New discovery makes fast-charging, better performing lithium-ion batteries possible

New Electrode 23c4a3_036cc463e8e9458d9a2070b7b7bb8c5c_mv2

April –  2019 – Rensselaer Polytechnic Institute – Material Science

Creating a lithium-ion battery that can charge in a matter of minutes but still operate at a high capacity is possible, according to research from Rensselaer Polytechnic Institute just published in Nature Communications. This development has the potential to improve battery performance for consumer electronics, solar grid storage, and electric vehicles.

A lithium-ion battery charges and discharges as lithium ions move between two electrodes, called an anode and a cathode. In a traditional lithium-ion battery, the anode is made of graphite, while the cathode is composed of lithium cobalt oxide.

These materials perform well together, which is why lithium-ion batteries have become increasingly popular, but researchers at Rensselaer believe the function can be enhanced further.

“The way to make batteries better is to improve the materials used for the electrodes,” said Nikhil Koratkar, professor of mechanical, aerospace, and nuclear engineering at Rensselaer, and corresponding author of the paper. “What we are trying to do is make lithium-ion technology even better in performance.”

Vanadium Sulfide download

Vanadium disulfide – a promising new monolayer material for Li-ion batteries

Koratkar’s extensive research into nanotechnology and energy storage has placed him among the most highly cited researchers in the world. In this most recent work, Koratkar and his team improved performance by substituting cobalt oxide with vanadium disulfide (VS2).

“It gives you higher energy density, because it’s light. And it gives you faster charging capability, because it’s highly conductive. From those points of view, we were attracted to this material,” said Koratkar, who is also a professor in the Department of Materials Science and Engineering.

Excitement surrounding the potential of VS2 has been growing in recent years, but until now, Koratkar said, researchers had been challenged by its instability–a characteristic that would lead to short battery life. The Rensselaer researchers not only established why that instability was happening, but also developed a way to combat it.

The team, which also included Vincent Meunier, head of the Department of Physics, Applied Physics, and Astronomy, and others, determined that lithium insertion caused an asymmetry in the spacing between vanadium atoms, known as Peierls distortion, which was responsible for the breakup of the VS2 flakes. They discovered that covering the flakes with a nanolayered coating of titanium disulfide (TiS2)–a material that does not Peierls distort–would stabilize the VS2 flakes and improve their performance within the battery.

“This was new. People hadn’t realized this was the underlying cause,” Koratkar said. “The TiS2 coating acts as a buffer layer. It holds the VS2 material together, providing mechanical support.”

Once that problem was solved, the team found that the VS2-TiS2 electrodes could operate at a high specific capacity, or store a lot of charge per unit mass. Koratkar said that vanadium and sulfur’s small size and weight allow them to deliver a high capacity and energy density. Their small size would also contribute to a compact battery.

When charging was done more quickly, Koratkar said, the capacity didn’t dip as significantly as it often does with other electrodes. The electrodes were able to maintain a reasonable capacity because, unlike cobalt oxide, the VS2-TiS2 material is electrically conductive.

Koratkar sees multiple applications for this discovery in improving car batteries, power for portable electronics, and solar energy storage where high capacity is important, but increased charging speed would also be attractive.

Rensselaer college-photo_3861

Rensselaer Polytechnic Institute


Vanadium disulfide flakes with nanolayered titanium disulfide coating as cathode materials in lithium-ion batteries Lu Li, Zhaodong Li, Anthony Yoshimura, Congli Sun, Tianmeng Wang, Yanwen Chen, Zhizhong Chen, Aaron Littlejohn, Yu Xiang, Prateek Hundekar, Stephen F. Bartolucci, Jian Shi, Su-Fei Shi, Vincent Meunier, Gwo-Ching Wang & Nikhil Koratkar Nature Communications volume 10, Article number: 1764 (2019)

Rensselaer Polytechnic Institute

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Stacked nanoparticle layers shine new light on optical thin films

Posted: November 5, 2012

Stacked nanoparticle layers shine new light on optical thin films(Nanowerk Spotlight) The refractive index is the property of a material that changes the speed of light and describes how light propagates through the material. The refractive index is an important property of solar cells – the higher it is, the more incident light gets reflected and is not converted to a photocurrent.

Air for instance, has a low refractive index very close to 1.0; but silicon, still the most common material used in today’s commercial solar cells, has a high refractive index which causes more than 30% of incident light to be reflected back from the surface of the silicon crystals.Solar cell manufacturers have therefore developed various kinds of anti reflection coatings to reduce the unwanted reflective losses (read more in our Nanowerk Spotlight“Moth eyes inspire self-cleaning antireflection nanotechnology coatings”). The purpose of these optical thin-films is to minimize the differences in the refractive indices between the ambient medium and the solar cells (or other opto-electronic devices).”For both solar cells and LEDs, coating with nano-particles can enhance the performance without harming the electrical properties of the devices, as can occur with etching or lithographic processing,” Hsuen-Li Chen, a professor in the Department of Materials Science and Engineering at National Taiwan University, tells Nanowerk.

nanoparticle multilayer stacksSchematic representation of: a) graded-refractive-index nanoparticle multilayer stacks, and b) scattering particles on graded-refractive-index nanoparticle stacks. (Reprinted with permission from Wiley-VCH Verlag)

In new work, reported in the October 16, 2012 online edition of Advanced Functional Materials (“Nanoparticle Stacks with Graded Refractive Indices Enhance the Omnidirectional Light Harvesting of Solar Cells and the Light Extraction of Light-Emitting Diodes”), Chen and his team have not only demonstrated this advantageous feature but also provided a strategy for optimizing the types and sizes of nanoparticles for use in both solar cells and LED’s. “Previous research did not mainly focus on the refractive indices of nanoparticles” says Chen. “Therefore, we wanted to know how these nanoparticles behave if they were spin-coated onto substrates. We assumed that nanoparticle stacks can be seen as optical thin films with refractive indices because of their little roughness and we successfully used both simulation and experimental measurement to prove our hypothesis.”The team’s main motivation has been to develop an easy and inexpensive method to construct optical thin films.

Traditionally, multi-layer optical thin-films with graded refractive indices were fabricated by PVD (physical vapor deposition) or CVD (chemical vapor deposition). However, using vacuum systems is both time consuming and expensive. In order to save money and processing time, Chen’s team therefore decided to spin-coat dielectric nanoparticle stacks with suitable refractive index to fabricate graded refractive indices multi-layers.”Our assumption was that, if the sizes of the nanoparticles are far less than the wavelength,they can be treated as optical thin films with effective refractive indices after they have been spin-coated onto the substrate,” Chen explains.

“Our rapid, low-cost, solution-based method allows the construction of graded-refractive-index nanoparticle stacks that function as broadband, omnidirectional antireflection coatings. This technique can minimize the reflectance of the silicon-air interface and increase the efficiency of silicon solar cells.””Moreover” he says, “if the sizes of the particles were to be close to the wavelength of incident light, these particles will behave as scattering centers, changing the direction of incident light and roughening the surface.

Different optoelectronic devices require different surface morphologies; some need a flat surface to avoid scattering and retain their electrical properties, while others need a moderately rough surface to enhance the light extraction or increase the optical path through a scattering effect.”Therefore, in their experiments, the researchers prepared both types of system readily through the selection of the types and size of the nanoparticles and their subsequent spincoating onto the substrates.Chen points out that the thickness of each spin-coated layer is critical and can be controlled by the rotational speed. “However, it took time to optimize them because we constructed multi-layers. The interference between individual layers as well as the solvent we chose would influence the thickness and roughness significantly. Fortunately, we were able to overcome these problems.”Because spin-coating is a rapid and cheap process, the main issue in this work was to choose nanoparticles with suitable refractive indices.

As long the refractive indices of the materials of optoelectronic devices are known, it is not difficult to coat nanoparticle stacks onto them to reduce the interfacial reflectance.The result of the team’s study is a novel strategy for arranging dielectric nanoparticles of various types and sizes to enhance both the omnidirectional light harvesting of solar cells and the light extraction of LEDs; and because the fabrication approach involves rapid and simple spin-coating and does not require any etching, the electrical properties of the devices will remain unimpaired.

“The underlying strategy of our process is to match the optical constants while considering the effects of scattering under different wavelengths,” Chen sums up the study. “We are convinced that our nanoparticle-based technique has great potential for application to other optoelectronic devices, including thin-film solar cells, organic solar cells, transparent conductors, and OLEDs.

By Michael Berger. Copyright © Nanowerk

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