The lithium-ion batteries that you find in many of your electronic gadgets, like your smartphone, typically consist of two electrodes connected by a liquid electrolyte. Apart from being prone to problems, including leakage, low charge retention and difficulties in operating at high and low temperature, this liquid electrolyte makes it difficult to reduce the size and weight of the battery.
Finding low-cost solid materials capable of efficiently and safely replacing liquid electrolytes in these batteries has been a considerable research interest over the past years.
Of the various types of solid electrolytes that have been developed so far, composite polymer electrolytes exhibit acceptable Li-ion conductivity due to the interaction between nanofillers and polymer.
Composite polymer electrolyte is typically a mixture of ceramic nanofiller and polymer electrolyte. The nanoscale ceramic fillers are known to be able to enhance mechanical or thermal stability as well as ionic conductivity of polymer electrolyte.
“Unfortunately, conventional composites suffer from agglomeration of nanofillers at high weight ratio which deteriorates the distribution of fillers and results in discontinuous ionic conduction pathways,” Guihua Yu, a professor in Materials Science and Engineering, Mechanical Engineering, at the Texas Materials Institute, University of Texas at Austin. “In our recent study, we fabricated a three-dimensionally (3D) interconnected framework consisting of ceramic nanofillers via a facile templating method using nanostructured hydrogels. The 3D ceramic framework can prevent aggregation of nanofillers and provide a continuous conduction pathway resulting in excellent ionic conductivity and stability.”
The conventional method for making composite polymer electrolytes is to physically mix nanoparticles with polymer electrolyte. The problem here is that the filler ratio is limited to only 10∼20wt%. At higher weight ratios the nanoparticles tend to agglomerate, resulting in deterioration of percolation and poor ionic conductivity.
“To solve this issue, we fabricated a pre-percolated network of ceramic filler instead of distributing particles in polymer,” Yu notes. “In our study, a 3D interconnected ceramic framework provides continuous pathways for ion conduction. We believe that our novel method will help to develop composite materials in a different but much improved way than conventional particle distributions.”
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The new method of making mixed halide-perovskites results in solar cells with improved stability and performance. The new method results in better control over perovskite crystallization rates. This means the crystal structure is more ordered, in part due to researchers understanding and taking advantage of the faster crystallization of bromide relative to iodide.
The result is a material with fewer defects and less halide migration and thus less segregation of the bromide and iodide. This in turn means uniform mixing of bromide and iodide across the material, which allows the material to absorb light evenly. The end result is that solar cells made using the new method will perform better under real-world conditions.
Typical halide perovskite solution deposition uses an anti-solvent drip procedure to initiate crystallization of the halide film. The standard anti-solvent method for producing bromide-iodide mixed halide perovskite films often leads to excessive defect formation (e.g., bromide vacancies) owing to the rapid crystallization of bromide vs. iodide-perovskite phases. Simulations show that halide migration is enhanced in the presence of a large population of halide vacancies. This limits the stability of bromide-iodide mixed halide perovskites under light and heat.
In comparison to the anti-solvent approach, the gentler gas-quench method better controls crystallization, first producing a bromide-rich surface layer that then induces top-down columnar growth to form a gradient structure with less bromide in the bulk than in the surface region. The anti-solvent method does not produce such a gradient structure.
In this study, researchers from the National Renewable Energy Laboratory, the University of Toledo, and the University of Colorado Boulder demonstrated that the gas-quench method also produces fewer bromide vacancies and results in materials with a higher quality opto-electronic performance. Solar cells made using the gas-quench method retain desirable light absorption properties and provide enhanced performance in the form of a higher charge carrier mobility, higher open circuit voltage, and enhanced stability.
Researchers from the University of Rostock and Technion Haifa have created the first three-dimensional topological insulator for light. A judiciously placed screw dislocation allows optical signals to wind around the surface of a synthetic lattice while keeping it protected from scattering.
Their discovery has recently been published in the journal Nature (“Photonic topological insulator induced by a dislocation in three dimensions”).
