Structural and electrical properties of Li2AlMn3O8

The spinel Li2AlMn3O8 was synthesized using a conventional solid-state method at 800°C for 12 h in air. The samples were analyzed using X-ray Diffraction (XRD) and Electrochemical Impedance Analyzer to study the structural and electrical properties of the samples. Well defined reflections were obtained with additional weak peaks of Al2O3. However, small values of χ2 indicating a good agreement between calculated and observed pattern. A single semicircle from the complex plane plot (Z” vs. Z’), and the capacitance value at the top point of semicircular arc was determined as a grain boundary region. The conductivity of Li2AlMn3O8 at room temperature is about 2.99 x 10-5 S cm-1. Meanwhile, the Arrhenius plot related from conductivity shows the activation energy also fairly similar at frequency range from 101 to 105 Hz.


Introduction
Lithium ion batteries (LIB) have been widely used in electronic products in the past decade. Inexpensive and environmentally benign spinel cubic structure of LiMn2O4 has Fd3m symmetry in which the Li + and the Mn 3+ /Mn 4+ are located in the 8a tetrahedral and 16d octahedral sites, respectively while O 2ions was occupy the 32e sites. The 8a sites share a face with the vacant 16c octahedral sites, offers 3D conduction pathways for Li + ions . [1] However, this composition has a bottleneck related to capacity fades issues on charge-discharge cycling. Widespread studies have been focused towards various strategies to improve the cycleability of LiMn2O4. Among these, one effective approach is to substitute small amount of dopant ions place of Mn ions. It is believed that, the structure of LiMn2O4 become more stable since the dopant ions are occupied in the 16d sites of the Mn ions in the spinel lattice. [2] The substitution of aluminium as a non-transition metal in LiMn2O4 has a potential of reducing strain during repeated cycling and facilitates a higher discharge capacity retention. [3] In this paper, the studied of the new composition of spinel structure, Li2AlMn3O8 was reported. Much attention .has been done to investigate the effect of non-transition metal that has fixed oxidation state in the 16d site of the spinel structure. Result of the structural and electrical property of Li2AlMn3O8 was reported. The structural property has been done using XRD, coupled with Rietveld refinement. Meanwhile, to meet the needs of analyzing the electrical properties of Li2AlMn3O8, electrochemical impedance analyzer has been used. The electrical properties was discussed with a set of impedance data measured between 303K and 26K.

Experimental
Li2CoMn3O8, Li2NiMn3O8, and Li2AlMn3O8 samples were prepared by the conventional solid-state. Reagent grade Li(CH3COO)·2H2O, Al(NO3)3·9H2O and Mn(CH3COO)2·4H2O were used as raw materials. The powder samples from conventional solid state synthesis were prepared using a stoichiometric amount of raw materials were mixed and ground by using pestle and mortar. After that, the samples were slowly heated to remove acetates at 400°C for 12 h. Then, samples were further heated at 800°C for 12 h in the air with intermittent grinding.
Bruker D2 Phaser benchtop X-ray diffractometer equipped with LYNXEYE 1D-detector with Cu-Kα radiation) was used to analyzed the purity of the final products at room temperature with a scan rate about 1.0 s/step and 0.02 of step size. The detector is the fast detector with 192 silicon strips. Rietveld refinements were carried out by using GSAS/EXPGUI software to determine the structural parameters.
For electrical measurements, a pellet was coldpressed uniaxially at 4 tons, sintered at 950°C for 12 hours in order to increase the mechanical strength, slowly cooled to 600°C and maintained at 600°C for 12 hours before slowly cooled to room temperature. In/Ga paste electrodes were coated on opposites sides of sintered pellet. Impedance measurements were carried out over the frequency range 10 Hz-100 kHz, using

Results and Discussion
The powder XRD pattern for Li2AlMn3O8 that were prepared at 800 °C in air was shown in Fig. 1. The XRD pattern of Li2AlMn3O8 shows very well defined reflections; refer to PDF Card 00-048-0261. All the peaks indexable in the Fd-3m space group with a cubic lattice. The lattice belongs to face-centered cubic, since the planes agreed with rule for reflection to be observed, which (h, k, l) either all odd or all even. However, a few extra peaks could be observed at 2θ=37.49°, 45.65° and 66.50°, indicating the presence of mixed phases. The additional peaks in these diffractogram are ascribed to Al2O3. Diffraction angle (2) Fig. 1. XRD pattern of Li2AlMn3O8 heated in air at 800°C for 12 h.
Replacing the transition metal with Al is not easy. It was reported that, at calcination temperatures above 600 °C, the inessential phases such as Al2O3, β-LiAlO2 and γ-LiAlO2, can easily form. [3] Manganese was expected to become Mn 3+/4+ in the composition of Li2AlMn3O8, since Al has fixed valence states as Al 3+ . Al 3+ mostly occupies in octahedral site (16d) and was predicted to stabilize the spinel structure. However, the presence of additional peaks of Al2O3 was indicated Mn oxidize as Mn 4+ .
The purpose of obtaining the details of crystallographic properties of sample was accomplished with structure refinement using Rietveld analysis. The refinement were performed regarding to the steps described elsewhere. [4][5][6] The spinel structure of Li2CoMn3O8 was used as the structural model with a space group Fd-3m. Li ions were located at 8a-site (0.125,0.125,0.125), transition metal ions (Al and Mn) located at 16d-site (0.5,0.5,0.5) and oxygen at the 32esite (0.2629,0.2629,0.2629). The initial structure model assumed that there is no cation disorder between Li and transition metal ions. Thus, the occupancy of Li ion at 8a-site was fixed at 100%, while transition metals at 16d-site were fixed with Al, Mn1 and Mn2 as 0.25, 0.25 and 0.5, respectively. The detail of crystallographic properties of Li2AlMn3O8 was summarized in Table 1. The disagreements between observed and calculated patterns were represented as Rwp, Rp, and χ 2 . Small values of χ 2 indicating a good agreement between calculated and observed pattern. The small weak peaks of Al2O3 were excluded and negligible during the refinement. Fig. 2 shows a set of typical impedance data measured for Li2AlMn3O8. An impedance complex plane, which is imaginary vs. real at 303K was plot in Fig. 2(a). The value of capacitance calculated from the relation ωRC=1 at the maximum point of semicircle is 5.88 x 10 -8 F. Meanwhile, Z", M" spectroscopic plots from the relation of Z* and M* at 303K was shown in Fig. 2(b). The plots shows Z" and M" peak at low frequencies and high frequencies, respectively. A capacitance vs. frequency plots (shown in Fig. 2(c)) shows the frequency and temperature dependence. The value of capacity was decreased with increasing the frequency, with at three orders of magnitude while decreased with decreasing of temperature between 303K and 268K. Other than that, the conductivity of Li2AlMn3O8 also shows clearly frequency dependence as shown in Fig. 2(d). A clear plateau was observed at the intermediate measured frequency (between 10 2 and 10 3 Hz). The plot also shows temperature dependent conductivity since the conductivity was decreased with decreasing the temperature from 303K to 268K. The conductivity at room temperature is about 2.99 x 10 -5 S cm -1 , while an Arrhenius plots in Fig. 2(e) shows average of activation energy at frequency between 10 Hz and 100 kHz is about 0.3 eV.  The magnitude and direction of the complex number can be expressed by real, Z' and imaginary, Z' components through the relation; From the top point of semicircular arc ( Fig. 2 (a)), the capacitance value was attributed to grain boundary regions of the sample. [7] The complex electric modulus, M*, is related to the complex impedance Z*, through the relation; Where M′ = ωCoZ″ and M″ = ωCoZ′; Co = εoA/Ɩ is the vacuum capacitance of the empty measuring cell with electrode area, A and separation, Ɩ. The permittivity of free space, εo = 8.854 x 10 -14 Fcm -1 . [8] Non-ideal RC element of Li2AlMn3O8 also observed from the Z", M" vs. frequency plot ( Fig. 2(b). The Z" spectra are broadened on the low frequency side of the peak maximum, while the M" spectra are broadened on the high frequency side. These two separated peak maxima contributed to the non-ideal RC element, with an additional parallel constant phase element (CPE). An accurate fit experimental data for a range of electrically homogeneous and heterogeneous materials also was provided by insertion of a CPE into equivalent circuits. [9,10] The frequency dependence capacitance of this composition also lies into grain boundary region. At low frequency, capacitance almost constant contributes more to grain boundary. Other than that, the capacitance was declined with increasing of frequency. The limiting high frequency capacitance corresponds to the bulk capacitance. However, the bulk capacitance was modified by the grain boundary capacitance. [10] a b c d e The electrical conductivity and lithium-ion diffusivity are most important properties responsible for the electrochemical performance of the LIB cathode material. The conductivity can be written as; where Ɩ is the thickness and A the area of the electrode deposited on the sample. Low conductivity may lead to a poor rate capability because it provides a kinetic limitation and induces polarization during lithium intercalation and deintercalation. Using Eq. (3) Fig. 2(d)), with existence of clear plateau between 10 2 and 10 3 Hz. In high-frequency region, the conductivity increases with frequency and become more sensitive. It is caused by the situation known as hopping frequency. It is shifted towards higher frequency with a rise in temperature. The conductivity increases due to long range movement of charge carriers and the material show the relaxor properties in high frequency. The difference of conductivity in the lowfrequency range is due to the polarization belongings. The accumulation of charge occurs at very low frequencies, hence drop the conductivity. An Arrhenius law that was related with conductivity can be presented as; where σ̥ is a pre-exponential factor and characteristics of the materials, Ea, k, and T are activation energy for conduction, Boltzmann's constant and the absolute temperature, respectively. [12] The Arrhenius plot from Eq. (4) in shows the activation energy also fairly similar at frequency range from 10 1 to 10 5 Hz.

Conclusions
The composition of spinel Li2AlMn3O8 was synthesized with the aim of investigates the structural and electrical properties. XRD studies shows that, this composition had existence of extra peaks belongs to Al2O3 at 2θ=37.49°, 45.65° and 66.50°. Meanwhile, the refinement data shows a good agreement between calculated and observed pattern. A single semicircle from the complex plane plot (Z" vs. Z'), and the capacitance value at the top point of semicircular arc was determined as a grain boundary region. Non-ideal RC element also observed from the Z", M" vs. frequency plot, with the presence of two separated peak maxima.
The conductivity of Li2AlMn3O8 at room temperature is about 2.99 x 10 -5 S cm -1 . An Arrhenius plot related from conductivity shows the activation energy also fairly similar at frequency range from 10 1 to 10 5 Hz with the average about 0.3 eV.