Research

Synthesis and characterization of nanomaterials for advanced Li-ion batteries

General ideas about nanomaterials in Li-ion batteries
The next generation of Li-ion batteries requires the development of new functional materials to achieve better performances. In particular, rechargeable batteries providing improved storage capacities (i.e. gravimetric and volumetric energy densities), higher voltages and longer cycle lives are desirable for several applications. Beside the well-known use in portable electronics, the application of Li-ion batteries to high-power systems (i.e. electric or hybrid-electric vehicles) is nowadays attracting considerable attention. In this context, the role of nano-structures and nano-particulates is crucial, since decreasing the size of the host materials can significantly improve the cell performances. For example, some important consequences of the reduction in size are: a) an increased ratio surface per weight (or per volume) allows a more efficient charge transfer process; b) reduced diffusion (migration) lengths of the charge carriers result in enhanced power delivery; c) a “novel” red-ox chemistry is responsible for relevant shift in thermodynamic properties; d) a reduced volume of the host particles favours both effective acceptance of local strain and more stable mechanical behaviour; e) the presence of an extended interface opens also the possibility for electron and ion storage in neighbouring nano-phases. Furthermore, in the near future, advanced Li-ion batteries should fulfil higher safety standards together with acceptable costs and suitable lifetimes. In this respect, nanostructures and nano-architectures are potential challenges to meet these requirements.

Our current work

Our investigation is mainly focussed on the synthesis and the characterization of nanomaterials in order to improve Li-ion battery electrodes. Various chemical and physical-chemical techniques are used to produce nanoparticles of different materials. Typical chemical routes are for example: Solid-State synthesis, Advanced Sol-Gel or Template syntheses, which are employed typically for the production of spinel-type intercalation hosts and mesoporous oxidic structures. Besides, a series of physical and physical-chemical techniques are exploited to produce metal and metal-oxides nanoparticles. Spark Discharge Generation (SDG), Electrostatic Spray Pyrolisis (ESP), and Laser-Assisted Chemical Vapour Pyrolysis (LA-CVP) are some examples of novel methods that are currently investigated. In all the cases the particles are generally collected in the form of nanopowders (or nanostructured thin films), which are used as positive or negative electrodes. Together with the development of anode and cathode materials, the study of new types of particle coatings and alternative electrolytes (i.e. Ionic Liquids) is being carried out for further improvements of the performances and the safety of the batteries.

Goals and outlooks

The goal of the research is to tailor the particles properties and to study their influence on the electrochemical behaviour of the cells. Understanding the fundamental mechanisms involved in intercalation and de-intercalation of lithium in and out of the electrodes is also of paramount importance, both from a theoretical and a technological point of view. For this purpose, several techniques (XRD, TEM, SEM, EDX, AFM, IS, etc…) are being employed to characterize the materials. Moreover, collaborations with other European research institutions allow complete characterization of the samples with dedicated in-situ measurements (i.e. EXAFS, Mossbauer, Raman etc.) during the electrochemical treatment.

Overview of the techniques

Spark Discharge Generation

SDG is a physical technique that relies on the atomization of two metal electrodes via a sudden spark, produced by the discharge of a capacitor across the gap between the electrodes in an inert atmosphere of flowing gas. Two cylindrical rods are connected to a 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 nanosized primary particles. The spark lasts a few nanoseconds giving a temperature near 20000 K. Cooling proceeds within nanoseconds at a rate of 10¹⁰ K/s, and occurs via radiation, expansion and mixing with the carrier gas. In addition, an unconventional densification technique, called Magnetic Pulse Compaction (MPC), is being exploited for self-manufacturing the cylindrical rods to be atomized. Mixed electrodes are being made via blending and tapping different powders.

Magnetic Pulse Compaction

MPC works on a simple concept: a high current through a coil induces a magnetic field that on its turn generates Lorentz forces directed to the centre of the coil. When a coil is filled with a metal tube, this last one will experience these forces, and as a consequence it will be deformed radially (i.e. to the centre of the coil). This method then is used for compacting materials which are placed in the metallic tube. This metal tube is initially filled with the material to be compacted, tapped several thousand times for pre-densification and finally placed inside a coil. By charging three Maxwell capacitors up to 10 kV, an amount of energy of 90 kJ can be stored. Then, via a fast discharge (50-70 μs) the same energy is released through the coil, producing a 300 kA current and an induced magnetic field of 35 T. The huge magnetic force (due to the Lenz’s law) acting on the walls of the metal tube causes a sudden (≈ 200 μs) radial compression (5 GPa) of the cylinder and the complete compaction of its content.

Electrostatic Spray Pyrolysis

ESP is a physical-chemical technique that is based on the atomization of a liquid by high voltage (i.e. Electrohydrodynamic Atomization - EHDA). A high electric field is applied between a grounded counter-electrode and a metallic nozzle, through which a liquid is pumped at low flow-rate. By increasing the voltage, the shape of droplet at the outlet of the nozzle is progressively deformed from a sphere to a cone. The emission of a thin jet from the tip of the liquid cone is normally accompanied by the break-up of the jet into micrometer-sized droplets with a narrow size distribution. When the generated aerosol contains for instance dissolved salts or sols, powders are formed during the evaporation (pyrolysis) of the solvents. By tuning the experimental parameters (i.e. temperature, distance of the nozzle, liquid viscosity, concentration, etc.) different kinds of products (i.e. nano-powders, nanostructured films, nano-fibres, nanotubes, etc..) and various morphologies (i.e. porous, reticular, dense) can be obtained. Moreover, a combination of the precursor atomization with red-ox reactions in liquid or gaseous phase allows further engineering of the desired products.     

Laser Assisted Chemical Vapour Pyrolysis

LA-CVP is a pyrolytic method for the production of nanoparticles from gaseous reactants. The major steps in this process are: gas phase reaction, nucleation, collision, and sintering. In the employed system a 10.6 μm infrared emission of a continuous wave CO₂ laser provides the reaction energy. Silane and acetylene are used as precursors, where silane absorbs the IR emission. Hard agglomeration and sintering of the nucleates are suppressed by confining the reaction zone to a narrow region of a few cubic millimetres in the reactor chamber. Consequently, the residence time in the reaction hot zone is limited and also the reaction time is controlled by mechanical chopping of the CO₂ laser beam. In this way Si particles of few nanometres capped by a carbon monolayer can be formed.

Advanced Sol-Gel Methods

Advanced Sol-Gel methods using surfactants (Block copolymer P123) or porogene (urea) are used in order to synthesise oxidic nanopowders. An important point of these syntheses is that the texture of the desired materials can be easily tuned (i.e. high surface area, porosity and particles size distribution). These processes have been applied to a number of different precursors. In particular, the synthesis of high-surface-area nano anatase TiO₂, nano LiCoO₂ and nano high voltage spinel (LiMg_0.05Ni_0.45Mn_1.5O₄) proved a simple and cost-effective way for producing potential electrode materials.

Template Filling Synthesis

Polycarbonate membranes with sub-micrometric pores are being used as templates for the synthesis of free-standing nanostructured architectures. The membranes are dipped in a precursor sol in order to infiltrate the pores and initiate the formation of particle networks. Subsequent thermal treatment burns the membrane off and consolidates the oxidic nanostructures left inside the pores. In this way, self-standing mesoporous structures of different materials (i.e. SnO₂, LiMn₂O₄) can be easily formed on a substrate and directly employed as 3D-electrodes. Furthermore, by proper control of the experimental parameters (i.e. precursor concentrations, dipping times, etc..) different morphologies can be obtained, spanning from hollow tubes to rod-like pillars.