Developing nanostructured electrodes for renewable energy applications

Abstract

The energy transition from fossil fuels to renewable energy sources is the approach globally adopted to address the climate change driven by emission of greenhouse gases. Owing to the intermittent nature of solar and wind energies, the development of energy storage technologies such as sustainable rechargeable batteries of high energy density beyond the current lithium-ion batteries is critical to accelerate this energy transition to achieve net-zero carbon emissions. Lithium metal batteries such as lithium-sulfur and lithium-air offer high theoretical energy densities of 2567 Wh/kg and 3505 Wh/kg, respectively. For battery energy storage applications, safer aqueous zinc-ion batteries (AZIBs) are another attractive option. The commercialization of next generation batteries is hindered by challenges such as poor cycle life, poor charge efficiency associated with the respective anode and cathode materials. This dissertation comprises of studies aimed towards improving our understanding on the working of various anode and cathode materials directed towards the development of high-performance next-generation batteries.

Implementing three-dimensional (3-D) hosts is one of the effective approaches to mitigate dendrite formation in lithium metal anodes (LMAs). However, the formation and evolution of electrodeposited lithium (e-Li) and solid electrolyte interphase (SEI) on 3-D LMA hosts remain understudied. To address this, mechanistic studies conducted using a model 3-D nanostructured host based on vertically aligned carbon nanofibers (VACNFs) grown on Cu foil (VACNF/Cu) are discussed in Chapter 2. Chemical and morphological analysis uncovers the different types of SEIs forming in lithiated VACNF/Cu. Upon plating, Li electrodeposits dominantly as dense, infiltrative, columnar micro-grains that help to enhance the electrochemical performance. The VACNFs that are not engulfed by infiltrative-Li are coaxially coated with nanoscale Li. The SEI accumulation during each cycle causes loss of electrolyte leading to cell failure. Further studies to elucidate the effect of plating and stripping kinetics on the morphology of e-Li and on the cycling performance of 3-D LMAs are discussed in Chapter 3. Microscopic analysis and electrochemical tests with decoupled Li plating/stripping rates reveal that a moderate current density (~ 1.0 mA/cm²) is necessary for obtaining dense, columnar e-Li morphology, governed by classical nucleation and growth theory, which exhibits the highest performance. Low current density (≤ 0.10 mA/cm²) results in sparsely distributed noodle-like Li with inferior stripping efficiency but could be converted into dense flat islands by applying a high pressure in the electrode/separator stacks.

In Chapter 4, the development of binder-free air cathodes toward Li-air batteries based on transition metal (Ni) and noble metal alloy (PtRu) deposited VACNF arrays grown on carbon substrates (VACNF-Ni and VACNF-PtRu respectively) is discussed. The VACNF-Ni and VACNF-PtRu air cathodes deliver enhanced discharge capacity by forming thin film-like Li₂O₂. The coverage of VACNFs’ surface with catalyst nanoparticles offers improved cycling stability by preventing degradation of carbon at oxidizing potentials. Chapter 5 discusses the development of nanostructured hybrids consisting of V₂O₅ nanoribbons and reduced graphene oxide as AZIB cathodes. The hybrids are synthesized using a facile coprecipitation method utilizing divalent cations (Zn²⁺ and Mn²⁺). The pre-intercalation of Zn²⁺ ions facilitates faster Zn²⁺ diffusion into the V₂O₅ structure thereby enhances the specific capacity of the hybrid material, while the inclusion of Mn²⁺ during coprecipitation benefits to improve the cycling stability of the cathode by providing stable pillars between V₂O₅ layers due to the stronger interaction between the Mn[superscript n+] ions and the V₂O₅ framework. Further studies on 3-D LMAs are discussed in Chapter 6. In conclusion, the scientific knowledge gained could aid in advancing high-performance energy storage technologies thereby supporting the shift towards renewable energy sources.