Unveiling the Role of Al2O3 in Preventing Surface Reconstruction

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Unveiling the Role of Al2O3 in Preventing Surface Reconstruction During High-Voltage Cycling of Lithium-Ion Batteries Lamuel A David, Kevin Dahlberg, Debasish Mohanty, Rose E. Ruther, Ashfia Huq, Miaofang Chi, Seong Jin An, Chengyu Mao, David King, Lisa Stevenson, and David Lee Wood ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01877 • Publication Date (Web): 11 Jan 2019 Downloaded from http://pubs.acs.org on January 14, 2019

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ACS Applied Energy Materials

Unveiling the Role of Al2O3 in Preventing Surface Reconstruction During High-Voltage Cycling of Lithium-Ion Batteries Lamuel David,a Kevin Dahlberg,e Debasish Mohanty,a Rose E. Ruther,a Ashfia Huq,c Miaofang Chi,d Seong Jin An,a Chengyu Mao,a David M. King,f Lisa Stevenson,e and David L. Wood IIIa,b,* aEnergy and Transportation Science Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA 37831 bBredesen Center for Interdisciplinary Research and Graduate Education, University of Tennessee, Knoxville, Tennessee, USA 37996 cNeutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA 37831 dMaterial Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA 37831 eXALT Energy, LLC, Midland, MI, USA 48640 fForgeNano, LLC, Louisville, CO, USA 80027

Correspondence and requests for materials should be addressed to D.L.W. ([email protected]) Keywords: Li-ion battery, full concentration gradient, NMC811, ALD coating, electrochemistry, EELS, XPS, neutron diffraction

Abstract Recent achievements in high energy batteries have been made by using Ni-rich NMC cathodes (LiNixMnyCo1-x-yO2 with x > 0.5) in conjunction with higher cell voltages. However, these gains have come at a cost of fast capacity fade and poor rate performace. In our previous study, we showed that Al2O3 ALD coatings on LiNi0.8Mn0.1Co0.1O2 (NMC811) and LiNi0.8Co0.15Al0.05O2 (NCA) cathodes prevented surface phase transitions, reduced impedance, and extended cycle life in high voltage cells. Here, neutron

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diffraction (ND), X-ray photoelectron spectroscopy (XPS), and electron energy loss spectroscopy (EELS) are used to fully investigate the mechanism by which ALD surface coatings mitigate NMC811 cathode degredation. Refinement of ND patterns indicated no changes in the bulk crystal structure of cycled cathodes with or without the Al2O3 coating. Rather, the improved performance of ALD-coated cathodes is clearly due to surface stabilization. EELS established that all three transition metal oxidation states were reduced at the surface of the uncoated cathode after cycling, whereas the coated cathode showed no changes in surface oxidation states relative to the bulk. The surface coatings also prevented transition metal dissolution and crossover. XPS analysis of the anode harvested from cycled cells with uncoated cathodes showed significant amounts of Mn deposited within the SEI. In contrast, no Mn could be detected on the anodes cycled with coated cathodes. These results affirm that ALD coatings can effectively reduce the reactivity of the NMC surface and prevent detrimental side reactions that shorten battery cycle life. Introduction Efforts to develop high energy, high voltage cathodes for lithium-ion batteries have mainly focused on a few materials.1-2 These include nickel-rich NMCs (LiNixMnyCo1-x-yO2 with x > 0.5), lithium-manganeserich NMCs (xLi2MnO3·(1–x)LiMO2 where M=Mn, Ni, Co), and the high voltage spinel (LiNi0.5Mn1.5O4).35

These cathode materials suffer poor coulombic efficiency and rapid capacity fade due to side reactions

and structural instablity at high voltage.6-7 Specifically, undesirable phase transformations occur on the particle surface.8 In particular, transition metal (TM) reduction and subsequent dissolution into the electrolyte is a well-documented mechanism that leads to impedance rise and capacity fade.9 In the layered NMC materials, the surface transforms from the layered (R3¯m) phase to spinel (Fd3¯m) or rock-salt (Fm3¯ m) structures.10 Various approaches to improve the structural stability in high-energy cathode materials have been suggested in the literature: a) Doping layered cathode materials with TMs like Ti and Ru to improve stability by preventing rock-salt formation at high voltages.11-12 However, capacity fade due to other side reactions is still evident in these materials.12 b) Reformulating the electrolyte with additives to form a stable cathode-electrolyte interface to prevent degrading side reactions.13 The effects of additives on TM dissolution, particle surface reconstruction, impedance rise, and gas formation at the cathode are not fully understood, and more effective additives need to be developed..


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c) Coating the particle surface with wide bandgap metal oxides to inhibit TM dissolution and improve surface structural stability.14-16 d) Engineering the cathode particle to have a gradient in transition metal concentration to reduce surface reactivity and improve high-voltage cycling stability. Concentration-gradient NMCs are more nickel-rich in the core and more manganese-rich at the surface.17 In our previous study, we used approaches c and d in tandem to achieve structural stability.18 LiNi0.8Mn0.1Co0.1O2 (NMC811) with a full concentration gradient was coated with Al2O3 using atomic layer deposition (ALD). The ALD coating significantly improved capacity retention in full cells with the Ni-rich cathode. Transmission electron microscopy showed that the Al2O3 coating remained intact after cycling, and no phase transformations occured on the surface of the coated particles. In this study, we further analyze the causes for improved electrochemical stability observed in Al2O3-coated NMC. Neutron diffraction was used to characterize the bulk structures, while EELS and XPS provide insights into the surface chemistry. The results confirm the Al2O3 coating prevents surface reactions and phase transformations, thereby mitigating the severe capacity loss that occurs with uncoated cathodes. Methods and Materials Materials. Full concentration gradient (FCG) NMC811 oxide powder was purchased from Posco ESM Co. Ltd. The composition of the NMC was determined to be LiNi0.77Mn0.11Co0.12O2 by inductively coupled plasma atomic emission spectroscopy (ICP-AES). ALD coating of Al2O3 on the NMC powders was performed by Forge Nano. Powders were loaded into a batch fluidized bed reactor, heated and dried under vacuum, then iteratively dosed with i) trimethylaluminum, ii) inert N2, iii) water vapor, and iv) inert N2. Uncoated NMC materials are denoted as NMC-Uncoated. Al2O3-coated samples are denoted as NMCAl2O3. Slurries for NMC cathodes and graphite anodes were mixed using NMP as the solvent and coated onto foil current collectors at XALT Energy’s R&D facility. The cathode formulation was 92 wt.% NMC, 4.5 wt.% carbon black, and 3.5 wt.% polyvinylidene difluoride (PVDF) binder, and the loading was 15.0 mgNMC/cm2. The anode formulation was 93 wt.% graphite and 7 wt.% PVDF binder, and the loading was 11.0 mgGraphite/cm2. Pouch cell assembly and testing. 2 Ah pouch cells were fabricated by XALT Energy by punching 95 × 64 mm electrodes, welding tabs, stacking alternately 9 cathodes and 10 anodes with Z-folded separator, and vacuum sealing. The cells were filled with 1.2 M LiPF6 in 1:9 ethylene carbonate:diethyl carbonate



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(EC:DEC) as the electrolyte before formation and aging. This electrolyte composition was used because it is the preferred choice for large-scale industry applications. The cycle life tests were performed in 2 Ah pouch cells at 1 C/−1 C cycling rates at 23 °C within a voltage window of of 3–4.35 V. The cells with Al2O3-coated NMC811 and uncoated NMC811 were cycled to 20% capacity fade, 760 and 530 cycles, respectively. Two pouch cell replicates were used for each cathode material. Further information can be found in our previous paper.18 Pouch cell disassembly. 2 Ah pouch cells were fully discharged and opened in an Ar-filled glove box. Care was taken to prevent shorting the cell by using ceramic scissors. The electrodes were rinsed in pure DEC for approximately 2 mins to remove electrolyte residue. The rinsed electrodes were dried and stored in the Ar-filled glovebox for further analysis.18 Half cell testing. The electrochemical performance of pristine and cycled cathodes was evaluted in coin cells with lithium metal counter electrodes. Half cells were assembeled in an Ar-filled glove box using a 12 mm diameter disk of NMC as the cathode, lithium metal as the anode, a Celgard 2325 seperator (19 mm diameter disk) and 1.2 M LiPF6 in 3:7 ethylene carbonate:diethyl carbonate (EC:DEC) as the electrolyte. This composition of electrolyte has been found to be better for cathode electrochemical studies at the half cell level, and it is also a standard among the U.S. national laboratories. Rate capability tests were performed between 3-4.8 V vs. Li/Li+ with a constant charge rate of C/20 and discharge rate of C/20 and 1C (where 1C = 180 mA/g). Two coin-cell replicates were used for each cathode material. Neutron Diffraction. Cathode materials from cycled electrodes were collected by scraping the material from the current collector inside an Ar-filled glovebox. These powders were sealed in airtight 6 mm vanadium sample cans under argon and transported to the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory (ORNL). ND patterns were collected at room-temperature on beamline 11A (POWGEN) using a beam of neutrons with a center wavelength of 1.066 Å. The patterns were refined by the Rietveld method using GSAS and the EXPGUI interface. The ND patterns in the lower d-spacing range (high Q) show some background which is due to the presence of hydrogen from the binder. XPS. X-ray photoelectron spectroscopy (XPS) was performed using a Model K-Alpha XPS instrument (Thermo Scientific, Waltham, MA, USA). The instrument utilizes a monochromated, micro-focusing, Al Kα X-ray source (1486.6 eV) with a variable spot size (i.e., 30-400 µm). Analyses of the samples were all conducted with a 400 µm X-ray spot size for maximum signal and to obtain an average surface


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composition over the largest possible area. The instrument has a hemispherical electron energy analyzer equipped with a 128 multi-channel detector system. The base pressure in the analysis chamber is typically 2 x 10-9 mbar or lower. The samples were mounted on a glass slide using double sided tape inside a glove box and transferred in a special vacuum transfer system to the XPS instrument to prevent air exposure. Areas were chosen for analysis by viewing the samples with a digital optical camera with a magnification of approximately 60-200×. For depth profiles, an Ar-ion gun operated at 2 kV and 10 nA was used. All the XPS data were obtained after the samples were sputtered for 120 s to remove surface contaminents. All spectra were acquired with the charge neutralization flood gun turned on to maintain stable analysis conditions. The flood gun uses a combination of low energy electrons and argon ions for optimum charge compensation. The typical pressure in the analysis chamber with the flood gun operating is 2 x 10-7 mbar. Data were collected using the Thermo Scientific Avantage XPS software package (v.4.61). EELS. Electron microscopy work was carried out on a Cs-corrected FEI Titan 80/300 kV TEM/STEM microscope, equipped with a Gatan Image Filter Quantum-865. All STEM images and EELS spectra were acquired at 300 kV and with a beam size of ∼0.7 Å. EELS spectra shown in this work were acquired from a square area of ∼0.5 × 0.5 nm2 using an acquisition time of 3 s and a collection angle of 35 mrad. To minimize possible electron beam irradiation effects, EELS presented in this work were acquired from areas without prebeam irradiation. Transition metal L3 to L2 intensity ratio analysis was performed using the method described by Wang et al.19 Results and Discussion In our previous study,18 we showed that Al2O3 coating significantly improves the electrochemical performance of NMC cathodes (Table 1). Post cycle HRTEM and SAED analysis showed the coating remained intact and no phase transformation was observed, consistent with significantly lower growth in the charge transfer resistance of the coated cathode as determined by post-cycle EIS analysis. In this study, we conducted further advanced characterization analysis of the cycled electrodes using various physical and chemical characterization techniques to better understand the mechanisms for improved cycling stability. The electrochemical performance of the pristine cathodes and cathodes harvested from cells extensively cycled at 1C/-1C were evaluated in half cells (Figure 1). During the first charge (delithiation at a rate of C/20) both cycled cathodes (coated and uncoated) had a capacity of 180 mAh/g. This is only 77% of the capacity of fresh cathodes, 235 mAh/g (Figure 1a). The loss of lithium is consistent with the



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fact that the full cells were cycled to ~80% of their capacity at the beginning of life. During relithiation, the cycled, Al2O3-coated NMC delivered 221±2.6 mAh/g, while the cycled, uncoated NMC delivered 215±0.7 mAh/g. Meanwhile, the initial discharge capacity for cells made from fresh cathodes with and without coating remained the same at 235±0.5 mAh/g. These capacity values were stable during the second charge/discharge step @ C/20 rate (Figure 1b). The uncoated NMC cathode lost 8.5% of its initial capacity after long-term cycling compared to only 6.0% for the cathode with Al2O3 coating. The coated cathode had better capacity retention even though it underwent over 40% more charge/discharge cycles. Even though Al2O3 coating is known to be electronically insulating, the ALD coating also improved rate performance (Figure 1c). Previous studies have shown that these surface coatings suppress the propagation of HF by acting as HF scavengers and reduce surface degradation during extensive cycling. Therefore, the impedance of the cells with coated cathodes was lower when compared to cells with bare cathode powder.20-21 Even after cycling, the Al2O3-coated NMC had a capacity of 170±1.3 mAh/g at 1C, which was 3.5% higher than uncoated NMC before cycling. More studies into the surface interactions between coated cathodes and electrolytes should be part of future work. Neutron Diffraction To understand the origins of the capacity fade in the cathodes, the bulk structure was characterized by neutron diffraction (Figure 2 and Table 2). Neutron diffraction is much more sensitive to lighter elements like lithium and oxygen than X-ray diffraction, and therefore ideally suited to monitor subtle changes in the crystal structure of cathode materials.22 Rietveld refinements of the diffraction patterns showed the expected layered R3¯m phase both before and after cycling, and no other phases were present. No significant peak broadening or change in peak position was observed for the (003) and (104) reflections as shown by the similarity in c- and a-lattice parameters before and after cycling. After cycling, the lithium occupancy in both the coated and uncoated cathode was reduced to ~0.8, which is consistant with the capacity loss measured in the full cells and half cells. Therefore, the lower performance of the uncoated cathodes is not due to changes in the bulk structure. Electron Energy Loss Spectroscopy Clear changes in the surface structure of uncoated, cycled cathodes were observed in our earlier work using transmission electron microscopy (TEM).18 To understand how the surface chemistry may also evolve, EELS analysis of the particle surface and bulk were compared (Figures 3 and 4). EELS allows


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differentiation between the NMC particle bulk, which remained largely unchanged even at shallow depths of 10-20 nm through cycling, and the particle surface which degraded significantly causing cell capacity loss. The most striking change was in the O K-edge spectra of the uncoated-NMC surface before and after cycling (Figure 3a and 3b). After cycling the intensity of the pre-peak decreased significantly at the surface. This transition is characteristic of Li2MnO3-based NMC materials, and has been associated with O vacancy formation and oxygen loss.23 Simultaneously, the intensity ratio of L3/L2 for Ni (Figure 3(c,d)) increased at the surface of the particle after cycling suggesting a decrease in oxidation state.19 The intensity ratios of L3/L2 in Co and Mn (Figure 3(e-h)) are difficult to ascertain due to their weak intensities. However, the peaks shifted towards lower energy loss at the surface relative to the bulk for all of the transition metals, indicating an overall decrease in oxidation state.24 In addition to the reduction of the transition metals, the surface was also found to be Mn-rich after cycling. This phenomenon, which is consistant with the surface reconstruction from layered to spinel phase was observed using HRTEM in an earlier paper.18 This cathode surface degradation mechanism and subsequent increase in impedance has been well documented through EIS results.20, 25-26 In contrast, the surface of the Al2O3-coated NMC remained stable after cycling. The O K-edge peak intensity was unchanged after cycling, confirming the layered structure and oxygen chemistry were preserved on the surface (Figure 4(a,b)). Further, the L3/L2 peak intensity ratios for Ni, Mn, and Co were similar after cycling both at the particle surface and in the interior, which indicates the oxidation states remained the same (Figure 4(c-h)). The Al2O3 coating also prevented the formation of a Mn-rich surface by preventing side reactions with the electrolyte, as has been observed using similar surface techniques like XANES.27, which allows for highly reversable Li cycling to occur primarily with charge compensation from the oxidation of Ni. The relative peak intensities for the transition metals remained constant at the surface and in the bulk. All these observations provide supporting evidence for cyclinginduced surface degradation in uncoated-NMC cathode particles, which is associated with the gradual decline in their electrochemical performance.23, 28 Al2O3 coating greatly improves the surface stability, eliminating these surface-induced failure modes. X-ray Photoelectron Spectroscopy Damage to the cathode surface is strongly correlated with transition metal dissolution and crossover to the anode.10 High resolution X-ray photoelectron spectroscopy (XPS) was employed to understand the impact



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of the cathode coating on the chemistry of the graphite anode (Figure 5). The anode from the cell with uncoated NMC had a large Mn 2p peak (0.43 at.%), while the anode from the cell with Al2O3-coated NMC showed no detectable presence of Mn. This indicates that the coating on the cathode acted as a protective barrier to prevent transition metal dissolution into the electrolyte and crossover to the anode. A stable solid electrolyte interface (SEI) on the anode is critical to increase the cycle life of the cell, and the presence of transition metals in the SEI has been shown to be a leading cause for capacity fade.9 Conclusion Many of the causes for performance loss in high voltage cells are related to surface structural instability of the cathode, and ALD coatings are an effective means to prevent surface phase transformations, transition metal migration, and subsequent capacity fade. Careful analysis of the bulk crystal structure using neutron diffraction showed no significant difference between coated and uncoated cathodes, indicating that the difference in electrochemical performance is related to surface phenomena. This finding was further confirmed by EELS, which showed that the particle interior remained chemically unchanged after extended electrochemical cycling. However, significant changes in the particle surfaces were observed by EELS for the uncoated cathode. Without the ALD coating, the transition metals at the cathode surface were reduced. This reduction in oxidation state is consistent with oxygen loss and the associated phase transition from layered to spinel. ALD Al2O3 coating completely prevented this change in surface chemistry and structure. Similarly, ALD Al2O3 coatings eliminated manganese dissolution and crossover to the anode. These studies show that the debilitating surface reconstruction that leads to TM dissolution and subsequent cell failure are prevented by the ALD Al2O3 coating, greatly increasing cycle life. Acknowledgement The research at Oak Ridge National Laboratory, managed by UT Battelle, LLC, for the U.S. Department of Energy (DOE) under contract No. DE-EE0005384, subcontracts Nos NFE-11-03678, TSA 14-587 and LS-111201AMMW, was sponsored by the Office of Energy Efficiency and Renewable Energy for the Vehicle Technologies Office (Deputy Director: Dave Howell; Peter Faguy: Program Manager). Work performed at PneumatiCoat Technologies under contract No. DE-SC0010230 was also sponsored by the Vehicle Technologies Office (Program Manager: Brian Cunningham). Microscopy was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. Part of this research was supported by the DOE Basic Energy Sciences (BES), Materials Sciences and Engineering


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Division. The authors also acknowledge BASF for supply of the electrolyte for pouch cells. A portion of this research used resources at the Spallation Neutron Source, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory. Author contributions All authors reviewed the manuscript. L.D. performed electrochemical characterization, analyzed the structural data, prepared figures, and wrote the manuscript. K.D. wrote/edited the manuscript, analyzed electrochemical data, designed ALD coating and 2 Ah pouch cell experiments, helped design other experiments. D.M.K. helped write/edit, designed and made ALD coatings. L.S. allocated company resources for project (in-kind resources) and aided in the data analysis/discussion. A.H., D.M. and K.D. performed neutron powder diffraction measurements. A.H. and C.M. were involved in structural refinement discussion. S.A. collected XPS data and was involved in the discussion. M.C. performed HRTEM and collected EELS measurement and was involved in the discussion. R.E.R. and D.L.W. helped write/edit the manuscript and aided in data analysis/discussion. Notes The authors declare no competing financial interest. This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

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Page 12 of 18

Figure 1. Electrochemical charge/discharge profiles of half-cells comparing capacities of pristine cathodes and harvested, cycled cathodes during (a) first cycle, (b) second cycle, and (c) rate capability test at 1C rate.


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Figure 2. Neutron diffraction patterns comparing (a, c) uncoated NMC and (b, d) Al2O3 coated NMC before cycling and the corresponding harvested cathode after long term cycling.



Page 14 of 18

Figure 3. EELS spectra showing the comparision of (a, b) O-K edge and L2,3 white lines of (c, d) Ni, (e, f) Mn and (g, h) Co at the particle surface (black) and interior (red) for NMC cathode with no ALD coating before cycling and the corresponding harvested cathode after long term cycling. For all spectra, the intensity is normalized to the O-K peak.


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Figure 4. EELS spectra showing the comparision of (a, b) O K-edge and L2,3 white lines of (c, d) Ni, (e, f) Mn and (g, h) Co at the particle surface (black) and interior (red) for Al2O3 coated NMC cathode before cycling and the corresponding harvested cathode after long term cycling. For all spectra, the intensity is normalized to the O K-edge peak.



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Figure 5. High resolution XPS spectra comparing Mn 2p peaks of cycled anodes corresponding to uncoated and Al2O3 coated NMC cathodes.


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Table 1. Electrochemical data from large format pouch cell cycled at 1C/-1C rate. The cells were stopped after the capacity decreased to ~80 % (1.8 Ah) of their initial capacity.18

Cell

Pouch cell capacity at the

Number of cycles to reach

beginning of life (Ah, avg. 

~80% of the initial reversible

std.dev.)

capacity 2.27 0.02

530

2.262  0.007

760

NMC-Uncoated NMC – Al2O3

Table 2. Neutron diffraction data comparing uncoated NMC and Al2O3 coated NMC before cycling with the corresponding harvested cathodes after long term cycling.

c-lattice

a-lattice

Li Occupancy

RT Volume unit cell Å3

c/3a

Before Cycling Uncoated-NMC

14.210

2.8723

1.0000

101.5235

1.6491

Al2O3 Coated-NMC

14.212

2.8736

1.0000

101.6319

1.6485

After Cycling Uncoated-NMC

14.2846

2.8612

0.8267

101.2709

1.6642

Al2O3 Coated-NMC

14.2940

2.8584

0.8246

101.1364

1.6669



TOC


Page 18 of 18

Unveiling the Role of Al2O3 in Preventing Surface Reconstruction

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