Triphenylphosphine Oxide as Highly Effective Electrolyte Additive for

Article Cite This: Chem. Mater. 2018, 30, 2726−2741

pubs.acs.org/cm

Triphenylphosphine Oxide as Highly Effective Electrolyte Additive for Graphite/NMC811 Lithium Ion Cells Kolja Beltrop,† Sven Klein,† Roman Nölle,† Andrea Wilken,† Juhyon J. Lee,‡ Thomas K.-J. Köster,‡ Jakub Reiter,‡ Liang Tao,‡ Chengdu Liang,§ Martin Winter,†,∥ Xin Qi,*,† and Tobias Placke*,† †

University of Münster, MEET Battery Research Center, Institute of Physical Chemistry, Corrensstrasse 46, 48149 Münster, Germany BMW Group, Research Battery Technology, Petuelring 130, 80788 München, Germany § Contemporary Amperex Technology Limited, No.1 Xingang Road, Jiaocheng District, Ningde 352100, Fujian Province, China ∥ Helmholtz Institute Münster, IEK-12, Forschungszentrum Jülich GmbH, Corrensstrasse 46, 48149 Münster, Germany ‡

S Supporting Information *

A BS T R A C T : N ic k e l- ri c h l a y er e d o x id e m at er i al s ( Li NixMnyCo1−x−yO2, x ≥ 0.8, LiNMC) attract great interest for application as positive electrode in lithium ion batteries (LIBs) due to high specific discharge capacities at moderate upper cutoff voltages below 4.4 V vs Li/Li+. However, the comparatively poor cycling stability as well as inferior safety characteristics prevent this material class from commercial application so far. Against this background, new electrolyte formulations including additives are a major prerequisite for a sufficient electrochemical performance of Ni-rich NMC materials. In this work, we introduce triphenylphosphine oxide (TPPO) as electrolyte additive for the application in graphite/LiNi0.8Mn0.1Co0.1O2 (NMC811) cells. The addition of only 0.5 wt % TPPO into a carbonate-based electrolyte (LiPF6 in EC:EMC) significantly increases the first cycle Coulombic efficiency as well as the reversible specific capacity and improves the capacity retention of the LIB full cell cycled between 2.8 and 4.3 V. Electrochemical results indicate that the full cell capacity fade is predominantly caused by active lithium loss at the negative electrode. In this contribution, X-ray photoelectron spectroscopy and inductively coupled plasma-mass spectrometry analysis confirm the participation of the electrolyte additive in the solid electrolyte interphase formation on the negative electrode as well as in the cathode electrolyte interphase formation on the positive electrode, thus, effectively reducing the active lithium loss during cycling. Furthermore, the performance of the TPPO additive is compared to literature known electrolyte additives including triphenylphosphine, vinylene carbonate, and diphenyl carbonate demonstrating the outstanding working ability of TPPO in graphite/NMC811 cells.

1. INTRODUCTION In order to advance lithium ion batteries (LIBs) with respect to energy density, power density, lifetime, and safety, many efforts have been made to substantially expand the application areas of LIBs. Especially the increasing demands on both high gravimetric and volumetric energy density in LIBs for automotive applications raise the research efforts all over the world.1−6 With respect to active material development, further development of cathode materials is still considered as the main driver to improve energy density, thus, novel cathode materials with enhanced capacity and/or operating voltage are pursued. Unfortunately, high voltage cathode materials are highly restricted by the narrow electrochemical stability window of the state-of-the-art LiPF6 in carbonate-based electrolytes (≈1.0−4.4 V vs Li/Li+)7−9 and related side reactions, such as metal dissolution.10,11 Instead of focusing only on the highvoltage cathode materials, an alternative way to enhance the energy density of the system is to use cathode materials with © 2018 American Chemical Society

high specific capacity at moderate working potentials (i.e., below 4.4 V vs Li/Li+). By changing the content of each transition metal, a variety of different LiNixMnyCo1−x−yO2 (LiNMC) materials have been synthesized and characterized, including, for example, LiNi 1/3 Mn 1/3 Co 1/3 O 2 (NMC111), LiNi 0.5 Mn 0.3 Co 0.2 O 2 (NMC532), LiNi 0.6 Mn 0.2 Co 0.2 O 2 (NMC622), and LiNi0.8Mn0.1Co0.1O2 (NMC811).12−15 While an increase in the Ni content leads to an enhanced capacity at the same cutoff potential, it is believed that the thermal stability will decrease in the opposite way and may result in safety issues.12,16 Kasnatscheew et al. emphasized the dependency of the structural stability of various LiMO2 electrode materials on the respective Li+ extraction ratio, which is related to the specific charge capacity and charge cutoff potential.7 Currently, Received: February 2, 2018 Revised: March 28, 2018 Published: March 28, 2018 2726

DOI: 10.1021/acs.chemmater.8b00413 Chem. Mater. 2018, 30, 2726−2741

Article

Chemistry of Materials

Figure 1. General chemical and physicochemical properties of the electrolyte additive triphenylphosphine oxide (TPPO).

and cathode surface to prevent the electrolyte from ongoing (electro-) chemical decomposition.39−44 In general, it has to be considered that the electrolyte can react with both, anode and cathode, and that these reactions mutually impact each other in the complete battery cell, depending on capacity and mass balancing.25,45 Against this backdrop, it has been shown that the NMC811 is a unique material and that a variety of famous electrolyte additives, like vinylene carbonate (VC), prop-1-ene-1,3-sultone (PES), or tris(trimethylsilyl) phosphite (TMSPi), which improve the cycling stability of graphite/NMC111 or graphite/NMC442 cells do not necessarily enhance the overall cell performance of graphite/NMC811 cells.46 Interestingly, it was proposed that the addition of VC and PES can effectively suppress the formation of a rock salt surface layer on the NMC811 particles, which is often claimed to be the major failure mechanism during long-term cycling.47 Recently, Qiu et al. stated on the application of methyl phenyl carbonate (MPC) and diphenyl carbonate (DPC) as electrolyte additives either as single component or in combination with methylene methyl disulfonate and TMSPi in graphite/NMC811 pouch cells.46 It could be shown that the addition of 1 wt % DPC leads to a decreased gas evolution, smaller voltage drop during storage, lower impedance increase, and better capacity retention during long-term cycling. Phenyl carbonates are supposed to act as a kind of solid electrolyte interphase (SEI) modifier, rather than SEI formers because of the absence of a clear reduction peak prior to EC. Furthermore, the additives seem to have an impact on the positive electrode, as the charge end point capacity slippage rate is reduced. The work of Li et al. focused on the failure mechanism of graphite/NMC811 cells with and without the use of different additives, including VC, PES, TMSPi, and methylene methanedisulfonate (MMDS).21 Even though the addition of 2 wt % VC to the electrolyte showed much better capacity retention and lower voltage drop compared to the PES system, severe impedance growth was monitored for all cells cycled above 4.2 V, indicating an unsatisfactory effect of these additives on the total cell performance. Yim et al. reported on the effect of a bifunctionalized divinyl sulfone (DVS) as electrolyte additive for NMC-712 that improves the stabilization of the electrolyte−electrode interface by forming a durable protective layer on the cathode surface.48 As stated above, several famous electrolyte additives such as VC, PES, and TMSPi were proven not to sufficiently work in graphite/NMC811 cells, demonstrating the unique surface chemistry in this system.46 In this contribution and to the best of our knowledge, DPC shows the best results among the literature reported electrolyte additives so far.46

NMC532 and/or NMC622 can be considered as state-of-theart cathode materials for commercial high-energy LIBs used for automotive applications.17 In particular, high Ni-content LiNixCoyMn1−x−yO2 materials with x ≥ 0.6 (i.e., NMC811) have attracted more attention recently due to their high specific capacity of ≈200 mAh g−1 at an upper cutoff potential of 4.3 V versus Li/Li+.12 However, the overall cell performance suffers from severe capacity fading caused by ongoing chemical electrolyte decomposition18,19 and/or intrinsic structural changes of the material, such as large c axis shrinkage at high cathode potentials, and limit its commercial application so far.20 The increased amount in highly reactive Ni4+ surface sites in the delithiated state of NMC811 are suggested to be the predominant cause for the parasitic side reactions between the electrolytes and the NMC surface.21 However, recent studies reveal that the release of reactive oxygen from the NMC surface is causing chemical oxidation of the electrolyte, yielding in increased CO2 and CO evolution and that a transition metal induced catalytic effect on the electrochemical electrolyte oxidation does not exist.19 Much effort has been made to improve the electrochemical performance of the LiNMC systems, including doping as well as coating of the active material with several components to increase the intrinsic structure and to circumvent the abovementioned failure mechanisms.22,23 Further attempts to stabilize the cathode/electrolyte interface have been made by tailoring the electrolyte formulations, in order to avoid ongoing electrolyte decomposition and transition metal ion dissolution. The state-of-the-art carbonate-based electrolyte solutions display only a compromise in terms of cycling stability, thermal stability, and safety issues because of the manifold advantages and disadvantages of each single component.24−27 A very promising approach to master the above-mentioned challenges lies in the addition of small amounts of other components into the electrolyte system to tailor specific targeted properties of the electrolyte without changing the bulk properties.28,29 These additives are able to create a specific electrode/electrolyte interface, thus reducing the chemical/electrochemical side reactions. In addition, the electrolyte properties in terms of electrochemical stability, ionic conductivity, and thermal stability can also be enhanced.28,30 Various literature reports deal with the influence of these electrolyte additives, such as HF-scavengers, oxidative stability enhancers, flame retardants, or overcharge protection additives, on the overall cell performance, demonstrating the great relevance of this specific research field.31−39 Furthermore, film forming additives are expected to be either oxidized or reduced prior to the electrolyte solvent in order to build a thin and homogeneous passivation film (SEI or CEI) on both the anode 2727


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weight (solvionic; purity: battery grade), which was defined as baseline electrolyte 1 (BL 1) and 1 M LiPF6 in EC/EMC 1:1 by wt (solvionic purity: battery grade), defined as baseline electrolyte 2 (BL 2), respectively. In each case, 0.5 wt % of the corresponding electrolyte additive [i.e., triphenylphosphine oxide (TPPO; Sigma-Aldrich; purity: 98.5%), triphenylphosphine (TPP; Sigma-Aldrich; purity: 99%), vinylene carbonate (VC; Sigma-Aldrich; purity: 99.5%), or diphenyl carbonate (DPC; Sigma-Aldrich; purity: 99%)] was used. The electrolyte preparation and storage as well as the cell manufacturing were carried out in an argon-filled glovebox (H2O and O2 contents < 0.1 ppm). 2.2. Cell Preparation and Electrochemical Characterization. In order to monitor the potential changes of the anode and cathode versus the reference electrode (lithium metal; Albemarle Corporation; purity: battery grade) in the charge/discharge process, a threeelectrode setup (Swagelok type T-cell) was applied for the electrochemical characterization. For the investigation of the graphite/NMC811 cells, a polypropylene separator (PP, Freudenberg 2190, 6 layers) soaked with 240 μL of the corresponding electrolyte was used. To ensure a sufficient wetting of the composite electrodes (surface area: 1.13 cm2) by the electrolytes, the assembled cells were equilibrated for 24 h at 20 °C. It has to be kept in mind that a much larger amount of electrolyte is used in these laboratory three-electrode cells compared to commercial cells, therefore, the percentage of the studied electrolyte additive most likely needs to be adjusted when going to commercial cells formats. The electrochemical charge/discharge cycling stability of the different electrolyte formulations was examined via constant current charge/discharge cycling (CCC), performed on a Maccor 4000 battery analysis system within climatic chambers set to 20 °C. The cells were charged/discharged between 2.8 and 4.3 V, with a specific current of 19.6 mA g−1 (0.2 mA cm−2, 0.1C) for two formation cycles in order to provide a homogeneous SEI formation. A current density of 2.0 mA cm−2 is defined as 1C. Afterward, the cells were cycled with 98 mA g−1 (1 mA cm−2, 0.5C). A constant voltage charge step at 4.3 V after each constant current charge process was applied until the current dropped below 0.01C. The calculation of the capacity retention after representative cycles is based on the discharge capacity of the third cycle after the formation process. The oxidative stability limits of various electrolyte compositions were obtained by means of linear sweep voltammetry (LSV) using a VSP instrument (Bio-Logic Science Instruments) with a scan rate of 1 mV s−1. All measurements were carried out using metallic lithium (Albemarle Corporation; purity: battery grade) as counter and reference electrodes. For the oxidative scan, a platinum working electrode (WE; geometry: round-shaped; surface area: 7.85 × 10−3 cm2) was used. The anodic limits were arbitrarily defined as the potential at which the current density reached 0.025 mA cm−2. Cyclic voltammetry (CV) of the graphite anodes was performed on a VSP instrument (Bio-Logic Science Instruments). The scan rate was set to 0.05 mV s−1 within a potential range of 0.005 and 1.8 V versus Li/Li+. Graphite was used as WE, and metallic lithium as counter and reference electrodes. 2.3. Characterization of the Specific Surface Composition via XPS Analysis. X-ray photoelectron spectroscopy (XPS) measurements were performed using a monochromatic Al Kα source (hν = 1486.6 eV) at a 10 mA filament current and a 12 kV filament voltage source energy. In order to compensate for the charging of the sample, a charge neutralizer was used. The measurement was carried out applying a pass energy of 40 eV at a 0° angle of emission. The measurement time was 600 s, and the lateral resolution was 3 μm. The pressure within the analysis chamber was 10−7 Pa. The fitting was performed using CasaXPS. The data fitting was carried out as described in detail elsewhere.50,51 For the positive electrodes, the energy of the C 1s peak [conductive carbon at a binding energy (BE) of 284.5 eV] was used as an internal standard for the calibration of the BE of the measured spectra. For the negative electrodes, the energy scale of the measured spectra was adjusted using the energy of the C 1s peak (graphite at BE = 284.5 eV) as an internal reference. The harvested positive and negative electrodes were taken from discharged

In this contribution, we introduce triphenylphoshine oxide (TPPO) from the phosphine oxide family as an electrolyte additive for the application in graphite/NMC811 lithium ion cells. The chemical structure as well as some major general properties of TPPO are illustrated in Figure 1. Against this background, TPPO combines the most beneficial properties such as low costs and nontoxicity (classified only as harmful)49 and results in a greatly enhanced cell performance. Therefore, it can be concluded that this additive is highly interesting for industry applications and further fundamental studies to advance LIBs.

2. EXPERIMENTAL SECTION 2.1. Electrode and Electrolyte Preparation. The electrode materials and formulations applied in this work are summarized in Table 1. The anode contains 95.4 wt % graphite (FSNC-1; Shanshan

Table 1. Electrode Composition and Constitution for the Graphite/NMC811 Cell Investigations properties composition processing solvent average areal capacity electrode loading current collector porosity

negative electrode

positive electrode

95.4 wt % graphite; 2.5 wt % SBR; 1.5 wt % Super C65; 0.6 wt % Na-CMC water

93 wt % NMC811; 4 wt % Super C65; 3 wt % PVdF N-methyl-2-pyrrolidone

≈2.6 mAh cm−2

≈2.1−2.2 mAh cm−2

≈7.7 mg cm−2

≈10.6 mg cm−2

dendritic copper (20 μm)

aluminum (15 μm)

30−35%

30−35%

Technology; D50 = 15.0 ± 2.0 μm; and BET surface area = 1.3 ± 0.3 m2 g−1), 1.5 wt % carbon black (Super C65; Imerys), 2.5 wt % SBR (SB 5521; LIPATON; Polymer latex GmbH), and 0.6 wt % Na-CMC (Walocel CRT 2000 PPA12; Dow Wolff Cellulosics) and was prepared in the laboratory. Deionized water was used as a dispersant, and the electrode paste was homogenized with a dispermat machine (10.000 rpm; 60 min; Dissolver Dispermat LC30, VMA-Getzmann GmbH) and cast onto the dendritic copper current collector (Carl Schlenk AG; purity: 99.9%). The areal capacity of the graphite anode was ≈2.6 mAh cm−2, and the density was 1.4 g cm−3, as determined by the capacity of the cathodes in order to achieve a suitable anode/ cathode capacity balancing. For the full cell investigations, the capacity ratio between the cathode and anode was set as 1:1.30, to avoid lithium metal plating at the graphite anode. The NMC811-based cathodes were prepared in large scale with the battery line in the MEET institute. Carbon black (Super C65; Imerys) and a PVdF solution in N-methyl-2-pyrrolidone (Solef 5130, Solvay; 6 wt %) were first added into an airtight container and homogenized by a shear mixer at 2500 rpm for half an hour. Afterward, the NMC811 material (provided by CATL; BET surface area = 0.64 m2 g−1; D90 = 18.3 μm) was added, and the paste was mixed for another 1.5 h. The solid loading of the electrode paste was 50%, and the viscosity after dispersion was 1500 Pa s at a shear rate of 10 s−1. The container was set under reduced pressure to avoid the generation of gas bubbles. A water-cooling system was applied to remove the heat generated during the mixing process. The paste was cast on the aluminum foil (Evonik Industries; purity: 99.9%), and the dimension of each electrode sheet was 20 × 20 cm2. The electrodes were cut into 12 mm ø disks (1.13 cm2), and the areal capacity of all the electrodes was controlled as ≈2.1−2.2 mAh cm−2 with a standard deviation of less than 0.03 mAh cm−2. The influence of the electrolyte additives on the cycling stability and Coulombic efficiency of the graphite/NMC811 system was evaluated within the benchmark electrolytes 1 M LiPF6 in EC/EMC 3:7 by 2728


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Figure 2. LSV diagrams for (a) BL 1 (=1 M LiPF6, EC/EMC 3:7 by wt, (b) BL 1 + 0.5 wt % TPPO, (c) BL 2 (=1 M LiPF6, EC/EMC 1:1 by wt, and (d) BL 2 + 0.5 wt % TPPO electrolyte. Scan rate: 1 mV s−1; anodic limit: 0.025 mA cm−2. cells after 102 cycles in BL 1 and BL 1 + 0.5 wt % TPPO. Prior to the measurement, the harvested electrodes were rinsed with dimethylcarbonate (3 × 1 mL) to remove electrolyte residue. 2.4. Determination of Active Lithium-Loss via ICP-MS Analysis. The harvested graphite electrodes were taken from discharged cells after two formation cycles in BL 1 and BL 1 + 0.5 wt % TPPO. Prior to the measurement, the harvested electrodes were rinsed with dimethyl carbonate (3 × 1 mL) to remove electrolyte residue. The gathered values for the lithium content display an average of three measurements. The graphite electrodes were weighed into PTFE vessels and digested in a Multiwave PRO microwave reaction system SOLV (Anton Paar, Graz, Austria) with 3 mL nitric acid and 3 mL hydrochloric acid. After cooling down, the samples were filled up to 25 mL with deionized water and diluted 1:10 for analysis. Calibration solutions were prepared in a range from 0.01 to 500 μg/L containing 1 vol % of nitric acid. An Agilent 7900 inductively coupled plasma-mass spectrometer (ICP-MS) with Pt-cones from Agilent Technologies was used for total determination of lithium. The system was controlled by the software Mass Hunter 4.3 Workstation Software for ICP 7900 (Agilent Technologies). For sample introduction, a SPS 4 auto sampler (Agilent Technologies) was preconnected to the peristaltic pump of the system combined with a MicroMist nebulizer. The torch position, ion lenses, gas output, resolution axis, and background were optimized daily with the tuning solution (1 mg/L Ce, Co, Li, Tl, Y) to perform a short-term stability analysis of the instrument, to maximize ion signals, and to minimize interference effects due to high oxide levels (CeO+/ Ce+ < 1.2%) and doubly charged ions (Ce2+/Ce+ < 1%). Linearity response in the pulsed and analogue modes (P/A factor determination) was verified weekly using PA tuning solutions. Detailed information about instrumental settings for ICP-MS analysis can be found in Table S1.

3. RESULTS AND DISCUSSION 3.1. Influence of TPPO as Electrolyte Additive on the Electrolyte Stability. The results from the linear sweep voltammetry measurements for BL 1 and BL 2 electrolytes (a, c) without and (b, d) with 0.5 wt % TPPO are depicted in Figure 2, respectively. The results indicate that the addition of TPPO to the baseline electrolytes decreases the oxidative stability of both electrolyte systems (stability decrease of ≈0.16−0.23 V). In the TPPO molecule, the phosphorus atom exhibits the highest possible oxidation state (+V). Thus, the observed increased current flow is most likely induced by the electrochemical oxidation of the phenyl moieties and/or P−C bonds. Due to the reduced electrolyte stability, an oxidative decomposition and contribution of TPPO in the CEI formation process appears reasonable and will be the subject of the following discussion. In accordance with literature, the current density increase, starting at a potential of ≈4.2 V versus Li/Li+, is attributed to the oxidative decomposition of the carbonates.4 Furthermore, a possible migration of electrochemical reduction products of TPPO from the negative electrode through the electrolyte and subsequent oxidation on the cathode surface, often called as cross-talk between anode and cathode,52,53 is conceivable, however, not terminable within this measurement. Nevertheless, it also has to be kept in mind that the validity of the stability data can be questioned, as it is uncertain whether the obtained results can be transferred from the LSV measurement setup to a practical LIB full cell setup, as discussed in a previous publication.54 The CV diagrams for the graphite negative electrodes in BL 1 and BL 2 electrolytes with and without 0.5 wt % TPPO are 2729


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Figure 3. Cyclic voltammograms of graphite/lithium metal cells with (a) BL 1 (=1 M LiPF6, EC/EMC 3:7 by wt, (b) BL 1 + 0.5 wt % TPPO, (c) BL 2 (=1 M LiPF6, EC/EMC 1:1 by wt., and (d) BL 2 + 0.5 wt % TPPO. Cut-off potential: 0.005−1.4 V vs Li/Li+; scan rate: 0.05 mV s−1.

Figure 4. Influence of the addition of 0.5 wt % TPPO on the charge/discharge cycling behavior of graphite/NMC811 cells in (a and b) BL 1 (=1 M LiPF6, EC/EMC 3:7 by wt and (c and d). BL 2 (=1 M LiPF6, EC/EMC 1:1 by wt electrolyte. Depicted are the (a and b) discharge capacities as well as the (c and d) CEs. Cut-off voltage: 2.8−4.3 V.

at which the specific current reached 0.003 mA mg−1 (as seen in the inset images). In general, the addition of TPPO to the baseline electrolytes leads to a shift of the onset potential for electrolyte reduction to higher values, indicating the reductive decomposition of TPPO prior to the electrolyte solvent molecules and effectively reducing the electrolyte decomposition in the ongoing scan. In this contribution, the current density from the reductive

illustrated in Figure 3. The CV diagrams for the BL 2 electrolyte were implemented to evaluate the influence of an increased ethylene carbonate (EC) content (50 wt % in BL 2 compared to 30 wt % in BL 1) on the reductive electrolyte stability and to further study the working principle of the TPPO additive. From the measurements, the onset potential for electrolyte reduction was arbitrarily defined as the potential 2730


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Table 2. Discharge Capacity and Coulombic Efficiency Data of Investigated Electrolytes Taken from CCC of Graphite/ NMC811 Cells of Selected Cycles (cf. Figure 4) and the Calculation of the Capacity Retention after 100 cycles, Based on the Third Cyclea discharge capacity (mAh g−1) electrolyte BL BL BL BL a

1 1 + TPPO 2 2 + TPPO

Coulombic efficiency (%)

1st cycle

5th cycle

100th cycle

1st cycle

5th cycle

100th cycle

capacity retention [100th / 3th] (%)

163 198 143 197

156 187 136 186

135 172 118 172

70.5 86.2 62.4 85.8

99.4 99.8 99.2 99.8

99.9 99.9 99.86 99.9

86 92 86 92

Error range for the discharge capacity values is ±4 mAh g−1 and for the Coulombic efficiency ±0.9% for the first and ±0.05% for ongoing cycles.

Figure 5. (a and b) Anode and cathode potential profiles vs Li/Li+ of graphite/NMC811 cells during charge (black) and discharge (red). Cut-off voltage: 2.8−4.3 V; electrolyte: (a) BL 1 (=1 M LiPF6, EC/EMC 3:7 by wt and (b) BL 1 + 0.5 wt % TPPO (b). (c and d) The corresponding anode potential profiles vs Li/Li+ during charge and discharge in BL 1 (black) and BL 1 + 0.5 wt % TPPO electrolyte (red).

decomposition of BL 2 (Figure 3c) in the first cycle (onset potential: ≈ 0.61 V vs Li/Li+) is significantly higher than for BL 1 (onset potential: ≈ 0.79 V vs Li/Li+; Figure 3a), indicating an enhanced decomposition of the BL 2 electrolyte, most likely caused by the increased EC content and further evidenced below. Against this background, the addition of TPPO to the baseline electrolytes has a strong impact on the lithiation/ delithiation behavior of graphite-based anodes as denoted by the reduced current peak area in the first charge process (Figure 3, panels b and d). For example, the current density increase during lithiation in the first reductive scan in the TPPOcontaining BL 2 electrolyte starts at ≈0.79 V vs Li/Li+ compared to 0.61 V vs Li/Li+ in the pure BL 2 electrolyte. The reductive peak at ≈0.8 V vs Li/Li+ is most likely caused by the electrochemical reduction of TPPO molecules on the electrode surface, whereas the broad reductive peak at ≈0.6 V vs Li/Li+ can be related to the electrochemical reduction of EC molecules, both forming a solid electrolyte interphase (SEI)

layer on the graphite active material29,55 and especially on the “non-basal plane” surfaces of graphite.56−59 A particularly conspicuous aspect in this contribution is the prevention of the severe electrolyte decomposition between 0.6 and 0.2 V vs Li/ Li+ by the addition of TPPO, evidencing the effective contribution of the additive in the SEI formation process on graphite. For all electrolyte systems, the discussed reduction peaks vanished in the second cycle, indicating the formation of an effective SEI on graphite. 3.2. Influence of TPPO Addition on the Coulombic Efficiency and Capacity Retention in Graphite/NMC811 Cells. In order to study the influence of the electrolyte additive TPPO on electrochemical performance of the graphite/ NMC811 cells, constant current cycling experiments were conducted. The cathode/anode capacity ratio is a critical parameter to achieve an optimized cell performance45 and was set as 1:1.30, with respect to the properties of the active materials and composite electrodes. In accordance with the CV measurements (Figure 3), the BL 2 electrolyte was studied to 2731


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Figure 6. Change of the upper cutoff potential of the NMC811 cathode during cycling (a) in the BL 1 electrolyte (=1 M LiPF6, EC/EMC 3:7 by wt and (b) in the TPPO-containing electrolyte taken from CCC experiments.

start decomposition from ≈0.8 V vs Li/Li+, resulting in the formation of the SEI layer on graphite. The anode potential incline between 0.8 and 0.2 V versus Li/Li+ in the BL 1 electrolyte is much lower than for the one with TPPO, indicating a higher degree of decomposition caused by the reduction of the BL 1 electrolyte (Figure 5, panels c and d). This phenomenon directly affects the cathode and anode potential profiles during discharge. As mentioned above, the decomposition of electrolyte components, mainly from the reduction of EC, is most likely accompanied by an increased active lithium loss during SEI formation. As a consequence, in the BL 1 electrolyte, insufficient active lithium is available to fully relithiate the NMC811, indicated by the rise in the anode potential together with the missing drop in the cathode potential at the end of discharge (Figure 5a). A closer look on the anode potential profiles in the low specific capacity area (0−100 mAh g−1) during charge is illustrated in Figure 5d. In the potential range between 0.8 and 0.55 V versus Li/Li+, the reductive current increase is more pronounced in the TPPO containing electrolyte as also observed in CV measurements, indicating the reductive decomposition of TPPO for SEI formation (cf. Figure 3c). However, in this electrolyte, the anode potential drops much faster to 0.2 V versus Li/Li+ and the overall first lithiation plateau (≈0.2−0.1 V vs Li/Li+) is enlarged compared to the BL 1 electrolyte, resulting in an increased Li+ intercalation into graphite and, thus, leading to a higher specific capacity during discharge. In accordance with the discharge potential profile, a specific discharge capacity of 197 mAh g−1 can be obtained for the electrolyte with 0.5 wt % TPPO, whereas the BL 1 electrolyte shows only a minor discharge capacity of 167 mAh g−1. Thus, it can be assumed that more active lithium is present in the TPPO system after SEI formation which, in turn, leads to a higher discharge capacity in the ongoing cycles. This assumption is further supported by comparing the first cycle CE of the electrolyte systems. The addition of TPPO leads to an increase of the first cycle CE from 73% to 86%. In this context, it is of great importance to distinguish between a specific capacity loss caused by active lithium loss (i.e., during SEI formation and/or kinetic limitations during the relithiation of the NMC active material).8 Consequently, if the observed specific capacity loss in the BL 1 electrolyte is due to kinetic limitations, a constant voltage step (CV-step) at the discharge cutoff voltage in the third cycle would result in an increased specific discharge capacity. In Figure S1), it becomes clearly visible that no

further evaluate the influence of a higher EC content on the overall cell performance. The corresponding results are depicted in Figure 4. In general, graphite/NMC811 cells with BL 1 electrolyte show a higher discharge capacity as well as higher Coulombic efficiency (CE) in the first and ongoing cycles compared to BL 2 electrolyte (cf. Table 2). A possible reason for this behavior can be assigned to the differences in the SEI formation at the graphite anode. As stated above, the BL 2 electrolyte contains a higher amount of EC and most likely forms a thicker SEI at the anode, which consumes more Li+ from the NMC811 cathode. As a result, less active lithium would remain in the system to participate in the electrochemical reaction in the following charge/discharge processes. Thus, the first cycle CE and the specific discharge capacity are directly affected [i.e., BL 1 displays an enhanced first cycle CE (≈70%) compared to BL 2 (≈62%, Table 2)]. By adding only 0.5 wt % TPPO to the electrolytes, the overall discharge capacity, CE and capacity retention can be greatly increased (≈26% discharge capacity gain in BL 1 and ≈44% discharge capacity gain in BL 2 electrolyte after 100 cycles), indicating a participation of the additive in the formation of the corresponding surface layers on the anode and/or cathode. Most likely, due to an increased active lithium loss during the first cycles in BL 2 compared to BL 1 electrolyte, the overall discharge capacity is reduced, and thus, the deviation in discharge capacity between BL 1 and BL 2 electrolytes with and without TPPO as additive is more severe (Figure 4). In general, active lithium loss which results in capacity fading is considered as one major failure mechanism for LIB full cells and has to be minimized, in particular, in the formation cycles but also during ongoing cycling.60−65 As a consequence, the BL 1 electrolyte was applied as benchmark electrolyte in this work, due to the superior cycling performance, and thus, the following discussion focuses only on the influence of TPPO as electrolyte additive in the BL 1 electrolyte (=1 M LiPF6, EC/EMC 3:7 by wt). The charge/discharge potential profiles of graphite/ NMC811 cells, taken from the first cycle in BL 1 and BL 1 + 0.5 wt % TPPO as electrolyte, are depicted in Figure 5 (panels a and b, respectively). Figure 5 (panels c and d) shows the enlarged potential profiles of the graphite anodes. In the first charge process, both cathodes reach nearly the same cutoff potential against the reference electrode (≈4.38 V vs Li/Li+), while both anodes reach ≈0.08 V versus Li/Li+. Judging from the anode potential profiles during charge, both electrolytes 2732


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Figure 7. Influence of the addition of 0.5 wt % TPPO on the long-term cycling behavior of graphite/NMC811 cells in BL 1 (=1 M LiPF6, EC/EMC 3:7 by wt) and BL 1 + 0.5 wt % TPPO electrolyte during charge and discharge. Depicted are (a) the discharge capacities as well as (b) the CEs. The discharge capacities until SOH 80% are depicted in green. The calculation of the capacity retention after representative cycles is based on the discharge capacity of the 3rd cycle after the formation process. Cut-off voltage: 2.8−4.3 V.

Figure 8. XPS spectra (F 1s, Mn 2p, O 1s, C 1s, P 2p, and Li 1s) of (top) pristine and harvested NMC811 electrodes taken from discharged cells after 102 cycles in (middle) BL 1 (=1 M LiPF6, EC/EMC 3:7 by wt) and (bottom) BL 1 + 0.5 wt % TPPO electrolyte.

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of great interest. Therefore, XPS analysis was conducted to prove the assumption made above of the SEI modifying ability and to verify a possible influence of TPPO on the cathode performance. The surface analysis may provide a deeper understanding of the working principle of TPPO as electrolyte additive in graphite/NMC811 cells. In the following, XPS spectra and elemental concentrations of pristine and harvested positive and negative electrodes from discharged cells after 102 cycles in BL 1 and BL 1 + 0.5 wt % TPPO are presented in their entirety. 3.3.1. XPS Analysis of the NMC811 Positive Electrodes. The F 1s, Mn 2p, O 1s, C 1s, P 2p, and Li 1s spectra of the pristine and harvested positive electrodes from discharged cells with and without 0.5 wt % TPPO are displayed in Figure 8, respectively. Additionally, the corresponding elemental concentrations (at %) are summarized in Table 3, and the mean

discharge capacity gain was achieved during the CV-step, evidencing that the observed specific capacity loss is caused by active lithium loss rather than by kinetic limitations. In conclusion, the electrolyte additive TPPO strongly suppresses faradaic parasitic side reactions, presumably by forming a more effective, less lithium-consuming and electronically-insulating but lithium ion conducting SEI layer. By comparing the constant voltage behavior during discharge in the work of Kasnatscheew et al.8 and in this work, it can be furthermore concluded that the electrolyte composition does have a strong influence on the impact of kinetic limitations on the observed specific capacity loss. In order to gain a better understanding of the working mechanism of TPPO in graphite/NMC811 cells, the change of the upper cutoff potential of the cathode material during cycling was monitored over time in two different electrolytes. The results for the BL 1 electrolyte and the TPPO-based electrolyte are depicted in Figure 6 (panels a and b, respectively). For the baseline electrolyte, an ongoing increase in the cathode potential is observed, indicating most likely a continuous loss of active lithium.53,61 The increased cathode potential (≈20 mV) is directly accompanied by an increased delithiation degree of the NMC structure during charge, which then may suffer from a surface reconstruction to a rock salt structure and/or oxidative electrolyte decomposition.20,21 By the addition of TPPO to the electrolyte, this increase in cathode potential can be effectively suppressed. After a small initial increase (≈5 mV), the cathode potential remains stable over the whole cycling (300 cycles) and helps to improve the stability of the system. The long-term cycling behavior of NMC 811/graphite cells with and without TPPO-containing electrolytes is depicted in Figure 7. By addition of TPPO, an overall discharge capacity gain of ≈30% based on the 350th cycle with a CE exceeding 99.9% can be achieved. Furthermore, the capacity retention after 295 cycles in the TPPO-containing electrolyte was still above 80% state of health (SOH, based on the discharge capacity after the second cycle), whereas the BL 1 electrolyte reached the end of life (EOL) criteria after 167 cycles. This results in a remarkable capacity retention increase of ≈77% for the TPPO-based cell. In accordance with the above-discussed electrochemical investigations, it can be concluded that TPPO effectively acts as a SEI modifier in order to decrease faradaic parasitic side reactions with, most likely, less active lithium loss in the first and ongoing cycles. Furthermore, a positive effect of TPPO on the overall cathode performance cannot be excluded, especially regarding the strong capacity retention increase. Further electrochemical and spectroscopic investigations were conducted and are presented in the following to elucidate the working mechanism of TPPO as electrolyte additive in graphite/NMC811 cells. 3.3. Analysis of the Interphases and Active Lithium Loss: XPS and ICP-MS Investigations of Pristine and Harvested Electrodes. The chemical and electrochemical anodic and cathodic decomposition of electrolyte components on the specific positive and negative electrode surface results in the formation of unique interphases at both electrode/ electrolyte interfaces, strongly affecting the overall cell performance.31 Against this background, the evaluation of a possible participation of TPPO in the corresponding cathode electrolyte interphase (CEI), as well as in the SEI formation is

Table 3. Mean Atomic Concentration Percentages (at %) (F 1s, Mn 2p, O 1s, C 1s, P 2p, and Li 1s) of (Top) Pristine and Harvested NMC811 Electrodes Taken from Discharged Cells After 102 Cycles in (Middle) BL 1 (=1 M LiPF6, EC/ EMC 3:7 by wt) and (Bottom) BL 1 + 0.5 wt % TPPO Electrolyte element region (%at) pristine electrode BL 1 BL 1 + TPPO

F 1s

Mn 2p

O 1s

C 1s

P 2p

Li 1s

21.78 23.16 22.74

0.33 0.22 0.22

3.94 5.22 6.46

73.94 67.33 66.47

0 0.24 0.31

0 3.83 3.81

atomic concentration percentages (at %) of the specific surface layers are listed in Table S2. The results from the pristine cathode surface are discussed for the purpose of quantitative interpretation of the electrochemically treated samples. The surface of the pristine NMC811 cathode is mainly composed of species related to the conductive carbon and to the PVdF binder. From the F 1s spectrum, the peak at ≈687.5 eV can be attributed to the C−F bonds in PVdF.50 In addition, also a small peak is observed at ≈684.7 eV, which is presumably caused by the presence of LiF on the cathode/electrolyte interface as a remaining impurity of the electrode paste preparation process and due to parasitic dehydrofluorination of PVdF.66,67 A small intensity signal at ≈642.3 eV is present in the Mn 2p spectrum and is related to the NMC811 active material. The O 1s spectrum displays two peaks at ≈531.4 and ≈529.1 eV and a small shoulder at ≈533.4 eV, which is presumably caused by chemically formed R2 CO 3, low coordination oxygen atoms,68 and possibly by Li2CO369 as the remaining surface impurity from the synthesis process. The peak at ≈532.4 eV can be related to oxidized conductive carbon, and the signal at ≈529.1 eV can be assigned to O2− anions in NMC811.70 The C 1s spectrum contains multiple contributions, in particular from the CF2 (CF2CH2; at ≈290.7 eV) and CH2 (CH2CF2; at ≈286.5 eV) groups of PVdF, respectively, and an additional large peak at ≈284.5 eV that results from C−H and C−C groups of the conductive carbon.71,72 As expected, no peaks are present in the P 2p and Li 1s spectra. After constant current cycling in BL 1 (middle) and BL 1 + 0.5 wt % TPPO-based electrolyte (bottom), the specific surface composition on NMC811 changes significantly. In this contribution, the decrease of the Mn 2p peak (≈643 eV) in combination with the decrease of the O 1s peak at ≈529 eV (O2− anions in the NMC811 active 2734


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Figure 9. XPS spectra (F 1s, O 1s, C 1s, P 2p, and Li 1s) of pristine (top) and harvested graphite negative electrodes taken from discharged cells after 102 cycles in (middle) BL 1 (=1 M LiPF6, EC/EMC 3:7 by wt) and (bottom) BL 1 + 0.5 wt % TPPO electrolyte.

material) and the decrease of the C 1s peak at ≈284 eV (conductive carbon) for the cycled electrodes compared to the pristine electrodes indicates the formation of a compact surface film on the cathode, thus, diminishing the peak intensities of the active and inactive cathode materials.71 Due to the same mean atomic concentration percentages (≈0.20 at %) of Mn 3/ 2, it can be assumed that the formed surface layers possess the same thickness for the different electrolyte formulations and allows, therefore, a precise quantification and interpretation of the specific elemental concentrations. From the F 1s spectrum as well as from the mean atomic concentration percentages (cf. Table S2), a distinct increase in the LiF content, which has been identified as a common decomposition product of LiPF6,73 on the surface of the cycled electrodes is clearly visible. However, no difference in the accumulated F 1s elemental concentration of both electrolytes is observed. Judging from the O 1s spectra of the cycled electrodes, the increased peak intensity at ≈533.4 eV arises from the electrochemical decomposition of the electrolyte solvent and salt.74 Even though, the mean atomic concentration percentages for Li2CO3 remain the same for the pristine and the cycled electrodes, a clear difference is found for the alkyl carbonates. With an alkyl carbonate mean value of ≈2.28 at % for the BL 1 compared to ≈2.97 at % for the TPPO-containing

electrolyte, the latter electrolyte formulation shows the highest value for the alkyl carbonates and indicates the CEI modifying ability of TPPO. These findings are further supported by the increased carbonate and phosphate content seen in the C 1s and P 2p spectra as well as in the corresponding mean atomic concentration percentages. The peak at ≈134 eV in the P 2p spectrum is assigned to a P atom that is bound to a less electronegative element than F, such as O, representing the TPPO molecule or its decomposition products. In this context, a peak shift to lower binding energy (BE) is present in the spectrum for the TPPO-based system. Furthermore, a significant increase in the LiPOxFy (≈134.3 eV) content from 0.14 at % in BL 1 to 0.23 at % in the TPPO-containing electrolyte was observed and clearly indicates the participation of the TPPO additive in the CEI layer composition. However, no clear oxidation of TPPO was observed in the LSV measurements (on platinum), as discussed above. Further, the F 1s spectra of LiPOxFy overlap with the peak coming from the PVdF binder and prevents a further identification of these decomposition species. As stated in literature, an increased amount in LiPOxFy species was proven to have a beneficial effect on the electrochemical stability of the CEI surface layer.75 The Li 1s spectra and the elemental concentration of lithium species show no difference for the harvested electrodes. Finally, 2735


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After cycling, the graphite peak intensity at ≈284.5 eV decreases in both electrolyte systems and shows the lowest value for the electrodes cycled in the BL 1 electrolyte. As the intensity of the C 1s peak at ≈284.5 eV is also affected by possible decomposition products, a reliable statement on the SEI thickness based on the graphite signal is not possible without additional sputter depth profiling measurements. The most prominent change in the C 1s spectra is observed in the BL 1 electrolyte. A new peak at ≈282.2 eV appears and is related to the formation of a lithium/graphite species (LiCx). The identification of a metal/carbon species on an electrochemically treated graphite negative electrode, owning a negative BE shift with respect to the graphite signal, was also reported by Niehoff et al.51 This may explain the increased irreversible active lithium loss in the first and ongoing cycles in the BL 1 electrolyte and further explains the reduced discharge capacity compared to the TPPO-containing electrolyte. Combining the results from the O 1s and C 1s spectra, it can be concluded that the nature of the formed SEI in the baseline electrolyte without TPPO contains more organic compounds due to increased solvent reduction products and supported by the higher amounts of R2CO3, carbonates, and PEO species, as found in the measured surface layer composition. The unique nature of the SEI formed in the TPPO-based electrolyte is more inorganic, as a result of increased salt reduction/decomposition products such as LiF. This assumption is further supported by analyzing the Li 1s spectra. The highest peak intensity at ≈55.2 eV together with the highest elemental concentration of Li 1s was observed for the TPPO-containing electrolyte. Finally, the following conclusion of the discussed XPS results for pristine and cycled negative electrodes in different electrolyte formulations appears reasonable: (i) the SEI formed in the baseline electrolyte (BL 1) consists of more organic reduction/decomposition species, predominantly from solvent molecules; (ii) metal/carbon species were found in the surface layer formed in the TPPO-free electrolyte, likely explaining the increased lithium-loss observed during cycling; and (iii) the SEI resulting from the BL 1+TPPO-based electrolyte is mainly composed of inorganic species such as LiF, resulting from the reduction/decomposition of conductive salt species. 3.3.3. Data from the ICP-MS Analysis. In accordance to the electrochemical investigations which identified the observed specific capacity loss after formation as a result of active lithium loss, rather than kinetic limitations during the relithiation of the NMC active material (cf. section 3.2 and Figure S1), also the XPS results from the negative electrodes indicate an increased active lithium loss during the SEI formation process. This process is assumed to be the main reason for the deviation in discharge capacity in the baseline electrolyte compared to the TPPO-based electrolyte. To further verify this suspicion, inductively coupled plasma-mass spectrometry investigations (ICP-MS) from harvested graphite negative electrodes after formation (2 cycles; completely discharged state) were conducted to quantify the active lithium loss and to correlate these results with the deviation of the discharge capacity in the specific electrolytes (Figure 10). The active lithium loss (defined as the remaining mean specific lithium content; average of three cells) of the anode is ≈62% higher in the BL 1 electrolyte (0.53%) compared to the TPPO-containing electrolyte (0.20%). This result clearly evidences an increased active lithium loss during SEI formation in the BL 1 electrolyte, negatively influencing the first cycle CE (≈70% in BL

the XPS results of pristine and harvested positive electrodes may be summarized as follows: (i) after 102 cycles in electrolytes with and without TPPO, a compact surface film on the cathode, formed by electrolyte solvent and salt decomposition products, was confirmed; (ii) the resulting surface films possess the same thickness for the different electrolyte formulations and allow, therefore, a precise quantification and interpretation of the specific elemental concentrations of the surface layers; (iii) the surface film resulting from the electrochemical decomposition of the TPPO containing electrolyte shows an increased amount of alkyl carbonates as well as LiPOxF y species, indicating the participation of TPPO in the CEI formation process. Furthermore, the resulting surface layers are still thin enough (

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