Understanding the Role of Temperature and Cathode Composition on

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Understanding the Role of Temperature and Cathode Composition on Interface and Bulk: Optimizing Aluminum Oxide Coatings for LiIon Cathodes Binghong Han,† Tadas Paulauskas,‡ Baris Key,† Cameron Peebles,† Joong Sun Park,† Robert F. Klie,‡ John T. Vaughey,† and Fulya Dogan*,† †

Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, United States Department of Physics, University of Illinois at Chicago, 845 W. Taylor Street, Chicago, Illinois 60607, United States

‡

S Supporting Information *

ABSTRACT: Surface coating of cathode materials with Al2O3 has been shown to be a promising method for cathode stabilization and improved cycling performance at high operating voltages. However, a detailed understanding on how coating process and cathode composition change the chemical composition, morphology, and distribution of coating within the cathode interface and bulk lattice is still missing. In this study, we use a wet-chemical method to synthesize a series of Al2O3-coated LiNi0.5Co0.2Mn0.3O2 and LiCoO2 cathodes treated under various annealing temperatures and a combination of structural characterization techniques to understand the composition, homogeneity, and morphology of the coating layer and the bulk cathode. Nuclear magnetic resonance and electron microscopy results reveal that the nature of the interface is highly dependent on the annealing temperature and cathode composition. For Al2O3-coated LiNi0.5Co0.2Mn0.3O2, higher annealing temperature leads to more homogeneous and more closely attached coating on cathode materials, corresponding to better electrochemical performance. Lower Al2O3 coating content is found to be helpful to further improve the initial capacity and cyclability, which can greatly outperform the pristine cathode material. For Al2O3-coated LiCoO2, the incorporation of Al into the cathode lattice is observed after annealing at high temperatures, implying the transformation from “surface coatings” to “dopants”, which is not observed for LiNi0.5Co0.2Mn0.3O2. As a result, Al2O3-coated LiCoO2 annealed at higher temperature shows similar initial capacity but lower retention compared to that annealed at a lower temperature, due to the intercalation of surface alumina into the bulk layered structure forming a solid solution. KEYWORDS: NMC, LCO, Al2O3 coating, coating vs doping, TEM, 27Al MAS NMR used Al2O317−19 can effectively improve the electrochemical performance and/or cyclability of LCO by suppressing the chemical and structural transitions during the electrochemical cycles.17,20 Similar coating approaches have also been applied to NCM system to decrease metal dissolution and protect electrode surface interacting with electrolyte.21−27 Two commonly used surface Al2O3 coatings methods are wetchemical process21−23 and atomic layer deposition (ALD) process.24−28 Compared to ALD, the wet-chemical method is cheaper, simpler, and easier to scale up; however, it has less control on the coating quality and requires post heat treatment.29,30 Although previous studies have shown improved performance on Al2O3-coated NCM and LCO cathodes through a wetchemical process,18,19,21−23 a detailed understanding on the effects of sintering conditions and cathode compositions to the

I. INTRODUCTION The fast development of portable electronic devices and zeroemission or hybrid electric vehicles has raised great research interests in developing new lithium-ion batteries with higher energy density and longer lifetime.1,2 In the last two decades, many Ni- and Mn-based layered compounds with the compositions of LiNixCoyMn1−x−yO2 (NCM) have been studied as promising cathode materials, with higher energy density, better thermal stability, and relatively low costs compared with the traditional LiCoO2 (LCO) cathode materials.3−11 It is believed that adding Ni can improve the capacity and adding Mn can enhance the stability of the layered cathode materials.3,4 However, when charged to a high voltage (4.6 V or higher), NCM types of materials still show instability due to increased electrode surface reactions (solid electrolyte interphase formation), electrolyte decomposition, transition metal dissolution, and cathode surface transformations due to transition metal rearrangement and oxygen loss.4 Previous studies on LCO have shown that the surface coating of MgO,12 SiO2,13 TiO2,14 ZrO2,15 ZnO,16 and particularly the commonly © XXXX American Chemical Society

Received: January 12, 2017 Accepted: April 7, 2017 Published: April 7, 2017 A

DOI: 10.1021/acsami.7b00595 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

carbon, and 10 wt % polyvinylidene difluoride (PVDF) binder in Nmethyl 2-pyrrolidone (NMP). The electrolyte was a 1.2 M solution of LiPF6 in a 3:7 mixture of ethylene carbonate (EC) and ethylmethyl carbonate (EMC). The separators used were glass fiber for the first method and Celgard-2325 for the second, respectively. Lithium metal anodes were used, and the cycling conditions are included in figure captions for electrochemical testing data accordingly. After electrochemical testing, the cells were disassembled in an Ar-filled glovebox. The cathodes were rinsed with anhydrous DMC to remove electrolyte and dried in Ar before further characterizations. II.C. Structural Characterizations. Powder XRD patterns for the synthesized samples were obtained using Bruker D8 diffractometer equipped with Cu−Kα radiation source (λ = 1.5418 Å). The lattice parameters were obtained by doing Rietveld refinement on the XRD patterns using Profex software (version 3.8.0). 27 Al magic angle spinning (MAS) NMR spectra were acquired at 11.7 T, 500 MHz on a Bruker Avance III spectrometer at a Larmor frequency of 130.25 MHz. A rotor-synchronized echo pulse sequence with the pulse width of 1.5 μs was used to acquire the spectra with a 2.5 mm probe at spinning speed of 30 kHz. The pulse delay time was 2 s. All spectra were collected at a constant temperature of 283 K. The spectra were referenced to 1 M Al(NO3)3 at 0 ppm. SEM images were taken using a Hitachi S-4700-II microscope in the Electron Microscopy Center of Argonne National Laboratory. The dry powders were directly sprayed onto the conductive carbon tape before imaging. Images were taken at 20 kV operating voltage. TEM studies were performed at 200 kV on a probe-side aberrationcorrected JEOL JEM ARM-200CF, equipped with an Oxford XMAX100TLE energy dispersive X-ray spectrometer (EDS). The images and EDS maps were taken in scanning transmission electron microscopy (STEM) mode and processed using Gatan Digital Micrograph v2.01 (Gatan Inc.) and AZtecEnergy (Oxford Instruments), respectively.

coating layers is still missing. Particularly, in previous cases the Al2O3-coated cathode materials were often characterized by long-range-order techniques, e.g., X-ray diffraction (XRD), and therefore little has been studied regarding the aluminum local environment and the composition of the interface between the coating layer and bulk cathode, which are important for the surface−electrolyte interaction and hence the solid electrolyte interface (SEI) formation. The effect of sintering conditions on homogeneity, morphology, and thickness of the coating layers is also critical as they can affect the overall electrochemical performance and stability. Understanding how the coating process and cathode composition affect the interfacial and bulk chemistry and hence the electrochemical performance of Nirich cathodes is important for the future development of surface-modified NCM cathodes. Our previous experimental study on aluminum-doped cathodes (LiNi1−y−zCoyAlzO 2 (NCA) and NCM) showed Al−Mn incompatibility and favored aluminum solubility in cobalt-rich systems.31 Therefore, it is critical to have compositional local characterization on coated cathodes to look for possible transformation from surface coatings to a dopant. In this study, a series of Al2O3-coated LiNi0.5Co0.2Mn0.3O2 (NCM523) and LCO are prepared through wet coating method and treated under various sintering conditions. XRD, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and solid state nuclear magnetic resonance (NMR) characterization techniques have been applied to study the evolution of surface species, possible aluminum diffusion into the lattice, changes in coating morphology and thickness, and changes on the long-range order of the coated materials upon high-temperature treatment. Local structural characterization data have shown the effect of cathode composition on aluminum diffusion into the lattice, in other words expected coating converting into a dopant with annealing process. These new findings can help us find out how the coating process, chemical structure, surface morphology, and layer thickness of surface coating can influence the electrochemical performance of the cathode materials, which is critical to develop new design principles of next-generation cathode materials with improved stability and capacity.

II. EXPERIMENTAL SECTION

III. RESULTS III.A. Structural Characterization Results. X-ray diffraction patterns for all Al2O3-coated NCM and LCO samples (annealed at different temperatures) conform to the R3̅m symmetry of the pristine materials. No new peak formation is observed by X-ray diffraction for the presence of additional phases formed, and the only detectable changes in diffraction analysis are unit cell volume changes. Figure 1 shows the unit cell volumes of pristine and 2% Al2O3-coated NCM523 and LCO heat treated under various temperatures, calculated from XRD refinement results. An example of XRD refinement and a

II.A. Material Preparation. The pristine NCM523 and LCO are commercial powders from TODA America Inc. During the coating process, the cathode powders were first added into the aqueous solution made of Al(NO3)3·9H2O (Sigma-Aldrich) and DI water. The ratios between Al and pristine cathode powders were determined to produce a final Al2O3 ratio of 2, 1, 0.5, or 0 wt % (the 0 wt % one is noted as the water-treated sample). The mixture was stirred at room temperature for 6 h. Then the temperature was raised to 80 °C, and the stirring was continued overnight to evaporate the water. The remaining mixture was further dried in the vacuum for 4 h. The obtained as-coated powders were annealed at various temperatures (200, 400, 600, or 800 °C) for 8 h to assess the reactivity of the coating with the cathode and its effect on cathode surface and bulk structure. II.B. Electrochemical Testing. Electrochemical tests were performed using 2032-type coin cells containing cathode electrodes prepared by two methods. The first method consisted of making a slurry from acetone and NCM523 powders (pristine and coated) that were mixed with Super P carbon (10 wt %). The slurry was drop-cast on the stainless steel spacers/current collectors of the coin cell assemblies, thoroughly dried, and then assembled into cells as usual. The second method consisted of laminating Al foil current collectors with a slurry containing 82 wt % of the oxide powder, 8 wt % Super P

Figure 1. Volumes of unit cells of 2% Al2O3-coated NCM523 and LCO annealed at various temperatures for 8 h. The points at 25 °C represent the as-coated particles. B


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role as a probe for Li-ion systems, providing useful information on chemical and structural local environment changes, chemical bonding, local dynamics of bulk Li-ions, and evolution of surface layers.20 Although it is generally considered as a bulk technique, it has been increasingly utilized to study interfacial species (solid-electrolyte interphase and surface coating composition and morphology) and provide information on bulk versus surface species for paramagnetic battery systems.20,31 Due to the hyperfine (or Fermi-contact) interaction, which is a measure of the unpaired spin density that is transferred from paramagnetic transition metals ions to the sorbital of the observed nuclei (through oxygen p-orbital) in the bulk, NMR peak positions are significantly shifted from that in diamagnetic surface environments. Therefore, solid state NMR is a unique technique to study whether a coating stays as a surface layer acting as a coating or diffuses into the lattice and works as a dopant.31 As seen in Figure 2a, with annealing temperature below 400 °C, only six-coordinate Al peaks for surface Al2O3 and/or Al(OH)6 are observed in 27Al MAS NMR spectra of 2% Al2O3-coated NCM523. When the annealing temperature is increased to 600 and 800 °C, formation of lithium aluminate (LiAlO2) is observed with a new aluminum27 peak emerging at ∼69 ppm (Figure 2a). Presence of LiAlO2 also suggests possible lithium removal from bulk and formation of surface lithium bearing species during the wet-chemical coating process. 27Al NMR data of pure LiAlO2 and Al2O3, as well as their physical mixture with NCM523 compared to Al2O3-coated NCM523, can be found in the Figures S2 and S3 in the Supporting Information for comparison. This observation is significant to understand the effect of a wet coating and temperature treatment on bulk and interface changes as well as their effect on electrochemical performance of coated cathode and will be discussed in more detail in later sections. In addition to the formation of new aluminum environments with increasing annealing temperature on Al2O3-coated NCM523, significant changes are observed for spinning sideband pattern and envelope, as shown in Figure 2b. The formation of spinning sidebands is partially due to electron− dipolar interaction and contains information on the interaction between a surface species (nucleus under observation) and the bulk active material with paramagnetic metal centers. The dipolar interaction has a (1/r)3 dependency, where r is the distance between the two dipoles involved. Large dipolar interaction leads to a broad NMR spectra line shape and a broad spinning sideband envelope, which corresponds to a small distance between the nucleus under observation and paramagnetic bulk.32,33 Therefore, the changes in spinning sideband pattern can provide information on the thickness and distribution of the surface species. On Al2O3-coated NCM523, with increasing annealing temperature, the sideband envelope gets broader and the intensity of sidebands for aluminum oxide peaks increases due to increased proximity of surface coating with paramagnetic bulk, which is likely caused by the formation of a denser, more closely attached and more uniform Al2O3 coating. These lineshapes observed are wider than the spinning sideband manifold observed for Al2O3 alone and can only be seen with strong interaction with paramagnetic centers distributed around aluminum nuclei (as shown with spectra comparison in Figure S3). The effect of annealing temperature on aluminum local environments is also studied for Al2O3-coated LCO samples by solid state NMR. Figure 2c shows 27Al MAS NMR spectra comparison of as-coated LCO, coated sample annealed at 400

full table of the lattice parameters can be found in Figure S1 and Table S1 in the Supporting Information. For NCM523, the as-coated sample and the coated samples annealed at 200 and 400 °C do not show significant change in the unit cell volume, indicating the bulk of NCM523 is merely influenced by the coating or low-temperature annealing process. When the annealing temperature is above 400 °C, there is a minor increase in the unit cell volume, indicating a possible aluminum insertion or lithium loss from the structure at higher temperatures. For aluminum-coated LCO, the same minor change in unit cell volume is seen even after annealing at 400 °C, indicating the crystal structure of LCO is easier to be influenced by alumina coating at lower temperatures compared to NCM523. The lack of significant observable difference in long-range order from XRD warrants investigation of the material with a local probe technique. Therefore, solid state NMR spectroscopy is used to study possible aluminum insertion, lithium loss, and formation of new phases that are not detectable by XRD. The effect of annealing temperature on aluminum local environment, possible aluminum diffusion into the lattice, and composition coating layer are studied by 27Al MAS NMR, as shown in Figure 2. Solid state NMR is a powerful characterization tool and has been playing a unique and complementary

Figure 2. (a) Near-0-ppm region of 27Al MAS NMR results of ascoated and annealed NCM523 (LiNi0.5Co0.2Mn0.3O2) samples. (b) The whole spectra of 27Al MAS NMR results of as-coated and annealed NCM523. (c) The near-0-ppm region of 27Al MAS NMR results of as-coated and annealed LCO (LiCoO2) samples, compared to LiAlCoO2 solid solution. C


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Figure 3. SEM images of (a) pristine NCM523, (b) as-coated NCM523, and (c)−(f) Al2O3-coated NCM523 annealed at various temperatures for 8 h.

Figure 4. Low-angle annular dark-field STEM images of (a) pristine NCM523, (b) as-coated NCM523, and (c)−(f) Al2O3-coated NCM523 annealed at various temperatures for 8 h. The green dash lines shown in panels (b)−(f) represent the rough interface between the bulk and the surface coating layer, which can be confirmed by comparing the dark-field images to the corresponding bright-field images in Figure S5. In this figure the contrast of the dark-field images has been inverted to compare to the bright-field images.

and 800 °C, and aluminum-doped lithium cobalt oxide. The ascoated sample shows a broad peak at ∼0 ppm for six-coordinate aluminum oxide. Different from the Al2O3-coated NCM523 prepared with the coating process, Al2O3-coated LCO shows the formation of new aluminum environments with annealing temperature of 400 °C for 8 h. As the temperature increased to 800 °C, formation of at least six different aluminum peaks is observed from 25 to 60 ppm, each separated ∼7 ppm apart (Figure 2c). These peak positions and increment values are consistent with the previous 27Al MAS NMR studies on LiAlxCo1−xO2 solid solutions,34 and the observed peaks can be assigned as octahedral aluminum environments with varying cobalt neighbors (Figure 2c). These peak observations and overlaps with our synthesized LiAlxCo1−xO2 sample prove that Al2O3-coated LCO gradually forms Al-doped LCO with

increasing annealing temperature, implying the Al2O3 coating layer is diffusing into the bulk and forming a solution of LiAlCoO2. Comparing to the Al2O3-coated NCM523 samples, we can find that the Al diffusion from the surface coating layer to the bulk is much easier in LCO, which will be discussed in a later section. The effect of annealing temperature and cathode composition on coating−cathode interface morphology and thickness is further studied by electron microscopy (Figures 3, 4, and 5). Figure 3 shows SEM images of pristine and Al2O3-coated NCM523 samples (as-coated and annealed at varying temperatures). The pristine sample has primary particles of 0.5−1.5 μm with smooth surfaces (Figure 3a) and secondary particles around 10 μm (image not shown). The as-coated material has a loose surface layer covering all secondary particles (Figure 3b). D


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4f). In addition, a semiamorphous interface layer can be observed for 600 °C annealed sample (pointed to in Figure S6a), which might be the misaligned interfacial layer between the bulk NCM523 and the surface Al-coating. On the contrary, for 800 °C annealed sample, continuous crystal fringes can be observed from the bulk to the surface (Figure S6b), implying that the coating layer is better aligned with the bulk layered structure after higher temperature annealing. These interfacial differences can influence the electrochemical performance and surface composition and morphology effects on electrochemistry will be discussed in the following section. On some selected particles, high-angle annular dark-field images and EDS mapping show some nonuniform Al- and O-rich regions for coated NCM523 samples annealed at 600 and 800 °C (Figure S7). These can represent excess segregated Al2O3 and/or LiAlO2 small particles, which are also observed in SEM images shown in Figures 3e and 3f. In the case of Al2O3-coated LCO, due to the feasibility of aluminum into the bulk cathode after annealing, the morphological change shows a different trend from that of coated NCM523. Figure 5 shows SEM images for pristine, ascoated LCO, and coated samples annealed at 400 and 800 °C. Pristine primary particle sizes are in the range of 1−3 μm, whereas secondary particle sizes are around 10 μm. As seen in Figure 5b, the as-coated LCO looks similar to that of as-coated NCM523 in Figure 3b with loose surface coating layers covering the secondary particle. With annealing the sample at 400 °C for 8 h, the coating more uniformly covers the primary particles separately as more similar to coated NCM523 primary particles annealed at 800 °C. When the annealing temperature is increased to 800 °C, the coated particle surface becomes more pristine-like and no surface layer can be identified from the SEM images, which is likely the result of the total intercalation of Al into the bulk as proven by the 27Al MAS NMR results. III.B. Electrochemical Test Results. Effect of Annealing Temperature. The effect of annealing temperature on electrochemical performance of 2% Al2O3-coated NCM523 are studied on loose powder half-cells. Figure 6 shows the cycling performance of pristine and coated NCM523 annealed at

Figure 5. SEM images of (a) pristine LCO, (b) as-coated LCO, and (c)−(d) Al2O3-coated LCO annealed at various temperatures for 8 h.

As the material is heated to 200 and 400 °C the loose surface layer gets denser and more closely attached to the surface, but the morphology change is very limited (Figure 3c and 3d). After annealing at 600 °C, the surface Al2O3 layer clearly transforms from loose coating to a more uniform layer attached more closely to the bulk NCM523 revealing primary particles again, as shown in Figure 3e. With heating the coated sample to 800 °C, the coating layer becomes more like a hard shell uniformly covering the bulk NCM primary particles (Figure 3f). There is no significant change observed in primary particle size with high-temperature treatment. For both 600 and 800 °C annealed samples, some small particles can be observed decorated on the surface, which is likely due to excess Al2O3 segregated under high temperatures. To further study the morphology and thickness of the surface coating layer, low-angle annular dark-field (LAADF) STEM images and EDS mapping are employed, and the results are shown in Figure 4 and Figure S4, respectively. The STEM images of as-coated and 200 or 400 °C-annealed NCM523 demonstrate a loose amorphous covering on top of the bulk NCM523 particle with the thickness of ∼20, ∼15, and ∼10 nm, respectively. The identity and thickness of such Al-rich surface layer can be confirmed using EDS mapping technique under STEM mode, as shown in Figures S4b−S4d, where Al-rich layers outside the bulk particle with a thickness from ∼20 to ∼10 nm can be observed. With increasing the annealing temperature to 600 and 800 °C, the surface Al2O3 coating forms very thin layers closely attached to the bulk NCM523 particle (Figures 4e and 4f). These thin coating layers can be identified by comparing the bright-field (BF) image to the LAADF image taken at the same time, as shown in Figures S5e and S5f, where the different contrasts in the surface layer between the corresponding BF and LAADF images come from the higher atomic-number sensitivity of the annular dark-field mode compared to the BF mode. Please note that in Figure S5 the contrast of the dark-field images has been inverted to compare to the bright-field images. The brighter outer rims in LAADF images in Figures S5e and S 5f represent the lighterelement-rich (i.e., Al-rich) coating layers, which is only ∼3 and ∼2 nm for samples annealed at 600 and 800 °C, respectively. Such coating is too thin and therefore cannot be separated within the spatial resolution of EDS mapping (Figure S4e and S

Figure 6. Evolution of discharge capacity for pristine NCM523 and 2% Al2O3-coated NCM523 annealed at various temperatures for 8 h as the cathode materials. The cells were cycled between 3 and 4.5 V at the rate of C/9. For each sample, two coin cells were made to ensure the repeatability of the results. E


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back into the lattice with heat treatment. The broadening observed is due to the formation of various lithium environments with temperature increase (such as LiAlO2 for 800 °C). For Al2O3-coated LCO, coated LCO cathodes annealed at 400 and 800 °C also show lower initial discharge capacity compared to the pristine LCO sample, but they show better cycle stability after 20 cycles (Figure 8). The lower initial

varying temperatures in the 3−4.5 V voltage range at a rate of C/9. Significant differences are observed in initial capacities and cycling stability for pristine and coated samples with changing annealing temperatures. As seen with the discharge capacity plot, all the coated samples show slightly low initial capacities in comparison to uncoated cathode, and the Al2O3-coated NCM523 cathodes annealed at higher temperatures demonstrate higher initial capacity and better cyclability compared to the low-temperature samples. These preliminary results on electrochemical performance and both bulk and surface characterization results suggest that coated samples prepared with high-temperature annealing (600 °C and higher) show better cycling performance and more uniform and thinner coating layer covering both primary and secondary particles. Therefore, further coating work on detailed electrochemical testing on effect of coating content and coating layer thickness has been focused on samples annealed at 800 °C. The fact that initial capacities of all Al2O3-coated NCM523 electrodes are lower than that of pristine NCM523 could be attributed to the existence of thick coating layer which may increase the resistance for Li transportation through the surface. It has to be noted that the initial coating study has been performed with relative high coating content (2% by weight) to improve the structural characterization results, and the effect of low coating levels on electrochemical performance will be discussed in the following section. One other possible explanation for lower initial capacities for coated samples might be possible lithium loss from bulk and formation of surface lithium bearing species after a wet coating process. This surface lithium segregation can be confirmed by comparison of normalized 7Li MAS NMR spectra for pristine, as-coated, and coated and annealed NCM523 samples as shown in Figure 7.

Figure 8. Evolution of discharge capacity for pristine LCO and 2% Al2O3-coated LCO annealed at 400 or 800 °C for 8 h as the cathode materials. The cells were cycled between 3 and 4.5 V at the rate of C/ 9.

capacities of Al2O3-coated LCO could be explained by the presence of electrochemically inactive Al3+ ions and the higher lithium intercalation potentials of LiCo1−xAlxO2 compounds compared to LiCoO2. Different from the Al2O3-coated NCM523, for Al2O3-coated LCO, annealing at 400 °C shows slight improvement in electrochemical performance compared to sample annealed at 800 °C. This could be due to the intercalation of surface Al into LCO starting at low temperature, unlike on NCM523 where the surface Al coating cannot intercalate into the bulk and will only start to form the dense coating at high temperatures. In addition, since aluminum intercalation is not complete (as in coated sample annealed at 800 °C) and there is still surface Al2O3 species, the stabilization effect of the surface Al coating is more pronounced. As shown in Figure 8 and Table S2, although the initial capacity of Al2O3coated LCO (∼180 mAh/g) is lower than that of the pristine LCO (∼200 mAh/g), after ∼20 cycles, the discharge capacity of the Al2O3-coated LCO surpasses that of pristine LCO due to the better cyclability. After 50 cycles at the rate of C/9, the relative capacity losses of the Al2O3-coated LCO are less than 20% (see Table S2), much lower than that of pristine LCO (36.4%). In comparison, due to the good cyclability of pristine NCM523, the performance of 2% Al2O3-coated NCM523 can hardly surpass its pristine counterpart even for the best coated one annealed at 800 °C. To obtain improvement in both capacity and cycling stability, a thinner coating layer with same interfacial composition (annealed at 800 °C) is also studied and will be discussed in the next section. The effect of cycling on aluminum local environment changes is studied by 27Al solid state NMR on cycled Al2O3coated NCM523 and LCO annealed at 800 °C (Figure 9). For NCM523 where aluminum is found as Al2O3 and LiAlO2 coatings, no significant change is observed for LiAlO2 phase intensity and local order. The 27Al peak for Al2O3 environment

Figure 7. Mass-normalized 7Li MAS NMR spectra of pristine NCM523 and Al2O3-coated NCM523 annealed at various temperatures. The inset shows full 7Li MAS NMR spectra with spinning sidebands included.

The as-coated sample has sharp and high-intensity lithium-7 peak at around 0 ppm for surface lithium bearing species in comparison to pristine/untreated NCM523. This shows that the aqueous coating process draws out a small amount of surface lithium ions (with possible proton exchange); therefore, the effect of water treatment (without any coating) needs to be studied to see its effect on electrochemical performance. As the annealing temperature increases, the peak gets broader and the area integration of the peaks shows decrease in surface lithium environments suggesting some of the surface lithium migrates F


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Figure 9. Effect of cycling on aluminum local environments. 27Al MAS NMR of coated pristine and cycled (a) Al2O3-coated NCM523 annealed at 800 °C and (b) Al2O3-coated LCO annealed at 800 °C.

gets broader as a sign of site distortion and presence of different aluminum environment distribution. However, these are still mostly aluminum oxide environments, and no significant formation of AlF or AlOF species is observed as proven with 19 F MAS NMR experiments performed on the cycled samples (Figure S8). 27Al MAS NMR data of cathode harvested from cycled LCO cell (coated and annealed at 800 °C) is compared with pristine coated LCO in Figure 9b. The data reveals that the aluminum local ordering observed in pristine sample is lost with cycling and a completely disordered aluminum phase is formed. This significant change also proves that aluminums are in the structure as a dopant, not on the surface as an oxide coating, and have been effected by electrochemical cycling. The Role of Thinner Coating on Improved Electrochemistry and Effect of Water Treatment on Initial Capacity Loss. As briefly discussed in the previous section, the initial capacity differences observed for Al2O3 coated cathodes can be due to the formation of surface species (as a result of water treatment) and/or the presence of a thick coating layer. To study the effect of these two concepts on capacity loss, new batches of Al2O3-coated NCM523 are prepared with lower coating loadings (0.5% and 1% by weight, all annealed at 800 °C for 8 h), and a water-treatment control group is prepared by immersing NCM523 into the water, stirring for the same duration as the coated samples (without adding Al salt), and then annealing at 800 °C for 8 h. Figure 10 and Table S3 demonstrate the electrochemical performance of pristine, water-treated, and 0.5%/1.0%/2.0% Al2O3‑-coated NCM523 particles, tested by the accelerate protocol (i.e., first 4 cycles at a slow rate of C/10 for SEI formation, then 80 cycles at a fast rate of C/3 for cyclability test). With decreasing the coating content from 2% to 1% and 0.5%, the initial capacity at C/10 increases from 168.5 to 176.4 and 179.4 mA/g (Table S3), implying the thinner coating can reduce the initial capacity loss caused by the Li conduction resistance from the surface coating. This is consistent with the previous study showing that when Li passes through the Al2O3 coating, it will first lithiate the surface Al2O3 layer to form a Li-conductive LixAl2O3 phase and then overflow through the lithiated surface coating layer, which can cause the extra capacitance loss.35 In comparison to the pristine sample, the water treatment decreases the initial capacity from 188.4 to 182.6 mAh/g, proving lithium loss from the bulk during the wet coating process also contributes to the initial capacity loss. However, this initial capacity loss is compensated with the use of thinner, well uniform coating covering both primary and

Figure 10. Evolution of discharge capacity for pristine, water-treated, and Al2O3-coated NCM523 with various coating loadings and annealed at 800 °C for 8 h. The half-cells were cycled between 3 and 4.5 V, first at the rate of C/10 for 4 cycles (formation cycles) and then at C/3 for 80 cycles.

secondary NCM523 particles as with the electrochemical performance stability for the cells prepared with 0.5 and 1% Al2O3 coating. The 0.5% Al2O3-coated NCM523 shows a much smaller capacity loss (5.6%) after 80 cycles at C/3 compared to both 2% Al2O3-coated NCM523 (22.0%) and pristine NCM523 (37.6%). The discharge capacity of 0.5% Al2O3coated NCM523 surpasses that of pristine NCM523 after ∼25 cycles at C/3, implying that a reasonable thin coating is an effective way to improve the electrochemical performance of NCM cathode materials. In our test, the water treatment does not show clear influence to the cyclability, implying the influence of water treatment might be confined to the initial Li leaching and have less continuous effects on long-term performance.

IV. DISCUSSION IV.A. Evolution of Surface Morphology and Composition with Varying Annealing Temperature: Al2O3Coated NCM523. With the combination of all the results from XRD, NMR, and electron microscopy studies discussed in previous sections, here we propose a scheme for the evolution of surface Al2O3 coating layer as a function of sintering temperature on NCM523, as shown in Figure 11a. The initial coating step without any temperature treatment first forms a G


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Figure 11. Schemes of the Al2O3 coating layer evolution on (a) NCM523 and (b) LCO, under different annealing temperatures.

(see Figure 1). The observation of broader spinning sideband envelope of 27Al MAS NMR signal at 600 and 800 °C shows increased intimacy of surface layer with paramagnetic bulk and hence reduced distance between surface Al coating and the bulk NCM particle. This observation points out that the coating layer is now covering primary particles and possibly accumulating at the grain boundaries. It has to be noted that the morphological uniformity of this interfacial layer (which is a mixture of both LiAlO2 and Al2O3) is yet to be proven with further experiments by identifying the precise distribution of this dense surface layer. All the findings on aluminum-based surface coating evolution on NCM523 cathode with changing annealing temperature are summarized within Figure 11a. This picture of surface coating evolution built on structural characterizations in combination with our electrochemical characterization data can help us to understand how the annealing temperature could influence the electrochemical performance of Al2O3-coated NCM523. The enhanced coating layer formed at high temperatures is observed to protect the bulk NCM523 much better from the structural change during electrochemical cycles, and therefore the Al2O3coated NCM523 annealed at higher temperatures (600 and 800 °C) demonstrates better cyclability compared to that annealed at lower temperatures (200 and 400 °C). When we gradually increase the annealing temperature, much denser and more uniform aluminum-based coating layer is formed, which is more closely attached to the bulk, covering even the primary particles. At this stage the composition of the interface is a mixture of LiAlO2 and Al2O3. The presence of LiAlO2 (being a better lithium ion conductor relative to Al2O3) could help to reduce the resistance of Li transportation during the charging/ discharging process. As a result, the samples heated at higher temperatures show higher initial discharge capacities (see Figure 6), and under the same coating conditions when the coating content is reduced (and the coating layer gets even thinner and more uniform) both the initial capacity and capacity retention in subsequent cycles improve (see Figure 10). IV.B. From Coating to Dopant: Temperature Effect on Al2O3-Coated LCO. Compared to the high-temperature

loose and nonuniformly distributed amorphous Al2O3 and/or Al(OH)3) layer covering just the surface of secondary particles with a thickness of ∼20 nm, as shown by the SEM and TEM images in Figures 3b and 4b. With annealing at lower temperatures (200 and 400 °C), the SEM and TEM images (Figures 3c, 3d, 4c, and 4d) reveal that the loose oxide layer gets thinner (∼10 nm) and denser covering over the secondary particles but interacting more with the bulk cathode particles. 27 Al MAS NMR characterization shows the only presence of octahedral aluminum oxide as coating composition, and lithium-7 solid state NMR shows the formation of surface lithium bearing species such as Li2CO3, LiOH, LiOH·H2O, and Li2O. Therefore, the wet coating process draws out a small amount of surface lithium at the expense of bulk lithium. Despite these surface formation and lithium loss, the XRD results indicate that the bulk lattice is merely influenced by the coating process. When the annealing temperature is increased to 600 and 800 °C, surface lithium reacts with alumina forming a γ-LiAlO2 phase, where aluminums are at tethrahedral positions. At these temperatures, the interface and coating layer become a mixture of LiAlO2 and alumina. Magnetic coupling of transition metals in NMC composition inhibits aluminum diffusion into the transition metal layer (formation of lattice aluminum) at these annealing temperatures as no paramagnetic aluminum-27 NMR shift is observed for the hightemperature annealed samples. These findings are in agreement with earlier calculations from the Ceder group and our experimental findings on aluminum “doped” NMC cathodes.31,36 Since the surface Al cannot diffuse into the bulk even under high annealing temperatures, the expansion of unit cell observed from XRD results in Figure 1 on 2% Al2O3-coated NCM523 is likely caused by the Li loss from the structure rather than the Al insertion. When annealed at lower temperatures, the surface Al2O3 coating will not react with the bulk NCM523 to form new LiAlO2 phase, and therefore no lattice expansion was observed in XRD after annealing at lower temperatures. The high-temperature process also forms thinner and more uniformly distributed crystal layer on the surface cathode particles with a thickness of ∼3 nm (see Figures 4e and S5e), and the unit cell volume of the layered oxides increases H


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annealing temperatures, and the alumina can completely diffuse into the lattice at high temperatures. As a result, the Al2O3coated LCO samples annealed at higher temperatures show similar initial capacity but worse cyclability during the charge/ discharge cycles compared to that annealed at lower temperatures. The electrochemical characterizations show that the Al2O3coated NCM523 samples annealed at higher temperatures have higher initial capacities and better cyclabilities during the charge/discharge cycles compared to that annealed at lower temperatures, due to the thinner, denser, and more uniform coating that improves the Li transportation and bulk chemistry protection. However, even after annealing at 800 °C, with 2% Al2O3 coating content and ∼2 nm thickness of coating layer, the initial capacities of Al2O3-coated NCM523 are still lower than the pristine electrodes, due to the Li transportation barrier through the thick coating layer and the Li leaching during water treatment. Our studies on the role of lower coating content and hence thinner coating (after annealing at 800 °C) have shown that the discharge capacity over 80 cycles can be greatly improved and surpassed that of the pristine electrode when the coating content was down to 0.5%. This proves that a combination of optimized coating content and annealing temperature can provide an effective interfacial layer to improve the battery performance. The results provide significant understanding in choosing optimum coating conditions for specific cathode compositions and therefore will greatly impact development of better cathode materials for lithium-ion battery applications.

annealed Al2O3-coated NCM523 samples, where no aluminum diffusion into the lattice is observed, local structural characterization studies reveals that Al diffusion from the surface coating layer to the bulk is much easier for lithium cobalt oxide composition. 27Al MAS NMR data show that the LiAlCoO2 solid solution phase starts to form at only 400 °C (see Figure 2c), and at 800 °C the surface aluminum completely intercalates into the lattice. The transition of aluminum from “surface coating” to “bulk dopant” after high-temperature annealing (up to 800 °C) can only be observed for LCO but not for NCM523 composition, emphasizing that the bulk chemistry has large influence on the evolution and intercalation of surface Al-based coating layers. This observation is consistent with the previous simulation studies stating that the existence of Mn in the layered oxides would increase the energy penalty and prevent the intercalation of Al into the lattice of layered oxides,36 while the existence of M = Co, Ni, or Cr would not hinder the formation of Li(Al,M)O2 solid solution. Our hightemperature annealed coating studies on different NCM compositions with lower manganese content (NCM 811 and NCM 622) have also shown limited aluminum diffusion into the lattice forming a combination of coated/doped cathode with hindered electrochemical performance in comparison to the coated NCM523. These results will be the subject of another article. The intercalation of Al into the bulk of LCO after annealing leads to a different temperature effect on electrochemical performance compared to the Al2O3-coated NCM523. For Al2O3-coated LCO, the Al starts to intercalate into LCO even at 400 °C, and therefore the resistance of Li transportation from the coating layer had already reduced significantly for the sample annealed at 400 °C. As a result, the difference of initial capacities between 400 and 800 °C annealed Al2O3-coated LCO is smaller than that of NCM523, as shown in Figure 7. Meanwhile, the Al2O3-coated LCO annealed at 400 °C has a better cyclability than that annealed at 800 °C, which is opposite to the trend in Al2O3-coated NCM523. The total intercalation of Al into the bulk and the disappearing of protecting surface Al2O3 coating are likely the reasons for the poor cyclability of 800 °C annealed Al2O3-coated LCO.

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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/acsami.7b00595. Additional electrochemical results, NMR spectra, and lattice parameter information (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

V. CONCLUSIONS In this study, we show that the evolution of surface alumina coating layer after heat treatment is highly dependent on the compositions of bulk cathodes. Based on XRD, NMR, and electron microscopy results, two different models are proposed for evolution of the interface of alumina-coated NCM523 and LCO with annealing at different conditions. The proposed models can be used to understand the influence of annealing temperature on coating morphology and thickness and hence the electrochemical performance. For NCM523, after annealing at higher temperatures, the Al2O3 coating layers became denser, more closely attached, and more uniformly distributed, and a new LiAlO2 phase is formed. However, no solid solution forms by Al intercalation into the bulk even after annealing Al2O3coated NCM523 at high temperatures, proving Al intercalation is not preferred in the presence of both manganese and nickel. However, it has to be noted that this observation is only for NCM523 composition, and similar studies on different NCM compositions (e.g., 442, 622, and 811) are ongoing and will be the subject of a follow-up paper. In contrast to NCM523 cathodes, for Al2O3-coated LCO, a transition of Al2O3 from surface coating to a dopant can be observed even at low

ORCID

John T. Vaughey: 0000-0002-2556-6129 Fulya Dogan: 0000-0001-7997-266X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support from the Vehicle Technologies Program, Hybrid and Electric Systems, in particular, David Howell, Tien Duong, and Peter Faguy, at the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy is gratefully acknowledged. The submitted manuscript has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. This work used SEM from the Electron Microscopy Center at Argonne, which is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, also under Contract No. DE-AC0206CH11357. Dr. Jason Croy is acknowledged for valuable discussions. T.P and R.F.K acknowledge funding from the Joint I


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Center for Energy Storage Research (JCESR), Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences.

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