Article ID: 1003-6326(2005)06-1425-04

Synthesis and electrochemical characterization of

layered Li[Ni_1/3Co_1/3Mn_1/3]O₂ cathode material

for Li-ion batteries

YU Xiao-yuan(ÓíóãÔª)^1,2, HU Guo-rong(ºú¹úÈÙ)²,

PENG Zhong-dong(ÅíÖÒ¶«)², XIAO Jin(Ф ¾¢)², LIU Ye-xiang(ÁõÒµÏè)²

(1. School of Chemistry and Chemical Engineering, Central South University,

Changsha 410083, China;

2. School of Metallurgical Science and Engineering, Central South University,

Changsha 410083, China)

Abstract: Layered LiNi_1/3Co_1/3Mn_1/3O₂ materials were synthesized using a nickel-cobalt-manganese carbonate precursor obtained by chemical co-precipitation. The [Ni_1/3Co_1/3Mn_1/3]CO₃ precursor and the LiNi_1/3Co_1/3Mn_1/3O₂ powders were characterized by X-ray diffraction(XRD) and scanning electron micrograph(SEM). The SEM analysis shows that these particles possess uniform and spherical morphology. The electrochemical properties of the LiNi_1/3-Co_1/3Mn_1/3O₂ cathode material for rechargeable lithium-ion batteries such as the galvanostatic charge-discharge performance and cyclic voltammetry(CV) were measured. The results show that an initial discharge capacity of 190.29mA¡¤h¡¤g^-1 is obtained in the voltage range of 2.5-4.6V and at a current rate of 0.1C at 25¡æ.The discharge capacity increases linearly with the increase of the upper cut-off voltage limit.

Key words: lithium-ion batteries; cathode material; layered structure; nickel-cobalt-manganese oxides CLC

number: TM911.1                 Document code: A

1 INTRODUCTION

Due to the high cost of LiCoO₂, a commonly used cathode material in commercial rechargeable lithium-ion batteries, much efforts have been made to develop cheaper cathode materials than LiCoO₂, LiNiO₂ and LiMnO₂ have been studied extensively as possible alternatives to LiCoO₂^{[1-4 ]}. Stoichiometric LiNiO₂ is known to be difficult to synthesize and its multi-phase reaction during electrochemical cycling leads to structural degradation, and layered LiMnO₂ has a significant drawback in its crystallographic transformation to spinel structure during cycling^[5-7]. Recently, a concept of one-to-one solid solution of LiCoO₂, LiNiO₂ and LiMnO₂, i.e., LiNi_1/3Co_1/3Mn_1/3O₂, was adopted to overcome the disadvantage of LiNiO₂ and LiMnO₂^[8-12]. The layered LiNi_1/3Co_1/3Mn_1/3O₂ is an attractive cathode material for rechargeable lithium-ion batteries in several aspects. In this research, layered LiNi_1/3-Co_1/3Mn_1/3O₂ was prepared using the nickel-cobalt-manganese carbonate precursor, and the electrochemical properties of LiNi_1/3Co_1/3Mn_1/3O₂ were investigated.

2 EXPERIMENTAL

LiNi_1/3Co_1/3Mn_1/3O₂ powders were synthesized by mixed carbonate method, an aqueous solution of metal nitrates was made with a cation ratio, n(Ni)¡Ãn(Co)¡Ãn(Mn)=1¡Ã1¡Ã1, the precipitation of [Ni_1/3Co_1/3Mn_1/3]CO₃ was achieved by slowly dripping the nitrate solution to a NH₄HCO₃ solution with continuous stirring. The filtrated precipitate was washed with de-ionized water and dried in air, then mixed with stoichiometric amount of Li₂CO₃ by ball-milling. The mixed powders were heated at 480¡æ for 6h and then calcined at 950¡æ for 16h in air.

The thermal behavior of the precursor was examined by thermogravimetric analysis(TGA).The powder was characterized by X-ray powder diffraction measurements using a diffractometer PW 1710 with CuK¦Á radiation (Japan). The morphology of sample was observed using scanning electron microscopy (SEM, KYKY 2800, Japan). The electrochemical properties of LiNi_1/3Co_1/3Mn_1/3O₂ as cathode materials were evaluated using prototype cell on LAND-2001A battery program-control test system, using a lithium metal foil as the anode and 1mol/L LiPF₆ in a 1¡Ã1 solvent of Ethylene carbonate(EC) and Dimethyl carbonate(DMC) as electrolyte. The separator was made from a Celgard 2400 film microporous polypropylene membrane. The cells were assembled in argon gas filled glove-box.

The microelectrode was produced in glove-box with the mixture of the samples and carbon black as the working electrode in ratio of 8¡Ã1 and with the pure-lithium foil as the count-electrode. The cyclic voltammetry curves were measured by Potentiostat/Gallanostat Model (Perkin-Elmer 273A, EG&E).

3 RESULTS AND DISCUSSION

The thermal behavior of the [Ni_1/3Co_1/3-Mn_1/3]CO₃ precursor and Li₂CO₃ was examined by thermogravimetric analysis(TGA). From the TG and DTA results of the precursor, it reveals that below 350¡æ, there is a mass loss due to the decomposition process of the carbonate compound. The mass loss of the specimens stops at temperatures above 480¡æ until to 1080¡æ (Fig.1).

Fig.1  TG and DTA curves of mixture of  [Ni_1/3Co_1/3Mn_1/3]CO₃ precursor and Li₂CO₃

    The XRD pattern of [Ni_1/3Co_1/3Mn_1/3]CO₃ precursor obtained by co-precipitation method is shown in Fig.2(a). Although the XRD pattern of precursor has a low crystallinity, it is found that the precursor has a similar well-defined Ni_1/3Co_1/3-Mn_1/3CO₃ hexagonal structure (a=4.52¦@, c=15.6¦@) with no impurity phase, This would be attributed to the homogeneous powder precursor, in which Ni, Co and Mn are uniformly distributed in an atomic scale. The powder X-ray diffraction pattern of LiNi_1/3Co_1/3Mn_1/3O₂ finial sample is shown in Fig.2(b). The XRD pattern is well defined and shows the hexagonal doublets (006)/(102) and (108)/(110) a clear splitting, which indicate that they have a high degree of crystallization, good hexagonal ordering and greater layered characteristics. The integrated intensity ratio of the (003) peak to (104) peak(R) in the XRD patterns is shown to be a measure of ¡°cation mixing¡± and a value of R¡´1.2 is an indication of undesirable cation mixing^{[13, 14]}. The ratio of the intensity of the (003) peak to (104) peak of the LiNi_1/3Co_1/3-Mn_1/3O₂ sample reported here was calculated to be R=1.42, well above the values reported of undesirable cation mixing. The lattice parameters of LiNi_1/3Co_1/3Mn_1/3O₂ are: a=2.866¦@, c=14.262¦@ and match with the values observed by Shaju et al^[4] and Yabuuchi et al^[15] (a=2.867¦@ and c=14.246¦@), and the c/a ratio is 4.976. The high value of c/a means that the de-intercalation/intercalation of Li⁺ is more flexible.

Fig.2  XRD pattern of [Ni_1/3Co_1/3Mn_1/3]CO₃  precursor(a) and LiNi_1/3Co_1/3Mn_1/3O₂ final sample(b)

   The SEM images of precursor and final powders is shown in Fig.3. It can be seen that particles in Ni_1/3Co_1/3Mn_1/3CO₃ precursors and the LiNi_1/3-Co_1/3Mn_1/3O₂ powders possess spherical morphology. However, the size of LiNi_1/3Co_1/3Mn_1/3O₂ particles is more uniform and in the range of 1-2¦Ìm.

Fig.3  SEM images of [Ni_1/3Co_1/3Mn_1/3]CO₃  precursor(a) and LiNi_1/3Co_1/3Mn_1/3O₂ final sample(b)

Fig.4  Charge and discharge curves (at 0.1C) of LiNi_1/3Co_1/3Mn_1/3O₂ powder at 25¡æ

    Fig.4 shows the charge and discharge curves for the Li/ LiNi_1/3Co_1/3Mn_1/3O₂ cell at a current rate of 0.1C in voltage window 2.5-4.6V at room temperature. As seen in Fig.4, the initial discharged capacity of 190.29mA¡¤h¡¤g^-1 is obtained. On starting the current, the voltage suddenly increases to about 4V and then slowly decreases to 3.75V and stays along an almost horizontal line at 3.75V, until the charge capacity reaches about 95mA¡¤h¡¤g^-1. The slope in the voltage versus capacity curves increase at 95mA¡¤h¡¤g^-1 and voltage curves linearly increase until voltage reaches 4.6V, similar to that observed by Yabuuchi and Ohzuku^[15] .The irreversible capacity observed in the first cycle is about 40mA¡¤h¡¤g^-1.

Fig.5 shows the specific discharge capacity vs number of cycle for Li/ LiNi_1/3Co_1/3Mn_1/3O₂ cell at 25¡æ at a constant current density of 0.1mA/cm² in the different voltage range of 2.5-4.3, 2.5-4.4, 2.5-4.5 and 2.5-4.6V. The specific discharge capacity increases linearly with the increase of the upper cut-off voltage limit, the discharge capacities of LiNi_1/3Co_1/3Mn_1/3O₂ electrode are 190.29, 172.25,164.27 and 156.12mA¡¤h¡¤g^-1, respectively, with good cycleability. The discharge capacities remain at 158.73, 153.59, 149.35 and 146.86mA¡¤h¡¤g^-1 after 20 cycles, which are 83.42%, 89.17%, 90.92% and 94.07% of initial capacities, respectively.

Fig.6 shows the cyclic voltammetry curve of the Li/ LiNi_1/3Co_1/3Mn_1/3O₂ cell between 2.8V and  4.6V at a scan rate of 0.05mV/s at room

Fig.5  Discharge capacity vs number of cycle for Li/ LiNi_1/3Co_1/3Mn_1/3O₂ cell at 0.1C in different voltage range at 25¡æ

Fig.6  Cyclic voltammetry curve of Li/ LiNi_1/3Co_1/3Mn_1/3O₂ cell between 2.5V and 4.6V at scan rate of 0.05mV/s

 temperature. As can been seen from Fig.6, the main oxidation peak is observed at 3.9V, while the reduction peak appears at 3.7V, corresponding to Ni^2+/4+. The material has a couple of redox peak representing the de-intercalation of Li⁺ from the initial structure that is observed in a narrow potential range. This implies that the extraction of Li⁺ occurs easily from an ordered and stabilized layered structure of LiNi_1/3Co_1/3Mn_1/3O₂.

4 CONCLUSIONS

The layered LiNi_1/3Co_1/3Mn_1/3O₂ was synthesized using a nickel-cobalt-manganese carbonate precursor and characterized by means of XRD, SEM, galvanostatic charge-discharge performance and cyclic voltammetry(CV). The lattice parameters obtained are: a=2.866¦@, and c=14.262¦@. The nicely split (006)/(102) and (108)/(110) peak in the XRD patterns reveal the layered structure of the compound. The initial discharge capacity of 190.29mA¡¤h¡¤g^-1 was obtained in the range of 2.5-4.6V and at a current rate of 0.1C at 25¡æ, and the discharge capacity increases linearly with the increase of the upper cut-off voltage limit. Cyclic voltammetry shows the major redox process at 3.7-3.9V corresponding to Ni^2+/4+. The results indicate that the layered LiNi_1/3-Co_1/3Mn_1/3O₂ is an attractive cathode material for rechargeable lithium-ion batteries.

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(Edited by LI Xiang-qun)

Received date: 2005-03-10; Accepted date: 2005-06-03

Correspondence: YU Xiao-yuan, PhD; Tel: +86-731-8830474; Fax: +86-731-8876454; E-mail address: yxy7021@mail.csu.edu.cn