Morphology and Phase Changes in Iron Anodes Affecting their Capacity and Stability in Rechargeable Alkaline Batteries

Rechargeable alkaline batteries may become attractive nonflammable alternatives to lithium-ion (Li-ion) batteries for applications where achieving the highest energy density is less critical than safety, environmental friendliness, and low cost of energy storage. The broad abundance and low price of iron (Fe) make it attractive as a rechargeable anode material for aqueous batteries. Through cyclic voltammetry and post-mortem analysis, we revealed four distinct stages of Fe anode evolution: development, retention, fading, and failure, where each stage is associated with very specific changes in the morphology and phase of Fe anodes. We observed the Fe particle fragmentation resulted in the capacity increase during the initial cycles of charge–discharge. Most importantly, we discovered the irreversible formation of maghemite (γ-Fe₂O₃) with low reactivity is responsible for the eventual Fe anode capacity fading. This unexpected discovery changes the paradigm on possible routes to stabilize Fe anodes and contributes to future development of low-cost alkaline cells

Porous TiO_2–<i>x</i>/C Nanofibers with Axially Aligned Tunnel Pores as Effective Sulfur Hosts for Stabilized Lithium–Sulfur Batteries

Hierarchically porous TiO2–x/C nanofibers (NFs) with axially aligned cylindrical tunnel pore channels were synthesized as a sulfur (S) host for lithium–sulfur batteries (LSBs) by a microemulsion electrospinning method. We explored a synergistic chemical trapping reinforced by coordinatively unsaturated Ti3+ nuclei with oxygen deficiency (or more broadly via polar O–Ti–O units) in combination with physical trapping in both narrow pores (<5 nm) and larger ordered pore tunnels (20–100 nm) separated by thin walls to allow for a large volume of active material and rapid diffusion within the channels while effectively blocking out the diffusion of soluble lithium polysulfides. Due to this unique architecture and enhanced conductivity, the prepared materials enabled a high S loading (∼72 wt %) and significantly reduced the shuttle effect. Hence, the composite TiO2–x/C@S cathodes exhibited a high utilization of active materials, excellent rate performance, and promising cycling stability (retention of up to ∼1010 mAh g–1 after 150 cycles for the aerial capacity of 1.5 mAh cm–2, with very stable performance even for the high S loading of 2.5 mg cm–2). By designing control nanomaterials that lack either the pore tunnels or the desired chemical compositions, we elucidated the importance of the synergistic effect of both factors. This work demonstrates a successful exploration of oxide NFs with tunnel pores via a simple single-needle microemulsion electrospinning method, which should pave the way for similar nanomaterials engineering with other chemistries for improved LSB performance

Toward a Long-Chain Perfluoroalkyl Replacement: Water and Oil Repellency of Polyethylene Terephthalate (PET) Films Modified with Perfluoropolyether-Based Polyesters

Original perfluoropolyethers (PFPE)-based oligomeric polyesters (FOPs) of different macromolecular architecture were synthesized via polycondensation as low surface energy additives to engineering thermoplastics. The oligomers do not contain long-chain perfluoroalkyl segments, which are known to yield environmentally unsafe perfluoroalkyl carboxylic acids. To improve the compatibility of the materials with polyethylene terephthalate (PET) we introduced isophthalate segments into the polyesters and targeted the synthesis of lower molecular weight oligomeric macromolecules. The surface properties such as morphology, composition, and wettability of PET/FOP films fabricated from solution were investigated using atomic force microscopy, X-ray photoelectron spectroscopy, and contact angle measurements. It was demonstrated that FOPs, when added to PET film, readily migrate to the film surface and bring significant water and oil repellency to the thermoplastic boundary. We have established that the wettability of PET/FOP films depends on three main parameters: (i) end-groups of fluorinated polyesters, (ii) the concentration of fluorinated polyesters in the films, and (iii) equilibration via annealing. The most effective water/oil repellency FOP has two C₄F₉–PFPE-tails. The addition of this oligomeric polyester to PET allows (even at relatively low concentrations) reaching a level of oil repellency and surface energy comparable to that of polytetrafluorethylene (PTFE/Teflon). Therefore, the materials can be considered suitable replacements for additives containing long-chain perfluoroalkyl substances

Performance Enhancement and Side Reactions in Rechargeable Nickel–Iron Batteries with Nanostructured Electrodes

We report for the first time a solution-based synthesis of strongly coupled nanoFe/multiwalled carbon nanotube (MWCNT) and nanoNiO/MWCNT nanocomposite materials for use as anodes and cathodes in rechargeable alkaline Ni–Fe batteries. The produced aqueous batteries demonstrate very high discharge capacities (800 mAh g_Fe^–1 at 200 mA g^–1 current density), which exceed that of commercial Ni–Fe cells by nearly 1 order of magnitude at comparable current densities. These cells also showed the lack of any “activation”, typical in commercial batteries, where low initial capacity slowly increases during the initial 20–50 cycles. The use of a highly conductive MWCNT network allows for high-capacity utilization because of rapid and efficient electron transport to active metal nanoparticles in oxidized [such as Fe­(OH)₂ or Fe₃O₄] states. The flexible nature of MWCNTs accommodates significant volume changes taking place during phase transformation accompanying reduction–oxidation reactions in metal electrodes. At the same time, we report and discuss that high surface areas of active nanoparticles lead to multiple side reactions. Dissolution of Fe anodes leads to reprecipitation of significantly larger anode particles. Dissolution of Ni cathodes leads to precipitation of Ni metal on the anode, thus blocking transport of OH^– anions. The electrolyte molarity and composition have a significant impact on the capacity utilization and cycling stability

Ultra Strong Silicon-Coated Carbon Nanotube Nonwoven Fabric as a Multifunctional Lithium-Ion Battery Anode

Materials that can perform simultaneous functions allow for reductions in the total system mass and volume. Developing technologies to produce flexible batteries with good performance in combination with high specific strength is strongly desired for weight- and power-sensitive applications such as unmanned or aerospace vehicles, high-performance ground vehicles, robotics, and smart textiles. State of the art battery electrode fabrication techniques are not conducive to the development of multifunctional materials due to their inherently low strength and conductivities. Here, we present a scalable method utilizing carbon nanotube (CNT) nonwoven fabric-based technology to develop flexible, electrochemically stable (∼494 mAh·g^–1 for 150 cycles) battery anodes that can be produced on an industrial scale and demonstrate specific strength higher than that of titanium, copper, and even a structural steel. Similar methods can be utilized for the formation of various cathode and anode composites with tunable strength and energy and power densities

Enhancing the Stability of Sulfur Cathodes in Li–S Cells via in Situ Formation of a Solid Electrolyte Layer

Enhancing the performance of rechargeable lithium (Li)–sulfur (S) batteries is one of most popular topics in a battery field because of their low cost and high specific energy. However, S experiences dissolution during its electrochemical reactions; hence, maintaining its initial capacity is challenging. Protecting the S cathode with a Li ion conducting layer that acts as a barrier for polysulfide transport is an attractive strategy, but formation of such protective layers typically involves significant effort and cost. Here, we report a facile route to form a conformal solid electrolyte layer on S cathodes in situ using a carbonate solvent. The chemically and mechanically stable and Li ion conducting protective layer is formed by inducing electrolyte reduction and polymerization reactions on the cathode surface. The layer serves as a polysulfide’s barrier, successfully helping to retain S active material in the carbon pores. In addition, it helps to improve the performance of Li anodes

Influence of Binders, Carbons, and Solvents on the Stability of Phosphorus Anodes for Li-ion Batteries

Phosphorus (P) is an abundant element that exhibits one of the highest gravimetric and volumetric capacities for Li storage, making it a potentially attractive anode material for high capacity Li-ion batteries. However, while phosphorus carbon composite anodes have been previously explored, the influence of the inactive materials on electrode cycle performance is still poorly understood. Here, we report and explain the significant impacts of polymer binder chemistry, carbon conductive additives, and an under-layer between the Al current collector and ball milled P electrodes on cell stability. We focused our study on the commonly used polyvinylidene fluoride (PVDF) and poly­(acrylic acid) (PAA) binders as well as exfoliated graphite (ExG) and carbon nanotube (CNT) additives. The mechanical properties of the binders were found to change drastically because of interactions with both the slurry and electrolyte solvents, significantly effecting the electrochemical cycle stability of the electrodes. Binder adhesion was also found to be critical in achieving stable electrochemical cycling. The best anodes demonstrated ∼1400 mAh/g-P gravimetric capacity after 200 cycles at C/2 rates in Li half cells