Scalable synthesis of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂/LiNi_0.5Mn_1.5O₄ sphere composites as stable and high capacity cathodes for Li-ion batteries

Abstract

Li-rich materials have become a very promising kind of cathode materials due to their high specific capacities and working potentials. Herein, we successfully synthesized a high voltage cathode of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ spheres coated with thin LiNi_0.5Mn_1.5O₄ layers via a simple co-precipitation method. The finally obtained composite cathode was examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), clearly confirming that the inner core was completely uniformly coated by the outer shell. X-ray powder diffraction (XRD) and energy dispersive spectrometry (EDS) results show that the products have no other component. The coated composite cathode exhibited a superior rate capacity and stable cycle performance in a voltage range of 2.0–4.8 V. The Li_1.2Mn_0.54Ni_0.13Co_0.13O₂/LiNi_0.5Mn_1.5O₄ microsphere cathode delivers high capacities of 249.5 and 96 mA h g⁻¹ at rates of 0.2C and 5C, respectively.

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1. Introduction

With the problems of fossil energy exhaustion, global warming, and environment pollution, the improvement of our modern society has been hindered. Thus, expanding the use of clean renewable energy resources has gradually become a worldwide topic. Among the renewable energy resources, Li-ion batteries have been the most intensively used for their excellent energy and power density performance. Although Li-ion batteries have been widely used in a variety of applications, they have gradually failed to meet the need of mankind for high power density devices.¹ The key strategy to increase the energy density of a battery is to develop cathode materials with higher capacities or higher operating voltages,^2–4 since anode materials match the demand for high energy density well.^5–8 Layered LiCoO₂, LiNi_1/3Co_1/3Mn_1/3O₂, spinel LiMn₂O₄, and olivine LiFePO₄ have been commercially used in Li-ion batteries,⁹ but all of their available discharge capacities approach their limits (<180 mA h g⁻¹). Among the common candidates for Li-ion batteries, Li-rich layered materials, Li_1+xM_1−xO₂ (M = Ni, Co, Mn), play an important role in cathode materials with high capacities and operating voltages.¹⁰ Thackeray et al. previously reported Li-rich transition metal oxide composite cathodes of xLi₂MnO₃·(1 − x)LiMO₂ with a capacity of about 280 mA h g⁻¹ when charged over 4.5 V.^11,12 The higher capacity of this Li-rich material results from a two-step process: firstly lithium is extracted from the layered transition metal oxide below 4.4 V; after that begins the activation of Li₂MnO₃, forming Li₂O and MnO₂. Further decomposition of Li₂O gives extra Li which provides additional capacity, leaving MnO₂ at the end of the charge.^13,14

Although the layered Li-rich cathodes provide significantly high specific capacities, it is still very difficult to put them into practical applications due to some major unsolved issues, such as (a) phase transformation from a layered to a spinel-layered intergrowth structure which can result in the continuous decay of the discharge plateau during long-term cycling, (b) oxygen release during the initial charge, leading to serious safety problems and especially low initial coulombic efficiency, (c) Ni dissolution accompanied by increases in the oxidation states of the remaining transition metals, at surface regions of Li-rich particles, and (d) deterioration of the material surface caused by side reactions with the electrolyte at high potential.^15–19

To overcome these obstacles, various strategies have been successfully developed.^20–26 Surface modification with metal oxides, fluoride, and phosphates has been proven as an effective method to improve the electrochemical performance of Li-rich layered materials.¹⁵ The inert coating layer can restrict direct contact between the active material and the electrolyte, so the host structure was protected against HF attack in the electrolyte.²² Nevertheless, the thin coating layer is liable to flake off the cathode surfaces during cycling. Moreover, most of the coating layers show poor electron conduction and contribute no electrochemical properties, so the beneficial effect may be contrasted with an increase in battery impedance resulting from the insulating nature of the coating media. Carbon shows good electron conduction during cycling, and Li-ion positive materials exhibit enhanced performance with carbon encapsulation.^20,21 In this work, we successfully designed and fabricated Li-rich layered cathode material Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ coated with LiNi_0.5Mn_1.5O₄, which remains electrochemically active when charged over 4.5 V.²⁷ We expected that the combination of the two materials would produce a synergetic effect. Namely, a high rate capacity from the Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ and an excellent cycling stability at high voltages from the LiNi_0.5Mn_1.5O₂ could be expected.

2. Experimental

2.1 Synthesis of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ microspheres

All the reagents used in the experiments were from Aldrich and were used without any other purification. Li-rich layered cathode Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ microspheres were prepared by a co-precipitation method. MnSO₄·H₂O, NiSO₄·6H₂O and CoSO₄·7H₂O were dissolved in deionized water in a molar ratio of 0.54 : 0.13 : 0.13. After that, a stoichiometric amount of NaHCO₃ solution was added drop by drop. The pH of the mixture was adjusted between 7 and 8 with ammonia. Finally the mixture was kept at 60 °C for 15 h, and this process refers to the growth of the carbonate precursor.^28–30 The obtained Mn_0.54Ni_0.13Co_0.13[CO₃]_0.8 microspheres were filtered, washed, and dried at 80 °C overnight. After that the prepared carbonates were thoroughly mixed with an appropriate amount of LiOH·H₂O and preheated at 500 °C for 5 h, and were finally calcined at 850 °C for 15 h in air with an annealing rate of 5 °C min⁻¹ to obtain Li_1.2Mn_0.54Ni_0.13Co_0.13O₂.

2.2 Synthesis of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂/LiNi_0.5Mn_1.5O₄ composite spheres

Li_1.2Mn_0.54Ni_0.13Co_0.13O₂/LiNi_0.5Mn_1.5O₄ microspheres were fabricated by the hydrolytic action of manganese acetate and nickel acetate. In a typical synthesis, 16 mmol of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂, 1 mmol of Ni(CH₃COO)₂·4H₂O, and 3 mmol of Mn(CH₃COO)₂·4H₂O were added to a mixture of 10 mL of deionized water and 40 mL of ethyl alcohol with stirring overnight, and then 2 mmol of LiCH₃COOH was dissolved in the mixture. The mixture was well distributed and evaporated at 80 °C, then preheated at 450 °C for 3 h and annealed at 780 °C for 10 h under air to obtain the Li_1.2Mn_0.54Ni_0.13Co_0.13O₂/LiNi_0.5Mn_1.5O₄ composite.

2.3 Materials characterization

Phase identification of the samples was performed with a diffractometer (D/Max-3C, Rigaku) using Cu Kα radiation (λ = 1.54178 Å) and a graphite monochromator at 40 kV, 100 mA. The scanning rate was 8° min⁻¹ and the scanning range of the diffraction angle (2θ) was 5° ≤ 2θ ≤ 80°. The morphologies of the samples were observed using scanning electron microscopy (SEM) (JSM-6100LV, JEOL). The local compositions of the cathode composites were obtained with an energy-dispersive X-ray spectroscope (EDXS), while the micro-structural characterizations were observed with transmission electron microscopy (TEM) (JEM-2100).

2.4 Electrochemical test

To fabricate the positive electrodes, the as-prepared materials were mixed with carbon black and polyvinylidene fluoride (PVDF) in a weight ratio of 8 : 1 : 1 in N-methy1-2-pyrrolidone (NMP) as the solvent to make a slurry. The obtained slurry was coated onto Al foil and then dried overnight in vacuum, and each pruned electrode had a composite loading of about 2 mg cm⁻². To fabricate a 2025 coin-type cell, Li metal was used as the counter electrode and a solution of 1 M LiPF₆ in ethylene carbonate (EC)/dimethyl carbonate (DMC) (1 : 1 in volume) was used as the electrolyte. The charge–discharge curves and cycling capacity were tested using a NEWARE-BTS battery test system in a voltage range of 2.0–4.8 V. Electrochemical impedance spectroscopy (EIS) data were recorded using a CHI 660C electrochemical workstation in a frequency range of 200 kHz to 5 mHz with an AC amplitude of 5 mV.

3. Results and discussion

To prepare uniform core–shell structured particles, a second sintering method was used. Scheme 1 shows a schematic drawing of the detailed formation process for the spherical core–shell structure of the Li_1.2Mn_0.54Ni_0.13Co_0.13O₂/LiNi_0.5Mn_1.5O₄ cathode. Firstly, the precursor Mn_0.54Co_0.13Ni_0.13(CO₃)_0.8 was prepared by a co-precipitation method and the estimated average precursor particle diameter is about 3 μm (Fig. S1†). Then, these precursor particles were homogeneously mixed with Li₂CO₃ in an agate mortar, and annealed at 850 °C for 15 h; the pristine Li-rich cathode material Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ was obtained, and the Li-rich cathode particles’ shape still retains the precursor’s spherical structure, with an average diameter of about 4 μm. The aqueous solution containing Ni and Mn was then added to the dispersion solution of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ particles. After that, an appropriate amount of lithium acetate was added, and the coated materials were finally obtained by a second annealing treatment.

Scheme 1 Schematic illustration of the formation of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂/LiNi_0.5Mn_1.5O₄ cathode microspheres. The carbonate was synthesized by heating the mixture of MnSO₄, NiSO₄, CoSO₄ and NaHCO₃ at 60 °C for 15 h. Subsequently, the mixture of the carbonate precursor and Li₂CO₃ was heated at 850 °C for 15 h, and the pristine Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ cathode microspheres were obtained. Finally, the coated Li_1.2Mn_0.54Ni_0.13Co_0.13O₂/LiNi_0.5Mn_1.5O₄ microspheres were obtained by hydrolysis of NiAc₂ and MnAc₂ and subsequent annealing with LiAc at 780 °C for 10 h.

Fig. 1a shows the XRD patterns of the uncoated Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ cathode. All of the diffraction peaks can be indexed to the hexagonal α-NaFeO₂ structure with a space group of R3̄m except for the diffraction peaks between 20° and 25° which are caused by superlattice ordering of the Ni, Co and Mn in the 3a site. The superlattice peaks indicate a layered structure with Li₂MnO₃ character reflecting the degree of cation ordering in the transition metal layers.³¹ The calculated lattice parameters from Rietveld refinement of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ are a = 2.8621 Å and c = 14.2229 Å (Table S1, ESI†). Compared with pure Li_1.2Mn_0.54Ni_0.13Co_0.13O₂, the XRD patterns of the coated cathode (Fig. 1b) show no obvious difference, except for the existence of a small peak in the position 2θ = 71° (marked with *), which belongs to the cubic LiNi_0.5Mn_1.5O₄ (according to the standard card JCPDS: 32-0581). The second annealing was performed to acquire the outer layer, leading to mature superlattice peaks (illustrated in Fig. 1). The XRD patterns of the co-precipitated carbonate precursor were also detected (Fig. S2, ESI†). All the samples can be indexed to a typical hexagonal structure with a space group of R3̄c corresponding to MnCO₃ (JCPDS no. 44-1472).³² The diffraction peaks are broadened, which is probably due to the spherical secondary particle consisting of nanoscale sized primary particles.^33,34

Fig. 1 XRD patterns of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ microspheres (a) and Li_1.2Mn_0.54Ni_0.13Co_0.13O₂/LiNi_0.5Mn_1.5O₄ microspheres (b). The standard card of pure LiMn_1.5Ni_0.5O₄ is also exhibited in (c). The insets of (a) and (b) show the superlattices between 20° and 25° of the pristine and coated Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ cathode materials, respectively.

To further investigate the microstructure of the uncoated Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ and coated Li_1.2Mn_0.54Ni_0.13Co_0.13O₂/LiNi_0.5Mn_1.5O₄ cathode materials, TEM and HRTEM examinations were conducted. Fig. 2a and d show the low-magnification images of the uncoated Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ and coated Li_1.2Mn_0.54Ni_0.13Co_0.13O₂/LiNi_0.5Mn_1.5O₄ composites, which indicate that the products remain spherical and the diameter of the microsphere is about 4 μm. As marked in Fig. 2d, the outer shell of LiNi_0.5Mn_1.5O₄ adheres to the inner core of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ uniformly with an average thickness of about 300 nm. Fig. 2b and c show the HRTEM images of the pristine material Li_1.2Mn_0.54Ni_0.13Co_0.13O₂. There are two kinds of crystal lattice in the image: one lattice fringe width was 0.47 nm, corresponding to the (003) plane, and the other one was 0.238 nm, corresponding to the (006) plane of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂. Fig. 2e and f illustrate the details of the HRTEM images of the coated materials; the lattice fringe width was 0.47 nm, corresponding to the (111) plane of LiNi_0.5Mn_1.5O₄.

Fig. 2 (a–c) TEM images of the uncoated Li_1.2Mn_0.54Ni_0.13Co_0.13O₂; (d–f) TEM images of the coated Li_1.2Mn_0.54Ni_0.13Co_0.13O₂/LiNi_0.5Mn_1.5O₄ microsphere cathode; (b) and (c) are HRTEM images of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂; (e) and (f) are HRTEM images of the Li_1.2Mn_0.54Ni_0.13Co_0.13O₂/Li Ni_0.5Mn_1.5O₄ material.

Fig. 3a shows the SEM image of the coated Li-rich Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ with the corresponding EDX spectra of the interior part (Fig. 3b) and outer surface (Fig. 3c). The estimated Ni : Co : Mn atomic ratios are 0.13(4) : 0.11(2) : 0.54(0) (the interior part) and 1 : 0 : 4.6(9) (the outer surface), as shown in Fig. 3b and c. In fact, the atomic ratios may hardly be indicative of the chemical composition of each Li-rich manganese-based compound, but the selected zone can reflect well the existence of the expected elements of the prepared spherical product, and there is no Co element in the outer layer.

Fig. 3 To discuss the difference in elementary composition between the outer layer and the internal composite, we made an energy spectrum spot measurement in a cracked granulocyte. (a) SEM image of the selected cracked granulocyte (test spots are marked); EDX spectra showing (b) the internal spot and (c) the outer layer element content.

To explore the advantages of the current Li_1.2Mn_0.54Ni_0.13Co_0.13O₂/LiNi_0.5Mn_1.5O₄ composites as cathode active materials for high voltage Li-ion batteries, a coin cell was assembled using Li_1.2Mn_0.54Ni_0.13Co_0.13O₂/LiNi_0.5Mn_1.5O₄ as the cathode material and Li metal as the counter electrode. For comparison, the pristine Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ was also studied. The cells were cycled between 2.0 V and 4.8 V at room temperature. Fig. 4a shows the first charge and discharge curves of the pristine and coated materials at a rate of 0.2C (1C is equal to a current density of 300 mA g⁻¹). The coated Li_1.2Mn_0.54Ni_0.13Co_0.13O₂/LiNi_0.5Mn_1.5O₄ composite electrode delivered an initial discharge capacity of 225 mA h g⁻¹, which is larger than the pristine composite Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ (202 mA h g⁻¹). In addition, they both exhibit a long plateau on the charge curve which corresponds to the removal of Li-ions from the Li₂MnO₃ component accompanied by oxygen evolution.¹¹ As can be seen, the long plateau of the coated composite is relatively longer than that of the pristine one, which may be the reason for the higher specific capacity of the coated materials. The discharge voltage (Fig. 4a) of the coated electrode was apparently higher than that of the pristine electrode, suggesting the higher energy density of the battery.

Fig. 4 Electrochemical performances of the pristine Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ and coated Li_1.2Mn_0.54Ni_0.13Co_0.13O₂/LiNi_0.5Mn_1.5O₄ cathode materials: (a) the first charge and discharge curves of the pristine and coated Li-rich cathodes; (b) charge and discharge curves of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂/LiNi_0.5Mn_1.5O₄ microspheres at different rates; (c) discharge capacity comparison of the pristine (red) and coated (black) Li-rich cathodes at different rates; (d) the long cycle performance of the pristine (red) and coated (black) Li-rich cathodes at a high current density of 1C.

Fig. 4b presents the charge and discharge curves of the Li_1.2Mn_0.54Ni_0.13Co_0.13O₂/LiMn_1.5Ni_0.5O₄ coated microspheres at different rates. They delivered an average capacity of 221, 202, 168, 142 and 95 mA h g⁻¹ at a current density of 0.2C, 0.5C, 1C, 2C and 5C, respectively. After cycling at different rates, the cell was cycled back to a current density of 60 mA g⁻¹, and its discharge capacity was 245 mA h g⁻¹ (Fig. 4c). In contrast, the pristine Li-rich Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ composite delivered a capacity of 205, 142, 108, 77, and 48 mA h g⁻¹, at a current density of 0.2C, 0.5C, 1C, 2C and 5C, respectively. For the initial few cycles, the discharge capacities of both composites increased at a current density of 60 mA g⁻¹, which might be caused by activation of the cathode material while cycling. The variation of the discharge capacity as a function of the cycle number was performed to evaluate the capacity retention of the as-prepared Li-rich cathode materials. Fig. 4d demonstrates a comparison of the cycling performance between the conventional uncoated and coated Li-rich cathode materials at a rate of 1C. The coated cathode shows much better cycling stability with 87.7% capacity retention after 60 cycles, while the pristine one maintains 72.2% of its initial capacity.

With a coating layer of LiMn_1.5Ni_0.5O₄, the high voltage Li_1.2Mn_0.54Ni_0.13Co_0.13O₂/LiNi_0.5Mn_1.5O₄ cathode shows better rate capacity and cycling stability. Fig. 5a and b display the differential capacity versus voltage of the pristine and coated cathodes for the first charging process corresponding to the charging curves of Fig. 4a. The coin cells were cycled between 2.0 V and 4.8 V at a current density of 0.2C. Both the uncoated and coated electrodes exhibited two distinct and one weak dQ/dV peak; the weak peak is from 3.7 V to 4.25 V and one strong peak is from 4.25 V to 4.6 V. The much larger anodic peak at 4.5 V corresponds to the irreversible reaction in which the Li is extracted from the Li₂MnO₃ component with the simultaneous release of oxygen. As for the second strong peak occurring at about 4.74 V, this corresponds to the oxidation of Ni²⁺/Ni³⁺ and Ni³⁺/Ni⁴⁺ for the LiMn_1.5Ni_0.5O₄ ingredient.²⁷ After the cells were charged/discharged for 20 cycles, their EIS spectra were recorded (Fig. 5c). The Nyquist plots show a high frequency semicircle and a low frequency line inclined at a constant angle to the real axis. The semicircle indicates the double-layer response at the electrode/sample interface and the line shows the diffusion of Li-ions in the solid matrix.²² It is clear that the resistance of the coated cathode is much smaller than that of the pristine one. According to Chen et al. and our previous studies on the EIS of Li-ion batteries,^35–37 cell impedance is mainly attributed to the cathode impedance, especially charge-transfer resistance. Therefore, it can be assumed that the electrochemical impedance of the coated composition is suppressed by the presence of LiMn_1.5Ni_0.5O₄ and the discharge capacity of the coated material increased nearly 50 mA h g⁻¹ at a current density of 0.2C.

Fig. 5 (a) The differential capacity versus voltage of the pristine composite cathode cycled between 2.0 V and 4.8 V for the first cycling process at a rate of 0.2C, (b) the differential capacity versus voltage of the coated composite cathode for the first cycling process at a rate of 0.2C, and (c) the Nyquist plots of the pristine (red) and coated (black) Li-rich cathodes after the batteries performed 20 cycles.

4. Conclusion

A scalable synthesis of a high voltage Li_1.2Mn_0.54Ni_0.13Co_0.13O₂/LiNi_0.5Mn_1.5O₄ cathode has been successfully developed via a simple co-precipitation method. The coated cathode shows much better cycling behavior and rate capability compared with the pristine one. The improvement is mainly attributed to the protective LiNi_0.5Mn_1.5O₄ surface layer not only acting as an excellent Li-ion conductor to increase the rate capability of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂, but also protecting against HF attack of the Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ in the electrolyte. This design provides a new approach for the development of advanced Li-ion batteries with high energy density, good rate performance and high open operating voltage.

Acknowledgements

This work was financially supported by National Natural Science Foundation of China (11202177, 51202207).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra17804e

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