Nanostructured Non-carbonaceous Materials for Improvement of Sulfur Cathode in Lithium-Sulfur Battery

Lithium-sulfur battery based on sulfur cathode has the advantages of high specific capacity, high energy density, environmental benignity and natural abundance of sulfur. These advantages over conventional lithium-ion batteries have driven researchers to make a lot of efforts to understand the redox mechanisms and improve the cathode performance. In order to fully realize the potential of lithium-sulfur battery and to approach commercialization, there are still many problems to overcome. Among them are i) low conductivity of sulfur and the discharge product, ii) lithium polysulfide intermediates dissolution and shuttle phenomenon, iii) volumetric expansion upon discharge and iv) lithium metal dendrite formation on anode side. In this thesis, the work is mostly focused on the cathode materials in order to address the first two problems. In the first part of the thesis, reduced titanium oxide is used as both a highly conductive support and a polysulfide adsorbent to prevent the loss of active materials. Mesoporous Magnéli phase with high surface area is synthesized through a sol-gel method. The reduction to suboxide is realized by carbothermal reaction at high temperature and the porous architecture is attributed to the cross-linking of polymer-mediated gel precursor that undergoes decomposition. The strong interaction between oxide and Li2S4 is confirmed by X-ray photoelectron spectroscopy analysis, based on comparison with carbon materials. This is also visually observed from a polysulfide adsorption study where the oxide and carbons were in contact with Li2S4 solutions. With 60 wt% of sulfur introduced onto synthesized Magnéli phase material by melt-diffusion, the cathode experienced very low capacity fading rate of 0.072% per cycle over 500 cycles at a discharge rate of 2C (full discharge in half an hour). The electrode morphology evolution upon cycling reveled by scanning electron microscopy imaging demonstrated more uniform deposition of discharge products for conductive oxide than carbon, which is due to the in-situ adsorption of lithium polysulfides during discharge. In the second part, another class of metallic non-carbonaceous materials - metal boride - was explored as a sulfur host. This polar material is expected to adsorb hydrophilic polysulfide as well. Both bulk and nanosized borides were synthesized through either simple thermal decomposition of metal borohydride and solid state reaction between elements, respectively. In all samples prepared, crystalline metal boride was confirmed to be the dominant phase, with small amount of oxide forming on the surface. Especially, with the addition of carbon nanotubes for solid state reaction, the particle size of as-synthesized boride/carbon composite was effectively reduced to ~100 nm, which is vital for sulfur impregnation. With 60 wt% of sulfur impregnated, the boride/S nanocomposite exhibited a high initial capacity of 985 mA h/g with only 0.1 % of capacity fading per cycle over 200 cycles at C/2.1 yea

Advanced Electrodes and Electrolytes For Long-Lived and High-Energy-Density Lithium-Sulfur Batteries

The increasing demand on renewable but intermittent energy and the need for electrified transportation place great emphasis on energy storage. Lithium-sulfur (Li-S) batteries are promising systems due to the high theoretical energy density and natural abundance of sulfur. This thesis presents a thorough investigation on strategies to confine the polysulfides and to build long-lived and high-energy-density Li-S batteries. A series of sulfur host materials and a class of sparingly solvating electrolyte are presented. Chapter 3 presents an approach to confine polysulfides within a cobalt sulfide material, which exhibits both metallic conductivity and high polysulfide adsorptivity. First-principles calculations and X-ray photoelectron spectroscopy studies consistently demonstrate the coupled interaction between the ionic components of cobalt sulfide and lithium polysulfides. The interconnected nanosheets form 3D networks and enable high sulfur loading electrodes with stable cycling. Chapter 4 presents a novel dual-doping strategy on porous carbon for effective binding of polysulfides. The N and S heteroatoms respectively bond with the Li cations and S anions in the polysulfides. The synthesis is based on liquid-crystal driven self-assembly of bio-sustainable cellulose nanocrystals. Chapter 5 reports a light-weight graphitic carbon nitride material that incorporates high concentration of active N-doping sites for polysulfide binding. Excellent long-term cycling performance of the sulfur electrode is achieved with only 0.04% capacity fading per cycle over 1500 cycles. Chapter 6 further reports a comprehensive strategy on coupling a hybrid sulfur host with an in-situ cross-linked binder in order to construct high loading electrodes while using a low electrolyte volume. Alternative stacking of graphitic carbon nitride and graphene offers both Li-N based adsorption for polysulfide and high electronic conductivity. Benefiting from the high elasticity of the cross-linked binder, crack-free high loading electrodes are fabricated at an electrolyte/sulfur ratio of 3.5:1 (µl:mg). Chapter 7 presents a comprehensive study on the ACN2-LiTFSI-TTE electrolyte with sparing solubility for polysulfides at elevated temperature. A quasi-solid state reaction is demonstrated by the distinct Li-S voltage profiles and sulfur/lithiu sulfide phase evolution as probed by operando XRD measurements. This discovery will inspire further studies into modifying the local structure of electrolytes to control the reaction pathways of dissolution-precipitation electrochemistry

Efforts at Enhancing Bifunctional Electrocatalysis and Related Events for Rechargeable Zinc-Air Batteries

Invited for this month's cover picture is the group of Prof. Dr. Kenneth I. Ozoemena at the University of the Witwatersrand and collaborators from Qatar University and Peking University. The Front Cover illustrates the significance of bifunctional electrocatalysis (ORR / OER) and zinc anode as the key drivers for the development of rechargeable zinc-air batteries that promise to revolutionize electricity storage and applications (represented herein as electric vehicle charging point). Read the full text of the Review at 10.1002/celc.202100574

Enhanced room-temperature Na+ ionic conductivity in Na4.92_{4.92}Y0.92_{0.92}Zr0.08_{0.08}Si4_{4}O12_{12}

Developing cost-effective and reliable solid-state sodium batteries with superior performance is crucial for stationary energy storage. A key component in facilitating their application is a solid-state electrolyte with high conductivity and stability. Herein, we employed aliovalent cation substitution to enhance ionic conductivity while preserving the crystal structure. Optimized substitution of Y3+ with Zr4+ in Na5YSi4O12 introduced Na+ ​ion vacancies, resulting in high bulk and total conductivities of up to 6.5 and 3.3 ​mS ​cm−1, respectively, at room temperature with the composition Na4.92Y0.92Zr0.08Si4O12 (NYZS). NYZS shows exceptional electrochemical stability (up to 10 ​V vs. Na+/Na), favorable interfacial compatibility with Na, and an excellent critical current density of 2.4 ​mA ​cm−2. The enhanced conductivity of Na+ ​ions in NYZS was elucidated using solid-state nuclear magnetic resonance techniques and theoretical simulations, revealing two migration routes facilitated by the synergistic effect of increased Na+ ​ion vacancies and improved chemical environment due to Zr4+ substitution. NYZS extends the list of suitable solid-state electrolytes and enables the facile synthesis of stable, low-cost Na+ ion silicate electrolytes

Surface passivation for highly active, selective, stable, and scalable CO2 electroreduction

Electrochemical conversion of CO2 to formic acid using Bismuth catalysts is one the most promising pathways for industrialization. However, it is still difficult to achieve high formic acid production at wide voltage intervals and industrial current densities because the Bi catalysts are often poisoned by oxygenated species. Herein, we report a Bi3S2 nanowire-ascorbic acid hybrid catalyst that simultaneously improves formic acid selectivity, activity, and stability at high applied voltages. Specifically, a more than 95% faraday efficiency was achieved for the formate formation over a wide potential range above 1.0 V and at ampere-level current densities. The observed excellent catalytic performance was attributable to a unique reconstruction mechanism to form more defective sites while the ascorbic acid layer further stabilized the defective sites by trapping the poisoning hydroxyl groups. When used in an all-solid-state reactor system, the newly developed catalyst achieved efficient production of pure formic acid over 120 hours at 50 mA cm–2 (200 mA cell current)

Integrated energy storage and CO2 conversion using an aqueous battery with tamed asymmetric reactions

Abstract Developing a CO2-utilization and energy-storage integrated system possesses great advantages for carbon- and energy-intensive industries. Efforts have been made to developing the Zn-CO2 batteries, but access to long cycling life and low charging voltage remains a grand challenge. Here we unambiguously show such inefficiencies originate from the high-barrier oxygen evolution reaction on charge, and by recharging the battery via oxidation of reducing molecules, Faradaic efficiency-enhanced CO2 reduction and low-overpotential battery regeneration can be simultaneously achieved. Showcased by using hydrazine oxidation, our battery demonstrates a long life over 1000 hours with a charging voltage as low as 1.2 V. The low charging voltage and formation of gaseous product upon hydrazine oxidation are the key to stabilize the catalyst over cycling. Our findings suggest that by fundamentally taming the asymmetric reactions, aqueous batteries are viable tools to achieve integrated energy storage and CO2 conversion that is economical, highly energy efficient, and scalable

Generation of Color Images by Utilizing a Single Composite Diffractive Optical Element

This paper presents an approach that is capable of producing a color image using a single composite diffractive optical element (CDOE). In this approach, the imaging function of a DOE and the spectral deflection characteristics of a grating were combined together to obtain a color image at a certain position. The DOE was designed specially to image the red, green, and blue lights at the same distance along an optical axis, and the grating was designed to overlay the images to an off-axis position. We report the details of the design process of the DOE and the grating, and the relationship between the various parameters of the CDOE. Following the design and numerical simulations, a CDOE was fabricated, and imaging experiments were carried out. Both the numerical simulations and the experimental verifications demonstrated a successful operation of this new approach. As a platform based on coaxial illumination and off-axis imaging, this system is featured with simple structures and no cross-talk of the light fields, which has huge potentials in applications such as holographic imaging

Efforts at Enhancing Bifunctional Electrocatalysis and Related Events for Rechargeable Zinc-Air Batteries

Rechargeable zinc-air batteries (RZABs) are one of the most promising next-generation energy-storage technologies for stationary applications (home and industry), wearable and portable electronics, and transportation (including electric vehicles) due to their high energy density, environmental friendliness, safety, and low cost. However, RZABs still face serious challenges (such as sluggish oxygen reactions, poor durability, inferior reversibility of the zinc anode, and low cell energy efficiency) that conspire against their widespread commercialization. The reactions that occur at the three key components of the RZAB (air cathode, zinc anode, and electrolyte) co-operatively conspire against its performance. Thus, this review focuses on the bifunctional electrocatalytic events at the cathode (i. e., oxygen reduction reaction (ORR) and oxygen evolution reaction (OER)). That is in addition to the recent developments aimed at mitigating the performance-limiting events at the anode and the electrolytes. This review directs the attention of researchers and users to the critical areas for the development of the next-generation RZABs

Efforts at Enhancing Bifunctional Electrocatalysis and Related Events for Rechargeable Zinc‐Air Batteries

Rechargeable zinc-air batteries (RZABs) are one of the most promising next-generation energy-storage technologies for stationary applications (home and industry), wearable and portable electronics, and transportation (including electric vehicles) due to their high energy density, environmental friendliness, safety, and low cost. However, RZABs still face serious challenges (such as sluggish oxygen reactions, poor durability, inferior reversibility of the zinc anode, and low cell energy efficiency) that conspire against their widespread commercialization. The reactions that occur at the three key components of the RZAB (air cathode, zinc anode, and electrolyte) co-operatively conspire against its performance. Thus, this review focuses on the bifunctional electrocatalytic events at the cathode (i. e., oxygen reduction reaction (ORR) and oxygen evolution reaction (OER)). That is in addition to the recent developments aimed at mitigating the performance-limiting events at the anode and the electrolytes. This review directs the attention of researchers and users to the critical areas for the development of the next-generation RZABs

Enhanced room-temperature Na+ ionic conductivity in Na4.92Y0.92Zr0.08Si4O12

Developing cost-effective and reliable solid-state sodium batteries with superior performance is crucial for stationary energy storage. A key component in facilitating their application is a solid-state electrolyte with high conductivity and stability. Herein, we employed aliovalent cation substitution to enhance ionic conductivity while preserving the crystal structure. Optimized substitution of Y3+ with Zr4+ in Na5YSi4O12 introduced Na+ ​ion vacancies, resulting in high bulk and total conductivities of up to 6.5 and 3.3 ​mS ​cm−1, respectively, at room temperature with the composition Na4.92Y0.92Zr0.08Si4O12 (NYZS). NYZS shows exceptional electrochemical stability (up to 10 ​V vs. Na+/Na), favorable interfacial compatibility with Na, and an excellent critical current density of 2.4 ​mA ​cm−2. The enhanced conductivity of Na+ ​ions in NYZS was elucidated using solid-state nuclear magnetic resonance techniques and theoretical simulations, revealing two migration routes facilitated by the synergistic effect of increased Na+ ​ion vacancies and improved chemical environment due to Zr4+ substitution. NYZS extends the list of suitable solid-state electrolytes and enables the facile synthesis of stable, low-cost Na+ ion silicate electrolytes