Open crystal structured anode materials for superior aqueous metal-ion batteries

Abstract

Rechargeable batteries with aqueous electrolytes have safety and cost benefits over the flammable organic electrolytes used in current batteries. However, aqueous batteries suffer from lower energy and power densities. In the first part of this work, niobium tungsten oxides are shown to enable aqueous lithium-ion batteries that could be cycled stably with a volumetric capacity of ~200 Ah l-1 at 1C rate, which is much higher than state-of-art graphite (50 to 110 Ah l-1). This is attributed to the higher density of niobium tungsten oxide in the anode, as well as the abundance of tunnels within its particles that allow fast diffusion of ions. Facile synthesis, ease of handling, and high performance makes niobium tungsten oxide anodes an attractive alternative to traditional electrodes, especially in applications where high volumetric energy and power densities are desired. Scarcity, high prices and safety concerns limit the use of lithium. Calcium has been actively researched for batteries because of its abundance, but the large size of calcium-ion impairs diffusion kinetics and cyclic stability. In the second part of this work, an aqueous calcium-ion battery is demonstrated using molybdenum vanadium oxide (MoVO) as anode, 5m calcium triflate as electrolyte, and activated carbon as pseudo-reference electrode. MoVO is special as it provides large tunnels for easy calcium-ion diffusion. Three different polymorphs of molybdenum vanadium oxide (MoVO) have been employed as calcium host. Orthorhombic and trigonal structured MoVO outperformed tetragonal structure because of large hexagonal and heptagonal tunnels which provides easy calcium-ion diffusion pathways. For Tri MoVO, high specific capacity of 203 mAh g-1 was obtained at 0.2C and at 100 times faster rate of 20C, 60 mAh g-1 capacity was achieved. The open structure also happens to promote cyclic stability and reversibility, demonstrating a capacity fade rate of only 0.15% per cycle. Good cycling stability and cost-effectiveness of this battery make it a potential candidate for energy storage applications.