Optimising Electrochemical Properties of Spinel LiMn2O4 Cathode Materials for Lithium Ion Battery Using Microwave Irradiation
Authors: Nkosi Funeka Phumzile , Ozoemena Kenneth I. 
Published: February 2020

Lithium manganese oxide (LMO) is one of the most promising 4 V cathode materials for lithium ion batteries (LIBs). This is due to its availability, ease of preparation, non-toxicity, environmental safety, cheapness and high energy density compared to its counterpart materials like LiCoO2 and LiNiO2[1-2]. For these reasons, LMO LIBs are currently used in electric vehicles such as Chevy volt and Nissan leaf. However, its full utilization is hampered by capacity fade upon cycling. The two main causes of capacity fade in LMO are the Jahn-Teller distortion of Mn3+ ions and dissolution of the cathode in the electrolyte [3,4]. These main causes are a consequence of high concentration of Mn3+ ion in the spinel structure. There is therefore the need to find synthetic procedures that will be able to tune or optimise the concentration of the Mn3+to enhance the electrochemical performance of LMO.

Several methods are used to make LMO, including microwave-assisted strategy. The use of microwave is generally employed to quicken the reaction time. In our work, we introduce strategic microwave irradiation as an essential step in the synthesis of LMO to optimize the concentration for Mn3+.  In this work, we clearly show that microwave irradiation can be used as a curative tool to tune the Mn3+ ion concentration, average manganese ion valence (n
Mn), lattice parameter (a), lattice stability, particle size and morphology, in order to improve capacity and capacity retention. Microwave irradiation is currently used for quicker synthesis, preheating or annealing, and has been used to tune the electrochemistry of energy materials [5].

X-ray diffraction (XRD) showed that the powders were successfully prepared with spinel LMO structure belonging to Fd-3m space group. The XRD results also showed that the microwave irradiation decreased the lattice parameter. Scanning electron microscopy (SEM) and, transmission electron microscopy (TEM) results showed that microwave irradiation was able to shrink the particles and improve the morphological integrity of the LMO powders. X-ray photoelectron spectroscopy (XPS) data clearly showed that microwave irradiation can be used to tune the Mn3+ content, Mn3+/Mn4+ ratio and average manganese valence (n
Mn). The Mn3+ion content was found to be largest for pristine LMO (LMO-a) with 83.5 % and was decreased by microwave irradiation to 50.3 % and 54.2 % for LMO-am and LMO-ma, respectively.

The cycling performance of the LMO powders is shown in Figure 1. It is evident from Figure 1 that the microwave irradiation improved the capacity and capacity retention of LMO powders. LMO-a (n
Mn = 3.165+) with initial discharge capacity of 127 mAhg-1 retained only 78 % of its initial capacity after 50 cycles. LMO obtained by microwave irradiation post-annealing (LMO-am; n
Mn = 3.498+) with initial discharge capacity 94 mAhg-1 retained only 91 % of its capacity after 50 cycles. LMO obtained prior to high-temperature annealing (LMO-ma;  n
Mn = 3.541+) with initial discharge capacity of 131 mAhg-1retained 95% of its initial capacity after 50 cycles.

The increased average manganese valence by microwave irradiation also played an important role in the enhancement of the capacity retention. Therefore, the findings in this study can potentially revolutionize the use microwave irradiation in the preparation of spinel cathode materials and other energy storage materials.

Figure 1: Discharge capacity and coulombic efficiency vscycle number graphs for the pristine sample (LMO-a) and microwave irradiated samples prior and post-annealing stages (LMO-am and LMO-ma, respectively) at 0.1 C rate between 3.5-4.3 V at room temperature.

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Figure 1