A Bifunctional Electrolyte Additive for High-Voltage LiNi0.5Mn1.5O4

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A Bifunctional Electrolyte Additive for High-Voltage LiNi Mn O Positive Electrode 0.5

1.5

4

Tae Jin Lee, Jiyong Soon, Seulki Chae, Ji Heon Ryu, and Seung M. Oh ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19009 • Publication Date (Web): 04 Mar 2019 Downloaded from http://pubs.acs.org on March 4, 2019

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ACS Applied Materials & Interfaces

A Bifunctional Electrolyte Additive for High-Voltage LiNi0.5Mn1.5O4 Positive Electrode

Tae Jin Lee, † Jiyong Soon, † Seulki Chae, † Ji Heon Ryu,§ and Seung M. Oh*,†

†

Department of Chemical and Biological Engineering, and Institute of Chemical Processes,

Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 151-742, South Korea §

Graduate School of Knowledge-based Technology and Energy, Korea Polytechnic University,

Gyeonggi, 429-793, South Korea

Keywords: Lithium-ion batteries, electrolyte additives, hydrogen fluoride scavenger, filmforming agent, siloxane -1-

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ACS Applied Materials & Interfaces 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

ABSTRACT: 4-(trimethylsiloxy)-3-pentene-2-one (TMSPO) is tested as an electrolyte additive to enhance Coulombic efficiency and cycle retention for Li/LiNi0.5Mn1.5O4 (LNMO) half-cell and graphite/LNMO full-cell. TMSPO carries two functional groups; siloxane (-SiO-) and carbon-carbon (C=C) double bond. It is found that the siloxane group reacts with hydrogen fluoride (HF), which is generated by hydrolysis of lithium hexafluorophosphate (LiPF6) by impurity water in the electrolyte solution, to produce 4-hydroxypent-3-ene-2-one (HPO). The as-generated HPO, as well as TMSPO itself, is electrochemically oxidized to form a protective surface film on the LNMO electrode, in which it is inferred that the carbon-carbon (C=C) double bond initiates radical polymerization. The surface film derived from the TMSPO-added electrolyte shows a superior passivating ability to that generated from the pristine (TMSPO-free) electrolyte. The suppression of electrolyte oxidation enabled by the superior passivating ability offers two beneficial features to the half-cell and full-cell; the suppression of both HF generation and deposition of resistive surface film on LNMO. As a result, the metal dissolution by HF attack on LNMO appears to be smaller by addition of TMSPO. The cell polarization is also less significant due to the latter beneficial feature. In short, the bifunctional activity of TMSPO (HF scavenger and protective film former) allows an enhanced Coulombic efficiency and cycle retention to the half-cell and full-cell.

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■ INTRODUCTION Recently, the lithium-ion batteries (LIBs) have been required to much higher energy density and power for improved electric vehicles (EVs) and energy storage system (ESS) applications. Voltage is closely related to both energy density (E = ∫ Q × V) and power (P = V × I), and thus some high-voltage positive electrodes are reviewed to achieve these two properties. Among these materials, the nickel-doped manganese spinel (LiNi0.5Mn1.5O4, hereafter LNMO) reversibly reacts beyond 4.6 V (vs. Li/Li+) with stable structural stability,1-6 while the commercially used layered materials (e.g. LiCoO2, LiNi1/3Co1/3Mn1/3O2) are usually cut-off up to 4.3 V due to their structural instability beyond 4.3 V.7,8 The advantageous features of the LNMO positive electrode, however, are not highlighted by instability of the commercially used organic electrolytes over at 4.3 V, the electrochemical stability windows of the electrolytes.9,10 Unlike the passivating solid electrolyte interphase (SEI) on negative electrodes,11-14 the initially generated surface film on the positive electrode is poorly passivating.15-19 The high-voltage positive electrode is thus exposed to continual electrolyte oxidation, which gives rise to concomitant gas evolution and film deposition at electrolyte/electrode surface during long life cycling.20 Furthermore, hydrofluoric acid (HF) derived from hydrolysis of lithium hexafluorophosphate (LiPF6) by trace water (H2O) leads to transition metal dissolution into electrolyte,20-22 which causes capacity fading for the LNMO electrode and deteriorates SEI properties on negative electrodes (e.g. graphite) in full-cells.23 Some electrolyte additives could be employed to mitigate these undesirable parasitic reactions. There are various ways, in which electrolyte additives work; for example, filmforming type to prevent further electrolyte decomposition at electrolyte/electrode interface and -3-



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scavenger type to capture HF, a transition metal-corrosive acid, in electrolytes.24,25 It was previously reported that siloxane (-Si-O-) or silazane (-Si-N-) derivatives could capture water (H2O) or HF in electrolytes.26-31 Carbon-carbon double bond (-C=C-) is able to be electrochemically polymerized, in which whether the electrochemical reactions are anodic or cathodic.32 In this work, it was tested, if an electrolyte additive involving the two functional groups, siloxane and carbon-carbon double bond, could play roles in HF scavenging and passivating surface film-forming simultaneously for the LNMO positive electrode. As is seen in the previous abstract figure, the tested 4-(trimethylsiloxy)-3-pentene-2-one (hereafter TMSPO) has the two functional groups, siloxane and carbon-carbon double bond with carbonyl. For the effects of TMSPO, this electrolyte additive shows a much higher Coulombic efficiencies and improved capacity retention for the Li/LNMO half-cells and graphite/LNMO full-cells. The mechanisms, this additive works, were investigated in two respects; chemical and electrochemical reactions. The scavenging chemical reaction was examined by

19

F,

29

Si

nuclear magnetic resonance (NMR) and Fourier-transform infrared spectroscopy (FT-IR); the chemical bond changes of TMSPO with HF were monitored before and after its addition into electrolytes. After the chemical reaction, it was ascertained that the TMSPO-added electrolyte is electrochemically oxidized on the LNMO electrode during the first charging process through differential capacity plot. The relative electrochemical oxidation tendency and the first evolved radical intermediate were inferred by ab-initio calculation. X-ray photoelectron spectroscopy (XPS) informs that this TMSPO-added electrolyte generates surface film on the LNMO after the first charging process. The initially formed surface film by electrochemical oxidation of TMSPO and its chemical product 4-hydroxypent-3-ene-2-one (hereafter HPO) prevents additional oxidative electrolyte decomposition to mitigate increase in polarization of the -4-


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LNMO electrode, which is ascertained by chronoamperometry, voltage profiles, electrochemical impedance spectroscopy, and field emission-scanning electron microscope (FE-SEM). The overall accumulated HF concentrations, and their resulting dissolved transition metal ion quantities were examined by

19

F NMR and inductively coupled plasma-atomic

emission spectroscopy (ICP-AES). Finally, it was confirmed that this additive mitigates polarization increment at electrolyte/electrode interface and captures corrosive acid (HF) to reduce dissolution amounts of transition metal ions into electrolytes.

■ EXPERIMENTAL SECTIONS Electrolyte preparation. TMSPO (≥ 97 %, liquid) was purchased from Sigma-Aldrich corp. This reagent was added in 1.3 M lithium hexafluorophosphate (LiPF6) in ethylene carbonate (EC): ethyl methyl carbonate (EMC): diethyl carbonate (DEC) = 3:2:5 (v/v/v) at a weight percent (wt.%) of 0.2 (13.1 mM) and 1.0 (65.5 mM). NMR spectroscopy. The chemical reactions of TMSPO in a LiPF6/carbonates-based electrolyte was monitored through 19F, 29Si NMR spectroscopy. The additive and all electrolyte samples had been mixed in CDCl3 solvent, and the dissolved solutions were delivered to Bruker Avance-300 and 500 (19F; 300 MHz, 29Si; 500 MHz) NMR instruments. The used probes were 5 mm QNP probehead (19F) and 5 mm BBFO probehead (29Si). The resonance frequencies (frequency at chemical shift (δ) = 0 ppm) were followings; 19F: 282. 40 MHz, 29Si: 99.36 MHz. Chemical shifts in all spectra were shown through TopSpin 3.X software program. Additionally, the HF quantities in the cycled electrolytes were examined by

19

F NMR spectroscopy. The

cycled electrolytes were prepared; after the separators in the 100th cycled (at 60 ℃)-Li/LNMO -5-



coin-cells were immersed in 1 mL of the uncycled background electrolyte (1.3 M LiPF6 in EC:EMC:DEC = 3:2:5 (v/v/v)) for 24 hour, the obtained electrolyte solutions were delivered to the NMR instrument without air exposure. FT-IR spectroscopy. FT-IR spectroscopy for the TMSPO, the background electrolyte, and their mixed composition were observed. All liquid solutions had been delivered to FT-IR instrument (Bruker, TENSOR27) without air exposure, and scanned with an attenuated total reflection (ATR) mode. Preparation of electrodes and electrochemical cells. LiNi0.5Mn1.5O4-δ (LNMO, JCPDS #: 802162, Fd3̅m, Brunauer-Emmett-Teller (BET) surface area = 0.4279 m2 g-1, LG chem. Corp.) or artificial graphite (JCPDS #: 080415, P63/mmc, BET surface area = 3.17 m2 g-1, LG Chem. Corp.) were uniformly dispersed in N-methyl-2-pyrrolidone with a conducting carbon (SuperP, LG chem. Corp.) and poly(vinylidenefluoride) (PVdF, LG chem. Corp.). The dispersed slurries were loaded on Al (LNMO) and Cu (graphite) foils by using doctor blade. The composite electrodes were dried at 120 ℃, and the electrodes plates were followed to be pressed to increase contact and adhesion between electrode constituents. The LNMO electrodes (the loaded mass/thickness) were punched into 11 mm of diameter-circle (LNMO; 4.0 mg/45 μm, Al; 5.56 mg/21 μm), and the graphite electrodes were done into 13 mm of diameter-circle (graphite; 2.5 mg/35 μm, Cu; 11.74 mg/10 μm). The punched electrodes were dried at 120 ℃ under vacuum overnight. In full-cells, the N/P ratio was controlled to 1.1, in which the capacity was based on the reversible discharging capacities of LNMO and graphite electrodes in Li halfcells. The Li/LNMO half-cells and graphite/LNMO full-cells were assembled with polypropylene/polyethylene/polypropylene (PP/PE/PP) separator and the above-composition of electrolytes in CR2032 coin-type cell. -6-


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Electrochemical measurements. The galvanostatic charging/discharging cycling was conducted through WBCS 3000 cycler. The Li/LNMO half-cells were cycled at 3.5 ~ 4.9 V (vs. Li/Li+) with 65 mA g-1 (0.5 C) of current density at 25 / 60 ℃, and constant voltage was applied at 4.9 V to charge the LNMO electrode completely. The graphite/LNMO full-cells were cycled at 3.5 ~ 4.8 V without constant voltage (CV) charging at 4.8 V, the cut-off voltage. Ac impedance (CHI instrument 660B) was performed to examine surface resistances for the cycled LNMO electrodes. Alternative current was applied to the LNMO/LNMO symmetric cells from 100,000 to 0.005 Hz (frequency range) with 5 mV of amplitude at initial and later cycling. The LNMO electrodes were cycled with each electrolyte in the form of Li/LNMO half-cells to be cut-off at 4.9 V. After disassembling Li/LNMO half-cells, the cycled LNMO electrodes were cut in half, and followed to be re-assembled in each symmetric cell. The Nyquist plots of the cycled LNMO electrodes at 60 ℃ were obtained at 25 ℃, and that of the initially film-formed LNMO electrodes at 25 ℃ were done at 25 ℃. Passivating abilities for the initially film-formed LNMO electrodes by the TMSPO-added and its free electrolytes were examined through chronoamperometry and chronocoulometry at 4.9 V at 60 ℃. After the Li/LNMO half-cells were cycled in three times to be cut-offed at 4.9 V at 25 ℃, the resulting current and capacity were monitored with applying constant voltage to the electrochemical cells at 4.9 V at 60 ℃. Ab-initio calculation. The ab-initio calculations for highest occupied molecular orbital (HOMO) of the electrolyte components (carbonates, salt anion), TMSPO, and its chemical reaction products were performed by the General Atomic and Molecular Electronic Structure System (GAMESS), which runs in super computer owned by Korea Institute of Science and Technology Information (KiSTi). Their geometries were optimized by restricted open shell -7-



Hartree-Fock (ROHF) 6-31++G(d,p) in gas phase. Through Wxmacmolplt software program, their input files were created and the calculated HOMO / singly occupied molecular orbital (SOMO) were visualized. For visualizing the orbitals, after the geometries of the neutral molecules (charge = 0, spin multiplicity S=2s+1=1) had been first optimized to confirm HOMO, by creating input files from the first optimized neutral molecules with charge =+1 and S=2s+1=2, the one electron elimination structures such as radical cation were optimized to ascertain the corresponding SOMO. XPS. The initially generated surface film was observed by X-ray photoelectron spectroscopy (XPS) with depending on the TMSPO concentrations. The Li/LNMO half-cells were charged to 4.9 V, and followed to be disassembled in argon-filled glove box. After washing the disassembled LNMO electrodes with DEC to remove adsorbed electrolyte species, the obtained LNMO electrodes were delivered to XPS instrument with being sealed in vials not to be degraded by air or moisture contact. Al Kα (1486.6 eV) X-ray source was irradiated on a 400 μm of spot diameter in the LNMO electrodes. Based on the C 1s hydrocarbon peak (285.0 eV), all binding energies for each functional group were calibrated and fitted through using AVANTAGE 4.19 software program. The peak variations were just allowed within ±0.2 eV. For the fitting, the employed parameters are followings; the full-width at half-maximum (FWHM) (eV) = 0.5:3.5, and Lorentz/Gaussian = 20:40. FE-SEM. The cycled (at 60 ℃) LNMO electrode surface images were observed through FE-SEM (JEOL Ltd., JSM-6700F) instrument. The cycled LNMO electrodes were washed with DEC. Acid titration. Acid in the initial background and cycled electrolytes (at 60 ℃) were titrated with NaOH solution by using acid-base titrator. For the cycled electrolytes, the separators in -8-


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the Li/LNMO cells cycled 100 times at 60 ℃ were immersed in 1 mL of the uncycled background electrolyte (1.3 M LiPF6 in EC:EMC:DEC = 3:2:5 (v/v/v)) for 24 hours. After eliminating the separators, the resulting solutions were delivered to the instrument. Total acid concentrations in the diluted solutions were converted to concentrations for the coin-cells by considering the dilution factor (initial electrolyte amount in coin-cells; 120 μL). ICP-AES. Transition metals (Ni, Mn) dissolution of the LNMO electrodes after cycling at 60 ℃ were measured through inductively coupled plasma-atomic emission spectroscopy (ICPAES, OPTIMA 4300DV). Preparation procedure of the cycled electrolyte solutions and concentration conversion method are same as in acid titration.

■ RESULTS AND DISCUSSION Usually, electrolyte solutions inevitably seem to allow moisture contamination, even though they are handled in dried conditions (argon-filled glove box or dry room). The trace water reacts with PF5, LiPF6 (the conventionally used lithium salt) salt-decomposed product by equilibrium, to generate hydrofluoric acid (HF),22,28 which is corrosive acid toward transition metal of positive electrodes.20 Some previous literatures reported that the trace water and the resulting HF concentrations were below 50 ppm in fresh electrolytes.27,33 For the used background electrolyte (1.3 M LiPF6 in EC:EMC:DEC = 3:2:5 (v/v/v)) in this work, HF concentration was titrated to 46 ppm. It was examined, if HF is scavenged when TMSPO is added into the background electrolyte. The NMR and FT-IR spectra in Fig. 1 show changes of chemical species and chemical bonds in the background electrolyte before and after adding this additive. It was summarized for the chemical shifts and coupling constants of the previous references -9-



(ref.) and observed results (obs.) in table 1.27,30,34-41 In Fig. 1a, the previously titrated 46 ppm of HF in the background electrolyte solution is observed. After adding 0.2 wt.% or more of the TMSPO into the electrolyte, the HF peak disappears and trimethylsilyl fluoride ((CH3)3SiF, hereafter TMSF) peak evolves (Figs. 1bc). The HF concentration after TMSPO addition was analyzed to be; 8 ppm after 0.1 wt.% addition, and 0 ppm both after 0.2 wt.% and 1.0 wt.% addition. It is confirmed by 29Si NMR (Fig. 1cd), whether HF removal is done by TMSPO. The siloxane bond (-Si-O-) of the TMSPO (liquid at 25 ℃) solution is observed at 19.4 ppm with multiplet (Fig. 1d). The two bonds-connected 13C/1H from 29Si could spin-couple with 29Si.40,41 In detail, for the (CH3)3Si-X-C bond structure (X; O or N), the two bonds-connected 13C could lead to spin-coupling with 29Si (2JSi-C in table 1); the oxygen-connected C in Si-O-C of TMSPO, causes doublet (d) in

29

Si NMR. In addition, nine protons (1H) in trimethylsilyl group

((CH3)3CSi-, hereafter TMS), another two bonds-connection, could also lead to spin-coupling with 29Si (2JSi-H in table 1) in the (CH3)3Si-X-C bond structure, which is thus shown in multiplet (m) in

29

Si NMR spectra. Resultantly, siloxane bond (-Si-O-) in TMSPO is observed in

multiplet (m) in 29Si NMR spectra by these two spin couplings (by 1H and 13C) (Fig. 1d). After TMSPO is added in the background electrolyte over one week (time to deliver to the NMR instrument after its addition), TMSF is observed and siloxane bond intensity is lower than the TMSF one. For the

29

Si NMR, the chemical bond change from siloxane to TMSF was not

clearly observed at 0.2 wt.% (Fig. 1e), however, was certainly ascertained at 1.0 wt.% (Fig. 1f). Note that TMSF is observed in doublet (d) in 29Si NMR spectra by the neighboring 19F. This indicates that siloxane bond favorably cleavages to capture F- of HF with co-considering 19F NMR (Figs. 1abc). Proton (H+) in HF is also scavenged by TMSPO, which is ascertained by FT-IR (Fig. 1g). The hydroxyl bond (-OH), strong broad vibration from 3200 to 3550 cm-1,42,43 - 10 -


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is not observed in the TMSPO alone and background electrolyte, however, it is remarkably seen in the two TMSPO-added electrolytes. That is to say, the oxygen of siloxane bond scavenges proton of HF to form HPO (4-hydroxypent-3-ene-2-one, in the inset in Fig. 1g). Siloxane group reacts with HF, in which silicon scavenges fluoride (F-) and oxygen captures proton (H+) during electrolyte preparation. This means that acidity of electrolyte solution decreases. Note that pKa of HF is 3.2 while alcohols are usually in the ranges from 16 to 18 in aqueous solution.42,44 Furthermore, an alcohol (HPO) is much less corrosive toward transition metal oxide (LNMO electrode) than HF.

Figure 1. NMR and FT-IR spectra for the TMSPO additive and electrolyte compositions before and after its addition in 1.3 M LiPF6 in EC:EMC:DEC = 3:2:5 (v/v/v). (a), (b), (c); 19F NMR, (d), (e), (f); 29Si NMR, (g) FT-IRs. The detailed compositions are indicated in the subtitle or inset. - 11 -



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Table 1. Chemical shifts and coupling constants for the NMR spectra in Figs. 1a-1d. (s; singlet, d; doublet, m; multiplet, nJx-y: n; numbers of connected bonds, x-y; coupled atoms)

19

F

chemical shift δ / ppm

coupling constant J / Hz

Ref.

Obs.

Ref.

Obs.

PF6-

-72.7

-74.9

d, 708

d, 1JF-P / 669

TMSF

-158

-158.5

s

s

HF

-189

-190.9

s

s

29

Si

chemical shift δ / ppm Ref.

Obs.

-Si-O-

6~20

19.6

TMSF

30.5

33.2

coupling constant J / Hz Ref.

Obs.

2

JSi-C, 2JSi-H m, 2JSi-C, / ≤18, ≤10 2JSi-H / 99 d, 1JSi-F / 267

d, 1JSi-F / 276

∙

After this chemical reaction, the TMSPO-added electrolyte electrochemically reacts on the LNMO electrode during initial cycling. Differential capacity curves were compared between TMSPO-added and free electrolytes (Figs. 2ab). At 3.6 V (vs. Li/Li+), the TMSPO-added electrolyte initiates electrochemical anodic reaction on the LNMO electrode surface, and the main reaction peak is observed at 4.4 V (Fig. 2a). This is clearly ascertained by its 1.0 wt.%added electrolyte. This anodic reaction in the first charging is irreversible, and it does not occur in the second charging (Fig. 2b).

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Figure 2. Differential capacity plots for the Li/LNMO half-cells with the TMSPO-free and added electrolytes during initial cycling. (a); first charging, (b); second charging. Ab-initio calculations for the electrolyte components and TMSPO additive. (c); HOMO energies in gas phase, (d); the HOMO and corresponding SOMO for TMSPO and its chemical reaction product, HPO.

In order to understand the earlier oxidation of TMSPO-added electrolyte, HOMO energies were calculated for TMSPO-related species and electrolyte components. Fig. 2c exhibits HOMO energies for electrolyte components with TMSPO and its chemical reaction products (HPO and TMSF). HPO as well as TMSPO shows higher HOMO energies than other electrolyte components, carbonates and salt anion. Higher HOMO energies and the earlier - 13 -



anodic reaction with TMSPO adding, these two features indicate that TMSPO anodically electro-reacts prior to the background electrolyte on the LNMO electrode surface. In order to infer its electrochemical oxidation reaction mechanism; which bonds are involved in this electrochemical reaction, it was calculated, which radical cation is generated when one electron is removed from the TMSPO and HPO. Fig. 2d visualizes HOMO and the corresponding SOMO. First, whether TMSPO or HPO, carbon-carbon double bond and oxygen atoms (carbonyl/siloxane/alcohol) are involved in HOMO. When one electron leaves from TMSPO or HPO; when we observe the corresponding SOMO of HOMO, it is seen that π bond of the carbon-carbon double bond is severely distorted than oxygen atoms. In detail, in the SOMO, the orbital lobe for the β position carbon from the carbonyl group is slightly decreased while that for the α position carbon is still noticeable. It needs to be reminded that molecular orbital means the physical region or space where electrons are occupying in the bond. In other words, when TMSPO or HPO are electrochemically oxidized, an electron leaves from the β position carbon to generate a positive charge at the β position and a radical at the α position. Furthermore, new double bond is observed between the β position carbon and the neighboring oxygen (siloxane/alcohol) in the radical intermediates. This supports that the generated positive charge at the β position carbon could be delocalized by the lone pair electrons of the neighboring oxygen; the positive charge could be resonance stabilized. If the radical cation intermediate polymerizes to form surface film, it could be inferred that the α radical carbon would initiate polymerization reactions. After ascertaining its resulting surface film products (Fig. 3), these possible electrochemical mechanisms are finally summarized to be suggested in the latter Fig. 4.

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By using XPS instrument, it is observed for the TMSPO-added electrolyte to generate surface film on the LNMO electrode after the first charging (Fig. 3). It was summarized for the binding energies of the observable functional groups in surface films on LNMO electrode in table 2.17,19,33,45-57 Figs. 3a-3c show Si 2p XPS spectra, in which any intensities for silicon bonds are negligible. Figs. 3d-3f and 3g-3i show C 1s and O 1s XPS spectra. When the TMSPO additive is more added, organic surface film components, such as polyethylene oxide (-C-O-) or carbonyl (-C=O), becomes richer and the lattice oxygen intensities more decrease. In initial cycles, the TMSPO-added electrolytes derive thicker organic films on the LNMO electrodes than the background electrolyte. The absence of silicon moieties on the LNMO surface could be understood as followings. As is seen in the Fig. 1d, when TMSPO is added in electrolytes, most of siloxane bond cleavages to form TMSF, and thus the remained siloxane bond amounts are low. The HOMO energy for the TMSF is the lowest one among electrolyte compositions (Fig. 2c), it is thus likely that TMSF does not be electrochemically oxidized to deposit in a forms of surface films on the LNMO electrodes. In addition, the original TMSPO is not major component anymore in total additive molecules after the chemical reaction, and the major species are rather the siloxane cleavage forms (HPO, Figs 1.de). The HPO must be involved in this film-forming reaction. If they do not contribute this film formation, the increase for the oxidation current and concomitant film thickness would not be observed when increasing the additive quantity. Of course, TMSPO can also contribute to film generation, in which the silicon moiety is not also deposited.

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Figure 3. Si 2p; (a), (b), (c), C 1s; (d), (e), (f), and O 1s; (g), (h), (i) XPS spectra for the LNMO electrodes with the TMSPO-free and added electrolytes after the first charging. All LNMO electrodes were cut-off at 4.9 V (vs. Li/Li+), and washed with DEC before being delivered to the instrument.

Table 2. The binding energies for the observable functional groups in surface films on LNMO electrode (in eV). Si 2p -Si-C-Si-O-C-Si-O-Si-F

C 1s 100.8 102.2 103.0 104.5

-CC-/-CH-C-O-O-C=O C=O PVdF (-C-F)

O 1s 285.0 286.7 287.8 289.1 290.7

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Lattice O -C=O -C-OPFyOz

530.3 532.2 533.5 534.6

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The chemical and electrochemical two-step reactions are summarized in Fig. 4. First, chemical reaction occurs before the electrochemical cells are cycled. HF, generated from trace water and PF5 (LiPF6), which is scavenged by siloxane functional group of TMSPO; fluoride (F-) is captured by silicon (Si) to generate TMSF and proton (H+) is also taken by oxygen (O) to form HPO. After this chemical reaction, the as-generated HPO is electrochemically oxidized to be a radical cation, which carries positive charge on the β position carbon from carbonyl group and radical on the α carbon during the first charging process. This radical generation is reasonable, which is because the neighboring oxygen of alcohol in HPO provides its lone pair electrons to the positive β position carbon, and thus the positive charge could be delocalized so that the radical cation to be stabilized. Further reactions are difficult to be predicted precisely, however, it seems likely that the resulting α radical carbon would initiate polymerization with the adjacent solvating carbonates to form organic surface films on LNMO electrode. It is thus suggested as a possible electrochemical reaction mechanism such as in Fig. 4-2. In the case of the TMSPO electrochemical oxidation, it is inferred that the -Si-O- bond cleavage reaction is followed after the radical intermediate is generated (Fig. 4-3). The resulting trimethylsilyl+ (TMS+) fragment seems not to be involved in surface film components.

- 17 -



Figure 4. The reaction mechanisms of TMSPO in LiPF6/carbonates-based electrolytes. 1); chemical reaction, 2); possible electrochemical reaction (1st charging), and 3) possible electrochemical reaction of TMSPO itself (1st charging).

After initial chemical and electrochemical reactions, it was examined, if the TMSPO-added electrolyte improves cycle life for the LNMO electrode through Li/LNMO half-cells and graphite/LNMO full-cells. For the Li/LNMO half-cells, the Coulombic efficiencies and its resulting cycle performances are improved at 25 / 60 ℃ by adding 0.2 wt.% of TMSPO into the background electrolyte (1.3 M LiPF6 in EC:EMC:DEC = 3:2:5 (v/v/v)) (Figs. 5ab). For the graphite/LNMO full-cells, as well as for the Li/LNMO half-cells, it is also observed that Coulombic efficiencies and cycle lives are enhanced by the TMSPO-added electrolyte at 25 / 60 ℃ (Figs. 5cd). Note that TMSPO has a better beneficial effect on the cycle stability as compared to tris(pentafluorophenyl)silane, which has been tested as an electrolyte additive in - 18 -


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our previous work.56 Notably, the optimal quantity for this additive needs to be discussed. As is seen in Fig. 5b, the 1.0 wt.% of TMSPO-added electrolyte does not improve the Li/LNMO cycle life at 60 ℃. At lower concentration of 0.2 wt.%, the improvement is observed. It is suggested in this work that excessive addition of TMSPO (1.0 wt.%) has a harmful effect due to PF5 generation. Namely, the HF in the electrolyte solution (46 ppm initially) is removed by the scavenging action of TMSPO. The HF titration results indicate that HF concentration was 8 ppm upon addition of 0.1 wt.% of TMSPO. When the TMSPO addition was increased (0.2 wt.% and 1.0 wt.%), HF concentration was 0 ppm. This means that, when 1.0 wt.% is added, the extra amount of the unreacted TMSPO still remains in the electrolyte solution. One interesting feature in the

19

F NMR spectra (Figs. 1abc) is that the peak intensity for TMSF

increased up to 2.8 times when TMSPO concentration was 1.0 wt.%. TMSF is generated by Fattack on TMSPO. Hence, F- sources are needed. Obviously, one F- source is HF in the electrolyte solution. When 1.0 wt.% of TMSPO is added, HF is totally removed (0 ppm). Here, for the excessive formation of TMSF (2.8 times larger), other F- sources are needed. Only possible F- source in the electrolyte solution is PF6- in this work. Accordingly, the reaction between TMSPO and PF6- should be considered to explain the excessive formation of TMSF. It is known that PF6- readily decomposes to release F- and PF5.58 Hence, the reaction between TMSPO and PF6- to generate TMSF is very likely. A feasible reaction mechanism is proposed in Fig. S1. If this is really the case, PF5 formation should be assumed. PF5 is very reactive as a strong Lewis acid. This reactive gas can attack the organic carbonates (PC, EC, DEC etc.).59 It can attack surface films on electrodes to deteriorate cell performance.60 At present, the worse cycle life observed with 1.0 wt.% TMSPO addition is not clearly explained. However, a possible origin is suggested. Detailed study is needed. In addition, the HF concentration would - 19 -



depend on the used electrolytes, and it can also change during storage. Due to this uncertainty in HF concentration, the optimal concentration of TMSPO cannot be predetermined. Namely, if the HF concentration in the used electrolytes is less than that for this experiment (46 ppm), the optimal TMSPO concentration for this work (0.2 wt.%) would be excess, which decreases the cell performance instead of increase. Back to the performance improvement, it is ascertained that this improvement occurs on the LNMO electrode rather than the graphite electrode in the full-cell. Fig. S2 and S3 exhibit XPS spectra on the graphite electrode surface after the first charging and Coulombic efficiencies/cycle lives of the Li/graphite half-cells. Any special differences for SEI components are not observed on the graphite surfaces (Fig. S2) whether adding TMSPO additive or not, which indicates that electrolytes containing this additive generates surface films by oxidation reaction more preferentially than that by reduction reaction. Any noticeable improvement of Coulombic efficiencies and cycle lives are also not shown (Fig. S3). These results support that the TMSPO-added electrolyte has an effect on the LNMO positive electrode rather than graphite negative electrode in the full-cells. Therefore, the subsequent results are the analysis for the improvement effect on the LNMO positive electrode.

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Figure 5. The galvanostatic charging/discharging cycling for the LNMO electrodes with TMSPO-free and its 0.2 or 1.0 wt.%-added electrolytes at 25 and 60 ℃. (a), (b); the Li/LNMO half-cells, (c), (d); the graphite/LNMO full-cells.

It is exhibited for the voltage profiles for the Li/LNMO cells at 60 ℃ in Figs. 6ab. During charging/discharging cycling at high temperature for 100 times, server polarization and the concomitant capacity fading (capacity below 40 mA h g-1) are observed for the TMSPO-free electrolyte (Fig. 6a). In contrast, when the TMSPO is added in the electrolyte, the polarization increase is significantly mitigated to deliver more than 100 mA h g-1 of capacity even after 100 cycles (Fig. 6b). In order to confirm, what is responsible for the mitigation of polarization increment, ac impedance was measured for the LNMO electrodes before and after cycling (Figs. 6cd). All the cycled LNMO electrodes (SOC 100) were disassembled, and followed to be reassembled in each symmetric cell. Each semi-circle indicates their commonly accepted - 21 -



meaning.61-63 After the first cycle formation (before cycling at 60 ℃), film and charge transfer resistances for the TMSPO-added electrolyte tends to be a little larger than for the TMSPOfree electrolyte, however, the difference is not significant (Fig. 6c). This is very encouraging property, which is because TMSPO induces a little thicker organic film on the LNMO electrode than its free electrolyte composition in initial cycles (Fig. 3). Even though the initial resistances at interface is slightly larger, the electrolyte containing this additive shows much smaller increment for the film and charge transfer resistances after long term cycle life (Fig. 6d). This mitigation is numerically ascertained through their fitted values in the inset table in Fig. 6d.

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Figure 6. Voltage profiles for cycling of the Li/LNMO half-cells with (a) TMSPO-free and (b) added electrolytes at 60 ℃. Ac impedance Nyquist plots (measured at 25 ℃, SOC 100) for the re-assembled LNMO/LNMO symmetric cells (c) before and (d) after cycling at 60 ℃. The equivalent circuit and the fitted values are indicated in each (c)/(d) inset.

The cycled LNMO electrode surfaces were visually observed through FE-SEM (Fig. 7). Unlike the pristine LNMO surface in Figs. 7ab, heavy surface films are certainly visible on the LNMO surface by being cycling at 60 ℃ with TMSPO-free electrolyte (Fig. 7c). This indicates that the cycling at high temperature induces severe electrolyte decomposition and its resulting surface film growth, which is responsible for significant increase of the previous film and charge transfer resistances (Figs. 6cd). The thick surface film growth during cycling, however, is seen to be mitigated for the cycled LNMO electrode with the TMSPO-added electrolyte (Fig. 7d). The accumulated film does not seem to be as thick as for that of the additive-free electrolyte. The previous less increase of the film resistance by adding TMSPO (Fig. 6cd) is visually ascertained.

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Figure 7. FE-SEM images for the LNMO electrodes surfaces. (a); pristine LNMO particle, (b); pristine LNMO surface, (c); the 100th-cycled LNMO electrode with TMSPO-free electrolyte at 60 ℃, (d); the 100th-cycled LNMO electrode with TMSPO 0.2 wt.%-added electrolyte at 60 ℃.

The passivating abilities were examined for the initially generated surface films by the two electrolytes. The third charged LNMO electrodes by each electrolyte (TMSPO-free and 0.2 wt.% added) were applied to constant high-voltage (4.9 V) at 60 ℃ (Fig. 8). At the 4.9 V of highvoltage, which is beyond electrochemical stability of the background electrolyte7,8 additional electrolyte oxidation could evolve. It was thus tested, how the initially deposited two surface films are passivating. The monitored decay current for the TMSPO-added electrolyte is lower than that for the additive-free one; the surface film derived from TMSPO-added electrolyte is much more resistive against electrolyte oxidation (Fig. 8a). This ascertains that the surface film - 24 -


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is given a stronger passivation role by TMSPO adding (Fig. 3). The TMSPO-added electrolyte less allows further electrolyte oxidation and the concomitant film growth after its initial cycling. Furthermore, it is observed that the decay current increases a little after 10 h for the TMSPOfree cells, which indicates that the initial surface film is vulnerable in thermal condition so that it would be degraded to allow significant electrolyte decomposition. In contrast, this behavior does not occur for the TMSPO-added cells, which supports that the thermal stability of the initial generated film becomes improved by TMSPO-adding. The integrated capacities calculated from the two decay currents (Fig. 8a) were compared in Fig. 8b. After 18 h, the total accumulated capacity is over 100 C in the TMSPO-free cells, while the cumulative capacity is much low (below 80 C) in the TMSPO-added cells. This supports that electrolyte oxidation and the resulting film growth are less significant in the TMSPO-added cells than in the TMSPO-free one for the long-term cycles, as previously ascertained in Coulombic efficiencies (Fig. 5) and surface visual images (Fig. 7).

- 25 -



Figure 8. Chronoamperometry (a) and Chronocoulometry (b) for the initial films-deposited LNMO electrodes derived by TMSPO-free and added electrolytes. After the Li/LNMO halfcells had been charged to 4.9 V (the third cycle) at 25 ℃, the electrochemical cells were applied to 4.9 V of constant voltage at 60 ℃. The loaded-LNMO masses are equal to 4.0 mg.

When the LNMO electrodes are cycled, capacity fading also occurs by irreversible degradation as well as increase of resistances. The irreversible capacity degradation usually arises from transition metal dissolution by HF attack in electrolytes, 20-23 rather than the structural instability of the LNMO electrode. The

19

F NMR spectra in Fig. 9 show HF

intensities for the cycled Li/LNMO cells at 60 ℃. If Figs. 9a and 9b are compared, the - 26 -


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accumulated HF intensity during cycling becomes lower, when TMSPO is added in the Li/LNMO cells. The acid concentrations in electrolytes were quantified by acid titration, in which most of the titrated acid species were assumed to HF. As is seen in table 3, the generated HF concentration during cycling becomes lower by TMSPO-adding in the background electrolyte. This decrement is attributed to the chemically induced initial HF scavenge reaction (Figs. 1ab). This decrease, however, cannot be totally understood by this initial HF capture reaction. Hence, some other effects should be considered for the decreased acid concentration. There are two paths in which HF is generated; chemical hydrolysis of LiPF620-22, or electrochemical co-oxidation of PF6- and carbonates64 during charging. HF generation by the latter electrochemical reaction also could be prevented by the passivation role of surface film derived from the TMSPO-added electrolyte, as well as the initial chemical HF scavenging reaction. As explained in Fig. 8, the additive-added electrolyte improves initial passivation ability of film on LNMO electrode to lead to prevent additional electrolyte oxidation at highvoltage (4.9 V). This decay current involves the electrochemical co-oxidation of carbonates and PF6-, so that the total generated HF concentration after cycling could be reduced by the surface film. Additionally, here in table 3, it is re-ascertained for the high passivation ability of the initial film by TMSPO-added electrolyte. In detail, when the TMSPO is added in electrolytes, after long term cycling, the total evolved HF concentration decrease about to two thirds (2/3), while the resulting transition metal dissolution quantity is more reduced to less than one third (1/3). Namely, the additional generated HF amount was reduced, however, the transition metal dissolution amounts by its attack was much more reduced. This could be accounted for by the high passivation role of the generated film by TMSPO-added electrolyte.

- 27 -



The surface film, derived from the TMSPO-adding, prevents HF attack as well as electrochemical oxidation of electrolytes.

Figure 9. 19F NMR spectra for (a) TMSPO-free and (b) TMSPO 0.2 wt.%-added electrolytes collected from the Li/LNMO cells after 100 times cycling at 60 ℃. After the separators in the cycled Li/LNMO cells with and without TMSPO were immersed in 1 mL of the uncycled 1.3 M LiPF6 in EC:EMC:DEC = 3:2:5 (v/v/v) for 24 hour, the resulting electrolyte solutions were delivered to NMR instrument. - 28 -


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Table 3. The generated HF concentrations and the resulting transition metal dissolution quantities (in ppm) for the Li/LNMO cells after 100 cycles at 60 ℃. The prepared electrolytes solutions were same as in Fig. 9, and the concentrations were calibrated by the dilution factor.

HF concentration TMSPO-free

Transition metal dissolution quantity

TMSPO 0.2 wt.%

1792

TMSPO-free

TMSPO 0.2 wt.%

Ni

304

206

Mn

1017

661

1159

■ CONCLUSIONS TMSPO is tested as an electrolyte additive for Li/LNMO half-cell and graphite/LNMO fullcell. The effectiveness of TMSPO as a bifunctional additive (HF scavenger and protective film former) can be summarized as the following. (i) The siloxane (-Si-O-) group in TMSPO reacts with HF, which presents in the initial (uncycled) electrolyte solution as a result of hydrolysis of LiPF6, to generate HPO. The asgenerated HPO and TMSPO, which carry the carbon-carbon (C=C) double bond, are electrochemically oxidized during the charge/discharge cycling of the half-cell and full-cell to produce a protective film on the LNMO surface. (ii) The superior passivation ability imparted to the HPO and TMSPO-derived surface films on LNMO suppress further electrolyte oxidation and concomitant film growth on the LNMO surface. Resultantly, HF generation that is accompanied by the electrolyte oxidation is mitigated, which eventually decreases the metal dissolution from LNMO. In addition, the high passivation property enables the electrochemical cells to be cycled with a better Coulombic - 29 -



efficiency and cycle retention, which is achieved by less serious cell polarization due to the mitigated resistive film growth.

■ ASSOCIATED CONTENT Supporting Information. A feasible mechanism for TMSPO and PF6- is suggested. The SEI components for graphite electrode and cycle lives for the Li/graphite cells with and without TMSPO were attached in the supporting information.

■ AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected]

Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS The authors acknowledge LG Chem. Corp. and Korea Institute of Science and Technology Information (KiSTi). LG chem. provided electrode materials and financial support. KiSTi gave a free license for the GAMESS quantum calculation software program. - 30 -


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A Bifunctional Electrolyte Additive for High-Voltage LiNi0.5Mn1.5O4

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