Structural Analysis of LiNixMn 2-xO 4 Prepared by Irradiation Microwave-Assisted Reflux Technique

This study aims to synthesize Ni-doped LiNixMn2-xO4 (x = 0, 0.02, 0.04, 0.06, 0.08, 0.1) by irradiation microwave-assisted reflux technique. The microstructure of the products was investigated by x-ray diffraction equipped with U-FIT program package. Results of the structural analysis show that Ni doping changed the size, crystallinity, and microstructure of LiNixMn2-xO4. The LiNixMn2-xO4 solids have a cubic structure with a space group of Fd-3 m. Increasing the content of Ni doping does not affect the structure. The average volume of LiNixMn2-xO4 is around 555 Å3 to 580 Å3. The crystallinity of the solids tends to increase with the increasing Ni content accompanied by the decrease in unit cell lattice parameters.


INTRODUCTION
In the current era, innovations in the field of technology have been rapidly increased.One is the electronic portable items such as mobile phones, laptops, cameras, hybrid cars, and others.Battery as a power source was instrumental component in the technological innovation.One type of the power source is a lithium battery.Currently, the common source of lithium is LiCoO 2 that widely used as a cathode material for lithium-ion batteries.However, LiCoO 2 has several disadvantages which are difficult to be used as a large-sized cathode material, security issues in high power and high cost.This has an impact on the amount of research that focus on the development of other alternative cathode material in view of cost, safety and environment issues 1,2 .
Although many new cathode materials have been developed, LiMn 2 O 4 is recognized as a promising alternative positive electrode for lithiumion batteries 3 .Spinel structure of LiMn 2 O 4 offers an attractive alternative material beyond LiCoO 2 available commercially.The advantages of using LiMn 2 O 4 are that materials are safe, non-toxic, and low cost 4 .However, LiMn 2 O 4 has a constraint of life-time, being reduced levels of storage capacity in applications 5 .The biggest constraint is the life-time at high temperature.This happens because of the dissolution of manganese into the electrolyte which is then followed by the disproportionation of Mn 3+ ions, the decomposition of the electrolyte, and changes in the structure of the powder particles after Jahn-Teller distortion in the cells that consume the battery content 6 .These constraints can be reduced by other transition metals having structural conformity to give LiNi x Mn 2-x O 4 where M = Co, Mg, Al, Cr, Ni, Fe, Ti and Zn 7 .
Many methods of synthesis have been reported to prepare LiMn 2 O 4 compound.These includes sol-gel method 8 , hydrothermal 9 , spraypyrolysis 10 , and so forth.However, studies related to the synthesis of compounds of LiNi x Mn 2-x O 4 (x = 0-0.1)are hardly reported, particularly those that deals with the use of irradiation microwave-assisted reflux technique, followed by solid-state reaction.This paper repor ts the result of the synthesis of LiNi x Mn 2-x O 4 (x = 0-0.1)prepared by irradiation-microwave assisted reflux-technique and its microstructural characterization.By using this technique, it is expected that the complex reactions between lithium ion, nickel ion, manganese ion, and citric acid monohydrate (as a charge stabilizer and chelating agents) will occur.Irradiation microwave is used so that the collision among the molecules occur more rapidly and therefore the reaction will proceed more easily and efficiently to produce the manganese oxide.Hence, the solid-state reaction was used to lithiation process and doping nickel metal in oxide manganese compounds with the chelating agent.The effects of various mole ratio of Ni/Mn in the synthesis of the products are structurally reported in detail in this work.

Synthesis of Mn 2 O 3 and LiNi x Mn 2-x O 4
Analytical grade of Mn(CH 3 COO) 2 •4H 2 O and Na 2 S 2 O 8 (E.Merck) has been used.All other chemicals used without further purification.In a typical synthesis, Mn(CH 3 COO) 2 •4H 2 O and Na 2 S 2 O 8 with a mole ratio of 1:1 was dissolved at room temperature in 80 mL of deionized distilled water.The mixture was stirred to form a homogeneous solution.
The solution was refluxed with the aid of irradiation microwave intensity of 50% of 760 watts for 20 minutes.The precipitate obtained was separated and dried at 250°C for 1 h in an oven and calcination at 750°C for two hours.

Determination and Characterization of LiNi x Mn 2-x O 4
Powder Mn 2 O 3 and LiNi x Mn 2-x O 4 were analyzed using X-Ray Diffractometer.The XRD pattern obtained in XRD instrument Rigaku Miniflex 600-Benchtop using CuKα radiation (λ = 1.5406Å) at room temperature.XRD instrument was set to operate at 40 kV and 15 mA.XRD data obtained by 2θ interval ranging from 20 o to 80 o .Rietveld analysis was performed with the software package U-FIT to refine X-ray diffraction data.The parameters for refining are a unit cell, hkl, and volume.SEM image was obtained using a JEOL JSM-6510LASEM.Effect of nickel content on the structure of LiMn 2 O 4 was studied using Energy Dispersive X-ray spectroscopy.EDX analysis is also used to analyze the presence of Mn, O 2 and Ni elements in material prepared.

XRD Patterns
Figure 1 shows the XRD generated pattern of LiNi x Mn 2-x O 4 .High peak intensity indicates that the LiNi x Mn 2-x O 4 has a good crystallinity.Fig. 2 shows the variation of lattice constants of materials with different compositions.The refinement data obtained using the U-FIT indicates that the lattice parameter decreases with increase in doping content.
Replacement Mn 3+ by Ni 2+ in the octahedral site (16d) causes a decrease in the lattice parameter, while the lithium ions occupy tetrahedral sites (8a).After modification, lattice shrinkage caused by smaller ionic radius of Ni 2+ which replaces the Mn 3+ sites in 16d.Note that the ionic radius of Ni 2+ of 0.560 Å is smaller than that of Mn 3+ , which is 0.645 Å.The differences in the ionic radius between Ni 2+ and Mn 3+ is relatively small, therefore only a view of changing the lattice parameters is observed.On the other hand, the replacement of Mn 3+ by Ni 2+ induces the increase of the content of Mn 4+ to maintain the balance of the refill process (charge), since Mn 4+ has an ionic radius smaller than the Mn 3+ , this also contribute to lattice shrinkage.Oxygen atoms are arranged in a hermetically sealed packaging 32e 11,12 .
Table 1 shows the areas hkl various mole ratio of Ni/Mn in LiNi x Mn 2-x O 4 using U-FIT program.The products have a cubic phase with space group Fd-3m as shown by the results of the analysis using the U-FIT program in Table 2.There was no significant difference in the crystal structure after it is being doped.It shows that the Mn sites in LiMn 2 O 4 are replaced completely by Ni, although there are other phases of Mn 2 O 3 are formed.The emergence of this phase of Mn 2 O 3 may be due to the availability of the rest of Mn 2 O 3 which is not completely reacted with LiOH in the lithiation process.This phase is shown by the peak area of 32-33 o and 54-55 o attributed to Mn 2 O 3 .
Figure 3 shows the SEM-EDX Mn 2 O 3 , LiMn 2 O 4 , and the LiNi 0.08 Mn 1.92 O4.Based on the SEM image it can be seen that in general the three samples showing the non-homogeneous particle surface.Qualitatively, it appears that the particles forms agglomeration.It is indicated by the structure of particles that joined by each other compatibly, so that the grain/single particles do not appear clearly.It is noteworthy that the shape of the particle is basically irregular cubic.

Fig. 2 .
Fig. 2. Variation of lattice parameters with x in LiNi x Mn 2-x O 4

Fig. 3 . 4 CONCLUSION
Fig. 3. SEM-EDX of (a) Mn 2 O 3 (b) LiMn 2 O 4 and (c) the LiNi 0.08 Mn 1.92 O 4 Synthesis LiNi x Mn 2-x O 4 is as follows; onemole LiOH, 0.02 mol of Ni(CH 3 COO) 2 •4H 2 O, and 1.98 mol of Mn 2 O 3 synthesized, dispersed into citric acid monohydrate to form a thick slurry.It was dried in the oven at 250°C for one hour.Solids of LiNi 0.02 Mn 1.98 O 4 then was calcined at 750°C for two hours.The same procedure was carried out to synthesize materials with different mole ratios.