST), 1-8-31, Midorigaoka, Ikeda, Osaka 563-8577, Japan Innovation Center for Organic Electronics, Yamagata University, 1-808-48, Arcadia, Yonezawa, Yamagata 992-0119, Japan § Department of Greenergy, National University of Tainan, Tainan 70005, Taiwan ‡

S Supporting Information *

ABSTRACT: An analysis approach utilizing liquid chromatography mass spectrometry (LC-MS) and direct analysis in real time mass spectrometry (DART-MS) is applied to discern the influence of vinylene carbonate (VC) on the cycling performance of a LiNi0.5Mn0.3Co0.2O2 (NMC)/graphite cell at elevated temperature. The VC-containing cell exhibits much improved cycling performance against the elevated temperature rather than the VC-free one. On the basis of the LC-MS results, more decomposition compounds including carbonate oligomers and organophosphates are present in the electrolyte without VC after the initial cycling, while they are less identifiable in the VC-containing electrolyte. On the other hand, the DART-MS results show that a thermally resistant film, which is primarily composed of the cyclic organophosphates, favors forming on the graphite anode surface of the VC-containing cell rather than the VC-free one, preventing further electrolyte decomposition. Moreover, a reaction scheme is proposed to reasonably explain the formation of the decomposition compounds, in which it is understood that VC can trap the free alkoxide anions and meanwhile allow more EC to react with POF3 during the initial cycling, thus reducing the decomposition compounds in the electrolyte and facilitating the formation of thermally stable organophosphates. Consequently, the cell’s cycling performance is improved at elevated temperature.

1. INTRODUCTION Ensuring a long cycle life of a lithium ion battery (LIB) is always a pursued target for various electronic applications, from portable ones like laptops to large-scale ones such as electric vehicles (EVs), hybrid electric vehicles (HEVs), as well as other stationary battery storage systems. For these large-scale applications, extension of the battery’s cycle life, particularly at elevated temperature, is highly required because the largescale batteries are more often utilized under an environment with high temperature. Moreover, continuous operation of thermally unstable batteries would eventually lead to a thermal runaway, thus threatening our life seriously. To improve the thermal stability of LIBs, various development strategies are thus toward seeking the thermally stable active materials, binders, separators, substrates, and electrolytes. Since the commonly used electrolytes, which are mainly composed of LiPF6 salt and organic solvents, are more likely to decompose at elevated temperature than the other battery components, development of thermally stable electrolytes should be put in high priority. Regarding this issue, the use of electrolyte additives is one of the most effective and economical ways to © 2018 American Chemical Society

enhance the battery’s performances in terms of cycle life as well as rate capability.1 Among many electrolyte additives, vinylene carbonate (VC) is the most well-known one, which has been shown to have positive effects on the improvement of electrochemical performances for either the cathode2−5 or anode side.6−11 For example, in previous research of Aurbach’s group,8 it has been reported that VC can react with the cathode materials of LiNiO2 and LiMn2O4, and the anode material of graphite. Despite that no profound impact on the cycling behavior of the cathodes has been confirmed, VC substantially improves the cycling stability of the graphite anode together with a reduction of irreversible capacity, particularly at elevated temperature. The high reactivity of VC, corresponding to its polymerizable vinyl group, tends to induce polymerization with other compounds during cell cycling, resulting in the formation of a passivating film on the anode surface.8,9 Accordingly, various Received: October 20, 2017 Revised: February 25, 2018 Published: February 26, 2018 5864

DOI: 10.1021/acs.jpcc.7b10391 J. Phys. Chem. C 2018, 122, 5864−5870

Article

The Journal of Physical Chemistry C

min before it was moved to calendaring. After a final drying treatment was conducted under a vacuum at 160 °C for 6 h, the as-prepared cathode was obtained. As for the anode, a slurry mixing 83.7 wt % graphite (Osaka Gas Chemical Co., Ltd.) powders, 9.3 wt % soft carbon, 1 wt % vapor growth carbon fiber (VGCF), 2 wt % acetylene black (AB), and 4 wt % waterbased acrylic binder was coated on a copper foil sheet, after which the sheet was subjected to calendaring and heat treatment similar to those for the cathode. 2.2. Electrochemical Evaluation and Material Analysis. For the electrochemical evaluation, a coin-type test cell (CR2032) was used to accommodate the as-prepared electrodes together with a glass-fiber filter used as the separator in between the electrodes. In addition, the electrolyte used was 1 M LiPF6 in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) at a ratio of 1:1 (v/v) (Kishida Chemicals) with or without 1 wt % VC (Kishida Chemicals). The VC concentration was adopted in order to produce better electrode performances of graphite15 and to avoid possible oxidative polymerization on the cathode1 particularly under higher voltage and elevated temperature operation. The electrolyte volume in each test cell was 150 μL, allowing us to obtain a sufficient amount of electrolyte for analysis. The value of moles of VC to the specific surface area of graphite, which is calculated as an estimated relevant VC amount, is 8.43 × 10−6 mol g m−2. This value is close to that of a previous study15 using graphite with a specific surface area of 0.8 m2 g−1. A charge−discharge machine (BLS series, Keisokuki Center Co., Ltd.) was applied to galvanostatically carry out the charge/ discharge cycling tests for which the current density was 75 mA g−1; the cutoff voltage range was over 2.5−4.2 V; the operating temperature was at 60 °C. With regard to the material analysis, the cycled cells were decomposed in a dry room with an ultralow humidity environment to prudentially collect the samples of electrolytes and electrodes. LC-MS (Nexera, Shimadzu Corp. and Esquire 3000 plus, Bruker) and DARTMS (DART-SVP, AMR Inc. and LCMS-8030, Shimadzu Corp.) were respectively applied to analyze the collected electrolyte and the electrode surface. In the DART-MS experiments, the electrode surface was allowed to contact the ion beam ejected from the beam generator at an angle of approximately 45° without shielding the beam. Further, the detection was conducted at various beam temperatures of 150, 250, 350, and 450 °C.

polymerization products as well as nonpolymeric species can be produced during the film formation process.8,12,13 On the basis of a series of XPS analyses of the electrode surfaces for the LiCoO2(LCO)/graphite and LiFePO4(LFP)/graphite cells, it has been evidenced that there is no interaction between the cathode and anode during the VC polymerization process.14 In addition, different from the LFP/graphite cell, in which no polymeric products were found on the cathode surface, the same VC polymer can be identified on both the cathode and anode surfaces for the LCO/graphite cell as a result of a radical polymerization process. Another study has further pointed out that the improvement of Coulombic efficiency and reduction of charge/discharge end point slippage can be achieved by use of a VC additive for a Li/graphite half cell, which is more profound at elevated temperature.15 As for a Li/LiNi1/3Mn1/3Co1/3O2 (NMC) cell, VC was found to suppress the parasitic reactions occurring at the cathode surface, corresponding to a better cycle life. According to a recent research study, VC has also been proved to form a polymer at both NMC and graphite surfaces, resulting in more stable and protective solid electrolyte interface (SEI) films.16 Liquid-chromatography-based analysis methods17−20 and gas chromatography−mass spectrometry (GC-MS)21 have been utilized to detect neutral decomposition compounds, while FTIR8,11 and XPS2,14,16 are commonly used in order to identify the compounds of the films on the electrode surfaces on the basis of their functional groups. Recently, direct analysis in real time mass spectrometry (DART-MS) has been applied as a more direct analysis approach to specify the compounds deposited on the electrode surfaces of LFP/graphite22 and LFP/hard carbon23 cells after cycling at elevated temperature. It is supported from the studies that a combination of LC-MS and DART-MS analysis can allow one to simultaneously identify the decomposition compounds in the electrolyte and on the electrode surfaces, benefiting the elucidation of the formation mechanisms of a thermally stable film induced by the VC additive. Since the olivine LFP has already been known as a thermally stable cathode material, the impact of VC on the thermal stability of a cell composed of layered cathode materials with higher energy density, such as NMC, should be more worthy of investigating based on the above-mentioned analysis approach. In this study, we aim to apply the analysis approach combining LC-MS and DART-MS to comprehensively understand the influence of VC on the thermally resistant film formation for a NMC/graphite cell at 60 °C. The cell’s cycling stability is correlated to the identified decomposition products either in the electrolyte or on the electrode surface. On the basis of the analysis results, a series of reaction mechanisms of the decomposition compounds is proposed to explain the VCinduced film formation on the electrode surface, offering new insight into the VC contribution to the improved cycling stability at elevated temperature.

3. RESULTS AND DISCUSSION 3.1. Effect of VC on High-Temperature Cycling Performance of the NMC/Graphite Cell. Figure 1a shows the cycling performance at 60 °C for the NMC/graphite cells with/without VC additive. It is obvious that the VC-containing cell exhibits better cycling stability with a capacity retention of 79% at the 100th cycle. On the other hand, the capacity of the VC-free cell drastically decays in the initial cycling and then becomes stable with a gradual capacity fading, resulting in a lower capacity retention of 44% at the 100th cycle. This result differs from that for the cells evaluated at 30 °C, which shows a smaller difference in capacity fading during cycling (see Figure S1). Therefore, it is denoted that the electrochemical behaviors during the initial cycling at 60 °C dominate the thermal stability of the NMC/graphite cells, which is greatly influenced by the VC additive. The first and second charge/discharge curves for the two cells mentioned above are shown in Figure 1b. For the VC-containing cell (solid line), its first charge capacity reaches

2. EXPERIMENTAL SECTION 2.1. Preparation of NMC and Graphite Electrodes. The LiNi0.5Mn0.3Co0.2O2 (NMC) powders (Nihon Kagaku Sangyo Co., Ltd.) were used to prepare the cathode material. The powders were first mixed with a conductive agent of 5 wt % acetylene black (AB) and a 3.5 wt % water-based acrylic/ carboxyl methyl cellulose (3.5 wt %/1.5 wt %) binder to form the slurry, followed by casting it on a carbon coated aluminum foil sheet. The coated sheet was then preheated at 80 °C for 10 5865


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the first charge profile almost overlaps with that of the VCcontaining cell, whereas its first discharge capacity is lower, thus giving a poorer Coulombic efficiency. As for its second cycle, much reduced capacities in both charge and discharge sides can be observed from the profiles despite that the irreversible capacity becomes less. It is therefore indicated that VC plays an important role in preventing the capacity fading, which corresponds to the initial irreversible electrochemical reaction at the elevated temperature. 3.2. Electrolyte Analysis by LC-MS. Figure 2 shows the LC-MS chromatograms of the electrolytes collected from the NMC/graphite cells experiencing 2 charge/discharge cycling at 60 °C. According to the analysis results, six types of compounds, including two carbonate oligomers (C1 and C2) and four organophosphates (P1−P4), are identified and summarized in the right box. Other nonidentifying peaks in the chromatograms could be owing to the impurity substances in the sample solution. The electrolyte of case (a) contains C1, C2, and P3 with relatively low peak intensities. After 100 cycling (b), other types of organophosphates P1, P2, and P4 are formed in addition to the aforementioned compounds. It is also worth noting that, compared with the compounds C1 and C2, the peak intensity of P3 remarkably increases after the longterm cycling, indicating that the formation reaction of the organophosphates is more significant than that of the carbonate oligomers during the cycling. When introducing VC additive into the electrolyte, no compounds can be identified after 2 cycling, except for C1 with a much lower peak intensity, as shown in (c). With further cycling up to 100 cycles (d), despite that several carbonate oligomers and organophosphates are found in the electrolyte, they are all within much lower peak intensity compared with the VC-free case (b). Since the organophosphates and the carbonate oligomers should be attributed to the decomposition of LiPF6 and EC/DEC solvent, respectively, it is evidenced from the above analysis that adding VC offers the function of hindering those decomposition reactions at elevated temperature. A previous study reported

Figure 1. (a) Cycling performance of NMC/graphite cells with/ without VC at 60 °C and (b) corresponding initial charge/discharge curves.

189 mAh g−1 at 4.2 V through experiencing a plateau at ca. 3.6 V, while the discharge capacity reduces to 159 mAh g−1 at 2.5 V, leading to a Coulombic efficiency of 84%. After the first cycle, the second charge and discharge profiles look more symmetric and the discharge capacity is almost the same as the first one. The reduced irreversible capacity and symmetric profiles in the second cycle offer the evidence of improved electrochemical reversibility. In the case of VC-free (dot line),

Figure 2. LC-MS chromatograms of various electrolyte samples of VC-free after (a) 2 cycle and (b) 100 cycle charge/discharge tests; VC-containing after (c) 2 cycle and (d) 100 cycle charge/discharge tests. (The deduced chemical formulas for the identified compounds are summarized in the right box.) 5866


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The Journal of Physical Chemistry C that, for Li-rich HE-NCM-based lithium ion cells, the use of a glass fiber separator would lead to the formation of more decomposition compounds compared with a polypropylene (PP) separator.24 On the basis of their conclusions, electrode potentials higher than 4.2 V and activation of the Li2MnO3 domains of the Li-rich cathode would be the reasons accounting for further formation of decomposition compounds corresponding to phosphoryl trifluoride (POF3). However, compared with our previous report20 using a PP separator, the glass fiber separator tested here does not facilitate the formation of more organophosphates in electrolyte, indicating a limited amount of POF3. A lower cutoff voltage (4.2 V) and nonexistence of Li2MnO3 domains in cathode material may allow us to eliminate the concern of separator type in this study. 3.3. Electrode Surface Analysis by DART-MS. To further understand the influence of VC on the initial decomposition reactions, DART-MS is applied to analyze the electrode surfaces. The analysis results for the NMC cathodes of the cells with/without VC are shown in Figure 3. The

Table 1. Summary of the Compounds Identified by DARTMS

decomposition compounds are found on the LFP cathode surface except for the solvents.22 Similarly, the graphite anodes of the VC-free and VCcontaining cells are analyzed by DART-MS and the results are summarized in Figure 4. The representative spectra can be referred to Figures S4 (350 °C) and S5 (450 °C) for the VCfree cell and Figures S6 (350 °C) and S7 (450 °C) for the VCcontaining cell. For the VC-free graphite anode, the detected compounds are composed of the organic solvents EC and DEC together with the organophosphate P1. This is similar to the result of NMC cathode of Figure 3a, whereas P1 on the anode side is detected in a wider temperature range with a stronger intensity than that on the cathode side. On the other hand, the VC-containing case (b) reveals several types of organophosphates labeled from P5 to P12 and carbonate oligomers C1 at a beam temperature higher than 350 °C, showing a very different result from that of (a). Also, this result is different from that of our previous LFP/graphite cell where only P8 and P11 were detected except for EC.22 This finding suggests that NMC should be more electrochemically active than LFP to form the decomposition compounds induced by VC. Among

Figure 3. Summary of DART-MS spectra for NMC cathodes experiencing 2 cycle charge/discharge test: (a) VC-free sample; (b) VC-containing sample.

number for each compound near the figures represents the m/z value, and its deduced chemical structure is summarized in Table 1. The representative spectra at 450 °C can also be referred to Figures S2 and S3. In spite of the variation in intensity over the detected temperature range, most of the compounds can be referred to the solvents EC (m/z = 89/106/ 177) and DEC (119) for the VC-free cell (a), except for the minor compound of organophosphate P1 (153) that is present at higher temperature. The result of the VC-containing cell (b) is similar to the VC-free one, demonstrating EC and DEC are the primary compounds detected. Because most of the detectable compounds are the organic solvents, the NMC cathode surface may remain intact during the initial electrochemical cycling regardless of the presence of VC. This result is consistent with the previously reported one revealing that no 5867


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structurally stable against the elevated temperature. More importantly, compared with the NMC cathodes (see Figure 3), most of the decomposition compounds are detected on the graphite surface of the VC-containing cell, and they are different from those found in the electrolyte (see Figure 2), indicating these compounds could be much less soluble in the electrolyte. This suggests that adding VC substantially facilitates the formation of the compounds, mainly composed of the cyclic organophosphates, constituting a thermally resistant film on the graphite surface during the initial cycling, which corresponds to the reduced decomposition compounds in the electrolyte, as mentioned in Figure 2c. That is, further electrolyte decomposition can be prevented by forming the thermally resistant film;22 consequently, the capacity fading can be alleviated from the initial cycling for the VC-containing cell, as discussed in Figure 1. 3.4. Influence of VC on the Formation Mechanisms of Decomposition Compounds. It is well-known that LiPF6 dissolves in carbonate-based solvents and readily decomposes into LiF and PF5, reaching an equilibrium state. Once a trace amount of water or alcohol (ROH) is present in the electrolyte, PF5 further reacts with them to form POF3, which is the precursor for the organophosphates. Figure 5 shows the proposed formation mechanisms for the primary cyclic organophosphates, as shown in Figure 4b. All of the products P6, P8, and P11 are initiated from OPF2OC2H2F that is formed by the reaction of POF3 and EC, accompanied by the release of CO2, as proposed by Campion et al.25 A subsequent reaction of OPF2OC2H2F, either esterification with EC or hydrolysis, leads to the formation of P6 and P8. As for P11, it is derived from a further reaction of P6 and cyclic OPFO2C2H4. According to the reaction mechanisms, it is known that EC plays a very important role in triggering the formation of these organophosphates. On the other hand, EC also tends to react with alkoxide anion (C2H5O−), which is from the common electrochemical reduction product lithium alkoxide,26 to form the other decomposition compounds as detected by LC-MS. The probable reaction mechanisms for the compounds detected in the electrolyte are also summarized in Figure 6. These compounds are all induced by the reactions associated with

Figure 4. Summary of DART-MS spectra for graphite anodes experiencing 2 cycle charge/discharge test: (a) VC-free sample; (b) VC-containing sample.

these compounds, P6 (169), P8 (253/270), and P11 (275/ 292) exhibit a relative strong detection intensity, denoting that they are more likely to form on the graphite surface. It is also worth noting that, unlike the other organophosphates, they all consist of a cyclic phosphate group, which should be more

Figure 5. Proposed formation mechanisms for the primary cyclic organophosphates formed on the graphite anode surface of the VC-containing cell. 5868


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Figure 6. Proposed reaction mechanisms of the compounds detected in the electrolyte and the VC influence on the reaction pathways.

either the alkoxide anion or the intermediate (C2H5OCO2C2H4O−) that is derived from the reaction of EC and alkoxide anion. It has been reported that VC is capable of trapping the alkoxide anion, prohibiting further corresponding reactions.27 On the basis of this viewpoint, once the VC is added into the EC-based electrolyte, the alkoxide anion is trapped and the route to generate the intermediate is hindered, thus suppressing the formation of the compounds C1, C2, and P1−P4. Despite that a different reaction mechanism involving DEC and POF3 was previously reported to explain the formation of P3,22 this reaction scheme satisfactorily agrees with the analysis results of Figure 2. Further, since EC is less involved in the reaction associated with the alkoxide anions, it should be, instead, more likely to react with POF3, facilitating the formation of cyclic organophosphates, as discussed in Figure 5. This is considered the main reason for the induced thermally resistant film formation on the graphite anode surface and the reduced decomposition compounds in the electrolyte, which is greatly influenced by VC particularly at elevated temperature. The proposed reaction scheme can also support the explanation for the LC-MS and DART-MS results of our previous studies22,23 where the correlation between reduced decomposition compounds in the electrolyte and on the electrode surface may not be sufficiently elaborated.

results, it is known that a thermally resistant film, which mainly consists of cyclic organophosphates, prefers to form on the graphite anode rather than the NMC cathode surface in the VC-containing cell. Moreover, the formation mechanisms of the detected compounds are elucidated, allowing us to satisfactorily explain the reduced decomposition compounds in the electrolyte and the formation of the thermally resistant film induced by VC. That is, VC can trap the free alkoxide anions and meanwhile allow more EC to react with POF3, leading to the formation of the thermally stable organophosphates that is beneficial to the improved cycling performance at elevated temperature. As an extension of this research, future works regarding the effects of water content and VC quantity would be required to quantitatively evaluate the electrolyte decomposition process.

4. CONCLUSIONS The cycling stability of a NMC/graphite cell at 60 °C is investigated via an analysis approach incorporating the LC-MS and DART-MS techniques. The VC-containing cell exhibits much improved cycling performance against the elevated temperature rather than the VC-free one. According to the LC-MS analysis, more decomposition compounds of carbonate oligomers and organophosphates are identified in the electrolyte without VC, while the formation of these compounds can be suppressed by introducing 1 wt % VC during the initial cycling. On the other hand, from the DART-MS analysis

Corresponding Author

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b10391. Cycling test results at 30 °C and original data of DARTMS spectra (PDF)

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AUTHOR INFORMATION

*E-mail: [email protected]. Phone: +886-6-2606186. Fax: +886-6-2602205. ORCID

Yi-Hung Liu: 0000-0002-0800-7421 Yuria Saito: 0000-0002-9616-6309 Author Contributions

Y.-H.L. designed the experiments, performed the electrochemical evaluations, and carried out DART-MS measurements together with the data analysis. S.T. carried out the LC-MS measurements and analyzed the data. T.M. performed the 5869


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electrochemical measurements. The manuscript was written by Y.-H.L. All of the authors discussed and checked the results, giving approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Ms. Miki Okano of National Institute of Advanced Industrial Science and Technology (AIST) for the technical support on the experiment. Also, the authors would like to greatly thank ASIT for financial support.

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REFERENCES

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Understanding the Improved High-Temperature Cycling Stability of a

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