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Long-life nickel-rich layered oxide cathodes with a uniform Li2ZrO3 surface coating for lithium-ion batteries Bohang Song, Wangda Li, Seung-Min Oh, and Arumugam Manthiram ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00070 • Publication Date (Web): 01 Mar 2017 Downloaded from http://pubs.acs.org on March 2, 2017

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ACS Applied Materials & Interfaces

Long-life nickel-rich layered oxide cathodes with a uniform Li2ZrO3 surface coating for lithium-ion batteries Bohang Song, Wangda Li, Seung-Min Oh, and Arumugam Manthiram* Materials Science and Engineering Program & Texas Materials Institute The University of Texas at Austin, Austin, Texas 78712, USA

ABSTRACT As nickel-rich layered oxide cathode start to attract worldwide interests for the nextgeneration lithium-ion batteries, their long-term cyclability in full cells still remains a challenge for electric vehicles. Here we report a long-life Ni-rich layered oxide cathode (LiNi0.7Co0.15Mn0.15O2) with a uniform surface coating of the cathode particles with Li2ZrO3. A pouch-type full cell fabricated with the Li2ZrO3-coated cathode and a graphite anode displays a 73.3 % capacity retention after 1,500 cycles at C/3 rate. The Li2ZrO3 coating has been optimized with a systematic study with different synthesis approaches, annealing temperatures, and coating amounts. The complex relationship among the coating conditions, uniformity, and morphology of the coating layer, and their impacts on the electrochemical properties is discussed in detail.

KEYWORDS: lithium-ion batteries, layered oxide cathodes, nickel-rich oxides, Li2ZrO3 surface coating, full cells, long-term cyclability

___________________________________ *Corresponding author: Phone: (512) 471-1791. Fax: (512) 471-7681. E-mail: [email protected]

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INTRODUCTION Lithium-ion batteries (LIBs) are powering the portable devices, including laptops and smartphones, for more than two decades. Whether such dominance could be extended to the next contest of electric vehicles is highly dependent on the gravimetric and volumetric energy densities the next-generation LIBs could provide. To meet the demands of at least 300 Wh kg-1 and 750 Wh L-1, the storage capability of both the positive and negative electrodes need to be increased compared to the state-of-the-art LiCoO2 cathode and graphite anode. Among the cathode candidates, nickel-rich layered oxides (LiNi1-x-yCoxMnyO2, (1-x-y) > 0.5) have drawn worldwide interest mainly due to their high reversible capacity (> 200 mA h g-1) and high operating voltage (> 3.5 V vs. Li/Li+).1-2 LiNiO2 has long been recognized to be electrochemically active since the 1980s. Unfortunately, a large degree of cation disorder between Ni2+ and Li+ is inevitable during the synthesis of LiNiO2 because of the difficulty of maintaining all Ni3+, even under flowing oxygen.3 As a result, nonstoichiometric Li1-xNi1+xO2 is formed with a partial amount of Ni2+ residing at the lithium layers that blocks the effective diffusion pathway for Li+ ions during cell operation. Additionally, Li1-x-δNi1+xO2 at highly delithiated states undergoes several phase transitions, which could be accelarated at elevated temperatures.4 Based on the above considerations, the electrochemical properties of LiNiO2 are not satisfactory. Nonetheless, elemental substitutions in LiNi1-yMyO2 (M = Co, Mn, Ti, Mg, Al, etc.) has revived this family of cathode materials, among which cobalt and manganese co-substituted LiNi1-x-yCoxMnyO2 cobalt and aluminum co-substituted LiNi1-x-yCoxAlyO2 are the most successful compositions.5-9 Generally speaking, Co3+ suppresses the cation disorder between Ni2+ and Li+, while Mn4+ stabilizes the local structure. A recent study10 has reported that Al3+ is 2 ACS Paragon Plus Environment

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homogeneously distributed at a single particle level even at only

five mole percent of

substitution at the transition-metal layer in LiNi0.8Co0.15Al0.05O2. It has been speculated that Al3+ also reduces the Jahn-Teller distortion of Ni3+. Besides the bulk stabilization, the particle surface chemistry of nickel-rich layered oxides is also vital to the overall performance, especially the long-term cyclability. According to previous studies, Ni4+ formed under the charge tends to react with the organic electrolyte aggressively and yields undesirable side reaction products; these reactions are aggravated particularly at elevated temperatures and higher operating voltages (> 4.3 V vs. Li/Li+).11 In addition, a NiO rock-salt phase has been identified with transmission electron microscopy (TEM) after several cycles at the outmost region of a grain due to the chemical instability of the highly oxidized Ni4+.12-13 Both the undesirable side reactions with the electrolyte and the rock-salt phase formation degrade the diffusion kinetics of Li+ ions during cycling, resulting in capacity fade. In this regard, stabilization of the surface of nickel-rich layered oxides is essential to achieve longterm cyclability.14-19 The surface coating layer needs to be inert and chemically stable against the electrolyte throughout the whole applied voltage window, while offering good lithium-ion diffusion. Hellstrom et al.20 reported that Li2ZrO3 is a relatively good Li-ion conductor with a conductivity of 3.3 x 10-5 S m-1 at 573 K. More importantly, they claimed that Li2ZrO3 belong to a small group of ternary oxides that are thermodynamically stable against Li. Baklanova et al.21 studied the active diffusion pathways of polycrystalline Li2ZrO3 via

6,7

Li nuclear magnetic

resonance (NMR) and evidenced Li-ion diffusion pathways involving both Li(1) and Li(2) active sites in an equal proportion. Figure 1a represents these Li(1) and Li(2) sites in a monoclinic phase with the C2/c symmetry. Although the Li2ZrO3 phase has been previously reported as an effective coating agent to protect the surface of nickel-rich layered oxides, there is not much 3 ACS Paragon Plus Environment


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literature systematically addressing the complex relationships among coating conditions, uniformity and morphology of the coating layer, and their impacts on the electrochemical properties. In this study, we aim to explore the best coating conditions for Li2ZrO3 to achieve longterm cyclability for a Ni-rich layered oxide cathode, LiNi0.7Co0.15Mn0.15O2. Sol-gel and precipitation synthesis approaches are pursued with two different annealing temperatures of 650 and 800 °C, which essentially tune the crystallinity level of the Li2ZrO3 coating layer. In addition, various coating amounts of 1, 3, and 5 wt.% are pursued for each coating condition. More importantly, pouch full cells fabricated with an optimal Li2ZrO3-coated LiNi0.7Co0.15Mn0.15O2 cathode and a graphite anode are evaluated for 1,500 cycles to assess the practical potential of the coating method. EXPERIMENTAL SECTION Synthesis of LiNi0.7Co0.15Mn0.15O2 A continuously stirred tank reactor (CSTR) was used to produce first the Ni0.7Co0.15Mn0.15(OH)2 precursor, as reported in our previous paper.22 Ni0.7Co0.15Mn0.15(OH)2 was homogenously mixed with a corresponding amount of LiOH·H2O with a mortar and pestle. Then, the mixture was preheated at 650 °C for 12 h in air and calcined at 800 °C for 15 h under oxygen flow. The as-prepared LiNi0.7Co0.15Mn0.15O2 is hereafter termed as the bare sample. The bare LiNi0.7Co0.15Mn0.15O2 had a tap density of ~ 2.2 g cm-3.


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Preparation of Li2ZrO3-coated LiNi0.7Co0.15Mn0.15O2 To prepare the Li2ZrO3-coated LiNi0.7Co0.15Mn0.15O2, both a sol-gel and a precipitation methods were applied. For the sol-gel method, the Ni0.7Co0.15Mn0.15(OH)2 precursor was first mixed with a corresponding amount of LiOH·H2O and preheated at 650 °C for 12 h. 2 g of the preheated powder was then dispersed into an ethanol solution (200 mL) that contained a required amount of CH3COOLi·2H2O and ZrO(NO3)2 (molar ratio of Li/Zr = 2). The suspension was continuously stirred at 80 °C until all the ethanol solvent evaporated, and the residual mass was finally calcined at 800 °C for 15 h under oxygen flow to obtain the coated sample. Various coating amounts of 1, 3, and 5 wt.% were applied and the corresponding samples are hereafter termed as, respectively, SG800-1, SG800-3, and SG800-5. For the precipitation method, 2 g of the preheated powder (650 °C) was dispersed into an ethanol solution (200 mL) that contained only a required amount of ZrO(NO3)2. Then, ammonium hydroxide solution in ethanol (0.3 M) was added into it drop by drop. The added ammonium hydroxide was ten time larger than the OH- groups needed for the formation of Zr(OH)4. After stirring the mixed suspension for 2 h, the powder was collected by filtration and washed with de-ionized water before drying it overnight at 100 °C. The collected powder was further mixed with a required amount of LiOH·H2O, which was

calculated

based

on

the

weight

ratio

of

LiNi0.7Co0.15Mn0.15O2

in

the

LiNi0.7Co0.15Mn0.15O2/Zr(OH)4 composite, and calcined at 800 °C for 15 h under oxygen flow. The samples with 1, 3, and 5 wt.% of Li2ZrO3 are hereafter termed as, respectively, PC800-1, PC800-3, and PC800-5. It is important to note that LiNi0.7Co0.15Mn0.15O2 forms during the preheating process at 650 °C. For a comparison, bare LiNi0.7Co0.15Mn0.15O2 (calcinated at 800 °C) sample was also coated with 1, 3, and 5 wt.% of Li2ZrO3 with the same sol-gel approach as described previously, but post-annealed at 650 °C for 5 h, which are hereafter termed, 5 ACS Paragon Plus Environment


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respectively, as SG650-1, SG650-3, and SG650-5. Similarly, bare LiNi0.7Co0.15Mn0.15O2 (calcinated at 800 °C) sample was also coated with 1, 3, and 5 wt.% of Li2ZrO3 with the same precipitation approach as described previously, but post-annealed at 650 °C for 5 h, which are hereafter termed as PC650-1, PC650-3, and PC650-5. Structural characterization X-ray powder diffraction (XRD) was carried out with a Rigaku Miniflex 600 (Cu Kα radiation) within a 2θ range of 10 – 80° with a scan rate was 1° per min. Particle morphology and elemental distribution of the cross-section was examined by a FEI Quanta 650 scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). Detailed crystallographic information was also revealed by a transmission electron microscopy (TEM, JEOL-2010F). Electrochemical testing Galvanostatic charge/discharge properties were evaluated with CR2032 coin cell. The slurry was prepared by mixing the nickel-rich layered oxides, carbon black (Super P), and polyvinylidene fluoride (PVDF) in N-methyl-2-pyrrolidone (NMP) solution with a weight ratio of 80 : 10 : 10. After homogeneously mixing, the slurry was pasted onto an aluminum foil as a current collector and dried at 120 °C overnight before punching into small pieces as cathode electrodes. A typical loading of the as-prepared cathode was 4 mg·cm-2. All half coin cells were assembled in an Argon filled glove-box with the as-prepared cathode, a Li metal anode, a Celgard 2500 separator, and several drops of the electrolyte (1.2 M LiPF6 in a 3 : 7 mixture of ethylene carbonate (EC) and ethylmethyl carbonate (EMC)). All half cells were cycled in a voltage window of 3.0 – 4.5 V (vs. Li+/Li) except for the rate capability test (2.0 – 4.5 V to ensure full discharge at a large current density). A 1C rate refers to 180 mA·g-1. Laminated-type 6 ACS Paragon Plus Environment

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pouch cell was also assembled with the optimum nickel-rich oxide as the cathode and graphite (Mesocarbon microbead (MCMB) from Osaka Gas Co., Ltd) as the anode, and then cycled in a voltage window of 2.5 – 4.4 V. A solartron 1260A impedance analyzer was used to conduct the electrochemical impedance spectroscopy (EIS) study on half coin cells from 100 kHz to 0.01 Hz with an AC voltage amplitude of 5 mV. Various cycling stages (1st, 10th, 50th, and 100th cycles) were monitored. Before the EIS study, all cells were charged to 4.2 V and held for 3 h to reach a quasi-equilibrium state. RESULTS AND DISCUSSION Enlarged XRD patterns are shown in Figures 1b-1e to compare the crystallinity of the Li2ZrO3 coating layer (PDF: #04-011-5929). As the post annealing temperature of both the SG650 (Figure 1b) and PC650 (Figure 1c) samples was 650 °C, the crystallinity of Li2ZrO3 is inconspicuous regardless of the various amounts of coating. In comparison, as the annealing temperature increases from 650 to 800 °C, Figures 1d and 1e obviously show a highly-crystalline Li2ZrO3 phase in both the SG800 and PC800 samples as indicated by the expected reflections of Li2ZrO3. This observation is consistent with the previous literature.23-25 In addition, the precipitation coating clearly leads to a higher degree of crystallinity as the relative intensities of the peaks of PC800 samples are higher than those of SG800. Sato et al.26 systematically studied the thermal decomposition behaviors of zirconium hydroxide prepared from an aqueous solution and found that a tetragonal zirconium oxide formed at around 500 °C, but was metastable. As the temperature increases from 500 to 600 °C, this tetragonal ZrO2 phase transformed into a more stable monoclinic phase with the P21/c symmetry. Note that the final Li2ZrO3 phase in our case is also a monoclinic phase with the C2/c symmetry. It is likely that the formation of Li2ZrO3 by transforming the precipitated Zr(OH)4 is relatively easier in comparison with the sol-gel case 7 ACS Paragon Plus Environment


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using the CH3COOLi·2H2O and ZrO(NO3)2 as precursors. The full XRD patterns of all the samples are shown in Figure S1, where no other impurity peaks could be seen. Figure 1f shows the morphology of bare LiNi0.7Co0.15Mn0.15O2, representing a large, micron-scale secondary particle composed of smaller submicron-sized primary particles. The inset demonstrates a uniform distribution of secondary particles of ~ 10 µm size in average. Figure 1g compares similarly-scaled SEM images of the PC650-5 sample. Apart from the analogous size distribution of both the primary and secondary particles, it is also easy to identify additional shining nano-sized spots on the primary particle surface, corresponding to the Li2ZrO3 grains.27 However, as shown in Figure S2, such uniformly distributed nano-sized spots of Li2ZrO3 are barely seen in the SG650-5, SG800-5, and PC800-5 samples. This is possibly due to either the non-uniform coating or the larger grain size of the Li2ZrO3 phase as a result of various coating conditions and annealing temperatures. To further characterize the uniformity of the surface coating layer, we applied an elemental mapping of Zr on the cross-section of a large secondary particle using SEM-EDX. Note that such SEM-EDX mapping was usually conducted on the outmost surface of either nano-sized or micron-sized particles rather than their cross-section. One concern of the outmost surface mapping is that it is unable to distinguish the coating effect from doping. However, in our case, the large size of the secondary particles in addition to the cross-section mapping easily yields a good contrast of the coating element in terms of the spatial difference between the surface and bulk, as long as they are rich in surface and poor in the bulk. In fact, we indeed observed a Zrring at the particle surface of the PC650-5 sample, as clearly represented in Figure 2b’. It implies the formation of a surface Zr-rich phase Li2ZrO3 instead of elemental substitution to form Li(Ni0.7Co0.15Mn0.15)1-xZrxO2. Nonetheless, a small amount of Li(Ni0.7Co0.15Mn0.15)1-xZrxO2 is still 8 ACS Paragon Plus Environment

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possible only at the outmost surface region.28-29 Coating rather than doping is crucial for electrochemical cyclability as the surface sensitivity issue is more critical than the bulk stability. Based on the surface area of our Ni-rich layered oxide (~ 0.7 m2/g)30 and the theoretical density of Li2ZrO3 (4.16 g/cm3), the coverage is, for example, ~ 34% for the PC650-5 sample with 5 wt.% of Li2ZrO3. The EDX mapping of other elements (Ni, Co, and Mn) in the sample PC650-5 is shown in Figure S3. Apparently, all of them are uniformly distributed throughout the bulk region. Figure S4 provides additional evidence with EDX mapping to fully support the uniformity of Zr-ring formation for another particle. In contrast, none of the other samples exhibit a similar Zr-ring. For example, the SG800-5 sample (Figure 2c’) shows the most uniform distribution of Zr across the entire secondary particle. It is possibly due to a higher penetration rate of Li+ and Zr4+ ions into the bulk particle during the sol-gel synthesis, while in the case of precipitation, the insoluble Zr(OH)4 easily nucleates at the surface region rather than penetrating into the bulk. Nonetheless, for the SG800-5 sample, it is still important to note that Zr is in the form of Li2ZrO3 (Figure 1d) although penetration into the bulk occurs. As will be seen in the electrochemistry section, SG800-5 does not show improved cyclability. On the other hand, the PC800-5 sample exhibits Zr-rich clusters at the outmost surface, which might be explained by an excessive grain growth of Li2ZrO3 upon calcination at 800 °C. TEM was conducted to further unveil the distribution and detailed crystallographic information of coated-Li2ZrO3. Figures 3a-3c show the SG800-5 sample and Figures 3d-3f show the PC650-5 sample. Comparing Figure 3a with 3d, the SG800-5 sample has larger Li2ZrO3 particles (~ 200 nm average size) at the surface of secondary particles than the counterpart PC650-5 sample, where the average size of Li2ZrO3 is ~ 50 nm. This observation agrees well 9 ACS Paragon Plus Environment


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with the XRD results shown in Figures 1c and 1d. Not only the different nucleation seeds induced by sol-gel and precipitation methods but also the final annealing temperature plays a critical role to determine the average particle size of Li2ZrO3. 800 °C leads to a drastic grain growth by merging several small particles into bigger particles via high-temperature solid-state diffusion. This could explain why the PC800-5 (Figure 2d’) sample exhibits Zr-rich clusters rather than a uniform coating layer (Figure 2b’) although precipitation was applied in both cases. As expected, the PC650-5 sample shows a quite uniform coating layer with Li2ZrO3 agglomerates sticking together, as shown in Figure 3d. Combining this TEM observation with the previous cross-section EDX mapping result, it can be concluded that the sample prepared by the PC650 method possesses the best coating condition to fully protect the surface of the active material. Furthermore, high-resolution TEM image (Figure 3f) reveals that within a crystal of Li2ZrO3, highly crystallized domain is in the middle, which is fully wrapped by an amorphous region for the PC650-5 sample. Presumably, this configuration will help facilitate Li+-ion transport at the surface region because amorphous lattice contributes to faster ion transport than the crystallized one.31 In contrast, the Li2ZrO3 phase in the SG800-5 sample is completely crystallized, as shown in Figure 3c. The lattice mismatch between crystalline Ni-rich oxide and Li2ZrO3 could result in poor transport of Li+ ions due to the increased energy barrier for hopping. The coating could also peel off from the Ni-rich particle because of the strain repeatedly generated around the mismatch region especially when Li+ ions hop through. As seen in the TEM data in Fig. 3f, the amorphous region around the exterior of Li2ZrO3 for the best coating condition (PC650) may alleviate the lattice mismatch effects in terms of both ion transport and lattice strain around the grain boundary. In comparison, the highly crystallized Li2ZrO3 particle throughout all the regions 10 ACS Paragon Plus Environment

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(Figure 3d) for the worst coating condition (SG800) could amplify the lattice mismatch effects upon cycling. This difference between the PC650 and SG800 samples could possibly be one reason for the difference in their cycling performance. The charge/discharge curves of the initial cycle of different samples are shown in Figure S5. The various amounts of the inactive phase Li2ZrO3 in the composite lead to a small variation in the charge/discharge capacity and coulombic efficiency (CE). Nonetheless, taking advantage of a high cut-off voltage (4.5 V vs. Li+/Li), all samples delivered a ~ 200 mA h g-1 discharge capacity and ~ 88 % CE. A slightly higher discharge capacity of the 1 wt.% coated sample compared to the bare sample is possibly due to less formation of residual Li-containing compounds such as Li2CO3 and LiOH upon air storage.32 As the coating amount increases from 1 to 5 wt.%, the inactive Li2ZrO3 starts to degrade the reversible capacity. Figure 4 shows the cyclability of all the samples. The bare LiNi0.7Co0.15Mn0.15O2 delivers a discharge capacity of 190 mA h g-1 at C/3 rate with 82.4 % capacity retention after 100 cycles, while all the other coated samples exhibit enhanced cyclability to various extents. The SG650 and PC650 series of samples are better than the SG800 and PC800 series of samples. The better performance is attributed to different protection efficiency as a result of different coating conditions. As discussed in the previous sections, precipitation is better than sol-gel in terms of the coverage efficiency. Meanwhile, an increase in the annealing temperature from 650 to 800 °C induces a drastic grain growth that significantly reduces this coverage efficiency. Overall, the PC650 series of samples exhibit the best cyclability among all the samples. Within the PC650 samples, to balance between specific capacity and cyclability, the 3 wt.% Li2ZrO3-coated LiNi0.7Co0.15Mn0.15O2 is the best cathode material because of the high initial capacity of 190 mA h g-1 and a 92.8 % capacity retention after 100 cycles. The TEM images of the SG800-5 and 11 ACS Paragon Plus Environment


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PC650-5 samples after 100 cycles are shown in Figure S6. As can be seen in Figure S6, it is hard to find crystallized Li2ZrO3 particles with around 200 nm size for the SG800-5 sample after cycling. We speculate that it is because of the lattice mismatch (Figure 3) between the Ni-rich oxide and Li2ZrO3, which induces serious strain between grain boundaries. After repeatedly cycling, the Li2ZrO3 particles are peeled off from the Ni-rich particle surface. In contrast, an agglomerate of nano-sized Li2ZrO3 particles is still easily found for the PC650-5 sample after cycling. It strongly supports our assertion that the amorphous exterior region of Li2ZrO3 alleviates the strain evolution between the Ni-rich particle and the Li2ZrO3 coating and suppresses the peeling off of the coating, which is crucial in continuously protecting the surface of the Ni-rich oxide during cycling. To reveal the robustness of the Li2ZrO3 coating layer, the cathode electrodes were harvested from coin cells after 100 cycles and a cross-sectional SEM-EDX mapping of Zr of the secondary particle was obtained. The results are shown in Figure 5. Again, as clearly seen, the Zr-ring is preserved at the outmost particle surface for the PC650-5 sample. It implies strong chemical stability of the Li2ZrO3 phase against side reactions with the electrolyte. While in both cases, the inert Zr4+ is preserved at the Ni-rich particle surface throughout 100 charge/discharge cycles, which helps suppress the continuous corrosion of the particle surface as a result of electrolyte decomposition and transition-metal dissolution.33 To correlate the cyclability with impedance evolution, EIS study was conducted as a function of cycle number from 1st to 100th cycle on the bare and PC650-5 samples. The results are compared in Figure 6. Apparently, three semicircles in the order of decreasing frequency are observable from the initial cycle to the last for both samples. Zhuang et al.34 explained these three semicircles in accordance with the (de)intercalation type reaction in LiMn2O4 spinel. The 12 ACS Paragon Plus Environment

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first one in the high frequency range represents the resistance of the solid-electrolyte interphase (SEI) film (RSEI). The second one from the middle to a high-frequency range is correlated with the electronic properties of the material (Re). The third one in the middle frequency range is associated with the charge-transfer resistance (Rct). Based on these explanations, there are two important characteristics in our case. First, the Rct is significantly reduced throughout all the cycles (from 1st to 100th cycle) when comparing with the bare Ni-rich oxide. It implies less side reactions and formation of insulated products between the particle surface and electrolyte due to the protection by Li2ZrO3 coating layer. Second, the RSEI of the coated sample is always larger than the bare counterpart throughout cycling. It is another evidence of the varied SEI components before and after coating. Overall, the surface resistance is still significantly reduced as the Rct always contributes the most part of the resistance. These impedance observations agree well with the electrochemical performance obtained with the same cells (left image in Figure 6). The rate capabilities of all the samples are compared in Figure S7. All the coated samples exhibit better high-rate performance than the uncoated sample, especially at 10C rate. This is mainly because of the suppressed formation of insulating products due to side reactions with the electrolyte. In fact, Ni-rich layered oxides are excellent electronic and ionic conductors, which is why the large-secondary-particle morphology with 2.2 g cm-3 tap density still exhibits high-rate capability. On the other hand, the surface chemistry plays a key role in determining the electronic and ionic transport. In order to fully evaluate the potential of the coated Ni-rich oxides, we prepared a pouch-type full cell with the PC650-3 sample as a cathode and graphite as an anode (Figure 7). The electrochemical performance of a half cell using this graphite anode is given in Figure S8. The negative and positive electrode capacity ratio (N/P ratio) was kept at ~ 1.05. The total capacity of the pouch full cell is ~ 17 mA h with ~ 175 mA h g-1 specific capacity. 13 ACS Paragon Plus Environment


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Remarkably, the cyclability is superior with 73.3 % (12.5 mA h) capacity retention after 1,500 cycles. The initial coulombic efficiency is 77 %, which increases continuously to above 99 % and holds for the rest of the cycles. Such observation is common in full cell configuration that is reasonably attributed to the formation of stable SEI layer on both electrodes. The evolution of voltage midpoint upon this long-term cycling is shown in Figure S9. As clearly seen, although internal resistance of the cell induces voltage separation from 40 mV (1st cycle) to 206 mV (1500th cycle), the discharge plateau still exhibits a reasonably good voltage of 3.67 V after 1500 cycles. The ex-situ TEM image and XRD pattern of the PC650 cathode material is shown in Figure S10. Clearly, even after 1500 cycles, the nickel-rich layered structure with 𝑅𝑅3� 𝑚𝑚

symmetry is maintained whereas the protective Li2ZrO3 nanoparticles still adhere to the surface of a large nickel-rich particle. Most importantly, this cycling performance was tested under C/3 rate and using 4.4 V as cut-off voltage, that induced an extremely long-term duration time and aggressive condition for the cell testing. We believe such remarkable cycle performance proves that our Li2ZrO3 coating of LiNi0.7Co0.15Mn0.15O2 is a promising approach for next-generation Liion batteries when a long cycle life is needed. Figure 8 schematically summarizes the best coating condition (PC650) we explored for the Ni-rich LiNi0.7Co0.15Mn0.15O2 oxide. Note that the coated Li2ZrO3 phase is in the form of an island bead rather than a flake or a thin layer because 650 °C annealing temperature leads to a moderate grain growth. Nonetheless, this relatively low annealing temperature was unable to fully crystallize the Li2ZrO3 phase. Spatially, a highly crystalline domain is located within an amorphous lattice. This structural configuration might be beneficial for fast Li+-ion transport and adaptation of lattice strain generated around the grain boundary area between the coated phase and the Ni-rich phase. On the other hand, these Li2ZrO3 islands effectively protect the surface of 14 ACS Paragon Plus Environment

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the particle by minimizing the corrosion effects from the electrolyte upon cycling. Both effects determine that the PC650 sample delivers the best cyclability.

CONCLUSIONS

We have systematically investigated the correlation between surface coating conditions and the electrochemical properties of Ni-rich layered oxides. Taking Li2ZrO3 as an example for the coating agent, it has been found that the precipitation method via the formation of Zr(OH)4 as the nucleation seeds in addition to a relatively-low annealing temperature at 650 °C leads to the best electrochemical cyclability. As a result, a pouch-type full cell fabricated with the optimal Li2ZrO3-coated LiNi0.7Co0.15Mn0.15O2 as a cathode and graphite as an anode was capable of retaining 73.3 % of the initial capacity after long 1,500 cycles at C/3 rate. Through detailed characterizations with XRD, SEM-EDX cross-section mapping, and HRTEM, the remarkable long-term cyclability is attributed to the formation of Li2ZrO3 islands with large areas and excellent homogeneity to effectively protect the surface of the sample. Furthermore, a single Li2ZrO3 island was found to be partially amorphous and partially crystalline, while the crystalline domain is fully surrounded by the amorphous domain. The amorphous domain is believed to facilitate Li+-ion transport. We hope this study could help understand the complex relationships among coating conditions, uniformity and morphology of the coating layer, and their impacts on the electrochemical properties of Ni-rich layered oxides. ASSOCIATED CONTENT Supporting Information



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Full XRD patterns and SEM images of the bare and coated samples. SEM-EDX crosssection mapping of all elements. Initial charge/discharge profiles for the 1st cycle and rate capability of all samples. TEM images of SG800-5 and PC650-5 samples after 100 cycles. The electrochemical performance of graphite anode. Voltage midpoint evolution of the pouch cell. TEM image and XRD pattern of the cathode material from the pouch cell after 1500 cycles. AUTHOR INFORMATION Corresponding Author *Tel.: +1-512-471-1791. Fax: +1-512-471-7681. E-mail: [email protected] (A.M.). Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy (DOE) under grant number DE-EE0006447 and the Welch Foundation Grant F-1254. The authors thank Dr. Yubao Zhao and Michael Klein for their assistance with TEM operation and Dr. Pilgun Oh for his helpful discussion on the experimental design.


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