Synthesis, structure and electrochemical properties of lithium-rich cathode material Li_1.2Mn_0.6Ni_0.2O₂ microspheres

Abstract

Lithium-rich Li_1.2Mn_0.6Ni_0.2O₂ microspheres with a few mesopores have been successfully obtained. The results show that the Li_1.2Mn_0.6Ni_0.2O₂ material exhibits excellent cycling capability and rate performance. The microsphere morphology with a few mesopores could play a significant role in improving electrochemical performance.

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1. Introduction

Lithium-ion batteries (LIBs) with high energy and power density are imperative to develop for electric vehicles (EV), hybrid electric vehicles (HEV) and plug-in hybrid electric vehicles (PHEV). For LIBs, it is cathode materials that determine energy density, cycle life and the cost. Thus the development of advanced cathode materials with less cost, higher capacity and more environmental friendliness is crucial in every essence. Layered lithium cobalt oxide (LiCoO₂),¹ spinel lithium manganese oxide (LiMn₂O₄),² olivine lithium iron phosphate (LiFePO₄)³ have been widely studied as cathode materials for several decades. However, these materials cannot extricate themselves from relatively low capacity: 140 mA h g⁻¹ for LiCoO₂, 148 mA h g⁻¹ for LiMn₂O₄ and 170 mA h g⁻¹ for LiFePO₄.^4,5 In addition, for Co-containing cathode materials, the sustainable development is restricted due to the limited resource, high cost and toxicity of cobalt element.^6,7

Recently, lithium-rich Li[Li_1/3−2x/3Mn_2/3−x/3Ni_x]O₂, which is also denoted as xLi₂MnO₃·(1 − x)Li[Ni_1/2Mn_1/2]O₂, cathode material has attracted much attention owing to its higher capacity (over 250 mA h g⁻¹), lower cost, and less toxicity. It is considered as solid solution or integrated compound of layered Li₂MnO₃ phase (space group C2/m) and layered Li[Ni_1/2Mn_1/2]O₂ (space group R3̄m).^8–15 However, Li[Li_1/3−2x/3Mn_2/3−x/3Ni_x]O₂ have some intrinsic problems to be solved, like the relatively low cycling capability and poor rate performance.^16–18

It is well known that electrochemical properties are significantly influenced by the synthesis methods.^8,10,15,19 As a result, various synthesis methods, such as co-precipitation method,^11,19–21 sol–gel method^22–24 and hydrothermal method,^8,25–27 have been extensively exploited to improve electrochemical performance and overcome these mentioned drawbacks. Hydrothermal method is generally used for crystal growth control to obtain favorable morphology and enhanced correlative properties.

In this work, the precursor with mesoporous microspheres was successfully prepared by a facile hydrothermal method. Through calcined with a step procedure, lithium-rich cathode material Li_1.2Mn_0.6Ni_0.2O₂ (i.e., 0.5Li₂MnO₃·0.5Li[Ni_1/2Mn_1/2]O₂) with a few mesopores were obtained, which is proved beneficial for electrochemical performances, especially for rate capability. The structure, morphology and electrochemical properties were thoroughly investigated. The cycling capability and rate performance are improved compared to some previous reports without mesoporous microspheres structure.^28,29

2. Experimental

2.1 Synthesis of Li_1.2Mn_0.6Ni_0.2O₂

The precursor was prepared by a facile hydrothermal method. Stoichiometric amount of manganese(II) chloride, nickel(II) chloride with a molar ratio of 0.6 : 0.2 were dissolved in distilled water. Ammonium bicarbonate (NH₄HCO₃) was subsequently dropped in by stirring. The mixture was then transferred into a stainless steel autoclave and heated to 200 °C for 20 h in an oven to produce the precursor. The precursor was then dried at 120 °C and mixed with a certain amount of lithium carbonate. The dried mixture was preheated at 450 °C for 3 h and at 500 °C for 3 h, and then calcined at 700 °C for 1 h and at 900 °C for 6 h in the air. Ultimately, Li_1.2Mn_0.6Ni_0.2O₂ powder was obtained by cooling to room temperature in the furnace. All the raw materials were analytical purity grade and lithium carbonate was excess to compensate lithium loss during the calcination processes at high temperature.

2.2 Characterization and electrochemical tests of Li_1.2Mn_0.6Ni_0.2O₂

X-ray diffraction (XRD, Rigaku D/Max-2400, Japan) with Cu Kα radiation was used to identify the crystalline structure of materials. The XRD spectrum was collected in the range of 2θ value from 10° to 80°. The chemical stoichiometry was determined by Inductively Coupled Plasma-Atomic Emission Spectrometer (ICP-AES, PROFILE SPEC, Leeman). Scanning electron microscopy (SEM, Hitachi S-4800, Japan) and high-resolution transmission electron microscopy (TEM, Tecnai G2 F30, FEI) were engaged to observe the shape, size and distribution. Specific surface areas of the powders were measured by the multipoint Brunauer–Emmett–Teller (BET) method. The chemical valence state of all the elements were determined through X-ray photoelectron spectroscopy (XPS) by using ESCALAB250 (Thermo Fisher Scientific) with monochromatic Al Kα anode source with pass energy of 20 eV and energy step of 0.1 eV.

Electrochemical properties were investigated by assembling half-cells in an argon-filled glove box (MB-10 G with TP170b/mono, MBRAUN) with lithium metal as counter and reference electrode. The electrode was prepared by spreading a mixture of 80 wt% active material, 10 wt% carbon black, and 10 wt% polyvinylidene fluoride (PVDF) with n-methyl-2-pyrrolidone (NMP) onto an Al foil current collector. The electrode was then dried at 120 °C for 10 h in a vacuum oven. The electrolyte was formed by 1 M LiPF₆ dissolved in a mixture of ethylene carbonate (EC), dimethyl carbonate (DEC) and ethyl–methyl carbonate (DMC) (1 : 1 : 1 in volume). The charge–discharge measurements were galvanostatically carried out by using a battery test system (NEWARE BTS-610, Neware Technology Co., Ltd, China) in the voltage range of 2.0–4.8 V at room temperature. The current at 1C corresponds to 250 mA g⁻¹. The cyclic voltammograms (CV) were tested by using an electrochemical station (CHI660a) at a scanning rate of 0.1 mV s⁻¹ in the voltage range of 2.0–4.8 V. The electrochemical impedance spectra (EIS) measurements were carried out at frequencies from 100 kHz to 10 mHz with amplitude of 5 mV using an electrochemical station (CHI660a). All electrochemical measurements were conducted at room temperature.

3. Results and discussion

3.1 Material characterization

Fig.1 shows SEM images of the precursor (a), the intermediates (b–d) and final product (e and f). The precursor obtained by hydrothermal method displays uniform microspheres with a diameter of about 3 μm, as shown in Fig. 1a. The microsphere morphology is well maintained through a step procedure sintering from 400 °C to 900 °C, as shown in Fig. 1b–e. The microspheres almost retain the same diameter size (approximate 3 μm) before being calcined at 900 °C (Fig. 1a–d). The lithium-rich cathode material is obtained after being heated at 900 °C for 6 h and the diameter of microsphere is 6–7 μm, two times larger than that at previous state. The surface of microsphere turns obviously rough and the red region signed in Fig. 1e is magnified and shown in Fig. 1f. With the combination of Fig. 1e and f, the final product displays a secondary microsphere densely aggregated by nanoparticles of about 150 nm. The molecular formula of as-prepared lithium-rich cathode material obtained by ICP is Li_1.256Mn_0.595Ni_0.182O₂, listed in Table 1.

Fig. 1 SEM images of precursor (a) obtained by hydrothermal process, the intermediates preheated at 450 °C for 3 h (b), at 500 °C for 3 h (c) and calcined at 700 °C for 1 h (d), as-synthesized lithium-rich Li_1.2Mn_0.6Ni_0.2O₂ cathode material calcined at 900 °C for 6 h (e and f).

Table 1 Chemical composition of the as-prepared lithium-rich cathode material

	Element ratio in mole				Chemical formula
	Li	Mn	Ni	O
Designed	1.200	0.600	0.200	2.000	Li[Li0.2Mn0.6Ni0.2]O2
Measured	1.256	0.595	0.182	2.000	Li1.256Mn0.595Ni0.182O2

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The microspheres architecture of precursor (Fig. 2a) and Li_1.2Mn_0.6Ni_0.2O₂ powder (Fig. 2b–d) can also be clearly observed through TEM images. The HRTEM image (Fig. 2b) demonstrates that the distance of 0.47 nm agrees well with the (003) lattice spacing of the hexagonal Li[Ni_1/2Mn_1/2]O₂ and the (001) for monoclinic Li₂MnO₃, indicating the formation of highly crystalline oxide.³⁰ The shape and size are accordant with the results observed from the SEM images. Furthermore, some pores can be clearly observed in the microspheres of precursor. The microspheres of Li_1.2Mn_0.6Ni_0.2O₂ powder are larger and thicker than that of precursor.

Fig. 2 TEM images of precursor (a) and Li_1.2Mn_0.6Ni_0.2O₂ powder (b–d).

Nitrogen adsorption–desorption isotherms and pore distribution curves (Fig. 3a and b) demonstrate that the precursor exhibits mesoporous microspheres and the BET surface area is 87.179 m² g⁻¹. A large amount of mesopores and large BET surface area are mainly attributed to the gas release from the decomposition of NH₄HCO₃, and the evolution of NH₃ and CO₂ generated the pores in the microstructures. However, compared with precursor, the as-prepared microspheres, with the BET surface area of 1.528 m² g⁻¹, own fewer mesopores with smaller pore size, which is probably due to high surface energy and high levels of agglomeration, the particles are unceasingly densely aggregating to form a larger microsphere during high temperature heating processes. The primary nanoparticles and mesopores could effectively shorten the Li⁺ migration length. Meanwhile, moderate specific surface area (1.528 m² g⁻¹) could refrain from side reactions during the charge–discharge process. This kind of structure and morphology is beneficial to the electrochemical performances.

Fig. 3 Nitrogen adsorption–desorption isotherms and pore distribution curves of precursor (a and b) and Li_1.2Mn_0.6Ni_0.2O₂ powder (c and d).

Fig. 4 shows the XRD patterns of precursor (a), intermediate products (b–d) and Li_1.2Mn_0.6Ni_0.2O₂ (e). The precursor is obtained by hydrothermal process and the XRD peaks are attributed to Ni_0.25Mn_0.75CO₃ (Fig. 4a). The XRD patterns of the phase transformation during the solid state reaction between Ni_0.25Mn_0.75CO₃ and LiCO₃ at 450 °C, 500 °C, 700 °C and 900 °C are shown in Fig. 4b–e respectively. The thermal decomposition of the transition metal carbonate precursor produces NiMnO₃, Mn₂O₃ and CO₂ when the temperature is more than 200 °C.³¹ Its decomposition reaction follows the scheme:

4Ni_0.25Mn_0.75CO₃ → NiMnO₃ + Mn₂O₃ + 3CO₂↑ (1)

Fig. 4 XRD patterns of precursor (a), the intermediates preheated at 450 °C for 3 h (b), at 500 °C for 3 h (c) and calcined at 700 °C for 1 h (d), as-synthesized lithium-rich Li_1.2Mn_0.6Ni_0.2O₂ cathode material calcined at 900 °C for 6 h (e).

Then the reaction between NiMnO₃ and Li₂CO₃ takes place in the stoichiometric ratio (the total transition metals/Li = 1) to form the R3̄m phase (Fig. 4b–d), which follows the equation:

NiMnO₃ + Li₂CO₃ → 2LiNi_0.5Mn_0.5O₂ + CO₂↑ (2)

Further decomposition of the residual Li₂CO₃ engages at the temperature higher than 700 °C (Fig. 4e).

It can be observed from Fig. 4b–d that two major diffraction peaks at 18.64° and 44.54° (2θ, highlighted by blue lines) appear during the calcinations process. The two lines can be indexed to (003) and (104) peaks based on the R3̄m space group. In the meantime, the peaks at 21.3°, 30.5° and 31.9° (2θ, highlighted by red lines), corresponding to (110), (202̄) and (002) diffraction peaks of Li₂CO₃, fade during the heat-treatment from 450 to 900 °C.³¹ After high temperature treatment at 900 °C for 6 hours, as-synthesized lithium-rich Li_1.2Mn_0.6Ni_0.2O₂ cathode material is obtained and the XRD pattern is shown in Fig. 4e. All the diffraction peaks can be indexed to a hexagonal α-NaFeO₂ structure with the R3̄m space group except for the weak peaks between 20° and 23°. These weak peaks identified as the (020) reflection are attributed to the super lattice structure of Li₂MnO₃ component (C2/m space group).³² Lattice parameters (a = 2.860(3) Å and c = 14.228(3) Å) are close to the previous reported value. The c/a value is 4.97 (>4.9), indicating a satisfactory crystalline layered structure.³² The clear splitting between the adjacent peaks of (006)/(102) and (108)/(110) confirms the highly ordered layered structure.³³ The intensity ratio of (003)/(104) is used to denote the degree of cationic disorder in the Li-layers.³² The value of I(003)/I(104) is 1.377 (>1.2), reflecting a favorable layered structure without undesirable cationic disorder.

XPS was carried out to ascertain chemical valence of the surface for Li_1.2Mn_0.6Ni_0.2O₂ powder. The spectra of Ni 2p, Mn 2p, O 1s and Li 1s are shown in Fig. 5. It can be detected that Ni²⁺ (854.4 eV) and Ni³⁺ (855.3 eV) have co-existed on the surface of cathode, as shown in Fig. 5a and b.²⁴ There occurs oxidation reaction of partial amount of Ni²⁺ during the synthesis process, which is probably due to over-lithiation.¹⁷ According to Mn 2p (Fig. 5c and d) and Mn 3p spectra (Fig. 5f), Mn⁴⁺ is the majority constituent with respect to the peaks at 642.3 eV (Mn 2p_3/2), 653.7 eV (Mn 2p_1/2) and 50.3 eV (Mn 3p). The peaks at 641.8 eV (Mn 2p_3/2) and 49.7 eV (Mn 3p) are attributed to Mn³⁺,²⁴ indicating that a small amount of Mn³⁺ exists on the surface. The decrease of average valence for Mn may imply the formation of spinel structure. From Fig. 5e, the large peak of O 1s at 529.2 eV is assigned to the lattice oxygen. The small peak at 531.5 eV can probably be attributed to the residual Li₂CO₃.³⁷

Fig. 5 Ni 2p (a and b), Mn 2p (c and d), O 1s (e) and Li 1s (f) XPS spectra of Li_1.2Mn_0.6Ni_0.2O₂ powder.

3.2 Electrochemical properties

Fig. 6 displays the cyclic voltammetry measurements of Li_1.2Mn_0.6Ni_0.2O₂ electrode for the first 10 cycles. The oxidation and reduction peaks of the first cycle are different from those of the subsequent cycles, indicating a special charge and discharge mechanism in initial cycle, as is the character of Li-rich cathodes. In initial positive scanning (Fig. 6a), the oxidation peaks around 3.85 V and 4.14 V are ascribed to the oxidation of Ni²⁺/Ni³⁺ and Ni³⁺/Ni⁴⁺, respectively.¹² The sharp oxidation peak at 4.69 V can be assigned to the irreversible oxidation of O²⁻, corresponding to electrochemical activation of Li₂MnO₃, which disappears in subsequent cycles.^10,34–36 The reduction peaks at 3.83 V and 3.29 V are attributed to the reduction of Ni and Mn⁴⁺, respectively. The reduction of Mn⁴⁺ ions brings about extra lithium insertion, leading to improved discharge capacity in subsequent cycles. Although a small amount of Mn³⁺ is analyzed through the XPS spectra, oxidation peak of Mn³⁺ cannot be observed in the first oxidation cycle of CV curves, which probably indicates that only a small amount of Mn³⁺ exist on the surface. From 2nd to 10th cycles (Fig. 6b), the oxidation peaks of Mn³⁺ and Ni²⁺ appear at 3.3–3.4 V and 3.89–4.0 V, respectively.¹⁰ The oxidation peaks of Ni³⁺ shift to 4.37 V, suggesting that the electronic environment for high-valence nickel ions is affected by structural rearrangement which is originated from irreversible loss of oxygen in the initial cycle. The reduction peaks in the range of 4.3–3.6 V belong to the reduction of Ni⁴⁺ to Ni²⁺. The reduction peaks of Mn⁴⁺/Mn³⁺ (3.3–3.1 V) shift to the low voltage gradually with cycling, which is associated to a part of structure transformation from layer to spinel.³⁷

Fig. 6 Cyclic voltammogram of Li_1.2Mn_0.6Ni_0.2O₂ electrode for the first 10 cycles.

The charge–discharge profiles of Li_1.2Mn_0.6Ni_0.2O₂ electrode at 0.1C are shown in Fig. 7a. The initial charge curve shows two plateau located at 3.7–4.5 V and above 4.5 V, indicating two different Li-extraction processes during the first charge process.³⁸ The low-voltage plateau is associated with the Li-extraction from the structure of space group R3̄m at 3.7–4.5 V, corresponding to the oxidation of Ni²⁺ to Ni⁴⁺.³⁹ The second plateau (>4.5 V) is widely accepted as simultaneous oxygen release with Li-extraction from the layered Li₂MnO₃ lattice, which results in high irreversible capacity loss.^40,41 The plateau situated at ∼4.5 V region no longer appears reversibly after the first cycle. The first charge and discharge capacity are respectively 392.0 and 256.4 mA h g⁻¹ with a coulombic efficiency of 65.4%. The 2nd charge capacity drops to 257.7 mA h g⁻¹, which is due to irreversible extraction of Li₂O in the first charge process. The 2nd discharge capacity is 246.4 mA h g⁻¹ and the coulombic efficiency is 95.6%. After 10 cycles, charge and discharge capacity are respectively 235.4 and 225.1 mA h g⁻¹, corresponding to a coulombic efficiency of 95.6%. Fig. 7b shows the first discharge curves for Li_1.2Mn_0.6Ni_0.2O₂ electrode measured at 0.1C, 1C and 2C. The electrode delivers capacities of 256.4, 215.6 and 125.7 mA h g⁻¹ at 0.1C, 1C and 2C, respectively. The initial discharge capacity decreases with the increase of C-rates, which is probably owing to the ascent of polarization for electrodes, and it could impact the lithium ion intercalation/deintercalation.

Fig. 7 Charge–discharge profiles at 0.1C (a) and initial discharge profiles at 0.1C, 1C, 2C (b) of Li_1.2Mn_0.6Ni_0.2O₂ electrode.

Fig. 8a depicts the cycle performance of Li_1.2Mn_0.6Ni_0.2O₂ electrode for 100 cycles at different rates from 2 to 4.8 V.²⁹ The discharge capacities of 0.1C and 1C gradually decrease with cycling. After 100 cycles, the discharge capacities retain 47.8% (122.6 mA h g⁻¹) and 85.0% (183.3 mA h g⁻¹) at 0.1C and 1C, respectively. The electrode has relatively low capacity in initial cycles at 2C. With the increase of the cycle number, the discharge capacity shows an increasing trend and rises to as much as 163.3 mA h g⁻¹ after 100 cycles. It is probably attributed to gradually adequate activation of Li₂MnO₃. It can be observed that the capacity decreases with the increase of the C-rates and polarization. Since smaller rates bring about deeper charge and discharge reaction, quantities of lithium ions deinsert from their place, leading to many lithium vacancies. On the one hand, some nickel ions are likely to transport into lithium ion vacancies; on the other hand, vacancies may increase the chance of structural collapse in cycles that followed. So, lithium ions which can go back and insert into its position might be less than it should be. Therefore, the discharge capacity decreases more quickly at 0.1C than that at larger rates.

Fig. 8 Cycle performance of Li_1.2Mn_0.6Ni_0.2O₂ electrode for 100 cycles at different rates (a) and its rate performance from 0.5C to 3C (b).

Fig. 8b shows the rate performance of Li_1.2Mn_0.6Ni_0.2O₂ electrode from 0.5C to 3C. It can be seen that the electrode displays high capacity and excellent rate performance. The cathode delivers the average discharge capacities of 225 mA h g⁻¹, 200 mA h g⁻¹, 175.7 mA h g⁻¹ and 156.8 mA h g⁻¹ at 0.5C, 1C, 2C and 3C, respectively. About 92.7% (208.6 mA h g⁻¹) of the initial discharge capacity can be recovered back at 0.5C.

To further investigate the electrochemical properties, electrochemical impedance spectroscopy (EIS) measurements were carried out. The cells were fully charged to 4.8 V and cycled at 1C after 1, 50 and 100 times before being tested and the Nyquist plots, relevant equivalent circuit for them are shown in Fig. 9. In the equivalent circuit, R_s mean the resistance for the transportation of Li⁺ through the interfacial film, R_ct represents the charge transfer resistance, CPE refers to the constant phase-angle element, which depicts the nonideal characteristic of the double layer, Z_w indicates the Warburg impedance relating to Li⁺ diffusion in the bulk material. There exists a semicircle at high frequency and a slope line at low frequency. The diameter of the semicircle is an indication of the resistance when Li⁺ diffuses through the interfacial film, i.e., the charge transfer resistance (R_ct). The slope line is attributed to Li⁺ diffusion in the bulk material.⁶ A higher R_ct value reflects larger electrode polarization, thus causing worse rate capability. The R_ct value, which is subjected to large variation within extensive cycles, increases from 178.9 to 388.3 Ω from 1 cycle to 100 cycles. The relative low R_ct value and the deviation also imply an improved electrochemical performance.

Fig. 9 EIS spectra and equivalent circuits of Li_1.2Mn_0.6Ni_0.2O₂ electrode.

The excellent electrochemical properties could be ascribed to three aspects. On the one hand, the primary nanoparticles and a few mesopores could effectively shorten the Li⁺ migration length. On the other hand, relatively small specific surface area could refrain from partial side reaction during the charge–discharge process. In addition, a few spinel structures at the surface region originated from the small amount of Mn³⁺ is helpful to improve rate capability, which is due to the faster diffusion of Li⁺.^17,24

4. Conclusions

In summary, lithium-rich layered cathode material Li_1.2Mn_0.6Ni_0.2O₂ has been successfully synthesized and thoroughly investigated. The cathode material owns a desired crystalline layered structure and secondary microsphere morphology with a few mesopores. The primary nanoparticles and mesopores could effectively shorten the Li⁺ migration length. Meanwhile, relatively small specific surface area could refrain from side reactions during the charge–discharge process. The cathode material exhibits excellent cyclic capability and rate performance. It retains 85.0% of initial discharge capacity after 100 cycles at 1C. After the battery cycles at the increasing rate from 0.5C to 3C, about 92.7% (208.6 mA h g⁻¹) of the initial discharge capacity is recovered back at 0.5C. And it holds such desirable rate capability as to more than 90% of the initial capacity could be recovered after designed cycles. Thus, the hydrothermal method applied in this work has provided a new approach to synthesize a functional structure for lithium-rich layered cathode materials.

Acknowledgements

This work was supported by the National Basic Research Program of China (973 Program) (2013CB934001), National Natural Science Foundation of China (51274017), National 863 Program of China (2013AA050904), International S&T Cooperation Program of China (2012DFR60530), Shanghai Academy of Space Technology (SAST201467).

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