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Self-terminated Artificial SEI Layer for Nickel-rich Layered Cathode Material via Mixed Gas Chemical Vapor Deposition In Hyuk Son, Jong Hwan Park, Soonchul Kwon, Junyoung Mun, and Jang Wook Choi Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b03081 • Publication Date (Web): 12 Oct 2015 Downloaded from http://pubs.acs.org on October 21, 2015

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Chemistry of Materials

Self-terminated Artificial SEI Layer for Nickel-rich Layered Cathode Material via Mixed Gas Chemical Vapor Deposition

In Hyuk Son,*,†,# Jong Hwan Park,†,# Soonchul Kwon,† Junyoung Mun,‡ and Jang Wook Choi*,§

†

Energy Material Lab, Material Research Center, Samsung Advanced Institute of Technology, Samsung Electronics Co., LTD, 130 Samsung-ro, Yeongtong-gu, Suwon-si, Gyeonggi-do 443-803, Republic of Korea. ‡

Department of Energy and Chemical Engineering, Incheon National University, 12-1, Songdo-dong, Yeonsu-gu, Incheon 406-840, Republic of Korea.

§

Graduate School of Energy, Environment, Water, and Sustainability (EEWS), Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Republic of Korea.

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ABSTRACT Due to the higher specific capacity, nickel-rich layered cathode material has received much attention from the lithium-ion battery community. However, its cycle life is desired to improve further for practical applications, and unstable interface with electrolyte is one of the main capacity fading mechanisms. Here, we report a facile chemical vapor deposition process involving mixed gases of CO2 and CH4, which yields thin and conformal artificial solidelectrolyte-interphase (SEI) layer consisting of alkyl lithium carbonate (LiCO3R) and lithium carbonate (Li2CO3) on nickel-rich active cathode powder. The coating layer protects from side reactions and improves the cycle life and efficiency significantly. Remarkably, the coating process is self-terminated after the thickness reaches ~10 nm, leading to the coating layer to account for only 0.48 wt%, because of the growing binding energy between the gas mixture and the surface products. The self-termination is characterized by various analytical tools and is well explained by density functional theory calculations. The current gas phase coating process should be applicable to other battery materials that suffer from continuous side reactions with electrolyte.

INTRODUCTION The slow progress in improving the energy density is one of the most serious hurdles for timely advent and widespread adoption of upcoming lithium ion battery (LIB) applications represented by electrical vehicles.1-4 Among all of the key cell components, the cathode active material is currently a limiting factor in boosting the energy density because they usually exhibit less than two thirds the specific capacities of commonly used graphite anodes. After tremendous efforts to increase the specific capacity and operations voltage of diverse cathodes, high capacity layered phases, such as nickel (Ni)-rich or lithium (Li)-rich


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metal oxides,3-7 are considered the most viable candidates to overcome the limited energy density originating from the current cathode active materials. Both Ni-rich and Li-rich layered phases have their own unique structural properties that allow their specific capacities to surpass that of conventional LiCoO2 counterpart.3,4 Unlike LiCoO2 in which the filled t2g band is substantially overlapped with the O2p band, the filled eg band in LiNiO2 barely touches the O2p band so can reach a high degree of charging towards higher reversible capacity without perturbing the oxygen framework.8,9 In the case of the Lirich layered phases, the embedded Li2MO3 domains are activated in the first charge and provide additional sites for reversible Li storage.10,11 In spite of the attractive specific capacity enhancement, these layered phases commonly suffer from insufficient cycling performance due to the structural instability, especially during charged states. The failure mechanism shared with both layered phases is the known layer-to-spinel transition progressing from the particle surface to the core.3,12-14 During this structural transformation, transition metals (TMs) tend to migrate into the Li slabs generating a cation mixing,15 and the oxygen atoms in the lattice evolve into the electrolyte.16-20 The oxygen evolution successively decomposes the electrolyte, worsening the charge-discharge efficiency in each cycle, and also produces unwanted compounds, such as hydrofluoric acid (HF), that could attack the active material. Similarly, Ni-rich phases suffer from Ni dissolution that gives rise to surface side reactions.8,16-20 Surface coating21-26 and doping27,28 with other foreign elements are the most widely adopted remedies to alleviate the aforementioned fading mechanisms. The surface coating physically blocks the oxygen evolution and assists maintaining the original structure near the surface.29,30 Nevertheless, most surface coating processes start with a mechanical mixing of precursor compounds, making conformal and homogeneous coating entirely over individual 3 ACS Paragon Plus Environment


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particles difficult. Instead, the coating materials are deposited in the form of small nanoparticles due to surface energy mismatch, leaving a great chance for the undesirable structural deformation at uncoated local surface spots. The doping with foreign elements was also found to improve the cycling performance but inevitably struggles with a trade-off between the improvement of the cyclability and the sacrifice of the specific capacity.31 Herein, we report a facile chemical vapor deposition (CVD) process to uniformly coat a high capacity layered cathode, namely LiNi0.6Co0.1Mn0.3O2 (NCM), with artificial solid electrolyte interphase layer (SEI) consisting of organic species, such as alkyl lithium carbonate (LiCO3R) and lithium carbonate (Li2CO3). A CVD process based on CO2 and CH4 gas mixture32,33 generates a thin (~10 nm) artificial SEI layer in a self-limited fashion at a low temperature of 400 oC. The stabilized electrode-electrolyte interface by the ultra-thin artificial SEI layer improves long-term cycling performance under various voltage window and temperature conditions. EXPERIMENTAL SECTION Materials. LiNi0.6Co0.1Mn0.3O2 (NCM-613, Deajung chemicals & materials Co. Ltd.), Super P (TIMCAL Graphite & Carbon), and poly(vinylidene fluoride) binder (PVDF, SOLEF5130, Solvay) were used as received. CVD process. 1 g of the NCM powder was positioned in a fixed-bed SUS reactor (I.D. = 7 mm)34,35 where, to achieve uniform coating of powder, reacting powder is continuously rotated at constant pressure and temperature while reacting gases are being fed in. Pure N2 gas was fed into the reactor at a flow rate of 100 mL min-1, while the temperature was raised at 10 ºC min-1 and maintained at 400 ºC for 1 h. The mixed gases (CH4 100 mL min-1 and CO2 100 mL min-1) were then fed into the reactor for 2 h. Next, the reactor was cooled down


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to room temperature under pure N2 flow (100 mL min-1). When either of CH4 and CO2 was used for the control samples, the flow rate was still the same. Characterization of the coating layer. In-situ diffuse reflective infra-red Fourier transform spectra (in-situ DRIFTs) were recorded in the range of 1650-750 nm-1 using a Fourier transform spectrometer (Nicolet, 5700). The DRIFT cell chamber (Harricks, Praying Mantis) is compatible with the temperature ranges chosen for this study. Ex-situ attenuated total reflection Fourier transform infra-red (ATR-FTIR) spectra were additionally recorded on a Bruker VERITEX 70 spectrometer equipped with MiracleTM reflection accessory by using the ATR mode. The morphology of the NCM powder was examined before and after CVD processes using ultra-high-resolution field emission scanning electron microscopy (UHR-FESEM, Hitachi S-5500) and transmission electron microscopy (FEI Titan Cubed 60-300). The XPS spectra were obtained using a Physical Electronics (PHI) (quantum 2000 scanning ESCA microprobe) spectrometer. Photoelectrons were excited with an Al Kα (1486.6 eV) anode operating at a constant power of 100 W (15 kV and 10 mA) with an X-ray spot diameter of 400 µm. X-ray diffraction (XRD) analyses were carried out using an X’pert pro (PANalytical) diffractometer with Cu Kα radiation (1.54056 angstrom, 40 kV, 40 mA). The measurements were performed over the 2 theta range of 10-60º at a scanning rate of 0.02º min-1. To perform Auger electron measurements, NCM powder samples were transferred to a UHV scanning Auger nanoprobe chamber (PHI 710, ULVAC-PHI). After obtaining the Auger electron spectra and elemental signature with respect to Li, C, O, Ni, Co and Mn (sampling depth < 10 nm), elemental mapping was carried out with each pixel size of 20 nm x 20 nm. The electron beam voltage and current for Auger electron measurements were 3 kV and 1 nA, respectively. In depth profile experiments, the surface of NCM was etched with Ar+ sputtering. For characterizing the composition of the prepared NCM, thermal gravimetric



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analysis (METTLER TOLEDO TGA/DSC1) was conducted in the temperature range of 30500 ºC with a heating rate of 5 ºC min-1 under air condition. DFT Calculation. DFT calculations were carried out to examine the binding of CO2 or CH4 on the Li2CO3 (100) and CH3-LiCO3/Li2CO3 (100) surfaces using a periodic threedimensional model. The geometry-optimized unit cell parameters of Li2CO3 are in excellent agreement with its experimental lattice parameters (see Figure S1 in the Supporting Information). The geometry optimization was carried out to refine the model structure without constraints using the CASTEP v. 7.0 module36 under the following conditions: (i) the general gradient approximation (GGA) was used at the PBE level for functional options with spin polarized calculations, (ii) 340 eV was chosen for the basis set to estimate the properties of the carbonates and reactants, and (iii) ionic cores were used by the ultrasoft pseudopotential. The vacuum thickness (size of unit cell perpendicular to slab-slab thickness) was set to 30 Å for all calculations.37 The convergence SCF tolerance is 1x10-6 eV/atom, and a special 3x3x1 k point of structures was chosen. Dispersion correction was applied to the calculation. The binding energy, ∆Eb, between the reactant and the surface was determined by three single total energy calculations: (i) geometry optimization of the reactant, (ii) geometry optimization of the surface without the reactant, and (iii) geometry optimization of the surface with the reactant. The gas binding energy on the catalysts was determined using Equation (1): ∆E b =

E surface + reactant − (E surface + E CO2 /E CH4 ) n

(1)

where ∆Eb denotes the binding energy of the reactant gas on the surface; Esurface+reactant,

Esurface, and ECO2/ ECH4 are the formation energy of the reactant adsorbed on the surface, the formation energy of the surface without reactant, and the formation energy of the single


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reactant (CO2 or CH4) in the vacuum, respectively. n denotes the number of reaction molecules.

Electrochemical analysis. A slurry consisting of NCM, super P, and PVDF binder in a weight ratio of 90:6:4 was coated on an aluminum current collector. The electrode was then roll-

pressed to enhance the interparticle contact. Coin-type half-cells (2032 size, Hoshen) were prepared by assembling the composite electrode, a separator (polypropylene, Celgard) and Li foil. 1.3 M lithium hexafluorophosphate (LiPF6) in co-solvents of ethylene carbonate (EC), diethylene carbonate (DEC), and fluoroethylene carbonate (FEC) in 2:6:2=v:v:v was used as electrolyte. The galvanostatic cycling was carried out at 10 mA g-1 (0.05C, 1C = 200 mA g-1) in the potential range of 2.5-4.6 V (vs. Li/Li+) for the precycling, and at 90~100 mA g-1 (0.5C) over the same potential range for the subsequent cycles using a battery tester (model TOSCAT 3500U) at 25 and 60 oC. For charging, constant current constant voltage (CCCV) mode was used, and the cut-off currents were 5 mA and 20 mA for precycling and subsequent cycles, respectively. The loading amount of the NCM on the aluminum foil was ranged from 5.4 to 16.2 mg cm-2.

RESULTS AND DISCUSSION Figure 1(a) ~ (h) display the morphology of NCM particles before and after CVD processes at 400 oC for 2 hrs with various reacting gas conditions. The original NCM particles prepared by a co-precipitation38,39 process have an average diameter of ~10 µm, and each secondary particle is comprised of 100~200 nm primary nanoparticles with spheroidal shape (Figure 1(d) and (h)). While the NCM particles after CVD with carbon dioxide (CO2NCM, Figure 1 (a) and (e)) and methane (CH4-NCM, Figure 1(c) and (g)) exhibit rough morphologies, the particles after reacting with CO2 and CH4 mixed gas (CO2+CH4-NCM, 7 ACS Paragon Plus Environment


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Figure 1(b) and (f)) show smooth surface morphologies similar to those of the original particles, indicating the formation of uniform and thin coating layers. To elucidate the coating layers generated from the gas phase reactions, we performed X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses. The XRD results verify that all of the CVD processes maintain the initial hexagonal (R m) crystal structure5 of NCM (Figure 1(i)). However, closer inspection reveals that the (003) and (104) peaks in all the CVD samples are down-shifted (Figure 1(j)), reflecting the increased lattice parameters.40 The increase in the c-direction lattice parameter is more significant in the order of CO2+CH4-NCM < CH4-NCM < CO2-NCM. The series of results indicate that the surface reactions involve Li ion extraction, which increases the lattice distance, and this Li extraction is mildest for CO2+CH4-NCM. In the most serious case of CO2-NCM, the peaks assigned41,42 to monoclinic Li2CO3 were clearly detected at 2θ = 21.4°, 29.6°, 30.5°, 31.9° and 34.1° in the XRD spectrum (Figure 1(k)).

The formation of Li2CO3 in CO2-NCM is also evidenced by its XPS C1s

(Figure 1(l)) and O1s (Figure 1(m)) spectra, which exhibit the peaks corresponding to O=C and O=C-O.43-47 Based on these results, it can be concluded that the rough surface morphology observed in Figure 1(a) and (e) is ascribed to the formation of Li2CO3 nanocrystalline that involves Li ion extraction from the NCM host.48,49 By contrast, CH4-NCM shows lower intensities of the (003) and (104) peaks (Figure 1(j)) indicating weakened crystallinity of NCM during the CVD step. The impaired crystallinity is ascribed to the reduction of NCM by decomposed hydrogen from CH4 that perturbs the host framework and is also reflected in the reduced NiO peak at 529.2 eV in the XPS spectrum (Figure 1(m)). At the expense of the NiO peak, the Li2O peak at 528.7eV is


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enhanced, confirming the weakened crystallinity is accompanied with the Li ion extraction. On the other hand, the broad XRD peak around 2θ = 23o is attributed to formation of carbonaceous layers. In addition, as indicated by the XPS signal at 282.8-283.5 eV (Figure 1(l)), the Ni exposed by the Li extraction can react with the decomposed carbon radicals from CH4 to form NixC.50 The XPS peaks of CH4-NCM at C1s ~289.0 eV and O1s ~532.0 eV also indicate the presence of LiCO3R.43-47 From these combined results, it is concluded that the decomposed products of CH4 give rise to a variety of surface products, including pure carbon layer, NixC sub-layer, as well as LiCO3R. However, the addition of CO2 makes a dramatic difference in the coating materials and their properties. According to zoomed in XRD spectra (Figure 1(j)), the down-shift of the (003) peak of CO2+CH4-NCM was not as significant as that of CO2-NCM. Rather, the corresponding peak was split into two small peaks with the one located at the same position as that of the pristine (003) peak, indicating that CO2+CH4-NCM bears the crystal structure with two distinct lattice parameters along the c-axis. We anticipate that while the core of each secondary particle maintains the pristine crystal structure, the surface reaction involving Li extraction increases the lattice parameter of the host structure near the surface. The broader (104) peak of CO2+CH4-NCM can also be explained by the same phenomenon. The XPS C1s and O1s spectra (Figure 1(l) and (m)) suggest the formation of LiCO3R from the surface reaction, and LiCO3R could play as an artificial SEI layer to stabilize the interface. Notably, this artificial SEI layer is formed in a self-limiting manner and will be described in detail in the forthcoming paragraph presenting density functional theory (DFT) calculations. To better understand the surface reactions, in-situ DRIFTs and ex-situ ATR-FTIR analyses were carried out (Figure 2 and S2), and the following points are remarkable:



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1. Since Li2CO3 was used as a precipitation agent in the co-precipitation synthesis of the pristine NCM, surface sensitive FTIR spectrum clearly detects the signals of this precursor compound for all of the reaction conditions, such as two stretching C-O vibration peaks at 1490 and 1445 cm-1 and a OCO vibration peak at 867 cm-1.51 2. Consistent with the above XRD and XPS results, for the case of CO2-NCM, the aforementioned Li2CO3 peaks continuously grow with increased reaction time (Figure 2(a) and (d)). 3. Using the same analysis technique, in contrast with the aforementioned ex-situ results that indicate LiCO3R formation, CH4-NCM exhibits absorption bands at 1163, 1140 and 1104, cm-1 indicative of alkoxy lithium (LiOR) formation52-54 during the middle of reaction (Figure 2(b) and (d)). The distinct ex-situ XPS (Figure 1(l)) and ATR-FTIR (Figure S2) results are attributed to the fact that LiOR further reacts with CO2 in air to finally form LiCO3R.53 4. In the combined case of CO2+CH4-NCM, the DRIFT and ATR-FTIR spectra display the peaks assigned to both LiCO3R (1716, 1683, 1652, 1380, 1175, 1121 and 830 cm-1, Figure 2(c)-(d) and S2) and Li2CO3 peaks (1490, 1445, and 867 cm-1). The formation of these two products suggest the following stepwise process: LiOR is first produced by the reaction between Li and CHxO radical originating from decomposition of CH4 and CO2.55 Subsequently, LiOR reacts with excess CO2 to produce LiCO3R.53 As in the case of CO2NCM, excess amount of CO2 produces Li2CO3 formation, but balances with the competitive LiOR formation. More importantly, these two kinds of peaks grow simultaneously up to 10 min but then become saturated afterwards, directly evidencing the self-terminating nature of the surface reaction of CO2+CH4-NCM (Figure 2(c) and (d)). 10 ACS Paragon Plus Environment

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5. Based on these in-situ DRIFT analyses, the surface reactions of the three samples can be summarized as follows: 1) CH4-NCM: MeO (Me = Ni, Co, Mn) + CHx (radicals, x= 1-3) → CHxO + Li → LiOCHx (=LiOR) or MeO (Me = Ni, Co, Mn) + CHx (radicals, x= 1-3) → Me-C (=carbide) + H2↑ 2) CO2-NCM: LixMeO + 2CO2 → Li2CO3 + CO↑ 3) CO2+CH4-NCM: CO2 + CH4 → CHxO + H2↑ + CO↑ CHxO + LixMeO → LiOCHx (=LiOR) LiOCHx + CO2 → LiCO3R To elucidate the self-terminating mechanism and further clarify the role of the reactant gases, DFT calculations were carried out. DFT calculations estimate binding energies of reacting gas molecules with the host surfaces with designated crystalline orientations. Figure 3 and S1 (in the Support Information) portray optimal atomic arrangements of CO2 and CH4 (both separately and together) in interaction with Li2CO3 and LiCO3R surfaces. In this particular calculation, R=CH3 was taken into consideration as a representative case. First, it turned out that the binding energies of the LiCO3R are far lower than those of the Li2CO3 (100) surface for all of the three gas molecule cases (CH4, CO2, CH4+CO2), meaning that the binding onto the LiCO3R surface is energetically less favorable. Next, for the same surfaces, the binding energies of the CO2 alone and CO2+CH4 cases were found to be lower than that of the CH4 alone case. Hence, the self-terminated film formation of CO2+CH4-NCM can be explained by the formation of LiCO3R that is less reactive with incoming reacting gas molecules, especially when the reacting gases are a mixture of CO2 and CH4. The less



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preferred interaction onto the LiCO3R surface can also be correlated with its larger bandgap between the lowest unoccupied molecular orbital (LUMO) level and the highest occupied molecular orbital (HOMO) level as well as the longer bonding lengths between the gas molecules and the surface. While the surface reactivity plays a role in the self-terminated reaction, reaction temperature needs to be set properly; below 400oC, the surface layer formation was almost negligible, whereas above 500oC, the surface layer became too thick with excessive Li extraction (Figure S3 in the Supporting Information). The coating layers of pristine NCM and CO2+CH4-NCM were investigated further by using Auger electron microscopy (Figure 4).56 The point survey scans (spot size: 10 nm x 10 nm) of both samples show three representative spectra related to the pristine NCM, Li2CO3– rich and carbon-rich regions (Figure 4(g)). In addition, the elemental mapping with respect to carbon and lithium (Figures 4(c)-(f)) exhibits the atomic distributions of both atoms on those two surfaces. Although both SEM images appear almost same, carbon atoms are less uniformly distributed on the surface of pristine NCM, and its atomic concentration is mostly lower than 10%. On the contrary, the carbon atom distribution on the CO2+CH4-NCM surface is more uniform with higher atomic concentration of 20~30%, in agreement with the formation of Li2CO3 and LiCO3R. Regarding Li atom distribution, CO2+CH4-NCM exhibits slightly higher concentration, reflecting the thin surface layer generated involving the Li extraction. The depth profiles of Li and C in CO2+CH4-NCM reconfirm that the generated coating layer consists mainly of these two atoms and is also very thin (

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