Generation of Cathode Passivation Films via Oxidation of Lithium Bis

Article pubs.acs.org/JPCC

Generation of Cathode Passivation Films via Oxidation of Lithium Bis(oxalato) Borate on High Voltage Spinel (LiNi0.5Mn1.5O4) Mengqing Xu,† Nikolaos Tsiouvaras,‡ Arnd Garsuch,§ Hubert A. Gasteiger,‡ and Brett L. Lucht*,† †

Department of Chemistry, University of Rhode Island, Kingston, Rhode Island, 02881, United States Chair of Technical Electrochemistry, Technische Universität München, D-85748 Garching, Germany § BASF SE, GCN/E, Rheinland-Pfalz 67056, Germany ‡

ABSTRACT: The reactions of lithium ion battery electrolyte (LiPF6 in ethylene carbonate/ethyl methyl, EC/EMC, 3:7 v/v) with and without added lithium bis(oxalato) borate (LiBOB) on the surface of high voltage LiNi0.5Mn1.5O4 cathodes has been investigated via a combination of electrochemical measurements, in situ gas analysis, and ex situ surface analysis. The oxidation of LiBOB on the cathode results in the generation of CO2 and a cathode passivation film containing borate oxalates. The cathode passivation film inhibits oxidation of the bulk electrolyte at high potential (>4.8 V vs Li/Li+).

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INTRODUCTION There has been significant recent interest in increasing the energy density of lithium ion batteries. One method to improve the energy density of lithium ion batteries is to increase the operating potential of the cathode. One of the most interesting new high voltage cathode materials is LiNi0.5Mn1.5O4.1−4 However, this cathode material has problems associated with the stability of standard carbonate based electrolytes at the high operating potential, 4.8 V vs Li/Li+.5−7 In order to improve the performance of LiNi0.5Mn1.5O4 cycled to high potential, the use of novel electrolytes and electrolyte additives has been investigated. Sulfone electrolytes and electrolytes containing fluorinated ethers and carbonates have been investigated as novel high voltage electrolytes.8−12 In addition, fluorinated phosphates,13 phosphonates,14 and lithium bis(oxalato) borate15−18 have been investigated as electrolyte additives to improve the performance of standard carbonate electrolytes at high potential. However, each novel electrolyte system has drawbacks. One of the most promising methods to improve the performance of LiNi0.5Mn1.5O4 cathode is the use of cathode film forming additives, including LiBOB.15−18 The incorporation of additive concentrations of LiBOB has been reported to provide significant improvements in the performance of Li/LiNi0.5Mn1.5O4 and graphite/LiNi0.5Mn1.5O4 cells which is related to a combination of cathode surface film formation and the inhibition of Mn dissolution. However, the mechanism of the oxidation reactions of LiBOB at the cathode interface and the structure of the cathode surface films have been elusive. Thus, developing an improved understanding of the formation mechanism of a cathode passivation layer will lead to improved methods to passivate high voltage cathode materials. Herein, the performance of LiNi0.5Mn1.5O5 electrodes charged to high voltage has been investigated via a combination of electrochemical cycling, ex situ surface analysis (X-ray © 2014 American Chemical Society

photoelectron spectroscopy (XPS) and infrared (IR) spectroscopy), and in situ gas analysis (online electrochemical mass spectroscopy (OEMS)).19 The results provide the experimental evidence for CO2 generation and borate film formation upon oxidation of LiBOB supporting previous computational results.20

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EXPERIMENTAL SECTION Battery grade carbonate solvents, ethylene carbonate (EC) and ethyl (methyl) carbonate (EMC), and salts lithium hexafluorophosphonate (LiPF6) and lithium bis(oxalato) borate (LiBOB) were provided by BASF. The standard electrolyte is 1.0 M LiPF6 in EC/EMC (3/7, v/v). To detect the gas evolution of the electrolyte, a special electrode preparation method was devised. LiNi0.5Mn1.5O4 was coated on a Celgard trilayer (PP/PE/PP) C480 separator with a similar composition as that used for coin cell measurements (see below) at active material loadings of 9.3−9.5 mg cm−2. Coating onto the separator which directly exposes the cathode electrode to the free OEMS cell volume (connected electrically with a stainless steel grid)19 allowed for a fast diffusion of evolved gases to the mass spectrometer inlet. 160 μL of electrolyte was used. The cells were subsequently connected to the OEMS and after 2 h OCV the cell was charged at a rate of C/10. The upper cutoff potential limit was set to 6 V which was, however, never reached as the electrolyte decomposition rate was high enough at a lower potential (∼5.4 V) to sustain the demanded current corresponding to C/10. The OEMS cell was assembled in an argon filled glovebox according to the procedure described in our previous work.19 A Received: February 25, 2014 Revised: March 14, 2014 Published: March 18, 2014 7363

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17-mm-diameter lithium disc (0.45 μm thick, 99.9%; Chemetall, Germany) wetted with 40 μL electrolyte was placed on the anode current collector. It was then covered with two Celgard C480 separators, and another 80 μL of electrolyte was added. The cathode was then placed onto the wetted separators and also wetted with another 40 μL of electrolyte. The electrode was then covered with a 21-mm-diameter stainless steel mesh cathode current collector (316SS, Spörl KG, Germany). The cell was afterward sealed with four screws at a torque of 6 N m, removed from the glovebox, and connected to the OEMS. Calibration of the mass spectrometer signals and conversion into current-normalized molar evolution rates (in μmol (As)−1) was described previously.19 The composite LiNi0.5Mn1.5O4 electrodes supported on Alfoil current collector and provided by BASF contain active material (85%), conductive carbon (Super P 6%, graphite 3%), and PVDF binder (6%). Li/LiNi0.5Mn1.5O4 2025-type coin cells, aluminum coated stainless steel (Pred materials), with trilayer polypropylene/polyethylene (PP/PE/PP) separator (Celgard) were assembled in an argon glovebox for electrochemical performance measurements with and without LiBOB added to the electrolyte. The Li/LiNi0.5Mn1.5O4 cells were cycled at room temperature at C/10 for the three cycles with two different voltage ranges (3.5↔4.9 V and 3.5↔5.4 V). The cells were charged with a CC−CV mode, i.e., a constant current charge (C/10) to the upper potential limit at either 4.9 or 5.4 V vs Li/Li+ followed by a constant voltage charge step at the upper potential limit until the current decreased to 10% of the nominal charging current (CV mode). The cells were then discharged to 3.5 V at C/10 (CC mode). The charge− discharge performance tests were conducted with an Arbin instrument (BT2000). The cycled cells were disassembled in an argon glovebox, and the cycled cathodes were harvested and rinsed with anhydrous dimethyl carbonate (DMC) three times to remove residual LiPF6 or EC, followed by vacuum drying overnight at room temperature for XPS analysis. X-ray photoelectron spectroscopy (XPS) was performed on a PHI 5500 system using Al Ka radiation (hv = 1486.6 eV) under ultrahigh vacuum conditions. Lithium was not monitored due to its low inherent sensitivity and small change of binding energy. Calibration of XPS peak position was made by recording XPS spectra for reference compounds, which are present on the electrode surface: LiF, PVDF, and LixPOyFz. The graphite peak at 284.3 eV was used as a reference for the final adjustment of the energy scale in the spectra. The spectra obtained were analyzed by Multipack 6.1 A software. Line syntheses of elemental spectra were conducted using Gaussian−Lorentzian (80:20) curve fitting. Elemental concentration was calculated based on the equation: Cx = (Ix/ Sx)/(∑Ix/Sx), where Ix is the relative intensity of the element and Sx is the sensitivity value of the element.

Figure 1. Galvanostatic charge profile (a) and CO2 evolution rates (b) of LiNi0.5Mn1.5O4 electrodes in EC/EMC (3/7, v/v) with 1.0 M LiPF6 without LiBOB additive (STD; black lines) and with 1% (wt) LiBOB additive in the electrolyte (red lines).

minor CO2 gas evolution in the presence of LiBOB all throughout the charging process up to ca. 4.9 V (red line in Figure 1b), while no CO2 evolution is observed in this potential range without LiBOB additive (black line in Figure 1b). This indicates the slow decomposition of LiBOB during the first charge of LiNi0.5Mn1.5O4 electrodes, which we believe is accompanied by the generation of borate radicals which subsequently cross-link and form a passivating surface film (Scheme 1). The oxidation of LiBOB provides an additional Scheme 1

charge capacity which can be seen by comparing the charging curves with and without LiBOB (Figure 1b): without LiBOB additive, a charge capacity of ∼150 mAh g−1 is obtained (black line), while with LiBOB a slightly higher capacity of ∼160 mAh g−1 is obtained (red line) at a potential of 4.9 V (just above the charging plateau). Upon continuous charging after full de-lithiation (achieved at ∼4.8 V), the voltage of the cell containing standard electrolyte rises steeply to 5.4 V (vs Li/Li+), and then maintains a constant potential (black line in Figure 1a). Since the electrode is delithiated at 4.8 V, the additional capacity corresponds to electrolyte oxidation. As the potential rises to ∼5.4 V, CO2 evolution initiates at ∼5.0 V and its rate of formation gradually increases until it becomes constant as the potential achieves a constant value of ∼5.4 V (black line, Figure 1b). Continuous charging of the cell containing electrolyte with added LiBOB after full de-lithiation results in a more gradual increase in

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RESULTS AND DISCUSSION Gas Evolution Analysis (OEMS-cells). Galvanostatic charge profiles for Li/LiNi0.5Mn1.5O4 OEMS-cells for gas analysis containing standard electrolyte with and without added LiBOB are depicted in Figure 1. Upon charging from 3.5 to 4.9 V, the two plateaus at 4.7 and slightly below 4.8 V expected for the Ni2+/Ni3+ and Ni3+/Ni4+ transitions in LiNi0.5Mn1.5O4 electrodes are observed; the very small plateau at 4.0 V for the Mn3+/Mn4+ transition is not observed for the electrodes coated on Celgard. Over this typical potential range for de-lithiation of the LiNi0.5Mn1.5O4 electrode, there is some 7364

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Figure 2. Initial three charge−discharge curves of Li/LiNi0.5Mn1.5O4 cells with standard electrolyte (1.0 M LiPF6 EC/EMC (3/7)), and with 1.0% LiBOB (wt) containing electrolyte, at different charging cutoff voltages, 4.9 (a) and 5.4 V (b), respectively.

potential to ∼5.4 V (red line, Figure 1a). However, the CO2 evolution rate increases dramatically at ∼5.0 V, reaches a nearly constant value at ∼5.1 V, and after some time drops off to the same CO2 evolution rate observed in the standard electrolyte without LiBOB additive (red line, Figure 1b). While we are not aware of any gas evolution studies on LiNi0.5Mn1.5O4 in the literature, the CO2 evolution observed here starting at ∼4.8 V is in contrast to the reported absence of CO2 evolution up to 5.5 V on the closely related LiMn2O4 and LiCoO2 cathode material in a similar electrolyte (EC/DMC with 1 M LiTFSI).21 On the other hand, more consistent with our OEMS data (Figure 1b), CO2 evolution starting at ∼4.7−4.8 V was observed for Li1+x(Ni0.33Mn0.33Co0.33)1‑xO2 (x = 0 and 0.1) in EC/DMC with 1 M LiPF6.22 Interestingly, O2 evolution is not observed on LiNi0.5Mn1.5O4 for either electrolyte in our study, which parallels the behavior of stoichiometric NMC (Li1+x(Ni01/3Mn1/3Co1/3)1‑xO2; x = 0) and is different from the significant O2 evolution observed for over-lithiated NMC (x = 0.1);22 while only CO2 evolution is observed on Li-excess material Li[Li1/9Ni1/3Mn5/9]O2 upon cycling over 4.6 V in Gray’s work.23 Integration of the gas evolution curves provides quantification of the evolved gas. The additional CO2 evolution above ∼5.0 V in LiBOB containing electrolyte compared to the standard electrolyte (corresponding to the area under the red curve and the black curve in Figure 1b) amounts to 13.2 μmol CO2. Considering that the total quantity of LiBOB present in the electrolyte is approximately 12 μmol and that ∼1.1 mol of CO2 is liberated upon the anodic oxidation of LiBOB (Scheme 1), this means that a majority of the LiBOB additive is being oxidized rapidly at potentials exceeding ∼5.0 V. However, even at lower potentials LiBOB is being continuously oxidized at a low rate, which we believe creates a protective cathode surface coating according to Scheme 1 discussed below. Charge−Discharge Profiles of LiNi0.5Mn1.5O4 Electrodes (Coin Cells). In an effort to understand the surface reactions leading to significant differences in gas evolution for electrolyte with and without added LiBOB, additional electrochemical cycling and surface analysis have been conducted. The initial three charge−discharge profiles of Li/LiNi0.5Mn1.5O4 cells with standard electrolyte (1.0 M LiPF6 EC/EMC (3/7, v/v)) and electrolyte with 1.0% LiBOB (wt) at different

charging cutoff voltages, 4.9 and 5.4 V (vs Li/Li+), respectively, are depicted in Figure 2. The cycling profiles of the Li/ LiNi0.5Mn1.5O4 cells are very similar to the gas analysis cells described above (Figure 1). The first three cycles of the LiNi0.5Mn1.5O4 electrodes in standard electrolyte and electrolyte containing 1.0% LiBOB are very similar for cells cycled to 4.9 V, except for the Coulombic efficiency which is slightly higher for the standard electrolyte with 94.5%, 98.2%, and 98.4% (first, second, and third cycle, respectively) compared to the electrolyte containing LiBOB with 90.6%, 94.5%, and 95.3% (first, second, and third cycle, respectively). The lower Coulombic efficiency of the Li/LiNi0.5Mn1.5O4 cells cycling with LiBOB containing electrolyte is most likely due to the surface reactions of the added LiBOB on the electrodes as previously reported.15,16 Similar cycling was conducted with Li/LiNi0.5Mn1.5O4 cells with a higher cutoff voltage, 5.4 V, to develop a better understanding of the effect of the reaction of LiBOB with the surface of the LiNi0.5Mn1.5O4. Significant differences are observed between the electrolytes with and without added LiBOB, as depicted in Figure 1b. The Li/LiNi0.5Mn1.5O4 cells with both electrolytes have typical charge curves below 4.9 V. However, the potential of the cell with standard electrolyte rises to 5.4 V and then is maintained at 5.4 V, delivering significant additional current (70−100 mAh g−1) and resulting in electrolyte decomposition and low Coulombic efficiencies of 60.9%, 48.6%, and 49.1% (first, second, and third cycle, respectively). The electrolyte with added LiBOB has a sharper rise and much lower current at 5.4 V and significantly higher Coulombic efficiencies of 74.7%, 78.1%, and 86.2% (first, second, and third cycle, respectively). The enhanced Coulombic efficiency suggests that the oxidation of added LiBOB results in passivation of the cathode surface and inhibition of the oxidation of the bulk electrolyte. The differences in the sharpness of the increases to 5.4 V for Figures 1 and 2 may be due to continuous removal of CO2 in Figure 1 and retention of CO2 in Figure 2. A better understanding of the mechanism of passivation is afforded via ex situ surface analysis of the cycled electrodes, as discussed below. X-ray Photoelectron Spectroscopy (XPS) of LiNi0.5Mn1.5O4. The XPS spectra and surface elemental 7365

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Figure 3. 1s, O 1s, F 1s, B 1s, P 2p, Ni 2p, and Mn 2p XPS spectra of fresh LiNi0.5Mn1.5O4 electrode, and after cycling at 4.9 and 5.4 V with/without LiBOB added to the electrolyte.

slightly decreased, while O and P are increased, suggesting the formation of a thin surface film composed of electrolyte decomposition products covering the binder and metal oxide. Upon cycling to 5.4 V with the standard electrolyte, the concentration of O increases dramatically, while the concen-

concentrations of the fresh LiNi0.5Mn1.5O4 electrode and LiNi0.5Mn1.5O4 electrode after cycling at 4.9 and 5.4 V with and without added LiBOB are depicted in Figure 3 and Table 1. For the LiNi0.5Mn1.5O4 electrode cycled with standard electrolyte at 4.9 V, the concentrations of C, F, and Mn are 7366

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and LiF is observed in the F 1s spectrum. The XPS spectra of the LiNi0.5Mn1.5O4 electrodes cycled to higher potential (5.4 V) with electrolyte containing added LiBOB are very similar to the electrodes cycled at 4.9 V. There are minimal changes to the surface of the electrode suggesting that the initial oxidation products of the LiBOB inhibit further oxidation reactions of the electrolyte and the generation of a thicker surface film. FTIR Spectroscopy of Electrode Surfaces. The FTIRATR spectra of a fresh LiNi0.5Mn1.5O4 electrode and electrodes cycled with the standard electrolyte with and without added LiBOB at 4.9 and 5.4 V (vs Li/Li+) are depicted in Figure 4.

Table 1. Elemental Concentration (%) of Cathode Surfaces Determined by XPS

fresh cathode STD cycled at 4.9 V STD cycled at 5.4 V 1% LiBOB cycled at 4.9 V 1% LiBOB cycled at 5.4 V

C 1s

O 1s

F 1s

P 2p

Mn 2p

Ni 2p

B 1s

63.2 61.0 59.1 49.2

6.0 10.4 28.9 21.5

25.8 22.2 9.5 21.2

0 1.6 1.0 1.0

3.4 1.2 0 2.8

1.6 3.6 1.5 1.7

0 0 0 2.6

51.2

21.1

20.5

2.0

1.8

2.8

0.6

tration of Mn further decreases suggesting significant deposition of electrolyte decomposition products. For the LiNi0.5Mn1.5O4 electrodes cycled with LiBOB containing electrolyte at 4.9 V, decreases in the concentrations of C and F and increases in the concentrations of O and P are observed. However, upon cycling at 5.4 V the concentrations of C, O, F, and P remain relatively constant, suggesting that, after the initial oxidation of LiBOB, further oxidation of the bulk electrolyte is inhibited. In addition, B is detected on the electrode surface upon cycling at both 4.9 and 5.4 V, consistent with the presence of borates on the cathode surface, although surprisingly the concentration of B is lower for the sample cycled to 5.4 V. The C 1s spectrum of the fresh LiNi0.5Mn1.5O4 cathode contains three peaks. The peak at 284.3 eV is assigned to conductive carbon, and the peaks at 285.7 and 290.4 eV are characteristic of the PVDF binder. The F 1s peak of PVDF is also observed at 687.6 eV. The O 1s spectrum of fresh LiNi0.5Mn1.5O4 cathode is dominated by metal oxide (∼529 eV) and contains a very low concentration of Li2CO3 (531.5 eV), which is frequently present on fresh cathode particles.24 After cycling at 4.9 V with standard electrolyte, small changes can be observed in the XPS spectra of the LiNi0.5Mn1.5O4 electrode. The intensity of metal oxide (O 1s, 529.5 eV) is decreased and new peaks are observed in the C 1s and O 1s spectra characteristic of C−O and CO containing species (286 eV, 288 eV, and 533−534 eV and 532−533 eV, respectively) and in the F 1s spectrum characteristic of LiF (684.5 eV). Much larger changes are observed on the surface of the LiNi0.5Mn1.5O4 electrode after cycling at 5.4 V in standard electrolyte. The characteristic peaks corresponding to the PVDF binder (F 1s, 687.6 eV; C 1s 290.4 eV, 285.7 eV) and metal oxide (O 1s 529.5 eV) are no longer observable, suggesting that a thick layer of electrolyte decomposition products is formed on the electrode, which is further supported by the absence of Mn 2p and Ni 2p signals in the XPS spectra. The C 1s and O 1s spectra are dominated by peaks characteristic of CO and CO containing species, characteristic of poly(ethylene carbonate),5 while the F 1s spectrum contains a broad peak centered at 686 eV consistent with a combination of LiF and LixPFyOz (P 2p 134.5 eV). The dramatic increase in the presence of electrolyte decomposition products is consistent with the significant additional current observed at 5.4 V, as described above. The XPS spectra of the LiNi0.5Mn1.5O4 electrode cycled with electrolyte containing added LiBOB have some changes upon cycling at 4.9 V. The O 1s peak characteristic of the metal oxide (529 eV) is decreased in intensity while new peaks characteristic of CO and CO containing species are observed in the C 1s and O 1s spectra, consistent with the presence of carbonates and oxalates. In addition, a peak is observed at 193 eV in the B 1s spectrum, suggesting the presence of borates,

Figure 4. FTIR-ATR spectra of fresh LiNi0.5Mn1.5O4 electrode, and after cycling at 4.9 and 5.4 V with/without LiBOB added to the electrolyte.

The IR spectra are dominated by absorptions of the PVDF binder at 872, 1178, and 1400 cm−1. The changes to the IR spectra of electrodes cycled at 4.9 V with either the standard electrolyte or the electrolyte with added LiBOB are small, suggesting little reaction of the electrolyte with the surface of the electrodes. However, significant changes are observed for the electrodes cycled at 5.4 V. A strong absorption is observed at 1744 cm−1 characteristic of poly(ethylene carbonate), consistent with oxidation of the electrolyte upon cycling at 5.4 V.5 Incorporation of LiBOB results in a significant decrease in the intensity of polyethylene carbonate on the surface, especially when compared to the PVDF absorption at 1400 cm−1, and a new broad absorption is observed at ∼1600 cm−1 characteristic of lithium oxalate from the decomposition of LiBOB.15,16 The IR spectroscopic results provide further support that the LiBOB is sacrificially oxidized on the surface of LiNi0.5Mn1.5O4 and inhibits further electrolyte oxidation. The presence of oxalate supports the reaction of LiBOB with the cathode surface, while the decreased intensity of the poly(ethylene carbonate) supports the inhibition of electrolyte oxidation. 7367

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(8) Zhang, Z.; Hu, L.; Wu, H.; Weng, W.; Koh, M.; Redfern, P. C.; Curtiss, L. A.; Amine, K. Fluorinated Electrolytes for 5 V Lithium-ion Battery Chemistry. Energy Environ. Sci. 2013, 6, 1806−1810. (9) Xu, K.; Angell, C. A. Sulfone-Based Electrolytes for Lithium-Ion Batteries. J. Electrochem. Soc. 2002, 149, A920−A926. (10) Abouimrane, A.; Belharouak, I.; Amine, K. Sulfone-based Electrolytes for High-voltage Li-ion Batteries. Electrochem. Commun. 2009, 11, 1073−1076. (11) Felix; Cheng, J. −H.; Hy, S.; Rick, J.; Wang, F. −M; Hwang, B. −J. Mechanistic Basis of Enhanced Capacity Retention Found with Novel Sulfate-Based Additive in High-Voltage Li-Ion Batteries. J. Phys. Chem. C 2013, 117, 22619−22626. (12) Abu-Lebdeh, Y.; Davidson, I. High-Voltage Electrolytes Based on Adiponitrile for Li-Ion Batteries. J. Electrochem. Soc. 2009, 156, A60−A65. (13) Cresce, A. V.; Xu, K. Electrolyte Additive in Support of 5 V Li Ion Chemistry. J. Electrochem. Soc. 2011, 158, A337−A342. (14) Xu, M.; Lu, D.; Garsuch, A.; Lucht, B. L. Improved Performance of LiNi0.5Mn1.5O4 Cathodes with Electrolytes Containing Dimethylmethylphosphonate (DMMP). J. Electrochem. Soc. 2012, 159, A2130− A2134. (15) Dalavi, S.; Xu, M.; Knight, B.; Lucht, B. L. Effect of Added LiBOB on High Voltage (LiNi0.5Mn1.5O4) Spinel Cathodes. Electrochem. Solid-State Lett. 2012, 15, A28−A31. (16) 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. (17) Haa, S.−Y.; Hana, J. G.; Songa, Y. M.; Chuna, M.−J.; Hanb, S. I.; Shinb, W. C.; Choia, N. S. Using a Lithium bis(oxalato) borate Additive to Improve Electrochemical Performance of High-Voltage Spinel LiNi0.5Mn1.5O4 Cathodes at 60 °C. Electrochim. Acta 2013, 104, 170−177. (18) Pieczonka, N. P. W.; Yang, L.; Balogh, M. P.; Powell, B. R.; Chemlewski, K.; Manthiram, A.; Krachkovskiy, S. A.; Goward, G. R.; Liu, M.; Kim, J.−H. Impact of Lithium Bis(oxalate)borate Electrolyte Additive on the Performance of High-Voltage Spinel/Graphite Li-Ion Batteries. J. Phys. Chem. C 2013, 117, 22603−22612. (19) Tsiouvaras, N.; Meini, S.; Buchberger, I.; Gasteiger, H. A. A Novel On-Line Mass Spectrometer Design for the Study of Multiple Charging Cycles of a Li-O2 Battery. J. Electrochem. Soc. 2013, 160, A471−A477. (20) Shkrob, I. A.; Zhu, Y.; Marin, T. W.; Abraham, D. P. Mechanistic Insight into the Protective Action of Bis(oxalato)borate and Difluoro(oxalate)borate Anions in Li-Ion Batteries. J. Phys. Chem. C 2013, 117, 23750−23756. (21) Imhof, R.; Novak, P. Oxidative Electrolyte Solvent Degradation in Lithium-Ion Batteries: An In Situ Differential Electrochemical Mass Spectrometry Investigation. J. Electrochem. Soc. 1999, 146, 1702−1706. (22) La Mantia, F.; Rosciano, F.; Tran, N.; Novak, P. Direct Evidence of Oxygen Evolution from Li1+x (Ni1/3Mn1/3Co1/3)1‑x O2 at High Potentials. J. Appl. Electrochem. 2008, 38, 893−896. (23) Jiang, M.; Key, B.; Meng, Y. S.; Grey, C. P. Electrochemical and Structural Study of the Layered, “Li-Excess” Lithium-Ion Battery Electrode Material Li[Li1/9Ni1/3Mn5/9]O2. Chem. Mater. 2009, 21, 2733−2745. (24) Li, W.; Lucht, B. L. Lithium-Ion Batteries: Thermal Reactions of Electrolyte with the Surface of Metal Oxide Cathode Particles. J. Electrochem. Soc. 2006, 153, A1617−A1625.

4. SUMMARY AND CONCLUSIONS The combination of electrochemical measurements with ex situ surface analysis and in situ gas analysis has provided significant new insight into the mechanism of performance enhancement of LiNi0.5Mn1.5O4 cathodes cycled to high potential (4.8 V) in the presence of LiBOB. As previously suggested, LiBOB is sacrificially oxidized on the cathode surface to generate CO2 and to form a cathode passivation film. Oxidation of LiBOB results in the substantial decomposition of the added LiBOB, generating gaseous CO2 and forming a surface film on the cathode material. The latter significantly inhibits the oxidation of the carbonate solvents. Ex situ surface analysis of the LiNi0.5Mn1.5O4 electrodes by XPS reveals a thin surface film containing borates, carbonates, and oxalates. Thus, the gas and surface analysis of the oxidation of LiBOB is consistent with the mechanism shown in Scheme 1, which leads to the formation of a passivation film on the surface of the LiNi0.5Mn1.5O4 electrode: 1-electron oxidation of BOB− generates a neutral radical which eliminates CO2 to generate borate radical 1, which undergoes a radical coupling reaction with a second equivalent of 1 to generate the oxidized dimer 2. Similar reaction mechanisms have been reported recently.24 Further oxidation reactions of 2, and additional CO2 loss as evidenced by the ∼10% excess generation of CO2 per mole of LiBOB, would lead to cross-linking of the borates and stabilization of the surface film. In summary, we believe that the addition of LiBOB to LiPF6 electrolytes improves the retention capacity of LiNi0.5Mn1.5O4 cathodes during charge−discharge cycling via changes to the cathode material surface, thereby resulting in decreased Mn dissolution.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: (+) 1-401-874-5072. Present Address

Nikolaos Tsiouvaras, BMW AG, Munich 80788, Germany. Notes

The authors declare no competing financial interest.

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REFERENCES

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