Electrochemical properties of LiNi0.8Co0.15Al0.05O2–graphene composite as cathode materials for lithium-ion batteries

Electrochemical properties of LiNi0.8Co0.15Al0.05O2–graphene composite as cathode materials for lithium-ion batteries

Journal of Electroanalytical Chemistry 683 (2012) 88–93 Contents lists available at SciVerse ScienceDirect Journal of Electroanalytical Chemistry jo...

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Journal of Electroanalytical Chemistry 683 (2012) 88–93

Contents lists available at SciVerse ScienceDirect

Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem

Electrochemical properties of LiNi0.8Co0.15Al0.05O2–graphene composite as cathode materials for lithium-ion batteries Sukeun Yoon ⇑, Kyu-Nam Jung, Sun-Hwa Yeon, Chang Soo Jin, Kyung-Hee Shin New and Renewable Energy Research Division, Korea Institute of Energy Research, Daejeon 305-343, Republic of Korea

a r t i c l e

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Article history: Received 29 May 2012 Received in revised form 21 July 2012 Accepted 4 August 2012 Available online 16 August 2012 Keywords: Lithium-ion batteries Cathode LiNi0.8Co0.15Al0.05O2 Graphene Composite

a b s t r a c t A LiNi0.8Co0.15Al0.05O2–graphene composite has been synthesized by a high energy mechanical ball milling (HEMM) process. The structural and morphological features of the synthesized samples are characterized by various techniques. Electrochemical studies on the LiNi0.8Co0.15Al0.05O2–graphene composite are conducted using of the galvanostatic charge–discharge process and electrochemical impedance spectroscopy methods by constructing a lithium-ion cell. The LiNi0.8Co0.15Al0.05O2–graphene composite exhibits a high capacity of >180 mA h/g with good cyclability and high rate capability. The improved electrochemical performance is attributed to increased electrical contact provided by the graphene layer, which reduces the cell polarization. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Layered oxides including LiCoO2 have good electrochemical properties and are still used as cathode materials in most commercial lithium-ion batteries. The demand for medium and large size batteries for use in energy storage system and electric vehicles (EVs), however, has motivated research into developing new cathode materials with high specific energy densities [1,2]. Layered LiCoO2 cathode materials are typically charged up to only 4.2 V vs. Li (Li0.5CoO2) due to chemical instability arising from an overlap of the Co:3d band with the top of the O:2p band and consequently they have a limited specific capacity [3]. Accordingly, overcharging is often found to cause significant deterioration to the hexagonal phase transition, which induces extensive defects (micro-cracks) between and within the particles, and potential surface reactions such as cobalt dissolution occur at voltages >4.4 V [4,5]. Among the various possible cathode alternatives that have been pursued, lithium nickel cobalt mixed oxides have been suggested as one of the promising candidates to replace LiCoO2 on the basis of their high capacity and raw material cost [6–8]. Unfortunately, the use of nickel-rich cathodes in practical lithium-ion cells has generally been plagued by severe capacity fading and increased resistance, arising from structural instability [9,10]. To alleviate this problem, numerous research groups have focused on partially substituting Ni with other elements, such as Li, Mg, Al, Ti, Cr, Mn, Fe, and Co to improve the capacity retention and eliminate the ⇑ Corresponding author. Tel.: +82 42 860 3526; fax: +82 42 860 3133. E-mail address: [email protected] (S. Yoon). 1572-6657/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jelechem.2012.08.005

Li2CO3/or LiOH impurities on the surface. The radii of Co3+ and Al3+ are smaller than that of Ni3+ and the substitution of these ions result in the shrinkage of the a-axis, which is considered to be the origin of the layered structure’s stability [11–13]. Additionally, surface modification by coating with metal oxides such as TiO2, SiO2, and Al2O3 has been shown to enhance the cyclability and diminish side reactions between the electrode and electrolyte [14]. In this study, we present a LiNi0.8Co0.15Al0.05O2–graphene composite cathode that offers the following advantages: (i) dispersion of LiNi0.8Co0.15Al0.05O2 on the graphene surface, which acts as an electronic conductive matrix; and (ii) high structural stability at high temperature. The graphene has been gaining attention for energy storage applications due to its high conductivity, light weight, high mechanical strength, and structural flexibility [15,16]. Recent work has exhibited the improved specific capacities and rate capabilities of metal oxide nanomaterials, such as SnO2, Mn3O4, Co3O4, and Fe3O4, grown on graphene/or wrapped by graphene as anode materials as well as olivine-type cathode active materials for lithium-ion batteries [17–22]. The LiNi0.8Co0.15Al0.05O2–graphene composite in this work is prepared by a simple high energy mechanical milling (HEMM) technique. We demonstrate origin of the enhanced electrochemical performance using various characterizations and electrochemical measurements including an impedance analysis. 2. Experimental The LiNi0.8Co0.15Al0.05O2–graphene composite was prepared by HEMM technique. In a typical experiment, 200 mg of graphene

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Fig. 1. XRD patterns of LiNi0.8Co0.15Al0.05O2 and the LiNi0.8Co0.15Al0.05O2–graphene composite.

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Fig. 3. DCPs of LiNi0.8Co0.15Al0.05O2 and the LiNi0.8Co0.15Al0.05O2–graphene composite at the 2nd cycle.

Fig. 4. Lithium diffusivities of LiNi0.8Co0.15Al0.05O2 as a function of the cell potential during the 2nd cycle, which were determined by GITT (inset) during Li deintercalation (charging).

Fig. 2. SEM images of: (a) LiNi0.8Co0.15Al0.05O2 and (b) the LiNi0.8Co0.15Al0.05O2– graphene composite. The inset indicates EDS images of Ni, Al, Co, and O elements.

nanoplatelets (xGnP-M-15, XG Sciences) and LiNi0.8Co0.15Al0.05O2 (1.8 g; CA-NCA021, ECOPRO) were stirred in 20 mL isopropyl alcohol for 1 h and sonicated for 2 h. The resultant suspension was then filtered and dried in a vacuum oven at 80 °C. For preparation of homogeneously mixed LiNi0.8Co0.15Al0.05O2–graphene composite, the powders were put into a zirconium oxide vial with capacity of 250 mL with a ball-to-powder ratio of 10:1. The vial, which was assembled in an argon-filled glove box, was installed in a planetary mill (Fritsch P6), and the HEMM reaction was performed for 30 min at a rotation speed of 200 rpm. Phase analysis of LiNi0.8Co0.15Al0.05O2 and the LiNi0.8Co0.15Al0.05O2–graphene composite samples was performed with a Bruker X-ray diffractometer employing Cu Ka radiation and the morphol-

ogy was examined with a Hitachi S-4000 scanning electron microscope (SEM) in conjunction with energy dispersive X-ray spectroscopy (EDS). The electrodes for the electrochemical evaluation were prepared by mixing 85 wt.% active material powder, 7 wt.% carbon black (Super C65) as a conducting agent, and 8 wt.% polyvinylidene fluoride (PVDF) dissolved in N-methylpyrrolidone (NMP) as a binder to form a slurry, followed by coating on an aluminum foil, pressing, and drying at 80 °C for 2 h under vacuum. The CR2032 coin cells were assembled in an Ar-filled glove box using Celgard polypropylene as a separator, lithium foil as the counter electrode, and 1 M LiPF6 in ethylene carbonate (EC)/diethyl carbonate (DEC) (1:1 v/v) as the electrolyte. The charge–discharge experiments were performed galvanostatically at a constant current of 55.6 mA/g of active material within the voltage range of 4.3–3.0 V vs. Li+/Li. With an aim to understand the evolution of lithium diffusivity as a function of cell potential, the galvanostatic intermittent titration technique (GITT) was employed. A current pulse of 8.34 mA/g was applied for 40 min to take the closed-circuit voltage (CCV) and turned off for 10 h to obtain the quasiopen-circuit voltage (QOCV). The sequential current pulse was applied for both discharge and charge period in the range of 3–4.3 V vs. Li+/Li. The electrochemical impedance spectroscopic analysis (EIS) was carried out with a Zahner Zennium instrument by applying a 10 mV amplitude signal in the frequency range of 10 kHz– 0.02 Hz. In the EIS measurements, the LiNi0.8Co0.15Al0.05O2–graphene composite with an active material content of 8 mg served as the working electrode and lithium foil served as the counter and reference electrodes. The impedance response was measured after 30 cycles.

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Fig. 5. Discharge profiles illustrating the rate capabilities of: (a) LiNi0.8Co0.15Al0.05O2 and (b) the LiNi0.8Co0.15Al0.05O2–graphene composite at the 2nd and 20th cycles.

3. Results and discussion The X-ray diffraction patterns of LiNi0.8Co0.15Al0.05O2 and the LiNi0.8Co0.15Al0.05O2–graphene composite are shown in Fig. 1. All of the reflections of the LiNi0.8Co0.15Al0.05O2–graphene composite could be indexed based on the hexagonal structure of a-NaFeO2 (space group R3M) with the exception of an additional peak at around 26° attributed to the (0 0 2) of the hexagonal graphite, and no impurity phase is detected. The SEM images shown in Fig. 2a reveal an average aggregated LiNi0.8Co0.15Al0.05O2 particle size of 7 lm and a homogeneous distribution of Ni, Co, Al, and O elements. Each nanoparticle has an average size of 500 nm. On the other hands, as seen in Fig. 2b, the LiNi0.8Co0.15Al0.05O2–graphene composite presents that the particle is embedded in the graphene sheet. Fig. 3 presents differential capacity plots (DCPs) of LiNi0.8Co0.15Al0.05O2 and the LiNi0.8Co0.15Al0.05O2–graphene composite at the 2nd cycle. The DCPs of LiNi0.8Co0.15Al0.05O2 display features characteristic of voltage plateaus during charge process at 3.65, 3.73, 3.98, and 4.2 V, associated with complete oxidation. The main cathodic peaks at 3.7, 3.96, and 4.17 V during the discharge process are meanwhile ascribed to the reduction reaction. The major anodic/cathodic peaks at 3.65/3.7 V correspond to the oxidation/ reduction process of Ni3+/Ni4+. The anodic peak at 3.65 V has a shoulder at 3.73 V, which is a two-phase region corresponding to the phase transition of the hexagonal (h1) to monoclinic. Also

Fig. 6. Comparison of rate capabilities and rate capability retentions of LiNi0.8Co0.15Al0.05O2 and the LiNi0.8Co0.15Al0.05O2–graphene composite: (a) normalized discharge capacity at the 2nd cycle, (b) normalized discharge capacity at the 20th cycle, and (c) rate capability retention.

the anodic/cathodic peaks at 3.98/3.96 V and 4.2/4.17 V are assigned to the phase transition of the monoclinic to hexagonal (h2) and hexagonal (h2) to hexagonal (h3), respectively [23]. On the other hand, the DCPs of LiNi0.8Co0.15Al0.05O2–graphene exhibit the voltage plateaus that are almost constant excluding the main anodic peaks at 3.6 V, due to significantly reduced resistance by conductive graphene. Fig. 4 shows the lithium diffusivity as a function of cell potential of LiNi0.8Co0.15Al0.05O2 during the charge process. The lithium diffusivity of the 2nd cycle is between 1  109 and 9  1011 cm2/s. The GITT, as shown in inset of Fig. 4, was employed to investigate the evolution of lithium diffusivity as a function of cell potential. Weppner and Huggins derived a simple expression for lithium diffusivity in the electrode as given below [24]:

DLi ¼

 2 4L2 DEs ps DEs

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Fig. 7. Relationship between voltage and current of LiNi0.8Co0.15Al0.05O2 and the LiNi0.8Co0.15Al0.05O2–graphene composite at different DOD.

where L refers to the electrode thickness, s refers to the interval time of the current pulse (40 min), and DEs and DEs denote the voltage changes during, respectively, the applied current pulse and the turned off current pulse. The ideal density for LiNi0.8Co0.15Al0.05O2 was used in implementing this equation. Rate capability of electrode materials for lithium-ion batteries is an important issue in assessing their potential for high power applications. The rate capabilities of the LiNi0.8Co0.15Al0.05O2 and the LiNi0.8Co0.15Al0.05O2–graphene composite samples were assessed at the 2nd and 20th cycles. Fig. 5 compares the discharge profiles of LiNi0.8Co0.15Al0.05O2 and the LiNi0.8Co0.15Al0.05O2–graphene composite at various C-rates. To illustrate the differences between the LiNi0.8Co0.15Al0.05O2 and LiNi0.8Co0.15Al0.05O2–graphene composite samples, we normalized the discharge capacity values at various C-rates to the discharge capacity value at a C/5 rate (55.6 mA/g) and plotted the results in Fig. 6a and b for LiNi0.8Co0.15Al0.05O2 and the LiNi0.8Co0.15Al0.05O2–graphene composite, respectively. As shown in Fig. 6, the LiNi0.8Co0.15Al0.05O2–graphene composite sample exhibits considerably better rate capabilities than LiNi0.8Co0.15Al0.05O2 sample at the 20th cycle. A comparison of the data in Fig. 6a and b shows that the difference in rate capability between LiNi0.8Co0.15Al0.05O2 and the LiNi0.8Co0.15Al0.05O2– graphene composite becomes increasingly significant as increase of cycle. For simplicity, we present the change in rate capability upon cycling as the rate capability retention in Fig. 6c. The rate capability retention is defined as the ability to retain the rate capability upon cycling, and the rate capability retention values are calculated as the percentage ratio of the rate capability measured at the 20th cycle to that measured at the 2nd cycle. The data in Fig. 6c clearly demonstrate that the rate capability retention increases significantly for the LiNi0.8Co0.15Al0.05O2– graphene composite. Polarization resistance can play a meaningful role in the rate capability, as has been mentioned in the case of olivine materials [25,26]. To develop a relation between the rate capability and total polarization resistance (Rp), the discharge profiles (see Fig. 5) were

analyzed further for LiNi0.8Co0.15Al0.05O2 and the LiNi0.8Co0.15Al0.05O2–graphene composite. The voltage vs. mass current plots given in Fig. 7 indicate a linear domain range from 10% to 50% depth of discharge (DOD), while the total polarization resistance values can be extracted from the slopes of these curves. The total polarization resistance can be obtained by the polarization resistances of the active material cathode and lithium foil anode, the separator resistance, and the electrolyte resistance. Fig. 8 presents the total polarization resistance (Rp) vs. depth of discharge (DOD) for LiNi0.8Co0.15Al0.05O2 and the LiNi0.8Co0.15Al0.05O2–graphene composite at the 2nd and 20th cycles. The Rp values shown in Fig. 8a and b reduce with increasing DOD (until 50%), indicating that the kinetics of the discharge reaction becomes favorable. The increase in Rp proceeding from the 2nd to 20th cycle, which is termed DRp, is plotted vs. DOD in Fig. 8c. DRp of the LiNi0.8Co0.15Al0.05O2–graphene composite is less than that of LiNi0.8Co0.15Al0.05O2, thereby indicating that the surface chemical stability and electronic conductivity were increased. This is also confirmed by the rate capability retention data in Fig. 6c. In general, the total polarization resistance consists of ohmic resistance, activation resistance, and diffusion resistance. They can be more clearly indentified in electrochemical impedance studies. Fig. 9a shows the discharge capacity of LiNi0.8Co0.15Al0.05O2 and the LiNi0.8Co0.15Al0.05O2–graphene composite between 4.3 and 3 V at a constant current of 55.6 mA/g. While LiNi0.8Co0.15Al0.05O2 delivers a capacity of 172 mA h/g with a capacity retention of 91%, the LiNi0.8Co0.15Al0.05O2–graphene composite shows a capacity of 180 mA h/g with a capacity retention of 97% after 80 cycles at 25 °C. Accordingly, at 55 °C in the lithium-ion cell, the LiNi0.8Co0.15Al0.05O2–graphene composite exhibits stable capacity retention. Fig. 9b compares the rate capabilities at various C-rates (i.e., 1C = 278 mA/g). The LiNi0.8Co0.15Al0.05O2–graphene composite shows a stable capacity even at a high C-rate, where it retains a high capacity of 152 and 112 mA h/g, respectively, at 10 C and 20 C with stable cycling, which is approximately 2 times higher

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Fig. 9. Comparison of: (a) discharge capacity vs. cycle number at 25 °C and 55 °C and (b) capacity retentions at various C-rates of LiNi0.8Co0.15Al0.05O2 and the LiNi0.8Co0.15Al0.05O2–graphene composite.

Fig. 8. Variations of the polarization resistance (Rp) with DOD for LiNi0.8Co0.15Al0.05O2 and the LiNi0.8Co0.15Al0.05O2–graphene composite at: (a) the 2nd cycle and (b) the 20th cycle. (c) Variations of DRp (increase in DRp on going from the 2nd to 20th cycle) with DOD for LiNi0.8Co0.15Al0.05O2 and the LiNi0.8Co0.15Al0.05O2– graphene composite. Fig. 10. Electrochemical impedance spectra (EIS) of LiNi0.8Co0.15Al0.05O2 and the LiNi0.8Co0.15Al0.05O2–graphene composite after 30 cycles. The inset shows an equivalent circuit.

than the value for the LiNi0.8Co0.15Al0.05O2 electrode. The high rate capability of the LiNi0.8Co0.15Al0.05O2–graphene composite is due to the increased electronic conductivity, which reduces the cell polarization. In order to gain further insight into electrochemical performances, EIS measurements were carried out at 3.8 V vs. Li+/Li with LiNi0.8Co0.15Al0.05O2 and the LiNi0.8Co0.15Al0.05O2–graphene composite samples after 30 cycles. Before the EIS measurements, the samples were charged to 50% depth of discharge (DOD) to reach an identical status. The EIS data were analyzed based on the equivalent circuit given in the inset of Fig. 10. The EIS spectra of LiNi0.8Co0.15Al0.05O2 consist of three semicircles and a line. The small diameters of the first and second semicircles (at the high frequency region) are a measure of the native film resistance [27] and the surface layer resistance, respectively, which are ascribed to the formation of a complex native passivation film on the particle surface and

lithium-ion diffusion through the surface layer. The diameter of the second semicircle (at the medium–low frequency region) is a measure of the charge transfer resistance, which is related to the contact between the particles or between the electrode and the electrolyte [20]. After 30 cycles LiNi0.8Co0.15Al0.05O2 shows native film resistance of 6.2 X, surface resistance of 11.8 X, and charge transfer resistance of 21.2 X. The EIS spectra of the LiNi0.8Co0.15Al0.05O2– graphene composite recorded after 30 cycles, on the other hand, consist of two semicircles and a line. The small diameter of the first semicircle (at the high frequency region) is a measure of the surface layer resistance and the diameter of the second semicircle (at the medium–low frequency region) is a measure of the charge transfer resistance. The sloping line is related to lithium-ion diffusion in the

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bulk of the active material. The LiNi0.8Co0.15Al0.05O2–graphene composite show surface resistance of 7.7 X and a charge transfer resistance of 16.8 X. The diameters of both the surface resistance and charge transfer resistance semicircles are smaller than those of LiNi0.8Co0.15Al0.05O2 due to the graphene material, which has high electrical conductivity.

4. Conclusion We have successfully synthesized a LiNi0.8Co0.15Al0.05O2–graphene composite using a simple HEMM process. We have demonstrated that good rate performance of LiNi0.8Co0.15Al0.05O2 based lithium-ion batteries could be achieved by material design. The discharge capacity of the LiNi0.8Co0.15Al0.05O2–graphene composite retained 152 and 112 mA h/g, respectively, at 10 C and 20 C with stable cycling, which is nearly 2 times higher than the value for the LiNi0.8Co0.15Al0.05O2 electrode. The graphene provides high electrical conductivity along the surface, which leads to enhanced rate capability. The LiNi0.8Co0.15Al0.05O2–graphene composite shows considerable promise as a candidate for high performance lithium-ion batteries.

Acknowledgments This work was supported by Korea Institute for Advancement of Technology (KIAT) and the Internal Research Program (Secondary Battery Research) of Korea Institute of Energy Research (KIER) Granted by Korea government Ministry of Knowledge Economy (MKE).

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