Power/Performance Bits: May 2

Turning bottles into batteries

Researchers at the University of California, Riverside used waste glass bottles and a low-cost chemical process to create nanosilicon anodes for high-performance lithium-ion batteries.

Billions of glass bottles end up in landfills every year, prompting the researchers to ask whether silicon dioxide in waste beverage bottles could provide high purity silicon nanoparticles for lithium-ion batteries.

Silicon anodes can store up to 10 times more energy than conventional graphite anodes, but expansion and shrinkage during charge and discharge make them unstable. Downsizing silicon to the nanoscale has been shown to reduce this problem, and by combining an abundant and relatively pure form of silicon dioxide and a low-cost chemical reaction, the researchers created lithium-ion half-cell batteries that store almost four times more energy than conventional graphite anodes.

To create the anodes, the team used a three-step process that involved crushing and grinding the glass bottles into a fine white power, a magnesiothermic reduction to transform the silicon dioxide into nanostructured silicon, and coating the silicon nanoparticles with carbon to improve their stability and energy storage properties.

Waste glass bottles are turned into nanosilicon anodes using a low cost chemical process. (Source: UC Riverside)

As expected, coin cell batteries made using the glass bottle-based silicon anodes greatly outperformed traditional batteries in laboratory tests. Carbon-coated glass derived-silicon (gSi@C) electrodes demonstrated excellent electrochemical performance with a capacity of ~1420 mAh/g at C/2 rate after 400 cycles.

Changling Li, a graduate student in materials science and engineering at Riverside, said one glass bottle provides enough nanosilicon for hundreds of coin cell batteries or three-five pouch cell batteries.

“We started with a waste product that was headed for the landfill and created batteries that stored more energy, charged faster, and were more stable than commercial coin cell batteries. Hence, we have very promising candidates for next-generation lithium-ion batteries,” Li said.

Super-efficient solar

Engineers at Kobe University in Japan developed a solar cell design with exceptional efficiency. The design works by absorbing the spectral components of longer wavelengths that are usually lost during transmission through the cell.

In theory, 30% energy-conversion efficiency is the upper limit for traditional single-junction solar cells, as most of the solar energy that strikes the cell passes through without being absorbed, or becomes heat energy instead. Experiments have been taking place around the world to create various solar cell designs that can lift these limitations on conversion efficiency and reduce the loss of energy. The current world record is at 46% percent for a 4-junction solar cell.

A solar cell structure using a hetero interface and the up-conversion of the two-proton system (yellow and red arrows). The light represented by the red and yellow arrows normally passes through when only a semiconductor is used, but with this system the light is absorbed, greatly increasingly the flow of electricity. (Source: Kobe University)

The research team used two small photons from the energy transmitted through a single-junction solar cell containing a hetero-interface formed from semiconductors with different bandgaps, aluminium gallium arsenide (Al0.3Ga0.7As) and gallium arsenide (GaAs). Using the photons, they developed a new solar cell structure for generating photocurrents.

As well as demonstrating theoretical results of up to 63% conversion efficiency, it experimentally achieved up-conversion based on two photons, a mechanism unique to this solar cell. The reduction in energy loss is over 100 times more effective compared to previous methods that used intermediate bands.

Icy batteries

Engineers at Columbia Engineering developed an ice-templating method to control the structure of a solid electrolyte for lithium batteries, which could lead to batteries that are safer, have longer battery life, and are bendable.

The team explored the idea of using a solid electrolyte as a substitute for the liquid electrolyte to make all-solid-state lithium batteries. They were interested in using ice-templating to fabricate vertically aligned structures of ceramic solid electrolytes, which provide fast lithium ion pathways and are highly conductive.

“In portable electronic devices, as well as electric vehicles, flexible all-solid-state lithium batteries not only solve the safety issues, but they may also increase battery energy density for transportation and storage. And they show great promise in creating bendable devices,” said Yuan Yang, assistant professor of materials science and engineering at Columbia Engineering.

They cooled the aqueous electrolyte solution with ceramic particles from the bottom and then let ice grow and push away and concentrate the ceramic particles. They then applied a vacuum to transition the solid ice to a gas, leaving a vertically aligned structure. Finally, they combined this ceramic structure with polymer to provide mechanical support and flexibility to the electrolyte.

Schematic of vertically aligned and connected ceramic channels for enhancing ionic conduction. In the left figure, ceramic particles are randomly dispersed in the polymer matrix, where ion transport is blocked by the polymer matrix with a low conductivity. In the right one, vertically aligned and connected structure facilitates ion transport, which can be realized by the ice-templating method. (Source: Yuan Yang/Columbia Engineering)

Researchers in earlier studies used either randomly dispersed ceramic particles in polymer electrolyte or fiber-like ceramic electrolytes that are not vertically aligned. “We thought that if we combined the vertically aligned structure of the ceramic electrolyte with the polymer electrolyte, we would be able to provide a fast highway for lithium ions and thus enhance the conductivity,” said Haowei Zhai, aPhD student at Columbia. “We believe this is the first time anyone has used the ice-templating method to make flexible solid electrolyte, which is nonflammable and nontoxic, in lithium batteries. This opens a new approach to optimize ion conduction for next-generation rechargeable batteries.”

In addition, the researchers say, this technique could in principle improve the energy density of batteries: By using the solid electrolyte, the lithium battery’s negative electrode, currently a graphite layer, could be replaced by lithium metal, and this could improve the battery’s specific energy by 60% to 70%.

The team plans next to work on optimizing the qualities of the combined electrolyte and assembling the flexible solid electrolyte together with battery electrodes to construct a prototype of a full lithium battery.