In the past decade, there have been exciting developments in the field of lithium ion batteries as energy storage devices, resulting in the application of lithium ion batteries in areas ranging from small portable electric devices to large power systems such as hybrid electric vehicles. However, the maximum energy density of current lithium ion batteries having topatactic chemistry is not sufficient to meet the demands of new markets in such areas as electric vehicles. Therefore, new electrochemical systems with higher energy densities are being sought, and metal-air batteries with conversion chemistry are considered a promising candidate. More recently, promising electrochemical performance has driven much research interest in Li-air and Zn-air batteries. This review provides an overview of the fundamentals and recent progress in the area of Li-air and Zn-air batteries, with the aim of providing a better understanding of the new electrochemical systems.

https://www.onlinelibrary.wiley.com/doi/epdf/10.1002/aenm.201000010

Metal–Air Batteries with High Energy Density: Li–Air versus Zn–Air

A theoretical model for porous structure formation during electrochemical anodization of aluminium is developed taking into account anodic oxide growth at the oxide/electrolyte and oxide/aluminium interfaces, electrochemical and field-enhanced oxide dissolution, and a three-dimensional configuration of electric field and current in the scalloped barrier oxide layer at pore bottoms. A system of equations ensuring a general description of the porous structure formation is evolved and is further applied to the analysis of stationary pore growth on aluminium. In this particular case analytical expressions are obtained establishing the relations of pore geometry and electric field in oxide with the parameters of anodic process (voltage, temperature, pH). They are found to be in agreement with existing experimental data.

https://iopscience.iop.org/article/10.1088/0022-3727/25/8/017/pdf

Theoretical modelling of porous oxide growth on aluminium

The revived interest and research on the development of novel energy storage systems with exceptional inherent safety, environmentally benign and low cost for integration in large scale electricity grid and electric vehicles is now driven by the global energy policies. Within various technical challenges yet to be resolved and despite extensive studies, the low cycle life of the zinc anode is still hindering the implementation of rechargeable zinc batteries at industrial scale. This review presents an extensive overview of electrolytes for rechargeable zinc batteries in relation to the anode issues which are closely affected by the electrolyte nature. Widely studied aqueous electrolytes, from alkaline to acidic pH, as well as non-aqueous systems including polymeric and room temperature ionic liquids are reported. References from early rechargeable Zn-air research to recent results on novel Zn hybrid systems have been analyzed. The ambition is to identify the challenges of the electrolyte system and to compile the proposed improvements and solutions. Ultimately, all the technologies based on zinc, including the more recently proposed novel zinc hybrid batteries combining the strong points of lithium-ion, redox-flow and metal-air systems, can benefit from this compilation in order to improve secondary zinc based batteries performance.

An overview of progress in electrolytes for secondary zinc-air batteries and other storage systems based on zinc

https://spiral.imperial.ac.uk/bitstream/10044/1/66181/2/new_dendrite_paper_final.pdf

Operando Visualization and Multi-scale Tomography Studies of Dendrite Formation and Dissolution in Zinc Batteries

Summary
New applications and emerging markets in electromobility and large-scale stationary energy storage require the development of new electrochemical systems with higher energy density than current batteries. Rechargeable metal–air batteries, mainly lithium–air and zinc–air systems, are considered one of the most promising candidates. In contrast to lithium, zinc is abundant, inexpensive and its electrodeposition in aqueous electrolytes is relatively easy. Unfortunately, achieving a rechargeable zinc–air battery is still hindered by various technical problems related to the reversibility and lifetime of the electrodes.

The most widely used electrolyte in zinc–air batteries has been the classical aqueous alkaline. In this context and with the main objective of providing a complete overview, we studied a wide number of articles starting from the beginning of the development of secondary zinc–air batteries (1970–1980s) to more recent works, with the aim of compiling all available information. It is essential to revise older papers to find relevant information that may get otherwise forgotten and not taken into account to develop new solutions. This information could also be applied in other storage systems based on zinc as nickel–zinc, zinc hybrid or zinc-ion. Copyright © 2016 John Wiley & Sons, Ltd.

Alkaline aqueous electrolytes for secondary zinc–air batteries: an overview

Two types of emulsion-grade polytetrafluoroethylene particles have been studied. We refer to these as ribbons and rods. The ribbons consist of very thin ribbons or lamellae folded upon themselves a number of times. In typical emulsion-grade material prepared at Allied Chemical, the unraveled ribbon measures about 3.25 μ in length, 0.25 μ in width, and 60A in thickness. The folded ribbons, which form the particles, are about 0.5 μ long and 0.25 μ wide. Electron diffraction shows that the ribbons are single crystals with the chain axis parallel to the long axis of the ribbons thus forming extended chain crystals. This extended-chain packing is consistent with the observed cleavage or fibrillation of the ribbons and with the molecular weight. The rods are formed in low–yield polymerizations. Electron diffraction also shows that the rods are single crystals with the chain axis parallel to the long axis of the rods. Striations parallel to the long axis are believed to result from stacking of parallel segments. Considerable bending of the long axis of rods is observed.

Studies of the morphology of emulsion‐grade polytetrafluoroethylene

Electrolytic zinc dendrites grown potentiostatically from a range of alkaline zincate solutions were studied using X-ray diffraction and scanning electron microscopy (SEM) to reveal structural and morphological characteristics as a function of their growth rate. Conclusions from this work were as follows: Dendrites grown at a rate of 3 μm min−1 were identified as twinned monocrystalline “swords” with a noticeable absence of regular branching. Dendrites grown at a rate of 24 μm min−1 were basically twinned monocrystals with some polycrystalline character. As for the slowest growing dendrites, the morphological character was of a sword-like nature. Morphologically these first two types of dendrites show great similarity to the butterfly dendrites of DeVries. Dendrites grown at 67 μm min−1 were of the fern-like morphological type with a large degree of polycrystalline character. These dendrites are suggested as being composites of very small slow growth rate dendrites in a roof tile arrangement. The results for the ideal fern-like dendrites seemingly disagree with the earlier hcp dendrite work of Wranglen who identified this type of dendrite as a monocrystal. An explanation for this discrepancy is offered. The work presented here appears to be consistent with the twin plane re-entrant edge (TPRE) growth mechanism.

Crystallographic and morphological studies of electrolytic zinc dendrites grown from alkaline zincate solutions

The crystal structure of the α polymorph of nylon 4 has been determined from the x-ray diffraction patterns of uniaxially oriented monofilaments. In general the crystal structure of α nylon 4 is similar to that of α nylon 6. The unit cell is monoclinic with the following dimensions: a = 9.29 ± 0.05 A., b = 12.24 ± 0.05 A., c = 7.97 ± 0.05 A., and β = 114.5 ± 1.0°. There are eight monomeric units in the unit cell. The theoretical density is 1.37 g./cc. and the observed density 1.25 g./cc. The space group is P21. The nylon 4 chains are of the extended planar zigzag type, with the plane of the zigzag approximately parallel to the a axis of the unit cell. Along the a axis, every other chain is inverted—an antiparallel arrangement of chains—thus permitting complete hydrogen bonding and the formation of sheets of nylon 4 chains. Along the c axis of the unit cell, the second sheet is displaced by3/10 of the b axis, thus leading to a staggered arrangement of sheets. The sheets are held in place by van der Waals forces.

Crystallographic studies of nylon 4. I. Determination of the crystal structure of the α polymorph of nylon 4

In addition to the α polymorph, two crystalline forms of nylon 4, the β and δ polymorphs, have been observed and studied.

The β polymorph of nylon 4, which has not been found in a pure condition, appears to be similar to the β polymorph of nylon 6. The extended planar zigzag conformation of chains, found in the α polymorph, is present also in the β polymorph. Three models have been proposed to explain the observed data. In model 1, the staggered shear of van der Waals bonded sheets found in the α polymorph is abandoned; in model 2, the sheets are displaced by 1/2 the b axis, and in model 3 the chains are arranged in a parallel array. The β polymorph is converted to the α form in air upon heating for 11 min. at 227°C. and upon immersion in water for 2 hr. Models 1 and 2 would be converted to the α polymorph by a slippage of the van der Waals bonded sheets while the conversion from model 3 would involve a rupture of hydrogen bonds, a rotation of the chains through 90°, and the reformation of hydrogen bonds. The δ polymorph is formed by rapidly quenching extruded nylon 4 against chilled rolls. It is a metastable form which has only been observed in an unoriented condition. Upon orientation it is completely converted to the α polymorph. In addition to this, its conversion to the α form occurs under conditions similar to the β–α transition. The observed diffraction pattern can be indexed on the basis of a hexagonal packing of chains. High-temperature x-ray diffraction studies of the α polymorph suggest that the δ form, or a structure similar to it, is the high-temperature form of nylon 4.

Crystallographic studies of nylon 4. II. On the β and δ polymorphs of nylon 4

Fully methylated nylon 12 was prepared by the hydrolytic polymerization of N-methyllaurolactam. The polymer, with about 20% crystallinity, has a crystal structure different from that of nylon 12. The X-ray diffraction patterns of copolymers of N-methyllaurolactam and laurolactam were shown to be composition-dependent. Copolymers containing up to 15% laurolactam units exhibited a crystal structure essentially the same as poly-N-methyllaurolactam. Copolymers containing from 20 to 50% laurolactam are transition structures, whereas copolymers containing more than 50% laurolactam units exhibit a crystal structure characteristic of polylaurolactam. DSC data on the homopolymers and copolymers are also reported.

Robert J. Fredericks

Papers

Studies of the morphology of emulsion‐grade polytetrafluoroethylene

Crystallographic and morphological studies of electrolytic zinc dendrites grown from alkaline zincate solutions

Crystallographic studies of nylon 4. I. Determination of the crystal structure of the α polymorph of nylon 4

Crystallographic studies of nylon 4. II. On the β and δ polymorphs of nylon 4

Crystallinity in poly‐N‐methyllaurolactam and copolymers