Investigation on Li-ion and solid-state batteries with silicon nanostructured anodes and LFP cathodes

Solid-state batteries (SSBs) are gaining attention because of their enhanced safety, higher energy density and promising performance compared to traditional batteries with organic liquid electrolytes (LE). Replacing volatile organic liquid electrolytes with solid and, eventually, flexible solid electrolytes, SSBs mitigate flammability concerns and set the stage for advanced energy storage innovations. Among different solid electrolytes, polymer and hybrid electrolytes have gained popularity due to their flexibility, lightweight, mechanical strength, and high ionic conductivity. These properties, added to ease of use and good interfacial compatibility, make them a compelling choice for high-performance SSBs. However, compared to their liquid counterparts, challenges such as interface resistance and limited solid electrolyte conductivity limit their broad utilisation. Addressing these obstacles is crucial for unlocking the full potential of solid-state batteries and accelerating their integration into various applications, from portable electronics to electric vehicles. With the need to boost battery energy density, Li-alloying materials are the ideal choices for LIBs, particularly silicon, which has a theoretical capacity of 3579 mAh g-1 , nearly 10 times greater than graphite (372 mAh g-1 ). Unfortunately, volume expansion for silicon electrodes has yet to be fully addressed. The use of nanosized silicon electrodes, such as nanoparticles or nanowires, dramatically enhances cell stability, electrochemical performance, and coulombic efficiency by providing porosity and empty spaces to accommodate volume expansion while reducing mechanical stress.

This thesis focuses on the development of nanosized silicon active materials for use in Li-ion solid-state batteries (SSB). Hybrid and polymer solid electrolytes were developed, and their chemical and electrochemical properties were studied. The thesis addresses a knowledge gap in the literature, where nanoscale Si anodes have been sparingly studied for SSB applications (compared to conventional LE systems). The research chapters are structured as research articles with introductory summaries at the beginning of each chapter.

Chapter 3 details the synthesis of Si NWs directly from stainless steel current collectors utilising copper seeds as a catalyst within a solvent-vapour-growth mechanism. The production of electrodes without the requirement for binder or conductive materials reduces the cell's inactive weight, increasing overall energy density. The electrochemical performances were tested in a half-cell configuration with various electrolyte compositions, and the electrolyte 1 M LiPF6 in EC:DEC (1:1) + 10% (vol.%) FEC was found to perform best. The production of poly(VC) compounds during cycling was related to the increase in performance relative to other electrolytes, as revealed by post-cycling microscopy analysis. Further electrochemical data were obtained in a half-cell design with high charge-discharge rates (1 C) over 1000 cycles, as well as in a full-cell configuration using LFP cathodes. In all cases, the results demonstrated a highly stable capacity that was two to three times greater than that of commercial graphite.

Chapter 4 describes the use of Sn-seeded Si NW electrodes in SSBs. A new thin hybrid solid electrolyte (HSE) was created with high ionic conductivity at room temperature, a broad electrochemical stability window, and excellent thermal stability. SSB performance with the new HSE and Sn-seeded Si NWs was evaluated at room temperature with no external pressure applied. To guarantee good contact between the electrode and the electrolyte, the minimum amount of LE additive was identified and combined with the HSE disc. Electrochemical performance in half-cells showed a specific capacity of 1500 mAh g-1 after 100 cycles, with a capacity retention of 40%. Full-cell testing with LFP cathodes yielded good results for Si NWs electrodes without prelithiation, with specific capacities exceeding 500 mAh g-1 over 50 cycles.

In Chapter 5, SSBs with hybrid polymer electrolyte (HPE) membranes were built with a silicon anode and an LFP cathode, to investigate the role of prelithiation in achieving satisfactory electrochemical performances. Silicon electrodes were prelithiated in half-cells using a two-step process that included conditioning and prelithiation at varying capacity limitations. Electrodes with a combined conditioning and prelithiation (referred to as Cond-2000, Prelith-1000 based on the specific capacities of the related steps) demonstrated the best electrochemical results after 100 cycles, with an average coulombic efficiency of 98%. The wettability and permeability of prelithiated silicon electrodes with HPE material were studied using microscopy and spectroscopy elemental mapping. The results demonstrated the electrolyte's ability to infiltrate the silicon framework, filling its voids and increasing the electrode-electrolyte interface.