Synthesis of LiNi0.5Mn1.5O4 for Class 5V Batteries Using a Microwave Heating Method
Lithium-ion batteries are the main power source for most portable electronic devises. The faster the technologies develop the greater the worldwide demand for batteries with higher performance grows. The transition to new challenging applications such as hybrid and electrical vehicles require going beyond what the present state of batteries can achieve density wise. One of the solutions for achieving high energy density is by using cathode materials with high voltage output, such as lithium nickel manganese oxide LiNi0.5Mn1.5O4 (LNMO). It is considered as one of the most promising cathode materials for lithium-ion batteries due to its good rate capability, cycle performance and most importantly its high discharge plateau of about 4.7V .
Recently, many methods have been reported for synthesizing LNMO, which include solid-state reaction, sol-gel technique and precipitation . However, we developed a unique synthetic method by using microwaves, which is a promising method of preparation [3, 4]. In this technique, unlike other heating methods the microwave heating can affect higher temperatures inside the product than on its surface. Also due to the heating of the material throughout the whole volume, the high uniformity of warming is reached as well as more precise temperature regulation. Overheating of certain parts of the product can be better prevented this way. This method is utilized for mass cathode material production at low energy cost.
In this research, we report the synthesis of LNMO using our microwave heating method. Precursor material was prepared using a self-reaction method that was previously shown to be suitable for the preparation of complex metal oxide powders.
The LNMO precursor was prepared by dissolving Lithium Nitrate, Nickel (II) Nitrate Hexahydrate and Manganese (II) Nitrate Hexahydrate with a mol ratio of (0.1:0.05:0.15) in distilled water and mixed using a stone mortar to form a mixed aqueous solution. The solution was then heated up in a microwave with various conditions such as time and oxygen/air flow. The product was grinded and left to dry before various characterization tests were preformed. To clarify the influence of the many factors applied, the prepared materials were characterized by X-ray diffraction, Fourier transformation infrared spectroscopy, scanning electron microscopy and their electrochemical properties.
To perform the electrochemical measurements, LNMO was mixed with various amounts of the conductive material, acetylene black (AB), and the adhesive material, polytetrafluoroethylene (PTFE), to determine the right amount with the best results. The material was then rolled to form a thin sheet and cut into 6mm diameter discs to make the positive electrode. For the battery tests, the metal cell was assembled in an argon atmosphere with the LNMO cathode, the anode as lithium metal and the electrolyte 1M LiPF6 in a 1:1 solvent mixture of (EC)/(DMC). The current rate was 20mAg-1 with cut-off voltages of 3 and 4.9V.
Results and discussion
The results revealed that spinel LiNi0.5Mn1.5O4 powders can be directly synthesized by microwave heating. The X-ray diffraction patterns of the product showed more or less to have a single-phase spinal structure. With the increase in heating time, the samples showed rather sharper diffractions. The SEM photographs showed that the powders are composed of small particles and regular particles whose morphologies are almost the same as for the spinals reported in other papers [2, 5]. In the electrochemical analysis, the discharge curve had two plateaus, one with a high potential at the 4.7 V region, and the other is in the 4 V region. The plateau at 4.7 V has been ascribed to the two-steps (Ni4+/Ni3+, Ni3+/Ni2+) redox reactions . The plateau at 4V is due to the presence of residual amount of Mn3+ in the spinel .
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