Layered Li(Ni_0.2Mn_0.2Co_0.6)O₂ synthesized by a molten salt method for lithium-ion batteries

Abstract

Sub-micron size Li(Ni_0.2Mn_0.2Co_0.6)O₂ was synthesized by the molten salt method at 800 °C and 900 °C using LiOH:LiNO₃ eutectic salt for the first time and the phases were characterized by X-ray diffraction (XRD), SEM, density and BET surface area. Rietveld refinement of the XRD data showed 5 and 3% cation-mixing in the compound synthesized at 800 °C and 900 °C, respectively. Galvanostatic charge–discharge cycling at 30 mA g⁻¹ between 2.5 and 4.4 V vs. Li at room temperature showed the second cycle discharge capacities of 119 and 133 mA h g⁻¹ for the phases synthesized at 800 °C and 900 °C, respectively. The capacity retention was 81% and 87%, respectively between 2 and 50 cycles. After reheating the 900 °C sample for another 2 hours at 900 °C, the XRD pattern shifted obviously to low angle, which indicated a reduction of Ni³⁺ to Ni²⁺ and was further proved by X-ray photoelectron spectroscopy (XPS) measurement. The re-heated compound showed an improved discharge capacity of 159 mA h g⁻¹ (2^nd cycle) and it retained a capacity of 123 mA h g⁻¹ at the end of the 50^th cycle, corresponding to a capacity-retention of 77%. Cyclic voltammetry studies on the above compound clearly showed the redox peaks due to Ni^2+/4+ and Co^3+/4+ and the 4.5 V-structural transition was not suppressed. The cathodic performance of the phases improved upon cycling to the cut-off voltage of 4.3 V.

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1. Introduction

Layered LiCoO₂ has been used as the first generation cathode material for lithium-ion batteries from the 1990s.^1,2 However, because of the high cost and toxicity, much effort has been made to develop cheaper and non-toxic cathode materials to replace LiCoO₂. Ni, Mn doped compounds with the composition, Li(Ni_xMn_xCo_1−2x)O₂ (0 ≤ x ≤ 0.5),³ namely, Li(Ni_1/3Mn_1/3Co_1/3)O₂ and Li(Ni_0.5Mn_0.5)O₂ and other layered compositions^4–7 showed improved thermal stability in the charged state in comparison to LiCoO₂ and have been well-studied in the literature.

Jiang et al.⁸ prepared the composition with x = 0.2, namely Li(Ni_0.2Mn_0.2Co_0.6)O₂ and studied its Li-cycling properties. This composition is of interest since it has Li-ion diffusion coefficient (D_Li⁺) as high as ∼1 × 10⁻⁹ cm² s⁻¹, almost the same as LiCoO₂ whereas the compositions with x = 0.25–0.5 show much lower D_Li⁺ values. For example, at x = 0.5 and 0.38, the D_Li⁺ values are 1 × 10⁻¹² cm² s⁻¹ and 1 × 10⁻¹¹ cm² s⁻¹, respectively. In addition, the thermal stability of 4.2 V charged composition with x = 0.2 is improved by ∼40 °C in comparison to LiCoO₂ (180 °C vs. 140 °C). The compound with x = 0.2 prepared by mixed hydroxide method and solid state reaction at 900 °C for 3 to 10 h in air.⁸ They reported Li-cycling to 4.4 V cut-off and noted capacity fading of ∼20% in 200 cycles (capacity of ∼160 mA h g⁻¹ at 0.05 C-rate).⁸ Bentaleb et al.⁹ prepared Li(Ni_0.2Mn_0.2Co_0.6)O₂ by solution combustion method at 900 °C for 1 h in air and have shown that in the range, 2.5–4.3 V vs. Li, a fairly stable capacity of ∼135 mA h g⁻¹ is obtained at C/2 rate up to 50 cycles and ∼120 mA h g⁻¹ at 1 C-rate up to 50 cycles. Very recently Labrini et al.¹⁰ reported the magnetic and electron paramagnetic resonance (EPR) studies on the above solid solution.

Among the limited publication, only two of them did research on the material synthesis. Hydroxide method and solid state reaction⁸ take two synthesis steps and containing pH control. Combustion method⁹ may cause violent explosion if sucrose and nitrate mixture do not controlled at a critical level. Presently to simplify the synthesis process while at the same time using safe reactants, we adopted the molten salt synthesis method is a one pot method and has been used to synthesize and study cathode and anode materials for lithium-ion batteries,^11–19 and other materials are summarized by recent reviews,^20,21 this synthetic method have some advantages over other traditional methods which are: simple and versatile to prepare various binary^22–25 and ternary oxides.^26–28 This method gives better reactivity of initial reactants in molten media²⁰ and less cation mixing. In this work, for academic and applied aspects of research, Li(Ni_0.2Mn_0.2Co_0.6)O₂ was synthesized by molten salt method for the first time and its structural and electrochemical properties have been studied.

2. Experimental

The compound, Li(Ni_0.2Mn_0.2Co_0.6)O₂ was synthesized using NiCO₃ (Alfa Aesar, 98%), MnCO₃ (Merck, purity 98%), Co(OH)₂·H₂O (Aldrich, 95%), LiOH (Aldrich, 99.99%), LiNO₃ (Alfa Aesar 99%) as starting materials. Stoichiometric amounts of Ni-, Mn- and Co-salts along with 0.38 M LiOH–0.62 M LiNO₃ (eutectic mixture; melting point, 175 °C) were mixed in an agate mortar. The molar ratio of total metal ions to eutectic salt was fixed to 1 : 4. After grinding, the mixture was transferred into a recrystallized alumina cylindrical crucible (volume 50 mL) and heated in a box furnace (Carbolite, UK) at the temperature 800 or 900 °C in air for 6 h. The furnace heating rate of 3 °C min⁻¹ was used, the melt was cooled to room temperature. The contents were washed several times with de-ionized water to remove the excess Li-salts, then filtered and finally washed with ethanol, dried at 80 °C overnight and stored in a desiccator. One select sample prepared at 900 °C was reheated in air at 900 °C for 2 h and then quenched to room temperature.

The crystal structures of the compounds were identified by X-ray diffraction (XRD) (X'PERT MPD unit/Empyrean, PANalytical with Cu Kα radiation) and the unit-cell lattice parameters were obtained by Rietveld refinement of the powder XRD data using the software TOPAS Version 2.1. The morphology were examined by scanning electron microscope (SEM: JEOL-JSM-6700F). Tristar 3000 (Micromeritics, USA) and AccuPyc 1330 pycnometer (Micromeritics, USA) were used to determine the BET surface area and density of the compounds, respectively.

The electrochemical measurements were carried out using a CR-2016-type coin cell with a Celgard 2502 membrane separator and 1 M LiPF₆–ethylene carbonate (EC)–diethyl carbonate (DEC) (1 : 1 in volume) as electrolyte. The electrode was composed of 80 wt% active material, 10 wt% conductive carbon black (Super P) and 10 wt% polyvinyledene difluoride (PVDF) binder, which were mixed in N-methyl pyrrolidinone (NMP) solvent to make viscous slurry, and then coated on to etched aluminum foil using doctor blade method. The electrodes were dried in an oven, pressed between rollers, and cut into 16 mm circular disks. Geometrical area of the electrode is 2.0 cm² and weight of the active material is 4–5 mg. Cells were assembled in Argon-filled glove box (MBraun, Germany) using Li–metal foil as counter and reference electrode. Cyclic voltammetry was carried out by using computer controlled Mac-pile II system (Bio-logic, France). Charge–discharge cycling between 2.5–4.4 V and 2.5–4.3 V vs. Li at room temperature was carried out using Bitrode battery tester (Model SCN, Bitrode, USA) at a current of 30 mA g⁻¹. Further details on cell fabrication and instrumentation reported elsewhere.^16,29

3. Results and discussion

3.1. Structure and morphology

The prepared compounds, Li(Ni_0.2Mn_0.2Co_0.6)O₂ are free-flowing black crystalline powders. The Rietveld refined XRD patterns of the phases prepared at 800 and 900 °C are shown in Fig. 1a and b. Both XRD patterns can be indexed and the structure refined based on a hexagonal α-NaFeO₂ structure (space group: R-3m, 166).³⁰ The peak splits of (006)/(102) and (018)/(110) are known as an indicator of characteristic layered structure. The compositions with x > 0.1 in Li(Ni_xMn_xCo_2−x)O₂ are prone to cation mixing, the exchange of Li-ions at Ni-site and vice versa and cation-mixing of 3–6% are common depending on the preparation the method.^8,9,12,13 This is due to the close proximity of the ionic radii of Li⁺ and Ni²⁺ ions in octahedral-coordination.³¹ Rietveld refinements were done assuming Ni, Co, and Mn-ions randomly occupy the 3b site with the positional co-ordinates (x, y, z) of (0, 0, 0), Li-ions are in 3a (0, 0, 0.5) site, and O-ions are in 6c (0, 0, 0.253) site. The lattice parameters, together with other related data are summarized in Table 1.

Fig. 1 The Rietveld refinements of the XRD patterns of Li(Ni_0.2Mn_0.2Co_0.6)O₂ synthesized at (a) 800 °C, (b) 900 °C. Symbols represent experimental pattern, continuous line are fitted data. Difference patterns are shown and vertical bars represent (hkl) lines. (c) Comparative XRD patterns of the 900 °C-synthesized compound and that re-heated at 900 °C, 2 h in air. CuKα radiation. Miller indices are indicated. (d) The pattern of (c) in the expanded scale (2θ = 18–19°).

Table 1 Hexagonal lattice parameters, Rietveld parameters, surface area and density values of Li(Ni_0.2Mn_0.2Co_0.6)O₂ synthesized at 800 °C and 900 °C by molten salt method

Synthesis temperature (°C)	800	900	Re-900
a (Å)	2.827(2)	2.826(4)	2.831(9)
c (Å)	14.118(4)	14.122(4)	14.145(6)
c/a	4.994	4.997	4.995
Intensity ratio I(003)/I(104)	1.2	1.6	1.2
Cation-mixing (%)	5	3	2.5
Rwp (%)	1.07	1.31	1.2
Rp (%)	0.82	0.9	0.8
GOF	1.51	1.89	1.6
R-Bragg	0.42	0.52	0.35
BET surface area (m^2 g^−1)	8.4	2.7	2.4
Experimental density (g cm^−3)	4.652	4.590	4.672
Theoretical density (g cm^−3)	4.946	4.946	4.946

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The compounds synthesized at 800 and 900 °C have almost identical a lattice parameter, whereas the one synthesized at 900 °C has slightly larger c lattice parameter (14.122 Å) in comparison to the one synthesized at 800 °C. However, the observed a and c values are still smaller when compared with those reported by Jiang et al.⁸ and Bentaleb et al.⁹ The probable reason is due to the oxidation of part of Ni²⁺ (r_i: 0.69 Å) to Ni³⁺ (r_i: 0.56 Å)³¹ in the synthesized compounds due to the use of LiNO₃ as the molten salt which is an oxidizing melt.¹⁵ The c/a ratios of the compounds prepared at 800 °C and 900 °C are 4.994 and 4.997 respectively, which indicate that the compounds have almost ideal layered structure. Rietveld refinement studies shows the compounds synthesized at 800 °C and 900 °C have cation-mixing of 5% and 3%, respectively. For comparison, Jiang et al.⁸ found cation-mixing of 1.2 to 1.8% in Li(Ni_0.2Mn_0.2Co_0.6)O₂ depending on the preparation method, and Bentaleb et al.⁹ assumed only 1% cation-mixing in their compound for the Rietveld refinement of the XRD data.

The comparative XRD patterns of the as-prepared and re-heated compound at 900 °C are shown in Fig. 1c. The region, 2θ = 18–19° at high resolution, corresponding to hkl: 003 is shown in Fig. 1d. From the latter, it is clear that a slight shifting to the low angle occurs in the re-heated compound, which clearly proves reduction of Ni³⁺ content to form Ni²⁺ in the lattice. Accordingly, the lattice parameters of the re-heated compound are a = 2.832(1) Å, c = 14.145(6) Å which are larger when compared with the as-prepared compounds (Table 1) and are in very good agreement with those reported by Bentaleb et al.⁹ (a = 2.8399(2) Å, c = 14.165(1) Å). In order to further prove this reduction, X-ray photo electron spectroscopy (XPS) measurement was carried out to known the differences in the oxidation states. In Fig. 2a, the peaks of Ni2p_3/2 at 900 °C-re-heated compound shifted to lower binding energy value compared with 900 °C compound, which is an obvious indication of the Ni ion reduction. However, there is almost no peak shifting in Mn and Co XPS spectra.

Fig. 2 XPS spectra of Li(Ni_0.2Mn_0.2Co_0.6)O₂ synthesized at 900 °C and 900 °C-re-heated compound. (a) Ni 2p (b) Mn 2p and (c) Co 2p levels.

The SEM images of compounds synthesized at 800 °C and 900 °C show homogeneous distribution of aggregates sphere nanoparticles (Fig. 3a) and plate-shape morphology with a cubic structure (Fig. 3b). We note not much differences in the morphology of Li(Ni_0.2Mn_0.2Co_0.6)O₂ was noted upon reheat treatment (figures not shown). The BET surface area decreases from 8.4 to 2.7 m² g⁻¹, for the synthesis temperatures of 800 °C and 900 °C. The experimental densities match well with the calculated densities (Table 1).

Fig. 3 Scanning electron microscopy images of (SEM) images of Li(Ni_0.2Mn_0.2Co_0.6)O₂ of prepared at (a) 800 °C and (b) 900 °C.

3.2. Electrochemical studies

The cyclic voltammograms (CV) of Li(Ni_0.2Mn_0.2Co_0.6)O₂ in cells in the voltage range, 2.5–4.4 V vs. Li at room temperature are shown in Fig. 4a and b. For clarity, only second cycle CV are shown. Broad anodic and cathodic peaks at ∼4 V are clearly seen. Compared with the compound prepared at 800 °C, the compound prepared at 900 °C has much lower hysteresis in the peak potentials. The broad peaks can be explained as due to the overlap of the redox peaks of Ni^2+/4+, Ni^3+/4+ and Co^3+/4+. The CVs of 900 °C re-heated compound show a well-defined redox peak at 3.8/3.66 V and 3.96/3.86 V which corresponds to Ni^2+/4+ and Co^3+/4+ redox couples, respectively (Fig. 4c).^8,9,32,33 When cycled to 4.6 V cut-off, clear indication of additional anodic peak at 4.5 V is seen and can be assigned to the hexagonal phase transition similar to that shown by LiCoO₂.^11,33

Fig. 4 Cyclic voltammograms (2^nd cycle) of Li(Ni_0.2Mn_0.2Co_0.6)O₂ prepared at (a) 800 °C (b) 900 °C. (c and d) 900 °C-re-heated compound (6^th cycle), scan rate: 0.058 mV s⁻¹, voltage range: 2.5–4.4 or (2.5–4.6 V for (d)). Li–metal is the counter and reference electrode, measured at room temperature.

Galvanostatic charge–discharge curves of Li(Ni_0.2Mn_0.2Co_0.6)O₂ prepared at 900 °C are shown in Fig. 5a and b in the voltage range, 2.5–4.4 V and 2.5–4.3 V, respectively, at 30 mA g⁻¹. During charge reaction (Li-de-intercalation), the voltage suddenly increases to ∼3.75 V followed by a small voltage plateau and this region is due to equilibrium of Li_x(Ni_0.2Mn_0.2Co_0.6)O₂ phases, with x = 1.0 and x < 1.0. The charge profile increases in a continuous fashion up to upper cut-off voltage. There are obvious charge–discharge plateaus near 3.8 V, which is consistent with the redox peaks in the CV curves. Similar profiles are observed for the 800 °C-prepared and 900 °C re-heated compounds. The reversible capacities at the second cycle are 119 and 133 mA h g⁻¹ for the compounds prepared at 800 and 900 °C respectively with the 4.4 V cut-off. After 50 cycles, the discharge capacities decrease to 95 and 117 mA h g⁻¹ for each, and the capacity retention is 81% and 87%, respectively (Fig. 5c). From 50 to 100 cycles, only 5% capacity fading was noticed for the compound prepared at 900 °C and it has a better electrochemical performance. The coulombic efficiency improves after 10–20 cycles and is of the order 97–98%. We note that Jiang et al.⁸ obtained higher capacities for the same composition with 4.4 V cut-off at the rate, C/20. But, the capacity-retention is similar to that noted in the present study.

Fig. 5 Galvanostatic cycling curves of Li(Ni_0.2Mn_0.2Co_0.6)O₂. Voltage vs. capacity profiles of the 900 °C-synthesized compound: (a) voltage range, 2.5–4.4 V vs. Li, (b) voltage range, 2.5–4.3 V vs. Li. Capacity vs. cycle number plots: (c) voltage range, 2.5–4.4 V; (d) voltage range, 2.5–4.3 V; current: 30 mA g⁻¹.

The 900 °C re-heated compound showed higher reversible capacity in comparison to the 800 °C and 900 °C-synthesized compounds (Fig. 5c). For example, the second discharge capacity is 159 mA h g⁻¹, similar to that reported by Jiang et al.⁸ However, after 60 cycles, the discharge capacity decreased to 123 mA h g⁻¹ and the capacity-retention is 77%. The capacity-fading noted in the compounds in the present study and also by Jiang et al.⁸ when cycled up to the cut-off voltage 4.4 V can be ascribed to the cation-mixing in the compounds, up to 3% in the 900 °C-synthesized compound, and also due to the proximity to the 4.5 V structural transition in the compound, as is clear from the CVs of Fig. 4d.

The compound, Li(Ni_0.2Mn_0.2Co_0.6)O₂ prepared at 800 °C and 900 °C have also been cycled in the voltage range, 2.5–4.3 V at 30 mA g⁻¹ up to 40 cycles. The galvanostatic profiles and the capacity vs. cycle number plots are shown in Fig. 5b and d, respectively. The second cycle charge capacities are 126 and 130 mA h g⁻¹ for the 800 °C and 900 °C-synthesized compounds, respectively. At the end of 38 cycles, the capacities are 108 and 120 mA h g⁻¹, respectively, corresponding to 86% and 92% capacity-retention. Bentaleb et al.⁹ reported that when cycled in the range, 2.5–4.3 V at 0.2 C rate, capacity retention was 92% between 2 and 40 cycles, in good agreement with the values noted in the present study.

4. Conclusions

The cathode material, Li(Ni_0.2Mn_0.2Co_0.6)O₂ is synthesized by molten salt method using LiOH:LiNO₃ eutectic salt at 800 °C and 900 °C and characterized by X-ray diffraction (XRD), SEM, surface area and density. Submicron size particles with platelike morphology and hexagonal α-NaFeO₂ structure showed 5% and 3% cation-mixing by Rietveld refinement of the XRD data for the compounds prepared at 800 °C and 900 °C, respectively. Galvanostatic cycling studies between 2.5 and 4.4 V and at a current of 30 mA g⁻¹ showed that the compound prepared at 900 °C delivered a second discharge capacity of 133 mA h g⁻¹ and after 50 cycles, the capacity retention is 87%, whereas, from 50 to 100 cycles, the capacity retention is 95%. Re-heating the compound at 900 °C for 2 h showed an improved 2^nd cycle discharge capacity of 159 mA h g⁻¹ and a capacity retention of 77% after 60 cycles. The cyclic performance of 800 °C-synthesized compound is slightly inferior. Lower reversible capacities and better capacity-retention, up to 40 cycles were noted with the upper cut-off voltage of 4.3 V. Cyclic voltammetry clearly delineated the redox peaks due to Ni^2+/4+ and Co^3+/4+ in the re-heated compound at 900 °C.

Acknowledgements

Authors thank to Chinese Research Council, National University of Singapore and Ministry of Education (MOE), Singapore, Grant no. R-284-000-076-112. M. V. R. thank to associate Prof. Stefan Adams, Department of Materials Science & Engineering for his support and National Research Foundation, Prime Minister’s Office, Singapore, under its Competitive Research Programme (CRP Award no. NRF-CRP 10-2012-6) for research support.

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