Because of their high energy and power density, lithium ion batteries (LIBs) are currently the most promising energy storage technology for mobile electronic devices and electric vehicles. To increase the energy density of LIBs, relentless research efforts are invested in the development of new electrode materials and optimizing the electrode formulation. New anode materials that allow for alloying or conversion energy storage mechanisms (e.g. Si, Sn, GeO2, SnO2, Fe3O4, etc.) offer high capacities but have disadvantages including poor cycling stability, large volume change during Li+ insertion/extraction, high voltage hysteresis, poor rate performance, poor coulombic efficiencies (CE) and low electrical conductivity.
Conductive carbons additives such as graphene and carbon nanotubes (CNTs) have been applied as additive materials for high capacity materials. However, these materials tend to cause side reactions and phase segregate after slurry mixing. This phase segregation during the electrode coating and drying, compromises electron conduction and results in poor material utilization and high ohmic losses. The phase segregation is particularly pronounced with high areal loading electrodes pursued in industry, which dry slower. Further, several researchers have shown that the interface between carbon additives and the active material can degrade over time, which further accentuates the above problems. Industrially, time consuming kneading and high intensity mixing processes are used to reduce the agglomeration of carbon additives, and academically, methods are developed to anchor active battery materials on the surface of carbon additives (e.g. by synthesizing the active material in the presence of CNTs or graphene). However, to nucleate the active material on the carbon additives, the carbon surface typically requires oxidation or other chemical modifications, which in turns decreases their electronic conductivity and is poorly scalable.
In this work we address several of the above challenges by fabricating advanced Silicon / Iron Silicide anodes coated with CNTs. In particular, we developed a continuous fabrication process that structures nanoparticles into secondary micrometer sized particles on which CNTs are synthesized. Here, we use spray drying to pack our Si nanoparticles into micrometer-sized spheres. The electrical resistance of micrometer-sized pure Si secondary particles would be too high for good battery operation, but this work shows that CNTs can be synthesized inside the crevices of the secondary particles hereby providing a good internal short range electrical network. In addition, CNTs extending from the surface of the secondary particles can help with the inter-particle long-range conductivity, which is particularly important as industry shifts to thicker electrode coatings.
