Hybrid annealing method synthesis of Li[Li_0.2Ni_0.2Mn_0.6]O₂ composites with enhanced electrochemical performance for lithium-ion batteries

Abstract

A lithium rich composite cathode electrode material Li[Li_0.2Ni_0.2Mn_0.6]O₂ was synthesized using the hybrid annealing method. Compared with the traditional annealing method, the hybrid annealing method added a cool treatment to the annealing process. Based on detailed characterizations from X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and electrochemical impedance spectroscopy (EIS), it is suggested that a good layer structure (axis ratio of c/a > 5.0), appropriate particle size of about 100 nm, and high lithium ion diffusion coefficient around 3.98 × 10⁻²⁴ cm² s⁻¹ are attained when the annealing temperature drops to 100 °C. Because of the above reasons the sample showed a high discharge capacity and excellent cycle stability. The first discharge capability and capacity retention after 60 cycles are 287.3 mA h g⁻¹ and 96.1%, respectively. The reasons for the electrochemical enhancement are systematically investigated.

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1. Introduction

The present dominant commercial cathode material LiCoO₂ has a practical discharge capacity value of 140 mA h g⁻¹ at low rates (C/10 or lower rates), only 50% of the theoretical capacity, as limited by its structural stability.^1–3 To overcome the critical disadvantages of the commercially used LiCoO₂ such as expensiveness,⁴ limited energy and power density,⁵ toxicity, etc., an alternative cathode with a layered structure is urgently needed to meet the rapid development of electric vehicles (EVs) and hybrid electric vehicles (HEVs).⁶ Therefore, the further development of cathode materials is necessary in order to attain high discharge capacities and good cycling stabilities. At present, olivine structure LiFePO₄,^7,8 spinel structure LiMn₂O₄,^9,10 and layered structure LiNi_xMn_yCo_zO₂ (x + y + z = 1)^11–15 are the alternatives with the double advantages of lower toxicity and cost. Nevertheless, these cathode materials have to face the challenge of greatly enhancing electrochemical performance.¹⁶

With these considerations in mind, more and more interests have been focused on the lithium-rich xLi₂MnO₃·(1 − x)LiMO₂ (M = Mn, Ni, and Co, etc.) cathode materials because of their capacity as high as 250 mA h g⁻¹ (close to their theoretical capacity), low cost, chemical stability and environmental friendliness.^17,18 However, several problems still block the commercial application of the xLi₂MnO₃·(1 − x)LiMO₂ cathode materials. On the one hand, such Li-rich layered oxides with manganese element in high valence state have a serious surface destruction due to the decomposition of electrolyte and the Jahn–Teller effect especially at the highly delithiated state up to 4.7 V. Therefore, this phenomenon is general and leads to a steady capacity decrease during cycling.¹⁹ On the other hand, the large irreversible capacity loss in the first cycle leads to a first Coulomb efficiency decrease.²⁰ So far, the investigations have mainly focused on the surface modification to improve these disadvantages.^21–24 However, the electrochemical performance of cathode materials is closely related to its structure which is influenced heavily by preparation conditions.²⁵ Hence, researchers have paid much attention to the synthesis and optimization of the material with co-precipitation method, solid state method, sol–gel method, etc.^26–30 Controlling and improving the experimental conditions (for example pH, annealing temperature, and time of mixing) are necessary to obtain a good electrochemical performance final product. As we all known, the reported optimized electrochemical performance is that the first discharge capacity is about 250 mA h g⁻¹ (0.1 C), the retained discharge capacity is about 200 mA h g⁻¹ after 40 cycles.^31–33 In the traditional two-step annealing method, the precursor is annealed at 500 °C for 4 h and annealed subsequently at 900 °C for 12 h in air, and then the products were obtained after being slowly cooled to room temperature.³⁴ This method can result in micron-sized large spherical aggregates, and thus hinder Li-ion transport.³⁵ All these reasons may give rise to low discharge capacity and poor cycling stability.

In this work, we prepared Li[Li_0.2Ni_0.2Mn_0.6]O₂, i.e., xLi₂MnO₃·(1 − x)LiNi_0.5Mn_0.5O₂ (x = 0.5), through a co-precipitation method and proposed a hybrid annealing method to improve its first discharge capacity and cycling performance, which is completely different from the traditional annealing method such as optimize annealing temperature or time. First the precursor was annealed at 500 °C for 3 h, and then the annealing temperature dropped from 500 °C to 100 °C. Subsequently, annealed at 900 °C for 6 h in air and then cooled to room temperature. The sample shows the first discharge capacity is 287.3 mA h g⁻¹ (0.1 C) and capacity retention ratios is 96.1% after 60 cycles. The Li[Li_0.2Ni_0.2Mn_0.6]O₂ prepared by the hybrid annealing method exhibits a excellent electrochemical performance because of the cool treatment mainly affects the formation of the good layered structure which contributes to enhance the value of lithium ion diffusion coefficient (D_Li⁺).

2. Experimental

2.1. A novel synthesis for Li[Li_0.2Ni_0.2Mn_0.6]O₂

Li[Li_0.2Ni_0.2Mn_0.6]O₂ was synthesized from mixed nickel–manganese carbonate and lithium carbonate precursors by a co-precipitation method. The preparation procedure can be briefly described as follows: firstly, a 0.2 mol L⁻¹ aqueous solution of nickel sulfate and manganese sulfate (Ni : Mn = 1 : 3) was added to a 1 mol L⁻¹, equivolume, aqueous solution of sodium carbonate, from which green precipitates formed instantly; the mixed solution was aged for 12 h. The co-precipitation and aging procedures were carried out under constant stirring (250 rpm) at 80 °C. Secondly, The (Ni_0.25Mn_0.75)CO₃ precipitate was filtered, washed, dried at 100 °C overnight, and thereafter intimately mixed with Li₂CO₃. Thirdly, the mixture was decomposed at 500 °C for 3 h in air. Then, the samples annealing temperature dropped from 500 °C to 200 °C, 100 °C and 20 °C, respectively, annealed subsequently at 900 °C for 6 h in air. Finally, the products were obtained after being slowly cooled to room temperature. The final products were named as T200, T100 and T20, respectively. The sample without cool treatment in the process of annealing was named as T500.

2.2. Structural characterization

Crystallite structures were determined by X-ray diffraction (XRD) using a Rigaku D/MAX 2400 diffractometer (Japan) with Cu Kα radiation (λ = 0.15418 nm) operating at 40 kV and 150 mA. The microstructure and morphology of the materials were characterized using a transmission electron microscope (TEM, JEOL, JEM-2010, Japan) operating at an accelerating voltage of 5.0 kV and a field emission scanning electron microscope (SEM, JEOL, JSM-6701F, Japan) working at an accelerating voltage of 200 kV.

2.3. Electrochemical performance tests

The electrodes were fabricated using a mixture of the prepared materials (85 wt%), acetylene black (10 wt%), and polyvinylidene fluoride (PVDF 5 wt%) in N-methyl-2-pyrrolidone (NMP) to form a slurry. The slurry was spread onto Al foil and dried in an oven at 50 °C overnight. The electrode area was 0.64 cm² and the loading of active materials was about 2.5 mg cm⁻². The electrochemical performance was tested by assembling CR2032 coin cells, using Li metal as the negative electrode. The electrolyte solution was 1 mol L⁻¹ LiPF₆ in EC–DEC (1 : 1 vol%). Charge/discharge performance was tested by LAND CT2001A battery testing system at a current density of 20 mA g⁻¹ (0.1 C rate) and a voltage range of 2.0 V–4.8 V. The electrochemical impedance spectroscopy (EIS) was measured using an electrochemical workstation (CHI 660C) and the applied frequency was from 100 kHz to 10 mHz.

3. Results and discussion

3.1. Structure analysis of Li[Li_0.2Ni_0.2Mn_0.6]O₂

X-ray diffraction (XRD) is employed to determine the crystallographic phases of the samples (Fig. 1a). All the strong diffraction peaks can be indexed as the O3 type layered structure based on a hexagonal α-NaFeO₂ with space group R3̄m.³⁶ The additional weak peaks with short-ranged superstructure reflections around 2θ = 20–25° can be attributed to the existence of monoclinic Li₂MnO₃ phase (JCPDS card no. 27-1252) (C2/m) and the ordering of Li, Ni, and Mn atoms in the transition metal layers.^22,37,38 The details of the atomic arrangement in the transition metal layers of these materials have not confirmed yet.³⁹ With the annealing temperature dropping, the splitting of the pair reflections (018)/(110) and (006)/(012) become more obvious, indicating the good structural compatibility between Li₂MnO₃ and LiNi_0.5Mn_0.5O₂.^40,41 It is reported that the c/a axis ratio is an indicator of the hexagonal ordering.⁴² So, the axis ratio of c/a is taken as an important structural parameter to examine the influences of the cool treatment temperature on the lithium de-intercalation in the lattice of the samples. The higher axis ratio of c/a makes the material a better hexagonal ordering. With the annealing temperature dropping from 500 °C to 20 °C, the axis ratios of c/a are 4.9524 (T500), 4.9791 (T200), 5.0215 (T100), and 4.9897 (T20) (Table 1). The trends of c/a axis ratio evolution are represented in Fig. 1b, which indicates the improved layered structure by cool treatment. Therefore, T100 has good hexagonal ordering, high crystallinity, and good layered structure.

Fig. 1 (a) X-ray diffraction (XRD) patterns and (b) axis ratio of c/a trends evolution of the samples synthesized at different cool treatment temperatures: T500, T200, T100, and T20.

Table 1 Lattice parameters of the samples synthesized at different cool temperatures

Samples	a (Å)	c (Å)	c/a	V (Å^3)
T500	2.8568	14.1486	4.9524	300.0215
T200	2.8566	14.2236	4.9791	301.5633
T100	2.8537	14.3301	5.0215	303.1962
T20	2.8566	14.2539	4.9897	302.2057

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3.2. Morphology of Li[Li_0.2Ni_0.2Mn_0.6]O₂

The size and morphology of the samples are characterized by scanning electron microscopy (SEM). The particle sizes of all the samples are hugely different (Fig. 2). The particle gradually formed inerratic prismatic structure with the annealing temperature dropping. T500 has a slightly big particle size, with an average particle size of 650 nm. The surface morphology is rough and the particle boundary is not clear (Fig. 2a). As can be seen from the Fig. 2b and c, T200 and T100 show nearly the same particle size, mainly distributed around 100–150 nm. The surface morphology is smoother than T500. The particle size of T100 (∼100 nm) is the smallest among all the samples. T20 has a slightly big particle size and the mean size is about 450 nm (Fig. 2d). It is thus concluded that the cool treatment has a great influence on the particle size. As the annealing temperature dropping, the particle distribution becomes more uniform and smaller. Therefore, the T100 has fine microstructure and small particle size, which can increase sufficient contact area of the material with the electrolyte and provide a large area for electrochemical reaction.

Fig. 2 Scanning electron microscopy (SEM) images of the samples synthesized at different cool treatment temperatures: (a) T500, (b) T200, (c) T100, and (d) T20.

The morphology and structure of the T100 are further studied by TEM and the results are given in Fig. 3. It is shown from TEM images that the grain sizes of the T100 are in the range of 100–200 nm (Fig. 3a), consistent with the above SEM observations. Fig. 3b displays high resolution transmission electron microscopy (HRTEM) image of T100. The high-resolution TEM image of T100 displays clear lattice fringes with a width of 2.34 Å corresponding to the (101) plane. Therefore, from the structural analyses, it is certain that there is a high crystallinity for the sample.

Fig. 3 Transmission electron microscopy (TEM) image (a) and the high-resolution TEM image (b) of the T100.

3.3. Electrochemical test

Fig. 4a shows the first charge–discharge curves of the samples at a current rate of 0.1 C between 2.0 V and 4.7 V. During the first charge to 4.7 V, the electrochemical reaction happens in two charging regions: a smoothly sloping voltage region from 3.8 V to 4.5 V, which is ascribed to the removal of lithium from the electrode structure accompanied with the oxidation of nickel ions from Ni²⁺ to Ni⁴⁺, and the voltage plateau region above 4.5 V, which could be ascribed to the activation of Li₂MnO₃.^19,43 This activation of Li₂MnO₃ process has been considered to be associated with the irreversible removal of Li₂O from the electrode material.⁴⁴ With the annealing temperature dropping from 500 °C to 100 °C, the first discharge capacity gradually increased, 201.8 mA h g⁻¹ (T500), 217.3 mA h g⁻¹ (T200), 287.3 mA h g⁻¹ (T100), and the first Coulomb efficiency also gradually increased, 65% (T500), 66.2% (T200), 75% (T100). These results indicate that the cool treatment is beneficial for achieving a high discharge capacity. However, when the annealing temperature further decreases to 20 °C, the electrochemical performance becomes poor. The first discharge capacity and first Coulomb efficiency are only 214.4 mA h g⁻¹ and 63.8%, respectively. Therefore, it is believed that the cool treatment temperature plays an important role to obtain the high discharge capacity and first Coulomb efficiency values. The cycle performance of the samples at different cool treatment temperatures is further discussed, as indicated in Fig. 4b. For T500 and T200, the cycling stability become poor with the cycle number increasing. After 60 cycles for T100, the discharge capacity and the capacity retention ratios are 276.2 mA h g⁻¹ and 96.1%. The T20 exhibits a relatively high cycle stability, but with a low discharge capacity. Obviously T100 displays the most excellent electrochemical performance among all of the samples.

Fig. 4 (a) First charge–discharge curves and (b) cycle performance at 0.1 C (20 mA g⁻¹) for T500, T200, T100, and T20; inset shows Coulombic efficiency for T100.

The rate capability of T100 exhibits a remarkable improvement compared to the T500. The first charge–discharge curves of the T500 and T100 are presented in Fig. 5a, measured at 0.5 C. The first discharge capacities of the T500 and T100 are 152.7 mA h g⁻¹ and 182.6 mA h g⁻¹, respectively. Fig. 5b shows the cyclic performances of T500 and T100 at 0.5 C between 2.0 V and 4.7 V. While the T500 delivers a capacity of 111.5 mA h g⁻¹ with capacity retention of 73.8% after 200 cycles, the T100 exhibits a capacity of 178.4 mA h g⁻¹ with capacity retention of 97.6% after 200 cycles. The results indicate that rate capability and cyclic stability of Li[Li_0.2Ni_0.2Mn_0.6]O₂ are improved effectively by cool treatment.

Fig. 5 (a) First charge–discharge curves and (b) cycle performance at 0.5 C (100 mA g⁻¹) for T500 and T100.

Electrochemical impedance spectroscopy (EIS) measurements were conducted to provide further information on the effect of cool treatment on electrode kinetics and lithium-ion diffusion coefficient in Li-ion battery materials.⁴⁵ Impedance spectra for the samples produced at different cool treatment temperatures were investigated to get insight into the electrochemical process. All the Nyquist plots show two semicircles in the high and middle frequency regions and a slope in the low frequency region (Fig. 6a). The semicircles in the high and middle frequency regions correspond to lithium ion diffusion through the surface layer and charge transfer reaction, respectively, while the slope in the low frequency region is attributed to lithium ion diffusion in the bulk material.⁴⁶ These Nyquist plots are fitted with the equivalent electrical circuit inserting in Fig. 6a, where R_e stands for internal resistance of the cell, R_sf and C_sl represent the resistance and the capacitance of solid electrolyte interface (SEI) film. R_ct and C_dl correspond to charge transfer resistance and double layer capacitance. W is the Warburg impedance related to the solid state lithium ions diffusion inside the active materials.⁴⁷

Fig. 6 (a) Electrochemical impedance spectroscopy (EIS) of different samples measured, and (b) profiles of the real parts of impedance (Z_r) νs. ω^−1/2 from 0.1 to 0.01 Hz and corresponding linear fitting curves for T500, T200, T100, and T20.

The lithium ion diffusion coefficient (D_Li⁺) is calculated from the straight sloping line in low frequency region in terms of the following eqn (1),^48,49

where R is the ideal gas constant, T is the absolute temperature, F is the Faraday constant, A is the surface area of the electrode, C is the concentration of Li⁺ in the material, and σ is the Warburg factor which has the relationship with Z_r (Z_r is the real part of impedance) in the following eqn (2),

Z_r = σω^−1/2 (2)

From eqn (2), the Warburg factor (σ) can be obtained from the linear fitting of (Z_r) νs. ω^−1/2 in the low frequency range of 0.1–0.01 Hz (Fig. 6b).

The calculated results of lithium ion diffusion coefficient and the values of R_e, R_sf, R_ct, and R_total for T500, T200, T100, and T20 are shown in Table 2. As can be seen, it is obviously that T100 has the lowest R_sf and R_ct, while T500 has the highest R_sf and R_ct. In addition, the lithium ion diffusion coefficient of the T100 is 3.98 × 10⁻²⁴ cm² s⁻¹, which is greater than the T500 of 6.82 × 10⁻²⁵ cm² s⁻¹. Therefore, T100 exhibits the best electrochemical performance compared with other samples, which are consistent with above analyses results. Cool treatment decreases both R_sf and R_ct, indicating an enhancement in the kinetics of lithium ion diffusion through surface layer and charge transfer reaction, which increase the electrochemical performance consequently. As a consequence, cool treatment contributes to decrease the R_sf and R_ct and increase lithium ion diffusion coefficient, which also result in high discharge capacity, excellent cycling stability and high Coulombic efficiency.

Table 2 Fitted parameters for EIS obtained by Zswinpwin software of different samples

Samples	Re (Ω)	Rsf (Ω)	Rct (Ω)	Rtotal (Ω)	DLi^+ (cm^2 s^−1)
T500	5.41	60.11	219.39	284.91	6.82 × 10^−25
T200	5.66	52.28	180.83	238.77	1.65 × 10^−24
T100	5.76	44.59	146.36	196.71	3.98 × 10^−24
T20	5.19	47.13	201.87	254.19	3.01 × 10^−24

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To analysis the relation between structure and electrochemical performance of the electrode materials, the trends of c/a axis ratio and values of D_Li⁺ evolution are shown in Fig. 7. With the axis ratio of c/a increasing, the value of D_Li⁺ gradually increasing, T100 has the maximum value of c/a and D_Li⁺, which indicates that the sample has a good layered structure and the maximum lithium ion diffusion coefficient. From above results, the cool treatment mainly affects the formation of the good layered structure which contributes to enhance the value of D_Li⁺, and further influences electrochemical performance of the electrode materials.

Fig. 7 Axis ratio of c/a and D_Li⁺ changes of T500, T200, T100 and T20.

4. Conclusions

In conclusion, Li[Li_0.2Ni_0.2Mn_0.6]O₂ electrode material is successfully synthesized by a novel hybrid annealing method. The resulting material delivers excellent performance (first discharge capacity is 287.3 mA h g⁻¹ and capacity retention ratios is 96.1% after 60 cycles) as a cathode material for Li-ion batteries. The novel hybrid annealing method facilitates the formation of good layered structure which leads to high lithium ion diffusion coefficient, and then improves the electrochemical performance. This excellent property makes the Li[Li_0.2Ni_0.2Mn_0.6]O₂ be a potential practical application material of high energy Li-ion batteries.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (no. 51362018, 21163010), the Key Project of Chinese Ministry of Education (no. 212183), and the Natural Science Funds for Distinguished Young Scholars of Gansu Province (no. 1111RJDA012).

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