Abstract
Zn-doped LiNi1/3Co1/3Mn1/3O2 composite, Li(Ni1/3Co1/3Mn1/3)1–xZnxO2 (x = 0.02; 0.05; 0.08), is synthesized by the sol-gel method. The crystal structure, morphology, and electrochemical performance are investigated via X-ray diffraction (XRD), scanning electron microscope (SEM), cyclic voltammetry (CV), and constant current charge/discharge experiment. The result reveals that Zn-doping cathode material can reach the initial charge/discharge capacity of 188.8/162.9 mAh·g−1 for Li(Ni1/3Co1/3Mn1/3)0.98Zn0.02O2 and 179.0/154.1 mAh·g−1 for Li(Ni1/3Co1/3Mn1/3)0.95Zn0.05O2 with the high voltage of 4.4 V at 0.1 C. Furthermore, the capacity retention of Li(Ni1/3Co1/3Mn1/3)0.98Zn0.02O2 is 95.1% at 0.5 C after 50 cycles at room temperature. The improved electrochemical properties of Zn-doped LiNi1/3Co1/3Mn1/3O2 are attributed to reduced electrode polarization, enhanced capacity reversibility, and excellent cyclic performance.
1. Introduction
Nowadays, the lithium ion battery as a fairly new member in the battery technology has been widely studied due to its promising applications in the field of vehicle, military, space, and medical devices [1]. Of the reported lithium based cathode materials, the LiCoO2 has excellent discharge capacity and cyclic performance and is still the most popular candidate for the lithium ion battery application. However, the toxicity and high cost of LiCoO2 have restricted its further practical utilization. Recently, the LiNi1/3Co1/3Mn1/3O2 has attracted intensive attentions as an ideal alternative cathode material candidate to the traditional LiCoO2 [2]. The layered LiNi1/3Co1/3Mn1/3O2 has a typical hexagonal α-NaFeO2 structure with a space group of R3m. Moreover, the LiNi1/3Co1/3Mn1/3O2 held some prominent advantages, such as high capacity, thermal stability, safety, and low cost [3], which make it as a promising cathode material for high power application, for instance, the electric vehicle and portable device. But there still exist two shortcomings limiting further application of LiNi1/3Co1/3Mn1/3O2 in lithium ion battery.
One is the poor lithium ion diffusion efficiency that would cause low electronic conductivity [4]; another is the severe capacity decay, especially working under the high voltage [5]. Doping the parent LiNi1/3Co1/3Mn1/3O2 with another transit metal has been regarded as an effective approach to improve the electrochemical properties of cathode materials. So far, some transition metals such as La [6] and Ti [7] or nontransition metals such as Al [8] and Mg [9] have been reported as the doping element for the LiNi1/3Co1/3Mn1/3O2. It is found that the doping can avoid the unfavorable cation mixing of active materials under high voltage.
Different from the typical synthesized method, including coprecipitation method [10], solid-state method [11], and hydrothermal method [12], herein, the Zn-doped LiNi1/3Co1/3Mn1/3O2 is prepared by a facile sol-gel method, with purpose to modify the electrochemical properties of this cathode materials. The crystal structure, morphology and electrochemical performance are investigated via X-ray diffraction (XRD), scanning electron microscope (SEM), cyclic voltammetry (CV), and constant current charge/discharge experiment.
2. Experimental
2.1. Preparation of Li(Ni1/3Co1/3Mn1/3)1–xZnxO2 Samples
All chemicals (analytical grade or better) were purchased commercially and used without any further purification. The cathode material was synthesized by sol-gel method. Metal precursors, including Co(CH3COO)2·4H2O, Ni(CH3COO)2·4H2O, Mn(CH3COO)2·4H2O, CH3COOLi·2H2O, and Zn(CH3COO)2·4H2O, were stoichiometrically mixed ((Li+) : (Ni2+ + Co2+ + Mn2+) : (Zn2+) = 1 : (1–x) : (; 0.05; 0.08)) as raw materials; C6H8O7·H2O was used as complexing agent. Firstly, the mixture was dissolved into aqueous solutions and then the C6H8O7·H2O was dropped slowly into the aqueous solution under stirring at 100°C oil bath. The mixtures were kept under vigorous stirring to obtain the gel-like form. The wet gel was dried at 100°C for 24 h to get an amaranth dry gel and then ground into powder. Finally, the powder was preheated at 400°C for 3 h and then calcined at 800°C for 10 h in a muffle.
2.2. Characterization and Electrochemical Measurement
The crystallinity and structure of the samples was characterized by X-ray diffraction (XRD, D-MAY iiA, Japan) with Cu Kα radiation. The morphology of synthesized products was observed on a scanning electron microscopy (SEM, Hitachi, S-3400N, Japan).
The electrochemical characteristics of samples were investigated using CR2430 coin cells assembled in an argon-filled glove box. To prepare the working electrode, the , acetylene black, and polyvinylidene fluoride (PVDF) with a weight ratio of 8 : 1 : 1 were mixed in N-methylpyrrolidone (NMP) to obtain a homogeneous slurry, which was spread on the Al foil. A metallic lithium foil was used as the counter and reference electrode. The celgard 2400 polypropylene porous film served as the separator and electrolyte consisted of a solution of 1 M LiPF6 in ethylene carbonate (EC) and dimethyl carbonate (DMC) (1 : 1 in volume).
The assembled cells were charged and discharged with constant current on the NEWARE battery program-control test system (Shenzhen, China) in a potential range of 2.8–4.4 V (versus Li+/Li) at room temperature (25°C).
The cyclic voltammetry was performed on CHI660C electrochemical workstation (Wuhan, China) between 2.5 and 4.5 V at room temperature, at the scanning rate of 0.1 mVs−1.
3. Results and Discussion
3.1. Characterization of Zn-Doped LiNi1/3Co1/3Mn1/3O2 Composite
The samples were synthesized with different doping content of Zn, so they are referred to as Zn-X. Figure 1 shows the XRD patterns of . All the samples have a typical hexagonal α-NaFeO2 structure (JCPDS card number 50-0653); no diffraction peaks of impurity can be found, revealing that partial substitution of Zn in the Li(Ni1/3Co1/3Mn1/3)O2 cannot change the crystal structure. This result could indicate that the Zn element is totally inserted into the lattice of Li(Ni1/3Co1/3Mn1/3)O2. Moreover, the diffraction patterns show clear splitting of the hexagonal characteristic doublets of (006)/(102) and (108)/(110); this can be ascribed to the layered structure of Li(Ni1/3Co1/3Mn1/3)O2 [13]. Table 1 gives the refined lattice parameters of . The lattice expansions are slightly increased with Zn-doped content, which further illustrates that the Zn2+ ions have doped into the lattice of Li(Ni1/3Co1/3Mn1/3)O2 during the calcined process. Since the metal ions, for example, Ni2+ (0.069 nm), Co3+ (0.0545 nm), and Mn4+ (0.054 nm), have smaller ion radius than that of Zn2+, when they are replaced by the Zn2+ (0.074 nm), Zn2+ ions would enlarge the lattice parameter. Similar results have observed on the effect of Mg doping into lithium nickel cobalt oxides by Pouillerie et al. [14]. Besides, the values of (003)/(104) are ≥1.2, indicating that the samples have low cation mixing [15]. The cation mixing will lead to an increasement of disorder, making an undesirable electrochemical performance of Li(Ni1/3Co1/3Mn1/3)O2, for example, low lithium conductivity, low capacity, and poor cyclic performance [4].
The morphologies of the layered composites before and after Zn modification are shown in Figure 2. It is found that the Zn-doping does not change their morphologies. The bare and modified materials are uniformly distributed with particles size of 200–300 nm, which would facilitate the intercalate and deintercalate process for Li+ and improve ion conductivity and rate performance.
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3.2. Electrochemical Performances
The galvanostatic charge/discharge tests were conducted to study the electrochemical performances. Figure 3 shows the initial charge/discharge curves of the (, 0.02, 0.05, 0.08) with the current rates of 0.1 C between 2.8 V and 4.4 V at room temperature. Increasing the Zn-doping content, the charge/discharge capacities declined slightly, which can be ascribed to the fact that the Zn2+ with larger radius block the Li+ intercalation-deintercalation path or decrease volume concentration of Ni2+ [16]. The charge/discharge capacities are 195.4/168.7, 188.8/162.9, 179.0/154.1, and 177.7/142.3 mAh·g−1 for Zn-0, Zn-2, Zn-5, and Zn-8, respectively. Correspondingly, the irreversible capacities are 26.7, 25.9, 24.9, and 35.4 mAh·g−1, respectively. Though Zn-doping decreases the charge/discharge capacities, Zn-2 and Zn-5 show greater efficiency than Zn-0, owing to the Zn-doping. Therefore, the capacity retention of Li(Ni1/3Co1/3Mn1/3)O2 cathode material at high voltage (4.4 V) is improved.
Cycling performance is one of the significant electrochemical characteristics of lithium ion battery for high-voltage application. Figure 4 shows the relationship of discharge capacities and cycle times at different discharge rates. As shown from Figure 4, owing to the increase of discharge current, the capacity of discharge declines slightly during the cycling process. The initial discharge capacity of Li(Ni1/3Co1/3Mn1/3)0.98Zn0.02O2 is 163.6 mAh·g−1; additionally, the discharge capacities of Li(Ni1/3Co1/3Mn1/3)0.98Zn0.02O2 maintain 157.7 mAh·g−1 at 0.5 C after 20 cycles, and the capacity retention is 96.4%. After 40 cycles, the discharge capacity declines by 7 mAh·g−1, and the capacity retention is 95.7%. At the 50th cycle, the electrode still obtains high discharge capacity of 155.6 mAh·g−1, and the capacity retention is 95.1%. Compared to Li(Ni1/3Co1/3Mn1/3)0.98Zn0.02O2, Li(Ni1/3Co1/3Mn1/3)O2 shows an obvious drop of discharge capacity from 169.2 mAh·g−1 to 158.1 mAh·g−1 at 0.5 C after 50 cycles; then the capacity retention is 93.4%. Furthermore, with the increase of C-rate, the cyclic performance of these two samples has a sharp decline because of the capacity irreversibility. Therefore, the cyclic performance of Li(Ni1/3Co1/3Mn1/3)0.98Zn0.02O2 is enhanced at low C-rate. On the one hand, Zn-doping is beneficial to stabilize the structure of cathode materials and reduces the cation mixing in the electrolyte at high voltage. On the other hand, the stronger the Zn-O bond, the weaker the Li-O bond, which is beneficial to lithium ion migration [17].
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Cyclic voltammogram is carried out to investigate the electrochemical performances of Li(Ni1/3Co1/3Mn1/3)0.98Zn0.02O2 cathode materials. Figure 5 exhibits the cyclic voltammogram of (, 0.02) between 2.5 V and 4.5 V at a scan rate of 0.1 mV/s. As is shown in Figure 5, the curves of Li(Ni1/3Co1/3Mn1/3)0.98Zn0.02O2 are similar to that of pristine LiNi1/3Co1/3Mn1/3O2, without any impure peaks, which indicates that Zn-doping does not change the crystal structure of Li(Ni1/3Co1/3Mn1/3)0.98Zn0.02O2, as the XRD pattern (Figure 1) shows. Furthermore, Zn-doping leads to the potential translation, and the higher the redox potential peaks, the wider the application in the high working potential. The anodic and cathodic peaks center at around 3.882 and 3.681 V for Zn-0 and 3.941 and 3.791 V for Zn-2, respectively, and the voltage differences between the oxidation and reduction are 0.15 V for Zn-2, which is less than that (0.201 V) for Zn-0. Thus, bits of Zn-doping reduce the polarization and enhance the reversibility possibly due to improved order structure of the cathode materials.
4. Conclusion
In this work, (, 0.02, 0.05, 0.08) is synthesized by doping the Li(Ni1/3Co1/3Mn1/3)O2 with Zn via sol-gel route. The Zn-doping does not change the crystal structure and morphology of composite; it can enhance the electrochemical performance of the as cathode material assembled in form of lithium ion battery. With various Zn-doping content, the cathode materials can reach an initial charge/discharge capacity of 188.8/162.9 and 179.0/154.1 mAh·g−1 for Zn-2 and Zn-5, and the Zn-2 has the greatest capacity retention at high voltage of 4.4 V at 0.1 C. Moreover, no obvious capacity decay after 50 cycles at room temperature was observed for the Li(Ni1/3Co1/3Mn1/3)0.98Zn0.02O2. Reduced electrode polarization, enhanced capacity reversibility, and excellent cyclic performance make the be a promising lithium ion cathode material.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.