Synthesis and electrochemical properties of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ cathode material for lithium-ion battery

Abstract

The layered Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ lithium-rich manganese-based solid solution cathode material has been synthesized by a simple solid-state method. The as-prepared material has a typical layered structure with R-3m and C2/m space group. The synthesized Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ has an irregular shape with the size range from 200 to 500 nm, and the primary particle of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ has regular sphere morphology with a diameter of 320 nm. Electrochemical performances also have been investigated. The results show that the cathode material Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ prepared at 900 °C for 12 h has a good electrochemical performance, which can deliver a high initial discharge capacity of 233.5, 214.2, 199.3, and 168.1 mAh g⁻¹ at 0.1, 0.2, 0.5, and 1 C, respectively. After 50 cycles, the capacity retains 178.0, 166.3, 162.1, and 155.9 mAh g⁻¹ at 0.1, 0.2, 0.5, and 1 C, respectively. The results indicate that the simple method has a great potential in synthesizing manganese-based cathode materials for Li-ion batteries.

Introduction

Recently, lithium-ion batteries have been considered as the dominant power sources for large-scale applications, such as electric vehicles (EV) and hybrid electric vehicles (HEV) [1–3]. While conventional cathode materials, such as olivine LiFePO₄ [4], spinel LiMn₂O₄ [5], and layered LiMO₂ (M = Ni, Co, Mn, etc.) [6–8], cannot meet the increasing demands for high-energy and high-power batteries due to limited theoretical capacity. Searching for high-capacity and safe alternatives has become one of the most important subjects. The material system based on the layered-layered compositions of xLi₂MnO₃ · (1−x)LiMO₂ (M = Ni, Mn, etc.) has recently attracted more and more attention due to low cost, environmental friendliness, and higher capacity [9–11]. Lithium-excess layered oxides xLi₂MnO₃ · (1−x)LiMO₂ (M = Ni, Mn, etc.) are composed of the intergrowth of LiMO₂ (R3m) and Li₂MnO₃ (C2/m) phases and can deliver higher capacity (>250 mA h g⁻¹) with relatively high operating voltage (3.5 V versus Li/Li + on average). However, there are several disadvantages such as high first cycle irreversible capacity and inherently low rate capability, which limit its application. Thus, various strategies have been used to improve its electrochemical performances, most of which involves reducing particle size [12, 13] or coating with other materials [14–16] and doping with different elements [10, 17, 18]. Compared with doping and coating methods, reducing particle size is an effective way to improve the properties of as-prepared materials without degrading the intrinsic properties [19]. Meanwhile, synthesis method has significant effects on the morphology and particle size of materials [19, 20]. Solid-state approach is considered to be one of the simplest methods, which has been applied widely in the synthesis of electrode materials [21–24].

During conventional synthesis of Li₂MnO₃-LiMO₂, the raw materials usually have micrometer scale with irregular morphology and mixed by grinding or ball-milling methods. After high-temperature sintering process, the resultant products are usually composed of irregular nano- or microparticles due to undesirable particle growth, although the undesirable particle growth can be partly inhibited by adding growth inhibitors [25, 26]. However, morphology and size control of Li₂MnO₃-LiMO₂ nanostructures remain a great challenge. Herein, we present a simple and feasible synthesis of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ microspheres with nano-sized subunits by a precipitation method followed by a simple solid-state reaction. As the distance for atomic migration is shortened significantly to several nanometers, the undesired particle growth during the annealing is effectively suppressed in the present synthesis. The resultant Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ exhibit a reasonable discharge capacity and fairly good cycling stability.

Experiments and characterization

Preparation of MnCO₃ and MnO₂ microspheres

The MnCO₃ microspheres were prepared by a precipitation method. 7.5 mmol of MnSO₄·H₂O (A.R., Guangdong Guanghua Sci-Tech Co., Ltd.), and 75 mmol of NH₄HCO₃ (A.R., Guangdong Guanghua Sci-Tech Co., Ltd.) was separately dissolved in 500 mL of distilled water. Fifty milliliters of ethanol and the NH₄HCO₃ solution was then added to the MnSO₄·H₂O solution in sequence under stirring. The mixture was kept under stirring for 1 h at room temperature, then centrifuged, and washed with water for several times. The as-obtained MnCO₃ microspheres were dried at 80 °C. The Mn₂O₃ microspheres were synthesized by thermal decomposition of the MnCO₃ microspheres at 400 °C for 5 h at air atmosphere.

Preparation of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂

To obtain Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ microspheres, stoichiometric amounts of Mn₂O₃ microspheres, Ni(NO₃)₂·6H₂O (A.R., Guangdong Guanghua Sci-Tech Co., Ltd.), LiOH·H₂O (A.R., Guangdong Guanghua Sci-Tech Co., Ltd.), and Co(NO₃)₂·6H₂O (A.R., Guangdong Guanghua Sci-Tech Co., Ltd.) were mixed and ball-milled for 4 h at a speed of 300 rpm. The Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ sample was formed by heating the milled precursors at 900 °C for different times in air atmosphere in a muffle furnace.

Material characterization

The crystal structures of the Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ samples were characterized employing an X-ray diffractometer (Rigaku RINT2000) with Cu Ka radiation and recorded between 10 and 80° with a scanning speed of 3° min⁻¹. The morphologies of the precursors and as-prepared materials were observed using scanning electron microscope (SEM) (Hitachi S4700). Transmission electron microscopy studies of the samples were conducted using a JEM-2100F transmission electron microscope operated at 200 kV.

The electrochemical tests were performed with CR2032 coin-type cells. To fabricate the cathodes, 80 wt% active material, 10 wt% Super P, and 10 wt% polyvinylidene difluoride (PVDF) binder in N-methylpyrrolidone (NMP) solution were mixed homogeneously. The resulting slurry was coated on aluminum foil, dried at 120 °C, and pressed with 10 MPa. The resulting cathodes had an active material loading of about 2.5∼2.7 mg cm⁻². The cells were assembled in an argon-filled box with the moisture content and oxygen levels less than 5 ppm. A metallic lithium foil served as counter and reference electrodes. The electrolyte was 1 M LiPF₆ in a mixture of ethylene carbonate and dimethyl carbonate (1:1 in volume). Galvanostatic charge and discharge tests were performed between 2.0 and 4.8 V at room temperature on a battery test system (Neware BTS-5V5mA, China). Cyclic voltammetry tests were carried out at a scanning rate of 0.1 mV s⁻¹ between 2.0 and 4.8 V on an electrochemical workstation (ChenHua CHI1040B, China). The electrochemical impedance spectroscopy (EIS) analysis of the fresh cell with the open circuit voltage of the cell about 3.0 V was carried out by applying an AC voltage of 5 mV over the frequency range from 100 kHz to 10 MHz on an electrochemical workstation (Gamery PC14-750, USA). All electrical measurements have been performed at room temperature.

Results and discussion

Figure 1 shows the XRD patterns of the synthesized microsphere MnCO₃ and Mn₂O₃ precursor. As shown in Fig. 1a, the diffraction peaks of the precursor MnCO₃ are matched well with MnCO₃ (JCPDF No. 44-1472) and no impurity is detected, which indicates the MnCO₃ microspheres were prepared by a precipitation method. As shown in Fig. 1b, the diffraction peaks of the precursor Mn₂O₃ are matched well with Mn₂O₃ (JCPDF No. 71-0636), which indicates that the Mn₂O₃ microspheres were synthesized by thermal decomposition of the MnCO₃ microsphere at 400 °C for 5 h. Thus, the MnCO₃ microspheres are converted into Mn₂O₃ by thermal decomposition at 400 °C according to 4MnCO₃ + O₂→2Mn₂O₃ + 4CO₂. Besides, Fig. 1b shows that a little impurity phases are indexed as Mn₃O₄ (JCPDF No. 75-0765), which responds with the side reaction 6MnCO₃ + 3O₂→3Mn₃O₄ + 6CO₂.

Fig. 1

XRD patterns of the MnCO₃ (a) and Mn₂O₃ (b) and cathode material Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ synthesized at 900° C for different time

In Fig. 2, the SEM images illustrate the morphology of the precursor MnCO₃ and Mn₂O₃. As shown in Fig. 2a, c, the precursors MnCO₃ exhibited spherical morphology with the size of about 3–6 μm. It is clearly observed from Fig. 2c that the spherical MnCO₃ particles have a rough surface and are composed of well-crystalized nanoparticles. Figure 2b, d show that precursor Mn₂O₃ obtained by thermal decomposition of spherical MnCO₃ particles retains the microsphere structure with the size of about 6–7 μm. Thus, the use of the pre-grown MnCO₃ as the precursor allows for the shape control of the resultant Li_1.2Mn_0.54Ni_0.13Co_0.13O₂.

Fig. 2

SEM images of the precursors MnCO₃ (a, c) and Mn₂O₃ (b, d)

Figure 3 shows the diffraction peaks of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ synthesized at 900 °C for various times. All the diffraction peaks are indexed by two-phase system consisting of rhombohedral LiMO₂ (R-3m, a-NaFeO₂ structure) and layered monoclinic Li₂MnO₃ (C2/m). The results were in good agreement with the previous reports [27]. The reflection peaks between 20° and 25° originated from the ordering of lithium ions with transition metal ions in the transition metal layers, corresponding to Li₂MnO₃ phase. Both the (006)/(102) and (108)/(110) doublets are clearly separated, indicating that the well-crystallized layered structures have been formed.

Fig. 3

XRD patterns and Rietveld refinement of cathode material Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ synthesized at 900° C for different time

The XRD patterns have been refined by the Rietveld method with General Structure Analysis Software (GSAS Los Alamos National Laboratory, USA) using the approach outlined in Ref. [28]. The refined lattice parameters are listed in Table 1. Because the crystallographic sites of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ are shared by more than two cations, it is hard to obtain reliable results on atomic occupancies from the Rietveld refinement. As shown in Ref. [28], the XRD diffraction patterns are refined by the sets of diffraction data: aNaFeO₂ type with R-3m and monoclinic Li₂MnO₃ type with C2/m. As listed in Table 1, the cell parameters firstly increase from t = 8 h and then reach the maximum till t = 12 h and finally decrease with the increasing of calcination time. The increasing lattice parameters can enhance lithium-ion migration in the crystal lattice, which improve its electrochemical performances. These results indicate that the sample synthesized at 900 °C for 12 h has a better electrochemical properties.

Table 1 Lattice parameters from the Rietveld refinement of Li_1.2Mn_0.53Ni_0.13Co_0.13O₂ synthesized at 900° C for different time

In Fig. 4, the SEM images illustrate the morphology of cathode material Li_1.2Mn_0.54Ni_0.13Co_0.13O₂. As shown in Fig. 4, the as-prepared Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ is composed of well-crystallized particles with size range from 200 to 500 nm. These nanoparticles have irregular shape. As the calcining time increases, the particles grow up and aggregate more seriously. In order to investigate the detailed morphology, transmission electron microscopy studies on the material synthesized at 900 °C for 12 h are shown in Fig. 5. As shown in Fig. 5a, the micrograph shows that the primary particle of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ has regular sphere morphology with a diameter of 320 nm. The Fig. 5b shows that the particles are high crystalline, and the lattice fringes are found to be about 0.204 nm, corresponding to the d104 and d202 spacing in the XRD patterns of rhombohedral LiMO₂ (R-3m) and layered monoclinic Li₂MnO₃ (C2/m), respectively. Furthermore, as shown in the Fig. 5c, the Fourier transform images exhibit spot patterns suggesting highly crystalline structures, which are in accordance with XRD results.

Fig. 4

SEM images of cathode material Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ synthesized at 900° C for different time: 8 h (a, e), 10 h (b, f), 12 h (c, g), and 14 h (d, h)

Fig. 5

TEM (a), HRTEM (b), and Fourier transform (c) images of cathode material Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ synthesized at 900° C for 12 h

Figure 6 exhibits the charge-discharge curves of the Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ synthesized at 900 °C for various times operated at 0.1 C (1 C = 180 mA g⁻¹). Similar charge-discharge profiles can be observed in Fig. 6. All the initial charge curves clearly displayed two plateaus located at 3.7–4.5 V and above 4.5 V. The first plateau is ascribed to lithium ion extraction from the layered LiCoO₂ and LiNiO₂ structure [28]. The second plateau could be attributed to the oxygen loss process from the Li₂MnO₃ component, which results in high irreversible capacity of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ electrode [28]. The initial discharge capacities are 181.4, 213.5, 233.6, and 229.4 mAh g⁻¹ for 8, 10, 12, and 14 h, respectively. After 10 cycles, the discharge capacities retain 143.3, 167.4, 214.3, 189.8 mAh g⁻¹, respectively. The capacity retentions are 78.4, 79.2, 91.7, and 82.7 %, respectively. The results indicate the sample synthesized at 900 °C for 12 h has the best electrochemical performances.

Fig. 6

The charge/discharge curves of cathode material Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ synthesized at 900° C for different time. The charge rate for fixed at a current of 0.1 C

To further investigate the electrochemical behaviors of the Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ synthesized at 900 °C for various time, cyclic voltammetry (CV) was conducted on the electrode at the scan rate of 0.1 mV s⁻¹ between 2.0 and 4.8 V and the results are shown in Fig. 7. The two oxidation peaks at 4.06 and 4.75 V in the first cycle corresponded to the oxidation of the Ni²⁺ and Co³⁺ ions and the release of oxygen, respectively. The reduction peaks at 4.45 and 3.79 V in the first cycle could be ascribed to the reduction of Ni⁴⁺ and Co⁴⁺ ions. In the second and third cycles, the CV profiles are significantly different from that observed in the first cycle. A new weak oxidation peak emerges at 4.6 V with the disappearance of the strong oxidation peak at 4.7 V. The two oxidation peaks at 3.9 and 4.6 V could be attributed to the oxidation of Ni²⁺ and Co³⁺, respectively. The negative shift of Mn⁴⁺/Mn³⁺ reduction peak with cycles evidenced the structure transformation from layered to a layered-spinel intergrowth structure [29].

Fig. 7

Cyclic voltammetry curves of cathode material Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ synthesized at 900° C for different time

Furthermore, electrochemical impedance spectroscopy (EIS) is used to analyze the electrochemical properties of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ synthesized at 900 °C for various time. As shown in Fig. 8a, the Nyquist plots of the cells consist of barely visible semicircles alike in appearance of a half ellipse (in the high frequency ranges) and a straight line with changing slope to the real axes (in the lower frequency range). The study of the EIS results has been performed by using the approach outlined in Ref. [19, 30, 31] by using an equivalent circuit model shown in Fig. 8b. The fitting results derived from the equivalent circuit are presented in Table 2. Generally, an intercept at the Zreal axis at a high frequency corresponds to the ohmic resistance (Rs), which represents the total resistance of the electrolyte, separator, and electrical contacts. The depressed semicircle in the high frequency range indicates the charge transfer resistance (Rct). The inclined line in the lower frequency range represents the Warburg impedance and corresponds to the lithium diffusion kinetics towards the electrodes. As shown in Table 2, the electrolyte resistance remained almost constant, which was expected since the variation of an electrolyte concentration was not so large as to affect the electrolyte conductivity. The transfer resistance (Rct) of the sample synthesized at 12 h is smaller than the other ones, indicating that the Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ synthesized at 900 °C 12 h has better electrochemical performances. These results are in accordance with above results.

Fig. 8

Nyquist plots (a) and the equivalent electric circuit (b) of cathode material Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ synthesized at 900° C for different time

Table 2 Impedance parameters derived from the equivalent circuit for the cathode material Li_1.2Mn_0.53Ni_0.13Co_0.13O₂ synthesized at 900° C for different time

The initial charge-discharge curve and cyclic performance of the sample prepared at 900 °C for 12 h at the rate of 0.1, 0.2, 0.5, and 1 C for 50 cycles are presented in Fig. 9a, b, respectively. As shown in Fig. 9a, the sample has the similar shape and delivers discharge capacities of 233.5, 214.2, 199.3, and 168.1 mAh g⁻¹ at 0.1, 0.2, 0.5, and 1 C, respectively. After 50 cycles, the capacity retains 178.0, 166.3, 162.1, and 155.9 mAh g⁻¹ at 0.1, 0.2, 0.5, and 1 C, respectively. The capacity retentions are 76.2, 77.6, 81.3, and 92.7 %, respectively. These results indicate that the simple and feasible morphology and size control synthesis has a great potential in manganese-based electrode materials. However, the high initial irreversible capacity and poor cycle performances especially in high rate still can be observed. It should be ascribed to the irreversible the oxygen loss process and the side reactions of the cathode surface with electrolyte [32, 33]. Further works, such as doping, surface coating, and electrolyte additive, are necessary to improve the electrochemical performance.

Fig. 9

The initial charge/discharge curves and cycle performances of cathode material Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ synthesized at 900° C for 12 h

Conclusions

Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ microspheres have been successfully synthesized by the morphology-controlled solid-state method. XRD, SEM results show well-crystallized Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ microspheres have been obtained. Electrochemical tests show the as-prepared material synthesized at 900 °C for 12 h displays better electrochemical performances. The initial discharge capacities are 233.5, 214.2, 199.3, and 168.1 mAh g⁻¹ at 0.1, 0.2, 0.5, and 1 C, respectively. After 50 cycles, the capacity retains 178.0, 166.3, 162.1, and 155.9 mAh g⁻¹ at 0.1, 0.2, 0.5, and 1 C, respectively. In summary, the results demonstrated that the state-of-art Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ can be synthesized with the unique solid-state method. The simple method could pave the road for the scale-up production of the advanced manganese-based cathode materials for Li-ion batteries.