Low-Cost Al2O3 Coating Layer As a Preformed SEI on Natural

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Low-Cost Al2O3 Coating Layer As a Preformed SEI on Natural Graphite Powder To Improve Coulombic Efficiency and High-Rate Cycling Stability of Lithium-Ion Batteries Tianyu Feng,†,‡,§ Youlong Xu,*,†,‡ Zhengwei Zhang,† Xianfeng Du,†,‡ Xiaofei Sun,†,‡ Lilong Xiong,†,‡ Raul Rodriguez,∥ and Rudolf Holze*,§ †

Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education & International Center for Dielectric Research, Xi’an Jiaotong University, Xi’an 710049, China ‡ Shaanxi Engineering Research Center of Advanced Energy Materials & Devices, Xi’an Jiaotong University, Xi’an 710049, China § Institut für Chemie, AG Elektrochemie, Technische Universität Chemnitz, 09111 Chemnitz, Germany ∥ Institut für Physik, Technische Universität Chemnitz, 09111 Chemnitz, Germany S Supporting Information *

ABSTRACT: Coulombic efficiency especially in the first cycle, cycling stability, and high-rate performance are crucial factors for commercial Li-ion batteries (LIBs). To improve them, in this work, Al2O3-coated natural graphite powder was obtained through a low-cost and facile sol−gel method. Based on a comparison of various coated amounts, 0.5 mol % Al(NO3)3 (vs mole of graphite) could bring about a smooth Al2O3 coating layer with proper thickness, which could act as a preformed solid electrolyte interface (SEI) to reduce the regeneration of SEI and lithium-ions consumption during subsequent cycling. Furthermore, we examined the advantages of Al2O3 coating by relating energy levels in LIBs using density functional theory calculations. Owing to its proper bandgap and lithium-ion conduction ability, the coating layer performs the same function as a SEI does, preventing an electron from getting to the outer electrode surface and allowing lithium-ion transport. Therefore, as a preformed SEI, the Al2O3 coating layer reduces extra cathode consumption observed in commercial LIBs. KEYWORDS: lithium-ion batteries, anodes, alumina coating, SEI, DFT calculations, energy levels

1. INTRODUCTION Commercial Li-ion batteries (LIBs) use a graphite-based carbon anode. During the initial lithiation cycles, a solid electrolyte interphase (SEI) forms on the graphite anode surface due to electrochemical instability of the electrolyte versus lithiated graphite.1−7 Ideally, the SEI inhibits further reduction of the electrolyte, allows Li-ion conduction, and is electronically insulating. However, in fact, the fragile and nonuniform SEI film will crack caused by surface defects and anisotropic rough edges and will be re-formed again and again during charging and discharging. This will consume the limited supply of Li ions continuously in a full battery and therefore result in capacity fading.7 Much effort has been devoted to overcome this problem, and one effective solution is the surface modification of anodes by methods such as mild oxidation8,9 and metal or medal oxide coating.10−12 He et al. deposited alumina on an etched silicon surface by atomic layer deposition (ALD) and obtained a great improvement of capacity retention and Coulombic efficiency (CE).10 His group also found that the open-circuit voltage for © XXXX American Chemical Society

the anode coating with alumina was lower than that without coating, showing that alumina can reduce side reactions between electrode and electrolyte.10 Riley et al. improved electrochemical performance of MoO3 nanoparticles as highcapacity/high-volume expansion anodes for Li-ion batteries by alumina surface coating using ALD.13 Jung et al. performed ALD alumina coating on natural graphite (NG) and LiCoO2 to enhance their long-term cycling performance.14,15 For the mechanism, most of the above concluded the protection effect for electrode by alumina coating. Kim et al. pointed out that amphoteric oxide alumina can protect the electrode surface from acid attack by scavenging HF and H2O from the electrolyte.16 Jung and Han found that, after the lithiation of the Al2O3 coating layer reached a thermodynamically stable phase, extra Li atoms overflowed into the electrode by passing through the coating layer.17 It was reported that Al2O3 coating Received: January 7, 2016 Accepted: February 25, 2016

A

DOI: 10.1021/acsami.6b00231 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Table 1. Sample Treatments and Names, CE, and Specific Capacity in First Cycle at Current Density of 15 mA/ga sample G0 G5A G05A G05U G005A G005U a

solution A 5 mol % Al(NO3)3 0.5 mol % Al(NO3)3 0.5 mol % Al(NO3)3 0.05 mol % Al(NO3)3 0.05 mol % Al(NO3)3

solution B

first CE, %

first specific charge capacity, mA h/g

first Sspecific discharge capacity, mA h/g

5 mol % ammonia 0.5 mol % ammonia 0.5 mol % urea 0.05 mol % ammonia 0.05 mol % urea

85.10 81.30 88.80 90.70 89.20 90.0

320.2 239.6 289.3 320.4 312.4 320.5

376.4 294.9 325.6 353.3 350.3 356.1

The molar ratios of Al(NO3)3, ammonia, and urea refer to the graphite content. black (3 wt %) was prepared by stirring for 180 min with a proper amount of NMP. The slurry was coated onto a copper foil to a thickness of 200 μm using the doctor-blade technique. The electrode was placed in a vacuum oven at 120 °C overnight to dry the electrode and stored in an Ar-atmosphere glovebox. CR2016 coin cells were fabricated to study the electrochemical performance. The rolled electrode film was cut into 2 cm2 circular discs as the anodes. The counter electrode was lithium metal foil. The electrolyte solution was 1 M LiPF6 in EC (ethylene carbonate):EMC (ethyl methyl carbonate) (1:1 by volume). Coin cells were assembled in an argon-filled glovebox (Mikrouna Super (1225/750)) with H2O and O2 concentrations below 0.1 ppm. Electrochemical performance of the cells was evaluated by galvanostatic discharge/charge measurement between 0.01 and 3 V using a computer controlled battery tester (LANHE CT2001A, Wuhan, China). The electrochemical impedance spectra (EIS) were measured at 0.01, 0.1, and 0.5 V in the fifth cycle with a sine voltage of 5 mV in the frequency range from 100 kHz to 10 mHz using a versatile multichannel galvanostat 2/Z (VMP2, Princeton Applied Research, Oak Ridge, TN, USA). 2.3. DFT Calculations. For all solvent molecules, Kohn−Sham DFT calculations were performed using Dmol3 code.21,22 The generalized-gradient approximation (GGA) with the Perdew− Burke−Ernzerhof functional (PBE)23 and Perdew−Wang generalized-gradient approximation (PW91)24 was used to describe the exchange-correlation effects. The descriptions of the valence states were obtained with the double numerical basis set augmented with polarization p-function (DNP)21 which has a computational precision being comparable with the Gaussian split-valence basis set 6-31G**. Global orbital cutoff was 3.7 Å for all the molecules. Spin-unrestricted wave functions were used for each molecule with charge of 0, −1, and +1, respectively. The energy convergence for geometry optimization was set to 1 × 10−5 Ha. A Gaussian smearing of 0.005 Ha was used to achieve self-consistent field convergence. For all bulk materials, calculations were performed using CASTEP code, which adopts fully self-consistent DFT calculations to solve Kohn−Sham equations. For graphite, van der Waals (vdW) interactions were taken into consideration using the Tkatchenko− Scheffler (TS) scheme25 of dispersion correction DFT (DFT-D). The GGA, with the functional PBE23 and PW9124 was employed. The electronic wave functions were expanded as a linear combination of plane waves, with a kinetic energy cutoff of 500 eV. The ultrasoft pseudopotentials for Li, C, Al, and O were used in all calculations. We used a method of Gaussian smearing to achieve self-consistent field convergence with a smearing value of 0.1 eV. The energy and force convergence tolerance was 5.0 × 10−6 eV/atom and 0.01 eV/Å, respectively. A five to ten atomic layer slab with a vacuum region of 35 Å in the vertical direction was employed as the surface model framework for bulk materials. All atoms were relaxed into their ground states by a conjugate-gradient algorithm except the middle layers which were kept frozen mimicking bulk materials.

may act as a solid electrolyte and may prevent direct contact between the cathode surface and the electrolyte.15,18,19 Based on first-principles calculations, Hao et al. suggested that in bulk Al2O3 the Li diffusivity is low, while in coating materials other factors, such as grain boundary and short length scale of coatings, might provide only a small resistance for Li-ion transport.20 However, (i) ALD is very costly and complex and is not suitable for mass production. (ii) The reason for the superiority of alumina coating was not investigated with respect to energy levels. Herein, we demonstrate a low-cost treatment to modify NG by coating with Al2O3 based on a sol−gel method to improve its CE (especially the CE in the first cycle), cycling stability, and rate performance. From a fundamental point of view, we examined the advantages of alumina coating with respect to relative energy levels in Li-ion batteries using first-principles density functional theory (DFT) calculations.

2. EXPERIMENTAL SECTION The natural graphite powder YAG-1 used in this study was obtained from UNION AMPEREX Co. (Henan Province, People’s Republic of China). The particle size D50 and Brunauer−Emmett−Teller (BET) surface area of the natural graphite are 18−24 μm and 5.0 m2/g, respectively. 2.1. Preparation. A 0.6 g amount of YAG-1 graphite particles (G0) was added into 50 mL of aqueous solution with various molar concentrations of Al(NO3)3 (vs graphite) (solution A). After 30 min of stirring, 50 mL of ammonia solution or urea solution with the same molar concentration of Al(NO3)3 (solution B) was added dropwise to solution A under vigorous stirring. The final solution was dried and concentrated at 70 °C on a water bath under stirring resulting in a gel. Subsequently the gel was transferred into a tube furnace and heated in argon atmosphere at 900 °C for 6 h. The treatments and names for each sample are listed in Table 1. 2.2. Characterization. The ζ potential of the graphite powder and the sol solution were measured with a Zetasizer NanoZS 90 (Malvern Instruments Ltd., Malvern, U.K.) with a measure angle of 90° and a laser wavelength of 633 nm at 25 °C. A suspension of 5 mg of graphite powder in 20 mL of DI water was used for graphite ζ potential characterization. A mixture of 10 mL of 0.2 M Al(NO3)3 and 10 mL of 0.2 M NH3·H2O or urea was used for colloidal ζ potential characterization. Field emission scanning electron microscopy (FESEM, Quanta 250FEG, FEI, Hillsboro, OR, USA) and transmission electron microscopy (TEM, JEM-2100, JEOL Ltd., Tokyo, Japan) were employed to investigate the surface morphology of graphite samples. An energy dispersive spectrometer (EDS, EDAX TEAM Apollo XL-SDD, Oxford Instrument) attached to the SEM was used to analyze elements on the surface. The crystalline structures were characterized by powder X-ray diffraction (XRD) measurements using an X’Pert PRO (PANalytical Ltd., Eindhoven, Holland) diffractometer equipped with Cu Ka radiation by step scanning in the 2θ range of 10−80°. Graphite electrodes were prepared using polyvinylidene fluoride (PVDF) as binder and N-methylpyrrolidinone (NMP) as solvent. A mixture of graphite powder (92 wt %), PVDF (5 wt %), and carbon

3. RESULTS AND DISCUSSION Figure 1 shows schematic relative energy levels in a typical Liion battery. The open-circuit voltage, UOC, of a cell is the difference between the electrochemical potentials μA and μC of the anode and cathode: B


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With Al2O3 coating, probably because of its proper bandgap and lithium-ions conduction ability,17,20,26 the coating layer performs a function similar to that of an SEI, preventing electron transfer and allowing lithium-ion transport. As a preformed SEI, the Al2O3 coating layer decreases the lithiumion consumption for generation of SEI. Therefore, it can improve CE and reduce required inventory of lithium ions in commercial batteries. ζ potential is the electric potential in the interfacial double layer (DL) at the location of the slipping plane relative to a point in the bulk fluid away from the interface. It has been widely used to characterize the interactions between colloids or particles. Figure 2a shows the ζ potential of the graphite particle G0 in water and the colloids of 0.1 M [Al(NO3)3 + NH3·H2O] and 0.1 M [Al(NO3)3 + urea], respectively. The results are consistent with previously reported ones.27−30 They suggest that colloids with positive charge will attach onto graphite particles with negative charge by electrostatic attraction spontaneously, as shown in Figure 2b, suggesting that surfactants are not necessary for this sol−gel coating method. In Figure 3, the samples with different coating amounts were investigated by FE-SEM analysis. The rough surface of pristine NG powders with many flakes would result in coarse and much SEI formation, consuming a lot of lithium ions while cycling. In the coating process, the ammonia added into the solution increased its pH so as to form Al(OH)3 sol to enhance the adhesion of Al3+ on graphite particles. With thick coating, as shown in Figure 3b, the cracks on the surface of G5A were harmful to formation of a smooth SEI, indicating that the coating was too thick. Cracks may be caused by the different strains of graphite and the coating layer. Its specific charge/ discharge capacity was lower (see below) than that of others, which could also be attributed to the thick coating. G05A and G05U (Figure 3e,f) showed a relatively smooth surface which could act as a preformed SEI. To further improve the coating method, urea was used instead of ammonia for the following reasons. First, an aqueous solution of urea has a lower pH than that of ammonia at the same concentration so that the initial colloidal particles will be much smaller and more uniform. In addition, urea will decompose gradually when heated to over 60 °C. The colloidal particles attached onto the graphite surface would grow larger and merge into a uniform layer finally (G05U in Figure 3e,f). Moreover, Figure 3g shows the relative atomic composition of the area of G05U in Figure 3f by EDS. In addition, as shown in Figure 3i,j,k,l, with 0.05 mol % coating, G005A and G005U had thinner coating layers. TEM was performed to study the morphology and lattice plane of G05U, as shown in Figure 4. To analyze the crystal structure clearly, FFT images for various areas were aquired to study the crystal plane from outside in. The lattice plane for each pattern was pointed out correspondingly, according to its symmetrical length. The patterns of Al2O3 (113) and graphite (002) indicated their good crystallinity and polycrystallinity of Al2O3. Figure 5 presents the XRD patterns of the pristine and the Al2O3-coated NG samples. A logarithmic y-axis is used to highlight the peaks of Al2O3 in the presence of the extremely high peak of graphite (002). The intensities of Al2O3 peaks in G05A and G05U were higher than those in G005A and G005U, indicating they contained a greater amount of Al2O3, which is consistent with their SEM images in Figure 1.

Figure 1. Schematic energy levels in a typical Li-ion battery. The LUMO of the electrolyte is lower than the Fermi energy of Li (ELi), the difference between which is noted as ΔEred. EIE stands for ionization energies of various organic solvent molecules.

UOC = (μA − μC )/e

(1)

Theoretically, μA and μC should be both between LUMO and HOMO of the components of the electrolyte solution so that electrolyte will not decompose. But, in fact, μA above the electrolyte LUMO reduces the electrolyte unless the anode− electrolyte reaction becomes blocked by the formation of a passivating SEI layer; similarly, a μC located below the HOMO oxidizes the electrolyte unless the reaction is blocked by an SEI layer.7 DFT calculations were used to supplement the insufficient experimental data (see Table 2). In Supplementary Information Table 2. ΔEred for Various Electrolyte Solvents by DFT Functionals of PBE and PW91, Respectively ΔEred (eV) VC (vinylene carbonate) FEC (fluoroethylene carbonate) EC (ethylene carbonate) DMC (dimethyl carbonate) DEC (diethyl carbonate) PC (propylene carbonate) MeOH (methanol) H2O

PBE

PW91

1.414 1.392 0.927 0.677 0.194 0.718 0.582 2.194

1.464 1.443 0.989 0.726 0.247 0.773 0.598 2.273

(SI) Table S1 and Table S2, details of the LUMO−HOMO gaps and ionization energies (EIE) of various organic solvent molecules and the work function φ of bulk Li, Al2O3, and graphite are collected. Experimental data are compared with calculated ones. The LUMO of the electrolyte is lower than the Fermi energy of Li (Ef,Li), the difference between which is noted as ΔEred. This is consistent with the view of Goodenough and Park that Ef,Li ∼ 1.1 eV above the LUMO of the common DMC/DEC electrolyte solutions.7 As the graphite anode becomes lithiated, Ef,graphite will shift up gradually to the end of the charging voltage (in our work, the end of lithiation potential was set to 0.01 V vs Li/Li+). Since the charge/ discharge plateau of graphite is ∼0.1 V vs Li/Li+, μA of lithiated graphite is ∼0.1 eV lower than Ef,Li. This suggests that the decomposition of electrolyte solution will still take place yielding a SEI film on graphite anodes. C


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Figure 2. (a) ζ potential of G0, 0.1 M [Al(NO3)3 + NH3·H2O] and 0.1 M [Al(NO3)3 + urea], respectively. (b) Scheme depicting adhesion of colloid onto graphite powder due to electrostatic attraction.

Figure 3. SEM/EDS analysis of pristine and Al2O3-coated graphite. (a) Pristine natural graphite powder G0. (b) G5A graphite powder. The dotted red rectangular shows a crack in the coating on the powder. (c, d) G05A graphite powder. (e, f) G05U graphite powder. (i, j) G005A graphite powder. (k, l) G005U graphite powder. The dotted red rectangular in (c, e, i, k) shows the area of magnification. The EDS area scanning result for G05U powder is shown in g. (h) Schematic of the coating.

and the hexagonal structure with space group P63/mmc, respectively. As shown in Figure 6, the open-circuit voltage (OCV) for each sample was measured 24 h later after cell assembly for relaxation before any electrochemical tests. The cell with pristine graphite powder G0 showed higher OCV than any

Accordingly, the smaller peaks of Al2O3 for G005A and G005U suggested their thinner coatings. Besides, all of the NG peaks did not show any evident changes, revealing that the coating treatment did not influence the crystal structure of core material. All diffraction peaks of Al2O3 and graphite can be indexed to the rhombohedral structure with space group R3c̅ D


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Figure 6. Comparison of the OCVs measured 24 h later after cell assembly for relaxation before any electrochemical tests.

layer effectively prevented some side reactions between the electrode and the electrolyte solution. It also implies that G05U with the lowest OCV had the most uniform and effective coating. Theoretical specific capacity of graphite is 372 mA h/g assuming a lithiated state LiC6 with a charge/discharge potential plateau of ∼0.1 V vs Li/Li+.31 The CE, specific charge, and discharge capacity in the first cycle for the samples with various treatments were summarized in Table 1. Commonly during lithiation of anode materials in the first cycle, most lithium ions are consumed to form the SEI layer on the surface. Therefore, the CE in the first cycle is more indicative than that in following cycles. Figure 7 shows initial charge/discharge profiles of the samples at a current density of 15 mA/g in the electrochemical

Figure 4. TEM images of G05U powder. Panels a1−a4 show FFT images of the red rectangular areas in panel a, respectively. In a2−a4, crystal faces and XRD reference card numbers are shown.

Figure 5. XRD patterns of G0, G05A, G05U, G005A, and G005U (a). The peaks around 36° and 67° corresponding to Al2O3 (104), (110), (214), and (300) are magnified and shown in insets b and c, respectively. The specific peaks of Al2O3 (113), (024), and (116) are also labeled in panel a. The reference patterns for Al2O3 and graphite are presented at the bottom of panel a.

other cells with Al2O3 coating anodes. G05U presented the lowest OCV. According to energy conservation during reaction (eq 2), higher U means higher ΔG, i.e., more “reactivity”. (2)

Figure 7. Initial voltage profiles of the samples obtained at 15 mA/g in the electrochemical window of 0.01−3 V vs Li/Li+.

where ΔG is the Gibbs free energy change, n is the number of electrons transferred, U is the cell voltage, and F is the Faraday constant. There is a negative sign because a spontaneous reaction has a negative ΔG and a positive U. This indicates that the interaction between G05U and the electrolyte was the weakest while that of G0 was the strongest. This agrees well with the report by He et al. that patterned silicon electrodes without alumina coating were more reactive toward the electrolyte solution than coated ones.10 The Al2O3

window of 0.01−3 V vs Li/Li+. The voltage step at ∼1.2 V versus Li/Li+ corresponding to side reactions including SEI formation32 was reduced with coated NG, which suggests that the coating method decreased the irreversible capacity loss for the electrodes. The much lower specific charge capacity of G5A than that of G0 might result from its thick coating, which can be seen in Figure 3b. G05U showed the smallest discharge plateau between 1.3 and 0.1 V and a relatively high specific

ΔG = −nFU

E


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ACS Applied Materials & Interfaces charge capacity, suggesting that its coating layer could decrease the consumption of lithium ions to form SEI and not provide much resistance for lithium ions to transport. The superiority performance of G05U could be attributed to its smooth coating, as shown in Figure 3e,f. Even the thinner coating of G005A and G005U apparently results in a relatively high first CE and specific capacity at a low current density of 15 mA/g (see also Table 1). To evaluate the impact of Al2O3 coating on rate performance and cycling stability, the cells were tested at rates varying from 15 to 480 mA/g, as presented in Figure 8. The specific charge

Figure 9. Series of mass-normalized impedance spectra for electrodes in the fifth cycle at 0.01, 0.1, and 0.5 V, respectively.

Table 3. Most reversible specific capacity and lithiation/ delithiation reactions took place at this potential plateau as Table 3. Rf and Rct in Each Electrode in the Fifth Cycle at 0.01, 0.1, and 0.5 V, Respectively, Corresponding to the Curves in Figure 9a

Figure 8. Rate performance of the samples obtained from 15 to 480 mA/g between 0.01 and 3 V vs Li/Li+.

R f ,# Ω

capacity of G005A and G005U is relatively high at low current density but low at higher current density, which can be attributed to their thinner coating that might not be able to act as strong and thick enough preformed SEI layers at high current density. As the thickness of SEI will increase and decrease while discharging (lithiation) and charging (delithiation), respectively,32 the thinner coating could not prevent the formation and deformation of SEI while cycling and resulted in deterioration of cycling stability at high current density finally. G05U displayed superior rate performance and cycling stability, especially at relatively high current density, which could result from its relatively smooth Al2O3 coating with optimum thickness. The coating with proper thickness could act as a preformed SEI reducing regeneration of SEI and lithium-ion consumption during cycling. Less regeneration of SEI would produce less SEI fragments, therefore resulting in its best rate performance. Although G05A shows similar charge behavior at high current density as G05U does, its cycling stability deteriorated quickly at relatively high current density of 120 mA/g. The cycling stability of G0 deteriorated dramatically at high current density, revealing its instable SEI causing low CE. In addition, G5A exhibited the lowest specific charge capacity, which can be probably due to its thick and unsmooth coating with cracks (Figure S2). Despite the cracks, thick coating provides an insufficient number of transport channels for lithium ions, resulting in low specific capacity at both low-rate and relatively high-rate cycling. EIS was used to better understand how the cycling stability and high-rate performance of the Al2O3-coated NG were improved greatly. Figure 9 shows the Nyquist plots of the samples in the fifth cycle at 0.01, 0.1, and 0.5 V, respectively. The detailed data from EIS experiments are summarized in

a

Rct,* Ω

sample

0.01 V

0.1 V

0.5 V

0.01 V

G0 G05A G05U G005A G005U

107 53 47 42 38

21 27 34 17 20

21 27 34 17 20

108

Symbols “#” and “*” reference the indicators given in Figure 9.

shown in Figure 7. Therefore, EIS measurement at ∼0.1 V is of great significance. The high-frequency semicircle region is related to lithium-ion diffusion in the SEI and coating layer (Rf, #); the middle-frequency semicircle region is related to the charge transfer resistance (Rct, *) between the active material and the surface film; the low-frequency slope region represents lithium-ion diffusion in the bulk material.15,33−36 As Zhang et al. pointed out for anodes the thickness of SEI will increase and decrease while discharging (lithiation) and charging (delithiation), respectively;32 Rf at low potential would be larger than that at high potential accordingly. The Rf of G0 was the highest (∼107 Ω) at 0.01 V while almost the lowest (∼21 Ω) at 0.5 V, indicating its highly variable thickness of SEI, as shown in Table 3. In contrast, the Rf of G05U stayed at around 40 Ω (∼47 to ∼34 Ω) in the range of 0.01 to 0.5 V, suggesting that its Al2O3 coating provides a relatively stable SEI, preventing generation and decomposition of SEI during cycling. Due to incomplete reversibility of SEI regeneration,7 more thickness variation of SEI would generate more fragments of SEI and consume more lithium ions, which would increase the resistance and result in degradation of cycling stability and high-rate performance finally. F


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ACS Applied Materials & Interfaces G005A and G005U possessed relatively low Rf, but their Rct was high especially at 0.01 V. This could be attributed to its thinner coating that might not be insulating and strong enough to act as a proper preformed SEI, so that SEI regeneration would still happen and therefore lead to high Rct. At low current density, low ohmic polarization of thinner coating could support relatively high first cycle CE and specific capacity. At higher current density, however, it displayed relatively low specific capacity and deterioration of cycling stability probably because of required regeneration of SEI.32 This explanation agrees well with cycling performance in Table 1 and Figure 8. Consequently, it implies that a smooth Al2O3 layer with a proper thickness could act as a preformed SEI, reducing regeneration of SEI and associated lithium-ion consumption and increasing cycling stability especially at high current density.

Shaanxi Province (Grant Nos. 2014JQ2079, 2014JM6231, and 2014JQ2-2007), the Fundamental Research Funds for the Central Universities of China (Grant No. xjj2014044), and the Deutscher Akademischer Austausch Dienst (German Academic Exchange Service, DAAD).

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(1) Verma, P.; Maire, P.; Novák, P. A Review of the Features and Analyses of the Solid Electrolyte Interphase in Li-Ion Batteries. Electrochim. Acta 2010, 55 (22), 6332−6341. (2) Peled, E. The Electrochemical Behavior of Alkali and Alkaline Earth Metals in Nonaqueous Battery Systems - The Solid Electrolyte Interphase Model. J. Electrochem. Soc. 1979, 126 (12), 2047−2051. (3) Aurbach, D. Review of Selected Electrode−Solution Interactions Which Determine the Performance of Li and Li Ion Batteries. J. Power Sources 2000, 89 (2), 206−218. (4) Edström, K.; Herstedt, M.; Abraham, D. P. A New Look at the Solid Electrolyte Interphase on Graphite Anodes in Li-Ion Batteries. J. Power Sources 2006, 153 (2), 380−384. (5) Xu, K.; von Cresce, A. Interfacing Electrolytes with Electrodes in Li Ion Batteries. J. Mater. Chem. 2011, 21 (27), 9849−9864. (6) Aurbach, D.; Markovsky, B.; Shechter, A.; Ein-Eli, Y.; Cohen, H. A Comparative Study of Synthetic Graphite and Li Electrodes in Electrolyte Solutions Based on Ethylene Carbonate-Dimethyl Carbonate Mixtures. J. Electrochem. Soc. 1996, 143 (12), 3809−3820. (7) Goodenough, J. B.; Park, K.-S. The Li-Ion Rechargeable Battery: A Perspective. J. Am. Chem. Soc. 2013, 135 (4), 1167−1176. (8) Wang, H.; Abe, T.; Maruyama, S.; Iriyama, Y.; Ogumi, Z.; Yoshikawa, K. Graphitized Carbon Nanobeads with an Onion Texture as a Lithium-Ion Battery Negative Electrode for High-Rate Use. Adv. Mater. 2005, 17 (23), 2857−2860. (9) Wu, Y. P.; Jiang, C.; Wan, C.; Holze, R. Modified Natural Graphite as Anode Material for Lithium Ion Batteries. J. Power Sources 2002, 111 (2), 329−334. (10) He, Y.; Yu, X.; Wang, Y.; Li, H.; Huang, X. Alumina-Coated Patterned Amorphous Silicon as the Anode for a Lithium-Ion Battery with High Coulombic Efficiency. Adv. Mater. 2011, 23 (42), 4938− 4941. (11) Kim, S.-S.; Kadoma, Y.; Ikuta, H.; Uchimoto, Y.; Wakihara, M. Electrochemical Performance of Natural Graphite by Surface Modification Using Aluminum. Electrochem. Solid-State Lett. 2001, 4 (8), A109. (12) Park, S. B.; Shin, H. C.; Lee, W.-G.; Cho, W. I.; Jang, H. Improvement of Capacity Fading Resistance of LiMn2O4 by Amphoteric Oxides. J. Power Sources 2008, 180 (1), 597−601. (13) Riley, L. A.; Cavanagh, A. S.; George, S. M.; Jung, Y. S.; Yan, Y.; Lee, S.-H.; Dillon, A. C. Conformal Surface Coatings to Enable High Volume Expansion Li-Ion Anode Materials. ChemPhysChem 2010, 11 (10), 2124−2130. (14) 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 (19), 2172−2176. (15) 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 (1), A75−A81. (16) Kim, J.-S.; Johnson, C. S.; Vaughey, J. T.; Hackney, S. A.; Walz, K. A.; Zeltner, W. A.; Anderson, M. A.; Thackeray, M. M. The Electrochemical Stability of Spinel Electrodes Coated with ZrO2, Al2O3, and SiO2 from Colloidal Suspensions. J. Electrochem. Soc. 2004, 151 (10), A1755−A1761. (17) Jung, S. C.; Han, Y.-K. How Do Li Atoms Pass through the Al2O3 Coating Layer During Lithiation in Li-Ion Batteries? J. Phys. Chem. Lett. 2013, 4 (16), 2681−2685. (18) Li, C.; Zhang, H. P.; Fu, L. J.; Liu, H.; Wu, Y. P.; Rahm, E.; Holze, R.; Wu, H. Q. Cathode Materials Modified by Surface Coating for Lithium Ion Batteries. Electrochim. Acta 2006, 51 (19), 3872−3883.

4. CONCLUSIONS A low-cost treatment to modify NG by coating with Al2O3 based on sol−gel method was demonstrated to improve its CE (especially the CE in first cycle), cycling stability, and high-rate performance. A smooth Al2O3 coating layer with proper thickness can act as a preformed SEI reducing regeneration of SEI and lithium-ion consumption during cycling. Advantages of Al2O3 coating were theoretically examined by inspecting relative energy levels in Li-ion batteries based on first-principles DFT calculations. Due to its suitable bandgap and favorable lithium-ions conduction ability,17,20,26 the Al2O3 coating performs the functions as an SEI, preventing electron transfer and allowing lithium-ion transport. Therefore, as a preformed SEI, the Al2O3 coating reduces extra lithium-ion consumption in commercial batteries. Possible future research topics can be extended to the application for other anode materials and further confirmation for the SEI change on the Al2O3 coating layer by in situ Fourier transform infrared attenuated total reflection spectroscopy or Raman spectroscopy.

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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.6b00231. Details of work functions and bandgaps for lithium, Al2O3, and graphite, EIE, LUMO−HOMO gap, and ΔEredof electrolyte solvents (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*(Y.X.) E-mail: [email protected]. *(R.H.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The SEM and TEM work was done at the International Center for Dielectric Research (ICDR), Xi’an Jiaotong University, Xi’an, China; we also thank Ms. Dai and Dr. Lu Lu and for their help in using SEM and TEM. We acknowledge Professor Xiang Zhao in Xi’an Jiaotong University for his support with use of the calculation code. This project is financially supported by the National 111 Project of China (Grant B14040), the National Natural Science Foundation of China (Grant Nos. 21203145, 21343011, and 21503158), the Natural Science Foundation of G


Research Article

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Low-Cost Al2O3 Coating Layer As a Preformed SEI on Natural

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