A Versatile Coating Strategy to Highly Improve the Electrochemical

Nov 24, 2015 - College of Materials Science and Optoelectronic Technology, University of Chinese Academy of Sciences, Beijing 100049, People's Republi...
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A Versatile Coating Strategy to Highly Improve the Electrochemical Properties of Layered Oxide LiMO2 (M = Ni0.5Mn0.5 and Ni1/3Mn1/3Co1/3) Enyue Zhao,† Minmin Chen,† Dongfeng Chen,‡ Xiaoling Xiao,*,† and Zhongbo Hu*,† †

College of Materials Science and Optoelectronic Technology, University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China ‡ China Institute of Atomic Energy, Beijing 102413, People’s Republic of China S Supporting Information *

ABSTRACT: This work provides a convenient, effective and highly versatile coating strategy for the layered oxide LiMO2 (M = Ni0.5Mn0.5 and Ni1/3Mn1/3Co1/3). Here, layered oxide LiMO2 (M = Ni0.5Mn0.5 and Ni1/3Mn1/3Co1/3) has been successfully coated with ion conductor of Li2SiO3 by in situ hydrolysis of tetraethyl orthosilicate (TEOS) followed by the lithiation process. The discharge capacity, cycle stability, rate capability, and some other electrochemical performances of layered cathode materials LiMO2 can be highly enhanced through surface-modification by coating appropriate content of Li2SiO3. Particularly, the 3 mol % Li2SiO3 coated LiNi1/3Mn1/3Co1/3O2 exhibits approximately a discharge capacity of 111 mAh/g after 300 cycles at the current density of 800 mA/g (5 C). Potentiostatic intermittent titration technique (PITT) test was carried out to investigate the mechanism of the improvement in the electrochemical properties. The diffusion coefficient of Li+-ion (DLi) of Li2SiO3 coated layered oxide materials has been greatly increased. We believe our methodology provides a convenient, effective and highly versatile coating strategy, which can be expected to open the way to ameliorate the electrochemical properties of electrode materials for lithium ion batteries. KEYWORDS: lithium ion batteries, layered oxide materials, coating strategy, Li+-ion conductor, Li2SiO3

1. INTRODUCTION The demands for clean and renewable energy sources are becoming more and more urgent with the worsening of energy shortages and environmental pollution. Lithium-ion batteries (LIBs) with high energy densities, long cycling life, low cost and environmental friendly have received a lot of attentions.1−8 At present, LIBs, whose main cathode material is LiCoO2, have been widely used in microelectronic products such as laptops, cameras and mobile phones. Much effort has been made to develop cheaper cathode materials than LiCoO2 due to its high cost used in commercial rechargeable lithium-ion batteries.7−15 Layered cathode materials LiNixMnyCo1−x−yO2, in which Co is substituted by Ni and Mn elements, have been widely researched for their lower cost, higher specific capacity and better thermal stability. 16−24 LiNi 0.5 Mn 0.5 O 2 and LiNi1/3Mn1/3Co1/3O2, as the two representatives, have been applied in many commercial cells.24−29 However, there are still two major drawbacks for LiNi0.5Mn0.5O2 and LiNi1/3Mn1/3Co1/3O2 to restrict their sustainable and high-power applications.25−28 First, the highvalent Ni4+ or Co4+ ions formed at the charged state of the layered cathode materials are active when in direct contact with the electrolyte and can induce the oxidative degradation of the © XXXX American Chemical Society

electrolyte, which is the primary reason for the capacity fading of LiMO2 during cycle process.28,29 Second, compared with the LiCoO2, there is no Co ions in the LiNi0.5Mn0.5O2 or less content of Co ions in the LiNi1/3Mn1/3Co1/3O2, which would lead to the low electrical conductivity of LiMO2, and the increased content of Ni ions produce the issue of Li/Ni mixing, which would reduce the diffusion rate of the lithium ions.30−32 The low Li+-ion and electrical conductivity of LiMO2 could impede their rate capability. For the first existing problem of LiMO2, surface modification by coating layered cathode materials with electrochemically inert oxides, such as ZrO2, Al2O3, La0.4Ca0.6CoO3, AlF3, NiO, H3PO4, and others, has been extensively reported as a simple and effective method to improve the cycle performance of layered oxide materials.21,33−39 For instance, Yan et al. coated LiNi1/3Mn1/3Co1/3O2 with ZrO2 and found that the cycling stability of the ZrO2 coated material remarkably improved.30 Unfortunately, these coating materials are usually insulative for Li+-ion conduction and increase the Li+-diffusion length which Received: June 30, 2015 Accepted: November 24, 2015

A

DOI: 10.1021/acsami.5b08777 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. (a) Schematic illustration of the preparation process for LiMO2@Li2SiO3. X-ray diffraction patterns for different molar ratios of (b) LiNi0.5Mn0.5O2@Li2SiO3 and (c) LiNi1/3Mn1/3Co1/3O2@Li2SiO3.

are not beneficial for Li+-ion conduction and interfacial charge transfer of the electrode. Li2SiO3, as a novel coating material, has some unique advantages compared with the aforementioned coating materials. It can not only act as a protective layer to stabilize the surface structure of cathode materials, suppress the metalion dissolution, and unfavorable interfacial side reactions, but it also has three-dimensional path for Li+-ion diffusion which can effectively increase Li+-ion conduction, and thus ameliorate the rate capability of the layered oxide materials.40 In addition, in our previous work, lithium-rich layered oxide materials were chosen as an ideal building block to demonstrate the effectiveness of the Li2SiO3 coating layer in improving the electrochemical performance of electrode materials.40 To further demonstrate the effectiveness of this in situ coating approach and novel coating material, in this work, layered oxide LiNi0.5Mn0.5O2 and LiNi1/3Mn1/3Co1/3O2 were selected as representative building blocks coating with Li+-ion conductor Li2SiO3. The discharge capacity, cycle stability, rate capability, and some other electrochemical performances of the layered cathode materials can be highly enhanced through this surface-modification strategy. Furthermore, our work shows that the content of Li2SiO3 on the surface of the bulk material has a significant influence on the electrochemical performance of the layered cathode materials. On the other hand, potentiostatic intermittent titration technique (PITT) test was carried out to investigate the mechanism of the improvement in the electrochemical properties of the Li2SiO3-coated layered oxide materials. The diffusion coefficient of Li+-ion (DLi) obtained by PITT gives a convincing explanation for the excellent electrochemical performances of the coated cathode materials. A series of results proved that our methodology is highly effective to layered oxide LiMO2 (M = Ni0.5Mn0.5 and Ni1/3Mn1/3Co1/3). So, we believe we provide a convenient, effective, and highly versatile coating strategy, which can be expected to open the way to ameliorate the electrochemical properties of electrode materials for lithium ion batteries.

2. EXPERIMENTAL SECTION 2.1. Materials Synthesis. Li2SiO3-coated layered cathode materials (LiMO2) were synthesized by a facile process according to our previous work.40 First, oxalate precursors (MC2O4·xH2O) were prepared by a coprecipitation (CP) method. Stoichiometric amounts of nickel sulfate, manganese sulfate, or cobalt sulfate were dissolved in deionized water. The Na2C2O4 solution was added dropwise into the above solution under violently stirring. The obtained oxalate precursor was then filtered, washed, and dried overnight at 70 °C. Second, a solvothermal process was carried out to coat different molar ratios of SiO2 on the MC2O4·yH2O samples. Then, 2 mmol MC2O4·xH2O was dispersed into 20 mL absolute ethanol, followed by addition of different molar ratios (Si:M) of Si(OC2H5)4 diluted in absolute ethanol. The mixtures were maintained at 180 °C for 5 h. Next, the products of SiO2 coated MC2O4·yH2O (MC2O4·yH2O@SiO2) were washed with ethanol and collected by centrifugation. Finally, the different molar ratios of SiO2-coated MC2O4·yH2O were mixed with stoichiometric amounts of LiOH. One part of LiOH was used to lithiate oxalate precursors to form LiMO2, the other part was used to lithiate SiO2 to form Li2SiO3. The mixtures were then calcinated at 800 °C for 12 h with a heating rate of 5 °C·min−1 to get different molar ratios of Li2SiO3-coated LiMO2 compounds (LiMO2@Li2SiO3). To get the none coated layered cathode material LiMO2, the same synthesis process without addition of Si(OC2H5)4 was used. Layered cathode material LiMO2 coated with molar ratio of Si:M = 0/100, 1/ 100, 3/100, 5/100, and 8/100 are hereafter referred as 0 (pristine), 1, 3, 5, and 8 mol %, respectively. 2.2. Materials Characterizations. The crystal structures of all the samples were characterized by X-ray diffractometer (Persee XD2) equipped with Cu Kα radiation. The FTIR spectra were measured on a Nicolet Avatar 360 FTIR instrument by a transmission mode (Thermo Electron). Particle morphologies of the prepared powders were observed by scanning electron microscope (SEM, Hitachi S4800) and transmission electron microscopy (TEM, FEI Tecnai G2 F20). Thermogravimetric analyzer 2050 (TGA 2050) was applied on the metal oxalates precursors from room temperature to 600 °C at a heating rate of 10 °C min−1 under air flow. XPS spectra were obtained using a Kratos Axis ULTRA X-ray photoelectron Spectrometer under UHV conditions. Inductively coupled plasma (ICP) measurements were carried out on Agilent 7500ce. 2.3. Electrochemical Measurement. To analyze the electrochemical performance of the samples, we used a two-electrode cell in the electrochemical characterization by galvanostatic cycling. The working electrodes were prepared by mixing 80 wt % active materials, B

DOI: 10.1021/acsami.5b08777 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 2. (a) HRTEM images of LiNi0.5Mn0.5O2@Li2SiO3 (5 mol %) and (inset) the lattice fringes; (b) high-angle annular dark-field scanning TEM (HAADF-STEM) of LiNi0.5Mn0.5O2@Li2SiO3 (5 mol %) sample and elemental mapping results of Ni, Mn, and Si; (c) HRTEM images of LiNi1/3Mn1/3Co1/3O2@Li2SiO3 (3 mol %) and (inset) the lattice fringes; and (d) high-angle annular dark-field scanning TEM (HAADF-STEM) of LiNi1/3Mn1/3Co1/3O2@Li2SiO3 (3 mol %) sample and elemental mapping results of Ni, Mn, Co, and Si. 10 wt % carbon black, and 10 wt % polyvinylidene fluoride (PVDF) in N-methylpyrrolidinone (NMP) to form a homogeneous slurry. The slurry was spread uniformly on an aluminum foil current collector and dried under vacuum at 120 °C for about 12 h. Then 2016-type coin cells were assembled in an Ar-filled glovebox using Li foil as the counter electrode. The electrolyte was 1 mol/L LiPF6 in a 1:1 mixture of ethylene carbonate (EC)/dimethyl carbonate (DMC) and the separator was Celgard 2500. Galvanostatic charge−discharge cycling was tested between 3.0 and 4.3 V (vs Li/Li+) using automatic galvanostat (NEWARE) at room temperature. The electrochemical spectroscopy (EIS) was performed with an instrument (PGSTAT 302N, Metrohm Autolab) using an amplitude of 5 mV and a frequency range from 100 kHz to 0.1 Hz. The PITT tests were also carried out on the Metrohm Autolab instrument.

MC2O4 ·y H2 O@SiO2 + LiOH 800 ° C

⎯⎯⎯⎯⎯→ LiMO2 @Li 2SiO3 + CO2 + H 2O

The mixed metal oxalate compounds are selected as precursors because its dehydration reaction can occur at around 150 °C (see TGA curves in Figure S1). The SiO2 coating layer on the surface of MC2O4·yH2O is due to the highly reactive Si(OC2H5)4 reacts with the released H2O from MC2O4·xH2O at 180 °C during solvothermal condition. The unique in situ coating procedure creates an excellent interface bond between the bulk material and surface coating layer. The coating strategy is convenient and can be used to prepare other electrode materials. The structural informations of the different molar ratios of Li2SiO3 coated LiNi0.5Mn0.5O2 and LiNi1/3Mn1/3Co1/3O2 layered cathode materials are determined by XRD analysis as displayed in Figure 1b,c. It can be seen that the diffraction patterns of both the pristine and Li2SiO3 coated LiMO2 materials show hexagonal system and a signal phase of welldefined α-NaFeO2 structure (space group R3m).25,26 All the diffraction peaks in Figure 1b,c are sharp and well-defined, which suggests that the compounds are well crystallized. In addition, it is worth noting that the splitting of paired diffraction peaks (018)/(110) and (006)/(102) is more noticeable in the Li2SiO3 coated LiNi1/3Mn1/3Co1/3O2 cathode materials compared with that of the Li2SiO3-coated LiNi0.5Mn0.5O2 samples. The phenomenon indicates that the LiNi1/3Mn1/3Co1/3O2 cathode materials are easier to form a well-developed layered structure than the LiNi0.5Mn0.5O2 materials during the synthesis process. 27,28 No peaks

3. RESULTS AND DISCUSSION 3.1. Characterization of LiMO2@Li2SiO3. The synthetic route of Li2SiO3 coated layered oxide LiMO2 can be described in three steps, in which oxalate precursors were first prepared, coated with SiO2, and further lithiated to obtain the coated products. (Figure 1a) The microstructure evolution of all the samples during the synthesis process of LiMO2@Li2SiO3 can be described by the following three pivotal chemical reaction equations: 180 ° C

MC2O4 ·x H 2O ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ MC2O4 ·y H2 O + z H 2O

(1)

zH 2O + Si(OC2H5)4 → SiO2 + C2H5OH

(2)

solvothermal

(3)

C

DOI: 10.1021/acsami.5b08777 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 3. XPS spectra for (a) Ni, (b) Mn, (c) Co, and (d) Si elements of the LiNi1/3Mn1/3Co1/3O2@Li2SiO3 (3 mol %) sample.

electrode materials and electrolyte, which is favorable for the conduction of electrons and ions. It can be seen in Figure S5c,d that the Li2SiO3 coated LiNi1/3Mn1/3Co1/3O2 have the lower agglomeration and more uniform particle size compared with the pristine material. The results can be ascribed to the Li2SiO3 coating layer on the surface of the host materials which can prevent aggregation between the particles during the calcining process at 800 °C. Furthermore, it can also be concluded from the scanning electron micrographs that there is not obvious differences on the morphology between the pristine and Li2SiO3 coated layered cathode materials which indicates that the existence of Li2SiO3 coating layer does not affect the mainly morphology of the particles. To identify the surface microstructure, we analyzed the 5 mol % Li2SiO3 coated LiNi0.5Mn0.5O2 and 3 mol % Li2SiO3 coated LiNi1/3Mn1/3Co1/3O2 samples under high-resolution TEM (HRTEM). In Figure 2, panels a and c display the HRTEM images of the LiNi0.5Mn0.5O2@Li2SiO3 (5 mol %) and LiNi1/3Mn1/3Co1/3O2@Li2SiO3 (3 mol %) samples, respectively. The bulk layered structure of LiNi0.5Mn0.5O2@Li2SiO3 (5 mol %) and LiNi1/3Mn1/3Co1/3O2@Li2SiO3 (3 mol %) samples are well preserved, and both of them present layered fringes with interplanar spacing of ca. 0.47 nm.25,26 Meanwhile, an obviously thin Li2SiO3 coating layer can be observed on the surface of them. It is evident from Figure 2a,c that the thickness of Li2SiO3 coating layer is about 2−3 nm distributed on the surface of the LiNi0.5Mn0.5O2, while the thickness of Li2SiO3 coating layer is about 2−6 nm distributed on the surface of the LiNi1/3Mn1/3Co1/3O2 sample. We know that the thinner coating layer not only could effectively segregate the host layered cathode materials from liquid electrolyte and thus suppress the side reactions, but also can lead to lower electronic resistance during the electrochemical reaction. Figure 2b,d present the elemental mapping images for the 5 mol % Li2SiO3 coated LiNi0.5Mn0.5O2 and 3 mol % Li2SiO3 coated LiNi1/3Mn1/3Co1/3O2 samples, respectively. Elemental

corresponding to Li2SiO3 can be observed in the XRD patterns, which indicates that the Li2SiO3 exist in an amorphous phase. Figure S3 show the FTIR spectra of the pristine and 3 mol % Li2SiO3 coated LiNi1/3Mn1/3Co1/3O2 materials. The absorption bands between 3500 and 3000 and 1650 cm−1 are assigned to the O−H vibrations. The absorption band around 984 cm−1, which can be seen only in the Li2SiO3-coated material is typical absorption (Si−O−Si) in the IR spectra for the SiO32− group.41,42 The results indirectly demonstrate the existence of the Li 2 SiO 3 . Rietveld refinements of the pristine LiNi 0.5 Mn0.5 O 2, 5 mol % Li 2SiO3 -coated LiNi 0.5Mn 0.5 O2 , LiNi1/3Mn1/3Co1/3O2 and 3 mol % Li2SiO3-coated LiNi1/3Mn1/3Co1/3O2 are performed to obtain the structural parameters of the samples, and the results are shown in Figure S4 and Table S1. No obvious change of the lattice parameters can be observed between the pristine and Li2SiO3 coated samples, implying that the structure of LiMO2 remains unchanged after surface modification and Si atoms adhered on the surface of LiMO2 particles as Li2SiO3 coating rather than diffused into LiMO2 lattice. The inductively coupled plasma (ICP) results show that the actual content of the elements is in good agreement with the theoretical content in the pristine and Li2SiO3 coated materials, and the content of Na in the samples is negligible. Scanning electron microscopy (SEM) observations were carried out to survey morphologies of the prepared samples. Figure S5a,b and c,d show pictures of 0 and 5 mol % Li2SiO3 coated LiNi0.5Mn0.5O2 and 0 and 3 mol % Li2SiO3 coated LiNi1/3Mn1/3Co1/3O2 samples in various magnifications, respectively. All the particles are well-crystallized which is consistent with the results of the sharp peaks in the X-ray diffraction (XRD) patterns (Figure 1b,c). The photographs in Figure S5 display that the average particle size of the samples is about 200−400 nm, and these small particles aggregated each other to form microsized irregular secondary particles. This micronano structure increases the contact area between the D

DOI: 10.1021/acsami.5b08777 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 4. (a) Cycle performance of different molar ratios of LiNi0.5Mn0.5O2@Li2SiO3 at the rate of 0.2C and (inset) the initial Coulombic efficiency of them; (b) discharge capacity vs voltage curves for the pristine and LiNi0.5Mn0.5O2@Li2SiO3 (5 mol %) electrodes at the 80th cycle; dQ/dV profiles for (c) pristine LiNi0.5Mn0.5O2 and (d) LiNi0.5Mn0.5O2@Li2SiO3 (5 mol %) calculated from numerical data observed after 3rd, 10th, 30th, 50th, and 80th cycles at 0.2C and (insets) the variation trend of discharge voltage after different cycles.

3.2. Electrochemical Properties. As discussed above, the layered LiMO2@Li2SiO3 cathode materials have been successfully prepared by controlling the growth of SiO2 on the surface of the oxalate precursors followed by a smart approach. Next, it is interesting to explore the influence of the Li2SiO3 coating on the electrochemical properties of the LiMO2 cathode materials. Different molar ratios of Li2SiO3 coating LiNi0.5Mn0.5O2 and LiNi1/3Mn1/3Co1/3O2 cells were charged/discharged galvanostatically between 3.0 and 4.3 V with various charge−discharge current densities at room temperature. The cycle performance of different molar ratios of LiNi0.5Mn0.5O2@Li2SiO3 at the rate of 0.2 C (1 C = 160 mAh/g) are shown in Figure 4a. It is clear that both the 3 and 5 mol % Li2SiO3 coated LiNi0.5Mn0.5O2 electrodes exhibit higher discharge capacity than that of the pristine LiNi0.5Mn0.5O2 sample, while the 8 mol % Li2SiO3 coated electrode shows the lowest capacity among them. Specifically, the pristine electrode approximately delivered a discharge capacity of 49 mAh/g after 100th cycle, while the 3 and 5 mol % Li2SiO3-coated LiNi0.5Mn0.5O2 electrodes approximately delivered a discharge capacity of 57 and 82 mAh/g, which increased by 16 and 67%, respectively, compared to that of the pristine electrode (Table S3). Meanwhile, the 3 and 5 mol % coated electrode also display better cycle stability compared with the pristine electrode. For instance, the capacity retention of LiNi0.5Mn0.5O2@Li2SiO3 (5 mol %) is about 72% which is higher than the 51% of the pristine electrode. The LiNi0.5Mn0.5O2@ Li2SiO3 (8 mol %) electrode shows a discharge capacity of only 39 mAh/g after 100th cycle. This phenomenon can be ascribed to the reason that the Li2SiO3 is not the electrode active material, and it will reduce the discharge capacity of the electrode material when the amount of which is larger. Until now, it can be summarized that the appropriate coating amount of Li2SiO3 is very important for enhancing the electrochemical properties of the LiNi0.5Mn0.5O2 material. Surprisingly, it can be

mapping of Ni, Mn, Co, and Si in Figure 2d shows that all the elements are distributed homogeneously throughout the particles, indicating that the Li2SiO3 coating layer deposited outside the LiNi1/3Mn1/3Co1/3O2 particle surface is uniform. However, the results of elemental mapping in Figure 2b display that the Ni and Mn distributed homogeneously throughout the particles, while the distribution of Si is inhomogeneous. So, the Li2SiO3 coating layer on the surface of LiNi0.5Mn0.5O2 may be not uniform. The same coating procedure was used for the LiNi0.5Mn0.5O2 and LiNi1/3Mn1/3Co1/3O2 cathode materials, but the coating result is a little difference. The reason is not clear, and further exploration may be needed. For further characterization, the 3 mol % Li2SiO3 coated LiNi1/3Mn1/3Co1/3O2 material is selected as the representative for all the coated samples. In order to investigate the valence state of the surface elements on the particles, X-ray photoelectron spectroscopy (XPS) experiments were carried out for the LiNi1/3Mn1/3Co1/3O2@Li2SiO3 (3 mol %) sample. The Ni 2p3/2, Mn 2p3/2, Co 2p3/2, and Si 2p3/2 photoelectron spectra for the layered LiNi1/3Mn1/3Co1/3O2@Li2SiO3 (3 mol %) are given in Figure 3. As shown in Figure 3a−c, the peak of Ni 2p3/2, Mn 2p3/2, and Co 2p3/2 is at 855.0, 642.4, and 780.1 eV, respectively, which indicate the valence of Ni, Mn, and Co element is +2, + 4, and +3 in the LiNi1/3Mn1/3Co1/3O2@ Li2SiO3 (3 mol %) sample.43 These results are in agreement with the previous XPS results for LiNi1/3Mn1/3Co1/3O2 cathode material.29 It can be concluded that the presence of Li2SiO3 on the surface of the bulk material does not affect the oxidation of Ni, Mn and Co elements in the layered LiNi1/3Mn1/3Co1/3O2. As to the element Si, the Si 2p3/2 peak in Figure 3d is located at 102.0 eV, which is in accordance with the oxidation state of Si4+ in the Li2SiO3 material.43 The Li 1s photoelectron spectra in Figure S7 suggests that there is Li elememt on the surface of the pristine and Li2SiO3 coated LiMO2 materials, and the valence of Li ions is +1.43 E

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Figure 5. (a) Cycle performance of different molar ratios of LiNi1/3Mn1/3Co1/3O2@Li2SiO3 at the rate of 1C. (b) The first specific capacity vs voltage curves for different molar ratios of LiNi1/3Mn1/3Co1/3O2@Li2SiO3 and (inset) the initial Coulombic efficiency of them. Specific capacity vs voltage curves for the (c) pristine LiNi1/3Mn1/3Co1/3O2 and (d) LiNi1/3Mn1/3Co1/3O2@Li2SiO3 (3 mol %) electrodes at the different cycles.

coated materials only the 3 mol % Li2SiO3 coated electrode exhibits higher discharge capacity after 100 cycles compared with that of the pristine LiNi1/3Mn1/3Co1/3O2. The LiNi1/3Mn1/3Co1/3O2@Li2SiO3 (3 mol %) electrode delivers a discharge capacity of 145 mAh/g after 100 cycles, which increases by 14%, compared with 127 mAh/g of the pristine electrode (Table S4). The capacity retention of the pristine and 3 mol % coated electrodes are 92% and 94%, respectively. Both the pristine and 3 mol % coated electrodes show greatly cyclic stability during cycling, and the capacity retention of LiNi1/3Mn1/3Co1/3O2 is slightly higher after coated by 3 mol % Li2SiO3. In addition, it can also be seen in Table S4 that the discharge capacity and capacity retention of the LiNi1/3Mn1/3Co1/3O2 materials does not show better potential when the coating amount of Li2SiO3 exceeds 3 mol %. The results prove the importance of the coating amount once again, and the corresponding reasons have been already mentioned. Figure 5b shows the first charge−discharge curves for the pristine and 3 mol % Li2SiO3 coated electrodes at the rate of 1 C. The reversibility of the initial cycle of all Li2SiO3-coated materials improved, and the relative results are that the initial Coulombic efficiency enhanced, which can be clearly seen in the inserted histogram. Figure 5c,d display the charge− discharge curves for the pristine and 3 mol % Li2SiO3 coated materials at different cycles, which can clearly exhibit the charge−discharge process of the layered cathode materials. It can be summarized from the analysis of Figures 4 and 5 that the electrochemical performances of layered LiMO2 cathode materials are obviously enhanced due to the existence of the Li2SiO3 coating layer. The schematic diagram of the LiMO2@Li2SiO3 structure in Figure 6 clearly explains the mechanism of improving its electrochemical performances. It has been reported that the high-valent Ni4+ or Co4+ formed at the charged state are unstable when in direct contact with the electrolyte, and can induce the oxidative degradation of the electrolyte, which is the main reason results the capacity attenuation of the cathode materials during the cycling

seen from the histogram inset in Figure 4a that the initial Coulombic efficiency of all the molar ratios of LiNi0.5Mn0.5O2@ Li2SiO3 is higher than that of the pristine electrode. The results indicate that the Li2SiO3 coating can effectively improve the reversibility of the first cycle. Moreover, it can be revealed from Figure 4a and Table S3 that the improved electrochemical properties of 5 mol % Li2SiO3 coated LiNi0.5Mn0.5O2 are most evident. Thus, to further understand the difference in the electrochemical performances between the pristine and Li2SiO3 modified materials, the 5 mol % coated sample is selected as the representative for the coated materials. The gaps between the charge and discharge plateau regions are displayed in Figure 4b. The gap for the LiNi0.5Mn0.5O2@Li2SiO3 (5 mol %) electrode is much smaller after 80 cycles than that for the pristine electrode, which indicates that the LiNi0.5Mn0.5O2 materials modified by Li2SiO3 have lower polarization. Meanwhile, the 5 mol % Li2SiO3 coated LiNi0.5Mn0.5O2 also shows slower polarization than that of the pristine electrode during the cycling, the phenomenon can be clearly observed in Figure S8b and c. The dQ/dV plots of pristine and 5 mol % coated electrodes were calculated from numerical data observed in discharge profiles of various cycles, which are shown in Figure 4c,d. It is clear that the reduction peaks of the pristine electrode shift continuously toward lower potential upon cycling, while the reduction peaks of the 5 mol % coated electrode are almost overlapping after 80 cycles. To some extent, the shift of the reduction peaks represents the voltage decline during cycling.44 More precisely, as shown in the insets of Figure 4c,d, the discharge voltage plateau of the LiNi0.5Mn0.5O2@Li2SiO3 (5 mol %) decreased by 0.5%, while that for the pristine electrode decreased by 3.6%. The results suggest that the Li2SiO3 coating layer alleviates the voltage drop of the LiNi0.5Mn0.5O2 sample during charge−discharge cycling. Figure 5a displays the cycle stability of different molar ratios of LiNi1/3Mn1/3Co1/3O2@Li2SiO3 at the rate of 1 C (160 mA/ g). Unlike the LiNi0.5Mn0.5O2@Li2SiO3 electrode, among all F

DOI: 10.1021/acsami.5b08777 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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capacity of the Li2SiO3 coated LiNi0.5Mn0.5O2 becomes larger with the increase of the charge−discharge current density. For instance, at the rate of 0.8 C (1 C = 160 mA/g), the layered LiNi0.5Mn0.5O2@Li2SiO3 (5 mol %) cathode material delivered a discharge capacity of 65 mAh/g, which increased by 41% compared with the 46 mAh/g of the pristine LiNi0.5Mn0.5O2 electrode. At the rate of 1.5 C, the LiNi0.5Mn0.5O2@Li2SiO3 (5 mol %) electrode delivered a discharge capacity of 33 mAh/g, which increased by 230% compared with the 10 mAh/g of the pristine LiNi0.5Mn0.5O2 electrode. Figure 7b displays the rate capability of the pristine LiNi1/3Mn1/3Co1/3O2 and 3 mol % Li2SiO3 coated LiNi1/3Mn1/3Co1/3O2 electrodes. It is similar to the case of the LiNi0.5Mn0.5O2@Li2SiO3 (5 mol %), the 3 mol % Li2SiO3 coated LiNi1/3Mn1/3Co1/3O2 electrode exhibits higher capacity compared with the pristine electrode at each current density, especially in high current density. The LiNi1/3Mn1/3Co1/3O2@ Li2SiO3 (3 mol %) sample delivered a discharge capacity of 147 mAh/g at the rate of 1 C (160 mA/g), whereas the pristine electrode showed the discharge capacity of 132 mAh/g. The gap of the discharge capacity between the pristine and Li2SiO3 coated LiNi1/3Mn1/3Co1/3O2 becomes larger with the increase of the charge−discharge current density. The Li2SiO3 coated LiNi1/3Mn1/3Co1/3O2 material shows the discharge capacity of 142, 118, and 86 mAh/g at the rate of 2 C, 5 and 10 C, respectively, while the pristine electrode exhibits the discharge capacity of 118, 83, and 25 mAh/g. The discharge capacity of the LiNi1/3Mn1/3Co1/3O2@Li2SiO3 (3 mol %) electrode increased by 20.3, 42.2, and 244% compared with the pristine electrode at the rate of 2, 5, and 10 C, respectively. In addition, it is worth noting that the coated material could show the discharge capacity of 33 mAh/g at a high discharge rate of 20 C, while the pristine electrode shows the discharge capacity of only 1 mAh/g. It can be concluded that the Li2SiO3 coating

Figure 6. Schematic diagram of the LiMO2@Li2SiO3 structure and the role of Li2SiO3 coating layer to improve the electrochemical performance.

process.25−28 In this work, Li2SiO3, as a protective layer on the surface of the bulk electrode, can effectively control the decomposition of the electrolyte, inhibit the unfavorable interfacial side reactions, and thus improve the cyclic stability of the electrodes. Furthermore, unlike other coating materials which may be unfavorable for the transportation of Li+, Li2SiO3, have three-dimensional channel for Li+, which acts as a “expressway” for Li+ ions and accelerate the transportation of Li+ between the host materials and electrolytes. It is known that the faster migration rate of lithium ions is beneficial for improving the rate capability of the electrodes. Therefore, to further explore the advantages of the Li+conductive Li2SiO3 coating layer, rate performances of the pristine and Li2SiO3 coated layered cathode materials were tested under various current densities. As shown in Figure 7a, the 5 mol % Li2SiO3 coated LiNi0.5Mn0.5O2 shows higher discharge capacity compared with that of the pristine electrode at each current density. The increased degree of the discharge

Figure 7. (a) Rate capability of the pristine LiNi0.5Mn0.5O2 and 5 mol % Li2SiO3 coated LiNi0.5Mn0.5O2 electrodes. (b) Rate performance of the pristine LiNi1/3Mn1/3Co1/3O2 and 3 mol % Li2SiO3 coated LiNi1/3Mn1/3Co1/3O2 electrodes. (c) High rate cyclic performance of the LiNi1/3Mn1/3Co1/3O2@Li2SiO3 (3 mol %) electrode and (d) the corresponding specific capacity vs voltage curves at the 100th cycle. G

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Figure 8. PITT curves of (a) pristine LiNi0.5Mn0.5O2 and (b) 5 mol % Li2SiO3 coated LiNi0.5Mn0.5O2 electrode as a function of time between 3.0 and 4.3 V; Lithium diffusion coefficients (DLi) determined by the PITT curves as a function of the voltage during (c) charge and (d) discharge processes.

electrolyte interface. The reduced resistances (Rs) of the Li2SiO3 coated layered materials in the high frequency region also further supports that the side reactions were reduced in the present electrode−electrolyte interface due to the stable surface of the LiMO2@Li2SiO3 materials (Table S6). Additionally, a potentiostatic intermittent titration technique (PITT) test, which is considered to be a reliable method for determining the lithium ions diffusion coefficient (DLi, an important kinetic parameter of intercalation materials) in electrode-active materials, was performed.45−48 The pristine LiNi0.5Mn0.5O2 and LiNi0.5Mn0.5O2@Li2SiO3 (5 mol %) electrodes were selected as the representatives for the layered cathode samples. Before measurement, the cells were galvanostatically charged and discharged in one cycle at the rate of 0.2 C between 3.0 and 4.3 V at room temperature. Details of the PITT tests are described in the Supporting Information. Assuming that lithium transport in the electrode obeys Fick’s second law, the lithium ions diffusion coefficients can be obtained by the following equation:

layer highly enhanced the rate capability of the layered LiMO2 cathode materials. To determine the cycle performance of the LiNi1/3Mn1/3Co1/3O2@Li2SiO3 (3 mol %) electrode at the high current density, LiNi1/3Mn1/3Co1/3O2@Li2SiO3 (3 mol %) cells with Li anodes were tested between 3.0 and 4.3 V at the rate of 2 C, 5 and 10 C, respectively, after activation at the current density of 40 mA/g for 2 cycles. Cycle performance of the LiNi1/3Mn1/3Co1/3O2@Li2SiO3 (3 mol %) electrode at different rates and corresponding specific capacity vs voltage curves at 100th cycle are presented in Figure 7c,d. It can be seen that the 3 mol % Li2SiO3 coated LiNi1/3Mn1/3Co1/3O2 can deliver approximately a discharge capacity of 122 mAh/g at the rate of 2 C and 111 mAh/g at the rate of 5 C after 300 cycles, respectively. In particular, it can still exhibit approximately a discharge capacity of 79 mAh/g after 600 cycles at the rate of 10 C. The capacity retention of the LiNi1/3Mn1/3Co1/3O2@ Li2SiO3 (3 mol %) sample is 67% at the rate of 10 C after 600 cycles (Table S5). These results indicate that the layered 3 mol % Li2SiO3 coated LiNi1/3Mn1/3Co1/3O2 material possesses excellent cyclic performance under higher charge−discharge rates. 3.3. Mechanism Analysis. In order to better identify the reasons that why layered Li2SiO3 coated LiMO2 materials present the higher rate performance and other excellent electrochemical capabilities, electrochemical impedance spectroscopy (EIS), which is a powerful technique of identifying the kinetics of Li ions in oxide cathodes, was performed, first. The Nyquist plots of the pristine and 5 mol % Li2SiO3 coated LiNi0.5Mn0.5O2 electrodes are displayed in Figure S9a. Figure S9b shows the Nyquist plots of the pristine and 3 mol % Li2SiO3 coated LiNi1/3Mn1/3Co1/3O2 electrode. It can be seen all the plots show the similar equivalent circuit (Figure S9 c). In addition, Table S6 clearly shows the charge transfer resistances (Rct) for the 5 mol % Li2SiO3 coated LiNi0.5Mn0.5O2 (389.26Ω) and 3 mol % Li2SiO3 coated LiNi1/3Mn1/3Co1/3O2 (366.98Ω) electrodes, and both of them are smaller than that for the pristine LiNi0.5Mn0.5O2 (753.59Ω) and LiNi1/3Mn1/3Co1/3O2 (489.70Ω). The results indicate that the Li2SiO3 coating layer effectively reduces the barrier for Li+ transfer at the electrode−

DLi = −

d ln(I ) 4L2 dt π 2

(4)

where L is the thickness of the electrode active material on the aluminum foil.47−49 Figure 8a,b show the PITT curves of the pristine and 5 mol % Li2SiO3 coated LiNi0.5Mn0.5O2 electrodes as a function of time between 3.0 and 4.3 V, respectively. On the basis of the PITT measurement and corresponding data calculation, the lithium diffusion coefficients (DLi) as a function of the charge− discharge voltage are presented in Figure 8c,d. It can be clearly seen that the lithium diffusion coefficient (DLi) of the LiNi0.5Mn0.5O2@Li2SiO3 (5 mol %) are higher than that of the pristine LiNi0.5Mn0.5O2 during both the charge and discharge processes. For instance, during the discharge process, the DLi average value is 7.1 × 10−10 cm2 s−1 which increased by 7 times, compared to 1.0 × 10−10 cm2 s−1 of the pristine electrode. The improved kinetic parameters, lithium diffusion coefficient of the LiNi0.5Mn0.5O2@Li2SiO3 (5 mol %) electrode are related to the existence of the Li2SiO3 coating layer on the surface of the bulk layered material. It has been reported that if H

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the coating material only acts as a physical protection layer with defect, the lithium diffusion coefficient should not be changed.50 So, the results of PITT not only further demonstrated that the Li2SiO3 coating layer improves the lithium diffusion coefficient but also verified from the side that the existence of the stable surface on the coated materials, which can reduce the side reactions and enhance the electrochemical performance of the layered cathode materials.

4. CONCLUSIONS In summary, a series of Li2SiO3 coated layered oxide LiMO2 have been prepared by in situ hydrolysis followed by the lithiation approach. The LiNi0.5Mn0.5O2@Li2SiO3 (5 mol %) and LiNi1/3Mn1/3Co1/3O2@Li2SiO3 (3 mol %) electrodes show superior discharge capacity, cycle stability, and rate capability. The improved electrochemical properties of the layered oxide LiMO2 can be ascribed to the following reasons: (1) Li2SiO3 coating layer stabilize the surface structure of cathode materials; (2) Li2SiO3 coating layer suppress Ni4+ or Co4+ dissolution and inhibit the unfavorable interfacial side reactions; and (3) Li2SiO3 coating layer has three-dimensional path for Li+-ion diffusion, which can effectively increase Li+-ion conduction. In addition, our work also discloses that the coating amount of Li2SiO3 has a large effect on the electrochemical performance of the layered cathode materials, and it is necessary to optimize the electrochemical performance through tuning the coating content of Li2SiO3. Importantly, the versatile coating strategy in our work can be extended to prepare other advanced electrode materials with superior electrochemical performance for lithium-ion batteries.

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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.5b08777. Details of TGA, SEM, XPS, EIS, and PITT characterization and detailed electrochemical performance of Li2SiO3 coated layered oxide materials. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel. +86 10 8825 6655. *E-mail: [email protected]. Tel. +86 10 8825 6328. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Beijing Nova Program (Z141103001814065), the National Science Foundation for Young Scholars of China (21201177), the State Key Project of Fundamental Research (2014CB931900 and 2012CB932504), the Chinese Academy of Sciences (“Hundred Talents Project”), and the Youth Innovation Promotion Association CAS.

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J

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