Modifying the Surface of a High-Voltage Lithium-Ion Cathode - ACS

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Modifying the Surface of a High-Voltage Lithium-Ion Cathode Han Gao,†,○ Xiaoqiao Zeng,†,○ Yixin Hu,‡ Vasiliki Tileli,§ Luxi Li,∥ Yang Ren,∥ Xiangbo Meng,⊥ Filippo Maglia,# Peter Lamp,# Sung-Jin Kim,# Khalil Amine,*,†,∇ and Zonghai Chen*,†

ACS Appl. Energy Mater. 2018.1:2254-2260. Downloaded from pubs.acs.org by UNIV OF TEXAS AT EL PASO on 11/06/18. For personal use only.

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Chemical Science and Engineering Division and ∥X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Lemont, Illinois 60439, United States ‡ Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27514, United States § Institute of Materials, École Polytechnique Fédérale de Lausanne, Lausanne, Vaud, Switzerland CH-1015 ⊥ Department of Mechanical Engineering, University of Arkansas, Fayetteville, Arkansas 72701, United States # BMW Group, Munich, Germany 80788 ∇ Institute for Research and Medical Consultations, Imam Abdulrahman Bin Faisal University, Dammam, Saudi Arabia 34212 S Supporting Information *

ABSTRACT: Ni-rich lithium nickel manganese cobalt oxides (LiNixMnyCo1−x−yO2, NMCs) suffer from poor cycling stability at potentials above 4.2 V vs Li/Li+. This degraded cyclability at high potentials has been largely ascribed to the parasitic reactions between the delithiated cathode and the nonaqueous electrolyte. In this study, we mitigated the performance degradation of high-voltage NMC 622 by designing a functional interfacial layer that consists of a surface doping by Ti4+ and a TiO2 coating at the same time. The doping of Ti4+ near the surface of NMC can suppress the irreversible phase transformation, while the TiO2 coating can kinetically reduce the rate of the electron-transfer reaction between the delithiated cathode and the solvent. It is revealed that this interfacial engineering approach significantly enhanced both the cycling stability and the rate performance of NMC 622. KEYWORDS: lithium-ion batteries, high-voltage cathode, nickel-rich NMC, Ti4+ doping, TiO2 coating

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INTRODUCTION The development of high-voltage cathodes is essential for highenergy density lithium-ion batteries, which are targeted for both automobile and stationary energy storage applications. Among the available cathode materials, nickel-rich lithium transition metal oxides (LiNixMnyCo1−x−yO2, x ≥ 0.5, y ≤ 0.5, NMC) are gaining more popularity primarily due to their high specific capacity up to 200 mAhg−1 and potentially high energy density.1−3 However, this class of nickel-rich NMC cathodes suffers from an accelerated degradation of their electrochemical performance when the upper cutoff potential is raised above 4.2 V vs Li/Li+. Major research effort has been devoted to improving the stability of the nickel-rich NMC cathodes at relatively high working potentials to unlock their available energy density. In general, the performance degradation of the cathode material is dominated by their irreversible phase transformation4−6 and the parasitic reactions between the cathode materials and the nonaqueous electrolyte during the charging/ discharging or storage of lithium-ion batteries.7−10 Post diagnosis study showed that nickel-rich cathode materials have a high tendency to form a rock salt and/or spinel-like structure when the materials were aged/cycled at a high potential.11−13 Apparently, these aging processes are associated with partial loss of oxygen from the surface layer of material, leading to an increase of the atomic ratio between transition © 2018 American Chemical Society

metal elements and oxygen. However, the mechanism of oxygen loss still remains debatable. It has been consistently reported that this type of irreversible phase transformation was primarily observed at the interfacial layer of the cathode materials,14−16 probably associated with some other interfacial reactions that facilitate the loss of oxygen from the surface of the material to the environment. A commonly used strategy to suppress the irreversible phase transformation is to partially replace the transition metal ions with another metal ion like Li+, Ca2+, Mg2+, Al3+, or Ti4+,17−21 which have a stable valence state within the normal working potential window of lithium-ion batteries. This doping strategy is efficient in stabilizing the chemical environment of the transition metal ions close to the dopant sites, and hence, it suppresses the irreversible structural change of materials during cycling/aging at the expense of the reversible specific capacity of the cathode material. The latest research effort also significantly questions the chemical/electrochemical compatibility between the widely used nonaqueous electrolyte and the high-voltage cathode materials. Dahn et al. pioneered in developing a high-precision Coulombic efficiency (HPC) measuring system to investigate the parasitic reactions between the nonaqueous electrolyte and Received: March 1, 2018 Accepted: May 3, 2018 Published: May 3, 2018 2254

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fluorescence (XRF) and high-energy X-ray diffraction (HEXRD) experiments were carried out at the beamlines 2-ID-E and 11-ID-C of the Advanced Photon Source (APS) of Argonne National Laboratory, respectively, at room temperature. The scanned area for the XRF elemental mapping of the coated sample was 31 × 27 μm with a 200 nm step size for oversampling. The spatial resolution was around 500 nm, and the X-ray energy for XRF was 16 keV. The wavelength for the HEXRD experiment was 0.1173 Å. A PerkinElmer amorphous Sibased area detector and a CeO2 calibration standard were used for all diffraction images. Rietveld refinement was performed using general structure analysis software (GSAS).30 Transmission electron microscope (TEM) and scanning transmission electron microscope (STEM)/electron energy-loss spectroscopy (EELS) data were taken at a FEI Titan Themis at 300 kV. Samples were dispersed in ethanol and dropped in a lacey carbon TEM copper grid. Electrochemical Characterizations. Charge−discharge tests of the coin cells were performed on a battery tester (MACCOR Series 4000) at room temperature. Galvanostatic intermittent titration technique (GITT) measurement was carried out by charging or discharging the half-cells under a C/10 current for 10 min followed by an open circuit relaxation of 40 min. This procedure was repeated for one full cycle between 2.8 and 4.4 V vs Li/Li+. Electrochemical impedance spectroscopy (EIS) of the half-cells was performed with the setup as described in our pervious papers.28,29 A home-built highprecision leakage current measuring system was used to obtain the leakage current for electrodes holding at specific potentials at 30 °C.2

the electrode materials during the normal operation of lithiumion full-cells.22,23 Dahn et al. reported that the reaction mechanism changed at a potential above 4.45 V, resulting in an accelerated parasitic reaction at higher potentials.24 Alternatively, Chen et al. also developed a prototype system to precisely measure the static leakage current of lithium-ion cells. The kinetic study by Chen et al. revealed that, when the potential is lower than 4.45 V vs Li/Li+, the parasitic reaction is dominated by the chemical reaction between the delithiated cathode and carbonate solvents,2 the rate of which reaction is fundamentally determined by the concentration of reactive delithiated intermediates (LixTMO2, TM = transition metal) at the material surface. When the potential is higher than 4.45 V, the side reaction is then dominated by the direct electrochemical oxidation of the carbonate solvents,25,26 the rate of which, in theory, increases exponentially with the applied working potential. Regardless of their difference in the chemical nature, both the electrochemical reaction and the chemical reaction lead to the oxidation of carbonate molecules into organic radical cations, which are energetically instable and can be stabilized by releasing one proton to the environment. These reactions lead to a highly concentrated and localized H+ at the material surface,25 causing the leaching of oxygen anions and transition metal cations from the surface layer of the cathode material.27 We believe that the parasitic reactions between the delithiated cathode and the nonaqueous electrolyte are the major contributors of the performance degradation of the highvoltage cathode materials and that the performance degradation can be mitigated by designing a functional interfacial layer to improve the chemical compatibility between the solvent and the cathode intermediates. In this work, LiNi0.6Mn0.2Co0.2O2 (NMC 622) is used as the model high-voltage cathode material to demonstrate the effectiveness of our design philosophy. Here we developed a one-step surface modification process to introduce (a) a surface doping by Ti4+ to suppress the irreversible phase transformation; and (b) a TiO2 coating to kinetically reduce the electron-transfer reaction rate between the delithiated cathode and the solvent. We will show that our modification approach can significantly improve the electrochemical performance of NMC 622.

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RESULTS AND DISCUSSIONS LiNi0.6Mn0.2Co0.2O2 (NMC 622) was used as the baseline material to investigate the effect of the nano-TiO2 protection. Figure 1 shows the SEM images of the bare and the coated

EXPERIMENTAL SECTION

Preparation of Electrodes and Cells. Stoichiometric amounts of titanium butoxide (Ti(OBu)4, Sigma-Aldrich) and NMC 622 (Ecopro Co., research grade with no surface coatings or doping) cathode powder were mixed and well-dispersed in a pure ethanol solvent with strong stirring for 2 h. Subsequently, a mixed solution of ethanol and deionized water was added into the suspension dropwise. Then, the mixture was further stirred for 3 h followed by collecting the precursor powders by vacuum filtration. The solvent was slowly evaporated at 100 °C, and the obtained material was further heat treated at 500 °C for 5 h. The amount of TiO2 was 1 wt % (out of the total mass of TiO2 and NMC 622). Pristine and coated NMC 622 working electrodes with a composition of 91.5 wt % NMC 622, 4.4 wt % C45 carbon black, and 4.1 wt % polyvinylidene fluoride binder were prepared as described in our pervious papers.28,29 The loading of NMC 622 on each electrode was ca. 20 mg. CR2032 half-cells were then constructed in an Ar-filled glovebox using Celgard 2325 separators, metallic Li counter electrodes, and a LiPF6-based electrolyte (LP-57, BASF, 1.0 M LiPF6 in EC/EMC, 3:7 by weight). Material Characterizations. Scanning electron microscopic (SEM) images of the pristine and coated NMC 622 were obtained from a field emission SEM (Hitachi S-4700II). Synchrotron X-ray

Figure 1. SEM images of (a,c) pristine bare NMC 622 and (b,d) TiO2-coated NMC 622 particles at low and high magnifications.

NMC 622 at different magnifications. Both samples exhibited a roughly spherical morphology with a secondary particle size of ∼10 μm (Figure 1a,b). The coating of TiO2 did not change the morphology of the secondary particles. Figure 1c,d shows that small pallet-shaped primary particles were closely aggregated together to form the secondary particles. Also shown in Figure 1d is that the surface of the primary particles became rougher after being covered with TiO2. To further confirm the presence 2255

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particles of the coated sample was performed using TEM. Figure 3a shows the TEM images of a small agglomeration of primary particles of the coated NMC 622. At a higher magnification, it can be observed that TiO2 layer is an agglomeration of small crystalline particles with uneven distribution (Figure 3b). In fact, there are areas look like a layer, but most areas consist of an agglomeration of particles. The thickness of the layer/particle agglomerations ranges from ∼5 to ∼20 nm. Figure 3c shows the elemental mapping of Ti (green) and Ni (blue). An uneven distribution of the surface coverage with varying thickness was observed. The presence of the pinholes (i.e., areas that were not coated) may still be observed. To confirm the successful diffusion of Ti into the lattice of layered NMC material during the calcination process, a series of EELS spectra was obtained across the surface of the coated NMC. Figure 3d shows the focused ion beam (FIB) lamella used for the STEM/EELS analysis, and Figure 3e labels the positions where the EELS spectra were obtained at a higher magnification. The EELS spectra are shown in Figure 3f. Indeed, a gradual reduction in the Ti L3,2 peak and increases in the Mn L3,2 and Co L3,2 peaks can be observed when scanning toward the bulk of the particle. More importantly, the fingerprint of the Ti signals (i.e., Ti L3,2) can still be detected when peaks of Mn L3,2 and Co L3,2 begin to emerge. It was estimated that Ti doping in these areas ranged from ∼7 to 20 nm inside the NMC. Unlike conventional doping methods, the Ti doping in our case occurred near the surface of the NMC particle instead of the bulk material. This may be particularly beneficial to retard the phase transformation from layered to into spinel-like or rock-salt structures, which mostly occurs at the surface of the particle. The successful doping of Ti was further confirmed by examining the structural information on bare and coated samples obtained from HEXRD patterns with Rietveld

of nano-TiO2 on the NMC particle, synchrotron XRF imaging was used to trace the distribution of Ti. Figure 2 shows the

Figure 2. XRF elemental mapping of the TiO2-coated NMC 622 particles for (a) Ti, (b) Ni, (c) Mn, and (d) Co.

elemental mapping of Ti, Ni, Mn, and Co for the coated material. The secondary particles showed a spherical morphology, agreeing with the observations from SEM. Because of the much higher content of Ni, Mn, and Co compared to that of Ti, the signal from Ti was much weaker. Nevertheless, the uniform distribution of Ti on the NMC secondary particles can still be observed. Since the spatial resolution of XRF analysis was limited to ∼500 nm, more detailed characterizations of the primary

Figure 3. TEM images of a TiO2-coated NMC 622 particle at (a) low and (b) high magnifications; (c) STEM elemental mapping of the particle (Ti: green, Ni: blue); FIB lamella used for the STEM/EELS analysis at (d) low and (e) high magnifications; and (f) EESL spectra taken across the surface of the TiO2-coated NMC 622 particle. 2256

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ACS Applied Energy Materials refinement (Figure 4). Due to the limited amount of TiO2 on the sample, only the sharp diffraction peaks from the NMC 622

Figure 4. HEXRD Rietveld refinement of (a) bare NCM 622 and (b) TiO2-coated NMC 622.

were observed. In both samples, the good separation of the peaks between (006)/(102) and (108)/(110) at 2θ of ∼2.85 and ∼4.65° agrees with the typical layered structure of αNaFeO2 with an R3̅m space group. Table 1 shows the results of

Figure 5. (a) Voltage profiles and (b) differential capacity profiles of NMC 622/Li half-cells; (c) rate performance of the NMC 622/Li halfcells (2.8 to 4.4 V vs Li/Li+); and (d) variation in the chemical diffusion coefficient of Li+ (DLi) as a function of potential during Li+ insertion.

Table 1. Rietveld Refinements of Bare NMC 622 and TiO2Coated NMC 622 a/b (Å) c (Å) unit cell volume (Å3) I(003)/I(104)a wRp (%) Rp (%) a

bare NMC 622

TiO2-coated NMC 622

2.870(0) 14.222(1) 101.456(5) 1.28 3.24 3.58

2.871(6) 14.225(7) 101.593(7) 1.32 3.43 3.70

decrease in the internal resistance of the cells as demonstrated by the increase in the discharge voltage as shown by the arrow in Figure 5b. This difference between the differential capacity profiles at the beginning of discharge suggests a faster kinetics for Li+ insertion. In fact, the coated sample showed much faster discharge rate performance than the bare sample (Figure 5c). The TiO2-coated sample can still deliver nearly 70% of its original capacity under a 5 C current. In this discharge rate test, the cells were always charged to 4.4 V vs Li/Li+ at C/10 and then discharged at C/10, C/3, C/2, 1, 2, 3, and 5 C for every three cycles, respectively. This indeed demonstrates the advantage of the nanosized TiO2 particles deposited on the discrete domains of NMC surface. Since this significant enhancement in the rate performance may originate from the faster Li+ diffusion in the coated electrode, GITT analysis was used to compare the chemical diffusion coefficient of Li+ (DLi) in both samples with identical active material loadings and electrode porosity. Figure 5d shows the SoC-dependent diffusion coefficient of Li+ during the discharging of the half-cells. The high DLi values (on the order of 10−8 cm2s−1) of both samples at high SoC suggest that the measured DLi corresponds to the Li+ diffusion in the liquid electrolyte phase.32 It should be pointed out that the obtained values were smaller than the diffusion coefficient of Li+ in the bulk LiPF6-based nonaqueous electrolyte (∼10−6 cm2s−1).33,34 This is expected, since the effective diffusion coefficient of Li+ becomes smaller in a porous medium. A significant drop in DLi was observed between 3.9 to 3.5 V vs Li/Li+. Nevertheless, both samples still showed very similar values with identical trends. This indicates that Li+ diffusion was not hindered by the nanoTiO2 domains. Interestingly, the coated sample exhibited slightly higher DLi values at low SoC. The extremely low DLi values (in the range of 10−13−10−12 cm2s−1) indicate that the measured DLi was the Li+ diffusion in the solid-state phase.32 Therefore, the slightly higher DLi values of the coated sample at

Peak intensity ratios were obtained from the observed patterns.

the XRD Rietveld refinement for the two samples. Due to the larger size of Ti4+ (0.68 Å) compared to Co3+ (0.545 Å) and Mn4+ (0.53 Å), the doping of Ti slightly increased the lattice constants (a and c) as well as the unit cell volume. This indeed confirms that Ti4+ ions have been incorporated into the NMC lattice. It is generally accepted that an integrated intensity ratio of I(003)/I(104) had a strong relation with intermixing between Li and Ni cations: the higher intensity ratio of the I(003)/I(104), the lower the cation mixing.31 As shown in Table 1, the intensity ratios of the (003) and (104) peaks increased from 1.28 to 1.32 by Ti doping. This suggests a lower mixing between Ni and Li, which can lead to a better Li+ intercalation stability. The findings from HEXRD were in good agreement with the results from the STEM/EELS analysis. Further investigations were focused on the electrochemical properties of the bare and coated NMC 622 materials, with the emphasis on their rate and cycling performance. Figure 5a shows the first cycle voltage profiles of the NMC/Li half-cells under a constant current of C/10 (1 C = 205 mAg−1). Both cells showed similar initial charge capacity (∼230 mAhg−1) and discharge capacity (∼207 mAhg−1) values. The coverage of TiO2 slightly increased the initial Coulombic efficiency (CE) from 88 to 89%. Both the overlapping voltage profiles (Figure 5a) and the similar differential capacity profiles (Figure 5b) suggest that the nature of the charge storage/release of the layered NMC 622 material was not affected by the presence of nano-TiO2 coating. Instead, the coating resulted in a slight 2257

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the electrode/electrolyte interface. This could lead to less surface film passivation and much reduced film resistance. On the other hand, the presence of TiO2 had very little influence on the charge-transfer resistance (Figure 6d). Both samples showed a continuous increase in the RCT values over cycling. This observation was in a good agreement with the results shown in Figure 5c: the similar capacity retention of the two samples at smaller discharge currents (e.g., ≤ 2 C) originated from their similar DLi (Figure 5d) and RCT (Figure 6d) values, while the significant enhancement in capacity retention of the coated sample at higher discharge currents actually resulted from the much lower electrical resistance of the surface film. It is already mentioned above that we believe the coverage of nano-TiO2 domains can mitigate the interfacial reactions between NMC and the nonaqueous electrolyte, leading to less surface film resistance. However, fundamental understanding of the observed improvement in the electrochemical performance is required. Since one major interfacial reaction is the oxidation of the electrolyte solvent, our approach is to quantify the rate of this electron-transfer reaction by using our high-precision leakage current measurement system. In this experiment, the SoC of the working electrode or the lithium concentration in the working electrode remained unchanged. Any gain of electrons from the oxidation of electrolyte solvent will be compensated by the external current (we refer to this as the leakage current).2,35 Therefore, this current value can be used to quantify the reaction rate of the oxidation of electrolyte solvent. Figure 7a shows the high-precision leakage current relaxation curves of the bare and coated NMC 622/Li cells held at 4.2 V vs Li/Li+ for 40 h. The initial high leakage current values originated from the charging of the half-cell as well as the double-layer capacitor. With increasing holding time, the

lower potentials may be a result from the improved Li ion diffusion in the layered crystal structure due to the doping of Ti cations. However, the improvement was not very significant due to the limited amount of Ti doping near the surface of NMC. Nevertheless, the above discussion suggests that the enhancement in Li+ diffusion was not the major contributor to the much improved rate performance. Therefore, we believe that the higher electronic conductivity, and/or the faster charge-transfer kinetics of the coated sample, should be responsible for the higher rate performance. EIS analysis was used to deconvolute and quantify the resistance of the surface film as well as the resistance of the charge-transfer reaction from the measured cell impedance. Figure 6a,b shows the Nyquist plots of the bare and coated cells

Figure 6. (a,b) Nyquist plots of the NMC 622/Li half-cells at 4.4 V vs Li/Li+ as a function of cycle number; the extracted (c) surface film resistance (Rfilm) and (d) charge-transfer resistance (RCT) as a function of cycle number.

at 4.4 V vs Li/Li+ obtained during the C/10 cycling (2.8−4.4 V vs Li/Li+). In general, the first semicircle in the Nyquist plots at high−mid frequencies can be assigned to the total surface film resistances (Rs) while the second semicircle at the mid−low frequency window is related to the charge-transfer resistance (RCT). Additionally, the “tail” at the low frequencies was an indicator for the Li+ diffusion process through the porous electrodes. In this study, a simple equivalent circuit (inset of Figure 6a) was used to quantify Rs and RCT for the frequency region before the appearance of the “tail”, and the extracted resistance values are shown in Figure 6c,d. Both samples showed an initial reduction in the film resistance followed by stabilization after 5 cycles (Figure 6c). Indeed, the coated sample showed much smaller Rfilm values compared to the bare sample. The higher Rfilm of the bare sample was attributed to the thicker surface films formed by the oxidative decomposition of the electrolyte at high potentials. On the other hand, we believe that the coverage of nano-TiO2 domains can mitigate the oxidative decomposition of the electrolyte solvent, thus suppressing the parasitic reactions at

Figure 7. (a) Typical current relaxation curves collected from NMC 622/Li half-cells during potentiostatic hold at 4.2 V vs Li/Li+; (b) variation of the static leakage current as a function of potential with and without TiO2 coating; and cycling performance of the NMC 622/ Li half-cells at (c) 4.4 V, (d) 4.5 V, and (e) 4.6 V vs Li/Li+ cutoff potentials (current = 0.1 C). 2258

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leakage current values of both cells decayed to less than 20 nAmg−1. As expected, the coated sample showed a slightly lower leakage current compared to the bare material. The smaller current indicates a slower electron-transfer reaction, and therefore, less oxidation of the electrolyte solvent. The static leakage current values (y0) were then extracted from the current relaxation curves by fitting the current data with an exponential decay function as shown in the inset of Figure 7a. In this case, different y0 values were obtained at different holding potentials (Figure 7b). Since the electrochemical oxidation of the electrolyte solvent would become more severe with increasing potentials, the y0 values indeed showed an increasing trend as shown in Figure 7b. Nevertheless, the modified NMC sample showed smaller static leakage current values compared to the bare sample, agreeing with the trend observed in Figure 7a. This finding successfully validates our hypothesis and provides direct evidence of the mitigation of solvent oxidation reactions by the nano-TiO2 coating. The electrochemical oxidation of solvent not only increases the impedance of the electrodes by thickening the surface film but also generates a highly acidic chemical environment, leading to the corrosion of the cathode surface and leaching of the transition metals in the NMC. The suppression of this oxidative decomposition reaction can consequently lower the generated protons and lessen the corrosion issue, providing good cycle life. Figure 7c−e compares the cyclability of the bare and coated NMC 622 at three different upper cutoff potentials under a C-rate of 0.1. Under this low current density, the negative effect of impedance growth by solvent oxidation was minimized. Therefore, we could focus on the corrosion of NMC and its structural instability as the major reasons behind any capacity fade. In general, higher upper cutoff potentials led to lower capacity retention. However, the incorporation of nano-TiO2 protection domains greatly enhance the cyclability under all the tested upper cutoff potentials. In fact, cycling of the full-cells consists of the modified NMC 622 cathode, and an artificial graphite anode showed much higher capacity retention compared to its nonmodified counterpart (Figure S1). This clearly demonstrated the beneficial effect of the dual-functional nano-TiO2 coating.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.8b00323. Cycling performance of NMC 622/artificial graphite fullcells with and without the nano-TiO2 modification (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K.A.) *E-mail: [email protected] (Z.C.) ORCID

Xiangbo Meng: 0000-0002-4631-7260 Khalil Amine: 0000-0001-9206-3719 Zonghai Chen: 0000-0001-5371-9463 Author Contributions ○

H.G. and X.Z. contributed equally to this work

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the BMW Corporation. Argonne National Laboratory is operated for the US Department of Energy by UChicago Argonne, LLC, under contract DE-AC02-06CH11357. The authors also acknowledge the use of the Advanced Photon Source (APS) and the Center for Nanoscale Materials (CNM) that are supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. Support from Tien Duong of the U.S. DOE’s Office of Vehicle Technologies Program is gratefully acknowledged. H.G. would also like to acknowledge the NSERC Canada Postdoctoral Fellowships Program.

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REFERENCES

(1) Noh, H.-J.; Youn, S.; Yoon, C. S.; Sun, Y.-K. Comparison of the Structural and Electrochemical Properties of Layered Li[NixCoyMnz]O2 (x = 1/3, 0.5, 0.6, 0.7, 0.8 and 0.85) Cathode Material for LithiumIon Batteries. J. Power Sources 2013, 233, 121−130. (2) Zeng, X.; Xu, G.-L.; Li, Y.; Luo, X.; Maglia, F.; Bauer, C.; Lux, S. F.; Paschos, O.; Kim, S.-J.; Lamp, P.; Lu, J.; Amine, K.; Chen, Z. Kinetic Study of Parasitic Reactions in Lithium-Ion Batteries: A Case Study on LiNi0.6Mn0.2Co0.2O2. ACS Appl. Mater. Interfaces 2016, 8 (5), 3446−3451. (3) Myung, S.-T.; Maglia, F.; Park, K.-J.; Yoon, C. S.; Lamp, P.; Kim, S.-J.; Sun, Y.-K. Nickel-Rich Layered Cathode Materials for Automotive Lithium-Ion Batteries: Achievements and Perspectives. ACS Energy Letters 2017, 2 (1), 196−223. (4) Yoon, C. S.; Choi, M. H.; Lim, B.-B.; Lee, E.-J.; Sun, Y.-K. ReviewHigh-Capacity Li[Ni1‑xCox/2Mnx/2]O2 (x = 0.1, 0.05, 0) Cathodes for Next-Generation Li-Ion Battery. J. Electrochem. Soc. 2015, 162 (14), A2483−A2489. (5) Whittingham, M. S. Lithium Batteries and Cathode Materials. Chem. Rev. 2004, 104 (10), 4271−4302. (6) Hausbrand, R.; Cherkashinin, G.; Ehrenberg, H.; Gröting, M.; Albe, K.; Hess, C.; Jaegermann, W. Fundamental Degradation Mechanisms of Layered Oxide Li-Ion Battery Cathode Materials: Methodology, Insights and Novel Approaches. Mater. Sci. Eng., B 2015, 192, 3−25. (7) Xu, K. Nonaqueous Liquid Electrolytes for Lithium-Based Rechargeable Batteries. Chem. Rev. 2004, 104 (10), 4303−4418.

CONCLUSIONS In this study, we mitigated the performance degradation of high-voltage NMC 622 by designing a functional interfacial layer that consists of (a) a surface doping by Ti4+ to suppress the irreversible phase transformation and (b) a TiO2 coating to suppress the parasitic reactions between the delithiated cathode and the electrolyte solvent. The doping of Ti4+ near the surface of NMC was confirmed by both EELS analysis and Rietveld refinement of HEXRD. Ti4+ doping slightly improved the Li+ diffusion in the layered NMC lattice. More importantly, such doping can also provide extra stability of the NMC lattice. On the other hand, the TiO2 coating can successfully suppress the oxidative decomposition of the electrolyte solvent, resulting in much faster rate performance and better cyclability. The much lower surface film resistance resulting from the suppressed interfacial reactions was revealed to be the major contributor to the improved rate performance. This work clearly shows that the parasitic reactions at the electrode/electrolyte interface are causing the performance degradation of the high-voltage cathode materials, and it is essential to improve the compatibility between the cathode intermediates and the electrolyte solvent. 2259

DOI: 10.1021/acsaem.8b00323 ACS Appl. Energy Mater. 2018, 1, 2254−2260

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DOI: 10.1021/acsaem.8b00323 ACS Appl. Energy Mater. 2018, 1, 2254−2260