Research Article www.acsami.org
Reduced Graphene Oxide-Wrapped Nickel-Rich Cathode Materials for Lithium Ion Batteries Jae-Hyun Shim,† Young-Min Kim,†,‡ Miji Park,§ Jongsik Kim,*,§ and Sanghun Lee*,∥ †
Department of Energy Science, Sungkyunkwan University (SKKU), Suwon, Gyunggido 16419, Republic of Korea Center for Integrated Nanostructure Physics, Institute for Basic Science (IBS), Suwon, Gyunggido 16419, Republic of Korea § Department of Chemistry, Dong-A University, Busan 49315, Republic of Korea ∥ Department of Nanochemistry, Gachon University, Seongnam, Gyunggido 13120, Republic of Korea ‡
S Supporting Information *
ABSTRACT: The encapsulation of Ni-rich cathode materials (LiNi0.6Co0.2Mn0.2O2) for lithium ion batteries in reduced graphene oxide (rGO) sheets is introduced to improve electrochemical performances. Using (3-aminopropyl)triethoxysilane, the active materials are completely wrapped with several rGO layers of ∼2 nm thickness. By virtue of the great electrical conductivity of graphene, the rGO-coated cathode materials exhibit much enhanced electrochemical performances of cycling property and rate capability. In addition, it is shown that the structural degradation of the active materials, which is from the rhombohedral layered structure (R3̅m) to the spinel (Fd3̅m) or rock-salt phase (Fm3̅m), is significantly reduced as well as delayed due to the protection of the active materials in the rGO layers from direct contact with electrolytes and the consequent suppression of side reactions. KEYWORDS: lithium ion battery, rGO-encapsulated cathode, structural degradation, electrical conductivity, Ni-rich cathode
1. INTRODUCTION
Meanwhile, owing to its great potentials, such as, large specific surface area, strong mechanical property, and excellent electrical conductivity, graphene-combined electrode materials have been investigated in a very large number of studies for energy storage materials, including LIBs.14−21 Among many candidates, several composite materials of LiNixCoyMn1−x−yO2 and graphene have also been suggested for LIBs.22−26 These composite materials, which were usually obtained by the simple sol−gel synthesis with the high-energy ball milling or spraydrying method with a postannealing process, showed much improved electrochemical performances. In addition, Jiang et al. also wrapped Li-rich layered materials of TMs (Ni, Co, and Mn) with graphene by simple mixing and exhibited excellent cell performances of rate capability and cycling ability.27 The preparation methods of these materials were basically based on mechanical mixing of the active materials and reduced graphene oxide (rGO) because the layer-structured materials need to be calcinated at high temperatures. Sometimes, graphene-encapsulated, that is, uniformly coated, cathode materials for LIBs exhibit an impressively good battery performance. In the early studies, investigations were mainly attempted on the LiFePO4 cathodes, probably due to the synthesis conditions, and the graphene-encapsulated materials showed a more enhanced
Rechargeable lithium ion batteries (LIBs), which have been the most popular energy storage devices for portable electronics, are being actively developed for employment in electric vehicles (EVs) and massive energy storage systems (ESSs).1,2 Among a large number of candidates, LiCoO2 has been widely used as a cathode material for commercialized LIBs due to its wellbalanced battery performances of good cycling property, small irreversible capacity, and excellent rate capability. However, the high cost and environmental concerns of cobalt have limited its application in large-capacity LIBs for EVs and ESSs.3 Therefore, simultaneous partial replacements of Co by Ni and Mn (LiNixCoyMn1−x−yO2) have been introduced and have shown great success.4−6 For the purpose of practical use of these materials, in particular, of high Ni contents, in the battery industry, the surface coatings with various materials of Al2O3,7 ZrO2,8 ZnO,9 TiO2,10 AlF3,11 or AlPO412 have been applied to enhance the cell performance by suppressing detrimental reactions at the interfaces and dissolution of transition metal (TM) ions despite a couple of drawbacks, for example, increase of costs caused by complicated manufacturing processes as well as the low electrical conductivity of the coating materials. Apart from these attempts, it has been found that the Ni-rich LiNixCoyMn1−x−yO2 materials have an important role in mitigating the voltage decay of Li-rich layer-structured cathode materials for high-energy LIBs.13 © 2017 American Chemical Society
Received: February 22, 2017 Accepted: May 18, 2017 Published: May 18, 2017 18720
DOI: 10.1021/acsami.7b02654 ACS Appl. Mater. Interfaces 2017, 9, 18720−18729
Research Article
ACS Applied Materials & Interfaces
Figure 1. SEM images of (a) NCM and (b) rGO-NCM of fresh (uncycled) samples. (c, d) Enlarged images of (a) and (b), respectively. HRTEM images of (e) NCM and (f) rGO-NCM.
battery performance.28−30 Meanwhile, rarely have there been studies reported on layer-structured or spinel cathode materials, with only a few exceptions. Oh et al.31 and Song et al.32 successfully wrapped Li-rich layer-structured materials with
rGO using chemical activation, which showed a remarkable battery performance. In addition, Kim et al. demonstrated that the Li-rich layer-structured materials can be coated with rGO and AlPO4 owing to electrostatic interactions between the 18721
DOI: 10.1021/acsami.7b02654 ACS Appl. Mater. Interfaces 2017, 9, 18720−18729
Research Article
ACS Applied Materials & Interfaces
Figure 2. XRD patterns of NCM and rGO-NCM. Miller indices of the Bragg peaks are indicated near each peak.
Figure 3. Raman spectra of NCM and rGO-NCM.
coating precursors and the active materials.33 For spinelstructured materials, Tang et al.34 and Xia et al.35 prepared LiM2O4 (M = Mn or mixed Mn and Ni) nanorods wrapped with graphene nanosheets or carbon nanotubes and demonstrated great potential as high-energy and high-power cathode materials for LIBs. In this study, following Oh et al.’s and Song et al.’s concept of rGO wrapping, we prepare high-quality rGO-encapsulated LiNi0.6Co0.2Mn0.2O2 by chemical activation. To the best of our knowledge, this work is the first report on the surface modification of LiNixCoyMn1−x−yO2 with rGO using (3aminopropyl)triethoxysilane (APTES), which is a popularly used chemical reagent to encapsulate anode materials for LIBs with rGO.36−40 The LiNi0.6Co0.2Mn0.2O2 particles are completely wrapped by the rGO sheets, and obviously, this surface modification improves the electrical conductivity of the cathode
materials. Using various experimental techniques, such as scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), scanning TEM (STEM) with electron energy loss spectroscopy (EELS), Xray diffraction (XRD), conductive atomic force microscopy (CAFM), and differential scanning calorimetry (DSC), the rGOwrapped materials, which show great improvement in the battery performance, are thoroughly investigated.
2. EXPERIMENTAL SECTION 2.1. Preparation of Materials. Ni0.6Co0.2Mn0.2(OH)2 (Reshine Co., China) and LiOH·H2O (>99%, Sigma Aldrich Co.) were used as precursors for the active materials with a 1:1.02 molar ratio of metal ions. They were preheated at 800 °C for 6 h in air and then reheated at 850 °C for 6 h in an O2 flow (pressure: 20 mL/min) to produce LiNi0.6Co0.2Mn0.2O2, which is denoted hereafter as NCM. The GO sheets were prepared from natural graphite powders (SFG6; Timcal
Figure 4. Height and current images of (a) NCM and (b) rGO-NCM from C-AFM. 18722
DOI: 10.1021/acsami.7b02654 ACS Appl. Mater. Interfaces 2017, 9, 18720−18729
Research Article
ACS Applied Materials & Interfaces
reached ∼5.0. Then, the obtained graphite oxide powders were exfoliated and dispersed in water (200 mL) by ultrasonication. The resulting solution was centrifuged to remove residual GO aggregates. Wrapping NCM with rGO was achieved as follows. First, 10 mL of APTES (>98%, Sigma Aldrich Co.) was dissolved in 50 mL of toluene with stirring for 5 min. Then, 10 g of NCM powder was dispersed in the solution with stirring for 6 h. After removing the solvents by filtration, the residue was added to the GO solution (0.05 g in 100 mL) with stirring at 80 °C for 2 h. After filtering, washing, and drying at 80 °C in a vacuum oven overnight, the resulting materials were heated at 300 °C for 4 h in a furnace under a H2 atmosphere to reduce GO to rGO (hereafter, rGO-NCM). 2.2. Characterization of Materials. XRD was performed using a Philips X-PERT PRO diffractometer (Cu Kα radiation, 2θ range: 15− 70°, step size: 0.013°, and scan rate: 0.67° min−1), and Rietveld refinement was carried out using the GSAS package42 with the EXPGUI interface.43 To observe the morphologies of the materials, SEM was used (Magellan XHR; FEI Co.). In addition, the HRTEM and STEM measurements were performed using a Cs-corrected STEM (JEM-2100F; JEOL Co., Japan) with a spherical aberration corrector (CEOS Gmbh, Germany) at the Korea Advanced Nano-Fab Center. For the HRTEM and STEM measurements, the samples were prepared by a focused ion beam (Hellios; FEI Co.). EELS was performed using a spectrometer (GIF 200 spectrometer, Gatan) attached to the STEM, in which the energy resolution calculated by the full width at half-maximum of the zero loss peaks was ∼0.8 eV and the energy dispersion was 0.05 eV/channel. Using AFM (Danish Microscope Engineering, Denmark), the electrical conductivity of the materials was analyzed,44 in which the force between the tip and the sample surface was set to ∼1.6 × 104 nN. The measurements were carried out in an ultrahigh vacuum (10−8 Torr). Micro-Raman scattering was performed in a quasi-backscattering geometry with a parallel polarization incident light (T64000; ISA Jobin-Yvon Inc., France). The excitation source was the 514.532 nm line from an Ar ion laser, and its incident power was 0.3 mW. 2.3. Evaluation of Electrochemical Performances. To evaluate the cell performances of the materials, CR2032 coin cells were fabricated as described in our previous work.45 First, the active materials for the cathode (NCM or rGO-NCM), conducting materials (Super-P carbon black), and binders (poly(vinylidene fluoride)) were mixed in N-methyl-2-pyrrolidone (96:2:2, wt ratio). After casting on an aluminum foil, the resulting slurry was dried in a vacuum oven (110 °C) for 2 h. The 1.15 M LiPF6 solution in ethylene carbonate/diethyl carbonate/ethyl methyl carbonate (30:30:40, vol ratio) was used as the electrolyte, and a small amount of fluoroethylene carbonate (5%) was added. The lithium metal was used as the counter electrode. To measure the charge/discharge capacities, the cells were cycled between 3.0 and 4.6 V. For electrochemical impedance spectroscopy (EIS) measurements, a Biologic VMP3 impedance analyzer was used in the frequency range of 500 kHz to 5 mHz with an amplitude of 10 mV. The cyclic voltammograms (CVs) were obtained between 3.0 and 4.5 V at a scan rate of 0.01 mV/s. To investigate thermal stability of the delithiated cathode materials in the charged cells, the DSC measurements were performed. The samples were prepared by charging the cells to 4.5 V at the rate of 0.1 C and disassembling the charged electrodes in the dry room. The dismantled electrodes were washed with dimethyl carbonate several times and soaked in the fresh electrolyte. The heating rate is 5 °C/min.
Figure 5. (a) Charge/discharge profiles at 1st and 100th cycles (0.1 C), (b) rate capability at incremental discharge rates from 0.1 to 10.0 C, and (c) cycling performance of discharge capacity at 1.0 C of NCM and rGO-NCM. The charge rates are 0.1 C for the rate capability and 0.5 C for the cycling test.
3. RESULTS AND DISCUSSION The outer morphologies of the materials from SEM are shown in Figure 1a−d. The size of the primary particles is ∼10 μm for samples of both NCM and rGO-NCM. In the highly magnified images, it is clearly observed that the entire particle of rGONCM is completely covered with very thin and transparent rGO sheets. In addition, to explore the detailed structures of the materials, the cross-sections of the particles were observed by HRTEM. As shown in Figure 1f, several rGO sheets are
Co., Switzerland) in a two-step process of oxidation and exfoliation by Hummer’s method.41 First, natural graphite (3.33 g and
