- Nano Express
- Open Access
Attachment of Li[Ni0.2Li0.2Mn0.6]O2 Nanoparticles to the Graphene Surface Using Electrostatic Interaction Without Deterioration of Phase Integrity
© Pyun and Park. 2016
- Received: 20 April 2016
- Accepted: 13 May 2016
- Published: 27 May 2016
In this article, we report a facile approach to enhance the electrochemical performance of Li-rich oxides with vulnerable phase stability. The Li-rich oxide nanoparticles were attached to the surface of graphene; the graphene surface acted as a matrix with high electronic conductivity that compensated for the low conductivity and enhanced the rate capability of the oxides. Our novel approach constitutes a direct assembly of two materials via electrostatic interaction, without a high-temperature heat treatment. The inevitable deterioration in phase integrity of previous composites between carbon and Li-rich oxides resulted from the reaction of oxygen in the structure with carbon during the heat-treatment process. However, our new method successfully attached Li-rich nanoparticles to the surface of graphene, without a phase change of the oxides. The resulting graphene/Li-rich oxide composites exhibited superior capacity and rate capability compared to their pristine Li-rich counterparts.
- Rate capability
- Li battery
Recently, Li-ion batteries have been used as the main energy storage system for electrical devices and electrical vehicles [1–5]. However, the energy densities of commercial Li-ion batteries using LiCoO2 as a cathode cannot satisfy the demand of many of these applications [6–10]; new high-capacity cathode materials are therefore required. The layered Li-rich materials in the Li-Mn-Ni oxide system, which have higher energy densities (~250 mAh g−1) than other cathode materials such as LiCoO2 [11–18], have attracted significant attention as promising new cathode materials. However, major drawbacks such as poor rate capability owing to insufficient electronic and ionic conductivities [19–21] have prevented their use in commercial applications. Many attempts have been made to enhance the rate capability of the cathode-like Li-rich layered oxide. For example, compositing the oxide with carbon-based materials that have high electronic conductivity has been used as a means of compensating for the low conductivity of the cathode materials [22–25]. In fact, graphene comprising two-dimensional carbon sheets has been used as a novel matrix in cathode/carbon composites that have high rate capability and stable cyclic performance; the use of graphene stems from its excellent electronic conductivity and mechanical flexibility as well as high specific surface area [26–29]. However, compositing with carbon materials (such as graphene) requires high-temperature (greater than ~400 °C) heat-treatment processes that produce strong bonds between the cathode and carbon. Unfortunately, Li-rich layered oxides are susceptible to phase changes and deterioration of the structural integrity during processing; this deterioration results from the reaction of carbon with oxygen (at over 400 °C) and the consequent loss of oxygen from the structure of the oxides [28, 29]. Since most of the cathode materials, such as LiCoO2, LiFePO4, and LiMn2O4, are relatively stable during the heating process [22–27], this susceptibility to deterioration is attributed to the structural instability of the oxides.
Li[Ni0.2Li0.2Mn0.6]O2 nanoparticles were prepared via the general combustion method using a dispersing agent to control the particle size of the cathode powder . The prepared Li[Ni0.2Li0.2Mn0.6]O2 nanoparticles were composited with graphene via electrostatic interactions. The commercial graphene (AMG Graphite/Graphit Kropfmühl) was immersed into the 1-N acidic solution (nitric acid/sulfuric acid = 3:1) for 2 h for surface modification and washed several times using distilled water. Then the Li[Ni0.2Li0.2Mn0.6]O2 and graphene particles were dispersed in an ethanol-based solution by ultra-sonication; the weight % of the graphene to the Li[Ni0.2Li0.2Mn0.6]O2 was controlled to amounts of 0.5, 2.0, and 5.0. The pH of the solution was adjusted to 4 using buffer solution (pH 4) in order to control the surface potential of the graphene and the Li[Ni0.2Li0.2Mn0.6]O2 particles. After a 10-min stirring for homogeneous mixing, the solution was vacuum filtered and the resulting samples were dried at 200 °C for 2 h under an air atmosphere. X-ray diffraction (XRD) measurements were then performed on the samples with a Rigaku X-ray diffractometer using monochromatized Cu-Kα radiation (λ = 1.5406 Å). In addition, the surface of the Li[Ni0.2Li0.2Mn0.6]O2 nanoparticle/graphene composite was examined using a transmission electron microscope (TEM, JEOL-4010) and X-ray photoelectron spectroscopy (XPS, PHI 5000 VersaProbe, Ulvac-PHI). The C content of the composites was determined via thermogravimetric analysis (TGA, Q500 V20.13 Build 39) by heating from 200 to 750 °C under an air atmosphere.
For electrochemical testing, a cathode slurry was prepared by mixing Li[Ni0.2Li0.2Mn0.6]O2 nanoparticle/graphene composite (or pristine Li[Ni0.2Li0.2Mn0.6]O2 nanoparticles) and carbon black (Super P) with polyvinylidene fluoride (PVDF) in a weight ratio of 80 (cathode):10 (super P):10(PVDF). After 24 h of ball-mill processing, the viscous slurry was coated onto an Al foil using a doctor blade and subsequently dried at 90 °C in an oven. A coin-type cell (2032) consisting of a cathode, Li-metal anode, separator (25 μm, SK Innovation), and an electrolyte (1 M LiPF6 in EC/DMC (1:1 vol%)) was used. The cells were subjected to galvanostatic cycling in the voltage range of 4.8–2.0 V and at various charge-discharge rates, using a WonATech voltammetry system. In addition, impedance measurements were performed by applying an AC voltage at an amplitude of 5 mV and a frequency range of 0.1 Hz to 100 KHz, using an electrochemical workstation (AMETEK, VersaSTAT 3).
To determine the actual graphene content, the results of the TGA of the pristine and composite samples were measured and compared as shown in Fig. 2b. If the composite sample is prepared through a high-temperature heat treatment, then some of the graphene will evaporate during the process. However, our composite samples were dried at 200 °C without heat treatment, and hence the original amount of graphene was maintained during the fabrication process. As Fig. 2b shows, composite 0.5, composite 2.0, and composite 5.0 exhibit weight loss of ~0.5, 2.0, and 5.0 wt.%, respectively, when the samples are heated to 750 °C. This weight loss stems from the evaporation of carbon and is therefore an indicator of the carbon content of the composites.
Discharge capacity and capacity retention of the pristine and composite samples at various current densities
44 mA g−1 (mAh g−1)
110 mA g−1 (mAh g−1)
220 mA g−1 (mAh g−1)
660 mA g−1 (mAh g−1)
1320 mA g−1 (mAh g−1)
Retention rate (%)
Figure 6b shows the results of electrochemical impedance spectroscopy measurements performed prior to the electrochemical tests. The Nyquist plots composed of a broad semicircle, which may be overlapped two semicircles. Generally, a semicircle located in high-frequency range represents the impedance due to a solid electrolyte interface, and a semicircle in relatively low-frequency range represents the charge-transfer resistance at the electrode/electrolyte interface [9, 30]. The size of the semicircle is dependent upon the impedance value of the cell. As shown in Fig. 6b, the semicircles associated with the composites had smaller diameters than the semicircle corresponding to the pristine sample. This indicates that graphene-containing composites are effective in reducing the impedance value of the Li[Ni0.2Li0.2Mn0.6]O2 cathode. Furthermore, the enhanced rate capability (Fig. 6a) of the composites results from this reduced impedance. The impedance value of the composite 5.0 was somewhat higher than composite 2.0, which may due to large amount of graphene. Too much graphene can block Li+ transport between liquid electrolyte and cathode surface since Li+ cannot penetrate through the graphene layer.
Li[Ni0.2Li0.2Mn0.6]O2 nanoparticles were successfully composited on the surface of graphene using electrostatic interaction without a high-temperature heat-treatment process. Carbon/Li-rich oxide composites (prepared via a heat-treatment process) are, in general, susceptible to phase changes and deterioration of the phase integrity; this deterioration results from the reaction of carbon with oxygen and consequent loss of oxygen from the structure of the vulnerable Li-rich oxide phase. However, we successfully fabricated graphene/Li[Ni0.2Li0.2Mn0.6]O2 composites using electrostatic interaction without deterioration of the phase integrity of the oxides, as confirmed by XRD and XPS analysis. The optimized graphene/Li[Ni0.2Li0.2Mn0.6]O2 composites (composite 2.0) exhibited higher discharge capacity and improved rate capability compared with those of pristine Li[Ni0.2Li0.2Mn0.6]O2. This improvement is attributed to the high conductivity of graphene, which compensates for the low conductivity of the pristine Li[Ni0.2Li0.2Mn0.6]O2.
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (No.2014R1A2A2A01003542) and by the National Strategic R&D Program for Industrial Technology (10043868), funded by the Ministry of Trade, Industry and Energy (MOTIE).
MH performed the synthesis and characterization in this study. YJ gave advice and guided the experiment. Both authors read and approved the final manuscript.
The authors declare that they have no competing financial interests.
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