Sonochemical synthesis of nanostructured VOPO₄ · 2H₂O/carbon nanotube composites with improved lithium ion battery performance

Transition metal phosphates have become of great interest as cathode materials for lithium ion batteries because of their high voltage, low cost and environmental friendliness. However, their low-intrinsic conductivity presents a major drawback to practical implementation. Here, nanocrystallization of VOPO₄ · 2H₂O was first realized by a sonication-assisted intercalation-split mechanism in order to increase its diffusion coefficient and surface area contacting with electrolyte thus improving its capacity and cyclability; then nanocompounding of the above split nanocrystals and acid-functionalized multiwalled carbon nanotubes to form the resulting nanocomposites was successfully achieved by an adsorption-reintercalation mechanism to increase their conductivity thus enabling them to discharge at high rate with high efficiency. As expected, nanosized VOPO₄ · 2H₂O possesses longer discharge plateau (average discharge voltage: 3.7 V), higher capacity (93.4% of the theoretical capacity) and much better cyclability (retain 95.1% of the first discharge capacity after 50 cycles) than microsized VOPO₄ · 2H₂O. Furthermore, the relatively high-rate capability of the nanocomposites, retaining 83% of the first discharge capacity, is remarkably improved compared with VOPO₄ · 2H₂O microcrystals (retain only 31.7%). In brief, the use of nanocrystallization and nanocompounding techniques enables the high voltage, low cost, environmentally benign VOPO₄ · 2H₂O to show the prospective signs for the future practical applications.