Revisiting the layered LiNi_0.4Mn_0.4Co_0.2O₂: a magnetic approach

Abstract

Layered LiNi_0.4Mn_0.4Co_0.2O₂ has been synthesized by the co-precipitation method, and the structural, electrochemical and magnetic properties were comprehensively studied by Rietveld analysis, charge–discharge potential profiles, X-ray photoelectron spectroscopy, and dc and ac susceptibilities. The material shows initial discharge capacities of 166 and 206 mA h g⁻¹ in potential windows of 2.5–4.4 V and 2.5–4.6 V, respectively, and a better capacity retention of 95% at 2.5–4.4 V after 50 cycles. The effective paramagnetic moment is calculated to be 3.02(3) μ_B/f.u. by fitting to the Curie–Weiss law, which is consistent with the averaged value, based on the specific contributions, as quantified by an analysis of the X-ray photoelectron spectroscopy data. The dc magnetization curves show irreversibility and spin freezing behavior at 77 K and 18 K, respectively. The evolution of irreversibility temperature under different applied fields indicates a spin-glass-like transition. The ac susceptibility data and the fitting using the frequency dependent spin-freezing temperatures also confirm this magnetic transition. In comparison with the previous results, the co-precipitation prepared sample shows a big difference in the magnetic parameters, coming from the different microscopic exchange interactions or the formation of a different scale of spin clusters, which is sensitive to the preparation procedure.

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1. Introduction

As a promising candidate for cathode materials for lithium-ion batteries, LiNi_yMn_yCo_1−2yO₂ has been investigated widely for its low cost and environmentally benign properties.^1–4 LiNi_yMn_yCo_1−2yO₂ has an α-NaFeO₂ structure, with the oxygen atoms in a cubic close packed arrangement, where Ni²⁺ serves as a two-electron redox center, Co³⁺ increases the electronic conductivity and the non-Jahn–Teller Mn⁴⁺ helps to stabilize the rhombohedral framework during charge–discharge cycling.^5,6 This system has been intensively studied by a number of experimental techniques, which usually focus on the long-range and short-range crystal structure, electronic structure, ionic and electronic transport.^7–10 Although magnetic characterization cannot provide a direct method for improving the electrochemical performance, it can contribute to understanding the electronic interaction between ion clusters at the microscopic level, since the charge capacity and capacity retention of a cathode are closely related to the cation ordering and phase stability of the material.

In an earlier report, Chernova et al.¹¹ synthesized the LiNi_yMn_yCo_1−2yO₂ system by a high temperature solid state reaction. They found that the Weiss constant increased with decreasing level of Co-substitution y, indicating that the nonmagnetic Co³⁺ ions destroy the strong antiferromagnetic 90° superexchange interaction between Ni²⁺, O²⁻ and Mn⁴⁺ ions, and a canonical spin glass transition was observed for the LiNi_yMn_yCo_1−2yO₂ system. However, another study on LiNi_0.4Mn_0.4Co_0.2O₂, synthesized by a citrate precursor method, showed cluster spin glass behavior with a higher irreversibility temperature and a more negative Weiss constant, in comparison with Chernova's results.¹² From a magnetic point of view, the nearly 90° superexchange via O²⁻ and the direct exchange between Ni²⁺, Mn⁴⁺ and Co³⁺ ions within the transition metal layers of LiNi_yMn_yCo_1−2yO₂ play a key role, because the transition metal layers are separated by nonmagnetic lithium layers.¹³ Within the transition metal layers, the geometrically frustrated triangle lattice would increase the quantum fluctuation of this system. On the other hand, the unavoidable disorder of Ni²⁺/Mn⁴⁺/Co³⁺ within the transition metal layers and the Li⁺/Ni²⁺ disorder in different layers would somewhat destroy the frustration effect. Note that the disordered arrangement is very sensitive to the preparation procedure, which is closely related to the formation of ion clusters. Hence, it is very interesting to study the competition between geometrical frustration and disorder and the influence of the preparation procedure on the magnetic and electrochemical properties, and highlight the link between the magnetism, electronic and atomic structure in the LiNi_yMn_yCo_1–2yO₂ system. In this work, a co-precipitation method was applied for the synthesis of the LiNi_0.4Mn_0.4Co_0.2O₂ compound, and the structural, electrochemical and magnetic properties are comprehensively studied and discussed in comparison with the previous reports.

2. Experimental

The precursor was synthesized by the co-precipitation method using transition-metal sulfate, sodium hydroxide and ammonia as starting materials. Stoichiometric amounts of NiSO₄·6H₂O, MnSO₄·H₂O and CoSO₄·7H₂O were dissolved with a molar ratio of 4 : 4 : 2 and stirred continuously in deionized water. NaOH and NH₃·H₂O (Na : NH₃ = 1 : 2) were dissolved in deionized water to reach a 1M hydroxide solution. Then the precipitation was achieved by slowly dripping the alkaline solution into the sulfate solution until pH = 12. After vigorous stirring at 50 °C under a nitrogen atmosphere for 12 h, the homogenously precipitated hydroxide powder (Ni_0.4Mn_0.4Co_0.2)(OH)₂ was washed several times with 3 L deionized water by centrifuging and dried at 120 °C for 12 h. Synthesis of LiNi_0.4Mn_0.4Co_0.2O₂ was carried out using a solid state reaction. The obtained precursor (Ni_0.4Mn_0.4Co_0.2)(OH)₂ was mixed with a 5% excess of Li₂CO₃ by ball milling. The excess amount of Li salts was used to compensate for possible Li loss during calcination. After heating at 450 °C for 5h, the mixture was ground and pressed into pellets, then heated again at 900 °C for 12h to obtain the final products.

The crystal structure of the material was studied by X-ray diffraction on a Bruker AXS diffractometer with Cu-Kα radiation. Diffraction data were recorded over the 2θ range 10°–80° with a step size of 0.01° and account time of 6s. The software package WINPLOTR was used for data analysis and Rietveld refinement of the structural model. X-ray photoelectron spectroscopy (XPS) was performed on an ESCALAB spectrometer (VG scientific) using a monochromatic Mg-Kα light source. All measured binding energies (BEs) were referenced to the C1s line at 284.6 eV. Magnetic characterization was performed using a superconducting quantum interference device magnetometer (Quantum Design MPMS-XL).

The electrochemical measurement was carried out using 2032 coin cells. A metallic lithium foil served as the anode electrode. The cathode electrode was composed of a mixture of LiNi_0.4Mn_0.4Co_0.2O₂ active material (80 wt%), super-P conductive additive (10 wt%), and polyvinylidenefluoride binder (PVDF, 10 wt%) dissolved in N-methylpyrrolidone (NMP). The slurry mixture was pasted on an aluminum foil, followed by drying at 120 °C for 24 h in a vacuum oven. Each electrode contained about 5 mg of active material. The electrolyte was 1 M lithium hexafluorophosphate (LiPF₆) dissolved in ethylene carbonate (EC) and diethyl carbonate (DEC) (EC : DEC = 1 : 1, v/v). The battery cell was assembled in an argon-filled glove box with H₂O and O₂ concentrations below 1 ppm. The charge–discharge cycle was performed on a Land-2001A battery cycler.

3. Results and discussions

In order to check the R-3m structure of the obtained LiNi_0.4Mn_0.4Co_0.2O₂ material and to determine the degree of Li⁺/Ni²⁺ cation disorder, a Rietveld refinement is performed, based on the powder X-ray diffraction data, as shown in Fig. 1. A structural model based on the α-NaFeO₂ structure is used, with the detailed refined parameters listed in Table 1. During the refinement, the occupancy of the 3a sites by Mn and Co is fixed at 0.4 and 0.2 for each ion, and the total amount of Li and Ni is also fixed. In agreement with other structural studies of the LiNi_yMn_yCo_1−2yO₂ system,^14,15 a partial Li⁺/Ni²⁺ mixing between different layers is also considered and allowed to vary. Such refinement results in a model in which 3.6% of Ni ions in the transition metal layers are exchanged with Li ions, being best described by the formula [Li_0.964Ni_0.036]_3b[Li_0.036Ni_0.364Mn_0.4Co_0.2]_3aO₂. The degree of Li⁺/Ni²⁺ cation disorder depends on many parameters during synthesis (time, temperature and atmosphere for annealing, particle morphology, residual organic components etc.),¹⁶ so that even very similar synthesis conditions can result in slightly different cation distributions. In general, the introduction of Co ions reduces the degree of Li⁺/Ni²⁺ mixing, for example from 8% in LiNi_0.5Mn_0.5O₂ to nearly 4%.¹¹ In comparison with previous reports on LiNi_0.4Mn_0.4Co_0.2O₂ material,¹⁷ our refined values exhibit good agreement among cell parameters, but a lower degree of Li⁺/Ni²⁺ cation disorder (3.64%).

Fig. 1 Results from the Rietveld refinement of the LiNi_0.4Mn_0.4Co_0.2O₂ structure model, based on power X-ray diffraction using FULLPROF.

Table 1 Structural parameters obtained from Rietveld refinement of the structural model of LiNi_0.4Mn_0.4Co_0.2O₂

Crystal system		Hexagonal
Space group		R-3m
Cell parameter		a = b = 2.874(8) Å, c = 14.278(5) Å
Atom	Site	x	y	z	Occupancy
Li(1)	3b	0.000	0.000	0.500	0.964
Ni(1)	3b	0.000	0.000	0.500	0.036
Li(2)	3a	0.000	0.000	0.000	0.036
Ni(2)	3a	0.000	0.000	0.000	0.364
Mn	3a	0.000	0.000	0.000	0.400
Co	3a	0.000	0.000	0.000	0.200
O	6c	0.000	0.000	0.241	1
R wp (%) = 7.67		R p (%) = 6.82		R f (%) = 1.32

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Fig. 2 displays the charge–discharge potential profiles and the cycling performance of LiNi_0.4Mn_0.4Co_0.2O₂ in different potential windows. The material exhibits an initial discharge capacity of 166 and 206 mA h g⁻¹ in the potential windows of 2.5–4.4 V and 2.5–4.6 V, respectively. The capacity retention is relatively poor when the material is cycled between 2.5 V and 4.6 V. This may be due to the formation of a solid electrolyte interface film on the particle surface or the dissolving of Co³⁺ into the electrolyte.^18,19 In spite of this possible disadvantage, the LiNi_0.4Mn_0.4Co_0.2O₂ material prepared by the described co-precipitation method demonstrates a high reversible capacity and capacity retention. Such an improvement in the observed electrochemical performance is probably related to the low level of Li⁺/Ni²⁺ cation disorder or a superior morphology of the material. Further electrochemical measurements are in progress to reveal the underlying mechanisms.

Fig. 2 Cycling performance of LiNi_0.4Mn_0.4Co_0.2O₂ in the voltage windows of 2.5–4.4 V and 2.5–4.6 V.

The valence states of the transition metals in LiNi_0.4Mn_0.4Co_0.2O₂ should be determined before a reliable magnetic property analysis is possible, since magnetic exchange interactions are very sensitive to the specific electron configuration of the involved ions. Fig. 3 shows the XPS of the Ni2p, Mn2p and Co2p of the LiNi_0.4Mn_0.4Co_0.2O₂ material. The fitting method is based on ref. 20, which uses the linear background and a constant full width at half maximum. The Ni2p_3/2 and Ni2p_1/2 spectra are located at 854.2 and 871.6 eV, respectively. After fitting to the Ni2p_3/2 peak, a mixed valence state of Ni is proposed, with two binding energies located at 853.6 eV and 855.0 eV, which correspond to Ni²⁺ and Ni³⁺, respectively.^21,22 The ratio of Ni²⁺ : Ni³⁺ is nearly 80 : 20, based on the corresponding integrated areas. A similar interpretation describes the Mn2p_3/2 doublet, which can also be separated into two peaks, located at 641.3 eV and 642.2 eV, corresponding to the Mn³⁺ and Mn⁴⁺ cations, respectively.²³ The Co2p_3/2 and Co2p_1/2 binding energies, which are localized at 779.4 eV and 794.4 eV, are consistent with those of the Co³⁺ ions.²⁴ In a typical LiNi_0.4Mn_0.4Co_0.2O₂ material, the oxidation states of nickel, cobalt and manganese are expected to be +2, +3 and +4, respectively. But the present material shows a mixed valence state for the Ni and Mn ions. This mixed valence state will not only affect the electrochemical performance strongly, but also plays an important role in the magnetic behavior, because the magnetic ordering in the LiNi_yMn_yCo_1−2yO₂ system is dependent on the magnetic exchange interaction via nearly 90° Ni^2+/3+–O²⁻–Mn^3+/4+ superexchange paths and 180° Ni^2+/3+–Mn^3+/4+ direct exchange.

Fig. 3 X-Ray photoelectron spectra for LiNi_0.4Mn_0.4Co_0.2O₂.

The magnetic properties of LiNi_0.4Mn_0.4Co_0.2O₂ are firstly studied by dc magnetization measurements, as shown in Fig. 4, including a de Almeida–Thouless (AT) line fitting and the inverse magnetization curve. Magnetization data in zero-field cooled (ZFC) and field-cooled (FC) modes in three different applied fields are compared in Fig. 4(a). Apparently the ZFC magnetization displays a sharp peak at the freezing temperature (T_f = ~18 K). Unlike ferromagnetism or canonical spin glass, the FC magnetization does not exhibit a “Brillouin-like” behavior or temperature-independent platform-like behavior.²⁵ At low temperature, a pronounced cusp is also observed in the FC mode and will be quenched by a stronger applied field of 1000 Oe. In addition the bifurcation between the ZFC and FC curves is still observed at the irreversibility temperature (T_irr = ~77 K) under 100 Oe. The irreversibility temperature shifts to a lower temperature with increasing magnetic field and can also be fitted by the de Almeida–Thouless (AT) line, which predicts one critical line for a spin glass in the mean-field theory.²⁶ The AT equation is expressed as follows

H_AT(T_irr)/ΔJ ∝ (1 − T_irr/T_F)^3/2 (1)

where T_F is the zero-field spin-glass freezing temperature and ΔJ is the width of the distribution of exchange interactions. As shown in Fig. 4(b), T_irr varies linearly with H^2/3, indicating that our data satisfies the AT line, which also confirms the spin-glass-like transition in LiNi_0.4Mn_0.4Co_0.2O₂.

Fig. 4 (a) ZFC and FC susceptibility as a function of temperature between 5 and 150 K in an applied magnetic field of 100, 500 and 1000 Oe; (b) fitting to the de Almeida–Thouless (AT) line with the irreversibility temperatures; (c) the inverse magnetization data with the Curie–Weiss fitting.

At higher temperatures the magnetization shows a paramagnetic dependence on temperature, as shown in Fig. 4(c). By fitting the high-temperature linear behavior to the Curie–Weiss law,²⁷ the Weiss constant is calculated to be −98 K, suggesting a strong antiferromagnetic interaction. The Curie constant is fitted to be 1.15 emu K mol⁻¹ Oe, with which the effective moment μ_eff = 3.02(3) μ_B/f.u. is obtained by evaluating the relation , where N is the number density of magnetic ions per mole and k_B is Boltzmann's constant.²⁸ Theoretically the effective moment μ_eff = 3.02(5) μ_B/f.u. can be obtained by , where the Mn⁴⁺/Mn³⁺ and Ni³⁺/Ni²⁺ ratios are taken from the XPS analysis. The consistency between the experimental and the theoretical moment not only confirms that the surface-sensitive XPS results are representative for the bulk material, but also suggests that nearly all the electrons are localized at the magnetic ions and are not itinerant.

The field dependence of the magnetization at four temperatures is also measured and shown in Fig. 5. It is obvious that the lower the temperature, the more the M(H) curve bends. However, magnetic saturation is not reached, even at 5 T. The magnetization becomes a nonlinear function of the field and displays ferromagnetic behavior with a small hysteresis in the low field region, whose coercive field and remanence at 5 K are listed in Table 2. Both the absence of magnetization saturation at high fields and the pronounced hysteresis loop in the low-field region are characteristics of spin-glass-like behavior.^13,29–32

Fig. 5 The magnetic field variation of magnetization at different fixed temperatures for LiNi_0.4Mn_0.4Co_0.2O₂.

Table 2 List of magnetic parameters of LiNi_0.4Mn_0.4Co_0.2O₂ prepared by different methods

	C	θ	μ eff	μ theo	M r	H c	T f	T irr
	(emu K mol^−1 Oe)	(K)	(μB)	(μB)	(emu mol^−1)	(Oe)	(K)	(K)
This paper	1.15	−98	3.02	3.07	49	294	18	77
Ref. 12	1.068	−65.8	2.92	3.03	4.9	83	11	−
Ref. 13	1.435	−112	3.38	3.03	295	510	28	140

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In order to confirm the spin-glass-like transition, the temperature dependence of the ac susceptibilities over the frequency range 10 to 1000 Hz is recorded as shown in Fig. 6(a). It can be seen that the real part of the ac susceptibility (χ′) exhibits a peak at ~20 K, corresponding to T_f in the dc curves. And with the increase in driving frequency the peak position shifts towards a higher temperature and the height of the ac peak decreases, which are characteristics of a spin glass or superparamagentic transition. In order to distinguish the spin glass from superparamagnetism, the criterion parameter δ is considered^33,34

δ = ΔT_f/T_fΔlog ω (2)

where ΔT_f and Δlog ω are the shifts of T_f and log ω with frequency, respectively. δ usually lies between 0.0045 and 0.06 for a spin glass system, while for a superparamagnetic system, δ exceeds 0.1 because spin clusters do not interact with each other. The δ value of the investigated LiNi_0.4Mn_0.4Co_0.2O₂ material is calculated to be around 0.045, which indicates a spin glass transition rather than superparamagnetism at low temperature.

Fig. 6 (a) Temperature dependence of the real part of ac susceptibility in the frequency range 10 to 1000 Hz; frequency dependence of the dynamic spin freezing temperature T_f in LiNi_0.4Co_0.2Mn_0.4O₂, analyzed using (b) the Néel–Arrhenius law, (c) the Vogel–Fulcher law, and (d) the power law.

So far, three kinds of models can be employed to quantitatively analyze the dynamic behavior near T_f: the Néel–Arrhenius law,^35–39 the Vogel–Fulcher law^34,40,41,43 and the power law^38,39,42. For a system of non-interacting superparamagnetic particles, the relaxation time follows the Néel–Arrhenius law

where E_a is the anisotropy energy barrier for the alignment of spin clusters, τ₀ the characteristic attempt frequency of the clusters, and k_B the Boltzmann constant. For a superparamagnetic relaxation, the value of τ₀ usually lies in the range ~10⁻¹⁰–10⁻¹³ s.³⁷ However, by plotting the τ versus T_f data using the Néel–Arrhenius law (Fig. 6(b)), an extremely small value of τ₀ ~ 10⁻⁴⁴ s is derived, which indicates that the Néel–Arrhenius law does not work well, and indicates the existence of magnetic interactions among the spin clusters.⁴⁰

The Vogel–Fulcher law is a modification of the Néel–Arrhenius law and takes the interactions among spin clusters into account, as expressed by

where T₀ is the Vogel–Fulcher temperature, a measure of the interaction strength among spin clusters in a spin glass. For τ₀ = 10⁻¹⁰ s as the commonly used value for a typical spin glass system, a linear variation of T_fversus 1/(ln τ – ln τ₀) is obtained for LiNi_0.4Mn_0.4Co_0.2O₂ (Fig. 6(c)). The best fit will give T₀ = 16 K, E_α/k_B = 58 K.

The power law, which supposes the existence of an equilibrium phase transition with a divergence of relaxation time near the transition temperature, has been used to explain the relaxation behavior in a spin glass system,

where T_g corresponds to the transition temperature, τ₀ is related to the relaxation time of the individual cluster magnetic moment, and zv is the dynamic critical exponent. Fig. 6(d) shows the fitting of eqn (5), and yields values τ₀ ≈ 1.9 × 10⁻¹¹ s and zv = 7.6. The value of zv agrees well with the range of zv ∼ 7.9 ± 1 K determined by the simulations of Ogielski for a three-dimensional SG with short-range magnetic interactions, which verifies the spin-glass-like transitions in LiNi_0.4Mn_0.4Co_0.2O₂.⁴²

The above experimental analysis confirms the spin-glass-like behavior in the LiNi_0.4Mn_0.4Co_0.2O₂ material prepared by co-precipitation. From the point of view of spin glass formation, geometrical frustration and random distribution of transition metal ions are regarded as the two main reasons. By calculating the frustration parameter , which is far below the limit where frustration plays an important role, we can rule out the influence of geometrical frustration.⁴⁴ On the other hand, in the local structure of LiNi_0.5Mn_0.5O₂, a superstructurally ordered “flower” structure has been suggested to be the ground state by first-principle calculations, where Li⁺ ions occupy the center surrounded by six Mn⁴⁺ ions, and the outer leaves are made up of twelve Ni²⁺ ions. In such a magnetic model, magnetic ordering with a short-range net magnetic moment formation happens at 95 K. When nonmagnetic Co³⁺ ions are introduced into the TM layers, they will act as a dilution of the magnetic network, destroy the superstructure ordering in [LiMn₆] “flowers”, and form ion clusters isolated by nonmagnetic Co³⁺ ions. Within the clusters there might be short-range ferromagnetic ordering coming from ferromagnetic 90° Mn⁴⁺–O–Mn⁴⁺ and direct 180° Ni²⁺–Mn⁴⁺ interactions,¹² as manifested by the small magnetic hysteresis in the low field range at 5 K. However, we cannot fully exclude the strong 180° Ni²⁺–O²⁻–Ni²⁺ antiferromagnetic interaction between different layers, although Co³⁺ doping can decrease the degree of Li⁺/Ni²⁺ disorder to 3.64%. The negative Weiss constant and no magnetic saturation, even at 5 T, strongly suggest dominant antiferromagnetic interactions among clusters. So the competition between ferromagnetic and antiferromagnetic interactions of clusters will induce a spin-glass-like transition, rather than the geometrical frustration effect.

Note that the specific chemical preparation method, including the sintering temperature, annealing time and so on, plays an important role in the electronic structure, ordered/disordered arrangement of transition metal ions and stoichiometric ratios in the transition metal oxides. For example, the A-site ordered RBaMn₂O₆ systems show a clearly separated charge-ordering and Néel transition temperature, instead of the spin glass transition observed in the disordered R_0.5Ba_0.5MnO₃.^45,46 As listed in Table 2, we summarize the magnetic parameters of three samples with different preparation conditions. It can be seen that our previous study of the LiNi_0.4Mn_0.4Co_0.2O₂ material prepared by the citrate precursor method shows the largest Curie constant and effective moment, which may come from the divergence of oxidation of bivalence in Ni and tetravalence in Mn, while having the largest coercive field and remanence, highest irreversibility, freezing temperature and negative Weiss constant imply a large size of the spin clusters with the strongest antiferromagnetic interactions, due to the high degree of Li⁺/Ni²⁺ disorder. In this paper we employ the co-precipitation method for synthesis, similar to the method used in ref. 12. But the magnetic parameters also show considerable differences. We think that although the preparation method, sintering temperature and time are nearly the same, the degree of disorder arrangement of the Ni²⁺/Mn⁴⁺/Co³⁺ ions is not exactly the same, in spite of the similar chemical preparation methods, and results in different sizes of ion clusters. The disorder in the LiNi_yMn_yCo_1−2yO₂ compounds would not only induce different magnetic behavior, but would undoubtedly influence the electrochemical performances in two possible ways. The first one is the disordered Li⁺/Ni²⁺ in the different layers, since rhombohedral LiNi_yMn_yCo_1−2yO₂ is characteristic of the layered structure, which provides two-dimensional channel transport of the Li ions during the discharge/charge cycles. The disorder means that some of the Li⁺ ions are introduced into the transition metal layers, which cannot be released in the discharge cycle. Hence, the electrochemical capacity will decrease correspondingly. The other cause lies in the disorder of the transition metal layers. It has been reported that in the LiNi_0.4Mn_0.4Co_0.2O₂ system, Ni²⁺ serves as a two-electron redox center, Co³⁺ increases the electronic conductivity and the non-Jahn–Teller Mn⁴⁺ helps to stabilize the rhombohedral layered framework. If large ion clusters were formed in the transition metal layers, the material would show the electronic phase separation state to some extent, and all the clusters would exhibit a separated electrochemical performance as LiNiO₂, LiMnO₂ or LiCoO₂. The effect of the three co-doped ions would be fully restrained during the electrochemical cycling. So reaching a perfect ordering of Ni²⁺/Mn⁴⁺/Co³⁺ ions in the transition metal layers might still be a serious challenge for future work.

4. Conclusion

LiNi_0.4Mn_0.4Co_0.2O₂ was prepared by the co-precipitation method. Powder X-ray diffraction confirmed the rhombohedral layered structure of the material. The sample exhibited an initial discharge capacity of 166 and 206 mA h g⁻¹ in different potential windows of 2.5–4.4 V and 2.5–4.6 V, respectively. X-ray photoelectron spectroscopy showed some divergence of +2 in Ni ions and +4 in Mn ions, which corresponds well to the high-temperature Curie–Weiss law fitting. A spin-glass-like transition was suggested at low temperature, based on the analysis of dc temperature and field dependent magnetization and ac frequency dependent susceptibility. In comparison with previous results, we considered that the disordered arrangement of Ni²⁺/Mn⁴⁺/Co³⁺ ions in the transition metal layer and of Li⁺/Nⁱ²⁺ in the different layers, induced by slight differences in the synthetic procedure, was one of the most important factors that influenced the magnetic and electrochemical behavior of the material.

Acknowledgements

This work was supported by the Special Funds for Major State Basic Research Project of China (2009CB220104), Jilin Province Project of Research and Development, China (20075007), the National Natural Science Foundation of China (Grand No.11004073) and Research Fund for the Doctoral Program of Higher Education of China (new teacher) 20090061120020. Parts of this work was also sponsored by the PhD. Candidates Interdiscipline Research Project of Jilin University (2011J015) and the Development Program of Science and Technology of Jilin Province, China (No. 201205035)

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