The optimal battery design includes thick electrode structures. This increases the energy density, but the design suffers from poor lithium-ion transport, an important step in the functioning of these electrodes. Various enhancement techniques have been tried, including building vertically aligned channels or creating pores of the correct size to facilitate the transport of lithium ions.
Another approach involves the use of carbon fillers that conduct electricity. Three types of fillers were examined in this study: single-walled carbon nanotubes (SWCNTs), graphene nanosheets, and a substance called Super P, a type of soot particles that are produced when petroleum precursors are oxidized. Super P is the most commonly used conductive filler in lithium-ion batteries.
The fillers have been added to a type of electrode material known as NCM, which contains nickel, cobalt and manganese. The researchers examined the resulting composites using scanning electron microscopy. It was found that the Super P and NCM particles were arranged in a point-to-point contact mode.
However, the SWCNTs were wrapped around the NCM particles and formed a conductive coating. In addition, networks of interconnected SWCNTs were observed in the spaces between NCM particles. The graphene nanosheets were also wrapped around the NCM electrode particles, but not as evenly as the SWCNTs.
The SWCNTs turned out to be the most conductive filler for NCM electrodes.
“The measured conductivity is consistent with percolation theory … When an electrically conductive filler is added to an insulating matrix, there is a significant increase in conductivity as soon as the first conductive path is formed through the composite,” said Guihua Yu, one of the authors.
Since percolation requires a full path through the filler, a sufficient amount of conductive filler is needed. Therefore, the researchers considered different amounts of filler and found that the combination of NCM electrodes with only 0.16 wt% SWCNT gave good electrical conductivity. To achieve the same results, higher amounts of Super P and graphene were required.
Researchers used a variety of spectroscopic techniques, including Raman and X-ray absorption spectroscopy, to study the resulting composites.
“This is a joint effort by the Center for Mesoscale Transport Properties, an Energy Frontier Research Center supported by the US Department of Energy Basic Energy Sciences. Our results suggest that integrating SWCNTs into the NCM electrode will facilitate ion and charge transfer resulting in higher electrochemical utilization, especially at high discharge rates, “said Yu.
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