A comparative Study of Physic-Chemical Properties of Certain Cerium Doped Li7La3-xCexZr2O12 Ceramic Oxides

Solid-state electrolytes have emerged as a promising alternative material for next-generation Li-ion batteries due to their safety and reliability. In this investigation, we report the synthesis of Cerium(Ce) doped Li 7 La 3 Zr 2 O 12 (LLZO) ceramic oxide which has a garnet-like structure and in which Ce 3+ typically occupies La 3+ sites. The synthesised LLZO ceramic oxide is doped with various weight percentages of cerium(Ce 3+ ) by sol-gel method using oxalic acid as a complexing agent and ethane-1,2-diol as a surface-active agent. The synthesised Li 7 La 3-x Ce x Zr 2 O 12 garnet is screened for surface morphology, chemical composition, and phase transition by various analytical techniques. The surface morphology and composition were analysed by HR-SEM with EDX analysis respectively. The cubic face formed was conrmed by XRD results. Thermogravimetric analysis indicates the thermal stability of the prepared materials. The effect of addition of various weight percentages of cerium with LLZO on ionic conductivity was analysed using ac impedance spectroscopy and compared. The maximum ionic conductivity measured was 6.34×10 -5 Scm -1 . The potential window was examined by cyclic voltammetry (CV), which showed that the lithium deposition and dissolution peak appeared around 0V.Li + /Li and no further reaction beyond 5.8V vs Li + /Li.The results showed that these materials could be used as a potential alternative material in the fabrication of lithium-ion batteries.


Introduction
In recent years, solid-state lithium-ion batteries, with high energy density and e ciency, have received considerable attention due to the increasing demand. The solid-state conductors expected to address the safety concerns of the traditional liquid electrolytes system. Therefore, storage systems such as Li-ion batteries (LiBs) are actively investigated in the context of large-scale power plants, as well as portable applications such as electronic devices, electric vehicles, and growing popularity for military and aerospace applications [1].
The Solid inorganic lithium-ion conductor is one of the lithium garnet families being widely studied is Li 7 La 3 Zr 2 O 12 (LLZO) garnet like ceramic oxide has been investigated in detail due to its chemical stability, high lithium-ionic conductivity, and wide electrochemical window as well as high thermal and mechanical stability [2][3][4][5]. The extensive investigation of these materials is considered a promising source for the development of solid-state lithium ions batteries with enhanced e ciency. There are two types of garnets with crystal phase tetragonal, and cubic structures are reported [6]. Further, it is observed that Li-ion conductivity of the cubic phase system is approximately two orders higher than that of the tetragonal phase system and the cubic structure could not be obtained low sintering temperature [7]. Many researchers have carried out the preparation of LLZO doped with aliovalent cations stabilise the cubic phase at room temperature with low sintering temperature and reported the materials with the conductivity of about 1 mS cm −1 , which is comparable to that of liquid electrolytes [9][10][11]. Various doping methods were a dopted such as Al 3+ [12], Ga 3+ [13]inLi + sites and Ta 5+ [14], Nb 5+ [15], Mo 6+ [16], Sb 5+ [17], Te 6+ [18], W 6+ [19], Y 3+ [20], Mg 2+ ,Sc 3+ [21] replacing Zr 4+ at the octahedrally coordinated position, simultaneous substitution of Li + and Y 3+ for Zr 4+ [22] and Ce 4+ in La 3+ [23]. The different synthesis methods for the doping were solid-state method, sol-gel method, modi ed Pechini sol-gel process, precipitation method, and wet chemical methods. Many of them reported on the cubic phase in the solidstate method with higher sintering temperature around 1000ºC above for 36 h in an alumina crucible. There are very few reports dealt with the cubic phase using a sol-gel method with low temperature [24].
Thus, developing a novel solid electrolyte system based on garnet based ceramic material with enhanced performances at room temperature is of great importance. Hence, we report on LLZO garnet doped with various weight percentage of cerium [Li 7  During the synthesis procedure. Fig.1 shows the detailed procedure followed in this work. The required quantities of LiNO 3 (Sigma-Aldrich) (10 wt % excess was added to compensate for the expected Li loss during sintering at high temperature), La(NO 3 ) 3 .6H 2 O, and ZrO(NO3)2.xH2O, Ce(NO3)3.6H2O were dissolved in de-ionised water, and then oxalic acid and ethane-1,2-ol were added to the above clear solution with continuous stirring. The amount of oxalic acid added was twice the total moles of the cation in the precursor solution.
The solution was heated to 60ºC with stirring initially to remove the excess water. The solvent was then slowly evaporated, and white gel precipitate was formed. The resulting gel precursor was heated at 150ºC for an hour in a hot oven, and tiny flakes of light yellow precursor powder were obtained. Then the powder was finely grounded and heated at 850ºC for 6 hours, powder of ceramic material was obtained. The surface morphology of ceramic oxide was analysed by a high-resolution scanning electron microscope (HRSEM; HITACH S-4800). The phase composition of the synthesised garnet powder was characterised with x-ray diffraction using cu kα radiation at 1.5406 A, scanned over 2θ range of 10-60θ at room temperature. Thermal analysis of the prepared materials was carried out by TG/DTA and thermograms were recorded using SDTQ600 (T.A.) in nitrogen flow with a heating rate of 10ºC/min. The lithium-ion conductivity of the synthesised Ce doped LLZOwas measured by using and thermograms were recorded using SDTQ600 (T.A.) in nitrogen flow with a heating rate of 10ºC electrochemical impedance spectroscopy (ESI, Bio-Logic SP200) the impedance of the fabricated cells was measured in the frequency range from 10 Hz to 7 MHz with a perturbation amplitude of100 mV. The potential window was examined by cyclic voltammetry (CV), which showed lithium deposition.

HRSEM study
The ne powder of the prepared sample was pale yellow in colour. The intensity of colour gradually intensi es to dark yellow on continuous doping with various weight percentages of cerium (Li 7 La 3-x Zr 2 Ce x O 12 ). The HR-SEM image of the Ce doped LLZO ceramic oxide powder calcinated at 850 C is shown in Figure.2. The SEM images of ceramic oxide demonstrated that the materials have large size irregular spherical and well -de ned cubic like structure. Figure. Figure.2(c) exhibited a large regular and homogeneous cubic morphology. Further, it is observed that the ceramic oxide with Ce concentration with above x=0.5, cubic morphology gradually reduced which may enhance the grain boundaries resistance and in uence lithium-ionsconductivity.

Energy dispersive X-ray spectroscopy
The EDX spectrum of Ce doped LLZO ceramic materials was recorded to analyse the elemental composition of the synthesised materials.The EDX spectrum of gure3,4,5,6 shows the elemental composition of x=0.

X-ray Diffraction Analysis
The x-ray diffractograms of ceramic garnet containing various weight percentages of (x= 0.1 to 0.75 of Ce) having the formula Li 7 La 3-x Ce x Zr 2 O 12 are shown in gure 7. First, it is clear that the addition of super valent cation stabilises the cubic LLZO phase, which may increase the Li + mobility due to the structural modi cation around the lithium sites [25][26]. The decrease in the intensity of peak doublet decrease as the weight percentage of cerium increases shown in gure 8, indicated that the addition of Ce results in the transformation of structure from tetragonal to cubic in cerium doped LLZO [23]. When the concentration of doped cerium is 0.1weight percentage (x=0.1of Ce), tetragonal with cubic structure is formed, and its respective formula is Li 7 La 2.9 Zr 2 Ce 0.1 O 12 . The intensity of peak doublet signi cantly decreases above 0.1 weight percentage of Ce and samples shows a peak at 2θ = 27.98° which indicated the presence of La 2 Zr 2 O 7 in both samples [27][28] and the XRD peak at 2θ = 27.38°,32.41° ,and 47.58° due to the CeO 2 in the samples.

Thermal analysis
The thermal behaviour of the synthesised ceramic oxide materials was studied using thermogravimetric analysis (TGA) and differential thermal analysis(DTA). The TGA and DTA thermograms recorded in the temperature range from 30ºC to 1000ºC in a nitrogen atmosphere for the synthesised Li 7 La 3-x Zr 2 Ce x O 12 ceramic material were presented in the Figure.

Conductivity Study of Ce doped ceramic oxide
The ionic conductivity property of the synthesised ceramic oxide material was characterised using electrochemical impedance spectroscopy. The synthesised samples calcined at 850ºC and the conductivity is measured at room temperature, for various weight percentage of cerium doped LLZO materials. The A.C. impedance spectra of Li 7 La 3-x Ce x Zr 2 O 12 tted through an equivalent circuit to obtain the ionic conductivities, as shown in Figure 11, and the plot can be well-resolved into bulk, grain-boundary, and electrode resistances. The high-frequency semicircle can be attributed to the bulk resistance of La 2.9 Ce 0.1 (G11), while the semicircle in the low frequency range correlates with the grain-boundary resistance [29]. From Fig. 11 and Table 1, several observations can be made. First, the observations showed that the addition of various weight percentages of Ce concentration increases in LLZO decreases the total resistance of ceramic electrolytes ,and hence, the total conductivity increases. Secondly,the conductivity of the Li 7 La 2.5 Ce 0.5 Zr 2 O 12 (6.34×10 -5 Scm 1 ) specimen is slightly higher compared to the conductivity of the Li 7

Conclusion
The garnet structured solid electrolytes were prepared according to the formula Li 7 La 3-x Zr 2 Ce x O 12 with different Ce content was successfully synthesised by a sol-gel method and calcinated at 850°C for 6 hours. It was noted that the addition of Cerium in LLZO could induce signi cant transformation of the structure from tetragonal to cubic phase and hence, increases ionic conductivity. It is, also, observed that single the cubic face phase was observed in the composition X ≥ 0.25 to 0.75, which is con rmed by XRD results. The ac impedance of Li 7 La 3-x Zr 2 Ce x O 12 has been investigated at room temperature and exhibits the highest ionic conductivity 6.34×10 -5 Scm -1 for Li 7 La 2.5 Zr 2 Ce 0.5 O 12 . The HR-SEM images revealed that cerium concentration in uences the size and the density of the cubic phase. The cubic phase with relatively dense morphology expected to show better ionic conductivity. The cyclic voltammogram of lithium deposition and dissolution peaks near 0V vs Li + /Li and indicated that there is no other peak beyond 5.9V vs. Li+/Li, demonstrated that Li 7 La 3 Zr 1.55 Ce 0.5 O 12 has a wide potential window. The materials prepared showed that they possess optimal conductivity for the fabrication of batteries.