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Understanding the Enhanced Kinetics of Gradient-Chemical-Doped Lithium-Rich Cathode Material Zhengping Ding,†,∥ Mingquan Xu,†,∥ Jiatu Liu,†,‡,∥ Qun Huang,† Libao Chen,† Peng Wang,*,‡ Douglas G. Ivey,§ and Weifeng Wei*,† †

State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan 410083, People’s Republic of China National Laboratory of Solid State Microstructures, College of Engineering and Applied Sciences, and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing, Jiangsu 210093, People’s Republic of China § Department of Chemical & Materials Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 1H9 ‡

S Supporting Information *

ABSTRACT: Although chemical doping has been extensively employed to improve the electrochemical performance of Li-rich layered oxide (LLO) cathodes for Li ion batteries, the correlation between the electrochemical kinetics and local structure and chemistry of these materials after chemical doping is still not fully understood. Herein, gradient surface Si/Sn-doped LLOs with improved kinetics are demonstrated. The atomic local structure and surface chemistry are determined using electron microscopy and spectroscopy techniques, and remarkably, the correlation of local structure-enhanced kinetics is clearly described in this work. The experimental results suggest that Si/Sn substitution decreases the TMO2 slab thickness and enlarges the interslab spacing, and the concentration gradient of Si/Sn affects the magnitude of these structural changes. The expanded interslab spacing accounts for the enhanced Li+ diffusivity and rate performance observed in Si/Sn-doped materials. The improved understanding of the local structure-enhanced kinetic relationship for doped LLOs demonstrates the potential for the design and development of other high-rate intercalated electrode materials. KEYWORDS: lithium-rich layered oxide, cathode materials, gradient chemical doping, interslab spacing, HAADF-STEM

1. INTRODUCTION Layered oxides of LiTMO2 (TM = Mn, Ni, Co, etc.) are extensively employed as cathode materials in commercial Li ion battery technology.1−3 The crystal structure of these layered oxides can be viewed as ordered rock salt derivatives, in which octahedral Li and TM cations form alternating layers confined to the (111) planes of cubic oxygen close packing. Through substitution of Li ions for TM ions in the metal layers, Li-rich layered oxides (LLOs), xLi2MnO3·(1−x)LiTMO2, are fabricated, and these LLOs exhibit high specific capacities (up to 250 mA h g−1) and are more cost-effective.4−9 In the electrochemistry of the LLOs, the Li2MnO3 component plays important roles; namely, it enhances the structural stability of LiTMO2 at high voltages and acts as an electrochemically active structure for Li extraction when charged above 4.5 V.10−15 However, the LLO cathode materials still suffer from cycling © 2017 American Chemical Society

instability and poor rate performance, which is attributed to the insulating nature and the activation process of the Li2MnO3 component.9,16−18 To address these technical challenges, chemical doping, such as Na,19 Mg,20,21 Al,22 Ti,23 Ru,24 Sn,25−27 B,28,29 Si,25,30 P,31,32 etc., has been employed to alter the electronic structure and stabilize the oxygen close-packed structure of LLO materials. Besides cycling durability, chemical doping also has a substantial influence on the lithium diffusion kinetics in the cathode materials. For instance, it has been demonstrated that the Sn-doping strategy can improve the lithium diffusion kinetics of LLO materials by expanding the tunnels for Li ion Received: February 28, 2017 Accepted: June 2, 2017 Published: June 2, 2017 20519

DOI: 10.1021/acsami.7b02944 ACS Appl. Mater. Interfaces 2017, 9, 20519−20526

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ACS Applied Materials & Interfaces

Figure 1. (a) Powder X-ray diffraction patterns of pristine, Si-doped, and Sn-doped materials synthesized at 900 °C. (b) Enlarged regions for the (003) and (104) peaks of the XRD patterns showing the peak shift.

diffusion.25,26 However, for Si-doped LLO materials, diverse tendencies were observed. Various research groups synthesized Si-doped LLO materials through solid-state reactions that exhibit inferior discharge capacity and rate behavior,25,30 while Xia et al. demonstrated improved Li ion kinetics in Si-doped material prepared by a sol−gel process.33 Moreover, dopant inhomogeneity, especially surface segregation, is evident in P-, Si-, and Na-doped LLO materials, giving rise to distinct effects on the subsequent electrochemical properties.19,25,31 It remains unclear whether the diverse performances of doped LLOs are a result of different synthesis processes and/or doped compositions employed in various research works, both of which profoundly influence the crystal structure and surface chemistry of the LLOs. Thus, a better understanding of both the local structure and chemistry of doped oxides is crucial, but a challenge, to elucidate the mechanism and behavior of chemical doping and the resulting rate performance of LLOs. Herein, we demonstrate a gradient doping strategy, driven by concentration diffusion of dopants, to enhance the electrochemical kinetics of LLO cathodes. To assess the importance of the dopant’s physicochemical characteristics on Li diffusion kinetics, two types of cations (Si4+ and Sn4+) with large differences in ionic radius and electronegativity were employed. The local chemistry and atomic arrangement from the surface to the bulk, revealed by X-ray photoelectron spectroscopy (XPS) depth profiling and aberration-corrected scanning transmission electron microscopy (STEM), clearly indicate that Si4+ and Sn4+ cations play distinct roles in changing the crystal structure and rate performance of LLO materials.

2.2. Materials Characterization. The crystallographic structure of the LLO materials was characterized by powder X-ray diffraction (PXRD) (Bruker AXS D8 Advance X-ray diffractometer, Germany) with a step of 0.02° at a dwell time of 2 s. Rietveld refinement was performed using GSAS+EXPGUI software. The morphology was characterized with a field emission scanning electron microscope (Nova Nano SEM230, United States). An FEI Titan G2 60-300 double Cs-corrected transmission electron microscope operated at 300 kV was employed to perform ABF/HAADF-STEM imaging (ABF/ HAADF = annular bright-field/high-angle annular dark-field). The overall compositions of all the materials was measured using inductively coupled plasma atomic emission spectrometry (ICPAES) (PS-6, Baird) and are shown in Table S1 (Supporting Information). Chemical-state analysis was performed by XPS (ESCALAB 250Xi, Thermo Scientific, United States). XPS measurements were carried out using monochromatic Al Kα X-rays (1489.6 eV). To collect the survey spectra, the energy step size was 1 eV and the pass energy was 100 eV. For each element, the high-resolution spectra were collected with an energy step size of 0.05 eV and a pass energy of 30 eV. Etching with Ar+ ions was applied to obtain depth profiles of Mn, Co, Ni, Sn, and Si, and the etching rate was estimated to be 5.4 nm min−1. The chemical composition was calculated by quantitative analysis with the instrument sensitivity factor. The XPS spectra were calibrated by using the C 1s peak with a binding energy of 284.8 eV. 2.3. Electrochemical Characterization. Electrochemical performance was determined on CR2025-type coin cells assembled in a glovebox filled with Ar gas. A lithium metal counter electrode, a Celgard 2400 separator, and 1 M LiPF6 in EC−DMC (1:1, w/w) electrolyte were employed. For the working electrode, 80 wt % active materials, 10 wt % acetylene black, and 10 wt % poly(vinylidene fluoride) (PVDF) were mixed with N-methyl-2-pyrrolidone (NMP) to form a slurry. The slurry was then pasted onto Al foil and dried at 120 °C for 12 h in a vacuum oven. Finally, it was cut into a circular electrode (diameter 12 mm) as the as-prepared cathode. Galvanostatic charge−discharge measurements were performed with a battery testing system (LANHE CT2001A, People’s Republic of China) in the potential range of 2.0−4.8 V. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were collected using an electrochemical workstation (PARSTAT 4000). The impedance spectra were recorded in the frequency range of 100 kHz to 0.1 Hz with an applied alternating voltage of 5 mV.

2. EXPERIMENTAL SECTION 2.1. Materials Synthesis. The pristine and doped Li1.2Mn0.54Ni0.13Co0.13X0.03O2 materials (X = Si and Sn) were synthesized via a sol−gel method.34 Typically, stoichiometric amounts of lithium acetate dihydrate (CH3COOLi·2H2O), manganese acetate tetrahydrate (Mn(CH3COO)2·4H2O), nickel acetate tetrahydrate (Ni(CH3COO)2· 4H2O), cobalt acetate tetrahydrate (Co(CH3COO)2·4H2O), and tetraethyl orthosilicate (TEOS) or stannous oxalate (SnC2O4) were dissolved in distilled water with an 8 atom % excess of Li salt to compensate for its evaporation during high-temperature calcination. Then a certain amount of citric acid was added to the solution as a chelating agent with the molar ratio of citric acid to metal ion of 1:1. The formed colloidal gel was then dried at 100 °C for 12 h. After grinding, the obtained powders were calcined first at 450 °C for 5 h and then at 900 °C for 12 h in air to obtain the pristine and Si/Sndoped materials.

3. RESULTS AND DISCUSSION To understand structural changes induced by Si/Sn substitution, PXRD analysis was carried out for as-prepared pristine and doped materials (Figure 1a). All diffraction peaks for the three samples can be indexed to a hexagonal R3m ̅ structure, except for the superlattice reflections appearing in the 2θ range of 20−25°, which are due to partial ordering of Li cations within the TM layers and are very similar to those 20520


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ACS Applied Materials & Interfaces Table 1. Refined Lattice Parameters and Li/Ni Disorder Degree (χ) for the Pristine and Doped Materials

a

sample

a (Å)

c (Å)

V (Å3)

I(003)/I(104)

χa

Rp (%)

Rwp (%)

pristine Si-3% Sn-3%

2.85709(9) 2.85576(6) 2.86058(5)

14.26076(6) 14.25116(3) 14.26233(8)

100.8147(5) 100.6528(7) 101.0720(4)

1.085 1.207 1.563

1.164 0.98 0.332

3.64 4.14 3.55

5.01 5.71 4.83

Li/Ni disorder degree χ = Occ.(Ni 3a)/Occ.(Li 3b).

Figure 2. (a−c) Charge−discharge curves for pristine and Si/Sn-doped materials at various C rates. (d) Rate performance for the pristine and Si/Sndoped materials. All of them were tested at 25 °C in the potential range of 2.0−4.8 V versus Li+/Li.

reported in the literature.24−26,32,35 No extra diffraction peaks for Si/Sn-containing samples were observed, suggesting that the Si and Sn cations enter the lattice. Compared with the pristine material, there is a significant increase in the d-spacings for the (003) and (104) planes of the Sn-doped material, whereas slightly smaller d-spacings are observed in the Si-doped material (Figure 1b). The (003)/(104) intensity ratios for Siand Sn-doped materials are 1.207 and 1.563, respectively (Table 1), which are much higher than that for the pristine material (1.085). This is indicative of lower Li+/Ni2+ interlayer mixing. A full Rietveld refinement was carried out to quantify differences in crystalline structures and lattice parameters, and the fitting results are presented in Tables S2−S4 and Figure S1 (Supporting Information). The enlarged lattice parameters associated with Sn substitution may be attributed to the larger ionic size of Sn4+ (0.69 Å) compared with those of Mn4+ (0.53 Å) and Si4+ (0.40 Å).36 As shown in Table 1, the Li/Ni disorder degree of Sn-doped materials is 0.332, while those of pristine and Si-doped are 1.164 and 0.980, respectively, indicating that Sn substitution also leads to the least Li+/Ni2+ interlayer mixing.37 It is generally believed that a larger d-spacing and a reduced amount of cation intermixing favor lithium diffusivity and voltage/capacity retention in layered oxide cathodes.38 Improved rate capability and cyclability are, therefore, expected in the Sn-doped material.

To identify the effect of structural changes on the electrochemical kinetics, galvanostatic charge−discharge testing was carried out on pristine and Si/Sn-doped materials, and the results are shown in Figure 2 and Figures S2 and S3 and Tables S5 and S6 (Supporting Information). Figure S2 shows the typical initial charge and discharge curves for the pristine and Si/Sn-doped materials, taken at a 0.1 C rate between 2 and 4.8 V. During the charge process, both of them exhibit a long plateau at ∼4.5 V (Figure S2), which is related to Li and O extracted from the Li2MnO3 phase and structural rearrangement.9,14,15 As shown in Table S5, the pristine material delivers the highest charge capacity of 359.1 mA h g−1 at 0.1 C, when compared with those of Si-doped (351.9 mA h g−1) and Sndoped (340.1 mA h g−1) materials. However, the pristine material delivers a discharge capacity of 283.0 mA h g−1, which is slightly lower than that of the Si-doped material (286.5 mA h g−1) and higher than that of the Sn-doped material (279.3 mA h g−1). Hence, improved first-cycle Coulombic efficiency was detected in Si-doped (81.41%) and Sn-doped (82.13%) materials when compared with that of the pristine material (78.80%). As indicated in Figure 2, compared to the pristine sample, the Si/Sn-doped samples exhibit an enhanced rate capacity. The Sn-doped material has specific capacities of 264.2, 226.2, 198.9, 177.5, 157.6, 142.5, 132.8, and 127.1 mA h g−1 at 0.2, 0.5, 1, 2, 4, 6, 8, and 10 C, respectively, and those of the Sidoped material are 274.1, 244.6, 210.5, 173.6, 139.6, 117.1, 85, 20521


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ACS Applied Materials & Interfaces 50 mA h g−1, respectively, while those of the pristine sample are 253.2, 195.9, 151, 107.4, 43, 19.2, 14.4, and 13.2 mA h g−1, respectively (Table S6). The Sn-doped material exhibits a slightly lower discharge capacity than the Si-doped material at a low C rate, which is likely due to the inactive and heavy Sn atoms incorporated into the LLO structure. As shown in Table S7 (Supporting Information), with the same doping level, the theoretical specific capacity of the Sn-doped material (369.0 mA h g−1) is slightly lower than that of the Si-doped material (379.4 mA h g−1). It is interesting to note that the rate capability of the Sn-doped material noticeably surpasses that of the Si-doped material at rates of 2 C or higher. At a very high current density of 10 C, the Sn-doped material still maintains a reversible specific capacity of 127.1 mA h g−1, while those for the pristine and Si-doped materials are 13.2 and 50 mA h g−1, respectively. Moreover, compared to that of the pristine material, the cycling performance is also enhanced after Si/ Sn-doping. As shown in Figure S3, the capacity retention of the Sn-doped material is 86.2% after 100 cycles, while those for the Si-doped and pristine materials are 70.4% and 46.0%, respectively. EIS was performed on the pristine and Si/Sn-doped materials to evaluate their charge transfer resistance, as shown in Figure S4 and Table S8 (Supporting Information). The charge transfer resistances for the pristine material, Si-doped material, and Sn-doped material are 299.9, 195, and 47.31 Ω, respectively (Table S8). Compared with the pristine material, both doped materials, especially the Sn-doped material, exhibit a much lower charge transfer resistance. The Li ion diffusion coefficient (DLi) for all materials was quantified using cyclic voltammograms collected at different scan rates, on the basis of eqs 1 and 2 in the Supporting Information. As shown in Figure S5 and Table S9 (Supporting Information), enhanced lithium ion diffusion coefficients were detected in both the Si-doped and Sn-doped samples, where the lithium ion diffusion coefficients (DLi+) for the pristine material, Si-doped material, and Sn-doped material are 6.889 × 10−11, 1.153 × 10−10, and 1.322 × 10−10 cm2 s−1, respectively (Table S9). A question that arises is what causes the anomaly in the structure−electrochemical kinetic correlation observed in the Si/Sn-doped material. To have a better understanding of the crystallographic changes induced by Si/Sn substitution, the interatomic distance and slab thickness (TMO2), as well as the interslab spacing (LiO2), were calculated and are shown in Table 2 and Figure 3. The interslab layer and slab layer alternatively arrange along the c-axis to form the layered structure.37 It is apparent that the changes in the TM−O distance are consistent with the large difference between the ionic radii of Si and Sn. It is worth

noting that the changes in the slab thickness (TMO2 layer) and interslab spacing (Li layer) do not follow a similar trend (Table 2). A larger shrinkage of the slab thickness is detected in the Sndoped material, which is probably due to the distortion induced by the large Sn4+ cations. This tendency is confirmed by the increased TM−O−TM bond angle between two adjacent TMO6 polyhedra along the a/b directions and expanded lattice parameter a (Tables 1 and 2). Even with the decrease in lattice parameter c, compression of the slab thickness can provide more open space in the interslab for Li diffusion, leading to the improved rate capability observed in the Si-doped material. On the other hand, the Sn-doped material possesses the largest Li layer spacing, which comes in part from the contraction of the TMO2 slab and the expansion of lattice parameter c. It is worth noting that the interslab spacing (Li layer) increases as the magnitude of cation intermixing decreases (Tables 1 and 2 and Figure 3), ensuring a significant enhancement of the rate capability, which is consistent with the tendency reported previously.39 However, the above XRD analysis can only provide average information regarding structural changes caused by Si/Sn substitution, so electron microscopy with high spatial resolution and XPS depth profiling were employed to obtain direct evidence of the local chemistry and atomic arrangement from the surface to the bulk. The morphology and chemical composition of the pristine and Si/Sn-doped materials were evaluated using high-resolution scanning electron microscopy (HR-SEM), STEM, and energydispersive X-ray spectroscopy (EDX) mapping (Figures S6 and S7, Supporting Information). It is apparent that Si/Sn dopants are uniformly distributed within the entire particles (Figure S7) and Si/Sn substitution exerts only a minor influence on the morphology of the doped materials (Figure S6). The survey spectra in Figure S8a−c (Supporting Information) show the characteristic peaks corresponding to Li 1s, Mn 2p, Co 2p, Ni 2p, and O 1s in all samples, an extra Si 2s peak for the Si-doped material, and extra Sn 3d and Sn 3p peaks for the Sn-doped material, which confirm the presence of Si and Sn elements. For the Si-doped material (Figure S8d), the binding energy of Si 2s (153.09) is in accordance with that of the Si4+ ion in Li2SiO3, suggesting the formation of the orthosilicate [SiO4] structure.40 Figure S8f shows the XPS spectra of Sn 3d for the Sn-doped material. The binding energies of Sn 3d5/2 and Sn 3d3/2 are about 486.25 and 494.75 eV, respectively, matching the values reported for Sn4+ in SnO2, indicating the 4+ valence of Sn.41 Figure 4a compares typical Mn 2p, Co 2p, and Ni 2p XPS spectra for the as-prepared pristine and Si/Sn-doped materials. Incorporation of Si/Sn leads to little change in the binding energies for the Mn, Ni, and Co cations, indicating that Si and Sn tend to substitute for the Mn4+ site with the same valence of 4+ to maintain charge balance in the system. XPS depth profiles by etching samples with Ar+ ion were used to provide further evidence for the gradient doping of Si/Sn. The XPS depth profiles (Figure 4b) show that the atomic ratios of Mn, Co, and Ni hardly change in the pristine material, whereas depletion of Mn (enrichment of Si/Sn) is evident in the surface region of the doped samples (the dotted box in Figure 4b and Tables S10−S12, Supporting Information). Specifically, the amount of Mn4+ in the Si-doped material is 53.73% in the surface, increases to 60.06% at a depth of 8.1 nm, and finally stabilizes at ∼58.30% after 16.2 nm. Oppositely, the amount of Si4+ is 6.85% in the surface, decreases to 2.39% at a depth of 8.1 nm, and finally levels off at ∼1.56% after 27 nm (Table S11). The phenomenon of depletion of Mn (enrichment of Sn) can also

Table 2. Refined Interatomic Distances and Slab Thicknesses (TMO2), as Well as the Interslab Spacings (Li Layer), of the Pristine and Doped Materials

TM−O (Å) interslab (Li layer) spacinga (Å) slab (TMO2) thicknessb (Å) TM−O−TM (deg)

pristine sample

Si-doped sample

Sn-doped sample

1.9794(13) 2.5632(1)

1.9755(9) 2.5770(3)

1.9611(8) 2.6390(0)

2.1903(8) 92.36(8)

2.1733(6) 92.63(6)

2.1151(1) 93.66(5)

Interslab space thickness: I(LiO2) = 2(1/3 − zox)c. bSlab thickness: S(TMO2) = c/3 − I(LiO2). a

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Figure 3. Crystal structure changes for (a) pristine, (b) Si-doped, and (c) Sn-doped materials. (d) Interslab (Li layer) and slab spacing (TMO2) variation for pristine, Si-doped, and Sn-doped materials.

Figure 4. Chemical information for pristine, Si-doped, and Sn-doped materials. (a) Mn 2p, Co 2p, and Ni 2p XPS spectra for the samples. The dashed lines are employed to show shifting of the Mn 2p, Co 2p, and Ni 2p peaks. (b) Compositional change for the transition metals and Si/Sn as a function of the etching depth.

be observed in the Sn-doped material (Table S12). The composition gradients may be ascribed to the substantial rearrangement of the Mn4+ ion induced by Si4+/Sn4+ ion substitution. To clarify the local structure changes, atomic resolution, ABF/HAADF-STEM images, and Z-contrast profiles were taken from the pristine and doped materials (Figure 5). In the HAADF-STEM images, only TM atoms appear as bright spots,

whereas TM, oxygen, and even lithium atoms can appear as dark contrast in the corresponding ABF-STEM images.42 Both periodic sequences of two bright spots and a dark spot, characteristic of Li2MnO3-like structural units, and continuous bright spots associated with LiTMO2 are observed, suggesting that Li2MnO3-like and LiTMO2 structures coexist in all these materials (shown as the schematics in the insets of Figure 5a,d).6 The Z-contrast profiles with the measured spacing of 20523


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Figure 5. Comparison of HAADF/ABF-STEM images of [001]-orientated pristine and doped LLOs. (a−c) HAADF-STEM images and Z-contrast profiles with measured spacing of the TM layers of pristine and Si/Sn-doped materials. (d−f) Corresponding ABF-STEM images and Z-contrast profiles of pristine and Si/Sn-doped materials.

thickness, which largely accounts for the expansion of the interslab spacing of the Li diffusion layer. The expanded interslab spacing reduces the lithium diffusion energy barrier and ensures enhanced Li+ diffusivity and rate performance for the doped materials, in particular the Sn-doped material. The reduced slab thickness may stabilize the oxygen close-packed lattice and have a positive impact on the electrochemical cyclability. We anticipate that this better understanding of enhanced kinetics of doped LLOs may enlighten the design and development of other high-rate intercalated electrode materials.

TM layers, compared in Figure 5a−c, indicate a gradual change in the spacings of the TM layers. It is apparent that considerable variations in the spacing of the TM layers are observed in the outermost surface regions, whereas only marginal changes were detected in the core parts of the doped materials. This behavior is closely associated with the concentration gradient for the Si/Sn dopants. The contrast histograms in the ABF-STEM images show a broad contrast peak for the TMO2 layer in the pristine material, while the introduction of Si/Sn narrows the contrast peak of the TM atom column positions, as depicted in the insets of Figure 5d−f. In particular, Sn dopants cause substantial broadening of the Li atom column (inset of Figure 5f), which may explain the improved kinetics observed in the Sn-doped material. Together, the XRD refinement, STEM, and XPS results confirm the idea that Si/Sn substitution may decrease the TMO2 slab thickness and enlarge the interslab spacing, and the magnitude of these structural changes is sensitive to the type and concentration of dopants. The expanded interslab spacing accounts for the enhanced Li+ diffusivity and rate performance observed in the doped materials. Also, the formation of a gradient surface layer with a thickness of ∼10 nm is closely related to Mn depletion (Figure 4b). Therefore, surface corrosion by hydrofluoric acid (HF) and subsequent Mn cation loss during the full discharge process42,43 can be suppressed effectively, leading to a more stable electrode/electrolyte interface.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b02944. ICP-AES data of the samples, results of XRD Rietveld refinement, initial charge−discharge curves, long cycling performance, detailed Nyquist plots and CV data of the samples, SEM and HAADF-STEM mapping, survey scan, Si 2s and Sn 3d XPS spectra for the samples, and detailed elemental composition for XPS depth profiles of the samples (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected].

4. CONCLUSIONS In summary, we have demonstrated improved kinetics for lithium-rich layered oxides via gradient surface Si/Sn-doping. Xray structural refinement, STEM, and XPS results verify that gradient Si/Sn-doping leads to the shrinkage of the TMO2 slab

ORCID

Weifeng Wei: 0000-0002-3088-6549 Author Contributions ∥

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Z.D., M.X., and J.L. contributed equally to this work. DOI: 10.1021/acsami.7b02944 ACS Appl. Mater. Interfaces 2017, 9, 20519−20526

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ACS Applied Materials & Interfaces Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support from the Recruitment Program of Global Youth Experts, the National Natural Science Foundation of China (Grant No. 51304248 and 11474147), the National Basic Research Program of China (Grant No. 2015CB654901), the Natural Science Foundation of Jiangsu Province (Grant No. BK20151383), the International Science & Technology Cooperation Program of China (Grant No. 2014DFE00200), the Innovation Program of Central South University (Grant No. 2016CXS003), the State Key Laboratory of Powder Metallurgy at Central South University, and Hunan Shenghua Technology Co., Ltd.

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Understanding the Enhanced Kinetics of Gradient ... - ACS Publications

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