­­Synthesis of LiNi_0.8Co_0.1Mn_0.1O₂ cathode material by chloride co-precipitation method

LI Ling-jun(李灵均), LI Xin-hai(李新海), WANG Zhi-xing(王志兴),

WU Ling(伍 凌), ZHENG Jun-chao(郑俊超), LI Jin-hui(李金辉)

School of Metallurgical Science and Engineering, Central South University, Changsha 410083, China

Received 6 July 2009; accepted 30 December 2009

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Abstract: LiNi_0.8Co_0.1Mn_0.1O₂ was prepared by a chloride co-precipitation method and characterized by thermogravimetric analysis, X-ray diffractometry with Rietveld refinement, electron scanning microscopy and electrochemical measurements. Effects of lithium ion content and sintering temperature on physical and electrochemical performance of LiNi_0.8Co_0.1Mn_0.1O₂ were also investigated. The results show that the sample synthesized at 750 ℃ with 105% lithium content has fine particle sizes around 200 nm and homogenous sizes distribution. The initial discharge capacity for the powder is 184 mA?h/g between 2.7 and 4.3 V at 0.1C and room temperature.

Key words: lithium ion battery; LiNi_0.8Co_0.1Mn_0.1O₂; chloride co-precipitation; Rietveld refinement

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1 Introduction

Recently, layered LiNi_0.8Co_0.1Mn_0.1O₂ has been intensively studied as a potential positive active electrode for application in batteries for hybrid electric vehicles (HEVs)[1-2]. It is reported that the mixed oxide inherits the merits of mono metal oxide such as LiCoO₂, LiNiO₂ and LiMnO₂, and exhibits high capacity, good cycling stability and excellent safety performance[3-6]. Nevertheless, compared with other lithium ion cathode materials, such as LiFePO₄, the cycling ability of LiNi_0.8Co_0.1Mn_0.1O₂ still needs to be improved, due to irreversible capacity loss caused by cation mixing[2, 7-9]. Several effective ways have been proposed to lower the mixing degrees, including adding excess Li ion to restrain Ni²⁺ moved to Li layer, and substitution of a small quantity of layered metal by Al and Mg ions[2, 10-11].

The tradition way to synthesize LiNi_0.8Co_0.1Mn_0.1O₂ is co-precipitation method with sulphate and nitrate as raw materials, and their particle sizes are 10-40 μm[9, 12]. As pointed out before, the electrochemical behavior of lithium ion cathode material is strongly influenced by preparation method, which includes different raw materials and particle size of final product[13-14]. It is also reported that submicron and nanometer particles are conducive to shorten the Li ion diffusion route, and finally improve the electrochemical performance of material[15]. Therefore, we assumed that adding excessive Li and decreasing particle size maybe benefit the capacity and cycling-life of LiNi_0.8Co_0.1Mn_0.1O₂. In this work, through a chloride co-precipitation method, submicron LiNi_0.8Co_0.1Mn_0.1O₂ was attained, and effects of lithium ion content and sintering temperature on physical and electrochemical performance of final products were also investigated.

LiNi_0.8Co_0.1Mn_0.1O₂ was synthesized with the starting materials of LiOH·H₂O, NiCl₂·6H₂O, CoCl₂·6H₂O, MnCl₂·4H₂O, NaOH and NH₃·H₂O. NiCl₂·6H₂O, CoCl₂·6H₂O and MnCl₂·4H₂O powders were dissolved in distilled water to obtain 2 mol/L solution (n(Ni):n(Co):n(Mn)=8:1:1). The mixtures were filled in beaker and heated at 50 ℃ in water bath kettle, then NH₃·H₂O and 2 mol/L NaOH were added to the solution to control the pH value between 11 and 12. After being stirred for 5 min, the solution was filtered to separate the precipitate powders, and the powder was washed with distilled water and dried at 100 ℃ for 8 h. Later, LiOH was milled with the powder for 0.5 h in stoichiometric ratios of Li to (Ni+Co+Mn) is 1.03, 1.05 and 1.07, respectively, and the obtained mixture was sintered at 480 ℃ for 5 h, and then sintered at 750 ℃ for 12 h in O₂ atmosphere, respectively. After being cooled to room temperature, the finale samples were obtained.

The TG—DTG analysis was tested by SDTQ600 with a step time of 10 (?)/min in O₂ atmosphere. The crystalline nature of the samples was identified with X-ray diffractometry (XRD, Dmax/2550VB+18 kW, Rigaku using monochromatic Cu K_α radiation with a step time of 2(?)/min), and X-ray Rietveld re?nement was performed by FULLPROF. Powder morphologies were observed by scanning electron microscope (SEM, JEOL JSM- 5600LV).

The electrochemical properties of LiNi_0.8Co_0.1Mn_0.1O₂ were measured by using 2025 button cell. The cathode consisted of 80% (mass fraction) active material, 10% acetylene black and 10% PVDF binder. A lithium metal foil was used as anode. LiPF6 (1 mol/L) in a 1:1:1(volume ratio) mixture of dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) and ethylene carbonate (EC) was used as electrolyte. Charge– discharge performances were evaluated using a battery test system (BTS-5V/1mA, Xinwei 2.7-4.3 V).

The TG—DTG curves of the mixture of LiOH·H₂O and Ni_0.8Co_0.1Mn_0.1(OH)₂ powders are shown in Fig.1. It is clear that three endothermic peaks and mass loss sections appear at 80-96℃, 226-273℃, 430℃, respectively, which represent the loss of absorbed water, dehydration reactions of LiOH·H₂O and Ni_0.8Co_0.1- Mn_0.1(OH)₂, respectively. With the increase of temperature, the dehydration products, Ni_0.8Co_0.1Mn_0.1O₂ and Li₂O, react slowly to LiNi_0.8Co_0.1Mn_0.1O₂. Owing to the fact that the reaction is composed by several sections including dehydration process as shown above, two- section sinter should be introduced to ensure sufficient decomposition and contact between LiOH·H₂O and precursor, and finally attain well electrochemical performance.

Fig.1 TG—DTG curves of mixture of LiOH·H₂O and Ni_0.8Co_0.1Mn_0.1(OH)₂ powders

Fig.2 shows the XRD patterns of LiNi_0.8Co_0.1- Mn_0.1O₂, synthesized at different temperatures or Li ion content. It is noted that all diffraction lines are indexed on the basis of the rhombohedral α-NaFeO₂ structure with a space group of R-3m. The samples with 105% Li ion content in Fig.2(a) were prepared at 700, 750 and 800 ℃, respectively. It is obvious that the ratio of I₀₀₃ to I₁₀₄ for LiNi_0.8Co_0.1Mn_0.1O₂ powder obtained at 750℃ is 1.51, larger than 1.01 and 1.20 obtained at 700 ℃ and 800℃, respectively, which scales lower mixing degree of Ni⁺ in Li layer than others. As we know, the excess of Li⁺, whether in Li layer or in transition metal layer, will suppress the migration of Ni⁺ into Li layer. Fig.2(b) shows the effect of various Li⁺ contents on the XRD pattern of LiNi_0.8Co_0.1Mn_0.1O₂ powders prepared at 750 ℃. It is found that the ratio of I₀₀₃ to I₁₀₄ for LiNi_0.8Co_0.1Mn_0.1O₂ powders is gradually enhanced from 1.29, 1.51 , to 1.52 with the increase of Li ion content.

Fig.2 XRD patterns of LiNi_0.8Co_0.1Mn_0.1O₂, synthesized at different temperatures (a) and Li ion contents (b)

The corresponding Rietveld refinement plot and parameters of LiNi_0.8Co_0.1Mn_0.1O₂ powder synthesized at 750 ℃ are shown in Fig.3 and Table 1, respectively.The refinements were performed by assuming a partial occupation of Ni in the Li layers. The refinement results are in good agreement between the observed and calculated patterns in Fig.3 with high reliability factor. As seen in Table 1, only 5.5% Ni moves to the 3b-site (lithium site), and the lattice parameters of LiNi_0.8Co_0.1Mn_0.1O₂ are smaller compared with the results from other research groups. WOO et al[11] reported that a=0.286 87 nm, c=1.425 31 nm, which demonstrated that well-ordering layered and smaller LiNi_0.8Co_0.1Mn_0.1O₂ powder can be prepared by chloride co-precipitation method.

Fig.3 Rietveld refinement patterns of XRD data for LiNi_0.8Co_0.1Mn_0.1O₂ (synthesized with 105% Li ion content at 750 ℃)

Table 1 Structural parameters obtained from Rietveld Refinement of XRD data of LiNi_0.8Co_0.1Mn_0.1O₂

Fig.4 presents the SEM images of the precursor and LiNi_0.8Co_0.1Mn_0.1O₂ powder synthesized with 105% of Li content at 750 ℃. As it can be seen that the particle morphologies of images are near-spherical and well distributed. And the particle sizes of precursor and LiNi_0.8Co_0.1Mn_0.1O₂ powders are fine, about 200 nm, which is conducive to shorten the lithium diffusion distance.

Fig.4 SEM images of Ni_0.8Co_0.1Mn_0.1(OH)₂ and LiNi_0.8Co_0.1- Mn_0.1O₂

The initial discharge curves of LiNi_0.8Co_0.1Mn_0.1O₂ prepared at different temperatures and Li ion contents are shown in Fig.5. The batteries were measured at about 10 ℃, 2.7-4.3 V and 0.1C (18 mA?h/g), respectively. All the samples exhibit a diagonal voltage plateau from 4.3 V to 3.58 V. In Fig.5, the sample obtained at 750 ℃ with 105% of Li ion content deliveries the best discharge capacity, 184 mA?h/g, which is consistent with that shown in Fig.2 or Fig.3, that lower mixing degree of Ni⁺ in Li layer results in better electrochemical performance. It is also noted that the capacities of sample synthesized with 107% and 103% are almost equal, which can be ascribed that too much Li ion might occupy the site of transition metals, while, the lack of Li ion might induce to cation mixing, both of which should take responsibility to the capacity irreversible loss of LiNi_0.8Co_0.1Mn_0.1O₂.

Fig.5 First discharge curves of LiNi_0.8Co_0.1Mn_0.1O₂ synthesized at different temperatures (a) and Li ion contents (b) at 18 mA?h/g (0.1C) and 2.7-4.3 V

Fig.6 shows the cycling abilities of LiNi_0.8Co_0.1Mn_0.1O₂ synthesized with different Li ion contents. After 30 cycles, all samples experience various capacity loss degrees. The sample prepared with 103% of Li ion content exhibits the worst cycle ability, and its discharge capacity was only 140 mA?h/g after 30 cycles. The capacity of sample prepared with 107% of Li ion content, on the other hand, exhibits 149 mA?h/g after 30 cycles. This demonstrated that excess Li ion can avoid cation mixing and improve cycle ability effectively. The sample prepared with 105% of Li ion content is found to be the optimum, which exhibited the highest discharge capacity and well cycle-life, with 169 mA?h/g after 30 cycles.

Fig.6 Cycling performance of LiNi_0.8Co_0.1Mn_0.1O₂ synthesized with various Li ion content tested at 0.1C and 2.7- 4.3 V

4 Conclusions

1) Layered LiNi_0.8Co_0.1Mn_0.1O₂ compounds are synthesized with various Li ion contents at different temperatures by a chloride co-precipitation method.

2) XRD, Rietveld refinement and SEM analyses indicate that well-ordering, near-spherical and well distributed precursor has attained, and the particle size of LiNi_0.8Co_0.1Mn_0.1O₂ compound is nearly 200 nm.

3) It is noted that the initial discharge capacity of the LiNi_0.8Co_0.1Mn_0.1O₂ synthesized with 105% of Li ion content at 750 ℃ is 184 mA?h/g, showing 169 mA?h/g after 30 cycles.

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Foundation item: Project(2007CB613607) supported by National Basic Research Program of China

Corresponding author: LI Xin-hai; Tel: +86-731-88836633; E-mail: csullj@hotmail.com

(Edited by ZHAO Jun)