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Atomic to Nanoscale Investigation of Functionalities of Al2O3 Coating Layer on Cathode for Enhanced Battery Performance Pengfei Yan, Jianming Zheng, Xiaofeng Zhang, Rui Xu, Khalil Amine, Jie Xiao, Ji-Guang Zhang, and Chong-Min Wang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b04301 • Publication Date (Web): 06 Jan 2016 Downloaded from http://pubs.acs.org on January 12, 2016

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Atomic to Nanoscale Investigation of Functionalities of Al2O3 Coating Layer on Cathode for Enhanced Battery Performance Pengfei Yan1, Jianming Zheng2, Xiaofeng Zhang3, Rui Xu3, Khalil Amine3, Jie Xiao2, Ji-Guang Zhang2, Chong-Min Wang1* 1

Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, 902 Battelle Boulevard, Richland, WA 99352, USA 2

Energy and Environmental Directorate, Pacific Northwest National Laboratory, 902 Battelle Boulevard, Richland, WA 99352, USA

3

Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439, USA

Abstract Surface coating of cathode has been identified as an effective approach for enhancing the capacity retention of layered structure cathode. However, the underlying operating mechanism of such thin coating layer, in terms of surface chemical functionality and capacity retention, remains unclear.

In this work, we use aberration corrected scanning transmission electron

microscopy and high efficient spectroscopy to probe the delicate functioning mechanism of Al2O3 coating layer on Li1.2Ni0.2Mn0.6O2 cathode.

We discovered that in terms of surface

chemical function, the Al2O3 coating suppresses the side reaction between cathode and the electrolyte upon the battery cycling. At the same time, the Al2O3 coating layer also eliminates the chemical reduction of Mn from the cathode particle surface, therefore avoiding the dissolution of the reduced Mn into the electrolyte. In terms of structural stability, we found that the Al2O3 coating layer can mitigate the layer to spinel phase transformation, which otherwise will initiate from the particle surface and propagate towards the interior of the particle with the progression of the battery cycling. The atomic to nanoscale effects of the coating layer observed here provide insight for optimized design of coating layer on cathode to enhance the battery properties.

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INTRODUCTION Rechargeable lithium ion batteries (LIBs) have penetrated into many aspects of our daily life in a dramatic speed during the past decades, especially as portable energy storage devices due to their high energy density and long cycle life. More recently, considerable efforts have been taken to develop next generation LIBs aiming on their utilization in electrified transportations. As the most critical part in LIBs, cathode materials have attracted intensive investigations to overcome various practical challenges, such as energy density, cycle stability, rate capability, and safety issues. As one of the most promising cathode candidates for next generation LIBs, Li-and-Mn rich (LMR) composite cathodes exhibited the highest capacity (usually > 250 mAh/g) among various cathode materials.1,

2

Despite of their advantage in

superior capacity, LMR cathodes suffer from poor cycle stability during prolonged battery test, including voltage fading and capacity decay. Such performance degradation has been found to be associated with the following factors: (1) The strong side reactions between LMR cathodes and electrolyte, which will lead to the consumption of electrolyte;3 (2) The lattice structural instability, featuring a gradual layered to spinel-like phase transformation, which is initiated from the particle surface and propagates towards the interior of the particle;4 and (3) migration of transition metal ions in the lattice and dissolution into the electrolyte.3, 4 It has been found that coating of cathode particle with a thin protective layer can lead to enhanced cycling properties. Various surface coating species and coating techniques have been applied to cathode materials and the reported performance improvement verified surface coating is an effective method to enhance cell performance as collectively summarized by recent review papers.5-8 Among various coating techniques, atomic layer deposition (ALD) is emerging as a powerful technique in developing next generation LIBs in recent years.7,

9, 10

The unique

formation reaction mechanism of ALD layer enables its many exclusive advantages over other coating techniques, such as uniform and conformal deposition, atomic level precision of control of thickness and a relatively low temperature deposition. In view of coating materials, representative examples are metal oxides and fluoride, such as Al2O3, TiO2, ZrO2, CeO2, ZnO, MgO, and AlF3.11 Among different coatings, it has been noticed that Al2O3 delivered the best cycle stability along with other property improvement.12-16 Therefore, Al2O3 material has been chosen for ALD coating for a variety of cathode materials.13-20


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Although it has been phenomenological observed that ALD-Al2O3 coating of cathode particle can significantly improve the battery performance, the exact functioning mechanism of such a thin layer of coating, in terms of surface chemistry between the cathode and electrolyte, is not

sufficiently understood. Published work on the elucidation of such a coating layer

functioning mechanism are limited to transmission electron microscopy (TEM) imaging of the coating layer,7,

11, 12, 15, 20-22

and no surface structural, chemical and electronic structural

information have ever been captured.

In this work, we use advanced scanning TEM (STEM)

and spectroscopy techniques, X-ray energy dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS), to probe the functioning mechanism of ALD-Al2O3 coating layer on LMR cathode (Li1.2Ni0.2Mn0.6O2).

For the first time we directly visualized that the solid

electrolyte interphase (SEI) layer on the cathode with Al2O3 coating is dramatically different in chemical composition from that without coating, demonstrating that the Al2O3 coating layer suppresses the cathode/electrolyte side reactions. In addition, the battery cycling induced severe Mn reduction that frequently occurred on uncoated cathode particle surface was significantly suppressed for the Al2O3 coated samples, therefore resulting in well-preserved particle surface and stabilized surface structure. 1. EXPERIMENTAL SECTION 2.1. Material synthesis. The Li1.2Ni0.2Mn0.6O2 was synthesized by a co-precipitation process.

Nickel

sulfate

hexahydrate

(NiSO4·6H2O),

manganese

sulfate

monohydrate

(MnSO4·H2O), sodium hydroxide (NaOH), and ammonium hydroxide (NH3·H2O) were used as the starting materials to prepare Ni0.25Mn0.75(OH)2

precursor. The precursor material was

thoroughly washed with de-ionized water to remove residual sodium and sulfate species, followed by filtering and drying. Afterwards, Ni0.25Mn0.75(OH)2 precursor was well mixed with Li2CO3 at a stoichiometric ratio for calcination (900 °C for 14 hrs) to get the final Li-excess Li1.2Ni0.2Mn0.6O2 cathode material. ALD Al2O3 film was grown on the Li1.2Ni0.2Mn0.6O2 cathode material prior to electrode assembly. Ultrathin Al2O3 film was prepared over Li1.2Ni0.2Mn0.6O2 powder surface in a continuous-flow ALD reactor operated under a base pressure of ∼ 1 Torr. Typically, 500 mg of substrate powders were well-spread in a flat stainless steel tray and loaded into the ALD reaction chamber. An Al2O3 thin film was deposited using ALD through 4 cycles



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of trimethylaluminum (TMA) and H2O at 150 °C. The precursor dose and purge time were 30 s and 60 s, respectively. 2.2. Electrochemical performance measurement. Electrochemical performance was evaluated in R2032 coin-type cells. The cathode electrodes were prepared by coating a slurry containing 80% active mass, 10% Super P, and 10% poly(vinylidene fluoride) binder onto Al foil current collector. Coin-type cells were assembled with the as-prepared uncoated and coated cathode materials, metallic lithium foil as counter electrodes, Celgard K1640 monolayer polyethylene (PE) membrane as separator, and 1M lithium hexafluorophosphate (LiPF6) dissolved in ethyl carbonate (EC) and dimethyl carbonate (DMC) (1:2 in volume) as electrolyte in an argon-filled MBraun glovebox. Detailed experimental set up for cathode materials synthesis and electrochemical test can be found in our previous report.4 Both uncoated and coated electrodes were electrochemically tested under the same condition: cycled at a rate of 0.1C between 2.0 and 4.7 V vs. Li/Li+ at the room temperature. A 1C rate corresponds to a current density of 200 mA g-1 in the present work. 2.3. TEM characterization. After 40 cycles, the obtained electrodes were first immersed in DMC for 12 h and then washed by DMC for three times and dried under vacuum for 12 h. The electrode were peeled off from the Al-foil and gently grounded to break the aggolomerated particles. The powder particles were dusted on lacy carbon TEM grids for TEM microanalysis. A probe aberration-corrected S/TEM microscope, JEOL JEM-ARM200CF, was used in this study with operation voltage 200 kV. The beam convergence angle is 27.5 mrad in STEM mode. The STEM-HAADF images were acquired at 68-280 mrad. The EDS elemental mapping was carried using the JEOL SDD-detector with a 100 mm2 X-ray sensor, featuring a collection angle of ~ 10 of traditional detector and therefore enabling high sensitivity and high performance analysis. STEM-EDS data were processed by Analysis Station (Version 3.8.0.52, JEOL Engineering, Co., Ltd.) The STEM-EELS data were collected in dual-EESL mode to obtain both zero-loss spectra and core-loss spectra. Core-loss EELS are calibrated by corresponding zero-loss EELS before further analysis using DigitalMicrograph (Version 2.11, Gatan Inc.).

2. RESULTS AND DISCUSSION 3.1. Electrochemical performance. The effect of the coating layer on the battery performance is illustrated in Figure 1 (a). Overall, the ALD-Al2O3 coated electrode exhibits slightly large


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electrode polarization during charge and discharge due to the presence of an Al2O3 coating layer which is a poor electron conductor. Obvious differences are observed at the end of charge and the end of discharge. The ALD-Al2O3 coated electrode shows lower charge capacity as compared to the uncoated one, indicating a suppression of side reactions between cathode and the electrolyte. The coated electrode shows slightly lower specific capacity at voltage lower than 3.5 V during discharge process. This could be attributed to the existence of Al2O3 coating layer, which may further increase the kinetic barrier for lithium ion intercalation into the MnO2 component (activated from Li2MnO3 component).23 Regarding the cycling stability as shown in Figure 1 (b), the uncoated cathode shows relatively fast capacity decay, presenting capacity lower than that of coated material after about 5 cycles. On the contrary, even though the coated material delivers relatively lower discharge capacity at the beginning, it exhibits negligible capacity fade during the subsequent cycling. The result further validates ALD-Al2O3 coating as a promising strategy in enhancing the cycle stability of LMR cathode materials, which is consistent with previous reports.12, 14, 15, 20, 24, 25 3.2. Sample characterization before cycling. The microstructure and chemical composition of the ALD-Al2O3 coated cathode particles were clearly revealed by the STEMHAADF imaging and STEM-EDS mapping before cycling. Based on the analysis of many particles, we confirmed that most of the particles were well conformably coated with a thin layer of Al2O3 as representatively shown in Figure 2 (a). At the same time, we also noticed that some particles were not well-coated as shown in Figure 2 (b) from which the low magnification STEM-EDS mapping reveals very weak Al-signal from part of the surfaces of the particle as indicated by the white arrows. Therefore, to achieve a uniform coating layer on all nano-sized particles, more efforts need to be done to optimize the ALD techniques. High resolution STEM-HAADF imaging and STEM-EDS mapping provide structural and chemical information of the interface between the coating layer and the cathode particle with atomic resolution. Figure 3 is a comparison of the surface layer structures of the coated and uncoated particles. EDS mapping of the coating layer on particle surface shows very similar in thickness, approximately 3 nm. Moreover, the EDS analysis indicates aluminum element was not only grown onto the outmost surface of the particles but also diffused into subsurface lattice as indicated by the aluminum distribution profile from EDS analysis in Figure 3 (a) and Figure S2



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in the supporting information. As a result of surface chemical modification, structural modification on the surface layer was also observed. As indicated in Figure 3 (a) and (c), the layered structure was partially disordered for the outmost few monolayers after ALD coating, which is contrasted by the surface structure of the corresponding uncoated samples as shown in Figure 3 (b) and (d). Therefore, the ALD Al2O3 coating not only physically deposited a thin protective oxide layer on the cathode particle surface but also modified the cathode material surface layer chemically and structurally. 3.3. Sample characterization after 40 cycles. To better understand how the ALD-Al2O3 coating layer improves battery performance, we conducted detailed STEM structural and chemical analysis of the cathode particle and their evolution with battery cycling. The degradation mechanisms of uncoated LMR materials have been comprehensively studied in previous reports.4, 7, 26-32 On the cathode side, there are three main contributors which are 1) Cathode surface phase transformation due to local chemical composition modification, such as TM cations enrichment, Li depletion and oxygen vacancy formation; 2) Cathode surface SEI formation due to side reactions with electrolyte; and 3) surface corrosion due to continuously acid species attack during battery cycling. Therefore, the effect of ALD-Al2O3 coating layer on the battery performance has been evaluated from these three aspects in the following. As aforementioned in Figure 2 (b), ALD coating techniques could not coat every single particle perfectly. This provides us the opportunity to compare the structural and chemical changes of both coated and uncoated particles from the same battery, which makes the results to be more reliable and comparable. Therefore, our strategy is to find both well-coated and uncoated particles in the same coated electrode and compare their difference after battery cycling. As shown in Figure 4, this particle was chosen from the coated material electrode after 40 cycles, but EDS mapping shows no alumina coating layer on its surface (Figure 4 (b) and (e)). Chemically, based on the EDS mapping (Figure 4 (c) and (d)) and X-ray spectrum (Figure 4 (e), carbonaceous and phosphorus rich SEI layer was formed on the surface of this particle, which is the consequences of the cathode-electrolyte reactions.3 Such an organic SEI layer is a direct evidence of electrolyte decomposition, where C is from the solvent and P is from Li-salt (LiPF6). Our recent observations have confirmed that P-rich surface layer on LMR cathodes is a direct result of high voltage cycling.3 Structurally, STEM-HAADF imaging clearly reveals that after


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the battery cycling, a surface reconstruction layer (SRL) were formed (Figure 4 (f) and (g)). To make a complete comparison, we also observed 40 cycles uncoated electrode. As shown in Figure 5 similar SEI layer and SRL were verified. All these observations are consistent with previous results on uncoated LMR cathodes.3, 4, 29-31 In contrast, it has been noticed that the surface of the well-coated particles is obviously different from that of the uncoated particle after 40 cycles. As representatively shown in Figure 6, the EDS mapping shows that the particle surface was conformally coated with a thin layer of Al2O3 (Figure 6 (c)). Chemically, after the cycling, the Al2O3 coating layer is still very well defined and the P-rich surface layer is not formed on the Al2O3 coated region (Figure 6 (d)). The whole area X-ray spectrum (Figure 6 (e)) shows significantly depressed C-peak and negligible level of P-peak, which indicates the cathode side reactions are significantly suppressed by the Al2O3 coating layer, especially between cathode and Li-salt. More examples can be found in Figure S2. Structurally, atomic level STEM-HAADF image (Figure 6 (f)) indicates that the original layered structure was well preserved even for the surface layer. Therefore, comparing the structural and chemical evolution of the coated particles with that of uncoated one upon cycling of the battery, we can firmly conclude that the Al2O3 coating layer can significantly suppress both the cathode-electrolyte side reactions and the formation of SRL on particle surface (the layer to spinel-like phase transformation which initiated from the particle surface).4 In previous investigations, cathode particle surface corrosion was considered to be one of the primary factors resulting in degradation of battery performance,4, 32, 33 because it not only consumes active cathode material but also introduces alien elements into electrolyte. Recent studies show that transition metals can migrate to anode side and deposit on anode (e.g. graphite anode), causing capacity fading of the battery.26, 34, 35 Dissolution of transition metals are mainly due to HF acid species attack and electrochemical reduction of Mn4+ to Mn2+ during battery cycling. In this work, particle morphology of the pristine samples, cycled uncoated-samples and cycled coated-samples. As shown in Figure 7, pristine sample shows smooth contrast, while both uncoated and coated samples after discharge/charge cycles show black dots, indicating mass loss occurred at black regions. Thus, even though EDS mappings indicate well preserved coating layer after cycling (Figure 6(c) and Figure 7(c)), the coating layer cannot fully prevent such mass loss. On the other hand, to our best knowledge, there is no direct evidence showing that such



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mass contrast is only due to surface corrosion. We speculate that such mass contrast probably comes from particle inside. Battery cycling induced surface Mn reduction was also perceived as one of the most important causes of cathode material degradation. Previous reports consistently showed that surface corrosion and surface layer structure transformation were closely related to Mn cation valence reduction in layered Mn-based cathode materials,7, 27, 28, 32 because reduced Mn cations (such as Mn2+) are believed to have higher mobility and can be easily dissolved into electrolyte.32,

36

Moreover, the well-known Jahn-Teller effect will occur and destabilize the

structure when Mn average valence state is reduced to 3+.37 Therefore, the stability of LMR cathode is heavily depended on Mn valence state. Thus, the Mn valence state and its spatial distribution on both uncoated and coated particle upon battery cycling were probed using STEMEELS. Previous studies have established two methods to estimate Mn valence state using EELS.38-40 One is through measuring Mn-L edge onset energy shift, so called chemical shift. The other is by calculating Mn L3/L2 ratio. In this work, we chose chemical shift to estimate Mn valence state, because it is more reliable due to its relative insensitive to sample thickness as well as the reliable processing protocol. In our case, Mn chemical shift was estimated by measuring L3 edge peak positions, because bulk EELS peak onset can be modified by the top and bottom surface layers, misleading the chemical shift measurement. Figure 8 (b) and (e) show the measured Mn valence states for the uncoated and coated particles after 40 cycles, respectively. Distinctive difference can be seen on the spatial distribution of Mn valence between uncoated and coated particles. For the uncoated particle, the Mn2+ is uniquely located at the outmost surface of particle followed by a thick Mn3+ layer. In contrast, for the Al2O3 coated particle, Mn reduction only occurred in a very thin surface layer and their valence states are well above 3+. Further, the EELS spectra acquired from surface and inner part of both uncoated and coated samples also support the mapping results (compare Figure 8(c) and (f)), where the chemical shift and Mn L3/L2 ratio are distinctively different for the coated and uncoated samples. The spatial mapping of Mn valence distribution and its apparent correlation with the Al2O3 coating layer clearly indicates the Al2O3 coating suppresses the reduction of Mn at the cathode/electrolyte interface, therefore, mitigating the dissolution of the Mn into the electrolyte.

3. CONCLUSIONS


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ALD-Al2O3 coating have been demonstrated to significantly improve the cycle stability of the Li1.2Ni0.2Mn0.6O2 cathode. Systematic STEM atomic level imaging and nanoscale chemical composition analysis directly reveals that the ALD-Al2O3 coating layer plays an important role in terms of the following three aspects: mitigating side reactions between the cathode and electrolyte; eliminating the surface structure transformation; and suppressing the reduction of Mn at the particle surface. We have shown that the advanced structural and chemical imaging techniques using TEM are able to diagnose cathode surface coating in atomic level and provide in-depth understanding of coating layer’s protecting mechanism. The fundamental understanding on the structure and composition changes observed in ALD-Al2O3 coated samples clearly demonstrate the effect of this surface coating and may also guide the design of new cathode materials

■ ASSOCIATED CONTENT Supporting Information STEM-EDS mappings on coated samples after 40 cycles (Figure S1) and coated sample surface structure (Figure S2). This material is available free of charge via the Internet at http://pubs.acs.org AUTHOR INFORMATION Corresponding Author *(C.-M.W.) E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS 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 under Contract No. DE-AC02-05CH11231, Subcontract No. 6951379 under the advanced Battery Materials Research (BMR) program. The work was conducted in the William R. Wiley Environmental



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Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by DOE’s Office of Biological and Environmental Research and located at PNNL. PNNL is operated by Battelle for the Department of Energy under Contract DE-AC05-76RLO1830.


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(15) Zhang, X.; Belharouak, I.; Li, L.; Lei, Y.; Elam, J. W.; Nie, A.; Chen, X.; Yassar, R. S.; Axelbaum, R. L. Structural and Electrochemical Study of Al2O3 and TiO2 Coated Li1.2Ni0.13Mn0.54Co0.13O2 Cathode Material Using ALD. Adv. Energy Mater. 2013, 3, 1299-1307. (16) Wang, D.; Belharouak, I.; Koenig, G. M.; Zhou, G.; Amine, K. Growth Mechanism of Ni0.3Mn0.7CO3 Precursor for High Capacity Li-Ion Battery Cathodes. J. Mater. Chem. 2011, 21, 9290–9295. (17) Jung, Y. S.; Cavanagh, A. S.; Dillon, A. C.; Groner, M. D.; George, S. M.; Lee, S.-H. Enhanced Stability of LiCoO2 Cathodes in Lithium-Ion Batteries Using Surface Modification by Atomic Layer Deposition. J. Electrochem. Soc. 2010, 157, A75-A81. (18) Jung, Y. S.; Cavanagh, A. S.; Riley, L. A.; Kang, S.-H.; Dillon, A. C.; Groner, M. D.; George, S. M.; Lee, S.-H. Ultrathin Direct Atomic Layer Deposition on Composite Electrodes for Highly Durable and Safe Li-Ion Batteries. Adv. Mater. 2010, 22, 2172-2176. (19) Bettge, M.; Li, Y.; Sankaran, B.; Rago, N. D.; Spila, T.; Haasch, R. T.; Petrov, I.; Abraham, D. P. Improving High-Capacity Li1.2Ni0.15Mn0.55Co0.1O2-Based Lithium-Ion Cells by Modifiying the Positive Electrode with Alumina. J. Power Sources 2013, 233, 346-357. (20) Kim, J. W.; Kim, D. H.; Oh, D. Y.; Lee, H.; Kim, J. H.; Lee, J. H.; Jung, Y. S. Surface Chemistry of LiNi0.5Mn1.5O4 Particles Coated by Al2O3 Using Atomic Layer Deposition for Lithium-Ion Batteries. J. Power Sources 2015, 274, 1254-1262. (21) Sun, S.; Yin, Y.; Wan, N.; Wu, Q.; Zhang, X.; Pan, D.; Bai, Y.; Lu, X. AlF3 SurfaceCoated Li[Li0.2Ni0.17Co0.07Mn0.56]O2 Nanoparticles with Superior Electrochemical Performance for Lithium-Ion Batteries. Chemsuschem 2015, 8, 2544-2550. (22) Sun, Y.-K.; Lee, M.-J.; Yoon, C. S.; Hassoun, J.; Amine, K.; Scrosati, B. The Role of AlF3 Coatings in Improving Electrochemical Cycling of Li-Enriched Nickel-Manganese Oxide Electrodes for Li-Ion Batteries. Adv. Mater. 2012, 24, 1192-1196. (23) Zheng, J.; Shi, W.; Gu, M.; Xiao, J.; Zuo, P.; Wang, C.; Zhang, J.-G. Electrochemical Kinetics and Performance of Layered Composite Cathode Material Li[Li0.2Ni0.2Mn0.6]O2. J. Electrochem. Soc. 2013, 160, A2212–A2219. (24) Zheng, J. M.; Zhang, Z. R.; Wu, X. B.; Dong, Z. X.; Zhu, Z.; Yang, Y. The Effects of AlF3 Coating on the Performance of Li[Li0.2Mn0.54Ni0.13Co0.13]O2 Positive Electrode Material for Lithium-Ion Battery. J. Electrochem. Soc. 2008, 155, A775-A782. (25) Bloom, I.; Trahey, L.; Abouimrane, A.; Belharouak, I.; Zhang, X.; Wu, Q.; Lu, W.; Abraham, D. P.; Bettge, M.; Elam, J. W.; Meng, X.; Burrell, A. K.; Ban, C.; Tenent, R.; Nanda, J.; Dudney, N. Effect of Interface Modifications on Voltage Fade in 0.5Li2MnO3·0.5LiNi0.375Mn0.375Co0.25O2 Cathode Materials. J. Power Sources 2014, 249, 509514. (26) Yu, X.; Lyu, Y.; Gu, L.; Wu, H.; Bak, S.-M.; Zhou, Y.; Amine, K.; Ehrlich, S. N.; Li, H.; Nam, K.-W.; Yang, X.-Q. Understanding the Rate Capability of High-Energy-Density Li-Rich Layered Li1.2Ni0.15Co0.1Mn0.55O2 Cathode Materials. Adv. Energy Mater. 2014, 4, 1300950. (27) Yan, P.; Xiao, L.; Zheng, J.; Zhou, Y.; He, Y.; Zu, X.; Mao, S. X.; Xiao, J.; Gao, F.; Zhang, J.-G.; Wang, C.-M. Probing the Degradation Mechanism of Li2MnO3 Cathode for Li-Ion Batteries. Chem. Mater. 2015, 27, 975-982. (28) Lin, F.; Markus, I. M.; Nordlund, D.; Weng, T. C.; Asta, M. D.; Xin, H. L.; Doeff, M. M. Surface Reconstruction and Chemical Evolution of Stoichiometric Layered Cathode Materials for Lithium-Ion Batteries. Nat. Commun. 2014, 5, 3529.


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(29) Boulineau, A.; Simonin, L.; Colin, J. F.; Bourbon, C.; Patoux, S. First Evidence of Manganese-Nickel Segregation and Densification upon Cycling in Li-Rich Layered Oxides for Lithium Batteries. Nano Lett. 2013, 13, 3857-3863. (30) Xu, B.; Fell, C. R.; Chi, M. F.; Meng, Y. S. Identifying Surface Structural Changes in Layered Li-Excess Nickel Manganese Oxides in High Voltage Lithium Ion Batteries: A Joint Experimental and Theoretical Study. Energ Environ. Sci. 2011, 4, 2223-2233. (31) Gu, M.; Belharouak, I.; Zheng, J. M.; Wu, H. M.; Xiao, J.; Genc, A.; Amine, K.; Thevuthasan, S.; Baer, D. R.; Zhang, J. G.; Browning, N. D.; Liu, J.; Wang, C. M. Formation of the Spinel Phase in the Layered Composite Cathode Used in Li-Ion Batteries. ACS nano 2013, 7, 760-767. (32) Zheng, J.; Gu, M.; Xiao, J.; Zuo, P.; Wang, C.; Zhang, J. G. Corrosion/Fragmentation of Layered Composite Cathode and Related Capacity/Voltage Fading during Cycling Process. Nano Lett. 2013, 13, 3824-3830. (33) Xiao, X.; Liu, Z.; Baggetto, L.; Veith, G. M.; More, K. L.; Unocic, R. R. Unraveling Manganese Dissolution/Deposition Mechanisms on the Negative Electrode in Lithium Ion Batteries. Phys. Chem. Chem. Phys. 2014, 16, 10398-10402. (34) Shkrob, I. A.; Kropf, A. J.; Marin, T. W.; Li, Y.; Poluektov, O. G.; Niklas, J.; Abraham, D. P. Manganese in Graphite Anode and Capacity Fade in Li Ion Batteries. J. Phys. Chem. C 2014, 118, 24335-24348. (35) Gu, M.; Genc, A.; Belharouak, I.; Wang, D. P.; Amine, K.; Thevuthasan, S.; Baer, D. R.; Zhang, J. G.; Browning, N. D.; Liu, J.; Wang, C. M. Nanoscale Phase Separation, Cation Ordering, and Surface Chemistry in Pristine Li1.2Ni0.2Mn0.6O2 for Li-Ion Batteries. Chem. Mater. 2013, 25, 2319-2326. (36) Xu, G. L.; Qin, Y.; Ren, Y.; Cai, L.; An, K.; Amine, K.; Chen, Z. H. The Migration Mechanism of Transition Metal Ions in LiNi0.5Mn1.5O4. J. Mater. Chem. A 2015, 3, 1303113038. (37) Kim, D.; Croy, J. R.; Thackeray, M. M. Comments on Stabilizing Layered Manganese Oxide Electrodes for Li Batteries. Electrochem. Commun. 2013, 36, 103-106. (38) Tan, H.; Verbeeck, J.; Abakumov, A.; Van Tendeloo, G. Oxidation State and Chemical Shift Investigation in Transition Metal Oxides by EELS. Ultramicroscopy 2012, 116, 24-33. (39) Loomer, D. B.; Al, T. A.; Weaver, L.; Cogswell, S. Manganese Valence Imaging in Mn Minerals at the Nanoscale Using STEM-EELS. Am. Mineral. 2007, 92, 72-79. (40) Wang, Z. L.; Yin, J. S.; Jiang, Y. D. EELS Analysis of Aation Valence States and Oxygen Vacancies in Magnetic Oxides. Micron 2000, 31, 571-580.



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Figure captions: Figure 1 (a) First cycle charge/discharge profile and (b) charge/discharge capacity as a function of cycle numbers for uncoated sample and 40 cycles ALD-Al2O3 coated sample. Both samples are cycled between 2.0 and 4.7 V vs. Li+/Li at rate of 20 mA/g. Figure 2. EDS maps showing the elemental distribution of the Al2O3 coated sample before the battery cycling. (a) A well-coated particle and (b) the surfaces coatings are not uniform as indicated by white arrows. Figure 3. Atomic scale STEM-HAADF image and STEM-EDS characterization on the chemical and structural modification of particle surface after ALD-Al2O3 coating. (a) and (c) are coated sample surfaces. (b) and (d) are uncoated sample surfaces. High resolution STEM-HAADF images show that coated sample surfaces were transformed into disordered structure ((a) and (c)), which is contrasted by the uncoated sample in (b) and (d). Figure 4. STEM-EDS maps and high resolution STEM-HAADF images of a particle without Al2O3 coating layer after 40 cycles. (a) STEM-HAADF image, (b) Al map, (c) C map and (d) P map. (e) X-ray spectrum from the surface area shown inset. (f) and (g) are lattice images from different particle surfaces in (a). Figure 5. STEM-EDS mapping on a 40 cycles particle from uncoated electrode. (a) STEMHAADF image, (b) O map, (c) Ni map, (d) Mn map, (e) C map and (f) P map. (g) high resolution image shows a surface reconstruction layer (SRL) formed after 40 cycles. Figure 6. STEM-EDS maps and high resolution STEM-HAADF images of a particle with alumina coating layer after 40 cycles. (a) STEM-HAADF image, (b) C map, (c) Al map and (d) P map. (e) X-ray spectrum from the whole mapping area shows negligible P signal and depressed C signal. (f) Lattice images from particle surfaces in (a). Figure 7. STEM-HAADF images show battery cycling induced mass loss. (a) pristine particle shows uniform contrast. After 40 cycles both (b) uncoated and (c) well-coated particles show considerable mass loss. Inset map in (c) is Al ratio map from EDS mapping. Figure 8. STEM-EELS analysis valence mapping of the 40 cycles and its correlation with the Al2O3 coating layer. (a-c) uncoated and (d-f) coated samples. (a) ADF image and (b) estimated Mn valence state map based on EELS chemical shift. (c) Mn EELS spectra from surface and inner region in (a). (d) ADF image and (e) estimated Mn valence state map from EELS chemical shift. (f) Mn EELS spectra from surface and inner region in (d).


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Figure 1 (a) First cycle charge/discharge profile and (b) charge/discharge capacity as a function of cycle numbers for uncoated sample and 40 cycles ALD-Al2O3 coated sample. Both samples are cycled between 2.0 and 4.7 V vs. Li+/Li at rate of 20 mA/g.



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Figure 2. EDS maps showing the elemental distribution of the Al2O3 coated sample before the battery cycling. (a) A well-coated particle and (b) the surfaces coatings are not uniform as indicated by white arrows.


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Figure 3. Atomic scale STEM-HAADF image and STEM-EDS characterization on the chemical and structural modification of particle surface after ALD-Al2O3 coating. (a) and (c) are coated sample surfaces. (b) and (d) are uncoated sample surfaces. High resolution STEM-HAADF images show that coated sample surfaces were transformed into disordered structure ((a) and (c)), which is contrasted by the uncoated sample in (b) and (d).



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Figure 4. STEM-EDS maps and high resolution STEM-HAADF images of a particle without Al2O3 coating layer after 40 cycles. (a) STEM-HAADF image, (b) Al map, (c) C map and (d) P map. (e) X-ray spectrum from the surface area shown inset. (f) and (g) are lattice images from different particle surfaces in (a).


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Figure 5. STEM-EDS mapping on a 40 cycles particle from uncoated electrode. (a) STEMHAADF image, (b) O map, (c) Ni map, (d) Mn map, (e) C map and (f) P map. (g) high resolution image shows a surface reconstruction layer (SRL) formed after 40 cycles.



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Figure 6. STEM-EDS maps and high resolution STEM-HAADF images of a particle with alumina coating layer after 40 cycles. (a) STEM-HAADF image, (b) C map, (c) Al map and (d) P map. (e) X-ray spectrum from the whole mapping area shows negligible P signal and depressed C signal. (f) Lattice images from particle surfaces in (a).


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Figure 7. STEM-HAADF images show battery cycling induced mass loss. (a) pristine particle shows uniform contrast. After 40 cycles both (b) uncoated and (c) well-coated particles show considerable mass loss. Inset map in (c) is Al ratio map from EDS mapping.



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Figure 8. STEM-EELS analysis valence mapping of the 40 cycles and its correlation with the Al2O3 coating layer. (a-c) uncoated and (d-f) coated samples. (a) ADF image and (b) estimated Mn valence state map based on EELS chemical shift. (c) Mn EELS spectra from surface and inner region in (a). (d) ADF image and (e) estimated Mn valence state map from EELS chemical shift. (f) Mn EELS spectra from surface and inner region in (d).


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