Effect of Doped Lithium and Thermal Treatments on the Structure and Conducting Properties of Perovskite LixCa1−xTiO3

In this paper, we present a detailed study of the influence of the thermal treatments, doped lithium on the crystal structure of perovskite lithium-ion conductor LixCa1−xTiO3 (x = 0-0.15), ionic conductivity and vibration behavior. These samples characterized by Differential Scanning Calorimetry (DSC), Raman scattering, X-Ray diffraction (XRD) and Complex impedance Spectroscopy (CIS). The results of Raman scattering have showed that Li ion was entering into the crystal lattice. A study of ionic conductivity by CIS implied that the conductivity of LixCa1−xTiO3 increased with the increase of substituted Li + ions and reached a maximum value of about 3.8×10−6 S/cm at x = 0.1, and then decreased for x > 0.1. [DOI: 10.1380/ejssnt.2012.255]


I. INTRODUCTION
Lithium ion batteries are widely used in our daily life.Capacity, life time, safety and size of a lithium battery are the major concerns for consumers and researchers.Since the lithium ion batteries were put into market, their capacity and life time have increased greatly with the improvement of various kinds of electrodes.However, the safety and size of commercial lithium ion batteries are still the bottleneck of their development.In commercial lithium ion batteries, liquid or organic electrolytes would cause a danger of explosion and are accidents; moreover, the essential separators and packaging limit the miniaturization of batteries.It was realized that the replacement of the liquid or organic electrolytes by solid state electrolytes would be an efficient approach to solve their safety and size issues.Among the solid electrolyte, calcium titanates doping Li, the Li x Ca 1−x TiO 3 is a disordered oxide with perovskite structure and have attracted much attention because of its high conductivity in the order of about 10 −4 S/cm at room temperature [1].The structure of Li x Ca 1−x TiO 3 is an orthorhomibic system about 600 • C, but it will turn into a cubic system at higher temperature about 1300 • C [2].The sol-gel method with the advantages of a low calcination temperature, shorter heat treatment time and homogeneities of the resulting has been introduced to prepare these electrolytes in this work.
In this work, we prepared series of Li x Ca 1−x TiO 3 solid electrolytes by a solgel method and investigated the phase, vibration behavior and ionic conductivity of

II. EXPERIMENTAL
The Li x Ca 1−x TiO 3 samples were made by a sol-gel method.For this method, the starting materials were Ca(NO  the need powders were obtained.Finally, the powders were pressed into pellets at a pressure of 30 MPa and sintered in air at 700 • C for 6h.The phase structure of the powder was analyzed by X-ray diffraction (XRD) on a Xpert-Pro XRD diffractometer with Cu Kα radiation (45 kV, 40 mA) using a 2θ step of 0.05 • .The diffractograms were recorded with a long scan rate of 0.5 degrees per second over the interval 20-80 • .The measurement of CIS was performed with an Impedance Analyzer HP4192A at a frequency range of 5Mh-13kHz at room temperature.

III. RESULTS AND DISCUSSION
DSC curve of the sol-gel Li x Ca 1−x TiO 3 sample is shown in Fig. 1.The curve consists of two endothermic peak followed by another very sharp endothermic peak.The first, two peaks at around 184 and 256 • C are due to the combustion of organic substances.The endothermic peak at around 613 • C is temperature of the formation Li x Ca 1−x TiO 3 phase.Phase formation temperature is much lower than it is required in conventional solid-state reaction [3].
Figure 2 displays the XRD patterns of the Li x Ca 1−x TiO 3 (x = 0) powders treated heat at different temperatures for 2h in air.The presence of diffraction peaks can be used to evaluate the structural order at long range or periodicity of the material.CaTiO 3 phase was confirmed by the comparison between the XRD patterns with the respective standard sample (SS) [2].All diffraction peaks can be assigned to the orthorhombic structure.Li x Ca 1−x TiO 3 (x = 0) powders heat treated at several temperatures for 2h in air exhibited characteristic diffraction peaks correspondent to an ordered structure at long range.
However, diffraction peaks appearance for the phase CaTiO 3 when treated heat at 400, 500 • C for 2h in air but weakness intensity.This behavior is typical of amorphous or disordered materials and some phases (Fig. 2).We can found that when temperature increases up to 700 • C or 800 • C, the appearance and evolution of the 310, 421, 233, When Ca 2+ ions in structure of CaTiO 3 were substituted by Li + ions partially, the phase Li x Ca 1−x TiO 3 was formed.Because the amount of Li + ions is not much and the ionic radius of Li + (0.078 nm) is much smaller than that of Ca 2+ (0.1 nm), it is why this change is not strong enough to modify the CaTiO 3 structure, and Li x Ca 1−x TiO 3 remains a orthorhombic perovskite structure similar to CaTiO 3 .The size of the prepared powders was calculated on the Scherrer formular and this is about 50 nm.There are 24 Raman-active modes for orthorhombic structure with space group Pbnm (ZB = 4) with four molecular units in the primitive cell, which can be described by the representation GRaman, Pbnm = 7Ag + 5B1g + 7B2g + 5B3g.The nine Raman modes observed in the range from 145 to 815 cm −1 are attributed to the orthorhombic structure, in agreement with the literature [4].P1 Raman mode is related to the CaTiO 3 lattice mode.P2, P3, P4, P5 and P6 are attributed to the O-Ti-O bending modes.P7 and P8 are ascribed to the torsional mode and P9 is assigned to the Ti-O symmetric stretching vibration.It can found that with increasing Li content, all the bands observed shift toward lower frequency, broaden and weaken in intensity.We can conclusion that Li + entering the crystal lattice.When Li + content increase, peak P1 relative to lattice mode may be disappear and weaken in intensity.Figure 5 is the CIS of Li x Ca 1−x TiO 3 pellets sintered at 700 • C for 2h.Each spectrum consists of an arc and long straight line shown as on Fig. 5.
The ionic conductivities of serious sample Li x Ca 1−x TiO 3 can be estimated by equation where L is the thickness of the pellet, S is the area of the pellet, and R is the resistance of the sample.The arc in the high frequency range is due to the ionic conductivity of the ceramic samples.The long straight line in the low frequency range corresponds to the blocking effect of the Ag electrodes.Usually, an equivalent circuit made of two RC elements in series, describing grain bulk and grain boundary contributions, is used for modeling the impedance spectrum of polycrystals [4].For such structure of Ag∥polycrystalline LCTO∥Ag, a simplified equivalent circuit as shown in the inset of Figure 6 can be used [4], i.e., the grain component is described by a  I indicate that the Li x Ca 1−x TiO 3 ionic conductivity increases with the increase of substitutegd Li + ions, reaches a maximum value at x = 0.1, and then decreases for x > 0.1.When the oxygen vacancies were substituted by Li + ions, the ionic conductivities of Li x Ca 1−x TiO 3 solid electrolytes were significantly improved, but when the amount of the substituted Li ions exceeded a certain value, the interactions among dopant cations (Li + ) and higher concentrations of oxygen vacancies caused the aggregating of oxygen vacancies, so the effective oxygen vacancies concentration did not increase and the ionic conductivity was lower for x > 0.1 [1].

IV. CONCLUSION
In summary, the solid electrolyte Li x Ca 1−x TiO 3 was successfully synthesized by a sol-gel method.The XRD shows that the sample Li x Ca 1−x TiO 3 when sintered over 700 • C for 2h are orthorhomibic perovskite structure and the mean size of Li x Ca 1−x TiO 3 powders is about 50nm.The results of Raman spectra has showed that Li + enter the crystal lattice.The ionic conductivity of Li x Ca 1−x TiO 3 increases with the increase of substituted Li + ions, reaches a maximum value at x = 0.1, and then decreases for x > 0.1.
) 2 .4H 2 O, Ti(OC 4 H 9 ) 4 , Li(CH 3 COO) 2 •2H 2 O, C 2 H 5 OH and C 5 H 8 O 2 .The solution was heated at 60 • C under constant stirring.After reaction at 60 • C for 6h, Li x Ca 1−x TiO 3 gel was obtained.Then the gel was kept in an oven at 70 • C for 24h to get the Li x Ca 1−x TiO 3 dried gel.This gel was pre-treated at 200 • C for 1h.After that the dried gel was sintered in air at 400-800 • C for 2h and