Effect of Lithium Borate Additives on Cathode Film ... - ACS Publications

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Effect of Lithium Borate Additives on Cathode Film Formation in LiNi0.5Mn1.5O4/Li Cells Yingnan Dong,† Benjamin T. Young,§ Yuzi Zhang,† Taeho Yoon,† David R. Heskett,‡ Yongfeng Hu,∥ and Brett L. Lucht*,† †

Department of Chemistry and ‡Department of Physics, University of Rhode Island, Kingston, Rhode Island 02881, United States Department of Physical Sciences, Rhode Island College, Providence, Rhode Island 02908, United States ∥ Canadian Light Source, Saskatoon, Saskatchewan S7N 2V3, Canada §

ABSTRACT: A direct comparison of the cathode−electrolyte interface (CEI) generated on high-voltage LiNi0.5Mn1.5O4 cathodes with three different lithium borate electrolyte additives, lithium bis(oxalato)borate (LiBOB), lithium 4-pyridyl trimethyl borate (LPTB), and lithium catechol dimethyl borate (LiCDMB), has been conducted. The lithium borate electrolyte additives have been previously reported to improve the capacity retention and efficiency of graphite/LiNi0.5Mn1.5O4 cells due to the formation of passivating CEI. Linear sweep voltammetry (LSV) suggests that incorporation of the lithium borates into 1.2 M LiPF6 in EC/EMC (3/7) electrolyte results in borate oxidation on the cathode surface at high potential. The reaction of the borates on the cathode surface leads to an increase in impedance as determined by electrochemical impedance spectroscopy (EIS), consistent with the formation of a cathode surface film. Ex-situ surface analysis of the electrode via a combination of SEM, TEM, IR-ATR, XPS, and high energy XPS (HAXPES) suggests that oxidation of all borate additives results in deposition of a passivation layer on the surface of LiNi0.5Mn1.5O4 which inhibits transition metal ion dissolution from the cathode. The passivation layer thickness increases as a function of additive structure LiCDMB > LPTB > LiBOB. The results suggest that the CEI thickness can be controlled by the structure and reactivity of the electrolyte additive. KEYWORDS: lithium ion battery, electrolyte, additive, cathode−electrolyte interface (CEI), high energy XPS (HAXPES)

1. INTRODUCTION An extensive desire for portable power has been driven by smartphones and laptop computers.1,2 The most popular portable power sources are lithium ion batteries which has resulted in significant research efforts. Lithium-ion battery research has intensified as automotive manufactures have expanded interest into lithium ion batteries for electric vehicles.3,4 However, one of the main obstacles limiting application of lithium ion batteries in electric vehicles is energy density, which leads to unsatisfactory driving range. Exploring novel materials with higher energy density is critical to overcoming this challenge.5 One method for improving the energy density of lithium ion batteries is by increasing the operating potential of the cathode material. Most commercial lithium-ion batteries contain a lithiated transition metal oxide cathode which typically operates at ∼4.1 V6 (or less) vs Li/Li+, such as LiCoO2,7 LiMn2O4,8 and LiFePO4.9 In recent years, LiNi0.5Mn1.5O4 spinel cathode has received significant attention due to its high operating potential (4.8 V vs Li/Li +). However, commercialization of the LiNi0.5Mn1.5O4 spinel cathode has been hampered by severe capacity fade and poor Coulombic efficiency, especially at moderately elevated temperature (>45 °C).10 Previous reports © 2017 American Chemical Society

have suggested that there are two main mechanisms leading to capacity fade for LiNi0.5Mn1.5O4 cathodes. The first is oxidation of electrolyte on the cathode surface, and the second is transition metal dissolution due to acidic electrolyte decomposition products, such as HF and POF3, which leads to damage of the anode SEI.11,12 Incorporation of cathode film forming additives into the electrolyte has been reported to improve capacity retention and Coulombic efficiency. The additives are reported to generate a cathode−electrolyte interface (CEI) which passivates the cathode surface, inhibiting further electrolyte decomposition and transition metal dissolution.13−15 Inert metal oxide surface coatings have been previously reported to significantly improve the performance of LiNi0.5Mn1.5O4 cathodes.10,16−18 In an effort to generate similar protective surface coatings via electrolyte additives, the development of additives for designed surface modification (ADSM) has been under investigation.13 Several lithium borates, including lithium bis(oxalate borate) (LiBOB), lithium alkyl trimethyl borates, lithium aryl trimethyl borates, and lithium catechol Received: February 2, 2017 Accepted: May 31, 2017 Published: May 31, 2017 20467

DOI: 10.1021/acsami.7b01481 ACS Appl. Mater. Interfaces 2017, 9, 20467−20475

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ACS Applied Materials & Interfaces

cathodes were harvested and rinsed with anhydrous dimethyl carbonate (DMC, Sigma, extra dry, 99%) three times to remove residual electrolyte, followed by vacuum drying overnight at room temperature. Symmetric LiNi0.5Mn1.5O4/LiNi0.5Mn1.5O4 cells were built with cathodes harvested from LiNi0.5Mn1.5O4/Li cells, after two cycles between 3.3 and 4.9 V vs Li/Li+, and then charged to a 50% state of charge (50% SOC, ca. 4.68 V vs Li/Li+). Electrochemical impedance spectroscopy (EIS) of symmetric LiNi0.5Mn1.5O4/LiNi0.5Mn1.5O4 cells was performed on a Bio-Logic potentiostat/galvanostat. Perturbation is 10 mV with the frequency range 1000 kHz−20 mHz. Symmetric cells were produced in duplicate. Ex-situ surface analysis of the discharged electrodes was conducted. Electrodes were analyzed after 10 cycles to afford characterization of the initially formed cathode surface films. X-ray photoelectron spectra were carried out using a ThermoFisher K-Alpha XPS, under a focused monochromatized Al Kα radiation (1486.6 eV). Cells were disassembled in the glovebox, and electrode samples were rinsed three times with DMC and dried under vacuum at room temperature for 10 min. Samples were then sealed in a vial under controlled atmosphere of the glovebox and stored for 24 h. A transfer case (ThermoFisher) was used to avoid any contact with air/moisture. Peaks were recorded with a constant pass energy of 50 eV with an energy resolution of 50 meV and charge neutralization. Peak positions and areas were optimized by a weighted least-squares fitting method using 70% Gaussian and 30% Lorentzian line shapes using Avantage (ThermoFisher) software. High energy X-ray photoelectron spectroscopy (HAXPES) was conducted at the SXRMB beamline of the third-generation synchrotron at the Canadian Light Source (CLS) in Saskatoon, Saskatchewan. The experimental setup and treatment of data are nearly identical to those of our previous investigation utilizing the decommissioned NSLS.21 The CEI thickness may be approximated from the Beer−Lambert law, using the metal oxide peak in the O 1s spectra similar to the procedure outlined recently by Malmgren et al.22 using the equation

dimethyl borate (LiCDMB) (Figure 1), have been investigated as electrolyte additives which generate an effective CEI

Figure 1. Chemical structure of lithium salt anions: (a) B(O(CO)2O)2− (LiBOB), (b) B(OMe)3(O(C5H4N))− (LPTB), and (c) a new anion B(OMe)2(O(C6H4)O)− (LiCDMB).

passivation film.13,19−21 The anionic borate additives are electron-rich, easily oxidized, and polarized toward the cathode surface upon initial cell charging. All of the lithium borate electrolyte additives have been reported to generate a borate-rich stable passivating CEI which improves the performance of LiNi0.5Mn1.5O4 cathodes. In an effort to develop a better understanding of the structure, function, and formation of cathode surface films, a direct comparison of the three different lithium borate additives has been conducted. The passivation layers on LiNi0.5Mn1.5O4 cathodes have been analyzed via a combination of in-situ electrochemical and ex-situ surface analysis methods including hard X-ray photoelectron spectroscopy (HAXPES), XPS, FT-IR, SEM, and TEM.

∞

2. EXPERIMENTAL SECTION

∫ e−x / λ sin θ dx Ci = d∞ −x / λ sin θ ≈ e−d / λ Cf dx ∫0 e

The standard electrolyte (STD) is 1.2 M LiPF6 in EC/EMC (3:7 by volume). Battery-grade carbonates solvents and lithium hexafluorophosphate (LiPF6) were obtained from BASF. Additives are added in weight percentage based on the total mass of electrolyte. Lithium bis(oxalato) borate (LiBOB), lithium 4-pyridyl trimethyl borate (LPTB), and lithium catechol dimethyl borate (LiCDMB) were synthesized and added as 2 wt % to the STD electrolyte. The composite cathodes were obtained from a commercial supplier. The composite LiNi0.5Mn1.5O4 electrode is composed of active material (92%), conductive carbon (4%), and PVDF binder (4%). The cathode loading is 15.9 mg/cm2. The 2032-type coin cells were constructed in an argonfilled glovebox with a trilayer polypropylene/polyethylene (PP/PE/PP) separator (d = 19 mm, Celgard) and one layer of glass fiber separator (d = 16 mm, thickness = 0.67 mm, Whatman) and 100 μL of electrolyte. Carbon black electrode (Super C65, d = 15 mm) half-cells were built with a trilayer polypropylene/polyethylene (PP/PE/PP) separator (d = 19 mm, Celgard) and one layer of glass fiber separator (d = 16 mm, thickness = 0.67 mm, Whatman) and 100 μL of electrolyte. Comparable cycling data have been obtained using only polyolefin separators. Cells were scanned from open circuit potential (OCV) to 6.0 V (high potential) or 0.01 V (low potential) vs Li/Li+ at a rate of 0.1 mV s−1 with Bio-Logic potentiostat/galvanostat at a controlled temperature of 25.0 °C. A uniform concentration of 2 wt % for each additive was used to afford better surface film characterization. The HOMO and LUMO energy calculations were performed on the anions of LiBOB, LPTB, and LiCDMB using the Gaussian 03 package (B3LYP/6-311+G(2d,p)). LiNi0.5Mn1.5O4/Li cells were cycled at 25 °C at a C/10 rate for ten cycles at 25 °C. Cells were charged with a CC-CV mode and constant current charge to 4.9 V followed with a constant voltage charge step at 4.9 V vs Li/Li+ until the current decreases to 10% of the applied charging current. The cells were discharged to 3.3 V vs Li/Li+ at the same constant current (CC mode). Cells were built in triplicate. Cell to cell variation was approximately 3%. LiNi0.5Mn1.5O4/Li cells were used in an effort to isolate the reaction of the additives with the cathode surface. Cycled cells were disassembled in an argon glovebox, and cycled

This model assumes the composition of the fresh sample remains unchanged beneath the CEI of thickness d. The ratio of the concentration Ci in a given sample to the concentration Cf in the fresh sample equates to an exponential which is readily solved for d. The mean free path of polyethylene was used for λ.23 The angle between the analyzer line of sight and the sample normal defines θ. For our analysis of the O 1s spectra, the concentration was restricted to include only the area of the metal oxide peak contribution, as obtained through fits to the experimental data making similar peak shape assumptions as described for the XPS analysis. Surface morphology of the cycled electrodes was characterized by scanning electron microscopy (SEM, JEOL5900). For transmission electron microscopic analysis (TEM), cycled electrodes were exposed to ultrasound in DMC solvent for 3 h to allow homogeneous dispersion of the active materials in the solution, and then the dispersed solution was cast on a copper TEM grid (500 mesh) and dried overnight in a vacuum oven. The TEM grids were quickly transferred into the TEM chamber. Imaging was conducted using a JEOL JEM-2100F TEM (Peabody, MA) at 160 eV. The diameter of the beam was 5 nm, and low-dose imaging was employed to minimize electron-beam-induced changes to organic components in the surface layer. The discharged electrodes were briefly (15 s) exposed to air during transfer to the SEM and TEM vacuum chambers. FTIR spectra were acquired on Bruker Tensor 27 with attenuated total reflectance (ATR) accessory with germanium crystal and 512 scans with a resolution of 4 cm−1. The IR spectrometer is inside of a N2-filled glovebox, and samples were transferred without any exposure to air.

3. RESULTS AND DISCUSSION 3.1. Electrochemical Stability. Electrochemical stability of both the STD electrolyte and electrolyte with 2% LiBOB, 2% 20468


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Figure 2. Anodic linear sweep voltammetry (LSV) at 25 °C of Super C65/Li cells (sweep rate of 0.1 mV s−1) using (in black) the EC/EMC (3/7) 1.2 M LiPF6, (in red) EC/EMC (3/7) 1.2 M LiPF6 + 2% LiBOB electrolytes, (in blue) EC/EMC (3/7) 1.2 M LiPF6 + 2% LPTB electrolytes, and (in pink) EC/EMC (3/7) 1.2 M LiPF6 + 2% LiCDMB electrolytes.

Table 1. Calculated HOMO and LUMO Energies (eV) of LiBOB, LPTB, and LiCDMB molecules

HOMO

LUMO

LiBOB LPTB LiCDMB

−3.93 −2.56 −1.45

1.83 2.70 2.88

Figure 3. (a) Cycling retention and (b) Coulombic efficiency of LiNi0.5Mn1.5O4/Li cells (C/10, cutoff potentials at 25 °C: 4.80−3.3 V vs LiC6/C6) using the STD electrolyte (in black), STD with 2 wt % added LiBOB (in red), STD with 2 wt % added LPTB, and STD with 2 wt % added LiCDMB (in pink).

LPTB, and 2% LiCDMB have been evaluated on carbon black electrodes with linear sweep voltammetry at high potential.24,25 Anodic linear sweep voltammetry of Super C65/Li cells is presented in Figure 2. Cells containing added LiCDMB have additional electrolyte oxidation above 3.5 V vs Li/Li+ as evidenced by increased current; additional oxidation peaks are observed at 3.7, 4.0, and 4.4 V vs Li/Li+, which is consistent with previously published results.20 Cells containing added LPTB contain new oxidation peaks at 4.25 and 5.5 V vs Li/Li+ with comparable intensity to those observed for LiCDMB. While the intensity of the additional oxidation peak is strongest for cells containing added LiBOB, the peak is also at the highest potential, 5.5 V vs Li/Li+. All lithium borate additives are oxidized at lower potential than the STD electrolyte, and initiation of additive oxidation occurs in the following order LiCDMB > LPTB > LiBOB. All three borate additives have been previously investigated in graphite/LiNi0.5Mn1.5O4 cells, confirming compatibility of the additives with graphite anodes.13,19,20 The HOMO and LUMO energy calculations are performed on LiBOB, LPTB, and LiCDMB using the Gaussian 03, B3LYP/ 6-311+G(2d,p), and are listed in Table 1. The order of the calculated HOMO energies follows the same trend observed experimentally via anodic LSV. The borate with the highest HOMO is LiCDMB, −1.45 eV, which is the borate with the lowest oxidation potential. The HOMO of LPTB, −2.56 eV, is slightly lower and the HOMO of LiBOB is the lowest at −3.93 eV. 3.2. Cycling Performance of the Electrolytes. The capacity retention and Coulombic efficiency for LiNi0.5Mn1.5O4/Li cells cycled at 25 °C containing the STD,

Figure 4. Equivalent circuit (a) and EIS spectra (b) of LiNi0.5Mn1.5O4/ LiNi0.5Mn1.5O4 symmetric cells using the (in black) STD, (in red) STD + 2% LiBOB electrolyte, (in blue) STD + 2% LPTB electrolyte, and (in pink) STD + 2% LiCDMB electrolyte.

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Figure 5. C 1s and O 1s XPS and HAXPES spectra of fresh LiNi0.5Mn1.5O4 cathode (top) cycled with standard electrolyte (second) and electrolyte with 2% of LiBOB (third), 2% LPTB (fourth), and 2% LiCDMB (bottom) versus electron binding energy in eV.

Figure 7. FT-IR spectra of fresh LiNi0.5Mn1.5O4 cathode, after cycling in standard electrolyte (STD), 2% LiBOB electrolyte, 2% LPTB electrolyte, and 2% LiCDMB electrolyte.

Figure 6. B 1s (top) XPS and HAXPES spectra of LiNi0.5Mn1.5O4 cathodes cycled with borate additives.

Table 2. Elemental Concentration of LiNi0.5Mn1.5O4 Cathode Surface from XPS Analysis with 1.5 keV Excitation

fresh STD LiBOB LPTB LiCDMB

C 1s (%)

O 1s (%)

F 1s (%)

P 2p (%)

Mn 2p (%)

B 1s (%)

N 1s (%)

64 54 43 50 50

10 17 31 15 30

22 25 16 22 5

1 1 2 4

4 3 1 1 1

7 7 10

3

This suggests that there is significant oxidation of the added LPTB or LiCDMB on the first cycle. After the first cycle the Coulombic efficiencies are improved for all electrolytes (>95%), suggesting that the cathode surface has been passivated. Interestingly, all cells containing LPTB have a slight decrease in efficiency for cycles 4−6, suggesting that additional reactions of the additive with the cathode surface may continue to occur on these cycles. The cells containing the STD electrolyte have the poorest efficiency for cycles 7−10 consistent with the poorest passivation of the cathode surface. However, all the cells have a similar reversible capacity of approximately 150 mAh/g for cycles 7−10. 3.3. Electrochemical Impedance Spectroscopy. After two full cycles between 3.3 and 4.9 V vs Li/Li+, LiNi0.5Mn1.5O4/ Li cells were charged to a 50% state of charge (50% SOC, ca. 4.68

LiBOB, LPTB, and LiCDMB electrolytes are presented in Figure 3. The first cycle efficiencies are very similar for the STD and LiBOB electrolytes (93%) while the efficiencies are lower for the LPTB and LiCDMB electrolytes (86% and 57%, respectively). 20470


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Figure 8. SEM micrographs of the LiNi0.5Mn1.5O4 cathodes fresh and extracted from cells cycled using the STD, STD + 2% LiBOB, STD + 2% LPTB, and STD + 2% LiCDMB electrolytes.

the smallest surface film resistance (Rfilm = 7.5 Ω), consistent with very thin surface films on the cathodes. Cathodes cycled with electrolyte containing 2% LiBOB show both greater surface film resistance (Rfilm = 11.5 Ω) and charge transfer resistance, suggesting that addition of LiBOB results in the formation of a thicker CEI than the STD electrolyte. Thus, the reactivity of LiBOB with the surface of the LiNi0.5Mn1.5O4 cathode occurs at slightly lower potential than observed in the LSV data. The surface film resistance is further increased for cathodes cycled

V vs Li/Li+) and disassembled, and the cathodes were extracted and used to construct symmetric LiNi0.5Mn1.5O4/LiNi0.5Mn1.5O4 cells. The electrochemical impedance spectra of symmetric cells are measured; equivalent circuit and the corresponding EIS Nyquist plots are provided in Figure 4.26 The Nyquist plots contain two semicircles. The first semicircle is typically attributed to the surface film resistance (Rfilm), while second semicircle is typically attributed to charge transfer resistance (Rct).26 LiNi0.5Mn1.5O4 electrodes cycled with the STD electrolyte have 20471


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Figure 9. TEM micrographs of LiNi0.5Mn1.5O4 cathodes harvested from cells cycled using the STD (a), STD + 2% LiBOB (b), STD + 2% LPTB (c), and STD + 2% LiCDMB (d) electrolytes.

with electrolyte containing 2% LPTB, which is around 19 Ω, suggesting that surface film formed by LPTB is thicker. Interestingly, the charge transfer resistance is smaller for the electrode cycled with electrolyte containing 2% LPTB, but the source for this difference is unclear at this time. Cathodes cycled with electrolyte containing 2% LiCDMB have the greatest surface film resistance (Rfilm = 24.5 Ω) and also largest charge transfer resistance (data provided in inset to Figure 4), indicating that the surface film formed by LiCDMB is the most resistive and thus most likely the thickest. 3.4. High Energy X-ray Photoelectron Spectroscopy (HAXPES). 3.4.1. XPS and HAPES Spectra. The C 1s and O 1s core level XPS and HAXPES spectra of the cycled LiNi0.5Mn1.5O4 cathodes acquired with photon energies of 1.5 (lab XPS), 3, and 6 keV are provided in Figure 5. The higher photon energies result in greater mean escape depths for photoelectrons, thus providing depth-dependent information for the composition of the CEI. The C 1s spectrum of the fresh LiNi0.5Mn1.5O4 cathode upon 1.5 keV excitation is dominated by −CF2− (291 eV) and −CH2− (286.2 eV) from PVDF and C−C (284.3 eV) from conductive carbon.27−30 Higher energy excitation (3 and 6 keV) reveals very similar spectra, indicating that there is little depth dependence, as expected, for the blend of binder and active material. The C 1s spectrum of the LiNi0.5Mn1.5O4 cathode cycled with the standard electrolyte with 1.5 keV excitation contains more C−O (287 eV) and CO3 (290 eV) species from electrolyte decomposition. However, increasing the energy of excitation results in spectra

similar to those of fresh LiNi0.5Mn1.5O4 cathodes, consistent with the formation of a thin surface film composed of electrolyte decomposition products. The changes to the C 1s spectra are greater for the cathode cycled with electrolyte with added LiBOB. New peaks are observed at 287 eV (C−O) and 290 eV (−CO3) for samples excited at 1.5 keV. Interestingly, the spectra for samples analyzed at higher energy (3 and 6 keV) are similar to the fresh LiNi0.5Mn1.5O4 cathode. Thus, while the CEI formed with the LiBOB-containing electrolyte is thicker than that of the standard electrolyte, it is still relatively thin. The C 1s spectrum of the electrode cycled with electrolyte containing added LPTB are similar for all excitation energies and contain a very strong peak characteristic of C−O (287 eV) and weak peaks characteristic of −CO3 (290 eV) groups consistent with the presence of electrolyte decomposition products.28,29 The bottom row contains the C 1s spectra of the cathode cycled with electrolyte containing added LiCDMB. The peaks characteristic of PVdF are no longer distinguishable even for the highest energy excitation, 6 keV, and thus deepest penetration, suggesting the generation of a very thick surface film on the cathode material. The C 1s data suggest that the CEI formed with the electrolyte containing added LiCDMB is the thickest of all the samples. The O 1s spectrum of the fresh cathode is dominated by the metal oxide peak at 529.0 eV but also contains a low concentration of C−O and CO (532−534 eV) due to residual Li2CO3, which is typically present on the fresh cathode surface.27,31 The cathode cycled with the STD electrolyte has a 20472


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cathode cycled with electrolyte containing LiCDMB has the largest change in elemental concentration when compare to electrode cycled with the STD electrolyte. High concentrations of O, P, and B and low concentrations of F and Mn are observed consistent with a thick CEI. The trends in elemental concentrations are in agreement with the trends observed for the XPS and HAXPES element spectra, suggesting that the CEI film thickness follows the order STD < LiBOB < LPTB < LiCDMB. 3.5. FT-IR Spectroscopy of Electrodes Surface. The FTIR-ATR spectra of the LiNi0.5Mn1.5O4 electrodes, fresh and after cycling with electrolyte with and without added borates, are depicted in Figure 7. All of the FT-IR spectra are dominated by the peaks associated with PVDF binder at 1072, 1180, and 1400 cm−1.35,36 The changes to the IR spectra of the electrode surface after cycling with the standard electrolyte are minor. A small new peak characteristic of poly(ethylene carbonate) is observed at 1750 cm−1,37 consistent with a cathode with a very thin surface film. The IR spectrum of the cathode cycled with electrolyte containing LiBOB has an increase in intensity of the peak associated with poly(ethylene carbonate) along with a new peak at 1620−1650 cm−1 which is assigned to lithium oxalate from the decomposition of LiBOB.38 A similar increase in intensity of the peak associated with poly(ethylene carbonate) is observed for the cathode cycled with electrolyte containing LPTB. The largest changes in the IR spectra are associated with cathodes cycled with electrolyte containing LiCDMB. The peak characteristic of poly(ethylene carbonate) is very strong, and a small new peak is observed at 1430 cm−1, consistent with the presence of low concentrations of Li2CO3. The IR data are consistent with the XPS data as discussed above. 3.6. SEM/TEM. SEM imaging of the LiNi0.5Mn1.5O4 cathodes after 10 cycles at 25 °C has been conducted in order to investigate electrode surface morphology. SEM micrographs of the fresh LiNi0.5Mn1.5O4 cathodes and cathodes cycled with and without lithium borate additives are depicted in Figure 8. The fresh electrode consists of ∼8 μm secondary spherical particles. The spherical particles are composed of primary rodlike particles several hundred nanometers in length. The surface of the fresh cathode particle is clean and smooth. After cycling with STD electrolyte, the changes to the surface are small with a slight dulling of the edges of the particles. Electrodes cycled with electrolyte containing LiBOB reveal larger changes in morphology, a film is present on the surface, but the primary rod structure is still observed, suggesting the surface film is thin. After cycling with the electrolyte containing LPTB, a thicker surface film is formed. The primary rod structure is further rounded due to the presence of more electrolyte decomposition products on the electrode. The greatest morphology change can be observed on electrode cycled with LiCDMB. A thicker film covers cathode surface. The CEI formed by LiCDMB appears more uniform, consistent with the XPS data discussed above. Electrodes extracted from LiNi0.5Mn1.5O4/Li cells after 10 cycles at 25 °C with and without lithium borate additives have been analyzed by TEM (Figure 9). After cycling with the standard electrolyte, the cathode surface maintains a sharp and clean edge (Figure 9a); there is no apparent surface film present. However, after cycling with electrolyte containing LiBOB the surface of cathode particles has been modified and includes a thin inhomogeneous surface film (Figure 9b), the thickness of CEI formed by LiBOB is approximately 1−2 nm, which is consistent with the HAXPES results, discussed above. The cathodes cycled with either LPTB or LiCDMB have significantly thicker surface

significant O−M (M = Mn, Ni) peak at 529.0 eV, along with new broad peak at 532−534 eV characteristic of C−O and CO bonds.27,29,30,32 The relative intensity of the M−O peak is increased compared to the broad C−O and CO peak as the excitation energy and depth of penetration increases, consistent with a thin film of electrolyte decomposition products on the surface of a metal oxide cathode. The differences are greater for the cathode cycled with LiBOBthe characteristic peak of the metal oxide at 529.0 eV is very weak while the peaks characteristic of electrolyte decomposition products have high intensity. At higher excitation energies, the intensity of the M−O peak increases, consistent with a thicker surface film than observed with the standard electrolyte. A similar trend is observed for the electrode cycled with electrolyte containing LPTB. However, the intensity of the M−O peak is slightly weaker at all excitation energies, consistent with a surface film which is slightly thicker than the surface film for the electrode cycled with the LiBOB electrolyte. The O 1s spectra of the electrodes cycled with electrolyte containing added LiCDMB at 1.5 keV excitation have no observable M−O peak while at higher energy excitation (3−6 keV) a weak M−O peak is observed, consistent with a thick surface film. The relative thickness of the CEI layers has been estimated for the cathodes based on fits to the metal oxide peak in the 3 keV spectra, using the ratio of the fitted peak area in each spectrum to the area of the fresh metal oxide peak as described in the Experimental Section. The analysis suggests that the CEI thicknesses on the cycled cathodes are 0.6, 1.2, 3.3, and 8.4 nm for the standard, LiBOB, LPTB, and LiCDMB, respectively. All of the F 1s spectra (not shown) are dominated by the peak characteristic of PVdF at 688 eV, suggesting the reaction of the borate additives primarily occurs on the surface of the metal oxide particles. In addition, most samples contain a low concentration of LiF (685 eV), consistent with the presence of LiPF6 decomposition products. The B 1s XPS and HAXPES spectra of the cycled LiNi0.5Mn1.5O4 cathodes taken with photon energies of 1.5 (lab XPS), 3, and 6 keV are provided in Figure 6. The B 1s spectra contain a peak at ∼193 eV, characteristic of previously reported cross-linked borates, which confirms that boron is incorporated into the CEI.33,34 The electrode cycled with the LPTB electrolyte contains a N 1s peak at 402 eV (not shown), consistent with the presence of N from the oxidation of LPTB and incorporation into the CEI. 3.4.2. Relative Atomic Concentrations. The elemental concentrations of the LiNi0.5Mn1.5O4 cathode fresh and after cycling with STD electrolyte, electrolyte with 2% LiBOB, 2% LPTB, and 2% LiCDMB upon excitation at 1.5 keV are provided in Table 2. The fresh electrode has a high concentration of Mn and O from the metal oxide and C and F from the PVDF binder. After cycling with the STD electrolyte, the concentrations of C and Mn are slightly decreased, while O, F, and P are slightly increased, indicating deposition of electrolyte decomposition products. However, the small change in concentration suggests a thin surface film. The cathode cycled with electrolyte containing added LiBOB has a further decrease in the concentration of C and Mn along with F, while the concentrations of O, B, and P are increased, suggesting more electrolyte decomposition products are deposited on the surface of LiNi0.5Mn1.5O4. A similar trend is observed for the cathode cycled with electrolyte containing added LPTB when compared to the cathode cycled with the STD electrolyte. The Mn concentration is very low while the concentrations of B and N are high, consistent with the reaction of LPTB on the cathode surface and subsequent incorporation into the CEI. Among all three lithium borate additives, the 20473


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ACS Applied Materials & Interfaces films as depicted in Figures 9c and 9d, respectively. The film thicknesses as observed by TEM range from ∼0 to 20 nm, which is slightly thicker than that estimated by HAXPES, as discussed above. However, the trend in surface film thickness, STD < LiBOB < LPTB < LiCDMB, is in agreement with the data from EIS, XPS, and HAXPES, as discussed above.

the University of Saskatchewan. Further support was provided to BTY by the Rhode Island College Faculty Research Fund.

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(1) Tarascon, J. M.; Armand, M. Issues and Challenges Facing Rechargeable Lithium Batteries. Nature 2001, 414, 359−367. (2) Armand, M.; Tarascon, J. M. Building Better Batteries. Nature 2008, 451, 652−657. (3) Dunn, B.; Kamath, H.; Tarascon, J. M. Electrical Energy Storage for the Grid: A Battery of Choices. Science 2011, 334, 928−935. (4) Jung, H.-G.; Jang, M. W.; Hassoun, J.; Sun, Y.-K.; Scrosati, B. A High-Rate Long-Life Li4Ti5O12/Li[Ni0.45Co0.1Mn1.45]O4 LithiumIon Battery. Nat. Commun. 2011, 2, 516−520. (5) Manthiram, A. Materials Challenges and Opportunities of Lithium Ion Batteries. J. Phys. Chem. Lett. 2011, 2, 176−184. (6) Kraytsberg, A.; Ein-Eli, Y. Higher, Stronger, Better··· A Review of 5 V Cathode Materials for Advanced Lithium-Ion Batteries. Adv. Energy Mater. 2012, 2, 922−939. (7) Wang, B.; Bates, J. B.; Hart, F. X.; Sales, B. C.; Zuhr, R. A.; Robertson, J. D. Characterization of Thin-Film Rechargeable Lithium Batteries with Lithium Cobalt Oxide Cathodes. J. Electrochem. Soc. 1996, 143, 3203−3213. (8) Huang, T.; Zheng, X.; Wu, M.; Wang, W.; Pan, Y.; Fang, G. Ethyl 3,3,3-Trifluoropropanoate as An Additive to Improve the Cycling Performance of LiMn2O4 Cathode on Lithium-Ion Batteries at Elevated Temperature. J. Power Sources 2016, 318, 264−269. (9) Yamada, A.; Chung, S. C.; Hinokuma, K. Optimized LiFePO4 for Lithium Battery Cathodes. J. Electrochem. Soc. 2001, 148, A224−A229. (10) Manthiram, A.; Chemelewski, K.; Lee, E.-S. A Perspective on the High-Voltage LiMn1.5Ni0.5O4 Spinel Cathode for Lithium-Ion Batteries. Energy Environ. Sci. 2014, 7, 1339−1350. (11) Yoon, T.; Park, S. J.; Mun, J. Y.; Ryu, J. H.; Choi, W. C.; Kang, Y. S.; Park, J.-H.; Oh, S. M. Failure Mechanisms of LiNi0.5Mn1.5O4 Electrode at Elevated Temperature. J. Power Sources 2012, 215, 312− 316. (12) Lu, D.; Xu, M.; Zhou, L.; Garsuch, A.; Lucht, B. L. Failure Mechanism of Graphite/ LiNi0.5Mn1.5O4 Cells at High Voltage and Elevated Temperature. J. Electrochem. Soc. 2013, 160, A3138−A3143. (13) Xu, M.; Zhou, L.; Dong, Y.; Chen, Y.; Demeaux, J.; MacIntosh, A. D.; Garsuch, A.; Lucht, B. L. Development of Novel Lithium Borate Additives for Designed Surface Modification of High Voltage LiNi0.5Mn1.5O4 Cathodes. Energy Environ. Sci. 2016, 9, 1308−1319. (14) Choi, N.-S.; Han, J.-G.; Ha, S.-Y.; Park, I.; Back, C.-K. Recent Advances in the Electrolytes for Interfacial Stability of High-Voltage Cathodes in Lithium-Ion Batteries. RSC Adv. 2015, 5, 2732−2748. (15) Xu, K. Electrolytes and Interphases in Li-Ion Batteries and Beyond. Chem. Rev. 2014, 114, 11503−11618. (16) Liu, J.; Manthiram, A. Kinetics Study of the 5 V Spinel Cathode LiMn1.5Ni0.5O4 Before and After Surface Modifications. J. Electrochem. Soc. 2009, 156, A833−A838. (17) Liu, J.; Manthiram, A. Understanding the Improvement in the Electrochemical Properties of Surface Modified 5 V LiMn1.42Ni0.42Co0.16O4 Spinel Cathodes in Lithium-Ion Cells. Chem. Mater. 2009, 21, 1695−1707. (18) Liu, J.; Manthiram, A. Improved Electrochemical Performance of the 5 V Spinel Cathode LiMn1.5Ni0.42Zn0.08O4 by Surface Modification. J. Electrochem. Soc. 2009, 156, A66−A72. (19) Xu, M.; Zhou, L.; Dong, Y.; Chen, Y.; Garsuch, A.; Lucht, B. L. Improving the Performance of Graphite/LiNi0.5Mn1.5O4 Cells at High Voltage and Elevated Temperature with Added Lithium Bis(oxalato) Borate (LiBOB). J. Electrochem. Soc. 2013, 160, A2005−A2013. (20) Dong, Y.; Demeaux, J.; Zhang, Y.; Xu, M.; Zhou, L.; MacIntosh, A. D.; Lucht, B. L. Improving the Performance at Elevated Temperature of High Voltage Graphite/ LiNi0.5Mn1.5O4 Cells with Added Lithium Catechol Dimethyl Borate. J. Electrochem. Soc. 2017, 164, A128−A136. (21) Young, B. T.; Heskett, D. R.; Nguyen, C. C.; Nie, M.; Woicik, J. C.; Lucht, B. Hard X-ray Photoelectron Spectroscopy (HAXPES) Investigation of the Silicon Solid Electrolyte Interphase (SEI) in

4. CONCLUSIONS The generation of a cathode−electrolyte interface (CEI) from the oxidation of different lithium borate electrolyte additives has been investigated. A direct comparison of lithium bis(oxalato) borate (LiBOB), lithium 4-pyridyl trimethyl borate (LPTB), and lithium catechol dimethyl borate (LiCDMB) as 2% (wt) electrolyte additives in 1.2 M LiPF6, EC/EMC 3/7 has been conducted to probe effects of different lithium borate additives on cathode film formation on the surface of LiNi0.5Mn1.5O4 cathodes. Electrochemical measurements suggest that all three lithium borate additives are preferentially oxidized compared to the standard electrolyte and inhibit further electrolyte oxidation and transition metal dissolution. The structure, composition, and thickness of the CEI have been investigated via a combination of XPS, HAXPES, IR-ATR, SEM, and TEM. While the composition of the CEI is dependent upon the structure of the additive, all cathode surface films generated in the presence of lithium borate additives are primarily composed of polycarbonates and crosslinked borates. The surface films generated with added LiBOB also contain lithium oxalate while the surface films with added LPTB have N-containing species, consistent with reaction of the anionic borate additives on the cathode surface. All of the additives investigated in this article improve the capacity retention of graphite/LiNi0.5Mn1.5O4 cells cycled under accelerated aging conditions, as previously reported.13,19,20 The borate additives reactivity with the cathode surface and calculated HOMO energy follow the order LiCDMB > LPTB > LiBOB. In addition, the thickness of the cathode surface films and the electrode impedance follow the same order as the additive reactivity: LiCDMB > LPTB > LiBOB. Thus, a strong correlation is observed between reactivity of the additive, surface film thickness, and impedance, suggesting that the properties of the cathode surface film can be tailored by additive structure and reactivity. The performance of high-voltage cathode materials can be improved via the development of additives for designed surface modification (ADSM).

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. ORCID

Benjamin T. Young: 0000-0001-5778-9274 Brett L. Lucht: 0000-0002-4660-0840 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Department of Energy Office of Basic Energy Sciences EPSCoR Implementation award (DESC0007074) for financial support. HAXPES research described in this work was performed at the Canadian Light Source, which is supported by the Natural Sciences and Engineering Research Council of Canada, the National Research Council Canada, the Canadian Institutes of Health Research, the Province of Saskatchewan, Western Economic Diversification Canada, and 20474


Research Article

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