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Facile Synthesis of Platelike Hierarchical Li1.2Mn0.54Ni0.13Co0.13O2 with Exposed {010} Planes for High-Rate and Long Cycling-Stable Lithium Ion Batteries Jiong Zeng,† Yanhui Cui,† Deyang Qu,‡,⊥ Qian Zhang,† Junwei Wu,*,† Xiaomeng Zhu,*,∥ Zuohua Li,*,§ and Xinhe Zhang⊥ †

Shenzhen Key Laboratory of Advanced Materials, Department of Materials Science and Engineering, Harbin Institute of Technology Shenzhen Graduate School, Shenzhen 518055, China ‡ Department of Mechanical Engineering, College of Engineering and Applied Science, University of Wisconsin Milwaukee, 3200 N. Cramer Street, Milwaukee, Wisconsin 53211, United States ∥ School of Materials Science and Engineering, Wuhan University of Technology, 122-loushi Road, Wuhan 430070, China § Department of Civil and Environmental Engineering, Harbin Institute of Technology Shenzhen Graduate School, Shenzhen 518055, China ⊥ Dongguan Mcnair Technology Co., Ltd., Dongguan 523800, China S Supporting Information *

ABSTRACT: Lithium-rich layered oxides are promising cathode candidates for the production of high-energy and high-power electronic devices with high specific capacity and high discharge voltage. However, unstable cycling performance, especially at high charge−recharge rate, is the most challenge issue which needs to be solved to foster the diffusion of these materials. In this paper, hierarchical platelike Li1.2Mn0.54Ni0.13Co0.13O2 cathode materials were synthesized by a facile solvothermal method followed by calcination. Calcination time was found to be a key parameter to obtain pure layered oxide phase and tailor its hierarchical morphology. The Li-rich material consists of primary nanoparticles with exposed {010} planes assembled to form platelike layers which exhibit low resistance to Li+ diffusion. In detail, the product by calcination at 900 °C for 12 h exhibits specific capacity of 228, 218, and 204 mA h g−1 at 200, 400, and 1000 mA g−1, respectively, whereas after 100 cycles at 1000 mA g−1 rate of charge and recharge the specific capacity was retained by about 91%. KEYWORDS: hierarchical morphology, solvothermal, exposed {010} plans, Li-rich cathode, Li ion battery

1. INTRODUCTION

transformation can also lead to capacity fading and poor rate performance of the cathode material.13−17 A great amount of scientific effort has been devoted to overcome the above issues.18−22 Hierarchical layered materials with fine tailoring of the micro-nanostructure have been demonstrated to enhance the electrochemical reactions effectively because of the short pathway of lithium ions insertion/extraction within the nanoparticles.23 Engineering of nanoparticle assembly can also improve the structural stability and limit side reactions. For example, the lithium-rich materials with rodlike hierarchical nano/microstructure prepared by Zhang et al. presented excellent capacity of 212.5 mA h g−1 after 30 cycles at 1C between 2.0 and 4.8 V.24 Wang et al. have synthesized a nanoarchitecture multistructural cathode materials by a coprecipitation method, which exhibited outstanding

High-performance cathode materials play a crucial role in the mass application of Li ion battery in the electric vehicle and large-scale energy storage.1−3 However, conventional cathode materials such as LiCoO2 and LiMn2O4 can hardly meet the demand of high specific energy. Recently, layered lithium-rich cathode materials,4−6 xLi2MnO3·(1−x)LiMO2 (M = Ni, Co, Mn, etc.), have been widely investigated because of their high specific capacity (more than 250 mA h g−1) and low cost. Moreover, by reducing the Co content, it may be also possible to produce environmentally sustainable lithium-rich materials. However, despite these technological advantages, the large irreversible capacity loss and voltage decay are the most critical issues which limit the diffusion of lithium-rich materials. The first drawback is due to the oxygen ion vacancies generation which realizes during the activation process of Li2MnO3 above 4.5 V,7−12 and the latter one is due to the phase transformation from layered to spinel structure. In addition, the phase © 2016 American Chemical Society

Received: July 21, 2016 Accepted: September 12, 2016 Published: September 12, 2016 26082

DOI: 10.1021/acsami.6b08835 ACS Appl. Mater. Interfaces 2016, 8, 26082−26090

Research Article

ACS Applied Materials & Interfaces capabilities of 200 mA h g−1 at 1C.25 Zhang et al. reported that a peanut-like hierarchical micro/nanostructure Li-rich material can deliver 229.9 mA h g−1 at a current density of 200 mA g−1 in the voltage range of 2.0−4.8 V, and exhibited a high capacity of 216.5 mA h g−1 after 100 cycles.26 Fu et al. have successfully synthesized 3D porous hierarchical microstructure lithium-rich materials, which presented outstanding capacity of 292.3 mA h g−1 at 0.2C, but only 131.1 mA h g−1 at 10C.27 All of the reported materials showed high discharge capacity and outstanding capacity retention at low rate; unfortunately, their performance at high rate was still unsatisfactory. Additionally, lithium-rich oxides characterized by crystalline orientation were reported to improve the performance. Chen and Grey had reported that crystal faces such as (100) and (010), which are perpendicular to the (001) face, were more prone to facilitate the Li+ deintercalation/insertion.28 Wu’s group also synthesized hierarchical lithium-rich nanoplates with exposed {010} planes, which included (010), (1̅10), (1̅00), (010), (11̅0), and (100) facets, the material showing excellent electrochemical performance, especially in the case of high-rate charge/discharge.29,30 However, the high surface energy possessed by (001) and (010) planes makes them unstable during the process of crystal growth.31 Therefore, the approaches to synthesize lithium-rich materials with high content of exposed {010} planes need to be further investigated. Especially, the simultaneously achievement of both hierarchical structure and electrochemically active {010} planes still remains a big challenge. In this work, we reported on the hierarchical Li-rich Li1.2Mn0.54Ni0.13Co0.13O2 (LMNCO) platelets materials characterized by a fine tailoring of the micro/nanomorphology and obtained by a facile solvothermal route. Such platelets consisting of nanoparticles opportunely assembled in a porous-layered morphology possess exposed electrochemically active {010} planes on the surface of primary nanoparticles. Therefore, the as-prepared LMNCO showed excellent rate capability. Specifically, the initial capacity was as high as 211 mA h g−1 at a discharge rate of 1000 mA g−1. Good cycling stability was achieved as well, the discharge capacity is maintained as high as 183 mA h g−1 with capacity retention of 91% after 100 cycles. Such Li-rich material represents a promising candidate for next-generation cathode material.

Kα radiation (λ = 1.5418 Å). Spectra were collected over the 2θ range 10−80° with scan rate of 1° min−1. Scanning electron microscopy (SEM) was performed by using a SEM, Hitachi S4700 with EDX (energy-dispersive X-ray spectroscopy) analysis. Thermal gravimetric analysis of the samples were carried out by using a Shimadzu thermoanalyzer DTG-60. The thermograms (TG) and differential thermal analysis (DTA) were collected at a constant heating rate (10 °C min−1) in the temperature range 0−1000 °C. High-resolution transmission electron microscopy (HRTEM) was performed by using JEOL JEM-2100 equipment to investigate the microstructure of samples. X-ray photoelectron spectroscopy (XPS) analysis was conducted with an Escalab 250Xi for evaluating the elemental chemical state analysis. Raman spectra were performed by using an Xplora (Horiba Corporation) with a 532 nm excitation line and a laser power of 0.1 mW. 2.3. Electrochemical Tests. The electrochemical performances were measured in 2032 type coin cells. The active materials, super P carbon and polyvinylidene fluoride (PVDF), were slurried in the weight ratio of 80:10:10 in N-methyl-2-pyrrolidone (NMP) solution. The slurry was then cast onto a polished Al foil. The coated electrode was dried at 80 °C for 12 h under vacuum. The electrolyte was 1 M LiPF6 dissolved in ethylene carbonate (EC) and dimethyl carbonate (DMC) with a volume ratio of 1:1. Celgard 2400 was used as the separator. The cells were assembled in an argon-filled glovebox (Mikrouna). Constant current-constant voltage charge and discharge tests were performed on the battery test system (Neware Instrument, Shenzhen, China) in the cutoff voltage between 2.0 and 4.8 V (vs Li/ Li+) at current densities from 0.2C to 5C (1C = 200 mA g−1) at room temperature. The dQ/dV curves were calculated from the charge/ discharge curves of the initial cycle at 0.2C. Electrochemical impedance spectroscopy (EIS) measurements were performed on CHI760D electrochemical workstation (Shanghai, China) with the frequency range from 100 kHz to 0.01 Hz, and the ac voltage amplitude was set to 5 mV.

3. RESULTS AND DISCUSSION 3.1. Synthesis of Hierarchical Porous Li1.2Mn0.54Ni0.13Co0.13O2 Platelets. LMNCO hierarchical platelets were synthesized by forming first the corresponding metal oxalate precursors and then submitting them to heat treatments (i.e., annealing and calcination treatments). The formation of the precursors in the solvothermal and the final porous platelets is schematically illustrated in Scheme 1. Before describing the process which brings to the formation of the porous layered material, it is worth taking into account the following: (1) the oxalic groups have a quite strong alcohol-

2. EXPERIMENTAL SECTION

Scheme 1. Illustration of the LCMNO Preparation Processa

2.1. Synthesis. Li1.2Mn0.54Ni0.13Co0.13O2 platelets were prepared via solvothermal step followed by a double treatment at high temperature (i.e., annealing and calcination treatments). In detail, CH3COOLi·2H2O (0.0252 mol) was dissolved in 50 mL of ethanol to form solution A, whereas Mn(CH3COO)2·4H2O (0.0108 mol), Ni(CH3COO)2·4H2O (0.0026 mol), and Co(CH3COO)2·4H2O (0.0026 mol) were dissolved in ethanol to form solution B. Solution A was slowly added (5 drops per min) into 1.0 M oxalic acid alcoholic solution under vigorous stirring. Solution B was added (30 drops per min) to the above mixture afterward. Then the resulting solution was transferred to a 100 mL Teflon-lined autoclave and heated at 180 °C for 10 h. Afterward, the resulting mixture was cooled down to room temperature. The white oxalate precursor was obtained by solvent evaporation at 80 °C under vigorous stirring. Then the prepared precursor was submitted to double thermal treatments; in particular, it was first annealed at 450 °C for 6 h in air and then calcined at 900 °C for 8, 12, and 16 h in the air, with heating rate of 5 °C min−1. According to the different calcination times, the obtained samples were labeled as T8, T12, and T16, respectively. 2.2. Material Characterization. X-ray diffraction (XRD) was performed by using a Rigaku Ultima IV (Rigaku Corporation) with Cu

a

(a) Solvothermal process with precipitation of nanorods; (b) structure of oxalate precursor; (c) structure of LMNCO after calcination; (d) crystal structure of the LMNCO’s surface (TM = transition-metal element).

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ACS Applied Materials & Interfaces philicity, while metal ions, such as Mn2+, are alcohol-resistant;32 (2) weak magnetic interactions were likely to take place between the cobalt and nickel ions.33,34 Therefore, as shown in Scheme 1a, the nickel oxalate and cobalt oxalate form the 1D nanorods in the presence of alcohol solvents (the corresponding SEM image and EDX spectra are shown in Figure S1 in the Supporting Information). The positively charged Li+ and Mn2+ can be immediately adsorbed onto the negatively charged (001) surfaces,35,36 which would impede further growth along the [001] direction, so the (001)-plane-dominated nanoplates were formed, as exhibited in Scheme 1b. The precursors were finally transformed to LMNCO phase during the calcinations. Meanwhile, CO2 gas was released to leave numerous pores on the surface, as displayed in Scheme 1c. Figure 1a shows XRD patterns of the oxalate precursor collected before thermal treatments at high temperature. The

T16 samples does not change significantly. The typical width and length of the platelets were about 1 and 6 μm, respectively. It was noted that there were more fragments in T16 than in T12 (marked by the red ring), which was ascribed to the cleavage of platelets during the prolonged calcination. From the high-magnification insets in Figure 2, it is also possible to observe that such platelets are composed of small primary particles with size roughly estimated in the range from 200 to 300 nm. EDX spectra of the sample T12 and sample T16 (Figure 2c,d) indicate that both samples contained a homogeneous distribution of Mn, Ni, Co, and O, and their molar ratio was close to the designed ratio of 0.54:0.13:0.13 (see Figure S2 in the Supporting Information). The chemical states of the Mn, Ni, and Co are +4, +2, and +3, respectively, which were consistent with those reported in the literature (see Figure S3 in the Supporting Information).29 TEM and HRTEM analysis was performed to further confirm the structure and morphology of the sample T12. As shown in Figure 3a,b, the porous platelets were composed of primary particles with size ranges from 200 to 300 nm, according to SEM results. Figure 3d shows the HRTEM image from the region marked in Figure 3c. A distinct lattice spacing of 0.47 nm was observed, corresponding to the (003) plane of the hexagonal layered phase. As is known, the (003) plane is parallel to the c-axial direction; therefore, the projected image planes of the region (I), marked by a black line, were perpendicular to the [001] direction which are part of the {010} planes. Figure 3f showed another highly magnified region marked in Figure 3e in which the apparent lattice fringes with interplanar spacing of 0.42 nm were clearly observed. These could be assigned to the (020) plane of the hexagonal layered phase. Hence, we confirm that the projected image planes of the region (II), marking by a black line, were parallel to (100) planes. In conclusion, the frontal plane of primary nanoparticles belong to {010} planes which are significantly active planes in supporting the rapid lithium ion intercalation/ deintercalation. The XRD patterns of calcinated precursors (i.e., T12, calcinated for 12 h, and T16, calcinated for 16 h) are shown in Figure 4a. The main peaks of sample T12 and T16 matched well with that of α-NaFeO2 type layered structure with space group R3̅m. The weak peak between 20° and 25° was caused by LiMn6 cation ordering of Li2MnO3.37 The second phase in the sample T8 marked by “*” is tentatively ascribed to insufficient calcination time (see Figure S4a in the Supporting Information). It is clear that sufficient calcinations time (>12 h) is necessary for the oxalate precursors to transform to the desired LMNCO phase. The clear splitting between the adjacent peaks of (006)/(012) and (108)/(110) indicates a well-defined structure of the sample.38 Moreover, the ratio of the (003) to (104) intensity was widely used to indicate the degree of cation mixing in the Li layers.39 It was 1.37 and 1.32 for samples T12 and T16, respectively. The value decreased with the increase of calcination time due to the fact that more energy can be supplied to assist the migration of cation in the Li layers. The enlarged XRD patterns are presented in Figure 4b. From the spectra it is evident that the lack of shoulders around (101), (006), (104), and (108) peaks which confirmed the inexistence of spinel phase. Furthermore, the positive shift of the peaks to larger 2θ was evident, indicating the larger d spacing for the sample T12 according to the Bragg equation. Raman spectroscopy was used to further verify the structure of the sample since it is sensitive to the difference between

Figure 1. XRD pattern (a) of the precursor; TG and DTA curves (b) of the obtained precursor; SEM images ((c) and (d)) of the oxalate precursor.

diffraction peaks can be indexed to Li2C2O4 (PDF#24-0646), MnC2O4·2H2O (PDF#32-064), NiC2O4·2H2O (PDF#250581), and CoC2O4 (PDF#37-0719) phases. The SEM images are displayed in Figure 1c,d. It can be clearly observed that the platelets are composed of nanorods with diameters of ∼100 nm. The average widths and lengths of the platelets were about 2 and 10 μm, respectively. Such structure was consistent with our hypothesis in Scheme 1. TG-DTA was used to monitor the transformation of oxalate precursors into the porous LMNCO platelets and the result is shown in Figure 1b. The first weight loss of about 8% in the temperature ranges of 120−250 °C was mainly ascribed to the evaporation of H2O. The second weight loss of about 30% between 300 and 360 °C was ascribed to the decomposition of the MnC2O4, CoC2O4, NiC2O4, and part of LiC2O4. The last weight loss which occurred at the temperature range of 550− 590 °C with about 18% weight loss is attributed to the decomposition of LiC2O4. The calcination time has a great influence on the morphology and crystalline phase of the product, which will be discussed later. 3.2. Morphology and Structure Characterization. Figure 2 shows SEM and EDX results of sample T12 and sample T16. As can be seen in Figure 2a,b, the morphology and size of primary nanoparticles which assemble into the T12 and 26084


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Figure 2. FE-SEM images with magnified images of T12 (a) and T16 (b) samples; EDX spectra of the sample T12 (c) and sample T16 (d).

vibrational bands around 630−636 cm−1; therefore, the absence of such bands confirmed the inexistence of spinel phase.29,41 3.3. Electrochemical Performances. Figure 5a,b shows the initial charge−discharge curves of T12 and T16 at 0.2C, 0.5C, 1C, 2C, and 5C (1C = 200 mA g−1) between 2.0 and 4.8 V. Two different stages can be observed in all initial charge curves. The stage below 4.5 V could be attributed to the Li+ extraction from layered LiMO2 structure whereas the long flat plateau in 4.5−4.8 V is related to the activation process of electrochemically inert Li2MnO3 that leads to the low initial Coulombic efficiency. As shown in Figure 5a,b, the sample T12 exhibits initial discharge specific capacities as high as 252, 242, 227, 218, and 203 mA h g−1 at 0.2C, 0.5C, 1C, 2C, and 5C, respectively, while the initial discharge specific capacities of sample T16 were 229, 224, 209, 190, and 170 mA h g−1 at the same conditions. The lower initial discharge specific capacity of sample T16 compared to that of T12 can be attributed to the higher degree of cation mixing, to the smaller d spacing, and to having more fragments. In comparison, the lower discharge capacity observed for sample T8 at various discharge rates could be ascribed to the presence of the second phase (see Figure S4b,c in the Supporting Information). The dQ/dV curves of the initial cycle for both samples T12 and T16 are shown in Figure 5f in which both samples showed the same redox peaks. The oxidation peak at 4.0 V can be attributed to oxidation of Ni2+ to Ni4+, and the second peak at 4.5 V was related to the lithium extraction from Li2MnO3.42 The reduction peak at around 3.7 V was due to the reduction of Ni4+ and Co4+, and the reduction peak of Mn4+ was located at about 3.4 V.43,44

Figure 3. Typical TEM images (a and b); HRTEM image of sample T12 (c−f).

spinel and layered phases. As shown in the Figure 4c, the weak band around 430 cm−1 is caused by the fingerprint vibration of Li2MnO3, and the peaks around 490 and 594 cm−1 can be assigned to the layered structure with space group R3̅m.40,41 The spinel phase was generally detected by the presence of 26085


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Figure 4. XRD patterns (a) and local magnification (b) of sample T12 and sample T16; Raman spectra (c) of sample T12 and sample T16.

Figure 5. Initial charge−discharge curves for samples T12 (a) and T16 (b); cycle performance at high rate for sample T12 (c) and sample T16 (d); the cycle performance (e) at 0.5C of both samples; the dQ/dV curves (f) of the initial cycle at 0.5C.

Table 1. Comparative Data of Cycling Performance and Capacity Retention Ratio at Different Rates of Sample T12 and T16 T12

T16

rates

1st discharge capacity (mA h g−1)

100th discharge capacity (mA h g−1)

capacity retention ratio

first discharge capacity (mA h g−1)

100th discharge capacity (mA h g−1)

capacity retention ratio

1C 2C 5C

228 218 204

191 192 182

84% 88% 91%

210 191 170

179 171 141

85% 90% 83%

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Figure 6. FE-SEM image (a) and EDX (c) of cycled electrode of T12 for 100 cycles at 5C; FE-SEM image (b) and EDX (d) of cycled electrode of T16 after 100 cycles at 5C.

Figure 7. Electrochemical impedance spectra (a) and insert is the equivalent circuit; real parts of the complex impedance versus ω−1/2 from 0.1 to 0.01 Hz (b).

was ascribed to the generation of SEI films. It was also observed that rate capability of sample T12 was quite remarkable. For example, the discharge capacity maintained 182 mA h g−1 after 100 cycles at 1000 mA g−1, with a capacity retention of 91%, which is up to now the highest reported value according to our knowledge.18−20,24−32,37,38,42−44 The main reason for the excellent rate capability was due to the exposure of electrochemical active {010} plane of sample T12 being able to enhance the Li+ intercalation/deintercalation. Furthermore, the hierarchical morphology with the presence of pores improved

The long-term cycling performances of samples T12 and T16 at different C-rates are given in Figure 5c−e. High capacity retention after 100 cycles can be clearly observed for both samples T12 and T16. The discharge capacity at different Crates after 100 cycles and capacity retention ratio are listed in Table 1. Obviously, the specific capacity of both samples T12 and T16 increased in the first few cycles and then decreased up to reaching a plateau. The initial capacity increase during the first few cycles was ascribed to the gradual activation of Li2MnO3, while the capacity decrease in the subsequent cycles 26087


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In general, for Li ion battery cathode materials, electronic and ionic conductivity are two key aspects to the chargetransfer resistance. In detail, the materials able to exhibit the higher electronic and ionic conductivity possess better rate performances. Therefore, for sample T12, the low Rct and high lithium ion diffusion coefficients are accountable for the high capacity and excellent rate performances.

the structural stability. Therefore, the platelike hierarchical cathode material with exposed {010} planes indeed demonstrated excellent cycling and rating performances of lithium-rich layered materials. To determine the structure stability, SEM and EDX characterizations were carried out for both samples T12 and T16 after 100 cycles at 5C. The results are shown in Figure 6. The platelike morphology was maintained for both electrodes after 100 cycles. Furthermore, the molar ratios of Mn:Ni:Co for the samples T12 and T16 were 0.51:0.15:0.13 and 0.52:0.14:0.13, respectively, both of which remained close to the original values of 0.54:0.13:0.13. EIS measurements were conducted at 2.5 V before cycles to further understand the performance differences of samples T12 and T16. Figure 7 shows the impedance spectra. The Nyquist plots and the equivalent circuit used for the numerical fitting are displayed in Figure 7a. The intercept with the x-axis of the semicircle at high frequency represented the uncompensated ohmic resistance (Re) between the working electrode and the reference electrode which included the contact resistance, electric resistance of the material, and ohmic resistance of the electrolyte. The semicircle in the high to medium frequency referred to charge-transfer resistance (Rct). The slope line at low frequency corresponded to the Warburg impedance (Zw), which was related to the Li+ diffusion in the electrode. CEPs and CEPdl were used to express the nonideal capacitance of the surface layer and double layer, respectively. The nonlinearity was the result of the porous structure of the electrode. Re and Rct values are summarized in Table 2. The difference of Re

4. CONCLUSIONS We have successfully synthesized a group of hierarchical platelike Li1.2Mn0.54Ni0.13Co0.13O2 with the exposed {010} planes via a facile solvothermal synthesis strategy coupled with aging and calcination thermal treatments at high temperature. The sample calcinated at 900 °C for 12 h delivered 228, 218, and 204 mA h g−1 at 1C, 2C, and 5C rates, respectively. Furthermore, the capacity retention of the material can reach 91% at 5C after 100 cycles. The excellent rate performance was due to the presence of exposed electrochemically active {010} planes on the surface of the primary nanoparticles. In addition, the hierarchical structure also can improve the rate capability by improving the lithium diffusion rate and retaining the cycling stability. The excellent performance at the high rate made the layered Li-rich materials promising for applications in the high-rate and cycling stable Li ion battery.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b08835. EDX elements mapping of compacted powder of samples T12 and T16; XPS spectra of Ni 2p, Co 2p, Mn 2p, and C 1s for sample T12; XRD pattern (a) of sample T8; and the initial charge−discharge curves and cycle performance at high rate for sample T12 (PDF)

Table 2. Simulation Results of Figure 7a samples

Re (Ω)

Rct (Ω)

σ

DLi+(cm2 s−1)

T12 T16

4.6 4.1

535.3 980.2

289.9 571.4

1.3 × 10−16 2.7 × 10−17

■

between sample T12 and T16 can be negligible. However, the Rct (535.3 Ω) of sample T12 was about half of that of T16 (980.2 Ω). This suggested that the Faraday charge-transfer kinetics on the surface of T12 was much better than that of T16, which could result from the lower degree cation mixing in the Li layer and larger d spacing. The diffusion coefficient of the lithium ion in the platelet cathode can also be calculated on the basis of EIS results. The lithium ion diffusion coefficients were estimated by the computational formula45,46

*E-mail: [email protected] (W.J.). *E-mail: address:[email protected] (Z.X.). *E-mail: [email protected] (L.Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Shenzhen Peacock Plan Program (KQCX20140521144358003), International Collaboration Program of Shenzhen (GJHZ20150312114008636), Shenzhen Technology-Innovation Program Technology Development Projects (Grant No.: CXZZ20140904154839135) and Guangdong Innovative and Entrepreneurial Research Team Program (No.: 2013N079).

4 2 4 2 2

Z′ = RD + RL + σω−1/2

AUTHOR INFORMATION

Corresponding Authors

0.5R2T 2 (1) n AF Cσ where R is the gas constant, T represents the absolute temperature, A is the surface area of the electrode, F denotes the Faraday constant, C is the concentration of lithium ion, and σ reflects the Warburg factor calculated by the formula D(Li+) =

ASSOCIATED CONTENT

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(2) −1/2

REFERENCES

(1) Armand, M.; Tarascon, J. M. Building better batteries. Nature 2008, 451, 652−657. (2) Li, X.; Liu, J.; Banis, M. N.; Lushington, A.; Li, R.; Cai, M.; Sun, X. Atomic layer deposition of solid-state electrolyte coated cathode materials with superior high-voltage cycling behavior for lithium ion battery application. Energy Environ. Sci. 2014, 7 (2), 768−778. (3) Li, X.; Liu, J.; Meng, X.; Tang, Y.; Banis, M. N.; Yang, J.; Hu, Y.; Li, R.; Cai, M.; Sun, X. Significant impact on cathode performance of

Figure 7b show the curves of Z′ vs ω in the region of 0.1−0.01 Hz. The obtained σ and D(Li+) values were tabulated in Table 2. Li ion diffusion coefficients (1.3 × 10−16 cm2 s−1) of sample T12 were much greater than those of sample T16 (2.7 × 10−17 cm2 s−1). The results further confirm the lithium ion diffusion in sample T12 is much higher than that of sample T16. 26088


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