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Impact of Selected LiPF Hydrolysis Products on the High Voltage Stability of Lithium-Ion Battery Cells Ralf Wagner, Martin Korth, Benjamin Streipert, Johannes Kasnatscheew, Dennis Roman Gallus, Sebastian Brox, Marius Amereller, Isidora Cekic-Laskovic, and Martin Winter ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09164 • Publication Date (Web): 28 Oct 2016 Downloaded from http://pubs.acs.org on October 29, 2016

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ACS Applied Materials & Interfaces

Impact of Selected LiPF6 Hydrolysis Products on the High Voltage Stability of Lithium-Ion Battery Cells Ralf Wagner*,a, Martin Korthc, Benjamin Streiperta, Johannes Kasnatscheewa, Dennis R. Gallusa, Sebastian Broxa, Marius Amerellera, Isidora Cekic-Laskovica and Martin Winter*,a,b

a

MEET Battery Research Center / Institute of Physical Chemistry, University of Münster,

Corrensstrasse 46, 48149 Münster, Germany b

Helmholtz-Institute Münster, IEK-12, Forschungszentrum Jülich GmbH, Corrensstrasse 46,

48149 Münster, Germany c

Institute for Theoretical Chemistry, Ulm University, Albert-Einstein-Allee 11, 89069 Ulm,

Germany

KEYWORDS: lithium-ion batteries, high voltage application, electrolyte aging, electrolyte additives, LiNi1/3Mn1/3Co1/3O2 (NMC) cathode

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ABSTRACT Diverse LiPF6 hydrolysis products evolve during lithium-ion battery cell operation at elevated operation temperatures and high operation voltages. However, their impact on the formation and stability of the electrode/electrolyte interfaces is not yet investigated and understood. In this work, literature-known hydrolysis products of LiPF6 dimethyl fluorophosphate (DMFP) and diethyl fluorophosphate (DEFP) were synthesized and characterized. The use of DMFP and DEFP as electrolyte additive in 1 M LiPF6 in EC:EMC (1:1, by wt.) was investigated in LiNi1/3Mn1/3Co1/3O2/Li half cells. When charged to a cut-off potential of 4.6 V vs. Li/Li+, the additive containing cells showed improved cycling stability, increased Coulombic efficiencies and prolonged shelf life. Furthermore, low amounts (1 wt.% in this study) of the aforementioned additives did not show any negative effect on the cycling stability of graphite/Li half cells. DMFP and DEFP are susceptible to oxidation and contribute to the formation of an effective cathode/electrolyte interphase as confirmed by means of electrochemical stability window determination, X-ray photoelectron spectroscopy characterization of pristine and cycled electrodes as well as supported by computational calculations.


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1. INTRODUCTION Since the commercial launch of lithium-ion batteries (LIBs) by Sony and the work by Dahn et al. 1 as well as Tarascon and Guyomard 2-3 in the early 1990s, the skeleton electrolyte based on lithium hexafluorophosphate (LiPF6) as conducting salt, which is dissolved in mixtures of ethylene carbonate and linear carbonates has not significantly changed 4-7. However, performance deterioration and safety risks caused by electrolyte degradation limit the application of this electrolyte in LIBs to operation temperatures < 50 °C and cut-off potentials < 4.5 V vs. Li/Li+ 8-10. Thereby, the decomposition rate of the LiPF6 is strongly dependent on temperature, applied potential and amount of protic impurities 11-17. Kraft et al. quantified the evolution of dialkyl fluorophosphates in 1 M LiPF6 in ethylene carbonate : ethyl methyl carbonate (EC:EMC; 1:1, by wt.) electrolyte after electrochemical aging in LiNi0.5Mn1.5O4/Li coin cells at 5.5 V for 72 h and thermal aging in aluminum vials at a storage temperature of 95 °C for 13 days.18 The concentration of dimethyl fluorophosphate (DMFP) as well as diethyl fluorophosphate (DEFP) in the aged electrolyte samples amounted up to ≈ 0.15 wt.% 18. So far, only scarce information regarding a potential contribution of LiPF6 hydrolysis products in the formation of the cathode/electrolyte interphase (CEI) is available in literature 19-22. In general, pristine cathode materials are covered by native surface species Li2CO3 and LiOH, which originate from the reaction of the transition metal oxides with atmospheric CO2 and H2O, respectively 23-27. When immersed into the electrolyte and especially after constant current cycling, these surface species are replaced by surface films originating from the intrinsic reaction of the active cathode particles with electrolyte solvents, conducting salt and hydrolysis products, such as HF 28-30. The oxidation of a conventional LiPF6/organic carbonate-based electrolyte starts above 4.5 V vs. Li/Li+, thus leading to a poly(ethylene carbonate) (PEC)-enriched CEI 9. Furthermore, the presence of hardly soluble conducting salt derived species LiPFxOy in the CEI composition were reported 19, 21-22. In this work, DMFP and DEFP were synthesized in high purity (> 99.4% 1H-NMR) and investigated 3 ACS Paragon Plus Environment


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as electrolyte additives in conventional LiPF6/organic carbonate-based electrolyte regarding their influence on the cycling performance of LiNi1/3Mn1/3Co1/3O2 (NMC111)-based cathodes at high voltage (HV). DMFP and DEFP significantly improve the kinetic stability of the formed CEI in terms of prolonged cycle life and shelf life of the additive containing cells. These findings do have a major influence on the understanding of the fundamental aging phenomena in LIB cells containing LiPF6-based electrolytes.

2. MATERIALS AND METHODS SECTION 2.1

Synthesis of DMFP and DEFP. Due to the sensitivity of the P-F bond against

hydrolysis, synthesis was conducted either in a dry room (dew point: -65 °C), or under argon atmosphere (Argon 4.6, Westfalen AG) using the Schlenk technique, or in an argon-filled glove box (MBRAUN). Note that fluorinated organophosphates are extremely toxic and should be handled only by skilled persons using full-face respirator and chemical resistant gloves 10. The dialkyl fluorophosphates, DMFP and DEFP, were synthesized from dialkyl phosphites following the procedure of Acharya et al. 31. The synthetic approach depicted in Scheme 1 uses trichloroisocyanuric acid (TCICA) in combination with potassium fluoride (KF) in a one-pot reaction.

2.1.1 Synthesis Procedure of DMFP and Characterization of the Obtained Product. Potassium fluoride (6.86 g, 118.1 mmol, 1.3 eq., Alfa Aesar, anhydrous 99%) was dried for at least 12 h under vacuum (rotary vane pump, Vacuubrand) at 90 °C. Trichloroisocyanuric acid (TCICA, abcr, 97%) (8.45 g, 36.4 mmol, 0.4 eq.) and anhydrous acetonitrile (20 mL, Carl Roth, ≥ 99.5%, ≤ 10 ppm H2O) were added in dry atmosphere. Dimethyl phosphite (DMP, Sigma-Aldrich, 98%) (10.00 g, 90.9 mmol, 1.0 eq.), freshly distilled and dried over molecular sieves 3Å (Carl Roth) for at least 12 h, was mixed with acetonitrile (20 mL) and added slowly 4 ACS Paragon Plus Environment

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to the KF-TCICA suspension. The mixture was heated under reflux at 60 °C in an argon atmosphere for 1 h and further stirred at room temperature (RT ≈ 20 °C) for at least 12 h. After filtration and washing with acetonitrile (10 mL), the crude product was subjected to vacuum fractional distillation (20 Torr, 80 °C) to isolate a colorless liquid. Drying over phosphorous pentoxide (P2O5, Sigma-Aldrich, ≥ 99.99 trace metals basis) and subsequent vacuum distillation afforded 4.65 g (36.3 mmol, 40%) pure target product as a colorless liquid.

IR (KBr, [cm-1]): 1303.9 (P=O); 859.9 (P-F) NMR: δ (400 MHz, [ppm], CDCl3): 1H: 3.89 (d, 3JH-P =11.5 Hz, 6H). 13C (101 MHz): 55.71 (d, 2JC-P = 5.9 Hz). 19F (376 MHz): -85.24 (d, JF-P = 979.7 Hz). 31P (162 MHz): -6.97 (dseptet, 3

JP-H =11.5 Hz, JP-F = 979.6 Hz).

MS (EI): m/z =128 [M+ ]. The NMR and GC/MS spectra of DMFP are depicted in the supporting information (Figure S1-S5).

2.1.2 Synthesis Procedure of DEFP and Characterization of the Obtained Product. Diethyl phosphite (DEP, Sigma-Aldrich, 98%) was freshly distilled and dried over molecular sieves 3Å for at least 12 h. KF (5.47 g, 79.6 mmol, 1.3 eq.) was dried for at least 12 h under vacuum (rotary vane pump, Vacuubrand) at 90 °C. TCICA (6.73 g, 28.9 mmol, 0.4 eq.) and anhydrous acetonitrile (40 mL) were added in dry atmosphere to KF. DEP (10.00 g, 72.4 mmol, 1.0 eq.) was added slowly through a syringe attached with a filter (PTFE membrane, 0.2 µm) to the suspension. Thereafter, the mixture was heated under reflux at 60 °C in an argon atmosphere for 1 h and further stirred at RT for 12 h. After filtration and washing with acetonitrile (10 mL), the crude product was subjected to vacuum fractional distillation (10 Torr, 80 °C) to isolate a colorless liquid. Drying over P2O5 and subsequent 5 ACS Paragon Plus Environment


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vacuum distillation afforded 6.86 g (43.9 mmol, 61%) pure target product as a colorless liquid.

IR (KBr, [cm-1]): 1299.7 (P=O); 870.0 (P-F). NMR: δ (400 MHz, [ppm], CDCl3): 1H: 1.38 (t, 3JH-H = 7.1 Hz, 6H); 4.24 (quint (dq could not be resolved), J = 7.4 Hz, 4H). 13C (101 MHz): 16.12 (d, 3JC-P = 6.3 Hz); 65.80 (d, 2JC-P = 5.8 Hz). 19F (376 MHz): -80.83 (d, JF-P = 979.3 Hz). 31P (162 MHz): -8.99 (dquint, 3JP-H =8.9 Hz, JP-F = 978.8 Hz). MS (EI): m/z =155 [M-H]+. The NMR and GC/MS spectra of DEFP are presented in the supporting information (Figure S6-S10).

2.2

Electrolyte Preparation. 1 M LiPF6 in EC:EMC (1:1, by wt.) electrolyte (LP50

Selectilyte™, BASF, battery grade) and conducting salt LiPF6 (BASF, battery grade) were used as received. All electrolytes were formulated in an argon-filled glove box. The H2O content in all electrolytes was determined to be less than 20 ppm by means of coulometric Karl-Fischer titration (Mitsubishi CA 200).

2.3

Electrode Preparation. LiMn2O4 (LMO) electrodes were composed of 80 wt.% LMO

(TODA) active material. The composition of graphite electrodes was 90 wt.% SFG6 graphite (Imerys). The preparation procedure has been described in detail elsewhere 32. The active mass loading of all investigated electrodes (Ø12 mm) was in the range between 2 and 3 mg cm-2. Calendered NMC111 electrodes (Ø12 mm) with an active mass loading of ≈14 mg cm-2 were provided by the BMW Group.

2.4

Cell Setup and Electrochemical measurements. Electrochemical investigations were

performed at 20 °C in a climate chamber (Binder) using a three-electrode cell setup 6 ACS Paragon Plus Environment

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(Swagelok® cells). The reference and counter electrode were made of lithium metal (Rockwood Lithium). Glass filters (GF/D Whatman) were used as separator. Cell assembly was conducted in an argon-filled glove box. The constant current experiments were performed on a battery test system (Series 4000, MACCOR). NMC111/Li half cells were charged and discharged in the potential range from 3.0 V to 4.6 V vs. Li/Li+ at a specific current of 30 mA g-1 for three formation cycles and subsequently at a specific current of 150 mA g-1 for 80 cycles. For the NMC111/Li half cell charged to only 4.3 V vs. Li/Li+, a specific current of 27 mA g-1 and 135 mA g-1 was applied for formation and ongoing cycling, respectively. Self-discharge tests were carried out at 20 °C as described in the following. After three formation cycles at a specific current of 30 mA g-1, the cells were charged to 4.6 V vs. Li/Li+ using a specific current of 30 mA g-1. Afterwards, the cells were hold at open circuit potential for 500 h and the potential of the working electrode was recorded. SFG6 graphite/Li half cells were charged and discharged in the potential range between 0.025 V and 1.5 V vs. Li/Li+ at a specific current of 74 mA g-1 for three formation cycles and at 372 mA g-1 for ongoing cycles. In order to achieve complete Li+ ion intercalation during charge, the potential was held for one hour at 0.025 V vs. Li/Li+. The oxidative stability of investigated electrolyte formulations was determined by means of cyclic voltammetry (CV) in LMO/Li half cells using a sweep rate of 0.1 mV s-1 on a VMP potentiostat (Bio-Logic).

2.5

Ex Situ Surface Analysis. The X-ray photoelectron spectroscopy (XPS) measurement

setup and data fitting is described in detail in the supporting information.

2.6

Computational Studies. B3LYP 33 DFT calculations were carried out with TURBO-

MOLE 6.4 34 including D2 dispersion correction 35 and the COSMO or COSMO-RS solvation 7 ACS Paragon Plus Environment


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models, 36 employing def2-TZVP basis sets 37. Enthalpic and entropic contributions are taken into account via normal mode analysis within the harmonic oscillator/rigid rotor approximation. Minor differences and no qualitative changes were observed when applying COSMO with a dielectric constant of 20.7 (corresponding to acetone) instead of ∞, as it was done for the data shown. The value of 20.7 is often used as a good approximation for typical battery electrolyte mixtures.

3. RESULTS AND DISCUSSION 3.1

Evolution of DMFP and DEFP Contents during LIB Cell Operation. In Scheme 2,

the reaction scheme adapted from literature 10 and calculated reaction energy values in kcal mol-1 at B3LYP-D3/TZVP/COSMO-RS level for the reaction of LiPF6 to DMFP and DEFP are depicted. LiPF6 reversibly reacts to LiF and PF5 38-39. PF5 as strong Lewis acid reacts instantly with trace impurities, such as H2O, to form POF3 and HF. The POF3 reacts with organic carbonates, followed by decarboxylation to generate alkyl difluorophosphates (P(=O)(F)2OR) and dialkyl fluorophosphates (P(=O)(F)(OR)2) 12. The consecutive reaction steps of PF5 to form DEFP are exothermic, whereas the reaction of P(=O)(F)2OMe to form DMFP is weakly endothermic. The replacement of the last F substituent in DMFP and DEFP by OR groups is thermodynamically unfavorable. According to these theoretical findings, Kraft et al. reported the evolution of DEFP in higher concentrations compared to DMFP after electrolyte aging of 1 M LiPF6 in EC:EMC (1:1, by wt.) at 95 °C 18. The trialkyl phosphates (P(=O)(OR)3) were observed only in much lower concentrations 18.

3.2

Oxidative Stability of DMFP and DEFP-Based Electrolytes. To ex ante elucidate

the oxidation tendency of the investigated dialkyl fluorophosphates, the oxidation stability (in eV) was calculated at B3LYP-D3/TZVP/COSMO level with respect to the isolated molecule and with respect to the complexation with Li+ ion. The calculated oxidative stability of the 8 ACS Paragon Plus Environment

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isolated structure of the literature reported HV stable dimethyl methyl phosphonate (DMMP) 40-42

amounts to 7.49 eV, whereas for DMFP and DEFP the calculated oxidative stability is

even increased to 8.23 and 8.14 eV, respectively. However, considering a complexation of Li+ ion by the investigated compounds, this trend is reversed and DMFP and DEFP display a far smaller oxidative stability as summarized in Table 1. For this reason, it is assumed that DMFP and DEFP are susceptible to oxidation in the lithium salt containing electrolyte. The oxidative stability of the LiPF6/dialkyl fluorophosphate-based electrolytes was investigated by means of CV on a LMO working electrode (Figure 1). The observed peaks in the range from 3.9-4.3 V vs. Li/Li+ correspond to a two-step Li+ ion extraction from the spinel structure 43. The DMFP containing cell displays a third reversible Faradaic process at ≈ 4.5 V vs. Li/Li+, which is caused by delithiation of remnant Li+ ions from the LMO working electrode 44. DMFP and DEFP-based electrolytes show high background currents after delithiation of the LMO host material and before the potential is further raised and bulk electrolyte decomposition becomes significant at potentials around 5.3 V and 5.0 V vs. Li/Li+, respectively, as indicated by an exponential rise in the specific current values. This high level of background current is mainly assigned to parasitic reactions, namely oxidative electrolyte decomposition. In line with the oxidative stability predicted by the DFT calculation, DMFP and DEFP-based electrolytes show continuous oxidative decomposition on LMO electrodes already at moderate oxidation potentials of ≈ 4.5 V vs. Li/Li+.

3.3

Dialkyl Fluorophosphates as HV Electrolyte Additives. DMFP and DEFP showed

insufficient oxidative stability in the CV measurement on LMO. Therefore, both compounds are investigated as CEI-forming electrolyte additives to stabilize the interface between the charged NMC111 surface and the 1 M LiPF6 in EC:EMC (1:1, by wt.) electrolyte. As seen in Figures 2a-b, the addition of 1 wt.% DMFP or DEFP to the benchmark electrolyte leads to 9 ACS Paragon Plus Environment


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improved cycling performance and higher CEs of the NMC111/Li half cells, compared to the benchmark system, when charged to 4.6 V vs. Li/Li+. The capacity retention and specific discharge capacities of selected cycles are listed in Table 2. The addition of DMFP or DEFP to the benchmark electrolyte leads to a significantly increased capacity retention after 50 cycles. The data of NMC111/Li half cell containing the benchmark electrolyte charged to only 4.3 V vs. Li/Li+ are added both in Figures 2a-b and Table 2 to allow comparison. In literature, the observed capacity fading of NMC111 at HV is mainly ascribed to kinetics, such as an increase in overpotential, thus resulting in higher cell internal resistance 45-47. The increase in over-potential involves a decrease in mean discharge potential. As seen in Figure 2c, the benchmark electrolyte shows a strong decrease of the mean discharge potential over 80 cycles, thus indicating the constant increase in internal cell resistance with each cycle. The rise in internal cell resistance originates from the growth of a more resistive CEI.47-48 The addition of 1 wt.% DMFP or DEFP to the benchmark electrolyte leads to a reduced fading of the mean discharge potential over 80 cycles, thus indicating less internal cell resistance. The observed fading rate for the DMFP containing electrolyte is even comparable to the benchmark electrolyte containing cell charged to only 4.3 V vs. Li/Li+. Considering the insufficient oxidative stability of 1 M LiPF6 in DMFP and DEFP electrolytes in the CV measurement, it can be assumed that both additives decompose on the charged cathode prior to the bulk electrolyte and lead to the formation of a kinetically more stable CEI, thus protecting the bulk electrolyte form further decomposition during cycling. The mechanism will be further investigated in the following at the example of DEFP, since it is the thermodynamically favored decomposition product. The kinetic stability of the formed CEI was further investigated by measuring the selfdischarge of NMC111/Li half cells containing the benchmark electrolyte with and without 1 wt.% DEFP (Figure 2d). In general, the self-discharge arises from the oxidation of the electrolyte or by transition metal dissolution from the cathode in the electrolyte 49-51. The self10 ACS Paragon Plus Environment

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discharge rate is distinctly decreased in case of the electrolyte containing 1 wt.% DEFP. This indicates the formation of a CEI with a protecting function on the charged NMC111 electrode surface in presence of DEFP in the electrolyte. The formed CEI suppresses electrolyte decomposition as one of the main parasitic reactions taking place.

3.4

Studies of the Electrode Surface by XPS. To gain better understanding of the CEI

formation, NMC111 electrodes before and after cycling in benchmark electrolyte with and without DEFP were analyzed by means of XPS. In Figure 3, F 1s, P 2p and Mn 2p surface spectra are depicted. The corresponding Li 1s, C 1s O 1s and Co 2p spectra are displayed in Figure S11 in the supporting information. Table 3 lists the respective atomic surface concentrations (%at.) 52. In comparison to the fresh electrode, the cycled samples show higher fluorine and phosphorous surface atomic concentration as well as a decrease in the manganese signals. These findings indicate the presence of a passivation film on the NMC111 surface after electrochemical formation. Close inspection reveals that the Mn and Co surface atomic concentration is reduced for the DEFP additive containing electrolyte compared to the benchmark electrolyte as shown in Table 3. Therefore, the use of DEFP as electrolyte additive results in a much thicker and/or more homogenous CEI after electrochemical formation. The peak at 137 eV in the P 2p spectra of cycled electrodes indicates remaining LiPF6 and its derived species LiPFx. The peak additionally represents species originating from DEFP. The peak at ≈135 eV indicates a P atom that is bound to a less electronegative element than F, such as O 53. The strong P-O signal in the DEFP containing sample clearly shows the contribution of the DEFP additive in the measured surface layer composition. At the same time, the signal of LiF (685 eV) in the F 1s spectrum is strongly increased relatively to the peak at 687 eV, which represents PVdF binder, LiPF6, LiPFx and DEFP-originated species 53. Pristine electrodes also show a small impurity of LiF, which arises from a parasitic dehydrofluorination of PVdF 22, 54. From the increased LiF, LiPFx and LiPFxOy intensity and 11 ACS Paragon Plus Environment


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the oxidative stability determination, it can be concluded, that the use of DEFP as electrolyte additive results in the oxidation of DEFP and its contribution in the formation of a CEI. As seen in Table 3, the concentration of the polyethylene oxide (PEO) and PEC surface species are slightly decreased for the NMC111 electrode cycled in the DEFP additive containing electrolyte, which indicated less oxidative decomposition of the bulk electrolyte compared to the benchmark electrolyte without additive.

3.5

Cycling Stability in SFG6 Graphite/Li Half Cells. Additionally to their beneficial

influence on the interfacial stability of the bulk electrolyte towards the charged NMC111 cathode surface at high potentials, the use of dialkyl fluorophosphates as electrolyte additives should not deteriorate the film-forming properties of the bulk electrolyte on graphite anodes. As reported for phosphorous-based electrolytes, such as DMMP, 55-58 1 M LiPF6 in either DMFP or DEFP show poor compatibility on graphite as well, most likely due to the reduction and co-intercalation of the solvent as well as exfoliation of the graphite (not shown) 59. Recently, however, novel phosphorous-based compounds, lithium difluorophosphate P(=O)(F)2OLi and lithium dimethylphosphate P(=O)(OMe)2OLi, have been reported as solid electrolyte interphase (SEI) 60-61 forming electrolyte additives 62-63. In Figure 4, the cycling performance of benchmark electrolyte with 1 wt.% DEFP in SFG6 graphite/Li half cells is depicted. The capacity retention after 50 cycles amounts to 98.9% and the CE in the first cycle amounts to 67%. The first cycle CE for SFG6 graphite/Li half cells containing only the benchmark electrolyte without additive amounts to 67% as well. In summary, the addition of low amounts of DEFP has no visible negative impact on the cycling stability of graphite/Li half cell.

4. CONCLUSION 12 ACS Paragon Plus Environment

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LiPF6 decomposition products form during operation at high voltages and high temperatures in lithium-ion cells containing conventional LiPF6/organic carbonate-based electrolyte. Among various parasitic reaction products, DMFP and DEFP were selected for further investigations and synthesized in battery grade purity. Both compounds were shown to be suitable as electrolyte additives to stabilize the cathode/electrolyte interface against degradation at high voltage applications. DMFP and DEFP are susceptible towards oxidative decomposition and participate in the formation of a CEI. The addition of already 1 wt.% DMFP or DEFP to the 1 M LiPF6 in EC:EMC (1:1, by wt.) electrolyte results in better capacity retentions, higher Coulombic efficiencies and prolonged shelf life of NMC111/Li half cells charged to 4.6 V vs. Li/Li+. Used in low amounts (1 wt.% in this study), these additives show no negative influence on the graphite SEI formation.



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ASSOCIATED CONTENT Supporting Information The supporting information is available free of charge on the ACS Publications website (http://pubs.acs.org) Detailed characterization of DMFP and DEFP by means of NMR and GC/MS as well as further XPS spectra and detailed information on XPS fitting parameters.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors R.W., B.S., J.K. and M.A. thank the BMW Group and BASF for the financial support of this work and the material support, respectively. M.K. acknowledges the “Dr. Barbara Mez-Starck-Foundation”. D.R.G., S.B. and I.C. received funding from German Federal Ministry for Education and Research (BMBF) within the project Electrolyte Lab 4E (project reference 03X4632). M.W. received funding from the Federal Government of Germany.


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50. Li, S. R.; Sinha, N. N.; Chen, C. H.; Xu, K.; Dahn, J. R., A Consideration of Electrolyte Additives for LiNi0.5Mn1.5O4/Li4Ti5O12 Li-Ion Cells. J. Electrochem. Soc. 2013, 160 (11), A2014-A2020. 51. Pistoia, G.; Antonini, A.; Rosati, R.; Zane, D., Storage Characteristics of Cathodes for Li-Ion Batteries. Electrochim. Acta 1996, 41 (17), 2683-2689. 52. Niehoff, P.; Winter, M., Composition and Growth Behavior of the Surface and Electrolyte Decomposition Layer of/on a Commercial Lithium Ion Battery LixNi1/3Mn1/3Co1/3O2 Cathode Determined by Sputter Depth Profile X-ray Photoelectron Spectroscopy. Langmuir 2013, 29 (51), 15813-15821. 53. Herstedt, M.; Stjerndahl, M.; Nytén, A.; Gustafsson, T.; Rensmo, H.; Siegbahn, H.; Ravet, N.; Armand, M.; Thomas, J. O.; Edström, K., Surface Chemistry of Carbon-Treated LiFePO4 Particles for Li-Ion Battery Cathodes Studied by PES. Electrochem. Solid-State Lett. 2003, 6 (9), A202-A206. 54. Edström, K.; Gustafsson, T.; Thomas, J. O., The Cathode–Electrolyte Interface in the Li-Ion Battery. Electrochim. Acta 2004, 50 (2–3), 397-403. 55. Feng, J.; Ma, P.; Yang, H.; Lu, L., Understanding the Interactions of PhosphonateBased Flame-Retarding Additives with Graphitic Anode for Lithium Ion Batteries. Electrochim. Acta 2013, 114 (0), 688-692. 56. Xiang, H. F.; Wang, Q.; Wang, D. Z.; Zhang, D. W.; Wang, H. H.; Chen, C. H., Optimizing the Compatibility Between Dimethyl Methylphosphonate (DMMP)-Based Electrolytes and Carbonaceous Anodes. J. Appl. Electrochem. 2011, 41 (8), 965-971. 57. Chung, G.-C.; Kim, H.-J.; Yu, S.-I.; Jun, S.-H.; Choi, J.-w.; Kim, M.-H., Origin of Graphite Exfoliation. J. Electrochem. Soc. 2000, 147 (12), 4391-4398. 58. Wagner, R.; Brox, S.; Kasnatscheew, J.; Gallus, D. R.; Amereller, M.; Cekic-Laskovic, I.; Winter, M., Vinyl Sulfones as SEI-Forming Additives in Propylene Carbonate Based Electrolytes for Lithium-Ion Batteries. Electrochem. Commun. 2014, 40 (0), 80-83. 59. Wagner, M. R.; Albering, J. H.; Moeller, K. C.; Besenhard, J. O.; Winter, M., XRD Evidence for the Electrochemical Formation of Li+(PC)yCn- in PC-Based Electrolytes. Electrochem. Commun. 2005, 7 (9), 947-952. 60. Winter, M., The Solid Electrolyte Interphase – The Most Important and the Least Understood Solid Electrolyte in Rechargeable Li Batteries. Z. Phys. Chem. 2009, 223 (10-11), 1395-1406. 61. Schranzhofer, H.; Bugajski, J.; Santner, H. J.; Korepp, C.; Möller, K. C.; Besenhard, J. O.; Winter, M.; Sitte, W., Electrochemical Impedance Spectroscopy Study of the SEI Formation on Graphite and Metal Electrodes. J. Power Sources 2006, 153 (2), 391-395. 62. Kim, K.-E.; Jang, J. Y.; Park, I.; Woo, M.-H.; Jeong, M.-H.; Shin, W. C.; Ue, M.; Choi, N.-S., A Combination of Lithium Difluorophosphate and Vinylene Carbonate as Reducible Additives to Improve Cycling Performance of Graphite Electrodes at High Rates. Electrochem. Commun. 2015, 61, 121-124. 63. Milien, M. S.; Tottempudi, U.; Son, M.; Ue, M.; Lucht, B. L., Development of Lithium Dimethyl Phosphate as an Electrolyte Additive for Lithium Ion Batteries. J. Electrochem. Soc. 2016, 163 (7), A1369-A1372.


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Scheme 1. Synthesis route of dialkyl fluorophosphates from diakyl phosphites using TCICA and KF.

Scheme 2. Calculated free reaction energies at B3LYP-D2/def2-TZVP/COSMO-RS level in kcal mol-1 for the hydrolysis of LiPF6 to form DMFP and DEFP.

Figure 1. Oxidative electrochemical stability window of 1 M LiPF6 in (a) DMFP and (b) DEFP. Measurements were performed in LMO/Li half cells at a scan rate of 0.1 mV s-1. The dashed line indicates the zero current value level.



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Figure 2. (a and b) Cycling stability and Coulombic efficiencies of 1 M LiPF6 in EC:EMC (1:1, by wt.) with and without 1 wt.% DMFP or 1 wt.% DEFP in NMC111/Li half cells charged to 4.3 and 4.6 V vs. Li/Li+. (c) Mean discharge potential for cycles 5-80. (d) Open circuit potential vs. time curves for 1 M LiPF6 in EC:EMC (1:1, by wt.) with and without 1 wt.% DEFP in NMC111/Li half cells after electrochemical formation.


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Figure 3. (a) F 1s, (b) P 2p and (c) Mn 2p spectra of a pristine NMC111 cathode and harvested NMC111 electrodes cycled in half cells using 1 M LiPF6 in EC:EMC (1:1, by wt.) with and without 1 wt.% DEFP after three formation cycles.



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Figure 4. Cycling stability of 1 M LiPF6 in EC:EMC (1:1, by wt.) + 1 wt.% DEFP in SFG6 graphite/Li half cells.


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Table 1. Calculated oxidative stability values at B3LYP-D3/TZVP/COSMO level for DMMP, DMFP and DEFP. Only uncharged complexes had been calculated. Complexation Isolated molecule Li attached to F Li attached to O

Calculated oxidative stability /eV DMMP DMFP DEFP 7.49 8.23 8.14 2.45 2.64 5.11 2.48 2.52



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Table 2. Summary of the electrochemical performance of investigated additives (1 wt.%) DMFP and DEFP in 1 M LiPF6 in EC:EMC (1:1, by wt.) electrolyte as measured by the discharge capacities in the 5th and 50th cycle as well as the capacity retentions in NMC111/Li half cells charged to 4.3 V and 4.6 V vs. Li/Li+. Investigated electrolyte additives No additive No additive 1 wt.% DMFP 1 wt.% DEFP

Upper cut-off potential vs. Li/Li+ /V 4.3 4.6 4.6 4.6

5th cycle specific discharge cap. / mAh g-1 141.3 167.3 171.4 162.0

50th cycle specific discharge cap. / mAh g-1 135.3 140.3 159.0 147.1


Cap. retention [50th/5th] /% 95.8 83.8 92.7 90.8

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Table 3. Atomic concentrations (%at.) and corresponding mean absolute deviations (MAD) determined from three spatially different measuring spots of a pristine NMC111 electrode and harvested NMC111 electrodes cycled in half cells using 1 M LiPF6 in EC:EMC (1:1, by wt.) with and without 1 wt.% DEFP after three formation cycles. Pristine NMC111 %at. MAD 0.40 0.01

NMC111 after three formation cycles in LP50 LP50 + 1% DEFP %at. MAD %at. MAD 0.29 0.02 0.22 0.05

Region Co 3/2

Component NMC111

Co 1/2

NMC111

0.16

0.01

0.09

0.02

0.08

0.02

F 1s

PVdF/ LiPFx

15.82

0.07

17.33

2.13

20.67

1.66

F 1s

LiF

1.19

0.08

5.88

0.70

5.79

0.19

Mn 3/2

NMC111

0.70

0.02

0.61

0.03

0.29

0.09

Mn 1/2

NMC111

0.24

0.01

0.19

0.03

0.07

0.04

O 1s

R2CO3/DEFP

0.02

0.02

0.09

0.07

1.26

0.30

O 1s

Li2CO3

4.94

0.20

12.25

0.76

15.56

0.72

O 1s

NMC111/Li2O

4.69

0.12

1.96

0.14

0.33

0.14

C 1s

Con. carbon

25.32

4.20

18.56

1.41

13.64

2.82

C 1s

CH2-CF2

7.21

1.28

3.71

0.46

1.97

0.33

C 1s

CF2-CH2

7.20

1.28

3.70

0.46

1.97

0.33

C 1s

Shake up

5.99

2.93

2.44

0.53

1.21

0.27

C 1s

PEO/PEC

3.28

0.85

9.19

0.66

8.84

0.43

C 1s

Am. carbon

18.70

3.00

15.98

2.11

11.38

0.38

P 2p

LiPFx/DEFP

0.00

0.00

1.89

0.26

4.27

0.97

P 2p

LiPFxOy/DEFP

0.00

0.00

0.73

0.14

1.37

0.35

4.14

0.70

5.10

0.83

11.09

1.75

Li 1s



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