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Pentafluorophenyl isocyanate as effective electrolyte additive for improved performance of silicon-based lithium ion full cells Roman Nölle, Andreas J. Achazi, Payam Kaghazchi, Martin Winter, and Tobias Placke ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07683 • Publication Date (Web): 25 Jul 2018 Downloaded from http://pubs.acs.org on July 25, 2018

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ACS Applied Materials & Interfaces

Pentafluorophenyl Isocyanate as Effective Electrolyte Additive for Improved Performance of Silicon-based Lithium Ion Full Cells Roman Nölle 1, Andreas J. Achazi 2, Payam Kaghazchi 2,3, Martin Winter 1,4, **, Tobias Placke 1, * 1

University of Münster, MEET Battery Research Center, Institute of Physical Chemistry, Corrensstr. 46, 48149 Münster, Germany 2

Physikalische und Theoretische Chemie, Institut für Chemie und Biochemie, Freie Universität Berlin, Takustraße 3, 14195 Berlin, Germany

3

Forschungszentrum Jülich GmbH, Institute of Energy and Climate Research (IEK-1), Materials Synthesis and Processing, Wilhelm-Johnen-Straße, 52425 Jülich, Germany

4

Helmholtz Institute Münster, IEK-12, Forschungszentrum Jülich GmbH, Corrensstraße 46, 48149 Münster, Germany

Abstract Due to its high specific and volumetric capacity and relatively low operation potential, silicon (Si) has attracted much attention to be utilized as high-capacity anode material for lithium ion batteries (LIBs) with increased energy density. However, the application of Si within commercial LIBs is still hindered by its poor cycling stability related to the huge volume changes of Si upon lithiation/de-lithiation, followed by continuous electrolyte decomposition and active lithium loss at the anode side. In this work, we present the application of pentafluorophenyl isocyanate (PFPI) as an effective electrolyte additive for lithium ion full cells, containing a pure, magnetron-sputtered Si anode and a LiNi1/3Mn1/3Co1/3O2 (NMC-111) cathode. The performance of the Si/NMC-111 full cells is significantly improved in terms of capacity retention and Coulombic efficiency (CE) by the addition of 2 wt.% of PFPI to the baseline electrolyte and is compared to the literature known additives vinylene carbonate and fluoroethylene carbonate. Furthermore, it is revealed, that the additive is able to reduce the active lithium losses by forming an effective solid electrolyte interphase (SEI) on the Si anode. X-ray photoelectron spectroscopy investigations unveil that PFPI is a main part of the SEI layer, leading to less active lithium immobilized within the interphase. Overall, our results pave the path for a broad range of different isocyanate compounds, which have not been studied for Si-based anodes in lithium ion full cells so far. These compounds can be easily adjusted by modifying the chemical structure and/or functional groups incorporated within the molecule, to specifically tailor the SEI-layer for Si-based anodes in LIBs. 1 ACS Paragon Plus Environment

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KEYWORDS silicon; LIB full cell; solid electrolyte interphase; isocyanate; electrolyte additive; lithium ion batteries

*Corresponding author:

**Co-Corresponding author:

Dr. Tobias Placke

Prof. Dr. Martin Winter

[email protected]

[email protected] [email protected]

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1. Introduction Since the commercial launch by Sony in 1991, lithium ion batteries (LIBs) have become the dominating power source for nearly all consumer electronics like laptops and smartphones, moreover also for domestic appliances, electric bicycles or medical applications.1-3 Due to the successful implementation and continuous improvements over the last 30 years, i.e. an increase in energy density of up to 700 Wh L-1 in 2015 (cell level) starting from 200 Wh L-1 in 1991, the LIB is expected to be the key technology for a substantial break-through of electro-mobility within the next 5-10 years.4 LIBs are indeed already widely applied plug-in hybrid electric vehicles (PHEVs), hybrid electric vehicles (HEVs) and also in all electric vehicles (EVs), however, especially EVs cannot compete with cars based on conventional combustion engines in terms of driving range so far. To achieve an increase of the driving range of EVs to more than 500 km, therefore, gaining greater consumer acceptance, the energy of state-of-the-art LIBs needs to be further increased up to values of ≈350 Wh kg-1 and ≈750 Wh L-1 at cell level.5 As the capacity of the today available, commercial relevant cathode materials is currently limited to a maximum value of ≈200 mAh g-1, the energy density on cell level needs to be boosted by improvements of the anode side. When considering different material factors like high capacity, low average de-lithitation potential (≈0.4 V vs. Li/Li+), low potential hysteresis and high energy efficiency, sufficient rate capability and low cost, silicon (Si) turns out to be the most promising anode material of choice to replace the state-of-the-art graphite-based anode.6-7 Si is able to provide a theoretical capacity of 3579 mAh g-1 or 2194 Ah L-1 (related to Li15Si4) based on an alloying mechanism compared to 372 mAh g-1 or 890 Ah L-1 for graphite (related to LiC6), which is based on a lithium intercalation mechanism.8 However, the use of Si-based anode materials with Si contents of more than 5-10 wt.% (based on the weight of the composite anode) in commercial cells is still hampered because of the huge volume expansion/contraction of up to 300% during lithium insertion/de-insertion into/from Si. Several issues for Si-based negative electrodes are related to the huge volume changes, that are9-12: (i)

cracking and pulverization of the Si particles and therefore loss of active Si.

(ii)

deterioration of the electrode structure and the conductive network of Si, binder and conductive agent alongside with contact loss to the current collector, leading to isolated Si particles.

(iii)

continuous parasitic reactions at the Si/electrolyte interface, i.e. ongoing (re-)formation of a solid electrolyte interphase (SEI)13, due to repeated exposure of 3 ACS Paragon Plus Environment


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the fresh Si surface to the electrolyte at potentials below the reductive stability of the electrolyte, which gradually consumes active lithium and electrolyte.14-15 The issues (i) and (ii) can be successfully mitigated by reducing the active particle size to the nanoscale 16, i.e. nanoparticles17, nanowires18 or thin films19-20. Another successful strategy is the usage of intermetallic phases of Si21-23 or Si/carbon composite materials24-26, therefore, decreasing the overall volume changes due to reduced active material content in an electrode and a buffer effect of the composite material.27 The third issue (iii) is typically addressed by artificial protection layers28-29, pre-lithiation techniques for partial compensation of active lithium losses30-31 or the addition of additives to the electrolyte solution. Significant improvements in terms of capacity retention and Coulombic efficiency (CE) are reported for Si-based anode materials by the addition of small amounts, i.e. up to 10 wt.%,

of

fluoroethylene carbonate (FEC) or vinylene carbonate (VC) to the electrolyte.32-34 However, most of the reports performed the electrochemical investigations in Li metal/Si cells (often reffered to as “half-cells”) with a theoretically unlimited lithium reservoir, therefore, lithium consuming parasitic reactions are neglected in these studies, though, it was shown by several groups that the dominating failure mechanism in Si-based full cells is the consumption of active lithium from the cathode due to continuous electrolyte reduction at the anode side, followed by depletion of active lithium.35-37 Therefore, we strongly recommend other researches to study Si or similar alloying-type anode materials in a full cell set-up for reasonable predictions of practical validity of the results. Several reports, focusing on the investigation of the origin of the beneficial effect of FEC or VC as electrolyte additive for Si-based anode materials, revealed that polymeric species arising from the reductive decomposition may be the crucial aspect for effectivity of the electrolyte additives.33-34,

38-42

Isocyanate compounds are known to be able to undergo

reductive polymerization reactions and, therefore, might be considered as SEI forming additives for Si-based anode materials.43 Actually, isocyanates were reported to function as effective SEI-forming additive for graphite44-49 or LTO-based anode materials50. Furthermore, isocyanate compounds also have been utilizied as electrolyte additives for forming an improved cathode electrolyte interphase (CEI) for different cathode materials, such as LiNi0.5Mn0.3Co0.2O2 and LiMn2O4. 51-53 In this report, pentafluorophenyl isocyanate (PFPI, see Error! Reference source not found.) was investigated as SEI-forming electrolyte additive for Si-based LIB full cells containing a LiNi1/3Mn1/3Co1/3O2 (NMC-111) cathode and is compared to the literature known additives VC and FEC. Therefore, magnetron-sputtered Si thin film anodes without binder or 4 ACS Paragon Plus Environment

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conductive agent were used as a model electrodes within this study to directly correlate the influence of the investigated electrolyte additive to the Si active material itself. Furthermore, the absence of any inactive material inside of the Si anode allowed for simplified post-mortem analysis, namely X-ray photoelectron spectroscopy (XPS) for elucidation of the working mechanism of the PFPI additive.

Figure 1: Chemical structure of the pentafluorophenyl isocyanate (PFPI) molecule.

2. Experimental 2.1 Electrode preparation of Si anodes Pure Si thin film electrodes (≈750 nm, ≈0.17 mg cm-2) were produced via magnetron sputter deposition. A RF-power of 90 W was applied to a high-purity n-type Si (99.99%, Ø4 in., FST GmbH) target for a sputter time of 90 min at an Ar plasma working pressure of 5 x 10-3 mbar. Dendritic copper foil (thickness = 20 µm, Ø3 in., Carl Schlenk AG) served as substrate during sputter deposition. The sputtered substrate was weighed and electrodes with a diameter of 12 mm were punched out inside a dryroom (dew point of at least -50 °C). The active material content of the as-prepared Si thin film electrodes was calculated by determining the Si mass loading of the whole copper substrate. The electrodes were dried at 120 °C for 12 h under reduced pressure (1 x 10-3 mbar) and afterwards stored in an argonfilled glovebox (H2O and O2 contents below 0.1 ppm).

2.2 DFT calculations The DFT calculations were performed using Gaussian 16 D3(BJ)/def2-TZVP level.

55-60

54

at the B3LYP-

The oxidation potentials correspond to the difference in the

Gibbs energy between the oxidized and neutral species divided by the Faraday constant and reduction potentials correspond to the difference in the Gibbs energy between the reduced and neutral species divided by the Faraday constant. Ro-vibrational contributions were evaluated by the rigid-rotor-harmonic-oscillator approximation at 298.15 K. The potentials were referenced with respect to Li/Li+ by subtracting 1.4 V from the oxidation/reduction potentials. 5 ACS Paragon Plus Environment


This is indicated by using VLi instead of V for the potentials determined by DFT.61 Solvent effects were included with the aid of the polarizable continuum model (PCM).62 PCM was used to model implicitly an acetone solvent (relative permittivity=20.493) for additives and solvent molecules. The reason of choosing the acetone solvent is that in most Li-based batteries a 7:3 mixture of linear and cyclic carbonates with a relative permittivity of about 20 are used.61

2.3 Electrochemical investigations The Si thin film electrodes were paired with NMC-111 electrodes (Ø12 mm, 0.53 mAh cm-2 for at cut-off potential of 4.3 V vs. Li/Li+, 90% active material content, Customcells Itzehoe) to Si/NMC-111 full cells with a N/P ratio of ≈1.13 (considering a capacity of 0.60 mAh cm-2 for the Si electrodes, based on the active material content and a theoretical capacity of 3,579 mAh g-1 for Si). Electrochemical investigations were carried out in three-electrode Swagelok-type T-cells assembled in an argon-filled glovebox (MBraun). High-purity lithium metal foil (Albemarle Corporation, battery grade) was used for the reference electrode (Ø6 mm) and Celgard 2500 (polypropylene, one layer, Ø13 mm and Ø10 mm for reference) was applied as separator. An overall amount of 120 µL electrolyte, i.e. (a) 1 M LiPF6 in EC/DEC 3:7 (w/w), (b) 1 M LiPF6 in EC/DEC 3:7 (w/w) + 2 wt.% VC, (c) 1 M LiPF6 in EC/DEC 3:7 (w/w) + 2 wt.% FEC (all UBE, battery grade) or (d) 1 M LiPF6 in EC/DEC 3:7 (w/w) + 2 wt.% PFPI (Alfa Aesar, 96%) was added to the cell. It should be noted, that a higher amount of electrolyte is used in these laboratory three-electrode T-cells in comparison to commercial cells. Therefore, the percentage of electrolyte additive or the actual amount of additive molecules probably needs to be adjusted within commercial cell set-ups. Charge/discharge cycling of the Si/NMC-111 cells was performed in a voltage range of 3.0 – 4.3 V at 20 °C on a Maccor series 4000 battery tester. The cycling protocol started by charging the cell up to 4.3 V with a current of 0.05C (1C = 150 mA g-1), followed by discharging down to 3.0 V at 0.1C. Two additional charge/discharge steps at 0.1C completed the formation process of the Si/NMC-111 full cells. Afterwards, the cells were cycled for 100 cycles at a constant charge/discharge current of 1C, additionally applying a constant voltage step at the charge and discharge cut-off voltage until the current dropped below 0.05C. The specific capacities shown in this report are related to the active material weight of the NMC111 electrode. The presented data are average values of three cells for each electrolyte formulation, demonstrating reproducibility of the obtained results. Oxidative stability limits of the different electrolytes were investigated by means of 6 ACS Paragon Plus Environment

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linear sweep voltammetry at a VMP3 (Biologic Science Instruments) with a scan-rate of 25 µV s-1 up to a potential of 6 V vs. Li/Li+ at 25 °C. Platinum was applied as working electrode, whereas metallic lithium served as counter and reference electrodes in the Swagelok three-electrode set-up.

2.4 Post-mortem investigations of Si anodes The Si thin film electrodes were harvested from the cycled cells in an argon-filled glovebox and washed two times via dipping the cycled electrodes in 200 µL DMC (BASF, battery grade) prior to surface analysis, in order to remove electrolyte residues and in particular LiPF6 salt. X-ray photoelectron spectroscopy (XPS) was performed to analyze the SEI formed on the Si thin film electrodes in different electrolyte solutions after 103 cycles. The samples were transferred to the XPS device (Axis Ultra DLD, Kratos) without exposure to ambient air in a sealed container. Monochromatic Al Kα X-rays (hν = 1486.6 eV) with a 10 mA emission current and 12 kV accelerating voltage was applied. A charge neutralizer was used to compensate for charging of the samples. The measurement was performed at a 0° angle of emission and a pass energy of 20 eV. The fitting was carried out with CasaXPS. Calibration of the binding energy (BE) of the recorded spectra was performed by using the C 1s C-H/C-C peak (BE = 284.5 eV) as an internal reference. To guarantee reproducibility, three different measurement spots per sample and two samples for each electrolyte formulation were investigated.

3. Results and discussion 3.1 DFT calculations for reductive and oxidative stability predictions for solvent and additive molecules DFT calculations were performed prior to electrochemical investigations to determine the reductive as well as oxidative stability tendency of the electrolyte solvents and additives . The

oxidation

potential

and

reduction

potential

were

calculated

at

the

B3LYP-D3(BJ)/def2-TZVP level, the data for the different molecules are depicted in Error! Reference source not found.. The reductive stability of a molecule is described by the reduction potential, i.e. a lower the value means a lower the reductive stability. As the electrolyte additives are applied to form an effective SEI layer on the silicon anode, they are intended to be reduced prior to the electrolyte solvent molecules. The PFPI (-3.92 VLi) exhibits the lowest reduction potential, followed by FEC (-3.05 VLi) and VC (-3.02 VLi). As 7 ACS Paragon Plus Environment


ethylene carbonate (EC, -2.69 VLi) and diethyl carbonate (DEC, -2.45 VLi) possess distinct higher reduction potentials, the three electrolyte additives are expected to be reduced prior to the solvent molecules and, therefore, can most likely prevent the reduction of EC and DEC. Table 1: Calculated oxidation potential and reduction potential values at B3LYP-D3(BJ)/def2-TZVP level of electrolyte solvent and additive molecules. The potential values are given in VLi in order to indicate that the values are referenced with respect to Li/Li+ by subtracting 1.4 V.

Molecule

oxidation potential / VLi

reduction potential / VLi

DEC

6.67

-2.45

EC

7.01

-2.69

VC

5.65

-3.02

FEC

7.33

-3.05

PFPI

5.59

-3.92

The oxidation potential values in Error! Reference source not found. give information about the stability of the different molecules against an oxidative decomposition reaction. FEC (7.33 VLi) exhibits the highest oxidation potential, followed by EC (7.01 VLi) and DEC (6.67 VLi). The investigated additive PFPI (5.59 VLi) possesses the lowest oxidation potential, which is actually lower than for the solvent molecules EC and DEC. Therefore, an oxidation of the PFPI molecule is expected to take place prior to the solvent molecules and at similar potential like VC (5.65 VLi). It should be noted that the calculated values do not represent the exact oxidation and reduction potentials of the electrolytes in the experimental part since the influence of the conductive salt was not included in the calculations. However, the DFTcalculated oxidation and reduction potentials can still help us to understand the relative stabilities of molecules, because the error for neglecting the influence of the conductive salt is approximately the same for all calculated oxidation and reduction potentials, respectively. To validate the reductive and oxidative stability tendencies of the different molecules determined via DFT calculations, the intrinsic reductive and oxidative stabilities of the entire electrolyte solutions, including conductive salt, solvent molecules and a potential additive, are experimentally investigated in the following.

3.2 Experimentally determined reductive and oxidative stabilities of the electrolyte solutions Differential capacity vs. potential plots of Si thin film negative electrodes during the first charge process of the Si/NMC-111 cells in the baseline electrolyte, namely 1 M LiPF6 in EC:DEC (3:7, by wt.), and the baseline electrolyte with additional 2 wt.% of either PFPI, VC or FEC are displayed in Error! Reference source not found.. The depicted area highlights 8 ACS Paragon Plus Environment

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the current flow related to irreversible electrolyte reduction processes, as lithium insertion into the amorphous Si anode during the first charge takes place below potentials of ≈0.3 V vs. Li/Li+ (see Figure S1, supporting information). An arbitrarily defined threshold value of -3.5 mAh g-1V-1 is added to the graph to qualitatively compare the onset potentials for reduction of the four different electrolytes. The electrolyte with PFPI additive exhibits the highest onset value of ≈1.4 V vs. Li/Li+ for a reductive decomposition reaction, whereas the VC- and FEC-based electrolytes show similar onset values of ≈1.2 V vs. Li/Li+. The baseline electrolyte without additive possesses the lowest onset potential of ≈1.0 V vs. Li/Li+, which is mainly related to the reduction of the carbonate solvent molecules.63 These experimentally determined reduction potentials of the different electrolyte solutions are consistent with the theoretical determined reductive stability tendencies obtained via DFT calculations (Table 1).

0

(dQ/dV) / mAh g-1V-1

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


-10 -20 -30 -40 Baseline Baseline + 2% PFPI Baseline + 2% FEC Baseline + 2% VC

-50 -60 0.5

1.0

1.5

2.0

Potential vs. Li/Li+ / V Figure 2: Differential capacity vs. potential plots of the Si negative electrodes in the Si/NMC-111 cells during 1st charge process for the four different electrolyte formulations. The depicted potential region focuses the reductive decomposition of the different electrolytes on the Si electrode. An arbitrary defined threshold value of -3.5 mAh g-1V-1 is added to the graph to qualitatively compare the onset potentials for reduction of the four different electrolytes.

The investigated PFPI additive shows a promising reductive instability and, therefore, the ability to form an SEI layer on the Si negative electrode prior to the reduction of the base electrolyte. 9 ACS Paragon Plus Environment


An additional reductive peak in the differential capacity plots is observed for all four electrolyte formulations at a potential of ≈0.55 V vs. Li/Li+, which can be related to the formation of Li2O from SiO264, as the electrodes have been handled in oxygen containing dryroom atmosphere, leading to an Si-O surface layer on top of the Si electrodes, prior to electrochemical investigations. The oxidative stability of the four different electrolytes was investigated by means of potentiodynamic linear sweep voltammetry (LSV) measurements up to 6 V vs. Li/Li+ against a Pt electrode, to exclude a possible overlapping of current induced by oxidative electrolyte decomposition and current due to lithium de-insertion of conventional cathode materials like NMC. We want to emphasize that this technique is only valid for a qualitative comparison of the oxidative stability tendency between the different electrolytes, not for an exact determination of the practical oxidation potential, as this is strongly dependent on the used electrode material, the electrode surface area or the applied scan-rate.65-66

0.30 0.25

Current / µA

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

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0.20

Baseline Baseline + 2% PFPI Baseline + 2% FEC Baseline + 2% VC

0.15 0.10 0.05 0.00 -0.05 3.5

4.0

4.5

5.0

5.5

6.0

Potential vs. Li/Li+ / V Figure 3: Linear sweep voltammetry for oxidative stability investigation of the four different electrolyte solutions on a Pt electrode up to 6 V vs. Li/Li+ at a scan rate of 25 µV s-1 and a temperature of 25 °C. A current of 0.015 µA is arbitrary defined as threshold value for current flow related to oxidative electrolyte decomposition reactions on the Pt electrode.

A current of 0.015 µA is arbitrary defined as threshold value for current flow related to oxidative electrolyte decomposition reactions on the Pt electrode. Based on the DFT 10 ACS Paragon Plus Environment

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calculations, the oxidative stability is expected to be in the order FEC > EC > DEC > VC > PFPI (Table 1). This tendency is confirmed via LSV measurements (see Error! Reference source not found.), as the PFPI-based electrolyte shows the lowest oxidative stability up to a potential of 4.23 V vs. Li/Li+, within the used measurement set-up. This value for an oxidative decomposition of the PFPI molecule is in good agreement with other isocyanate compounds, such as p-toluenesulfonyl isocyanate or benzyl isocyanate, that have been investigated in a similar set-up.52-53, 67 The potential of 4.23 V vs. Li/Li+ for an oxidation of PFPI determined via this method lies inside of the operation voltage range of the Si/NMC-111 cells during charge/discharge cycling within this investigation, i.e. the upper charge cut-off voltage of 4.3 V. Therefore, it has to be taken into account that the PFPI additive may not only be reduced on the negative electrode, but also experiences an oxidative decomposition reaction on the positive electrode. This issue will be checked in Si/NMC-111 cells operated during galvanostatic charge/discharge cycling experiments in the following sub-chapter. The electrolyte with VC as additive exhibits a slightly higher oxidative stability (i.e. 4.48 V vs. Li/Li+) in comparison to the PFPI-based electrolyte. The highest oxidative stability is observed for the baseline and the FEC-based electrolyte, which both possess a potential of ≈5.12 V vs. Li/Li+ for reaching the threshold value. It can be concluded, that the theoretical DFT calculations for the oxidative stability tendency are

successfully validated via

experimental LSV measurements.

3.3 Electrochemical performance of Si/NMC-111 cells Long-term charge/discharge cycling experiments were performed to investigate the influence of PFPI as electrolyte additive on the performance of Si/NMC-111 cells in comparison to cells containing either no additive or the well-known additives VC and FEC. Specific discharge capacities and Coulombic efficiencies (CEs) of the Si/NMC-111 cells for the four different electrolytes during prolonged charge/discharge cycling are depicted in Error! Reference source not found., and corresponding data are displayed in Error! Reference source not found.. The highest discharge capacities of 131 and 132 mAh g-1 and highest CEs of 72.9 and 72.4% in the 1st cycle are observed for the baseline and FEC-based electrolytes, respectively. The cells cycled in the PFPI or the VC-based electrolyte show slightly lowered initial discharge capacities of 122 and 123 mAh g-1, respectively. These lowered capacities are simultaneously associated with a lower CE of 62.4% for the PFPI-based electrolyte and 66.7% for the VC-based electrolyte, which most likely is related to higher irreversible capacity losses due to electrolyte reduction on the Si anode for the PFPI and VC electrolytes. 11 ACS Paragon Plus Environment


This can be observed in the potential versus capacity plots in Figure S2, showing higher capacities for VC and PFPI electrolytes during the first charge related to SEI formation, i.e. an enhanced capacity before the Si anode reaches a potential below 0.3 V vs. Li/Li+ as starting point of lithium insertion into Si.

0

10

20

30

40

50

60

70

80

90

100

Discharge -1 capacity / mAh g

140 120 100 80 60 40 20

a)

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

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95 90 70

60

Baseline Baseline + 2% PFPI Baseline + 2% VC Baseline + 2% FEC

b) 0

10

20

30

40

50

60

70

80

90

100

Cycle number Figure 4: Specific discharge capacities (a) and Coulombic efficiencies (b) during long-term charge/discharge cycling of Si/NMC-111 cells in a voltage range of 3.0 – 4.3 V at 20 °C for the different electrolyte solutions. The data are average values of three cells for each electrolyte formulation, the error bars representing the reproducibility of the results.


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Table 2: Discharge capacities, capacity retention and Coulombic efficiencies in selected cycles of the Si/NMC-111 cells cycled in four different electrolyte solutions.

Discharge st

Electrolyte

Baseline Baseline + 2 wt.% PFPI Baseline + 2 wt.% VC Baseline + 2 wt.% FEC

capacity (1 cycle) /

Discharge

Capacity

capacity

retention

rd

(103 cycle)

Coulombic efficiency / %

(103rd cycle) st

th

103rd

mAh g-1

/ mAh g-1

/%

1 cycle

5 cycle

131 ± 3

16 ± 3

12.0 ± 2.0

72.9 ± 1.5

98.7 ± 0.2

95.4 ± 0.6

122 ± 5

54 ± 2

44.6 ± 1.4

62.4 ± 2.6

97.8 ± 0.5

97.7 ± 0.2

123 ± 3

64 ± 4

52.4 ± 1.5

66.7 ± 1.5

99.2 ± 0.1

99.1 ± 0

132 ± 2

52 ± 7

39.2 ± 4.7

72.4 ± 1.0

99.1 ± 0.1

98.6 ± 0.2

cycle

The cells using the baseline electrolyte exhibit a rapid decay in the discharge capacity upon cycling, delivering a discharge capacity of 16 mAh g-1 and a capacity retention of only 12.0% after 103 cycles. This capacity fading goes along with a steady declining CE, leading to a value of as low as 95.4% in the 103rd cycle. The addition of 2 wt.% of an electrolyte additive out of either PFPI, VC or FEC significantly improves the cycling performance, i.e. results in increased CEs and capacity retentions after 103 charge/discharge cycles. Interestingly, the addition of VC to the baseline electrolyte leads to a higher capacity retention of 52.4% in comparison to 39.2% for the FEC-based electrolyte. This effect may partially be related to the higher initial capacity in the FEC electrolyte, accompanied with higher utilization and, therefore, larger volume changes of the Si anode. However, when taking a closer look on the CE, it can be observed that cells with the VC-based electrolyte exhibit a slightly higher CE compared to the FEC electrolyte throughout cycling, indicating improved passivation of the Si anode by the SEI layer formed in the VC electrolyte. Moreover, similar results were recently reported by Abraham et al. for Si-based full cells.35, 68 They observed slightly enhanced capacity retentions for Si/graphite composite based full cells cycled in a VC-containing electrolyte in comparison to a FEC-containing electrolyte. These findings were related to an improved CE and an associated lower impedance increase for cells cycled in a VC-based electrolyte. It should be noted, that results obtained for electrolyte additive investigations in Li metal/Si cells (“half cells”) might not be directly transferable to LIB full cells, as Si actually is referred as positive electrode when metallic lithium is used as counter 13 ACS Paragon Plus Environment


electrode. Therefore, reductive decomposition of the electrolyte additive will not only take place at the Si electrode, but as well at the surface of metallic lithium.69-70 This effect may be the reason for the widespread use of FEC as electrolyte additive compared to VC for Si-based anodes in Li metal/Si cell investigations, but also in full cell set-ups. When PFPI is added to the baseline electrolyte, a discharge capacity of 54 mAh g-1 , accounting to a capacity retention of 44.6%, can be obtained from the Si/NMC-111 cells after 103 cycles. This reveals, that cells with PFPI additive show a similar discharge capacity and capacity retention like cells cycled in a FEC-based electrolyte after 103 cycles. Only cells cycled in the VC electrolyte exhibit a slightly better performance in terms of discharge capacity and capacity retention. These results emphasize how huge the impact of electrolyte additives on the cycling performance of Si-based lithium ion full cells can be and that new classes of additives might be able to further significantly increase the performance of Si-based LIB cells. However, it is conspicuous that the cells cycled in PFPI-based electrolyte show ≈1% lower CEs throughout the whole charge/discharge cycling compared to the VC and FEC electrolytes (see Error! Reference source not found. and Error! Reference source not found.). As the discharge capacity and capacity retention after the 103rd cycle are similar to those of VC and FEC though, these Coulombic inefficiencies cannot be related solely to active lithium consuming parasitic reactions at the anode side. Besides active lithium loss through the formation of a SEI at the anode side, lowered CEs of lithium ion cells can also be induced by oxidative parasitic reactions at the cathode side. Dahn and co-workers introduced an inventory model for differentiating various possible parasitic reactions inside a LIB and their effect on the CE. The charge endpoint slippage, which is defined as the charge capacity of a cycle minus the discharge capacity of the previous cycle, can be used as an indicator for oxidative or shuttle-type parasitic reactions at the cathode side.71 As the DFT calculations as well as the LSV investigations revealed, that the PFPI might not be stable towards oxidative decomposition within the applied voltage range of the Si/NMC-111 cells in this study, we calculated the charge endpoint slippages for the four different electrolytes and plotted the cumulative charge endpoint slippages versus the cycle number in Error! Reference source not found..


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180 Baseline Baseline + 2% PFPI Baseline + 2% VC Baseline + 2% FEC

160

Charge endpoint slippage / mAh g-1

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


140 120 100 80 60 40 20 0 0

20

40

60

80

100

Cycle number Figure 5: Cumulative charge endpoint slippage vs. cycle number of the Si/NMC-111 cells for the different electrolytes upon 103 cycles.

The cells containing PFPI additive clearly exhibit the highest charge endpoint slippage of all different electrolytes, moreover, a quite high deviation between the three cells is observed, as indicated by the error bars. This finding confirms the assumption of an oxidative decomposition of the PFPI on the cathode side besides the reduction on the Si anode surface. As oxidative or shuttle-type parasitic reactions contribute to the charge capacity, though not to the discharge capacity, this results in an lowered overall CE of the LIB full cells. Therefore, the oxidation of the PFPI accounts for the lowered CE but similar discharge capacities compared to the VC and FEC-based electrolytes. The lowest charge endpoint slippage, slightly lower than for the FEC electrolyte, is observed for the VC-based electrolyte. This is in good agreement with results from Wang et al., showing the beneficial effect of VC and FEC in LiCoO2/graphite cells related to reduced parasitic reactions on the cathode side.72 These results again point out the importance of using full cell set-ups for electrolyte additive investigations, as the additive might not only have an effect at the anode side but also at the cathode side. Moreover, it becomes clear that LSV measurements are a viable method for determination of bulk oxidation processes, however, effects/reactions of the electrolyte additive molecules at the cathode side still might take place, despite of sufficient enough 15 ACS Paragon Plus Environment


oxidative stability determined via LSV.72-73 As it became clear that the PFPI additive undergoes an oxidative reaction at the cathode side, one can ask whether this oxidation might have an influence on the performance of the NMC-111 cathode and, therefore, on the Si/NMC-111 cells. Hence, Li metal/NMC-111 cells with the baseline and PFPI-based electrolyte are investigated upon charge/discharge cycling in a potential range of 3.0 – 4.4 V vs. Li/Li+ for 103 cycles, the data is depicted in Figure S3. A higher cut-off of 4.4 V compared to 4.3 V in Si/NMC-111 cells was chosen, because the potential of the NMC-111 cathode in the Si/NMC-111 cell reaches a value of ≈ 4.4 V vs. Li/Li+ already during the first charge process (see Figure S2), related to charge cutoff potential of ≈0.1 V vs. Li/Li+ for the Si anode. No significant effect, neither positive nor negative on the discharge capacity upon cycling the Li metal/NMC-111 cells under these conditions isobserved by the addition of PFPI to the baseline electrolyte. It can be concluded, that the oxidation of PFPI at the cathode side is responsible for the lowered CE of Si/NMC111 cells in PFPI compared to VC and FEC electrolyte (Figure S3). However, the improved performance in terms of capacity retention and Coulombic efficiencies of Si/NMC-111 cells cycled in the PFPI electrolyte compared to the baseline electrolyte can be assigned to the beneficial effect at the Si anode side. To get a deeper insight in the failure mechanism of the Si/NMC-111 cells upon charge/discharge cycling, the development of the individual electrode potentials, i.e. of the Si anode and the NMC-111 cathode, was recorded via the Li metal reference electrode. The electrode potentials are plotted versus the time throughout the 103 cycles in Error! Reference source not found. for the baseline and PFPI-based electrolyte. 5.0

5.0 a)

Potential vs. Li/Li+ / V

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 16 of 34

b)

4.5

4.5

4.0

4.0 3.5

3.5 Cathode Potential Anode Potential

3.0

3.0

1.0

1.0

0.5

0.5

0.0 0

50

100

150

200

250

50

100

150

200

0.0 250

Time / h Figure 6: Development of the cathode and anode potentials upon charge/discharge cycling of Si/NMC-111 cells for 103 cycles in the baseline (a) and PFPI (b) electrolyte.


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During the first cycle of the Si/NMC-111 cells no significant difference in the electrode potential curves is observed for the different electrolytes. The potential of the Si anode gradually decreases in the first charge process due to the insertion of Li ions into the Si, until reaching a potential of ≈0.1 V vs. Li/Li+ (exact values are depicted in Table S1). Simultaneously, the potential of the NMC-111 cathode increases, related to the de-insertion of Li ions from the structure. As the cells are cycled within a cell voltage range of 3.0 to 4.3 V, and considering that the cell voltage is defined as the difference between cathode potential and anode potential, the cathode reaches a potential of ≈4.4 V vs. Li/Li+ at the end of charge. When discharging the cells, the end of discharge potentials for the cathode and anode are ≈3.7 and ≈0.7 V vs. Li/Li+ for both electrolytes, respectively. Upon the repeated volume changes of the Si anode during lithiation/de-lithiation, a continuous SEI re-formation and repair takes place, most likely going along with irreversible active lithium consumption. These active lithium losses lead to a gradual shift of the individual electrode cut-off potentials towards higher values, as the electrode potentials are dependent on the lithiation degree.74 The Si anode exhibits a lower degree of lithiation upon cycling, observable by the increase of the charge cut-off potential higher than 0.2 V vs. Li/Li+, i.e. after 51 and 66 cycles or 161 and 191 h for the baseline and PFPI-based electrolyte, respectively. Due to the cross-talk of anode and cathode in a full cell set-up cycled via the cell voltage, the cathode charge cut-off potential simultaneously shifts to values of more than 4.5 V vs. Li/Li+ for both electrolytes. However, after 170 h the cathode potential of the cells cycled in the baseline electrolyte shows anomalous sharp increases up to 4.8 V vs. Li/Li+, what is likely related to intense overcharging of the NMC-111 cathode, going along with structural instabilities. This effect is not observed for Si/NMC-111 cells cycled in the PFPI electrolyte, as the isocyanate additive reduces the active lithium consumption at the anode side related to SEI re-formation. Major differences between cells cycled in either the baseline or the PFPI-based electrolyte can be noticed for the end of discharge electrode potentials within the last cycles, which are enlarged in Figure S4. Within the last discharge process (103rd cycle) in the baseline electrolyte, the anode and cathode reach cut-off potentials of ≈1.3 and ≈4.3 V vs. Li/Li+, respectively. This reveals that only little amounts of lithium are still left in the NMC-111 cathode after 103 cycles in the baseline electrolyte, which was already proven by the low discharge capacity of 16 mAh g-1. Additionally, high overpotentials are observed at the cathode side in the baseline electrolyte, what can be seen by the increase of the cathode potential when changing from constant current (CC) to constant voltage (CV) discharging (see Figure S4, Table S2). During discharge in the 103rd cycle in the baseline electrolyte, the NMC-111 cathode displays a 17 ACS Paragon Plus Environment


potential of 3.87 V vs. Li/Li+ at the end of the CC discharge step, which gradually increases within the CV discharge step up to 4.27 V vs. Li/Li+. As overpotentials are proportional to the current, the overpotential at the cathode side decreases within the CV discharge step, in which the current is steadily declining until the end criteria, i.e. dropping below 0.05C, is reached.75 The electrode potentials for cells cycled in the PFPI electrolyte after the last discharge process (103rd cycle) are significantly lower, namely ≈0.9 vs. Li/Li+ for the Si anode and ≈3.9 V vs. Li/Li+ for the NMC-111 cathode. The decreased cathode potential indicates a higher degree of lithiation of the NMC-111 material after 103 cycles in the PFPI compared to the baseline electrolyte. In addition, the overpotential on the cathode side is clearly reduced for the PFPI electrolyte, i.e. an increase of the cathode potential of 100 mV within the CV discharge step in the 103rd cycle compared to 400 mV for the baseline electrolyte (see Table S2). This is most likely related to better structural stability of the NMC-111 due to higher re-lithiation amounts after 103 charge/discharge cycles.75 A similar trend of the development of the electrode potentials upon cycling as for the PFPI electrolyte was observed by the addition of VC or FEC to the baseline electrolyte (see Figure S5, Table S2). These results reveal that the loss of active lithium in Si/NMC-111 cells, mainly induced by continuous electrolyte reduction and SEI re-formation at the Si anode, can be clearly displayed by the development of the individual electrode potentials upon cycling within a three-electrode cell set-up. In Si/NMC-111 cells, the usabale active lithium is limited by the amount of NMC-111 material, in difference to Li metal/Si cells where mostly a huge excess of metallic lithium is used. Therefore, lithium consuming parasitic reactions are not that crucial for Li metal/Si cells but strongly decisive for Si/NMC-111 cells, where anode induced active lithium losses have a direct impact on the NMC-111 cathode. Continuous lithium consumption at the anode side leads to higher delithiation and lower relithiation ratios of the NMC-111 cathode, going along with overcharging and increased structural instabilities. The PFPI additive is able to significantly reduce these active lithium losses upon charge/discharge cycling, proving the enhanced passivation of the Si anode by the SEI layer formed in the PFPI electrolyte.

3.4 Investigation of the SEI formed in the PFPI-based, VC-based and baseline electrolyte by means of XPS To elucidate the beneficial effect of the addition of VC or PFPI to the baseline electrolyte, XPS was employed to study the SEI layer formed on the Si anodes after 103 cycles in either the baseline, VC-based or the PFPI-based electrolyte. Since the Si/NMC-111 18 ACS Paragon Plus Environment

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cells cycled in the VC-based electrolyte showed an even slightly improved performance in terms of capacity retention compared to the PFPI-based electrolyte, the composition of the SEI formed on the Si anodes in the VC-based electrolyte was also investigated via XPS. The relative elemental composition of the SEI on the Si anode formed in either the baseline, VCbased or PFPI-based electrolyte is depicted in Error! Reference source not found.. The incorporation of the PFPI additive into the SEI can be easily verified by the detection of nitrogen at the surface of Si anode, and an additional increased amount of fluorine and carbon compared to the baseline electrolyte, as the aromatic backbone of PFPI molecule is build up of carbon and fluorine atoms (Error! Reference source not found.). A major difference is observed for the lithium content within the SEI, which is approximately twice as high for the baseline electrolyte. This is in good agreement with the electrochemical data, namely the lower capacity retention related to higher active lithium losses for Si/NMC-111 cells cycled without an electrolyte additive. Therefore, XPS results confirm the incorporation of higher amounts of lithium in the SEI layer in the baseline electrolyte related to reductive parasitic reactions at the Si anode surface. When taking a look at the elemental composition of the surface of the Si anodes cycled in the VC-based electrolyte (Table 3), it becomes clear that the elemental distribution of the SEI, considering the mean deviation, is quite similar to the SEI formed in the baseline electrolyte. This result shows that not the elemental composition is crucial for the effectiveness of a SEI, but rather the molecular composition and/or the morphology of the formed SEI. Additionally, it needs to be mentioned that XPS is a surface-sensitive method, giving information about a depth of ≈10 nm. Therefore, the elemental compositions determined via XPS will just give information about the first few nanometers of the SEI formed in the different electrolytes. Table 3: Relative atomic concentrations of the SEI on the Si anodes after charge/discharge cycling for 103 cycles in the baseline, VC-based or PFPI-based electrolyte determined via XPS.

F/%

O/%

19.48 ±

14.76 ±

1.44

1.76

Baseline + 2%

23.57 ±

16.10 ±

VC

1.72

3.00

Baseline + 2%

23.34 ±

13.18 ±

PFPI

1.55

1.33

Baseline

N/%

C/%

P/%

Si / %

Li / %

41.23 ±

0.90 ±

0.56 ±

23.06 ±

2.48

0.10

0.29

1.42

38.84 ±

0.97 ±

2.21

0.17

3.18 ±

49.28 ±

0.42 ±

0.23

1.10

0.09

0

0

0

0

20.52 ± 1.10 10.61 ± 0.55

XPS core spectra of the surface of Si anodes cycled in the three different electrolytes 19 ACS Paragon Plus Environment


are illustrated in Error! Reference source not found.. In the F 1s spectra, a distinct peak at ≈685 eV is present for the Si electrode cycled in the baseline electrolyte, which can be attributed to the presence of LiF.41, 76. Additionally, the peak exhibits a small shoulder at ≈687 eV indicating the presence of small quantities of LixPOyFz.36, 77 The SEI formed on the Si anode in the PFPI-based electrolyte also contains LiF, but the peak shows significantly lower intensity compared to the baseline electrolyte. Furthermore, the spectra exhibits an additional peak at 687.7 eV what can be assigned to the presence of fluorinated organic species41,

78

arising from the fluorinated phenyl group of the PFPI molecule. Residues or

decomposition products of the conductive salt LiPF6 may also be present in the SEI formed in the PFPI electrolyte, however, the detection is hampered by overlapping of the strong peak at 687.7 eV. The F 1s spectra for the Si anode cycled in the VC-based electrolyte is quite similar to the one cycled in the baseline electrolyte, with a slightly increased intensity of the shoulder related to LixPOyFz. In the O 1s spectra, a dominant peak centered at 531.4 eV (C=O) including a shoulder at 533.6 eV (C-O) is observed for the baseline electrolyte, which can be assigned to the presence of high amounts of lithium carbonate and/or alkylcarbonates, consistent with the literature.39, 79 Additionally, a small peak at 527.9 eV can be observed, related to the presence of small quantities of Li2O in the SEI formed in the baseline electrolyte. In contrast, the PFPI sample shows a peak centered at 532.5 eV, consistent with a mixture of C-O and C=O environments. Consequently, the addition of PFPI to the baseline electrolyte reduces the relative amount of carbonate species in the SEI formed on the Si anode and also prevents the formation of Li2O. When cycled in the VC-based electrolyte, the O 1s spectra of the Si anodes exhibit a broad peak centered at 533 eV, indicating a drastically increased contribution of C-O bonds compared to the baseline electrolyte. The N 1s spectrum exhibits a sharp, symmetric peak at 400.8 eV for the PFPI sample, indicating that there is only one nitrogen environment present in the SEI, which can be assigned to nitrogen within an imide group.80-81


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F 1s

O 1s

C 1s

N 1s C=O

LiF

C-C / C-H N-C=O

C-N

a)

LixPOyFz

C-O Li2O

C-F a)

a)

b)

b)

c)

c)

692

540

C-O C-F CO3 CO2

Poly(VC) a)

b) b)

c)

688 684 680 Binding Energy / eV

c)

536 532 528 524 Binding Energy / eV

408


294


Figure 7: XPS core spectra of the surface of the SEI on the Si anodes after 103 cycles in the baseline (a), VC-based (b) or PFPI-based (c) electrolyte.

In the C 1s spectrum for the baseline electrolyte a major peak at 284.5 eV, attributed to alkane species, and small peaks at 286.7 and 289.9 eV, related C-O and O-(C=O)-O environments can be found, respectively. These peaks are consistent with the presence of alkylcarbonates like lithium ethylene dicarbonate (LEDC), arising from the reduction of EC, as already observed in the O 1s spectrum.78, 82 The C 1s spectrum for the Si anode cycled in the PFPI electrolyte shows a peak at 284.5 eV (C-C/C-H) and a broad peak between 286 and 288 eV, which can be related to the presence of several different carbon environments including C-N, C-O, C-F and C=O/CO2.80-81, 83 This is consistent with observations for the F 1s, O 1s and N 1s spectra, verifying the incorporation of high amounts of PFPI within the SEI on the Si anode. The C 1s spectra for the VC sample exhibits two distinct peaks at 284.5 eV (C-C / C-H), 286.7 eV (C-O) and an additional peak at 290.6 eV. Peaks at these high binding energies can be related to lithium-free carbonate environments like for the poly(VC), formed by a reductive decomposition/polymerization of the VC molecule.39, 84-85 The increased intensity of the peak at 286.7 eV in the C 1s as well as the bigger shoulder at 533.6 eV in the O 1s spectra, both related to C-O bonds, compared to the baseline electrolyte, are in good agreement with the peak at 290.6 eV and the assignment to poly(VC). The formation of such polymeric species by the addition of VC is most likely the reason for the improved performance compared to the baseline electrolyte. This could be related to the formation of a SEI layer, which is more stable and ensures a more effective passivation of the Si anode and, therefore, reducing active lithium losses due to continuous SEI (re-)formation. A recent study by Michan et al., investigating the reduction products of the pure molecules VC and FEC, discovered the in-situ formation of VC during the reduction of FEC. They suggested a reduction mechanism of FEC to form LiF and VC and a subsequent reduction of 21 ACS Paragon Plus Environment


Page 22 of 34

the formed VC.39 Therefore, the positive influence of the FEC additive may be, at least partially, related to the beneficial effect of the polymeric VC reduction products. Considering that the reduction of FEC to LiF and VC would consume one equivalent lithium, this may explain the lowered CEs for the Si/NMC-111 cells cycled in the FEC-based compared to the VC-based electrolyte. Based on the XPS

results, and as the possibility of anionic

induced

homopolymerization of isocyanate monomers was reported in the literature43, 86, an electroninduced, reductive polymerization of the PFPI molecule is proposed, forming an effective SEI on the Si anode (see Error! Reference source not found.). Similar mechanisms were proposed for isocyanate compounds used as electrolyte additives for graphite anodes.46-47 The SEI formed via this mechanism ensures an enhanced passivation of the Si anode compared to the baseline electrolyte, followed by less active lithium consumption related to continuous electrolyte reduction. The polymeric nature of the SEI formed by PFPI might be the crucial aspect for the beneficial effect, as polymers are supposed to be able to withstand the huge volume changes of the Si upon cycling. This is in good agreement to the findings of polymeric species formed in the VC-based electrolyte.

e-

R

2n N

C

R

R

N

N

O

C

C

O

O

n

Figure 8: Proposed electron-induced reductive polymerization of the PFPI molecule.

4. Conclusion Within this work, we evaluated the effect of pentafluorophenyl isocyanate (PFPI) as an electrolyte additive on the electrochemical performance of Si-based lithium ion full cells using pure Si thin film anodes as model electrodes. DFT calculations were performed to investigate reductive and oxidative stability tendencies of the PFPI and well-known VC and FEC additives and the used electrolyte solvent molecules (EC, DEC). The promising result for a lowered reductive instability of PFPI via DFT was successfully proven for the electrolyte solution within the real LIB full cell set-up. The addition of only 2 wt.% of PFPI to the baseline electrolyte significantly improved the performance of Si/NMC-111 cells in terms of discharge capacity, capacity retention and 22 ACS Paragon Plus Environment

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Coulombic efficiency (CE) after 103 charge/discharge cycles, which is comparable to the effect of the widespread used electrolyte additives VC and FEC. Besides reductive decomposition, an oxidation of the PFPI on the cathode was observed for the Si/NMC-111 cells, leading to decreased CEs compared cells using VC and FEC as additives. However, it was pointed out that this oxidative parasitic reaction does not influence the discharge capacities of the NMC-111 cathode or the Si/NMC-111 cells in a negative manner, at least in the used lab-scale cells. If these parasitic reactions may have a negative influence on the operation of large scale LIBs has to investigated in future studies. It was revealed that the addition of PFPI significantly reduced the consumption of active lithium by decreasing the parasitic side reactions at the anode side upon charge/discharge cycling. Therefore, lower delithiation and higher relithiation degrees of the NMC-111 cathode can be achieved, preventing detrimental overcharge and structural instabilities of the NMC-111 cathode. Further, XPS investigations clearly pointed out the incorporation of PFPI as a main part of the SEI and additionally proved the incorporation of lower amounts of lithium compared to the baseline electrolyte. According to these results, we propose a reductive polymerization of the PFPI forming an improved, presumably more flexible SEI on the Si anode to be the basis of the effectiveness of this isocyanate electrolyte additive. Therefore, we believe that our results can give new insights into the degradation mechanism of Si-based lithium ion full cells and how these can be effectively mitigated by the addition of different types of electrolyte additives. As isocyanate compounds are also known to exhibit a beneficial effect as film forming electrolyte additive for different anode materials and cathode materials, this substance class might be highly interesting for application in different LIB cell chemistries in order to tailor the interphases (SEI, CEI). Additionally, electrochemical properties of isocyanate compounds can be easily adjusted by modifying the chemical structure and/or functional groups incorporated within the molecule to customize the properties of the isocyanate molecule to the used cell set-up.

Supporting Information The Supporting Information is available free of charge on the ACS publications website at DOI: XXXXXX. Cyclic voltammetry of Si anodes, potential vs. specific capacity plots, Li/NMC-111 cells, additional electrode potential vs. time plots, table of electrode potentials during cycling.



Acknowledgements The authors from the WWU Münster wish to thank the German Federal Ministry for Economic Affairs and Energy (BMWi) for funding this work in the project “Go3” (03ETE002D). The authors from Berlin gratefully acknowledge support from the “Bundesministerium für Bildung und Forschung” (BMBF) and the computing time granted on Zentraleinrichtung (ZEDAT) at the Freie Universität Berlin. Dr. Johannes Kasnatscheew and Dr. Kolja Beltrop are gratefully acknowledged for fruitful discussions during preparation of this manuscript.


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References (1) Wagner, R.; Preschitschek, N.; Passerini, S.; Leker, J.; Winter, M. Current Research Trends and Prospects among the various Materials and Designs used in Lithium-based Batteries. Journal of Applied Electrochemistry 2013, 43 (5), 481-496, DOI: 10.1007/s10800013-0533-6. (2) Crabtree, G.; Kócs, E.; Trahey, L. The Energy-storage Frontier: Lithium-ion Batteries and Beyond. MRS Bulletin 2015, 40 (12), 1067-1078, DOI: doi:10.1557/mrs.2015.259. (3) Placke, T.; Winter, M. Batterien für Medizinische Anwendungen. Z Herz- ThoraxGefäßchir 2015, 29 (2), 139-149, DOI: 10.1007/s00398-014-1129-0. (4) Placke, T.; Kloepsch, R.; Dühnen, S.; Winter, M. Lithium Ion, Lithium Metal, and Alternative Rechargeable Battery Technologies: The Odyssey for high Energy Density. Journal of Solid State Electrochemistry 2017, 21 (7), 1939-1964, DOI: 10.1007/s10008-0173610-7. (5) Schmuch, R.; Wagner, R.; Hörpel, G.; Placke, T.; Winter, M. Performance and Cost of Materials for Lithium-based Rechargeable Automotive Batteries. Nature Energy 2018, 3, 267278, DOI: https://doi.org/10.1038/s41560-018-0107-2 (6) Andre, D.; Hain, H.; Lamp, P.; Maglia, F.; Stiaszny, B. Future High-energy Density Anode Materials from an Automotive Application Perspective. J. Mater. Chem. A 2017, 5 (33), 17174-17198, DOI: 10.1039/c7ta03108d. (7) Meister, P.; Jia, H.; Li, J.; Kloepsch, R.; Winter, M.; Placke, T. Best Practice: Performance and Cost Evaluation of Lithium Ion Battery Active Materials with Special Emphasis on Energy

Efficiency.

Chemistry

of

Materials

2016,

28

(20),

7203-7217,

DOI:

10.1021/acs.chemmater.6b02895. (8) Chevrier, V. L.; Liu, L.; Dinh Ba, L.; Lund, J.; Molla, B.; Reimer, K.; Krause, L. J.; Jensen, L. D.; Figgemeier, E.; Eberman, K. W. Evaluating Si-Based Materials for Li-Ion Batteries in Commercially Relevant Negative Electrodes. Journal of the Electrochemical Society 2014, 161 (5), A783-A791, DOI: 10.1149/2.066405jes. (9) Obrovac, M. N.; Chevrier, V. L. Alloy Negative Electrodes for Li-Ion Batteries. Chemical Reviews 2014, 114 (23), 11444-11502, DOI: 10.1021/cr500207g. (10) Zhang, W.-J. A review of the Electrochemical Performance of Alloy Anodes for Lithium-ion Batteries. Journal of Power Sources 2011, 196 (1), 13-24, DOI: 10.1016/j.jpowsour.2010.07.020. (11) Peled, E.; Menkin, S. Review—SEI: Past, Present and Future. Journal of The Electrochemical Society 2017, 164 (7), A1703-A1719, DOI: 10.1149/2.1441707jes. 25 ACS Paragon Plus Environment


Page 26 of 34

(12) Winter, M.; Besenhard, J. O.; Albering, J. H.; Yang, J.; Wachtler, M. Lithium Storage Alloys as Anode Materials for Lithium Ion Batteries. Progress in Batteries & Battery Materials 1998, 17, 208-214. (13) Winter, M. The Solid Electrolyte Interphase - The Most Important and the Least Understood Solid Electrolyte in Rechargeable Li Batteries. Zeitschrift Für Physikalische Chemie 2009, 223 (10-11), 1395-1406. (14) Holtstiege, F.; Wilken, A.; Winter, M.; Placke, T. Running out of Lithium? A Route to Differentiate between Capacity Losses and Active Lithium Losses in Lithium-ion Batteries. Physical

Chemistry

Chemical

Physics

2017,

19

(38),

25905-25918,

DOI:

10.1039/C7CP05405J. (15) Winter, M.; Appel, W. K.; Evers, B.; Hodal, T.; Moller, K. C.; Schneider, I.; Wachtler, M.; Wagner, M. R.; Wrodnigg, G. H.; Besenhard, J. O. Studies on the Anode/Electrolyte Interface in Lithium Ion Batteries. Chemical Monthly 2001, 132 (4), 473-486. (16) Szczech, J. R.; Jin, S. Nanostructured Silicon for high Capacity Lithium Battery Anodes. Energy & Environmental Science 2011, 4 (1), 56-72. (17) Mazouzi, D.; Lestriez, B.; Roué, L.; Guyomard, D. Silicon Composite Electrode with High Capacity and Long Cycle Life. Electrochemical and Solid-State Letters 2009, 12 (11), A215-A218, DOI: 10.1149/1.3212894. (18) Green, M.; Fielder, E.; Scrosati, B.; Wachtler, M.; Moreno, J. S. Structured Silicon Anodes for Lithium Battery Applications. Electrochemical and Solid-State Letters 2003, 6 (5), A75-A79, DOI: 10.1149/1.1563094. (19) Ohara, S.; Suzuki, J.; Sekine, K.; Takamura, T. A Thin Film Silicon Anode for Li-ion Batteries having a very large Specific Capacity and long Cycle Life. Journal of Power Sources 2004, 136 (2), 303-306, DOI: https://doi.org/10.1016/j.jpowsour.2004.03.014. (20) Reyes Jiménez, A.; Klöpsch, R.; Wagner, R.; Rodehorst, U. C.; Kolek, M.; Nölle, R.; Winter, M.; Placke, T. A Step toward High-Energy Silicon-Based Thin Film Lithium Ion Batteries. ACS Nano 2017, 11 (5), 4731-4744, DOI: 10.1021/acsnano.7b00922. (21) Schmuelling, G.; Winter, M.; Placke, T. Investigating the Mg–Si Binary System via Combinatorial Sputter Deposition As High Energy Density Anodes for Lithium-Ion Batteries. ACS

Applied

Materials

&

Interfaces

2015,

7

(36),

20124–20133,

DOI:

10.1021/acsami.5b05382. (22) Jia, H.; Stock, C.; Kloepsch, R.; He, X.; Badillo, J. P.; Fromm, O.; Vortmann, B.; Winter, M.; Placke, T. Facile Synthesis and Lithium Storage Properties of a Porous NiSi2/Si/Carbon Composite Anode Material for Lithium-Ion Batteries. ACS Applied Materials & Interfaces 26 ACS Paragon Plus Environment

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


2015, 7 (3), 1508-1515, DOI: 10.1021/am506486w. (23) Liu, W.-R.; Wu, N.-L.; Shieh, D.-T.; Wu, H.-C.; Yang, M.-H.; Korepp, C.; Besenhard, J. O.; Winter, M. Synthesis and Characterization of Nanoporous NiSi-Si Composite Anode for Lithium-ion Batteries. Journal of the Electrochemical Society 2007, 154 (2), A97-A102, DOI: 10.1149/1.2402106. (24) Yen, Y.-C.; Chao, S.-C.; Wu, H.-C.; Wu, N.-L. Study on Solid-Electrolyte-Interphase of Si and C-Coated Si Electrodes in Lithium Cells. Journal of The Electrochemical Society 2009, 156 (2), A95-A102, DOI: 10.1149/1.3032230. (25) Agyeman, D. A.; Song, K.; Lee, G.-H.; Park, M.; Kang, Y.-M. Carbon-Coated Si Nanoparticles Anchored between Reduced Graphene Oxides as an Extremely Reversible Anode Material for High Energy-Density Li-Ion Battery. Adv. Energy Mater. 2016, 6 (20), 1600904-1600914, DOI: 10.1002/aenm.201600904. (26) Hochgatterer, N.; Schweiger, M.; Koller, S.; Raimann, P.; Wöhrle, T.; Wurm, C.; Winter, M. Silicon/Graphite Composite Electrodes for High-capacity Anodes: Influence of Binder Chemistry on Cycling Stability. Electrochemical and solid-state letters 2008, 11 (5), A76A80. (27) Besenhard, J.; Yang, J.; Winter, M. Will advanced Lithium-alloy Anodes have a chance in Lithium-ion Batteries? Journal of Power Sources 1997, 68 (1), 87-90. (28) Reyes Jimenez, A.; Nölle, R.; Wagner, R.; Hüsker, J.; Kolek, M.; Schmuch, R.; Winter, M.; Placke, T. A Step towards Understanding the beneficial Influence of a LIPON-based Artificial SEI on Silicon Thin Film Anodes in Lithium-ion Batteries. Nanoscale 2018, 10 (4), 2128-2137, DOI: 10.1039/C7NR06568J. (29) Jin, Y.; Li, S.; Kushima, A.; Zheng, X.; Sun, Y.; Xie, J.; Sun, J.; Xue, W.; Zhou, G.; Wu, J.; Shi, F.; Zhang, R.; Zhu, Z.; So, K.; Cui, Y.; Li, J. Self-healing SEI enables Full-cell Cycling of a Silicon-majority Anode with a Coulombic Efficiency exceeding 99.9%. Energy & Environmental Science 2017, 10 (2), 580-592, DOI: 10.1039/C6EE02685K. (30) Holtstiege, F.; Bärmann, P.; Nölle, R.; Winter, M.; Placke, T. Pre-Lithiation Strategies for Rechargeable Energy Storage Technologies: Concepts, Promises and Challenges. Batteries 2018, 4 (1), 4. (31) Aravindan, V.; Lee, Y.-S.; Madhavi, S. Best Practices for Mitigating Irreversible Capacity Loss of Negative Electrodes in Li-Ion Batteries. Adv. Energy Mater. 2017, 7 (17), 1602607, DOI: 10.1002/aenm.201602607. (32) Dalavi, S.; Guduru, P.; Lucht, B. L. Performance Enhancing Electrolyte Additives for Lithium Ion Batteries with Silicon Anodes. Journal of The Electrochemical Society 2012, 159 27 ACS Paragon Plus Environment


Page 28 of 34

(5), A642-A646, DOI: 10.1149/2.076205jes. (33) Nguyen, C. C.; Lucht, B. L. Comparative Study of Fluoroethylene Carbonate and Vinylene Carbonate for Silicon Anodes in Lithium Ion Batteries. Journal of the Electrochemical Society 2014, 161 (12), A1933-A1938, DOI: 10.1149/2.0731412jes. (34) Profatilova, I. A.; Stock, C.; Schmitz, A.; Passerini, S.; Winter, M. Enhanced Thermal Stability of a Lithiated Nano-silicon Electrode by Fluoroethylene Carbonate and Vinylene Carbonate. Journal of Power Sources 2013, 222, 140-149. (35) Klett, M.; Gilbert, J. A.; Trask, S. E.; Polzin, B. J.; Jansen, A. N.; Dees, D. W.; Abraham, D. P. Electrode Behavior RE-Visited: Monitoring Potential Windows, Capacity Loss, and Impedance Changes in Li1.03(Ni0.5Co0.2Mn0.3)0.97O2/Silicon-Graphite Full Cells. Journal of The Electrochemical Society 2016, 163 (6), A875-A887, DOI: 10.1149/2.0271606jes. (36) Dupré, N.; Moreau, P.; De Vito, E.; Quazuguel, L.; Boniface, M.; Bordes, A.; Rudisch, C.; Bayle-Guillemaud, P.; Guyomard, D. Multiprobe Study of the Solid Electrolyte Interphase on Silicon-Based Electrodes in Full-Cell Configuration. Chemistry of Materials 2016, 28 (8), 2557-2572, DOI: 10.1021/acs.chemmater.5b04461. (37) Delpuech, N.; Dupre, N.; Moreau, P.; Bridel, J.-S.; Gaubicher, J.; Lestriez, B.; Guyomard, D. Mechanism of Silicon Electrode Aging upon Cycling in Full Lithium-Ion Batteries. ChemSusChem 2016, 9 (8), 841-848, DOI: 10.1002/cssc.201501628. (38) Jin, Y.; Kneusels, N.-J. H.; Magusin, P. C. M. M.; Kim, G.; Castillo-Martínez, E.; Marbella, L. E.; Kerber, R. N.; Howe, D. J.; Paul, S.; Liu, T.; Grey, C. P. Identifying the Structural Basis for the Increased Stability of the Solid Electrolyte Interphase Formed on Silicon with the Additive Fluoroethylene Carbonate. Journal of the American Chemical Society 2017, 139 (42), 14992-15004, DOI: 10.1021/jacs.7b06834. (39) Michan, A. L.; Parimalam, B. S.; Leskes, M.; Kerber, R. N.; Yoon, T.; Grey, C. P.; Lucht, B. L. Fluoroethylene Carbonate and Vinylene Carbonate Reduction: Understanding Lithium-Ion Battery Electrolyte Additives and Solid Electrolyte Interphase Formation. Chemistry of Materials 2016, 28 (22), 8149-8159, DOI: 10.1021/acs.chemmater.6b02282. (40) Young, B. T.; Heskett, D. R.; Nguyen, C. C.; Nie, M.; Woicik, J. C.; Lucht, B. L. Hard X-ray Photoelectron Spectroscopy (HAXPES) Investigation of the Silicon Solid Electrolyte Interphase (SEI) in Lithium-Ion Batteries. ACS Applied Materials & Interfaces 2015, 7 (36), 20004-20011, DOI: 10.1021/acsami.5b04845. (41) Etacheri, V.; Haik, O.; Goffer, Y.; Roberts, G. A.; Stefan, I. C.; Fasching, R.; Aurbach, D. Effect of Fluoroethylene Carbonate (FEC) on the Performance and Surface Chemistry of Si-Nanowire

Li-Ion

Battery

Anodes.

Langmuir

2012,


28

(1),

965-976,

DOI:

Page 29 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


10.1021/la203712s. (42) Shkrob, I. A.; Wishart, J. F.; Abraham, D. P. What Makes Fluoroethylene Carbonate Different? Journal of Physical Chemistry C 2015, 119 (27), 14954-14964, DOI: 10.1021/acs.jpcc.5b03591. (43) Akbulut, U. Electroinitiated Trimerization and Polymerization of Phenyl Isocyanate. Die Makromolekulare Chemie 1979, 180 (4), 1073-1075, DOI: 10.1002/macp.1979.021800427. (44) Zhang, S. S. Aromatic Isocyanate as a new Type of Electrolyte Additive for the improved Performance of Li-ion Batteries. Journal of Power Sources 2006, 163 (1), 567-572, DOI: https://doi.org/10.1016/j.jpowsour.2006.09.046. (45) Korepp, C.; Kern, W.; Lanzer, E. A.; Raimann, P. R.; Besenhard, J. O.; Yang, M.; Möller, K. C.; Shieh, D. T.; Winter, M. 4-Bromobenzyl Isocyanate versus Benzyl Isocyanate—New Film-forming Electrolyte Additives and Overcharge Protection Additives for Lithium Ion Batteries. Journal of Power Sources 2007, 174 (2), 637-642, DOI: https://doi.org/10.1016/j.jpowsour.2007.06.142. (46) Korepp, C.; Kern, W.; Lanzer, E. A.; Raimann, P. R.; Besenhard, J. O.; Yang, M.; Möller, K. C.; Shieh, D. T.; Winter, M. Ethyl Isocyanate—An Electrolyte Additive from the large Family of Isocyanates for PC-based Electrolytes in Lithium-ion Batteries. Journal of Power Sources 2007, 174 (2), 628-631, DOI: https://doi.org/10.1016/j.jpowsour.2007.06.140. (47) Korepp, C.; Kern, W.; Lanzer, E. A.; Raimann, P. R.; Besenhard, J. O.; Yang, M. H.; Möller, K. C.; Shieh, D. T.; Winter, M. Isocyanate Compounds as Electrolyte Additives for Lithium-ion Batteries. Journal of Power Sources 2007, 174 (2), 387-393, DOI: https://doi.org/10.1016/j.jpowsour.2007.06.141. (48) Wu, F.; Xiang, J.; Li, L.; Chen, J.; Tan, G.; Chen, R. Study of the Electrochemical Characteristics of Sulfonyl Isocyanate/Sulfone Binary Electrolytes for Use in Lithium-ion Batteries. Journal of Power Sources 2012, 202 (Supplement C), 322-331, DOI: https://doi.org/10.1016/j.jpowsour.2011.11.065. (49) Wang, R.; Li, X.; Wang, Z.; Zhang, H. Electrochemical Analysis Graphite/Electrolyte Interface in Lithium-ion Batteries: p-Toluenesulfonyl Isocyanate as Electrolyte Additive. Nano

Energy

2017,

34

(Supplement

C),

131-140,

DOI:

https://doi.org/10.1016/j.nanoen.2017.02.037. (50) Wang, R.-H.; Li, X.-H.; Wang, Z.-X.; Guo, H.-J.; He, Z.-J. Electrochemical Analysis for Enhancing Interface Layer of Spinel Li4Ti5O12: p-Toluenesulfonyl Isocyanate as Electrolyte Additive. ACS Applied Materials & Interfaces 2015, 7 (42), 23605-23614, DOI: 10.1021/acsami.5b07047. 29 ACS Paragon Plus Environment


Page 30 of 34

(51) Liu, Y.; Qin, Y.; Peng, Z.; Zhou, J.; Wan, C.; Wang, D. Hexamethylene Diisocyanate as an Electrolyte Additive for High-energy Density Lithium Ion Batteries. J. Mater. Chem. A 2015, 3 (16), 8246-8249, DOI: 10.1039/C4TA07055K. (52) Wang, R.; Li, X.; Wang, Z.; Guo, H.; Wang, J. Electrochemical Analysis for Cycle Performance and Capacity Fading of Lithium Manganese Oxide Spinel Cathode at elevated Temperature using p-Toluenesulfonyl Isocyanate as Electrolyte Additive. Electrochimica Acta

2015,

180

(Supplement

C),

815-823,

DOI:

https://doi.org/10.1016/j.electacta.2015.09.019. (53) Dong, P.; Wang, D.; Yao, Y.; Li, X.; Zhang, Y.; Ru, J.; Ren, T. Stabilizing Interface Layer of LiNi0.5Co0.2Mn0.3O2 Cathode Materials under high Voltage using p-Toluenesulfonyl Isocyanate as Film Forming Additive. Journal of Power Sources 2017, 344 (Supplement C), 111-118, DOI: https://doi.org/10.1016/j.jpowsour.2017.01.116. (54) Frisch, M. J. Gaussian 16, Revision A.03, Gaussian Inc: Wallingfort CT, 2016. (55) Becke, A. D. Density‐Functional Thermochemistry. III. The Role of exact Exchange. The Journal of Chemical Physics 1993, 98 (7), 5648-5652, DOI: 10.1063/1.464913. (56) Vosko, S. H.; Wilk, L.; Nusair, M. Accurate Spin-dependent Electron Liquid Correlation Energies for Local Spin Density Calculations: A Critical Analysis. Can. J. Phys. 1980, 58 (8), 1200-1211, DOI: 10.1139/p80-159. (57) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti Correlation-energy Formula into a Functional of the Electron Density. Physical Review B 1988, 37 (2), 785-789. (58) Miehlich, B.; Savin, A.; Stoll, H.; Preuss, H. Results obtained with the Correlation Energy Density Functionals of Becke and Lee, Yang and Parr. Chemical Physics Letters 1989, 157 (3), 200-206, DOI: https://doi.org/10.1016/0009-2614(89)87234-3. (59) Grimme, S.; Ehrlich, S.; Goerigk, L. Effect of the Damping Function in Dispersion Corrected Density Functional Theory. Journal of Computational Chemistry 2011, 32 (7), 1456-1465, DOI: 10.1002/jcc.21759. (60) Weigend, F.; Ahlrichs, R. Balanced Basis Sets of Split Valence, Triple Zeta Valence and Quadruple Zeta Valence Quality for H to Rn: Design and Assessment of Accuracy. Physical Chemistry Chemical Physics 2005, 7 (18), 3297-3305, DOI: 10.1039/B508541A. (61) Xing, L.; Borodin, O.; Smith, G. D.; Li, W. Density Functional Theory Study of the Role of Anions on the Oxidative Decomposition Reaction of Propylene Carbonate. The Journal of Physical Chemistry A 2011, 115 (47), 13896-13905, DOI: 10.1021/jp206153n. (62) Tomasi, J.; Mennucci, B.; Cammi, R. Quantum Mechanical Continuum Solvation Models. Chemical Reviews 2005, 105 (8), 2999-3094, DOI: 10.1021/cr9904009. 30 ACS Paragon Plus Environment

Page 31 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


(63) Xu, K. Nonaqueous Liquid Electrolytes for Lithium-based Rechargeable Batteries. Chemical Reviews 2004, 104 (10), 4303-4417. (64) Omampuliyur, R. S.; Bhuiyan, M.; Han, Z.; Jing, Z.; Li, L.; Fitzgerald, E. A.; Thompson, C. V.; Choi, W. K. Nanostructured Thin Film Silicon Anodes for Li-Ion Microbatteries. Journal

of

Nanoscience

and

Nanotechnology

2015,

15

(7),

4926-4933,

DOI:

10.1166/jnn.2015.9831. (65) Xu, K.; Ding, S. P.; Jow, T. R. Toward Reliable Values of Electrochemical Stability Limits for Electrolytes. Journal of the Electrochemical Society 1999, 146 (11), 4172-4178. (66) Kasnatscheew, J.; Streipert, B.; Roser, S.; Wagner, R.; Cekic Laskovic, I.; Winter, M. Determining Oxidative Stability of Battery Electrolytes: Validity of Common Electrochemical Stability Window (ESW) Data and Alternative Strategies. Physical Chemistry Chemical Physics 2017, 19 (24), 16078-16086, DOI: 10.1039/C7CP03072J. (67) Wu, F.; Zhu, Q.; Li, L.; Chen, R.; Chen, S. A Diisocyanate/Sulfone Binary Electrolyte based on Lithium Difluoro(oxalate)borate for Lithium Batteries. J. Mater. Chem. A 2013, 1 (11), 3659-3666, DOI: 10.1039/C3TA01182H. (68) Trask, S. E.; Pupek, K. Z.; Gilbert, J. A.; Klett, M.; Polzin, B. J.; Jansen, A. N.; Abraham, D. P. Performance of Full Cells Containing Carbonate-Based LiFSI Electrolytes and Silicon-Graphite Negative Electrodes. Journal of The Electrochemical Society 2015, 163 (3), A345-A350, DOI: 10.1149/2.0981602jes. (69) Jung, R.; Metzger, M.; Haering, D.; Solchenbach, S.; Marino, C.; Tsiouvaras, N.; Stinner, C.; Gasteiger, H. A. Consumption of Fluoroethylene Carbonate (FEC) on Si-C Composite Electrodes for Li-Ion Batteries. Journal of The Electrochemical Society 2016, 163 (8), A1705-A1716, DOI: 10.1149/2.0951608jes. (70) Qian, Y.; Schultz, C.; Niehoff, P.; Schwieters, T.; Nowak, S.; Schappacher, F. M.; Winter, M. Investigations on the Electrochemical Decomposition of the Electrolyte Additive Vinylene Carbonate in Li Metal Half Cells and Lithium Ion Full Cells. Journal of Power Sources 2016, 332, 60-71, DOI: https://doi.org/10.1016/j.jpowsour.2016.09.100. (71) Smith, A. J.; Burns, J. C.; Xiong, D.; Dahn, J. R. Interpreting High Precision Coulometry Results on Li-ion Cells. Journal of The Electrochemical Society 2011, 158 (10), A1136A1142, DOI: 10.1149/1.3625232. (72) Wang, D. Y.; Sinha, N. N.; Burns, J. C.; Aiken, C. P.; Petibon, R.; Dahn, J. R. A Comparative Study of Vinylene Carbonate and Fluoroethylene Carbonate Additives for LiCoO2/Graphite Pouch Cells. Journal of The Electrochemical Society 2014, 161 (4), A467A472, DOI: 10.1149/2.001404jes. 31 ACS Paragon Plus Environment


(73) Liu, D.; Qian, K.; He, Y.-B.; Luo, D.; Li, H.; Wu, M.; Kang, F.; Li, B. Positive FilmForming Effect of Fluoroethylene Carbonate (FEC) on High-voltage Cycling with ThreeElectrode LiCoO2/Graphite Pouch Cell. Electrochimica Acta 2018, 269, 378-387, DOI: https://doi.org/10.1016/j.electacta.2018.02.151. (74) Krueger, S.; Kloepsch, R.; Li, J.; Nowak, S.; Passerini, S.; Winter, M. How Do Reactions at the Anode/Electrolyte Interface Determine the Cathode Performance in Lithium-Ion Batteries? Journal of the Electrochemical Society 2013, 160 (4), A542-A548. (75) Kasnatscheew, J.; Rodehorst, U.; Streipert, B.; Wiemers-Meyer, S.; Jakelski, R.; Wagner, R.; Laskovic, I. C.; Winter, M. Learning from Overpotentials in Lithium Ion Batteries: A Case Study on the LiNi1/3Co1/3Mn1/3O2 (NCM) Cathode. Journal of The Electrochemical Society 2016, 163 (14), A2943-A2950, DOI: 10.1149/2.0461614jes. (76) Niehoff, P.; Passerini, S.; Winter, M. Interface Investigations of a Commercial Lithium Ion Battery Graphite Anode Material by Sputter Depth Profile X-ray Photoelectron Spectroscopy. Langmuir 2013, 29 (19), 5806-5816, DOI: 10.1021/la400764r. (77) 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, DOI: 10.1021/la403276p. (78) Philippe, B.; Dedryvère, R.; Allouche, J.; Lindgren, F.; Gorgoi, M.; Rensmo, H.; Gonbeau, D.; Edström, K. Nanosilicon Electrodes for Lithium-Ion Batteries: Interfacial Mechanisms Studied by Hard and Soft X-ray Photoelectron Spectroscopy. Chemistry of Materials 2012, 24 (6), 1107-1115, DOI: 10.1021/cm2034195. (79) Characterization of Lithium Alkyl Carbonates by X-ray Photoelectron Spectroscopy: Experimental and Theoretical Study. Journal of Physical Chemistry B 2005, 109, 1586815875. (80) Lamb, R. N.; Baxter, J.; Grunze, M.; Kong, C. W.; Unertl, W. N. An XPS Study of the Composition of Thin Polyimide Films Formed by Vapor Deposition. Langmuir 1988, 4, 149256. (81) Luong, N. D.; Hippi, U.; Korhonen, J. T.; Soininen, A. J.; Ruokolainen, J.; Johansson, L.-S.; Nam, J.-D.; Sinh, L. H.; Seppälä, J. Enhanced Mechanical and Electrical Properties of Polyimide Film by Graphene Sheets via In Situ Polymerization. Polymer 2011, 52 (23), 52375242, DOI: https://doi.org/10.1016/j.polymer.2011.09.033. (82) Nie, M.; Abraham, D. P.; Chen, Y.; Bose, A.; Lucht, B. L. Silicon Solid Electrolyte Interphase (SEI) of Lithium Ion Battery Characterized by Microscopy and Spectroscopy. The 32 ACS Paragon Plus Environment

Page 32 of 34

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


Journal of Physical Chemistry C 2013, 117 (26), 13403-13412, DOI: 10.1021/jp404155y. (83) McEvoy, N.; Peltekis, N.; Kumar, S.; Rezvani, E.; Nolan, H.; Keeley, G. P.; Blau, W. J.; Duesberg, G. S. Synthesis and Analysis of Thin Conducting Pyrolytic Carbon Films. Carbon 2012, 50 (3), 1216-1226, DOI: https://doi.org/10.1016/j.carbon.2011.10.036. (84) Ota, H.; Sakata, Y.; Inoue, A.; Yamaguchi, S. Analysis of Vinylene Carbonate Derived SEI Layers on Graphite Anode. Journal of The Electrochemical Society 2004, 151 (10), A1659-A1669, DOI: 10.1149/1.1785795. (85) El Ouatani, L.; Dedryvère, R.; Siret, C.; Biensan, P.; Reynaud, S.; Iratçabal, P.; Gonbeau, D. The Effect of Vinylene Carbonate Additive on Surface Film Formation on Both Electrodes in Li-Ion Batteries. Journal of The Electrochemical Society 2009, 156 (2), A103-A113, DOI: 10.1149/1.3029674. (86) Shashoua, V. E.; Sweeny, W.; Tietz, R. F. The Homopolymerization of Monoisocyanates. Journal of the American Chemical Society 1959, 84 (4), 866-873.



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