Facile synthesis routes of silicon nanowires for Li-ion battery applications using a zinc catalyst

The work presented in this thesis describes the development of new synthesis routes for Zn catalyzed Si nanostructures used for the fabrication of high-capacity Li-ion battery electrodes on stainless-steel current collectors. The results chapters are arranged as research articles with introductory summaries at the beginning of each. As an anode material, Si boasts a capacity multiple times higher compared to graphite which currently is predominantly used for commercial Li-ion battery anodes. The high capacity of Si is facilitated by its ability to form Li-rich alloys where Li15Si4 is its fully lithiated state and results in a volume expansion of Chapter 3 describes a fabrication method of NW electrodes whereby electrodeposited Zn is used to directly catalyse the growth of Si, Ge and Si-Ge axial heterostructure nanowires. The study demonstrates that LiBH4 is a suitable reducing agent for removing the Zn oxide layer that forms after air exposure which dramatically enhances overall nanowire growth density. It was also found that Zn is a highly suitable catalyst for Si-Ge axial heterostructure NWs which had an atomically abrupt interface. The Zn-seeded Si NW electrode exhibited an initial discharge capacity of 1772 mAh/g retaining 85.5% of it’s initial capacity after 100 cycles. Notably, we demonstrate that the Zn seeds actively participate in the cycling process causing them to alloy with the Si to form a Zn-Si mesh. Chapter 4 describes a solution-based synthesis method whereby ZnO is reduced in-situ to form a Zn catalyst which can facilitate the growth of Si NWs. The approach was shown to allow for Si NW growth using ZnO both in bulk and powder form. Following NW synthesis, the NWs were washed in acidified IPA to remove any residue ZnO as well as the Zn seeds. The use of ZnO powder allowed for the synthesis of up to 200 mg of NWs per reaction with a chemical efficiency of 25%. Chapter 5 seeks to apply the nanowire growth mechanism described in chapter 4 and use this to produce high mass loading Si NW anodes. This was achieved by electrodepositing ZnO platelets onto a stainless-steel current collector which are placed in the solution phase of a refluxing solvent and reduced to generate the Zn catalyst to facilitate Si NW growth. This allowed for production of mass loadings of up to 1 mg/cm2 on planar substrates. To mitigate the effects of delamination during cycling, the stainless-steel current collector was modified using an etching procedure capable of creating either a micro and nano textured surface both of which were tested using the 1 mg/cm2 mass loading electrodes and show improvements in cycling performance. In chapter 6 the ability to grow Si NWs via both the VLS and VSS growth mechanism by using reaction temperatures above or below its melting point of Zn is reported. A larger mean NW diameter is observed when synthesizing at sub-eutectic temperatures via the VSS mechanism compared to NW synthesized via the VLS mechanism. This was used to vary the diameter along the axial length of individual NWs by transitioning between VLS and VSS growth temperatures in single reactions. </p

Uncovering electrochemistries of rechargeable magnesium-ion batteries at low and high temperatures

Rechargeable magnesium ion batteries, which possess the advantages of low cost, high safety, high volumetric capacity, and dendrite free cycling, have emerged as one of the potential contenders alleviate the burden on ex isting lithium ion battery technologies. Within this context, the electrochemical performance of Mg-ion batteries at high and ultra-low temperatures have attracted research attention due to their suitability for use in extreme environments (i.e. military and space station purposes). To meet the requirements for operation over wide tem perature ranges, extensive studies are being conducted to explore different cathodes, anodes, electrolytes, and interfacial phenomena. There is no review that compares the characteristics of magnesium ion batteries in terms of their working mechanism, current challenges, working voltages, possible cathode materials, and resultant elec trochemistry at different temperatures. To fulfil this research gap, we summarize the recent advances made in the development of magnesium ion batteries, including high-capacity cathodes, nucleophilic and non-nucleophilic electrolytes, hybrid ion tactics, working mechanisms, their high temperature and ultra-low temperature electrochemical performances. Future recommendations for the development of magnesium ion batteries with high energy densities capable of operating under extreme environmental conditions are also presented

A nanowire nest structure comprising copper silicide and silicon nanowires for lithium-ion battery anodes with high areal loading

High loading (>1.6 mg cm−2) of Si nanowires (NWs) is achieved by seeding the growth from a dense array of Cu15Si4 NWs using tin seeds. A one-pot synthetic approach involves the direct growth of CuSi NWs on Cu foil that acts as a textured surface for Sn adhesion and Si NW nucleation. The high achievable Si NW loading is enabled by the high surface area of CuSi NWs and bolstered by secondary growth of Si NWs as branches from both Si and CuSi NW stems, forming a dense Si active layer, interconnected with an electrically conducting CuSi array (denoted Si/CuSi). When employed as Li-ion battery anodes, the Si/CuSi nest structure demonstrates impressive rate performance, reaching 4.1 mAh cm−2 at C/20, 3.1 mAh cm−2 at C/5, and 0.8 mAh cm−2 at 6C. Also, Si/CuSi shows remarkable long-term stability, delivering a stable areal capacity of 2.2 mAh cm−2 after 300 cycles. Overall, complete anode fabrication is achieved within a single reaction by employing an inexpensive Sn powder approac

Alloying germanium nanowire anodes dramatically outperform graphite anodes in full-cell chemistries over a wide temperature range

The electrochemical performance of Ge, an alloying anode in the form of directly grown nanowires (NWs), in Li-ion full cells (vs LiCoO2) was analyzed over a wide temperature range (−40 to 40 °C). LiCoO2||Ge cells in a standard electrolyte exhibited specific capacities 30× and 50× those of LiCoO2||C cells at −20 and −40 °C, respectively. We further show that propylene carbonate addition further improved the low-temperature perform ance of LiCoO2||Ge cells, achieving a specific capacity of 1091 mA h g−1 after 400 cycles when charged/discharged at −20 °C. At 40 °C, an additive mixture of ethyl methyl carbonate and lithium bis(oxalato)borate stabilized the capacity fade from 0.22 to 0.07% cycle−1 . Similar electrolyte additives in LiCoO2||C cells did not allow for any gains in performance. Interestingly, the capacity retention of LiCoO2||Ge improved at low temperatures due to delayed amorphization of crystalline NWs, suppressing complete lithiation and high-order Li15Ge4 phase formation. The results show that alloying anodes in suitably configured electrolytes can deliver high performance at the extremes of temperature ranges where electric vehicles operate, conditions that are currently not viable for commercial batteries without energy-inefficient temperature regulation

Temperature induced diameter variation of silicon nanowires via a liquid–solid phase transition in the Zn seed

Herein, we demonstrate the ability of Zn to catalyze the growth of Si nanowires via reaction temperature determined, vapour–liquid–solid (VLS) or vapour–solid–solid (VSS) growth mechanisms. This is the first reported use of a type B catalyst to grow Si nanowires via the VSS mechanism to our knowledge whereby the highly faceted Zn seeds resulted in an increased NW diameter. This was used to induce diameter variations along the axial length of individual nanowires by transitioning between VLS and VSS growth

Amorphization driven Na-Alloying in SixGe1-x alloy nanowires for Na-ion batteries

Here we report the use of 1D SixGe1−x (x = 0.25, 0.5, 0.75) alloy nanowires (NWs) as anode materials for Na-ion batteries (NIBs). The strategy involves the synthesis of crystalline SixGe1−x NWs via the solution–liquid–solid (SLS) mechanism, followed by amorphization to activate the material for Na-ion cycling within a NIB. This study demonstrates the successful activation of SixGe1−x amorphous NW alloys, with a-Si0.5Ge0.5 delivering 250 mA h g−1 as compared to a-Ge NWs delivering only 107 mA h g−1 after 100 cycles. Also, amorphization proved to be a critical step, since crystalline NWs failed to activate in NIBs. However, Si NWs performed poorly during Na-ion cycling even after amorphization, and this behavior was explained by poor comparative Na-ion diffusivity. Further investigations on the impact of the relative content of Ge within the amorphized SixGe1−x NWs, Na-ion diffusivity and electrode degradation during cycling were also performed. Notably, the incorporation of Ge in the a-SixGe1−x alloy boosted Na ion diffusivity in the amorphized alloy, resulting in improved cycling performance and rate capability as compared to parent a-Si and a-Ge NWs.</p

Silicon nanowire growth on carbon cloth for flexible Li-ion battery anodes

Binder and conductive additive-free Si nanowires (NWs) grown directly on the current collector have shown great potential as next generation Li-ion battery anodes. However, low active material mass loadings and consequentially low areal capacities have remained a challenge in their development. Herein, we report the high-density growth of Si NWs on carbon cloth (CC) for use as Li-ion battery anodes. The NW growth reactions were carried out using a modified, glassware-based solvent vapor growth (SVG) process. Optimized growth conditions were applied to CC substrates to generate flexible Si NW anodes for Li-ion batteries. Battery testing revealed high areal charge and discharge capacities (>2 mAh/cm2) compared to Si NWs grown on stainless steel (SS) substrates (~0.3 mAh/cm2) and stable long-term cycling with 80% capacity retention after 200 cycles. The findings reported herein represent a significant advancement in the field in terms of achievable areal capacity enabled by a low-cost glassware-based system.</p

Lithiophilic nanowire guided Li deposition in Li metal batteries

Lithium (Li) metal batteries (LMBs) provide superior energy densities far  beyond current Li-ion batteries (LIBs) but practical applications are hindered  by uncontrolled dendrite formation and the build-up of dead Li in “hostless”  Li metal anodes. To circumvent these issues, we created a 3D framework of  a carbon paper (CP) substrate decorated with lithiophilic nanowires (silicon  (Si), germanium (Ge), and SiGe alloy NWs) that provides a robust host for  efficient stripping/plating of Li metal. The lithiophilic Li22Si5, Li22(Si0.5Ge0.5)5, and Li22Ge5 formed during rapid Li melt infiltration prevented the forma?tion of dead Li and dendrites. Li22Ge5/Li covered CP hosts delivered the best  performance, with the lowest overpotentials of 40 mV (three times lower  than pristine Li) when cycled at 1 mA cm−2 /1 mAh cm−2  for 1000 h and at  3 mA cm−2 /3 mAh cm−2  for 500 h. Ex situ analysis confirmed the ability of  the lithiophilic Li22Ge5 decorated samples to facilitate uniform Li deposi?tion. When paired with sulfur, LiFePO4, and NMC811 cathodes, the CP-LiGe/ Li anodes delivered 200 cycles with 82%, 93%, and 90% capacity retention,  respectively. The discovery of the highly stable, lithiophilic NW decorated CP  hosts is a promising route toward stable cycling LMBs and provides a new  design motif for hosted Li metal anodes. </p

Direct growth of Si, Ge, and Si–Ge heterostructure nanowires using electroplated Zn: an inexpensive seeding technique for Li-Ion alloying anodes

A scalable and cost-effective process is used to electroplate metallic Zn seeds on stainless steel substrates. Si and Ge nanowires (NWs) are subsequently grown by placing the electroplated substrates in the solution phase of a refluxing organic solvent at temperatures >430 °C and injecting the respective liquid precursors. The native oxide layer formed on reactive metals such as Zn can obstruct NW growth and is removed in situ by injecting the reducing agent LiBH4. The findings show that the use of Zn as a catalyst produces defect-rich Si NWs that can be extended to the synthesis of Si–Ge axial heterostructure NWs with an atomically abrupt Si–Ge interface. As an anode material, the as grown Zn seeded Si NWs yield an initial discharge capacity of 1772 mAh g−1 and a high capacity retention of 85% after 100 cycles with the active participation of both Si and Zn during cycling. Notably, the Zn seeds actively participate in the Li-cycling activities by incorporating into the Si NWs body via a Li-assisted welding process, resulting in restructuring the NWs into a highly porous network structure that maintains a stable cycling performance