Power/Performance Bits: Dec. 6

Tunable 2D semiconductors; preventing Li-ion battery fires.


Tunable 2D semiconductors
Researchers from the Singapore University of Technology and Design (SUTD), Hengyang Normal University, Nanjing University, National University of Singapore, and Zhejiang University identified a family of 2D semiconductors that could have lower resistance and enable further scaling.

“Due to the quantum tunnelling effect, shrinking a silicon-based transistor too small will lead to highly uncontrollable device behaviors,” said SUTD Assistant Professor Ang Yee Sin. “People are now looking for new materials beyond the ‘silicon era’, and 2D semiconductors are a promising candidate.”

“When you form a contact between metal and semiconductor, often there will be what we call a Schottky barrier,” continued Ang. “In order to force electricity through this barrier, you need to apply a strong voltage, which wastes electricity and generates waste heat.”

The team investigated Ohmic contacts, or metal-semiconductor contacts with no Schottky barrier. The found that a recently discovered family of 2D semiconductors, namely MoSi2N4 and WSi2N4, form Ohmic contacts with the metals titanium, scandium, and nickel. The new materials are free from Fermi level pinning (FLP), a problem that severely limits the application potential of other 2D semiconductors.

“FLP is an adverse effect that happens in many metal-semiconductor contacts and is caused by defects and complex materials interactions at the contact interface,” Ang said. “Such an effect ‘pins’ the electrical properties of the contact to a narrow range regardless of the metal used in the contact.”

In the study, they found that MoSi2N4 and WSi2N4 are naturally protected from FLP due to an inert Si-N outer layer that shields the underlying semiconducting layer from defects and material interactions at the contact interface. Because of this protection, the Schottky barrier is ‘unpinned’ and can be tuned to match a wide array of application requirements.

The researchers hope the work will prompt more investigation of this 2D semiconductor family for interesting properties. “Some of them might be very poor in terms of electronics applications but very good for spintronics, photocatalysts or as a building block for solar cells,” said Ang. “Our next challenge is to systematically scan through all of these 2D materials and categorize them according to their potential applications.”

Preventing Li-ion battery fires
Materials scientists from Nanyang Technological University propose a way to prevent internal short circuits in lithium-ion batteries, which are a major cause of battery fires.

Internal short circuits occur when dendrites, which are thin metal whiskers formed from the build up of lithium deposits, cross the separator between the cathode and anode electrodes of the battery when it is being charged.

To prevent this, the team developed an additional “anti-short layer” on the separator that can keep a dendrite from reaching the cathode.

“We know that for a Li-ion battery to work, Li-ions must be able to travel between the positive and negative sides during charge and discharge cycles,” explained Professor Xu Zhichuan of NTU, who is also the Cluster Director of Energy Storage and Renewables & Low Carbon Generation: Solar, at the Energy Research Institute @ NTU. “However, the transfer of the Li-ions also means the formation of dendrites is inevitable for current commercial Li-ion batteries.

“Instead of preventing the formation of dendrites, we decided to make use of their intrinsic properties by coating an additional layer of conductive material on the separator for these dendrites to connect with. Once the dendrites make the connection it will not be able to continue their growth further, thus preventing them from ever reaching the other side.”

The researchers tested the technology in the laboratory on over 50 cells with different Li-ion battery compositions and no short-circuits were detected during the charging phase even when the battery cells were used beyond their lifecycle.

They said that the anti-short layer is a common material used in battery manufacturing and can be easily integrated into the current separator manufacturing process. The team estimates that the cost increase after adopting this technology would be around 5% more than the existing production cost of a Li-ion battery.

It is now patent-pending and is being commercialized by NTUitive, NTU’s innovation and enterprise company. The researchers said that several battery companies have expressed interest in the anti-short layer.

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