ON THE AEROSOL SYNTHESIS OF HIERARCHICAL HYBRID CARBON NANOTUBE NANOMATERIALS FOR LITHIUM-ION BATTERIES

Abstract

Achieving the ambitious performance and cost goals for Li-ion batteries set by the industry at the 2030 horizon requires significant progress in the design and mass-production of active electrode materials. The aim of this dissertation is to progress the field of advanced battery materials by investigating novel materials designs and characterisation tools enabling the potential scale-up of their production process. In particular, we focus on the aerosol-based production of a novel hierarchical hybrid carbon nanotube (CNT) nanomaterial, the suitability of common aerosol neutralisers to monitor its production process, and its application to Li-ion batteries. This material is an ‘urchin-like’ nanostructure, whereby CNTs are grown radially from a central metal oxide core, which we refer to as Carbon Nanotube Sea-Urchin (CNTSU). We investigated the aerosol synthesis and characterisation of CNTSUs, providing new insights into the mechanisms of CNTSU synthesis following a continuous aerosol production process combining spray-pyrolysis to form CNTSU metal oxide cores, and growth of CNTs from these cores by Chemical Vapor Deposition (CVD). Further insights were gained on the physical and chemical characterisation of CNTSUs, which may be useful for their application as Li-ion battery anodes. Furthermore, it was found that mass-mobility relationship in-line aerosol measurements of the CNTSU process could generate valuable insights into process scale-up and fundamental CNTSU properties. However, these measurements rely on the use of an aerosol neutraliser, operated with the assumption that charging equilibrium is reached within the neutraliser. That assumption has not been challenged by existing academic literature in the context of purposeful nanomaterial production, where, typically, aerosol particle concentrations are of the same order as neutraliser ionic concentrations, particles are small, and neutraliser residence times are low. We find that the commonly-accepted neutraliser equilibrium indicator, the 𝑛 ∙ 𝑡 product, rule fails to predict equilibrium behaviour in that context. We propose a nondimensional approach to predict equilibrium behaviour as a function of two nondimensional groups: 𝑁̂ the non-dimensional ion concentration, and 𝜏̂the non-dimensional residence time. This approach was successfully applied to the CNTSU synthesis process discussed above, and it was found that, fortunately, the specific neutraliser used in this work was likely to generate an equilibrium charge distribution, but that it may not be the case of other common aerosol neutralisers. Finally, we investigate the application of CNTSUs as conversion anode for Li-ion batteries. Our results confirm the relevance of CNTSUs as interesting candidates for next-generation Li-ion battery anode materials, thanks to their high specific capacity, high lithiation average potential avoiding dendrite formation, encouraging specific capacity retention at high rates, and initial indications of their compatibility with a common commercial cathode material. Some of these features may be attributed to hierarchical nature of the CNTSU electrode conductive network compared to a network of randomly-mixed, weakly contacted CNTs, such as those reported in conventional electrodes where CNTs are externally added as a dry powder during the electrode slurry mixing step. Similarly, we suggest that using CNTSU-type structures for electrode preparation may be a viable route to addressing the known issue of CNT segregation during electrode mixing and coating. However, CNTSUs do not address other common issues associated with conversion anodes, including a sloping voltage profile with high average potential vs. Li/Li+ , voltage hysteresis, and excessive Solid-Electrolyte Interface (SEI) formation leading to poor cell efficiencies and cycle life in a full cell. We suggest that further investigating the lithiation mechanism of CNTSUs, and modifying the CNTSU core composition may be a valuable future research direction.