Lithium-ion batteries are hot commodities. They are rechargeable and portable, making them inseparable from the booming electronic and electric vehicle industries. Different research groups are working on making them better, addressing the most perceptible hurdle: charge capacity.
Lithium ion batteries have found increasing usage in recent times, finding use in everything from portable electronics to heavy transportation. While they have their own set of problems, they’re not unsolvable. And when they are solved, they’ll also have to find other reasons to persist in a market whose demands are soaring.
The simplest upgrade that can be mounted on it is to increase its charge capacity. It will then last longer per application, reducing the frequency of replacement. During charging, electrical energy from a chemical reaction is stored in a material, inside the battery. So, the battery’s charge capacity is this material’s charge capacity.
At the moment, the material is graphite. It is widely available and easy to handle. Replacing it without disrupting how a battery is made or in what conditions it has to be stored will be helpful. Thus, a material as ‘easy’ as graphite would be the ideal substitute. Like silicon.
Silicon v. graphite
Studies have shown that silicon has 400 times the charge capacity of graphite. It is abundantly available, very resilient to heat, and is easy to produce, store and dispose. However, there’s a big problem. “The lithium-silicon system has a much higher capacity than Li-graphite, but shows a strong volume change during charging and discharging,” said Dr. Thomas Fassler, Chair of Inorganic Chemistry, Technical University of Munich.
When charging, an external voltage is provided that overpowers the battery’s internal voltage, forcing lithium ions to migrate from the positive to the negative electrode, where they’re stored in the material in question. When discharging, the ions move out of the negative electrode and into the positive, generating a current that a connected appliance draws.
If the storage material at the negative electrode is made of silicon, lithium ions entering the silicon atomic lattice stretch the lattice, making it taut. With further charging, its volume could change, fracturing then breaking the lattice. At the same time, silicon’s abundance and ubiquity are enticing attributes for materials scientists.
Two recent studies, from June 4 and June 6, propose workarounds to this problem. The earlier one was from researchers in Stanford University, Yi Cui and Zhenan Bao, assisted by scientists from Tsinghua University, Beijing, and the University of Texas, Austin. Use silicon, they say, but bolster its ability to withstand expansion while charging.
The hydrogel bolster
“Our team has used silicon-hydrogel composites to replace carbon to increase charge storage capacity by many times,” said Dr. Yi Cui. He is the David Filo and Jerry Yang Faculty Scholar, Department of Materials Science and Engineering.
Using a process called in situ synthesis polymerization, they gave silicon nanoparticles a uniform coating of a hydrogel, which is a network of polyaniline polymer chains dispersed in water. This substance is porous and flexible yet strong. When lithium ions enter the silicon lattice, it expands into space created by the hydrogel pores while being held in place.
Cui and Bao also found that the network of polymer chains formed a pathway through which the lithium ions could be transported. At the same time, because the hydrogel contains water, with which lithium is highly reactive, the battery could be ignited if not handled properly.
For such a significant problem, the scientists found a very simple solution. “We baked the water off before sealing the battery,” Bao said.
Hard to make, hard to break
The second study, from June 6, was published in the Angewandte Chemie International Edition. Instead of the elegant and industrially reproducible hydrogel solution, Dr. Fassler, who led the study, synthesized a new, sophisticated material called lithium borosilicide. He’s calling it ‘tum’ after his university.
Tum is a unique material. It is as hard as diamond. Unlike the allotrope, however, the arrangement of molecules in the tum lattice forms channels, like tubes, throughout the crystal. This facilitates an increased storage of lithium ions as well as assists in their transportation.
About the choice of boron to go with silicon, Fassler said, “Intuition and extended experimental experience is necessary to find out the proper ratio of starting materials as well as the correct parameters.” To test their out-of-the-box solution, Fassler, and his student Michael Zeilinger, went to Arizona State University and used their high-pressure chemistry lab to apply 100,000 atmospheres of pressure and 900 degrees Celsius to synthesize tum.
They found that it was stable to air and moisture, and could withstand up to 800 degrees Celsius. However, they still don’t know what the charge capacity of this new compound is. “We will build a so-called electrochemical half-cell and test it versus elemental lithium,” Fassler said.
The synthesis mechanism is no doubt inhibiting. Such high pressures and temperatures required to produce industrially commensurate quantities of tum will clearly be incompatible with the ubiquity that lithium-ion batteries enjoy. Fassler is hopeful, though. “In case the electrochemical performance turns out good, chemists will look for other, cheaper, synthetic approaches,” he said.
Rethinking the battery
Another solution to increasing the performance of lithium-ion batteries was proposed at Oak Ridge National Laboratory (ORNL), Tennessee, in the first week of June.
Led by Chengdu Lian, the team reinvented the internal structure of the battery and replaced the liquid electrolyte with a solid, sulphur-based one. This eliminated the risk of flammability and increased the charge capacity of the setup by almost 100 times, but necessitated elevated temperatures to enhance the ionic conductivity of the materials.
Commenting on the ORNL solution, Yi Cui said, “Recently, high ionic conductivity of solid electrolytes was discovered, it looks promising down the road. However, the high inter-facial resistance at the solid-solid interface still needs to be addressed. Also, the new electrode materials have very large deadweight.” He added that the cyclic performance was good – at 300 charge-discharge cycles – but not outstanding.
A battery of power
As the Stanford team continues testing its hydrogel solution, and awaits commercial deployment, the Munich team will verify tum’s electrochemical capability, and the ORNL team will try to up its battery’s performance. These solutions are important for American because, in many other countries, the battery industry is a critical part of the economy. As The Economist is quick to detail, Japan, South Korea and China are great examples.
Knowing that rechargeable and portable sources of power will play a critical role in the then-emerging electronics industry, Japan invested big in lithium-ion batteries in the 1990s. Soon, South Korea and China followed suit. America, on the other hand, kept away because manufacturing these batteries provided low return on investment at a time when it only wanted its economy to grow. Now, it's playing catch up.
All because it didn’t see coming how lithium-ion batteries would become sources of power – electrochemical and economic.