HYDROTHERMAL SYNTHESIS AND CHARACTERIZATION OF LiMnPO 4 CATHODE MATERIALS. Bilen Aküzüm 1

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1 MATTER VOLUME 1 ISSUE1 1 HYDROTHERMAL SYNTHESIS AND CHARACTERIZATION OF LiMnPO 4 CATHODE MATERIALS Bilen Aküzüm 1 1 Department of Metallurgical and Materials Engineering, METU, Ankara, Turkey ABSTRACT Lithium batteries are of interest for many portable applications due to their high energy density and low weight properties. In this manner, finding new cathode materials for lithium-based batteries are in the interest of many researchers. Especially the olivine-structured transition metal phosphates for cathode materials are promising for many applications in the field due to their low cost, non-toxic and environmentally friendly nature. This paper reports the early investigations on the hydrothermal synthesis of the olivine structured LiMnPO 4 powders for cathode materials and the effects of the two additives namely glucose and citric acid to the processing route of the synthesized LiMnPO 4 powders. X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM) techniques are used to characterize the synthesized powders. Using glucose additive resulted in formation of submicron-sized particles with a yield of 73wt% primary LiMnPO 4 phase whereas; citric acid additive resulted in formation of micron-sized particles with a yield of 98wt% primary phase in the powder. These initial results indicated that the hydrothermal synthesis is a promising processing route for the synthesis of olivine structured cathode materials for Li-ion batteries. Keywords: Hydrothermal Synthesis, LiMnPO 4 Cathode, Additives, Citric Acid, Glucose, Lithium Batteries INTRODUCTION 1 As we are in the age of global industrialization and population growth, demands for new energy resources are increasing exponentially. Unfortunately, this increase in the energy demand cannot be compensated with the conventional energy resources anymore and this in fact makes the energy area one of the most critical research fields of today. More specifically, portable power applications such as cellphones, laptops and even automobiles drive the research and development of advanced battery systems. Moreover, the extra energy content and considerations of portability have outweighed the economics when a system like that is considered [1]. Nowadays, due to minimization of the portable electronic devices, request for portable power has increased drastically. In this manner, lithium ion batteries are in use for the past 30 years and they are still being improved and developed for better performance. Lithium ion batteries provide the best properties required for a battery so far. This is due to its highest electronegativity among all other elements. In addition, combining its low-density property with its highest electronegativity, lithium batteries provide the largest amount of energy per unit mass, which is very beneficial for light- 1 Senior Undergraduate student at Middle East Technical University, Ankara, Turkey Address: e167796@metu.edu.tr weight applications. Although there are many different components and factors that affect the properties of Li-ion batteries, finding new cathode materials are in the interest of many researchers. As again, although there are vast amount of various cathode materials present today, since Radhi et al. had first introduced LiFePO 4 olivine as a cathode for rechargeable lithium batteries, the olivine structured transition metal phosphates, LiMPO 4 (M= Co, Fe, Mn) have attracted tremendous attention of the researchers worldwide due to its low cost, nontoxic and environmentally friendly nature. [2] Before LiMnPO 4 batteries were developed, the first metal-phosphate olivine structured Li-ion battery to be commercialized was the LiFePO4 battery, which was also showing similar properties with LiMnPO 4 batteries. In phospho-olivines, all of the oxygen ions form strong covalent bonds with P 5+ to form the PO4 3- tetrahedral polyanion and stabilize the entire three-dimensional framework, which provides improved stability and extreme safety under abusive conditions. However, this structure restricts the electrochemical reaction kinetics in phospho-olivines due to the insulation effect of the polyanion. Therefore, almost a decade past before LiFePO 4 olivine cathode for lithium batteries was developed and successfully commercialized [2]. There were systematic works of many research groups worldwide to adopt various processes and methods to overcome the main disadvantage of LiFePO 4 restricting its application, which is low electronic conductivi-

2 MATTER VOLUME 1 ISSUE1 2 ty. Various methods have been applied by different researchers, such as using various synthesis techniques, coating by a conductive layer of carbon and dispersed metal particles, preparation of LiFePO 4 /carbon composites, producing smaller particles of cathode material etc. Nowadays, LiFePO 4 cathode has become one of the main commercial cathode materials for lithium batteries and further research is in progress for commercialization of LiMnPO 4 batteries. Actually, the successful development of a battery like LiFePO 4 have also encouraged the great interest of researchers to another olivine structured cathode, LiMnPO 4, which is even more attractive than lithium iron phosphate because of its higher theoretical energy density due to the higher operating voltage of 4.1 V [2]. This high energy density is also promising a huge potential for the large-scale applications such as electric transportation and power back-up systems. Olivine-type LiMnPO 4 compounds have an orthorhombic structure (with lattice parameters a= , b=6.1016, c= ) comprised of close-packed phosphate anions with Mn 2+ ions in corner-sharing MO 6 sites and Li ions in edge sharing LiO 6 sites. The strong P O covalent bond gives these materials good stability and makes them attractive as positive electrodes for lithium batteries in vehicle applications [2]. LiMnPO 4 (lithiophilite) cycles at a nearly constant potential of 4.1 V vs. Li/Li +, which is well suited to use in standard battery configurations [2]. In contrast with LiFePO 4, however, the manganese compound suffers from poor Li extraction and insertion kinetics. Many factors have been considered to contribute to the slow phase transition, including the electronic and ionic conductivities, the Jahn Teller effect in Mn 3+, interface strain due to the large volume change between LiMnPO 4 and MnPO 4, and the metastable nature of the delithiated phase. Although the mechanism of the twophase reaction has not been identified, it is likely that it is similar to that of the LiFePO 4 /FePO 4 system. Previous efforts to improve the performance of stoichiometric LiMnPO 4 have mostly been limited to particle size minimization, which increases the rate and utilization, but inevitably decreases the volumetric energy density of the electrode. [2] As mentioned before, critical success of cathode materials is in their preparation, which controls the morphology, particle size and cation order amongst other critical parameters [3]. Although traditionally high-temperature methods have been used, they are both energy intensive and cannot readily produce many potentially metastable structures that might result in high lithium ion diffusivity [3]. However, they do have the advantage of being hydroxyl/water free. There are many possible approaches to the synthesis of active materials, but in the end a commercially viable approach must be used. Soft chemical approaches, such as hydrothermal/solvothermal or ion exchange offer several advantages. Such methods are used on the tonnage quantities today, and so the chemical industry considers them viable. Hydrothermal synthesis has been extensively studied for simple oxides such as those of tungsten, molybdenum and vanadium, and today many of the key parameters are understood. Various phosphates have also been successfully prepared [3]. Hence, our experimental procedure will also follow the hydrothermal synthesis of the LiMnPO 4 cathodes. EXPERIMENTAL PROCEDURE The LiMnPO 4 olivine structured powder was prepared by hydrothermal reaction in a Parr reactor. Specifically, the starting materials were LiOH, Mn(NO 3 ) 2, NH 4.H 2 PO 4 with molar ratios 1.2 Molar LiOH/ 1 Molar Mn(NO 3 ) 2 / 1 Molar (NH 4.H 2 PO 4 ). These were prepared to obtain 1 liter of solution as follows: LiOH: 23.59*0.18=4.311, 500mL (1) Mn (NO 3 ) 2 : *0.15= , 250mL (2) NH 4.H 2 PO 4 : *0.15=17.253, 250mL (3) They were hydrothermally synthesized at 200 o C, 8 hours and then furnace cooled. After this, the solution was placed into centrifuge machine in order to eliminate the physical water. The remnant-centrifuged particles are dried overnight at 60 o C in an oven to remove the residual water in the powder content. Although the main steps of the powder synthesize was the same for all powder types, 3 different experiments are conducted, which includes a reference powder with no additives, powder with glucose as an additive and powder with citric acid as an additive. X- ray (Rigaku) and SEM (FEI) analyses were conducted to all corresponding experiments. RESULTS AND DISCUSSION An initial experiment with no additives has been conducted in accordance with the experimental procedure stated above in order to understand the basics of the hydrothermal synthesis mechanism (at 160 o C).

3 MATTER VOLUME 1 ISSUE1 3 Figure 1. Plot showing the XRD pattern of the synthesized powder w/o additives. The main aim of the synthesis for the sample was to obtain data for a reference sample. As in figure 1, the x-ray diffraction data was found to be consistent with the actual LiMnPO 4 peak data. However, the same XRD pattern also revealed the presence of an unwanted second phase, which is Li 3 PO 4. As in figure 2, the particle size for LiMnPO 4 can be estimated to be around 2 microns in average according to the SEM image and 68wt% LiMnPO 4 was produced. A second experiment conducted to decrease the amount of Li 3 PO 4 phase by using additives prior to reaction. Glucose is used as a reducing agent. The main purpose of this addition was to prevent the oxidation of manganese by taking advantage of the aldehyde group in glucose. Figure 2. SEM image showing the synthesized powder w/o additives. Figure 3. Plot showing the XRD pattern of the synthesized powder w/ glucose additive. Addition of glucose as an additive did not result in a significant decrease (figure 3) in unwanted phase of Li 3 PO 4 by providing a yield of 73wt% LiMnPO 4. However, as in Figure 4, it caused a drastic drop in the particle size of the

4 MATTER VOLUME 1 ISSUE1 4 Figure 4. SEM image showing the synthesized powder w/ glucose additive Figure 5. Plot showing the XRD pattern of the synthesized powder w/ citric acid LiMnPO 4 powder resulting in a submicron particle size with preferred orientation and agglomeration present in the system. One should not forget that small particle size is another significant parameter for the charge / discharge kinetics of the final product. A third experiment has been conducted by using citric acid as an additive in order to investigate the effect of ph in direction of the reactions and also to make use of the chelating properties of citric acid. Citric acid forms a complex molecule where metal ions can dissolve and hence it affects the reaction mechanism drastically: As in figure 5, the x-ray diffraction data obtained from the powder synthesized with citric acid showed almost no characteristic peaks for the undesired phase Li 3 PO 4 with a yield of 98wt% LiMnPO 4 phase present in the powder. Additionally, the particle size is found to be around 1 micron in average according to the SEM images on figure 6. This result directly reveals the importance of chelating power of citric acid in inhibiting the formation of undesired phase Li 3 PO 4. CONCLUSION To conclude, hydrothermal synthesis promises a good production route for the synthesis of cathode materials for battery applications in large quantities. It is indeed possible to produce the desired LiMnPO 4 cathode

5 MATTER VOLUME 1 ISSUE1 5 Figure 6. SEM image showing the synthesized powder w/ citric acid additive material with the provided route in previous sections. Additionally, one can definitely state that the presence of precursors/additives and surfactants are very crucial for the reaction mechanism and they affect the properties of the final product drastically. Considering the fact that conductivity and fast charge/discharge kinetics are important parameters for the battery applications, various additives should be utilized and tested in future studies in order to optimize and enhance the outcomes from the promising LiMnPO 4 cathodes. AUTHOR INFORMATION REFERENCES [1] B Scrosati & WAV Schalkwjick, Advances in lithium ion batteries, 1-5, [2] Z Bakenov and I Taniguchi, Open Materials Science Journal, vol. 5, , [3] J Chen, MJ Vacchio, S Wang, N Chernova, PY Zavali and MS Whittingham. Solid State Ionics, Vol. 178, , Bilen Aküzüm is a senior from METU, TURKEY majoring in metallurgical and materials engineering. His advisor for this project is M. Kadri Aydınol, Professor, METU Metallurgical and Materials Engineering Department. His academic interests include materials chemistry, reaction kinetics, electrochemistry, and materials synthesis. He studied abroad at University of California, Berkeley for his junior year and He has research experience at Harvard University working for Prof. Michael J. Aziz on organic based flow batteries. In his free time, he likes to read history, listen to music, go on long hikes with friends, and basically enjoy life.