Highly Effective Solid Electrolyte Interphase-Forming Electrolyte

Aug 22, 2017 - Highly Effective Solid Electrolyte Interphase-Forming Electrolyte Additive Enabling High Voltage Lithium-Ion Batteries. Stephan Röserâ...
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Highly Effective Solid Electrolyte Interphase-Forming Electrolyte Additive Enabling High Voltage Lithium-Ion Batteries Stephan Röser,† Andreas Lerchen,§ Lukas Ibing,† Xia Cao,† Johannes Kasnatscheew,‡ Frank Glorius,*,§ Martin Winter,*,†,‡ and Ralf Wagner*,† †

MEET Battery Research Center/Institute of Physical Chemistry, University of Münster, Corrensstrasse 46, 48149 Münster, Germany ‡ Helmholtz-Institute Münster, IEK-12, Forschungszentrum Jülich GmbH, Corrensstrasse 46, 48149 Münster, Germany § Institute of Organic Chemistry, University of Münster, Corrensstrasse 40, 48149 Münster, Germany S Supporting Information *

ABSTRACT: The electrochemical and thermal stabilities of commonly used LiPF6/organic carbonate-based electrolytes are still a bottleneck for the development of high energy density lithium-ion batteries (LIBs) operating at elevated cell voltage and elevated temperature. The use of intrinsic electrochemically stable electrolyte solvents, e.g. sulfones or dinitriles, has been reported as one approach to enable high voltage LIBs. However, the major challenge of these solvents is related to their poor reductive stability and lack of solid electrolyte interphase (SEI)-forming ability on the graphite electrode. Here, 3-methyl-1,4,2-dioxazol-5-one (MDO) is synthesized and investigated as new highly effective SEI-forming electrolyte additive which can sufficiently suppress electrolyte reduction and graphite exfoliation in propylene carbonate (PC)based electrolytes. With the addition of only 2 wt % MDO, LiNi0.5Mn0.3Co0.2O2 (NMC532)/graphite full cells containing a 1 M LiPF6 in PC electrolyte reach a cycle life of more than 450 cycles while still having a capacity retention of 80%. In addition, MDO has proven to be oxidatively stable until potentials as high as 5.3 V vs Li/Li+. Further development of MDO and its derivatives as electrolyte additives is a step forward to high voltage stable electrolyte formulations based on alternative electrolyte solvents and high energy density LIBs.

1. INTRODUCTION Lithium-ion batteries (LIBs) are the energy storage system of choice for the electrification of the powertrain due to their large range of power to energy ratio (P/E) design options, thus enabling them to power the whole range of electric vehicles with different P/E ratios such as hybrid (HEV, P/E ≈ 15), plugin hybrid (PHEV, P/E ≈ 8), and fully battery electric vehicle (BEV, P/E ≈ 3).1 Whereas the actual power density values can be considered quite satisfactory, an increase in energy density is absolutely mandatory to increase the electric driving range of BEVs.1 In general, the energy density at the materials level is determined by the product of capacity and voltage.2,3 Lithium transition-metal oxides based on the LiNixMnyCo1−x−yO2 (NMC) composition are regarded as the most suitable positive electrode materials for the next generation of high energy LIBs.4 Generally, the characteristic NMC potential profile arising from the solid solution reaction upon delithiation (charge) as well as lithiation (discharge) leads to an almost linear rise in capacity with increasing cutoff potential.4−6 However, performance deterioration and safety risks caused by electrolyte degradation limit the application of the commonly used electrolyte based on lithium hexafluorophosphate (LiPF6) as conducting salt, dissolved in mixtures of ethylene carbonate © 2017 American Chemical Society

(EC) and linear carbonates, e.g. dimethyl carbonate (DMC), in LIBs to cutoff potentials 99.4% 1H NMR) and investigated as SEI-forming electrolyte additive on graphite. Given the fact that an effective SEI cannot be formed on highly graphitic materials in electrolytes based on propylene carbonate (PC) due to solvent cointercalation,43 the new electrolyte additive was evaluated experimentally under these PC-rich conditions to prove its effectiveness.21 The use of the new SEI-forming additive MDO significantly improves the cycling performance of LiPF6/PC-based electrolyte in NMC532/graphite full cells in terms of Coulombic efficiency and cycle life.

2. EXPERIMENTAL SECTION 2.1. Synthesis of 3-Methyl-1,4,2-dioxazol-5-one. 3-Methyl1,4,2-dioxazol-5-one (MDO) was prepared according to a procedure of Chang et al. (Scheme 1).44 Acetohydroxamic acid (3.75 g, 50.0

Scheme 1. Synthesis Route of 3-Methyl-1,4,2-dioxazol-5-one (2) from Acetohydroxamic Acid (1) and 1,1′Carbonyldiimidazole (CDI)

mmol, 1.00 equiv) was dissolved in dichloromethane (500 mL), and 1,1′-carbonyldiimidazole (8.11 g, 50.0 mmol, 1.00 equiv) was added in one portion to the reaction mixture. After the solution was stirred for 16 h, the mixture was quenched with 1 M HCl (300 mL), extracted 3 times with dichloromethane (3 × 150 mL), and dried over MgSO4 followed by filtration. The solvent was removed under reduced pressure to afford the desired product MDO as a clear slightly yellow oil in 75% yield (3.77 g, 37.3 mmol). NMR: δ: 1H (400 MHz, [ppm], chloroform-d): 2.35 (s, 3H). 13C (101 MHz, [ppm], chloroform-d): 163.9, 154.2, 10.7. The analytical data is in accordance with the reported data.44 The NMR spectra of MDO are depicted in the Supporting Information (Figures S1 and S2). 2.2. Electrolyte Preparation. Battery grade electrolyte and electrolyte components 1 M LiPF6 in EC:DMC (1:1, by wt) (LP30 Selectilyte, BASF, battery grade), LiPF6 (BASF, battery grade), and PC (BASF, battery grade) were used as received. MDO was dried over 3 Å molecular sieves for at least 12 h before use. All electrolyte formulations were prepared in an argon-filled glovebox. The water content in the electrolyte was detected to be less than 20 ppm by coulometric Karl Fischer titration (Mitsubishi CA 200). 2.3. Cell Setup and Electrochemical Investigations. The electrochemical stability window of the investigated electrolyte formulations was determined by means of linear sweep voltammetry in LiMn2O4/Li half cells using three-electrode T-type Swagelok cells

3. RESULTS AND DISCUSSION 3.1. Oxidative and Reductive Stability Tendency. To ex ante elucidate the oxidation and reduction tendency of the investigated SEI-forming electrolyte additives in comparison to that of PC, the HOMO and LUMO energies (in eV) of the Li+ ion−solvent (or additive) complexes were calculated at B3LYP/6-311G++ (d,p) DFT level. Figure 1 shows the representative Li+ ion−solvent complexes after geometry optimization. The calculated HOMO and LUMO energies are depicted in Table 1. Basically, a decrease in the |HOMO| energy level value leads to an increased oxidative stability, whereas an increase in the |LUMO| energy level value elevates the reduction potential of the complex. Considering the LUMO energy level, the following trend in reduction onset potential is expected: MDO-Li > FEC-Li ≈ VC-Li > PC-Li. Regarding the |HOMO| energy level values, the trend in oxidative stability is VC-Li < MDO-Li ≈ PC-Li < FEC-Li. With the assumption that 7734

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graphite full cells comprising 1 M LiPF6 in PC with and without 2 wt % of either VC, FEC, or MDO as SEI-forming additive are compared with the commonly used 1 M LiPF6 in EC:DMC (1:1, by wt.) electrolyte. In case of the 1 M LiPF6 in PC electrolyte, no reversible Li+ ion intercalation/deintercalation was possible, as expected.33,46 Even in the case of the addition of either 2 wt % VC or 2 wt % FEC, the voltage profile exhibits a large voltage plateau at ≈3 V, which is associated with the electrolyte reduction and graphite exfoliation. The degree of exfoliation of the graphite electrode, which was charged/ discharged for only 1 cycle in the 1 M LiPF6 in PC + 2 wt % FEC electrolyte, can be seen in the broken-up graphite structure in the top-view SEM micrographs in Figures 6a and c and the intragranular cracking of secondary graphite particles in the cross-sectional SEM images in Figure 6c. The Coulombic efficiency47 in the first cycle amounts to only 35.8 and 40.7% for cells containing the VC and FEC electrolyte additive, respectively. Only the 2 wt % MDO containing electrolyte formulation can compete with the cycling performance of the commonly used 1 M LiPF6 in EC:DMC (1:1, by wt.) electrolyte. The voltage profiles of both electrolytes as well as the Coulombic efficiency of the first cycle, amounting to 86.3%, are identical. In line with the cycling results, the SEM micrographs of graphite electrodes obtained from the NMC532/graphite full cells containing the 1 M LiPF6 in PC + 2 wt % MDO electrolyte show no sign of graphite exfoliation (Figure 3). 3.3. Investigation of the Long-Term Cycling Stability. In Figure 4, the long-term cycling stabilities of NMC532/ graphite full cells comprising 1 M LiPF6 in PC + 2 wt % of

Figure 1. DFT-optimized structures of the Li+ ion−solvent complexes derived from the B3LYP/6-311G++ (d,p) DFT calculations. (a) PC, (b) VC, (c) FEC, and (d) MDO.

Table 1. Calculated Oxidative and Reductive Stability Values at B3LYP/6-311G++ (d,p) DFT Level for Li+ Ion−Solvent Complexes Comprising PC, VC, FEC, and MDO Li+ ion−solvent complex

|HOMO| (eV)

|LUMO| (eV)

PC-Li VC-Li FEC-Li MDO-Li

12.79 11.73 13.82 12.62

4.95 5.16 5.16 5.27

the decomposition products form an effective SEI, from the theoretical predictions, MDO should be a promising SEIforming electrolyte additive. On the one hand, it has a low reductive stability and thus should be reduced prior to the bulk electrolyte. On the other hand, it has a high oxidative stability. 3.2. Effectiveness of SEI Formation of Investigated Additives. The effect of different SEI-forming additives, viz. VC, FEC, and MDO, on the compatibility of graphite electrodes in combination with a 1 M LiPF6 in PC electrolyte was investigated in NMC532/graphite full cells. Because VC is commonly used in a concentration of 2 wt % in commercial electrolyte formulations,45 also FEC and MDO were used in this concentration to allow a fair comparison. MDO has a molecular weight of M = 101.06 g mol−1, whereas VC has a lower and FEC has a slightly higher molecular weight of M = 86.05 and 106.05 g mol−1, respectively. Therefore, with regard to specific energy improvement on the cell level, VC would be the optimal electrolyte additive followed by MDO and FEC when equimolar concentrations of these additives are used. In Figure 2, the voltage profile of the first cycle of NMC532/

Figure 3. Top-view SEM micrographs of harvested graphite electrodes after first charge/discharge in NMC532/graphite full cells using (a and c) 1 M LiPF6 in PC + 2 wt % FEC and (b and d) 1 M LiPF6 in PC + 2 wt % MDO. Cross-sectional micrographs obtained after focused ion beam cutting of harvested graphite electrodes using (e) 1 M LiPF6 in PC + 2 wt % FEC and (f) 1 M LiPF6 in PC + 2 wt % MDO.

Figure 2. Voltage profile of the first cycle of NMC532/graphite full cells comprising 1 M LiPF6 in EC:DMC (1:1, by wt) as well as 1 M LiPF6 in PC with and without 2 wt % of either VC, FEC, or MDO at 25 °C. The cycling procedure starts with a 3.5 h rest step at open circuit voltage. Cells were cycled in the voltage range from 2.8 to 4.2 V. 7735

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either VC, FEC, or MDO at 25 °C are compared. The cells comprising the electrolyte formulations containing VC or FEC as SEI-forming electrolyte additive show only poor cycling performance in terms of low discharge capacity and short cycle life. The end of life criterion of 80% discharge capacity retention is already reached after 36 and 87 cycles for the 1 M LiPF6 in PC electrolyte with 2 wt % VC and 2 wt % FEC, respectively. In contrast, the cell containing the 1 M LiPF6 in PC electrolyte with 2 wt % MDO shows high discharge capacities and reaches the 80% discharge capacity retention criterion after 460 cycles. Therefore, MDO is a highly effective SEI-forming electrolyte additive which can lead to good cycling performance even in the case of electrolyte solvents which without additive tend to exfoliate the graphite structure and normally cannot be used in LIB. The main drawback of the kinetic stabilization at the electrode/electrolyte interface is its limited use at elevated

Figure 4. Long-term cycling stability of NMC532/graphite full cells comprising 1 M LiPF6 in PC with and without 2 wt % of either VC, FEC, or MDO at 25 °C. Cells were cycled in the voltage range from 2.8 to 4.2 V.

Figure 5. N 1s XPS depth profiling spectra of harvested graphite electrodes after first charge/discharge in NMC532/graphite full cells using (a) 1 M LiPF6 in PC + 2 wt % FEC and (b) 1 M LiPF6 in PC + 2 wt % MDO electrolyte formulation. (c) Comparison between SEI thickness of organic and inorganic layers obtained in the investigated electrolytes. 7736

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The observed peaks in the potential range from 3.9 to 4.3 V vs Li/Li+ for all cells can be attributed to the typical two-step reversible Li+ ion extraction from the LiMn2O4 structure.34,49 A further reversible Faradaic reaction in the potential range from 5.0 to 5.3 V vs Li/Li+ is related to the deinsertion of remaining Li+ ions.50 The 1 M LiPF6 in PC + 2 wt % VC electrolyte shows an oxidative stability of ≈4.8 V vs Li/Li+ because of the decomposition of the VC electrolyte additive. In contrast, 1 M LiPF6 in PC, 1 M LiPF6 in PC + 2 wt % MDO, and 1 M LiPF6 in PC + 2 wt % FEC have a much higher oxidative stability, showing an exponential current increase at the potentials of 5.3, 5.3, and 5.5 V vs Li/Li+, respectively. The obtained oxidative stability limits are in perfect agreement with the calculated oxidation tendency in Table 1. Due to its high electrochemical oxidative stability, MDO is regarded as a highly promising SEIforming electrolyte additive for the application in cells comprising graphite, a 5 V cathode material, and a high voltage stable electrolyte formulation based on alternative electrolyte solvents.51

temperatures. In Figure S3 (Supporting Information), the longterm cycling stability of NMC532/graphite full cell comprising 1 M LiPF6 in PC + 2 wt % MDO at 60 °C is depicted. The Coulombic efficiency in the first cycle amounts to 69.5%, and 80% of the discharge capacity of the first cycle at 1.0 C is reached after 74 cycles. At elevated temperature, the cycling stability is greatly reduced; however, the overall cycling performance is still higher compared to the cells containing the electrolyte formulations with VC or FEC at 25 °C. 3.4. Investigation of the Graphite Electrode Surface. Due to the observed cycling performance improvement in case of the MDO electrolyte additive, it is assumed that MDO contributes to the SEI formation. In the top-view SEM image in Figure 3d, a dense surface layer which covers the graphite secondary particles can be seen. This surface layer was studied by means of XPS and compared to the surface layer on graphite electrode formed in the presence of the 1 M LiPF6 in PC + 2 wt % FEC electrolyte. In Figure 5, the N 1s XPS depth profiling spectra of the surface of both graphite electrodes are depicted. The respective XPS survey spectra as well as Li 1s, P 2p, C 1s, O 1s, and F 1s depth profiling spectra are displayed in Figures S4−S10 (Supporting Information). In case of the MDO electrolyte additive, a peak is observed in the N 1s spectra, which is not present in case of the FEC containing electrolyte sample. This peak, despite the fact that it is decreasing in atomic concentration with sputtering time, is even present after 120 s of Ar ion sputtering, thus showing a clear contribution of MDO in the SEI formation on graphite. The calculation of the SEI thickness of organic and inorganic layers was performed as described in detail in our previous publication and is depicted in Figure 5c.48 By analyzing the calculated SEI thicknesses, it can be seen that the formed SEI in case of the MDO electrolyte additive is much thinner compared to the SEI formed in the presence of FEC. Moreover, as can be derived from the error bars the MDO electrolyte additive leads to a much more homogeneously formed SEI on the graphite surface. 3.5. Oxidative Stability of Investigated SEI-Forming Electrolyte Additives. The oxidative stability of the 1 M LiPF6 in PC electrolyte with and without 2 wt % of either VC, FEC, or MDO was studied by means of linear sweep voltammetry in LiMn2O4/Li half cells (Figure 6).

4. CONCLUSION In summary, we demonstrate that MDO can be employed as a highly effective SEI-forming electrolyte additive in LIBs to enable the application of high voltage electrolyte solvents, which tend to exfoliate the graphite negative electrode. With the use of only 2 wt % of MDO, NMC532/graphite full cells comprising a 1 M LiPF6 in PC electrolyte reached an impressive cycle life of more than 450 cycles (80% capacity retention), whereas other commonly reported SEI-forming electrolyte additives such as VC and FEC failed. Furthermore, the MDO electrolyte additive showed no influence on the oxidative stability limit of the electrolyte formulation. Therefore, it can be envisioned that the obtained results will contribute to the development of new electrolyte formulations based on alternative high voltage stable solvent classes to enable high energy LIBs operating at elevated cell voltages.



ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b01977. 1 H and 13NMR spectra of MDO; cycling stability of NMC532/graphite full cells at 60 °C; XPS survey spectra; and Li 1s, P 2p, C 1s, O 1s, and F 1s depth profiling spectra (PDF)



AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]; Tel.: +49 251 8336686; Fax: +49 251 83-36032 (R.W.). *E-mail: [email protected] or [email protected]; Tel.: +49 251 83-36031; Fax: +49 251 83-36032 (M.W.). *E-mail: [email protected]; Tel.: +49 251 83-33248; Fax: +49 251 83-33202 (F.G.). ORCID

Figure 6. Oxidative electrochemical stability window of 1 M LiPF6 in PC electrolyte with and without 2 wt % of either VC, FEC, or MDO. Measurements were performed in LiMn2O4/Li half cells at a scan rate of 0.025 mV s−1 and a temperature of 25 °C.

Johannes Kasnatscheew: 0000-0002-8885-8591 Frank Glorius: 0000-0002-0648-956X Ralf Wagner: 0000-0002-5801-9260 7737

DOI: 10.1021/acs.chemmater.7b01977 Chem. Mater. 2017, 29, 7733−7739

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The authors declare no competing financial interest.



ACKNOWLEDGMENTS The Federal Ministry of Education and Research of the Federal Republic of Germany is gratefully acknowledged for financial support in the framework of the Electrolyte Lab project (4E) (Project 03X4632). In addition, generous financial support from the Deutsche Forschungsgemeinschaft (Leibniz Award) is gratefully acknowledged.



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DOI: 10.1021/acs.chemmater.7b01977 Chem. Mater. 2017, 29, 7733−7739

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DOI: 10.1021/acs.chemmater.7b01977 Chem. Mater. 2017, 29, 7733−7739