Novel routes to nano-materials for Li-ion batteries
published: Jan. 18, 2008, recorded: October 2007, views: 5893
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Advanced Li-ion batteries providing enhanced storage capacities and improved power performances are currently required not only by the fast-growing market of portable electronics, but also by emerging electric or hybrid-electric vehicles. We are investigating two novel techniques for this purpose: Spark Discharge Generation (SDG) and Electrostatic Spray Reductive Precipitation (ESRP). SDG uses a physical “top down” approach that relies on the atomization of two metal rods via a sudden spark. Two cylindrical rods are connected to high voltage and parallel to a variable capacitance. The capacitors are periodically charged to the break-down voltage of the system determined by the gap between the rods. Through the high temperature of the generated spark, electrode material is rapidly evaporated, and the vapour condenses to form nano-sized metallic particles. In addition, an unconventional densification technique, called Magnetic Pulse Compaction (MPC), is being used for self-manufacturing metal or alloy rods to be atomized. ESRP is a physical-chemical technique relying on a combined “top-down” and “bottom-up” approach, which bridges aerosol generation with chemical precipitation in order to form nanoparticles. Electro-Spraying of liquids consists in the creation of charged aerosols by applying a high voltage between a nozzle, through which the liquid to be sprayed is fed, and a counter-electrode. Interesting properties of this phenomenon are the narrow size distribution of the emitted droplets, as well as tuning the droplet size by controlling experimental parameters. Moreover, high net surface charge on the generated droplets causes repulsive interaction, preventing droplet coalescence. These beneficial aspects have been exploited in combination with a well-known technique for the synthesis of metallic and alloyed particles, namely Reductive Precipitation (RP) of metal chlorides by NaBH4. Dissolved metal chlorides precursors are forced to flow by a syringe pump which provides a constant supply to the nozzle with a controlled flow rate. Under the influence of the high electric field small charged droplets are formed and attracted towards a ring-shaped counter electrode, which is placed in the reductive solution. In this way, the droplets containing precursor metal ions are driven into the reductive bath, where they are immediately reduced to their zero-valent state. Primary particles with size in the range of 2-5 nm can be obtained by proper selection of the experimental parameters.
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