Rectification of Plasma Flow for Homogeneous Formation of Si Nanoparticles and Its Effect on Lithium-Ion Battery Performance

Next generation lithium-ion batteries (LiBs) are anticipated ideally to carry nearly 10 times higher electric capacity than the conventional LiB, by employing silicon as negative electrode active material replacing the graphite. Practically to make use the high potential of silicon, this material has to be processed smaller than 150nm and in a functional composite structure. This is because silicon changes its volume up to 400% during charge/discharge reaction cycles and tends to fracture in several cycles, casing a loss of electric and ionic conducting paths within the electrode and appreciable capacity decay in the end. From the engineering point view, these nanocomposite structures are to be produced at low cost and at high-process throughput to meet the ever-growing large LiB market demands. In this respect, plasma spray with which nanoparticles are produced from $2-3 powder feedstock and at a speed faster than 1kg/h is recognized as one potential candidate for the industrial method for the silicon electrode production. We have successfully produced silicon nanoparticles with various functionalities by plasma spray and demonstrated their high potentials for liquid-electrolyte LiB as well as for all-solid-state LiB. However, due to the presence of various distributions involved in the plasma spray process, the structural characteristics of nanoparticles have also a distribution which leads to unstable yield and battery performances.
To tackle this issue, we have attempted to homogenize the condensation process for nanoparticle formation by rectifying the thermal history of high silicon vapor i.e. plasma gas [1]. In the present work, we have equipped the inline axial cyclone underneath the ICP plasma torch within the plasma spray reactor. This cyclone has no mobile turbine so that the plasma flow is rectified spontaneously with no additional energy by its design. The fluid dynamics simulation have indicated that the recirculation flow within the plasma reactor that is commonly observed without cyclone disappears. That is, the total plasma gas flow time from the ICP torch exit to the particle collection filter is significantly shortened and the temperature of any gas streamline reduces monotonously, i.e. rapid and straightforward cooling. The estimated transport entropy of these streamlines suggests that the distribution of the gas thermal history is significantly reduced by an introduction of the cyclone. The nanoparticles so produced has also rather sharp particle size distribution and the resultant LiB cycle capacity is found to be improved appreciably.