Luminescent Silicon Nanoparticles: An improved inverse micelle synthesis and oxide-dependent photoluminescence studies

The luminescence of silicon nanoparticles has been an active area of study for a number of years. These nanoparticles luminesce when excited by ultraviolet (UV) light in the ~340 nm region, displaying either a blueish or a reddish colour. The control and origin of these colours has been debated, with researchers such as Veinot, Kauzlarich and Tilley claiming that the luminescence can be controlled with the use of nitrogen. In this thesis, a new synthetic procedure is presented that improves upon the established inversemicelle technique by replacing the methanol or ethanol quenching agent with copper chloride. It is found that this new technique lowers the level of surface oxide present in the form of alcohols on the surface of the nanoparticles, giving blue luminescent particles with improved surfaces for further chemistry. Photoluminescence studies are also performed on samples of silicon nanoparticles synthesised by electrochemical etching, using a variety of oxygen-containing compounds to probe the behaviour of the photoluminescence spectrum. It is found that there are typically five Gaussian peaks present in a typical spectrum; occurring at 405 nm, 430 nm, 460 nm 485-90 nm, 500 nm, and 640 nm, subsequently labelled a, b, g, e, and z . It is also found that molecular oxygen increases the b peak and suppresses the z peak, and that water has the opposite effect on these two peaks. It is shown that the peaks a, b, and z are all due to a single site on the surface of the silicon nanoparticle, corresponding to the cases when the site is oxide-free, occupied by molecular oxygen, and water, respectively. It is also hypothesised that the luminescence can be controlled via the use of compounds of different sizes and polarities, with electron-withdrawing molecules increasing the blue luminescence, and electron-donating molecules increasing the red. g and e are used as normalising peaks to compare between samples, and are not affected by oxygen

Unravelling the electrochemical impedance spectroscopy of silicon half cells with commercial loading

Silicon (Si) is an important anode material for lithium ion batteries (LIBs), and increasing the loading of Si electrodes is an important step towards commercialization. However, half cells commonly used for Si studies are limited by polarization of the lithium (Li) counter electrode, especially at high Si loading. To study the interplay between Si and Li electrodes, a set of electrochemical impedance spectroscopy (EIS) spectra are generated using cycled Si half cells at four different potentials in the charge–discharge profile, and then repeated using symmetric Si/Si and Li/Li cells assembled from half cells cycled to equivalent stages in the cycle. Distribution of relaxation times (DRT) analysis is used to design equivalent circuits (ECs) for both Si/Si and Li/Li symmetric cells incorporating both electrolyte and electrode-related diffusion, and these are applied to the half cells. The results demonstrate that the behaviour of half cells is dominated by the solid electrolyte interphase (SEI) impedances at the Li counter electrode at the low and high potentials where the Li+ mobility signal in Si is limited, while the Si electrode is dominant at intermediate potentials where the signal from mobile Li+ is strong. EIS studies of Si half cells should therefore be performed at intermediate potentials, or as symmetric cells.publishedVersio

Revisiting Polytetrafluorethylene Binder for Solvent-Free Lithium-Ion Battery Anode Fabrication

Solvent-free (SF) anodes with different carbon materials (graphite, hard carbon, and soft carbon) were fabricated to investigate the stability of different anodes with polytetrafluorethylene (PTFE) degradation. The graphite anode with large volume variation during the charge/discharge process showed poor cycle life performance, while hard carbon and soft carbon with low-volume expansion showed good cycle life. The SF hard carbon electrodes with a high loading of 10.7 mg/cm2 revealed good long-term cycling performance similar to conventional slurry-casting (CSC) electrodes. It demonstrated nearly 90% capacity retention after 120 cycles under a current of 1/3 C with LiNi0.5Co0.2Mn0.3O2 (NCM523) as cathode in coin cell. The rate capability of the high-loading SF electrodes also is comparable to the CSC electrodes. The high stability of SF hard carbon and soft carbon anodes was attributed to its low-volume variation, which could maintain their integrity even though PTFE was defluorinated to amorphous carbon irreversibly. However, the reduced amorphous carbon cannot tolerate huge volume variation of graphite during cycling, resulting in poor stability.publishedVersio

Enabling Increased Delithiation Rates in Silicon-Based Anodes through Alloying with Phosphorus

The capability of battery materials to deliver not only high lithium storage capacity, but also the ability to operate at high charge/discharge rates is an essential property for development of new batteries. In the present work, the influence on the charge/discharge rate behaviour of substoichiometric concentrations of phosphorus (P) in silicon (Si) nanoparticles was studied. The results revealed an increase in rate capability as a function of the P concentration between 0 and 5.2 at %, particularly during delithiation. The stoichiometry of the nanoparticles was found to strongly affect the formation of the Li3.5Si phase during lithiation. Cyclic stability experiments demonstrated an initial increase in capacity for the SiPx materials. Galvanostatic intermittent titration technique and electrochemical impedance spectroscopy demonstrated the increased lithium diffusivity with inclusion of P. Density functional theory and ab initio molecular dynamics were deployed to provide a rationale for the electrochemical behaviour of SiPx.publishedVersio

Enabling Increased Delithiation Rates in Silicon-Based Anodes through Alloying with Phosphorus

The capability of battery materials to deliver not only high lithium storage capacity, but also the ability to operate at high charge/discharge rates is an essential property for development of new batteries. In the present work, the influence on the charge/discharge rate behaviour of substoichiometric concentrations of phosphorus (P) in silicon (Si) nanoparticles was studied. The results revealed an increase in rate capability as a function of the P concentration between 0 and 5.2 at %, particularly during delithiation. The stoichiometry of the nanoparticles was found to strongly affect the formation of the Li3.5Si phase during lithiation. Cyclic stability experiments demonstrated an initial increase in capacity for the SiPx materials. Galvanostatic intermittent titration technique and electrochemical impedance spectroscopy demonstrated the increased lithium diffusivity with inclusion of P. Density functional theory and ab initio molecular dynamics were deployed to provide a rationale for the electrochemical behaviour of SiPx.publishedVersio

Revisiting Polytetrafluorethylene Binder for Solvent-Free Lithium-Ion Battery Anode Fabrication

Solvent-free (SF) anodes with different carbon materials (graphite, hard carbon, and soft carbon) were fabricated to investigate the stability of different anodes with polytetrafluorethylene (PTFE) degradation. The graphite anode with large volume variation during the charge/discharge process showed poor cycle life performance, while hard carbon and soft carbon with low-volume expansion showed good cycle life. The SF hard carbon electrodes with a high loading of 10.7 mg/cm2 revealed good long-term cycling performance similar to conventional slurry-casting (CSC) electrodes. It demonstrated nearly 90% capacity retention after 120 cycles under a current of 1/3 C with LiNi0.5Co0.2Mn0.3O2 (NCM523) as cathode in coin cell. The rate capability of the high-loading SF electrodes also is comparable to the CSC electrodes. The high stability of SF hard carbon and soft carbon anodes was attributed to its low-volume variation, which could maintain their integrity even though PTFE was defluorinated to amorphous carbon irreversibly. However, the reduced amorphous carbon cannot tolerate huge volume variation of graphite during cycling, resulting in poor stability

New Insights into the Behaviour of Commercial Silicon Electrode Materials via Empirical Fitting of Galvanostatic Charge‐Discharge Curves

Abstract Silicon (Si) materials for use in Lithium ion batteries (LIBs) are of continued interest to battery manufacturers. With an increasing number of commercially available Si materials, evaluating their performance becomes a challenge. Here, we use an empirical fitting function presented earlier to aid in the analysis of galvanostatic charge‐discharge data of commercial Si half‐cells with relatively high loading. We find that the fitting procedure is capable of detecting dynamic changes in the cell, such as reversible capacity fade of the Si electrode. This fading is found to be due to the highly lithiated Li2Si ↽⇀ Li3.5Si phase and that the behaviour is strongly dependent on the potential of this phase. EIS reveals that the Si electrode is responsible for the reversible behaviour due to progressive loss of Li+ leading to increasing resistance. SEM/EDX and XPS characterization are also employed to determine the origin of the irreversible resistance growth on the Si electrodes