Influence of Li₃PO₄ addition on properties of lithium ion-conductive electrolyte Li_1.3Al_0.3Ti_1.7(PO₄)₃

XIAO Zhuo-bing ¹, CHEN Shang², GUO Man-man¹

1. Key Laboratory of Hunan Forest Products and Chemical Industry Engineering, Zhangjiajie 427000, China;

2. College of Chemistry and Chemical Engineering, Jishou University, Jishou 416000, China

Received 18 April 2010; accepted 19 September 2011

Abstract: The sintered Li_1.3Al_0.3Ti_1.7(PO₄)₃  pellets with different mole fractions of Li₃PO₄ were prepared by sol-gel method. The structure, surface morphology, electrochemical window, ionic conductivity and activation energy of the sintered pellets were studied by X-ray diffraction, scan electron microscopy, cyclic voltammetry, and electrochemical impedance spectroscopy. The results show that all the sintered Li_1.3Al_0.3Ti_1.7(PO₄)₃ pellets with different mole fractions of Li₃PO₄ have similar X-ray diffraction patterns. The sintered Li_1.3Al_0.3Ti_1.7(PO₄)₃ pellet with Li₃PO₄ shows a lower porosity and is denser than the one without Li₃PO₄. The addition of Li₃PO₄ has a large effect on the porosity and density of the sintered Li_1.3Al_0.3Ti_1.7(PO₃)₄ pellet, and little effect on its electrochemical window. Among the sintered Li_1.3Al_0.3Ti_1.7(PO₃)₄ pellets with different mole fractions of Li₃PO₄, the one with 1% Li₃PO₄ shows the activation energy of 0.314 2 eV and the highest ionic conductivity of 6.15?10^-4 S/cm.

Key words: sol-gel; electrolyte; ionic conduction

1 Introduction

Lithium titanium phosphate, LiTi₂(PO₄)₃, which is based upon the NASICON (Na+super-ionic+conductor) framework with a space group of RC, has been studied as a lithium-ion-conductive solid electrolyte [1-2]. It is one of the promising lithium ion-conductive solid electrolytes. However, its conductivity is not high enough for practical applications [3-4


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]. In order to obtain LiTi₂(PO₄)₃ with a higher ionic conductivity, some researchers replaced Ti with Zr [2], Nb [5], La [6], Y [7], and Ge [8]. However, these studies did not result in satisfied ionic conductivity. AONO et al [9] reported that Al-doping was effective for the enhancement of lithium ion conduction. Since then, a lot of attention has been paid to Li_1.3Al_0.3Ti_1.7(PO₄)₃ for possible use in solid-state lithium-ion batteries [10-15].

Sol-gel method is one of the most frequently employed techniques to synthesize various materials. The technique has several advantages including good mixture of the reagents, relatively low crystallization temperature, homogenous and small grain size of product. The small grained powders can result in sintered pellet with high density, small voids and grains, which leads to a high ionic conductivity [16].

Flux can influence the sinterability and therefore affect the ionic conductivity and strength of the sintered electrolyte pellet [17]. Considering Li₃PO₄ a good flux with a relatively low melting temperature of 837 °C, the characterization of sol-gel derived Li_1.3Al_0.3Ti_1.7(PO₄)₃ with different mole fractions of Li₃PO₄ flux is studied in the present work to obtain Li_1.3Al_0.3Ti_1.7(PO₄)₃ pellet with high ionic conductivity and strength, which can be used in thin-film battery as both electrolyte and substrate. To the author’s knowledge, there are few reports about the influence of Li₃PO₄ addition on the properties of lithium ion-conductive electrolyte Li_1.3Al_0.3Ti_1.7(PO₄)₃.

2 Experimental

Li(CH₃COO)·2H₂O and Al(NO₃)₃·9H₂O at stoichiometric amount were dissolved in CH₃OCH₂CH₂OH. Then PO(OC₄H₉)₄ and Ti(OC₄H₉)₄ were consecutively dripped into the mixture under constant stirring to form Li_1.3Al_0.3Ti_1.7(PO₄)₃ precursor solution. Then several drops of deionized water were added into the solution followed by drying in an oven at 140 °C for 4 h to get Li_1.3Al_0.3Ti_1.7(PO₄)₃ gel. The gel was calcined at 900 °C for 2 h to obtain Li_1.3Al_0.3Ti_1.7(PO₄)₃ powders. The powders were mixed with different mole fractions of Li₃PO₄ and pressed into pellets at the pressure of 100 MPa using 3% polyvinyl-alcohol solution as the binder. The pressed pellets were calcined at 900 °C for 2 h to obtain Li_1.3Al_0.3Ti_1.7(PO₄)₃ pellets with different mole fractions of Li₃PO₄.

The structural identification of the sintered Li_1.3Al_0.3Ti_1.7(PO₄)₃ pellets with different mole fractions of Li₃PO₄ was conducted by X-ray diffraction. The surface morphology was studied by scan electron microscopy. The compositions of the synthesized powders were determined by the inductively coupled plasma (ICP) method. Porosity of the sintered pellets was determined by the Archimedes’s method. For the measurement of electrochemical window, cyclic voltammetry was carried out using micro-electrode fabricated from grounded Li_1.3Al_0.3Ti_1.7(PO₄)₃ pellet. The micro-electrode was placed in an beaker cell containing 1 mol/L LiPF6 dissolved in ethylene and dimethyl carbonate (1:1, volumetric ratio). Metal lithium was used as both counter and reference electrodes. The ionic conductivity and activation energy were measured by electrochemical impedance spectroscopy. Both sides of the sintered pellet were painted with silver slurry followed by drying at 150 °C for 30 min before the electrochemical measurements were carried out. Electrochemical impedance measurement was conducted in an Ar-flowed tube over the frequency range of 1 Hz to 100 kHz at different temperatures.

3 Results and discussion

The X-ray diffraction patterns of the sintered Li_1.3Al_0.3Ti_1.7(PO₄)₃ pellets with different mole fractions of Li₃PO₄ are displayed in Fig. 1. As observed in Fig. 1, all the sintered pellets show similar diffraction patterns belonging to rhombohedral symmetry (space group RC) with six formula units in the unit cell. No diffraction peaks of impure phase are identified. All the diffraction peaks of the sintered pellets agree well with the reports for Li_1.3Al_0.3Ti_1.7(PO₄)₃ [11], suggesting that the sintered pellets are pure Li_1.3Al_0.3Ti_1.7(PO₄)₃ phase. There are no obviously difference in the lattice parameters between Li_1.3Al_0.3Ti_1.7(PO₄)₃ pellets with and without Li₃PO₄, indicating that the addition of Li₃PO₄ flux has little effect on the crystal structure of Li_1.3Al_0.3Ti_1.7(PO₄)₃.

The scan electron micrographs of the sintered Li_1.3Al_0.3Ti_1.7(PO₄)₃ pellets with different mole fractions of Li₃PO₄ are presented in Fig. 2. As seen in Fig. 2, the contact between the particles of the sintered Li_1.3Al_0.3Ti_1.7(PO₄)₃ pellets with Li₃PO₄ flux is better than the one without Li₃PO₄, which can be attributed to the relatively low melting temperature of Li₃PO₄, namely, Li₃PO₄ can melt and fill the space between Li_1.3Al_0.3Ti_1.7(PO₄)₃ particles during the sintering at 900 °C, leading to better sinterability and densification. However, there are no obvious differences between the sintered pellets with Li₃PO₄.

Fig. 1 XRD patterns and lattice parameters of sintered Li_1.3Al_0.3Ti_1.7(PO₄)₃ pellets with different mole fractions of Li₃PO₄: (a) 0; (b) 1%; (c) 2%; (d) 3%; (e) 4%

The porosity of sintered Li_1.3Al_0.3Ti_1.7(PO₄)₃ pellets with different mole fractions of Li₃PO₄ is displayed in Fig. 3. As shown in Fig. 3, the porosity of sintered Li_1.3Al_0.3Ti_1.7(PO₃)₄ pellets with 1%, 2%, 3%, and 4% Li₃PO₄ are 2.45%, 2.38%, 2.17%, and 2.15%, respectively, which are lower than 3.78% for the one without Li₃PO₄. The reason why the sintered Li_1.3Al_0.3Ti_1.7(PO₄)₃ pellets with Li₃PO₄ shows lower porosity than the one without Li₃PO₄ is that Li₃PO₄ melts and fills the space between Li_1.3Al_0.3Ti_1.7(PO₄)₃ particles during the sintering at 900 °C, resulting in the pellets with better sinterability and densification.

Electrochemical window of electrolyte is an important factor for its practical application, especially in the field of batteries. Figure 4 shows the redox voltages of the sintered pellet with different mole fractions of Li₃PO₄. As seen in Fig. 4, there are no significant differences between these sintered pellets. All the pellets show the redox voltage beyond 2.4 V. This indicates that the addition of Li₃PO₄ has little effect on the electrochemical window of Li_1.3Al_0.3Ti_1.7(PO₄)₃.

Electrochemical impedance spectra of the sintered Li_1.3Al_0.3Ti_1.7(PO₃)₄ pellets with different mole fractions of Li₃PO₄ conducted at 25 °C are shown in Fig. 5. All the sintered pellets show an incomplete semicircle and an inclined line. The incomplete semicircle in the spectrum is caused by the insufficient frequency upper limit of electrochemical workstation CHI660B. The high- frequency limit should be more than 1 MHz because the time constant of the bulk process is 1 ns assuming a geometric capacity (c_geo) of 1-10 pF. Obviously, the high-frequency limit in this study of 100 kHz is insufficient and causes the incomplete semicircle, which makes it difficult to determine the bulk ionic conductivity. However, from the intercept of the inclined line in the diagram the total ionic conductivity of the sintered pellets with different mole fractions of Li₃PO₄ can be calculated.

Fig. 2 SEM images of sintered Li_1.3Al_0.3Ti_1.7(PO₄)₃ pellets with different mole fractions of Li₃PO₄: (a) 0; (b) 1%; (c) 2%; (d) 3%; (e) 4%

Fig. 3 Effect of mole fraction of Li₃PO₄ on porosity of sintered Li_1.3Al_0.3Ti_1.7(PO₄)₃ pellets

Fig. 4 Redox voltages of sintered Li_1.3Al_0.3Ti_1.7(PO₄)₃ pellets with different mole fractions of Li₃PO₄

    Temperature dependence of ionic conductivity for the sintered Li_1.3Al_0.3Ti_1.7(PO₃)₄ pellets with different mole fractions of Li₃PO₄ is presented in Fig. 6. All the sintered pellets result in a linear temperature-dependence of conductivity followed on Arrhnius’s law. According to the results in the figure, the activation energy E_a can be obtained from the following equation:

where σ₀ is the pre-exponential factor; E_a is the activation energy for conductor; R is the mole gas constant; and T is the thermodynamic temperature.

Fig. 5 Electrochemical impedance spectra of sintered Li_1.3Al_0.3Ti_1.7(PO₄)₃ pellets with different mole fractions of Li₃PO₄ conducted at 25 °C

The ionic conductivity and activation energies of sintered Li_1.3Al_0.3Ti_1.7(PO₄)₃ pellets with different mole fractions of Li₃PO₄ are displayed in Fig. 7. As shown   in Fig. 7, the ionic conductivities of the sintered Li_1.3Al_0.3Ti_1.7(PO₄)₃ with 1% and 2% Li₃PO₄ are higher than the one without Li₃PO₄. When the mole fraction of Li₃PO₄ increases from 0 to 1%, the ionic conductivity increases from 3.46?10^-4 S/cm to 6.15?10^-3 S/cm. However, the ionic conductivity decreases with further increase of Li₃PO₄. This effect of Li₃PO₄ addition on the ionic conductivity of Li_1.3Al_0.3Ti_1.7(PO₄)₃ may be caused by the following reasons: The increase of Li₃PO₄ leads to the pellet with better sinterability and densification because Li₃PO₄ is a good flux, which results in closer contact of Li_1.3Al_0.3Ti_1.7(PO₄)₃ particles and decreases the resistance at grain boundaries, leading to higher ionic conductivity. However, too much Li₃PO₄ addition also decreases the ionic conductivity of Li_1.3Al_0.3Ti_1.7(PO₄)₃ because the ionic conductivity of Li₃PO₄ is considerably lower than Li_1.3Al_0.3Ti_1.7(PO₄)₃. Figure 7 also shows the activation energies of sintered Li_1.3Al_0.3Ti_1.7(PO₃)₄ pellets with 0, 1%, 2%, 3%, and 4% Li₃PO₄ are 0.282 1, 0.314 2, 0.296 3, 0.331 5, and 0.323 7 eV, respectively. The activation energy of sintered Li_1.3Al_0.3Ti_1.7(PO₃)₄ with Li₃PO₄ is slightly higher than the one without Li₃PO₄. This means that the temperature has greater influence on the ionic conductivity of the sintered Li_1.3Al_0.3Ti_1.7(PO₃)₄ pellets with Li₃PO₄.

Fig. 6 Temperature dependence of conductivity for sintered Li_1.3Al_0.3Ti_1.7(PO₃)₄ pellets with different mole fractions of Li₃PO₄

Fig. 7 Ionic conductivity and activation energies of sintered Li_1.3Al_0.3Ti_1.7(PO₄)₃ pellets with different mole fractions of Li₃PO₄ conducted at 25 °C

4 Conclusions

1) The sintered Li_1.3Al_0.3Ti_1.7(PO₄)₃  pellets with different mole fractions of Li₃PO₄ show similar X-ray diffraction patterns. The sintered Li_1.3Al_0.3Ti_1.7(PO₄)₃ pellets with Li₃PO₄ show lower porosity and are denser than the one without Li₃PO₄.

2) The addition of Li₃PO₄ has little effect on the redox voltages of Li_1.3Al_0.3Ti_1.7(PO₄)₃. When the Li₃PO₄ addition increases from 0 to 1%, the ionic conductivity increases from 3.46?10^-4 S/cm to 3.95?10^-4 S/cm. However, the ionic conductivity decreases with further increase of Li₃PO₄.

3) The activation energies of sintered Li_1.3Al_0.3Ti_1.7(PO₃)₄ pellets with 0, 1%, 2%, 3%, and 4% are 0.282 1, 0.246 9, 0.297 1, 0.327 3, and 0.341 9 eV, respectively. The activation of the sintered Li_1.3Al_0.3Ti_1.7(PO₃)₄ pellet with Li₃PO₄ is slightly higher than the one without Li₃PO₄.

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添加剂Li₃PO₄对锂离子固体电解质

Li_1.3Al_0.3Ti_1.7(PO₄)₃性能的影响

肖卓炳¹，陈 上²，郭满满¹

1. 林产化工工程湖南省重点实验室，张家界 427000；

2. 吉首大学 化学化工学院，吉首 416000

摘  要：采用溶胶-凝胶法，添加不同比例的Li₃PO₄助熔剂，合成Li_1.3Al_0.3Ti_1.7(PO₄)₃锂离子固体电解质烧结片，采用X射线衍射、扫描电子显微镜研究合成产物的结构与形貌，采用循环伏安及交流阻抗技术研究添加不同摩尔分数的Li_1.3Al_0.3Ti_1.7(PO₄)₃固体电解质烧结片的结构、氧化-还原电位、离子电导率和活化能。结果表明：添加与未添加Li₃PO₄助熔剂的Li_1.3Al_0.3Ti_1.7(PO₄)₃烧结片具有相似的X射线衍射结果。添加Li₃PO₄的Li_1.3Al_0.3Ti_1.7(PO₄)₃烧结片的空隙率较小，更为致密。添加Li₃PO₄对Li_1.3Al_0.3Ti_1.7(PO₄)₃的氧化-还原电位影响不大。在所有添加Li₃PO₄助熔剂的Li_1.3Al_0.3Ti_1.7(PO₄)₃烧结片中，添加1%(摩尔分数)Li₃PO₄ 的烧结片具有最高的离子电导率6.15?10^-4 S/cm和最低的活化能0.314 2 eV。

关键词：溶胶-凝胶；电解质；离子电导

(Edited by YANG Hua)

Foundation item: Project supported by Aid Program for Science and Technology Innovative Research Team (Chemicals of Forestry Resources and Development of Forest Products) in Higher Educational Institutions of Hunan Province; Project (2009FJ3045) supported by the Fund of Science and Technology Department of Hunan Province, China

Corresponding author: XIAO Zhuo-bing; Tel: +86-744-8231386; E-mail: xiaoyddd@163.com

DOI: 10.1016/S1003-6326(11)61036-4