Synthesis and electrochemical properties of LiMn₂O₄/Li₄Ti₅O₁₂ composite

HE Ze-qiang(何则强)^{1, 2}, XIONG Li-zhi(熊利芝)^{1, 2},

WU Xian-ming(吴显明)¹, CHEN Shang(陈 上)¹, HUANG Ke-long(黄可龙)²

1. College of Biology and Environmental Sciences, Jishou University, Jishou 416000, China;

2. School of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China

Received 6 July 2009; accepted 30 December 2009

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Abstract: LiMn₂O₄/Li₄Ti₅O₁₂ composite was synthesized by in-situ composite technique using LiMn₂O₄, lithium acetate, tetrabutyl titanate as starting materials and characterized by various electrochemical methods in combination with X-ray diffractometry (XRD), infrared (IR) spectroscopy and scanning electron microscopy (SEM). The results show that Li₄Ti₅O₁₂ is coated on the surface of crystalline LiMn₂O₄ to form LiMn₂O₄/Li₄Ti₅O₁₂ composite. The structure of LiMn₂O₄ does not change due to the introduction of Li₄Ti₅O_12. By being coated with Li₄Ti₅O₁₂, the rate capability and high temperature cyclability of LiMn₂O₄ is improved greatly. At room temperature, the discharge capacity of LiMn₂O₄/Li₄Ti₅O₁₂ composite is more than 108.4 mA?h/g and the capacity loss per cycle is only 0.053% after 20 cycles at 2.0C.While at 55 ℃, the discharge capacity of LiMn₂O₄/Li₄Ti₅O₁₂ composite is more than 109.9 mA?h/g and the capacity loss per cycle is only 0.036% after 60 cycles at 1.0C.

Key words: Li₄Ti₅O₁₂; LiMn₂O₄; in-situ composite technique; lithium ion batteries

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LiMn₂O₄ is of great interest for the replacement of LiCoO₂ in Li ion batteries due to its low cost, nontoxicity and environmentally-friendly properties[1-2]. However, there are still some problems in terms of the wider application of LiMn₂O₄. The main problem is the capacity loss upon cycling at elevated temperature above 50 ℃[3-6]. Recently, this problem has partially been resolved by metal cation doping[7-8] and surface modification treatment[9-10].

Besides the long cycling life and safety, the high rate performance is also important for commercial batteries. In spite of the success in improving cycle stability, little has been reported on the enhanced rate capability of LiMn₂O₄. In particular, it is necessary to improve the rate capability of LiMn₂O₄ in order to be used as a cathode material of Li ion batteries for electric vehicles. The reason for poor rate capability of LiMn₂O₄ is not clear but it may be attributed to its low chemical diffusion coefficient (10^-9-10^-12 cm²/s)[11-12]. The low diffusion coefficient of LiMn₂O₄ can limit the current flow between particles, which may decrease its rate capability. Therefore, the increase in diffusion coefficient of LiMn₂O₄ probably improves its rate property.

LiMn₂O₄ was surface modified by nano-Li₄Ti₅O₁₂ particles to improve the high rate capability, in consideration of the following factors: 1) the chemical diffusion coefficient of Li₄Ti₅O₁₂ (10^-6 cm²/s)[13] being larger than that of LiMn₂O₄; 2) both Li₄Ti₅O₁₂ and LiMn₂O₄ sharing the same spinel structure and space group of Fd3m[14-15]; 3) Li₄Ti₅O₁₂ showing exceptional rate capacity retention even as high as 10C[16]; 4) Li₄Ti₅O₁₂ being a zero-strain material, which means that there is no structural change during the insertion/extraction of lithium ions[15].

In the present work, LiMn₂O₄/Li₄Ti₅O₁₂ composite was synthesized by in-situ composite technique and its electrochemical properties were investigated in detail.

2 Experimental

2.1 Preparation of LiMn₂O₄/Li₄Ti₅O₁₂ composite

A certain quantity of tetrabutyl titanate was solved in ethanol absolute with the volume ratio of tetrabutyl titanate) to ethanol absolute of 1?5. The solution of tetrabutyl titanate was added to an aqueous ethanol solution of lithium acetate by stirring strongly according to the stoichiometric ratio of Li₄Ti₅O₁₂ to get a clear yellow sol. A certain quantity of LiMn₂O₄ (the mass ratio of Li₄Ti₅O₁₂ to LiMn₂O₄ is 1?19) used in lithium ion batteries was dispersed uniformly in this sol with ultrasonic wave and stirrers. After stirring for 2-3 h, a black gel was obtained. The gel was dried at 105 ℃ for 4 h in vacuum to get a black precursor. The precursor was calcinated at 800 ℃ in Ar atmosphere for 6 h to obtain LiMn₂O₄/Li₄Ti₅O₁₂ composite.

2.2 Characterization of LiMn₂O₄/Li₄Ti₅O₁₂ composite

Phase identification studies of the samples were carried out by an X-ray diffractometer (Rigaku D/MAX-gA) with Cu K_α radiation. The surface morphology was done by scanning electron microscope (JSM 5600LV). The IR spectra of samples were conducted on a Fourier transform infrared spectrometer (Nicolet).

2.3 Electrochemical tests of LiMn₂O₄/Li₄Ti₅O₁₂ composite

A slurry containing 80% LiMn₂O₄/Li₄Ti₅O₁₂ composite, 10% acetylene black and 10% PVDF (polyvinylidene fluoride) was made using N-methylprrolidinone (NMP) as the solvent. Electrodes with an area of 1 cm² for the measurements of electrochemical characterization were prepared by coating slurries (about 100 μm in thickness) on aluminum foils followed by drying in vacuum at 60 ℃ for 12 h. Electrochemical tests were conducted using a conventional coin-type cell, employing lithium foil as a counter electrode and utilizing 1.0 mol/L LiPF₆ in ethylene carbonate/dimethyl carbonate (EC/DMC) (with an volume ratio of 1?1 of EC to DMC) as the electrolyte. The assembly was carried out in an Ar-filled glove box. The electrochemical analyses were carried out with an electrochemical analysis system.

3 Results and discussion

3.1 XRD analysis

Fig.1 shows the X-ray diffraction patterns of LiMn₂O₄ and LiMn₂O₄/Li₄Ti₅O₁₂. As can be seen, all those materials display nearly the same XRD patterns without any peak shift, which indicates that there is no structural change after the Li₄Ti₅O₁₂ surface modification treatment. In the modified samples, no impurity phase is detected, suggesting that Li₄Ti₅O₁₂ just exists on the surface of LiMn₂O₄ and is not incorporated into the host structure since no changes are detected in the XRD patterns.

Fig.1 XRD patterns of LiMn₂O₄ (a) and LiMn₂O₄/Li₄Ti₅O₁₂ (b)

3.2 IR analysis

Fig.2 shows the IR spectra of LiMn₂O₄ and LiMn₂O₄/Li₄Ti₅O₁₂. It is obvious that the spectra of LiMn₂O₄ and LiMn₂O₄/Li₄Ti₅O₁₂ agrees well, suggesting that Li₄Ti₅O₁₂ just exists on the surface of LiMn₂O₄.The peaks at 3 200-3 600 cm^-1 and 1 600 cm^-1 are due to the absorbed water on the surface of samples. The peaks at  2 300-2 500 cm^-1 is caused by CO₂ in air. The peaks between 400 and 1 200 cm^-1 are attributed to the metal-oxygen bond in samples.

Fig.2 IR spectra of LiMn₂O₄ (a) and LiMn₂O₄/Li₄Ti₅O₁₂ (b)

3.3 SEM analysis

The SEM images of LiMn₂O₄ and LiMn₂O₄/ Li₄Ti₅O₁₂ are shown in Fig.3. It can be seen clearly that the surface of the bare LiMn₂O₄ is smooth, but after surface modification treatment, a lot of nanoparticles are observed on the surface of the core materials. Therefore, it is speculated that Li₄Ti₅O₁₂ particles coated on LiMn₂O₄ and Li₄Ti₅O₁₂ coating would minimize the direct contact area between the cathode and the electrolyte.

Fig.3 SEM images of LiMn₂O₄ (a) and LiMn₂O₄/Li₄Ti₅O₁₂ (b)

3.4 Electrochemical properties

Fig.4 shows the initial discharge curves of the uncoated LiMn₂O₄ and LiMn₂O₄/LiMn₂O₄/Li₄Ti₅O₁₂ electrode at 1C between 3.4 and 4.3 V. It is evident that all these electrodes display the characteristic two 4 V plateaus, and similar results are also observed under 2C and 5C, which means that Li₄Ti₅O₁₂ coating does not change the intrinsic properties of LiMn₂O₄, such as charge/discharge behavior and crystal structure. In addition, the first-cycle discharge capacity of the LiMn₂O₄ is 122.2 mA?h/g, while that of the LiMn₂O₄/Li₄Ti₅O₁₂ is 119.3 mA?h/g. The capacity decreases with the increase of Li₄Ti₅O₁₂ coating amount, which can be attributed to the less active spinel in the electrode.

Fig.4 Initial discharge capacities of LiMn₂O₄ and LiMn₂O₄/Li₄Ti₅O₁₂ at 0.1C

Fig.5 compares the cycling performance of LiMn₂O₄ and LiMn₂O₄/Li₄Ti₅O₁₂ at 0.1C. After 100 cycles at 0.1C, the discharge capacity of LiMn₂O₄ decreases from 122.2 mA?h/g to 118.3 mA?h/g and the capacity loss per cycle is 0.032%. Under the same conditions, the discharge capacity of LiMn₂O₄/Li₄Ti₅O₁₂ decreases from 119.3 mA?h/g to 117.1 mA?h/g and the capacity loss per cycle is 0.018%, suggesting that the loading of Li₄Ti₅O₁₂ on LiMn₂O₄ helps to improve the cycling stability of LiMn₂O_4.

Fig.5 Cycling performance of LiMn₂O₄ and LiMn₂O₄/Li₄Ti₅O₁₂ at 0.1C

Table 1 presents the comparison of cycling performance of LiMn₂O₄ and LiMn₂O₄/Li₄Ti₅O₁₂ at various current rates. With the increase of discharge current rate, the discharge capacities of both samples decrease. This may be due to the increase of the electrode polarization at higher discharge current rate. In the point of capacity loss after 20 cycles, it can be seen obviously that the discharge capacity loss of LiMn₂O₄/Li₄Ti₅O₁₂ is lower than that of LiMn₂O₄. After 20 cycles at 2.0C, the discharge capacity of LiMn₂O₄ is 108.3 mA?h/g and the capacity loss per cycle is 0.108%. Under the same conditions, the discharge capacity of LiMn₂O₄/Li₄Ti₅O₁₂ keeps at 108.4 mA?h/g and the capacity loss per cycle is more than 0.053%, suggesting that the loading of Li₄Ti₅O₁₂ on LiMn₂O₄ improves the rate stability of LiMn₂O_4. This may be explained that Li₄Ti₅O₁₂ coating minimizes the direct contact area between the cathode and the electrolyte[17] and increases the chemical diffusion coefficient of lithium ion, accordingly improves the high rate performance of LiMn₂O₄.

Table 1 Comparison of cycling performance for LiMn₂O₄ and LiMn₂O₄/Li₄Ti₅O₁₂ composite at various current rates

Fig.6 shows the high temperature cycling performance of LiMn₂O₄ and LiMn₂O₄/Li₄Ti₅O₁₂ at 55 ℃ and 1.0C. The discharge capacities of LiMn₂O₄ and LiMn₂O₄/Li₄Ti₅O₁₂ are 113.1 mA?h/g and 112.3 mA?h/g at 55 ℃, respectively, lower than their discharge capacities at room temperature (as shown in Table 1).

Fig.6 High temperature cycling performance of LiMn₂O₄ and LiMn₂O₄/Li₄Ti₅O₁₂ at 1.0C

After 20 cycles at 55 ℃, the discharge capacities of LiMn₂O₄ and LiMn₂O₄/Li₄Ti₅O₁₂ are 111.3 mA?h/g and 111.5 mA?h/g, and the capacity losses are 0.080% and 0.036% per cycle, respectively. After 60 cycles at 55 ℃, the discharge capacities of LiMn₂O₄ and LiMn₂O₄/Li₄Ti₅O₁₂ are 108.0 mA?h/g and 109.9 mA?h/g, and the capacity losses are 0.075% and 0.036% per cycle, respectively. The high temperature cycling performance of LiMn₂O₄ is improved significantly by coating Li₄Ti₅O₁₂. Surface modification of Li₄Ti₅O₁₂ can obstruct the direct contact and suppress the side reactions between the electrode and electrolyte to prevent the spinel LiMn₂O₄ from dissolving into the electrolyte, resulting in improved high temperature cycling performance of LiMn₂O₄[8, 18-20].

4 Conclusions

1) LiMn₂O₄/Li₄Ti₅O₁₂ composite was prepared by in-situ composite technique using LiMn₂O₄, lithium acetate, tetrabutyl titanate as starting materials. Li₄Ti₅O₁₂ just exists on the surface of LiMn₂O₄ and is not incorporated into the host structure of LiMn₂O₄.

2) By coating Li₄Ti₅O₁₂ on the surface of LiMn₂O₄, the rate capability and high temperature cyclability of LiMn₂O₄ are improved greatly. This is because that the surface modification of Li₄Ti₅O₁₂ can minimize the direct contact area between the cathode and the electrolyte, increase the chemical diffusion coefficient of lithium ion and suppress the side reactions between the electrode and electrolyte to prevent the spinel LiMn₂O₄ from dissolving into the electrolyte.

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Foundation item: Project(20376086) supported by the National Natural Science Foundation of China; Project(2005037700) supported by Postdoctoral Science Foundation of China; Project(07JJ3014) supported by Hunan Provincial Natural Science Foundation of China; Project(07A058) supported by Scientific Research Fund of Hunan Provincial Education Department; Project(2004107) supported by Postdoctoral Science Foundation of Central South University

Corresponding author: HE Ze-qiang; Tel: +86-743-8564416; Fax: +86-743-8564416; E-mail: csuhzq@163.com

(Edited by CHEN Ai-hua)