Studies of Surface-Modified Molybdenum Dioxide as Anode Material for Lithium-Ion Batteries
López Pérez, José Ismael
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Lithium ion batteries have attracted much interest in the field of fundamental study and applied research, where the greatest challenge lies in the design and development of electrode materials with high power density and long cycle life. Recently MoO2, with a theoretical capacity of ~ 838 mAh g-1, has received much attention and has been considered as a promising anode material in lithium ion batteries due to its low electrical resistivity, high electrochemical activity and great chemical stability. But the intrinsic drawback of MoO2 for lithium ion battery applications is its volume expansion during Li+ insertion/extraction process. The irreversible volume change causes MoO2 particles to crack and pulverize, causing the detachment of the active material from the current collector, and consequently leading to a substantial loss in capacity. In order to improve structural stability and electrochemical behavior, many groups have demonstrated that the addition of a thin coating of metal phosphates, fluorides, oxides, or other analogous materials onto the electrode material results in reduced irreversible capacity, improved rate capability, and cycle life. Aluminum phosphate (AlPO4), an environmentally friendly, low cost, and thermally stable material, is of great interest in both environmental and technological fields. Considering the applications of AlPO4 for lithium ion battery technology, a significant improvement concerning the safety and the electrochemical properties of electrode materials can be achieved by applying a direct coating of AlPO4 nanoparticles from an aqueous solution. In this context, we hereby present a doctoral research project based on the effects of surface modification with AlPO4 on the structural and electrochemical properties of MoO2 anode material. MoO2 anode material was successfully coated with AlPO4 nanoparticles and electrodes were fabricated by spray coating technique. The AlPO4-coated electrodes displayed a significant improvement in life-cycle performance. Cyclic voltammetry studies indicated that surface modification with AlPO4 nanoparticles significantly reduced the formation of surface cracks that are provoked by the volume expansion of MoO2 anode material, diminishing the repetitive formation of electrode/electrolyte interfaces that affect the capacity fading. Such improvement in cycling performance of the AlPO4-coated MoO2 was attributed to the stabilization of the lattice structure due to the suppression of the elimination of oxygen vacancies in the anode material. Galvanostatic charge and discharge measurements, at a current density of 50 mA g-1, revealed that uncoated MoO2 exhibits an initial discharge capacity of 650 mAh g-1 and 54% capacity loss in 50 cycles; while the AlPO4-coated MoO2 exhibits an initial discharge capacity of 1,015 mAh g-1 and only 22% capacity loss at 50 cycles. Electrochemical impedance spectroscopy data were simulated with a Randles circuit. The results of the computational model indicate that the AlPO4 nanoparticle coating notably reduced the surface layer (Rsl) and charge transfer (Rct) resistances by 28% and 26%, respectively. The computational results are consistent with the experimental data, confirming that surface modification with AlPO4 nanoparticles is an effective way to improve the electrochemical performance of MoO2 as anode material for lithium ion batteries.