Carbon Nanostructures for Ionic Batteries

Lithium-ion rechargeable batteries are currently the state-of-the-art energy storage technology for devices as cell phones, laptops and electric cars. Despite their top perfomances, some problems have yet to be solved. The first issue is availability: lithium is diffcult to recycle and, while still available in large quantities, not enough to support a future economy based on electric vehicles. The second problem is performances: despite Li-ion batteries are state-of-the-art among other battery technologies, their performances are still not performing as fossil fuels, in terms of energy density and power density. The last issue is safety: Li-ion batteries have been reported to heat up, catch fire or even explode. Most of such safety issues come from the electrolyte, which is the most flammable element in a battery. Typical electrolytes are also toxic, and can easily leak out of a battery. Among the possible alternatives to Li-ion batteries, sodium-ion batteries and magnesium-ion batteries are particularly relevant, since both sodium and magnesium are far more abundant in nature and hence less expensive than lithium. One of the biggest challenges in realizing sodium- or magnesium-ion batteries is represented by finding materials suitable to intercalate such ions with competitive performances with respect to lithium-ion ones, especially as far as the negative electrode is concerned; this is due to the fact that graphite, the most commonly used material for lithium-ion batteries, cannot intercalate sodium or magnesium to a signi cative extent. A promising substitute is C60 fullerene, which can host in its crystal structure Na ions and Mg ions up to a stoichiometry of respecitvely Na11C60 and Mg5C60. This is favored by the large intersitial voids between fullerene molecules in the crystal and by the tendency of C60 to accept electrons from alkali and alkali-earth metals. Sodium-ion and magnesium-ion batteries with anodes based on fullerene and two derivatives, fullerene mixture and hydrogenated fullerene, were assembled and tested. Each material was successfully electrochemically intercalated with sodium and magnesium, with a specific capacity respectively up to 320 mA h / g and 125 mA h / g, although with a reversibility loss after the first cycles. A different approach is developing novel, more performing materials as electrodes for Li-ion batteries. Graphene, thanks to its similarity with graphite, good electrical conductivity and high specific surface area, is a good candidate to improve performances, especially fast rate of charge and discharge, of lithium-ion batteries, and former studies demostrated capability of intercalating graphene and hydrogenated graphene up to 500 mA h / g. In this study, diffusion mechanisms of graphene and hydrogenated graphene are characterized by means of electrochemical impedance spectroscopy, and are coupled, after a proper pre-lithiation, to three different cathodes, in order to obtain high performance full-cells. In particular, hydrogenated graphene boasted an impressive reversible specific capacity with fast charge/discharge capabilities, exceeding 370 mA h g􀀀1 even at the high C-rate of 25C. Moreover, for the first time, thermally exfoliated graphene was coupled to proper cathode materials, in order to study and develop full-cell prototypes, more similar to a commercially viable product. Lastly, one possible approach to solve liquid electrolyte-related safety issues in batteries, is the development of solid electrolytes, which cannot leak out of the batteries, and are more stable at high temperatures or if exposted to air. However, Solid electrolytes cannot compete with liquid electrolytes in terms of room temperature ionic conductivity, which is the capability of allowing the passage of ions. Lithium borohydride is a good solid electrolyte at high temperature, and some derivatives of C60, like Li4C60 and Mg2C60, are superionic conductors at room temperature. Fullerene-borohydiride composite materials with high ionic conductivity were synthesised and characterized. LiBH4-Li6C60 composite showed an ionic conducitivity of 6*10^-6 S / cm at room temperature, which is 100 times higher than pure LiBH4, while NaBH4- Na6C60 boasted an ionic conductivity of 5*10^-5 S / cm at room temperature.